 Good day everyone. I'm Assistant Professor Ushaal Mistry from the Mechanical Engine Department, Valjan Institute of Technology, Sholapur, and for today's session, I'll be discussing on capacitive sensors. The learning outcome of this session is that the learner will be able to list types of capacitive sensors, explain principle of operation of capacitive sensors, and animate applications of these capacitive sensors. In the previous class, we studied the measurement systems, different types of sensor classification, and also inductive sensors. So we continue from there, and today we'll be discussing capacitive sensors or capacitive transducers. Now, capacitive transducers typically are based on the fact that there's a variation in capacitance between two separated electrodes is used for the measurement. Capacitance is thus a function of the effective area of the conductors, the separation between the conductors, and the dielectric strength of the material. So a change in capacitance can thus be bought about by varying any one of the three parameters. That is, either you can change the distance between the two parallel electrodes, or by changing the dielectric constant, the permittivity, or the dielectric medium, or changing the area between the electrodes. So we'll look up all these three cases in a minute. So the ratio of the amount of charge stored on the plates to the amount of voltage across the capacitor is called the capacitance, and the capacitance is thus directly proportional to the area of plates, inversely proportional to the distance between, and inversely proportional to the distance between them, given by the formula, as given on the screen measured in farads. Now, the three cases that we were discussing is, you could alter the capacitance by separating the two electrodes or by altering D. So if you look at the formula, and if you change D, you get a curve typically, which is, since D is in the denominator, the relation actually ends up being non-linear. Now, another way would be actually to change the area. Now, the formula actually is rewritten like this, in terms of LW by D, where area is given as in the form of L and W. So if you move the areas, if you move the plates across like this, you can effectively change the capacitance, and this relation is very linear. Another way obviously would be to change the dielectric. Now, these different ways are used by industry in different contexts, and the best example actually would be the cell phone, where the third option that is capacitance variation using change of dielectric is very widely used. Now, some food for thought, of all the three techniques which are discussed about, which one do you think has a definitive advantage over the other? If you use it, now you use a device which has a capacity sensor every day, that is the cell phone, I'll give you the hint. So what is actually the principle which is being basically used over here? Now think of it, you use a device which has a capacity sensor, and you use it every day. So what device is it? Now, coming back to capacity sensors, what are the features of capacity sensors? Let's say vis-a-vis some of the other sensors which you have discussed before. One thing is that they require extremely small forces to operate. They are very sensitive and require very low power to operate as well. Frequency response is quite good up to 50 kilohertz, and they are very good candidates for applications involving dynamic measurements. So they can be used in accelerometers up to a certain obviously frequency range extensively. So this is one advantage capacity sensors have over elevators which have a very small application when it comes to dynamic measurements. So they offer extremely high resolution. Definitely more than potential metric sensors and some say almost in parallel or sometimes exceeding elevators sensors. One more thing is you need to insulate metallic parts from each other. There is some loss of sensitivity to error sources associated with cable connecting. They are susceptible to moisture and other environmental factors. These are typically the negative features which have been attributed in most textbooks. These sensors are remarkably resistant to falls. This is one of the reasons you see your cell phone operates extremely well in different conditions, moisture, et cetera, even very long period of time. But depending upon the application and depending upon the construction, impurities, contamination, dust, usually don't have effect on them, nor does electromagnetic induction. Now when it comes to features, let me tell you, there may be a bit of contradiction in textbooks. Some of the older textbooks tend to indicate that capacity sensors are somehow not tolerant to impurities, contamination, which may be true at that time, but things have changed and today if you actually look at features as advertised by the sensor manufacturers, they tend to claim that their sensors, especially capacity sensors, are quite tolerant to dust, impurities, moisture, et cetera, et cetera. So we will go with the latest data when it comes to such features. So what are the typical application then of capacity sensors? Now, remember the applications overlap with LVDTs and other sensors as well, but they do have a certain niche when it comes to the applications. The ability of capacity sensors to detect virtually all materials makes them an attractive choice in many applications. It's basically used as secondary transducer in tactile sensors, in force sensors, in pressure sensors. Take for example, a pressure sensor involving a deflection of a diaphragm, but the displacement is measured by a capacity sensor in the diaphragm, actually being the primary transducer and the capacity sensor, obviously being the secondary transducer. They're extensively used in track pads and touch screens in almost every tablet, the cell phone, or even laptop screen. They're almost always capacitive sensors. Other applications typically are proximity sensors in level and in feed monitoring. They are used in paper, wood, granules, which would detect the status of the product during the production process and final inspection. So again, these applications have been compiled from various sources. This is in addition to the traditional displacement sensing under which these sensors are grouped. Now, one application is actually in very high precision measurement where capacity sensors has been used. Physical instrument is a very renowned manufacturer of high-tech precision instrument involving nano-scale measurements. And this is where I found one application of capacity sensors, especially as nano sensors for nano measurements. The picture will also give you an idea of how these sensors look and how they are packaged. Again, if you take a look at some very general applications of capacity sensors are, like I said, in position measurement, displacement measurement, in measurement of thickness of plates in several applications, including rolling wells, et cetera, et cetera. To measure run-out and eccentricity, which is also one very wide application area of capacity sensors. Another application, again, is to measure deformation surface irregularities. So this application kind of overlaps the application area of the LVDT. In part sorting to assess the presence or absence of part, they can be used for vibration measurement as well because we saw this capacity sensors can be used for dynamic measurement, and they definitely have a wider application when it comes to dynamic measurement compared to LVDT. So one thing I always recommend is you look up how these sensors are actually available in the market, how they look like, how they're packaged. So a good Google image view gives us an idea regarding how these sensors are available and sold by manufacturers. So this gives you the different ways in which these sensors are manufactured and sold. References for this particular session. I have referred mechatronics handbook by Bishop. I have referred Shetty and Colk mechatronics system design and introduction to mechatronics by Bolton. Along with these textbooks, I recommend that you visit the websites of top manufacturers. One of them is Physics Instrument. Another website which I've referred to is from Line Instruments, which makes capacity sensors. The website I found is a website to be very informative with good explanation regarding principles of operations, applications, features, et cetera. So I definitely recommend that you look up these websites apart from the general reading that you will be doing via the textbooks. Thank you very much.