Servo-Torque control via load DAC and pos./vel. encoding

[izoard056]

I am implementing a virtual spring with the UMI 7764. I set the open-loop torque (I don't use a torque sensor) with the load DAC vi. I have read that I need to set the axis where I output the torque command as stepper (because load DAC requires an unmapped axis).

Now I need to read position and velocity which means that I have to use another axis because I use a servo and not a stepper. Using the same axis will give me incorrect or no encoder signal. (It works only if I set the stepper settings to closed loop - with all other settings to custom)

My question is if there is a possibility to use one axis for the load DAC and the reading of pos. and velocity as well, because I have 4 motors to be controlled with the PCI7354 that is limited to 4 axes.

Thank you very much.

[izoard056]

Ok, I finally found a way of doing it. I enable axis 1, enable encoder 1 (Enable Encoders.flx), axis 1 is set to stepper  (open loop), then I can read the encoder position by using Read Encoder Position.flx. I input the torque command with Load DAC on DAC Channel 1. I am not sure why it works this way but not if I use the Read Position VI or Read Velocity VI. Apparently the only reason is that these VI's only read signals from an axis and not directly from an encoder.

Is there a way of using the Read Velocity VI in my case to differentiate the position signal I get from Read Encoder Position?

Thank you!

[Jochen]

Even though the PCI-7354 supports only 4 axes, it provides 8 analog inputs and 8 analog outputs, so if you want to seperate the DAC operation from the active axes, you can choose DAC 5 to 8.

Regards,

Jochen

[izoard056]

I have tried this, Jochen, but with negative result. Nevertheless I have found a bad setting in one of the amplifiers, which could have been the cause. Before I rewire the device, are you sure that the PCI 7354 has 8 DAC's that can be used independently?

Thank you very much.

[Jochen]

Hi,

I was pretty sure, that my statement about the 8 DACs was correct, but now I feel a bit uncertain and I don't have 7354 to test (I only have a 7358). As I will be out of office for a while, an Applications Engineer from NI in Austin will answer your question.

Thanks,

Jochen

[ymerej]

Jochen,

Did you ever resolve this question - does the PCI 7354 have 8 ADC's/DAC's?  I'm working with one right now, and it would appear so.  So if I want to use Load DAC I should specify DAC 5 thru 8?

Jeremy

[Jochen]

Hi Jeremy,

unfortunately for some reason the hand-off didn't work, so there was obviously no follow-up to this thread.

I have found references in some presentations, that all 735x devices provide 8 DACs and as this seems to match with your experience, I would take that for granted now.

Your assumption is right. to use the upper four DACs, you should use numbers 5 to 8.

Regards,

Jochen