EMBEDDED SENSORS FORCRITICAL COMPACT APPLICATIONSPRESSURE SENSORS

Karmjit Sidhu, Director Sales & Marketing, Global Business Development

Pressure sensors, second only to temperature sensors inusage, are experiencing growth in hydraulics, water,
medical, and many other applications where size andperformance are of key importance. As the pressure
generated inside systems are increasing to improve
efficiency, the size of the system is decreasing. The costsavings of a more compact system is forcing pressuresensor manufacturers to develop a smarter solution.
Standalone sensors that offer integral electronics, EMC
protection and temperature compensation are acceptable
for applications with adequate space, however, they are
not suitable for compact and miniature systems.
Embedded pressure sensors can be designed to provide
either a compensated or uncompensated output based on
the price or overall performance targeted for the system.
Certain people have the capability within their electronics
to characterize the uncompensated sensors (learn theperformance characteristics of the sensor over pressure
and temperature) to optimize use within the application.
For uncompensated sensors, the user needs pressure andtemperature inputs to read precisely how the sensor isreacting before the data can be used. Uncompensatedsensors tend to be lower cost and offer flexibility if
someone can characterize within his or her electronics.
Compensated sensors are easy to use since these arecharacterized over pressure and temperature at thefactory. An amplifier module is required to get the desired
output. Since the sensor is supplied to the user to perform
to a specific accuracy over pressure and temperature, theuser has less to test or program.
In most cases, the use of remote electronics tends to offer
the best options when using embedded sensors.
Depending upon the technology and media, embedded
sensors can be used in high temperature, vibration, andradiation environments with the electronics isolated fromthese hostile environments. When using low impedance(i.e., less than 2 kΩ, high-output silicon piezo-resistive
strain gages), the user can allow the electronics to belocated several feet away from the sensor. The wetted
material and clamping configurations for the embedded
sensors must be carefully selected to avoid costly failures.
Using 316L stainless steel is well suited for applications

Figure 1: Compensated sensor (left) with mV/V output and SAE port with
O-ring seal for hydraulic manifolds. Uncompensated (right) sensor on aflat disc for medical and semiconductor valves.

such as water, oxygen, hydrogen, and many other hostileand critical media. Titanium and nickel alloys are preferredin medical and toxic media such as body fluids, hydrogensulfide, and bleach. Figure 1 shows typical configurationsof pressure embedded sensors for use in hydraulics andmedical OEM equipment.
Pressure sensing technologies play a vital part when
considering the integration of an embedded device.
Reliability and longevity are the two keys factors that
dictate system performance over time. In critical processes
such as medical, semiconductor and industrial, which
involve gases such as hydrogen and oxygen, it is very
important that the pressure sensor does not introduce
contamination into the process. Figure 2 shows two typesof sensor technologies that are popular with embedding;each technology measures pressure in a unique way, withdifferent capabilities to contain pressure. The main
variation is the potential for contamination within theprocess.
As valves and actuators are being used in criticalapplications such as nuclear, hydraulics, and automation,embedded position sensors are growing in importance andusage for position feedback. The linear position
measurement can be from a few millimeters to meters
long. Sensing valve seating is becoming a key area due tosafety in the nuclear and hydraulics industry. Similar to thepressure sensing technology, choosing the correcttechnology for position sensors plays a vital part of thesystem performance and reliability. For years, position
sensors had relied on either contacting or non-contacting

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Figure 2: Sensor with thin welded diaphragm and oil filled cavity (left), amajor source of contamination if the thin diaphragm ruptures. One piece
design by TE (right) with thicker diaphragm and no fluid filled cavity.

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 anoutput that can go from 0 to 100% of the applied voltage
as the slider travels from one end of the device to theother. These devices tend to be lower cost, however, they
are not suitable for high vibration and need to be protectedfrom dust and liquids. Non-contacting technologies, suchas optical, variable reluctance, eddy current, magneto
restrictive, and linear variable differential transformer(LVDT) have been successfully employed. These devices
offer much better performance and reliability incomparison to potentiometers. Optical sensors based onlaser interferometer, wavelength, and intensity modulation
are only used in lab type conditions and are suitable forharsh media. Variable reluctance type sensors are
excellent choices for wide media and temperature,
however these are highly non-linear and only operate overa short sensing range. Eddy current devices tend tooperate at higher frequencies and require the signal
processing electronics in close proximity to the sensor.
This limits the device's capability to operate over a wide
range of temperatures and radiation. Magneto restrictive
sensors while offering excellent performance are also
limited to operating temperature due to the close proximityof the signal processing electronics.

For years, LVDTs have been used in commercial andmilitary aircraft applications such as wing flaps, fuelpumps, and landing gear where reliability is critical. These
devices employ low frequency (3- to 5-kHz) magnetic
circuits and do not generate any radio frequency (RF)noise in comparison to the Eddy current or other highfrequency operated linear sensors. The low frequency
operation allows these sensors to be separated from theelectronics by several feet. Since the LVDT device utilizes
a magnetic coupling from the primary to secondary coils
and no physical connections, these sensors can behermetically sealed against water, dust, and ice. 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 allows 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 and subsea chokes in cylinder
sensing. Figure 3 shows two non-contacting position
sensors that can be embedded in valves andtransmissions.

Figure 3: Down hole high temperature and pressure (+200°C and20,000 psi) position sensor for drilling. Position sensor (right) withremote electronics for use in hostile environment.

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