Integrated travel measurement system for machine tools

In machine tools and other production machinery, in order to accurately move, control and position tools and workpieces, it is necessary to determine travel distances and changes in travel or position. Travel measurement systems in combination with linear guidance systems are often used for this purpose. If these measurement systems can be integrated into the guidance system, both functions - guidance and measurement - can be matched to each other for optimum performance.

The LMSA (length measurement system analogue) series from Schaeffler uses an inductive analogue measuring principle to achieve a high accuracy of +/- 3um/m. The system is characterised by its high robustness and insensitivity to electromagnetic interference. The system also operates without hysteresis, i.e. without any magnetism or residual magnetism.

Similar to the LMST and LMSD series of integrated measurement systems from Schaeffler, the new KUVE..LMSA series of fully integrated measuring system and linear recirculating ball bearing and guideway assembly, comprises a carriage with an adaptable housing for the measuring head and a guideway to accommodate the measuring scale.

Measurements are carried out according to the inductive principle (similar to a transformer) with a moveable reluctance core. A planar coil structure in the sensor head scans the incremental measuring scale in the head of the guideway using a high-frequency alternating field and generates two sensor signals with a 90 degree phase offset. A high-grade steel strip with a photo-lithographically-etched high-precision scale forms the dimensional measuring scale. The movement between the sensor head and the measuring strip generates a mutual inductance in the individual coils and therefore generates sine wave signals. This means the LMSA measurement system has an analogue output of 1Vss with sub-divided signal periods of up to 10um. This allows the measurement system to be easily integrated to existing control systems.

A precise, minimum distance must be maintained between the measuring head on the carriage and the measuring scale in the guideway. In order to ensure a high accuracy of +/- 3um/m, the dimensional scale is therefore integrated in a special three-layer measuring strip. This strip comprises a backing strip, the actual dimensional measuring scale and a cover strip. The backing strip supports the individual layers, maintains an exact distance to the measuring head and provides the necessary robustness. The dimensional measuring scale is applied to the backing strip under tension to ensure high accuracy measurements. The thin hard cover strip is used to protect the surface.

The three-layer measuring strip has an adhesive strip on its underside for easy handling and mounting. The sensor head is protected inside a metal housing, which is directly connected to the steel saddle plate of the carriage. The exact distance between the measuring head and the measuring strip is factory calibrated. However, the housing is designed so that the sensor head distance can be reset if required using just four adjustable screws.

For more information on the LMSA series of integrated travel measurement systems, please go to www.schaeffler.co.uk.