Introduction to computer control role of computers in automation

Introduction to computer control role of computers in automation


welcome viewers today we are going to start the first lecture of the just moment computer numerical control in of machine tools and processes my name is Eric Audrey and I am a professor in the mechanical engineering department of IIT Kharagpur so first of all computer numerical control what does it mean it means that control of a particular machine tool or a process is achieved by insertion of our use of numbers symbols signals letters codes words instructions in other words in short the communication with the machine in the form of a language previously machine and human interaction used to take place through physical devices like cams tracers templates so it was not very sophisticated but now with the advent of numerical control and computer numerical control a sort of language communication was started with machine tools and processes so in what we can this happen it can happen in the form of input to the machine daytime put through numbers letters codes this data can be processed inside the machine through numerical calculations logic operations and execution of operations can be carried out through generated signals from the computer or from some control circuit in the form of voltage signals etcetera etcetera so what’s the connection between numerical control computer numerical control etc so in the beginning what happened was there was no computer to start with and we had numerical control machines in which numbers and alphabets were used in the form of codes to feed information into a machine and for that at first these machines were called or they were referred to as numerical control machines inside they were primarily consisting of hardwired logic circuitry and after the advent of computers data entry became easier through the computer hardware’s wired circuitry logic circuits etc sometimes were replaced by software and control inside the control loops the computer also used to participate in order to improve the control characteristics so it became known as computer numerical control or simply computer control what is the area of application of computer control do we understand that say in mass production that is high volume production lots of pieces made in a particular span of time say 1 lakh pieces in one year so in mass production wood CNC or computer numerical control be suitable here surprisingly it is not so right because mass production finds the application of fixed automation that means fixed means machines which are you know they can only carry out certain operations and nothing else special purpose machines special purpose machines find wide application in mass production together with automated material handling devices why so this is because in case of mass production there is not much change in the part design over extended periods of time there what we find is a huge number of pieces have to be made again and again repeatedly over long lengths or long periods of time so a computer which is primarily meant for you know flexible communication or changeable communication with machines it does not find wide application in this particular area not much change is there with time so we generally go for systems which are exactly dedicated for that particular purpose special purpose machines for example camshaft grinding machine it can only grind camshafts and nothing else so that’s why in mass production CNC is not very suitable CNC strong point is it can be changed very easily why because it’s consisting of language instructions it’s consisting of numerical calculations and you know logic operations inside the machine these things are not very difficult to change if the requirements change and why not fixed automation for you know low volume production by low volume production I mean small lot and batch production is even piece production this is because fixed automation has dedicated machines so if you dedicate some machines build them for you know a very small number of pieces after that part design becomes gets changed to a different part you have no use for that particular fixed automation it becomes obsolete that is why if you have low volume production you cannot implement fixed automation or dedicated material handling system in that case you cannot automate it with fixed automation in that case CNC becomes highly applicable because if you are wanting automation make programs on the CNC machine which and you know which can produce those parts and then the part changes part design changes because it’s low volume small amount small numbers of different jobs one after the other so when the part design changes make a separate program for the next part and start running the machine very you know the costs will be cause for changeover will be much much less in this case so we understand that for low volume production small lot and batch production etc CNC would prove to be highly appropriate also we should not I think I should mention here at this moment that CNC also has the ability to manufacture complex shapes okay without parts specific tooling part specific tooling means they form tools if you want to make a very difficult profile make a form tool the conjugate profile and machine that particular part so you incur a lot of costs for the tooling in CNC it’s not the tool shape that produces the conjugate shape on the part but the two movements properly controlled by the computer can produce very complex shapes this is an edit ability of the CNC machine let’s see how the control is achieved in case of CNC there you can find I have drawn a monitor to represent this here the computer and from the computer there is some signal which can directly control a motor but is this a CNC machine it is not I have intentionally made drawn a conventionally controlled lead okay operator run leave on which you will see some equipment like speed gearbox feed gearbox carriage not etc it’s not a CNC machine so in in order to achieve control over a machine first of all a computer has to be there it has to be interfaced with the machine and the conventional machine tool architecture has to be changed so we will find some changes taking in this particular architecture and apart from that there should be the addition or incorporation of some devices which can control the motion and the extent of motion I mean rate of motion and the extent of motion and also we have to ultimately write a program on the computer and execute it so that it develops proper signals to the motor and other devices of the machine in order to control it this is how computer control is achieved so comes the question actually what is a CNC program a CNC program is a scene a sequence of commands just like you know we give a sequence of commands to someone to carry out some operation it is just a sequence of commands it’s written in a suitable language and it is meant for controlling the operations of a machine when executed it makes a machine tool carry out some motions and auxiliary operations generally by motions I have meant here say fast motion to approach a particular point and then say starting a linear cut at a controlled rate then taking circular cuts either clockwise or counterclockwise at a definite feed rate and also there can be optionally operations for tool changing for you know for for changing a job and bringing in another pallet containing another blank like that and this way when a computer program is executed it will ultimately lead to a part being successfully produced from a blank and CNC program ultimately does not necessarily mean that it is only for controlling the operations of a machine tool but it can be used employed for controlling other general purpose machines other general machines as well this is an example of a typical you know some command blocks taken from a CNC machine what does it show the first column shows the line number in zero zero six and zero zero seven etc after that we have a number of codes placed one after the other what are these codes for example g9 t g9 t means that whenever there is a coordinate system existing on that coordinate system we are supposed to refer to the absolute X&Y coordinate values of a particular point so we have a coordinate system in front of us and we can see that x and y is written there and in that XY coordinate space G zero zero says G zero zero means that we are supposed to reach a target coordinate position given by X 20 y 30 so it basically means move fast G 0 0 means rapid Traverse to the target point no velocity is mentioned because generally the Machine moves at the highest possible velocity so move fast to the point X 20 y 30 after that we are supposed to move then in the next command we are supposed to move to the point y 50 y is X not mention because it’s not changed so X remains the same and we change the y coordinate to 50 from 30 which means we are now at the point 20 comma 50 and we have executed a vertical motion after this vertical motion at 20 50 after that it’s written G 0 1 X hundred 500 F 200 what does this mean G 0 1 is a code for linear motion that means straight-line motion at a particular feed rate that means at a particular rate of motion the rate of motion is mentioned as F feed it’s called the feed world 200 means if not otherwise defined 200 millimetres per minute so the target point is mentioned as X hundred 500 so we reach the target point hundred comma hundred from the point 20 come 50 at the rate of 200 millimeters per minute next line is g0 to g0 to means circular interpolation clockwise so we move from the present position of 100 comma hundred to the point 1 140 x 140 okay and as Y is not mentioned by remains the same and the radius with which we move is 30 millimeters okay so we reach this particular point at the same feed define in the previous line to the point 140 comma 100 from the point hundred comma hundred at radius of with a circular radius of 30 millimeters in the clockwise direction is there any other possibility of reaching this particular point there is but unless otherwise mentioned it will take this particular what we call it minor arc there’s a major are possible also with 30 millimeters clockwise that is not taken M 30 means end of program so this is a typical example of a few command blocks on the scenes machine now now that we have started discussing on CNC machines computers were not there you know throughout the ages and we did not have NC machines also you know beyond in in the past say 70 years back or even more than that so at that time however automatic operations executed on machine tools this is one example of mechanical automation it means some sort of clockwork mechanized motions were carried out without the help of computers without the help of you know numerical instructions codes language etc simply physical devices were used it means something like if you want to command someone you can tell him or her or you can push him or push her to do that particular task physical devices so here we see a cam cam is a device in this case we have shown a disc like cam which has a profile on its periphery which deviates from a circle so that if it is rotating it will move anything in connection with it radially outwards so here we have something called the follower which is pressed against the cam with the help of a compression spring and when the cam moves in a uniform uniform circular motion in a counterclockwise direction it will tend to push the follower and then suddenly the follower will fall back when the fall of the cam or drop of the cam along the radius takes place so it will push the cutting tool also towards the left side continuously and then suddenly it will retract so this is the scheme of motion of the cutting tool which has been planned with the help of this particular cam is is there a computer controlling it no is there automatic motion getting repeated again and again and again yes what is the purpose of this motion maybe it’s moving forward parting some parting some job and then again retracting very fast the job is getting fed again so this continues unless something wears out either the tool wears out or power goes off or the cam undergoes wear and tear so this is a perfect example of an automatic machine by mechanical controls okay so this is also existing you know without computers automation and there’s a question given that is what should be the profile of the cam in order that we will have a constant forward feed of the cutting tool what should be the profile of the cam the answer is Archimedean spiral you can try this out yourselves so coming back to computer control machines now we have seen the example of mechanical controls being achieved in case of automation so how do computer control machines carry out this particular control in computer control machines instead of devices like cans etc there are programmed instructions which go inside the Machine and develop I mean when when they are executed these programmed instructions they develop digital signals which might be ultimately rotating motors at a definite rate and this leads to achieving a definite extent of motion and achieving a particular ratio of the access velocities okay and also it results in the attainment of a programmed feed velocity along the cutter path why we so much you know concerned about the program ratio of axis velocities because that is ultimately the particular parameter which which determines the profile taken I sorry determines the path taken by the tool which ultimately defines the profile of the part being machined here there are no physical devices but the path of the tool defines the profile of the job so we have been talking about digital signals binary logic logic gates digital circuitry quite a lot what do we exactly mean many of you might be conversant with digital circuitry digital logic in case you don’t have an introduction formal introduction to this particular subject we will be taking up in the next lecture some aspects of digital logic and hopefully you’ll be able to follow the later parts of the other lectures which involves some discussions on logic circuitry what we have shown here is this that digital circuitry will be employed in many aspects of CNC control like data input data storage processing interpol motion execution etc in on the in the figure we have shown two typical logic gates and how do we read them like a and B if there are two inputs which can only take up two values like say high value and low value and gate is a gate which controls the you know in relation of a and B in such a way that C is going to get a high value only if both a and B are simultaneously high this is called end gate that means you and say x and y if both are high then only Z is high that is the output in the same way or gate is a little more lenient it says that if is high and B is high or any of any one of them is high then the output is high in three cases a and B both are high is high knees high in these three cases the output will be high when both are low then the output will be low so we will have some more discussions on logic gates yeah not exactly mathematical operations but logic operations now that we are a little we have gone into the you know discussions of CNC machines let’s see what’s the architecture the control unit that is what is there inside which does all the control the control unit is frequently referred to as the machine control unit which can take contains two different you know modules one is the data processing unit and another is the control loops unit what is the data processing unit do it it is concerned with data entry data processing that means all sorts of calculations involving the data and also interpolation interpolation means that your you are instructing the machine to move and Honus a circular path or a linear path from one point to another but you are not instructing it about what sorts of velocities and positions it’s supposed to attain in between so somebody has to do the calculation and there’s a device called interpolator inside the data processing unit which does all these calculations and finds out the intermediate positions and velocities of the tool okay in order to cover the intermediate path between two programmed points the control loops unit it contains all the devices achieve for achieving you know required motion along an axis for example it might involve the actual motor or the prime mover the lead screw not the gearbox all these things are comprise are together referred to as the control loops unit and now that we are talking of we had referred to the modifications of the conventional machine tool in order to you know achieve CNC machining so let’s have a quick look at that what are these for example the machine has to be more robust and rigid for the same power level why because deflections excetera should be less so that errors are less our deflections there in other machines there is but there are physical devices to ensure that the the movements etcetera are exactly carried out here we are depending upon in CNC we are depending upon the accuracy of the movements etc in order to achieve a particular accuracy on the job backlash elimination has to be rare motions here should have to be completely free of backlash and for that a number of devices need to be incorporated for example the recirculating ball screw not mechanism gearboxes which establish definite ratios of motion between axis of motion in our particular machine these gearboxes limit the ratios of between access speeds and therefore they are generally eliminated but gearbox is for attainment of a particular range of speeds that has to be there feedback is sometimes necessary where the prime mover is not capable of carrying out exact motions as required by the program and generally from complex or compound chains or structures we generally go for simple kinematic chains which means we will be discussing these things in more detail and motors with lower time constants are preferred which have faster response interpolators are necessary in case of continuous control and we need to have you know machines which have a more you know where overshoots and sluggish response can be avoided that we that means the dynamic constants of the machine have to be so designed so that over overshoots are less and sluggish response is avoided etcetera so in this machine if we want to implement computer control we can see that there’s a chain moving from the top up to the spindle and there’s also power flowing this way so that the same motor through gearboxes is controlling two motions this has to be removed and a separate motor has to be put for the feed motion so one motor for the feed motion one motor for the speed motion and yet and the motor would be required for the cross feed one for cross feed one for longitudinal feed okay for example here we see a milling machine in which we have three motions XY and Z and there’s a motion there’s a motor which is also providing the cutting speed instead of this on the CNC machine we will find one motor for X 1 motor for Y 1 motor for Z and also a separate motor for the rotation of the cutter so finally let’s look at the advantages of CNC machines it is more flexible it is possible to incorporate automation in low-level production we have the ability to cut complex profiles and in many applications we can attain higher productivity and accuracy the disadvantage is that the initial investment is very high and required skill level of machinist operator etc is high there’s one practice question main advantage of CNC machining over fixed automation is flexibility accuracy speed none of the others the answer is flexibility very much

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