To gain an advantage in today's fiercely competitive international markets, process engineers must continuously strive to improve the productivity of their manufacturing equipment in terms of efficiency, reliability, safety and environmental impact. To achieve these goals, accurate, reliable and cost effective pressure measurement is not a luxury, it is vital.
To help process engineers appreciate the features and benefits of the latest pressure measurement technologies, Yokogawa product specialist, Tony Farah, explores resonant silicon pressure sensors (the enabling technology behind the company's DPharp transmitter) and then speculates about the future trends in pressure measurement technology.
History
About 70 years ago, quartz clock technology was discovered at Bell Laboratories in the US. The heart of the clock was the crystal resonator, an element that, thanks to quartz's elastic property, vibrates at a constant frequency. Due to their accuracy and stability, quartz clocks were quickly applied to engineering tasks including astronomy and international standard time measurement.
Now
Decades later, crystal resonators have been extended to pressure measurement. Unlike clocks, where the resonator sees atmospheric pressure, one side of crystal in a pressure sensor is in contact with the fluid being measured. When pressure increases, the crystal strains, altering the oscillation frequency.
In operation, differential pressure is transferred to the detection system via a diaphragm which protects the crystal from process fluids, overpressure, water hammer and other stresses. The resonator, shaped like an 'H', is immersed in a magnetic field and spontaneously oscillates due to the left arm developing an orthogonal Lorenz force whenever a current pulse passes through. Both resonator arms vibrate together at the same frequency because they are mechanically bonded at the centre.
Due to Faraday's law, the right arm - acting as a moving wire in a magnetic field - produces an electromotive force (EMF) which represents the variable being measured. As pressure changes, the tension in the arm alters, varying the resonant frequency. This output is directly linked to a microprocessor, avoiding losses through an analog to digital converter. Consequently the resonator is extremely sensitive, delivering high resolution, linearity and rangeability that cannot be found in other instruments. Two resonators are mounted on the same silicon chip.
When the unknown pressure increases, the central resonator is tensioned so the oscillation frequency increases, while the lateral resonator is compressed and its frequency decreases. The measured variable is represented by the difference of these two frequencies. Using this technique, the sensitivity to pressure change is doubled, while the effects of static pressure, temperature or other possible disturbances are automatically compensated.
To avoid interference caused by changing air density, the resonator is housed in a vacuum sealed silicon chamber. The two resonators, their shells, the diaphragm base and all other elements of the design are manufactured from the same silicon crystal, using micro-machining techniques. Thanks to silicon's molecular structure, the elastic property of the monocrystal remains unaltered for many years. This explains why the single crystal sensor offers levels of long term repeatability that are impossible to achieve using metal elements.
Unlike instruments using analog detectors, crystal resonant transmitters are termed digital because the frequency sensor output is connected directly to the CPU. By eliminating resistors, reference voltage generators and analog to digital converters, instability errors and reliability problems are eliminated. The resulting configurable transmitters offer useful functions such as: output signal reversal, bi-directional flow measurement, self diagnosis, digital communication, square root, output damping, elevation and suppression zero, linear conversion to engineering units and predefined output value for emergency conditions.
Another benefit of the resonant crystal transmitters is increased rangeability, allowing a small range of models to cover a wide range of pressures. Users benefit from greater single unit flexibility, and cost savings thanks to a consolidation of models and a subsequent reduction in spares.
The transmitters are ready to run with Foundation Fieldbus. As soon as this standard is universally accepted, the digital-analog converter will be removed, leaving DPharp transmitters to operate only in fully digital mode.
The future
Looking forward, trends in pressure measurement are characterised by a continuous move from analog to digital technologies and a demand for ever smaller sensors. Once again, clock developments light the way. In 1948, the Washington National Bureau of Standards succeeded in associating time measurement with the vibration of an ammonia molecule, resulting in a hysteresis free molecular resonator. The oscillation frequency of molecular resonators is stable over time because they are completely free from micro-creep caused by atomic displacements. Moving on, the sixties saw the arrival of rubidium and caesium atomic clocks, where the electron fluctuates with an oscillation frequency measured in GHz.
If these are the technological trends, it is safe to assume that in years to come new pressure transmitters, equipped with smaller, more stable molecular sensors, will start to replace the crystal resonant transmitters that typify today's state of the art.
For more information contact Yokogawa, 011 831 6300, [email protected],
Tel: | +27 11 831 6300 |
Email: | [email protected] |
www: | www.yokogawa.com/za |
Articles: | More information and articles about Yokogawa South Africa |
© Technews Publishing (Pty) Ltd | All Rights Reserved