Digital robot control having an improved pulse width modulator

• US 4,894,598 A
• Filed: 08/05/1988
• Issued: 01/16/1990
• Est. Priority Date: 11/20/1986
• Status: Expired due to Term

• Current Assignee: Stäubli International AG
• Original Assignee: Stäubli International AG
• Inventors: Daggett, Kenneth E.
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): IP, SHIK LUEN PAUL
• Application Number:
  US07/231,627
• Time in Patent Office: 529 Days
• Field of Search: 318/310-345, 318/562-577, 318/434, 318/599, 364/513, 901/20, 901/21, 901/22, 901/23, 901/24
• US Class Current: 318/568.16
• CPC Class Codes: B25J 13/088 with position, velocity or acceleration sensors
• CPC Class Codes: G05B 19/237 with a combination of feedback covered by G05B19/232 - G05B19/235
• CPC Class Codes: G05B 2219/33337 For each axis a processor, microprocessor
• CPC Class Codes: G05B 2219/34466 Bad circuits, watchdog, alarm, indication
• CPC Class Codes: G05B 2219/37313 Derive speed from position
• CPC Class Codes: G05B 2219/41123 Correction inertia of servo
• CPC Class Codes: G05B 2219/41426 Feedforward of torque
• CPC Class Codes: G05B 2219/42237 Pwm pulse width modulation, pulse to position modulation ppm
• CPC Class Codes: G05B 2219/42256 Sampling the signal
• CPC Class Codes: G05B 2219/45083 Manipulators, robot

Abstract

A pulse width moduation (PWM) scheme provides for digital implementation of robot control commands. For this purpose, it provides time stabilized current sampling synchronized to the sampling frequency of the position and velocity loops in the robot control. The PWM scheme enables accurate motor current measurement through usage of a single current sense resistor independent of the number of legs in a power amplifier configuration. This scheme permits commercially available, low cost, power blocks to be used for implementation of robot control system power amplifiers.

The PWM scheme is cost effectively implemented in a semi-custom large scale integrated (LSI) circuit which provides the digital interface between a torque loop microprocessor and the drive circuitry for the power handling devices used to apply voltage to the robot axis actuator. Interface and control for three axes is provided by a single LSI device.

Claims

1. A digital pulse width modulator circuit for operating power amplifiers and joint motors during positive, negative and non-forcing motor drive requests by measuring the motor current in an associated sense resistor for each joint motor in a robot control comprising:

up/down counter means for generating a sawtooth waveform, having positive and negative apexes adapted for timing the current measurement within a control pulse interval, by counting up to and down from an upper endpoint to a lower endpoint as a function of time;

means for comparing the count of the counter means for the sawtooth waveform to a signal representing a control command so as to define a time span needed to operate a power amplifier and energize an associated joint drive motor in accordance with the control command;

means for generating a first output control pulse for power amplifier switching when one drive polarity is commanded, said first output control pulse having a width corresponding to the defined time span and based on a count comparison referenced to the upper endpoint of the sawtooth waveform; and

means for generating a second output control pulse for power amplifier switching when the other polarity is commanded, said second output control pulse having a width corresponding to the defined time span and based on a count comparison referenced to the lower endpoint of the sawtooth waveform.

2. A digital pulse width modulator circuit as set forth in claim 1 wherein:

means are provided for generating an end stop output control pulse for a short period of time centered on the upper waveform endpoint and another end stop output control pulse for an equal short period of time centered on the lower waveform endpoint.

3. A digital pulse width modulator circuit as set forth in claim 2 wherein:

means are provided for generating a third output control pulse corresponding to said one command polarity and generally being on when said second output control pulse is off so as to provide for circulation of regenerative motor currents through the power amplifier; and

means are provided for generating a fourth output control pulse corresponding to said other command polarity and generally being on when said first output control pulse is off so as to provide for circulation of regenerative motor currents through the power amplifier.

4. A digital pulse width modulator circuit as set forth in claim 3 wherein:

underlap control means are provided to maintain a minimum off time between said first and fourth output control pulses and between said second and third output control pulses so as to allow for switching time of switching devices controlled by such output control pulses.

5. A digital pulse width modulator circuit as set forth in claim 1 wherein:

watchdog timer means are included to count a number of consecutive counter cycles over which no command update is received and to signal a shutdown if the number of such cycles reaches a predetermined value.

6. A digital pulse width modulator circuit as set forth in claim 1 wherein a large scale integrated gate array chip is employed to embody the circuit.

7. A digital pulse width modulator circuit as set forth in claim 2 wherein a large scale integrated gate array chip is employed to embody the circuit.

8. A digital pulse width modulator circuit as set forth in claim 4 wherein a large scale integrated gate array chip is employed to embody the circuit.

9. A digital pulse width modulator circuit as set forth in claim 5 wherein a large scale integrated gate array chip is employed to embody the circuit.

10. A digital pulse width generator as set forth in claim 7 wherein three sets of said comparing means and said generating means are provided within said chip to provide three channels for controlling three respective motors associated with three respective axes of said robot.

11. A digital control for a robot having a plurality of arm joints, said control comprising:

an electronic motor for driving each of the robot arm joints;

a power amplifier for each of said motors having power switching devices operable by sensing the current in an external sense resistor for each motor and power amplifier pair, to supply current to the motor for forward, non-forcing and reverse operation;

a digital pulse width modulator circuit for operating the power amplifier switching devices for each aarm joint;

said modulator circuit including up/down counter means for generating a sawtooth waveform, and congruently imposing a time interval centered on an apex of said sawtooth waveform defining a control pulse width for a current sensing measurement, by counting up to and down from predetermined endpoints as a function of time;

means for comparing the count of the counter means for the sawtooth waveform to a signal representing a control command so as to define a time span needed to operate a power amplifier and energize an associated joint drive motor in accordance with the control command;

means for generating a first output control pulse for power amplifier switching when one drive polarity is commanded, said first output control pulse having a width corresponding to the defined time span and based on a count comparison referenced to the upper endpoint of the sawtooth waveform; and

means for generating a second output control pulse for power amplifier switching when the other polarity is commanded, said second output control pulse having a width corresponding to the defined time span and based on a count comparison referenced to the lower endpoint of the sawtooth waveform; and

feedback control loop means for each arm joint and including at least digital position, velocity and current control loops looping through and operating the associated pulse width modulator circuit.

12. A digital robot control as set forth in claim 11 wherein:

each motor is a brush type DC motor;

each power amplifier is an H bridge amplifier circuit having a power switching device in each bridge leg and having the associated DC motor connected in a path joining across and central to the bridge legs;

said first and second output control pulses are coupled to first and second switching devices in the bridge legs on first and second sides of the bridge; and

said digital pulse width modulator circuit further includes means for generating third and fourth output control pulses corresponding respectively to said one and said other command polarities and generally being on respectively when said second output or said first control pulse is off so as to provide for circulation of regenerative motor currents through the power amplifier; and

said third pulse is coupled to a switching device in the other bridge leg on the first bridge side and said fourth pulse is coupled to a switching device in the other bridge leg on the second bridge side.

13. A digital robot control as set forth in claim 11 wherein:

each motor is a brushless type DC motor having three interconnected phase windings; and

each power amplifier includes six power switching devices interconnected between a power supply and the associated motor windings to energize said motor windings under control of said pulse width modulator circuit.

14. A digital robot control as set forth in claim 11 wherein:

means are provided for generating an end stop output control pulse for a short period of time centered on the upper waveform endpoint and another end stop output control pulse for an equal short period of time centered on the lower waveform endpoint.

15. A digital control as set forth in claim 12 wherein:

underlap control means are provided to maintain a minimum off time between said first and fourth output control pulses and between said second and third output control pulses so as to allow for switching time of switching devices controlled by such output control pulses.

16. A robot having:

an arm with a plurality of joints;

each of said joints having an electric drive motor associated therewith;

a power amplifier for operating each of said motors by sampling the motor current in a sense resistor remotely sensitive to each associated said motor;

digital pulse width modulator circuit means for operating each power amplifier;

each pulse width modulator circuit means including up/down counter means for generating a sawtooth waveform by counting to upper and lower endpoints, each endpoint is adapted to be symmetrically straddled by respective upper and lower control pulse intervals for defining a sensing interval for sampling motor current as a function of time;

means for comparing the counter means count for the sawtooth waveform to a signal representing a control command so as to define a time span needed to operate a power amplifier and energize an associated joint drive motor in accordance with the control command;

means for generating a first output control pulse for power amplifier switching when one drive polarity is commanded, said first output control pulse having a width corresponding to the defined time span and based on a count comparison referenced to the upper endpoint of the sawtooth waveform; and

control loop means for each arm joint and including at least digital position, velocity and current control loops looping through and operating the associated pulse width modulator circuit.

17. A robot as set forth in claim 16 wherein:

means are provided for generating an end stop output control pulse for a short period of time centered on the upper waveform endpoint and another end stop output control pulse for an equal short period of time centered on the lower waveform endpoint.

18. A robot as set forth in claim 16 wherein:

means are provided for generating a third output control pulse corresponding to said one command polarity and generally being on when said second output control pulse is off so as to provide for circulation of regenerative motor currents through the power amplifier; and

means are provided for generating a fourth output control pulse corresponding to said other command polarity and generally being on when said first output control pulse is off so as to provide for circulation of regenerative motor currents through the power amplifier.

19. A robot as set forth in claim 18 wherein:

underlap control means are provided to maintain a minimum off time between said first and fourth output control pulses and between said second and third output control pulses so as to allow for switching time of switching devices controlled by such output control pulses.

20. A system for energizing and controlling a robot arm joint motor comprising:

at least one winding for said motor;

a power amplifier for energizing said motor winding;

said power amplifier having first switching means for energizing said motor winding in the forward direction and second switching means for energizing said motor winding in the reverse direction and a sense resistor responsive to the current in both of said motor winding directions;

a digital pulse width modulator circuit having up/down counter means for generating a counting period as a function of time;

means for comparing the counter means counting period to a signal representing a control command so as to define a time width of at least a first output control pulse for operating said first switching means and a time width of at least a second output control pulse for operating said second switching means according to the control command magnitude and a command polarity; and

means for generating respective signals for operating said first and second switching means to force current to be conducted through said motor winding in each direction for a short period of time stationed at the count extremities, in each cycle of said counter.

21. A system as set forth in claim 20 wherein:

said counter means generates a counting period corresponding to substantially a cycle width of a sawtooth waveform; and

said forced current signals are end stop signals generated over equal time spans at and centered on so as to be equally timed from the respective apexes of the upper and lower endpoints of the sawtooth waveform in each counter cycle.

22. A system as set forth in claim 21 wherein:

means are provided for generating a strobe signal at least once each counter cycle at one of the waveform endpoints.

23. A system as set forth in claim 20 wherein:

watchdog timer means are included to count the number of consecutive counter cycles over which no command update is received and to signal a shutdown if the number of such cycles reaches a predetermined value.

24. A digital pulse width modulator circuit for operating power amplifiers and joint motors during positive and negative drive polarity as well as non-forcing request commands, by measuring the motor current during a sensing interval in an associated sense resistor for each joint motor in a robot control comprising:

up/down counter means for generating a sawtooth waveform, having positive and negative apexes adapted for control pulse positioning for defining the sensing interval in which to measure the current from each joint motor through said associated sense resistor, by counting up to and down from an upper endpoint to a lower endpoint as a function of time;

means for comparing the count of the counter means for the sawtooth waveform to a signal representing a control command so as to define a time span needed to operate a power amplifier and energize an associated joint drive motor in accordance with the control command request and for sampling the motor current in the sense resistor at a point in time midway between the beginning and end of the control pulse sensing intervals, the current sampling time occurring only at the apexes of said sawtooth waveform having one predefined polarity;

means for generating a first output control pulse for power amplifier switching when one drive polarity is commanded, said first output control pulse having a width corresponding to the defined time span and based on a count comparison referenced to the upper endpoint of the sawtooth waveform; and

means for generating a second output control pulse for power amplifier switching when the other polarity is commanded, said second output control pulse having a width corresponding to the defined time span and based on a count comparison referenced to the lower endpoint of the sawtooth waveform, said pulse width modulator circuit thereby measuring the motor current synchronously to control said robot, including at those times when the motor is regenerating while a non-forcing command is requested.