Sensors And Transducers

SENSORS and TRANSDUCERS

INTRODUCTION • Transducer – a device that converts a primary form of energy into a corresponding signal with a different energy form • Primary Energy Forms: mechanical, thermal, electromagnetic, optical, chemical, etc. – take form of a sensor or an actuator • Sensor (e.g., thermometer) – a device that detects/measures a signal or stimulus – acquires information from the “real world” • Actuator (e.g., heater) – a device that generates a signal or stimulus real world

sensor actuator

intelligent feedback system

Sensor • A Device that receives and respond to a signal or stimulus. • Sensors are sophisticated devices that are frequently used to detect and respond to electrical or optical signals. • A Sensor converts the physical parameter (for example: temperature, blood pressure, humidity, speed, etc.) into a signal which can be measured electrically..

Sensor Systems Typically interested in electronic sensor – convert desired parameter into electrically measurable signal • General Electronic Sensor – primary transducer: changes “real world” parameter into electrical signal – secondary transducer: converts electrical signal into analog or digital values

real world

primary transducer

analog

secondary transducer

usable values

Sensor •

Typical Electronic Sensor System input signal (measurand)

sensor

sensor data analog/digital

microcontroller signal processing communication

network display

Sensor characteristics • Span or Full scale input – A dynamic range of stimuli which may be converted by a sensor – represents the highest possible input value that can be applied to the sensor without causing an unacceptably large inaccuracy

• Full scale output – algebraic difference between the electrical output signals measured with maximum input stimulus and the lowest input stimulus applied

Sensor characteristics • Accuracy – Accuracy is measured as a highest deviation of a value represented by the sensor from the ideal or true value at its input – accuracy limits generally are used in the worst-case analysis to determine the worst possible performance of the system – The inaccuracy rating may be represented in a number of forms: • Directly in terms of measured value () • In percent of input span (full scale) • In terms of output signal

Sensor characteristics • Calibration – determination of specific variables that describe the overall transfer function

• Overall means of the entire circuit, including the sensor, the interface circuit, and the A/D converter

– E.g. use of forward biased diode for temperature measurement

• Transfer function v=a+bt • Take measurement at two T’s and solve and determine a and b – V1=a+bt1 and V2=a+bt2

– For Non-linear function more than one point can be required depending on the transfer function

Sensor characteristics • Calibration error – inaccuracy permitted by a manufacturer when

a sensor is calibrated in the factory Error is systematic in nature

Sensor characteristics • Hysteresis –

deviation of the sensor’s output at a specified point of the input signal when it is approached from the opposite directions

Sensor characteristics • Non-linearity error – specified for sensors whose transfer function may be approximated by a straight line Nonlinearity (%) = Maximum deviation in input ⁄ Maximum full scale input

Sensor characteristics • Repeatability – caused by the inability of a sensor to represent the same value under identical conditions – It is expressed as the maximum difference between output readings as determined by two calibrating cycles – It is usually represented as % of FS Repeatability = (maximum – minimum values given)/full range

Sensor characteristics • Resolution – the smallest increments of stimulus which can be sensed

• Response time - Response time describes the speed of change in the output on a step-wise change of the measurand

Classification of Sensors • The sensors are classified into the following criteria: 1. Primary Input quantity (Measurand) 2. Transduction principles (Using physical and chemical effects) 3. Material and Technology 4. Property 5. Application

Quantity being Measured

Input Device (Sensor)

Output Device (Actuator)

Light Level

Light Dependant Resistor (LDR) Photodiode Photo-transistor Solar Cell

Lights & Lamps LED’s & Displays Fibre Optics

Temperature

Thermocouple Thermistor Thermostat Resistive Temperature Detectors

Heater Fan

Position

Potentiometer Encoders Reflective/Slotted Opto-switch LVDT

Motor Solenoid Panel Meters

Force/Pressure

Strain Gauge Pressure Switch Load Cells

Lifts & Jacks Electromagnet Vibration

Speed

Tacho-generator Reflective/Slotted Opto-coupler Doppler Effect Sensors

AC and DC Motors Stepper Motor Brake

Sound

Carbon Microphone Piezo-electric Crystal

Bell Buzzer Loudspeaker

Sensor Classification Passive •

Doesn’t need any additional energy source

•

Directly generate an electric signal in response to an external stimuli

•

E.g. Thermocouple, photodiode, Piezoelectric sensor

Active •

Require external power called excitation signal

•

Sensor modify excitation signal to provide output

•

E.g. thermistor, resistive strain gauge

Light Sensors • A Light Sensor generates an output signal indicating the intensity of light by measuring the radiant energy that exists in a very narrow range of frequencies basically called “light”, and which ranges in frequency from “Infra-red” to “Visible” up to “Ultraviolet” light spectrum. • Commonly known as Photoelectric Devices or Photo Sensors. • Photo-emissive Cells, Photo-conductive Cells, Photo-voltaic Cells, Phototransistors

PhotoTransistors An alternative photo-junction device to the photodiode is the Phototransistor which is basically a photodiode with amplification. The Phototransistor light sensor has its collector-base PNjunction reverse biased exposing it to the radiant light source.

PhotoTransistors

PhotoTransistors In the NPN transistor the collector is biased positively with respect to the emitter so that the base/collector junction is reverse biased. therefore, with no light on the junction normal leakage or dark current flows which is very small. When light falls on the base more electron/hole pairs are formed in this region and the current produced by this action is amplified by the transistor.

Photodarlington Transistors • Photodarlington transistors use a second bipolar NPN transistor to provide additional amplification or when higher sensitivity of a photodetector is required due to low light levels or selective sensitivity, but its response is slower than that of an ordinary NPN phototransistor.

Temperature Sensors • Temperature Sensors measure the amount of heat energy or even coldness that is generated by an object or system, allowing us to “sense” or detect any physical change to that temperature producing either an analogue or digital output.

Temperature Sensors • A temperature sensor consists of two basic physical types: • Contact Temperature Sensor Types – These types of temperature sensor are required to be in physical contact with the object being sensed and use conduction to monitor changes in temperature. • Non-contact Temperature Sensor Types – These types of temperature sensor use convection and radiation to monitor changes in temperature.

Temperature Sensors • The two basic types of contact or even noncontact temperature sensors can also be sub-divided into the following three groups of sensors, • Electro-mechanical, • Resistive, and • Electronic

Thermostat • The Thermostat is a contact type electromechanical temperature sensor or switch, that basically consists of two different metals such as nickel, copper, tungsten or aluminium etc, that are bonded together to form a Bi-metallic strip. • The different linear expansion rates of the two dissimilar metals produces a mechanical bending movement when the strip is subjected to heat.

Thermostat

Thermostat • There are two main types of bi-metallic strips based mainly upon their movement when subjected to temperature changes. There are the “snap-action” types that produce an instantaneous “ON/OFF” or “OFF/ON” type action on the electrical contacts at a set temperature point, and the slower “creep-action” types that gradually change their position as the temperature changes. Snap-action type - commonly used in our homes for controlling the temperature set point of ovens, irons, immersion hot water tanks and they can also be found on walls to control the domestic heating system. Creeper type - generally consists of a bimetallic coil or spiral that slowly unwinds or coils-up as the temperature changes.

Thermostat • Generally, creeper type bi-metallic strips are more sensitive to temperature changes than the standard snap ON/OFF types as the strip is longer and thinner making them ideal for use in temperature gauges and dials etc. • Although very cheap and are available over a wide operating range, one main disadvantage of the standard snap-action type thermostats when used as a temperature sensor, is that they have a large hysteresis range from when the electrical contacts open until when they close again. For example, it may be set to 20oC but may not open until 22oC or close again until 18oC.

Thermistor • The Thermistor is another type of temperature sensor, whose name is a combination of the words THERM-ally sensitive resISTOR. A thermistor is a special type of resistor which changes its physical resistance when exposed to changes in temperature.

Thermistor •Thermistors are generally made from ceramic materials such as oxides of nickel, manganese or cobalt coated in glass which makes them easily damaged.

Thermistor Most types of thermistor’s have a Negative Temperature Coefficient of resistance or (NTC), that is their resistance value goes DOWN with an increase in the temperature, and of course there are some which have a Positive Temperature Coefficient, (PTC), in that their resistance value goes UP with an increase in temperature.

Thermistor • The following thermistor has a resistance value of 10KΩ at 25oC and a resistance value of 100Ω at 100oC. Calculate the voltage drop across the thermistor and hence its output voltage (Vout) for both temperatures when connected in series with a 1kΩ resistor across a 12v power supply.

Resistive Temperature Detectors (RTD) • Another type of electrical resistance temperature sensor • RTD’s are precision temperature sensors made from high-purity conducting metals such as platinum, copper or nickel wound into a coil and whose electrical resistance changes as a function of temperature, similar to that of the thermistor.

Resistive Temperature Detectors (RTD) • Also known as Resistance Thermometers • RTDs have positive temperature coefficients (PTC) but unlike the thermistor their output is extremely linear producing very accurate measurements of temperature. • Passive resistive devices

Resistive Temperature Detectors (RTD) • Carbon resistors – Most reliable at extremely low temperatures • Strain free elements – Uses a wire coil minimally supported within sealed housing filled with inert gas. • Thin Film elements – have a sensing element that is formed by depositing a thin layer resistive material (platinum), on a ceramic substrate. • Wire Wound elements – Have greater accuracy, especially for wide temperature ranges. • Coiled elements – Has a wire coil that expand freely over temperature and is strain free.

Resistive Temperature Detectors (RTD) • Measuring temperatures of materials and equipment in industrial manufacturing facilities • Measuring air temperature in laboratories, clean rooms, and other locations • Measuring temperature inside air ducts

• Measuring temperatures in harsh or highly pressurized environments, including plating baths • Measuring temperatures inside ovens, stoves and food processing equipment

Resistive Temperature Detectors (RTD) Advantages: • High accuracy • Low drift • Wide operating range • Suitability for precision applications.

Limitations • Rarely used above 660 °C • Less sensitive to small temperature changes • Slower response time

Thermocouple • The Thermocouple is by far the most commonly used type of all the temperature sensor types. • Thermocouples are popular due to its simplicity, ease of use and their speed of response to changes in temperature, due mainly to their small size. • Thermocouples also have the widest temperature range of all the temperature sensors from below -200oC to well over 2000oC.

Thermocouple Based on the “SEEBECK EFFECT”

Thermocouple • Uses “Seebeck effect” • When fused together the junction of the two dissimilar metals such as copper or iron and constantan produces a “thermoelectric” effect which gives a constant potential difference of only a few millivolts (mV) between them. • Then the output voltage from a thermocouple is a function of the temperature changes.

Thermocouple Sensor Colour CodesExtension and Compensating Leads Code Type

Conductors (+/-)

Sensitivity

E

Nickel Chromium / Constantan

-200 to 900oC

J

Iron / Constantan

0 to 750oC

K

Nickel Chromium / Nickel Aluminium

-200 to 1250oC

N

Nicrosil / Nisil

0 to 1250oC

T

Copper / Constantan

-200 to 350oC

U

Copper / Copper Nickel Compensating for “S” and “R”

0 to 1450oC

British BS 1843:1952

Temperature Sensors • Other Temperature Sensor Types not mentioned here include, Semiconductor Junction Sensors, Infra-red and Thermal Radiation Sensors, Medical type Thermometers, Indicators and Colour Changing Inks or Dyes.

Position Sensors • Position Sensors detect the position of something which means that they are referenced either to or from some fixed point or position.

Potentiometer • The most commonly used of all the “Position Sensors”. • It relates the change in position (linear or rotary) into the change in resistance, as shown in Figure a and b

Potentiometer • The resistance change is then converted to a proportional voltage change in the electrical circuit of the sensor. • Hence, the relationship between the measured physical variable, translational displacement x or rotary displacement θ, and the output voltage for a ideal potentiometer is

where the sensitivity, (k * Vr ) , of the potentiometer is a function of the winding resistance and physical shape of the winding.

Potentiometer

Potentiometer • It has a wiper contact linked to a mechanical shaft that can be either angular (rotational) or linear (slider type) in its movement, and which causes the resistance value between the wiper/slider and the two end connections to change giving an electrical signal output that has a proportional relationship between the actual wiper position on the resistive track and its resistance value.

Positional Sensing Circuit

Potentiometer Advantages • Low cost • Low tech • Easy to use Disadvantages • Wear due to moving parts • Low accuracy • Low repeatability • Limited Frequency response • Output voltage is limited to the physical size

Linear Variable Differential Transformer (LVDT) Principle of LVDT: LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is converted into an electrical energy. And the way how the energy is getting converted is described in working of LVDT in a detailed manner.

Cont.… • Construction of LVDT:

• LVDT consists of a cylindrical former where it is surrounded by one primary winding in the centre of the former and the two secondary windings at the sides.

Working of LVDT:

Case 1:

On applying an external force which is the displacement, if the core reminds in the null position itself without providing any movement then the voltage induced in both the secondary windings are equal which results in net output is equal to zero

Esec1-Esec2=0

Working of LVDT:

Case 2:

When an external force is applied and if the steel iron core tends to move in the left hand side direction then the emf voltage induced in the secondary coil 1 is greater when compared to the emf induced in the secondary coil 2. Therefore the net output will be Esec1-Esec2

Working of LVDT:

Case 3:

When an external force is applied and if the steel iron core moves in the right hand side direction then the emf induced in the secondary coil 2 is greater when compared to the emf voltage induced in the secondary coil 1. The net output voltage will be Esec2-Esec1

Advantages of LVDT: 1. 2. 3. 4. 5. 6. 7. 8.

Infinite resolution is present in LVDT High output LVDT gives High sensitivity Very good linearity Ruggedness LVDT Provides Less friction Low hysteresis LVDT gives Low power consumption.

Applications of LVDT: 1. LVDT is used to measure displacement ranging from fraction millimeter to centimeter. 2. Acting as a secondary transducer, LVDT can be used as a device to measure force, weight and pressure, etc..

Inductive Proximity Sensor • Also called as Eddy Current Sensor • Mainly used to detect the presence of an object within close proximity • Proximity sensors, are non-contact position sensors that use a magnetic field for detection with the simplest magnetic sensor being the reed switch.

Inductive Proximity Sensor An inductive proximity sensor has four main components; The oscillator which produces the electromagnetic field, the coil which generates the magnetic field, the detection circuit which detects any change in the field when an object enters it and the output circuit which produces the output signal, either with normally closed (NC) or normally open (NO) contacts.

The “sensing” range of proximity sensors is very small, typically 0.1mm to 12mm.

Inductive Proximity Sensor Applications 1. Metal Detectors 2. Traffic Lights 3. Car washes 4. Automated industrial processes Disadvantage 1. Omnidirectional

Optical Encoder What is an encoder? • An encoder is a device which converts a mechanical information of a shaft or position into an electrical signal.

Encoder

Cont.… How is this accomplished? As the code disc rotates, it shutters light from the LED and is received and transmitted as square\sine waveforms

Receiver / Sensor Code disc

Lens Transmitter / LED Shaft

Cont.… Types of Encoders Encoder

Rotary Encoder

Incremental

Linear Encoder

Absolute

Wire Draw

Incremental

Linear with Scale

Absolute

Rotary Encoders • An electromechanical, non-contact optical devices used for converting the angular position of a rotating shaft into an analog or digital data code/signals.

Rotary Encoders • All optical encoders work on the same basic principle. Light from an LED or infra-red light source is passed through a rotating high-resolution encoded disk that contains the required code patterns, either binary, grey code or BCD. Photo detectors scan the disk as it rotates and an electronic circuit processes the information into a digital form as a stream of binary output pulses that are fed to counters or controllers which determine the actual angular position of the shaft. • There are two basic types of rotary optical encoders, Incremental Encoders and Absolute Position Encoders.

Cont.… incremental encoders

Absolute encoders

Cont.… diode

diode Logical 0

Logical 1

Photo sensor

Photo sensor

• Rotating disk

•Rotating disk

blocking light

allowing light through

• Photo sensor has a

•Photo sensor has a

low logic output

high logic output

•Continuous train of pulses formed as the photo sensor traverses

between logic high and logic low in response to the rotation of the disk

Absolute Optical Encoder

Absolute Optical Encoder

Slotted Optical Switch • The slotted optical switch, sometimes known as opto switch or optical switch but not to be confused with the optical component, is a device comprising a photoemitter (e.g. LED) and a photodetector (e.g. photodiode) mounted in a single package so that the photoemitter normally illuminates the photodetector, but an opaque object can be inserted in a slot between them so as to break the beam.

A diagram of a mechanical computer mouse showing two slotted optical switches

Pressure Sensor • A pressure sensor is a device or pressure measurement of gases or liquids. • A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed.

• Pressure sensors are used for control and monitoring in thousands of everyday applications. • Pressure sensors can also be used to indirectly measure other variables such as fluid/gas flow, speed, water level, and altitude.

Pressure Sensor Pressure sensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the type of pressure they measure.

Pressure Sensor • Absolute pressure sensor This sensor measures the pressure relative to perfect vacuum.

Absolute pressure sensors are used to measure atmospheric pressure barometers or altimeters.

Pressure Sensor • Gauge pressure sensor This sensor measures the pressure relative to ambient atmospheric pressure. The average atmospheric pressure at sea level is 1013.25 mbar.

Gage pressure sensors only offer one pressure port. The ambient air pressure is directed through a vent hole or a vent tube to the back side of the sensing element and thus compensated.

Pressure Sensor • Differential pressure sensor This sensor measures the difference between two pressures, one connected to each side of the sensor. These sensors are called bidirectional differential pressure sensors with ranges of e.g. 1...+1 bar or -2.5...+2.5 mbar.

Pressure Sensor • Vacuum pressure sensor Measures pressures below atmospheric pressure, or absolute pressure relative to a vacuum. • Sealed pressure sensor This sensor is similar to a gauge pressure sensor except that it measures pressure relative to some fixed pressure rather than the ambient atmospheric pressure (which varies according to the location and the weather).

Pressure Sensor LVDT Strain Gauge Sensor

LVDT WITH DIAPHRAGM

Pressure Sensor Bellows

Pressure Sensor TUBE PRESSURE SENSOR

BOURDON TUBE

Bourdon tube pressure gauges are used for the measurement of relative pressures from 0.6 ... 7,000 bar. They are classified as mechanical pressure measuring instruments, and thus operate without any electrical power.

Pressure Sensor Piezoelectric actuators and sensors

Piezoelectric effect (sensor) An electric field is generated due to a change in dimensions of a material (Curie brothers 1880)

Converse Piezoelectric effect (actuator)

+

-

-

+

A change in dimensions of a material due to the Application of an electric field

Piezoelectric Force Transducer Preload stud

Electrical output

Piezoelectric element • Can be used in tension and compression

• Fragile to moments

PIEZOELECTRIC SENSORS

Pressure Sensor Tactile Sensor

• The term tactile sensor usually refers to a transducer that is sensitive to touch, force, or pressure. • The term tactile sensor usually refers to a transducer that is sensitive to touch, force, or pressure. • Tactile sensors are useful in a wide variety of applications for robotics and computer hardware and even security systems.

Pressure Sensor Strain Gauge Load Cell • A load cell is a “load transducer” which converts the weight or load acting on it into electrical signals. • A load cell is composed of an aluminum alloy spring element, strain gauges (serving as sensors) and a bridge circuit. • The strain gauges themselves are bonded onto four areas which become considerably distorted in the spring element. The load cell detects the force of the distortion as voltage change.

Speed Sensors According to physics, speed is related to motion. Distance travelled by a moving body in a given time is called speed. Other related terms are angular speed, velocity, and angular velocity. Angular speed is the angular distance per unit time. (Linear) Velocity and angular velocity are vector quantities and are defined as displacement (linear/angular) per unit time.

Speed Sensors Various kinds of principles are employed in speed sensors. Some of them are listed below 1. Variable reluctance based 2. Hall effect based 3. Eddy Current based 4. Radar Doppler based 5. LIDAR based 6. Accelerometer based 7. Pitot tube based. 8. Pitometer based.

Speed Sensors • VARIABLE RELUCTANCE SPEED SENSORS Variable Reluctance (VR) sensors convert mechanical motion to electric energy without direct contact when positioned near a gear, shaft, rotor, or other regularly moving device. As the ferromagnetic material, e.g., gear teeth, flywheel or other target features, pass by the sensor tip, the magnetic field is disrupted. The amount of magnetic flux passing through the magnet, and consequently the coil, to varies. Due to the time varying flux, a voltage is induced in the coil.

Speed Sensors • Hall Effect Sensors Hall Effect speed sensor uses a Hall Effect transducer element between the magnet and the target. As in VR sensors, the induced flux due to movement of the target is detected. But, a Hall transducer is sensitive only to the magnitude of flux; it does not sense its rate of change.

Speed Sensors • Eddy Current Speed Sensors Eddy Current principle is utilized in speed sensors the same way as Variable reluctance or Hall Effect. However, they need a preamplifier or a signal conditioner to operate. When a target is present, eddy currents are formed causing decrease in signal amplitude. The preamplifier demodulates the signal, detects the changes in voltage and produces an output whose frequency is directly proportional to the target speed

Speed Sensors • Radar Doppler Speed Sensors Echo of a radar signal sent toward a moving target will have its frequency shifted and the shift in the frequency is related to the velocity of the moving target. This occurs due to Doppler Effect.

Speed Sensors • LIDAR SPEED TRAPS LIDARs use the standard pulsed time of flight measurement technique using a micro-controller that processes the measured data to estimate the position and speed of the target. Velocity estimates are made using a tracking filter or by measuring the change in range as determined by the varying time of flight between successive pulses.

Speed Sensors • ACCELEROMETERS A very common method for measuring velocities in situations where magnetic field based sensors are not practical is to use accelerometers.

Speed Sensors Tachogenerator



The semiconductor element and the magnetic source are fixed relative to one another in a single package.



By moving the ferromagnetic member into the air gap between the magnetic source and the semiconductor element, the flux linkage can be altered. This changes Vo.



Suitable both as an analog proximity sensor and as a limit switch.



The relationship between the output voltage Vo and the distance of a Hall effect sensor measured from the moving member is non linear. Linear Hall effect sensors use calibration to linearize their outputs.

DIGITAL TACHOMETER

WHERE USED:

DIGITAL TACHOMETER AS A MEASUREMENT DEVICE

Speed Sensors • PITOT TUBES Pitot tubes are used by the aircrafts for measurement of their flying speeds. Pitot based system functions as the speedometer of the aircraft.

Speed Sensors • PITOMETERS Pitot tubes are used for aircrafts; Pitometers are used for ships and submarines. Pitometers measure the speed of the ship with respect to the water

Speed Sensors SELECTION OF SPEED SENSORS 1. Application Scenario Though selection of a speed sensor depends upon number of factors, it is primarily dependent upon the application. 2. Type of Output One of the factors may be the output format; digital or analog. 3. Target Details(for sensors operating on principle of magnetic field) The output of a speed sensor is highly dependent upon application details such as the size, shape and material of the target. 4. Operational temperature range. Selected sensor must be able to operate in the complete operating range