We know that the generalized measuring system consists of three functional elements(transducer is first stage):
- primary detector-transducer stage,
- intermediate modifying stage, and
- the output or terminating stage.
Each stage performs certain functions so that the value of the physical variable to be measured (measurand) is displayed as output
What is Transducer?
A transducer is a device that converts one form of energy into another form.
The transducer, which may be electrical, mechanical, optical, magnetic, piezoelectric, etc., converts the sensed information into a more convenient form.
The detecting or sensing element of a measuring system first makes contact with the quantity to be measured, and the sensed information is immediately transduced into an analogous form.
Classification of Transducers
Analog and Digital Transducers
In the case of analogue transducers, the input quantity is converted into an analogue output, which is a continuous function of time.
LVDT, strain gauge, thermocouple, and thermistor are some examples of analogue transducers.
If a transducer converts the input quantity into an electrical signal that is in the form of pulses, as output, it is called a digital transducer.
These pulses are not continuous functions of time but are discrete in nature. Examples are shaft encoders, linear displacement transducers.
Terminating Devices
Meter Indicators
Meter indicators can be categorized as follows:
- Simple D’Arsonval type to measure current or voltage
- Volt–Ohm milliammeters or multimeters
- Vacuum tube voltmeters
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Simple D’Arsonval Type to Measure Current or Voltage
It essentially consists of components like a permanent
magnet, coil, hairsprings, pointer, and scale. The coil assembly is mounted on a shaft, which in turn is hinged on a hair spring.
To concentrate the magnetic fields, initially, the iron core is placed inside the coil.
The curved pole pieces are then attached to the magnet to ascertain if the torque increases as the current increases.
This means that electromagnetic energy is used to move the meter.
Volt-Ohm Milliammetersor Multimeters
The typical volt–ohm milliammeter employs switching provisions for connecting multipliers, shunt resistors, and rectifier circuits.
The current flowing through the resistor can easily be determined by switching over to the ohm-meter function, wherein the leads can be connected to the unknown resistance that causes the meter movement.
The advantage is that direct measurement is possible since the current flow indication is calibrated in terms of resistance
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Vacuum Tube Voltmeters
The vacuum tube voltmeter, popularly known as VTVM, is basically used to measure voltage.
To control the grid of the tube, proper multiplier resistors are employed through which the AC or DC input is given to the meter.
The input resistance of a VTVM is usually very high, when compared to that of a simple meter, due to high grid circuit resistance. This minimizes the loading of the signal source.
Cathode Ray Oscilloscope
A CRO is a voltage-sensitive device that is used to view, measure, and analyse waveforms.
Its advantage is that a beam of electrons with low inertia strikes the fluorescent screen, generating an image that can rapidly change with varying voltage inputs to the system.
The cathode ray tube (CRT) is the basic functional unit of CRO. The electron gun assembly comprises a heater, a cathode, a control grid, and accelerating anodes. When a cathode is heated, electrons are generated.
The accelerating anodes, which are positively charged, provide the necessary striking velocity to the emitted electron stream.
The electron beam, after gaining necessary acceleration, passes through horizontal and vertical deflection plates, which provide the basic movements in the X and Y directions.
XY Plotters
The XY plotters generate Cartesian graphs, which are obtained by two DC inputs, in X and in the Y-axis. It consists of two self-balancing potentiometers, one to control the position of the paper and others to control the position of the pen.
In XY plotters, the output is generated by plotting one emf as a function of the other emf.
Further, the emf, which is the output signal of transducers, may be a value of displacement, force, pressure, strain, or any other parameter.
