Sine wave generation with PID using cRIO9024 and cRIO-9113 Chassis (FPGA)

[etuzuner]

Hi,

I've created a PID controlled hydraulic application by using Sine Generator.vi and PID.vi. I'm also using NI 9264 and NI 9203.

Although my linear displacement sensor is 4-20 mA output, I'm getting serious fluctuations in current signal. I've been trying to use Butterworth Filter.vi. My PID.vi, Sine Generator.vi and Butterworth Filter.vi configurations are shown below.

Sample rate of the loop which includes all express vi's and module I/O's is 2 kHz. When I apply Butterworth filter to displacements sensor's corresponding channel in 9203, PID gives a huge output value to NI 9264 which is really a big problem for me.(I tried 0.05, 0.01, etc. for proportional constant, still overshoots.) I guess there is a problem with sampling rates in general but I can't see a problem. Could you please tell me where I'm wrong with these configurations?

Another thing that I would like share with you. After running the system, a vibration begins to occur in cylinder. First, I thought the reason of vibration is the fluctuations of process variable of PID. Later I connected oscilloscope to the voltage probes of the servo valve. Voltage range is between 0.18 and -0.18. The servo valve's voltage which is supplied from NI 9264 is very noisy in that range. When I manually apply a sine wave having 2 V amplitude, the ratio of noise to supplied voltage is low. But when the range is below 0.5 V, the ratio gets high. I tried a voltage follower circuit by using OPAMP (LM324), but the response of OPAMP was not efficient as I expected. What would you do if you had a problem like this? Do you have any suggestions?

Any help or idea will be appreciated.

Thanks in advance,

Best regards,

Emre