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Lake Shore Model 325 Temperature Controller User’s Manual
4-18 Operation
4.8.4 Setting Manual Heater Power (MHP) Output
Manual Heater Power (MHP) output is a manual setting of control output. It can function in two different ways
depending on control mode. In open loop control mode, the MHP output is the only output to the load. The user can
directly set control output from the front panel or over computer interface. In closed loop control mode, the MHP output
is added directly to the output of the PID control equation. In effect, the control equation operates about the MHP output
setting.
Manual heater power output setting is in percent of full scale. Percent of full scale is defined as percent of full-scale
current or power on the selected heater range. Manual heater power output setting range is 0% to 100% with a resolution
of 0.001%.
To enter a MHP Output setting, press the
Manual Heater key. The following display appears.
Enter for Loop 1
Manual Out 0.00%
The MHP Output setting is entered using the numeric keypad, which includes the numbers 0
–
9, +/–, and decimal point.
Press the Enter key, then the Escape key to return to the normal display.
4.9 AUTOTUNE (Closed-Loop PID Control)
The Model 325 automates the tuning process of typical cryogenic systems with the AutoTune feature. For additional
information about the algorithm refer to Section 2.8.
Before initiating AutoTune the cooling system must be set up properly with control sensor and heater making it capable
of closed-loop control. AutoTune works only with one control loop at a time and does not set the manual heater power
output or heater range. The control sensor must have a valid temperature response curve assigned to it. An appropriate
heater range must also be determined as described in Section 2.7.1. Choosing good initial control parameters by
experimenting with Manual PID tuning can speed up the AutoTune process. If no initial parameters are known start with
the default values of P = 50 and I = 20. It is better to set an initial P value that causes the system to be more active than
desired. Starting with a low P value can increase the time and number of attempts required to tune.
There are three AutoTune modes available. They result in slightly different system characteristics. Auto PI is
recommended for most applications.
Auto P – Sets only the P parameter value. I and D are set to 0 no matter what the initial values are. This mode is
recommended for systems that have very long lag times or nonlinearity that prevents stable PI control. Expect some
overshoot or undershoot of the setpoint and stable temperature control below the setpoint value.
Auto PI – Sets values for both P and I parameters. D is set to zero. This mode is recommended for stable control at a
constant temperature. It may take slightly longer to stabilize after setpoint change than Auto PID. Expect some
overshoot or undershoot of the setpoint and stable temperature control at the setpoint value.
Auto PID – Sets values for P, I and D parameters. D is always set to 100%. This mode is recommended when setpoint
changes are frequent but temperature is allowed to stabilize between changes. Stability at setpoint may be worse than
Auto PI in noisy systems. Expect slightly less overshoot or undershoot than the other modes and control at the
setpoint value.