Selection of Displacement, Position & Proximity Sensor

SELECTION OF DISPLACEMENT, POSITION & PROXIMITY SENSOR:

Size of the displacement (mm)

Displacement type (Linear or angular)

Resolution required

Accuracy Required

Material of the object

Cost

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

PROXIMITY SENSORS

Proximity sensors are non – contact type sensor.

Types of Proximity Sensor:

Eddy current proximity sensor

Inductive proximity sensor

Pneumatic proximity sensor

Proximity switches

EDDY CURRENT PROXIMITY SENSOR:

PRINCIPLE:

When a coil is supplied with alternating current, an alternating  magnetic  field  is produced which

induces an EMF on it. If there is a metal near to this alternating magnetic field, on EMF is induced in it.

The EMF cause current to flow. This current flow is eddy current.

CONSTRUCTION & WORKING:

It has two identical coils.

One reference coil & another sensing coil which senses the magnetic current in the object. Eddy current start to flow due to AC(conducting object) close to sensor

Eddy current produce a magnetic field to oppose the magnetic field generated by sensing coil. Due to this opposition reduction flux is created. To detect 0.001mm

INDUCTIVE PROXIMITY SENSORS:

It consists of coil wound round a core.

Metal is close to coil Inductance changes occurs.

It is suitable for ferrous metals

PNEUMATIC PROXIMITY SWITCHES:

It is suitable for sensing non conducting materials

Air is allowed to escape from the front side of the sensor.

When there is no object air escapes freely.

When there is an object, the escaping air is blocked and return backed to system.

It is used to measure the range 3mm to 12mm

PROXIMITY SWITCHES:

It is used in robotics for sensing elements

It is also used in NC machines, material handling systems and assembly lines. Micro switch

Reed switch

Photo sensitive switch Mechanical switch

Micro Switch:

It is limit switch operated by levers, rollers & cams

It is switch which requires physical contact and small force to close the contacts. Example a belt conveyor.

Reed Switch:

It is a non – contact proximity switch that consists of two magnetic switch contacts enclosed in a glass tube fined with an inert gas.

When magnet is closed switch is operated. Used for high speed applications.

Photo Sensitive Devices:

It is used to sense opaque object.

Photo detector receives a beam of light produced by the LED.

Object is passed the beam gets broken or reflected when is detected.