The latest chemical and electronic technology is producing pressure transducers that can enhance the performance of machines across industry, writes Mike Powers, product marketing director for Gems Sensors & Controls
Advances in the design and manufacture of pressure transducers have resulted in high quality components with enhanced functionality that can withstand aggressive process conditions, extremes of temperature, mechanical shock and vibration. Pressure transducers can therefore be used in a wide range of process and industrial applications. But how has this been achieved and what are the resulting possibilities for design engineers?
A pressure transducer converts pressure – typically that of fluid or gas – into an electrical output signal, offering the potential for sophisticated predictive control within a system. For example, a gradual decrease in pressure measured by a transducer might trigger a series of escalating alarm signals, allowing for appropriate action to be taken before the ultimate level is reached and an automatic shutdown is triggered.
The latest devices comprise a sensitive pressure sensing element combined with a sophisticated electronics package. Testing of these has shown a response of 1ms or less to changes in pressure, accuracy with almost zero drift over time, and an operating life in excess of 100 million cycles. Such levels of performance are the result of highly innovative and carefully controlled construction methods.
Construction
Pressure sensors contain a thin sealed sensing element or diaphragm that is in direct contact with the pressure media. Displacement of this diaphragm causes the strain gauge to flex, either in compression or under tension, with the electrical output being directly proportional to the pressure or vacuum applied. Output from the sensor is connected to onboard electronics, with the entire unit contained in a compact and sealed stainless steel housing. Strain gauge sensors, which contain this pressure sensitive diaphragm, can be effectively manufactured using the sputtered thin film process, in which a molecular layer of material is atomically fused to the beam or diaphragm.
With sputtering, a solid target material is bombarded by energised particles, causing it to release atoms which are then deposited in a layer on a stainless steel beam to form the base insulating layer for the strain gauge. Using the same process this is then coated with further layers of a suitable gauge material, before being patterned using photoresist techniques. The unwanted areas are removed by sputter etching, to create a dielectrically isolated strain gauge in a conventional Wheatstone bridge arrangement, which is mounted on the reverse of a stainless steel diaphragm, resulting in a robust sensor that is suitable for direct contact with almost all liquids, oils and gases.
Chemical vapour deposition (CVD) technology is another method that produces compact, accurate devices. CVD sensors can also be mass produced at low cost, since they are produced on wafers in large batches, using polysilicon deposited on a stainless steel substrate, with the strain gauge patterns being chemically milled. The wafer is then divided to produce individual sensor beams, which are laser-welded to a stainless steel summing diaphragm and pressure port, before being connected to internal electronics for signal conditioning and amplification.
Advances in electronics technology have also made a positive impact on the efficacy of pressure transducers. The integral electronic signal conditioning that has been supplied with these devices over recent years often incorporates advanced ASIC (application specific integrated circuits) technology, a factor that has opened up opportunities for many markets as the performance and functionality of each transducer can be tuned to meet specific customer requirements. ASIC has, in many instances, also reduced the unit cost of transducers by a factor of ten.
As a result, pressure transducers are now being built into a variety of machine systems. While transducers are a powerful tool that can be used to increase efficiency, in some processes these devices can make a critical difference, including in the manufacture of electronic circuits such as those contained within the pressure transducers themselves. In the clean room environments where these circuits are produced, ion implantation and ion beam etching requires fine pressure control over the chamber’s internal climate and air quality. To ensure process integrity, a distributed control system incorporating pressure transducers is used to monitor air recirculation and handling units.
Applications
Another important role played by pressure transducers is in managing the pitch control system on wind turbines. Adjusting the pitch of the blade as the wind speed changes not only ensures optimal turbine efficiency but also avoids damage. When the wind speed rises above the turbine’s rate capacity, it is the speed at which the blades can be edged parallel with the wind that determines whether such damage can be avoided. Transducers enable the swift response of the hydraulic pitch control, minimising damage to wind turbines.
Furthermore, in clean or waste water processing, the use of submersible pressure transducers to measure level can provide a vital method of maintaining and protecting systems. For example, a constant measurement of pump discharge pressure via transducers will reveal whether a system is coping with a blockage. If so, the transducer can trigger appropriate action that can prevent the pump from generating excessively high pressure in excess of the operating limits of the piping system.
