Understanding Sensors: Types and Characteristics

Sensors

• A sensor measures a physical quantity and converts it into an electrical signal
• Examples are temperature, displacement, position, motion, velocity, fluid, liquid flow, liquid level and light sensors

Characteristics of Sensors

• Static and dynamic are the two types of sensor characteristics

Static Characteristics

• Static characteristics relate to the steady state relationship between sensor input and output
• Range indicates the limits between which the input can vary; for example, a thermocouple for temperature measurement has a range of 25-225 °C
• Span refers to the difference between the maximum and minimum input values; a thermocouple with a range of 25-225°C has a span of 200°C
• Error is the difference between the measured and the true value; a sensor reading 29.8 mm when the actual displacement is 30 mm has an error of 0.2 mm
• Sensitivity equals the ratio of change in output value per unit change in input value; a temperature sensor may have a sensitivity of 10 mV/°C, so a 1°C rise results in 10mV
• Non-linearity indicates the maximum deviation of the actual measured curve from the ideal curve
• Hysteresis is an error that defines the maximum difference in output at any measurement value within a sensor's specified range when approaching the point first increasing, then decreasing the input parameter
• Stability is the ability of a sensor device to give the same output with a constant input over a period
• Dead band, or dead space, signifies the range of input values for which there is no output
• Repeatability specifies a sensor's ability to give the same output for repeated applications of the same input value under the same conditions
• Accuracy refers to the closeness of a measurement to the actual value
• Precision is a sensor's ability to reproduce a certain set of readings within given accuracy and it depends upon repeatability
• Output impedance is the impedance measured at the output of a sensor and knowledge of this is needed because the electrical output interfaces with an electronic circuit

Dynamic Characteristics

• Dynamic characteristics relate to the relationship between sensor input and output when the measured quantity varies rapidly
• Response time is the time elapsed for a sensor to give an output corresponding to a specified percentage (90-95%) of its steady value after a constant, step input
• Time constant measures a sensor's inertia, representing how it reacts to input changes; it equals 63.2% of the response time
• Rise time is the time taken for the output to rise from 10% to 90% of its steady value
• Settling time is the time taken for the output to settle within a small percentage (2%) of its steady-state value

Classification of Sensors

• Sensors are classified as active or passive
• Active sensors (self-generating) do not require power for operation; an example is a thermocouple
• Passive sensors (external supply) require external power to operate; an example is a photodiode
• Another classification divides them into analog and digital
• Analog sensors produce an analog output, which is a continuous output signal with respect to the measured quantity; examples include LDR and strain gauge
• Digital sensors work with discrete or digital data used for conversion and transmission; examples include IR and PIR
• Sensors are further divided into primary and secondary types
• Primary sensors contain mechanical and electrical components, typically converting a physical quantity into a mechanical signal
• Secondary sensors are deployed in cascade with primary ones, converting the mechanical signal into a more comprehensible electrical signal
• Examples are a Bourdon tube (primary sensor) and LVDT (secondary sensor)

Selection of Sensors

• When selecting a sensor for an application, operating principle, availability, cost and performance figures are important parameters

Displacement Sensors

• These sensors measure the movement of an object, converting displacement into an electrical signal of resistance, capacitance, or inductance
• Based on the electrical output, displacement sensors are classified into three types: resistive, capacitive and inductive

Resistive Displacement Sensors

• Potentiometers and strain gauges are examples of these sensors

Potentiometer Construction

• Consists of a resistance element with a sliding contact that moves over the element's length
• The resistive element is either a wire wound track (0.5mm resolution) or conductive plastic (0.1µm resolution)

Potentiometer Operation

• It is a common sensor for position measurements, relating a change in position (linear or rotary) into a change in resistance
• The resistance change converts to a proportional voltage change in the sensor's electrical circuit
• For an ideal potentiometer, the relationship between the measured physical variable (translational/linear displacement x or rotary/angular displacement θ) and the output voltage is defined by equations involving supply voltage, displacement, and a sensitivity constant

Strain Gauge

• An example of a passive transducer that converts a mechanical displacement into a change of resistance, used for force, torque, pressure, and acceleration
• The basic principle of operation is simple: when strain is applied to a thin metallic wire, its dimension changes, thus changing the resistance of the wire

Working Principle of Stain Gauge

• It can be mechanical, electrical or piezoelectric
• Mounting can be bonded or unbonded (electrical is most common)
• A universal strain gauge features a grid-shaped sensing element of thin metallic resistive foil (3 to 6µm thick) on a thin plastic film (15 to 16µm thick), laminated with a thin film
• The strain gauge tightly bonds to a measuring object to cause sensing element (ex: metallic foil) to stretch or contract per the strain on the object
• The change in length changes the resistance ( R = ρl/A where, R=Resistance, l=length, A=Area)
• Applied strain is directly proportional to the change in resistance
• Where: G= Proportional constant (called gage factor), ε = Strain (Δl/l), R=Original resistance of strain gage, Ω (ohm) and ΔR= Elongation- or contraction-initiated resistance change, Ω (ohm)
• Depending on material: G= 2 for metal wire or metal foil strain gauge; G= -100 or less for N-type semiconductor; G= +100 or more for N-type semiconductor

Application of Strain Gauge

• Application include displacement, force, residual stress, vibration, and torque measurement, as well as bending/deflection and compression/tension measurement

Displacement Measurement using Strain Gauge

• A form of displacement sensor featuring a strain gauge attached to a flexible element such as cantilevers, rings and U-shapes.
• Stress or force application leads to beam displacement, straining the gauges, and causing one gauge to undergo tension while the other undergoes compression
• Can measure linear displacement from 1mm to 30mm , with non-linearity error of about ±1%

Inductive Displacement Sensor

• A linear Variable Differential Transducer/Transformer (LVDT) is the best example

Principles of LVDT

• LVDT operates on mutual induction, converting displacement (non-electrical energy) into electrical energy

Construction of LVDT

• LVDT features a cylindrical former surrounded by a primary winding in the center and two secondary windings at the sides
• The secondary windings have equal turns but are opposite to each other, The two secondary coils represented are S1 and S2
• An esteem iron core is placed in the center that can move to and fro and an AC excitation voltage of 5 to 12V is used
• The operating frequency = 50 to 400 HZ.

Working of LVDT

• The LVDT works by splitting in 3 based on the iron core position inside the insulated former:
  • Case 1: No external force. Core stays at null position. Voltage results in net output is = zero (V0=V1-V2=0)
  • Case 2: External force causing the steel iron core tends to move left. Voltage induced in the secondary coil1 is greater (V0=V1-V2= +ve)
  • Case 3: External force causing the steel iron core moves right. Voltage induced in secondary coil 2 is greater. Output voltage will be negative (V0=V1-V2= -ve)

Application of LVDT

• LVDTs measure displacement ranging from fraction millimeters to centimeters.
• Acting as a secondary transducer, LVDTs can measure force, weight and pressure

Force Sensor (Load Cell)

• A force sensor (or load cell), especially a strain gauge load cell operates on pressure from an objective

Basic Principle of Strain Gauge Load Cell

• When steel cylinder is subjected to a force, there is change in dimension
• There is stretch and compression, dimensional change causes resistance to change
• The measure of the applied force becomes either resistance or output voltage

Construction of Strain Gauge Load Cell

• The main parts includes cylinder made out of steel, with four identical strain gauges mounted
• Two of the four gauges are mounted in the direction of applied load and 2 are horizontal at right angles to the gauges

Operation of Strain Gauge Load Cell

• The four gauges connect in the bridge to change resistance to voltage, the voltage is the output
• Case 1: No force on cylinder, gauges have same resistance so the output voltage = 0
• Case 2: Load is measured when applied to the cylinder. This causes 1 to + R
• Can be used for vehicle weight bridges, force dynamo meters, and wire tension

Temperature Sensors

• They detect/measure changes in temperature and convert to electrical signal and include:
  • Bi-metallic strips (420°C)
  • Thermocouples (-200°C to over +2000°C)
  • Resistance Temperature Detectors (RTDs) (-200 to +600°C)
  • Thermistors (-50 to 200°C), Thermodiodes and thermotransistors ( -50 to 150°C)

Bimetallic Strips

• Two metals bonded with dissimilar thermal expansion coefficients can detect temperature by bending in one direction
• Has advantages(no power source, cheap) and disadvantages(not accurate, low temperature reading)

Resistance Temperature Detectors (RTD)

• Work on the increase of resistance with linear relationship
• Where:: Rt is the resistance at temperature T (°C), R0 is the temperature at 0°C and is the temperature coefficient of resistance
• Air and liquid sensing is the application

Thermistors

• Combines words thermal+register and the symbol is
• It is a register that measures differences of temperature
• Commonly used in: food, textiles, micro electronics

Thermocouple

• Thomas Seebeck discovered with the heating of two dissimilar metals there is electric potential
• Voltage is a function of temp
• Generally, Chromel (90% nickel and 10% chromium) and Alumel (95% nickel, 2% manganese, 2% aluminium and 1% silicon) are used

Thermodiodes and Thermotransistors

• A junction semiconductor pn junction diode is widely used as temperature sensor
• The effect is a linear function of temperature
• Has calibrated readings in Celsius, 0.5 ensured accuracy and a low cost

Light Sensors

• Convert from light to electrical energy
• Include sensors like LDR, Photodiode and phototransistor

Light Dependent Resistor (LDR)

• Measures resistance from light
• Composed of led sulphate or selenide
• Street lights, scanners and alarms are the uses
• The function is when photons hit, then electrons are fired and electricity is increased

Photodiode

• Converts light energy into electrical energy with an exposed UV sensor
• The made up GaAs and InGaAs
• Has pin, pn and avalanche types
• With no light, has a saturation current

Phototransistor

• It is a two or three terminal semiconductor with a light sensitive base region
• Has small sizes
• Used for printers, readers, audio and surveillance systems

Proximity Sensors

• Detect object in proximity that emit radiation infrared sensing
• Usually Inductive, Capacitive, Optical or Ultrasonic
• Applications are in automated systems and production lines

Ultrasound, capacitative and optical sensors

• Used in place of optical sensors
• Are of high frequency with 40khz

Distances calculation

• Calculated by time delays between the object in question