TOPIC 2 Measurement System Sensors and Transducers
Manufacturing Measurement System
Measurement system is primarily developed to collect the information on the system status. There are various building blocks of an automated system. Measurement system collects the information from the ground, from the application and it sends that information to the microprocessor, the microprocessor processes that information and it gives the decision to control the process. The information is to be collected from the ground and from the environment of the processing. Ultimately, we have to collect the information and we need to feed this information to microprocessors. Collection of the information, feeding the information to microprocessor are the basic functions of the measurement system. Based on this information itself, the controlling operation would be done. The accuracy of the information and efficient feeding to the microprocessor system, are the two at most important requirements of the efficient measurement system. The measurement system has various elements sensors, transducers and signal processing devices. What is the meaning of a sensor? The definition of a sensor is a physical element which produces a signal relating to the quantity being measured. Sensor is a physical element which senses the measurement, and it generates a signal, and produces a signal. This is called a sensor. What may be the physical variables? A temperature can be called a physical variable or displacement or we may also call the noise or the vibrations. These are the input variables, or the physical variables which needs to be measured. Sensor is a physical element which senses these variables, and it generates certain signals. These signals may be change in resistance, change in inductance or change in capacitance. But change in resistance or inductance and capacitance may not be useful for microprocessor application. The microprocessor understand the language of 0’s and 1 and these 0’s and 1’s are nothing, but the voltages. We need to have a signal in terms of change in voltage or a sequence of pulses and these pulses are nothing, but change in voltage. When a sensor produces the change in voltage or change in current which can be understood by the microprocessor, then that system is called as the transducer. Transducer by definition is a device which converts one form of energy into the other form of energy. Again, the physical variable is temperature, but it is generating the signal into delta V and delta I. So, delta V is voltage and delta I is the current. To have a transducer, we need to attach or integrate sensors plus the signal processing devices. Thus, all the sensors are transducers, but all transducers are not sensors. A simple example is a wire of constantan alloy, a metal alloy and it has two elements; copper and nickel. The proportion is 55 percentage of copper and 45 percentage of the nickel. This alloy can be called a sensor as it is generating a signal when there is a change in physical variable. Let us consider the change in physical variable to be measured is mechanical displacement.
Assessment Question#1.Match the following terms with their appropriate descriptions. Choose the correct answer from the drop-down list.
Correct answer: Sensor: Device that measures physical input from its conditions and converts it into data that can be read by a human/machine. Transducer: Electronic device that converts energy from one form to another.
Potentiometer sensor has a resistance element and a sliding contact. These are available in linear or rotary format. As the length of the sliding contact changes, the resistance of the system changes and that resistance is producing further voltage output. The length of the sliding contact is affecting the resistance of the system and it is affecting the potential difference across the connections. If the sliding contact is connected with the physical element of which the displacement is to be measured, the displacement can be easily computed by calibrating the change in potential difference inside the circuit. A typical linear potentiometer has a long wire and a slider. Such a long wire based linear potentiometer is difficult to handleand tedious, and we cannot use these as a sensor. For that purpose, the rotary modes or rotary type of potentiometers are used as sensors. These rotary sensors has wire wound track or a film of conductive plastic. This wire wound track has number of turns over the core of the sensor. And, as the wiper or the slider is getting contacted with the turns, we are measuring the resistance according to the contact of the slider with a typical number or a specified number of turns on the circular wound track. The resistance we can get, when the wiper is connecting to or is contacting to a wire. Nowadays, a film of conductive plastic is also used. This film is nothing, but a plastic resin which is embedded with carbon powder. Instead of having a number of turn coils, the carbon powder is embedded with plastic resin and the wiper is moving over the plastic resin. The carbon powder is conducting so, wherever the wiper is connecting, we are getting the resistance accordingly. A typical application of potentiometer sensor is shown on the screen. Here, we need to measure the linear displacement by using a rotary type potentiometer sensor. For that purpose, we are getting a string or a cable, we are winding this string or the cable over a threaded drum. This drum is mounted on a shaft and on the shaft, we are having a coil spring. At the end of this shaft, we are having the potentiometer sensor. The potentiometer sensor has a resistive strip and a wiper arrangement. There are three terminals; two terminals of the resistive strip are attached to two ends of the wire. This is pulling element or the contact element of this sensor. This element is in contact with the application of which we need to measure the displacement. We consider there is a pool in this conduct element. As there is pulling moment, the drum is rotating in clockwise direction and as the drum rotates in clockwise direction, the wiper also rotates in the clockwise direction. As the wiper is rotating along the clockwise direction, there is a change in length of contact with the resistive strip. The linear distance is the function of the change in contact length of the potentiometer sensor, and further it is a function of the resistance. So, this delta R will further be used to generate the appropriate signals that is a delta V or delta I current signals by using a Wheatstone bridge device. Delta R signals cannot be used by the microprocessor to process the information. We need to convert these signals from one form of energy that is resistance to the voltage and then, the potentiometer sensors will be converted into a transducer. The correlation will be shown with a very simple circuit. There is a resistance element, sliding contact. This resistance element has two ends A and B. A has resistance RA, and the node B has a resistance RB. We are applying a supply voltage VS across these two terminals, two ends A and B. The voltage at the sliding contacts is VO. Now, let us try to find out the correlation. Va=IRa (1) but I=Vs/(Ra+Rb) (2) Substituting equation 2 in equation 1, we get Va=VsRa/(Ra+Rb). The voltage at point A is a function of the supply voltage, which is the constant supply voltage that we are applying, resistance at node A and resistance at node B. The resistance R is directly proportional to length of the contact and is inversely proportional to the area of the cross section of the wire. Thus, R=ρL/A where ρ is nothing, but the constant of proportionality and is called as electrical resistivity. Thus, resistance is directly proportional to the length and inversely proportional to the area. Let us use this correlation and modify the equation of Va. Thus, Va=VsLa/(La+Lb) Thus, from point A, if the sliding contact length is varied, then there is a change in potential difference, which is nothing, but the potentiometer. What are the various applications of the potentiometer? Potentiometers are used in throttle valve. Throttle valves are used to control the flow of the gas or the fluid inside the combustion chamber. To monitor the operation or to monitor the displacement of the element of the throttle valve, these are either manually operated or automatically operated. When the manual operation is carrying out or when the automatic operations are carrying out, it needs to be monitored whether the specified or desired amount of displacement is there or not. The next is adjustment of voltage. This is a very common application. We also do have a lot of electrical appliances at home. To control the speed of the ceiling fan we are using a knob; we are using a device and that device is having the potentiometer sensors. We are just rotating the knob and accordingly there is a change in the speed. Then, for the acceleration as well. This is the pedal and when we apply pressure or a force on this pedal, this pedal is getting displaced. We need to continuously monitor the displacement of the pedal, because this displacement of the pedal is accelerating the automobile, or it is accelerating certain process. If it is beyond the limit, then we have to give certain alarm or control the acceleration. For that purpose, the control movement of this pedal is required. Earlier, it was used with the cables. The next type of application is electronic suspension. In this application, the lever which is attached to the axle of the automobile, the displacement of that lever will be sensed by using the potentiometer sensor. As the lever is moving, that movement can be sensed by the microprocessor. The lever is moving due to the unevenness on the road, as it is getting bumps or the vibrations from the road unevenness through the axle, that will be sensed by this lever and the microprocessor is taking certain decisions or it is giving alarm to the rider either to control the speed of the automobile or it itself will take the decisions to control it.
Assessment Question#2. Which of the following ARE the characteristics of a Potentiometer Sensor? Choose three answers. Correct answers: Displacement is measured based on the potential difference, The plastic resin is embedded with carbon powder, and It has a resistance element with sliding contact. Incorrect answer: It can perform temperature measurement, and It works as an Energy controller
Strain Gauge Element
The next important displacement sensor is strain gauge element sensor; it works on the principle of change in resistance when we are giving tension or compression to a mechanical element. We can see a simple arrangement of a strain gauge element. It has a foil which has two terminals and we are measuring resistance across these two terminals. This foil is attached to the mechanical element of which the strain is to be measured. When there is tension in the foil, then the resistance of that foil or the resistance of that mechanical element will increase; and when there is a compression, then the resistance will decrease. It works based upon the principle of electrical resistance and is particularly measuring the mechanical strain inside the work parts or the body parts of automated system. Strain gauge sensors are available either in metal wire or metal foil strip form; the strain gauges are also made up of semiconductor material. The advantage of semiconductor material is that it has high gauge factor. The resistance is directly proportional to the strain. In strain gauge element, the ratio of change in resistance of the mechanical element to its original resistance is directly proportional to the strain that is developed in the mechanical element. The change in resistance due to the strain of the original resistance is directly proportional to the strain, and the constant of proportionality is called as the gauge factor. In general the gauge factor is about 2 to 4 and it is decided by conducting the actual experiments in the laboratory, this is called the calibration process. In the calibration process, we are computing the gauge factor for known displacement and known strain inside the mechanical element. For a known strain, we are computing delta R that is change in resistance inside the coil or the foil and accordingly we are computing the G. In general a gauge factor of 2 to 4 is noticed for the variety of mechanical elements. The most common material which is used as the strain gauge material is constantan alloy; it has copper and nickel in the proportion of 55 percentage of copper and 45 percentage of nickel. Two more pictures are there on the screen; in the first picture we can see a thin foil and on the thin foil, a wire is wound, which is attached. In the second picture we are having metal foil.The strain gauge element is providing the passive output. Passive output as is producing the change in resistance; but for microprocessor processing, we need the active element, the voltage change in voltage; a pulse of voltage that is the digital signal that is useful for the microprocessor. For that purpose the resistance change have to be converted into voltage change. To carry out these operations, Wheatstone’s resistance bridge is used; a typical arrangement of the Wheatstone resistant bridge is shown on the screen. In this arrangement we are using four resistors R1, R2, R3 and Rx. Supply voltage is applied across a bridge of these four resistors and an output voltage is measured across the two connections A and B. The values of these four resistors are chosen in such a way that, the V0 that is output voltage should be 0. When the output voltage is 0, then the bridge is in balanced condition. To have output voltage is equal to 0 volt; if you try to get the correlation between this resistor values, we get R2/R1=Rx/R3. To get 0 voltage, we are choosing R2, R1, R3 and Rx. Now in the application of Wheatstone resistance bridge, we are attaching this Rx to our strain gauge leads. Let us attach our strain gauge at Rx. Whatever the output that is coming out from the strain gauge is the Rx value. For no load condition, the strain gauge is providing certain resistance that is Rx. For Rx, we are choosing the value of R1, R2, R3 such that this equation satisfies. But when this strain gauge is in action, when there is tension in the strain gauge; so the resistance of Rx is getting increased, there is change in Rx value due to the mechanical movement which is occurring that is strain. Thus, as Rx is increasing or it is changing; we are getting certain output at the Vo. The output at Vo will not be 0 volts, it would be a nonzero output at this. To make it 0, we have to change the resistance of one of the other resistors which are R1, R 2 or R3. We have to change resistance of either of R1 or R2 or R3. We can choose any resistor here. Let us consider if we have chosen; how much should be the change in R1, so that we can achieve our condition that, V output is 0 volts. This means we have to carry out the resistance change, it may be plus or minus; we have to add a resistance of plus or minus delta R into R1, so that we are getting this condition. The change in resistance in R1 resistor is the indicator of the strain. The change in resistance at R1 to get output voltage equal to 0 is indicating the strain value inside the strain gauge. In laboratory for known value of strain values, we can easily get the delta R1 and in this way we can calibrate the strain gauges. When the Wheatstone resistant bridge is attached, a simple strain gage element sensor will be converted into a transducer. The strain gauges are effective when there is a longitudinal strength along the length when the strain is occurring; then the strain gauges are working for the lateral application of the strain and are ineffective. The strain gauges are useful for measurement of displacement from 1 to 30 mm. However, they do have a nonlinearity error of 1 percentage and it is basically due to the temperature; because when the temperature is high, and the conditions are harsh, then the temperature will affect the material properties of the metal foil or the metal and that may lead to errors in the measurement of the strains. These type of strips are attached to cantilevers, pipes, U shaped elements, and accordingly the strains are measured. If suppose we are applying a repetitive load at the cantilever beam here, the load is applied intermittently; then the strain gauges which are pasted on the top surface are experiencing tension due to the tension in the fiber of the top surface. While the fibers which are there on the bottom surface of the cantilever are experiencing compression. The resistance value of the top strain gauges increases and the resistance values of the bottom strain gauges decrease.
Assessment Question#3.True/False:"The Strain-Gauge Displacement Sensor is used in manufacturing to gain data about experimental stress analysis, diagnosis on machines operations, and also for failure analysis". Correct answer: True
Capacitive Element Sensors
The next sensor is capacitive elements sensor; its advantage is that it is a non-contact type of sensor and used to monitor the displacement. When two charged plates are in the vicinity or when two charged plates are closer to each other; then the capacitance developed in between these two charge plates is directly proportional to their area of overlap, and it is inversely proportional to the distance between these two plates. The constant of proportionalities are called as the relative permittivity of dielectric between the plates and the permittivity of free space. Permittivity basically relates to the materials ability to transmit an electrical field. We are having two charge plates and there is a material or a medium in between these two charge plates. The ability of this medium to permit an electrical field is nothing, but the permittivity. Here we are considering ɛr is the relative permittivity of the dielectric medium between the plates and it is considered as 1 for vacuum; whereas ɛo is the permittivity of the free space. These useful principles can be utilized for measurement of the displacement in our domain that is manufacturing automation. On the screen we can see configuration, there is a top plate and there is a bottom plate and these two plates are separated by a distance d. There is overlap of A between these two plates. When this separation is increased by Δx i.e. d+Δx, then the capacitance is changing. Thus we get c-Δc. If we are increasing the overlap area or decreasing the overlap area that is also affecting on the capacitance. The area in this case is reduced. The area is reduced by an amount ΔA, thus A - Δ A will also reduce the capacitance. Let us consider one of these plates is attached to the mechanical element in our domain; then by changing the separation or the overlap area, we can easily measure the change in capacitance. Change in capacitance is the passive output; we have to convert the change in capacitance into the change in voltage value. The third case in capacitive element may be the moment of the dielectric medium itself. As we are moving the dielectric medium, we are getting the change in capacitance value. The capacitance is a function of separation distance, area of overlap and movement of dielectric medium. This change in capacitance is further utilized to get the displacement. We are taking three plates; plate number 1, plate number 2 and plate number 3. Now the second plate is attached to the element of which we need to measure the displacement. In this figure b, we can see the middle plate has been moved in a upward direction which is closer to the top plate. When we are moving it towards upward direction, there is increase in separation dip distance between plate number 2 and plate number 3 and there is decrease in separation distance between plate 1 and plate 2. As the principle of the capacitive element sensor is suggesting when the separation distance is decreasing; there is an increase in the capacitance between plate 1 and plate 2 and there is decrease in capacitance between plate 2 and plate 3 or somebody will use the movement of the plate number 2 in a lateral way. When plate is moving in a lateral way, the area of overlap is getting changed. In this case the complete area of plate 2 is being overlapped by plate 1 and plate 3; however, there is only half area which is being overlapped with plate 1 and plate 3. Thus, naturally there is a decrease in capacitance. The decrease in capacitance can directly be calibrated to the displacement. The capacitive element sensors are widely used as proximity sensors. In this case, we are using a simple arrangement. A typical industrial construction of these capacitive sensors are shown. This is the sensing area which is small; however the sensing area is guarded by the guard area, and the guard construction and the sensing construction are enclosed in a body. Capacitive proximity switches are working with target, only condition is that the target should be grounded. When these capacitive sensors are coming closer to the target, it produces the signals. A charge is applied to the coaxial cable and when they are coming closer to a target; then there is a change in capacitance which will be useful to generate the signal. What are the applications of capacitive element sensor? They are used to monitor the feed in hoppers. Hoppers are nothing, but a material handling device through which the commodities inside automated system are fed. Let us consider an injection molding; the hopper is nothing, but a conical flask which is mounted on the injection molding machine, through which we are feeding the granules of the polymers inside the mold area. The machine tools or the machineries which are used in automation, need to be continuously lubricated. For that purpose the grease is being used and to monitor the grease level, the capacitive element sensor is used. Then liquid level also can be monitored using capacitive element sensor.
Assessment Question#4. Which of the following IS the correct description for the "Capacitive Element Sensor"? Choose one answer. Correct answer: Displacement Sensor. Incorrect answer: Proximity Sensor, and Energy Transducer
Displacement Transducers used in Automated Systems
The next important and useful transducer which is used in automation is linear variable differential transformer and it is widely known as LVDTThese kind of sensors are used to measure the displacement between plus or minus 2 mm to say about 400 millimetres; these sensors has non-linearity error of 0.25 percentage. The principle of operation is very simple. In linear variable differential transformer, three coils are used; the first coil is the primary coil; then a set of secondary coils, secondary coil number 1 and secondary coil number 2. These three coils have equal number of turns; constant alternating current voltage is applied to the primary coil. And the secondary coil number 1 and second coil number 2 are attached or connected in such a way that their output voltage difference is zero. There is a phase change in the connection of secondary coil number 1 and secondary coil number 2. Inside these coils, a ferrous rod is moved . When this ferrous rod has equal overlap with secondary coil number 1 and secondary coil number 2, then 0 output voltage is obtained. When constant AC voltage input is applied to the primary coil, it generates alternating magnetic field; that alternating magnetic field will generate the electro motive force in secondary coil number 1 and secondary coil number 2 through the ferrous rod. When the overlap of ferrous rod one in secondary coil number 1 and secondary coil number 2 is equal; then the output voltage is obtained as 0. Why it is 0? Because the secondary coil 1 and secondary coil 2 are connected in opposite direction. Their magnitudes are same, but the signs are different opposite. Now, how the displacement is measured by using LVDT sensors? The ferrous rod which is generating the resultant emf across the leads or the connections of secondary coil 1 and secondary coil 2; that ferrous rod is attached to the mechanical element of which we want to measure the displacement.As the rod moves in a downward direction; then the overlap of ferrous rod one in secondary coil 1 is reduced, while the overlap in secondary coil number 2 is the same constant. As the overlap in coil one is reduced, the emf generated in coil number 1 will also be reduced. In this way nonzero voltage is obtained at the output. This nonzero voltage at the output is the function of the displacement. LVDT is again calibrated in the laboratories; for known displacement, we can get the generation of the voltages in these coils. A typical construction of LVDT which is used in the industry is shown in the next figure; we can see a set of secondary coils and a primary coil, and the core is attached to a shaft. The shaft is having a tip; the shaft is spring loaded so that we can measure the displacement. And after measuring the displacement of the tip, it will regain its original position. Thus, to go to its original position, the springs are used. The output will be taken through cables and further the output will be processed, signal conditioned and then it will be sent to the microprocessor. The tip is in contact with the mechanical elements; consider we are having a plate which is connected to some mechanical element. And as there is a moment of this mechanical element, this plate will push the tip and accordingly we are getting signals in the cables. The absolute position can be measured by using the LVDT sensors. These sensors have good repeatability and reproducibility and they are highly reliable. The construction has no contacts. What are various applications of the LCDT sensor? We can see a mill, the mill has two rollers and this mill is used to reduce the thickness of a metal sheet. There is an arrangement here to adjust the spacing between these two rollers.To adjust the spacing between these two rollers, we have to mechanically move the upper roller with respect to the second roller which is at the down side. When we move mechanically, we have to monitor whether the movement is appropriate, whether the movement is desired. To monitor this movement, LVDT sensors are used. If there is more movement or excessive movement of the upper roller, the quality of the product will be affected. To restrict that extra movement due to the inertia of these rollers, the LVDT sensors are used. The next example is an injection molding machine, this is used to manufacture plastic components. In injection molding machine, we have molds which are having two parts;, so we should have a precise opening and closing of the molds. For that purpose, we have to control its operation; the molds’ inertia and weights are also very high. To have the control movements, the LVDT sensors are used. In friction welding process, we need to control the distance between the two metal sheets which are to be welded. The precise moment of the one plate with respect to the other plate is monitored by using LVDT sensors.In the linear variable differential transformers, the lens are quite long. And in many cases, that long length linear variable differential transformers are not that convenient. There is another variation of linear variable differential transformer and that is used to measure the angular velocity. Instead of having the linear variation, the rotary variation is used, the cores are rotating instead of moving in a linear way. An arrangement of RVDT that is rotary variable differential transformer is shown on the screen; the construction is very interesting, it has cardioid shaped magnetic material core which is the cardioid shift magnetic material. We are having a primary coil and a set of secondary coil that is secondary coil number 1 and secondary coil number 2. The principle of operation is very similar to the LVDTs. As we rotate the core, we may get difference in the overlap of the core with secondary coil number 1 and secondary coil number 2. Due to the difference in overlap, there is nonzero voltage at the output, and that nonzero voltage is nothing, but the indication of angular motion of the shaft or the pin to which it is attached. This is the ideal position or the normal position. The portion 1 and portion 2 has the equal overlap with secondary coil 1 and secondary coil 2. As the cardioid shaped magnetic core is rotating, then there is more overlap for the secondary coil 2 than secondary coil 1. A typical construction is shown in the second diagram; these sensors are having a linearity error of about plus or minus 0.5 percentage. Now, let us look at various sensors and transducers which are used in manufacturing automation. The first type of sensors are displacement, position and proximity sensors. The automated systems have various mechanisms, and these mechanisms have various linkagesand elements. During the process of operation, these linkages and elements displace, there is a movementand motion of these elements or linkages. We need to monitor the displacement of these linkages or the elements. Second is the position. The automated systems are moving inside a shop floor and the position of these systems inside the shop floor need to be tracked. We need to track the position and locate the devices in the specified space.
Assessment Question#5. Fill in the blank: The LVDT (Linear variable differential transformer) is an important and useful "_____" used in Automation. Correct answer: transducer
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