The sensors for automation and control that industry relies upon - proximity, motion, temperature, pressure, level, infrared etc – are all still there, but most are smaller, more precise and, surprisingly, cheaper in real terms than they were a few years ago. Networking of production systems has advanced rapidly, with wireless networking becoming the favoured approach. As sensor costs have reduced, the cost of networking has tended to increase, thus balancing any savings.
Condition monitoring of machinery has become more important as equipment contains more sealed units, and working parts become less accessible. One of the recent innovations in this area has been wear debris sensor technology, invented by researchers at the University of Edinburgh some eight years ago and developed by MACOM Technologies. This technology makes possible the detection and counting of metallic wear particles in the lubrication oil of engines, gearboxes and complex assemblies. A sudden increase in the number and size of particles indicates imminent failure.
Temperature sensors have changed in a big way, with thermocouples rapidly being overtaken as a result of the improvement of RTDs (Resistance Temperature Devices). RTDs rely on the principle that the electrical resistance of metallic films changes with variation in temperature. Most RTDs use a thin-film platinum detector, which offers an approximately linear relationship between temperature and resistance. RTDs are available in platinum, nickel or copper, each giving a different resistance-to-temperature coefficient curve. For cryogenic applications, RTDs are made with ceramic substrates.
Thin-film pressure sensor technology has advanced dramatically during the last few years, and has made possible dramatic reductions in the size of pressure sensors. One of the leading companies in this technology is Sensor-Technik (www.sensor-technik.co.uk) in Germany, whose production techniques for manufacturing the titanium nitride thin film strain gauges used in its pressure sensors are entirely the company’s own creation. The thin film can be deposited on, and bonded to various different substrates, including ceramics but more commonly high-grade special steels, to create an IP69 sealed transmitter filled with a dry inert gas. Sensor-Technik is now supplying this advanced technology in standard OEM sensors, mainly targeted at higher pressure environments such as hydraulics and vehicle applications.
Piezoelectric sensors for dynamic measurements of force, pressure and vibration use well-established principles but have been progressively refined, made more precise and reduced in size. As well as their well-known use as accelerometers in vehicles, particularly aircraft, piezoelectric high frequency miniature sensors can be used to detect very small component vibrations and shock. Force sensors using similar technology can be used for impact measurements and fatigue monitoring and piezoelectric pressure sensors are used to monitor very fast changes in pressure.
Vibration measurement in potentially explosive atmospheres has always been a tricky subject. However, new, intrinsically safe ATEX and IEC Ex certified vibration sensors recently announced by Hansford Sensors (www.hansfordsensors.com) allow engineers to monitor vibration levels of pumps, fans, compressors, motors, conveyors, drilling rigs and all types of rotating machinery, which operate in a potentially explosive working environment. Nothing new in that, you might say, but these sensors are now much smaller. Hansford Sensors has applied for certification on its HS-420 vibration sensors (offering 4-20mA outputs for direct connection to PLCs) and expects to have these available soon.
Another company strong in ATEX devices is Pressure Vacuum Level (www.pvl.co.uk), with ATEX-certified switches for pressure, level, flow and temperature. PVL level sensors include capacitance probes and liquid level switches, level sensors and fuel gauges, conductivity probes and controllers with a wide range of specifications. Other products include depth, pressure and sludge level transmitters, vibrating fork and optical level sensors and high temperature and pressure sensors working at up to 500bar and 400ºC.
There have been substantial advances in programmable sensors. One of the leaders in this field is Honsberg of Germany. Honsberg is a long-established family-owned company in Remscheid, which has come up with the Omni range of programmable sensors for flow, level, temperature and pressure. Each of the stainless steel Omni sensors is rated to IP67 and has an LCD display protected by toughened mineral glass. This shows the range of the sensor, set points and programming parameters, and includes an LED alarm signal. Around the circular LCD display is a programming ring containing an integral magnet that can be operated even when wearing protective gloves. The programming ring makes it possible to display the parameters (for example set points) at Position 1, and to alter them at Position 2. Turn the programming ring through 180º, or remove it altogether, and the programming is totally protected. The Omni range is available at www.pvl.co.uk.
In more exotic spheres, micro-machined silicon sensors exploit the piezo-resistive properties of a monolithic silicon diaphragm to achieve more precise aerospace pressure measurements. Silicon sensors give a high standard of accuracy, response and overload capability, yet are more or less immune to the effects of vibration, shock and acceleration. They are sensitive to applied pressures from only tens of millibars upwards and operate accurately across a very wide range of temperatures.
Even more exotic is biomimetic sensor technology, using biomaterials and information processing of a type found in biological systems. Biomimetic sensors can detect and quantify taste (the electronic tongue) and smell (the electronic nose), and potentially have applications in food science, drugs detection and many other fields. Fast developing, but still with a long way to go, biomimetic sensors are already improving man’s understanding of the reception mechanisms in our gustatory and olfactory systems.
Sensor technology is an area of constant advance and improvement, drawing on pure research in the world’s universities as well as development work by sensor manufacturers. We shall see a lot more exciting innovations during the next few years. Watch this space.
Ivor Matanle has been writing about automation, controls and engineering subjects for over thirty years and advises companies on marketing and public relations issues. Contact him at ivor@matanle.co.uk.