Lake Shore Model 332 Temperature Controller User’s Manual
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 332 controls
the load temperature below the setpoint. Setting the integral allows the Model 332 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 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 332 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). Derivative of zero 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 332 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 332 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 332 does not automate that step of the tuning process.
When the AutoTune mode is selected, the Model 332 evaluates the control loop similar to the manual
tuning section described in Paragraph 2.7. One difference is that the Model 332 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.
Cooling System Design
2-13