What is a PLC? PLC Basics Pt1

welcome to this presentation an introduction to programmable logic controllers this lecture will start at a very basic level it does not assume that you have any prior knowledge of programmable logic controllers however there are some sections of this presentation that do require a fundamental understanding of basic electricity and magnetism if your background is in computer science this presentation will quickly enter into territory that you are already familiar with after all PLC's our computers and logic is well logic I have attempted to make this presentation as generic as possible however it is the examples are based on a specific family of processors programmable logic controllers are the major component in industrial control systems they replaced something called electromagnetic relays the language used to program programmable controllers still uses symbology from the evolution from these relays to using computers to control manufacturing systems so this section we are titling relays two bits basically making the transition from using relays to using bits in memory a brief bit of history is needed to build a background for some of the terminology and symbols used in programming programmable logic controllers what you see here is a photo of one of the original relays invented by Edward Davi in the 1800s for the telegraph system without going into a lot of detailed explanations regarding the line losses of signal over long runs of telegraphy telegraph wire it is sufficient to say that to prevent the loss for the telegraphed signal over these great distances they installed relay stations to restore the signal level so the original relay and that's why it's called a relay was a device for relaying the contact closure of a telegraph key at intervals along the telegraph wire as it traveled across the country from one end of the other once the relay became known as a device it was not long before engineers realized that there were other applications for contact relays if you could use a relay to relay a contact closure into an isolated circuit to restore the voltage levels then you could use a relay to relay a contact closure into a circuit with an entirely different voltage type and level in other words using a switch that supplies 24 volts DC to the coil but have the contacts that are switched by the relay actually switching a higher voltage such as 480 volts AC and switching heavy contacts to handle much higher currents enter the modern relay which you see in the photo before you the most dominant feature of a contact relay is the coil the coil is also the load in the switching circuit so the contact closure being relayed is controlling the coil which controls the contact of the relay the coil is wound around a pole piece a rod of ferromagnetic material and you know from your studies and basic electricity magnetism that as current flows through the conductors of that coil the electrons which each are are each a little magnet in principle each electron has a north and a South Pole when they're all flown in the same direction all the free electrons and the copper atoms all of their magnetic fields join together and inside of the coil you have a very intense magnetic field magnetic fields don't pass through air very easily air has a high resistance to a magnetic field so if you place a ferromagnetic material inside of that coil that coil could also be called a solenoid if you place a pole a rod inside of that coil of ferromagnetic material that greatly intensifies the magnetic field the object of the electromagnets affection is the armature the armature is another piece of ferromagnetic material that is suspended near one into the coil by a spring that would oppose the magnetic field now keep in mind remember we said that air is not a good path for magnetic field ferromagnetic material is another thing to remember is that the magnetic field is always trying to shorten itself so when it sees that armature that piece of ferromagnetic material it is in the magnetic field and since the field is trying to reduce its size it pulls that ferromagnetic material hard against the end of that pole peas on that coil attached to the armature as you see the armature is suspended on a spring and on the opposite side of the armature from the spring you have a moving contact the moving contact travels are restricted in each direction by stationary contacts when current flows through the coil the armature is drawn to the pole piece of the coil against the tension of the spring as the armature travels towards the coil the moving contact travels as well when the circuit containing the coil is opened that is turned off the magnetic field collapses and the spring tension is now the dominant force in the armature and the moving contact moves away from the pole piece the movement away from the coil is limited by a stationary contact and the movement towards the coil is limited by stationary contact so as power is alternately applied to the coil the moving contact makes contact with the two stationary contacts one contact when the relay is de-energized and the other when the coil is energized let's look at this contact relay in a slightly different graphical manner we have a diagram in front of you and in the diagram you see a coil of conductor in two connectors to apply it in a circuit you see it's wrapped around a pole piece at the end of the pole piece you see an armature another piece of ferromagnetic material and it is attached on a lever that is spring-loaded to pull it in the upper direction the lever with the moving contacts that that terminal would be called the common because it's common to both normally open and normally closed the stationary contacts are called normally closed contacts normally open contacts the normal state for the relay is de-energized when the coil is de-energized the moving contacts are under the influence of the spring and held tightly against the normally closed contacts so you have a complete circuit from the common to the normally closed contact when the coil is de-energized when you energize the relay in other words you're on electrical electrical current through the coil it produces a magnetic field inside the coil intensified through the poll peace passes through the armature pulls the armature down against the spring completing the circuit from the common to the normally open contacts as shown here in this animation I also added a little flex in the contact to show that the contacts are designed to have a slight wiping action when they make an break contact that's to clean off any carbon caused by you know an arc and leaves a little carbon trace so these contact kind of are self-cleaning as this presentation continues it will become apparent why a short discussion on relays will bring a more clear understanding of programmable logic controllers the programming language used to create programs for programmable logic controllers is based on the relay circuit symbols the original control panels use relays for the logic and the most common programming language today is relay ladder logic and it evolved from the electrical prints and uses of the same symbols as a matter of fact if you look at an old electrical diagram for a relay control circuit if you're a PLC programmer you might at first glance think that it's an actual relay ladder logic program let's add our little relay diagram into an actual application an actual circuit we normally refer to that portion of the circuit which interacts with the operator as the public interface you could say that there are three public interfaces to this circuit the single pole single throw switch which the operator controls and when he closes the switch that energizes the relay when he opens it the relieved energizes then you have a red indicator and as you can see in the circuit that the battery powers the red LED when the relay is de-energized and then of course you have a green indicator which will illuminate when the relay is energized this is the operator interface so the operator has a switch to control the circuit and then he has two indicators to indicate the state of the circuit for power we have an alternating current supply for the relay coil but the relay contacts are actually controlling current from a DC source now I show a battery made up of four cells however that DC source could be a DC power supply that takes the 115 volts ac or any current rectifies it into DC filters it and then regulates it at some specific DC voltage like 24 volts DC 15 volts DC 12 volts DC 5 volts DC it regulates it at some specific direct current level to control these other devices and then of course the control circuit itself is a single single-pole double-throw relay okay looking at our circuit when the switch is closed coil the current flows through the coil magnetic field pulls the armature down opening the normally closed contact and closing the normally open so you see now with a switch closed the green light is on but the switch did not turn on the green light the switch that the operator cannot by hand turns the relay coil on and off the contacts controlled by the relay coil then switch between the red indicator and the green indicator supplying power from the DC source so you could say that the contacts the normally open normally closed contacts define the state of the relay the normally closed contacts are true or they conduct electricity when the relay is de-energized so the normally closed contacts are true if the coil is off the normally open contacts are true and conduct if the coil is energized so normally closed are true if the relay is off the normally open are true if the relay is on this is this is an illustration of an electrical circuit not an actual conventional circuit diagram you would not see documentation on a control circuit that was drawn in this fashion we've laid this out so it's easy to see the relationship of the power sources the switches the coil the contacts and the loads in this case the loads are the two indicators the red and green indicator before we change the look of the circuit into something more conventional from this kind of an illustration of a circuit let's break the circuit up into a couple of pieces first you have the hundred and fifteen volt AC section which has three components an alternating power supply alternating source a single pole single throw switch and a relay coil now I really realize that the relay physically is the coil and the contacts but the coil of the relay is the only part of this 115 volt AC circuit the other section is the 12 volt DC section which includes the contacts from the relay so the troubled DC or just the DC section because it could be 24 volts DC has 2 LEDs relay contacts and the DC power supply what I want you to remember is that there is a air space an air space between these two circuits so if you follow the dashed red line you will see that there is an air space between anything electrical in the bottom half and the upper half now let's move on do you recognize this circuit you should you were just looking at it what components do we have we have a single pole single throw switch and alternating current supply and a contact relay coil just the coil what will happen when the switch is closed when the switch is closed the alternating current voltage source will alternate in one polarity than the other sixty times a second whenever the switch is closed it will power the relay now I show the relay going off is the current reverses but remember that this is 60 cycles per second which means that 120 times a second the voltage goes to zero and then keeps on going right over to the opposite polarity so the relay doesn't have time to de-energize even though I show it de-energizing in the diagram okay do you recognize this circuit this is the other part of the circuit that has a DC power supply has to relay contacts with a common one normally open one normally closed and we've gone so far as to identify the contact relay as contact relay 1 and then we identify the contacts as 1 CR - 1 + 1 CR - 2 now remember that airspace in the previous diagram between the AC circuit and the DC circuit you can really see the airspace here but you can no longer see that these two relay contacts belong to that relay coil up there therefore we have to put nomenclature on them we have to specify so we wouldn't normally say contact relay 1 we would have to say 1 CR and then down below you have 1 CR - 1 1 CR - 2 there's your red and green LED indicators so when we close the switch we get alternating current and notice that the when is de-energized up above the DC power supply has a complete path for current flow through the red LED when we've dropped the switch up above and energized a relay contact we've closed the normally open contact and we now have a complete path through 1 CR - 2 - the green LED so you have seen now the a simple illustrated type diagram for electrical circuit and this is a more conventional style typically you divide up the AC from the DC so all the AC is located on one sheet of the drawing set and the DC is located on other sheets because the relay contacts very well will end up on a completely different sheet of the drawing set then the coil that controls them you have to have very good nomenclature to identify what coil these contacts belong to looking once again at our Illustrated diagram here you see those two hands the circuit the upper half is the AC part lower half is the DC part let's watch this circuit action one more time and we watched this diagram the animation of it one more time in the upper circuit the AC circuit you have a switch that energizes a contact relay the contact relay when an energizer is going to open the normally closed contact that's represented by the parallel lines with a slash through it that means normally closed and you can see right now because that contact is normally closed the relay is in its normal state D energized so the normally closed contact is completing the circuit for current to flow from the DC source through the red LED back to the DC source however when we power the contact relay in other words when we close the single pole single throw switch up there and energize the contact relay that is going to pull the armature down is going to open one CR - one and close once they are - - and then if we open the single pole single throw switch the spring is going to pull the moving contacts back up to the normally closed once they are - one red LED goes back on it opened once CR - - and the green LED is no longer illuminated returning to our illustration of a relay doing a little review relay coil pull piece electromagnet pulls the armature down against a spring opening the circuit between the common and the normally closed contact enclosing the circuit between the common and the normally open contact if we take our illustration and convert it to symbols the circle on the right is the symbol for a contact relay coil at the contacts just the coil and up above we have a normally closed and normally open contact with a common so and we call these forms contacts so I'm going to flip this around so you can see it basically forms a C so when you hear form C contact contacts that means that you have a normally open normally closed and a common you don't have to completely separate switches because both of them are attached to the common returning to our photo of a common relay and by the way relays come in much smaller formats than this and much larger this is a circuit board mount relay actually a fairly small one coil symbol for the coil normally open contact normally closed contact now at this point because we are transitioning slowly into using the bits in a computer memory instead of the coils and contacts I want to point out that these two contacts in a logical circuit are used to query the status of the coil in other words you have a circuit that when the circuit completes it energizes this coil that circuit could be made up of many individual devices wired in series meaning it could be three switches from three completely different locations in the process they're wired in series so all three of them have to be closed before the circuit is complete and energizes the coil so the coil then represents the state collectively of those three switches so if in some place else in the relay logic you want to query that coil to see if those three switches are all closed then you would use the normally closed contact to ask the question is the coil de-energized if the contact if the normally closed contacts are closed then yes the coil is de-energized and is true consequently on the other side the normally open contacts if you want to ask is the coil energized meaning that all three switches are closed you use the normally open contact to query the coil and if the contact is closed and you have continuity then the coil is energized and that represents collectively those three switches so the normally closed contact basically is true if off true if the relay is off true if the relay is de-energized normally open contacts are true if the coil is energized true if the coil is on or energized okay this is a good point to take a break so take a deep breath hold it in let it out there is more if you need to go get a cup of coffee go use the facilities get a soda pop a beverage sale to someone and make a call text somebody check your email now's the time another look at our illustration of a contact relay we have one form C set of contacts we have a common normally open normally closed and remember that these contacts are used to examine the state of the coil to see if what is controlling the coil is on or off and that could be one device or many devices in a separate circuit and remember the relay coil represents collectively the combined state of those devices so you can use the normally closed contact to query the coil to see if it's D energized or you can use the normally open to see if the coil is energized well let's take the situation where somewhere else in our relay control panel we need to query that coil so we use a set of the contacts and then a week later we need to query that same coil from a completely different piece of relay logic somewhere else on the panel well we've already used up one of our contacts and keep in mind that because they have a common that electrically you can't use that common in two different places in your circuit there's only one common terminal and then there's a normally open norm to close terminal so we need more contacts so we just drop in more contacts they're all operated mechanically in in cotton synchronous and we have a heavier switch and we're going to need actually a heavier relay coil with more windings and more current because remember that we have a heavier spring to pull back for form see sets of contacts instead of one and we have to lift against gravity the four armatures actually there's probably going to be one armature but four sets of contacts so now we have more contacts on the same relay coil so we have four normally opens and four normally closes closed but remember we only have four Commons so depending on how you're wearing up your circuit you cannot use any more than probably four of these sets of contacts you'll probably use either a normally open or normally closed from each set very pretty now this is a four pole double throw contact really I think you can see the coil there we've drawn a little phantom and we point to the two screw terminals that you would hook into the coil circuit and then we've got a moving contactor common and if you look closely you can see that from the screw terminal you go a little to the left down and then back to the right to a black conductor that goes all the way over the other side and then it rotates back and then into that copper blade you see there and you can see the upper lower stationary contacts and you can see the screw terminals that they connect to the normally closed and the normally open so if you just take a second and look at this you can see the similarity between this and everything we've shown you so far in our Illustrated relay the moving contact our common was on the other side in this case they brought it back over to the same side for convenience of connecting it up now I said this was a four pole double throw so you can see there are four sets of form C contacts on here so this would be the actual physical device that we were representing in the last slide that showed four sets of contacts it's time to make a transition in our actual physical device this is the original contact really design however most popular industrial relays have a slightly different design the difference between what you see on the left and the right you see that there is no common contact on the right instead you have a shorting bar that when you energize it pull down and when it's energized it travels up and the stationary contacts there's two sets what this gives you is two completely individual contacts for each position of the relay armature that means that if there's an arc the arc is divided between the two contacts so that's an advantage it reduces the possibility of the contacts welding together because of excessive current so here's our new relay our new contact relay with no longer with a common which means that the normally open which you see open right now and the normally closed which you see closed right now could be used in two completely separate circuits because there's not just one common contact for both so right now the normally closed is closed so if you query this relay coil to see if it's energized the normally closed contacts are closed and they would have continuity indicating that the relay is de-energized if you read energize the relay that pulls the armature down opens the normally closed and closes the normally open and now you have a set of contacts that have continuity when the coil is energized remember the only real big difference here is that we eliminated the common and now we have separate connections for both the normally open and normally closed contacts this is the symbol for the coil it's still a circle that says 1 CR or has nomenclature in it however notice the difference in the contacts member before we had formed C contacts so the contact symbols really needed to include that common connector because they could not have been put in two completely separate circuits the common has to be involved in both the normally open and normally closed with this newer design the shorting bar design the normally closed and normally open contacts are completely isolated electrically from each other once again our relay coil our symbol if we need more than one set of contacts one normally open normally when normally closed we simply add them in the stack of course it's going to take a stronger spring and a more powerful electromagnet to move the additional mass and move the heavier spring but notice now the symbols for our contacts we have eight separate symbols not four sets of two symbols but eight separate okay take a deep breath hold it let it out there's more so here's another opportunity to take a little break this is a long lecture and this is a lot of material to absorb in one sitting here we have just a tiny little piece of a much larger circuit and in this two rungs of relay logic and when I say relay logic I mean rule you know magnetic relays and you can see they call them rungs because it looks like a ladder with two steps on it we have two relays so the loads are these two rungs are two relays 2 CR and 1 CR 2 CR its function is when it's energized it's okay to start the machine once they are when it's energized it's an indication that all three homing switches are at home in other words on this machine there's three axes of motion and if they all are in the home position then all three of those limit switches will be on and once the R will energize which will then energize well it'll close the contact once the R - one so looking at this diagram the only connection really between these two rungs is that when the second rung goes true in other words once the R energizes it closes the contact in the upper rung this is part of the logic to say it's okay to start the machine now here's our three limit switches limit switches open it close by mechanical contact right now and by the way they're always drawn in the normal position and they're normally open they are held open by a spring and they have a little arm or a roller probe that sticks out and when they make physical contact with something it depresses the mechanical device and closes the switch if the mechanical contact moves way than the switch is pulled back open by the spring so let's take and assume that all three of these switches have been closed because all three of the axes of motion and they're probably hydraulic cylinders they are all back in what's called the home or the home start position because remember this is part of our okay to start logic okay to start off the machine so when all three limit switches are closed that energizes one CR and that closes the normally open contact one CR - one we have another permissive or condition four okay to start and that's the master control relate a master control relay is a relay that is energized when all of the main safety systems have been met such as lake curtains and emergency stops which is when they're all reset then you press a button on the main panel to start the system that closes or I should say 10:00 urge eise's the master control relay and this contact goes closed somewhere else in the circuit that you can't see right now when the MCR was set or reset if you want to put it that way it probably started a hydraulic pump and the hydraulic right now the hydraulic pump is pumping up the pressure in the system because you don't want to start the machine and tell you up to full hydraulic pressure so at some point that normally open switch is now going to closed because that switch has a an operator inside of the hydraulic system and the pressure pushes it closed against spring tension that spring tension is adjustable very much like the regulator that we showed you in another lecture on basic electricity magnetism so the master control relay is reset so it's safe to start the hydraulic pressures is is up all of the axes are home we're just waiting for the operator now to close the gate to show that there's no one in that location when the gates which when the gate is closed then this normally closed contact goes close so actually this switch was held open while the gate was open okay at that point to see our Energizer's saying it's okay to start now you don't see any contacts marked to see our in this diagram that's because there are many other locations in the electrical schematic and they are now permissive Zoar conditions in other rungs of circuit logic to say that it's okay to start let's take another look at a real real a diagram if you've never seen one of these before you're probably seen too much information right now so we're going to simplify this diagram the first thing we're going to do is we're going to take the power source up at the top there you see it says l2 l3 goes through a couple fuses to a transformer this supplies power to this real real a ladder electrical diagram so we're going to take that electrical source and we're going to simplify it it's a fuse 220 volt 120 volt AC source we're going to simplify it into a line cord that goes over and plugs into the wall because it would do the exact same job and after all there are fuses in the fuse box in your home that would replace the fuses shown on the diagram so we've simplified this to show an electrical circuit and if you scan down through it you'll see that there are electrical conductors that go from left to right through fuses switches and lights switches and lights relay coils etc so what you're looking at right now is actually three separate circuits that all share this power source the first one is very simple you've got a fuse there that's just some case there's a short and then you have a switch that says system lights and then it shows a light L t10 literally that is the light inside of the control panel so you can turn it on to do maintenance in there the second diagram is a starter circuit for a motor for an oven recirculation fan and you can see the nomenclature over on the right-hand side it says motor starter oven recirculation fan now a motor starter is nothing more than a large contact relay with additional devices called motor overloads and if you look just to the right of the circle for the coil you'll see a normally closed contact that says O L under it that's the overload so if too much current is drawn through this line the overload pops open that's why they call the motor starter so the only difference between a starter and a contactor is a starter has the overload elements so if you look at this rung you've got a selector switch you know going from left to right now you've got a selector switch and it goes over and starts it energizes the motor starter so that's one circuit and then here's another identical circuit only it's a motor starter for an oven exhaust fan but you see how these form steps in a ladder so this is a classic relay ladder diagram this is an electrical circuit this is not a PLC program but later on you're going to see how PLC programs look a lot like this now we're going to make this a little more interesting what you are looking at is a pneumatic cylinder and pneumatic means air so this selling the cylinder is operated it right now send the retract position it's operated by air pressure through something called a solenoid valve a solenoid valve has two electromagnetic coils that are used to pull the inward working of the valve called a spool back and forth to control airflow to one side of the cylinder or the other and inside of this cylinder is a piston and at each end of the cylinder is a port if you put compressed air in the front it pushes the piston to the rear if you put compressed air in the back it pushes it to the front what's of interest for us as controls people are the two sensors and you can see that one of them has two LEDs lit up power LED and status LED the power LED just means that there's power there to power up the sensor the status LED says that that sensor right now is sensing something notice that the other sensor it's status LED is not lumen ated and that's because the sensor that's not lumen ated is the cylinder extended sensor right now the cylinder is retracted so the status LED at the back of the cylinder is now true now remember in an earlier lecture on basic electricity that if we took a magnetic field and we brought ferromagnetic material into close proximity that it would disturb the magnetic field and we were there by sense the presence of something so right now that sensor is sensing the present presence of the piston in the full retracted position now those sensors slide in those grooves that you see them mounted in so they're adjusted to a right position to sense the presence of the piston now these sensors aren't necessarily magnetic proxies they could be inductive praxis or something called Hall sensors and we're not going to go off discussing the difference of these sensors but it's keep in mind these sensors have a field that is disturbed when the presence of a particular element shows up in close proximity a closer look you can see that the status LED below is dark in the status LED above the retract position is illuminated okay now if we apply compressed air to the backside of the piston that pushes it forward now look at the status LEDs the one in the back of the cylinder is now dark and the one in the front is lit up so these two status LEDs are part of two cylinder sensors these sense sensors act like switches that are used in a circuit and when the switch closes it's saying in this case that the cylinder is in one position or the other and of course if you put compressed air on the other end of the cylinder it retracts back and you see our status LEDs are back to the original state this is a pneumatic valve this is a solenoid valve and on each end of it you see electrical connectors so in inside the black plastic we'll call them boxes that have the little knobs on the end our coils of conductor and then in between the two black boxes going through the entire body of that valve body there is a spool it's basically a rod that has grooves in it and as the two solenoids pull the rod back and forth it connects chambers between the ports you can see two the ports on the top of this valve where you thread in the fittings so you have two solenoids that can pull the spool in opposite directions and this makes up an air valve so these two solenoids remember a solenoid is a coil like a relay coil so when you look at electrical diagram we have a slightly different symbol for a solenoid than we do for a coil so you can recognize the difference and you'll see in a minute okay here we a circuit and we have three rungs of we'll call it real a ladder diagram this is an actual electrical circuit not a PLC program but they look a lot alike in the first run we have a normally open push-button and for a load we have one CRO relay coil in the second run we have a relay contact normally open contact and for a load nose of that contact closes it's going to energize the solenoid over there one Sol and it's known its nomenclature says that its extend and then on the third rung the normally closed contact from one CR if it's conducting will energize two solenoid and will retract the cylinder of course we recognize that inside the dashed blue line that is one device one CR coil and one CR - one - two relay contacts are all one device so when you push the push button one PB you are affecting the other two runs in the circuit so when it's de-energized this contact will be open and one Sol will be off when it's de-energized this will be closed because it's normally open and 2 SL will be energized looking at this right now in the normal state you're not pushing the push button is anything in this circuit currently energized look at it very carefully you have live voltage on the two vertical lines so over on the far left that vertical line is the height of power and the one over on the left the vertical line is neutral if this were 120 volt circuit if it was 24 volts DC you probably have plus voltage over on the vertical line on the left and negative on the vertical line on the right so following the path for current flow you see a complete path of course you do because once CR is de-energized once CR - 2 is closed because it's normally closed and it gives a complete path of conduction from the power rung on the left to the power ring on the right passing through - Sol so that solenoid is energized right now and it has shifted the spool and the valve to allow air compressed air to go to the front of the cylinder and push the piston all the way the back so it's in the retracted position or any of the contacts currently true nope no push button push just the way you see it are any of the contacts currently true remember you get Trufant roof off once CR - 2 is true because it's off in other words it will have continuity it's true because it's off are there any that are false look real closely push button 1 + 1 CR - 1 they're both normally open and right now they're open no one's pushing the button the relay is de-energized so once they are - one normally open is open so it's false with one push button close ok pretend like right now somebody's pushing that push button that's going to allow current to flow through and energized 1c are in that state where that scenario is anything energized I just gave you one yes once the R is energized that closes the normally open contact 1 CR - 1 which allows current to flow through when Sol energizing that coil which is a solenoid it shifts the spool to the other side and allows compressed air to be pushed in the back of the zone pushing the rod out and extending it looking in the state that it's in right now with a button push are any contacts true one PB and one CR - one they're true because they're on are any of them false I think you're catching on here once CR - 2 let's look a little more extensively at this circuit now this is a very very simple circuit but but it embodies most of the typical logic for a relay electrical diagram you have switches that are operated from an external source like a human being pushing the push button you have relay coils that move the movable contacts between normally open and normally closed and then you have circuits controlled by these relay contacts now when it comes to electrical diagram or to standard relay ladder logic like you would have in a PLC program there's a line that divides two sections of this these rungs this line right here divides the actions from the permissive x' or you could say the actions from the conditions and per misses that need to be met to cause the action so on this side we have actions we have solenoids and we have a relay coil the solenoids are actions external to the control the coil one CR is internal to the coil to the control and then on this side we have conditions or permissive now actions are on or off conditions or permissive are always true and false the first one is it's a push button so it's true if pushed the second one is true if on for the relay coil and the third one is true if off for the relay coil so conditions and permissive 's are always true or false don't say on and off now you can see once the R - one normally open is you can't say it's on if once the RS energize you can only say it's closed allows continuity it's never on none of these conditions or permissive are ever on or off they're true or false the actions on the other hand are on or off so we have a push button and we have to relay contacts but these are all conditions or permissive that will cause actions when they're true so these are these are our external interfaces from the control system to the real world the push button is the external interface to the operator and the two solenoids are the external interface out to the public to cylinders that extend and retract and then we have internal interfaces the two relay contacts and the relay coil itself and of course that relay is an actual device we're making progress folks any minute now we're going to be moving into new territory actually this is new territory you are looking at the most common electrical industrial rung ever and it's called a start/stop circuit you see to push push buttons and normally closed and normally open and then you see a relay coil bypassing the normally open push button is a relay contact this is the standard start/stop circuit for the start button we have a normally open push button for the stop button we have a normally closed push-button and just jumping out for a minute here is the top as they normally closed contact and the bottoms are normally open so this could be a relay contact or could be a push button in this case we're going to say it's for the two push buttons because remember you for a start button you have a normally open which would be the one on the bottom and for a stop button you have a normally closed which is the one on the top so if you look at the two sets of context they are 100% identical the difference is how they are connected so normally when you buy a industrial push button all you get is the operator which is just the spring-loaded button that goes in and out no contacts separately you buy a contact block and the contact block that you're looking at here you're looking at two of them it is the most common contact block it has one normally open one normally closed the top contact is normally closed the bottom one is normally open I show the block here twice to show you in the upper one how you would wire connect up a normally closed contact so the push button is the push button but the contexts that are operated can be wired up as a normally closed or normally open actually could use both contacts but you wouldn't use this both in the same circuit so the upper one is wired up like the stop push button normally closed and the bottom one is wired up like the start push button normally open okay we also have a contact relay and the relay contact in parallel with the normally open push button I show a dashed line in electrical diagrams we did this quite often to show that there was a mechanical link between the contact relay and the relay contact we don't do this anymore I threw it in here just to emphasize that that contact is controlled by that relay coil so if you energize that relay that contact is going to close if you de energize the relay that contact is going to open so look at that circuit right now the normally closed push-button is closed the normally open is open the relay contact is open and the relay is off de-energize however if I push that normally open push button the start button that closes the circuit from the left power rail vertical line all the way over through the relay coil to the right power rail vertical line the relay coil energizes and forces the contacts to change state so the normally open contact in parallel with the start button now goes closed so now you have two paths to the relay relay coil through the normally closed push-button through the normally open push button that you're holding closed to the relay coil or the current can flow around through the relay contact which is now held closed by the relay coil to keep the relay coil energized so if you let go of the normally open push button you still have the path for continuity through the relay contact to hold the relay closed so we call this seal in logic that's called a seal in context so you energize the relay and the relay holds itself on through that contact well how would you get this to shut off if the normally open push button is now open because you let go but current is flowing through that relay contact over to the relay how would you break this circuit or open it yes you push the stop button the normally closed button when you push that now there's no continuity anywhere through that circuit the relay coil D energizes the relay contact opens and when you let go the stop button it goes closed the normally open push button is still open and the relay contact is still open this is your classic start/stop circuit used to control millions of motors out there in the industry okay there's your stop button and there's your start button the red and green buttons that's the operator and directly behind it you see some plastic and some screw terminals and both of them have the identical screw terminals that's because they are those contact blocks I just showed you and then of course you have a relay now I'll just throw in another point here I show this earlier but I'll show it again this design provides better protection against contact welding than a single break design this feature also provides separation than normally open and normally closed circuits double break contacts open the circuit in two places creating two air gaps reducing the possibility of welded contacts compared to this single break design now I'm very familiar with these relays and I realize you might not be however I can tell by looking at the numbers on the screw terminals which two terminals you would wire up in parallel with a normally open push-button for a normally open contact to bypass the normally open push-button to seal in the coil now I have a mark there okay everybody take a deep breath let it out the next transition is really big we're going to make a huge quantum jump here in the next section but this is also a good time for you to take a breather grab something to drink pause if you're watching this you know in a video otherwise take a quick break here and then we'll get right back

Channel: plcprofessor

Views: 2,099,383

Rating: 4.8253207 out of 5

Keywords: Manufacturing (Industry), PLC, Programmable, Logic, Controller, training, tutorial, learning, course, relay, programming, ladder, logic, diagrams, PLC Programming, PLC Training, Programmable Logic Controller, PLC Tutorial, Allen Bradley, rslogix500

Id: PLYosK87D8E

Length: 62min 34sec (3754 seconds)

Published: Mon Nov 05 2012