Senors 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.

 

History of the sensor

Humankind has been experimenting with sensors of various kinds since at least the third century BCE, when Philo of Byzantine constructed a device capable of demonstrating how much air expanded or contacted in response to changes in temperature. By the seventeenth century, the Italian astronomer and physicist Galileo Galilei was building the first version of a thermometer. A few decades later in 1784, an English engineer named George Atwood had designed the first accelerometer, although it remained merely an instrument for demonstrating the truths of Newtonian physics until it was rediscovered in the late twentieth century as a gadget capable of multiple applications (the auto-rotate function on smartphone and tablet devices depends upon accelerometers).

Occasionally discoveries made by creative scientists lie fallow for decades and even centuries until applications for them have been imagined. For example, infrared radiation (literally radiation of a wavelength below that of visible red light), was discovered in 1800 by the German astronomer William Herschel. Three decades later in 1831, the Italian physicist Melloni created a thermopile capable of detecting warmth (i.e. receiving infrared radiation) from a human subject standing ten metres away. But it wasn’t until the 1970s that infrared sensors capable of creating “warmth” images of humans or animals in specially designed cameras really developed.  

The years of World War Two sponsored the inventiveness of necessity in many respects. The pre-cursors of the Radio Frequency Identification or RFID chip and the motion sensor were developed with great urgency during this period, with radar capitalizing on a discovery actually made in the nineteenth century and RFID technology growing out of basic IFF transponders used to detect the distinctive sounds of enemy and friendly aircraft.

Anyone who has ever stepped onto an electronic bathroom scale has, probably without being aware of it, used a strain gauge sensor. This particular type of transducer converts mechanical forces such as stretching or compression into changes in electrical resistance – as the strain gauge foil is squeezed or elongated, it undergoes changes in electrical resistance. In 1938, long before contemporary electronic bathroom scales had come into the world, American engineers Arthur Ruge and Edward Simmons crafted the world’s first variable resistor strain gauge.

 

Important technical elements
 

The core technical elements common to all sensors is that they should display linearity, (so that they provide measures accurately corresponding to real changes in the medium they are monitoring), they should be calibrated to conform to recognised and established measurement scales and should not be used on either side of the tolerance range they have been designed to measure accurately (otherwise their linearity and sensitivity will be distorted). They should have the minimum impact on the medium they are measuring: plunging the bulb of a mercury-in-glass thermometer into a beaker of hot water will, for example, affect the medium the thermometer is attempting to measure by cooling it.

 

Technical/scientific illustration of sensors

Type of Sensor

Versions

What it detects

Description

RFID Chip

Passive (receiving interrogation signal from an RFID reader); battery-operated active (transmitting ID signal at set intervals)

The RFID chip reacts to an incoming interrogation signal from an RFID reader

Radio Frequency Identification chips can store simple electronically coded data such as a single serial number or more complex product descriptions, which can be detected and displayed with an RFID reader

Temperature Sensor

Thermometers, thermocouples and resistance temperature detectors (TDs), each of which may be contact or non-contact sensors

Changes in heat energy in an object or medium, either by the expansion and contraction of a physical material or by changes in the electrical conductivity of a conductor

Different temperature sensors are designed to be applied in different heat environments, Some, such as thermocouples and RTDs, remain sensitive and accurate at very high temperatures, while others, such as many thermometers, are sensitive at lower temperatures

Accelerometer

Piezoelectric, piezoresistive,  magnetoresistive, Hall effect, Heat transfer, capacitative

Accelerative motion caused by vibration, rotation, tilt, gravity, free-fall or collision. Changes in the deflection of a spring-loaded proof load within the sensor in response to accelerative forces

Proof load deflection is converted by the sensor into a measure of the accelerative force that has caused it. The sensor also takes account of the plane in which the acceleration occurs

 

 

Infrared Sensor

Passive (receiving infrared thermal emissions from an object); active (emitting IR via an LED or IR laser).

Infrared radiation in the wavelength range 0.7 - 14µ

IR sensors detect radiation below the wavelength of visible red light, which is essentially heat radiation. They may be used to convert the IR radiation into images as in IR cameras, or as part of security systems to detect the presence of moving intruders

Motion Sensor

Local sensing (visible IR light beam from LED or laser, piezoelectric, piezoresistive, contact switch); Area sensing (video, active or passive IR motion sensor, ultrasonic motion detector,  microwave Doppler sensor)

Motion by objects, human beings or animals.

Motion sensors convert motion by an intruder in a designated area or field of view into an electrical signal, which can then function as an input to various applications and systems such as security, automated light systems, automatic doors and safety shutdown switches in machinery

 

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 (read more here).

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 (read more here).

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.

 

 

Short overview of the above different sensor types

RFID sensors

RFID sensors wirelessly read coded data electronically encrypted into specially designed RFID chips using radio waves instead of physical or visual contact. They can detect information at short range (such as the pet or livestock identification chips injected sub-dermally in veterinary science) or at much greater ranges in warehouses or freight transport, for example, to identify products. (Read more about RFID Sensors here).

Temperature sensors

Temperature sensors, from mercury-in-glass thermometers to thermostats to thermocouples, accurately measure changes in heat energy in a body or fluid, converting the measures into an established temperature scale such as degrees Kelvin, Celsius or Fahrenheit. Differently designed temperature sensors are sensitive over specific ranges, above or below, which can yield inaccurate and distorted readings. (Read more about temperature sensors here)

Accelerometers

Accelerometers detect changes in rotation or speed in an object relative to gravity, usually by means of electrical changes in piezoelectrical crystals or capacitance variations between electrically sensitive microstructures (read more about accelerometers here).

Infrared sensors

Infrared sensors detect radiation at the non-visible end of the red light spectrum associated with the emission of heat energy. The most commonly used IR sensors are ‘passive’, so called because they simply receive heat emissions from an intruding warm body into a designated field. They are widely used in domestic and commercial security systems, (read more here), and in thermosensitive cameras.

Motion sensors

Also widely used in security systems, as well as manufacturing processes, are motion sensors, all of which are designed to detect the movement of either objects, (such as mobile machine parts or vehicles), or human intruders into a specified field of vision or monitoring. Some emit signals and then “read” the presence of intrusion by means of alterations in the echo received, while others pick up signals emitted naturally by the intruder, such as infrared radiation emissions. They are ubiquitously deployed across the world in security systems, but can also be integrated with safety switches to shut down machinery if aberrant and potentially dangerous or damaging movement is detected by them (read more about motion sensors here).