noc18-me62 Lec 18-Comparators (Part 1 of 2)

So, the next topic of discussion is going to be comparator, moment you see the topic or title comparator, so then you will quickly realize that there is something to be compared. So, what is to be compared, unknown value to a known value, we compare and we try to get some data. So, this is what we are going to get the, we are going to go through this in this particular lecture. So, here the contents will be functional requirements of comparator, then classifications of comparator, dial gauge, so we will look into mechanical, then we will look into mechanical optical, then we will look into pneumatic, then electronic and then we will complete this chapter. So, we will see dial gauge, dial indicators, which is exhaustively used in workshop as well as in tool room, then Johnson's microcarotor, then sigma comparator, mechanical optical comparators, linear variable differential transformers LVDT, these are also comparators, electronic comparators and pneumatic comparators. So, at the end of this chapter or end of this lecture, you will be able to understand and appreciate how are these instruments working. All measurements require an unknown quantity to be compared with the known quantity called the standard. On the other hand, in certain devices, the standards are separated from the instrument. It compares the unknown length with the standard, such measurement is known as comparison measurement and the instrument which is used to provide that comparison is called as a comparator. The instrument is comparator and what we do is a comparison measurement. So, accuracy of a comparison measurement primarily depends on four factors, these are very important, four factors. First, it depends on the accuracy of the standard used for setting the comparator. If the standard itself is not correct or if there is a small error, then whatever you set on this comparator will be with that error, base data plus that error and when you measure it, it will always going to be this error also is going to get magnified and then you see the data. And especially when you do linear, it is okay, we will be able to control it, but when it is at an angle, it is very difficult. The least count of the standard is a important comparison. The sensitivity of the comparator, sensitivity is how sensitive you are. For example, you try to call up somebody, you call up five times, then they turn and then they realize that okay, you have made a call, so that is sensitivity. And second thing is sensitivity also depends upon one, what is the small change can you measure to how quick can you measure. These two are very important with as part of sensitivity. Then accuracy of the reading on the scale, all these things are very important factors which are to be considered for a comparator. It gives only dimensional difference in relation to the basic dimension or master set. A comparator gives only a dimensional difference in relation to a basic dimension. For example, if you have 20 plus or minus 0.1 millimeter diameter to be measured, so the question is, so why do we want to always measure this 20, we will measure only this tolerance, that deviation alone. So, you can have a dial gauge which has a 0, which this is a 0 limit, which tries to have positive 0.1 and this can be negative 0.1 and you try to measure, this is a dial gauge. Okay, this is the center value, this is minus, this is positive. So, all you have to do is measure this, so it tries to, it gives only the dimensional difference in relationship to the basic size, okay. Comparator are generally used for linear measurement. For example, the slip gauges when you stack it and then you make an end standard, you just have to check the end standard, you put a dial gauge, dial it all through or take a surface plate, dial it at 10 places, you get the deviation, so it is more of linear measurement. Various comparators currently available basically differs in their method of amplifying and recording the variation measured. So, when you talk about any device today, instrument, I covered in the beginning itself instrument. First is measurement, second thing is display of the data, the third thing is it has to be amplified and displayed and then fourth thing is it has to be recorded. So, now when we look at any instruments, we always try to see all the four data are capable in that instrument because this is with respect to time, okay, variation with time we can try to measure the data. So, in general requirement, a comparator must have a high degree of accuracy and precision. We can safely say that in general comparison measurement provides better accuracy and precision than the direct measurement because here we are not worried about this 20 plus or minus 0.1, we are not worried about this 20, we are only worried about this small measurement. So, this small measurement can be easily done because this will be magnified and then you try to take. It is better than the direct measurement, a comparison measurement, it is dependent on the least count of the standard and the mean for comparison. Least count means suppose let us assume you have to measure, you are trying to make a shaft of 20 plus or minus 0.1 mm. So, now you have, so you are asking for a tolerance of 0.1. So, the least count for this instrument should be one-tenth of its measurement, so that means to say you should have a instrument or a dial gauge which has a least count of 0.01 at least. Then only you can try to measure this tolerance variation. The thumb rule says one-tenth of the tolerance whatever is given you should have the least count for your equipment. If you do not have this, then it is very hard for measuring this variation. The direct measurement instrument such as vernier caliper and micrometer have a standard built into it with the result that measured is done by the displacement method. The scale should be linear and has to have a wide range. The comparator be it mechanical, be it pneumatic, be it electrical, so that means to say the source for comparator can the instrument can work on mechanical. Mechanical means it can be working with respect to spring, you use some levers, so you can magnify 10 times, demagnify 10 times, both are possible. So, generally we magnify and then we have a coiled spring or a torsion spring or we have a leaf spring to do this. Next you can use pressure, air pressure, so difference in air pressure can be used for measuring comparator. Then electrical also can be used has a means of amplification of the signal, linearity scale within the measurement range should be assured. So, what we are trying to say is suppose let us try to say this is the voltage output and this is the displacement. The response can be something like this, I am just amplifying it, then the response can be linear and then the response can fall down like this ok. So, now what they are saying is whenever you try to use an instrument, look at the linearity range and try to use the comparator or the instrument in this range. Why? Because in this range it is easy for you to define an equation for what volt displacement you will have this voltage ok. Here you can still write, but you will have error terms also, here it will be much more easier that is what we are trying to say. So, linearity of the scale within the measuring range should be assured ok. The comparator is required to have high amplification. It should be able to amplify changes in the input, so that the reading can be taken and recorded accurately. This puts load on the system resulting in in the system being unable to sense small changes in the input signals. Therefore, one has to strike a comparison between the two, so input output. So, if you have a very sensitive output, then if there is a very small change in the input, then it becomes as a mismatch. This input load on the system resulting in the system being unable to sense a small change in the input signal too sensitive, so it will try to give you lot of signal and noise. So, when you try to use it in electrical, it will get a signal and then you will also get one more term called as noise. And if it is too sensitive, the noise becomes dominating than the signal. If the input small variation, then it becomes very difficult for you to find out. The functional requirement last point is going to be, you should have the amplification demand used of more number of linkage in mechanical system and a more elaborate circuit in terms of electrical system is required when we try to do amplification right. So, this number of links can also lead you to error, we should be careful. Amplification of comparator. We can classify comparator into mechanical device and electrical device on the basis of the means used for comparison. In recent times, engineer prefers to classify comparator as low and high amplification comparators, which also reflects the sophistication of the technology that is behind these devices. So, they are classified like mechanical comparator, they are classified as mechanical comparator, mechanical optical comparator, electrical and electronics comparator, pneumatic comparator and finally, projection comparator and multi check comparator. So, these are the classifications which are done on the principle for amplifying and recording measurements. Dial gauge, which now we are talking about the mechanical comparator. So, mechanical comparator dial gauge, the dial gauge indicator or a dial gauge is one of the simplest and the most widely used comparator. It primarily used to compare workpiece as against a master. The basic feature of a dial gauge consists of a body with a circular graduated dial, a contact point connected to a gear train and an indicating hand that directly indicates the linear displacement of the contact point. Let us see the figure. So, this is the figure, right. So, you have a plunger which moves, the plunger is now, there is a spring which is loading to G and here the displacement will try to move this coil spring and you have a rack. So, from the rack you have a pointer or from here you can have a pointer which is displayed on a dial gauge. The contact point in a dial indicator is of an interchangeable type and provides versatility to the measurement. So, the contact point which touches the workpiece in a dial gauge is of an interchangeable type. It is available as a mounting and in a variety of hard wear resistant material. The heat treated steel, boron carbide, sapphire, diamond are also used at the tip, contact tip. The plunger and the spindle are one piece. The spindle attached to the bottom of the rack is the basic sensing element. So, where is the spindle? The spindle attached to the bottom of the rack. So, here is a rack, right, here is a pinion, here is a rack. So, when you push this, this rack moves, then the pinion moves. So, in the pinion if you have a pointer, then the pointer also moves. A coiled spring resists the measurement movement and thereby applies the necessary gauging pressure. So, when it moves, automatically if it goes, if it goes up and if it stands there, then it is of no meaning. So, there has to be a resistance given for this upward movement. So, we use a coil spring. Thus, the amplification of the gauging pressure is built into the mechanism rather than leaving it to the technician. It can also return the mechanism to the at rest position after each measurement that is because of the coiled spring whatever is available. The plunger carries a rack which meshes with the gear marked gear A in the figure. So, this is the figure which is marked A in the gear figure. The rack guides, prevents the rotation of the plunger about its own axis. A small movement of the plunger causes the rack to turn a gear A. So, if you go back gear A, I have said, when a large gear B mounted on the spring swindle as gear A rotates by the same amount and transfers to the motion C. So, this gear, this moves up and down. So, this is pivoted, this gear is also pivoted on the same point. So, when A rotates, B also rotates. Now, when B rotates, C also rotates. When C rotates, D also rotates and D in turn rotates E. So, a large gear B mounted on the same spindle as the gear A rotates by a small amount and transfers the motion to C. Attached to the gear C is another gear D which meshes with the gear E. Gear F is mounted on the same spindle as the indicator point, gear F is mounted on the same spindle of the indicator point. So, the overall magnification obtained in the gear train A, B, C, D and E is given by T D by T E into T B by T C, where T D, T E, T B and T C are the number of teeths on the gear D, E, B and C respectively. So, it is all a gear train gear ratios. So, if you try to do the overall magnification, it is all between D and E. If you go back to the figure D and E that is one. So, D, E is one set, then the next set is going to be B, C which is one other set. So, these two sets are there. So, we try to get the amplification based on this. So, I have explained this figure. Here is a plunger rack, this gear rotates, then this gear rotates, then this E rotates, this in turn is attached to the coil spring head. So, the contact point, the dial, indicator are versatile instrument because their mounting adapts them to many methods of support. The interchangeability of the contact point adapts them for varying measurement. So, the tip can be changed, you can have a circular tip, you can have a flat tip, you can have a tip which is like inward like a plunger, you can have multiple tips. The standard or a spherical contact is the most preferred one because present point contact on the mating surface irrespective of whether it is flat or not. What we are trying to say is, we are trying to say when we have a circular contact, then the point of contact is going to be a single point. So, the button type contact points can be used if light contact pressures of small components are used. So, to start up, these indicators is moved up and the standard is placed on the reference surface while measuring the spindle of the indicator does not make in contact with the surface. So, first we keep, we put it as a dial and then this dial is moved, this dial is moved against say for example, there is a stand, then we put all these standard measurement whatever it is, then here on this stand we mount a dial gauge. To start up, the indicator is moved up and the standard piece is placed on the reference surface while ensuring that the spindle of the indicator does not make in contact with the standard. Next, the standard clamp is loosened and the spindle of the indicator is gently lowered to the surface of the standard such that the spindle is under the given required gauge pressure. Then, the indicator is held in position by tightening the stand clamp, the bezel clamp is loosened and the bezel is rotated and the reading is set to 0. So, now here is the reading which is set to 0. The dial indicator should be set to the dimensions that is approximately in the center of the spread over which the actual object size is varied. So, the dial indicator is a delicate instrument and as a slender spindle can damage easily. So, the user should avoid sudden contact with the workpiece surface, overall tightening of the contact point. Any sharp fall or blow can damage the contact point. Standard reference surfaces should be used for measuring and dialing it to 0. The dial indicator should be kept should be cleaned thoroughly before use and periodic calibration of the dial gauge is must. These are all for dial gauge type indicator. So, next one we will see the Jackson's micro curator. The basic element of this type of comparator is a light pointer made of glass fixed to a thin twisted metal spring is a light pointed made of glass fixed to a thin twisted metal spring. Most of us during childhood would be familiar with a simple toy having buttons spinning on a loop of a string. The same concept is used here by Jonason. The basic principle is also referred as Abramson movement after H. Abramson was developed this comparator. Two halves ok, this is the twisted spring, this is the this is a pointer, this is the twisted string which is there and this is a bell crank lever, this is a bell crank lever. So, this is attached to a plunger, this is a light pointer right. So, here is this is to guide the pointer we use it and here is a cantilever strip. So, this is basically to hold and here when this is moved this is when this is moved up, this is pulled. So, then there is a deflection which is happening this is against the fixed cantilever strip ok. So, the two halves of a thin metal strip which carries the light pointer are twisted in opposite direction. Two halves of a thin metal strip which carries a light pointer is which carries are twisted in opposite direction. Therefore, any pull on the string will cause the pointer to rotate, the pointer will rotate. We can easily see the relationship of the length and the width of the strip with a degree of amplification. Thus dq by dl is inversely proportional to l by n w square omega square. So, here d theta by dl. So, this is d theta, d theta by dl is the amplification of the instrument, l is the length of the metal strip measured along the neutral axis, n is the number of turns of the metal strip and w is the width of the metal strip. So, with this we can try to we can try to see what is the amplification happens when this Johansson micro crater. So, we are talking about this right this rotates, the pointer rotates that is what we say here. Therefore, any pull on the string will cause the pointer to rotate. So, the next comparator is called as sigma comparator, sigma was developed by sigma instrument company in USA. The linear displacement of the plunger is translated into the movement of a pointer over a calibrated scale. This is the sigma comparator. So, here is a plunger. So, here is the instrument you place. So, when this is pushed, when this is pushed, this arm this is also moved ok, when this is moved. So, then you can see here that this translation happens and then finally, the pointer moves this way or this way, the pointer is mounted ok. So, this is attached to a band, this is attached to a band and this band in turn is attached to a fixed point and here is at moment the plunger moves this a knife edge also moves. So, from that knife edge we get transfer this to the to the other pointer. The plunger is a sensitive element that is in contact with the workpiece, it moves on a slit washer, the slit washer is everywhere, the slit washer is everywhere which is used to guide the plunger movement which provides frictionless linear movement and also arrest the rotation of the plunger. The magnification of this instrument is obtained in two stages. In the first stage, if the effective length of y arm is l and the distance from the hinge pivot of the knife edge is x, then the magnification is l by x. So, this is l and this is x. The second stage of magnification is obtained with respect to the pointer length r and driving drum radius r, this is r and this is r. So, then the overall magnification is one time here and one time here. So, this is how we try to get the magnification of the measurement whatever is done to get the response. So, you can have it 10 times, you can have even 100 times response is possible. The next comparator is going to be mechanical optical comparator which was the term also called as Cooke's optical comparator. As the name of the comparator itself suggest, it has a mechanical part as well as an optical part. Small displacement of a measurement measuring plunger are initially amplified by a mechanical system the lever mechanism pivoted about a point. The plunger is spring loaded such that it is biased to exert a downward force on the workpiece. So, for example, here is the workpiece we keep ok. So, here is a plunger which in turn tries to move this lever. So, this lever tries to move this mirror here ok, it is pivoted here. So, this is moved. So, then what happens there is a light source, a condenser, a projector which hits at the surface and then this gets reflected on a scale and then we try to measure the value ok. So, here you this is the this is the workpiece which is kept a plunger. A plunger in turn is pivoted here, this the other end of the plunger is attached to the mirror. So, when the plunger moves down, this moves up. So, once this moves up, there is a change in the mirror angle. So, here it can move up this way that way. So, here is a pivot point. So, light source optical system, mechanical system put together called as optical comparator. So, the magnification amplification mechanical amplification is L 2 by L 1 and the optical one is L 4 by L 3 into 2 times. So, the multiplication factor 2 figures the optical amplification because if the mirror is tilted by an angle theta, then the image is tilted by an angle 2 theta over the scale. Thus the overall magnification is going to be twice L 4 by L 3 L then multiplied by L 2 by L 1. The scale is set to 0 by inserting the reference gauge below the plunger and the amplified mechanical moment is further amplified by by optical system due to a tilt in the plane mirror. The condensed beam of light passes through the index which normally comprises of a set of cross wires. So, the image of the cross wires is measured and we try to take the amplification. We can try a small problem if the value of L 1 is given, L 2 is given, L 3 is given and L 4 is given. So, to find out the overall magnification in units. So, what we do is we try to do L 1 by L 2, then L 4 by L 3, then the multiplication is twice. So, if you see it will be 200, 2000 times magnification can be done and you try to get the output. So, why is it 2 times? It is 2 time into del theta, the del theta is very small angle displacement we try to get this. The next one is going to be optical projector. The optical projector is a versatile projector which is widely used for inspection purpose. It is essentially used in tool room application and the other thing is if you I gave you an example for O ring in the first lecture task for the students. If you remember, I gave you an example of a O ring to measure the diameter of the O ring. So, optical projection is the most suitable technique to measure the O ring dimensions because the material is elastic in nature. Moment you push or touch against an instrument, it is going to deflect or deviate. So, optical projection is the technique which is used for that. It projects a two dimensional magnified image of the workpiece on to a screen to facilitate the measurement. So, you can have an O ring there and this can be optically magnified may be 20 times, 200 times and you try to see a very large image you can try to see, but even now the biggest challenge is on a O ring suppose if there is a crack, on a O ring if there is a crack this cannot be detected by the optical projector because optical projector projects these dimensions only, but on the surface if there is a crack this technique cannot be used ok. It comprises of three main elements, a projector itself comprise a light source and a set of lenses house inside it, then a working table to hold the workpiece, then a transparent screen with or without a chart. So, what we say is this is a O ring and the transparent screen will be something like this. So, if you see a transparent screen, so here you can have graduations. So, the most preferred light source is tungsten filament light although mercury and xenon lights are also used sometime, achromatic collimated lens is placed in the path of a light beam coming through the lamp. The collimated lens will reoriented the light rays into a parallel beam, beam large enough in diameter to provide the coverage of the entire workpiece. Because if you try to project an object and you have to see the full dimensions full features of the object, then we use a collimated lens. So, what it does is which gets all focused on top the light into a parallel beam large enough in the diameter to provide the coverage of the entire workpiece. So, this is how an optical projector works. Next topic of discussion is going to be electrical comparator. The electrical and electronic comparators are in wide range used because of their instantaneous response and convenience in amplifying the input. The electronic amplification is very very easy and it is very very reliable as compared to mechanical because mechanical ET is prone to have a wear and tear. An electronic comparator in particular can achieve an exceptional high magnification of the order of 105 times is to 1 quite easily which is not possible in mechanical. So, mechanical if you want to do such amount of high magnification, we do it on stages. So, that is why if you remember in the discussion itself we when we discussed about the dial gauge, we said using multiple levers lever mechanism, we can start magnification. But if there is a error in one of the mechanism, it further amplifies and what the reading you get is not correct. So, that is a problem with dial gauge, but in electronic you can easily go for very high magnification. The electrical and electronic comparator mainly differs with respect to magnification and the type of output. Both rely on mechanical contact with the work to be measured, but finally here it is only the display, but mechanical contact is always used. The electrical comparator generally depends on Wheatstone bridge principle. This we have studied in your basic engineering courses Wheatstone bridge principle for measurement. A DC circuit comprising of four resistors each on both arms is balanced when the ratio of the resistance in the both arms are equal. So, this is nothing but Wheatstone bridge principle. So, we use this Wheatstone bridge principle generally in electrical comparator a plunger. So, I told you earlier mechanical system is used and then we attach it to an electrical system. So, plunger is the sensing element the movement of this displace the which displaces an armature inside a pair of coil. So, this is now converted into electrical, so mechanical to electrical. The movement of the armature causes the change in the inductance in the two coils resulting in a net change in the inductance. This change in the inductance brings an imbalance in the bridge circuit resulting in an output. So, the basic principle of electrical comparator works on Wheatstone bridge principle. So, this is the electrical comparator you have a mechanical plunger. So, when you have a long arm or a long shaft you should always make sure it is supported at several places such that it is guided and the plunger does not deviate from its axis ok. So, this is done. So, this is what you see in the older ones we have for every plunger we had a we had a comparator. So, or we had see you see here we have a slit diaphragm this is only to make sure it goes along a straight line. So, then here this moves then this is trying to push this fellow then this fellow armature might move this might move like this or it can move like this depending upon the displacement here whatever you give. So, based on this there is a deflection which is coming this in turn is attached to the Wheatstone bridge then we try to measure the take the measurement. The LVDT is an provides an AC current. So, here we talked about DC the next one is LVDT which is nothing, but linear variable differential transformer it provides an alternative current voltage output proportional to the relative displacement of a transformer core with respect to a pair of electrical winding. So, core with respect to electrical winding. So, see it is left to you. So, if you have a change in the construction the armature can slide up and down in this direction or it can deviate LVDT is for displacement. It provides high degree of amplification and it is very popular and it is easy to use. Moreover, it is a non contact type device where there is no physical contact between the plunger and the sensing element plunger and the sensing element. As a consequence this friction is avoided result in better accuracy and long life of comparator. So, it is it is used it can be conveniently packaged into small cartridges and LVDT produces an output proportional to the displacement of the movement of a core within the field of within the field of several coils. The motion of the core varies the mutual inductance as I told you in Wheatstone bridge the change in the inductance determines the voltage induced from the primary coil to the secondary coil. Since the secondary coil are in series a net differential output results and gives the position of the core the output voltage is generated when the core moves on either sides of the null. So, theoretically the output voltage magnitudes are of are very small or are the same for equal a core displacement. So, this is how an LVDT works. So, you can see here this is a primary coil, these are secondary coil, this is the in voltage voltage you apply, this is the out voltage. So, this is a this R transformer right. So, this is an armature when it moves up or when it moves down you can see that there is a displacement change and this is the voltage output there is a change. So, you see here voltage output and again here it is between these two transformers it always try to display a null and when it is moved up or down then you see a change in voltage. So, this is how a linear variable differential transformer works and here the displacements can be from few microns it can go up to few millimeter this itself is too huge few millimeter it can measure, but we are more more interested in the lower sensitivity. So, the electronic comparator electronic gauges are more accurate and reliable which has made them preferred choice in many applications. So, they are highly accurate, reliable, highly sensitive, sensitive in terms of speed, it easily provides multiple amplification, it is versatile a large number of measurement situations can be handled with a standard accessory and it can be easily integrated to any automated system. So, this is how an electronic comparator looks like. So, you can set values you can also see the displacement here. So, you can see here it displays to the third decimal point. Next topic of discussion is going to be pneumatic comparator. Thank you very much.