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PID Controls
y(t)
P u
Illustration 16.5 Marginally Stable System
Measure P
when the amplitude of oscillation is quite
u
small. Then back off from this gain again, as shown in
Table 16.7.
K
is the gain at which the oscillation is obtained.
u
Type of
Proportional
control
gain
PI-control
0.45 x K
u
PID tight
0.6 x K
u
control
PID some
0.33 x K
u
overshoot
Table 16.7 Ziegler Nichols Tuning for Regulator,
Based on a Stability Boundary
Use the control settings described below for initial tuning.
The process operator can then fine-tune the control as
necessary.
Step-by-step description
1.
Select only proportional control (integral time is
selected to the maximum value, while the differ-
entiation time is selected to 0).
2.
Increase the value of the proportional gain until
the point of instability is reached (sustained
oscillations) and the critical value of gain, K
reached.
3.
To obtain the critical time constant, measure the
period of oscillation: P
4.
To calculate the necessary PID control parameters,
use Table 16.7.
MG37N102
Design Guide
t
Integral time
Differentiation
time
0.833 x P
–
u
0.5 x P
0.125 x P
u
u
0.5 x P
0.33 x P
u
u
, is
u
.
u
Danfoss A/S © 6/2016 All rights reserved.
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