Draw wire sensors position mechanical moving parts on X-ray machines

A manufacturer of digital X-ray machines is using high precision draw wire (string pot) sensors from Micro-Epsilon to accurately position mechanical moving parts of the X-ray machine and table.

X-ray machines must provide high quality images in a variety of different positions. Previously, an X-ray cassette with film had to be manually aligned with the X-ray tubes, but today, this is performed digitally and is a fully automatic process.

Modern equipment functions with a camera that digitalises the recordings directly. This saves time and development costs. The camera must be exactly aligned with the X-ray tubes so that high-resolution recordings are produced for digital equipment. The cameras, the X-ray tubes, the table and the wall stands can be moved on several axes, providing as much flexibility as possible.

Roesys GmbH, a developer and manufacturer of digital X-ray systems is using draw-wire sensors from Micro-Epsilon's WPS-MK series to position the mechanical moving parts on its machines.

The synchronisation controller in the X-ray machine uses the displacement information from the draw-wire sensors to enable the camera and X-ray tubes to move parallel to one another. This parallel running means that the best possible focussing of the X-ray tubes for the camera is achieved. A total of five sensors are located in the wall stands, in the X-ray table and in the vertical traversing unit for the X-ray table.

Measuring ranges of the sensors are from 500mm up to 1,700mm, with linearity of 0.25% of the measurement value.

Due to ease of installation, the customer can use the sensor without having to modify existing systems. It is critical that optimum focussing of the X-ray image is achieved using intelligent software and precise displacement measurement of the draw-wire sensors. This results in reduced radiation exposure for the patient and perfect images for more extensive diagnoses.

Micro-Epsilon's wireSENSOR range of draw wire sensors uses three main measurement methods: a multi-turn potentiometer, an incremental encoder or an absolute encoder. One of these devices is mounted onto the drum axle, converting the rotary motion of the drum into a signal that can be evaluated. The measured displacement can be output as an analogue current, voltage or potentiometric signal. For a digital output, there is a choice of HTL or TTL, with interfaces for most of the common fieldbus systems, including CANopen, SSI and Profibus.

December 2011

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