Commutation Settings Of The Automatic Commutation - Parker Compax3 Series Operating Instructions Manual

Setting up Compax3
218
I(t): Torque-forming motor current
Kt: Torque constant
ML(t): External disturbance torque
Jtotal: Total mass moment of inertia (motor + load)
a(t): Acceleration
n(t): Velocity
x(t): Position
Index b: Observed signal quantities
h0...h2: Controller coefficients of the tracking controller
The figure shows that an additional I element is connected for interference com-
pensation to correct external disturbance forces in the observer. Therefore the
speed and the acceleration observed are statically precise. The same applies to
the output of the integrator in the tracking controller which is a statically precise
determination of an external interference torque ML. For this reason, the I compo-
nent is not required in the speed controller, and the entire control can be set up as
a state cascade control, which increases the bandwidth of the speed and position
controlled member by factor 2. As a consequence, the interference stiffness of the
drive and the tracking error behavior improve.
Here the quantisation of the speed signal is proportional to the sampling time TAR,
hence there is no longer any conflict between the requirements for minimum sam-
pling time and minimum quantisation noise. For the integral velocity acquisition, the
motor current variable, which is proportional to the acceleration, can be used. This
approach is particularly advantageous in direct drive engineering; due to the ab-
sence of a mechanical drive train, there is a very good match between the mathe-
matical model of the observer and the real physical control section in the funda-
mental frequency range of the control. This applies in particular to direct drive sy-
stems with fixed moving masses, as otherwise the mismatch between model and
the physical drive system has a destabilising influence on the transfer behaviour of
the speed control. A remedy is to increase the observer dynamics, however this
increases the noise of the observed signals. Therefore in the case of variable mo-
ving masses a compromise has to be found between the dynamics of the observer
and the maximum stiffness of the drive.

Commutation settings of the automatic commutation

In this chapter you can read about:
Display of the commutation error in incremental feedback systems................................ 219
Prerequisites for the automatic commutation .................................................................. 220
Course of the automatic commutation function ............................................................... 220
Other ................................................................................................................................ 222
Permanently excited synchronous motors can only be operated with an absolute
feedback system (at least for electric motor rotation). The reason is the necessary
commutation information (position assignment of the magnet field generated by the
motor to the motor magnets). Without the commutation information, there is inevi-
tably the possibility of a positive feedback between position and velocity loop ("run-
ning away" of the motor) or of bad motor efficiency (reduced force constant).
Digital hall sensors are the most common aid to prevent this. Due to the mechani-
cal design it is however impossible or very hard to integrate these sensors in some
motors. The Compax3 automatic commutation function (in the F12 direct drive
device) described below allows however to use incremental feedback systems
without hall sensors.
The functionality implemented in the servo controller establishes the necessary
reference between motor stator field and permanent magnetic field without additio-
nal aids.
The incremental feedback devices are, in contrast to absolute feedback devices,
able to acquire relative distances. It is true that any position can be approached
from a starting point, there would be however no consistency between these positi-
on values and a fixed virtual absolute system. Other than with an absolute feed-
back, the correlation between rotor and stator is lost if the position acquisition is
switched off ("the position acquisition zero is lost"). When switching on, the actual
position is randomly taken as zero. A commutation angle error can therefore abso-
192-120114 N5 C3I22T11 June 2008
C3I22T11