System and method for velocity control of a D.C. Motor

• US 5,811,946 A
• Filed: 06/07/1995
• Issued: 09/22/1998
• Est. Priority Date: 03/16/1992
• Status: Expired due to Term

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Bader, Clifford J., Mullin, Eugene T.
• Primary Examiner(s): Martin, David S.
• Application Number:
  US08/485,651
• Time in Patent Office: 1,203 Days
• Field of Search: 318/254, 318/439, 318/138, 388/809-815, 388/901, 388/928.1
• US Class Current: 388/811
• CPC Class Codes: H02P 7/2805 whereby the speed is regulated by measuring the motor speed and comparing it with a given physical value
• CPC Class Codes: Y10S 388/901 Sample and hold

Abstract

A system and method is disclosed for feedback velocity control of a D.C. motor. The system includes a motor (30) which is represented by its circuit equivalent--a series combination of an inductance (32), a resistor (34) and a back EMF source (36). The drive to the motor (30), supplied by a controller, is periodically inhibited by timing logic (22) and AND gate (20). Once the inductive transient decays via clamping diode (38) such that the motor current is zero, switch (40) is closed and the internally generated motor voltage is sampled by a capacitor (42). A/D converter (44) transforms the sampled value to a digital value and sends it to the controller. The controller compares it to a predetermined desired value then makes the necessary adjustments to the drive signal.

Claims

1. A system for controlling the velocity of a brush-commutated D.C. motor, comprising:

adjustable voltage means for applying a first voltage across said D.C. motor;

means for inhibiting the first voltage applied across the D.C. motor, where the duration of said inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, the duration of the inhibition being selected to be sufficiently long so that a motor current decays to substantially zero, and said inhibition is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

means for sampling and holding a voltage amplitude value proportional to a second voltage across said D.C. motor during each time interval that said first voltage is inhibited, and after the motor current decays to substantially zero;

means for comparing the second voltage with a value representing a desired velocity; and

means for controlling, in real-time, the adjustable voltage means only in response to the comparing means to adjust the first voltage to a value relating to the desired velocity.

2. Apparatus as defined in claim 1 wherein said means for sampling and holding comprises means for sampling and holding the voltage amplitude value during each time interval that said first voltage is inhibited but after a sufficient delay from commencement of the inhibition to permit decay of motor current to substantially zero and to permit the second voltage across said D.C. motor to reach a steady-state level, wherein the delay is less than the time interval that said first voltage is inhibited.

3. Apparatus as defined in claim 2 wherein each of the inhibitions is sufficiently short to avoid causing any substantial motor velocity change, but is sufficiently long that the second voltage reaches a steady-state magnitude and can be sensed.

4. Apparatus as defined in claim 3, further comprising means for dissipating, during the inhibition interval, energy stored in the motor inductance.

5. Apparatus as defined in claim 4 wherein the inhibitions of the first applied voltage are separated by a time interval that is sufficiently long, relative to the duration of said inhibitions,to allow sufficient energy to be imparted to the motor, by the adjustable voltage means, for operation of the motor.

6. A method for controlling the velocity of a brush-commutated D.C. motor, comprising the steps of:

applying an adjustable drive voltage to the D.C. motor;

inhibiting the adjustable drive voltage, where the duration of said inhibition is constant and predetermined as a function of an electrical time constant of the motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, the duration of the inhibition being selected to be sufficiently long so that a motor current decays to substantially zero, and said inhibition is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

sampling and storing during each said regularly repeated inhibit time interval a value proportional to an amplitude of a voltage across the D.C. motor, and after the motor current decays to substantially zero;

comparing the stored value with a predetermined desired value; and

controlling the magnitude of the adjustable drive voltage only in response to the comparison of the stored value and the desired value.

7. A method as defined in claim 6 wherein said step of sampling and storing comprises sampling and storing a voltage proportional to the magnitude of the back emf during each inhibit time interval but after sufficient delay from commencement of the inhibit interval to permit decay of motor current to substantially zero and to permit the voltage across the D.C. motor to reach a steady-state level, wherein the delay is less than the inhibit time interval.

8. A method as defined in claim 7 wherein the inhibit time interval is sufficiently short to avoid causing any substantial motor velocity change, but is sufficiently long that the voltage across the D.C. motor reaches a steady-state magnitude and can be sensed.

9. A method as defined in claim 8, further comprising the step of dissipating, during the inhibition, energy stored in the motor inductance.

10. A method as defined in claim 9 wherein successive said steps of sampling and holding are separated by a time interval that is sufficiently long relative to the inhibit duration to allow sufficient energy to be imparted to the D.C. motor by said applying step, for operation of the motor.

11. A system for controlling the rotational velocity of a brush-type, direct-current motor, wherein the stator of the motor produces a stationary magnetic field, and the rotor of the motor contains one or more windings which terminate in a rotating commutator, the commutator being contacted by conductive brushes connected to a unidirectional current source, the commutator being arranged to supply current to the windings in a manner which causes rotation of the rotor, comprising:

(a) adjustable voltage means for applying voltage across the rotor windings of the D.C. motor wherein said adjustable voltage means is sufficient to cause rotation of the rotor of the motor;

(b) means for inhibiting said adjustable voltage means, where the duration of said inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, the duration of the inhibition being selected to be sufficiently long so that a motor current decays to substantially zero, and said inhibition is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control, thus inhibiting the voltage applied to the rotor windings of the D.C. motor where the duration of each such inhibition is such that the impact to the rotor velocity, of said D.C. motor, is negligible;

(c) a back emf magnitude detector, comprised of a means for sampling and holding a voltage value proportional to the voltage detected across the rotor windings of the D.C. motor during each inhibition of said adjustable voltage means, and after the motor current decays to substantially zero;

(d) means for comparing the voltage representing the magnitude of the sampled back emf, and thus representing the actual rotor velocity of the D.C. motor, with a value representing a desired rotor velocity to produce a velocity error signal; and

(e) means for controlling, in real-time, said adjustable voltage means, in response to the velocity error signal, to adjust the voltage applied across the rotor windings of the D.C. motor to minimize the velocity error signal and achieve the desired rotor velocity of the D.C. motor.

12. A system as defined in claim 11 wherein said means for sampling and holding comprises means for sampling and holding the voltage value during each inhibition such that said adjustable voltage means is inhibited by said inhibiting means but after a sufficient delay from commencement of the inhibition to permit the voltage across the rotor windings to reach a steady-state level, wherein the delay is less than the inhibit duration.

13. A system as defined in claim 12 wherein the inhibition duration is sufficiently short to avoid causing any substantial motor velocity change, but is sufficiently long such that the voltage across the rotor windings reaches a steady-state magnitude and can be sensed.

14. A system as defined in claim 13, further comprising means for dissipating, during the inhibition, energy stored in the motor inductance.

15. A system as defined in claim 14 wherein the inhibitions are separated by a time interval that is sufficiently long, relative to the duration of said inhibitions, to allow sufficient energy to be imparted to the motor, by said adjustable voltage means, for operation of the motor.

16. A method for controlling the rotational velocity of a brush-type, direct-current motor, wherein the stator of the motor produces a stationary magnetic field, and the rotor of the motor contains one or more windings which terminate in a rotating commutator, the commutator being contacted by conductive brushes connected to a unidirectional current source, the commutator being arranged to supply current to the windings in a manner which causes rotation of the rotor, comprising the steps of:

(a) applying an adjustable voltage across the rotor windings of the D.C. motor wherein the adjustable voltage is sufficient to cause rotation of the rotor of the D.C. motor;

(b) inhibiting the adjustable voltage where the duration of said inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, the duration of the inhibition being selected to be sufficiently long so that a motor current decays to substantially zero, and said inhibition is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control, thus inhibiting the voltage applied to the rotor windings of the D.C. motor, where the duration of each inhibition is such that the impact to the rotor velocity, of the D.C. motor, is negligible;

(c) sampling and holding the magnitude of the back emf voltage detected across the rotor windings of the D.C. motor during each said inhibition such that the adjustable voltage is inhibited by said inhibiting step, and after the motor current decays to substantially zero;

(d) comparing the stored, sampled voltage representing the magnitude of the sampled back emf, and thus representing the actual rotor velocity of the D.C. motor, with a value representing a desired rotor velocity to produce a rotor velocity error signal; and

(e) controlling, in real-time, the adjustable voltage, in response to the rotor velocity error signal, to adjust the voltage applied across the rotor windings of the D.C. motor to minimize the velocity error signal and achieve the desired rotor velocity of the D.C. motor.

17. Apparatus for controlling angular velocity of a brush-commutated direct current motor operated by a single adjustable electric input, said apparatus comprising:

means for inhibiting application of the adjustable electric input where the duration of said inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, the duration of the inhibition being selected to be sufficiently long so that a motor current decays to substantially zero, and said inhibition is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

means for sampling and holding the magnitude of the back emf of the motor during each said inhibition and after a sufficient delay from commencement of the said inhibition to permit decay of motor current to substantially zero and to permit the back emf to reach a steady-state level, wherein the delay is less than the predetermined duration of the inhibition;

means for comparing the sampled and held back emf magnitude with a preset fixed value; and

means for adjusting a magnitude of the single electric input proportional to any difference found by said comparing means between the sampled and held back emf magnitude and the preset fixed value.

18. Apparatus as defined in claim 17 wherein the duration of inhibition is sufficiently short to avoid causing any substantial motor velocity change, but is sufficiently long that the back emf reaches a steady-state magnitude and can be sensed.

19. Apparatus as defined in claim 18, further comprising means for dissipating, during the inhibition, energy stored in the motor inductance.

20. Apparatus as defined in claim 19 wherein the inhibitions are separated by a time interval that is sufficiently long, relative to the time duration of any of the inhibitions, to allow sufficient energy to be imparted to the motor, by the single adjustable electric input, for operation of the motor.

21. A method for controlling angular velocity of a brush-commutated direct current motor operated by a single adjustable electric input, said method comprising the steps of:

inhibiting application of the adjustable electric input where the duration of said inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, the duration of the inhibition being selected to be sufficiently long so that a motor current decays to substantially zero, and inhibition is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

sampling and holding the magnitude of the back emf of the motor during the inhibition but after a sufficient delay from commencement of the inhibition to permit decay of motor current to substantially zero and to permit the back emf to reach a steady-state level, wherein the delay is less than the predetermined duration of the inhibition;

comparing the sampled and held back emf magnitude with a preset fixed value; and

adjusting a magnitude of the single electric input proportional to any difference found by said comparing step between the sampled and held back emf magnitude and the preset fixed value.

22. A method as defined in claim 21, wherein the duration of inhibition is sufficiently short to avoid causing any substantial motor velocity change, but is sufficiently long that the back emf reaches a steady-state magnitude and can be sensed.

23. A method as defined in claim 22, further comprising the step of dissipating, during the inhibition, energy stored in the motor inductance.

24. A method as defined in claim 23 wherein successive said inhibiting steps are separated by a time interval that is sufficiently long, relative to the predetermined time duration of said inhibiting step, to allow sufficient energy to be imparted to the motor, by the single adjustable electric input, for operation of the motor.

25. A method as defined in claim 21 wherein the inhibition is a temporary, brief interruption of the applied voltage.

26. A method as defined in claim 21, further comprising repeating said inhibiting, sampling and holding, comparing and adjusting steps.

27. A method as defined in claim 21 wherein:

said inhibiting step comprises removing a voltage; and

said step of sampling and holding the back emf comprises sampling and holding the back emf upon each removal of the applied voltage.

28. A system for controlling the velocity of a brush-commutated D.C. motor, comprising:

adjustable voltage means for applying a first voltage across said D.C. motor;

means for periodically inhibiting, for regularly repeated time intervals of predetermined duration, the first voltage applied across the D.C. motor, where the duration of each inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor and where the duration of the inhibition is selected to be sufficiently long so that a motor current decays to substantially zero, and where the inhibition of the first voltage is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

means for sampling and holding a voltage amplitude value proportional to a second voltage across said D.C. motor during each time interval that said first voltage is inhibited, and after the motor current decays to substantially zero;

means for comparing the second voltage with a value representing a desired velocity; and

means for controlling, in real-time, the adjustable voltage means only in response to the comparing means to adjust the first voltage to a value relating to the desired velocity.

29. A method for controlling the velocity of a brush-commutated D.C. motor, comprising the steps of:

applying an adjustable drive voltage to the D.C. motor;

periodically inhibiting, for a regularly repeated time interval of predetermined duration, the adjustable drive voltage, where the duration of each inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, and where the duration of the inhibition is selected to be sufficiently long so that a motor current decays to substantially zero, and where the inhibition of the drive voltage is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

sampling and storing during each said regularly repeated inhibit time interval a value proportional to amplitude of a voltage across the D.C. motor, and after the motor current decays to substantially zero;

comparing the stored value with a predetermined desired value; and

controlling the magnitude of the adjustable drive voltage only in response to the comparison of the stored value and the desired value.

30. A system for controlling the rotational velocity of a brush-type, direct-current motor, wherein the stator of the motor produces a stationary magnetic field, and the rotor of the motor contains one or more windings which terminate in a rotating commutator, the commutator being contacted by conductive brushes connected to a unidirectional current source, the commutator being arranged to supply current to the windings in a manner which causes rotation of the rotor, comprising:

(a) adjustable voltage means for applying voltage across the rotor windings of the D.C. motor wherein said adjustable voltage means is sufficient to cause rotation of the rotor of the motor;

(b) means for periodically inhibiting said adjustable voltage means at preset time intervals of predetermined duration, thus inhibiting the voltage applied to the rotor windings of the D.C. motor for each such time interval where the duration of each such interval of inhibition is such that the impact to the rotor velocity, of said D.C. motor, is negligible, and where the duration of each inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, where the duration of the inhibition is selected to be sufficiently long so that a motor current decays to substantially zero, and where the inhibition of the voltage is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

(c) a back emf magnitude detector, comprised of a means for sampling and holding a voltage value proportional to the voltage detected across the rotor windings of the D.C. motor during each time interval that said adjustable voltage means is inhibited by said inhibiting means, and after the motor current decays to substantially zero;

(d) means for comparing the voltage representing the magnitude of the sampled back emf, and thus representing the actual rotor velocity of the D.C. motor, with a value representing a desired rotor velocity to produce a velocity error signal; and

(e) means for controlling, in real-time, said adjustable voltage means, in response to the velocity error signal, to adjust the voltage applied across the rotor windings of the D.C. motor to minimize the velocity error signal and achieve the desired rotor velocity of the D.C. motor.

31. A method for controlling the rotational velocity of a brush-type, direct-current motor, wherein the stator of the motor produces a stationary magnetic field, and the rotor of the motor contains one or more windings which terminate in a rotating commutator, the commutator being contacted by conductive brushes connected to a unidirectional current source, the commutator being arranged to supply current to the windings in a manner which causes rotation of the rotor, comprising the steps of:

(a) applying an adjustable voltage across the rotor windings of the D.C. motor wherein the adjustable voltage is sufficient to cause rotation of the rotor of the D.C. motor;

(b) periodically inhibiting the adjustable voltage for regularly repeated time intervals of predetermined duration, thus inhibiting the voltage applied to the rotor windings of the D.C. motor for each such time interval where the duration of each such time interval is such that the impact to the rotor velocity, of the D.C. motor, is negligible, and where the duration of each inhibition is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, where the duration of the inhibition is selected to be sufficiently long so that a motor current decays to substantially zero, and where the inhibition of the adjustable voltage is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

(c) sampling and holding the magnitude of the back emf voltage detected across the rotor windings of the D.C. motor during each said time interval that the adjustable voltage is inhibited by said inhibiting step, and after the motor current decays to substantially zero;

(d) comparing the stored, sampled voltage representing the magnitude of the sampled back emf, and thus representing the actual rotor velocity of the D.C. motor, with a value representing a desired rotor velocity to produce a rotor velocity error signal; and

(e) controlling, in real-time, the adjustable voltage, in response to the rotor velocity error signal, to adjust the voltage applied across the rotor windings of the D.C. motor to minimize the velocity error signal and achieve the desired rotor velocity of the D.C. motor.

32. Apparatus for controlling angular velocity of a brush-commutated direct current motor operated by a single adjustable electric input, said apparatus comprising:

means for periodically interrupting application of the adjustable electric input for a regularly repeated interval of a predetermined time duration, where the duration of each interruption is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, where the duration of the interruption is selected to be sufficiently long so that a motor current decays to substantially zero, and where the interruption of the adjustable electric input is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

means for sampling and holding the magnitude of the back emf of the motor during each said interval but after a sufficient delay from commencement of the interval to permit decay of motor current to substantially zero and to permit the back emf to reach a steady-state level, wherein the delay is less than the predetermined time duration;

means for comparing the sampled and held back emf magnitude with a preset fixed value; and

means for adjusting a magnitude of the single electric input proportional to any difference found by said comparing means between the sampled and held back emf magnitude and the preset fixed value.

33. A method for controlling angular velocity of a brush-commutated direct current motor operated by a single adjustable electric input, said method comprising the steps of:

periodically interrupting application of the adjustable electric input for a regularly repeated interval of a predetermined time duration, where the duration of each interruption is constant and predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, where the duration of the interruption is selected to be sufficiently long so that a motor current decays to substantially zero, and where the interruption of the adjustable electric input is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control;

sampling and holding the magnitude of the back emf of the motor during the interval but after a sufficient delay from commencement of the interval to permit decay of motor current to substantially zero and to permit the back emf to reach a steady-state level, wherein the delay is less than the predetermined time duration;

comparing the sampled and held back emf magnitude with a preset fixed value; and

adjusting a magnitude of the single electric input proportional to any difference found by said comparing step between the sampled and held back emf magnitude and the preset fixed value.

34. A method for controlling an angular velocity of a brush-commutated direct current (D.C.) motor that is operated by a pulse width modulated (PWM) train of electric drive pulses, said method comprising the steps of:

continuously generating PWM electric drive pulses for application to the D.C. motor;

periodically asserting a constant pulse width signal to interrupt the application of the PWM train of electric drive pulses to the D.C. motor for a predetermined interval of time, where the duration of each interruption is the same and is predetermined as a function of an electrical time constant of the D.C. motor expressed as L/R, where L is an inductance of the D.C. motor and R is a resistance of the D.C. motor, where the duration of the interruption is selected to be sufficiently long so that a motor current decays to substantially zero, and where the interruption of the adjustable electric input is set to repeat at a predetermined, constant rate that is chosen to maximize available torque while maintaining a desired accuracy of motor speed control, and where a plurality of the PWM electric drive pulses are applied to the motor between assertions of the constant pulse width signal;

during each interruption of the PWM train of electric drive pulses, waiting for the motor current to decay to substantially zero and then sampling and holding a magnitude of the back emf of the motor;

deasserting the constant pulse width signal, thereby terminating the interruption of the PWM train of electric drive pulses;

comparing the sampled and held back emf magnitude with a reference value that is indicative of a desired angular velocity of the D.C. motor; and

adjusting the pulse width of the PWM train of electric drive pulses by an amount that is proportional to any difference found by said comparing step between the sampled and held back emf magnitude and the reference value.