Varian Medical Systems is using 11 Heidenhain rotary and linear encoders (www.heidenhain.co.uk) for the ultra-precise positioning of cancer patients being treated by its TruBeam radio-therapy system. Fitted by Varian engineers at the company’s 250-employee site in Sussex, the encoders help to ensure that sub-micron accuracy is achieved in a system that integrates imaging, beam delivery and motion management.

Critically for Varian, the encoders’ EnDat serial interface technology provides secure data transmission of absolute position values. The decision to use Heidenhain encoders instead of potentiometers, which have traditionally been used for system positioning, was taken at the beginning of the project to create the ultimate radio-therapy system.

Project manager Jeff Dowrick says: “To guarantee that we had the best patient-positioning system, we had to have the best absolute rotary encoders available. We took a standard Varian couch design, then fitted the encoders and brushless DC motors and integrated them with the new Maestro control system developed by our colleagues in the USA.”

The couch has three independent axes of movement, and each of these uses Heidenhain EQN 1125 units, which can run at up to 12,000rev/min, as both its primary ‘master’ encoder and its secondary ‘fail-safe’ encoder.

In addition, the new Perfect Pitch couch system has two more axes — pitch and roll — where four more Heidenhain encoders are used. Built-in lead shields protect the encoders from the effects of radiation.