COMPARATORS

Introduction A device used for comparing a measurable

property or thing with a reference or standard.

It is a precision instrument employed to compare the dimension of a given component with a working standard.

It does not measure the actual dimension but indicates how much it differs from the basic dimension.

The general principle of comparator is to indicate the differences in size between the standard and the work.

Need For A Comparator 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. For inspecting newly purchased gauges. Attached with some machines, comparators can

be used as working gauges to prevent work spoilage and to maintain required tolerances at all stages of manufacturing.

In selective assembly of parts, where parts are graded in three or more groups depending upon their tolerances.

Desirable features of Comparators Robust design and construction Linear characteristics of scale High magnification Quick in Results Versatility Minimum Wear of contact Point Free from Oscillations Free from backlash Quick insertion of work piece Adjustable table Compensation from temperature effects Means to prevent damage

Principle of ComparatorsAll comparators consist of three basic features:

1)A sensing deviceWhich faithfully senses the input signal

2) A Magnifying or amplifying systemTo increase the signal to suitable magnitude. Mechanical, Optical, Pneumatic, hydraulic and electronic methods are used for this purpose.

3) A display system (usually a scale and pointer)Which utilizes the amplified signal to provide a

suitable readout.

Classification of ComparatorsMechanical comparators Optical comparatorsElectrical and Electronic comparators Pneumatic comparatorsFluid displacement comparators Projection comparatorsMulti-check comparators Automatic gauging machinesCMM

Mechanical Comparator The mechanical comparator is also called as “microcator”.

In mechanical comparators, magnification is achieved by various mechanical devices and mechanical linkages. 

Mechanical comparator is a measuring instrument.

The mechanism of this instrument is in the form of tape spring twisted, and that is positioned in the middle part.

The middle part of the mechanical comparator rotates through a fixed angle upon tension.

Working principle of a Mechanical Comparator

A mechanical comparator employees mechanical means for magnifying the small movement of the stylus.

the magnification of the small stylus movement is obtained by means of levers, gear trains, rack and pinion or a combination.

The usual magnification obtained by these comparators ranges from about 250 to 1000.

Types of Mechanical Comparators

Dial Indicator

Johansson Mikrokator

Sigma Mechanical Comparator

Dial Thickness Gauge

Reeds Type Comparator

External & internal Groove Comparator

Plate Gauge Type comparator

Dial Indicator Instrument which sense the measuring distance variations.

It has a rack & pinion Mechanism Which Convert the axial displacement of a measuring spindle into rotational movement.

The movement is then amplified & displayed on a pointer graduated scale or digital display.

It has following Types: Mechanical Dial Indicator Mechanical Dial indicator with Limit Contacts Micrometer Dial Comparator Lever Type dial Indicator

Johansson Mikrokator

Reeds Type Comparator

Advantages of Mechanical Comparator

Cheaper than all the other type of comparators

Does not require any external source of power or air supply

These comparators use a linear scale that can be easily understood.

Usually these comparators are robust and compact but are very easy to handle

These are small in size and can are portable from one place to other very easily without much difficulty

Limitations of Mechanical Comparator

Contains more number of moving parts so there develops friction which in turn reduces the accuracy

Slackness in the moving parts reduces accuracy very drastically

These have more inertia so the instrument is prone to vibrational effects

Limited range of the instrument is another drawback as the pointer moves over a fixed scale

Parallax error may also arise when proper scale is used

Optical Comparators In this comparator a small dislocation of the

measuring plunger is first amplified by a mechanical system comprising of pivoted levers.

The mechanical movement which is amplified by an optical system connecting the outcrop an image. 

The general arrangement of mechanical system causes a reflector to angle near the image 

An axis of an index estimated on a scale on the inner glass screen. 

Optical magnification offers a high degree of precision measuring.

Principle Of Optical Comparator

Types of Optical Comparator Profile (optical) Projector

Mechanical Optical Comparator

Zesis Optical Comparator

Electrical Optical Comparator

Mechanical Optical Comparator

Zesis Ultra Comparator It gives very high magnification, as it

works on double magnification principle.

Advantages of Optical Comparators

It has small number of moving parts and hence a higher accuracy.

 In the optical comparators, the scale can be made to move past a datum line and thus have high range and no parallax errors.

It has very high magnification.

Optical lever is weightless

Limitations of Optical Comparators

As the instrument has high magnification, heat from the lamp, transformer etc. may cause the setting to drift.

An electrical supply is necessary.

The apparatus is usually large and expensive.

When the scale is projected on a screen, then it is essential to use the instrument to a dark room in order to take the readings easily.

The instruments in which the scale is viewed through the eyepiece of a microscope are not convenient for continuous use.

Pneumatic Comparator In pneumatic comparators, either air

flow or air pressure is measured to give measurement deviations from a standard.

Basic Principle of Pneumatic

Types of Pneumatic Comparator Differential Back-Pressure-Type

Comparator

Solex Comparator

Velocity Differential-Type Air gauge with Barr Graph & Digital Display

Advantages of Pneumatic Comparator

The gauging member does not come into contact with the part to be measured and hence practically no wear takes place on the gauging member.

It has usually very small number of moving parts and in some cases none. Thus the accuracy is more due to less friction and less inertia.

Measuring pressure is very small and the jet of air helps in cleaning the dust, if any, from the part to be measured.

It is possible to have very high magnification. The indicating instrument can be remote from the

measuring unit. It is very suitable device for measuring diameter of

holes where the diameter is small compared with the length.

It is probably the best method for determining the ovality and taperness of the circular bores.

Limitations of Pneumatic Comparators

It requires elaborate auxiliary equipment such as accurate pressure regulator.

The scale is generally not uniform.

When indicating device is the glass tube, then high magnification is necessary in order to avoid the meniscus errors.

The apparatus is not easily portable and is rather elaborate for many industrial applications.

Different gauging heads are required for different dimensions.

Electric & Electronic Comparators

It is essentially consist of a transducer for converting a displacement into a corresponding change in current or Potential difference.

The change in displacement is calibrated in three ways: Using Inductive Principle Using Capacitive Principle Using Resistive principle

It requires elaborate auxiliary equipment such as accurate pressure regulator.

The scale is generally not uniform. When indicating device is the glass tube, then high

magnification is necessary in order to avoid the meniscus errors.

The apparatus is not easily portable and is rather elaborate for many industrial applications.

Different gauging heads are required for different dimensions.

It breaks against the W and spring and W1. If the armature is located between the coils W & W1,

then the inductance of these coils is equal. The Wheatstone bridge is stable and forms the

datum line.

Working Principle Electrical Comparators

When the work piece is located under the stylus for the measurement purpose due to the difference in datum. 

The armature, component size would either be raised up or down.

It defeats the Wheatstone bridge balance that results the unbalanced current flow. 

This current is directly adjusted into difference in size of the component which is expanded by an amplifier specified by the galvanometer.

This current is directly adjusted into difference in size of the component which is expanded by an amplifier specified by the galvanometer.

The main advantages of these comparators are no moving parts, sensitivity and accuracy over long periods.

Working Principle Electrical Comparators

Linear Variable Differential Transducer

This instrument works on inductive principle.

Advantages & Limitations of Electrical Comparators Advantages:

It has got small number of moving parts. It is possible to have high & more number of

magnification by an instrument. The mechanism carrying the core is very light &

is not sensitive to vibrations. Limitations:

Heating of the coil in measuring unit may cause zero drift and hence alter the calibration.

This is generally more expensive than others comparators.

External Power source is required.