GE AF-650 GP Design And Installation Manual page 70

Application Set-Up Examples
Illustration 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 Stability Boundary
Parameter
Parameter PI-20 Process CL Feedback 1
Resource
Parameter PI-22 Process CL Feedback 2
Resource
Parameter PI-30 Process PID Normal/ Inverse
Control
Parameter PI-31 Process PID Anti Windup
Parameter PI-32 Process PID Start Speed
Parameter PI-33 Process PID Proportional Gain
Parameter PI-34 Process PID Integral Time
Parameter PI-35 Process PID Differentiation
Time
MG35M402
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 (that is, analog or pulse input) the Process PID should get 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 2 feedback signals are added
before being used in the Process PID Control.
Under [0] Normal operation, the Process Control responds 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 responds 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 is set to a gain that corresponds to the actual frequency. This
avoids integrating on an error that cannot in any case be compensated for with a speed
change. This function can be disabled by selecting [0] Off.
In some applications, reaching the required speed/set point can take a long time. In such
applications, it might be an advantage to set a fixed motor speed from the frequency
converter before the process control is activated. This is done by setting a Process PID
Start Value (speed) in parameter PI-32 Process PID Start Speed.
The higher the value - the quicker the control. However, too large value may lead to
oscillations.
Eliminates steady state speed error. Lower value means quick reaction. However, too small
value may lead to oscillations.
Provides a gain proportional to the rate of change of the feedback. A setting of zero
disables the differentiator.
DET-767c All rights reserved.
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
67
6 6