DAC voltage capped when operating in torque mode with load cell feedback

[yanxinix]

Hi, i'm doing a tension control on a cable driven system using servo motor. I had connected a load cell as analogue feedback for cable tension into the labview programme suggested in http://www.ni.com/pdf/manuals/371242c.pdf (Figure 13-3. Torque Control Using Analog Feedback Using LabVIEW)

Problem 1: The response between desired tension and actual tension is good until about 80N, when the motor stops responding to higher torque changes. The voltage output in DAC is only 4-5V (max voltage output is suppose to be 10V) and the motor and amplifier had been verified to run up to 200N. What is limiting my DAC?

Settings:

Card: PCI-7350

Analogue input: 0-5V

Problem 2: I had implemented a low pass filter of 30Hz in MAX for the load cells. The effect is quite good for the low tension. However, as the tension increases, the noise increases proportionally too, causing vibrations and switching sounds to be heard at higher tension. Is there anyway to overcome this?

Please help. Thanks 

[Jochen]

Hi yanxinx,

is there a chance, that you have set a torque limit in MAX? This could explain the behavior.

If not, could you please post your PID settings?

Thanks,

Jochen Klier

National Instruments

[yanxinix]

Hi Jochen,

The torque limit is set to the default (+10v/-10v).

My pid values are Kp=70, Kd=1000, Ki=15.

Thanks for helping! =]

Yanxin

[Brian_Beal]

What value are you using for the setpoint?  What is the range of your load cell?  A setpoint of 0 to 65,535 would be equal to 0 to "max force" of the load cell.

[Jochen]

I can't see anything special with your settings, so I currently don't have a clear understanding, why the output voltage is limited.

Could you please test the analog output? You could do this configuring the axis as an open loop stepper axis. This will allow you to set the analog output with Load DAC.

Does this allow you to output higher voltages?

Could you please do the test with and without signal connection to the analog output? Maybe the input impedance of your drive is too low and pulls down the board's output voltage.

I hope that helps,

Jochen

[yanxinix]

Brian, i'm sorry, but what do you mean by the setpoint? if it means the input, it is an increment of 5N step starting from 10N onwards. My system response starts to saturate at about 80N. My loadcell range is up to 110lb (445 N)

Jochen, I tried the analogue output using load DAC vi, it is able to output up to 10V. Oh, to sort of push the tension limit up, I tried adding another PID (Kp=90, Kd=1200, Ki=35). The new PID is able to reach up to 120N. However, the readings are so noisy that I cant figure out if its just noise, or the system is oscillating. Is it normal to be like that?

Thanks for helping!