25 Jul 2024


Linear axes have integrated inductive measuring system

The German bearing manufacturer Schaeffler has developed an integrated system for measuring the travel of linear axes in machine tools and other precision machinery. The LMSA (length measurement system analogue) uses an inductive measuring principle to achieve accuracies of ±3µm/m.

Schaeffler says that integrating the guidance and measurement functions allows them to be matched to each other for optimum performance. The robust, hysteresis-free measurement system is not affected by electromagnetic interference. 

The measuring head is carried in a housing on the carriage of a KUVE linear recirculating ball bearing and guideway assembly. The guideway accommodates a steel strip with a photolithographically-etched precision scale.

Measurements are carried out using a moveable reluctance core. A coil in the sensor head scans the incremental measuring scale in the head of the guideway using a high-frequency alternating field, and generates two signals with a 90° phase offset.

The movement between the sensor head and the measuring strip generates a mutual inductance in the coils, thus generating sinewave signals. The system has an analogue output of 1Vss with sub-divided signal periods of up to 10µm, and can be integrated easily with control systems.

A precise minimum distance needs to be maintained between the measuring head on the carriage and the measuring scale in the guideway. To ensure an accuracy of ±3µm/m, the scale is integrated in a three-layer measuring strip, comprising a backing strip, the scale and a protective cover strip. The backing strip maintains an exact distance to the measuring head. The measuring scale is applied to this backing strip under tension to ensure accurate measurements.

The sensor head is protected inside a metal housing connected to the steel saddle plate of the carriage. The exact distance between the measuring head and the measuring strip is factory calibrated.