Technology Option F01 08.2012
6SE7087-6QX70 (Version AN) Siemens AG
9-48 Compendium Motion Control SIMOVERT MASTERDRIVES
9.4.9 Using absolute encoders for positioning of motors with load-side
gearing and rotary axis
This chapter describes the procedure required when mechanical
gearing
is interposed between a rotary axis and the motor and
position control is to be performed by an
absolute value encoder
located on the motor. In this case, an additional function block is
required which is shown in function diagram 327 for the motor encoder
and function diagram 333 for the external encoder.
Absolute encoders capable of counting a definite number of encoder
revolutions (such as 4096) are used for angular synchronism and in
order to avoid having to start from a home position.
As a rotary encoder turns endlessly in on particular direction, the
representation range of the encoder is exceeded. The result is an
encoder overflow which means that the encoder starts counting from 0
again after, for example, 4096 rotations.
This is illustrated by the diagram below.
Load position
Absolute encoder
Encoder overflow
Max. number of revolutions
countable by absolute encoder
To save costs, an absolute encoder is fitted on the motor and used both
for torque and speed control and for position control (EQN 1325). One
advantage of this is that fitting the encoder on the motor is much less
critical but much more precise than fitting it on the load side.
Mechanical gearing is usually located between the motor and the load
for speed adjustment. Depending on the gear ratio, an offset between
the zero position of the load and the motor occurs on every encoder
overflow.
Basic problems
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