Chapter 6 Comparators 1
Chapter 6
Comparators
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All measurements require the unknown quantity to be compared with a known quantity, called a standard.
There are certain devices in which the standards are separated from the instrument. It compares the unknown length with the standard. Such measurement is known as comparison measurement and the instrument, which provides such comparison, is called a comparator.
Comparators are generally used for linear measurements, and various comparators available differ basically in the methods employed for amplifying and recording the variations measured.
Introduction
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Any instrument used to compare size of work piece to known standard
High degree of accuracy and precision
The scale should be linear and have a wide range
High amplification
Good resolution
Comparator should be versatile
Functional Requirements
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Mechanical comparators
Mechanical‐optical comparator
Electrical and electronic comparators
Pneumatic comparators
Other types such as projection comparators, multi‐check comparators, etc.
Classification of Comparators
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Mechanical Comparators
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Dial Indicators
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Dial Indicators
It is primarily used to compare work‐pieces against a master
It consists of a body with a circular graduated dial, a contact point connected to a gear train and an indicating hand, which directly indicates the linear displacement of the contact point.
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With the plunger set to approximately mid‐position, the face dial is set to read zero.
Dial Indicators
From this zero reference point, two rules apply:
• As the plunger moves out of the case, the needle travels counter‐clockwise...giving a NEGATIVE reading.
• As the plunger moves into the case, the needle travels clockwise...giving a POSITIVE reading.
Johansson Mikrokator
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A light pointer made of glass fixed to a thin twisted metal strip
While one end of the strip is fixed to an adjustable cantilever link, the other end is anchored to a bell crank lever
Any linear motion of the plunger will result in a movement of the bell crank lever, which exerts either a push or pull force on the metal strip.
Accordingly the glass pointer will rotate either clockwise or anti‐clockwise depending on the direction of plunger movement
A calibrated scale is employed with the pointer, so that any axial movement of the plunger can be conveniently recorded.
Johansson Mikrokator
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Mechanical Optical Comparators
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As the name of the comparator itself suggests, this is a part mechanical and part optical comparator. Small displacements of a measuring plunger are initially amplified by a lever mechanism pivoted about a point as shown in figure.
Mechanical Optical Comparator
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Optical projector is a versatile comparator, which is widely used for inspection purpose.
It is especially used in tool room applications.
It projects a two‐dimensional magnified image of the work‐piece on to a viewing screen to facilitate measurement.
Optical Projector
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Profile optical projector comparator
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Electrical Comparators
They are in widespread use because of their instantaneous response and convenience to amplify the input.
Electronic comparator, in particular, can achieve exceptionally high magnification.
The mechanism carrying the pointer is very light and not sensitive to vibrations.
As the instrument is usually operated on A.C. supply, the cyclic vibration substantially reduces errors due to sliding friction.
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Disadvantages Heating of coils in the measuring unit may cause zero drift and alter
the calibration. This is usually more expensive than mechanical instrument.
The plunger is the sensing element, the movement of which displaces an armature inside a pair of coils. Movement of the armature causes change in inductance in the two coils, resulting in a net change in inductance.
This change causes imbalance in the bridge circuit, resulting in an output.
The output display device, whether it is analog or digital, is calibrated to show the readings in units of length, that is, linear displacement.
Elements of Electrical Comparator
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Electrical ComparatorsAURA MAKE ELECTRONIC COMPARATOR
COUNTER: A single line Display ‐Counter unit is provided which displays Signal from probe in digital form. Resolutionof this unit is Selectable ‐ 0.0001 m.m, 0.001 mm.
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Electrical Comparators
Electronic Gage
An electronic gage for measuring bore diameter. The measuring head is equipped with three carbide‐tipped steel pins for wear resistance. The LED display reads 29.158 mm. Source: Courtesy of TESA SA.
The movement at the probe tip actuates inductance transducer which is supplied with an a.c. source from the oscillator.
The transducer converts this movement into an electrical signal which is then amplified and fed via an oscillator to the demodulator.
The current in D.C. form, then passes to the meter and the probe tip movement is displayed as a linear measurement over a circular scale.
Sigma Electronic Comparator
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Sigma Electronic Comparator
Air Gauges or Pneumatic Comparators
Used to compare work piece dimensions with master gauge by means of air pressure or flow.
Pneumatic gauge provide gauging of several features at once, it has become essential part of production inspection in the industry.
It is possible to gauge length, diameter, parallelism, concentricity, etc using a simple set up.
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Two types:1. Flow ( Indicates air velocity)2. Pressure (indicates air pressure in system)
Compressed air with a pressure in the range 1.5 to 2 bars is passed through a tapered glass column, which contains a small metal float.
The air then passes through a rubber or plastic hose and exits to the atmosphere through the orifice in the gauging head.
Since the gauging head is inserted inside the work part, which is being inspected, there is a small clearance between the gauging head and the work part.
This restricts the flow of air, thereby changing the position of the float inside the tapered glass column.
Flow Air Gauge
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Rate of flow proportional to clearance Gage set to master Work piece larger than
hole size, float rises Smaller, float falls
Flow Air Gauge
Source: Mahr, S 1840 PE
This system uses a two orifice arrangement as shown in figure above. While the orifice O1 is called the control orifice, the orifice O2 is referred to as the measuring orifice.
The measuring head gets compressed air supply at a constant pressure ‘P’, which is called the source pressure
Depending upon the gap d, the back pressure Pb changes, thereby providing a means for measuring dimension ‘d’.
Back Pressure Gauge
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In the set up shown in figure above, the back pressure is let into a bourdon tube, which undergone deflection depending on the magnitude of air pressure. This deflection of the bourdon tube is amplified by lever and gear arrangement and indicated on a dial.
Back Pressure Gauge
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Back Pressure Gauge
Mahr Federal Micro‐Dimensionair
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• Holes may be checked for taper, out-of-roundness, concentricity, and irregularity
• Gage does not touch workpiece• Gaging heads last longer than fixed gages• More than one diameter may be checked at same time.
Advantages of Air Gauges
Pneumatic gauging is one of the widely used methods for inspection of holes.
The gauging elements can be adapted to measure nearly any feature of the hole including diameter, roundness, squareness and straightness.
Applications of Pneumatic Comparators
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