GE AF-650 GP Design And Installation Manual page 74

Application Setup Examples
Figure 6.21 Marginally Stable System
P should be measured when the amplitude of oscillation
u
is quite small. Then we "back off" from this gain again, as
shown in Table 1.
K is the gain at which the oscillation is obtained.
u
Type of Proportional
Control Gain
PI-control 0.45 * K
u
PID tight 0.6 * K
u
control
PID some 0.33 * K
u
overshoot
Table 6.19 Ziegler Nichols Tuning for Regulator, Based on a
The following parameters are relevant for process control.
Parameter
PI-20 Process CL Feedback 1 Resource
PI-22 Process CL Feedback 2 Resource
PI-30 Process PID Normal/ Inverse Control
PI-31 Process PID Anti Windup
PI-32 Process PID Start Speed
PI-33 Process PID Proportional Gain
TM
AF-650 GP Design and Installation Guide
Integral Time Differentiation
Time
0.833 * P -
u
0.5 * P 0.125 * P
u u
0.5 * P 0.33 * P
u u
Description of function
Select from which source (i.e. analog or pulse input) the process PID should receive its
feedback
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.
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.
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".
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.
The higher the value, the quicker the control. However, a value that is too large may lead
to oscillations.
DET-767A
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
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.
, is reached.
u
6-19
6 6