Sensors & Transducers


Magnetostrictive position sensors: a revolution in linear position measurement

May 2008 Sensors & Transducers

Traditionally, linear displacements were measured using linear voltage displacement transducers, LVDTs. More recent developments in magnetostrictive technologies can attain accurate absolute linear position feedback in virtually every application. Further, the position information is available at fast repetition rates, even in high vibration and high pressure applications.

Temposonics, an MTS Systems company, is a leading supplier of magnetostrictive sensors, which are rapidly becoming the transducers of choice in many applications.

Magnetostriction – the principles

In the transducer a strain pulse is induced in a specially designed magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from a moving magnet, which passes along the outside of the transducer tube, and the other field is generated from a current pulse which is applied to the waveguide. The interaction between these two magnetic fields produces a strain pulse which travels along the waveguide until the pulse is detected at the head of the transducer. The position of the moving magnet is precisely determined by measuring the elapsed time between the application of the current pulse and the arrival of the strain pulse. As a result, accurate non-contact position sensing is achieved with absolutely no wear to any of the sensing elements.

A detailed description of the physics can be found at www.mtssensor.de

The characteristics

Magnetostrictive sensors combine the strengths and overcome the weaknesses of other technologies in one measuring system. Temposonics sensors offer non-contact absolute measurement at great accuracy and repeatability in extreme environments. This includes areas of mechanical extremes like high shock loads vibration and pressures. Samples of position information are available at very high updates, allowing for realtime control. Various mechanical variants are available, and lengths of more than 20 metres are possible. The units also come equipped with analogue, fieldbus and Ethernet interfaces.

A world Of applications

Level measurement

Many interesting applications for the technology exist outside general industrial use. Who would think a level sensor used in a petrol tank could be used in an operating theatre in a hospital?

A leading manufacturer of hospital and general medical appliances uses the name Neptune for a portable instrument for collecting body liquids. Unlike many other uncritical applications, liquids from the human body in the operating room may require handling with special care, ie, utmost accuracy of volume measurement is a must – liquid lost by a human body during operation must be replaced by transfusion. Moreover, an excessively filled collecting tank could be dangerous for septic reasons.

Factors that make magnetostrictive transducers suitable in these applications include:

* A temperature coefficient better than 50 ppm.

* Fully encapsulated stainless steel construction.

* Easy sterilisation.

* High measurement accuracy and repeatability.

* Battery operation.

* High inherent system reliability.

* EMC features.

Electromagnetic compatibility is a feature which had to be given particular attention for these special environments. Transducers must be immune to interference from the multitude of other hospital equipment and must not affect other laboratory instrumentation or even a pacemaker.

Redundant measurement capabilities

Applications exist where a transducer failure will have disastrous effects on safety and cost. For these applications critical variables are measured by means of two or three independent, functionally identical measurement systems. Each measurement system comprises a basic sensor element, evaluation electronics with separate output signal and supply voltage. The basic elements are accommodated in a pressure-resistant stainless steel pipe for direct stroke measurement, which can be in a hydraulic cylinder. The position magnet travels freely along the sensor rod which emerges into the open piston and marks the measurement point through its wall. The position measurement validity is checked by totals formation with inverse output signal programming (4–20 mA, 20–4 mA; total = 24). Alternatively, the control system monitors the difference between the measured position values (difference = 0). If variations are detected, remedial measures can be taken immediately. The measurement systems are completely independent and can be switched on individually. The sensor version with 3 redundant measurement systems allows redundant measurement without immediate replacement.

Linearity is better than 0,02% and repeatability is better than 0,001%. Due to high linearity even minimum measurement differences are identified, whereby the error tolerance can be adjusted individually and adapted to the application.

Shock resistance

In harsh shock and vibration applications, the mechanical noise frequently causes measurement problems in sensing systems, and without good sensor data, it is impossible to control motion accuracy.

MTS has developed a new Temposonics sensor with increased resistance to shock and vibration for heavy duty machinery. These sensors offer best-in-class performance with vibration up to 30 gav and single shock up to 100 g, without affecting the accuracy of the measurement signal. This high level of resistance to external influences is due to a sophisticated waveguide system, which is the heart of magnetostrictive measurement technology. In conjunction with the advantages of non-contact measurement physics, the new shock and vibration resistant sensors open a multitude of accurate measurement application not possible with other technologies.

The forces applied during punching, pressing, nibbling or cutting in the metal working industry now have little or no influence on measurement accuracy. Previously linear encoders were used, with their known limitations on shock and vibration at machine interface level.

Pressure-resistance

Magnetostriction sensors are available for high pressure applications. Normal operating pressure up to 800 bar is possible with peak pressures reaching 1200 bar. Protected by a specially reinforced stainless steel pipe, Temposonics sensors are now able to measure positions with high reliability and accuracy under high pressure.

These high pressure options have the advantage of low sensor deflection, eliminating the need for supporting structures. The unique high resistance to pressure offered by Temposonics permits operation of the sensor in industrial and mobile applications where the use of high-accuracy measuring instrumentation was not possible before. Lifting building blocks by means of heavy load lifting equipment, intelligent building monitoring, deep-sea applications and heavy machinery construction are only a few examples of the possibilities.

Features

Magnetostrictive sensors are normally packaged as a sensor head attached to a pipe which houses the measurement wave guide. Magnetostrictive sensors are available in a flexible format in lengths up to 20 metres or more. The Temposonics flexible version, is a teflon-sheathed stainless steel hose of 8,7 mm diameter with a maximum bending radius of 250 mm, ideal for curved measurements or applications where space conditions are restricted.

Multiposition measurement

From one sensor using 16 magnets it is possible to get 16 independent position measurements. Machine manufacturers, for example, need only a few sensors to measure and process all axes and displacements. This highly accurate sensor is ideal for machine tools, milling and metal presses where high-accuracy measurement of each individual position is a required.

For more information contact ATI Systems, +27 (0)11 383 8300, technical@atisystems.co.za, www.atisystems.co.za



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