How Position Sensors Are Used in Hazardous Areas

How Position Sensors Are Used in Hazardous Areas

Motion control devices in equipment operating in rugged environments need to withstand flammable or explosive conditions and help mitigate worker risk.

By Matt Hankinson, global market segment leader, MTS Systems Corp., Sensors Division

Editor's Note: This article is adapted from the white paper, "Implementing Position Sensors for Hazardous Areas & Safety," from Rockwell Automation EncompassTM Product Partner MTS Systems Corp., Sensors Division. Download the free, complete white paper that includes more details about industry standards and IEC 61508; key protection methods for hazardous environments; and principles of magnetostrictive sensing and how to use the technology.

A significant proportion of industrial equipment is deployed in hazardous areas or applications that require a functional safety assessment. Hazardous area-rated devices are designed to operate in a potentially flammable or explosive environment, while functional safety products are designed for applications where equipment or people may be at risk. These two parameters aren’t necessarily interrelated.

Therefore, components that make up the equipment must be suitably constructed to function in these uncompromising environments. Let’s look at what that means.

Defining Hazardous Areas

There are many examples of hazardous areas. The most common include oil exploration rigs, gas utility plants, chemical and pharmaceutical fabrication facilities, sewage treatment sites and large-scale dry-cleaning operations.

Places where industrial chemicals are processed or produced, powders (such as magnesium and aluminum) are stored, or chemically-active products (such as fertilizers) are manufactured also are typical hazardous areas.

IEC 61508 Functional Safety

The concept of functional safety is that having detected a potentially hazardous situation, measures can be taken to help keep a hazardous event from occurring, or to verify that if it does occur, its effect is contained to an acceptable degree so that employees aren’t put at risk and equipment isn’t damaged.

The IEC 61508 standard provides a standard to assess that a safety function performs to the required level, including failure modes. Adherence to IEC 61508 makes it possible to lower the risk of failure a particular hazard has associated with it, via safety functions that allow its detection. In addition, the standard allows an assessment of the probability of failure to perform the required function. Through this, risk analysis can be undertaken.

Devices are categorized in accordance with a specific Safety Integrity Level (SIL), which relates to the probability of failure occurring. A SIL 1 rated device has a probability of failure between 0.01 and 0.1 for low-demand operation, which translates into a probability of failure lower than 0.00001 per hour for high demand operation.

Likewise, a SIL 2 rated device has a failure probability between 0.001 and 0.01 for low-demand operation, equating a probability of failure that is within the confines of 0.000001 per hour for high-demand operation.

Failures are classified as either safe or dangerous, and they can either be detected or undetected. The safe failure fraction defines the ratio of failures that are either safe or detectable over the total number of failures. From this, it’s possible to determine the likelihood of dangerous undetectable failures. Even in the best-case scenario equipment or instrumentation failure will lead to operational downtime.

Through functional safety, it’s possible to quantify probability of a hazardous event taking place and the possible consequences. When developing industrial systems for hazardous areas, engineers therefore need to include functional safety aspects in their thought process.

Download the Free Position Sensing White Paper!

Register to download the free, complete white paper, "Implementing Position Sensors for Hazardous Areas & Safety" from MTS Systems Corp., Sensors Division, to get more details about industry standards and IEC 61508; key protection methods for hazardous environments; and principles of magnetostrictive sensing and how to use the technology.

Position Sensing in Hazardous Areas

Sensing equipment is deployed in many applications so positioning feedback can be delivered. Often, this has to be done in hazardous scenarios. Examples include the gas or steam turbines found inside power generation plants, oil/gas drilling apparatus, steel and wood presses, the equipment in fuel-servicing depots and oil drilling rigs, but the list goes beyond this.

A variety of sensing mechanisms are used to accomplish position measurement in such settings, including:

  1. Potentiometers. These act as voltage dividers. Position measurement is established via a voltage signal that’s proportional to the point in which a wiper resides on a linear transducer element. This method has been popular in the past, but it does have shortfalls. Most notably, potentiometers are prone to mechanical wear because of the contact between the transducer and wiper.
  2. Encoders. These use a reader head to scan a marked scale, and so indicate incremental changes in position. Long-term operation of this kind of mechanism tends to be subject to failure because of the presence of vibrational movement and high temperatures. Furthermore, oil, grease and other substances often found in heavy industrial environments mean the encoders require cleaning and maintenance regularly. They also require a homing move after start-up.
  3. Linear Variable Differential Transformer (LVDT) Devices. LVDTs rely on movement of a ferromagnetic core that varies the magnetic coupling between primary and secondary coils. Thanks to their high temperature rating, they’ve been deployed widely in hazardous areas. However, they tend to exhibit a poor linearity, and they need to be recalibrated periodically.
  4. Magnetostrictive Linear Sensor Devices. Using the principles of magnetostriction, where a magnetic field can alter the physical properties of a ferromagnetic material, magnetostrictive sensors have been effective at delivering accurate position measurement in hazardous applications. Because these devices give an absolute position, rather than a relative position, they don’t require recalibration.

In addition, they dispense with the need for reader heads, so the time allocated and costs relating to cleaning and maintenance work can be taken out of the equation. They have much stronger resilience to shock and vibration, as well as greater immunity the electromagnetic interference immunity (EMI) than other measurement options. Finally, it’s relatively straightforward to integrate functional-safety mechanisms into these devices.

Do the Homework

Appropriate sensor selection is critical to position sensing system deployment. Many different factors must be considered when specifying a suitable device. It’s worthwhile to engage with a sensor manufacturer that offers a comprehensive sensor portfolio capable of meeting a range of requirements and has a strong understanding of functional safety issues.  

MTS Systems Corp., Sensors Division, based in Cary, North Carolina, is a participating EncompassTM Product Partner in the Rockwell Automation PartnerNetwork ProgramTM program. The company supplies magnetostrictive linear position and liquid level sensors.

The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.


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