Technology in optical scan linear encoders is changing to support more demanding applications in which motion control performance largely depends on the type of encoder used.
Linear motors have made irreversible inroads into highly dynamic applications such as manufacturing and measuring equipment in the semiconductor industry, PCB assembly machines, textile machines, and automation. Direct drives for open- and closed-loop control require continuous real-time information on the position of the slide. The accuracy, performance, and reliability of the individual axes depend to a great degree on the linear measuring devices on the slides. This task is performed by compact, contact-free measuring devices called exposed linear encoders, which function according to the incremental measuring process.
High-quality position signals
Encoders that use photoelectric scanning are ideally suited for this task because very fine graduations can be used as measuring standards. Encoders with optical scanning methods (such as the one shown in Figure 1) show benefits in the position accuracy, speed stability, and thermal behavior of a direct drive, and thus help realize the potential of direct drives.
Up to now, absolute position value formation in exposed linear encoders could not be attained, although absolute encoders offered decisive benefits:
· Position value available immediately upon encoder switch-on, which permits fast position reapproach, especially in concatenated systems or machines with several axes
· No need to move the axes to find the reference position
· Prevention of collisions when initializing multiple axes
Because high acceleration capabilities and compact designs are required for applications on direct drives, exposed encoders usually come into question instead of sealed encoders. Exposed encoders operate without friction and, thanks to the lack of an enclosure, are very small and can be designed with little weight. Until recently, designers could only use exposed linear encoders operating according to the incremental measuring method.
Realizing an optical absolute encoder in an exposed design required new technologies and carefully selected manufacturing processes to integrate “absolute technology” into the mechanical design of the LIDA 400, which was already proven in the market. The goal was to meet essential criteria such as high accuracy, high traversing speeds, and high reliability in the smallest possible space.
Step by step to an exposed absolute encoder
Linear encoders that generate a high-quality position signal with low interpolation error place high demands on their optical, mechanical, and electronic performance. The external dimensions, defined in the technical specifications as the maximum for an exposed absolute encoder, demand innovations for the final realization. Both a high-quality graduation and the scanning method share responsibility for low interpolation error.
The METALLUR process was developed to manufacture graduations on glass, glass ceramic, or steel. The quasi-planar graduation structure is extremely tolerant to contamination and thereby greatly enhances encoder reliability. The manufacturing process is environmentally friendly and entirely forgoes chemicals such as those needed for etching. A design using a pulsed laser (Figure 2) produces the graduation structure. The graduation’s high edge definition and homogeneity permit low interpolation error to ensure smooth operation and high controller gain of the linear drive.
In the LIC 4000 series, the position value is available from the encoder immediately upon equipment switch-on and can be called at any time by the subsequent electronics. There is no need to move the axes individually to find the reference position. The absolute position information is scanned from the scale graduation, which is configured as a pseudorandom-coded track with a separate incremental track (Figure 3). An innovative scanning method with integrated opto-ASIC contributes to setting new benchmarks for low interpolation error and reliability.
Actual measurement results
Based on the high quality of the graduation, the newly developed scanning method provides absolute position values with a specified interpolation error of less than ±40 nm. Measurements resulted in an actual value significantly lower than the specification (Figure 4).
The value of the position noise, which is critical for servo control, lies below 20 nm RMS (Figure 5).
This series of exposed linear encoders is optimized for use on fast, precise machines. In spite of their exposed design, the encoders are extremely tolerant of contamination and therefore contribute greatly to reliability in diverse applications over a long period of time. The large scanning field and the single-field scanning also reduce sensitivity to contamination. Contamination applied in a laboratory (such as oil, dust, fingerprints, hair, and wires), which can easily hamper the everyday function of an exposed encoder, shows only small effects on functional safety and measuring accuracy, as illustrated in Figure 6.
Serial position transmission with EnDat 2.2
The EnDat interface, a bidirectional digital interface for encoders, is capable of transmitting position values as well as transmitting or updating information stored in the encoder and saving new information. Thanks to the serial transmission method, only four signal lines are required. The data is transmitted in sync with the clock signal from the subsequent electronics. The type of transmission (position values, parameters, diagnostics, and more) is selected through mode commands that the subsequent electronics send to the encoder.
Exposed absolute encoders with the EnDat 2.2 interface offer 1 nm position resolution and, unlike TTL devices, are not limited in traversing speed. Because the analog[
] scanning signals are digitized and subdivided directly at the place of measurement, the advantages of new absolute scanning methods can be efficiently translated into improved servo motor speed stability and positioning behavior. Moreover, these very short analog transmission paths provide greater stability in signal quality under the influence of electromagnetic noise. Especially on highly dynamic drives, this permits enhanced accuracy and reduces acoustic noise.
The high clock frequency of the EnDat interface also permits very short read-out times for the position data. EnDat 2.2 is now the fastest purely serial interface for position encoders based on RS-485 transmission characteristics. More information on EnDat is available at www.endat.de.
Better diagnostic capability
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 both at rest and in motion. Mounting tolerances in conjunction with valuation numbers make reliable mounting possible without having to relinquish a safety margin in the field. The valuation numbers are also used for online diagnosis over the EnDat 2.2 interface. The diagnostic system generates error messages and warnings and is an essential prerequisite for a high level of overall system availability. It enables machine tool builders and plant manufacturers to communicate directly with a problem machine or system, whether it is in Munich, Chicago, Tokyo, or Singapore, and gain unambiguous information on its condition to initiate appropriate measures on-site. It can also collect statistical information and use it for preventive maintenance.
Accuracy, performance combined
The technology in the LIC 4000 compact exposed linear encoder delivers high resolution that sets a new standard for control behavior and positioning accuracy. The EnDat 2.2 serial interface allows for fast data transfer and makes it possible to transfer various data besides position information.
In addition to the familiar linear encoder using the incremental measuring principle, demanding positioning and control tasks in the semiconductor industry, metrology, medical technology, automation and textile machines, machine tool builders, and plant manufacturers can now benefit from an exposed absolute encoder that contributes to the accuracy, performance, and reliability of individual axes today and in future generations.
Reinhard Kuhn is senior product manager at the worldwide headquarters of Dr. Johannes Heidenhain GmbH in Traunreut, Germany, with North American headquarters in Schaumburg, Illinois.
DR. JOHANNES HEIDENHAIN GmbH
800-233-0388
ProductManagement@heidenhain.com
www.heidenhain.com
