Embedded sensors for critical compact applications

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Sensing pressure

As system integrators and OEMs strive to improve reliability, safety and performance while reducing costs, the need to embed sensors into manifolds, balancing valves, actuators and pumps becomes critical. By Karmjit Sidhu, Senior Director, Oil & Gas/Marine Sensors

Pressure sensors are increasingly used in hydraulics, water, medical and many other fields where size and performance are critical, and are second only to temperature sensors in terms of market size. To improve efficiency, the pressure generated inside the system is increasing and the size of the system is shrinking. The cost savings from more compact systems are forcing pressure sensor manufacturers to develop smarter solutions. Standalone sensors that offer integrated electronics, EMC protection and temperature compensation are acceptable for applications with ample space, but they are not suitable for small and micro systems.

Embedded sensors can be used in environments with high temperatures, vibrations, and radiation, provided the electronics are isolated from these harsh environments.

Embedded pressure sensors can be designed to provide either compensated or uncompensated outputs depending on the target price and overall performance of the system. Some have built-in capabilities to characterize uncompensated sensors (understand the sensor's performance characteristics in terms of pressure and temperature) in their electronics to optimize for the application. With uncompensated sensors, users need pressure and temperature inputs to accurately read the sensor's response before they can use the data. Uncompensated sensors tend to be less expensive and more flexible if designers can characterize the sensor in their electronics. Compensated sensors are easier to use because their pressure and temperature characteristics are determined at the factory. An amplifier module is required to obtain the desired output. Because the sensor is provided to the user to operate to a specific pressure and temperature accuracy, the user does not have to do much testing or programming.

In most cases, using an embedded sensor with remote electronics is often the best option. Depending on the technology and media, embedded sensors can be used in environments with high temperature, vibration, and radiation, provided the electronics are isolated from these harsh environments. When using low impedance (i.e., less than 2 kΩ, high output silicon piezoresistive strain gauges), the user can place the electronics several feet away from the sensor. The materials and mounting structure of the embedded sensor contacting the fluid parts must be carefully selected to avoid costly failures. 316L stainless steel is well suited for applications in water, oxygen, hydrogen, and many other harsh and critical media. Titanium and nickel alloys are the materials of choice for medical and toxic media such as body fluids, hydrogen sulfide, and bleach. Figure 2 shows the typical structure of an embedded pressure sensor when used in hydraulic and medical OEM equipment.

Figure 2: Compensated sensor (left) with mV/V output and SAE ports with O-ring seals, suitable for use in hydraulic manifolds. Flat-surface, uncompensated sensor (right) suitable for use in medical and semiconductor valves.

Pressure sensing technology is a critical component when considering embedded device integration. Reliability and longevity are two key factors that determine system performance over time. In fields such as medical, semiconductor, and industrial where gases such as hydrogen and oxygen are involved, it is important to prevent pressure sensors from introducing contamination into critical processes. Figure 3 shows two common types of embedded sensor technologies; each uses a unique way to measure pressure and has different pressure control capabilities. The main difference is the different chances of exposing the process to contamination.

Figure 3: Sensor with ultra-thin welded diaphragm and oil-filled cavity (left), which becomes a major source of contamination in the event of a diaphragm rupture. TE’s one-piece molded design (right), with thicker diaphragm and no fluid-filled cavity.

Critical applications such as nuclear, hydraulics and automation are using valves and actuators, which has made embedded position sensors increasingly important and used for position feedback. Linear position measurements can range from a few millimeters to several meters. Sensing valve seats is evolving into a key area given the importance of safety in the nuclear and hydraulic industries. As with pressure sensing technology, choosing the right technology for position sensors is critical in terms of system performance and reliability. For many years, position sensors have relied on contact or non-contact technology. In contact technology, such as linear potentiometers, there is a slider attached to the moving member, which forms direct contact with the resistive device. The potentiometer acts as a voltage divider and provides a certain output - the applied voltage changes from 0% to 100% when the slider moves from one end of the device to the other. These devices are generally lower cost, but they are not suitable for high vibration environments and need to be dust and waterproof. Non-contact technologies such as optical, variable reluctance, eddy current, magnetoresistive and linear variable differential transformer (LVDT) have been widely used. Compared with potentiometers, these devices provide much better performance and reliability. Optical sensors based on laser interferometry, wavelength and intensity modulation are only used in laboratory type environments and for harsh media. Variable reluctance sensors are an excellent choice for a wide range of media and temperatures, but they are highly nonlinear and only operate over a short sensing range. Eddy current devices typically operate at higher frequencies and require the signal processing electronics to be close to the sensor. This limits the device's ability to operate over a wide temperature and radiation range. Magnetoresistive sensors, while offering excellent performance, are also limited in operating temperature due to their proximity to the signal processing electronics.

LVDTs have been used for many years in commercial and military aircraft in locations such as flaps, fuel pumps, and landing gear where reliability is critical. These devices use a low frequency (3 to 5 kHz) magnetic circuit and do not generate any radio frequency (RF) noise compared to eddy current or other high frequency linear sensors. Because of the low operating frequency, you can separate these sensors from the electronics by several feet. LVDT devices use a magnetic connection from the primary to the secondary coil without any physical connection, so these sensors can be hermetically sealed to prevent the intrusion of water, dust, ice, etc. By using the latest integrated circuit (ASIC) based electronics, the length of the sensor can be significantly reduced while maintaining performance. ASICs allow full compensation for a wide temperature range while performing digital signal processing. This technology has opened up the market for the use of linear non-contact sensors in tight space envelopes such as valve seats and subsea chokes in cylinder sensing. Figure 4 shows two types of non-contact position sensors that can be embedded in valves and transmissions.

Figure 4: Downhole high-temperature and high-pressure (+200°C, 20,000 psi) displacement sensor for drilling activities. Displacement sensor with remote electronics (right) for harsh environments.

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