ECE 03 Midterm 2 (1) PDF

ECE 03 – ELECTRONICS 3: ELECTRONICS SYSTEM & DESIGN BSECE III ENGR. JOEL ANTHONY L. SEVILLA NOVEMBER 2024 SENSORS AND TRANSDUCERS SENSORS - is a device that detects a change in a physical stimulus and turns it into a signal which can be measured or recorded. SEN...

ECE 03 – ELECTRONICS 3: ELECTRONICS SYSTEM & DESIGN BSECE III ENGR. JOEL ANTHONY L. SEVILLA NOVEMBER 2024 SENSORS AND TRANSDUCERS SENSORS - is a device that detects a change in a physical stimulus and turns it into a signal which can be measured or recorded. SENSORS AND TRANSDUCERS TRANSDUCERS - is any device that converts energy in one form to another. The majority either convert electrical energy to mechanical displacement or convert some non-electrical physical quantity, such as temperature, sound or light to an electrical signal. Transducer can be classified according to their application, based primarily on the physical quantity, property, or condition that is measured. The transducer can be categories into: A) Passive transducers: requires an external power output is a measure of some variation, such resistance and capacitance. ex: condenser microphone B) Self generating transducers: not requiring an external power and they produce analog voltage or current when stimulated by some physical form of energy. ex: Thermocouple I. TEMPERATURE TRANSDUCERS 1. THERMOCOUPLES When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small electric current is produced proportional to the difference in the temperature. The relationship is nearly linear over the operating range. The actual characteristic and suitable operating temperatures depends upon the metals used in the wires. The various types are designated in international and national standards. Typical linear operating ranges are shown for standard types. It is important that thermocouples are standard so that the same EMF will always represent the same temperature. Thermocouples come in several forms. They may be wires insulated from each other with plastic or fiber glass materials. For high temperature work, the wire pairs are put inside a tube with mineral insulation. For industrial uses the sensor comes in a metal enclosure such as stainless steel. Thermocouple have a wide range of temperature from as low as – 270°C up to as high as 2700°C. Magnitude of the thermal EMF depends on the wire materials used and on the temperature difference between the junctions. Standard metals and pairs for thermocouple constructions Chromel – Alumel (type K) Iron – Constantan (type J) Copper – Constantan (type T) Platinum – Rhodium-Platinum (type S, R & B)  Thermocouple Alloy composition Chromel - 90% nickel and 10% chromium Alumel - 95% nickel, 2% manganese, 2% aluminium and 1% silicon Constantan - 45 % nickel - 55% copper The effective EMF of the thermocouple is given as: E = c(T1 – T2) + k(T1² – T2²) where c & k = constants of the thermocouple materials T1 = temp of the hot junction T2 = temp of the cold or reference junction Example: During experiments with a copper – constantan thermocouple, it was found that c = 3.75 x 10¯² mV/°C and k = 4.5 x 10¯⁵ mV/°C². if T1 = 100 °C and the cold junction, T2 is kept in ice. Compute for the resulting EMF. 2. RESISTANCE TYPE SENSORS These work on the principle that the electrical resistance of a conductor change with temperature. If a constant voltage is applied to the conductor then the current flowing through it will change with temperature. The resistivity of the conductor change with temperature. This usually means the resistance gets bigger as the conductor gets hotter. A basic temperature sensor is made by winding a thin resistance wire into a small sensor head. The resistance of the wire then represents the temperature. This has an advantage over a thermocouple in that it is unaffected by the temperature of the gauge end. The main type of wire used is Platinum. The sensors are usually manufactured to have a resistance of 100 W at 0°C and a typical operating range of -200°C to 400°C. A special type of resistance sensor is called a Thermistor. They are made from a small piece of semiconductor material. The material is special because the resistance changes a lot for a small change in temperature and so can be made into a small sensor and it costs less than platinum wire. The temperature range is limited. They are only used for a typical range of -20°C to 120°C and are commonly used in small hand held thermometers for every day use. Thermistors are made of oxides of nickel, manganese or cobalt platinum. The output is not linearly proportional to any temperature scale. 2 types of Thermistor NTC – Negative Temperature Coefficient Resistance goes down when temp goes down PTC – Positive Temperature Coefficient Resistance goes up when temp goes up THERMISTOR NTC Thermistor Temperature dependent semiconductor resistors. Operates at a range of -200°C to 1000°C, supplied in glass bead, disc, chips or probe formats. applications: temperature measurement and control temperature compensation surge suppression fluid flow measurement PTC Thermistor Temperature dependent resistors manufactured from barium titanate and should be chosen when a drastic change in resistance is required at a specific temperature or current level. applications: temperature sensing switching of temperature liquid level sensor The relationship between temperature and resistance of conductors can be calculated by: R = Ro (1 + αΔT) where R = resistance of the conductor at temperature °C Ro = resistance of the reference temperature, usually 20 °C α = temperature coefficient of resistance ΔT = difference between operating and the reference temperature Example: A platinum resistance thermometer has a resistance of 150 Ω at 20°C. Calculate its resistance at 50°C, if α20 = 0.00392. 3. LIQUID EXPANSION and VAPOUR PRESSURE SENSORS These are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators. In both cases the inside of the sensor head and the connecting tube are completely full. Any rise in temperature produces expansion or evaporation of the liquid so the sensor becomes pressurized. The pressure is related to the temperature and it may be indicated on a simple pressure gauge. The movement may also directly operate a thermostat. These instruments are robust and used over a wide range. They can be fitted with electric switches to set off alarms. 4. BIMETALLIC TYPES It is a well-known principle that if two metals are rigidly joined together as a two-layer strip and heated, the difference in the expansion rate causes the strip to bend. The strip is twisted into a long thin coil inside a tube. One end is fixed at the bottom of the tube and the other turns and moves a pointer on a dial. The outward appearance is very similar to the pressure type. They can be made to operate limit switches and set off alarms or act as a thermostat. II. PRESSURE TRANSDUCERS 1. BOURDON TUBE It is a hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. The movement is usually coupled to a needle on a dial to make a complete gauge. It can also be connected to a secondary device such as an air nozzle to control air pressure or to a suitable transducer to convert it into an electric signal. This type can be used for measuring pressure difference. 2. PISTON TYPE The pressure acts directly on the piston and compresses the spring. The position of the piston is directly related to the pressure. A window in the outer case allows the pressure to be indicated. This type is usually used in hydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge). The piston movement may be connected to a secondary device to convert movement into an electrical signal. 3. BELLOWS A bellows is made of several capsules. These are hollow flattened structures made from thin metal plate. When pressurized the bellows expand and produce mechanical movement. If the bellows is encapsulated inside an outer container, then the movement is proportional to the difference between the pressure on the inside and outside. Bellows and single capsules are used in many instruments. They are very useful for measuring small pressures. 4. DIAPHRAGMS These are similar in principle to the bellows but the diaphragm is usually very thin and perhaps made of rubber. The diaphragm expands when very small pressures are applied. The movement is transmitted to a pointer on a dial through a fine mechanical linkage. III. SPEED TRANSDUCERS 1. OPTICAL TYPES These use a light beam and a light sensitive cell. The beam is either reflected or interrupted so that pulses are produced for each revolution. The pulses are then counted over a fixed time and the speed obtained. Electronic processing is required to time the pulses and turn the result into an analog or digital signal. 2. MAGNETIC PICK UPS Uses an inductive coil placed near to the rotating body. A small magnet on the body generates a pulse every time it passes the coil. If the body is made of ferrous material, it will work without a magnet. A discontinuity in the surface such as a notch will cause a change in the magnetic field and generate a pulse. 3. TACHOMETERS Very often the tachometer is built into electric motors to measure their speed. The frequency of the voltage represents the speed of rotation. The frequency must be counted and processed. IV. FLOW METERS A flow meter or a flow sensor is a type of flow instrument that is used to indicate the amount of liquid, gas, or vapor moving through a pipe or conduit by measuring linear, non-linear, mass, or volumetric flow rates. Since flow control is often essential, measuring the flow of liquids and gasses is a critical need for many industrial applications – and there are many different types of flow meters that can be utilized depending on the nature of the application. They may be classified as follows: POSITIVE DISPLACEMENT TYPES INFERENTIAL TYPES VARIABLE AREA TYPES DIFFERENTIAL PRESSURE TYPES 1. POSITIVE DISPLACEMENT TYPES These types have a mechanical element that makes the shaft of the meter rotate once for an exact known quantity of fluid. The quantity of fluid hence depends on the number of revolutions of the meter shaft and the flow rate depends upon the speed of rotation. Both the revolutions and speed may be measured with mechanical or electronic devices. Meshing Motor type consists of two rotors with lobes. When fluid is forced in, the rotors turn and operate the indicating system. 2. INFERENTIAL TYPE METERS The flow of the fluid is inferred from some effect produced by the flow. Usually this is a rotor which is made to spin and the speed of the rotor is sensed mechanically or electronically. The main types are: Turbine rotor types Rotary shunt types Rotating vane types Helical turbine types TURBINE TYPE The turbine type has an axial rotor which is made to spin by the fluid and the speed represents the flow rate. This may be sensed electrically by coupling the shaft to a small electric tachometer. Often this consists of a magnetic slug on the rotor which generates a pulse of electricity each time it passes the sensor. ROTATING VANE TYPE The jet of fluid spins around the rotating vane and the speed of the rotor is measured mechanically or electronically. 3. VARIABLE AREA TYPES FLOAT TYPE The float is inside a tapered tube. The fluid flows through the annular gap around the edge of the float. The restriction causes a pressure drop over the float and the pressure forces the float upwards. Because the tube is tapered, the restriction is decreased as the float moves up. Eventually a level is reached where the restriction is just right to produce a pressure force that counteracts the weight of the float. The level of the float indicates the flow rate. If the flow changes the float moves up or down to find a new balance position. TAPERED PLUG TYPE A tapered plug is aligned inside a hole or orifice. A spring holds it in place. The flow is restricted as it passes through the gap and a force is produced which moves the plug. Because it is tapered the restriction changes and the plug takes up a position where the pressure force just balances the spring force. The movement of the plug is transmitted with a magnet to an indicator on the outside. 4. DIFFERENTIAL PRESSURE FLOW METERS These are a range of meters that convert flow rate into a differential pressure: ORIFICE METERS VENTURI METERS NOZZLE METERS PITOT TUBES Cross section view The working principle for all these is that something makes the velocity of the fluid change and this produces a change in the pressure so that a difference ΔP = P2 – P1 is created. It can be shown for all these meters that the volume flow rate Q is related to ΔP by the following formula: Q = k Δ𝑃 k is the meter constant. Extra instrumentation heads can be fitted to produce an: electrical output (4 mA – 20 mA) or a pneumatic output (0.2 bar – 1 bar). V. FORCE SENSORS MECHANICAL TYPES It is a basic mechanical principle that the deflection of a spring is directly proportional to the applied force so if the movement is shown on a scale, the scale represents force. HYDRAULICS TYPE Are often referred to as hydraulic load cells. The cell is a capsule filled with liquid. When the capsule is squeezed, the liquid becomes pressurized. The pressure represents the force and may be indicated with a calibrated pressure gauge. The capsule is often a short cylinder with a piston. ELECTRICAL STRAIN GAUGE TYPE A typical load cell consists of a metal cylinder with strain gauges fixed to it. When the cylinder is stretched or compressed, the strain gauges convert the force into a change in resistance and voltage. Since the elements require a supply voltage, the cell usually has 4 wires, two for the supply and two for the output. VI. POSITION SENSORS Position sensors are essential elements in the control of actuators. The position of both linear and rotary actuators is needed in robotic type mechanisms. Three principle types: RESISTIVE OPTICAL INDUCTIVE RESISTIVE TYPES A potentiometer is a variable electrical resistance. A length of resistance material has a voltage applied over its ends. A slider moves along it (either linear or rotary) and picks off the voltage at its position or angle. The tracks may be made from carbon, resistance wire or piezo resistive material. The latter is the best because it gives a good analog output. The wire wound type produces small step changes in the output depending on how fine the wire is and how closely it is coiled on the track. OPTICAL TYPES Optical types are mainly used for producing digital outputs. A common example is found on machine tools where they measure the position of the work table and display it in digits on the gauge head. Light is emitted through a transparent strip or disc onto a photo electric cell. The strip or disc has very fine lines engraved on it which interrupt the beam. The number of interruptions are counted electronically and this represents the position or angle. INDUCTIVE TYPES The most common of these is the Linear Variable Differential transformer or LVDT. The transformer is made with one primary coil and two secondary coils, one placed above and the other below the primary. The coils are formed into a long narrow hollow tube. A magnetic core slides in the tube and is attached to the mechanism being monitored with a non magnetic stem (e.g. brass). A constant alternating voltage is applied to the primary coil. This induces a voltage in both secondary coils. When the core is exactly in the middle, equal voltages are induced and they cancel each other out. When the core moves, the voltage in one secondary coil grows but reduces in the other. The result is an output voltage which represents the position of the core and the mechanism to which it is attached. With suitable electronic equipment for phase detection, it is possible to detect which direction the core moves and to switch the dc voltage from plus to minus as the core passes the center position. VII. DEPTH GAUGES Depth gauges measure the depth of liquids and powder in tanks. They use a variety of principles and produce outputs in electrical and pneumatic forms. The type to use depends on the substance in the tank. The ultrasonic system reflects sound waves from the surface and determines the depth from the time taken to receive the reflected sound. The electronic version uses a variety of electrical affects including conduction of the fluid and capacitance. The pneumatic version bubbles air through the liquid and the pressure of the air is related to the depth. A simple pressure gauge attached to a tank also indicates the depth since depth is proportional to pressure.