초록 ▼

A control apparatus, a control method, and an engine control unit are provided for controlling an output of a controlled object which has a relatively large response delay and/or dead time to rapidly and accurately converge to a target value. When the output of the controlled object is chosen to be

A control apparatus, a control method, and an engine control unit are provided for controlling an output of a controlled object which has a relatively large response delay and/or dead time to rapidly and accurately converge to a target value. When the output of the controlled object is chosen to be that of an air/fuel ratio sensor in an internal combustion engine, the output of the air/fuel ratio sensor can be controlled to rapidly and accurately converge to a target value even in an extremely light load operation mode.

대표청구항 ▼

1. A control apparatus comprising:deviation calculating means for calculating a deviation of an output of a controlled object from a predetermined target value; and control input calculating means for calculating a control input to said controlled object based on one modulation algorithm selected fr

1. A control apparatus comprising:deviation calculating means for calculating a deviation of an output of a controlled object from a predetermined target value; and control input calculating means for calculating a control input to said controlled object based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm for converging the output of said controlled object to said target value in accordance with the calculated deviation; wherein said control input calculating means calculates a first intermediate value in accordance with said deviation based on said one modulation algorithm, and calculates said control input based on a product of said calculated first intermediate value and a predetermined gain. 2. A control apparatus according to claim 1, further comprising:gain parameter detecting means for detecting a gain parameter indicative of a gain characteristic of said controlled object; and gain setting means for setting said gain in accordance with the detected gain parameter. 3. A control apparatus according to claim 1, wherein said control input calculating means calculates a second intermediate value in accordance with said deviation based on said one modulation algorithm, and calculates said control input by adding a predetermined value to said calculated second intermediate value.4. A control apparatus according to claim 1, wherein:said deviation calculating means includes predicted value calculating means for calculating a predicted value of said deviation in accordance with said deviation based on a prediction algorithm, wherein said control input calculating means calculates said control input in accordance with the calculated predicted value of said deviation based on said one modulation algorithm. 5. A control apparatus according to claim 1, wherein:said control input calculating means calculates said control input in accordance with said deviation based further on a controlled object model which models said controlled object. 6. A control apparatus according to claim 5, further comprising identifying means for identifying a model parameter for said controlled object model in accordance with one of said calculated control input and a value reflecting the control input inputted to said controlled object, and an output of said controlled object.7. A control apparatus according to claim 6, wherein:said controlled object model comprises a discrete time system model, wherein said identifying means identifies a model parameter for said discrete time system model in accordance with one of discrete data of said control input and discrete data of the value reflecting the control input inputted to said controlled object, and discrete data of the output of said controlled object. 8. A control apparatus according to claim 5, further comprising:dynamic characteristic parameter detecting means for detecting a dynamic characteristic parameter indicative of a change in a dynamic characteristic of said controlled object; and model parameter setting means for setting a model parameter for said controlled object model in accordance with the detected dynamic characteristic parameter. 9. A control apparatus comprising:deviation calculating means for calculating a deviation of an output of a controlled object from a predetermined target value; first control input calculating means for calculating a control input to said controlled object based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm for converging the output of said controlled object to said target value in accordance with the calculated deviation; second control input calculating means for calculating a control input to said controlled object in accordance with the calculated deviation based on a response specifying control algorithm for converging the output of said controlled object to said target value; controlled object state detecting means for detecting a state of said controlled object; and control input selecting means for selecting one of the control input calculated by said first control input calculating means and he control input calculated by said second control input calculating means in accordance with the detected state of said controlled object as a control input which should be inputted to said controlled object. 10. A control apparatus according to claim 9, wherein said first control input calculating means calculates a first intermediate value in accordance with said deviation based on said one modulation algorithm, and calculates said control input based on a product of said calculated first intermediate value and a predetermined gain.11. A control apparatus according to claim 10, wherein:said controlled object state detecting means includes gain parameter detecting means for detecting a gain parameter indicative of a gain characteristic of said controlled object, and said control apparatus further comprises gain setting means for setting said gain in accordance with the detected gain parameter. 12. A control apparatus according to claim 9, wherein said first control input calculating means calculates a second intermediate value in accordance with said deviation based on said one modulation algorithm, and calculates said control input by adding a predetermined value to said calculated second intermediate value.13. A control apparatus according to claim 9, wherein said deviation calculating means includes predicted value calculating means for calculating a predicted value of said deviation in accordance with said deviation based on a prediction algorithm,wherein said first control input calculating mans calculates said control input in accordance with the calculated predicted value of said deviation based on said one modulation algorithm; and said second control input calculating means calculates said control input in accordance with the calculated predicted value of said deviation based on said response specifying control algorithm. 14. A control apparatus according to claim 9, wherein said first control input calculating means and said second control input calculating means calculate said control input in accordance with said deviation based further on a controlled object model which models said controlled object.15. A control apparatus according to claim 14, further comprising identifying means for identifying a model parameter for said controlled object model in accordance with one of said calculated control input and a value reflecting the control input inputted to said controlled object, and an output of said controlled object.16. A control apparatus according to claim 15, wherein:said controlled object model comprises a discrete time system model, wherein said identifying means identifies a model parameter for said discrete time system model in accordance with one of discrete data of said control input and discrete data of the value reflecting the control input inputted to said controlled object, and discrete data of the output of said controlled object. 17. A control apparatus according to claim 14, further comprising:dynamic characteristic parameter detecting mans for detecting a dynamic characteristic parameter indicative of a change in a dynamic characteristic of said controlled object; and model parameter setting means for setting a model parameter for said controlled object model in accordance with the detected dynamic characteristic parameter. 18. A control apparatus according to claim 9, wherein said response specifying control algorithm is a sliding mode control algorithm.19. A control apparatus comprising:a downstream air/fuel ratio sensor for outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyzer in an exhaust pipe of an internal combustion engine; output deviation calculating means for calculating an output deviation of an output of said downstream air/fuel ratio sensor from a predetermined target value; and air/fuel ratio control means for controlling an air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value. 20. A control apparatus according to claim 19, wherein said air/fuel ratio control means includes:air/fuel ratio calculating means for calculating a first intermediate value in accordance with said output deviation based on said one modulation algorithm, and calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value based on a product of said calculated first intermediate value and a predetermined gain, wherein said air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 21. A control apparatus according to claim 20, further comprising:operating condition parameter detecting means for detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and gain setting means for setting said gain in accordance with the detected operating condition parameter. 22. A control apparatus according to claim 19, wherein:said air/fuel ratio control means includes air/fuel ratio calculating means for calculating a second intermediate value in accordance with said output deviation based on said one modulation algorithm, and calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value by adding a predetermined value to the calculated second intermediate value, wherein said air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 23. A control apparatus according to claim 19, wherein:said output deviation calculating means includes predicted value calculating means for calculating a predicted value of said output deviation in accordance with said output deviation based on a prediction algorithm, wherein said air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said one modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value. 24. A control apparatus according to claim 19, wherein:said air/fuel ratio control means includes air/fuel ratio calculating means for calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor, wherein said air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 25. A control apparatus according to claim 24, further comprising identifying means for identifying a model parameter for said controlled object model in accordance with said target air/fuel ratio and the output of said downstream air/fuel ratio sensor.26. A control apparatus according to claim 25, wherein:said controlled object model comprises a discrete time system model, wherein said identifying means identifies a model parameter for said discrete time system model in accordance with discrete data of said target air/fuel ratio, and discrete data of the output of said downstream air/fuel ratio sensor. 27. A control apparatus according to claim 24, further comprising:operating condition parameter detecting means for detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and model parameter setting means for setting a model parameter for said controlled object model in accordance with the detected operating condition parameter. 28. A control apparatus according to claim 19, further comprising:an upstream air/fuel ratio sensor for outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of said catalyzer in said exhaust pipe of said internal combustion engine, wherein said air/fuel ratio control means includes air/fuel ratio calculating means for calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor, and said air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture supplied to said internal combustion engine in accordance with the calculated target air/fuel ratio. 29. A control apparatus according to claim 28, further comprising identifying means for identifying a model parameter for said controlled object model in accordance with one of said target air/fuel ratio and the output of said upstream air/fuel ratio sensor, and the output of said downstream air/fuel ratio sensor.30. A control apparatus according to claim 29, wherein:said controlled object model comprises a discrete time system model, wherein said identifying means identifies the model parameter for said discrete time system model in accordance with one of discrete data of said target air/fuel ratio and discrete data of the output of said upstream air/fuel ratio sensor, and discrete data of the output of said downstream air/fuel ratio sensor. 31. A control apparatus according to claim 28, further comprising:operating condition parameter detecting means for detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and model parameter setting means for setting a model parameter for said controlled object model in accordance with the detected operating condition parameter. 32. A control apparatus comprising:a downstream air/fuel ratio sensor for outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyzer in an exhaust pipe of an internal combustion engine; output deviation calculating means for calculating an output deviation of an output of said downstream air/fuel ratio sensor from a predetermined target value; first air/fuel ratio control means for controlling an air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; second air/fuel ratio control means for controlling the air/fuel ratio of the air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on a response specifying control algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; operating condition parameter detecting means for detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and selecting means for selecting one of said first air/fuel ratio control means and said second air/fuel ratio control means in accordance with the detected operating condition parameter to control the air/fuel ratio of the air/fuel mixture. 33. A control apparatus according to claim 32, wherein said selecting means selects one of said first air/fuel ratio control means and said second air/fuel ratio control means in accordance with at least one of whether or not said catalyzer has a total amount of catalyst loaded therein equal to or larger than a predetermined load amount, and whether or not a carrier for carrying said catalyst in said catalyzer has a length in a direction in which said exhaust pipe extends equal to or longer than a predetermined length.34. A control apparatus according to claim 32, wherein:said selecting means includes operation mode discriminating means for discriminating an operation mode of said internal combustion engine in accordance with said operating condition parameter, and said selecting means selects said first air/fuel ratio control means when the discriminated operation mode is a predetermined first operation mode, and selects said second air/fuel ratio control means when the discriminated operation mode is a predetermined second operation mode. 35. A control apparatus according to claim 32, wherein:said catalyzer comprises a plurality of catalyzers disposed along said exhaust pipe of said internal combustion engine and spaced apart from each other, and said selecting means includes operation mode discriminating means for discriminating an operation mode of said internal combustion engine in accordance with said operating condition parameter, wherein said selecting means selects said first air/fuel ratio control means when the discriminated operation mode is a predetermined first operation mode, and selects said second air/fuel ratio control means when the discriminated operation mode is a predetermined second operation mode. 36. A control apparatus according to claim 32, wherein:said internal combustion engine comprises a downstream catalyzer in said exhaust pipe at a location downstream of said downstream air/fuel ratio sensor, and said selecting means includes operation mode discriminating means for discriminating an operation mode of said internal combustion engine in accordance with said operating condition parameter, wherein said selecting means selects said first air/fuel ratio control means when the discriminated operation mode is a predetermined first operation mode, and selects said second air/fuel ratio control means when the discriminated operation mode is a predetermined second operation mode. 37. A control apparatus according to claim 36, wherein:the total amount of catalyst loaded in said downstream catalyzer is set smaller than the total amount of catalyst loaded in said catalyzer, and said target value is set to provide the highest purification percentage for said catalyzer. 38. A control apparatus according to claim 34, wherein said first operation mode includes a transient operation mode of said internal combustion engine.39. A control apparatus according to claim 34, wherein said first operation mode includes an idle operation mode.40. A control apparatus according to claim 34, wherein said second operation mode includes a steady-state operation mode in which said internal combustion engine is loaded substantially in a steady-state.41. A control apparatus according to claim 32, wherein:said first air/fuel ratio control means includes first air/fuel ratio calculating means for calculating a first intermediate value in accordance with said output deviation based on said one modulation algorithm, and calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value based on a product of said calculated first intermediate value and a predetermined gain, wherein said first air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 42. A control apparatus according to claim 41, further comprising gain setting means for setting said gain in accordance with said operating condition parameter.43. A control apparatus according to claim 32, wherein:said first air/fuel ratio control means include first air/fuel ratio calculating means for calculating a second intermediate value in accordance with said output deviation based on said one modulation algorithm, and calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value by adding a predetermined value to the calculated second intermediate value, and said first air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 44. A control apparatus according to claim 32, wherein:said output deviation calculating means includes predicted value calculating means for calculating a predicted value of said output deviation in accordance with said output deviation based on a prediction algorithm, wherein said first air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said one modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value, and said second air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said response specifying control algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value. 45. A control apparatus according to claim 32, wherein:said first air/fuel ratio control means includes first air/fuel ratio calculating means for calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, ad a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor, wherein said first air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio, and said second air/fuel ratio control means includes second air/fuel ratio calculating means for calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said response specifying control algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor, wherein said second air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 46. A control apparatus according to claim 45, further comprising identifying means for identifying a model parameter for said controlled object model in accordance with said target air/fuel ratio and the output of said downstream air/fuel ratio sensor.47. A control apparatus according to claim 46, wherein:said controlled object model comprises a discrete time system model, wherein said identifying means identifies the model parameter for said discrete time system model in accordance with discrete data of said target air/fuel ratio and discrete data of the output of said downstream air/fuel ratio sensor. 48. A control apparatus according to claim 45, further comprising model parameter setting means for setting the model parameter for said controlled object model in accordance with said operating condition parameter.49. A control apparatus according to claim 32, further comprising:an upstream air/fuel ratio sensor for outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location upstream of said catalyzer in said exhaust pipe of said internal combustion engine, said first air/fuel ratio control means includes first air/fuel ratio calculating means for calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor, wherein said first air/fuel ratio control means controls he air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio, and said second air/fuel ratio control means includes second air/fuel ratio calculating means for calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said response specifying control algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor, wherein said second air/fuel ratio control means controls the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 50. A control apparatus according to claim 49, further comprising identifying means for identifying the model parameter for said controlled object model in accordance with one of said target air/fuel ratio and the output of said upstream air/fuel ratio sensor, and the output of said downstream air/fuel ratio sensor.51. A control apparatus according to claim 50, wherein:said controlled object model comprises a discrete time system model, wherein said identifying means identifies the model parameter for said discrete time system model in accordance with one of discrete data of said target air/fuel ratio and discrete data of the output of said upstream air/fuel ratio sensor, and discrete data of the output of said downstream air/fuel ratio sensor. 52. A control apparatus according to claim 49, further comprising a model parameter setting means for setting a model parameter for said controlled object model in accordance with said operating condition parameter.53. A control apparatus according to claim 32, wherein said response specifying control algorithm is a sliding mode control algorithm.54. A control method comprising the steps of:calculating a deviation of an output of a controlled object from a predetermined target value; and calculating a control input to said controlled object based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm for converging the output of said controlled object to said target value in accordance with the calculated deviation; wherein said step of calculating a control input includes calculating a first intermediate value in accordance with said deviation based on said one modulation algorithm, and calculating said control input based on a product of said calculated first intermediate value and a predetermined gain. 55. A control method according to claim 54, further comprising the step of:detecting a gain parameter indicative of a gain characteristic of said controlled object; and setting said gain in accordance with the detected gain parameter. 56. A control method according to claim 54, wherein said step of calculating a control input includes calculating a second intermediate value in accordance with said deviation based on said one modulation algorithm, and calculating said control input by adding a predetermined value to said calculated second intermediate value.57. A control method according to claim 54, wherein:said step of calculating a deviation includes calculating a predicted value of said deviation in accordance with said deviation based on a prediction algorithm, and said step of calculating a control input includes calculating said control input in accordance with the calculated predicted value of said deviation based on said one modulation algorithm. 58. A control method according to claim 54, wherein:said step of calculating a control input includes calculating said control input in accordance with said deviation based further on a controlled object model which models said controlled object. 59. A control method according to claim 58, further comprising the step of identifying a model parameter for said controlled object model in accordance with one of said calculated control input and a value reflecting the control input inputted to said controlled object, and an output of said controlled object.60. A control method according to claim 59, wherein:said controlled object model comprises a discrete time system model, wherein said step of identifying includes identifying a model parameter for said discrete time system model in accordance with one of discrete data of said control input and discrete data of the value reflecting the control input inputted to said controlled object, and discrete data of the output of said controlled object. 61. A control method according to claim 58, further comprising the steps of:detecting a dynamic characteristic parameter indicative of a change in a dynamic characteristic of said controlled object; and setting a model parameter for said controlled object model in accordance with the detected dynamic characteristic parameter. 62. A control method comprising the steps of:calculating a deviation of an output of a controlled object from a predetermined target value; calculating a control input to said controlled object based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm for converging the output of said controlled object to said target value in accordance with the calculated deviation; calculating a control input to said controlled object in accordance with the calculated deviation based on a response specifying control algorithm for converging the output of said controlled object to said target value; detecting a state of said controlled object; and selecting one of the control input calculated based on the one modulation algorithm and the control input calculated based on the response specifying control algorithm in accordance with the detected state of said controlled object as a control input which should be inputted to said controlled object. 63. A control method according to claim 62, wherein said step of calculating a control input to said controlled object based on one modulation algorithm includes calculating a first intermediate value in accordance with said deviation based on said one modulation algorithm, and calculating said control input based on a product of said calculated first intermediate value and a predetermined gain.64. A control method according to claim 63, wherein:said step of detecting a controlled object state includes detecting a gain parameter indicative of a gain characteristic of said controlled object, and said control method further comprises the step of setting said gain in accordance with the detected gain parameter. 65. A control method according to claim 62, wherein said step of calculating a control input to said controlled object based on one modulation algorithm includes calculating a second intermediate value in accordance with said deviation based on said one modulation algorithm, and calculating said control input by adding a predetermined value to said calculated second intermediate value.66. A control method according to claim 62, wherein said step of calculating a deviation includes calculating a predicted value of said deviation in accordance with said deviation based on a prediction algorithm,wherein said step of calculating a control input to said controlled object based on one modulation algorithm includes calculating said control input in accordance with the calculated predicted value of said deviation based on said one modulation algorithm; and said step of calculating a control input to said controlled object based on a response specifying control algorithm includes calculating said control input in accordance with the calculated predicted value of said deviation based on said response specifying control algorithm. 67. A control method according to claim 62, wherein:said step of calculating a control input to said controlled object based on one modulation algorithm includes calculating said control input in accordance with said deviation based further on a controlled object model which models said controlled object; and said step of calculating a control input to said controlled object based on a response specifying control algorithm includes calculating said control input in accordance with said deviation based further on a controlled object model which models said controlled object. 68. A control method according to claim 67, further comprising the step of identifying a model parameter for said controlled object model in accordance with one of said calculated control input and a value reflecting the control input inputted to said controlled object, and an output of said controlled object.69. A control method according to claim 68, wherein:said controlled object model comprises a discrete time system model, wherein said step of identifying includes identifying a model parameter for said discrete time system model in accordance with one of discrete data of said control input and discrete data of the value reflecting the control input inputted to said controlled object, and discrete data of the output of said controlled object. 70. A control method according to claim 67, further comprising the steps of:detecting a dynamic characteristic parameter indicative of a change in a dynamic characteristic of said controlled object; and setting a model parameter for said controlled object model in accordance with the detected dynamic characteristic parameter. 71. A control method according to claim 62, wherein said response specifying control algorithm is a sliding mode control algorithm.72. A control method comprising the steps of:outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyzer in an exhaust pipe of an internal combustion engine; calculating an output deviation of an output of said downstream air/fuel ratio sensor from a predetermined target value; and controlling an air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value. 73. A control method according to claim 72, wherein said step of controlling an air/fuel ratio includes:calculating a first intermediate value in accordance with said output deviation based on said one modulation algorithm; calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value based on a product of said calculated first intermediate value and a predetermined gain; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 74. A control method according to claim 73, further comprising the steps of:detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and setting said gain in accordance with the detected operating condition parameter. 75. A control method according to claim 72, wherein said step of controlling an air/fuel ratio includes:calculating a second intermediate value in accordance with said output deviation based on said one modulation algorithm; calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value by adding a predetermined value to the calculated second intermediate value; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 76. A control method according to claim 72, wherein:said step of calculating an output deviation includes calculating a predicted value of said output deviation in accordance with said output deviation based on a prediction algorithm, and said step of controlling an air/fuel ratio includes controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said one modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value. 77. A control method according to claim 72, wherein:said step of controlling an air/fuel ratio includes calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 78. A control method according to claim 77, further comprising the step of identifying a model parameter for said controlled object model in accordance with said target air/fuel ratio and the output of said downstream air/fuel ratio sensor.79. A control method according to claim 78, wherein:said controlled object model comprises a discrete time system model, wherein said step of identifying includes identifying a model parameter for said discrete time system model in accordance with discrete data of said target air/fuel ratio, and discrete data of the output of said downstream air/fuel ratio sensor. 80. A control method according to claim 77, further comprising the steps of:detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and setting a model parameter for said controlled object model in accordance with the detected operating condition parameter. 81. A control method according to claim 72, further comprising the step of:outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of said catalyzer in said exhaust pipe of said internal combustion engine, wherein said step of controlling an air/fuel ratio includes: calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and controlling the air/fuel ratio of the air/fuel mixture supplied to said internal combustion engine in accordance with the calculated target air/fuel ratio. 82. A control method according to claim 81, further comprising the step of identifying a model parameter for said controlled object model in accordance with one of said target air/fuel ratio and the output of said upstream air/fuel ratio sensor, and the output of said downstream air/fuel ratio sensor.83. A control method according to claim 82, wherein:said controlled object model comprises a discrete time system model, wherein said step of identifying includes identifying the model parameter for said discrete time system model in accordance with one of discrete data of said target air/fuel ratio and discrete data of the output of said upstream air/fuel ratio sensor, and discrete data of the output of said downstream air/fuel ratio sensor. 84. A control method according to claim 81, further comprising the steps of:detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and setting a model parameter for said controlled object model in accordance with the detected operating condition parameter. 85. A control method comprising the steps of:outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyzer in an exhaust pipe of an internal combustion engine; calculating an output deviation of an output of said downstream air/fuel ratio sensor from a predetermined target value; controlling an air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; controlling the air/fuel ratio of the air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on a response specifying control algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; detecting an operating condition parameter indicative of an operating condition of said internal combustion engine; and selecting one of the air/fuel ratio controlled based on the one modulation algorithm and the air/fuel ratio controlled based on the response specifying control algorithm in accordance with the detected operating condition parameter to control the air/fuel ratio of the air/fuel mixture. 86. A control method according to claim 85, wherein said step of selecting includes selecting one of the air/fuel ratio controlled based on the one modulation algorithm and the air/fuel ratio controlled based on the response specifying control algorithm in accordance with at least one of whether or not said catalyzer has a total amount of catalyst loaded therein equal to or larger than a predetermined load amount, and whether or not a carrier for carrying said catalyst in said catalyzer has a length in a direction in which said exhaust pipe extends equal to or longer than a predetermined length.87. A control method according to claim 85, wherein said step of selecting includes:discriminating an operation mode of said internal combustion engine in accordance with said operating condition parameter; and selecting the air/fuel ratio controlled based on the one modulation algorithm when the discriminated operation mode is a predetermined first operation mode, and selecting the air/fuel ratio controlled based on the response specifying control algorithm when the discriminated operation mode is a predetermined second operation mode. 88. A control method according to claim 85, wherein:said catalyzer comprises a plurality of catalyzers disposed along said exhaust pipe of said internal combustion engine and spaced apart from each other, and said step of selecting includes discriminating an operation mode of said internal combustion engine in accordance with said operating condition parameter; and selecting the air/fuel ratio controlled based on the one modulation algorithm when the discriminated operation mode is a predetermined first operation mode, and selecting the air/fuel ratio controlled based on the response specifying control algorithm when the discriminated operation mode is a predetermined second operation mode. 89. A control method according to claim 85, wherein:said internal combustion engine comprises a downstream catalyzer in said exhaust pipe at a location downstream of said downstream air/fuel ratio sensor, and said step of selecting includes: discriminating an operation mode of said internal combustion engine in accordance with said operating condition parameter; and selecting the air/fuel ratio controlled based or the one modulation algorithm when the discriminated operation mode is a predetermined first operation mode, and selecting the air/fuel ratio controlled based on the response specifying control algorithm when the discriminated operation mode is a predetermined second operation mode. 90. A control method according to claim 89, wherein:the total amount of catalyst loaded in said downstream catalyzer is set smaller than the total amount of catalyst loaded in said catalyzer, and said target value is set to provide the highest purification percentage for said catalyzer. 91. A control method according to claim 87, wherein said first operation mode includes a transient operation mode of said internal combustion engine.92. A control method according to claim 87, wherein said first operation mode includes an idle operation mode.93. A control method according to claim 87, wherein said second operation mode includes a steady-state operation mode in which said internal combustion engine is loaded substantially in a steady-state.94. A control method according to claim 85, wherein said step of controlling an air/fuel ratio based on one modulation algorithm includes:calculating a first intermediate value in accordance with said output deviation based on said one modulation algorithm; calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value based on a product of said calculated first intermediate value and a predetermined gain; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 95. A control method according to claim 94, further comprising the step of setting said gain in accordance with said operating condition parameter.96. A control method according to claim 85, wherein said step of controlling an air/fuel ratio control based on one modulation algorithm includes:calculating a second intermediate value in accordance with said output deviation based on said one modulation algorithm; calculating a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value by adding a predetermined value to the calculated second intermediate value; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 97. A control method according to claim 85, wherein:said step of calculating an output deviation includes calculating a predicted value of said output deviation in accordance with said output deviation based on a prediction algorithm; said step of controlling an air/fuel ratio based on one modulation algorithm includes controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said one modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; and said step of controlling the air/fuel ratio based on a response specifying control algorithm includes controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said response specifying control algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value. 98. A control method according to claim 85, wherein:said step of controlling an air/fuel ratio based pn one modulation algorithm includes: calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, aid a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio, and said step of controlling the air/fuel ratio control based on a response specifying control algorithm includes: calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said response specifying control algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 99. A control method according to claim 98, further comprising the step of identifying a model parameter for said controlled object model in accordance with said target air/fuel ratio and the output of said downstream air/fuel ratio sensor.100. A control method according to claim 99, wherein:said controlled object model comprises a discrete time system model, wherein said step of identifying includes identifying the model parameter for said discrete time system model in accordance with discrete data of said target air/fuel ratio and discrete data of the output of said downstream air/fuel ratio sensor. 101. A control method according to claim 98, further comprising the step of setting the model parameter for said controlled object model in accordance with said operating condition parameter.102. A control method according to claim 85, further comprising the step of outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location upstream of said catalyzer in said exhaust pipe of said internal combustion engine,wherein said step of controlling an air/fuel ratio based on one modulation algorithm includes: calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio, and said step of controlling the air/fuel ratio based on a response specifying control algorithm includes: calculating said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said response specifying control algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and controlling the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio. 103. A control method according to claim 102, further comprising the step of identifying the model parameter for said controlled object model in accordance with one of said target air/fuel ratio and the output of said upstream air/fuel ratio sensor, and the output of said downstream air/fuel ratio sensor.104. A control method according to claim 103, wherein:said controlled object model comprises a discrete time system model, wherein said step of identifying includes identifying the model parameter for said discrete time system model in accordance with one of discrete data of said target air/fuel ratio and discrete data of the output of said upstream air/fuel ratio sensor, and discrete data of the output of said downstream air/fuel ratio sensor. 105. A control method according to claim 102, further comprising the step of setting a model parameter for said controlled object model in accordance with said operating condition parameter.106. A control method according to claim 85, wherein said response specifying control algorithm is a sliding mode control algorithm.107. An engine control unit includinga control program for causing a computer to calculate a deviation of an output of a controlled object from a predetermined target value; and calculate a control input to said controlled object based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm for converging the output of said controlled object to said target value in accordance with the calculated deviation wherein said control program further causes the computer to calculate a first intermediate value in accordance with said deviation based on said one modulation algorithm, and calculate said control input based on a product of said calculated first intermediate value and a predetermined gain. 108. An engine control unit according to claim 107, wherein said control program further causes the computer to detect a gain parameter indicative of a gain characteristic of said controlled object; and set said gain in accordance with the detected gain parameter.109. An engine control unit according to claim 107, wherein said control program causes the computer to calculate a second intermediate value in accordance with said deviation based on said one modulation algorithm, and calculating said control input by adding a predetermined value to said calculated second intermediate value.110. An engine control unit according to claim 107, wherein said control program causes the computer to calculate a predicted value of said deviation in accordance with said deviation based on a prediction algorithm; and calculate said control input in accordance with the calculated predicted value of said deviation based on said one modulation algorithm.111. An engine control unit according to claim 107, wherein said control program causes the computer to calculate said control input in accordance with said deviation based further on a controlled object model which models said controlled object.112. An engine control unit according to claim 111, wherein said control program further causes the computer to identify a model parameter for said controlled object model in accordance with one of said calculated control input and a value reflecting the control input inputted to said controlled object, and an output of said controlled object.113. An engine control unit according to claim 112, wherein:said controlled object model comprises a discrete time system model, wherein said control program causes the computer to identify a model parameter for said discrete time system model in accordance with one of discrete data of said control input and discrete data of the value reflecting the control input inputted to said controlled object, and discrete data of the output of said controlled object. 114. An engine control unit according to claim 111, wherein said control program further causes the computer to detect a dynamic characteristic parameter indicative of a change in a dynamic characteristic of said controlled object; and set a model parameter for said controlled object model in accordance with the detected dynamic characteristic parameter.115. An engine control unit including a control program for causing a computer to calculate a deviation of an output of a controlled object from a predetermined target value; calculate a control input to said controlled object based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm for converging the output of said controlled object to said target value in accordance with the calculated deviation; calculate a control input to said controlled object in accordance with the calculated deviation based on a response specifying control algorithm for converging the output of said controlled object to said target value; detect a state of said controlled object; and select one of the control input calculated based on the one modulation algorithm and the control input calculated based on the response specifying control algorithm in accordance with the detected state of said controlled object as a control input which should be inputted to said controlled object.116. An engine control unit according to claim 115, wherein said control program causes the computer to calculate a first intermediate value in accordance with said deviation based on said one modulation algorithm, and calculate said control input based on a product of said calculated first intermediate value and a predetermined gain.117. An engine control unit according to claim 116, wherein said control program causes the computer to detect a controlled object state includes detecting a gain parameter indicative of a gain characteristic of said controlled object, and set said gain in accordance with the detected gain parameter.118. An engine control unit according to claim 115, wherein said control program causes the computer to calculate a second intermediate value in accordance with said deviation based on said one modulation algorithm, and calculate said control input by adding a predetermined value to said calculated second intermediate value.119. An engine control unit according to claim 115, wherein said control program causes the computer to calculate a predicted value of said deviation in accordance with said deviation based on a prediction algorithm; calculate said control input in accordance with the calculated predicted value of said deviation based on said one modulation algorithm; and calculate a control input to said controlled object based on a response specifying control algorithm includes calculating said control input in accordance with the calculated predicted value of said deviation based on said response specifying control algorithm.120. An engine control unit according to claim 115, wherein said control program causes the computer to calculate said control input in accordance with said deviation based on said one modulation algorithm, and a controlled object model which models said controlled object; and calculate said control input in accordance with said deviation based on said response specifying control algorithm, and a controlled object model which models said controlled object.121. An engine control unit according claim 120, wherein said control program further causes the computer to identify a model parameter for said controlled object model in accordance with one of said calculated control input and a value reflecting the control input inputted to said controlled object, and an output of said controlled object.122. An engine control unit according to claim 121, wherein:said controlled object model comprises a discrete time system model, wherein said control program causes the computer to identify a model parameter for said discrete time system model in accordance with one of discrete data of said control input and discrete data of the value reflecting the control input inputted to said controlled object, and discrete data of the output of said controlled object. 123. An engine control unit according to claim 120, wherein said control program further causes the computer to detect a dynamic characteristic parameter indicative of a change in a dynamic characteristic of said controlled object; and set a model parameter for said controlled object model in accordance with the detected dynamic characteristic parameter.124. An engine control unit according to claim 115, wherein said response specifying control algorithm is a sliding mode control algorithm.125. An engine control unit including a control program for causing a computer to output a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyzer in an exhaust pipe of an internal combustion engine; calculate an output deviation of an output of said downstream air/fuel ratio sensor from a predetermined target value; and control an air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value.126. An engine control unit according to claim 125, wherein said control program causes the computer to calculate a first intermediate value in accordance with said output deviation based on said one modulation algorithm, and calculate a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value based on a product of said calculated first intermediate value and a predetermined gain; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio.127. An engine control unit according to claim 126, wherein said control program further causes the computer to detect an operating condition parameter indicative of an operating condition of said internal combustion engine; and set said gain in accordance with the detected operating condition parameter.128. An engine control unit according to claim 125, wherein said control program causes the computer to calculate a second intermediate value in accordance with said output deviation based on said one modulation algorithm; calculate a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value by adding a predetermined value to the calculated second intermediate value; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio.129. An engine control unit according to claim 125, wherein said control program causes the computer to calculate a predicted value of said output deviation in accordance with said output deviation based on a prediction algorithm; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said one modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value.130. An engine control unit according to claim 125, wherein said control program causes the computer to calculate said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio.131. An engine control unit according to claim 130, wherein said control program further causes the computer to identify a model parameter for said controlled object model in accordance with said target air/fuel ratio and the output of said downstream air/fuel ratio sensor.132. An engine control unit according to claim 131, wherein:said controlled object model comprises a discrete time system model, wherein said control program causes the computer to identify a model parameter for said discrete time system model in accordance with discrete data of said target air/fuel ratio, and discrete data of the output of said downstream air/fuel ratio sensor. 133. An engine control unit according to claim 130, wherein said control program further causes the computer to detect an operating condition parameter indicative of an operating condition of said internal combustion engine; and set a model parameter for said controlled object model in accordance with the detected operating condition parameter.134. An engine control unit according to claim 125, wherein said control program further causes the computer to output a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of said catalyzer in said exhaust pipe of said internal combustion engine; calculate said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and control the air/fuel ratio of the air/fuel mixture supplied to said internal combustion engine in accordance with the calculated target air/fuel ratio.135. An engine control unit according to claim 134, wherein said control program further causes the computer to identify a model parameter for said controlled object model in accordance with one of said target air/fuel ratio and the output of said upstream air/fuel ratio sensor, and the output of said downstream air/fuel ratio sensor.136. An engine control unit according to claim 135, wherein:said controlled object model comprises a discrete time system model, wherein said control program causes the computer to identify the model parameter for said discrete time system model in accordance with one of discrete data of said target air/fuel ratio and discrete data of the output of said upstream air/fuel ratio sensor, and discrete data of the output of said downstream air/fuel ratio sensor. 137. An engine control unit according to claim 134, wherein said control program further causes the computer to detect an operating condition parameter indicative of an operating condition of said internal combustion engine; and set a model parameter for said controlled object model in accordance with the detected operating condition parameter.138. An engine control unit including a control program for causing a computer to output a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyzer in an exhaust pipe of an internal combustion engine; calculate an output deviation of an output of said downstream air/fuel ratio sensor from a predetermined target value; control an air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on one modulation algorithm selected from a Δ modulation algorithm, a ΔΣ modulation algorithm, and a ΣΔ modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; control the air/fuel ratio of the air/fuel mixture supplied to said internal combustion engine in accordance with the calculated output deviation based on a response specifying control algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; detect an operating condition parameter indicative of an operating condition of said internal combustion engine; and select one of the air/fuel ratio controlled based on the one modulation algorithm and the air/fuel ratio controlled based on the response specifying control algorithm in accordance with the detected operating condition parameter to control the air/fuel ratio of the air/fuel mixture.139. An engine control unit according to claim 138, wherein said control program causes the computer to select one of the air/fuel ratio controlled based on the one modulation algorithm and the air/fuel ratio controlled based on the response specifying control algorithm in accordance with at least one of whether or not said catalyzer has a total amount of catalyst loaded therein equal to or larger than a predetermined load amount, and whether or not a carrier for carrying said catalyst in said catalyzer has a length in a direction in which said exhaust pipe extends equal to or longer than a predetermined length.140. An engine control unit according to claim 138, wherein said control program causes the computer to discriminate an operation mode of said internal combustion engine in accordance with said operating condition parameter; select the air/fuel ratio controlled based on the one modulation algorithm when the discriminated operation mode is a predetermined first operation mode; and select the air/fuel ratio controlled based on the response specifying control algorithm when the discriminated operation mode is a predetermined second operation mode.141. An engine control unit according to claim 138, wherein said catalyzer comprises a plurality of catalyzers disposed along said exhaust pipe of said internal combustion engine and spaced apart from each other,wherein said control program causes the computer to discriminate an operation mode of said internal combustion engine in accordance with said operating condition parameter; select the air/fuel ratio controlled based on the one modulation algorithm when the discriminated operation mode is a predetermined first operation mode; and select the air/fuel ratio controlled based on the response specifying control algorithm when the discriminated operation mode is a predetermined second operation mode. 142. An engine control unit according to claim 138, wherein said internal combustion engine comprises a downstream catalyzer in said exhaust pipe at a location downstream of said downstream air/fuel ratio sensor,wherein said control program causes the computer to discriminate an operation mode of said internal combustion engine in accordance with said operating condition parameter; select the air/fuel ratio controlled based on the one modulation algorithm when the discriminated operation mode is a predetermined first operation mode; and select the air/fuel ratio controlled based on the response specifying control algorithm when the discriminated operation mode is a predetermined second operation mode. 143. An engine control unit according to claim 142, wherein:the total amount of catalyst loaded in said downstream catalyzer is set smaller than the total amount of catalyst loaded in said catalyzer, and said target value is set to provide the highest purification percentage for said catalyzer. 144. An engine control unit according to claim 140, wherein said first operation mode includes a transient operation mode of said internal combustion engine.145. An engine control unit according to claim 140, wherein said first operation mode includes an idle operation mode.146. An engine control unit according to claim 140, wherein said second operation mode includes a steady-state operation mode in which said internal combustion engine is loaded substantially in a steady-state.147. An engine control unit according to claim 138, wherein said control program causes the computer to calculate a first intermediate value in accordance with said output deviation based on said one modulation algorithm; calculate a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value based on a product of said calculated first intermediate value and a predetermined gain; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio.148. An engine control unit according to claim 147, wherein said control program further causes the computer to set said gain in accordance with said operating condition parameter.149. An engine control unit according to claim 138, wherein said control program causes the computer to calculate a second intermediate value in accordance with said output deviation based on said one modulation algorithm; calculate a target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value by adding a predetermined value to the calculated second intermediate value; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio.150. An engine control unit according to claim 138, wherein said control program causes the computer to calculate a predicted value of said output deviation in accordance with said output deviation based on a prediction algorithm; control the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said one modulation algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated predicted value of said output deviation based on said response specifying control algorithm such that the output of said downstream air/fuel ratio sensor converges to said target value.151. An engine control unit according to claim 138, wherein said control program causes the computer to calculate said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio; calculate said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said response specifying control algorithm, said controlled object model having a variable associated with a value indicative of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio.152. An engine control unit according to claim 151, wherein said control program further causes the computer to identify a model parameter for said controlled object model in accordance with said target air/fuel ratio and the output of said downstream air/fuel ratio sensor.153. An engine control unit according to claim 152, wherein:said controlled object model comprises a discrete time system model, wherein said control program causes the computer to identify the model parameter for said discrete time system model in accordance with discrete data of said target air/fuel ratio and discrete data of the output of said downstream air/fuel ratio sensor. 154. An engine control unit according to claim 151, wherein said control program further causes the computer to set the model parameter for said controlled object model in accordance with said operating condition parameter.155. An engine control unit according to claim 138, wherein said control program further causes the computer to output a detection signal indicative of an air/fuel ratio of exhaust gases at a location upstream of said catalyzer in said exhaust pipe of said internal combustion engine; calculate said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said one modulation algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio; and calculate said target air/fuel ratio in accordance with said output deviation based on an algorithm which applies a controlled object model and said response specifying control algorithm, said controlled object model having a variable associated with a value indicative of one of said target air/fuel ratio of the air/fuel mixture for converging the output of said downstream air/fuel ratio sensor to said target value and a value indicative of an output of said upstream air/fuel ratio sensor, and a variable associated with a value indicative of the output of said downstream air/fuel ratio sensor; and control the air/fuel ratio of the air/fuel mixture in accordance with the calculated target air/fuel ratio.156. An engine control unit according to claim 155, wherein said control program further causes the computer to identify the model parameter for said controlled object model in accordance with one of said target air/fuel ratio and the output of said upstream air/fuel ratio sensor, and the output of said downstream air/fuel ratio sensor.157. An engine control unit according to claim 156, wherein:said controlled object model comprises a discrete time system model, wherein said control program causes the computer to identify the model parameter for said discrete time system model in accordance with one of discrete data of said target air/fuel ratio and discrete data of the output of said upstream air/fuel ratio sensor, and discrete data of the output of said downstream air/fuel ratio sensor. 158. An engine control unit according to claim 155, wherein said control program further causes the computer to set a model parameter for said controlled object model in accordance with said operating condition parameter.159. An engine control unit according to claim 138, wherein said response specifying control algorithm is a sliding mode control algorithm.