Lake Shore Model 340 Temperature Controller User’s Manual
Cooling System Design
2-11
2.6.4 Manual Heater Output (MHP)
The Model 340 has a control setting that is not a normal part of a PID control loop. Manual heater output can
be used for open loop control, meaning feedback is ignored and the heater output stays at the users manual
setting. This is a good way to put constant heating power into a load when needed. The manual output term
can also be added to the PID output. Some users prefer to set a power near that necessary to control at a
setpoint and let the closed loop make up the small difference. Manual output is set in percent of full scale
current or power for a given heater range.
2.7 MANUAL
TUNING
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 of thumb 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 the user has worked through the operation sections of this
manual, has a good temperature reading from the sensor chosen as a control sensor, and is operating
Loop 1. It is also a good idea to begin at the center of the temperature range of the cooling system (not close
to its highest or lowest temperature). AutoTune (Paragraph 2.8) is another good place to begin, and do not
forget the power of trial and error.
Setting the heater range is discussed in Paragraph 2.7.1. Manually tuning Proportional (P) is discussed in
Paragraph 2.7.2. Manually tuning Integral (I) is discussed in Paragraph 2.7.3. Finally, manually tuning
Derivative (D) is discussed in Paragraph 2.7.4.
2.7.1 Setting
Heater
Range
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 overcome 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. 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 heater 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. If the load never reaches room temperature, some adjustment may be needed in heater
resistance or maximum heater current.
The list of heater range versus load temperature is a good reference for selection 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 340 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.
2.7.2 Tuning
Proportional
The proportional setting is so closely tied to heater range that it 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.
Begin this part of the tuning process by letting the cooling system cool and stabilize with the heater off. Place
the Model 340 in Manual PID control mode, then turn integral, derivative and manual output settings off. Enter
a setpoint several degrees above the cooling systems lowest temperature. Enter a low proportional setting of
approximately 5 or 10 and then enter the appropriate heater range as described above. The heater display
should show a value greater than zero and less than 100%. The load temperature should stabilize at a
temperature below the setpoint. If the load temperature and heater meter swing rapidly, the heater range may
be set too high and should be reduced. Very slow changes in load temperature that could be described as
drifting are an indication of a proportional setting that is too low (which is addressed in the next step).