Burkert Type 8635 - Manual (Page 151)

8635 - 149
G
ENERAL
R
ULES
(A
PPENDIX
)
Curve of controller output at the limit of stability
The literature on control technology contains a number of rules by which a favourable setting of the
controller parameters can be determined experimentally. In order to avoid incorrect settings, the
conditions under which the rules were set up in each case must be kept in mind. Apart from the
characteristics of the controlled member and the controller itself, it makes a difference whether a change
in disturbance or a command variable is to be compensated.
Adjustment rules of Ziegler and Nichols (oscillation method)
With this method, the controller parameters are set on the basis of the behaviour of the control loop at
the limit of stability. These parameters are initially set such that the control loop begins to oscillate.
Critical characteristic values occurring allow one to deduce a favourable setting of the control
parameters. A prerequisite for using this method is naturally that the control loop is permitted to
oscillate.
Procedure:
• Set the controller to P control (i.e. Tn = 999, Tv = 0), Kp initially small.
• Set the desired set point.
• Increase Kp until the controlled variable executes continuous, undamped oscillation).
The proportional action factor (amplification factor) set at the limit of stability is designated K
crit
.
The resulting oscillation period is designated T
crit
.
Rules for adjusting PID controllers
Istwert X
T krit
Zeit
T
crit
Time
Actual value
From K
crit
and T
crit
, the controller parameters can then be calculated using the following table.
Parameter setting according to Ziegler and Nichols:
Controller type Parameter setting
P Kp = 0.5 K
crit
- -
PI Kp = 0.45 K
crit
Tn = 0.85 T
crit
-
PID Kp = 0.6 K
crit
Tn = 0.5 T
crit
Tv = 0.12 T
crit
The adjustment rules of Ziegler and Nichols have been determined for P members with first order time
increase and dead time. However, they apply only for controllers with disturbance behaviour and not for
those with command behaviour.