transducer

A substance or device, such as a piezoelectric crystal, microphone, or photoelectric cell, that converts input energy of one form into output energy of another.

[From Latin trānsdūcere, to transfer : trāns-, trans- + dūcere, to lead.]

A device that converts variations in one energy form into corresponding variations in another, usually electrical form. Measurement transducers or input transducers may exploit a wide range of physical, chemical, or biological effects to achieve transduction, and their design principles usually revolve around high sensitivity and minimum disturbance to the measurand, that is, the quantity to be measured. Output transducers or actuators are designed to achieve some end effect, for example, opening of a valve or deflection of a control surface on an aircraft. Actuators, therefore, normally operate at high power levels. The term sensor is often used instead of transducer, but strictly a sensor does not involve energy transformation; the term should be reserved for devices such as a thermistor, which is not energy-changing but simply changes its intrinsic electrical resistance in response to changes in temperature.

Both input and output transducers, together with the instrumentation to which they are connected, may be called upon to respond to both slowly varying or dynamic signals. This means that the transducer, together with its instrumentation system, must be designed to meet such a specification. Some prior knowledge is therefore required of the type of signal to be transduced, and the bandwidth of the transducer and instrumentation system must be suitably matched to this signal.

Transducers are often described in terms of their sensitivity to input signals (responsivity). This is simply defined as the ratio of the output signal to the corresponding input signal. Once again, the responsivity of a transducer must be matched to the expected levels of signal to be transduced. See also Sensitivity (engineering).

The measurement of force is very often accomplished by allowing an elastic member (spring or cantilever beam) to deflect and then measuring the deflection by using some form of displacement transducer. Transducers designed to measure acceleration are frequently based on the simple equation below, where f is force, m is mass, and a is acceleration. Thus, if the force due to the movement of a known mass can be measured, it is possible to derive the acceleration. Very often, the measurement technique employed uses piezoelectric, magnetostrictive, or mechanoresistive materials. Acceleration transducers or accelerometers are frequently employed for the measurement of vibration. See also Accelerometer; Force; Magnetostriction.

Transducers for a wide range of chemical species are available, but probably the most widely applied is the pH transducer for the measurement of hydrogen-ion concentration. The traditional method has relied on a glass membrane electrode used to make up an electrochemical cell. See also Hydrogen ion; Ion-selective membranes and electrodes; pH.

Measurements of the partial pressure of oxygen (pO₂) may be accomplished by the use of a Clark oxygen cell, which comprises a gas-permeable membrane controlling the rate of arrival of oxygen molecules at a noble-metal cathode that is held at 600–800 mV potential with respect to the anode. The ensuing reduction process gives rise to a cathode current from which oxygen concentration can be derived.

Other electrochemical transducers are used in such applications as voltametry, polarography, and amperometry. Chemical transduction is also possible by adsorbing a species onto a surface and detecting its presence by mass change, electrical property change, color change, and so on. See also Electrochemical techniques; Polarographic analysis; Titration.

Measurements of the partial pressure of oxygen and the partial pressure of carbon dioxide (pCO₂) are also of particular importance in the context of blood gas analysis in medicine, and by using the Clark cell they can be performed without removing the blood from the body and noninvasively, that is, without puncturing the skin.

There have been remarkable advances in the area of biological transducers or biosensors. Examples are the ion-selective field-effect transducer (ISFET), the insulated-gate field-effect transducer (IGFET), and the chemically sensitive field-effect transducer (CHEMFET).

A smart transducer or smart sensor is a device that not only undertakes measurement but also can adapt to the environment in which it is placed. Such adaptation may range from simple changes in the characteristics of the transducer in response to changes in temperature, to more complex procedures such as adaptation of the transducer's performance to conform to overall system requirements. In integrated transducers, much of the signal processing that might previously be done remotely is brought into the transducer packaging.

The development of inexpensive fiber-optic materials for communications has led to an examination of the potential for using these devices as the basis for transduction. Two major types of devices have resulted: fiber-optic transducers for physical variables and similar devices devoted to chemical and biological determinations. The advantages of the all-optical transducer are its lack of susceptibility to electrical interference and its intrinsic safety. Small deformations of an optical-fiber waveguide cause a change in the light transmission of the fiber, and this has been exploited to produce force and pressure transducers. Alternatively, miniature transducers based on color chemistry can be fabricated at the end of a fiber and the color change can be sensed remotely. Devices of this type have been developed for measuring pH, the partial pressures of oxygen and carbon dioxide, and glucose. See also Fiber-optic sensor.

The most important recent technological development in the area of transducers, sensors, and actuators is micro-electro-mechanical systems (MEMS). There are a wide variety of MEMS devices, mostly fabricated in silicon. See also Micro-electro-mechanical systems (MEMS).

A device that converts one energy into another. There are myriad types of transducers; for example, a read/write head converts magnetic energy into electrical energy and vice versa. A loudspeaker converts electronic signals into air pressure, and a microphone does the reverse. An antenna converts electronic signals into electromagnetic waves and vice versa.

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A device that is activated by power from one system and then supplies a different form of power to a second system; used to convert electric energy into mechanical energy in ultrasonic and sonic scalers.

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A device which converts power in one kind of system to power in another form, e.g., a loudspeaker which converts electric power to acoustic power.

A device that transforms energy from one form into another.

A device that translates one physical quantity to another, e.g. pressure or temperature to an electrical signal. In ultrasonography, the device that emits sound waves.

• annular array t. — an ultrasound transducer with crystals arranged in concentric rings with different frequencies of sound produced. It allows for a greater depth of focus.
• linear array t. — an ultrasound transducer with crystals arranged in a line. It gives a rectangular field of view.
• neuroendocrine t. — a neuron, such as a neurohypophyseal neuron, that on stimulation secretes a hormone, thereby translating neural information into hormonal information.
• sector t. — an ultrasound transducer which produces a fan-shaped field of view.

Device that converts energy from one form to another.

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