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Introduction to Comparator : 

• Comparators are one form of the linear measurement device.
• It is quick and more convenient for checking a larger number of identical dimensions.
• Comparators normally will not show the actual dimensions of the workpiece. They will show only the deviation in size.
• This cannot be used as an absolute measuring device but can only compare two dimensions.

Principle of comparator

The general principle of the comparator is to indicate the difference in size between the standard and work being measured by means of some pointer on the scale with sufficient magnification.

Comparators can give precision measurements, with consistent accuracy by eliminating human error. They are employed to find out, by how much the dimensions of the given component differ from that of a known datum. If the indicated difference is small, a suitable magnification device is selected to obtain the desired accuracy of measurements. It is an indirect type of instrument and used for linear measurement. If the dimension is less or greater, than the standard, then the difference will be shown on the dial. It gives only the difference between the actual and standard dimension of the workpiece. To check the height of the job H2 ,with the standard job of height H1

Initially, the comparator is adjusted to zero on its dial with a standard job in position as shown in Figure(a). The reading H1 is taken with the help of a plunger.

Then the standard job is replaced by the work-piece to be checked and the reading H2 is taken. If H1and H2 are different, then the change in the dimension will be shown on the dial of the comparator. Thus the difference is then magnified 1000 to 3000 X to get a clear variation in the standard and actual job.

Definition of Comparators :

In short, Comparator is a device which

(1) Picks up small variations in dimensions.

(2) Magnifies it.

(3) Displays it by using indicating devices, by which comparison can be made with some standard value.

Need for a comparator

• A comparator is used in mass production to inspect the components to close tolerance with a high degree of precision and speed
• Use of line standards such as vernier caliper and micrometer required considerable skill
• Many dimensions can be checked in a very short time.

Mechanical Comparators : 

Working Principle of Mechanical comparators:

JOHANSSON “MIKROKATOR:

• Johansson “Mikrokator‟ is a mechanical comparator having a magnification of about 5000.
• It works on the principle of a button spinning on a loop of string.
• The instrument consists of a plunger, twisted thin metal strip, spring elbow, pointer, etc.
• A very light glass pointer is attached to the Centre of the twisted strip.
• The two halves of the strip from the Centre are twisted in opposite directions, so that any pull in the strip causes the Centre and hence the pointer to rotate.

• One end of the strip is fixed to an adjustable cantilever strip and the other end is attached to an arm of spring elbow.
• The measuring plunger is mounted on a flexible diaphragm. Its inner end is attached to the other arm of spring elbow.
• Thus the vertical movement of the plunger transmitted to the metal strip through the elbow.
• Any vertical movements of the plunger make it to twist or untwist.
• This will cause the pointer to rotate by an amount proportional to the change in the length of the strip
• Magnification of the instrument depends upon the length, width, and a number of twists of the twisted strip.
• It can vary by changing the length of the strip with screws provided on adjustable cantilever strip.

SIGMA COMPARATOR:

• This is a mechanical comparator providing magnification in 300 to5000.
• It consists of a plunger mounted on two steel strings (slit diaphragms). This provides a frictionless linear movement for the plunger.
• The plunger carries a knife-edge, which bears upon the face of the moving block of a cross-strip hinge.
• The cross-strip hinge is formed by pieces of flat steel springs arrange at right angles and is a very efficient pivot for smaller angular movements.
• The moving block carries light metal Y-forked arms. A thin phosphor bronze ribbon is fastened to the ends of the forked arms and wrapped around a small drum, mounted on a spindle carrying the pointer.
• Any vertical displacement of measuring plunger and hence that of the knife-edge makes the moving block of the cross-strip hinge to pivot.

• This causes the rotation of the Y-arms. The metallic band attached to the arms makes the driving drum and hence the pointer to rotate.
• The ratio of the effective length (L) of the arm and the distance (X) of the knife edge from the pivot gives the first stage magnification
• The ratio of pointer length (R) and radius r of the driving drum gives second stage magnification of the instrument.
• The total magnification of the instrument is thus (L/X× R/r).
• The magnification of the instrument can be varied by changing the distance (X) of knife-edge by tightening or slackening of the adjusting screws.

Advantages of Mechanical Comparators

• They are cheaper compared to other amplifying devices.
• Do not require electricity or air and such the variations in the outside sources do not affect the accuracy.
• They have a linear scale robust and easy to handle.
• It is suitable for ordinary workshop and also easily portable.

Disadvantages of Mechanical Comparators

• They have more moving linkages, due to which friction is more and accuracy is low.
• Any wear, dimensional faults in the mechanical devices used will also be magnified.
• The range of the instrument is limited because the pointer moves over a fixed scale

OPTICAL COMPARATOR

Introduction

There are no pure optical comparators but the instruments classed as optical comparators obtain large magnification in these instruments contributes principles through mechanical magnification
All-optical comparators are capable of giving a high degree of measuring precision.

Working principle of Optical comparators:

The operating principle of this type, of the comparator, is based on the laws of light reflection and refraction. The magnification system depends on the tilting of a mirror, deflects a beam of light, thus providing an optical lever.

Principle of the optical lever

If a beam of light AC is directed on to a mirror as shown in the figure, it will be reflected onto the screen at O as a dot.The angle Ɵ at which the beam strikes the mirror is equal to the angle Ɵ at which the beam is reflected from the mirror. When the plunger moves upwards vertically, causing the mirror to tilt by an angle „α‟ as shown in the figure.

Then the reflected light beam moves through an angle “2α” which is twice the angle of tilt produced by the plunger movement. The illuminated dot moves to “B” thus a linear movement “h” of the plunger produces a movement of the dot equivalent to the distance OB on the screen. It also clear that as the distance (OC) of the screen from tilting mirror increases, greater will be the magnification and is called the principle of enlarge image.

Zeiss ultra- Optimeter

• The optical system of this instrument involves a double reflection of light and thus gives a higher degree of magnification.
• A lamp sends light rays through the green filter to filter all rays except green light, which causes less fatigue to the eye.
• The green light then passes through a condenser which via an index mark projects it on to a movable mirror M1. It is then reflected to another fixed mirror M2 and back again to the first movable mirror.
• The objective lens brings the reflected beam from the movable mirror to a focus at a transparent graticule containing a precise scale that is viewed by eye-piece.
• The projected image of the index line on the graticule can be adjusted by means of a screw in order to set the initial zero reading.
• When correctly adjusted, the image of the index line is seen against that of the graticule scale.
• The end of the contact plunger rests against the other end of the first movable mirror so that any vertical movement of the plunger will tilt the mirror.
• This causes a shift in the position of the reflected index line on the eyepiece graticule scale, which in turn measures the displacement of the plunger.

Advantages of optical comparators : 

• Optical comparators have few moving linkages and hence are not subjected to friction, wear, and tear.
• High accuracy of the measurement.
• The magnification is usually high.

Disadvantages of optical comparators 

• An electrical supply is necessary to operate these types of comparators.
• The size of these comparators are highly, and costly.
• Since the scale is projected on a screen, it is essential to use these instruments in a dark room in order to take the readings easily.

Pneumatic Comparators :

• These instruments utilize the variations in the air pressure or velocity as an amplifying medium.
• A jet or jets of air are applied to the surface being measured and the variations in the backpressure or velocity of air caused due to variations in loused to amplify the output signals.
• Based on the physical phenomena, the pneumatic comparators are classified into two types.
  Flow or velocity type
  Backpressure type.

Solex Pneumatic Comparator:

• This instrument was first commercially introduced by Solex Air. Gauges Ltd. It uses a water manometer for the indication of backpressure.
• It consists of a vertical metal cylinder filled with water up to a certain level and a dip tube immersed into it up to a depth corresponding to the air pressure required.

• A calibrated manometer tube is connected between the cylinder and control orifice as shown in the fig.
• The pressure of the air supplied is higher than the desired pressure, some air will bubble out from the bottom of the dip tube and air moving to the control volume will be at the desired constant pressure.
• The constant pressure air then passes through the control orifice and escapes from the measuring jets.
• When there is no restriction to the escape of air, the level of water in the manometer tube will coincide with that in the cylinder.
• But, if there is a restriction to the escape of air through the jets, back pressure will be induced in the circuit and level of water in the manometer tube will fall.
• The restriction to the escape of air depends upon the variations in the dimensions to be measured.
• Thus the variations in the dimensions to be measured are converted into corresponding pressure variations, which can be read from the calibrated scale provided with the manometer.

Advantages of Pneumatic Comparators  :

1. Very high magnification
2. Less friction, wear, and inertia
3. Less measuring pressure
4. Determines ovality and taper of circular bores

Disadvantages of pneumatic comparators :

1. Scale is generally not uniform
2. Requires compressor and accurate pressure regulator
3. Nonportable
4. Less sensitivity

Difference between Mechanical Comparator and Pneumatic Comparators

Electrical comparator

Working principle of Electrical comparators:

These instruments are based on the theory of Wheatstone A.C. Bridge. When the bridge is electrically balanced, no current will flow through the galvanometer connected to the bridge, and the pointer will not deflect. Any upset in the inductances of the arms will produce unbalance and cause deflection of the pointer.

Introduction

• Electrical comparators are also called as electromechanical measuring systems.
• This is because they use an electro-mechanical device that converts mechanical displacement into an electrical signal.

LVDT

Linear Variable Differential Transformer (LVDT) is the most popular electro-mechanical device used to convert mechanical displacement into an electrical signal. It is used to measure displacement.

Read More about LVDT: LVDT – Diagram, working, Characteristics, Advantages, Application

Advantage of electrical comparator

• A small number of moving parts.
• Possible to have very high magnification.
• Used for a variety of ranges.
• Remote operation can also be done.

The disadvantage of electrical comparator

• Required an external agency to operate i.e., A.C .power supply. 10
• Heating coils may cause zero drift.
• More expansive than the mechanical comparator.

Difference between Gauges and Comparators : 

Comparison between gauges and comparators are as follows, 

Difference between Measuring Instruments and Comparators :

Comparison between measuring instrument and comparators are as follows,

Applications of Comparators : 

Comparators are used for Following purposes : 

1) Comparators are used as laboratory standards.
2) Used as working gauges to prevent work spoilage and to maintain required tolerance at all
important stages of manufacture.
3) Used as final inspection gauges.
4) Used as a receiving inspection gauge for checking parts received from outside sources.
5) For checking newly purchase gauges.

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