Method and apparatus for time-based dc motor commutation
View Patent ↗A motor control circuit uses a stored commutation table to effect time-based commutation of a brushless dc motor. Each entry in the commutation table defines a motor winding commutation state, and an exemplary method of open-loop motor control based on the commutation table comprises commutating the motor based on sequentially selecting table entries at a desired selection rate. That is, motor speed is controlled precisely by controlling the sequential selection rate rather than by relying on feedback from the motor. However, motor feedback for closed-loop control may be used in some modes of operation. For example, the method may include closed-loop control or open-loop control depending on the motor speed range, and may transition from closed-loop to open-loop control.
1. A method of controlling a brushless dc motor comprising:
commutating the motor under open-loop control based on energizing its windings according to a stored commutation table whose table entries define sequential commutation states for the windings; and
controlling motor speed by setting a selection rate for sequentially selecting the table entries;
wherein the motor comprises an image forming apparatus drive motor, and wherein controlling motor speed by setting a selection rate for sequentially selecting table entries comprises setting the motor speed according to a desired printing process speed.
2. The method of claim 1 , further comprising selecting one of the table entries as a commutation starting point based on positional feedback from the motor.
3. The method of claim 2 , wherein the positional feedback comprises an indication of rotor angle for the motor, and wherein selecting one of the table entries as a commutation starting point based on positional feedback from the motor comprises selecting the table entry corresponding to a commutation state matching the indicated rotor angle.
4. The method of claim 1 , further comprising transitioning from a closed-loop control method based on motor feedback to the open-loop control method based on the stored commutation table as a function of the motor speed.
5. The method of claim 4 , wherein transitioning from a closed-loop control method based on motor feedback to the open-loop control method based on the stored commutation table as a function of motor speed comprises using the closed-loop control method for a first motor speed range and using the open-loop control method for a second motor speed range.
6. The method of claim 1 , further comprising controlling a torque margin of the motor to avoid motor slippage during open-loop control of the motor.
7. The method of claim 6 , wherein controlling a torque margin of the motor to avoid motor slippage during open-loop control of the motor comprises controlling an average winding voltage of the motor to maintain motor torque at a level above expected or measured motor drive loads.
8. The method of claim 1 , further comprising, in a jog mode of motor operation, determining a number of commutation states equal to a desired rotational jog of the motor, and commutating the motor under open-loop control based on selecting that number of table entries from the stored commutation table, and wherein the selection rate is set based on a desired jog speed profile.
9. The method of claim 8 , wherein, in the jog mode of motor operation, the method further comprises determining a starting entry in the table based on positional feedback from the motor.
10. The method of claim 1 , further comprising effecting a desired motor velocity profile based on varying the selection rate according to the desired motor velocity profile.
11. The method of claim 1 , further comprising controlling average winding voltages of the motor while commutating the motor under open-loop control to maintain a torque margin for the motor relative to expected or measured image forming apparatus drive loads to avoid motor slippage.
12. The method of claim 1 , further comprising setting an average winding voltage for the motor responsive to closed-loop control of the motor, increasing that average winding voltage by an amount corresponding to a desired torque margin for commutating the motor under open-loop control, and transitioning from the closed-loop control to the open-loop control.
13. The method of claim 1 , wherein the stored commutation table comprise a first set of entries corresponding to a first motor direction and a second set of entries corresponding to a second motor direction, and further comprising sequentially selecting table entries from the first set or the second set depending on a desired motor direction.
14. A method of motor control in an image forming apparatus comprising:
driving an image forming subassembly of the image forming apparatus with a brushless dc motor; and
controlling the motor based on open-loop commutation comprising commutating the motor by energizing its windings according to a stored commutation table whose table entries define sequential commutation states for the windings, and controlling motor speed by setting a selection rate for sequentially selecting the table entries;
further comprising starting the motor in the first mode and transitioning to the second mode and selectively operating in either the first mode or the second mode as a function of a desired process speed for the image forming apparatus.
15. The method of claim 14 , wherein controlling the motor based on open-loop commutation comprises determining a starting table entry in the stored commutation table based on a motor feedback signal that indicates motor position.
16. The method of claim 14 , further comprising controlling the motor based on closed-loop commutation in a first mode, and controlling the motor based on the open-loop commutation in a second mode, wherein controlling the motor based on closed-loop commutation comprises commutating the motor under closed-loop control responsive to one or more motor feedback signals.
17. The method of claim 14 , further comprising determining an average motor winding voltage used for driving the motor in the first mode, and calculating a higher average motor winding voltage to be used for the second mode based on a torque margin desired for open-loop commutation of the motor.
18. The method of claim 14 , wherein selectively operating in either the first mode or the second mode as a function of a desired process speed for the image forming apparatus.
19. A motor control circuit for controlling a brushless dc motor comprising:
a logic circuit configured to obtain sequential commutation states for the motor from a stored commutation table whose table entries define sequential commutation states for the windings of the motor; and
an output circuit configured to output commutation signals for commutating motor according to the sequential commutation states;
said logic circuit further configured to control motor speed by setting a selection rate for sequentially selecting table entries;
wherein the motor comprises an image forming apparatus drive motor, and wherein the motor control circuit is configured to control the motor speed as a function of printing process operations.
20. The motor control circuit of claim 19 , wherein the logic circuit is configured to select one of the table entries as a commutation starting point based on positional feedback from the motor.
21. The motor control circuit of claim 20 , further comprising an input circuit configured to receive a feedback signal providing the positional feedback from the motor.
22. The motor control circuit of claim 20 , wherein the positional feedback comprises a indication of rotor angle for the motor, and wherein the logic circuit is configured to select the table entry corresponding to a commutation state matching the indicated rotor angle.
23. The motor control circuit of claim 19 , wherein the motor control circuit is configured to control the motor under closed-loop control based on motor feedback in a first mode of operation, and configured to control the motor under open-loop control based on the stored commutation table in a second mode of operation.
24. The motor control circuit of claim 19 , wherein the motor control circuit is configured to operate in the first mode for a first range of motor speeds, and is configured to operate in the second mode for a second range of motor speeds.
25. The motor control circuit of claim 19 , wherein the motor control circuit is configured to control a torque margin of the motor to avoid motor slippage during open-loop control of the motor.
26. The motor control circuit of claim 25 , wherein the motor control circuit is configured to control the torque margin of the motor by controlling an average winding voltage of the motor to maintain motor torque at a level above expected or measured motor drive loads.
27. The motor control circuit of claim 19 , wherein the motor control circuit is configured to operate in a jog mode of motor operation wherein it determines a number of commutation states equal to a desired rotational jog of the motor, and commutates the motor under open-loop control based on selecting that number of table entries from the stored commutation table, and wherein the motor control circuit controls the selection rate based on a desired jog rate profile.
28. The motor control circuit of claim 27 , wherein, for jog mode operation, the motor control circuit is configured to determine a starting entry in the table based on positional feedback from the motor.
29. The motor control circuit of claim 19 , wherein the motor control circuit is configured to effect a desired motor velocity profile based on varying the selection rate according to the desired motor velocity profile.
30. The motor control circuit of claim 19 , wherein the motor comprises an image forming apparatus drive motor, and wherein the motor control circuit is configured to set the selection rate as a function of printing process requirements of the image forming apparatus.
31. The motor control circuit of claim 19 , wherein the motor control circuit is configured to control average winding voltages of the motor while commutating the motor based on the stored commutation table to maintain a torque margin for the motor relative to expected or measured motor drive loads to avoid motor slippage.
32. The motor control circuit of claim 31 , wherein the motor control circuit is configured to determine a desired average winding voltage for the motor based on a measured drive load and a desired torque margin.
33. The motor control circuit of claim 31 , wherein the motor control circuit is configured to set the average winding voltages based on a desired torque margin relative to estimated motor drive loads.
34. The motor control circuit of claim 19 , wherein the stored commutation table comprise a first set of entries corresponding to a first motor direction and a second set of entries corresponding to a second motor direction, and wherein the motor control circuit is configured to control motor direction by sequentially selecting table entries from the first set or the second set depending on a desired motor direction.
35. The motor control circuit of claim 19 , wherein the motor control circuit comprises a processor circuit configured to execute stored program instructions.
36. The motor control circuit of claim 19 , wherein the motor control circuit comprises at least a portion of an Application Specific Integrated Circuit.
37. The motor control circuit of claim 19 , wherein the motor control circuit comprises at least a portion of a Field Programmable Gate Array.
38. An image forming apparatus comprising:
a printer subassembly used in an image forming process of the image forming apparatus;
a brushless do motor configured to drive the printer subassembly; and
a motor control circuit configured to commutate the motor under open-loop control based on energizing its windings according to a stored commutation table whose table entries define sequential commutation states for the windings;
said motor control circuit configured to control motor speed by setting a selection rate for sequentially selecting table entries;
wherein the motor control circuit is associated with a memory that is configured to store the commutation table.
39. The image forming apparatus of claim 38 , wherein, in a jog mode of operation, the motor control circuit is configured to determine a number of commutation states equal to a desired rotational jog of the motor, and commutate the motor under open-loop control based on selecting that number of table entries from the stored commutation table.
40. The image forming apparatus of claim 39 , wherein the motor control circuit is configured to jog the motor a desired rotational amount in a reverse direction relative to image forming process direction of the motor to relieve binding in the printer subassembly.
41. The image forming apparatus of claim 38 , wherein the motor control circuit is configured to commutate the motor under open-loop control based on the stored commutation table in a first mode of operation, and is configured to commutate the motor under closed-loop control based on a motor feedback signal in a second mode of operation, and wherein the motor control circuit is configured to transition between the first and second modes of operation as a function of motor speed.
42. The image forming apparatus of claim 41 , wherein the motor control circuit is configured to operate in the first mode if a desired motor speed is below a defined threshold, and is configured to operate in the second mode if the desired motor speed is above a defined threshold.