GE AF-650 GP Series, Design And Installation Manual

Page: 74 / 166

Figure 6.21 Marginally Stable System
P
u
should be measured when the amplitude of oscillation
is quite small. Then we “back off” from this gain again, as
shown in Table 1.
K
u
is the gain at which the oscillation is obtained.
Type of
Control
Proportional
Gain
Integral Time Differentiation
Time
PI-control 0.45 * K
u
0.833 * P
u
-
PID tight
control
0.6 * K
u
0.5 * P
u
0.125 * P
u
PID some
overshoot
0.33 * K
u
0.5 * P
u
0.33 * P
u
Table 6.19 Ziegler Nichols Tuning for Regulator, Based on a
Stability Boundary
Step-by-step Description:
Step 1: Select only proportional control, meaning that the
integral time is selected to the maximum value, while the
differentiation time is selected to zero.
Step 2: Increase the value of the proportional gain until
the point of instability is reached (sustained oscillations)
and the critical value of gain,
K
u
, is reached.
Step 3: Measure the period of oscillation to obtain the
critical time constant,
P
u
.
Step 4: Use Table 6.19 to calculate the necessary PID
control parameters.
The following parameters are relevant for process control.
Parameter Description of function
PI-20 Process CL Feedback 1 Resource Select from which source (i.e. analog or pulse input) the process PID should receive its
feedback
PI-22 Process CL Feedback 2 Resource Optional: Determine if (and from where) the process PID should get an additional
feedback signal. If an additional feedback source is selected, the two feedback signals will
be added together before being used in process PID control.
PI-30 Process PID Normal/ Inverse Control Under [0] Normal operation, the process control will respond with an increase of the
motor speed if the feedback is getting lower than the reference. In the same situation, but
under [1] Inverse operation, the process control will respond with a decreasing motor
speed instead.
PI-31 Process PID Anti Windup The anti-windup function ensures that when either a frequency limit or a torque limit is
reached, the integrator will be set to a gain that corresponds to the actual frequency. This
avoids integrating on an error that cannot in any case be compensated for by means of a
speed change. This function can be disabled by selecting [0] “Off”.
PI-32 Process PID Start Speed In some applications, reaching the required speed/set point can take a very long time. In
such applications, it might be an advantage to set a fixed motor speed from the
adjustable frequency drive before the process control is activated. This is done by setting
a Process PID Start Value (speed) in PI-32 Process PID Start Speed .
PI-33 Process PID Proportional Gain The higher the value, the quicker the control. However, a value that is too large may lead
to oscillations.
Application Setup Examples AF-650 GP
TM
Design and Installation Guide
DET-767A 6-19
6 6