1. What is Comparator ?
A comparator is a precision instrument used to compare the dimensions of a workpiece with a standard reference. Unlike direct measuring instruments (like vernier calipers or micrometers), comparators do not give the actual dimension but indicate the deviation from the standard size.
They are widely used in mass production and quality control where high accuracy and repeatability are required.
Table of Contents
1.2. Principle of Operation
Comparators work on the principle of:
- Magnification of small displacement
When a workpiece is placed under the measuring probe:
- Any small deviation from the standard size
- Is amplified mechanically, optically, or electronically
- And displayed on a scale or digital display
1.3. Characteristics of a Good Comparator
- High magnification
- High sensitivity
- Quick response
- Stability and repeatability
- Low friction and backlash
- Easy to read scale
2. Types of Comparators
2.1 Mechanical Comparators
A mechanical comparator is a precision instrument used to compare the dimensions of a workpiece against a known standard (like a slip gauge). Unlike measuring instruments that give an absolute rating, a comparator shows the deviation from the target size with high magnification.
Example: Dial Indicator
Working Principle:
The basic principle involves a plunger that contacts the workpiece. Any linear displacement of this plunger is magnified through a series of mechanical linkages (gears, levers, or racks and pinions) and transferred to a pointer that moves across a graduated scale.
Features
- Plunger movement is amplified by gears
- Reading shown on circular dial
Parts of a Dial Indicator & Their Functions:
- Plunger: Moves in and out in linear direction with the contact point.
- Dial Face: Graduated scale showing the measurement.
- Bezel: Holds the dial face and protects it; can be rotated to set reference.
- Main Pointer (Hand): Indicates the measurement on the dial.
- Revolution Counter: Counts the number of full rotations of the pointer.
- Locking Screw: Locks the plunger in any position.
- Stem: Supports and guides the plunger and contact point.
- Contact Point: Touches the work surface whose movement is to be measured.
Advantages of Dial Indicator
- Simple and robust
- No external power required
Disadvantages of Dial Indicator
- Friction and wear affect accuracy
2.2 Mechanical-Optical Comparators
Example: Sigma Comparator
A Sigma Comparator is a highly sensitive mechanical comparator used in metrology to measure very small linear dimensions or deviations from a standard size. It works on a mechanical magnification principle, giving accurate results without electrical components.
Mechanical Construction & Working
The instrument consists of a plunger mounted on two flat diaphragms to ensure purely linear movement. The core magnification happens in two distinct stages:
- The Knife-Edge & Block: The plunger is attached to a knife-edge. As the plunger moves up, this knife-edge pushes against a “movable block.”
- The Cross-Strip Hinge: This block is attached to a fixed member by a cross-strip hinge (two thin metallic strips crossing at right angles). When the knife-edge pushes the block, the block pivots around the hinge.
- The Y-Arm & Driving Drum: A long “Y-shaped” arm is attached to this pivoting block. The end of the Y-arm is connected to a thin phosphor-bronze band wrapped around a small driving drum (the pointer spindle).
- Final Magnification: The movement of the Y-arm pulls the band, rotating the drum and moving the pointer across a graduated scale.
Magnification Calculation
The total magnification is the product of the two stages of the leverage system:
- First Stage: The ratio of the length of the Y-arm to the distance from the hinge to the knife-edge contact point.
- Second Stage: The ratio of the pointer length to the radius of the driving drum.
Typically, Sigma comparators offer magnifications ranging from 300x to 5,000x.
Working
- Small movement of probe tilts a mirror
- Light beam is reflected and magnified on a scale
Advantages
- High magnification
- Less mechanical wear
2.3 Optical Comparators (Profile Projector)
A Profile Projector (also called an Optical Comparator) is a non-contact measuring instrument used to inspect and measure the shape, size, and profile of small components by projecting a magnified image onto a screen. Use optical projection to enlarge the image of a component.
Working Principle
- A light source projects light onto the workpiece.
- The object blocks the light, creating a shadow or silhouette.
- This image is magnified using lenses.
- The enlarged profile is displayed on a screen.
- Measurements are taken by comparing the image with a standard chart or digital scale.
Main Parts
- Light Source → Produces illumination
- Condenser Lens → Focuses light on the object
- Work Table (Stage) → Holds the component
- Projection Lens → Magnifies the image
- Mirror System → Directs light to the screen
- Screen (Glass with Scale) → Displays enlarged profile
- Digital Readout / Vernier → Measures dimensions
- Base & Frame → Provides stability
Applications
- Checking profiles, threads, gears
Advantages
- No contact measurement
- High accuracy
2.4 Electrical Comparators
An electrical comparator is a precision measuring instrument that converts small linear displacements into electrical signals, which are then amplified and displayed for accurate measurement.
Working Principle
- The plunger touches the workpiece and moves slightly.
- This displacement is converted into an electrical signal (voltage/current).
- The signal is amplified electronically.
- The output is shown on a meter, digital display, or oscilloscope.
Main Components
- Transducer (e.g., LVDT): Converts mechanical movement into electrical signal.
- Amplifier: Increases the strength of the signal.
- Display Unit: Shows the measured deviation (analog meter or digital readout).
- Power Supply: Provides necessary electrical energy.
- Plunger / Probe: Senses the dimensional variation.
Description
- Use electrical transducers (e.g., LVDT)
Advantages
- Very high sensitivity
- Fast response
Disadvantages
- Requires power supply
2.5 Electronic Comparators
Electronic comparator is a advanced version of electrical comparator. Electronic comparator gives high accuracy with high sensitivity.
Features
- Digital display
- High accuracy and reliability
Advantages
- No parallax error
- Easy data recording
2.6 Pneumatic Comparators
Description
- Use air pressure variation for measurement
Working
- Air is passed through a nozzle
- Gap between nozzle and workpiece affects pressure
Advantages
- No mechanical contact
- Suitable for delicate parts
Disadvantages
- Sensitive to air pressure variations
3. Advantages of Comparators
- High accuracy and precision
- Suitable for mass production
- Reduces human error
- Quick inspection
- Detects small deviations
4. Disadvantages of Comparators
- Do not give absolute measurement
- Require standard reference
- Calibration needed
- Some types are costly
5. Applications of Comparators
- Inspection of manufactured parts
- Quality control in industries
- Checking tolerances
- Gauge calibration
- Mass production inspection
8. Difference: Comparator vs Measuring Instrument
| Feature | Comparator | Measuring Instrument |
|---|---|---|
| Output | Deviation only | Actual dimension |
| Accuracy | Very high | Moderate |
| Use | Comparison | Direct measurement |
| Example | Dial indicator | Vernier caliper |
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Related Resource
- Standard of Measurements
- Limit Gauges