April, 2012 Continued from page 61
uring standards of periodic struc- tures known as graduations. They are characterized by their high edge definition and excellent homogeneity — a basic prerequisite for low inter- polation error — which makes them suitable for smooth operating per- formance and high control loop gain. The quasi-planar graduation
structure, which is applied in the Metallur process, tolerates contami- nation 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 evalu- ation of the PRC track and the incre- mental track. A highly integrated opto-ASIC makes it possible to achieve new dimensions in terms of
Encoders that use optical scanning and provide small signal periods have proven to be particularly effective.
accuracy and reliability of position information. The newly developed scanning
method is based on the high quality of the graduation and provides absolute position values with a spec- ified 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 con- tamination, the LIC 4000 is a key component of machine availability in many applications. Ordinary contamination result-
ing from small quantities of oil, dust, fingerprints, hair or small metal objects were intensively laboratory tested resulting in few effects on operational reliability. The measur- ing 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 accura- cy. The very quietly operating dynamic drives run realize very high proportional gain in the veloci- ty controller. The high control loop gain would not have been possible without the high clock frequency of the EnDat interface and the result- ing short read-out times. EnDat 2.2 is now the fastest purely serial
Design and functional principle of the optical scanning method of the LIC 4000.
www.us-tech.com Linear Encoders for Direct Drives
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 opera- tions, such as system diagnostics. The system is well suited for demanding positioning and control tasks in such applications as semi- conductor 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:
info@heidenhain.com Web:
www.heidenhain.us r
Page 63
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