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Publication Date: 04/1/2012
Archive >  April 2012 Issue >  Special Features: Assembly and Packaging > 

Linear Encoders for Direct Drives
Scale tape of the LIC.

Linear motors have made serious inroads into highly dynamic applications such as manufacturing and measuring equipment in the semiconductor industry, in PCB assembly machines, textile machines and in automation. Direct drives for open and closed-loop control require continuous real-time and exact information on the position of the slide. The accuracy, performance and reliability of the individual axes depend on the linear measuring devices used. In connection with linear motors, this task is now performed primarily by compact, contact-free measuring devices called exposed linear encoders.

The decisive advantage of direct drive technology is the very stiff coupling of the drive to the feed component without any other mechanical transfer elements. This allows significantly 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.
Design and functional principle of the optical scanning method of the LIC 4000.

High control-loop gain can only be reached if the encoder provides high-quality position signals. With the high gain required in the control loops, even minor disturbances in the encoder output signal can cause serious trouble in drive performance. The higher quality of the position information noticeably improves velocity control and positioning. In addition, the motor operates quietly and develops only a small amount of heat.

Velocity Measurement
On direct drives, there is no additional encoder needed to measure the speed. Both position and speed are measured by the position encoder: linear 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 distance traversed per unit of time. This method — which is also applied to conventional axes — represents a numerical differentiation that amplifies periodic 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 only small interpolation error are therefore essential for the meaningful operation of direct drives. Encoders that use photoelectric scanning 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 previously 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 Metallur scale tape encoder with optical scanning incorporates measuring standards of periodic structures known as graduations. They are characterized by their high edge definition and excellent homogeneity — a basic prerequisite for low interpolation error — which makes them suitable for smooth operating performance and high control loop gain.

The quasi-planar graduation structure, which is applied in the Metallur process, tolerates contamination well, which greatly enhances encoder reliability.

In the LIC 4000 series with its absolute graduation, the position value is available from the encoder immediately upon machine turn-on. There is no need to move the axes over the reference marks to find the reference position. The absolute position information is scanned from the scale graduation, which is configured as a pseudo-random-coded track (PRC) and a separate incremental track. The position information is ascertained with a newly developed scanning method through the evaluation of the PRC track and the incremental track. A highly integrated opto-ASIC makes it possible to achieve new dimensions in terms of accuracy and reliability of position information.
Contamination on the graduation: measuring accuracy of the LIC without the application of electronic corrective measures.

The newly developed scanning method is based on the high quality of the graduation and provides absolute position values with a specified interpolation error of less than ±40nm. Exposed linear encoders of the LIC 4000 series are optimized for use on fast, precise machines. In spite of its exposed design, because of its extremely high tolerance of contamination, the LIC 4000 is a key component of machine availability in many applications. Ordinary contamination resulting from small quantities of oil, dust, fingerprints, hair or small metal objects were intensively laboratory tested resulting in few effects on operational reliability. The measuring accuracy of the LIC 4000 is also hardly influenced by the tested types of contamination.

Serial Position Transmission
The scanning signals of the LIC 4000 are digitized directly in the scanning head and are converted into a high-resolution position value. This eliminates the previously prevailing transmission of analog signals from the scanning head for drive control. With digital position transmission via EnDat 2.2, the new absolute scanning process provides a high degree of accuracy and resolution. Both the speed stability and positioning behavior of linear direct drives can be increased significantly because of this new level of accuracy. The very quietly operating dynamic drives run realize very high proportional gain in the velocity controller. The high control loop gain would not have been possible without the high clock frequency of the EnDat interface and the resulting short read-out times. EnDat 2.2 is now the fastest purely serial interface for position encoders based on the RS-485 interface.

The EnDat 2.2 serial interface provides fast data transfer — making it possible to transfer various data that can be used, for related operations, such as system diagnostics. The system is well suited for demanding positioning and control tasks in such applications as semiconductor manufacturing, metrology, medical technology, automation and textile machines, and machine tool builders.

Contact: Heidenhain Corp., 333 E. State Parkway, Schaumburg, IL 60173 847-490-1191 fax: 847-490-3931 E-mail: Web:

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