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Methods and apparatus for controlling an actuator to provide linear and continuous force output to a user of a force feedback device. To provide continuous and smooth force output in a zero crossover region of operation, two drive signals are used, each causing current to flow in a different directi

Methods and apparatus for controlling an actuator to provide linear and continuous force output to a user of a force feedback device. To provide continuous and smooth force output in a zero crossover region of operation, two drive signals are used, each causing current to flow in a different direction in the actuator. When a desired output force is in the crossover region, the two drive signals are alternated to cause the output force to quickly switch directions. When the desired force is outside the crossover region, only one drive signal is used to cause output force in one direction. To compensate for a nonlinear output of the actuator, a desired command current is correlated with an approximated point of a characterization curve of the actuator, where the curve includes points determined in a previously performed actuator characterization. The approximated point is determined between two successive points using a linear approximation, and a drive signal duty cycle is determined from the approximated point. Other features can be implemented to compensate for power supply voltage variation, back EMF effect, and temperature.

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1. A circuit and method for commanding a desired force from an actuator provided in a force feedback device, the method comprising providing two drive signals, each of said drive signals operative to cause current to flow in a different direction in said actuator, thereby causing force to be output

1. A circuit and method for commanding a desired force from an actuator provided in a force feedback device, the method comprising providing two drive signals, each of said drive signals operative to cause current to flow in a different direction in said actuator, thereby causing force to be output by the actuator in two different corresponding directions; when a desired force to be output is below said predetermined threshold force, alternating between said two drive signals for each period of said drive signals to cause a corresponding current in the actuator, thereby switching said direction of said desired force for each of said periods; when said desired force is above said predetermined threshold force, using only one of said drive signals to cause current in said actuator in one direction and said desired force to be output in one direction; and wherein said drive signal are PWM (PULSE-WIDTH MODULATOR) drive signals. 2. A method as recited in claim 1 wherein a select signal is provided to select between using said two PWM drive signals and using only one of said PWM drive signals. 3. A method as recited in claim 2 wherein said select signal has three states, such that a first of said states causes only a first of said PWM drive signals to be used, a second of said states causes only a second of said PWM drive signals to be used, and a third of said states causes said alternating between said first and second PWM drive signals. 4. A method as recited in claim 1 wherein each of said PWM drive signals controls two switches in an actuator bridge circuit. 5. A method as recited in claim 4 wherein said select signal is passed through a flip flop and through a resistor in parallel with said flip flop, wherein an output of said flip flop is used to select whether said first PWM signal is provided to said actuator, and wherein an inverted output of said flip flop is used to select whether said second PWM signal is provided to said actuator. 6. A method as recited in claim 1 further comprising correlating said desired force to be output by said actuator with an approximated linear section of a characterization curve of drive signal duty cycles for said actuator to determine a required duty cycle for said drive signals. 7. A method as recited in claim 6 wherein said characterization curve includes a plurality of points obtained from a characterization performed before run time of said actuator. 8. A circuit for commanding a desired force from an actuator provided in a force feedback device, the circuit comprising:a selection circuit receiving two drive signals, said selection circuit selecting one of said drive signals or selecting both of said drive signals to be output from said selection circuit, wherein only one of said drive signals is selected when a desired force to be output by said actuator is above a predetermined threshold force, and wherein both of said drive signals are selected when said desired force is between said threshold force and zero such that said two drive signals are output by alternating each drive signal for each period of said drive signals; anda bridge circuit coupled between said selection circuit and said actuator and receiving said drive signal selected by said selection circuit, wherein said bridge circuit provides a current to flow in said actuator corresponding to said drive signal, thereby causing a force to be output by said actuator in a corresponding direction, wherein if said bridge circuit receives said one selected drive signal, said force is output in one direction, and wherein if said bridge circuit receives said alternating drive signals, said direction of said desired force is switched for each of said periods of said drive signals. 9. A circuit as recited in claim 8 wherein said drive signals are PWM drive signals. 10. A circuit as recited in claim 9 wherein a select signal is provided to select between using said two PWM drive signals and using only one of said PWM drive signals. 11 . A circuit as recited in claim 10 wherein said select signal has three states, such that a first of said states causes only a first of said PWM drive signals to be used, a second of said states causes only a second of said PWM drive signals to be used, and a third of said states causes said alternating between said first and second PWM drive signals. 12. A circuit as recited in claim 9 wherein said bridge circuit includes four switches, and wherein each of said PWM drive signals controls two of said switches. 13. A circuit as recited in claim 12 wherein said selection circuit includes a flip flop and a resistor in parallel with said flip flop, wherein an output of said flip flop is used to select whether said first PWM signal is provided to said bridge circuit, and wherein an inverted output of said flip flop is used to select whether said second PWM signal is provided to said bridge circuit. 14. A circuit as recited in claim 8 further comprising a memory storing a look-up table of PWM duty cycle values, said look-up table correlating said desired force to be output by said actuator with an approximated linear section of a characterization curve of drive signal duty cycles for said actuator to determine a required duty cycle for said drive signals. 15. A method for controlling an actuator to compensate for a nonlinear output of said actuator, the method comprising:determining a desired command current for an actuator, said command current causing a desired output force to be output by said actuator; andcorrelating said desired command current with an approximated point of a characterization curve of said actuator, said characterization curve including a plurality of points determined in a previously performed characterization of said actuator, wherein said approximated point is determined between two successive points of said plurality of points using a linear approximation, and wherein a required command duty cycle is determined from said approximated point of said characterization curve, wherein a drive signal having said determined command duty cycle is applied to said actuator to cause said actuator to output said desired force. 16. A method as recited in claim 15 wherein said plurality of points of said characterization curve are stored in a look-up table. 17. A method as recited in claim 15 wherein said plurality of points includes at least three points to provide at least two linear sections of said curve. 18. A method as recited in claim 15 wherein said drive signal is a first drive signal, and wherein a second drive signal is also output to drive said actuator when said desired output force is between zero and a threshold force. 19. A method as recited in claim 18 wherein when said first and second drive signals are both output, said first and second drive signals are alternated at each period of said drive signals to provide a switching current in said actuator and a switching output force. 20. A method as recited in claim 18 wherein a linearly-approximated duty cycle is obtained for both of said drive signals. 21. A method as recited in claim 18 wherein said characterization curve includes a step at said threshold force. 22. A method as recited in claim 15 further comprising compensating for voltage variations supplied to said actuator from a power supply such that said actuator output is linear regardless of said variations. 23. A method as recited in claim 15 wherein said actuator is provided in a force feedback device that also includes a manipulandum moved by a user, wherein said actuator outputs said force on said manipulandum, and further comprising:predicting a change in actuator current caused by a back EMF effect induced by motion of said manipulandum by said user, said predicted change in motor current based on a determined velocity of said manipulandum; andcompensating said drive signal to said actuator in accordance with said predicted change in current so as to reduce nonlinear output of said actuator caused by said back EMF effect. 24. A method as recited in claim 15 further comprising receiving temperature information indicating a temperature of a coil winding of said actuator, and adjusting said drive signal based on said temperature information to compensate for a change in current in said actuator caused by a temperature variation in said coil winding and to cause said desired force to be output. 25. A circuit for providing force feedback from an activator, wherein the force applied to the actuator is variable through a range of forces including a first range of forces comprising forces in a first direction and a second range of forces comprising forces in a second direction opposite the first direction and a third range of forces between the first and second ranges of forces, the circuit comprising:a selection circuit for selecting a first drive signal when a force in the first range of forces is desired and a second drive signal when a force in the second range of forces is desired; andan alternator for alternating between the first drive signal and the second drive signal when a force in the third range of forces is desired. 26. The circuit of claim 25, wherein the first drive signal generation circuit and the second drive signal generation circuit are configured such that a duty cycle of the second drive signal is not strictly determined by a duty cycle of the first drive signal and the duty cycle of the first drive signal is not strictly determined by the duty cycle of the second drive signal. 27. The circuit of claim 25, further comprising a first drive signal generation circuit that generates a PWM signal for the first drive signal and a second drive signal generation circuit that generates a PWM signal for the second drive signal. 28. The circuit of claim 27, wherein the first drive signal generation circuit and the second drive signal generation circuit are configured such that a duty cycle of the second drive signal is not strictly determined by a duty cycle of the first drive signal and the duty cycle of the first drive signal is not strictly determined by the duty cycle of the second drive signal. 29. The circuit of claim 27, wherein the alternator alternates between the first drive signal and the second drive signal each cycle of a PWM clock. 30. The circuit of claim 25, wherein the first and second forces are along one degree of freedom, the circuit further comprising circuitry for providing force feedback in a second degree of freedom. 31. The circuit of claim 25, wherein the third range is defined by a threshold such that forces below the threshold in the first direction form one boundary of the third range and forces below the threshold in the second direction form the other boundary of the third range. 32. The circuit of claim 25, further comprising a signal transfer circuit for each of the first drive signal and the second drive signal, wherein the signal transfer circuit includes an input indicating a desired force to be applied and is configured to generate a drive signal from the input such that the drive signal results in at least approximately the desired force. 33. The circuit of claim 32, wherein the signal transfer circuit includes linearization means for translating the input to the drive signal such that the resulting force is approximately linearly related to the input. 34. The circuit of claim 33, wherein linearization means is a lookup table containing correspondences between input values and drive signal values. 35. The circuit of claim 34, wherein the correspondences are correspondences measured from prior application of input signals to determine the corresponding force output for each input signal. 36. A circuit for providing force feedback from an activator, wherein the desired force applied to the actuator is variable through a range of forces including forces in one direction for a degree of freedom, forces in an opposite direction from the one direction and a zero net force for t he degree of freedom, the circuit comprising:a first drive signal generator for generating a drive signal to impart a force in the one direction;a second drive signal generator for generating a drive signal to impart a force in the opposite direction; anddrive signal selection logic for enabling force application based on the first drive signal generator when the desired force is in the one direction and greater than a first threshold in the one direction, enabling force application based on the second drive signal generator when the desired force is in the opposite direction and greater than a second threshold in the opposite direction, and enabling force application based on both the first drive signal generator and the second drive signal generator when the desired force is between the first threshold and the second threshold. 37. The circuit of claim 36, wherein the first threshold and the second threshold are equal and opposite. 38. The circuit of claim 36, wherein drive signal selection logic alternates between applying the first drive signal and the second drive signal.