CAPACITIVE POSITION SENSOR

• US 3,760,392 A
• Filed: 05/15/1972
• Issued: 09/18/1973
• Est. Priority Date: 05/15/1972
• Status: Expired due to Term

• Current Assignee: Frederick A. Stich
• Original Assignee: Frederick A. Stich
• Inventors: Stich, Frederick A.
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Mooney, Robert J.
• Application Number:
  US05/253,418
• Time in Patent Office: 491 Days
• Field of Search: 340/200 318/254 317/249 323/93
• US Class Current: 340/870.37
• CPC Class Codes: H02P 6/00 Arrangements for controlling synchronous motors or other dynamo-electric motors using electronic commutation dependent on the rotor position; Electronic commutators therefor vector control H02P21/00
• CPC Class Codes: H02P 6/16 Circuit arrangements for detecting position
• CPC Class Codes: H02P 6/32 Arrangements for controlling wound field motors, e.g. motors with exciter coils

Abstract

A capacitive rotor position sensor generates three phase output pulses indicative of the position of the rotor of a three phase commutatorless D.C. motor. A radially elongated capacitor plate connected to the motor rotor is rotatably mounted adjacent 12 circumferentially displaced stationary capacitor plates, and a high frequency oscillator is coupled to the rotatable plate. A differential amplifier for each phase has one input coupled to three adjacent first stationary capacitor plates which together subtend 180 electrical degrees and the other input coupled to three adjacent second stationary capacitor plates which together subtend 180 electrical degrees and are displaced 180 electrical degrees from the first plates, and the corresponding inputs of the three differential amplifiers are coupled to first stationary plates displaced 120 electrical degrees apart and to second stationary plates displaced 120 electrical degrees apart. The high frequency pulses from the oscillator are coupled through the rotatable plate to the stationary plates, and the differential amplifiers enhance the one-to-zero ratio of (1) a logic one signal from a stationary plate opposite the rotatable plate at a given instant; and (2) a logic zero signal from a stationary plate simultaneously displaced 180 electrical degrees from the rotatable plate, to derive square wave output pulses at a frequency proportional to motor speed indicative of the rotor position.

Claims

1. A capacitive position sensor for indicating the position of a rotatable member comprising, in combination, a rotatable, radially elongated capacitor plate operatively connected to said member for rotation therewith, a plurality of stationary capacitor plates circumferentially displaced from each other in a plane adjacent to said rotatable plate and perpendicular to the axis thereof and each of which subtends a substantially greater arc about said axis of rotation than said rotatable plate, an oscillator adapted to generate high frequency pulses, means for coupling the output of said oscillator to said rotatable plate, and indicating means coupled to said stationary capacitor plates and responsive to the high frequency pulses from said oscillator passing through the capacitance between said rotatable plate and each said stationary plate as said rotatable plate revolves for generating an output pulse when said rotatable plate is opposite each stationary plate indicative of the instantaneous position of said rotatable member.

2. A capacitive position sensor in accordance with claim 1 wherein said indicating means includes a plurality of differential amplifiers each having its inputs coupled to a pair of stationary capacitor plates displaced 180 electrical degrees apart, whereby a logic one voltage signal is coupled to one of said inputs from the stationary plate of said pair opposite said rotatable plate at a given instant and a logic zero voltage signal is simultaneously coupled to the other input from the other stationary plate of said pair.

3. A capacitive position sensor in accordance with claim 2 wherein said indicating means includes a plurality of bistable circuit means each of which receives the output from one of said differential amplifiers for deriving square wave pulses indicative of the position of said rotatable member, said bistable circuit means providing positive feedback to the inputs of said differential amplifier to enhance discrimination between stationary capacitor plates.

4. A capacitive position sensor in accordance with claim 1 wherein said oscillator is adapted to generate fast-rise pulses and has a frequency substantially higher than the r.p.m. of said rotatable member.

5. A capacitive position sensor in accordance with claim 1 for generating an n-phase train of output pulses indicative of the position of said rotatable member and at a frequency proportional to the angular velocity of said rotatable member, where n is an integer, and wherein said indicating means includes n pulse amplifier means each of which is electrically coupled to a plurality of adjacent first stationary capacitor plates which together subtend 180 electrical degrees and are displaced 360/n electrical degrees from the first stationary plates coupled to the other pulse amplifier means.

6. A capacitive position sensor in accordance with claim 1 for generating an n-phase train oF output pulses indicative of the position of said rotatable member and at a frequency which is a function of the angular velocity of said rotatable member, where n is an integer, and wherein said indicating means includes n differential amplifiers each of which has one of its inputs electrically coupled to a plurality of adjacent first stationary capacitor plates which together subtend 180 electrical degrees and its other input coupled to a plurality of adjacent second stationary capacitor plates which are displaced 180 electrical degrees from said first stationary plates, and wherein said first stationary capacitor plates coupled to said one input of each differential amplifier are displaced 360/n electrical degrees from the first stationary plates coupled to said one input of each of the other differential amplifiers.

7. A capacitive position sensor in accordance with claim 1 wherein said rotatable member is the rotor of an electric motor having n phases and p rotor pole pairs, where n and p are integers, and wherein said rotor position sensor includes at least n times p of said stationary capacitor plates and derives an n-phase train of output pulses indicative of the instantaneous position of said rotor having a frequency which is a function of the angular velocity of said rotor.

8. A capacitive rotor position sensor in accordance with claim 7 including n times 2p stationary capacitor plates and said indicating means includes n differential amplifiers each of which derives the output pulses for one of said phases and has its inputs coupled to stationary capacitor plates displaced 180 electrical degrees apart.

9. A capacitive rotor position sensor in accordance with claim 8 wherein a plurality of first stationary capacitor plates which together subtend 180 electrical degrees are electrically coupled to one input of each of said differential amplifiers and are displaced 360/n electrical degrees from the first stationary capacitor plates which are coupled to said one input of each of the other differential amplifiers, whereby each said differential amplifier derives output pulses of 180 electrical degrees duration for one phase displaced 360/n electrical degrees from the other phase output pulses.

10. A capacitive rotor position sensor in accordance with claim 9 wherein a plurality of second stationary capacitor plates which together subtend 180 electrical degrees are electrically coupled to the other input of each of said differential amplifiers and are displaced 180 electrical degrees from said first stationary plates which are coupled to said one input, and wherein each of said first and said second stationary plates subtend 360/2n electrical degrees, whereby a phase output pulse of 180 electrical degrees duration is generated when said rotatable plate is opposite said first stationary plates and its complement is generated when said rotatable plate is opposite said second stationary plates.

11. A capacitive rotor position sensor in accordance with claim 7 including n times 2p stationary capacitor plates and said indicating means includes n pulse amplifier means each of which is associated with one phase of said motor and is electrically coupled to a plurality of first stationary capacitor plates each of which subtends 360/2n electrical degrees and which together subtend 180 electrical degrees and which are displaced 360/n electrical degrees from the first stationary plates coupled to each of the other pulse amplifying means.

12. A capacitive rotor position sensor in accordance with claim 9 wherein p is equal to at least two and said rotatable plate has a radially elongated arm for each pole pair and said radially elongated arms are disposed 360 electrical degrees apart and also wherein stationary plates displaced 360 electrical degrees apart are Associated with the same phase and are coupled to the same differential amplifier, whereby the signals from said stationary capacitor plates displaced 360 electrical degrees apart are additive.

13. A capacitive position sensor in accordance with claim 1 and including grounded conductor means extending radially between adjacent stationary capacitor plates for electrically isolating them from each other.

14. A capacitive position sensor in accordance with claim 1 and including a coupling capacitor having a stationary electrode disposed in said plane radially inward from said stationary capacitor plates, and a rotatable electrode spaced by an air gap in an axial direction from said stationary electrode and having a radially extending portion integral therewith opposite said stationary capacitor plates constituting said rotatable capacitor plate, said oscillator being coupled to said rotatable plate through said coupling capacitor.

15. A capacitive rotor position sensor in accordance with claim 10 and including grounded conductor means extending radially between adjacent stationary capacitor plates for electrically isolating them from each other.

16. A capacitive rotor position sensor in accordance with claim 10 and including a coupling capacitor having a stationary electrode disposed in said plane radially inward from said stationary capacitor plates and a rotatable electrode spaced by an air gap in an axial direction from said stationary electrode and having a narrow radially extending portion integral therewith constituting said rotatable capacitor plate, said oscillator being coupled to said rotatable capacitive plate through said coupling capacitor.

17. A capacitive rotor position sensor for generating an n-phase train of pulses indicative of the position of the rotor of an electric motor having n phases and p pairs of rotor poles and at a frequency proportional to the angular velocity of said rotor, where n and p are integers, comprising, in combination, a rotatable, radially elongated capacitor plate operatively connected to said rotor for rotation therewith, 2pn stationary capacitor plates each of which subtends an arc of approximately 360/2n electrical degrees disposed adjacent said rotatable plate in a plane perpendicular to the axis of rotation of said rotatable plate and being circumferentially displaced from each other in said plane, a high frequency oscillator adapted to generate steep wavefront pulses electrically coupled to said rotatable plate, and n indicating means each associated with one phase of said motor and including a load impedance coupled to a plurality of adjacent first stationary capacitor plates for generating an output pulse in response to the high frequency pulses from said oscillator coupled through the capacitance between said rotatable and stationary plates when said rotatable plate is opposite each of said stationary plates as it revolves, the plurality of adjacent first stationary plates of each said indicating means coupled to said load impedance of one phase together subtending 180 electrical degrees and being displaced 360/n electrical degrees from the first stationary plates coupled to the load impedances associated with the other phases.

18. A capacitive rotor position sensor in accordance with claim 17 wherein each said indicating means includes a differential amplifier having one input coupled to said load impedance and to said first stationary plates of the associated phase and having its other input coupled to a plurality of adjacent second stationary plates which together subtend 180 electrical degrees and are displaced 180 electrical degrees from said first plates.

19. A capacitive rotor position sensor in accordance with claim 17 and including grounded conductor means extending radially between adjacent stationary plates for electrically isolating them from each other.

20. A capacitive rotor position sensor in accordance with claim 18 wherein each said indicating means also includes bistable circuit means coupled to the output of the associated differential amplifier for generating square wave pulses indicative of the position of said rotor, said bistable circuit means providing positive feedback to the inputs to the associated differential amplifier to enhance discrimination between stationary capacitor plates.

21. A capacitive rotor position sensor in accordance with claim 17 and including a coupling capacitor having a stationary electrode disposed in said plane radially inward from said stationary capacitor plates and a rotatable electrode spaced by an air gap in an axial direction from said stationary electrode and having a narrow radially extending portion integral therewith constituting said rotatable capacitor plate, said oscillator being coupled to said rotatable capacitive plate through said coupling capacitor.

22. A capacitor rotor position sensor in accordance with claim 17 wherein p is equal to at least two and said rotatable capacitor plate has a radially elongated arm for each pole pair and said radially elongated arms are disposed 360 electrical degrees apart, and also wherein stationary capacitor plates displaced 360 electrical degrees apart are associated with the same phase and are coupled to the same differential amplifier, whereby the signals from said stationary plates displaced 360 electrical degrees apart are additive.

23. A capacitive rotor position sensor for generating a three-phase train of pulses indicative of the position of the rotor of an electric motor having three pairs of rotor poles comprising, in combination, a rotatable, diametrically elongated capacitor plate operatively connected to said rotor for rotation therewith, 12 stationary capacitor plates circumferentially displaced from each other in a plane adjacent to said rotatable plate and perpendicular to the axis of rotation thereof, each of said stationary plates being in the shape of a sector of a ring and subtending an arc of approximately 30*, a high frequency oscillator coupled to said rotatable plate, and three differential amplifiers, one input of each of said differential amplifiers being coupled to three adjacent first stationary plates together subtending approximately 90* and to the three stationary plates diametrically opposed thereto, and the other input being coupled to three adjacent second stationary plates circumferentially displaced 90* from said first stationary plates and also to the stationary plates diametrically opposed to said second stationary plates, the corresponding inputs of said three differential amplifiers being coupled to stationary plates displaced 120* apart.

24. A capacitive rotor position sensor in accordance with claim 23 and including, a coupling capacitor having a stationary electrode disposed in said plane radially inward from said stationary capacitor plates and a rotatable electrode spaced in an axial direction by an air gap from said stationary electrode and having a pair of diametrically opposed, radially extending, narrow portions integral therewith positioned opposite said stationary capacitor plates and together constituting said rotatable capacitor plate, said oscillator being coupled to said rotatable capacitor plate by said coupling capacitor.

25. A capacitive rotor position sensor in accordance with claim 24 and including grounded conductor means extending radially between adjacent stationary plates and also between the inner periphery of said stationary plates and said coupling capacitor stationary electrode for electrically isolating said stationary capacitor plates from each other.