Watlow MINICHEF 2000 - Manual (Page 23)

2 2 ■ Wa t l o w M I N I C H E F 2 0 0 0 H a r d w a r e & S o f t w a r e S e t u p G u i d e
On/off Temperature control
On/off control switches the heat outputs either full on or full off, depending on the tem-
perature sensor input, set point and hysteresis values. The hysteresis value creates a
buffer zone that increases the time interval that the output is off or on. With hysteresis
set to 1º the process value would stay closer to the set point, but the output would
switch on and off more frequently.
PID Temperature control (Proportional, Integral, Derivative)
Proportional control: Some processes need to maintain a temperature or process
value closer to the set point than On/off control can provide. Proportional control pro-
vides closer control by adjusting the output when the temperature or process value is
within a proportional band. When the value is in the band, the controller adjusts the
output based on how close the process value is to the set point: the closer to set point
the lower the output. This is similar to backing off on the gas pedal of a car as you
approach the speed limit. It keeps the temperature or process value from swinging as
widely as it would with simple On/off control. However, when a system stabilizes, the
temperature or process value tends to “droop” short of the set point.
Proportional plus Integral: The droop caused by proportional control can be correct-
ed by adding integral (reset) control to the system. When the system has settled down,
the integral (reset) value is tuned to bring the temperature or process value closer to
the set point. However, this increases the overshoot that occurs at startup or when the
set point is changed.
Proportional, Integral, Derivative control (PID): Use derivative (rate) control to
minimize the overshoot in a Proportional-Integral controlled system. Derivative (rate)
adjusts the output based on the rate of change in the temperature or process value.
Auto-tuning
Auto-tuning is a feature that simplifies the determination of PID values (an otherwise
tedious, time-consuming system tuning process.) Auto-tuning allows the controller to
automatically explore the responsiveness of the complete system to determine an effec-
tive set of parameters for PID control. To do this it crosses an auto-tune set point three
times, then controls at the normal set point using the new parameters. Once the auto-
tuning cycle is complete, the optimized PID values are stored in the controller memory
automatically.
S t e p 3 C o n f i g u r e t h e C o n t r o l l e r