Unit 57: Mechatronic System Unit code: F/60/46 QCF level: 4 Credit value: 5 OUTCOME 2 TUTORIAL 2 - SENSOR TECHNOLOGIES 2 Understand electro-mechanical models and components in mechatronic systems and products Simple mathematical models: mechanical system building blocks; electrical system building blocks; electrical-mechanical analogies; fluid and thermal systems Sensor technologies: sensor and actuator technologies for mechatronic system e.g. resistive, inductive, capacitive, optical/fibre-optic, wireless, ultrasonic, piezoelectric Actuator technologies: electric motors; stepper motors; motor control; fluid power; integrated actuators and sensors; embedded systems CONTENTS. Introduction 2. Electrical Resistance Effect Variable Resistance Temperature-Resistance Strain-Resistance Strain Gauge 3. Capacitive Effect 4. Inductive Effect 5. Seebek Effect 6. Piezoelectric Effect 7. Sensor Examples Pressure Sensors Force Gauge 8. Digital Note Accelerometers Vibration Audio Devices Temperature Sensors Speed Transducers Optical Types Magnetic Pick Up Tachometers Flow Meters Positive Displacement Types Inferential Types Position Sensors Resistive Type Optical Types Linear Rotary Absolute Encoders Inductive Types Depth Types www.freestudy.co.uk
. INTRODUCTION Mechatronic systems rely on information gathered by a range of sensors. The information is processed and used to adjust the system. The purpose of this tutorial is to make you familiar with the basic types, their principles and their applications. Things that we commonly measure are: Temperature Light Sound Distance Movement Velocity Acceleration Angle Speed of rotation Force Stress and Strain Mass or Weight Size or Volume Pressure Flow rate Level or Depth Density Acidity/Alkalinity Voltage Current Frequency Resistance Capacitance Inductance Magnetism Sensors may be simple on/off switches or relays activated by the following: Objects (proximity switch) Hot or cold (thermostat) Light or dark (light switch) Empty or full (level switch) Pressure high or low (pressure switch) Voltage High or Low (voltage switch) The sensor is also called a primary transducer. Sometimes the sensor is used in a secondary function such as changing the form of the signal but this is not part of this tutorial. A simple block diagram is shown. 2. ELECTRICAL RESISTANCE You should be familiar with the property of electrical resistance. This is an important property used in a wide variety of sensors. The electrical resistance of a conductor can be made to change by changing its temperature or by changing its dimensions. VARIABLE RESISTANCE A variable resistance is often used to produce a voltage that changes with the position of the slider. This is used in a variety of sensors from volume controls to measuring movement. A potentiometer is a variable resistance as shown. Linear Potentiometer Angular Potentiometer 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 analogue output. www.freestudy.co.uk 2
TEMPERATURE- RESISTANCE AFFECT If a constant voltage is applied across a resistance the current flowing through it will change with temperature. The resistivity of the conductor changes with temperature. This usually means the resistance gets bigger as the conductor gets hotter. The following law relates the resistance and temperature. R = R o ( + ) is the temperature coefficient of resistance. R o is the resistance at 0 o C. Sometimes the equation is given as R = R o ( + - 2 ) A special type of resistance material is a semi conductor material used to make things like a THERMISTOR (Thermally Sensitive Resistor). 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 is cheap. The temperature range is limited. There are many shapes and sizes as shown. There are two types NTC and RTC. The NTC works in a negative way in that R drops as T rises. The simplified relationship between resistance and temperature for the NTC type is R = Ae / R = thermistor resistance at temp T A = constant of equation T = thermistor temperature (K) β = beta is the material constant and is based on the resistance at two temperatures STRAIN - RESISTANCE AFFECT The resistance of a conductor depends on the cross sectional area and length. When it is stretched (tensile strain) it gets thinner and the resistance increases. When it is compressed the reverse happens. Let the length of the conductor be L and the change in length be L. The mechanical strain = L/L Let the resistance of the conductor be R and the change in resistance be R. The electrical strain = R/R. For most materials it is found that / = L R/R L = constant STRAIN GAUGES Strain gauges are elements or sensors specifically designed to measure strain. The strain gauge element is basically a very thin long wire all in one direction but bent to fit it on a small surface area. The element is often formed by etching a thin foil on a plastic backing. The completed element is glued to the surface of the material or component that will be strained. The axis of the strain gauge is aligned with the direction of the strain. When the component is stretched or compressed, the length of the resistance wire is changed. This produces a corresponding change in the electrical resistance. The resistance R is typically 20 The electrical and mechanical strain is directly proportional and the constant relating them is called the gauge factor (typically 2). Gauge Factor = Electrical Strain/Mechanical strain = / = L R/R L A common material used for the strain element is quartz (covered further down). www.freestudy.co.uk 3
3. CAPACITIVE EFFECT You should be familiar with the basics of capacitance in electrical work. The capacitor can be part of an electronic circuit and if its value changes, some property of the circuit will respond. The capacitance is basically two parallel plates separated by a dielectric. The capacitance C is given by the formulae: C = (A/d) o r o and r are permittivity constants of the dielectric. The capacitance is usually changed by changing the distance d, the area A or the material of the dielectric (e.g. a tank filling with a substance). Sensors come in many shapes and sizes and have many applications. 4. INDUCTIVE EFFECT You should be familiar with the basics of inductance in electrical work. The inductor can be part of an electronic circuit and if its value changes, some property of the circuit will respond. An inductor is basically a coil of wire carrying a current wound on a magnetic core. The inductance L is given by the formula: L = o r An 2 / l o and r are permeability constants for the core material. The value can be changed by moving the core in or out of the coil. 5. SEEBEK EFFECT 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. Seebeck discovered this effect. It is true no matter how the ends are joined so one end may be joined at a sensitive millivolt meter or an electronic amplifier. The sensor end is hotter or colder than the other end. This principle is used to measure temperatures and could also be part of an overheating alarm system for example. 6. PIEZOELECTRIC EFFECT The element used here is a piece of crystalline material (usually Quartz otherwise called Silicon Dioxide). This produces an electric charge on its surface when it is mechanically stressed or made to change dimensions when electrically charged. The electric charge may be converted into voltage. It is used widely for sound detection, emitting sound, detecting strain or anything that causes it to vibrate or change its dimensions. The applications are very wide and it is used for resistance its resistance effect as described in forgoing examples. www.freestudy.co.uk 4
WORKED EXAMPLE No. A Platinum resistance thermometer has a resistance of 00 at 0 o C and the value of is 0.00385. In operation the resistance is 05. Calculate the temperature. SOLUTION Rearrange the formula to make the subject and evaluate. R 05 R o 00 o θ 2.987 C α 0.00385 WORKED EXAMPLE No.2 A thermocouple produces an e.m.f. in mv according to the temperature difference between the sensor tip and the gauge head 2 such that e = ( - 2 ) + ( 2-2 2 ) = 3.5 x 0-2 and = 8.2 x 0-6 The gauge head is at 20 o C. The mv output is 0 mv. Calculate the temperature at the sensor. SOLUTION 8.2 x 0 θ 0 0.035 θ 20 8.2 x 0 0 0.035θ 0.7 8.2 x 0 0 8.2x0 6 2 6 θ 2 0.035θ 0.70328 0.035θ 0.7038 0 6 6 θ (θ 2 2 20 2 ) 0.00328 Solving the quadratic equation yields = 286.6 o C WORKED EXAMPLE No.3 A strain gauge is glued to a structure. It has a gauge factor of 2. and a resistance of 20.2. The structure is stressed and the resistance changes to 20.25. Calculate the strain and convert this into stress. Take E = 205 GPa SOLUTION R = 20.25 20.2 = 0.05 = R/R = 0.05/20.2 = 4.6 x 0-4 = /G = 4.6 x 0-4 /2. =.98 x 0-4 = E =.98 x 0-4 x 205 x 0 9 = 40.6 MPa www.freestudy.co.uk 5
SELF ASSESSMENT EXERCISE No.. A thermocouple produces an e.m.f. in mv according to the temperature difference between the sensor tip and the gauge head 2 such that e = ( - 2 ) + ( 2-2 2 ) Given = 3.5 x 0-2 and = 8.2 x 0-6 determine the mv output when the tip is at 220 o C and the gauge head at 20 o C. (Answer 7.394 mv) 3. A strain gauge is glued to a structure. It has a gauge factor of 2. and a resistance of 9.85. The structure is stressed and the resistance changes to 20.5. Calculate the strain and convert this into stress. Take E = 205 GPa (Answer 244.35 MPa) 4. A strain gauge has a resistance of 20.6 Ohms at 20 o C. Calculate its resistance at 30 o C. = 8 x 0-6 / o C. (Answer 20.6 ) 5. A STRAIN GAUGE has a gauge factor of 2.2. It is glued to tensile test piece and the resistance before straining is 9.8. The test piece is stretched and the resistance goes up to 20. Calculate the following. The modulus of elasticity E for the test piece is 200 GPa. i. The strain in the test piece. (758.8 x 0-6 ) ii. The stress in the test piece. (5.8 MPa) 6. The resistance of an NTC Thermistor temperature sensor is given by R = Ae / Given A = 2. x 0-6 Ohms and = 3936 K determine the resistance at 40 o C. (Answer 0.607 ) www.freestudy.co.uk 6
7. SENSOR EXAMPLES Pressure Sensors These may be based on Strain, Piezoelectric or Capacitance. The picture shows a typical transducer. STRAIN TYPE PIEZO TYPE CAPACITOR TYPE A typical pressure transducer would contain a metal diaphragm which bends under pressure. This can change the resistance of any strain gauge glued to it or deform the quartz crystal or change the capacitance as shown above. Similar technology is used in Force gauges and Accelerometers. Force Gauge - Force produces strain so the strain gauge is the basis of many weighing systems. The gauge can be incorporated into many systems such as Torque measurement. The picture shows a button load cell. Accelerometers Acceleration produces a force on any object being accelerated hence a force sensor can be an accelerometer. This forms the basis of many systems ranging from navigation to computer toys. Vibration monitor This is fundamentally a microphone that converts vibrations into a proportional electric signal. Audio Devices Microphones pick up air vibrations. This can be done with a diaphragm and converted with piezoelectric elements, resistance elements, capacitive elements or inductive elements. Sound emitting devices might be piezoelectric material that vibrates according to the electric charge. Traditional loudspeakers use inductive coils that interact with a magnetic core to cause movement of the diaphragm. www.freestudy.co.uk 7
Temperature Sensors These are based on many technologies such as resistance and infra red radiation. A typical resistance thermometer is shown next. 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 changes with temperature. 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 00 at 0 o C and the value of is 0.00385 to 0.00390. A typical operating range is -200 to 400 o 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 to 20 o C and are commonly used in small hand held thermometers for every day use. Speed Transducers Speed transducers are widely used for measuring the output speed of a rotating object. There are many types using different principles and most of them produce an electrical output. 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 analogue or digital signal. Magnetic Pick Ups These use 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. The pulses must be processed to produce an analogue or digital output. www.freestudy.co.uk 8
Tachometers There are two types, A.C. and D.C. The A.C. type generates a sinusoidal output. The frequency of the voltage represents the speed of rotation. The frequency must be counted and processed. The D.C. type generates a voltage directly proportional to the speed. Both types must be coupled to the rotating body. Very often the tachometer is built into electric motors to measure their speed. Flow Meters There are many hundreds of types of flow meters depending on the make and application. This covers a few that are most likely to occur in a mechatronic system. 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 measurement 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. Some of the most common listed below. These are used for systems that need a precise measurement. An example is the Meshing Rotor type shown below. Inferential Types 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 drives a tachometer. An example is the turbine type meter shown below. The turbine type shown 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. www.freestudy.co.uk 9
Position Sensors Position sensors are essential elements in many systems where precise control of movement is required. The position of both linear and rotary motion is needed in robotic type mechanisms. There are three principle types. RESISTIVE OPTICAL INDUCTIVE Resistive Types This has been covered earlier. Basically a potentiometer is moved by the mechanism and the voltage output represents the position. Optical Types Linear 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. Digital micrometers and verniers also use this idea. The basic principle is as follows. Light is emitted through a transparent strip or disc onto a photo electric cell. Often reflected light is used as shown. The strip or disc has very fine lines engraved on it which interrupt the beam. The number of interruptions is counted electronically and this represents the position or angle. This is very much over simplified and you should refer to more advanced text to find out how very accurate measurements are obtained and also the direction of movement. Rotary Rotary types can produce either absolute or incremental measurements. Incremental encoders use a disc with slots in it as shown. Light sensitive detectors pick up light from two sensors. As the disc revolves, a series of pulses are generated. The angular (or linear) position is found by counting the pulses. The sensors are normally offset so that the two sets of pulses are displaced in time by a half increment. This enables the direction of rotation to be determined by checking the order in which they change. www.freestudy.co.uk 0
Absolute Encoders (for example) use four rings on the disc divided into opaque and transparent sectors. Four detectors produce a binary digital code for each sector. This produces an absolute code for each position in the form of a 4 bit number. The grey coded scale is usually preferred because only one bit changes from one transition to the next producing less likelihood of switching errors. If the disc revolves more than once, the revolutions must be counted as well. 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 when connected as shown, 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 hence the mechanism to which it is attached. The output voltage is usually converted into D.C. 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 centre position. These can be very accurate and are widely used for gauging the dimensions of machined components. www.freestudy.co.uk
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. Here are a few. 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 is also indicates the depth since depth is proportional to pressure. 8. DIGITAL NOTE In order to integrate sensors into a modern digital set up (i.e. computer technology) analogue signals must be converted into digital form with Analogue to Digital converters. Many sensors (e.g. the optical encoder) have a digital output but many have an analogue output and so conversion is essential. www.freestudy.co.uk 2
SELF ASSESSMENT EXERCISE No. 2. Solid objects passing down a conveyor system have to be checked if they are tall or short so that they can be sorted as part of an integrated packaging system. Suggest a suitable sensor type for detecting this giving your reasons. 2. A robotic device is part of an integrated manufacturing system. Its purpose is to accurately check the diameter of machined parts in order to produce a statistical analysis so that the cutting system can be automatically adjusted to any changes. Suggest a suitable type of sensor giving your reasons. 3. A modern surveying system measures distance and angles with sensors that are part of an integrated logging and computational system. Suggest a suitable sensor for finding the distance to an object and for measuring the angles giving your reasons. 4. The temperature of a solid state semi conductor device has to measured and used to trip a cooling fan when it exceeds a set value. Suggest a suitable type of sensor giving your reasons. 5. The picture shows a home weather station with a touch screen. Do some research and describe the technology used to sense the following: Wind Direction Wind Speed(0 to 50 km/h) Temperature sensor (-50 o C to 60 o C) Air Pressure (800 to 200 mbar) Rain Fall Humidity What would be a suitable device for making bleeps when a critical value of any of these is exceeded? www.freestudy.co.uk 3