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Wednesday, February 1, 2012

Displacement Measurements


Displacement Measurement
These types of transducers are used to measure small displacements. Normally there are two types of Displacement Transducers: contact and noncontact types. Besides the measurement principles can be classified into two categories: electrical sensing and optical sensing. In electrical sensing, passive electrical sensors are used variation of either inductance or capacitance with displacement is measured. On the other hand the optical method mainly works on the principle of intensity variation of light with distance. Interferometric technique is also used for measurement of very small displacement in order of nanometers. But this technique is more suitable for laboratory purpose, not very useful for industrial applications.
Here I am explaining about contact type displacement measuring equipments. The commonly used types are:
Resistive Types:
·        Potentiometer
Inductive Types:
·        LVDT
·        RVDT
·        Resolver
Capacitive Type:
·        Rotary Type Capacitive transducer
·        Linear Type Capacitive Transducer

Potentiometer
Potentiometers are simplest type of displacement sensors. They can be used for linear as well as angular displacement measurement, as shown in Fig. 1. They are the resistive type of transducers and the output voltage is proportional to the displacement and is given by:
E0= (Xi/Xt)E ,
Where Xi is the input displacement, Xt is the total displacement and E is the supply voltage.
The major problem with potentiometers is the contact problem resulting out of wear and tear between the moving and the fixed parts. Because of friction and moving parts, we can’t expect an accurate measurement. But for simple applications potentiometers are very useful and are of low cost.

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