Transducers

Definition: Transducers are the device that is used for the conversion of energy from one form to another different but equivalent form. In terms of instrumentation, more specifically we can say, transducers are the devices that change non-electrical quantity into its electrical equivalent form.

The converted form of energy by any transducer can also be mechanical energy, optical energy, chemical energy, electromagnetic energy etc. This depends on the type and usage of the transducer.

transducer

Mostly, a transducer finds its applications in measuring devices. A speaker is also an example of transducer that has the ability to change an electrical signal into an equivalent sound signal.

Types of Transducers

A transducer is the heart of the instrumentation system and can be majorly classified as:

types of transducer

Basically, transducers are classified according to the operating principle, uses and the required output.

Active and Passive transducer

These two basically shows variation according to the energy conversion technique.

Active transducers are the type of transducer that does not make use of any external source in order to generate the output. These actually generate the driving energy from the system under operation.

As against, a passive transducer is a transducer that requires (or makes use of) an external source of energy in order to drive the system. Some energy may be driven from the system under measurement.

Analog and Digital Transducer

These two transducers differ from each other in the way of providing output.

Analog transducer produces analog signal as an output at the time of energy conversion. Analog signal is a signal which is a continuous function of time. On the contrary, the digital transducer generates a digitized signal at the output i.e., in the form of pulses.

Primary and Secondary Transducer

Primary and secondary transducers are categorized on the criteria of methods of applications.

A primary transducer is the one that does not require an individual input sensing unit. This means that the applied input can be directly detected by these transducers.

However, a secondary transducer requires an additional unit that can detect the applied input. The output of this detector is then fed to the transducer for energy conversion.

Transducer and Inverse Transducer

As transducers change any physical unit into its electrical equivalent. While inverse transducer reverses this operation and converts electrical quantity into a non-electrical equivalent.

An advancement of transducer emerges with the evolution of VLSI technology. These are sometimes known as smart transducers lets understand how it operates.

Smart Transducer

Smart transducers are somewhat enhancement of transducer circuit that is formed when a combination of sensor, transducer, signal conditioning circuit and display devices are inferred in a single chip.

With the evolution of VLSI technology embedding these components in a single chip becomes somewhat an easier task.

The figure shown below represents the block diagram of a smart transducer comprising of various components:

smart transducer

Suppose, the input provided to the bimetallic strip (sensor) is temperature that generates displacement as its output. This displacement output acts as input to the potentiometer (transducer) that resultantly provides an equivalent electrical quantity of the applied input in mV.

As we know that mV is a small unit and it cannot drive the other circuits of the instrumentation system. Thus, a signal conditioning circuit is employed here.

This signal conditioning circuit is composed of an op-amp that is here operating in open loop mode (means its gain is high), that produces output in volts.

This analog amplified quantity is then fed to an analog to digital converter, that generates an equivalent digital signal at the output. This display device then shows the digital signal at the output.

Let us now move further to determine the overall sensitivity of the system:

Sensitivity of bimetallic strip,

eq1

: Δx is the change in the displacement output of the strip and

ΔT is the change in input temperature applied

Sensitivity of potentiometer,

eq2

: ΔV is the change in the output of the potentiometer

Sensitivity of op-amp,

eq3

: ΔE is the change in the output of the op-amp

Therefore, the overall sensitivity of the system,

eq4

Parameters for selecting a Transducer

There exist several criteria, in order to have proper energy conversion by any transducer. These parameters are as follows:

  • Sensitivity: The system must be sensitive to any change in the input applied to it. A sensitive system will provide accurate results at the output.
  • Accuracy: Accuracy of the system is the ability to generate proper or errorless results. The unit must be properly calibrated in order to generate accurate results.
  • Operating range: Operating range must be properly chosen in order to have errorless results. Also, the range must be selected according to the instrumentation system. Its operating range must be high so that it can operate up to a large range.
  • Ruggedness: The system must be rugged in terms of its usage. As it must be rugged enough to handle high electrical as well as mechanical intensities.
  • Linearity: The input and output characteristic of any transducer must be linear in order to have desired results.
  • Repeatability: A transducer should generate a repeatedly same output when same input is applied at its terminals.
  • Electrical parameters: Electrical parameters of the system must be properly selected. This relates in terms of SNR at the time of amplification of the signal, its frequency response and required cable length etc.

Advantages of Transducer

  1. It simplifies the amplification and attenuation of the signal.
  2. It is a low power consuming device.
  3. Due to IC fabrication technique, it can be implemented in the small chip. Thus, minimizes its size.

Key terms related to Transducer

  1. Sensitivity: The ability of a system to produce the desired change in output with the change in input is known as sensitivity. A transducer must be sufficiently sensitive in order to generate accurate results at the output.
  2. Linearity: Linearity is the property by which proportional change in output is noticed with any variation in the input applied.
  3. Repeatability: It is the property of producing the same output all the time when the applied input is same keeping the environmental conditions constant.

So, a transducer can be an electrical, mechanical, optical, acoustic, thermal or chemical transducer or it can be formed by the combination of these.

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