Analog Output Problem with Piezo Actuator Loaded - USB6259

[anli5210]

Hi everyone,

I'm a student just start using DAQ USB6259 recently. I'm trying to generate two sine wave signals with 90 deg difference and amplitude ranging from 1V to 3V. The signals are used to drive two stack piezo actuators. After I configured every parameter in SignalExpress, everything works very well when piezos are not loaded. But after I connected the signal to piezos, only AO 0 functions well (sine wave, amplitude as desired), the signal output from either AO 1 or AO 2 or AO 3 distorted to a saw wave with amplitude insensitive to my settings. But when I disconnect piezos, everything just changes back to normal.

I read the specs for USB6259, I didn't find any note saying any difference between AO 0 and other analog output ports.

Thanks in advance!

-Li

[Paul-D]

Hey there, this is Paul with Applications Engineering National Instruments.

This sounds like you may be exceeding the current drive of your USB 6259's analog output.  Our multifunction DAQ cards are generally not made to drive significant amounts of current, they are only meant to provide a signal that can be amplified or used in some other fashion.  As such, checking out the specification sheet for your device (http://www.ni.com/pdf/manuals/371291h.pdf), we see that it's rated output current drive is +/- 5mA.  You may also see that it has a rated output current drive of 20mA.  This means that you are able to drive upto 20 mA current with your output. However if you drive current above 5 mA, the output voltage is no longer guaranteed to be what you set it at.

Overdriving your analog outputs seems like a likely culprit here.  I would recommend investigating how much current your piezo devices are expected to be pulling at the voltage ranges you are using, either by checking out their specs, or or measuring the actual current being pulled during your operation.  If you are overdriving them, then you will have to build an amplifier that can provide the current you require for your application.

If you are sure you are not overdriving these outputs, then we can look into the issue further.

Regards,