PID control with 0-10Voltage LVDT

[Roy19]

Hi everyone,

In my case, I have a LVDT with 0-10V output range, which causes a problem that if I set the start point to be at some Voltage between 0-10 V (say 9 V) and then start moving, then the actuator would immediately drop to some value below 9 V (blue line left top waveform) until meet the gradual increasing error (right top waveform) due to the zero initial error and hence zero output.

The only way to solve this problem is to buy another LVDT that ranges from -10~10V and the neutral position would be 0 V.

Are there any methods to solve this problem?

Any help would be much appreciated.

Attached please find a schematic diagram of the immediately drop of the  

[RIObotics]

Hi,

In my experience the range of that LVDT output shouldn't be an issue.  Think of it this way, the LVDT is your feedback sensor.  If the system was a temperature controller, you really don't need neutral position to be 0 degrees C.  The sharp drop in the LVDT output is kind of strange.  I would first check, in an open-loop condition, whether the drop happens just by turning on the system, motor turning, etc.  If you can pinpoint the root cause of the voltage drop and make sure it's not caused by the PID, that would be good.  If you verify that the voltage drop is not due to hardware, next steps I would take is tuning the PID a little bit at a time.  Use 0's for everything, and then increase the P gain gradually, if it looks stable then add a little bit of I and D.

[Roy19]

Hi,

Thank you very much for your reply.

In my case, the start position is 9 Voltage and my setpoint is 9 V as well. So the initial error (difference between my setpoint and measured value) is zero, then the output is zero at the very beginning. When the ouput becomes zero, the actuator tends to go to the zero position measured by LVDT. This is why we have a drop.

We can refer to the top right waveform, which is the output voltage from PID to the Servo-valve. At the very beginning, the ouput is zero, as the drop produces the output gradually increases until meet with my setpoint.

If my start point is at zero Voltage, then everything is fine becasue the intial error is zero and output is zero at the first place. So the pid can control the motion when error 

I dont know if I explain this issue clearly.

Roy

[RIObotics]

Just a really quick idea ... could you apply a offset to your LVDT voltage to zero it out?  Just subtract 9 to your voltage and see what happens.  🙂  I would be curious to know if this works!