Basic Troubleshooting [PID Temperature Controller]

Discussion in 'The Projects Forum' started by polymathematical, Jun 9, 2009.

  1. polymathematical

    Thread Starter New Member

    Jun 8, 2009

    I am attempting to modify a commercial temperature controller (Thorlabs ITC133: in order to make it more suitable for our particular application, namely stabilizing the temperature of a diode/mount setup. The problem with the circuit as it was given to me was that with the integrator turned on, the temperature would oscillate continually over a range of greater than 10 degrees Celsius. Without the integrator, the temperature would not settle at exactly the correct setting.

    I have done a huge amount of reading and searching online, so I have a pretty good idea of PID tuning and generally how the circuit works. However, I'm quite new to circuit modification and soldering and so I have had a difficult time troubleshooting. Some modifications to similar circuits (eg. ITC102; I believe the technical specs are available at the above link) have been made in our lab and I have been using these as a guideline. However, the problem is that the people who made these modifications no longer work here, and the changes they made were not properly documented, so there is a lot of guesswork on my end.

    Modifications I have made have mostly been to add capacitors and to increase the value of resistors. From what I can gather, I have mostly been changing the integrator portion of the circuit, and also modifying the differential portion to a small extent.

    Basically, the proportional part of the circuit seems to be functioning just fine. However, there is a big problem with the integrator. (The integrator can be turned on or off with a jumper/switch, which is how I was able to isolate the problem.) With the integrator off, the temperature will settle to an error of ~0.3 degrees. Turning the integrator on results in a huge steady state error of about -7 degrees. The temperature will actually settle around this point, and I can't figure out what part of the integrator circuit would be causing this strange behaviour.

    I've managed to find an agreeable configuration, i.e. where the temperature will settle (slowly), with the I-share turned on, with an offset of only 0.1 degree. I made some modifications afterward in order to improve these results, which is when the huge negative offset started showing up. I went back to the previous configuration, but the error was still there, which leads me to suspect that I may have damaged something.

    I am stuck for what to do. Any suggestions as to what might be causing the error, or some troubleshotting tips, would be very much appreciated!

    Also, it would be very helpful to be able to identify the value of some capacitors without removing them from the circuit. I don't have access to a "capacitance metre", i.e. I have no way of simply probing the value. Are there any techniques out there for divining the value of a capacitor in a fast and painless manner?

    Thanks in advance!
  2. leichner

    New Member

    Mar 4, 2010
    did you ask Thorlabs for a custom PID circuit? they might be able to help