Using compactRIO to visualize Torque-speed characteristic

[Lindex]

Hi,

I have an asynchrounous motor and a Siemens sinamics drive for speed control. I use the compactRIO to send a 0-10V signal to the drive, so the motor has a speed between 0 and 1400RPM. This works fine.

Now, I use a Leroy Somer Active Load for operating the motor's brake. The Leroy Somer gives me a torque signal (10V = 10 Nm) and a speed signal (10V = 2000RPM). I read this signal real-time with the compactRIO and store the data in an array, so I can visualize the torque-speed characteristic.

The Leroy-somer gives an adequate,  voltage signal to the compactRIO (I checked this with a multimeter), but the compactRIO sees the signal as an alternating (for constant 1400RPM, it gives back values between +-1350 and +-1450) signal. I already used the "mean" function in LabVIEW, but this is also not working.

Does anyone has an explanation/solution for this problem?

Kind Regards,

Dries

[Chris.Hyde@NI]

Hi Lindex,

Thanks for posting on the NI forum!

The description you've given on the problem is a bit vague, I'll need some more information to find out if something is going wrong here.

• Is the cRIO operating in a high noise environment? This could be messing with the signal
• Check all connections to the cRIO for errors 
• Are you applying any signal conditioning to the signal which could be affecting it? (screenshot of code would be useful)

Our cRIOs are considerably more accurate than DMMs so unless the cRIO is malfunctioning, the cRIO is outputting correct values. It may be that the module you are using needs calibrating or repairing.

If this is a big issue for you, I'd recommend opening a formal service request with our support team.

Many Thanks,

Chris

[nanocyte]

• How much of a (voltage?) change causes a ±50 RPM change. It might be useful to see a graph of the "alternating" signal. Is it sinusoidal for example or more random?
• What module are you using? Have you tried both single ended and differential mode along with the appropriate digital gain setting?
• One thing to understand about a DMM is the measurements usually have a large aperture. You might be able to simulate that behavior by using a low pass analog filter. If that works, you might be able to simulate that large aperture by averaging on the FPGA.

[Lindex]

Hi,

First of all I'm not using the FPGA-function. I'm only displaying the real time value of the speed and the torque on a VI.

The noise looks to be a sinusoidal noise, but has no specific frequency. 

I'll try to use a low pass filter. Do I have to use it in the LABVIEW-software, or do I have to use a hardware lowpass filter?

Thanks a lot for ur answer!

[nanocyte]

Again, what module are you using?

Yes, please, show us the noise. Maybe an FFT would help too.

My suggestion is to first try a hardware filter to see if that's your problem. If that solves your problem, then we know it's worth it to use either FPGA or hybrid mode.  If it doesn't solve your problem, it still might be worth it to use the FPGA but it's less clear and the cause could be a grounding  issue, or, as Chris suggested possibly an issue with that particular serial number, or something else.

I got one more trick. If you measure with the DMM at the same time as with LabVIEW, does that change what either LabVIEW or the DMM see?