Exposed Linear Encoders for Direct Drives
January 24, 2012
Linear motors have made irreversible 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. 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
On direct drives, there is no additional encoder for measuring 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 here 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 particularly suited here for this task, since very fine graduations can be used as measuring standards by this method. These encoders provide benefits in the positioning, speed stability, and thermal behavior of a direct drive. Up to now, an absolute position value calculation and the associated availability of the position value immediately after encoder switch-on without any axis movement were not yet available with exposed linear encoders. Sealed absolute linear encoders are not always used on direct drives because they often required compact dimensions. Exposed encoders, which thanks to their lack of an enclosure 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 and exposed linear encoder with EnDat 2.2 serial interface.
The graduation carrier—a METALLUR scale tape
HEIDENHAIN encoders with optical scanning incorporate measuring standards of periodic structures known as graduations. They are characterized by their high edge definition and excellent homogeneity—a fundamental prerequisite for low interpolation error, and therefore for smooth operating performance and high control loop gain.
The quasi-planar graduation structure, which is applied in the METALLUR process, is extremely tolerant to contamination and thereby greatly enhances encoder reliability.
Figure 1: Scale tape of the LIC
In the LIC 4000 series with its absolute graduation, the position value is available from the encoder immediately upon machine switch-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.
Figure 2: Design and functional principle of the optical scanning method of the LIC 4000
Measuring accuracy and high reliability contribute to machine safety 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 ± 40 nm. Exposed linear encoders of the LIC 4000 series are therefore optimized for use on fast, precise machines. In spite of its exposed design, the LIC 4000 is extremely tolerant of contamination and therefore contributes greatly to the optimization of machine availability in its various applications. Everyday contamination resulting from small quantities of oil, dust, fingerprints, hair or small metal objects applied were tested intensively in a laboratory and show few effects on operational reliability. The measuring accuracy of the LIC 4000 is also hardly influenced by the tested types of contamination (Figure 3).
Figure 3: Contamination on the graduation: measuring accuracy of the LIC without the application of electronic corrective measures
Benefits of serial position transmission with EnDat 2.2
The scanning signals of the LIC 4000 series 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, these advantages of the new absolute scanning process in terms of accuracy and resolution can be exploited without loss in the transmission. Both the speed stability and positioning behavior of linear direct drives can therefore be increased significantly because of the LIC 4000. Highly dynamic drives running on position signals from the LIC 4000, also with very high proportional gain in the velocity controller, feature quietness in operation. 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 transmission characteristics.
Simple installation with diagnostic capabilities
The absolute encoders of the LIC 4000 series are particularly easy to install. The valuation numbers provided over EnDat 2.2 for the absolute track, incremental track and position value calculation provide a real-time status report on the condition of the encoder. Well proven mounting tolerances in conjunction with valuation numbers make reliable mounting possible without having to do without a safety margin in the field. The valuation numbers can be called through EnDat 2.2 during motor operation without sacrifices in the performance of the servo control. The diagnostic system generates error messages and warnings, and therefore makes an important contribution to ensuring the availability of the overall system.
Applications with direct drives often place special requirements on control and measuring technology. Encoders for acquisition of position and drive velocity need to provide highquality signals. Short-range errors in the position measuring signal are particularly critical for direct-driven feed axes because they can result in positioning error, speed ripple, loud noise and additional heat generation. Encoders that use optical scanning and provide small signal periods have proven to be particularly effective by enabling machines to operate with high accuracy and resolution values.
An absolute encoder with high resolution of one nanometer (1 nm) is now available in the LIC 4000 compact exposed linear encoder. The well proven EnDat 2.2 serial interface provides fast data transfer. Besides position information, the EnDat protocol makes it possible to transfer various data that can be used, for example, for system diagnostics. For demanding positioning and control tasks in various applications such as in the semiconductor industry, metrology, medical technology, automation and textile machines, machine tool builders and plant manufacturers are now flanked by an exposed absolute encoder that contributes to a decisive degree to the accuracy, performance and reliability of individual axes both for today’s and for future generations.