Difference between SCXI AO board and HP dc power supply

[Panthera395]

I am using LV 7.1 and DAQmx 8.5 running a SCXI-1000 Chassis and a SCXI-1600 A/D board. I use a variable dc voltage as a speed reference signal for a servodrive. Previously, I've used a DC power supply (HP E3631A) and manually controlled the output voltage using the knob. To simplfy tests and reduce the amount of equipment needed for offsite experiments, I wanted to replace the dc power supply with an AO module for the SCXI chassis. I have wired up a SCXI-1124 module with a single +/- 10V output. When plugged into a multimeter, the output voltage reads what it is supposed to. However, when I connect the output to the servodrive and test the motor control the motor does not behave correctly (excessive vibration), the multimeter still reads the correct output. The problem does not occur when using the dc power supply, even when using the same voltage ranges. Are there any differences (electrical or signal) between an AO board and a dc power supply that might change the signal that the servodrive is seeing? I was thinking that it might be a ground issue, but the servodrive is on an independent circuit from all of the instrumentation. Thanks! -Sarah

[Dennis_Knutson]

The most obvious thing to check is the current draw of the drive. The SCXI module is only capable of 5ma, according to the specs.

[Panthera395]

The drive specs give the impedence of the reference channel as 10Kohm, so that should yield a max current drive of only 1mA at max ouput (10V).  I have been testing as anywhere near max ouput.  Assuming my math is correct, electrical theory was never my strongest area.

The puzzling thing is that the motor responds to the first command (a fpr loop that ramps the voltage from 0-0.5V in discreet steps) just fine and will hold speed fine.  But when I send a second command (similar ramp down to 0V or higher to 1V) the motor has problems.

Thanks for you input!

-Sarah 

[Margaret F.]

Hello Sarah,

Thank you for posting to the NI forums. As Dennis said, the SCXI module you are using can only output 5mA. I think it would be good to use a multimeter and measure the actuall current that you are drawing just to check that it is not in excess. Also, could you please reply with the specs page for your motor? Please take a look at the link that I am attaching to make sure that you are within the specs of the 1600.

http://www.ni.com/pdf/manuals/320712b.pdf#page=42