BECKHOFF AL2000 - Manual (Page 12)

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12
Practical considerations
A PART OF A TOTAL MACHINE CONCEPT
A linear motor of Beckhoff Industrie Elektronik is not a system on itsself. It should be build within a total ma-
chine concept or a working unit. Depending on the application a choice has to be made between different sys-
tems. Therefore it is important to know the specifications of both the total machine concept and the linear motor
types. The machine should meet all the appilcable CE requirements
MOUNTING FRAME: SOLIDITY AND STABILITY
For two main reasons the mounting frame has to meet special requirements. The propelling peak forces of a
linear motor are high, so the frame needs sufficient dynamic stiffness. Because of the accuracy the frame
should be insensible to shocks and vibrations. Usually the magnetplate is horizontally fixed to the base of a
machine or working unit. The slide will bear a functional load, such as a measurement unit or a laser cutter.
Because of this load, the cabling and optional cooling lines the ratio of peak force to motor inertia could be in-
fluenced a little. It will be obvious that a vertical position of a linear motor system demands special considera-
tions. If desired contact Beckhoff Industrie Elektronik.
RAILS AND BEARINGS
To assure a free movement the slide is provided with bearings that run smoothly on two rails or with air bear-
ings. The rails are mounted aside the magnetplate. In addition this construction ensures the right airgap be-
tween the coil unit and the magnet plate. For the sideward positioning of the coil unit to the magnetplates a
small tolerance is acceptable.
HEAT DISSIPATION
Especially in the coil unit the heat dissipation can be significant. Heatflow causes temperature differences. In
some cases this could be unacceptable for accuracy or other reasons. So, special attention has to be given to
the heatflow to the ambient. In some cases (water) cooling is needed.
ACCURACY
The accuracy of the positioningsystem is crucial, since the linear motor is supposed to be controlled at submi-
cron level. For instance the accuracy can be influenced by heat expansion of the coil unit, by vibrations and
shocks from the surroundings or by a lack of stiffness of the mounting frame. Generally the main restriction for
the accuracy arises from this lack of stability.
Like most mechanical systems a linear motor deals with resistance, elasticity and inertia. Because of the feed-
back of position and velocity it also deals with measuring accuracy and certain delay times. For instance the
controller has to deal with some delay because of processor and update times. Accuracy could also be limited
by the fixed pulse frequency of the servo-amplifier.
Of course the linear ruler and the measurement probe should meet the desired accuracy. This requires a so-
phisticated ruler-probe combination. The probe should be mounted as close as possible to the working point of
the complete moving unit (slide + load). The probe sends signals to the linear encoder, whereby the encoder
has a resolution time. At last the linear motor itsself is an electromagnetic device. It deals with resistance and
selfinductance. This implies that the current in the coils always lags behind the provided voltagesignal. Since
the magnetic force is directly linked to the current this means that the motorforce also lags behind.