DIGIVEX Little Drive Servoamplifier
60
PVD 3530 GB 01/2005
Choosing PI (proportional and integral action)
Compared with P action alone, PI provides the following two modifications:
The gain (open loop) at zero frequency is infinite. If there is a torque surge, there will be an
angular discrepancy of the motor shaft compared with the state at rest. This angle will be
proportional to the torque applied and there will not be any permanent speed drift. The system
can be said to be "rigid". This rigidity is strictly proportional to the integration stop frequency.
The proportional gain P sets the bandwidth f
0
(system rapidity). The integral action entails -90°
phase shift, which creates instability. This phase shift is not troublesome at low frequencies, but
may make the system unstable at higher frequencies. It is therefore best to adjust the "integral
stop frequency" correctly (0.2 - 0.3 times the bandwidth f
0
).
Choosing PI
2
action (proportional and double integration)
Compared with P action alone, PI
2
provides the following two modifications:
Rigidity when stopped is infinite. When motor torque surges and after a transient period, the
motor shaft returns to the position it was in at rest (there is no longer any permanent position
discrepancy).
The double integral action entails a -180° phase shift at low frequencies. Poor adjustment of the
integral stop frequency may entail instability in the system. Restrict to 0.1 to 0.2 times the
bandwidth f
0
.
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