BECKHOFF AL2000 Application - Manual (Page 11)

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11
Measurement unit
The series AL2000 motors use a wireless linear encoder for communication, speed and position control. The
performance of the linear motor depends on the characteristics of the applied linear encoder. The use of an
encoder system with a wire is not possible due to the bad dynamic characteristics. The unit is to be connected
and shielded with care. Any disturbance of the positioning signal could lead to positioning failures and system
oscillations.
Several types of measurement units can be applied, such as encoder kits by Heidenhain, Renishaw, Siko and
Numerik Jena. Mostly the resolution should lie between 0.1 and 5 µm. The accuracy of the measurement unit
must at least be better than the required accuracy of the motor system. Depending on the dynamics of the ap-
plication it can be a factor 2 to 10.
The position of a moving body is measured with respect to the frame. Depending on its stiffness the frame acts
more or less as a reaction force body. In the measurement appearing rotations or vibrations should be dis-
garded. Therefore the mounting of the measurement unit is critical. The probe should be mounted as close as
possible to the mass centre of the complete moving unit (slide + load), whereas the ruler needs to be placed
near the centroid of the frame. The presence of rotations or vibrations is surmountable but their measurement
and feed back should be avoided.
In exceptional cases stick or slip effects are present. If so the measurement can be placed best between the
bodies between that cause these effects.