 we can come or come to this now like you know classification of these different subsystems that are normally there. So, we have seen this gross positioning that is happening there ok. So, this is a gross tracking that happens, but that is not sufficient ok. So, if you see the positioning accuracy that we had talked about was about 100 nanometer and you can imagine like you know no stepper motor drive can produce that kind of a high precision positioning ok. Whatever like you know fine micro stepping that you will think it can have simply because of this mechanical ok maybe you think about what prevents like know the positioning to happen so, finally. See like know even if you have very fine positioning sensors available would you be able to position like know by using this kind of a stepper motor the system the head or the laser spot on the surface of CD within the accuracy of 100 nanometers ok that is certainly not possible ok. So, so what we do ok so, then there must be some kind of a fine tracking or some kind of a fine motion system that is that will be there inside the CD. And as we open up then we will get more on to that we will observe that ok. So, we should look for now what is the fine like know fine tracking or fine positioning system there. Then there needs to be something needs to be getting like the laser see the CD surface may have some undulations ok small undulations in the range of few microns or you may put like know some kind of a small little scratch on the surface of CD and still like it scratches almost in the microns kind of a dimension, but still that will be harmful for the data to be read on this from the surface. So, the surface may have some few microns undulations and then like know you need to kind of tightly focus the laser on to the surface from where the data is to be picked up. There must be some there must be some kind of a focusing servo or it is like auto focusing that is happening inside inside the CD. So, there must be some kind of a small fine motion that is given for focusing direction. Then we have seen this tray handling system, tray operation system and things like that ok. Now and of course, you need to have a CD rotation system here. So, this motor should kind of drive the CD to make sure that it rotates in some control kind of a fashion. So, let us explore little more now. Now here again under thought that ok so, this all these pauses are where like know you need to really pause the video and think about yourself do not rush through just this lecture. Otherwise like know you will miss out on things see this thought thinking and keep preparing your mind to get to understand is very very important part of the process which I am giving you the cues, but I mean if you follow them only then things will happen ok. So, please do that. So, now let us think of these two subsystems only, this cross positioning system that you already seen some part of it and then fine positioning system ok fine tracking ok. How can with this be possible? So, suppose we know now we need that fine kind of a tracking there. How do you do? Suppose you are supposed to do that, how would you think based on your knowledge so far what kind of a sensors actuators that you will use in this system ok. So, do some thinking you may say ok oh look this is very fine positioning which is required I may use piezo actuators which will give you fine positioning ok fine like know that can be one possibility. How would you kind of set up such kind of a drive? Piezos typically need very high voltage to operate ok. We have such a high voltage operation possibility ok. Any other kind of a actuator comes to your mind which can do this job ok like that you need to kind of think about ok what kind of a way we sense that ok what it is really on the track or not on the track. So, you say for example, if you put a sensor on the system side here is that sufficient to kind of get us know that ok it is on the track or not on the track those kind of things. Or how do you kind of get a feedback from the from the CD surface which can tell us that. So, so many many different kind of questions can come here ok. So, we should kind of you know based on whatever our knowledge think about these different kind of questions and lease them down or like think about ok what comes to your mind as a thought to give a solution to these different questions that are coming up ok. So, let us move on I mean after your thinking is done you can move on ok to see further now what what we open up and actually check out ok. So, so these questions you know we can have say sensors for the sensors some encoder possibility or you know we can use make use of the track on the surface of CD itself to see if we can use that as a sensor. So, actuation what kind of actuation you can have ok. So, you can have some CD round drives have actually this servo motor possibility servo motors they use as actuators. Some some others they use stepper motor as actuators the things like that can be possible for gross positioning. So, again I am flashing the same picture here. So, you can see this is from some some view from some other CD round drive and this is from like know the the currently open CD round drive that we are talking about. So, this is the same picture as we saw. So, do you see any sensors up here or positioning sensing ok. So, you see that the stepper motor typically does not have any sensors that are that are needed for sensing the position ok. So, you observe that there is only like only the shaft coming out of the sensor and there is no sensor up here. See typically the motor shaft we should need to have a sensor if at all the sensor is there in the system ok. So, either motor shaft or like know the slider moving like know back and forth needs to have some some slight position sensing possibility, but if you observe carefully that is that is not there in the system. So, there is no sensor in this in this gross positioning kind of a system only stepper motor and we rely on the steps of the stepper motor to make sure the position desired position is achieved and then of course actuator is this stepper motor along with this leads screw and this mechanism this is a way is is useful for preventing the backlash you heard this term backlash ok. See if I have only like know say for example, only one tip going into the into these slots here then depending upon like know this is some kind of a manufacturing tolerances and errors the slot size and my teeth size may not match perfectly ok. There will be some kind of a within some kind of tolerance accuracy few microns kind of accuracy it will be not matching that means like know that much kind of a back and forth motion I will have around in the slot ok. So, imagine like know you have a hole in which there is a rod which is smaller in size is pushed and you know you can move the rod in the linear direction any radial linear direction back and forth in the in the hole that much kind of a you know you will see that kind of a thing is is a backlash in the system ok. So, these clearances that are given typically for like know smooth motion happening like we will we will result into some kind of a backlash in the system ok. So, this backlash is some problem which in mechanical systems you know you think of gears will have a backlash you think of any kind of like know this motion transmission systems they will have some backlash that is going to be there. And now what what we do here is like know we are using these two stubs here ok which are which are like know pushed against two opposite surfaces of the screw ok that way and then the spring is making sure that ok that is pushed and that force is maintained and that takes care of this backlash that will not allow the backlash to come into into play ok. So, that is how like know this backlash is handled by using this kind of a spring loaded stuff here ok. We will see in other cases like know when you have a gear transmission system instead of screw transmission how do how you take care of like backlash ok. So, this is just to focus on the on this you know you have this little kind of a you know shock absorbing element here ok. This is another system like this has this if you see this has only two wires coming out of this motor and then there is a gear system this is like a bevel gear system and then there is this gear think much what is missing in this location A ok. So, think about that ok what is missing I have removed something from there what it should have been to kind of like know can think about so that the motion from the bevel gear is transmitted to this rack here and if you observe carefully this area you see the small little spring sitting there and this rack is having like know two parts the top rack and the bottom rack ok. They are exactly same pitch and everything, but they are in the two parts like know the top part and the bottom parts are separate there ok. So, what are they for? So, think about that this spring there and then top part and bottom part and what is there in this location A ok. Think about these issues this is another kind of a mechanism in which like know you see the same the spring loaded stuff for the cross positioning system. Now on the guide rails ok so you see this guiding system. So, this is like know I would say one view and this is opposite side view ok. So, this is a view from the opposite side than this side ok. So, say this is like you know head which is seeing the surface this is a lens which is seeing the surface of a CD and let me get a pointer properly ok. So, this is a lens of a CD and you observe like know these locations ok. These are the locations where the head is touching these rods ok. This is one on this side only one kind of a touch is there on this side there are two touches are there ok two places it is touching. Now carefully if you observe ok then you will find that this touch is happening here on all the sides as you see here. The rod is covered on all the sides by this hole ok and this hole also ok. But on this side there is only one support here and then that too the rod is not completely covering ok. The rod the support is only like you know covering half side ok that you cannot see here actually the cross section view actually will show you that ok maybe where somewhere we have this separately open mechanism though this removed from the rod kind of a thing we will be able to see that ok. But here you you presume that know you take my words that this is only like know up to half of a distance. Now you think why this is done in this way ok. Again I am not going to kind of give you answer right now I want you to kind of like know think about this question why this construction is happening this way only ok. Like that you need to have now you can see from the pictures only but if you have actual drive you can kind of like know see this many of such kind of observations and then like you know think about and come up with ok why that people have done it in that way ok. So, that is where like know the our like nice concepts are going to get developed ok. So, think about this issue and you see here in detail how this again the same thing I am saying like know this is a spring here and then there is two racks are like one on the top of each other ok. So, lot of things to think about. So, how do you kind of like know get this spring loaded rack done what is the purpose here that you think about. So, this is like know little larger view of the same thing you can see the spring very clearly here and then there is a top and bottom rack and then this top rack is like you know moving with respect to bottom rack in the direction of like this laser pointer in this you know direction of motion of the rack it is they can relatively move with respect to each other. Now let us think about the fine tracking ok what are the requirements what are the sensors what are the actuators and we can like know open up and talk about that ok. So, so sensor that is used here is some kind of a photo detector here and actuator is some small kind of a little voice coil actuator that is sitting up here. So, so we will see this you know as you open up this will be evident, but now we need to think about how we can create this kind of a closed loop system here so that you can have a fine tracking and fine focusing that is happening here. What kind of a sensors should be needed what kind of actuators will be needed for that thing to happen ok. So, maybe I will leave you with with a small kind of you know actual observations and then so we can remove the head from the guide rod and like know start observing what is there inside the head means like know the optical pickup head we will start looking at. So, if you see the head you will find is there is this block which is held in the held by these wires and you see these are the wires up here ok this is a block ok the block has the lens inside it. So, the lens moves up and down direction means this is a focusing kind of a direction ok this block can move up and down can this block get stretched front and back in this direction think about that can this block move sideways in this side. So, there are three directions it can move or it can tilt ok can this block tilt also think about that. So, tilting is possible or not or like know tilting of this block is possible in any direction twisting is possible. So, so you think about the six degrees of freedom any rigid body has and what are the degrees of freedom this block will be able to move into ok given the constraint that you know you have these wires which are holding this block here ok these wires are simply like know some kind of a you know small compliant kind of a wires as you operate them you see like know how so, they may not be stretchable, but they certainly are bendable ok. So, you can see these wires operating actually ok. So, here you can you see that this is a block up here this is a lens here and then this is wires are connected to this block from inside and coming out here and getting into this small little black portion here. What is there in this black portion we will see we will we will think about ok. It is very interesting construction done for some purpose ok what is there inside the why this. So, it is the wire is not fixed here there is some small space and then wire is fixed inside this black part somewhere towards the end. So, we will see why why that is done in that way and if you push the lens down you can see that ok this wire is you can see this wire like know it goes like from here it goes and like know comes here. So, there are these three wires that are used in in parallel and they are doing they are getting bend when this block is pushed down and up ok. Now this pushing down and up action will happen not by by somebody moving it up and down, but there there are some kind of a coils you can see here and then there is some magnet that is there ok. So, the coils are suspended in the magnetic field and when the current is passed they will kind of start doing this kind of a small up and down or some kind of a sidewise motions ok and that is what is kind of important the fine positioning. Now think why they have like know put such a kind of a mechanism there what is this mechanism how this mechanism is helping us to to do the positioning with the with a such a you know nanometric kind of a accuracy. So, I would leave you right now at this point and then like really discuss things more. So, so you can see this these are coils up here and then there is there are these two magnets ok. So, so this these two little dots that you see are actually a glue to kind of secure that magnet in position or thing like that ok. So, this is the do not worry about this these two black spots are actually glue and then like know you have these coils the magnets are fixed and the coil and this this lens assembly is actually moving ok. So, I will leave you with this right now and think about now how do you kind of like know make sense of all these components together to really think about ok system which will have this kind of a nano positioning possible. So, we will stop here for for this lecture.