DC motor drive system and method

• US 5,220,259 A
• Filed: 10/03/1991
• Issued: 06/15/1993
• Est. Priority Date: 10/03/1991
• Status: Expired due to Term

• Current Assignee: Graco Incorporated
• Original Assignee: Graco Incorporated
• Inventors: Werner, Neal A., Box, Gary W.
• Primary Examiner(s): Ro, Bentsu
• Application Number:
  US07/770,554
• Time in Patent Office: 621 Days
• Field of Search: 318/138, 318/254, 318/432, 318/433, 318/434, 318/439
• US Class Current: 318/432
• CPC Class Codes: F04B 2203/0207 Torque
• CPC Class Codes: F04B 2203/0209 Rotational speed
• CPC Class Codes: F04B 49/065 and making use of computers
• CPC Class Codes: H02P 6/06 Arrangements for speed regulation of a single motor wherein the motor speed is measured and compared with a given physical value so as to adjust the motor speed
• CPC Class Codes: H02P 6/34 Modelling or simulation for control purposes

Abstract

A DC motor drive system for providing an enhanced motor speed/torque relationship, where circuits modify the motor field winding control switching sequence to compensate for speed effects on motor torque output and to compensate on transient loading effects on motor torque output, and to compensate for rotor inertial effects caused by transient motor load changes.

Claims

1. An apparatus for controlling the driver bus current in a DC motor drive system by selectively controlling the switching intervals of a plurality of inverter switches coupled to the motor field windings, comprising:

means for manually selecting a set-point torque value defining the desired motor output torque;

means for monitoring the instantaneous motor driver bus current and developing a motor driver bus current control signal therefrom;

means for monitoring the motor rotor rotational speed and developing a motor rotor speed control signal therefrom;

means for modifying said set-point torque value, responsive to said motor rotor speed control signal, to produce a modified set-point torque value;

means for simulating motor winding phase current by utilizing said motor driver bus current and rotor speed control signals to generate a simulated motor phase current signal;

means for utilizing said modified set-point torque value and said simulated motor phase current signal, and means for developing an inverter control signal therefrom; and

means for turning off said inverter switches with said inverter control signal.

2. The apparatus of claim 1, wherein said means for monitoring instantaneous motor driver bus current and developing a motor driver bus current control signal therefrom further comprises a current sensor and first circuit means for developing a signal I_BUS representative or motor driver bus current.

3. The apparatus of claim 2, wherein said means for manually selecting a set-point torque value further comprises second circuit means for developing a signal I_COM representative of said set-point torque value.

4. The apparatus of claim 3, wherein said means for monitoring motor rotor speed and developing a motor rotor speed control signal therefrom further comprises third circuit means for developing a signal I_SPD representative of the motor rotor speed.

5. The apparatus of claim 4, wherein said means for modifying said set-point torque value further comprises fourth circuit means for developing a signal I_MOD which is the sum of I_COM and I_SPD.

6. The apparatus of claim 5, wherein said means for monitoring the instantaneous motor driver bus current and developing a motor driver bus current control signal therefrom further comprises fifth circuit means coupled to receive said I_BUS signal and for developing a signal I_PEAK representative of the peak value of I_BUS.

7. The apparatus of claim 6, wherein said means for monitoring the instantaneous motor driver bus current and developing a motor driver bus current control signal therefrom further comprises sixth circuit means coupled to receive said I_PEAK signal and said I_SPD signal, for developing a signal I_PHASE representative of said motor winding phase current.

8. The apparatus of claim 7, wherein said means for utilizing said modified set-point torque value and said simulated motor phase current signal further comprises seventh circuit means for comparing said I_PHASE signal with said I_MOD signal.

9. The apparatus of claim 8, wherein said means for utilizing said modified set-point torque value and said simulated motor phase current signal further comprises eighth circuit means for comparing said I_PHASE to said I_MOD, and ninth circuit means for developing an error signal I_ERROR if said I_PHASE is greater than said I_MOD, the magnitude of said I_ERROR being proportional to the difference between said I_PHASE and said I_MOD ;

and tenth circuit means for developing an error signal I_ERROR equal to zero if said I_PHASE is not greater than said I_MOD.

10. The apparatus of claim 9, wherein said means for utilizing said modified set-point torque value and said simulated motor phase current signal further comprises eleventh circuit means coupled to receive said I_MOD signal and said I_ERROR signal, and for developing a signal I_SET representative of the sum of said I_MOD and said I_ERROR.

11. The apparatus of claim 10, wherein said means for turning off said inverter switches further comprises twelfth circuit means for comparing said I_BUS signal with said I_SET signal, and for generating a turn-off signal to said inverter switches when said I_BUS equals said I_SET.

12. The apparatus of claim 1, further comprising means for calculating the rate of change of motor rotor speed, and means for comparing the calculated rate of change against a predetermined value.

13. The apparatus of claim 12, further comprising means for generating a BRK 1 and a BRK 2 signal when the means for comparing shows the calculated rate of change to be equal to or greater than the predetermined value.

14. The apparatus of claim 13, further comprising first means for receiving said BRK 1 signal and means for reversing the commutation sequence of said motor in response thereto.

15. The apparatus of claim 14, further comprising second means for receiving said BRK 2 signal and means responsive thereto for setting said simulated motor phase current signal equal to said motor driver bus current control signal.

16. The apparatus of claim 15, further comprising further means for changing said set-point torque value to a value equal to a constant multiplied by said motor rotor rotational speed control signal.

17. The apparatus of claim 16, wherein said means for modifying said set-point torque value, responsive to said motor rotor speed control signal, further comprises means for developing a modified signal I_MOD equal to the sum of said motor rotor speed control signal and said changed set-point torque value.

18. The apparatus of claim 17, wherein said means for simulating phase current further comprises means for simulating a phase current equal to said instantaneous motor driver bus current.

19. The apparatus of claim 18, wherein said means for utilizing said modified set-point torque value and said simulated motor phase signal further comprises means for utilizing said I_MOD signal and said instantaneous motor driver bus current, and said means for developing an inverter control signal comprises means for turning off said inverter switches when said I_MOD signal equals said instantaneous motor driver bus current.

20. In a DC motor drive system having variable loading conditions and comprising semiconductor switches interposed between a direct current supply and the DC motor field windings, the improvement in controlling said switches to provide a constant motor torque output corresponding to a manually-set torque set-point value, comprising:

a) means for monitoring the DC motor driver bus peak current and generating a first control signal (I_PEAK) responsive thereto;

b) means for monitoring the DC motor rotor speed and generating a second control signal (I_SPD) responsive thereto;

c) means for generating a third control signal (I_MOD) by additively combining said second control signal with said set-point value;

d) means for generating a fourth control signal (I_PHASE) by linearly reducing said first control signal over time;

e) means for comparing said third control signal and said fourth control signal, including means for generating an error signal if said fourth control signal exceeds said third control signal;

f) means for generating a fifth control signal by additively combining said error signal and said third control signal; and

g) means for shutting off said switches when said fifth control signal becomes equal to the motor field winding current.

21. A method for controlling a DC motor drive system to produce a constant output torque under variable loading conditions, responsive to a preselected set-point torque, by controlling the DC motor driver bus current I_BUS, comprising the steps of:

a) selecting a set-point current I_COM corresonding to said preselected set-point torque;

b) monitoring the instantaneous motor drive bus current I_BUS ;

c) monitoring the motor rotational speed and developing a current I_SPD responsive thereto;

d) modifying I_COM in correspondence with I_SPD to form I_MOD =I_COM +I_SPD ;

e) simulating the motor phase current I_PHASE by linearly reducing the peak value of I_BUS at a time rate corresponding to I_SPD ;

f) comparing I_PHASE with I_MOD and developing an error signal proportional to the difference (I_PHASE -I_MOD);

g) forming a new set-point signal I_SET equal to the sum of I_MOD plus said error signal; and

h) turning off the current I_BUS when I_SET equals I_BUS.

22. The method of claim 21, wherein the step of simulating the motor phase current I_PHASE further comprises capturing the peak value I_PEAK of I_BUS, and linearly reducing I_PEAK to form I_PHASE according to the relation I_PHASE =I_PEAK (1-nt) where "n" is a constant value and "t" is the reciprocal of I_SPD.

23. The method of claim 22, wherein the step of comparing I_PHASE with I_MOD further comprises developing a zero error signal I_ERROR =0 if I_PHASE is not greater than I_MOD, and developing an error signal I_ERROR =(I_PHASE -I_MOD)×

10 if I_PHASE is greater than I_MOD.

24. The method of claim 23, wherein the step of turning off the current I_BUS further comprises turning off inverter driver transistors in the DC motor drive system.

25. A method for controlling a DC motor drive system under variable loading conditions, comprising the steps of:

a) manually setting a set-point motor torque output value I_COM ;

b) monitoring the instantaneous motor driver bus current I_BUS ;

c) monitoring the motor rotational speed I_SPD ;

d) modifying the set-point value I_COM in correspondence with increases in motor rotational speed I_SPD, to form a modified set-point value I_MOD ;

e) capturing the peak value I_PEAK of the motor driver bus current I_BUS ;

f) linearly reducing the captured peak value at a time rate corresponding to motor rotational speed to simulate a motor phase current I_PHASE ;

g) comparing the simulated motor phase current I_PHASE with I_MOD, and developing an error signal if the simulated motor phase current is greater than the modified set-point value, the magnitude of the error signal being substantially greater than the difference between the simulated motor phase current and the modified set-point value;

h) adding the error signal to the I_MOD to create a new signal I_SET ;

i) comparing the new signal I_SET to the driver bus current I_BUS ; and

j) shutting off the driver bus drive current I_BUS when the driver bus current signal I_BUS becomes equal to the new signal I_SET.