Protecting linear positioning sensors in harsh environments

We examine considerations that affect optimal performance of the sensors, and therefore production machine performance.

All industrial manufacturing, ranging from refining raw materials to plastic bottles to aerospace vehicles, is driven by a balance of econom­ics and performance. Companies that can increase performance of their operations and products without unrealistic costs can create a true value differentiation from their competitors.

One of the most reliable ways to improve operations is implementing higher precision measurement and control of the machines. In a fast-paced environment, every inch — or micron in some cases — matters. High-speed precision manufacturing is now the standard.

This simple need has driven the development of linear positioning sensors for almost half a century. Few production processes exist that don’t rely on some type of positioning measurement method, whether it’s a simple encoder or a more advanced solution such as magnetostriction. Better equipment control and quality assurance means less waste and greater productivity.

However, there are still areas where installation and use of linear positioning sensors is problematic. Because of the mechanical and environmental sensitivity of many positioning technologies, many manufacturers struggle to find a good solution in applications where the sensor is close to or exposed to corrosive materials, high temperatures, electromagnetic interference (EMI), continual shock and other factors that could damage the sensor or compromise performance.

As a result, these manufacturers often must consider suboptimal or less precise measurement techniques, which can result in slower production and less efficient operations. In many instances, it can even affect safety and environmental compliance.

Defining Problems

In most of these applications, the culprits can be classified as excessive shock, vibration or temperatures. Many standards and regulations exist to define varying levels of damaging conditions and assist in product specification.

Shock and vibration often lead to a reduced position sensor lifetime. Specifications for shock typically address the survivability of a single shock event. There may be a momentary measurement anomaly, but the device should survive the event and continue operation after the event. A vibration specification, however, typically is an operational specification, meaning that the sensor should continue operating under the sinusoidal vibration.

High temperature is another important concern to consider. Many position sensor technologies require electronics that might fail at high temperatures. Performance might also be affected if the measurement drifts with temperature variation.

To survive high temperatures, some technologies may offer housing options to cool the electronics or remotely mount the electronics to a protected location where the temperatures are acceptable. Otherwise, electronic components must be selected to operate at the required temperature.

Embeddable Products Where Design Matters. The simplest and often most effective way to ensure the integrity of a sensor is to fully embed the electronics inside of a hydraulic or pneumatic cylinder. This has become the standard in mobile hydraulics, where moving parts on construction and mining equipment are continually exposed to the elements. This approach is also used in other industrial applications.

Detached Electronics Where Space Matters. Embedded solutions do have another drawback, however. They have to enclose both the sensing element and the electronics to perform the measurements. When more advanced controller interfaces are needed, such as industrial Ethernet, a different approach is required to accommodate the electronics.

High Temperature Products Where Heat Matters. In many applications, such as steel mills or turbines, the environment requires elevated temperatures that are damaging to common electronics. Special component selection and heat mitigation are required to produce linear position sensors that can reliability operate under these conditions.

Redundancy Where Reliability Matters. Even the best designs and protective housings can experience failures, however. Many machine designers address this by installing multiple redundant sensors across the equipment. While effective, it’s typically the bulkiest and most expensive solution.

Special Considerations

Potential hazardous conditions exist in almost every manufacturing facility, but some require special considerations. Gas and steam turbine valves, for example, require sensors that can withstand high temperatures and typically require hazardous area approvals. These industries are continually evolving and looking for new sensor technologies to improve reliability and accuracy.

In steel fabrication, rolling mills have requirements for high accuracy, but also experience high temperature, shock and vibration. These environments also can generate a lot of contamination that would affect sensor performance and reliability. Many steel applications use a combination of technologies, but are beginning to transition machines to higher performance sensors to add redundancy, reliability and contamination tolerance.

Equipment with high levels of shock and vibration, such as sawmill machinery, often require additional vibration mitigation even for the most robust technologies. This migration may include remotely mounting electronics or providing other high vibration options.

By Matt Hankinson, Ph.D., technical marketing manager, MTS Systems, Sensors Division

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