Lakeshore 325, Руководство пользователя, Страница: 35 / 166

Lake Shore Model 325 Temperature Controller User’s Manual
Cooling System Design 2-13
AutoTuning (Continued)
When the user selects a new setpoint, the Model 325 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 325 looks at the logged data to calculate the best P, I, and D settings values.
Those values are then loaded and used as the control parameters so the control loop can stabilize at the new setpoint.
AutoTune does not function during a ramp because the dominant time constant of the load is disguised by the ramp rate.
The control channel annunciator blinks to indicate that tuning data is being logged. The control channel annunciator
stops blinking when the tuning process is complete. The control channel annunciator will not blink 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 325 go until the control channel annunciator stops blinking. In many cases, AutoTune is
able to arrive at a better set of control settings.
There are situations where AutoTune is not the answer. The algorithm can be fooled when cooling systems are 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 under 10 seconds (with an appropriate I setting >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 is unappealing to
users who need the speed for fast cycle times. Manual tuning is not difficult on these systems because 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 at a new setpoint is too slow (with an appropriate I setting <5).
Thermal lag can be improved by using the sensor and heater installation techniques discussed above. Lag times up to a
few seconds should be expected; much larger lags can be a problem. System nonlinearity 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 periodic temperature fluctuations, the mass is too
small or temperature too low to AutoTune.
2.9 ZONE TUNING
Once the PID tuning parameters have been chosen for a given setpoint the whole process may have to be done again for
other setpoints significantly far away that have different tuning needs. Trying to remember when to use which set of
tuning parameters can be frustrating. The Model 325 has a Zone feature as one of its tuning modes that can help.
To use the Zone feature the user must determine the best tuning parameters for each part of the temperature range of
interest. The parameters are then entered into the Model 325 where up to ten zones can be defined with different P, I, D,
heater range, and manual heater settings. A setpoint setting is assigned as the maximum temperature for that zone. The
minimum temperature for a zone is the setpoint for the previous zone, 0 K is the starting point for the first zone. When
Zone tuning is on, each time the setpoint changes, appropriate control parameters are chosen automatically.
Control parameters can be determined manually or by using the AutoTune feature. AutoTune is a good way to determine
a set of tuning parameters for the control system that can then be entered as zones. Once the parameters are chosen,
AutoTune is turned off and zone tuning takes over.
Zone tuning has advantages over AutoTune during normal operation. When a new setpoint is set the zone tuning
automatically sets the appropriate control parameters for the destination. Approach to the new setpoint is controlled with
the best parameters. AutoTune, on the other hand, is not able to learn enough about the system to change the control
parameters until after the temperature gets near or to the new setpoint. Approach to the new setpoint is controlled with
the old parameters because they are the best available.