Displacement is a basic variable whose
value is measured and involved in many other physical parameters such as
velocity, force, acceleration, torque and so on. The transducer used
for the measurement of displacement can be classified in many ways. One
of the most common classification is given below.
- Mechanical
- Pneumatic
- Electrical
- Optical
In order to obtain an electrical output,
a mixture of two or more methods is also used. For example, optical
methods using photo-detectors present the output as an electrical
quantity like voltage, current and so on. Thus, the combined mechanical
and optical method is desired.
Measurements can be made in the direct
and indirect way. In direct method, the displacement is measured
directly. But indirect methods are mostly used as the associated
variables like force, acceleration, torque, velocity and so on can be
obtained.
In electrical conversion method, the
displacement is converted to an electrical quantity like voltage or
current. This value is then recorded or displayed on a screen.
A basic displacement scheme is shown in the figure below.
Displacement Transducer
Some of the most commonly used methods are listed and explained below. Though some of these methods can be used for the measurement of other physical quantities, the electrical signals derived from such transducers always depend on a displacement parameter.
Linear Potentiometer Transducer
Linear Motion Variable Inductance Transducer
Proximity Inductance Transducer
Capacitive Transducer
Linear Voltage Differential Transformer (LVDT)
Piezoelectric Transducer
Photo-Electric Transducers
Photo-Electric Transducers
Before explaining about the different photo-electric transducers, it is important to know the basics of photo-electric effect.
Photo-Electric Effect – It can be explained as the electric current produced when a light of certain intensity strikes on a piece of metal. The energy contained in the light is transferred to the surface of the metal and thus produce a movement of electrons. This movement produces a current. This effect cannot be produced by all colors in the spectrum. A
bright red colored light will not produce a current flow in the metal. But, a dim blue light can cause current flow. Thus, the only way to decide the colors producing the photo-electric effect is through the concept of photons. The idea was brought forward by Albert Einstein, and according to him, light is made up of small packets of energy that had the behavior of particles. These packets of energy were named photons. A red light does not move the electrons, as their individual photons does not have much energy. But a blue light can move electrons as their individual photons have more energy than that of red light. The electrons thus emitted in this manner are called photo electrons.
Some of the most commonly used displacement transducers through the application of photo-electric effect are explained below.
1. Vacuum Photo-Tube as Transducer
A schematic of the vacuum photo-tube transducer is shown in the figure below.
Vaccum Photo-Tube Transducer
A displacement produced will modulate or change the intensity of the light intensity of the light incident on the photo-cathode. This changes the amount of voltage and thus, the proportional anode current is given to the resistor R. This changes the output voltage. The output voltage produced will be proportional to the amount of displacement given as input. This transducer is appropriate when there is an availability of a stable light source or an ac modulated light.
Advantages
Efficiency is fairly high.
Can take both static and dynamic measurements.
Disadvantages
Stability is achieved only for a short period.
If the light variations are subjected to high temperatures, there will be very little response.
Only suitable for applications having large displacements.
2. Photo-Diode as Transducer
The circuit for a photo-diode transducer is shown below.
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