Lakeshore 336, User Manual

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28 c
HAPTER
2: Cooling System Design and Temperature Control
Model 336 Temperature Controller
7. Gradually increase the proportional setting by doubling it each time. At each new
setting, allow time for the temperature of the load to stabilize.
8. Repeat step 7 until you reach a setting in which the load temperature begins a
sustained and predictable oscillation, rising and falling in a consistent period of
time. See FIGURE 2-2(a).
The goal is to find the proportional value in which the oscillation begins, do not turn the
setting so high that temperature and heater output changes become extreme.
9. If step 8 is achieved, complete steps 10 and 11, if not skip to step 12.
10. Record the proportional setting and the amount of time it takes for the load to
change from one temperature peak to the next. The time is called the oscillation
period of the load. It helps describe the dominant time constant of the load,
which is used in setting integral.
11. Reduce the proportional setting by half. The appropriate proportional setting is
one half of the value required for sustained oscillation in step 8. See
FIGURE 2-2(b). Continue to Tuning Integral section 2.8.3.
12. There are a few systems that will stabilize and not oscillate with a very high pro-
portional setting and a proper heater range setting. For these systems, setting a
proportional setting of one half of the highest setting is a good starting point.
Continue to the Tuning Integral section 2.8.3.
2.8.3 Tuning Integral When the proportional setting is chosen and the integral is set to 0 (off), the
Model 336 controls the load temperature below the setpoint. Setting the integral
allows the Model 336 control algorithm to gradually eliminate the difference in tem-
perature by integrating the error over time. See FIGURE 2-2(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.
1. Begin this part of the tuning process with the system controlling in proportional
only mode.
2. Use the oscillation period of the load that was measured in section 2.8.2 in sec-
onds. Divide 1000 by the oscillation period to get the integral setting.
3. Enter the integral setting into the Model 336 and watch the load temperature
approach the setpoint.
4. Adjust the integral setting if necessary:
a. 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.
b. If the temperature is stable but never reaches the setpoint, the integral
setting is too low and should be doubled.
5. Verify the integral setting by making a few small (2 K to 5 K) 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.