servo command polarity

[scoobydont]

Hi,

I'm using a PXI-7358 to interface a third party drive and servo motor in a bench test proir to installation.

When outputing a positive command signal for clockwise motion the motor ran in reverse i.e. counter clockwise. Can the polarity of the command signal be changed in MAX?

This has been solved by changing the motor setup to have a ccw forward direction but I would prefer to change the polarity in MAX as that way the encoder feedback would agree between MAX and third party motor driver software.

Also when bench testing a motor is it neccesary to provide an appropraite level of resistance?

I have resorted to reducing the max and min torque voltage outputs from -+ 10v to 0.5v.

Wthout this measure when using One-D interactive MAX output the maximum 10v no matter what rpm was requested in velocity opetration mode. When using position mode, output would initailly be maximum in the wrong direction (question 1) or a square wave of +-10V. Auto tune also produced maximum PID values. I'm guessing MAX starts by outputting maximum voltage at the start of a move inorder to delivery maximum torque then adjust according to encoder feedback. The reason I'm bench testing is the device that will be motored is v. expensive so i would like to be confident with the system before i install it.

many thanks

Darren

[Hillman]

Hi scoobydont,

I hope your well.

I was wondering if you could give me an update of your application?

Im more familiar with stepper motors, which you can change the polarity of the direction signal (which inverts both the step and the dir lines). 

Could you also state again what your trying to do, and I will try to answer the points,

Are you trying to tune your servo motor? there are some excellent KBs on this. 

With the benefiting, I would have thought you would need to test it at appropriate level of resistance, otherwise your values maybe erroneous. 

Kind Regards

[scoobydont]

Hi Hillman thanks for your reply. I'm bench testing a motion system proir to installation. I am using LV8.5 and motion assistant with a PXI-7358 card talking via a single AO line to a third party driver that controls a servo motor. Feedback is via an encoder. When configured the system with the driver software we had a clockwise rotation as forward motion with the encoder counting up. When connected to NI a positive command produced a reverse in the motor direction and lose of control as the encoder was then counting whilst targetting a positive step number. We have solved this by reconfiguring the motor to have a forward direction as ccw. However, now the NI side of things behaves perfectly the 3rd party encoder count is the opposite to that in NI. Ideally i would like to change the polarity of the NI command signal to have a negative output as forward with an increasing encoder count. This is not entirly neccesary but we may monitor the system with the 3rd party software whilst NI is in command. As for motor resistance, we reduced the output voltage and successfully tuned the motor before we perfomed a trial with the system installed. With the system mounted against the correct resistance it performed well. best regards Darren

[Jochen]

If you want to reverse the direction of rotation, you will have to reverse both, the motor cabling and the encoder signals. To reverse the motor direction, you should swap the Motor+ and Motor- connection of your drive. To adjust the encoder to this change, you could either swap the cabling for Phase A and Phase B of your encoder or you could reverse the polarity of one of the two phases in MAX (encoder settings).

I hope this helps,

Jochen

[scoobydont]

Thanks for your reply Jochen

we are using a reference single ended command line connected to AO1 and AOGnd that didn't like being reversed so we are content to continue with the system as it is.

However I am now working on a second motor that had the same problem that we have resolved by applying your recommendation.

many thanks

Darren