Compact embedded sensors fit the bill for critical applications

Industrial Embedded Systems — March 19, 2009

5Sensor technology continues to evolve, with both the physical sensing mechanism and processing electronics making strides. This overview highlights some of the latest technology for small embedded sensors in critical applications.

As system integrators and OEMs strive to increase reliability, safety, and performance while decreasing cost, the need for sensors embedded in manifolds, load-holding valves, actuators, pumps, and similar critical monitoring applications is becoming vitally important. The following discussion will present the technologies in play for pressure and position sensing and explain the primary issues involved in choosing the right sensor technology.

Pressure sensing

Second only to temperature sensors in usage, pressure sensors are experiencing growth in hydraulics, water, medical, and other applications where size and performance are significant concerns. As the pressure generated inside a system is increasing to improve efficiency, the size of the system is decreasing. The cost savings of using more compact systems is forcing manufacturers to develop smarter solutions. Stand-alone sensors that offer integral electronics, EMC protection, and temperature compensation such as those shown in Figure 1 are acceptable for applications with adequate space; however, they are not suitable for compact and miniature systems.

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Figure 1: Stand-alone pressure sensors with a connector (left) and cable (right) are useful in some applications but do not fulfill the requirements for compact systems.
(Click graphic to zoom by 1.9x)

 

Embedded pressure sensors can be designed to provide either compensated or uncompensated outputs based on the price and overall performance targeted for the system. Certain users have the capability within their electronics to characterize the uncompensated sensors (learn the performance characteristics of the sensor over pressure and temperature) for optimum use within the application.

For uncompensated sensors, users need pressure and temperature inputs to read precisely how the sensor is reacting before the data can be used. Uncompensated sensors tend to be lower cost and offer flexibility if users can characterize within their electronics. Compensated sensors are easy to use because they are characterized over pressure and temperature at the factory. An amplifier module is required to achieve the desired output. Because the sensors perform to a specific accuracy over pressure and temperature, users have less to test or program.

In most cases, remote electronics offer the best options when using embedded sensors. Depending on the technology and media, embedded sensors can be used in high-temperature, vibration, and radiation environments with the electronics isolated from these hostile conditions. When using low-impedance (<2,000 ohms), high-output, silicon piezoresistive strain gages, the electronics can be located several feet away from the sensor. The wetted material and clamping configurations for the embedded sensors must be carefully selected to avoid costly failures. For example, 316L stainless steel is ideal in applications such as water, oxygen, hydrogen, and other hostile and critical media. Titanium and nickel alloys are preferred in medical and toxic media such as bodily fluids, hydrogen sulfide, and bleach. Figure 2 shows typical configurations of pressure embedded sensors for use in hydraulics and medical OEM equipment.

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Figure 2: A compensated sensor (left) has a mV/V output and an SAE port with an O-ring seal for hydraulic manifolds. An uncompensated (right) sensor is located on a flat disc for medical and semiconductor valves.
(Click graphic to zoom by 1.8x)

 

Pressure-sensing technologies also play a vital part in embedded device integration. Reliability and longevity are the two keys factors that dictate system performance over time. In critical processes such as medical systems, semiconductor fabrication, and industrial gas (such as hydrogen and oxygen) handling, it is important that the pressure sensor does not introduce contamination in the process. Figure 3 shows two types of popular sensor technologies, one that can lead to contamination and another with a more reliable design.

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Figure 3: A sensor with a thin, welded diaphragm and oil-filled cavity (left) presents a potential source of contamination if the thin diaphragm ruptures. Another sensor (right) with a thicker diaphragm and no fluid-filled cavity provides more reliable operation.
(Click graphic to zoom by 1.9x)

 

Position sensing

As valves and actuators are being used in critical applications such as nuclear, hydraulics, and automation, embedded position sensors are growing in importance and usage for position feedback. Linear position measurement can be from a few millimeters to several meters long. Sensing valve seating is becoming a key area due to the need for safety in the nuclear and hydraulics industry. As is the case in pressure-sensing technology, choosing the correct technology for position sensors is essential for proper system performance and reliability.

Position sensors typically rely on either contacting or noncontacting technology. In contacting technologies such as a linear potentiometer, a slider attaches to a moving member, which makes direct contact with resistive device. The potentiometer, acting as a voltage divider, provides an output that can go from 0 to 100 percent of the applied voltage as the slider travels from one end of the device to the other. While these devices are lower cost, they are not ideal for high-vibration environments and must be protected from dust and liquids.

Noncontacting technologies such as optical, variable reluctance, eddy current, magnetorestrictive, and Linear Variable Differential Transformer (LVDT), have been deployed successfully. These devices offer better performance and reliability compared to potentiometers. Optical sensors based on laser interferometer, wavelength, and intensity modulation are used only in lab conditions and are not suitable for harsh media. Variable reluctance sensors are excellent choices for wide media and temperature; however, they are highly nonlinear and only operate over a short sensing range. Eddy current devices tend to operate at higher frequencies and require the electronics to be located near the sensor, limiting the device’s capability to operate over a wide range of temperatures and radiation. Though magnetorestrictive sensors offer excellent performance, they are limited in operating temperature due to the close proximity of the signal processing electronics.

LVDTs are used in commercial and military aircraft applications such as wing flaps, fuel pumps, and landing gear. These devices employ low-frequency (3-5 kHz) magnetic circuits and do not generate any RF noise compared to eddy current or other high-frequency operated linear sensors. Low-frequency operation allows these sensors to be separated from the electronics by several feet. Because the LVDT device utilizes a magnetic coupling from the primary to secondary coils and no physical connections, these sensors can be hermetically sealed against water, dust, and ice. Figure 4 shows two noncontacting position sensors that can be embedded in valves and transmissions.

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Figure 4: A down-hole high-temperature and high-pressure (200 °C and 20,000 PSI) with remote electronics can be used in hostile environments.
(Click graphic to zoom by 1.9x)

 

By using modern Application-Specific Integrated Circuit (ASIC)-based electronics, the length of the sensor can be reduced significantly in size while maintaining maximum performance. The ASIC enables with full compensation over a wide range of temperatures. This technology has opened the market for linear noncontacting sensors to be used in tight space envelopes such as valve seating, subsea chokes, and in-cylinder sensing.

Karmjit Sidhu is VP of business development and cofounder of American Sensor Technologies, Inc., based in Mt. Olive, New Jersey. He has more than 25 years of experience in pressure and position sensing sales and engineering. Karmjit is currently completing his PhD in Material Science.

American Sensor Technologies

973-448-1901

kssidhu@astsensors.com

www.astsensors.com