Comparator

•It is a device that gives dimensional differences with respect to a basic dimension.

•It compares the unknown dimension of a part with some standard or master setting and those dimensional variations are amplified and measured.

•They are generally used for linear measurements, and they can be classified according to what principle they use for amplifying and recording the variations measured.

Characteristics of good Comparators

• Should be able to record variations of 0.0025 mm.

• Should be of robust design and construction, so as to withstand the

effect of ordinary usage without damaging its measuring accuracy.

• Should be of light weight so as to minimize inertia.

• The scale should be linear and straight.

• Should have maximum versatility, i.e. its design must be such that it

can be used for a wide range of operations.

Applications

• In mass production where components are to be checked at a very

fast rate.

• As laboratory standards from which working or inspection gauges

are set and correlated.

• Online inspection-Attached with some machines, they can be used

as working gauges to prevent work spoilage and to maintain

required tolerance at all stages of manufacturing.

Classification:

Electrical Comparators

• Also known as the electro-mechanical measuring system, these comparators employ an electro mechanical device which converts a mechanical displacement into electrical signal.

• The most popular electro-mechanical device used is ‘Linear variable differential transformer (LVDT)’

Electro-mechanical transducer

Signal conditioning

Readout or display

Information processing

Electricalsignal

Mechanicaldisplacement

Informationoutput

Observer

Linear variable differential transformer

Unique features of LVDT:•Due to no physical contact b/w the core and the coil its mechanical components do not wear out; as a result there is no friction and true resolution is obtained.•The small core mass and the lack of friction enhance response capabilities for dynamic measurements and thus it becomes very suitable for taking on-line machining measurements.

It works on the principle of mutual inductance

Advantages and Disadvantages

Advantages-System has a high magnification with small number of moving parts.Measurement unit can be small.Disadvantages-These comparators require an external agency to operate, i.e. an

a.c. supply. Thus the fluctuations in voltage or frequency of electric supply may affect the results.

More expensive than mechanical instruments.

Mechanical Comparators

These utilize mechanical methods of magnifying the movement of the contact plunger.

Some of the mechanical comparators are:Dial IndicatorReed type comparatorSigma comparatorJohansson MikrokratorEden Rolt Millionth comparator

Dial Indicator

Checking out of roundness of a cylindrical component using a dial indicator and a V-block

attachment

Advantages and DisadvantagesAdvantages-Usually cheaper than electrical and pneumatic.Do not require any external supply such as electricity or air.Robust and easy to handle.Disadvantages-Posses more moving parts than other types and hence the

friction is more and less accuracy.The range of the instrument is limited as the pointer moves

over a fixed scale.The mechanism of these instruments has more inertia and

this may cause the instrument to be sensitive to vibrations.

Checking out of roundness of a cylindrical component using a dial indicator and a V-block

attachment

Pneumatic Comparators• Use of controlled air as an amplifying medium.• Can be used to measure diameter, length, squareness, parallelism,

concentricity, taper, etc.Characteristics-• Independent of operator skill.• Very high amplifications are possible.• As no contact is made either with the setting gauge or the part being

measured, there is no loss of accuracy because of gauge wear. Hence, they last very long.

• Due to high pressure gauging, parts are cleansed which helps to eliminate errors due to dirt.

• It is accurate, flexible, reliable, universal and speedy device foe inspecting parts in mass production.

• Best suited for checking multiple dimensions or conditions in least possible time.

Working of a Pneumatic Comparator

• The air will now pass through the control jet at the full controlled pressure and will reach the measuring jet S.

• If this jet S cannot pass the full volume of the air from the control jet, then a pressure will tend to develop between them.

• This backpressure is instantly released through the opening in the manometer tube where it will change the height of the liquid, which indicates the amount of backpressure built up.

• The back pressure is the result of restriction at the measuring jet due to the effect of variations in the dimension of the work being checked so that the variations in the height of the liquid of the manometer are a measure of the dimension variations.

Advantages and Disadvantages• Advantages-• The gauging member does not come in contact with the part to be

measured and hence practically no wear takes place on the gauging member.

• Usually small no. of moving parts and in some cases none. Thus the accuracy is more due to less friction and less inertia.

• Jet of air helps in cleaning the dust on the work piece being measured.

• Disadvantages-• The scale is generally not uniform.• Requires elaborate auxiliary equipment such as accurate pressure

regulators.• The apparatus is not easily portable.