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Lake Shore Model 370 AC Resistance Bridge User’s Manual
2.10.3 Tuning a Closed Loop PID Controller
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 and has a stable temperature reading from the
control sensor. 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).
2.10.3.1 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 through its operating range. Do not leave the
system unattended; the heater may have to be turned off manually to prevent overheating. If the load never reaches the
top of its operating range, some adjustment may be needed in heater resistance or an external power supply may be
necessary to boost the output power of the instrument.
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 370 is of no use controlling at or below the temperature reached
when the heater was off.
2.10.3.2 Tuning Proportional
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.
Begin this part of the tuning process by letting the cooling system cool and stabilize with the heater off. Place the Model
370 in closed loop PID control mode, then turn integral, derivative and manual output settings off. Enter a setpoint 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% when temperature stabilizes. The load temperature should stabilize at a temperature
below
the setpoint. If the load
temperature and heater display swing rapidly, the heater range or proportional value 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).
Gradually increase the proportional setting by doubling it each time. At each new setting, allow time for the temperature
of the load to stabilize. As the proportional setting is increased, there should be a setting in which the load temperature
begins a sustained and predictable oscillation rising and falling in a consistent period of time. See Figure 2-4(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 violent. In many systems it is difficult to differentiate oscillation and noise. Operating the
control sensor at higher than normal excitation power can help.
Record the proportional setting and the amount of time it takes for the load 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. If all has gone well,
the appropriate proportional setting is
one half
of the value required for
sustained oscillation. See Figure 2-4(b).
Theory of Operation 2-21