Digital robot control providing pulse width modulation for a brushless DC drive

• US 4,874,997 A
• Filed: 04/01/1988
• Issued: 10/17/1989
• Est. Priority Date: 11/20/1986
• Status: Expired due to Term

• Current Assignee: Unimation, Inc. (Stäubli Holding AG)
• Original Assignee: Unimation, Inc. (Stäubli Holding AG)
• Inventors: Johnson, Richard A., Casler, Richard J. Jr., Daggett, Kenneth E., Onaga, Eimei M.
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): Ip, Paul
• Application Number:
  US07/178,813
• Time in Patent Office: 564 Days
• Field of Search: 318/254, 318/138, 318/313, 318/314, 318/317, 318/326, 318/327, 318/341, 318/345 R, 318/345 B, 318/439, 318/563, 318/568 R, 318/568 M, 318/560, 318/565, 318/566 , 318/567, 318/574, 318/590, 318/599, 318/603, 318/608, 318/615, 318/616, 318/617, 318/618, 318/685, 364/478, 364/513, 364/573, 364/169, 364/167.01, 364/174, 901/19, 901/20, 901/22, 901/23, 901/24
• US Class Current: 318/568.1
• 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

Circuitry is provided for applying a pulse width modulation (PWM) scheme to a brushless DC motor that operates as a robot axis drive. A pulse width modulation (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 brushless drive application circuitry provides for application of the PWM control outputs to commutate the motor energization from winding pair to winding pair.

Claims

1. A system for digitally controlling and energizing a brushless DC arm joint motor in a robot control, said system comprising:

three interconnected phase windings for said motor;

a power amplifier having first through sixth switching means interconnected with said windings to direct drive current through series paired windings corresponding to the switching operation of pairs of said switching means;

a digital pulse width modulator circuit having up/down counter means for generating a sawtooth voltage waveform representing counts as a function of time;

means for comparing the counter means count to a voltage signal representing control command so as to digitally control and define the time width of forward and reverse reference control pulses for said power amplifier switching means in accordance with the compared control command voltage signal and the polarity of the control command signal;

means for generating an enabling reference control pulse for each of said reference control pulses;

means for generating a plurality of phase displaced signals representative of motor rotor position;

means for generating respective control pulses which operate the six power amplifier switching means and correspond in time width to said reference control pulses; and

commutating means responsive to said position signals and the polarity of the control command signal to apply successive pairs of said switch control pulses to said switching means so as to produce digitally commutated, pulse width modulated, motor drive current for the command time as the motor current path is commutated from winding pair to winding pair in accordance with the polarity of the control command signal and a predetermined sequence of phase rotation.

2. A system as set forth in claim 1 wherein:

a first reference control pulse represents one control command signal polarity and is based on a count comparison referenced to the upper endpoint of the sawtooth waveform; and

a second reference control pulse represents the other control command signal polarity and is based on a count comparison referenced to the lower endpoint of the sawtooth waveform.

3. A system as set forth in claim 1 wherein said commutating and control pulse generating means are embodied in a programmed logic array (PAL) device programmed in accordance with the following equations:

##EQU2## wherein U1 U2, V1, V2, W1 and W2 are the switching means control pulses, A1, A2 and B1, B2 are the reference control pulses, and HA, HB and HC are the overlapping position signals which are phase shifted sixty electrical degrees with respect to each other.

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

a brushless DC motor for driving each of the arm joints;

three interconnected phase windings for said motor;

a power amplifier having first through sixth switching means interconnected with said windings to direct drive current through series paired windings corresponding to the switching operation of pairs of said switching means;

a digital pulse width modulator circuit having up/down counter means for generating a sawtooth voltage waveform by counting up and down as a function of time;

means for comparing the voltage of the counter means count for the sawtooth waveform to a voltage signal representing a drive control command so as to digitally define the time width of forward and reverse reference control pulses for said power amplifier switching means in accordance with the drive control command voltage and the polarity thereof of the control command;

means for generating an enabling reference control pulse for each of said reference control pulses;

means for generating a plurality of phase displaced signals representative of motor rotor position;

means for generating respective control pulses which operate the six power amplifier switching means and correspond in time width to said reference control pulses;

commutating means responsive to said position signals and the polarity of the drive control command to apply successive pairs of said switch control pulses to said switching means so as to digitally rotate the pulse width modulated motor drive current for the command time as the motor current path is commutated from winding pair to winding pair in accordance with the command polarity and a predetermined rotational sequence; and

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

5. A robot having:

an arm with a plurality of joints;

each of said joints having a brushless DC motor as a drive;

three interconnected phase windings for said motor;

a power amplifier having first through sixth switching means interconnected with said windings to direct drive current through series paired windings corresponding to the switching operation of pairs of said switching means;

a digital pulse width modulator circuit having up/down counter means for generating a sawtooth waveform representing counts as a function of time;

means for comparing the counter means count level to a signal magnitude representing a control command so as to define the time width of forward and reverse reference control pulses for said power amplifier switching means in accordance with the magnitude of the control command signal having control command polarity;

means for generating an enabling reference control pulse for each of said reference control pulses;

means for generating a plurality of phase displaced signals representative of motor rotor position;

means for generating respective control pulse which operate the six power amplifier switching means and correspond in time width to said reference control pulses;

commutating means responsive to said position signals and the polarity of the control command signal to apply successive pairs of said switch control pulses to said switching means so as to produce digitally commutated motor drive current for the command time as the motor current path is commutated from winding pair to winding pair in accordance with the command polarity and a pedetermined phase sequence; and

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