stepper accuracy in velocity mode

[EEburnin]

I am usin the 7334 board to control 3 open loop stepper axes using Labview. I configured the system to home one of the axes to the edge of a home switch then reset the position to 0 steps. The axis is connected to a 2mm pitch ballscrew and I place the application into velocity mode and move some distance off of the home position. I then change the mode to absolute position and command the axis to return to the 0 position, the problem is that the move seems to hit the home switch before reaching 0 steps. The displayed distance in mm (after I convert the steps to mm based on the ballscrew pitch) varies. The first iteration I may end up at 1.2mm, the next iteration I end up at 2mm and the error seems to grow. It appears as though the steps being recorded while in velocity mode are not correct. Each time I reach the home switch before hitting the 0 position. I assume I'm not accounting for something while in velocity mode, but I'm not sure what it is. I set the mode to velocity, I load a velocity, I enable the axis, Then I use the multistart to start the motion, I sit in a while loop until the user releases a button on the GUI. After the user releases the button I use the stop axis command to decelerate the axis to 0.

Any suggestions would be appreciated.

[Jochen]

I can think of two reasons that could cause this behavior:

1. The axes loose steps either during acceleration or when travelling with constant speed. Please run your sequence with very low acceleration and velocity values and with low load. Does the error persist? If not there might be an issue with your mechanical setup or your motors are not strong enough or your drive doesn't provide enough power.
2. If you run the find reference sequence with very high velocity values there is a chance that the home switch position can't be found accurately.

There are no known issues with the internal position counters of the 7334 so it's very unlikely that the issue you see is caused by a technical problem of the board.

If you can't eliminate the problem please answer the following questions:
What version of NI-Motion are you using?
What are your motor specs?
What's the microstepping factor of your drive?

Best regards

Jochen Klier
National Instruments Germany

[EEburnin]

Thanks for the quick response Jochen. It turns out that I had the set operating mode and multistart command in the while loop and that was causing the problem, I was loosing steps as a result. I change the while loop to only include the read status control and the status of my GUI button. This resulted in a smooth velocity mode profile and no loss of steps.