A transducer is a device that takes a useful signal in one form, say electrical, and converts it to another useful form, perhaps optical. Transducers have an input quantity (or signal) and an output quantity (or signal); the output quantity varies as the input quantity varies and is related to it by a more or less complex mathematical function called the transducer characteristic or transducer transfer function: by knowing the transfer function and the output quantity, it is therefore possible to know the value of the input quantity and vice versa.

Some examples of transducer input quantities are: acceleration, electric field, magnetic field, density, force, level, weight, pH, flow rate, angular position, linear position, pressure, radiation, electrical signal, temperature, velocity, viscosity, humidity. The output quantities of transducers are generally: force, displacement, impedance change, or electrical/electronic signal.

Note that “sensors” and “actuators” are “transducers” in a more general sense, but common usage restricts the meaning of the term. Transducers are therefore, by definition, different from sensors, which are usually technological devices capable of “feeling” or being sensitive to the physical quantity in question and transmitting a signal (output) to the measuring instrument. Transducers are connected to electrical systems to provide electrical signals that indicate the state of the perceived phenomenon. They therefore make it possible to measure and control, by means of electronic equipment, the variations that physical quantities of various types undergo, such as speed, temperature, pressure, etcetera.

A transducer can be active or passive, mechanical or electrical, and can be characterized by certain parameters such as sensitivity, linearity, frequency response, internal impedance, and stability. Transducers affect electroacoustics, the field of measurement, and the field of control and regulation equipment; in practice, they can be divided into only two categories, since the functional principle related to the second and third is basically the same.

In rare cases, the output of a transducer can be directly connected to a measuring, processing or display instrument. The electrical signal output from the sensor/transducer, in addition to containing unwanted components, is usually too noisy and too weak (values on the order of millivolts or picoamperes) to be transmitted remotely. This requires the presence of an interface circuit that optimizes the connection between the transducer and the load.

Electroacoustic transducers convert a sound signal into a corresponding electrical signal or vice versa, keeping the waveform from input to output as similar as possible. The first category includes microphones, laryngophones, and systems that drive the recording heads of magnetic recorders; the second category includes loudspeakers, telephone receivers, and bone vibrators in hearing aids. Electroacoustic transducers also include transducers that convert an electronic signal into a magnetic recording and transducers that perform the reverse conversion, which are part of the recording and playback heads of magnetic recorders.

Electroacoustic transducers are most often reversible, i.e. the principle of their operation is such that signal conversion can occur in either direction. In general, transducers are characterized by a coupling between the oscillating mechanical part and the electrical circuit, a coupling that can be based on an electric field (e.g., electrostatic and piezoelectric microphones, electrostatic and corona effect loudspeakers) or on a magnetic field, such as moving-coil microphones and loudspeakers. In addition to fidelity, a characteristic quantitative measure of a transducer is its sensitivity, which is the ratio of the amplitude (or rms value) of the outgoing signal to the same magnitude of the incoming signal, under specified measurement conditions: for example, in the case of microphones, sensitivity is measured with the electrical output circuit open.

Measuring transducers fall within the first definition, but the purpose of converting the signal from one form to another is to provide an output signal suitable for feeding a measuring instrument; in this case, the essential requirement of the conversion is that the amount of information associated with the signal is retained.

The simplest transducers include tachometers, pressure gauges, thermocouples, bimetals and thermistors for temperature measurement. These transducers do not differ essentially from the transducers used in control and regulation systems, but in the latter the purpose of the measurement, e.g. of a temperature, is not, as in the former, to provide the value to an observer, but to constitute the input signal of a reaction loop, the output of which is connected to the comparator.

Control transducers can be classified into resistive, electromagnetic, magnetostrictive, electrodynamic, capacitive, and piezoelectric transducers according to their physical operating principles. Resistive transducers are based on the variation of the electrical resistance of a conductor as a function of its length, e.g. for displacement measurements, the moving organ moves a moving slider along a resistor: the displacement is proportional to the change in resistance.

Electromagnetic transducers are based on the change in the ratio between the inductances due to the rotation of the armature around the fulcrum; the measurement of the change in the ratio between the inductances makes it possible to determine the angle of rotation, which is the quantity to be measured.

Magnetostrictive transducers are based on the change in permeability, for example of a nickel core, due to magnetostriction; the transducer can be used to measure the force acting on the core; electrodynamic transducers are based on the electromotive force induced in a voice coil by its movement in a magnetic field; Capacitive transducers are based on the principle of electrostatic microphone, suitable for pressure measurement; piezoelectric transducers are based on the principle of piezoelectric microphone, also suitable for pressure measurement.

Transducers can be divided into passive and active transducers; resistive transducers are considered passive, as neither electromotive force nor pressure is generated in the transducer by direct action of the input signal; electrodynamic transducers and thermoelectric torque transducers, which are suitable for temperature measurement, are considered active.

Transducers used specifically for measurement are divided into analog and digital. They are analog when the continuously variable input quantity is matched by a continuously variable output quantity; they are digital when each value of the input quantity is matched by an output quantity represented by a number with a definite number of digits visible in a frame.

Dependence on environmental variables

Virtually all transducers are affected to a greater or lesser degree by one or more of the environmental variables. That is, their proper operation may depend on and be affected by temperature, humidity, pressure, airborne dust, ionizing radiation, magnetic fields, electric fields, brightness, substances in the atmosphere, etc. Therefore, it will be necessary to select with due care the most suitable transducer for the purpose and location where it will be used or, alternatively, to provide adequate protection.

Some examples may illustrate this point.

  • In a potentially explosive environment, due to the presence of dust and/or explosive chemicals, properly protected electrical/electronic components must be used to prevent sparks from causing an explosion in the working environment.
  • In a dusty environment, protected gears should be used to prevent dust from blocking their smooth operation.
  • In an environment where ionizing radiation is present, properly shielded electronic components should be used to prevent the radiation from damaging the various circuits.
  • In an environment where strong magnetic fields and/or strong electric fields are present, it will be necessary to use electrical instruments that are insensitive to these fields or, alternatively, to use adequately shielded instruments.

Transducer classification

Transducers can be classified according to the type of input and output energy. In this case, there are two different families of transducers:

  • homogeneous transducers: transducers where the input energy is homogeneous (identical in nature) with respect to the output energy; some examples of homogeneous transducers are:
    • mechanical transducers (including gears)
    • hydraulic transducers (including pressure pipes)
    • electrical transducers (including transformers)
    • electronic transducers (including the transistor)
  • non-homogeneous (or hybrid) transducers: transducers where the input energy is of a different (non-homogeneous) nature than the output energy; some examples of non-homogeneous transducers are:
    • electromechanical transducers (electrical quantity → mechanical quantity and vice versa)
    • electro-optical transducers (electrical quantity → optical quantity and vice versa)
    • magnetoelectric transducers (magnetic quantity → electrical quantity and vice versa)
    • piezoelectric transducers (pressure magnitude → electrical magnitude and vice versa).

Transducers can also be classified by the ratio of output energy to input energy:

  • passive (or inert) transducers: when the output energy is less than the input energy (e.g. gears, transformers, springs, etc.). An example would be the piezoceramic transducer, which senses vibrations and shocks experienced by the structure by converting mechanical energy into an electrical signal).
  • active transducers: when the output energy is equal to or greater than the input energy. This means that the transducer must be powered by an external auxiliary power source (e.g., triode, transistor, etc.).

Series or Cascade coupling

Multiple transducers can be coupled in cascade or series to create a single transducer with very specific characteristics. In this case, the output quantity of the first transducer must be of the same type as the input quantity of the second transducer in order for the coupling to take place. Finally, the overall output of the transducers is usually pneumatic or electric/electronic.

The reason transducers have a pneumatic output is that, until not too many years ago, it was the only way to make complex operations on a machine fully automatic.

Recently, electronics have replaced virtually all pneumatic controls, which is why so many modern transducers have an electric/electronic output. The characteristics of the output signal are normalized so that the signal can be processed by other components or a computer. In this case, there is a clear separation of meaning between “sensor” and “transducer”.

The sensor is the element that senses the input energy, transduces it (i.e., changes some of its characteristics), and transmits it to the module that reprocesses it to make it compatible with the normalized electrical/electronic output. The transducer is the device as a whole and thus includes the sensor and the normalization module. Of course, the normalization module is a transducer in the broadest sense, since it converts the sensor output to the normalized electrical/electronic signal.

Parallel coupling

It is theoretically possible to couple several identical transducers, i.e. transducers with the same input and output magnitude, in parallel. In this case, the transducers share the same input magnitude with the goal of having a more energetic output. In practice, this almost never happens (there are very few cases where transducers are connected in parallel in practice) because no two transducers are truly identical.

By connecting two transducers in parallel, there is a risk that one will work in overload and the other will work under load. The real risk is that the transducer operating under overload will fail due to overload. Once that happens, the other transducer can immediately fail due to overload because the first transducer is no longer contributing to the power transfer. Thus, the failure of all transducers coupled in parallel can occur at the same time.

There are two solutions to avoid this problem. The first solution is to require a total transfer energy that is less than the sum of the transfer energies of the individual transducers, precisely to prevent the overloaded transducer from failing. The second solution is to use a single transducer with appropriately enhanced characteristics. That is, to be clear, either two or more transducers are used by having them work under load, or one transducer is used that is adequate for the increased requirements.

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