ER5000 —
36
How It Works
Figure 3 shows four typical response curves to a step
(instantaneous) change in setpoint.
A
B
D
C
Overshoot
Ringing
Offset
Settling Time
Rise Time (10%–90%)
Figure 3: Typical Response Curves to Programmed Setpoint Change
The controller in Curve A responds with a Rise Time that is short
and sharp. It also overcorrects (Overshoots) the error and oscillates
around the new setpoint, a condition known as Ringing. There is a
prolonged Settling Time before the system reaches its new stable
state. The overall response is quick but unstable.
The controller in Curve D has a long, gradual rise time and ends
up in Steady State Error, settling into a stable state that is slightly
below the new setpoint.
Curve A demonstrates the unwanted effects of a fast, but also
unstable, response. Tuning this controller will entail lowering the
Proportional setting and perhaps the Integral setting as well.
Alternatively, increasing the Derivative will help to stabilize the
response. Curve D demonstrates the unwanted effects of a
stable, but overly slow, response. Tuning this controller will entail
increasing Proportional. Integral also should be increased to
eliminate the offset. Derivative should be checked to see if it is
overdamping the response.
The controllers in Curves B and C have been tuned to achieve a
balance between speed and stability. Curve B allows a small amount
of overshoot and ringing in exchange for the fastest rise time
possible, with a settling time that is short and stable. Curve C is
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