Watlow Controls
6
EHG CL User’s Guide
Proportional Control
Some processes need to maintain a temperature or
process value closer to the set point than on-off con-
trol can provide. Proportional control provides 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 out-
put based on how close the process value is to the set
point.
The closer the process value is to the set point, the
lower the output power. This is similar to backing off
on the gas pedal of a car as you approach a stop sign.
It keeps the temperature or process value from swing-
ing as widely as it would with simple on-off control.
However, when the system settles down, the tempera-
ture or process value tends to “droop” short of the set
point.
With proportional control the output power level
equals (set point minus process value) divided by the
proportional band value.
Adjust the proportional band with Proportional
[`Pb]
.
Time
Temperature
Proportional Control
Set Point
Proportional Band
Droop
Overshoot
Proportional plus Integral (PI) Control
The droop caused by proportional control can be cor-
rected by adding integral control. When the system
settles down, the integral value is tuned to bring the
temperature or process value closer to the set point.
Integral determines the speed of the correction, but
this may increase the overshoot at startup or when
the set point is changed. Too much integral action will
make the system unstable. Integral is cleared when
the process value is outside of the proportional band.
Integral
[Int]
is measured in minutes per repeat. A
low integral value causes a fast integrating action.
Proportional plus Integral plus Derivative (PID)
Control
Use derivative control to minimize the overshoot in
a PI-controlled system. Derivative
[dEu]
adjusts the
output based on the rate of change in the temperature
or process value. Too much derivative will make the
system sluggish.
Time
Temperature
PID Control
Set Point
Reduced Overshoot
Proportional Band
Proportional Band x 2
Heating Slows
On-Off Control
On-off control switches the output either full on or full
off, depending on the input, set point and hysteresis
values. The hysteresis value indicates the amount the
process value must deviate from the set point to turn on
the output. Increasing the value decreases the number
of times the output will cycle. Decreasing hysteresis im-
proves controllability. With hysteresis set to the lowest
value of 3
°
C or 5
°
F, the process value would stay closer
to the set point, but the output would switch on and off
more frequently, and may result in the output “ chatter-
ing.” Both the control mode (
[Cnt]
prompt) and hyster-
esis (
[Hys]
prompt) values can be changed either using
the front panel or via Modbus communications.
Set Point
Time
Temperature
The heating action switches off when the process
temperature rises above the set point.
The heating action
switches on at startup.
Hysteresis
Process Temperature
On/Off System Cycles
The heating action switches on when the process temperature
drops below the set point minus the hysteresis.
Mounting the Series EHG CL
88.8 mm
(3.496 in)
55.8 mm
(2.196 in)
Base Controlller
front
48.4 mm
(1.907 in)
Base Controller
side
