Sensors Guide

The key function common to all sensors is conversion: sensors, (or “detectors”), detect and measure physical objects or quantities, which can be as diverse as an electronic identification code on a specially designed label known as an RFID chip, (where RFID stand for Radio Frequency Identification), the quantity of heat in an object, fluid or person, the movement of an object, person or animal into an electronically monitored field of vision, or the type of acceleration an object is undergoing, such as free-fall or rotation. Upon measurement, a sensor converts the data it has received into a signal or visual display which can then be meaningfully interpreted by either a human agent or by another electronic device. A sensor, in other words, is also always a transducer – a device that converts one form of energy or stimulus into another.

One form of motion sensor, for example, can be integrated into industrial machinery and wired to a safety switch. This allows safe shutdown to occur in the event of the detector signalling to the switch an aberrant mechanical movement which may damage the equipment if permitted to continue or pose a danger to nearby human operatives. This is an example of measurement being converted into a signal for input into another non-human device, but of course many sensors convert the measurements into scales or displays intended for human eyes. The mercury-in-glass thermometer, for example, is a ubiquitous form of temperature sensor which converts the expansion or contraction of a small bulb of mercury into a readable scale (Celsius or Fahrenheit): as the mercury expands or contracts, it rises or falls inside a narrow hollow filament within the glass, which has a calibrated temperature scale engraved on its outer surface.

The mercury-in-glass thermometer, within the temperature ranges it is designed to measure, displays an important feature required of all sensors: linearity. In other words, the physical changes in the sensor’s detector material, in this case mercury, are in direct proportion to changes in the object, force, movement or radiation under measurement. Another type of sensor, the thermocouple, will similarly respond to temperature changes in a linear fashion, in this case generating changes in output voltage that are in proportion to changes in heat. To ensure accuracy, sensors are carefully calibrated to conform to established, tried and tested scales.

In our electronically mediated civilisation, sensors play a pivotal role in ensuring the proper functioning of a vast number of machines, gadgets, vehicles and manufacturing processes. Most people may be completely unaware that sensors lie behind many things that they take for granted, such as the accelerometer, which ensures the display on a mobile phone or tablet is always the right way up whatever movement or rotation the device undergoes, or that sensors help cars and aeroplanes function safely. They are widely deployed in medical equipment, in aerospace engineering, in innumerable manufacturing processes and in robotics, to mention but a few applications.

Sensor sensitivity determines many of their applications. When a sensor responds to a relatively large change in a medium with a relatively small change in its detector material and consequent output, it shows low sensitivity. But sensors are sometimes required to measure tiny changes, in which case they are required to show high sensitivity, responding significantly to minute changes in the medium under measurement. Often, the linearity of such sensors is confined to a tightly delimited range, beyond which it will respond far less accurately.

Sensor manufacturers have to take account of the affect a sensor will have on whatever it is detecting or measuring: plunging the bulb of a mercury-in-glass thermometer into a hot cup of tea, for example, will in itself cool the liquid because it absorbs heat energy from the medium into which it immersed. Some degree of sensor impact is inevitable most of the time, but considerable care and ingenuity goes into ensuring that the effect is as small as possible. One way of minimising this effect is to aim for as much miniaturisation as possible in sensor design: the smaller a sensor is physically, the less physical impact it will have on its medium. Today, Microelectromechanical Systems (MEMS) technology is transforming the manufacture of sensors, enabling the construction of micro-detectors with are literally microscopic in scale. These sensors are typically faster in their response times and considerably more sensitive than their larger counterparts.

Technical/scientific illustration of sensors

Sensor stories from a manufacturing perspective

Sensors have been put to work by engineers in the most imaginative and labour-saving ways. RFID sensors, for example, have not only helped create a paperless bill payment system when integrated into smartphones, they have also massively simplified security procedures by enabling automated gate-and door-opening systems in secure zones for people bearing the requisite RFID chip, and they have revolutionised logistics and transportation as well as cross-docking processes.

Accelerometer sensors are not only ubiquitously used to “auto-rotate” displays on digital cameras, smartphones and tablets, they are also widely used in the navigation systems of modern aerospace equipment.

Meanwhile, contemporary infrared sensors are frequently integrated into heavy machinery and electrical equipment, where they activate safe shutdown in the event of aberrant and potentially dangerous temperatures.

Motion sensors, when wired to shutdown switches, perform a similar function, this time by detecting potentially hazardous movement in mechanical moving parts which may suggest worn bearings or other faults which could have enormous cost implications if not deactivated swiftly. They are also widely deployed in domestic and commercial security systems to detect intruders.

Temperature sensors in the form of thermocouples are seen frequently in chemical engineering plants, where they can withstand very high temperatures and monitor small heat changes indicative of chemical reactions. The new field of nanothermometry has enabled the measurement of temperature differentials between particles measuring less than a single magnitude in size to be gauged.