Manual Tuning; Setting Heater Range; Tuning Proportional - Lakeshore 335 User Manual

2.8 Manual Tuning

2.8.1 Setting Heater
Range
2.8.2 Tuning
Proportional
There has been a lot written about tuning closed loop control systems and specifically
PID control loops. This section does not attempt to compete with control theory
experts. It describes a few basic rules to help less experienced users get started. This
technique will not solve every problem, but it has worked for many others in the field.
This section assumes you have worked through the operation sections of this manual,
you have a good temperature reading from the sensor chosen as a control sensor, and
you are operating Loop 1. It is also a good idea to begin at the center of the tempera-
ture range of the cooling system (not close to its highest or lowest temperature).
Autotune (section 2.9) is another good place to begin, and trial and error can be help-
ful as well.
Setting an appropriate heater output range is an important first part of the tuning
process. The heater range should allow enough heater power to comfortably over-
come the cooling power of the cooling system. If the heater range will not provide
enough power, the load will not be able to reach the setpoint temperature. Con-
versely, if the range is set too high, the load may have very large temperature changes
that take a long time to settle out. Delicate loads can even be damaged by too
much power.
Often there is little information on the cooling power of the cooling system at the
desired setpoint. If this is the case, try the following: allow the load to cool completely
with the heater off. Set Manual Output to 50% while in Open Loop control mode. Turn
the heater to the lowest range and write down the temperature rise (if any). Select
the next highest heater range and continue the process until the load warms up to
room temperature. Do not leave the system unattended; the heater may have to be
turned off manually to prevent overheating. If the load never reaches room tempera-
ture, some adjustment may be needed in heater resistance or load.
The list of heater range versus load temperature is a good reference for selecting the
proper heater range. It is common for systems to require two or more heater ranges
for good control over their full temperature. Lower heater ranges are normally
needed for lower temperature. The Model 335 is of no use controlling at or below the
temperature reached when the heater was off. Many systems can be tuned to control
within a degree or two above that temperature.
The proportional setting is so closely tied to heater range that they can be thought of
as fine and course adjustments of the same setting. An appropriate heater range must
be known before moving on to the proportional setting.
1. Allow the cooling system to cool and stabilize with the heater off.
2. Place the Model 335 in closed loop PID mode tuning.
3. Turn integral, derivative and manual output settings to 0.
4. Enter a setpoint several degrees above the cooling system's lowest temperature.
5. Enter a low proportional setting of approximately 5 or 10, and enter the appro-
priate heater range as described in section 2.8.1.
6. The load temperature should stabilize at a temperature below the setpoint. The
heater display should show a value greater than 0% and less than 100%. If the
load temperature does not stabilize below the setpoint, do one of the following:
a. If the load temperature and heater display reading swing rapidly, the
proportional setting or possibly the heater range may be set too high.
Reduce the proportional setting or the heater range, and go back
to step 6.
b. If the load temperature and heater display reading change very slowly, a
condition described as drift, it is an indication of a proportional setting
that is too low. Increase the proportional setting and go back to step 6.

2.8.1 Setting Heater Range

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