 resistive sensors ok might have already like you know seen or maybe you might be seeing for the first time I do not know. So, so these are typically the strain gauges ok are resistive sensors. So, what what people do is like you know they they lay down this kind of a architecture of like a wire which is running across this patch of the sensor and this wire is actually some kind of a metal ok which is laid on the some small very thin kind of a plastic substrate. And when you when you like you know stick it to some metal or some place where you want to sense the strain then when it is stuck there and you start like bending that beam or say for example, you have your steel ruler that you use for measurement that steel ruler like know if you imagine this patch is stuck on that steel ruler at one end and you hold that end formally and then like know you apply some kind of a bending force at the other end then like know the strain that will happen at this end will get sense by using this resistive strain gauges sensor. Now you might have seen this formula principle and so the principle of operation here is just like know that the resistance value ok R which is equal to rho L by A ok where rho is resistivity as a length changes delta L change in the length will cause a change in the resistance ok that is the basic principle on which this typical strain gauges or metallic strain gauges are based on ok. So, typically the gauge factor will be two for the metals but then there are some other kind of a ways the resistance in the metal can change. So, if you have the resistivity change itself rho change itself happening then your gauge factor can be enhanced to higher values ok. So, nowadays there are sensors that are coming up in the market which will have much higher like know gauge factor. So, so these are what kind of sensors can you think they are analogal digital yeah they are analog sensors right there is nothing in like know digital here. So, you apply the voltage and like know in some kind of a western bridge kind of a configuration and you keep the strain and then continuous change in the voltage will happen at the output of the sensor. And then to kind of interface it with micro rotor you need some kind of conversion of analog to digital and then you can interface it with that. There will be some of the issues when you start practically using these sensors there will be temperature considerations there will be noise issues and then if you want to use whether you want to do single axis kind of a single sensor kind of a measurement or you need to kind of use what kind of a configuration that you want to use like you know that depends. So, we will see some I will post some material about that in the thing to kind of you to refer to and see half bridge configuration full bridge configuration you know what are the formulas that are available they are there are directly available you one can derive them by using kickoff's laws, but like no you don't need to get into probably in details I mean it just application of kickoff's laws and like using that for finding out some some output for the strain gauge that kind of exercise. So, this table I will post in the in the noodles you know our notes. Then you can have this more advanced now semiconductor based in say silicon for example when used as a as a strain sensor the basic difference is that we are not just dependent upon the length change there ok. So, suppose we lay like know the silicon strain gauge then the difference from the metal is that like you know in the formula you have a rho L by A as a resistance right. So, rho is a resistivity now this resistivity itself changes in the in the semiconductor strain gauges ok. So, it is basically based on the principle of like know that the flow of electrons that happen inside the semiconductor ok. So, that the distance it change you know for the electron to jump from one one kind of a molecule to other molecule to kind of like know create a flow path that change itself affects the resistivity ok in the in case of semiconductors in case of metals you will have a free electron. So, there is no such kind of a need there, but in the semiconductors you will have that kind of a possibility happening and based on that you can create like know in very like know very very highly sensitive strain gauge sensors based on the semiconductor principle and that sensitivity is important for the noise consideration. So, if you have very highly sensitive sensor then like know typically the the sensor to noise ratio can be minimized ok. So, that is a principle that one can use for having a deal with noise in some way. Then we can have particles of conducting material in some non-conducting polymer ok that kind also creates some kind of a resistive sensor which will have a very high gauge factor possibility ok. The same principle like you have a polymer matrix in which the carbon particles for example are there and carbon particles are conducting or compared carbon nanotubes for example or like graphene for example that is conducting domains in non-conducting material. So, the the I mean the boundary is very very small I mean it is not like know very far away kind of a boundary it is like a dense packing of these particles in the polymer matrix and just electrons have to jump between the in the gap that is created by the polymer between the two say carbon nanotubes for example and this jump will be kind of creating resistance for the path and as you bend or as you kind of give a strain the length of the jump starts increasing ok. So, amount by which the electron has to jump to kind of like you know get to the next kind of a conducting path that increases and that because of that you can imagine that you know small increase in the distance will have a large kind of a you know possibility for the large voltage needed for this jump to happen in some sense. So, the resistance change would be like know very drastic for small even small change in the strain. So, this is how the newer ways of creating these strain gauge sensors are typically coming up in the market ok. So, think about this question like know can you have a digital strain gauge possibility or like know the output of the strain gauge can be digital. So, think about this ok. And so, once you have some kind of a ways of like know thinking this classification can we see ok or can I have like know this digital domain sensing possible that can open up some interesting kind of innovative ideas in your mind. So, these are like know some pictures of actually the strain and strain gauge sensors for like know different kind of configurations and making sure like you know we want to measure something say torque measurement if you want to do then like know you place it in some. So, these are kind of called rosettes strain rosettes ok. So, the multiple sensors put together to get a specific kind of a measurement and then you can have a capacitive principle to operate ok. The basic principle of capacitance you know is epsilon a by d. So, this a is the area of the capacitor d is a gap between the two electrodes that are using capacitor. So, you can have many different kind of configurations here flat plate configuration cylindrical configuration and things like that. And typical application capacitive sensors I use is in the displacement ok. So, there is a normally this formula is applicable when there are no fringe fields, but like know typically there will be like a fringe field from here to here from plates the side of the plates like know the the electric lines ok or lines of electric field may not be completely going straight from the corners. In the corners may they may have some small bend that is happening ok. So, this bend will like know contribute to some some change in the capacitance which is not captured by this formula, but typically that may that may have to be done in in or dealt within some way in the in the application depending upon how much accuracy that you would need in in in these measurements ok. So, one of the examples of this capacitor same sensor is this this way of doing things where like know you have a surface which is electrical conductor and, but it is not participating really in a in a in a capacitive measurement activity in in some in some way. So, for example, you you have this measurement electrode which is kind of giving the field ok. So, to avoid fringe fields they have this additional kind of a secondary electric screening electrodes around here ok and then there is a ground ok. So, this is like how the they place typically the the sensor elements in the in the system and this electrical conductor is where like know the surface which from which we want to measure the distance of the probe. See this is a probe here ok. So, this electrical conductor is is is grounded. So, that you form a capacitance between these two. So, that way it is it is active part of the sensor, but it is not really like know you do not need it to be constructed with sensor you can kind of have this probe separately and then like know just put a ground on this electrode and then like know you you form this capacitance here and this can be extremely like know high sensitivity displacement measurement possible. So, typical displacement measurement with these units is can be possible to the extent of some few nanometers ok. Not just tens of nanometers, but like know some some 5 or 10 kind of a nanometer based on 10 nanometer can offer positioning can be sensing can be possible with such sensors. We do have such sensor in our lab I mean whenever you want to in the future you want to kind of like know take a look at you will be able to kind of amazed to see what how accurately and nicely it gives the positioning data. Maybe I would post like know the the data sheet also for such a capacitive sensor I will post it to the model to by the to watch ok. These are like inductive sensors where you might have seen these already this is called linear variable displacement transducer ok LBDTs ok. They have this primary coil and primary coil is excited and secondary coils is where we are measuring the voltage and this this you know the magnetic core when it is moved inside then the coupling of the voltages to these secondary coils changes that is the principle these LBDTs are based on based on ok. So, these are like know some industrial LBDTs that are there in the in the market and these are also very extremely high resolution kind of sensors can be possible with LBDTs ok. Then in the optical domain you you can have this is a very very what you say interesting or important sensor for a lot of mech atomic systems ok encoders ok. So, these are different from like the encoder word that is used in the electronics domain ok. So, we need to be careful about this mechanical this is mechanical encoders where you want to sense a position then like know you do this based on the principle as as demonstrated in this figure. What we do here is like know we have a light source here and then the light detector on the opposite side and there is a wheel which is having these slits ok. There are some dark portion and some bright you know the transparent portions. So, some opaque parts and some transparent parts ok or some gaps ok either way. So, when the light like when so this is powered up you know light is powered up and like know it receives a detector receives a right to begin with maybe and then like know you start rotating the wheel and the light will get cut by the wheel ok. As the light gets cut the detector will respond to that this is a light detector. So, it will when the light is not falling it will kind of respond and give some signal and that is how like know one can get get a signal out of these different sensors. So, you count these number of pulses that are coming on this light detector here and once you count like that that count will indicate the position of the wheel. Now, the question is like how do you kind of sense the direction of rotation ok. So, we will come to that in a minute, but think about like know how do we sense the direction ok. If you rotate in this direction or this direction only the thing is that we have happening is that you know the light is getting cut I mean and you are seeing some pulses there ok. So, for very interesting ideas that are used for getting the direction sensed ok we will come to that. So, this is one kind of a sensor which is called which is of the incremental type ok. It just gives you incremental information about the displacement. So, you can count the number of pulses you can get from some reference you will get to know the total displacement, but you do not get absolute displacement here. So, there could be possibility of absolute encoders also, but I mean it is very people are I mean that is not very widely used it is very rare to use these absolute encoders. Although they are they are construction is little difficult, but we maybe will have a chance of looking at some kind of a principle and like know how they are absolute encoders are designed. So, you can have a linear possibility for sensing or you can have a rotary possibility for sensing ok with the encoders ok. So, in the linear case there is a scale which is having these strips on the surface. Now, as you can see here we are having this transmission kind of a principle. And it is the same thing can be possible if I have a reflective principle here. So, that like that is the light reflects from some surfaces and comes back. So, that I can I do not need to have this kind of a complete pathway for the light. So, that this rotating wheel needs to come in the path rather than like know I can put this light detector on the same side as a light source and have this source and detector act in a reflective kind of a mode. So, the light falls on some surface and reflects from the surface and getting registered. That is a principle typically used for the linear encoders ok where they have a linear encoder head which is running on the scale that encodes the light that that use some kind of a that encodes the strips I mean the position scales ok. So, the idea is like you shine a light on this scale and you take the reflection there are some reflective parts and some transparent non-reflecting parts on the surface of the scale. And that is how they know you have a distinction between the two parts. Now, there are many different additional principles that can be used where this reflection happens as a as a reflective grating kind of ok. If you have these reflective parts very very tiny then the light starts behaving in a very different way ok. So, this has different kind of a repetitions for I know actually this becomes a little more complicated to do the encoding in a sense. So, the thing that is coming out from the all the strips has like say one micron kind of a line spacing. The spacing that is registered by encoders will be of your sensors may be different based on like know the grating of the diffraction pattern that are produced by reflection from such a small distance. See when the distances are larger you do not have the light diffraction effects, but when the distances get smaller and smaller then you will have typically you will have to deal with the light diffraction effects. And then consider that in your analysis or your calibration or your detecting your output of the sensor whatever you want to say. In rotary there are these two possibilities I have been told you absolute and incremental. The linear encoders can also have absolute encoding possibility, but not very widely used at all. Now, we come to this question how we sense direction of these pulses ok. Now, just pulses that are coming then like know how we sense direction. So, the idea is to use instead of one we use two sensors. And the two sensors are put in such a way that outputs of them are 90 degree out of phase with each other ok. So, that has two advantages. One is that it is it enables the detection of direction and other advantage is it enhances the resolution. You know we are putting instead of one sensor we are putting two sensors which are out of phase then like know you are expecting like know that resolution also will increase because we have two sensors now ok. The third sensor may be put if they want to know like that is called the incremental pulse that you want or sorry this is a word one you complete evolution you get one single pulse ok. So, that is a output that you get it is defined as I output I for the sensor. And like know these two phase like know outputs that are coming in their phase A phase B that is what is up there standard terminology that is used in the input as to mean ok. So, we will see now this A and B pulses that are coming and this increment of a revolution that is I pulse will typically be a simple single pulse that is coming also we do not need to look at it more. We look at this 90 degree how this 90 degree phase difference helps us detection of detection is what we need to kind of focus on now ok. So, imagine that you have this two sensors which are two pulses which are coming as the as a rotation happens. So, in this particular case like if you see at position 1 you have like you know you have 0 0 output A and B switching to this, but like let us start with some other position where they are both 0. Yeah maybe we can start here itself. So, say it is just like know you can consider that you know just before this red line ok what is the position what is the thing that you get recorded ok. So, that will be 0 0 for this ok. So, if you imagine like now here this is 0 here and this also is 0 just before this red line ok this timer timing marks I put it in a. So, in a way that you know we can just kind of like you think in a same kind of a logic in every red line case ok. So, here this is 0 0 here then if you go for the next position ok then A is still 0 ok, but B has gone to 1 ok. We are now reading just before the red line ok. So, red lines are drawn where the transitions happen, but we need to kind of be consistent with where we kind of consider this phase output ok. So, second third part like know you will have 1 1 here and fourth part you have 1 0 here ok this is how like know these pulses are coming. Now, you need to imagine that when the direction is reversed ok. So, when the direction is 1 in the forward direction these are the pulses coming when the reverse direction happens ok. So, say for example, you have these strips which are this requires little bit of imagination or like know you need to do little bit exercise you can take a pause here and then like know use the slits as I have shown here in this diagram and like know put actually the two sensors know such that like know you have the A and B output coming as shown in the first kind of a way ok when the forward direction motion is happening. And now if the direction of the wheel is only rotated the sensor position is just kept the same then what kind of a signal that you expect to come here is what we want to understand ok. So, do that little bit of exercise by pausing up here and then like know you will find this this we will again take this little bit of a discussion when we when we talk about in the discussion class ok. So, this is this you may or may not be able to crack it at this point, but just give a fair track and then we will come back in a in a class to talk about this ok. So, we will do that. So, is like a clockwise direction of rotation and then like know the the anti clockwise from direction of rotation will have like know the output produced in this kind of a fashion 0 0 followed by 1 0 now and then 1 1 and 0 1. So, based on these like ok 0 0 is followed by 0 1 or 1 0 you would you you can say that the direction of the incremental encoder can be found out ok. So, this is how one can go about hitting the direction ok by using these 90 degree phase shift head signals and seeing like know how they are coming with respect to each other. So, so you think about and then like know if you have doubts we can take up in the discussion class. Then you can have something called moir fringes you might have seen like know if you have this gate which will have so say basically some kind of a mattress for example or gate which is having these bars which are vertical and like know you run run like know the other two other kind of a set of bars which are slightly at an angle to it and you start kind of moving around you can see this kind of a fringes fringe pattern coming ok. So, you can simply like maybe we have a experiment maybe I can show some experiment or see the or maybe is it difficult or maybe some video maybe you can see ok. See it is basically like know you have this parallel kind of a lines drawn on the surface and then you are moving with an angle these lines with respect to each other and then there are some kind of a pattern that you see called moir fringes. Now this effect also is used in many different ways in the research these days ok. So, these diffraction effects will give these kind of a patterns ok. So, one can use these patterns to kind of do the sensing ok. For example, for a very small motion at a very small angle between them these patterns will start moving very fast actually ok. So, even for the small motion these patterns will move in the perpendicular direction fast and that can be a source of information or a sensing in the optical encoder based on this principle ok alright. Then this is another kind of like we can have more kind of a complicated examples where we have using some kind of a light path with additional light elements reflecting elements or you know the mirrors and things like that or beams fitters and things like that to put together like know very highly sophisticated sensors ok. So, that is the that is possible ok. So, this is the sensors we we use in long back like in my PhD days we have used such a kind of a sensor ok. Then in autofocus camera as you can have these different kinds of optical sensors. You can see these are this is a SLR camera in which you you have this array of you know CCD ok and the CCD will record like image and if the if the image if you are. So, this is an example of some pattern that is the image by the CCD ok. It may not be you may not have always this pattern you can have whatever image you are focusing on at that time, but I am just kind of like know this is both for such a kind of a pattern you you see the the left part is a blur part of the image and you do not have very sharp distinction between like you know see you see bottom side that is a that is an array of CCDs ok. So, these CCDs are actually acting giving this output which is blur in the sense like know you do not have a sharp kind of a distinction between the two adjacent CCDs ok. So, as you start moving the lens and the image starts becoming clearer you get this distinction very sharply. So, sharp boundary is found and that is why that is when like know the the spawnness can be built in the in the system to say that ok or now it is in the focus ok. So, the more is the contrast in the in the CCDs and like know sharp kind of a line that you find in the on the CCD then you can say that ok this is images in the focus ok. So, like that you can have this auto focus sense in possibility. Then there are these magnetic sensors which are based on the hall effect principle the principle is very simple you have the semiconductor which is like you know flowing current in one direction. So, this is the direction of current in the in the semiconductor and then you have a magnetic flux which is applied in a direction perpendicular to it and then you will find that like you know because of the flow of current and in this magnetic field which is perpendicular to it the electrons will get deflected in in in in the in in one direction ok and that deflection of electron will create a charge on that side ok. So, that charge is basically sense for the this is called a hall effect ok. So, this is a principle that is that the hall effect sensors are are based on ok. So, CD-ROM spindle motors these hall effect sensors are used in a lot of these BLDC motors to sense like know the magnetic field magnetic flux happening in the system to some kind of a feedback mechanism is is there based on that ok. So, we will see some some examples as we go along when we open the CD-ROM drive you know the spindle motor we will be able to see these hall effect sensors. Then these are which is another example for the car window opening operation ok. Then then we can have a piezoelectric sensors based on the piezoelectricity principle. So, this is like typically like the charge that is generated on the surface of a piezoelectric element when when some kind of a force is applied ok. So, you have two possibilities one is you can apply the force and generate a charge or you can have a give a charge and like non-generate the force ok both based this piezoelectric sensors work typically ok. So, there are these typical material city which are used as quartz or PZT or the PVDF these are different kind of a material that are used and nowadays people are using this piezoelectric as a energy harvesting elements. So, you can have a some kind of a small micro cantilever that is sitting in the on the sensor itself and it senses ambient it is not for sensing with the ambient vibration it is vibrating and because that vibrations the charge is generated ok. So, the energy of this vibration is converted into the electric energy in form of this charge and then like you know you can use this charge to store this electricity and use it for the for the powering of the sensors ok. So, one can have this wireless sensor nodes based on this kind of idea. So, idea is that the sensor is sitting in some industrial environment where some vibrations are based on those vibrations it is generating energy to sensor to power itself ok. So, the sensor does not need a extra battery or any you know solar or other kind of a power source to be there with the sensor ok. In the industrial environment whatever vibrations are it is generating a power for itself from those vibrations ok that is a kind of idea that is used in the piezoelectric energy harvesters ok and that is where like you know this is principles of piezoelectricity is going to be important and they are used of course, they are used as a sensors in the in the accelerometers for example and they are used as sensors in say pressure sensing kind of applications ok. So, I think this is one of the applications of piezoelectric kind of a pressure sensing ok. Then maybe we can now pause here for this lecture I think there is we have spent good amount of time over you know monitoring over these fundamental principles and then maybe we can like you know start in the next class with some other kind of a aspects or other elements of mechatronic system ok. So, and then MEM space sensor I will talk briefly about some in the next class and you know we can move on to the actuators from there ok. So, far here we will stop now.