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VOLUME -24 NUMBER 12
Publication Date: 12/1/2009
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Special Features: Test and Measurement
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December 2009 Issue
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Linear Encoders for Production Environments
LIDA 400 series exposed linear encoder.
By Reinhard Kuhn, Heidenhain, Traunreut, Germany
The typical manufacturing environment can be harsh, meaning that measuring machines have to meet new requirements that either did not exist or were less critical in the previously sheltered surroundings of a measuring room. Measuring machines on the shop floor are exposed to changing temperatures and other, even more difficult ambient conditions. Shock, vibration and contamination are nothing unusual on the production floor. Today's measuring machines have to be designed to work in this difficult ambience, and their manufacturers are responding to these requirements with various designs and solutions.
Deviations from the 20°C reference temperature specified in DIN 102 result in changes of length and angle on both the workpiece and the measuring machine, and these changes have to be compensated for mathematically. A defined, reproducible thermal behavior of the encoder is indispensible — creating an added level of responsibility for encoder accuracy. The encoder's coefficient of expansion and its tolerance will play a more significant role in future ISO standards for classifying coordinate measuring machines (see ISO TC 213-WG 10).
Coefficient of expansion must be allowed for when using encoders that serve as the basis for measuring machines. Encoders usually use measuring standards made of steel, glass or glass/ceramic. The relevant literature provides data for the coefficients of expansion; however the data provided often differ significantly from one source to another.
Their utility as a basis for length compensation is therefore limited, as becomes visible in the data for steel, for example. A temperature change of only a few degrees can result in deviations of several micrometers in compensation values that have been calculated from an inaccurate coefficient of expansion.
A coefficient of expansion can be measured exactly by a dilatometer, which is a device for measuring thermal expansion. With a well-designed dilatometer, it is possible to obtain exact data on a material's coefficient of expansion by measuring a test object and then using these results to manufacture encoders. An example is the "alpha measuring station" for measuring the thermal length expansion of bar-shaped bodies. Such a measuring station has been set up at the Physikalisch-Technische Bundesanstalt, Germany's national metrological institute in Brunswick.
This exactly measured value can then be applied to calculate length compensation. In most cases, companies manage as best they can with data from the literature or the material manufacturer. This inevitably creates uncertainty in the result.
Special care must be taken in setting up a shop-floor measuring system. The quest for high reliability is possible because of the manufacturer's long experience. It is important to be able to assure high reliability and accuracy in spite of harsh environmental conditions. No compromises in accuracy are made compared with machines in measuring rooms. Any thermal effects must be dealt with appropriately, including selecting suitable materials and providing for thermal requirements. Because temperature increases expand diferent materials differently and these materials absorb ambient temperatures at different speeds, complex calculations are made to compensate for the effects of temperature and accuracy. One known basis for mathematical compensation is very important — the linear encoder.
Steel, glass and glass-ceramic encoder scales.
Thermally stable encoders are indispensible for calculations that will result in accurate measurement data, thereby achieving accurate compensation. The selection of encoder material for the shop-floor measuring machine is therefore particularly important. While glass or steel scales permit only an approximate value for calculation, the coefficient of expansion of 0±0.1 x 10
for glass ceramic scales remains accurate over a large temperature range, and the scales have proven to be very durable. As an example, the material is used worldwide for astronomical telescopes because of the need for imperviousless to temperature changes and distortion-free imaging.
Thermally Stable Encoders
Having the right encoders enhances machine characteristics and contributes significantly to the reliability of the measuring machine. These are the parameters needed for a high-accuracy operation:
Use encoders with defined coefficients of expansion.
System accuracy must account for deviation between compensation points.
Area must be free of contamination for disturbance-free measurement.
Predictably high reliability over a long time period.
Encoders must be cost-effective.
The LIDA 400 exposed incremental encoder from Heidenhain is characterized by high accuracy and customer-friendly mounting tolerances, high traversing speed and the small height of the scanning head. These attributes make it well suited for use on production equipment, in automation engineering, and the electronics industry as well as for applications on linear drives and in many areas of metrology. Now with the introduction of new graduation carriers of glass and glass ceramics (Zerodur, Robax
) the range of potential applications for these encoders has been greatly expanded.
The encoders are especially suitable for applications in shop-floor measuring machines. They are easily installed using the Precimet
adhesive film on the back. Because the standard scanning heads for the LIDA 400 meet all requirements for reading the scales of glass and glass ceramic, no special scanning heads are needed. In addition, because of the identical cross section of the scales, the graduation carriers can be swapped with no loss in accuracy. Logistically, this is a great advantage because the standard Lida 48 (1 VPP) und Lida 47 (TTL) scanning heads can be combined with glass ceramic and glass scales as well as with steel scale tapes — all interchangeably. In addition, the identical carrier cross-section of glass ceramic and glass scales make it possible to easily upgrade existing measuring machines. All designs have the identical scanning surface of 14.5mm
, which ensures high tolerance to contamination while generating very clean scanning signals, which can readily be interpolated.
The Lida 400 encoders incorporate a grating period of 20 micrometers. They are available in the widely used 1VPP and TTL interfaces and for measuring lengths of up to 30 meters (steel) or 3 meters (glass and glass ceramic). Traversing velocities up to 480 m/min are easily possible. The encoders are available with reference marks as well as integrated magnetic limit switches.
Today's changing requirements for capital equipment such as measuring or production equipment in the electronics industry call for encoders that are capable of meeting increasingly stringent demands. The problem of thermal expansion can be solved by the proper selection of different graduation carriers that are uniformly capable of using the same model of scanning head. In conjunction with measuring standards of glass and glass ceramic, the new generation of Lida 400 exposed linear encoders offer exceptional properties for accurate measurement even on theshop-floor and in production-integrated machines.
Contact: Heidenhain Corp., 333 E. State Parkway, Schaumburg, IL 60173
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