Control methods for the reduction of limit cycle oscillations for haptic devices with displacement quantization

• US 20030076297A1
• Filed: 10/23/2002
• Published: 04/24/2003
• Est. Priority Date: 10/24/2001
• Status: Active Grant

• Current Assignee: Immersion Corporation
• Original Assignee: Immersion Corporation
• Inventors: Hasser, Christopher J.
• Granted Patent: US 6,833,846 B2
• Time in Patent Office: Days
• US Class Current: 345/156
• CPC Class Codes: G06F 3/016 Input arrangements with force or tactile feedback as computer generated output to the user

Abstract

Systems and methods for reducing limit cycle oscillations of a haptic device. A net force applied to the device is a combination of a primary force computed in a main haptic loop and a secondary force computed in a damping loop that cancels or minimizes the oscillations. Various algorithms for computing the secondary force are provided. In one algorithm, the secondary force is determined from the momentum error associated with crossing of a wall position by the manipulandum and is applied immediately after the crossing is detected. In another algorithm, a periodically varying secondary force with a phase shift relative to the phase of the oscillating manipulandum is computed.

Claims

1. In a haptic device having a manipulandum, wherein a position of the manipulandum is determined at regular sampling times, a method for reducing limit cycle oscillations of the manipulandum, comprising:

in a first servo loop operating at a first servo rate, determining a primary force based on the position of the manipulandum at a current sampling time;

in a second servo loop operating at a second servo rate;

detecting an oscillation of the manipulandum; and

determining a secondary force in response to the oscillation;

combining the primary force and the secondary force; and

applying the combined force to the manipulandum.

2. The method of claim 1, wherein the first servo rate is slower than the second servo rate.

3. The method of claim 1, wherein detecting an oscillation includes detecting a crossing of a virtual wall position by the manipulandum.

4. The method of claim 3, wherein determining a secondary force includes:

computing a momentum error associated with the crossing of the virtual wall position; and

computing a secondary force offsetting the momentum error.

5. The method of claim 4, wherein computing a momentum error associated with the crossing includes:

computing an applicable force for a position inside the virtual wall region;

computing an error time corresponding to a time spent inside the virtual wall region before the applicable force is applied; and

multiplying the applicable force by the error time.

6. The method of claim 5, wherein computing an applicable force for a position inside the virtual wall region includes:

computing the product of a spring constant associated with the virtual wall times a displacement resolution associated with the position of the manipulandum.

7. The method of claim 5, wherein computing an error time includes:

setting the error time equal to half an interval between sampling times.

8. The method of claim 4, wherein computing a secondary force offsetting the momentum error includes:

dividing the momentum error by a period of the second servo loop.

9. The method of claim 1, wherein detecting an oscillation includes detecting at least two crossings of a zero position by the manipulandum within a predetermined time interval.

10. The method of claim 9, wherein determining a secondary force in response to the oscillation includes:

computing a period of the oscillation;

identifying a last zero crossing time;

computing a displacement phase of a periodic displacement function based on the oscillation period and the last zero crossing time;

computing a secondary force from the displacement phase, the secondary force having a damping phase offset from the displacement phase by a constant offset.

11. The method of claim 10, wherein computing an oscillation period includes:

identifying two successive zero crossing times; and

computing an oscillation period equal to twice the difference between the two successive zero crossing times.

12. The method of claim 9, wherein the zero position corresponds to a position of a virtual wall.

13. A control system for a haptic device having a manipulandum, comprising:

a processor configured to generate a primary force signal at a main haptic loop rate;

a damping circuit configured to detect an oscillation of the manipulandum and to generate a secondary force signal at a damping loop rate in response to the detected oscillation; and

a combining circuit configured to combine the primary force signal and the secondary force signal, thereby generating a control signal for controlling a haptic feedback force of the haptic device.

14. The control system of claim 13, wherein the main loop rate is slower than the damping loop rate.

15. The control system of claim 13, wherein the damping circuit is further configured to detect an oscillation of the manipulandum by detecting a crossing of a virtual wall position by the manipulandum.

16. The control system of claim 15, wherein the damping circuit is further configured to compute a momentum error associated with the crossing of the virtual wall position and to compute a secondary force offsetting the momentum error.

17. The control system of claim 16, wherein the momentum error is equal to the product of a spring constant associated with the virtual wall times a displacement resolution associated with the manipulandum times half a time interval between successive determinations of the manipulandum position.

18. The control system of claim 16, wherein the secondary force is substantially equal to the momentum error divided by a period of the damping loop.

19. The control system of claim 13, wherein the damping circuit is further configured to detect an oscillation of the manipulandum by detecting at least two crossings of a zero position by the manipulandum within a predetermined time interval.

20. The control system of claim 19, wherein the damping circuit is further configured to compute a periodic secondary force having a phase offset from an estimated phase of the detected oscillation.

21. The control system of claim 20, wherein the estimated phase of the detected oscillation is substantially equal to the quotient of the difference between a current time and a last zero-crossing time divided by an estimated period of the detected oscillation.

22. The control system of claim 21, wherein the estimated period of the detected oscillation is substantially equal to twice the difference between two successive zero-crossing times.

23. A haptic feedback device, comprising:

a manipulandum;

an actuator coupled to the manipulandum and configured to exert a force on the manipulandum in response to an actuator control signal;

a processor configured to generate a primary force signal at a main haptic loop rate;

a damping circuit configured to detect an oscillation of the manipulandum and to generate a secondary force signal in response to the detected oscillation; and

a combining circuit configured to combine the primary force signal and the secondary force signal, thereby generating the actuator control signal.

24. The haptic feedback device of claim 23, wherein the main loop rate is slower than the damping loop rate.

25. The haptic feedback device of claim 23, wherein the damping circuit is further configured to detect an oscillation of the manipulandum by detecting a crossing of a virtual wall position by the manipulandum.

26. The haptic feedback device of claim 25, wherein the damping circuit is further configured to compute a momentum error associated with the crossing of the virtual wall position and to compute a secondary force offsetting the momentum error.

27. The haptic feedback device of claim 26, wherein the momentum error is equal to the product of a spring constant associated with the virtual wall times a displacement resolution associated with the manipulandum times half a time interval between successive determinations of the manipulandum position.

28. The haptic feedback device of claim 26, wherein the secondary force is substantially equal to the momentum error divided by a period of the damping loop.

29. The haptic feedback device of claim 23, wherein the damping circuit is further configured to detect an oscillation of the manipulandum by detecting at least two crossings of a zero position by the manipulandum within a predetermined time interval.

30. The haptic feedback device of claim 29, wherein the damping circuit is further configured to compute a periodic secondary force having a phase offset from an estimated phase of the detected oscillation.