Essential Metrology and Measurement Systems Explained

1. Instruments for Angular Measurement

The following four instruments are commonly used for angular measurement:

Bevel Protractor: Used for measuring angles between two surfaces with a vernier scale.
Sine Bar: Used in conjunction with slip gauges to measure angles accurately based on trigonometry.
Angle Gauges: Hardened steel blocks of precise angles used as reference standards.
Clinometer: A specialized version of a spirit level used for measuring angles of inclination or slope.

Principle: It works on the trigonometric sine principle. In a right-angled triangle, sin θ = Opposite / Hypotenuse.
Procedure:
1. Place the sine bar on a high-precision surface plate.
2. Place the component on the upper surface of the sine bar.
3. Place a stack of slip gauges under one roller of the sine bar until the top surface of the component becomes perfectly horizontal (verified using a dial indicator).
4. Note the height of the slip gauges (h) and the distance between the roller centers (L, usually 100mm or 200mm).
Formula: Calculate the angle using: θ = sin⁻¹(h / L)

High Precision and Accuracy: Eliminates human error and provides sub-micron level measurement.
Reduced Inspection Time: Faster than manual methods, especially for complex 3D shapes.
Versatility: Can measure a wide variety of dimensions (linear, angular, geometric) in one setup.
Automation: Can be programmed for repetitive tasks and provides digital reports.

The most common method is using a Gear Tooth Vernier Caliper.

Principle: It measures the chordal thickness of the gear tooth at the pitch circle.
Procedure:
1. The vertical slide of the caliper is set to the chordal addendum distance.
2. The caliper is placed over the gear tooth so that the tongue rests on the top of the tooth.
3. The horizontal jaws are moved to touch both sides of the tooth at the pitch line.
4. The reading on the horizontal scale gives the chordal thickness of the gear tooth.

Construction: Consists of one primary winding (P) and two secondary windings (S1, S2) wound on a hollow cylinder, with a movable soft iron core inside.
Working:
1. When an AC supply is given to the primary winding, it induces a voltage in both secondary windings.
2. Null Position: When the core is central, induced voltages E1 and E2 are equal, and the output (Eout = E1 – E2) is zero.
3. Displacement: When the core moves toward S1, E1 > E2, resulting in a positive output. When it moves toward S2, E2 > E1, resulting in a negative output.
4. The output voltage is proportional to the linear displacement of the core.

A generalized measurement system consists of three main functional elements:

Primary Sensing Element: Receives energy from the measured medium and produces an output depending on the measured quantity (e.g., a diaphragm sensing pressure).
Variable Conversion/Manipulation Element: Converts the sensed signal into a more suitable form (e.g., converting mechanical displacement into an electrical signal) or amplifies it.
Data Transmission & Presentation Element: Transmits the signal to a remote location and presents it in a human-readable format, such as a digital display or printed chart.

Principle: Similar to LVDT, but used for measuring angular displacement. It consists of a primary winding and two symmetrical secondary windings.
Working:
1. A cam-shaped magnetic core is rotated by the input shaft.
2. When the core is in the null position, the flux linkage with both secondary windings is equal, resulting in zero output voltage.
3. As the core rotates, the inductance of one secondary winding increases while the other decreases.
4. The differential output voltage (Eout = E1 – E2) is proportional to the angular position of the shaft.

Non-contact Measurement: Does not require physical contact with the rotating shaft, adding no load to the machine.
Versatility: Can measure the speed of parts that are difficult to access.
Visual Inspection: Allows for the observation of periodic motion as if they were stationary.
Portability: Modern electronic stroboscopes are compact and easy to use in various industrial environments.

Strain Gauge Load Cell: Uses the change in electrical resistance of a foil.
Hydraulic Load Cell: Operates on the principle of fluid pressure change.
Pneumatic Load Cell: Uses air pressure to balance the applied force.
Capacitive Load Cell: Measures the change in capacitance between two plates under load.

Select two wires of identical diameter (d) such that they touch the flanks of the thread at the pitch line.
Place the wires in the thread grooves on opposite sides of the screw.
Measure the distance over the wires (M) using a micrometer.
Calculation: The effective diameter (E) is calculated using: E = M – d(1 + csc(α/2)) + (P/2)cot(α/2), where P is pitch and α is thread angle.

Principle: Resistance Temperature Detector (RTD) works on the principle that the electrical resistance of a metal increases linearly with an increase in temperature (Rt = R0[1 + α ΔT]).
Working:
1. A fine wire (usually Platinum) is wound around a ceramic or glass core.
2. As the temperature increases, the vibration of atoms in the metal increases, hindering electron flow and raising resistance.
3. This change in resistance is measured using a Wheatstone bridge circuit and converted into a temperature reading.

Definition: A load cell is a transducer that converts a mechanical force or weight into a measurable electrical signal.

Applications:

Industrial Weighing: Used in platform scales and truck weighbridges.
Material Testing: Used in Universal Testing Machines (UTM) to measure tensile or compressive force.
Aerospace: Measuring thrust in engine testing.
Process Control: Monitoring the weight of materials in hoppers and silos.
Safety: Crane scales for heavy lifting and in-motion weighing of rail wagons.