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FAQs - Frequently Asked Questions

How to find the best parameters for the PID-controller?

To find the right parameters and to optimize the PID-controller to have the most accurate and fastest feedback control of your laser can be a real challenge sometimes. We hereby describe a quick procedure finding the parameters P, I, D and the time constant Ta for the I- and D-calculation. This procedure is based on our PID-Simulator which estimates these parameters according to your setup- and laser-conditions.



First, you need to setup your Wavelength Meter:
Set an appropriate exposure time (the smaller the better). (e.g. 5 ms)
Start the measurement and determine the repetition rate (simply type "RepetitionRate" in the "Start | Run..." dialog box). (e.g. 40 Hz)

Now, you need to know your laser:
Find a working range where the laser is operating mode hop free in a certain range. This working range later must cover the wavelength range you want to work/regulate at.
Apply 0 V on the LC-output of the wlm using the "Calibration"-tab in the laser control settings window and press "Send". Which wavelength does the wlm display? (e.g. 780,311 nm)
Increase the output voltage step by step and observe the max. mode hop free range and the amplification V/nm.
Decrease the output voltage step by step and observe the max. mode hop free range and the amplification V/nm.
Now calculate the amplification (e.g. 7 V/100 pm)
It might be also necessary to limit the maximum/minimum voltage of the LC-output in order not to cause the laser to jump during the regulation (mode hop). Therefor set these values in the "Bounds"-tab in the laser control settings window.
Note: The "Test"-output voltage might be not available in older software versions. Then you need to disconnect the LC-cable and apply the desired voltage by an external powersupply to find out the 0 V-wavelength and the amplification.

Run the PID-Simulator:
You need to adjust the General- and Laser-settings to fit your experimental conditions:

General settings:
Time/Measurement = 1/Repetition Rate (e.g. 1/40 Hz = 25 ms = 0,025 s)
Response count = 2 (The PID gets an answer to its action after the next measurement shot, do not change this!)
Measurement count is only for displaying the function, it has no influence on the calculation (-speed).
V_out maximum = +/- 4096 mV (this is the max. range the wlm can give out)
V_out resolution. = 0,5 mV (14 bit resolution on max. range, do not change this!)

Laser settings:
Type in the amplification and 0 V-wavelength you determined above.
Noise and Sudden hops can be left "0" usually.

Regulation:
Formula: Use whatever you desire, within the mode hop free wavelength range (e.g. f(t) = 780,311+ 0,001 * (triangle(t/2)-0,5) to see how the laser reacts)
Ta, P, I and D, you can leave blank as these values are estimated automatically.
The sensitivity should be the found laser amplification.

Now you've entered all desired values. Press "Auto TPID" to start the iteration and wait until the "Auto TPID"-button is solid black again. The simulator now tries to find the best settings corresponding to your initial parameters. You can use these parameters in the laser control settings window and maybe adjust them a bit to optimize your feedback loop.
If the controller does not work properly with the found parameters, you might need to repeat the procedure with slightly different initial values. The better you know your conditions and the laser properties, the better are the estimated values.
If you change the formula in your laser control settings, you need to use the same formula in the simulator and run it again!