M1.pdf - Introduction to Transducers and Sensors PDF

# Module - 1 ## Introduction. Transducers and Sensors **Introduction:** - Definition - Multidisciplinary Scenario - Evolution of Mechatronics - Design of Mechatronics system - Objectives - Advantages and disadvantages of Mechatronics **Transducers and Sensors:** - Definition and classification...

# Module - 1 ## Introduction. Transducers and Sensors **Introduction:** - Definition - Multidisciplinary Scenario - Evolution of Mechatronics - Design of Mechatronics system - Objectives - Advantages and disadvantages of Mechatronics **Transducers and Sensors:** - Definition and classification of transducers - Difference between transducer and sensor - Definition and classification of sensors - Principle of working and applications of light sensors, proximity switches and Hall Effect sensors. ## 1.1 Introduction to Mechatronics In recent years, the application of *digital microelectronics* and *computers in design and manufacturing sectors* has considerably improved *productivity and quality of many mechanical products*. Automation and control methods, *adopting integrated embedded technology*, has become relevant to *industries, machineries and consumer products*. Automation and control represent a broad topic with applications in: - Production - Industrial and manufacturing engineering - Process control - Robotics - Instrumentation - Home automation - And many others based upon sophistication, flexibility and state-of-the-art technology. Mechatronics, an enabling discipline, has already emerged to cater to the need for *sophistication and flexibility* and in fact has become a preferred choice for the current generation of *real-time automation and control solutions* for traditional mechanical systems. ## 1.1.1 Definitions of Mechatronics Mechatronics is a word originated in *Japan in and around 1980s* to denote the combination of technologies which go together to produce *industrial robots*. A formal definition of Mechatronics is "the synergistic integration of Mechanics and Mechanical Engineering, Electronics, Computer technology, and IT to produce or enhance products and systems". Mechatronics is the synergistic integration of: - Sensors - Actuators - Signal conditioning - Power electronics - Decision and control algorithms - Computer hardware and software To manage complexity, uncertainty, and communication in engineered systems. According to the Mechatronics Forum (UK), a precise and common definition is as follows. "Mechatronics is the synergistic integration of *mechanical engineering* with *electronics and intelligent control algorithms* in the design and manufacture of products process". ## 1.2 Multidisciplinary Scenario The worldwide technical education curriculum has been a *reverse pyramid structure*. in contrast to traditional teaching methodology, which are separated by an axis. This figure shows how the original *electrical and mechanical disciplines* have given birth to *new disciplines* like electronics and production engineering, respectively, which further encouraged many other branches to emerge over a period of time, however, diverging. This is because of the requirement of *interdisciplinary knowledge* at the production place. For manufacturers, adoption of *modern and matured technology* with improved capability is paramount in order to meet their competitive challenges in the technology marketplace. *In fact, new multi-disciplinary subject* in the name of "mechatronics" has been attracting not only manufacturers, but also engineers, developers, researchers, and academicians. With the advent of *digital technology*, low cost VLSI, embedded technology, control networking systems (field bus technology), microcontrollers, industrial computers, information technology (data networks such as LAN, WAN, MAN, etc.), advanced software tools and techniques, real-time tools and techniques, soft computing methods, and Agile Manufacturing Engineering (AME), the engineering field is being radically altered. Technological design has become a *high-risk endeavor* due to the lack of knowledge and experiences on interdisciplinary subjects and methods. Advanced technological designs are highly complex and interdisciplinary nature involving synergistic integration of *mechatronics, photonics, computronics, and communication*. Technological development and innovations would thus require simultaneous knowledge of discrete fundamentals already developed to date. Synergistic integration is solitarily *logic-based integration*. Combined action and cooperation *increases effectiveness and productivity*. ## 1.3 Origin and Evolution of Mechatronics The field of mechatronics has received *international recognition* only within the last few years, although it is rooted in efforts on sensors and actuators that go back thirty years or more. The field has been driven *by rapid global progress in the field of microelectronics*, where solid-state *microprocessors and memory* have revolutionized many aspects of instrumentation and control, and have facilitated *explosive growth in data processing and communications.* The word *mechatronics* originated in Japan in and around 1980s to describe a philosophy adopted in designing subsystems of *electro-mechanical products*. Since those early days there have been *major advances in the technology* and the methods that have become available to *manufacturing industries*. Mechatronics, although *still a relatively new term* compared with many of the traditional branches of Engineering, now appears firmly established. Individuals, industries, and universities around the world are now using the term freely. At the research and development (R&D) level, ten *technical areas are classified* under the mechatronics discipline. They are, - Motion Control - Robotics - Automotive Systems - Intelligent Control - Actuators and Sensors - Modelling and Design - System Integration - Manufacturing - Micro Devices and Optical electronics - Vibrations and Noise Control ## 1.3.1 Evolution of Mechatronics Recently, technology sectors all over the world have *recognized the importance* of the mechatronics discipline for their product design for which it has become a buzzword. The *Japanese technocrats* such as control system integrators, *consumer electronics* manufacturers, university researchers, etc originated the concept. Later, Scandinavian, American, and European engineers experienced the significance and applicability in the design of electromechanical systems and also *in the other notable application areas*. In fact, the mechatronics-based design concept now exists in the domains such as *consumer products*, automatic cash machines, robots, printers, heavy and light vehicle *engines*, air craft engines, door locks, surgical instruments, industrial machines, agricultural equipment, and household equipment, to name a few. It is apparent that the *knowledge of mechatronics* is a prime requirement and is *considered fundamental* to engineers of all fields. Mechatronics has been defined variously. According to the *Mechatronics Forum (UK)* a precise and common definition is as follows: “Mechatronics is the synergistic integration of *mechanical engineering* with *electronics and intelligent control algorithms* in the design and manufacture of products process.” The term *synergistic* plays an important role in framing the *syllabus for mechatronics subjects*. Synergistic integration means the *mechatronics engineers* do not have to study selected but *definitive portions* of mechanical engineering, *electronics/electrical engineering*, computer engineering, and control engineering. This is because of the fact that some subjects are redundant as far as *the design and manufacture of products process* is concerned. Aspects of engineering that are vital for the design and manufacture of *products process is to be included.* ## 1.4 Design of Mechatronics System With regard to the design of mechatronics systems, the concentration is on studying *fundamental aspects of sensors, actuators, control and integration methods*. Depending upon the *load involved and sophistication required*, actuating devices, sensing elements, and control algorithms are implemented. A system can be thought of as a box or a bounded whole which has *input and output elements*, and a set of relationships between *these elements*. Figure 1.3 shows a *typical spring system*. It has 'force' as an input which produces an 'extension'. The input and output of this system follows the *Hooke's law F = -kx*. Where F is force in N. x is distance in m and k is stiffness of the spring. A Mechatronics system integrates *various technologies* involving sensors, measurement systems, drives, *actuation systems*, microprocessor systems and software engineering. *Consider the example of a simple spring-mass system* as shown in figure above. To replace the mechanics of this mechanical system with an equivalent *mechatronics based system*, we need to have the basic controlling element, a *microprocessor*. Microprocessor processes or utilizes the information gathered from the *sensor system* and generates the signals of appropriate level and suitable kind (current or voltage) which will be used to *actuate the required actuator*. *A hydraulic piston-cylinder device* for extension of piston rod in this case. The microprocessor *is programmed on the basis of the principle of Hooks' Law*. The schematic of *microprocessor based equivalent spring mass system* is shown in figure 1.4. The input to the system *is a force which can be sensed by suitable electro-mechanical sensors* viz. piezo-electric device or strain gauges. *These sensors generate either digital signals (0 or 1) or analogue signals (milli-volts or milli-amperes)*. These signals are then converted into right form and *are attenuated to a right level* which can properly be used by the microprocessor *to take generate the actuation signals*. Various *electronics based auxiliary devices* viz. Analogue-to-Digital Converter (ADC), Digital-to-Analogue Converter (DAC), Op-amps, Modulators, *Linearization circuits*, etc. are used to condition the signals which are *either received by the microprocessor from the sensors* or are sent to *the actuators from the microprocessor*. This mechatronics-based spring-mass system has the input signals in the *digital form which are received from the ADC and Piezo-electric sensor*. The digital actuation signals generated by the *microprocessors are converted* into appropriate analogues signals. *These analogue signals operate the hydraulic pump and control valves* to achieve the desired displacement of the piston-rod. ## 1.5 Objectives, Advantages and disadvantages Mechatronics is an *interdisciplinary field* in which the following disciplines act together: *electronic systems, information technology and mechanical systems*. Nowadays, mechatronic systems can be *found everywhere:* in medical applications (medical robotics, dental machines), in *vehicular applications* (automatic gear, engines with electronics, automatic and driver-less vehicles), in production industry (industrial robotics, production lines), in *aerospace* (mobile robotics, robotic arms, actuated wings tillers), in precision systems (atomic force microscopes, hard disk driver, electronic-mechanical watches, microrobotics), *in petroleum engineering* (extraction machines, actuated networks of pipelines). The objectives of mechatronics are the following. - To improve products and processes - To develop novel mechanisms - To design new products - To create new technology using novel concepts *Earlier the domestic washing machine used cam-operated switches in order to control the washing cycle.* Such mechanical switches have now been replaced by microprocessors. A microprocessor is a *collection of logic gates and memory elements whose logical functions* are implemented by means of software. The application of mechatronics has helped to improve many *mass-produced products* such as the domestic washing machine, dishwasher, microwave oven, cameras, watches, and so on. *Mechatronic systems are also used in cars* for active suspension, antiskid brakes, engine control, speedometers, etc. *Large-scale improvements have been made using mechatronic systems in flexible manufacturing engineering systems (FMS)* involving computer controlled machines, robots, automatic material conveying and, *overall supervisory control.* There are many advantages of mechatronic systems. Mechatronic systems have made it *very easy to design processes and products.* Application of mechatronics facilitates rapid setting up and *cost effective operation* of manufacturing facilities. Mechatronic systems help in optimizing performance and quality. These can be adopted to changing needs. Mechatronic systems are *not without their disadvantages*. One disadvantage is that the field of mechatronics *requires a knowledge of different disciplines*. Also, the design cannot be finalized and *safety issues are complicated* in mechatronic systems. Such systems also require *more parts than others, and involve a greater risk of component failure.* ### Advantages of Mechatronics: - Simplified mechanical design. - Rapid machine setup. - Rapid development trials. - Adaptation possibilities. - Optimized performance, productivity, reliability. ### Disadvantages of Mechatronics: - Different expertise required - Real-time calculations/mathematical models - Increased power requirements - Lifetimes change/vary - Increase in component failures - More complex safety issues ## Illustrative Examples ### Example 1.5.1 The automatic camera The modem camera is likely to have automatic focusing and exposure. Figure illustrate the baste aspects of a micro-prossesor-basic system that can be used to control the focusing and exposure. - When the switch is operated to activate the system and the camera pointed at the object being photographed, the micro-processor takes in the input from the range sensor and sends an output to the lens position drive to move the lens to achieve focusing. The lens position is fed back to the microprocessor so that the feedback signal can be used to modify the lens position according to the input from the range senior. - The light sensor gives an input to the microprocessor which then gives an output to determine, if the photographer has selected the shutter controlled rather than aperture controlled mode, the time for which the shutter will be opened. - When the photograph has been taken, the microprocessor gives an output to the motor drive to advance the film ready for the next photograph. ### 1.5.2 Engine Monitoring system The power and speed of the *engine are controlled* by varying the *ignition timing* and the *air-fuel mixture*. With modern car engines this is *done by a microprocessor*. Figure 1.6 *shows the basic elements of a microprocessor control system*. For ignition timing, the crankshaft drives a *distributor which makes electrical contacts* for each spark plug in turn and a liramg wheel. This timing wheel *generates puises* to indicate the crankshaft position. The microprocessor *then adjusts the timing* in which high voltage pulses *are sent to the distributor* so they occur at the 'right' moments of time. To control the *amount of air-fuel mixture entering a cylinder* during the intake strokes, *the microprocessor varies the time for which a solenoid is activated* to open the intake valve on the basis of *inputs received of the engine temperature and the throttle position*. The amount of fuel to be injected into the air stream can be *determined by an input from a sensor of the mass rate of air flow* or computed from other measurements, and *the microprocessor then gives an output to control a fuel injection valve.* ### 1.5.3 Tool monitoring systems Tool wear is a critical factor which affects the *productivity of a machining operation*. Complete automation *of a machining process realizes* when there is a *successful prediction of tool* (wear) state during the course of machining operation. *Mechatronics based cutting tool-wear condition monitoring system* is an integral part of *automated tool rooms and unmanned factories.* These systems *predict the tool wear* and give alarms to the system operator to prevent *any damage to the machine tool and workpiece.* Tool wear can be observed in a variety of ways. These can be classified in two groups: | **Direct Methods** | **Indirect Methods** | |---------------------------|-------------------------------------------------------------| | Electrical resistance | Torque and power | | Optical measurements | Temperature and acoustic emission | | Machining hours | Vibration and strain measurements | | Contact sensing | Cutting forces | *Direct methods deal with the application of various sensing and measurement instruments such as micro-scope, machine/camera vision; radioactive techniques to measure the tool wear.* The used or worn-out *cutting tools will be taken* to the metrology or inspection section of the tool room or *shop floor where they will be examined* by using one of direct methods. However, these methods *can easily be applied in practice* when the cutting tool is not in contact with the work piece. Therefore they are called as *offline tool monitoring system*. Figure shows *a schematic of tool edge grinding or replacement scheme based on the measurement carried out using offline tool monitoring system.* Offline methods are *time consuming and difficult to employ during the course of an actual machining operation at the shop floor.* Indirect methods *predict the condition of the cutting tool by analyzing the relationship between cutting conditions and response of machining process* as a measurable quantity through sensor signals output such as force, acoustic emission, vibration, or current. Figure 1.7 shows a *typical example of an on-line tool monitoring system*. It employs the cutting forces recoded *during the real-time cutting operation to predict the tool wear.* The cutting forces can be sensed by *using either piezo-electric or strain gauge based force transducer*. A micro-processor based control system *continuously monitors 'conditioned signals received from the Data Acquisition System (DAS)*. It is generally programmed/trained with the past recorded empirical data for a *wide range of process conditions* for a variety of materials, and takes the decision to *change the tool or gives an alarm to the operator*. An electrical switch is a *man-made mechatronic system*, used to control the flow of electricity. The toggle of the switch is a *mechanical system* and the *human brain or a control system used to actuate the switch* acts as an informatics system. *The brain or informatics system decides whether we need to turn on the switch.* If we do, the brain controls the movement of our limbs and *we turn on the switch.* When the switch is on, the *resistance of contact is nearly zero and energy flow takes place.* When the switch is off, the *resistance is infinity and no current flows.* ## 1.5.4 Thermostatically Controlled Heater A thermostatically controlled heater or furnace is a mechatronic system. *The input to the system is the reference temperature.* The output is the actual temperature. *When the thermostat detects that the output is less than the input*, the furnace provides heat until the *temperature of the enclosure becomes equal to the reference temperature.* Then the furnace is automatically turned off. Here, the *bimetallic strip of the thermostat acts as informatics* since it automatically turns the switch on or off. *The lever-type switch is mechanical system whereas the heater acts as an electrical system*. ## 1.5.5 Mechatronic Washing Machines Most washing machines are operated in the following manner. After *the clothes to be washed have been put in the machine*, the soap, detergent, bleach, and water are put in required amounts. *The washing and wringing cycle time is then set on a timer and the washer is energized.* When the *required amount of detergent*, bleach, water, and appropriate temperature are predetermined *and poured automatically by the machine itself*, then the machine is a mechatronic system. *The microprocessor used for this purpose acts as the informatics system.* The electrical motor actuated for wriggling is *an electrical system*. The agitator and timer are *mechanical systems*. The washing machine is an ideal example of a *mechatronic system*. ## 1.5.6 Automatic Bread Toaster The automatic bread toaster is a mechatronic system, in which two heating elements *supply the same amount of heat to both sides of the bread*. The *quality of the toast can be determined by its surface colors'*. When the bread is toasted, *the color detector sees the desired color, and the switch automatically opens and a mechanical lever makes the bread pop up*. Mechanical, electrical, and informatics systems are involved in the *operation of the bread toaster*. ## 1.6 Introduction to Transducers and Sensors Measurement *is an important subsystem of a mechatronics system*. Its main function is to collect the information *on system status and to feed it* to the micro-processor(s) for controlling the whole system. *Measurement system comprises* of sensors, transducers and signal processing devices. ### Sensor It is defined as *an element which produces signal* relating to the quantity being measured. According to the *Instrument Society of America*, sensor can be *defined as "A device which provides a usable output in response to a specified measurand."* Here, the output is usually an 'electrical quantity' and measurand is a ' *physical quantity, property or condition which is to be measured'*. Thus, in the case of, say, a variable inductance displacement element, the *quantity being measured is displacement and the sensor transforms* an input of displacement into a change in inductance. ### Transducer *It is defined as an element when subjected to some physical change* experiences a related change or an element which converts a *specified measurand into a usable output* by using a transduction principle. It *can also be defined as a device* that converts a signal from one form of energy to another form. *A wire of Constantan alloy* (copper-nickel 55-45% alloy) can be called as a sensor because *variation in mechanical displacement (tension or compression) can be sensed as change in electric resistance.* This wire becomes a transducer with appropriate electrodes and *input-output mechanism attached to it*. Thus, we can say that "sensors are transducers" The term *transducer and sensor* have been used synonymously although the concepts are different. *Transducers are the physical element*, which is a part of a sensor. In fact, transducer is an essential *element of a sensor.* A sensor is merely a sophisticated transducer in the sense that *it contains some signal conditioning circuits* capable of *amplifying and refining the weak and raw signal* that is available at the output *of the transducer.* Some of the commonly used signal conditioning Circuits is *amplifiers, filters, analog to Digital Converter (ADC), etc.* Fig. 1.8 gives a illustration of a sensor. The input signal is referred to as *measurands.* The output of the transducer is referred to as *equivalence*. ## 1.6.1 Sensor/transducers specifications : Transducers or *measurement systems are not perfect systems*. Mechatronics design engineer must *know the capability and shortcoming* of a transducer or measurement system to properly assess its performance. *There are a number of performance related parameters* of a transducer or measurement system. These parameters are called *sensor specifications*. Sensor specifications inform *the user to the about deviations from the ideal behaviour* of the sensors. Following are the various specifications of *a sensor/transducer system*. 1. Range: The range of a sensor indicates the *limits between which the input can vary*. 2. Span: The span is *difference between the maximum and minimum values* of the input. 3. Error: Error is the difference between the *result of the measurement and the true value* of the quantity being measured. 4. Accuracy: The accuracy *defines the closeness of the agreement between the actual measurement result* and a true value of the measurand. It is often expressed as a *percentage of the full range output* or full-scale deflection. 5. Sensitivity: Sensitivity of a sensor is defined as the *ratio of change in output value of a sensor* to the per unit change in input value that causes the output change. 6. Resolution: Resolution is the *smallest detectable incremental change* of input parameter that can be detected in the output signal. Resolution can be expressed either as *a proportion of the full-scale reading* or in absolute terms. 7. Stability: Stability is the *ability of a sensor device to give same output* when used to measure a constant input over a period of time. 8. Repeatability: It specifies the *ability of a sensor to give same output* for repeated applications of same input value. It is usually expressed *as a percentage of the full range output* 9. Response time: Response time *describes the speed of change in the output on a step-wise change of the measurand*. It is always specified *with an indication of input step and the output range for which the response time is defined.* 10. Hysteresis : *It is an error of a sensor*, which is defined as the maximum difference in output at *any measurement value within the sensor's specified range* when approaching the point *first with increasing and then with decreasing the input parameter.* ## 1.7 Definition and Classification of Transducers Transducer is a device which *transforms a nonelectrical physical quantity* (i.e. temperature, sound or light) into an electrical signal (i.e. voltage, current, capacity...). In other word it is a device that *is capable of converting the physical quantity into a proportional electrical quantity* such as voltage or current. ## 1.8 Definition & Classification of sensors : The input device which *provides an output (signal) with respect to a specific physical quantity (input)*. The term "input device” in the *definition of a Sensor* means that it is part of a bigger system *which provides input to a main control system* (like a Processor or a Microcontroller). *Another unique definition of a Sensor* is as follows: It is a device that converts signals from *one energy domain to electrical domain.* Sensors *can be classified into various groups* according to the factors such as measurand, application fields, *conversion principle*, energy domain of the measurand and thermodynamic considerations. Detail classific*ation of sensors in view of their applications in manufacturing is as follows.* **A. Displacement, position and proximity sensors** - Potentiometer - Strain-gauged element - Capacitive element - Differential transformers - Eddy current proximity sensors - Inductive proximity switch - Optical encoders - Pneumatic sensors - Proximity switches (magnetic) - Hall effect sensors **B. Velocity and motion** - Incremental encoder - Tachogenerator sensors **C. Force** - Strain gauge load cell **D. Fluid pressure** - Diaphragm pressure gauge - Capsules, bellows, pressure tubes. Piezoelectric sensors - Tactile sensor **E. Liquid flow** - Orifice plate - Turbine meter **F. Liquid level** - Floats - Differential pressure **G. Temperature** - Bimetallic strips - Resistance temperature detectors - Thermistors - Thermo-diodes and transistors - Thermocouples - Light sensors - Photo diodes - Photo resistors ## 1.9 Difference between Transducers and Sensors | | **Sensor** | **Transducer** | |:-----------|:----------------------------------------------------------------|-------------------------------------------------------------------| | **Definition** | A sensor is a device which detects one form of energy and converts the data to electrical energy. | A transducer is a *device which converts one form of energy into another*. So sensors are, *in fact, a type of transducer*. | | **Function** | A sensor is a device which detects a physical quantity *and produces an electric signal based on the strength of the quantity measured* | A transducer is a device which *converts one form of energy into another form plus any associated sensing element*. | | **Sensing Element** | Sensing element itself | Any *forms of energy* can be used *transducers can convert* between any forms of energy, *they can be used to provide feedback to the system*. | | **Feedback** | A sensor *merely measures a quantity and cannot, by itself, give feedback* to the system | | ## 1.10 PRINCIPLE AND WORKING OF LIGHT SENSOR The Light sensors are *semiconductor devices and this operation* is based on the change in the resistance and current flow in the circuit when light falls on them. There are *Three basic types of light sensors.* - Photo Diode - Photo transistor - Photo Register Photodiode is a *two terminal electronic device* which, when exposed to light the *current starts flowing in the diode*. It is operated *in reverse biased mode only*. It converts *light energy into electrical energy*. ### Working Principle of Photodiode The junction of Photodiode is illuminated by the light source, the photons *strike the junction surface.* The photons impart *their energy in the form* of light to the junction. Due to which electrons *from valence band get the energy to jump into the conduction band* and contribute to current. *In this way*, the photodiode converts light energy into *electrical energy.* Figure 1.14 *shows the working principle of Light sensor (Photo diode)*. ### Applications of Photodiodes - It is used for detection *of both visible as well as invisible light rays*. - Photodiodes *are used for the communication system* for encoding & demodulation purpose. - It is also *used for digital and logic circuits* which require fast switching and high-speed operation. - These diodes also *find application in character recognition techniques and IR remote control circuits*. - Photodiodes are considered *as one of the significant optoelectronics devices* which is extensively used in the *optical fiber communication system*. Phototransistors resemble *normal transistor except the fact that the base terminal is not present in case of* the phototransistor. Phototransistors convert *the incident light into photocurrent*. Instead of providing the base current for triggering the transistor, *the light rays are used to illuminate the base region*. The *symbol and circuit of the phototransistor* is described in the Figure 1.15. ### Working of the Phototransistor The base terminal is *made up of the material which shows sensitivity towards the light*. The circuit symbol of the phototransistor *is similar to that of the conventional transistor but the base terminal can be omitted*. The two arrows *point towards phototransistor* indicates that the phototransistor is triggered by the light incident on it. *The output of the phototransistor is taken from the emitter terminal and the light rays* are allowed to enter the base region. The magnitude of the *photocurrent generated by the phototransistor depends on the light intensity* of the light falling on the transistor. ### Applications of Phototransistors - Counting Systems: *The phototransistors are commonly used in counting systems*. As this device *works with the help of incident light*, thus it is much easy to utilize such device in the *computing system, as we don't need to worry about power supply*. - Encoder sensing and object detection: *The phototransistors can be used to detect the object or for encoding*. - Printers and Optical control remotes: *Due to its high light to current conversion efficiency*, it is commonly used *in optical devices such as remotes, printers etc*. - Light detector: *The most crucial application of phototransistor* is to use it as the light detector. *This is because it can detect even a small amount* of light because it is highly efficient. - Level Indication and Relays: *The phototransistors are also used to indicate the level* in the various systems. They also play a *vital role in relays and punch cards*. - Phototransistors are *the crucial optoelectronics device*, it is also used in optical fibres. ## Photoresistor The name photoresistor is the *combination of words: photon (light particles) and resistor*. A photoresistor is a type of *resistor whose resistance decreases when the intensity of light increases.* In other words, the *flow of electric current through the photoresistor increases when the intensity of light increases*. Photoresistors *are also sometimes referred as LDR (Light Dependent Resistor), semiconductor photoresistor, photoconductor, or photocell*. Photoresistor *changes its resistance only when it is exposed to light*. The American standard symbol and *the international standard symbol of the photoresistor* is shown in the below Figure 1.16. ### Working of the Photoresistor When *the light falls on the photoresistor*, some of the valence electrons *absorbs energy from the light and breaks the bonding with the atoms*. The valence electrons, which break the bonding with the atoms, *are called free electrons.* Figure 1.17 shows the working principle of Photoresister. ### Applications of photoresistors - Photoresistors *are used in streetlights to control when the light should turn on and when the light should turn off*. When *the surrounding light falls* on the photo resistor, *it causes the streetlight to turnoff*. When there is no light, *the photoresistor causes the street light to turn on*. This reduces *the wastage of electricity*. - They are also *used in various devices* such as alarm devices, solar street lamps *night-lights, and clock radios*. ## 1.11 PROXIMITY SENSORS A proximity sensor is a sensor *able to detect the presence of nearby objects without any physical contact*. While ambient *light sensors use the visible part of the spectrum*, proximity sensors use *the infrared (IR) wavelengths*. Instead of measuring the surrounding light, *a proximity sensor gauges the closeness of an object - like your face by detecting the IR reflection strength.* ### Working of the Proximity sensors Proximity sensors operate *with reflections, as shown in Figure 1.18*. an IR LED is chosen *to send out an infrared signal*. Any object in front of the IR LED *causes some of that signal to be reflected back to proximity sensor*. More signal is reflected *when an object is closer*. After calibrating (for losses like the filtering of the cell phone glass), the output *of the proximity sensor reveals the distance of an object.* There are *many design considerations when using a proximity sensor*. The first is obviously the *choice of the IR LED to be used*. The second is *the composition of the object to be sensed*. Most sensors *are calibrated with one of the standard reflective surfaces*, like 18% reflective gray paper. More signal will be returned to the sensor *if the surface has a higher reflectivity*. ### Applications of Proximity sensor - Parking sensors, systems mounted on car bumpers that sense *distance to nearby cars for parking* - Ground proximity warning system for *aviation safety* - Vibration *measurements of rotating shafts in machinery* - Top dead centre (TDC)/camshaft sensor *in reciprocating engines*. - Sheet break sensing in *paper machine*. - Anti-aircraft warfare - Roller coasters - Conveyor systems - Beverage and food *can making lines Mobile devices* - Touch screens that *come in close proximity to the face.* - Attenuating radio power *in close proximity to the body, in oder to reduce radiation exposure.* Proximity sensors *include all sensor that perform non contact detection in comparison to sensors such as limit switch*, that detects the object *by physically contacting them.* Proximity sensors are used *in various facets of manufacturing for detecting the approach of metal and non metal objects.* ## Different types of PROXIMITY sensors are: *Proximity switches are classified as:* 1. Inductive Proximity Switch es 2. Capacitive Proximity Switches 3. Photoelectrical Proximity Switches 4. Ultrasonic Proximity Sensor - Switch ### 1. Inductive Proximity switch *produces high frequency alternating field at the sensing face.* When any metallic material *enters in the sensing zone of the switch*, the field gets disturbed. A Sensitive detector circuit *senses this change which* is further processed by amplifier circuit to produce output signal ### Working principle Inductive proximity sensors *detect the presence of metallic objects*. Their 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 the operating area causes a change in the output of the sensor*. The operating distance *of the sensor depends on the coil's size as well as the target's shape, size and material.* An inductive sensor *is an electronic proximity sensor,* which *detects metallic objects without touching them*. Electric current *generates a magnetic field*, which *collapses generating a current* that falls asymptotically toward zero from its initial level when the input electricity ceases. ### Construction The proximity *inductive sensor basically consists of a wound coil located in front of a permanent magnet*. The permanent magnet is encased in a rugged housing. The *change in current in the coil* is output through the leads embedded in the resin. The *leads connected to the display through a connector gives signal for the presence* of an object in the vicinity. ### Application of Inductive Proximity Switches - Railroad yard position sensing - Nut placement *on transformer* - Sort ferrous & non *ferrous can tops* - Detecting incorrect shape of target - Position sensor - Speed sensing of *machines* - Tablet Counting - Positioning of bottles - Valve position *control* - Position sensor for *galvanising plant* - Drill *break sensor* - Part counter ### 2. Capacitive Proximity switches: When the object *enters in the sensing zone of the switch, capacitance between* two plates of capacitor *one plate is represented by electrode at sensing face of the switch* and another by all surrounding *material which is connected to the earth) changes*. As soon as *capacitance value crosses present level,* oscillator starts. This change is detected and *resulted in an output signal*. ### Working principle Capacitive sensors *are used for non-contact detection* of metallic objects & nonmetallic objects * (liquid, plastic, wooden materials and so on)*. Capacitive proximity sensors use the *variation of capacitance between the sensor and the object being detected.* When the object is at a preset distance *from the sensitive side of the sensor,* an electronic circuit inside the sensor begins to oscillate. *The rise or fall of such oscillation* is identified by a threshold circuit that drives *an amplifier for the operation of an external load*. ### Application of Capacitive Proximity Switches - Detecting *flow of material* - Belt breakage detection - Detecting *empty cartons* - Sensing level of *material from outside the plastic tank* ### 3. Photoelectrical Proximity Switches: The *proximity of the object is detected by the action of the travelling light move.* The light emitted by *the transmitter focuses on the object which reflects to be received by the receiver photo diode*. The light from the emitting diode is *focused by the transmitter lens,* on to the object surface. *The reflected waves travel back and received* by the solid state photo diode, *through the receiver lens*. The object within the range of the sensor *can detect the presence.* The focal length of the sensor lenses decide the range within which *the proximity of the object is detected*. ### Application of Ultrasonic Proximity Sensor - Switch - People detection - Height & *weight measurement* - Level control - Loop control - Glass detection - Stack *height control* - Monitoring foil - Checking *diameter* ## 1.