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A detection system for detecting a dangerous condition for an operator using a power tool of the type which has an exposed blade relative to a work surface and a protection system for minimizing, if not eliminating the possibility of a user being injured by contacting the blade. In one preferred emb

A detection system for detecting a dangerous condition for an operator using a power tool of the type which has an exposed blade relative to a work surface and a protection system for minimizing, if not eliminating the possibility of a user being injured by contacting the blade. In one preferred embodiment of the present invention, a proximity detection system is capable of detecting the presence of a user near the blade of a table saw and a protection system that can either retract the blade below the work surface of the table saw or terminate the drive torque to the blade which can result in rapid stopping of the saw blade by a work piece that is being cut.

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1. A table saw comprising:a frame structure with a generally flat work surface; a rotatable circular blade mounted on an arbor and being adapted to have at least a portion of said blade extending above the flat work surface in position to cut a work piece; a drive motor with an output shaft mounted

1. A table saw comprising:a frame structure with a generally flat work surface; a rotatable circular blade mounted on an arbor and being adapted to have at least a portion of said blade extending above the flat work surface in position to cut a work piece; a drive motor with an output shaft mounted to the frame structure for powering the saw blade; a sensing mechanism for generating at least a first actuation signal responsive to predetermined sensed characteristics relating to the existence and/or movement of a person in close proximity thereto; and a retracting mechanism located beneath said work surface for rapidly moving said blade downwardly below the work surface responsive to said first actuation signal being received. 2. A table saw as defined in claim 1 wherein said retracting mechanism comprises:an arm structure that has a first pulley operatively coupled to said arbor, a second pulley operatively coupled to said output shaft of said drive motor and a flexible belt coupling said first and second pulleys, said arm structure pivoting about an axis concentric with said output shaft so that the tension in said belt is relatively constant regardless of the position of the blade relative to said work surface; a flywheel mounted on said axis concentric with said output shaft and being operatively connected to and driven by said motor in a direction of rotation that is downward at the portion of the arc nearer the first pulley; and an actuator connected to said arm structure for operatively engaging said rotating flywheel to stop the rotation, the inertia of said flywheel resulting in rapid downward movement of said arm structure so that said blade is moved below said work surface responsive to receiving said first actuation signal from said sensing mechanism. 3. A table saw as defined in claim 2 wherein said flywheel is located immediately adjacent one side of said arm structure, said flywheel being driven by said motor but is disengaged from said motor output shaft when said actuator operatively engages said flywheel.4. A table saw as defined in claim 1 wherein said circular blade is electrically isolated from said arbor, drive motor output shaft and frame structure work surface, so that an excitation field can be applied to said blade.5. A table saw comprising:a frame structure with a generally flat work surface; a rotatable circular blade mounted on an arbor and being adapted to have at least a portion of said blade extending above the flat work surface in position to cut a work piece; a drive motor with an output shaft mounted to the frame structure for powering the saw blade; a sensing mechanism for generating at least a first actuation signal responsive to predetermined sensed characteristics relating to the existence and/or movement of a person in close proximity thereto; and a retracting mechanism located beneath said work surface for rapidly moving said blade downwardly below the work surface responsive to said first actuation signal being received, wherein said retracting mechanism further comprises: an arm structure that has a first pulley operatively coupled to said arbor, a second pulley operatively coupled to said output shaft of said drive motor and a flexible belt coupling said first and second pulleys, said arm structure pivoting about an axis concentric with said output shaft so that the tension in said belt is relatively constant regardless of the position of the blade relative to said work surface; and an actuator connected to said frame structure and said arm structure for rapidly moving said arm structure to cause said blade to be moved below said work surface responsive to receiving said first actuation signal from said sensing mechanism. 6. A table saw as defined in claim 5 wherein said actuator is a pyrotechnic device having a housing and an elongated rod extending therefrom, actuation of said device causing said rod to move longitudinally relative to said housing, one of the end of said rod and housing being connected to said frame structure and the other of the end of said rod and housing being connected to said arm structure.7. A table saw as defined in claim 5 wherein said arm structure has a transverse extension extending from near said second pulley, said arm structure being generally L shaped, with said actuator being connected to said transverse extension.8. A table saw as defined in claim 7 wherein the outer end of said transverse extension has a curved surface containing a plurality of teeth for engaging a worm gear, said arm structure including an elongated shaft having a worm gear portion and an a elevation crank attached to one end thereof for incrementally adjusting the amount of said blade that is above the work surface, said elongated shaft being longitudinally movable relative to said frame structure, said actuator being operatively attached to said shaft such that said shaft can be longitudinally moved while permitting rotation thereof.9. A table saw as defined in claim 7 wherein said frame structure includes a stop pad located near the position where the blade is completely below the work surface to engage said transverse extension and stop the downward movement of said arm structure.10. A table saw as defined in claim 5 wherein said actuator generates a moving force to said arm structure that is within the range of about 1000 to about 10000 pounds.11. A table saw comprising:a frame structure with a generally flat work surface; a rotatable circular blade mounted on an arbor and being adapted to have at least a portion of said blade extending above the flat work surface in position to cut a work piece; a drive motor with an output shaft mounted to the frame structure for powering the saw blade; a sensing mechanism for generating at least a first actuation signal responsive to predetermined sensed characteristics relating to the existence and/or movement of a person in close proximity thereto; a retracting mechanism located beneath said work surface for rapidly moving the blade downwardly below the work surface responsive to said first actuation signal being received, wherein said retracting mechanism moves said blade beneath said work surface within less than approximately 7 milliseconds after said first actuation signal is received by said retracting mechanism. 12. A table saw comprising:a frame structure with a generally flat work surface; a rotatable circular blade mounted on an arbor and being adapted to have at least a portion of said blade extending above the flat work surface in position to cut a work piece, said circular blade being electrically isolated from said arbor, a drive motor output shaft and said frame structure work surface, so that an excitation field can be applied to said blade; a drive motor with an output shaft mounted to the frame structure for powering said blade; a sensing mechanism for generating at least a first actuation signal responsive to predetermined sensed characteristics relating to the existence and/or movement of a person in close proximity thereto; a retracting mechanism located beneath said work surface for rapidly moving the blade downwardly below the work surface responsive to said first actuation signal being received, wherein said sensing mechanism comprises: said work surface defining a ground plane and having a slot opening through which said blade can extend; at least one electrically conductive deflection plate electrically isolated from but attached to said work surface for shaping an electric field that emanates from said blade when an excitation field is applied thereto; circuitry for selectively generating and applying excitation signals to each of said deflection plates and to said blade to thereby define at least one sensing volume near said blade above said work surface; circuitry for sensing the dielectric constant of said sensing volume and generating sensed signals that are indicative of the sensed dielectric constant; a processor for receiving said generated signals and for producing said first actuation signal when predetermined sensed characteristics indicate that a person is in a highly dangerous position relative to said blade. 13. A table saw as defined in claim 12 wherein said sensing circuitry senses at least one of the intensity and phase of the dielectric characteristic, said processor being capable of determining changes in the intensity and phase of the dielectric characteristic from successively sensed dielectric characteristic values.14. A table saw as defined in claim 12 wherein said sensing circuitry comprises an amplifier connected to said blade relative to ground, said amplifier having an output with said sensed signals that are at least intermittently applied to said processor.15. A table saw as defined in claim 14 wherein said sensing circuitry further comprises circuitry for measuring the phase of the sensed signals relative to the phase of an excitation signal that is applied to said blade and for measuring the amplitude of the sensed signals and applying said sensed amplitude and sensed phase signals at least intermittently to said processor.16. A table saw as defined in claim 14 wherein said sensing mechanism further comprises additional sensing circuitry including an amplifier connected to said deflection plate, and circuitry for measuring the phase of sensed signals relative to the phase of an excitation signal that is applied to said deflection plate and for measuring the amplitude of the sensed signals and applying said sensed amplitude and sensed phase signals at least intermittently to said processor.17. A table saw as defined in claim 12 wherein said excitation signal generating circuitry is capable of generating excitation signals for each of said blade and deflection plate that has individual variable amplitude, phase and frequency characteristics.18. A table saw as defined in claim 17 wherein said processor is operatively connected to said excitation signal generating circuitry and is capable of variably controlling the characteristics of said excitation signals.19. A table saw as defined in claim 12 wherein said processor generates a second actuation signal for terminating said motor from driving said blade when predetermined sensed characteristics indicate that a person is in a moderately dangerous position relative to said blade.20. A table saw as defined in claim 19 further including a switch for controlling the application of electric power to said motor, wherein terminating said motor from driving said blade comprises operating said switch to remove power to said motor.21. A table saw as defined in claim 19 further including a clutch mechanism operatively installed between said motor and said blade for disconnecting application of power to said blade, wherein terminating the motor from driving said blade comprises actuating said clutch mechanism.22. A table saw as defined in claim 12 wherein said sensing mechanism comprises a plurality of deflection plates located on said work surface adjacent said slot opening, said deflection plates being bonded to an electrical insulating layer generally coextensive with the surface of said deflection plates, said insulting layer being bonded to said work surface.23. A table saw as defined in claim 12 wherein said sensing mechanism comprises at least one deflection plate that generally cover a substantial portion of the area of said work surface adjacent said slot opening, said deflection plates being bonded to an electrical insulating layer generally coextensive with the surface of said deflection plates, said insulting layer being bonded to said work surface, an array of conductive sensor plates positioned around said slot opening and being bonded to an insulating layer that is bonded to said deflection plate, each sensor plate in said array being connected to said excitation signal generating circuitry and said sensing circuitry, each sensor plate thereby defining a separate sensing volume when excitation signals are applied thereto, said sensing circuitry for each sensor plate generating signals indicative of the sensed dielectric characteristic in its sensed volume.24. A table saw as defined in claim 23 wherein said array of sensor plates comprises a plurality of sensor strips that are positioned at increasing distances from said slot opening in opposite directions from the lengthwise direction of said slot opening and in opposite directions from the ends of said slot opening.25. A table saw as defined in claim 23 wherein said array of sensor plates comprises a plurality of small square shaped sensor plates generally evenly spaced from one another from said slot opening outwardly in all directions therefrom.26. A table saw as defined in claim 23 wherein said array of sensor plates comprises a plurality of concentric increasing sized hollow rectangles extending from said slot opening.27. A table saw as defined in claim 26 wherein each of said rectangles comprises of a plurality of individual strips.28. A table saw as defined in claim 27 wherein the width of the sides of said rectangles are generally constant.29. A table saw as defined in claim 23 wherein said processor receives said signals of sensed dielectric characteristic within each volume defined by each sensor plate, said processor performing calculations regarding successive signals regarding each volume and thereby determining sensed characteristics relating to the existence and/or movement of a person in close proximity to the array of sensor plates.30. A table saw as defined in claim 29 wherein said processor is capable of determining the height of the saw blade relative to said work surface from said sensed signals.31. A table saw as defined in claim 29 wherein said processor is capable of determining the distance a portion of a human is from said work surface in any one or more of x, y or z coordinate directions from said sensed signals.32. A table saw as defined in claim 29 wherein said processor is capable of determining the distance of a portion of a human is from said work surface in any one or more of x, y or z coordinate directions, and generally simultaneously determining height of said blade above the work surface and determining the distance of a portion of a human is from said blade in any one or more of x, y or z coordinate directions from said sensed signals.33. A table saw as defined in claim 29 wherein said processor is capable of determining the direction that a portion of a human is moving relative to said blade and thereby determine whether the portion is moving toward said blade from said sensed signals.34. A table saw as defined in claim 29 wherein said processor is capable of determining the speed of movement of a portion of a human upon receipt of successive sensed signals.35. A table saw as defined in claim 29 wherein said processor is capable of determining contact of said blade by a portion of a human from said sensed signals.36. A table saw as defined in claim 29 wherein said processor is capable of determining the beginning of a kickback action from said sensed signals.37. A table saw as defined in claim 29 wherein said excitation signal generating circuitry is capable of varying the frequency of said excitation signals, said processor being capable of determining additional characteristics from said sensed signals that are generated as a result of varying the frequency of said excitation signals.38. A system for sensing an object in proximity to a surface, said system comprising:said surface defining an electrical ground; at least one electrically conductive deflection plate electrically isolated from but operatively attached to said surface; at least one electrically conductive sensor electrically isolated from but operatively associated with said deflection plate; said at least one deflection plate shaping an electric field that emanates from said at least one sensor when an excitation field is applied thereto; circuitry for selectively generating and applying excitation signals to each of said deflection plates and to said sensor to thereby define at least one sensing volume near said sensor away from said surface; circuitry for sensing the dielectric constant of said sensing volume and generating sensed signals that are indicative of the sensed dielectric characteristic; a digital processor for receiving said generated signals and for determining the presence of the object from an analysis of a plurality of said generated sensed signals. 39. A system as defined in claim 38 wherein said sensing circuitry senses at least one of the intensity and phase of the dielectric characteristic, said processor being capable of determining changes in the intensity and phase of the dielectric characteristics from successively sensed dielectric characteristic values.40. A system as defined in claim 39 wherein said sensing circuitry further comprises circuitry for measuring the phase of the sensed signals relative to the phase of an excitation signal that is applied to said sensor and for measuring the amplitude of the sensed signals and applying said sensed amplitude and sensed phase signals at least intermittently to said processor.41. A system as defined in claim 38 further comprising a plurality of sensors and wherein said excitation signal generating circuitry is capable of generating excitation signals for each of said sensors and said deflection plate that provide individual and optionally variable amplitude, phase and frequency characteristics to thereby define a plurality of sensing volumes that have individually defined characteristics.42. A system as defined in claim 41 further comprising a plurality of deflection plates and wherein said excitation signal generating circuitry is capable of generating excitation signals for each of said sensors and for each of said deflection plates that has individual and optionally variable amplitude, phase and frequency characteristics to thereby define a plurality of sensing volumes that have individually defined characteristics.43. A system as defined in claim 42 wherein said processor is operatively connected to said excitation signal generating circuitry and is capable of variably controlling the characteristics of said excitation signals.44. A system for sensing a human in proximity to a moving element of a structure which is in proximity to a sensor operatively associated with the structure, said system comprising:at least a portion of said structure defining an electrical ground; at least one electrically conductive deflection member electrically isolated from but operatively attached to said portion of said structure; at least one electrically conductive sensor electrically isolated from but operatively associated with said deflection member; said at least one deflection member shaping an electric field that emanates from said at least one sensor when an excitation field is applied thereto; circuitry for selectively generating and applying excitation signals to each of said deflection member and to said sensor to thereby define at least one sensing volume near said moving element; circuitry for sensing the dielectric characteristics of said sensing volume and generating sensed signals that are indicative of the sensed dielectric characteristics; a digital processor for receiving said generated signals and for determining the presence of the human in proximity to said moving element from an analysis of a plurality of said generated sensed signals. 45. A system as defined in claim 44 wherein said deflection members are in the form of a generally flat plate.