Draw Wire Sensors

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Sensor Technology

Draw Wire Sensors
Draw Wire Sensors

Chris Jones, Managing Director at Micro-Epsilon (UK) Ltd discusses how draw-wire displacement sensors are helping medical operating theatres in a number of critical applications, including X-ray machines, CT scanners, patient tables and flexible surgical arms.  

Utilisation and cost efficiency are now just as important measures to medical operating theatres as they have always been to manufacturing companies. The latest operating tables, for example, offer numerous functions for precise handling of the patient, but need to be efficient and cost-effective.  

Typically, operating tables are modular in design and offer multiple adjustment options for the precise positioning of the patient during an operation. As well as the height of the table, the horizontal position and several angular functions for the head, torso and legs also need to be adjustable.  

In order to enable these adjustments, suitable measuring devices are required. Draw-wire displacement sensors are ideal for this, combining a compact design with high precision and long service life. The sensors can integrate easily with the operating table, offering an excellent price/performance ratio for the customer.  

The principle of a draw-wire displacement sensor is relatively straightforward, hence its inherent reliability. The sensor works rather like a tape measure, except with a draw-wire sensor; the user does not have to read off the measurement of the extended tape. The rotation of the drum on which the steel wire is wound is measured automatically and the measurement signal is output in either analogue or digital formats. The drum itself incorporates a long-life spring drive, which ensures the steel wire is rewound.  

The latest X-ray machines also require precise positioning so that they can provide high quality images of patients. The digital camera on these machines has to be aligned exactly with the X-ray tubes so that high resolution recordings can be produced. The cameras, X-ray tubes, the table and the wall stands can be moved on several axes, providing maximum flexibility.  

Draw-wire sensors can be used to position the mechanical moving parts. The synchronisation controller in the X-ray unit then uses the displacement data from the sensors to ensure that the X-ray tube and camera move parallel to each other.   Similarly, Computer Topography (CT) equipment requires fast, precise but cost effective measurement systems. Spiral, helical and dual source CT systems all require more precise, faster sensors.  

The length measurement equipment for the horizontal reclined position is important here. In order to obtain the best possible overall image of the target, the individual X-ray sections, which normally travel through the object, have to be measured with smaller and smaller spacing. To do this, the sections are compiled in a 3D model to obtain voxels (volumetric and pixel). Based on this complete volume data set, any 3D views or sectional planes can be produced. In order to correctly align the sections, the horizontal position of the couch must be measured precisely. Therefore, a measuring system with very high resolution and long measuring range is required. Here, draw-wire sensors can offer resolutions down to 0.001% of the measuring range.     

Draw-wire sensors are not only suitable for horizontal and vertical axis measurements; they can also be used to measure angles on operating tables and flexible surgical arms.  

Flexible surgical arms are used to assist surgeons during surgical procedures, providing them with several degrees of freedom of movement. Starting from a calibrated zero point, the arm, which is attached to an operating table, automatically performs the required incremental movements. An endoscopic camera is guided by the robot arm, achieving a 360-degree view with up to 80-degrees incline from the perpendicular of the endoscope.  

Typically, the arm must be immune to MR and X-rays and so sensors cannot be used in the area directly above the operating table. Therefore, direct measurement of the angular movements of the arm with angle sensors is not possible. However, the angle of rotation can be measured indirectly by using draw-wire sensors, which are installed underneath the operating table. Several sensors are required, which provide either an angle proportional output signal (potentiometer) or a digital incremental output.

Posted on September 1, 2009 - (826 views)
Micro-Epsilon Messtechnik GmbH & Co.KG
Königbacher Straße 15
94496 Ortenburg - Germany
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Micro-Epsilon offers one of the broadest product ranges of high-precision displacement sensors, 2D/3D laser scanners, IR temperature sensors, colour sensors and inspection systems in Europe.

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