Dear Friends,
At present I am intrested in implementing a PID control loop for my spot welding machines. In this system the input is the thyristor firing angle and the output is welding current achieved in one half cycle.
1. Thus I will be firing my thyristor at an appropriate angle measure the RMS current achieved during the half cycle.
2. Find the error ie. diffrenece in the desired current and the achieved current.
3. Put the errors and inputs i.e. firing angles in PID control loop equation. To get the next firing angle.

Thus in my case I will calculating new firing angles 100 times a second using PID control loop equation. Input is thyristor firing angle and output is quasi sinusoidal current achieved the factors that can affect the output current are..
1. Supply voltage fluctuations.
2. Surface condition of sheet being welded.
3. Thickness variation of sheets.
4. Squeeze pressure [ Depends on pressure of compressed pressure used for pneumatic cylinder ].
5. Electrode conditions a new electrode has sharp tips and goes on mushrooming with use.
6. Sarrounding Temperature.
etc.

Please suggest me some reading material so that I can implement a PID control for my requirements.

Thanks & Regards,
Prahlad J. Purohit