Precision Measurement Techniques and Instruments
Interferometry
Interferometry is a measurement technique using light wave interference to measure minute differences in length, displacement, or surface irregularities. It relies on the wave nature of light to produce interference patterns, allowing for extremely precise measurements.
Laser Interferometer
Principle: Laser interferometers use coherent laser light to create an interference pattern, detecting displacement or distance changes with high accuracy.
Working:
- A laser beam is split into two using a beam splitter.
- One beam reflects off a reference mirror, and the other off a movable mirror (test object).
- The beams recombine, creating interference patterns based on the distance traveled.
- By counting the interference fringes, minute displacements are measured.
Generalized Measurement System
A Generalized Measurement System converts a physical quantity into a readable output.
Block Diagram
Primary Sensing Element → Transducer Stage → Signal Conditioning → Output Stage → Feedback/Control System (optional)
Function with Examples
Temperature Measurement: A thermocouple (primary sensor) detects temperature and produces a voltage. Signal conditioning amplifies the voltage, and the output stage displays it in degrees.
Pressure Measurement: A Bourdon tube (primary sensor) converts pressure into mechanical displacement, transduced into an electrical signal, conditioned, and displayed on a gauge.
Measurement System Inputs
Desired Input: The specific quantity a measuring instrument is intended to measure (e.g., temperature in a thermometer).
Modifying Input: Inputs that alter the relationship between the desired input and output (e.g., ambient temperature affecting a thermometer). Modifying inputs typically cause measurement errors.
Interfering Input: Inputs unrelated to the desired measurement but affect the output, leading to erroneous readings (e.g., vibrations affecting a pressure sensor).
Methods to Minimize Effects
- Temperature Compensation: For thermometers, use temperature-insensitive materials or digital compensations.
- Shielding: Shield the instrument from electromagnetic interference.
- Environmental Isolation: Isolate the system from vibrations or environmental factors using damping or protective housings.
LVDT (Linear Variable Differential Transformer)
An LVDT is an electromechanical sensor used to measure linear displacement.
Working Principle
Construction: An LVDT consists of a primary coil (center) and two secondary coils (placed symmetrically around the primary), with a movable magnetic core.
Operation: An AC voltage is applied to the primary coil, inducing voltages in the secondary coils. When the core is centered, the voltages in the secondaries are equal, and their difference is zero. When the core moves, the induced voltages in the secondaries become unequal, and the differential voltage indicates displacement direction and magnitude.
Control Systems
Open Loop
No feedback; the output is not compared to the desired input. Example: Electric toaster (heats for a set time without adjusting based on toast color).
Closed Loop
Uses feedback; the output is compared to the desired input and adjusts accordingly. Example: Thermostat-controlled heating system (adjusts temperature based on current reading).
Measurement Characteristics
Sensitivity: The ratio of the change in output to the change in input, showing how responsive a measuring instrument is.
Precision: The consistency or repeatability of measurements, showing how close multiple measurements are to each other under the same conditions.
Threshold: The minimum input quantity required to produce a measurable output.
Hysteresis: The difference in output when approaching the same measurement point from different directions, due to lag in response.
Resolution: The smallest change in input that a measuring instrument can detect or display.
Accuracy: The degree of closeness between a measured value and the true or accepted standard value.
Span: The difference between the maximum and minimum values an instrument can measure.
Range: The interval over which a measuring instrument can measure, from its minimum to maximum measurable value.
Specialized Measurement Instruments
Floating Carriage Micrometer
Measures the pitch diameter of screw threads with high accuracy. It uses a micrometer attached to a floating carriage to align precisely with the screw axis, ensuring consistent and repeatable measurements.
Repeatability and Reproducibility
Repeatability: The consistency of measurements under the same conditions and setup.
Reproducibility: The variation in measurements when different operators or setups are used. Together, they indicate the reliability of a measurement process.
Frequency Domain Specifications
Analyze systems based on frequency response, covering parameters like bandwidth, gain, and phase margin, essential for stability analysis in control and signal processing systems.
McLeod Gauge
Used to measure very low pressures (high vacuum). It compresses a known volume of gas into a smaller volume, increasing its pressure to a measurable range. The relationship between the original and compressed volume allows calculation of the vacuum pressure.
RTD (Resistance Temperature Detector)
A temperature sensor that relies on the principle that the electrical resistance of a metal (usually platinum) changes predictably with temperature. RTDs are highly accurate and stable, making them suitable for industrial applications.
Capacitive Pressure Transducer
Uses capacitance changes to measure pressure. When pressure is applied to a diaphragm, it changes the distance between two capacitor plates, altering capacitance. This change is converted to an electrical signal representing pressure.
Optical Encoder
Converts mechanical motion into digital signals using light. It has a rotating disk with transparent and opaque sections; an LED and sensor detect the interruptions, generating pulses. Optical encoders can be incremental (counts steps) or absolute (provides a unique position code).
Metrology
Metrology is the science of measurement, encompassing both experimental and theoretical determinations of the values and accuracy of various measurements.
Length Standards
Line Standard: Measures length by the distance between marked lines (e.g., ruler). Prone to parallax error.
End Standard: Measures length between two flat ends (e.g., gauge blocks). High precision, no parallax error.
Wavelength Standard: Uses the wavelength of light for ultra-precise length measurement (e.g., laser interferometers).
Strain Gauge Load Cell
Consists of an elastic material with strain gauges attached. When a force is applied, the material deforms, and the strain gauges detect this deformation as a change in resistance. This resistance change is converted into an electrical signal, correlating to the amount of force applied.
Types of Fits
Clearance Fit: The internal component can freely move within the outer component. Example: Shafts that rotate freely within bearings.
Interference Fit: The components are made slightly larger than the housing, requiring force to assemble them. Example: Press-fit of bearings on shafts.
Transition Fit: The fit can result in either a clearance or an interference. Example: Applications where a controlled amount of movement is needed, like a sliding fit in machinery.