Newport 350B, User Manual, Page: 58 / 66

Maintenance and Service 57
7.5.3 PI Loop
To eliminate the small errors that cannot be compensated by P loop alone, an integral
term can be added to the control loop. This term integrates (adds) the error continuously
and the value, multiplied by the
K
i
gain factor, is added to the control signal (Figure 20).
Figure 20 PI Temperature Controller Block Diagram
The result is that the contribution of integral term to the control signal will increase until
it reduces the temperature following error to zero. While this is desirable for many
situations, the integral term can have a severe destabilizing effect on the control loop if
increased too much.
7.5.4 PID Loop
The third term of the PID loop is the derivative term. This term amplifies the rate of
change of temperature following error by K
d
gain factor. If the following error does not
change, the contribution of derivative term to the control signal is zero. Figure 21 shows
the PID control loop diagram.
Figure 21 PID Temperature Controller Block Diagram
The derivative term adds a damping effect which helps prevent oscillations and
temperature overshoot. It also supports achieving optimal stability and settling time.
u
Temperature
Setpoint
Temperature
Controller
Thermo-Electric
(TE) Module
Temperature Sensor
x
K
p
e
x
K
i
∫e
+
+
+
-
u
Temperature
Setpoint
o
Temperature
Thermo-Electric
(TE) Module
Temperature Sensor
x
K
p
e
x
K
i
∫
e
+
+
+
-
de
dt
x
K
d
+