# PID control in position sensing

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#### Mark R Burford

Hey guys, not written before so sorry if I break any rules ;-) ( I am another 3rd year proj
student at uni of Soputhampton, UK)

I am creating a control system to essentially keep the distance between two points the same.
It's application is in the UV writing of silica waveguides that are not completely flat (bowed)

I am reflecting a low power laser off a silicon sample on the front of a speaker cone. The laser
spot is focussed and detected by a split photodiode (side A and side B). I take a difference of the voltage generated from A and B, Va - Vc then put this signal through a Low pass filter and then on into a PID controller.

This is where I am stuck. I want the PID controller to take the Va - Vc signal and from it,
force the speaker cone to move the silicon sample fwd or backwards to bring the laser spot back
to centre of the split photodiode (so Va - Vc = 0) after the bowed wafer has deflected the spot
to the left or right.

At present the system works as in the speaker cone moves in the right direction but doesn't bring the spot quite back to centre.

I need to know how I should go about calculting Res and Cap values for the PID controller using
opamps.

Many thanks

Mark

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#### Dr. Igor Boiko

Mark,
It looks like the integral component of your controller doesn't work. You can try the following (electronics is not my area of expertise but I have some experience in what you are doing now): the op-amp should have an input resistor R1 connected between the source of the
signal and the "-" of the op-amp and a feedback circuit comprising a capacitor C and of a resistor R2 connected in series, and the whole
circuit connected between the "-" of the op-amp and the output. The transfer function of this filter would be: W(s)=-(R2*C*s+1)/(R1*C*s).
From here, you can calculate R1, R2, and C with Kp and Ki available. This filter inverts the signal.
Hope this would help.

Regards,
Dr. Igor Boiko
Consulting in Control is available
(including modeling, simulations and control design)
[email protected]
Tel: 1-403-294-2745