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Lake Shore Model 325 Temperature Controller User’s Manual
2-12 Cooling System Design
Tuning Proportional (Continued)
If the load does not oscillate in a controlled manner, the heater range could be set too low. A constant heater reading of
100% on the display would be an indication of a low range setting. The heater range could also be too high, indicated by
rapid changes in the load temperature or heater output with a proportional setting of less than 5. There are a few systems
that will stabilize and not oscillate with a very high proportional setting and a proper heater range setting. For these
systems, setting a proportional setting of one half of the highest setting is the best choice.
2.7.3 Tuning Integral
When the proportional setting is chosen and the integral is set to zero (off), the Model 325 controls the load temperature
below the setpoint. Setting the integral allows the Model 325 control algorithm to gradually eliminate the difference in
temperature by integrating the error over time. See Figure 2-3(d). An integral setting that is too low causes the load to
take too long to reach the setpoint. An integral setting that is too high creates instability and can cause the load
temperature to oscillate.
Begin this part of the tuning process with the system controlling in proportional only mode. Use the oscillation period of
the load that was measured above in seconds.
Divide 1000 by the period to get the integral setting. Enter the integral
setting into the Model 325 and watch the load temperature approach the setpoint. If the temperature does not stabilize
and begins to oscillate around the setpoint, the integral setting is too high and should be reduced by one half. If the
temperature is stable but never reaches the setpoint, the integral setting is too low and should be doubled.
To verify the integral setting make a few small (2 to 5 degree) changes in setpoint and watch the load temperature react.
Trial and error can help improve the integral setting by optimizing for experimental needs. Faster integrals, for example,
get to the setpoint more quickly at the expense of greater overshoot. In most systems, setpoint changes that raise the
temperature act differently than changes that lower the temperature.
If it was not possible to measure the oscillation period of the load during proportional setting, start with an integral
setting of 20. If the load becomes unstable reduce the setting by half. If the load is stable make a series of small, two to
five degree, changes in the setpoint and watch the load react. Continue to increase the integral setting until the desired
response is achieved.
2.7.4 Tuning Derivative
If an experiment requires frequent changes in setpoint or data taking between changes in the setpoint, derivative should
be considered. See Figure 2-3(e). A derivative setting of zero, off, is recommended when the control system is seldom
changed and data is taken when the load is at steady state.
The derivative setting is entered into the Model 325 as a percentage of the integral time constant. The setting range is
0 – 200% where 100% = ¼ I seconds. Start with a setting of 50 to 100%.
Again, do not be afraid to make some small setpoint changes; halving or doubling this setting to watch the affect. Expect
positive setpoint changes to react differently from negative setpoint changes.
2.8 AUTOTUNING
Choosing appropriate PID control settings can be tedious. Systems can take several minutes to complete a setpoint
change, making it difficult to watch the display for oscillation periods and signs of instability. With the AutoTune
feature, the Model 325 automates the tuning process by measuring system characteristics and, along with some
assumptions about typical cryogenic systems, computes setting values for P, I, and D. AutoTune works only with one
control loop at a time and does not set the manual heater power output or heater range. Setting an inappropriate heater
range is potentially dangerous to some loads, so the Model 325 does not automate that step of the tuning process.
When the AutoTune mode is selected, the Model 325 evaluates the control loop similar to the manual tuning section
described in Section 2.7. One difference is that the Model 325 does not initiate changes to control settings or setpoint for
the purpose of tuning.
It only gathers data and changes control settings after the user changes the setpoint. Unexpected
or unwanted disturbances to the control system can ruin experimental data being taken by the user.