Control Engineering BDA30703 Lecture 4: Transducers Prepared by: Ramhuzaini bin Abd. Rahman
Expected Outcomes At the end of this lecture, students should be able to; 1) Explain a basic measurement system. 2) Explain the basic working principle of various temperature, pressure, flow, motion and force sensors 2
Outlines Introduction Temperature measurement Pressure measurement Flow measurement Motion measurement Force measurement 3
1.0 Introduction A basic measurement system consists of: Sensors (Primary transducers) Signal processor Receivers (output device) Most modern analogue equipment works on the following standard signal ranges: Electric: 4 to 20mA Pneumatic: 0.2 to 1.0bar (20 to 100kPa) 4
1.0 Introduction (cont. d) Physical quantities that are commonly measured: Temperature Speed Force Stress and strain Mass or weight Size or volume Pressure Flow rate Position, velocity and acceleration Level or depth Density Acidity/Alkalinity Sensor may operate simple on/off switches to detect the following: Objects (proximity switch) Hot or cold (thermostat) Empty or full (proximity switch) Pressure high or low (pressure switch) 5
1.0 Introduction (cont. d) Three basic principle are commonly used in sensor: 1. Resistive 2. Capacitive 3. Inductive Selection of a sensor typically requires a consideration of : Operating range (e.g. ±100rad/s) Sensitivity (e.g. 0.01V/rad/s) Speed of response (e.g. Bandwidth of 50Hz) Environmental conditions (e.g. -10 to +80 deg. C) Accuracy (e.g. 3% FS) 6
2.0 Temperature measurement 2.1 Thermocouples the applied physical principle when two metal wires with dissimilar electrical properties are joined at both ends and one junction is made hot while other cold, a small electric current (e.m.f) that is proportional to the difference in the temperature is produced. This effect was discovered by Seebeck. The hot junction forms the sensor end while the cold end is joined at a sensitive millivolt meter. Simple Thermocouple circuit 7
2.0 Temperature measurement (cont. d) Most thermocouple metals produce a nearly linear relationship between temperature and the e.m.f as follows: e αt where e is the e.m.f. generated, α is the constant for type of thermocouple and T is the absolute temperature. A typical industrial thermocouple sensor with a flexible extension and standard plug E.m.f. temperature characteristics for some standard thermocouple materials 8
2.0 Temperature measurement (cont. d) 2.2 Resistance thermometers also known as resistance temperature devices (or RTDs). the applied physical principle electrical resistance of a metal varies with temperature according to the following relationship: R R 0 (1 + αt) where α is the temperature coefficient of resistance and R 0 is the resistance at 0 C. Typical resistance temperature characteristics of metals 9
2.0 Temperature measurement (cont. d) The sensor is made by winding a thin wire into a small sensor head. The resistance of the wire represents the temperature. The advantage of RTD over thermocouple is that the measurement is unaffected by the temperature of the gauge end. The main type of wire used is PLATINUM. A typical industrial RTD sensor with a flexible extension Schematic of RTD temperature measuring transducer 10
2.3 Thermistors 2.0 Temperature measurement (cont. d) Special type of resistance sensor and made of a small piece of semiconductor material. the applied physical principle electrical resistance of the semiconductor material decreases as the temperature increases according to the following relationship: R R 0 e β(1 T 1 T 0 ) where β is the temperature coefficient of resistance Typical resistance temperature characteristics of thermistor materials 11
2.0 Temperature measurement (cont. d) Advantage resistance of the thermistor material changes a lot for a small change in temperature so it can be made into a small sensor that costs less than platinum wire of RTD. Disadvantage the temperature range is between -20 to 120 C. For that, the application is limited to small handheld thermometers for everyday use A typical industrial thermistor sensor with a flexible extension Schematic of thermistor temperature measuring transducer 12
In class tutorial 1 2.0 Temperature measurement (cont. d) A Platinum resistance thermometer has a resistance of 100Ω at 0 C and the value of α is 0.00385. In operation the resistance is 101Ω. Calculate the temperature. In class tutorial 2 A thermocouple produces an e.m.f in mv according to the temperature difference between the sensor tip θ 1 and the gauge head θ 2 such that e αt = α θ 1 θ 2 α = 3.5 X 10 2. If the gauge head is at 20 C and the output is 12mV, calculate the temperature at the sensor. 13
In class tutorial 3 2.0 Temperature measurement (cont. d) A thermocouple produces an e.m.f in mv according to the temperature difference between the sensor tip θ 1 and the gauge head θ 2 such that e αt = α θ 1 θ 2 Given α = 3.5 X 10 2 determine the output in mv when the tip is at 220 C and the gauge heat at 20 C. 14
2.0 Temperature measurement (cont. d) 2.4 Pressure thermometers There are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators. In both cases, such fluids completely occupy the sensor head and connecting tube. the applied physical principle any rise in temperature produces expansion or evaporation of the liquid sensor becomes pressurized. The pressure is related to the temperature and may be indicated on a simple pressure gauge. 15
2.0 Temperature measurement (cont. d) 2.5 Bimetallic thermometer the applied physical principle if two metal are rigidly joined together as a two-layer trip and heated, the difference in the thermal expansion rate causes the strip to bend. Can be made to operate as limit switch to set off alarms or act as thermostat (e.g. on a boiler) 16
2.0 Temperature measurement (cont. d) 2.6 Liquid-in-glass thermometer The ordinary glass thermometer is an example of a complete measurement system. the applied physical principle any rise in temperature produces expansion of the liquid. The bulb is the sensor, column of liquid is the processor and the scale on the glass is the indicator. Mercury is used for hot and coloured alcohol for cold temperature. 17
2.0 Temperature measurement (cont. d) Problems with liquid-in-glass thermometers are: 1. Brittle 2. Mercury solidifies at -40 C 3. Alcohol boils at around 120 C 4. Accuracy less than ±1% of f.s. reading is hard to achieve. 5. The best industrial version of liquid-in-glass thermometer with accurate of ±0.15% is expensive 6. It is easy for people to make mistakes during reading. A typical industrial version of liquidin-glass thermometer 18
3.0 Pressure measurement Absolute pressure is the difference between the pressure of a fluid and the absolute zero of pressure. Gauge pressure describes the difference between the pressure of a fluid and atmospheric pressure. Absolute pressure = Gauge pressure + atmospheric pressure Differential pressure this term is used to describe the difference between two absolute pressure values, such as pressure at two different points within the same fluid (often between the two sides of a flow restrictor in a system measuring volume flow rate) 19
3.1 Bourdon tube 3.0 Pressure measurement (cont. d) is a hollow tube with an elliptical cross section. the applied physical principle 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. Bourdon tube 20
3.0 Pressure measurement (cont. d) 3.2 Bellows is made of several capsules (i.e., hollow flattened structures made from thin metal plate) the applied physical principle when pressurized, the bellows expand and produce mechanical movement. The movement is proportional to the difference between the pressure on the inside and outside of bellow. Very useful for measuring small pressures. 21
3.3 Diaphragms 3.0 Pressure measurement (cont. d) is the to the bellow but the diaphragm is usually very thin and normally made of rubber. the applied physical principle the diaphragm expands when very small pressures are applied. The movement is transmitted to a pointer on a dial through a fine mechanical linkage. 22
3.0 Pressure measurement (cont. d) 3.4 Electrical pressure transducers There are various ways of converting the mechanical movement of the preceding types into an electrical signal. The following methods are the most commonly applied: 1. Strain gauge type 2. Piezo electric type 3. Capacitive type 23
3.4.1 Strain gauge type 3.0 Pressure measurement (cont. d) Strain gauges are small elements that are fixed to a surface that is strained. the applied physical principle the change in length of the element produces changes in the electrical resistance which will be processed and converted into a voltage. A typical strain gauge type pressure transducer would contain a metal diaphragm which bends under pressure 24
3.4.2 Piezo electric type 3.0 Pressure measurement (cont. d) the applied physical principle the element used is a piece of crystalline material that produces an electric charge on its surface when it is mechanically stressed. The electric charge is converted into voltage 25
3.4.3 Capacitive type the applied physical principle pressure produces a change in the capacitance of an electronic component in the tranducer. This effect is used to change the frequency in an electronic oscillator. The frequency is then converted into a voltage to represent the pressure 3.0 Pressure measurement (cont. d) 26
4.0 Flow measurement The rate at which fluid flows through a closed pipe can be quantified by either measuring the mass flow rate or volume flow rate. Of these alternatives, mass flow measurement is more accurate, since mass, unlike volume, is invariant. There are many types of flow meters depending on the make and application. They can be classified generally as follows: 1. Positive displacement types 2. Inferential types 3. Variable area types 4. Differential pressure types 27
4.0 Flow measurement (cont. d) 4.1 Positive displacement types the applied physical principle consist of a mechanical element that makes the shaft of the meter rotates for an exact known quantity of fluid. The quantity of fluid is thus depends on the number of revolutions of the meter shaft and the flow rate depends upon the speed of rotation. The most common positive displacement flow sensors are: 1. Rotary piston type 4. reciprocating piston type 2. Vane type 5. fluted spiral gear 3. Meshing rotor The meshing rotor type consists of two rotors with lobes. When fluid is flowed in, the rotors turn and operate the indicating system 28
4.0 Flow measurement (cont. d) 4.2 Inferential types the applied physical principle the rotor is made to spin by the flow of fluid. The speed of the rotor is then sensed mechanically or electronically. The most common type of inferential flow sensor: 1. Turbine rotor types 2. Rotary shunt types 3. Rotating vane types 4. Helical turbine types Turbine rotor type 29
4.0 Flow measurement (cont. d) 4.4 Variable area types the applied physical principle measures fluid flow by allowing the cross sectional area of the device to vary in response to the flow. Examples: Tapered plug type Float type (rotameter) 30
4.0 Flow measurement (cont. d) 4.4 Differential pressure type the applied physical principle measures fluid flow by allowing the velocity of the fluid to change with respect to the cross sectional area of the device. Change in velocity will vary the pressure and subsequently create a pressure different P = P 2 P 1 The mass flow rate Q is related to P by the following formula: Q = K P 0.5 31
4.0 Flow measurement (cont. d) Pressure difference is created using the following principles: 32
5.1 Position sensors 5.0 Motion measurement Position sensors are essential elements in the control of actuators. The position of both linear and rotary actuators is needed especially in robotic type mechanisms. There are three main principles used: 1. Resistive 2. Inductive 3. Optical 33
5.0 Motion measurement (cont. d) 5.1.1 Resistive type A potentiometer is a variable electrical resistance. It consists of a length of resistance material with a voltage applied over its ends. the applied physical principle A slider moves along the resistance material during motion (either linear or rotary) and picks off the voltage at its position or angle. The resolution is determined by the wire construction how fine is the wire how closely it is coiled on the track Rotary potentiometer linear potentiometer 34
5.1.2 Inductive type 5.0 Motion measurement (cont. d) The most common position sensor of this type is the Linear Variable Differential transformer (LVDT). The transformer is made with one primary and two secondary coils. 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) 35
5.0 Motion measurement (cont. d) the applied physical principle a constant alternating voltage is applied to the primary coil. This induces a voltage in both secondary coils. When the core is exactly at the middle, equal voltages are induced and when connected as shown, cancel each other out. When the core moves, the voltage in one of the secondary coil grows while the other reduces. This result in 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, direction of the core movement can be detected (DC voltage switch from plus to minus as the core passes the center position. 36
5.0 Motion measurement (cont. d) 37
5.1.3 Optical type 5.0 Motion measurement (cont. d) Mainly used for producing digital output. the applied physical principle light is emitted through a transparent strip or disc onto a photo electric cell and get reflected as shown in the figure below. The strip or disc with very fine lines engraved on it interrupt the beam. The number of interruptions are counted electronically. This information is used to represent the position or angle. 38
5.2 Speed sensors 5.0 Motion measurement (cont. d) Speed sensors are widely used for measuring the output speed of a rotating object. Most of the speed sensors produce an electrical output. The most commonly used principles used: 1. Tachometers 2. Optical types 3. Magnetic pick ups 39
5.2.1 Tachometers 5.0 Motion measurement (cont. d) There are two types: A.C and D.C. Both types must be coupled to the rotating body. the applied physical principle The A.C generates a sinusoidal output voltage. The frequency of the voltage represents the speed of rotation. The D.C type generates a voltage that is directly proportional to the speed. 40
5.2.2 Optical types 5.0 Motion measurement (cont. d) Consists of a light beam and a light sensitive cell. the applied physical principle The beam is either reflected or interrupted so that pulses are produced for each revolution. The pulse are then counted over a fixed time to obtain the speed. Note: Electronic processing is required to time the pulses and turn into an analogue or digital signal. 41
5.2.3 Magnetic pick ups 5.0 Motion measurement (cont. d) An inductive coil is placed near the rotating body. the applied physical principle A small magnet on the body generates a pulse every time it passes the coil. However, if the body is made of ferrous material, no magnet is attached on the body. A discontinuity in the surface such as a notch will cause a change in the magnetic field and generate a pulse Note: Electronic processing is required to process the pulses and turn into an analogue or digital signal. 42
6.0 Force measurement 6.1 Force sensors Force sensors can be generally divided into 3 main categories: 1. Mechanical types 2. Hydraulic types 3. Electrical strain gauge types 43
6.1.1 Mechanical types The complete measuring system usually consists of a spring and simple scale. the applied physical principle deflection of a spring is directly proportional to the applied force and the magnitude is shown on the scale 6.0 Force measurement (cont. d) 44
6.1.2 Hydraulic types 6.0 Force measurement (cont. d) Often referred to as hydraulic load cells. The cell is a capsule filled with liquid. The capsule is consisted of a short cylinder with a piston. the applied physical principle when the capsule is squeezed, the liquid becomes pressurized. The pressure represents the force and the magnitude is indicated with a calibrated pressure gauge. Note that, the force can be calculated from the pressure equation given by P=F/A. F is the force and A is the piston area, 45
6.1.3 strain gauge types 6.0 Force measurement (cont. d) Often referred to as electronic load cells. The load cell consists of a metal cylinder with strain gauges fixed on it. the applied physical principle when the cylinder is stretched or compressed, the strain gauges convert the force into a change in resistance and hence voltage. The cell usually has 4 wires: two for supply voltage and two for the output. 46