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Achieving greater accuracy and stability in pressure sensing

04 January 2011

Pressure sensors are now required to measure with increasing accuracy, stability and repeatability, sometimes in difficult environments. As a result, new generations of pressure sensors have evolved to meet these new challenges. Steve Sajben and Russell Craddock describe evolving technologies, such as TERPS, against a background of landmark developments

A number of techniques and technologies have been developed over the years to improve the accuracy, stability and repeatability of pressure sensors. Techniques and technologies have also been developed to allow pressure sensing to be carried out at extremes of temperature, in harsh environments and in extremes of pressure.

The first advances were seen with the introduction of electrical pressure transducers, where the movements of springs, diaphragms and Bourdon tubes change an electrical parameter, such as capacitance, inductance or resistance. This concept was further developed with the appearance of strain gauges, but the overpressure performance of this last class of instruments was limited by material elastic limits and thermal hysteresis errors. The problem was solved in the 1960s with the introduction of thin-film transducers, a technology that still finds application at high pressures.

The gap for low-pressure sensing was filled in the 1970s by capacitive transducers, which can be also be used in absolute pressure measurement devices. The downside is that they provide a relatively weak output, have a low signal-to-noise ratio and suffer problems of long-term stability.

Today, brittle and crystalline materials are frequently used for the sensing element, as they avoid the pitfalls of material yield and plastic creep and can offer other significant benefits. Quartz elements, for example, can make use of the piezoelectric effect. Silicon crystals are also used but these have predominantly been used in piezoresistive mode.

Piezoresistive pressure sensors incorporate strain gages that are ion implanted or silicon diffused into silicon, typically in a Wheatstone bridge arrangement. When pressure is applied to the silicon sensor element, the resistors are strained, in either tension or compression. The deformation caused by the strain changes the resistance, and the subsequent change in voltage is proportional to the applied pressure.

Their long-term stability is generally good and can be improved by design and processing techniques, but the inherent instability of resistors is a performance limiting factor. However, silicon piezoresistive sensors can have a gauge factor of up to 200, 100 times better than conventional strain gauges, and they produce high signal levels using simple electronics. They are also relatively easy to manufacture, using wafer scale processing.

Quartz ‘mechanical’ sensors
By relying on the brittle nature of fused quartz, some interesting mechanical structures can be created. For instance, if a Bourdon tube of fused quartz is pressurised, a rotational force will be produced as the tube tries to ‘unroll’ and this force can be measured using capacitive or inductive techniques. Although precisions of 0.003% of reading can be achieved, the disadvantage of this technique is the size and fragility of the sensor, which limits it to application in precision laboratory instruments.

Quartz resonant sensors rely on the piezoelectric effect rather than the piezoresistive effect. Here the quartz sensor is driven into resonance much like a quartz crystal oscillator. Stretching or compressing the resonating structure using pressure-activated mechanical linkages or quartz diaphragm structures changes the resonance of the crystal and this change is proportional to the applied pressure.

These true crystal quartz devices achieve very high accuracy but tend to be bulky, often requiring significant transducer packaging to contain the mechanical linkages and to prepare them for use in harsh environments. The quartz sensor element is generally large compared with a silicon-based alternative, which can be more easily batch manufactured.

Silicon resonant sensors
Unlike many of the quartz sensors, silicon resonating pressure sensors employ a crystalline diaphragm structure ‘on-chip’, thus avoiding the need for complex mechanical linkages associated with some quartz devices. Silicon resonant sensors, which have been available for some years, typically offer a stability of 0.01%/year and accuracy of <0.01%fs, including temperature effects. GE’s RPT, for example, features a resonator and diaphragm manufactured entirely from single crystal silicon (which is perfectly elastic to its fracture point). The device is driven to oscillation by electrostatic means and frequency change sensing is by charge output.

The downsides of current silicon resonant sensors are their limited pressure range, their shock sensitivity and the fact that they are available in a non-media-isolated design only. However, the benefits of silicon resonant sensors can now be enjoyed over wider operating pressure and temperature ranges and in harsh environments thanks to the development of TERPS technology.

TERPS sensors operate in essentially the same way as existing resonant silicon pressure transducers. The silicon structure is driven into resonance by the application of an electrostatic field and when pressure is applied to a diaphragm, the silicon resonator is stretched, changing the frequency, much like a guitar string. This change in frequency relates directly to the applied pressure. However, TERPS sensors differ from conventional sensors (such as the RPT) in a number of ways.

In previous RPT designs, the frequency pick-up is capacitive. As the displacement of the RPT resonator is very small, the capacitance signal is correspondingly small and is effectively at the noise level of any nearby capacitive sources. As a result, the electronics have to be close to the resonator and there is no opportunity to use glass-to-metal seals.

TERPS frequency detection, on the other hand, is piezoresistive, which provides a much bigger signal with a small amount of movement via a patented ‘lever’ design. This signal is above the ambient noise level and allows a TERPS sensor to use glass to metal seals, enabling the construction of oil-filled isolated designs. The use of glass-to-metal seals allows the sensor to be packaged for applications in harsh environments. In addition, associated electronics can be located at a distance from a TERPS sensor, which permits operation at higher temperatures.

By optimising resonator geometry, the TERPS sensor oscillates in the horizontal plane unlike an RPT. Consequently, it can be made much more rigid and more mechanically balanced, minimising energy losses. As a result, a TERPS sensor can be oil isolated, as pressure can be effectively transmitted to the sensor element by overcoming the damping effect of the isolating oil. This optimised geometry is achieved by employing deep reactive ion etching (DRIE), a proven micro-machining process, which is used extensively in the manufacture of micro electromechanical systems. DRIE techniques used to manufacture TERPS can achieve aspect ratios greater than 15 and trenches 100 to 500um deep. As a result, the structure of a TERPS resonator is both mechanically and dynamically balanced.

TERPS sensors can operate at pressures of up to 700bar absolute thanks to silicon fusion bonding (SFB). A TERPS resonator element consists of three distinct layers: the diaphragm; the resonator; and the cap. With RPTs the resonator and the diaphragm layer are part of the same piece of silicon and are necessarily micro-machined together. This places restraints on the relative thicknesses of the two layers and hence the pressure range capability.

With SFB, the three layers are independently micro-machined to suit their complexity and then bonded together so that the crystalline structure of the complete assembly is maintained. With this method, the diaphragm layer can be made considerably thicker so that the pressure range is significantly increased.

By exploiting the mechanical properties of the single crystalline structure of silicon, with the development of MEMS manufacturing processes and the incorporation of innovative intrinsic design features, TERPS technology now provides an elegant and readily available solution to the problems of pressure measurement at its extremes and in difficult environments.

Steve Sajben and Russell Craddock, are with GE Measurement & Control Solutions

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