Understanding Sensors: Characteristics and Types

Sensors

• Sensors measure physical quantities and convert them into electrical signals.
• Examples include temperature, displacement, position, motion, velocity, fluid, liquid flow, liquid level, and light sensors.

Characteristics of Sensors

• Sensor characteristics are of two types: static and dynamic.
• Static characteristics relate to the steady-state relationship between sensor input and output.
• Dynamic characteristics relate to the sensor input and output when the measured quantity changes rapidly.

Static Characteristics

• Range: Limits within which the input can vary, such as 25-225°C for a thermocouple.
• Span: Difference between the maximum and minimum input values, like 200°C for a thermocouple with a 25-225°C range.
• Error: Difference between the measured and true value. For example, an error of 0.2 mm when a sensor reads 29.8 mm for an actual displacement of 30 mm.
• Sensitivity: Ratio of change in output value per unit change in input value. As an example, a temp sensor has sensitivity of 10 mV/°C and a 1°C increase results in 10mV.
• Non-Linearity: Maximum deviation of the actual measured curve from the ideal curve.
• Hysteresis: Maximum difference in output at any measurement value within the sensor's specified range when approaching the point with increasing, then decreasing, the input parameter.
• Stability: Ability of a sensor to provide the same output with a constant input over time.
• Dead band: Range of input values for which there is no output.
• Repeatability: Ability of a sensor to give the same output for repeated applications of same input value under the same conditions.
• Accuracy: Closeness to the actual value.
• Precision: Ability of a sensor to reproduce a certain set of readings within given accuracy; depends on repeatability.
• Output Impedance: Impedance measured at the sensor's output; important because the electrical output interfaces with an electronic circuit.

Dynamic Characteristics

• Response time: Time elapsed for a sensor to give an output corresponding to a specified percentage (90-95%) of its steady-state value after a constant input.
• Time constant: Measures a sensor's inertia and reaction to input changes; equals 63.2% of the response time.
• Rise time: Time for the output to rise from 10% to 90% of its steady value.
• Settling time: Time for the output to settle within a small percentage (2%) of its steady-state value.

Classification of Sensors

• Sensors are classified into active and passive, analog and digital, and primary and secondary types.
• Active sensors (self-generating): Do not require external power, e.g., Thermocouple.
• Passive Sensors (external supply): Require external power such as a Photodiode.
• Analog Sensors: Produce a continuous output signal relative to the measured quantity, e.g., LDR, Strain gauge.
• Digital Sensors: Work with discrete or digital data used for conversion and transmission, e.g., IR, PIR.
• Primary Sensor: Transducers contain mechanical and electrical devices, converting the physical quantity into a mechanical signal.
• Secondary Sensor: Deployed in cascade with a primary sensor, converting the mechanical signal into a more comprehensible electrical signal, e.g., Bourdon tube (Primary sensor) and LVDT (Secondary sensor).

Selection of Sensors

• Parameters considered are the operating principle, availability, cost, and performance figures.
• Availability includes source location, delivery schedule, payment options, and continuation of supply.
• Cost includes sensor and delivery costs.
• Performance figures include range, ease of use, power supply requirements, accuracy, and hysteresis effect.

Displacement Sensors

• Used for measuring the movement of an object, converting displacement into an electrical signal.
• Based on the electrical output, displacement sensors are classified into three types: resistive, capacitive, and inductive.
• Resistive Displacement sensors: Potentiometer, strain gauge
• Capacitive Displacement sensors: Capacitive element
• Inductive Displacement sensors: LVDT

Resistive Displacement Sensors

• Potentiometer: Contains a resistance element with a sliding contact that moves along its length.
• The resistive element is either a wire-wound track (resolution of 0.5mm) or conductive plastic (resolution of 0.1µm).
• Potentiometers relate changes in linear or rotary position to changes in resistance, which are then converted into proportional voltage changes.
• Strain gauge: A passive transducer that converts mechanical displacement into a change in resistance.
• Strain gauges measure force, torque, pressure, and acceleration.
• Their principle is when strain is applied to a thin metallic wire, its dimension changes, changing the resistance.
• Strain gauges can be mechanical, electrical, or piezoelectric, as well as bonded or unbonded.
• Electrical strain gauges feature a grid-shaped sensing element of thin metallic resistive foil (3 to 6µm thick) on a thin plastic film base (15 to 16µm thick).
• A strain gauge bonded to a measuring object stretches or contracts with the object's strain, resulting in a change in length of the strain gauge.
• The change in length causes a change in resistance.
• The equation is change in resistance = (gage factor) * (strain)
• Metal wire or foil strain gauges have a gage factor (G) of approximately 2.
• N-type semiconductors can have G values of -100 or less, or +100 or more.
• Used for displacement, force, residual stress, vibration, torque, bending, deflection, compression and tension measurement.
• Displacement Measurement using Strain gauge: Strain gauge attached to flexible element in the form of cantilevers, rings and U- shapes.
• When strain or force is applied on beam of cantilever, it is displaced, then the strain gauges mounted on the cantilever are also strained.

Inductive Displacement Sensor: Linear Variable Differential Transducer/Transformer (LVDT)

• The LVDT works on the principle of mutual inductance, converting non-electrical energy (displacement) into electrical energy.
• Construction: A cylindrical former with a primary winding in the center and two secondary windings on the sides, number of turns in secondaries is equal.
• The secondary windings are opposite to each other, output voltages will be the difference in voltages between the two-secondary coil.
• An esteem iron core is placed in the center of the cylindrical former which can move in to and fro motion.
• The AC excitation voltage is 5 to 12V, the operating frequency is 50 to 400 HZ.
• Working of LVDT:
  • Case 1: When no external force, the core reminds in the null position, voltage induced in both secondary windings are equal, makes V0=V1-V2=0
  • Case 2: With external force applies and the core moves to the left-hand side, the emf voltage induced in the secondary coil1 is greater, makes V0=V1-V2= +ve
  • Case 3: With external force applies and the core moves to the right-hand side, the emf voltage induced in the secondary coil1 is greater than secondary coil 2, makes V0=V1-V2= -ve
• LVDTs measure displacement ranging from fractions of a millimeter to centimeters; can also measure force, weight, and pressure.

Force Sensor (Load Cell)

• Load cells use strain gauges to measure force by detecting changes in dimension of a steel cylinder.
• Basic working principle of Strain gauge load cell: Bonded strain gauges on the cylinder stretch or compress, causing resistance changes.
• These changes in resistance or output voltage of the strain gauge are how force is measured.
• Construction: Main components are a steel cylinder and four identical strain gauges.
• Two strain gauges (R1 and R4) are mounted along the direction of the applied load, and the other two gauges (R2 and R3) are mounted circumferentially at right angles to gauges R1 and R4.
• Operation of strain gauge Load cell: -The four gauges are connected in the form of bridge to convert the change in resistance to voltage.
  • Case 1: When there is no load, gauges have same resistance, wheatstone bridge is balanced, output Vout =0 -Case 2: Vertical load applied, vertical gauges R1 and R4 will compress, decrease in resistance will occur. At the same time, horizontal gauges R2 and R3 will undergo tension, increase in resistance will occur. -When wheat stone bridge is unbalanced Vout≠0
• Applications: Vehicle Weigh Bridges, Too force dynamo meters and Tension measurement of wires.

Temperature Sensors

• Detect or measure temperature changes and convert into electrical signals.
• Types of temperature sensors:
  • Bi-metallic strips(420°C)
  • Thermocouples (-200°C to over +2000°C to be measured.)
  • Resistance Temperature Detectors (RTDs) (-200 to +600°C.)
  • Thermistors (-50 - 200°C)
  • Thermodiodes and thermotransistors (-50- 150°C)

Bi-Metallic Strips

• Bonding two metals with dissimilar thermal expansion coefficients can produce useful devices for detecting and measuring temperature changes,
• Brass and steel used as pair with thermal expansion coefficients of 19 and 13 parts per million per degree Celsius.
• A bimetallic strip is used as a thermal switch.
• Advantages
  • No power source required
  • Easy to use, but not very accurate. -Can be used to 500°C -Used in household Thermometers
• Disadvantages -Not suitable for low temperatures. -Device becomes insensitive Thermometer

Resistance Temperature Detectors (RTD)

• Electric resistance of a metal changes due to change in its temperature.
• Heating up metals, their resistance increases and follows a linear relationship.
• The correlation is Resistance at Temperature T (°C) = Resistance at 0°C [1 + (Temperature Coefficient of Resistance * Temperature)]
• Where Rt is the resistance at temperature T (°C) and R0 is the temperature at 0°C and α is the constant for the metal termed as temperature coefficient of resistance.
• Applications: Air conditioning and refrigeration servicing, food Processing, Stoves and grills etc

Thermistors

• Thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature.
• Thermistor is a combination of the words thermal and resistor.
• Relationship between resistance and temperature is ΔR = k * Δt
  • ΔR = Change In Resistance
  • k = Temperature Coefficient Of Resistance.
• Commercially available thermistors have nominal values of 1K,2K,10K,20K,100K etc,.
• Types of thermistors: Negative temperature co-efficient (NTC) and Positive temperature co-efficient (PTC).
  • Negative temperature co-efficient (NTC) thermistors: Mixture of metal oxides such as chromium, cobalt, iron, manganese and nickel pressed into a bead, disc or rod shape.
  • Positive temperature co-efficient (PTC) thermistors: Positive temperature co-efficient thermistors are made-up of barium, lead and strontium titanite and are used for protection of motors and transformer winding.

Thermocouple

• Thermocouple works on the fact that when a junction of dissimilar metals heated, it produces an electric potential related to temperature.
• As per Thomas Seebeck (1821), when two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, then there is a continuous current which flow in the thermoelectric circuit.
• ΔVAB= αΔΤ, Where the Seebeck coefficient is the constant of proportionality.
• Generally, Chromel(90% nickel and 10% chromium) and Alumel(95% nickel, 2% manganese, 2% aluminium and 1% silicon) are used in the manufacture of a thermocouple.
• Applications: Used to monitor temperatures and chemistry throughout the steel making process, temperature measurement of gas turbine and engine exhausts and Monitoring of temperatures throughout the production and smelting process in the steel,iron and aluminium industry.

Thermodiodes and Thermotransistors

• A junction semiconductor pn junction diode is widely used as temperature sensor.
• When temperature of doped semiconductor changes the mobility of their charge carries changes and this effect the rate at which electrons and holes can diffuse across a pn junction.
• The current through the junction is a function of the temperature.
• The LM35 shows relationship to temperature by an Integrated-circuit temperature sensor with an output voltage that is linearly-proportional to the Centigrade temperature.
• The LM35 Features are calibrated directly in Celsius, has linear scaling of 10mV/°C, and operates with 0.5°C accuracy, it's rated for Full -55°C to 150°C Range and suitable for remote Applications.

Light Sensors

• Light sensors are devices that are used to convert light energy into electrical energy. -Light Dependent Resister (LDR), Photo Diode and Photo transistor are the commonly used sensors.

Light Dependent Resistor (LDR)

• LDR is a device whose resistance is a function of amount of light falling on it.
• LDR is called as Photoresistor, Photoconductor, Photoconductive cell or simply Photocells.
• LDRs are Made up of with semiconductor materials like cadmium sulphide, lead sulphide (PbS), lead selenide (PbSe), indium antimonide (InSb).
• Applications: Automatic Street light systems, Bar code Scanners and Counting packages.

Photodiode

• A photodiode is a type of photo detector capable of converting light energy into electrical energy.
• They can be regular semiconductor diode except that they may be either exposed (to detect UV or X-rays) or packaged with a window or optical fiber to allow light to reach the sensitive part of the device.
• Photodiodes are made up with compounds of semiconductor materials like GaAs and InGaAs. -Photodiodes are classified into three types, they are PN Photodiode, PIN Photodiode and Avalanche Photodiode.
• Applications: CD Players, Smoke detectorsSpace and Optical Communication.

Phototransistor

• Phototransistors are either tri-terminal (emitter, base and collector) or bi-terminal (emitter and collector) semiconductor devices which have a light-sensitive base region.

• These are specially designed and optimized for photo applications made of diffusion or ion-implantation. -Phototransistors can be made of non-identical material such as (Group III-V materials like GaAs). However more often used devices are from homojunction

  • Collector current equation:
    • Ic = ßIB + (1 + β)Ісво (Transistor) -Іс = 1 + ẞ)Ісво, (phototransistor = open base).
  • In photodetector, ICBO is increased when collector base junction is illuminated by light. -Applications: Monitoring paper position and margin in printers, CD players, Night vision light systems and Security systems.

Proximity sensors

• A proximity sensor is sensor able to detect the presence of nearby objects without any physical contact.
• Proximity sensors emits or looks for electromagnetic radiation.
• Types of proximity sensors are Inductive, Capacitive, Optical and Ultrasonic.

Inductive Proximity sensor

• An Inductive proximity sensor is a type of non-contact electric proximity sensor that is used to detect the presence and position of metallic objects.
• Operating principle is based on a coil and high frequency oscillator that creates a field in the close surroundings of the sensing surface. The presence of metal in operating area causes a change in the oscillation amplitude.
• Applications metal detectors and car washes.

Capacitive proximity sensor

• Proximity sensors are used for detection of metallic objects & non-metallic objects (liquid, plastic, wooden materials and so on).
• Capacitive proximity sensors use the variation of capacitance between the sensor and the object being detected. -Applications: touch sensors and computer displays

Optical proximity sensor

• Optical proximity sensors work with all types of Through beam, Diffuse reflective and Retroreflective and are usually more expansive and have been available longer then most alternatives

Ultrasonic proximity sensor

• Ultrasonic sensors are sometimes used in place of optical sensors and uses high frequency sounds waves above normal hearing levels with 40Khz ranges are common
• the censor emits a pluse till an echo is received.
  • Subsonic sound is 20hz -Sonic sound ranges from range in 20 - 20khz -The high levels of frequency are above 20khz
• applications : Liquid level monitoring trash level monitoring and parking assist.