Motor controller providing position feedback correction

• US 7,696,712 B2
• Filed: 08/28/2007
• Issued: 04/13/2010
• Est. Priority Date: 08/28/2007
• Status: Active Grant

• Current Assignee: Rockwell Automation Technologies Incorporated (Allen-Bradley Co., Inc.)
• Original Assignee: Rockwell Automation Technologies Incorporated (Allen-Bradley Co., Inc.)
• Inventors: Cooper, Mark R., Lundell, Thomas D.
• Primary Examiner(s): Ro; Bentsu
• Assistant Examiner(s): DINH, THAI T
• Application Number:
  US11/846,403
• Publication Number:
  US 20090058345A1
• Time in Patent Office: 959 Days
• Field of Search: 318/602, 318/600, 318/603, 318/626, 318/632, 318/638, 318/560
• US Class Current: 318/602
• CPC Class Codes: G05B 19/21 using an incremental digital measuring device
• CPC Class Codes: G05B 19/27 using an absolute digital measuring device
• CPC Class Codes: G05B 19/4062 Monitoring servoloop, e.g. overload of servomotor, loss of feedback or reference
• CPC Class Codes: G05B 2219/37263 Absolute and incremental encoder, detector combined

Abstract

A motor controller employs absolute encoder signals to periodically assess the existence of cumulative error in a position signal derived from an incremental encoder signal. In one embodiment the absolute encoder signals are extracted from commutation switches of the motor eliminating the need for a separate absolute encoder.

Claims

1. A motor controller system for the control of an electric motor comprising:

a first input for receiving an incremental encoder signal from an incremental encoder driven by the electric motor;

a second input for receiving an absolute encoder signal from an absolute encoder driven by the electric motor;

a feedback circuit receiving a command signal for the control of a position of the electric motor and for receiving a position feedback signal indicating a position of the electric motor derived from the incremental encoder signal, the feedback circuit producing an error signal used to drive the electric motor; and

a correction circuit receiving the incremental encoder signal and the absolute encoder signal to compare the two to derive a feedback error signal on a regular basis for a detection of errors in the position feedback signal derived from the incremental encoder signal.

2. The motor controller system of claim 1 wherein the correction circuit further provides an output to a user indicating an error in the position feedback signal exceeding a predetermined magnitude.

3. The motor controller system of claim 1 wherein the correction circuit further corrects the feedback error signal based on detected error in the position feedback signal.

4. The motor controller system of claim 3 wherein the feedback error signal is corrected to maintain a change in the error signal caused by the correction below a predetermined magnitude.

5. The motor controller system of claim 4 wherein the feedback error signal is corrected incrementally over a predetermined period of time.

6. The motor controller system of claim 1 wherein the absolute encoder signal is obtained on a periodic basis at a rate lower than a periodic basis at which the incremental signal is obtained by the motor controller.

7. The motor controller system of claim 1 wherein the absolute encoder signal is a standard multi-bit position signal derived from an absolute encoder.

8. The motor controller system of claim 1 wherein in the absolute encoder signal is derived from commutation signals from the electric motor.

9. The motor controller system of claim 8 wherein in the absolute encoder signal is derived from Hall-effect commutation switches in the motor.

10. The motor controller system of claim 8 wherein the regular periodic basis is timed to coincide with a switching of a commutation electrical signal.

11. A method of controlling an electric motor comprising the steps of:

(a) receiving an incremental encoder signal from an incremental encoder driven by the electric motor;

(b) receiving an absolute encoder signal from an absolute encoder driven by the electric motor;

(c) receiving a command signal for the control of a position of the electric motor and a position feedback signal indicating a position of the electric motor and derived from the incremental encoder signal, the feedback circuit producing an error signal used to drive and the electric motor; and

(d) receiving the incremental encoder signal and the absolute encoder signal to compare the two to derive a feedback error signal on a regular basis for a detection of errors in the position feedback signal derived from the incremental encoder signal.

12. The method of claim 11 further including the step of correcting the feedback error signal based on detected error in the position feedback signal.

13. The method of claim 12 wherein the feedback error signal is corrected to maintain a change in the error signal caused by the correction below a predetermined magnitude.

14. The method of claim 12 wherein the feedback error signal is corrected incrementally over a predetermined period of time.

15. The method of claim 11 further including the step of providing an output to a user indicating an error in the position feedback signal exceeding a predetermined magnitude.

16. The method of claim 11 wherein the absolute encoder signal is obtained on a periodic basis at a rate lower than a periodic basis at which the incremental signal is obtained by the motor controller.

17. The method of claim 11 wherein the absolute encoder signal is a standard multi-bit position signal derived from an absolute encoder.

18. The method of claim 11 wherein in the absolute encoder signal is derived from commutation signals from the electric motor.

19. The method of claim 18 wherein in the absolute encoder signal is derived from Hall-effect commutation switches in the motor.

20. The method of claim 18 wherein the regular periodic basis is timed to coincide with a switching of a commutation electrical signal.