Transducer Sensor

Overview of the transducer

“Transducer” is the generic term for any device that is capable of converting one form of energy (the

so-called ‘input signal’) into another (the output) via an actuator, whilst retaining a recognisable and stable relationship between the two. The actuator is the part of the device that receives the incoming energy and converts it into some form of movement. The actuator in an electrical motor, for example, receives an electrical input and converts it into mechanical movement, as indeed does the actuator in a loudspeaker, although the form of motion is very different in each case.

At the most elementary level, transducers can be classified into three broad groups. Input transducers convert a source of physical energy (e.g., pressure, temperature or sonic waves) into a readable signal: microphones, for instance, transform the sonic waves that vibrate its diaphragm into electrical signals that can then be passed through conductive wires. Actuator transducers, by contrast, convert an electrical input signal into physical energy: the loudspeakers in a sound system, for example, will convert electrical input arising from a sound recording into physical sonic waves.

‘Combination transducers’ such as ultrasound machines are capable of creating and receiving motion. The ultrasonic sensor, for example, oscillates between its actuator function (producing ultrasonic waves and transmitting them into the body) and its sensor function (which receives the transmitted ultrasonic waves as they bounce back from anatomical structures within the body). This oscillation takes place many times every second. The transmission and reception signals are then processed and integrated into a visible image viewable on an electronic monitor screen.

 

History

A transducer is simply the generic name for any device that converts one form of energy into another and converts it into readable signal. One of the earliest forms of transducer is the famous invention of the English physicist Robert Hooke, which converted acoustic energy (the spoken word) into the kinetic vibration of a length of wire: a person spoke into a cup at one end and the voice would be converted into vibrations in the wire and reproduced as a voice at the “output” end of the wire, which was also a cup. In effect, he was creating an early version of the microphone.

During the nineteenth century, numerous discoveries were made about materials that were capable of creating a transducive effect. The German-Estonian physicist Thomas Johann Seebeck, for example, found in 1821 that two dissimilar metals arranged into a circuit would produce an electrical voltage capable of deflecting a magnet when heated at the point where they touched: the principle behind the modern thermocouple was born. The English physicist John P. Joule, a contemporary of Seebeck, found in 1842 that when a magnetic field was applied to an iron bar, the latter would undergo constriction, a phenomenon that became known as “Joule magnetostriction”. By 1880, Jacques and Marie Curie had discovered piezoelectricity: if a physical force were applied to a quartz crystal, it would produce an electrical charge.

By the twentieth century, numerous sophisticated transducive materials were being discovered and produced. Sonar equipment based on nickel and piezoelectric ceramic was becoming outdated by the end of the century, which led to the manufacture of new alloys based on lanthanide. The latter are capable of comparatively massive magnetostrictive effects. Today, most piezoelectric transducers are manufactured from lead zirconate titanate.

Today, hundreds of different transducers are used in countless commercial, industrial and domestic applications.

 

Technical aspects

Inevitably, when one form of energy is converted into another by a transducer’s operation, some of it will simply be lost. Some transducers, however, are more efficient in this respect than others. Radio antennae, in optimal conditions, will conserve over 80 per cent of the radio frequency power it receives when it converts it into an electromagnetic field. Electrical motors, on the other hand, are usually considerably less efficient, generally losing more than half of the energy they receive during conversion. Amongst the least efficient of transducers are standard incandescent light bulbs, which frequently lose more than 90 per cent of the electrical energy they convert into light in the form of heat.

There are now literally hundreds of different types of transducer. Electroacoustic, transducers, for example, either convert electrical signals into acoustic power, or acoustic power into electrical signals (e.g., the hydrophone, a device capable of picking up acoustic energy passing through water and converting it into electrical signals – an invaluable instrument in submarine sound detection). Piezoelectric transducers convert electrical voltages into motion or physical strain onto electrical signals via an integral piezoelectric element. They are typically used in strain gauges, piezoelectric vibration sensors and accelerometers. One of the most widely used types of transducer is the electromagnetic transducer, examples of which are the Hall-effect magnetic transducer (which coverts magnetic fields into electrical signals), the inductance transducer, and saturable reactors.

Electrical transducers such as the thermocouple convert heat energy into electrical voltage via two dissimilar conductive metals possessing different thermoelectric properties. This is an example of an “active” transducer, because it generates power (electrical in this case) in response to the input signal. Others, known as passive transducers, respond to the stimulation of an incoming signal by proportionately altering a passive electrical property such as inductance, resistance or capacitance. The linear-displacement transducer, for example, varies the position of a contact upon a length of electrically conductive material, altering the resistance to the electrical current passing through it in the process. Typically, the movement of the contact can be amplified by using a printed, thin-film or wire wound circuits so that a relatively small physical shift in the contact’s location produces a detectable change in resistance. Physical position changes, in other words, are converted into electrical signals.

Pneumatic transducers operate by means of compressed air, which they can convert into a readable signal. One such employs a stream of compressed air emitted from a nozzle and directed at a moveable baffle, which retreats from the baffle under the force of the air. As it does so, it creates air pressure behind it, which it then converts into a signal. Similar in principle to pneumatic transducers are hydraulic versions, which rely on changes in liquid pressure instead of air pressure to produce a signal.

 

Where the transducer is used in manufacturing

Electromechanical transducers have a vast range of applications in manufacturing, from accelerometers in motion-sensitive mobile phones to rotary and linear motors and strain gauges. Electro-optical (or photoelectric) diodes include light emitting diodes, which are increasingly being used in lighting systems in commercial and industrial buildings because of their low energy consumption and much improved efficiency over ordinary fluorescent lights. Thermocouples (thermoelectric transducers) are used to monitor ambient temperature in the heating systems of domestic and commercial buildings and, at the upper temperature range, to monitor intense temperatures in industrial kilns and chemical plants.

These are but a few of the countless examples of transducer use in the modern age.