Sensor 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
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.
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.
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 signal processing 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.
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 digital signal processing 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.
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