Autotuning - Lakeshore 340 User Manual

Lake Shore Model 340 Temperature Controller User's Manual
2.8

AUTOTUNING

Choosing appropriate PID control settings can be tedious. As systems may take several minutes to complete
a setpoint change, it is difficult to observe the display for that length of time watching for oscillation periods or
signs of instability. With the AutoTune feature, the Model 340 automates the tuning process by measuring
system characteristics. Then, combined with assumptions about typical cryogenic systems, it computes
setting values for P, I, and D. AutoTune works only with one control loop at a time and will not set manual
heater output or heater range. Setting an inappropriate heater range is potentially dangerous to some loads
so the Model 340 does not attempt to automate that step of the tuning process.
When AutoTune mode is selected, the Model 340 will evaluate the control loop similar to Manual Tuning
(Paragraph 2.7). The major difference is the Model 340 will not initiate changes to the control settings or
setpoint for the purpose of tuning. Since unexpected or unwanted disturbances to the control system can ruin
experimental data being taken by the user, the Model 340 only gathers data and changes control settings
after the user changes the setpoint.
When the user selects a new setpoint, the Model 340 logs the change in temperature at the load and the
change in heater output that was required to make the load temperature change. The old control settings are
used while data is being logged, so a good initial guess of settings can improve the efficiency of the AutoTune
feature. Once the load temperature is at or near the new setpoint, the Model 340 looks at the logged data to
calculate the best P, I, and D setting values. The values are then loaded and used as the control parameters
so the control loop can stabilize at the new setpoint. AutoTune does not work during a ramp because the
dominant time constant of the load is disguised by the ramp rate.
The user can tell when data is being logged because the Tune annunciator changes to Tuning. When the
annunciator returns to Tune, the process is complete and will not begin again until the user changes the
setpoint. If AutoTune does not give desired results the first time, make a few small (2 to 5 degree) changes in
setpoint and let the Model 340 complete the tuning cycle each time. This allows the AutoTune feature to zero
in on a better set of control settings.
There are situations where AutoTune is not the answer. The algorithm can be fooled when cooling systems
are either very fast, very slow, have a large thermal lag, or have a nonlinear relationship between heater
power and load temperature. If a load can reach a new setpoint in just under 10 seconds (with an appropriate
I setting of >500), the cooling system is too fast for AutoTuning. Systems with a very small thermal mass can
be this fast. Adding mass is a solution, but it is unappealing to users who need the speed for fast cycle times.
Manual tuning is not difficult on these systems since new settings can be tested very quickly. Some systems
are too slow for the AutoTune algorithm. Any system that takes more than 15 minutes to stabilize (with an
appropriate I setting of <5) is too slow.
Thermal lag can be improved by using the sensor and heater installation techniques discussed in this chapter.
Lag times of a few seconds should be expected; much larger lags can be a problem. System non-linearity is a
problem for both AutoTune and manual tuning. It is most commonly noticed when controlling near the
maximum or minimum temperature of a temperature control system. It is not uncommon, however, for a user
to buy a cryogenic cooling system specifically to operate near its minimum temperature. If this is the case, try
to tune the system at 5 degrees above the minimum temperature and gradually reduce the setpoint, manually
adjusting the control settings with each step. Any time the mechanical cooling action of a cryogenic
refrigerator can be seen as a periodic temperature fluctuation, the mass is too small or temperature too low to
AutoTune.
Cooling System Design 2-13