Temposonics
®
Magnetostrictive, Absolute, Non-contact
Linear-Position Sensors
Linear Position Sensors for
Safety Functions and Hazardous Areas
White Paper
Document Part Number
551666 Revision A
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 Laboratory 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 protection 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 temperature 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.
Technologies
There are numerous technologies to provide position sensing for hazardous environments, including potentiometers, encoders, linear variable differential 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 concerns 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 applications.
This new sensor is the first linear positioning sensor to meet Safety
Integrity Level (SIL2) standards. SIL is a measure of the failure probability 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 approvals. 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. Operations 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.
Temposonics® Linear-Position Sensors for Safety Functions and Hazardous Areas- White
Paper, Document Part No.: 551666 Revision A (EN) 09/2014
2
MTS Sensors
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 Sensors, 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 products, 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.
LEGAL NOTICES
Document Part Number:
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contact a local MTS representative.
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551666 Revision A (EN) 09/2014
MTS and Temposonics are registered trademarks of MTS Systems Corporation.
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Printed in USA. Copyright © 2014 MTS Systems Corporation. All Rights Reserved in all media.
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