Stability, Attenuation - Parker Compax3S025V2 Operating Instructions Manual

Setting up Compax3
Step response of a stable controller and of a controller approaching the stability limit
w
x
Stable
Well attenuated
138

stability, attenuation

In this chapter you can read about:
Stability problem in the high-frequency range: ................................................................................138
Stability problem in the low-frequency range:..................................................................................138
In general, two stability problems may occur in a servo drive control:
Stability problem in the high-frequency range:
The "control structure" figure shows that the reverse effect in the control loop
(negative feedback) is a prerequisite for the functioning of a control system. Due to
the delay in signal transmission, the effect of the negative feedback is diminished
or even compensated. The reason is that the corrective measures of the controller
are also delayed in the event of delayed signal transmission. This results in a
typical oscillating course of the control variable. In the worst case, the deviation of
the control variable and the effect of the corrective measures get in phase, if the
delays reach a defined value. The negative feedback passes into positive
feedback. If the product of the gain factors of all control loop components is higher
than 1, the oscillation amplitude will continually rise.
In this case the control loop is unstable. In the total gain of 1 the oscillation keeps
its amplitude and the control loop is within the limits of stability. The transient
response can be characterized by the attenuation and the transient time (velocity).
w
x
Stable
Poorly attenuated
W: Setpoint value
x: Actual value
Stability problem in the low-frequency range:
In this case the controller was set for a very inert control path, while the actual
control path is much more dynamic. The controller reacts to a disturbance variable
with a much too strong corrective measure so that the disturbance variable is
overcompensated and even an increasing oscillation may be the result. In this
case the mechanic system of the control path may be destroyed.
Velocity jerk response (low-frequency stability limit)
1: Setpoint speed value
2: Actual speed value
192-120101 N11 C3I11T11 November 2007
Positioning via digital I/Os
w
x
Stability limit
not attenuated
1
2