Sensors PDF

SENSORS  A sensor is an element in a mechatronic or measurement system that detects the magnitude of a physical parameter and changes it into a signal that can be processed by the system. Often the active element of a sensor is referred to as a transducer. Monitoring and control systems...

SENSORS  A sensor is an element in a mechatronic or measurement system that detects the magnitude of a physical parameter and changes it into a signal that can be processed by the system. Often the active element of a sensor is referred to as a transducer. Monitoring and control systems require sensors to measure physical quantities such as position, distance, force, strain, temperature, vibration, and acceleration. Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand  A transducer is a device that converts energy from one form to another. Transducers that convert physical quantities into mechanical ones are called mechanical transducers; Transducers that convert physical quantities into electrical are called electrical transducers. Examples are a thermocouple that changes temperature differences into a small voltage, or a Linear variable differential transformer (LVDT) used to measure displacement. Source: https://en.wikipedia.org/wiki/Transducer  Active vs Passive Sensors Active sensors require an external power source to operate, which is called an excitation signal. The signal is modulated by the sensor to produce an output signal. Example: Photoelectric sensor Passive sensors, in contrast, generate an electric current in response to an external stimulus which serves as the output signal without the need of an additional energy source. Examples: piezoelectric sensor and thermocouple. Source: https://en.wikipedia.org/wiki/Transducer  SENSORS Temperature Sensors Optical Sensors Electric and Magnetic Sensors Mechanical Sensors Microelectromechanical Systems(MEMS) TEMPERATURE SENSORS  Temperature Sensors The classical thermometer is clearly a sensor, but in its common configuration it does not provide an output signal. Its output is read directly and can’t be inputted to a controller for further use. Several temperature sensors in which the output can be used for further processing are outlined in succeeding slides. Thermocouple Two dissimilar metals in contact (see figure) form a thermoelectric junction that produces a voltage proportional to the temperature of the junction. This is known as the Seebeck effect. Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand  Thermocouple Because an electrical circuit must form a closed loop, thermoelectric junctions occur in pairs, resulting in what is called a thermocouple. We can represent a thermoelectric circuit containing two junctions as illustrated in Figure. CHROMEL – 90% nickel & 10% chromuim ALUMEL – 94% nickel, 3% manganese, 2% aliminum & 1% silicon Thermocouple Resistance Temperature Detector/Device A resistance temperature detector/device (RTD) is constructed of metallic wire wound around a ceramic or glass core and hermetically sealed. The resistance of the metallic wire increases with temperature. How RTDs are connected? Wheatstone Bridge Circuit The most common and simplest bridge network to find the resistance is the DC Wheatstone Bridge. This bridge is used where small changes in resistance such as sensors are to be measured. This is used to convert a resistance change to a voltage change. Wheatstone Bridge Circuit Example Wheatstone Bridge Example Thermistors A thermistor is a semiconductor device, available in probes of different shapes and sizes, whose resistance changes exponentially with temperature. Thermistors have much narrower operating ranges than RTDs. Thermistors Thermistors are available in two types: those with Negative Temperature Coefficients (NTC Thermistors) and those with Positive Temperature Coefficients (PTC Thermistors). An NTC Thermistor's resistance decreases as its temperature increases. A PTC Thermistor's resistance increases as its temperature increases. NTC Thermistors are more commonly used for temperature measurement while PTC Thermistors are primarily used for circuit protection. p-n JUNCTION TEMPERATURE SENSORS When a p-n junction is forward biased, the current through the diode is temperature dependent. This current can be measured and used to indicate temperature. Alternatively, the voltage across the diode can be measured (almost always a preferable approach) and its dependence on temperature used as the sensor’s output. This type of sensor is called a p-n junction temperature sensor or bandgap temperature sensor.  Ex: LM35 Temperature Sensor The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature. Some features: Calibrated directly in degrees centigrade Linear + 10.0 mV/˚C scale factor Operates from 4 to 30 volts  Bimetallic Strip It is composed of two or more metal layers having different coefficients of thermal expansion. The strip can be straight, as shown in the figure, or coiled for a more compact design. Because these layers are permanently bonded together, the structure will deform when the temperature changes. This is due to the difference in the thermal expansions of the two metal layers. The deflection can be related to the temperature of the strip. Bimetallic strips are used in household and industrial thermostats where the mechanical motion of the strip makes or breaks an electrical contact to turn a heating or cooling system on or off. Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand Bimetallic Strip OPTICAL SENSORS  Optical Sensors Optical sensors are those sensors that detect electromagnetic radiation in what is generally understood as the broad optical range—from far Infrared to Ultraviolet. The sensing methods may rely on direct methods of transduction from light to electrical quantities such as in photovoltaic sensors or indirect methods such as conversion first into temperature variation and then into electrical quantities. Light Dependent Resistor (LDR) or photoresistor LDR is one of the widely used photo sensors. They are basically resistors whose resistance depends on the intensity of light. Their resistance variation is continuous and inversely proportional to the intensity of light. In the dark, their resistance is very high, but when the LDR sensor is exposed to light, the resistance drops dramatically, depending on the light intensity. One simple use of a Light Dependent Resistor, is as a light sensitive switch as shown above. Photo-diode The construction of the Photodiode light sensor is similar to that of a conventional PN-junction diode except that the diodes outer casing is either transparent or has a clear lens to focus the light onto the PN junction for increased sensitivity. The junction will respond to light particularly longer wavelengths such as red and infra-red rather than visible light..  Photo-diode Construction and Characteristics When used as a light sensor, a photodiodes dark current (0 lux) is about 10uA for geranium and 1uA for silicon type diodes. When light falls upon the junction more hole/electron pairs are formed and the leakage current increases. This leakage current increases as the illumination of the junction increases. Phototransistor A phototransistor is a light-sensitive transistor. A common type of phototransistor, called a photobipolar transistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base– collector junction. OPTICAL ENCODER A digital optical encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or decoding a set of bits, the pulses can be converted to relative or absolute position measurements. Encoders have both linear and rotary configurations, but the most common type is rotary. Rotary encoders are manufactured in two basic forms: the absolute encoder where a unique digital word corresponds to each rotational position of the shaft, and the incremental encoder, which produces digital pulses as the shaft rotates, allowing measurement of relative displacement of the shaft. Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand Digital Optical Encoder Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand Digital Optical Encoder Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand Digital Optical Encoder Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand Photoelectric Sensors A photoelectric sensor emits a light beam (visible or infrared) from its light-emitting element. A reflective type photoelectric sensor is used to detect the light beam reflected from the target. A thrubeam type sensor is used to measure the change in light quantity caused by the target crossing the optical axis. In retroreflective type, the light from the emitting element hits the reflector and returns to the light receiving element. When a target is present, the light is interrupted. Types of photoelectric sensors configuration: (a) reflective, (b) thru beam, (c) retroreflective (a) (b) (c) Source: https://www.keyence.com/ss/products/sensor/sensorbasics/photoelectric/info/ ELECTRIC AND MAGNETIC SENSORS  Proximity Sensor A proximity sensor is a non-contact sensor that detects the presence of an object (often referred to as the “target”) when the target enters the sensor's field. Magnetic, electrical capacitance, inductance, and eddy current methods are particularly suited to the design of a proximity sensor.  Types of proximity sensors: Inductive Sensors - Detection of metallic objects Capacitive Sensors - Detection of metallic and non-metallic objects (Liquids, plastics, woods) Photoelectric Sensors - Use light sensitive elements to detect objects Magnetic Sensors - Detects the presence of permanent magnets REED SWITCH Reed Switches consist of two flat ferromagnetic reeds sealed in an inert atmosphere within a glass capsule. In the presence of a magnetic field the reeds are attracted to each other and close to complete the magnetic and electric circuit. MAGNETO RESISTOR A special kind of variable resistor, whose electrical resistance depends on the external magnetic force applied. MAGNETO RESISTOR – Working Principle When current-carrying material is placed in the absence of any magnetic force, the free electrons move in orderly fashion. As soon as it is subjected to magnetic force, the free electrons get excited an start moving in an indirect motion creating collision among them. Thus, the resistance of magneto resistor increases with increase in magnetic field strength. Linear Variable Differential Transformer/Transducer The linear variable differential transformer (LVDT) is a transducer for measuring linear displacement. It consists of primary and secondary windings and a movable iron core. It functions much like a transformer, where voltages are induced in the secondary coil in response to excitation in the primary coil. The LVDT must be excited by an AC signal to induce an AC response in the secondary. The core position can be determined by measuring the secondary response.  Output voltage of LVDT in different positions Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand HALL EFFECT SENSOR Hall Effect Sensors are devices which are activated by an external magnetic field. The output signal from a Hall effect sensor is the function of magnetic field density around the device. When the magnetic flux density around the sensor exceeds a certain pre-set threshold, the sensor detects it and generates an output voltage called the Hall Voltage. Hall Effect Sensor Principals MECHANICAL SENSORS MECHANICAL SWITCH The simplest kind of displacement sensor is a mechanical switch which returns one bit of information: touching or not touching. A typical Microswitch consists of a lever which, when depressed, creates a mechanical contact within the switch, which closes an electrical connection. Source: The Mechatronics Handbook, by: Robert Bishop Source: Programmable Logic Controllers by: Bolton STRAIN GAUGE / STRAIN GAGE The most common transducer for experimentally measuring strain in a mechanical component is the bonded metal foil strain gage illustrated in the figure. It consists of a thin foil of metal, usually constantan, deposited as a grid pattern onto a thin plastic backing material, usually polyimide. The foil pattern is terminated at both ends with Large metallic pads that allow leadwires to be easily attached with solder. The entire gage is usually very small, typically 5 to 15 mm long. Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand Source: Introduction to Mechatronics and Measurement, by: Alciatore and Histand Types of strain gage assembly: 1. uniaxial 2. biaxial 3. strain gage rosette Accelerometers Accelerometers are devices that measure acceleration, which is the rate of change of the velocity of an object. Piezoelectric Accelerometer Some materials (such as natural quartz crystal, silicon dioxide, barrium titanite, lead zirconate titanate (PZT)), called piezo crystals, produce a charge in response to a force (or deformation) applied to them. Magnetic Accelerometers A simple magnetic accelerometer can be built by using a permanent magnet as a mass on a spring or cantilever beam and sense the field of the permanent magnet using a Hall element or a magnetoresistive sensor as shown below.  Load Cell A load cell is a sensor or a transducer that converts a load or force acting on it into an electronic signal. This electronic signal can be a voltage change, current change or frequency change depending on the type of load cell and circuitry used Load Cell Micro-electromechanical systems(MEMS) MEMS Micro-electromechanical systems(MEMS) is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical components. They are fabricated using integrated circuit (IC) batch processing techniques and can range in size from a few micrometers to millimetres.  MEMS have the ability to sense, control and actuate on the micro scale, and generate effects on the macro scale. In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and microelectronics, all integrated onto the same silicon chip  OTHER SENSORS Potentiometer One of the simplest and least expensive ways to measure rotational or linear motion is using a variable resistor called a potentiometer or rheostat. The rotary potentiometer (aka pot ) is a variable resistance device that can be used to measure angular position. It consists of a wiper that makes contact with a resistive element, and as this point of contact moves, the resistance between the wiper and end leads of the device changes in proportion to the angular displacement.  PIR Sensor PIRs are basically made of a pyroelectric sensor (which you can see above as the round metal can with a rectangular crystal in the center), which can detect levels of infrared radiation. Everything emits some low level radiation, and the hotter something is, the more radiation is emitted. Triggering/retriggering?  The sensor in a motion detector is actually split in two halves. The reason for that is that we are looking to detect motion (change) not average IR levels. The two halves are wired up so that they cancel each other out. If one half sees more or less IR radiation than the other, the output will swing high or low. proximity_pir-diagram.png  Ultrasonic Distance Sensor The Ultrasonic Sensor sends out a high- frequency sound pulse and then times how long it takes for the echo of the sound to reflect back. The sensor has 2 openings on its front. One opening transmits ultrasonic waves, (like a tiny speaker), the other receives them, (like a tiny microphone). How it works:A chirp is emitted from the “speaker.” It bounces off of an object. The echo returns to the microphone. The time it takes to travel to the object and back is used to figure out the distance.