Temposonics®

Magnetostrictive, Absolute, Non-contact

Linear-Position Sensors

Document Part Number 551666 Revision A

Linear Position Sensors for Safety Functions and Hazardous Areas

White Paper

T-Series Model TH

Linear Position Sensors for Safety Functions and Hazardous Areas

By: Matt Hankinson, Technical Marketing Manager, MTS Sensors Division Introduction

Selecting the best product for operating in hazardous areas and safety related applications can be challenging. Hazardous areas include locations where fire or explosion hazards may exist due to flammable gases, vapors, dust, or liquids. Operating an electrical device, such as a linear position sensor, in these environments requires special consideration because of the potential source of ignition. Functional safety is an important design feature for applications that depend on the correct operation of equipment due to the potential risk for equipment damage or personal injury. Risk is assessed by the likelihood of a hazardous event and the consequence of a failure, ranging from negligible to catastrophic. This article reviews a number of solutions and describes a new technology designed to decrease the risk.

Safe Operation and Hazardous Environments

Concentrated flammable gases, vapors, liquids, or dust create a risk of fire or explosion. Electronic devices operating in these environments require approvals such as IECEx, ATEX, or NEC (depending on the region) to match the specific hazardous conditions. In North America, devices should be tested by a Nationally Recognized Testing Labora-tory and follow the labeling requirements. The ratings are classified by Class and Division to indicate the type of hazard ranging from unlikely presence to a continuous presence of explosive material. Many pro-tection methods are employed to achieve the desired approval rating depending on the type, condition and nature of the environment. For example, an environment with a continuously present explosive gas might use an explosion-proof enclosure for an electronic device to contain a possible internal explosion and must function at a tempera-ture below the ignition temperature for the surrounding environment.

Functional Safety is a design concept to ensure safe operation of systems in applications with risk of equipment damage or personal injury. Following the IEC 61508 standard reduces the risk of failure by providing a safety function to detect failures, and assesses the probability of failure to perform the desired function. Devices are classified according to a Safety Integrity Level (SIL) to determine the probability of failure. A SIL2 capable device, for example, has a probability of failure better than 0.01 for low demand operation and a probability of failure better than 0.000001 per hour for high demand operation. Failures are categorized as either safe or dangerous failures, and can either be detected or undetected. In addition to the probability of failure, the safe failure fraction is also required by determining the ratio of failures that are either safe or detectable over the total failures, leaving only the dangerous undetectable failures.

Temposonics® Linear-Position Sensors for Safety Functions and Hazardous Areas- White Paper, Document Part No.: 551666 Revision A (EN) 09/2014 MTS Sensors2

Technologies

There are numerous technologies to provide position sensing for hazardous environments, including potentiometers, encoders, linear variable dif-ferential transformers (LVDTs), and magnetostrictive linear displacement transducers.

Potentiometers operate as voltage dividers that indicate linear position as a voltage signal proportional to the location of a wiper across the transducer element. Though widely used, they are prone to wear because of the contact made by the wiper.

Encoders operate by using a reader head that scans a scale with markings to indicate incremental position changes. They require cleaning and maintenance and are subject to failure in the presence of high vibration and temperature.

LVDTs work by moving a ferromagnetic core that varies the magnetic coupling between primary and secondary coils. They are commonly used in hazardous work areas either because of their high temperature rating or hazardous approvals, but they have poor relative linearity and need periodic calibration.

Magnetostrictive linear displacement transducers operate by measuring the time it takes to receive a sonic strain pulse generated at the location of the moving position magnet when its magnetic field interacts with the field generated by an interrogation current pulse. The elapsed time is converted into an absolute position value down to a micron resolution. Magnetostrictive sensors are an effective solution for many hazardous area applications because they do not have reader heads that require cleaning and maintenance, they do not require recalibration over time, they have a high rating for shock and vibration, they are double-shielded for electromagnetic interference immunity, and they are available with options for safety functions and hazardous area approvals.

New Advances in Magnetostrictive Sensor Technologies

A sensor recently introduced to the market addresses many of the con-cerns and shortcomings of previous solutions. The Temposonics® T-Series product family from MTS Systems Corp., Sensors Division, provides high performance linear position feedback and is designed specifically for use in hazardous areas and safety –related applica-tions.

This new sensor is the first linear positioning sensor to meet Safety Integrity Level (SIL2) standards. SIL is a measure of the failure prob-ability of a safety function. As a SIL2 capable device, the T-Series provides a safety function that can be designed into a SIL rated safety system. Other position sensing technologies, such as LVDTs, do not offer a similar safety function.

The T-Series performs well in high pressure conditions and is explosion- and flame-proof. It offers an ATEX and IECEx Zone 0/1 flameproof and increased safety certification for hazardous areas. It also will offer an explosion-proof rating for Class I Division 1 and 2, as defined by the National Electric Code (NEC 500 and NEC 505 Zone 0/1).

The advantages of this solution are that it offers excellent measurement performance and a robust and reliable operation. This technology is also less susceptible to environmental factors, such as EM interference and contamination. This combination of hazardous area and functional safety certifications is unique among linear position feedback devices.

Application Examples

Many applications utilize position feedback in hazardous environments. Gas and steam turbine valves, for example, require sensors that can withstand high temperatures and typically require hazardous area ap-provals. These applications have often used LVDTs, but more recently, are switching to magnetostrictive sensors for improved reliability and accuracy. An increasing number of these applications also require functional safety certification.

There are many other examples of industries and environments that can benefit from this technology. Op-erations involving storage and handling of liquefied petroleum gas or natural gas and oil exploration can benefit, as can aircraft hangars, fuel service areas, and fuel servicing equipment. Producers of products with combustible dust as varied as starch, candies, plastics, medicines, fireworks, flour, feed, spices, sugar, cocoa, and fertilizer will find applications for this sensor. Those managing hazardous materials such as sewage treatment plants, hazardous chemical plants, dry cleaning plants, utility gas plants, coal preparation plants, and plants that manufacture, use, or store magnesium or aluminum powders can improve the safety and reliability of their operations with this technology.

GERMANYMTS Sensor TechnologieGmbH & Co. KGAuf dem Schüffel 958513 Lüdenscheid, GermanyTel. + 49 - 23 51- 95 87 0Fax + 49 - 23 51- 5 64 91info.de@mtssensors.comwww.mtssensor.de

USAMTS Systems CorporationSensors Division3001 Sheldon DriveCary, N.C. 27513, USATel. +1- 919 - 677- 0100Fax +1- 919 - 677- 0200info.us@mtssensors.comwww.mtssensors.com

JAPANMTS Sensors Technology Corp.737 Aihara-machi, Machida-shi, Tokyo 194-0211, JapanTel. + 81- 42 - 775 - 3838Fax + 81- 42 - 775 - 5512info.jp@mtssensors.comwww.mtssensor.co.jp

Document Part Number:

551666 Revision A (EN) 09/2014

MTS and Temposonics are registered trademarks of MTS Systems Corporation.All other trademarks are the property of their respective owners.Printed in USA. Copyright © 2014 MTS Systems Corporation. All Rights Reserved in all media.

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In Conclusion

Selecting a position sensor that meets the measurement performance of the application is clearly essential. Just as important, however, is selecting a position sensor that meets the safety requirements of both the user and relevant regulatory agencies. The magnetostrictive sensor described in this article is capable of meeting the performance requirements for a wide range of applications while providing the functional safety and hazardous area certification needed for safe operation.

About MTS Sensors

MTS Sensors is a global leader in sensing technologies and solutions that enable feedback control for automation and safety applications. MTS Sen-sors, a division of MTS Systems Corporation, serves its global customers with a focus on regional support to provide innovative and reliable sensing solutions. Through its research, development and production of various prod-ucts, MTS offers a large selection of sensor designs, outputs, performance levels and technologies to fit your application needs. MTS’ industrial sensors are commonly used for factory automation, fluid power, energy production markets as well as wood, metal, plastic and glass applications.

For more information on MTS Sensors, please visit www.mtssensors.com or contact a local MTS representative.