Robot control system having adaptive feedforward torque control for improved accuracy

• US 4,925,312 A
• Filed: 03/21/1988
• Issued: 05/15/1990
• Est. Priority Date: 03/21/1988
• Status: Expired due to Term

• Current Assignee: Westinghouse Electric Corporation (Paramount Global (f/k/a ViacomCBS Inc.))
• Original Assignee: Stäubli International AG
• Inventors: Lancraft, Roy E., Penkar, Rajan C., Casler, Richard J. Jr., Sha, Chi, Onaga, Eimei M.
• Primary Examiner(s): Ruggiero, Joseph
• Application Number:
  US07/171,500
• Time in Patent Office: 785 Days
• Field of Search: 364/513, 364/191-193, 364/174, 364/164, 364/165, 318/561, 318/568, 901/2-5, 901/9, 901/20, 901/23, 901/24, 901/41, 901/46, 414/730
• US Class Current: 700/261
• CPC Class Codes: B25J 9/1633 compliant, force, torque control, e.g. combined with position control
• CPC Class Codes: G05B 19/4163 Adaptive control of feed or cutting velocity without NC B23Q15/12
• CPC Class Codes: G05B 2219/36417 Programmed coarse position, fine position by alignment, follow line, path adaptive
• CPC Class Codes: G05B 2219/37357 Force, pressure, weight or deflection
• CPC Class Codes: G05B 2219/41434 Feedforward FFW

Abstract

A digital control provides adaptive feedforward torque control for a robot having a plurality of arm joints. An electric motor drives each of the robot arm joints and a power amplifier supplies drive current to each motor under controlled operation.

Each joint motor has feedback control loop means including position and velocity control loops driving a torque control loop in accordance with position commands to generate motor commands for controlling the associated power amplifier. The motion of said joint motor generates position and velocity feedback signals respectively for combination with the position and velocity commands to generate an error signal as a torque command for each of the torque control loops from the corresponding position and velocity control loops. Load force is sensed at the endmost robot joint. The actual load moment for each robot joint on the basis of the load force and the dynamic/kinematic data moment is computed for each robot joint on the basis of stored dynamic and kinematic data for the robot arm. The data and load moments are combined for each joint and applied as a feedforward torque for combination with the position and velocity errors in generating the torque command for each of the torque control loops. A feedback signal representative of the joint motor drive current is combined with the torque command to generate a torque error signal in each torque control loop.

Claims

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

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

a power amplifier operable to supply drive current to each motor;

means for sensing load force applied to at least a predetermined one of said robot joints;

means for computing the actual load moment for each robot joint on the basis of the output from said force sensing means;

means for computing the dynamic/kinematic data moment for each robot joint on the basis of stored dynamic and kinematic data for the robor arm;

means for combining the data and load moments for each joint to generate a feedforward torque therefor; and

each joint motor having feedback control loop means including a torque control loop to generate motor commands for the associated power amplifier in accordance with position commands and in accordance with the feedforward torque.

2. A robot control as set forth in claim 1 wherein said data moment computing means employs the Newton-Euler dynamic equations to compute the respective joint data moments.

3. A robot control as set forth in claim 1 wherein said force sensing means is disposed at the endmost joint linked to the robot and effector, said load moment computing means computes the respective joint load moments in a backward recursion, and said data moment computing means employs the Newton-Euler dynamic equations in a forward recursion to compute the respective joint data moments.

10. A robot control as set forth in claim 1 wherein said kinematic data includes joint offsets based on the length of each link and the twisting angle between adjacent links, and said dynamic data includes the inertia of each joint and the center of mass of each link.

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

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

a power amplifier operable to supply drive current to each motor;

each joint motor having feedback control loop means including position and velocity control loops driving a torque control loop in accordance with position commands to generate motor commands for controlling the associated power amplifier;

means for sensing the motion of said joint motor and for generating position and velocity feedback signals respectively for combination with the position and velocity commands to generate an error signal as a torque command for each of said torque control loops from the corresponding position and velocity control loops;

means for sensing load force applied to at least a predetermined one of said robot joints;

means for computing the actual load moment for each robot joint in the basis of the output from said force sensing means;

means for computing the dynamic/kinematic data moment for each robot joint on the basis of stored dynamic and kinematic data for the robot arm;

means for combining the data and load moments for each joint and applying the combined moment as a feedforward torque for combination with said position and velocity errors in generating said torque command for each of said torque control loops; and

means for generating a feedback signal representative of the joint motor drive current for combination with the torque command to generate a torque error signal in each torque control loop.

5. A robot control as set forth in claim 4 wherein said data moment computing means employs the Newton-Euler dynamic equations to compute the respective joint data moments.

6. A robot control as set forth in claim 5 wherein said force sensing means is disposed at the endmost joint linked to the end effector, said load moment computing means computes the respective joint load moments in a backward recursion, and said data moment computing means employs the Newton-Euler dynamic equations to compute the respective joint data moments in a forward recursion.

7. A robot control as set forth in claim 5 wherein the robot control is essentially fully digital, each feedback control loop means is digitally operable at a predetermined sampling rate to generate digital motor commands on the basis of digital position commands, each motor motion sensing means generates position and velocity feedback signals for control calculations in the position and velocity control loops, and each current feedback generating means generates digital signals for control calculations in the torque control loop.

8. A robot control as set forth in claim 7 wherein motion planning means generates said digital position commands, and the calculated torque for each joint is applied to said motion planner for comparison to available torque and reduction or increase in planned speed accordingly.

9. A robot control as set forth in claim 7 wherein said kinematic data includes joint offsets based on the length of each link and the twisting angle between adjacent links, and said dynamic data includes the inertia of each joint and the center of mass of each link.