Voltage drop to servo drive

[Bruce_Mac_Donald]

Am using a PCI-7340 to control a drive for a motor (drill spindle) with no feedback on one axis, so have the axis disabled and am using the DAC output to set specific voltages.  With the analog cable to the drive disconnected, I can set voltages and the DAC output reads correctly and is stable.  But when connected to the drive, the voltage drops.,  For instance setting 2.5V results in 1.95V (but is 2.5V when the cable is disconnected).  Setting 10V output yields 7.75V, not the max that I need.  The input impediance of the drive is specified as >50k, and I measured it at 114 kohms.  The Specs of the 7340 say it can provide 5 mA, and with a load of 114 k that is well within its range.  

Also leaving the DAC output fixed, the drive frequency slowly increases over time, but not consistently - probably just heating up, but the voltage does change slightly over time as well, even though the DAC output command has not changed.  

It seems like a loading issue, yet with an input impedance of 114k how can this be?

It isn't electrical noise in the machine as the voltage I get is consistently less than what I set, and is not too unstable, about +-50 mV fluctuation (more variability than I want to control the drive).

Any ideas?

[Reyquaza]

Hi Bruce,

What analog cable are you using in this setup? Also, why not just use an analog output module if you just need to set voltages?

[Bruce_Mac_Donald]

Hi Rey

Am using a shielded good quality analog cable, and the input impedance to the drive is over 110 k, so I don't expect the small current running on the cable to cause this voltage drop - the cable resistance is much less than the input impedance.  But your question has made me think and I will check the voltages at the drive end a bit more.

I am using the other 3 axes of the motion card for other drives, have this fourth channel available, and we don't want to add another card just for that when we have an available analog output already.

I may simply have to have a conversion, as long as the drop is consistent day-to-day, but would still like to know what is going on.  When I set fixed voltages on the other axes, they all work properly.

[RenKylo2]

Bruce,

How are you setting the AO using the DAC? Are you indirectly controlling the DAC in MAX by performing a 1D move when your axis is configured for a servo-motor? The better way is to use the Load DAC.flx motion function in LabVIEW, it is located in the function palette -> Vision and Motin -> NI 73xx -> Analog & Digital IO.

For the AO to work properly, the axis need to be disabled or configured as a stepper motor, not servo. This is becasue the DAC outputs servo commands and can't be used for general AO signals. You mentioned that you have the axis disabled. Was the axis still disabled it was connected to the drive?

[Bruce_Mac_Donald]

Hi Ren

Yes, I am using the Load DAC, with axis disabled.  Using a voltmeter the analog reading is stable and fine.  But the reading drops when connected to the drive.  I'm starting to wonder if there is damage to this motion card, although wiht a 114k ohm drive input impedance, the DAC output only needs to provide 40 uA with a 5V setting.

I will probably have to disconnect one of the other axes and see if the problem is consistent across the others.

[RenKylo2]

Hi Bruce,

That is a good idea, please let us know if you see the same behaviour on other axises.

[Bruce_Mac_Donald]

I won't get a chance to try it out at the customer's site until next week, but will let you know what I find.

Thanks.