Difference between open loop and closed loop mode when finding the index on a stepper motor axis.

[MarkMoss]

I have a test system which uses a PXI-7352 motion controller with a MID-7602 stepper motor drive to position a 200 step/revolution stepper motor with an attached 1000 line encoder.  The MID-7602 is configured for 64 microsteps per step, giving 12,800 microsteps and 4000 encoder counts per revolution.  The system is controlled using LabView 7.1 and NI-Motion 6.13.  To home the motor, I've defined a sequence in MAX which first finds the reverse limit switch and then moves forward to the encoder index pulse.

When finding its reference in open loop mode, the system its reference at the same microstep each time.  When findings its reference in closed loop mode, the system finds its reference somewhere within about a 20 microstep wide range.

I hope somebody out there with more experience with this controller can describe the difference between finding the encoder index in open loop mode and finding the encoder index in closed loop mode. 

Thanks,

Mark Moss

[Luke H]

Hello Mark,

I suspect that the behavior you are experiencing has today with the resolution difference between your encoder and your microstepping.  Let me illustrate with an example:

Example Setup

• Stepper Motor
• NI Motion Controller
• 10 steps per encoder pulse (Every 10 steps the controller gets an encoder pulse, therefore positions 0-9 all appear to be 0 to the controller)
• The system has a maximum of 3 pull in moves set

Open Loop Scenario

1. The system is commanded to go to position 9.
2. The controller sends out 9 pulses.
3. The motor moves to position 9.

Closed Loop Scenario

1. The system is commanded to go to position 9.
2. The controller sends out 9 pulses.
3. The motor moves to position 9.
4. The controller checks its position and sees that it is still at position 0 because it has not received an encoder pulse.
5. Because the controller thinks it is still at position 0, it sends another 9 pulses as its first pull in move.
6. The motor moves to position 18.
7. The controller checks its position and sees that it is at position 10 because it has only received one encoder pulse.
8. Because the controller thinks it is still at position 10, it sends1 reverse pulse as its second pull in move.
9. The motor moves to position 17.
10. The controller checks its position and sees that it is at position 10 because it has only received one encoder pulse.
11. Because the controller thinks it is still at position 10, it sends and additional single reverse pulse as its third pull in move.
12. The motor moves to its final position of 16.

I believe something similar is happening with your application as it looks for the index pulse.  Because the controller does not compensate for what it perceives as position error in open loop mode, the motor always goes to the same commanded position.  In closed loop mode, the motor is bouncing around inside the single index encoder pulse trying to find a certain position. There are several recommendations I can make towards correcting this behavior.  These recommendations are in order of effectiveness:

1. Setup your system so that there are more encoder counts per revolution than steps per revolution.
2. Turn off pull-in moves by setting them to 0 in MAX.
3. Use an open loop configuration.
4. Play around with the pull in window in MAX.

Regards,
Luke H

[tarique]

i have dots in open loop &close loop control please define and how to classified can i use binary signal in close loop control please tell your answer as soon as possible

thanking you tarique

[VGA_CD-ROM]

Hi tarique,
What do you mean by dots in closed loop and stepper loop? Can you post a screenshot of your Axis Configuration page in MAX?