Relative Measuring Systems - Bosch Rexroth IndraDrive MPH-02 Functional Description

5-66
Motor, Mechanical Axis System, Measuring Systems
Setting the Initial Position Value

Relative Measuring Systems

Evaluating Position
Measurement
Aspects of Use
Establishing Axis-Related
Absolute Distance
If the absolute encoder range of the control encoder is smaller than the
value of S-0-0278 you have to check whether the travel range was
correctly input or whether the default value is active!
If desired, it is possible to enter in P-0-0019, Initial position value
parameter a defined initial position value for the actual position value of
the encoder or the encoders. For encoders that can be evaluated in
absolute form this value is only active the first time the drive is switched
on. After an encoder that can be evaluated in absolute form was set "in
reference", this value is insignificant even when the drive is switched on
again!
Brief Description
IndraDrive controllers can evaluate the signals of both absolute and
relative measuring systems, if the encoder signals correspond to the
specification.
The disadvantages of relative encoders as opposed to encoders that can
be evaluated in absolute form are as follows:
• Axes with relative position encoder must go through a homing
procedure after switching on so that they can be operated in position
control.
• Relative encoders are unsuitable as motor encoders for synchronous
motors because each time the drive is restarted it has to go through a
procedure for setting the commutation offset. Therefore, the
immediate readiness for operation is not guaranteed for synchronous
motors!
There are, however, advantages of relative encoders as opposed to
encoders that can be evaluated in absolute form:
• Longer travel distances are possible for linear encoders!
• The costs of the encoder are mostly lower given equal absolute
precision and number of lines or division period length!
Due to the above disadvantages, relative measuring systems are not
used as motor encoders for synchronous Rexroth housing motors. For
asynchronous motors there aren't any disadvantages when using relative
motor encoders.
For kit motors it can be necessary, however, to use relative encoders as
motor encoders if absolute encoders of the required design are not
available:
• great encoder lengths for long travel distances in the case of linear
motors
• hollow-shaft encoders with special bore diameters or encoders for high
maximum speeds in the case of rotary kit motors
The actual position values of relative encoders first do not have any
position reference. In any axis position the actual position value of the
respective relative encoder, when switching the drive on, is written with
the so-called initial position value, if no other encoder being in reference
has been connected.
There are two ways to establish the axis-related absolute distance:
• move to a defined axis position with reproducible precision
- or -
• detecting a defined axis position by "passing" two distance-coded
reference marks of the encoder.
MPH-02, MPB-02, MPD-02
DOK-INDRV*-MP*-02VRS**-FK01-EN-P