April, 2012
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Page 61 Linear Encoders for Direct Drives By the Heidenhain Staff, Dr. Johannes Heidenhain GmbH L
inear motors have made seri- ous inroads into highly dynam- ic applications such as manu-
facturing and measuring equipment in the semiconductor industry, in PCB assembly machines, textile machines and in automation. Direct drives for open and closed-loop con- trol require continuous real-time and exact information on the position of the slide. The accuracy, performance
only small interpolation error are therefore essential for the meaning- ful operation of direct drives. Encoders that use photoelectric scan- ning are especially suitable for this task, since very fine graduations can be used as measuring standards. These encoders provide benefits in the positioning, speed stability, and thermal behavior of a direct drive. Until now, an absolute position value calculation and the position value without any axis movement were not possible with exposed linear encoders.
Sealed absolute linear encoders are not always used on direct drives
because they often require compact dimensions. Exposed encoders, which have very small dimensions and therefore low weight, were previ- ously available only in incremental versions. With the new LIC 4000 exposed linear encoder, Heidenhain now also offers an absolute exposed linear encoder with EnDat 2.2 serial interface.
Metallur Scale Tape The graduated carrier — a
Contamination on the gradua- tion: measuring accuracy of the LIC without the application of electronic corrective measures.
Metallur scale tape encoder with optical scanning incorporates meas-
Continued on page 63
Scale tape of the LIC.
and reliability of the individual axes depend on the linear measuring devices used. In connection with lin- ear motors, this task is now per- formed primarily by compact, con- tact-free measuring devices called exposed linear encoders. The decisive advantage of direct
drive technology is the very stiff cou- pling of the drive to the feed compo- nent without any other mechanical transfer elements. This allows signif- icantly higher gain in the control loop than with a conventional drive. Here, the efficiency of a linear motor is greatly influenced by the selection of the position encoder. High control-loop gain can only be reached if the encoder provides
Encoders for acquisition
of position and drive velocity need to provide high-quality signals.
high-quality position signals. With the high gain required in the control loops, even minor disturbances in the encoder output signal can cause seri- ous trouble in drive performance. The higher quality of the position informa- tion noticeably improves velocity con- trol and positioning. In addition, the motor operates quietly and develops only a small amount of heat.
Velocity Measurement On direct drives, there is no addi-
tional encoder needed to measure the speed. Both position and speed are measured by the position encoder: lin- ear encoders for linear motors, angle encoders for rotating motors. Since there is no mechanical transmission between the speed encoder and the feed unit, the position encoder must have a correspondingly high resolution in order to enable exact velocity control, particularly at slow traversing speeds. The velocity is calculated from the dis- tance traversed per unit of time. This method — which is also applied to con- ventional axes — represents a numeri- cal differentiation that amplifies peri- odic disturbances or noise in the signal. The significantly higher control loop gain on direct drives dramatically increases the influence of the signal quality on drive performance. Linear encoders that generate a high-quality position signal with
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