Types of Displacement sensors

DISPLACEMENT SENSORS

Displacement sensors are con tact type sensor

Types of Displacement sensors:

Potentiometer

Strain gauge

Capacitive sensors

Linear variable differential tr ansformer

POTENTIOMETER

PRINCIPLE:

It works on variable resistance transduction principle

Linear or Rotary potentiometer is a variable resistance displacement tran sducer which uses the variable resistance transduction prin ciple in which the displacement or rotation isco nverted into a potential differencedue to the movement ofsli ding contact over a resistiveelement

CONSTRUCTION & WORKING:

A resistor with three terminals.

Two end terminal & one mid dle terminal (wiper)

Two end terminal are connected to external input voltage

One middle and one end term inal as output voltage

The slider determines the ma gnitude of the potential difference developed

Characteristics:

Resistance element         =       Precision Drawn wire with a diameter o f about 25

          to      microns, and wad over a cylindrical or a flat

          50     

          mandrel of ceramic, glass or Anodized

Aluminium. 2 mm to 500 mm in case of linear pot.

= For high resolution, w ire is made by using ceramic   (cermet)  or  conductive  plastic  film  d ue  to  low  noise  levels.

Wipers (Sliders) = Tempered phosphor bronze, beryllium copper or other  precious alloys.

Wire Material = Strong, ductile and protected from surfa ce corrosion by  enamelling or oxidation.

Materials &e alloys  of copper nickel, Nickel chromium, and silver palladium.

 = Resistivity of wire ranges from 0.4 µΩm to 13 µΩm

Resistance range = 20Ω to 200KΩ and for plastic 500Ω to 80KΩ

Accuracy = Higher  temperature  coefficient  of  resistance  than  the  wire and so temperature changes have a greater effect Accuracy.

STRAIN GAUGE:

Strain  gauges  are  passive  type  resistance  sensor  whose  electrical  resistance  change  when  it  is

stretched or compressed (mechanically strained) under the application of force.

The electrical resistance is changed due to the change in length (increases) and cross sectional area

(decreases) of the strain gauge.

This change in resistance is then usually converted into voltage by connecting one, two or four

similar gauges as an arm of a Wheatstone bridge (known as Strain Gauge Bridge) and applying excitation to

the bridge. The bridge output voltage is then a measure of strain, sensed by each strain gauge.

Unbonded Type Strain Gauges:

In unbonded type, fine wire filaments (resistance wires) are stretched around rigid and electrically insulated pins on two frames.

One frame is fixed and the other is movable.

The frames are held close with a spring loaded mechanism.

Due to the relative motion between two frames, the resistance wires are strained.

This strain is then can be detected through measurement of the change in electrical resistance since they are not cemented with the surfaces, they can be detached and reused.

Bonded Type Strain Gauges:

Bonded type strain gauges consists of resistance elements arranged in the form of a grid of fine wire, which is cemented to a thin paper sheet or very thin Bakelite sheet, and covered with a protective sheet of paper or thin Bakelite.

The paper sheet is then bonded to the surface to be strained. The gauges have a bonding material which acts an adhesive material during bonding process of a surface with the gauge element.

Classification of Bonded Type Strain Gauges:

Fine wire gauges

Metal foil gauges

Semiconductor filament type

Fine Wire Gauges:

Wire of 3 to 25 microns diameter is arranged in the form of grid consisting of parallel loops

Metal Foil Gauges:

A thin foil of metal, deposited as a grid pattern onto a plastic backing material using polyimide

Foil pattern is terminated at both ends with large metallic pads Entire gauge size 5- 15mm

Adhesive directly bonded to the gauge usually epoxy

Semiconductor Filament Type:

The gauges are produced in wafers from silicon or germanium crystals

Special impurities such as boron is added

It is mounted on an epoxy resin backing with copper on nickel leads

Filament about 0.05mm thick 0.25mm wide and 1.25 to 12mm length

CAPACITIVE SENSORS:

It is used for measuring, displacement, velocity, force etc..

Principle:

It is passive type sensors in which equal and opposite charges are generated on the plates due to voltage applied across the plate which is separated by dielectric material.

Formula:

By Changing the Distance between Two Plates:

The displacement is measured due to the change in capacitance

By Varying the Area of Overlap:

The displacement causes the area of overlap to vary

The capacitance is directly proportional to the area of the plates and varies linearly with changes in the displacement between the plates

By Varying the Dielectric Constant:

The change in capacitance can be measured due to change in dielectric constant as a result of displacement.

When the dielectric material is moved due to the displacement, the material causes the dielectric constant to vary in the region where the two electrodes are separated that results in a charge in capacitance.

Push Pull Sensor:

Push pull displacement sensor is used to overcome the non-linearity error.

The sensor consists of three plates with the upper pair forming one capacitor and the lower pair forming another capacitor.

The displacement moves central plate between the two other plates.

If the central plate moves downwards.

The plate separation of the upper capacitor increases and the separation of the lower one decreases.

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER:

It consists of three symmetrically spaced coils.

The centre coil is primary coil and other two are secondary coil

Secondary coils are connected in series opposition and equally positioned with respect to primary coil

The output voltage is proportional to the displacement of the core from null position