Homework Help, Someone help please!

[Lancie1]

Smoking,
You are going in the right direction, but there are some errors. For example, in Rung 000, your PB2 Stop Pushbutton will not STOP the Open Motor from running. It should be moved, either to the traditional left outside of the seal-in contact branch, or to the right next to the Limit Switch. In either position, it will stop the motor. Your PB3 ALSO must be in this rung in series with PB2, because it must also STOP the motor from opening.

Your Rungs 001 and 002 are using the same OTE Output O:0/0 Door Open Motor. This is called a "double-coil" and means that the LAST one (Rung 002) will be doing the control, and Rung 001 will have no effect, because whether it is On (True) or Off (False), Rung 002 will then be evaluated, and its state will replace that of Rung 001. So you need to combine the left sides of both rungs (put the Stop functions in series and the Start functions in a parallel branch) into ONE rung with ONE version of O:0/0.

There is one problem you have not seen (and your instructor probably has not noticed this either). Each motor direction (Open and Close) has two or more operator stations that have both Maintained-contact and Momentary-contact (toggle) operator switches. That means that the maintained-contact STOP functions are going to have heavier weight in in the control rungs, because technically and logically they will override the momentary or toggle switch Open or Close pushbutton functions. The maintained-contact switches are labeled for two functions, either Close AND Stop, or Open and Stop. I "AssUMe" (you should ask your instructor to explain these labels in detail) that this means that the maintained contact switches must perform both functions AT ANY TIME. There is a conflict or paradox (if that assumption is true). If the logic for the momentary-contact pushbuttons in the vehicle for two functions (Open and Stop) is designed to start the door opening but then PB2 can also STOP the opening, AND the maintained-contact pushbutton PB3 can also STOP the opening, then these two functions must logically be closed switches in series. However then if PB3 is in the STOP mode, then PB1 in the vehicle CANNOT open the door, therefore does not have equal weight or control of the door. This may be the desired outcome, or not!

The same reasoning applies to the Close function. PB5 must be in series with PB4 (in order for each to be able to STOP the CLOSE Motor, then if maintained-contact PB4 is opened, then PB5 cannot close the door (does not have equal weight because it cannot do its job unless PB4 is in the closed position).

The question for your instructor is: Are PB3 and PB4 supposed to be able to override the other control switches (have preference or first-come first-served control of the door functions)? I would say that if you cannot get this cleared up, the normal answer would be to err on the side of safety, and make the door stop whenever the maintaied-contact Start-Stop switches are open. If not, then there is a paradox in the specifications. If they are, then I have worked out a program that seems to work and meet all the other parameters.

 

[Lancie1]

Looking at your Red and Green Lights, your Green light goes on when the motor starts OPENing. Now what if the door gets all the way open, but the car is not yet parked in its final position (as indicated by PhotoSwitch I:0/9)? In that case, your green light will go off, never to come on again until the door is closed and reopened. What you must do is use the Door Open to turn ON green, but then use I:1/9 to TURN OFF the green light. This is a common scheme in industrial applications where one parameters turns something on, but an entirely different parameter turns it off. Would a seal-in contact for Green Light O:0/3 be useful here? TIP: According to your instructions, you need an interlock from the red light on this rung also, even though in reality the Photoswitch contact will serve as an interlock keeping green off until red goes off.

Your Rung 5 Red Light looks pretty good, although you should add a XIO "interlock" for the green light to the rung branch next to the T4:0/DN bit.

 

[SmokingGun]

I have sent him the question about if he wants it to override or not and I think I have made all of your suggested corrections. Here is what I have.

 

[pal]

Smoking,
You are going in the right direction, but there are 1 or 2 things to think about. As I read the question the green light should come on when it is OK to enter i.e. the door is fully open.
Second point - There are a maintained open and a maintained close switch. what happens if they are both "on"!? Good work so far!
Paul

 

[Lancie1]

As I read the question the green light should come on when it is OK to enter i.e. the door is fully open.

Pal, here is what it says about the Green Light (rearranged, emphasis added):

-When the door is opening, a green light comes on saying the driver can come in.
-The green light will only shut off if the car is in the right place.
-When the car pulls in far enough, a normally closed photo switch detects it and turns on a red light. (green and red light should have interlock).

Technically, the green light must come on "when the door is opening" (not when it is fully open even though we all know that the top will be dinged if the driver happens to try to drive in before the door is all the way up. In the real world this would be a part of the spec to be resolved by a meeting or phone call with the customer. For a student problem we have to assume that the problem is to test the student's ability to follow and understand the instructions as written). This green light must only go off "when the car is in the right place. The Red Light must not go on until the green light goes off.

I think I have made all of your suggested corrections. Here is what I have.

For your "Project Revised", here are some comments:

Rung 0000: Now your Stop buttons both work to Stop the Open Motor. Your PB1 Open will Open the door - only IF PB2 is Closed (thus the contradictory clauses in the instructions). Similarly, PB3 is described as "Open and Stop", but really now it can not cause the door to open on its own, due to the Normally Open PB1 in the circuit (that can be fixed though - see my later post). Your Limit Switch Top I:0/2 needs to be an XIC instruction (not XIO) to cause a TRUE output for a closed physical switch.

Rung 0001: You probably should add an interlock for the Door Close Motor, to guarantee that in no case can the two motor directions EVER be energized at the same time (add XIO O:0/1 before the OTE). This is commom and usual on almost all motor circuits involving direction-reversing motors - you never want to be able give the two commands at the same time.

Rung 0002: This is a tough one. First let's get the easy stuff out in the open. You used both the Blocked Photoswitch I:0/8 and the Bottom Limit Switch, but both these physical switches are descibed as "NC", therefore if they are "not operated", they will be closed, and to get a TRUE from your PLC instuction looking at NC switches, you need to change both instructions in this rung to XIC's ("open" contacts).

You probably should add an interlock for the Door Open Motor, to guarantee that in no case can the two motor directions EVER be energized at the same time (add XIO O:0/2 before the OTE).

Now look at the harder, more arcane PB control stuff on Rung 0002. First, add these labels to your PB switches: for PB4, add "main. or maintained" to your address description; for PB5, add "toggle" or "alternate". Now if PB5 only alternates from closed to open each time it is pressed, how will you start the Close Motor with PB4, if PB5 happens to be open? You can't (with your present logic). You must depend on BOTH PB4 and PB5 to be closed before you can start the Close Motor (not a desirable or efficient way to run a garage). That can be fixed though by adding alternator logic for PB5 (probably what they are testing to see if you spotted that little glitch - see my later post).

Rung 0003: Now your Green Light will only go on (not while the door is opening as specified), but when the door is opening AND the Car Position Photoswitch detects a parked car (which cannot happen while the door is opening and the car is still outside). This little problem is the exact reason why the green light was specified to go on "while the door is opening", so they could hit you with this problem of how to get it on with one condition, yet much later turn it off with a second condition. The fix is, like I said earlier, Door Opening to turn ON the Green Light, and I:0/9 (sealed in on a rung branch by O:0/3) to Turn OFF the green light.

Rung 0004 : Great, I think this one will work.

Now think about these comments, and later I may post a LogixPro Simulator working version that handles all the specifications (except the question about the maintained-contact STOP switches also being able to also start the motors).

 

[pal]

Whoops - Sorry
Pal

 

[Lancie1]

By now, many of you know that the real problem is combining the various types of pushbutton switches to control the motors. All switches are not created equal, and for motor control, the momentary type is generally the best or preferred type. The maintained-contact type have one bad problem - they are hard to combine with other types and after a power failure, they can restart dangerous equipment unexpectedly.

So how do you solve the problem where your customer (your instructor in the is case) demands that you combine incompatible devices safely? HINT: Use OSRs to convert the maintained states of the maintained-contact and toggle (alternating) switches to momentary pulses, which are then compatible with the momentary switches and work well for motor controls.

 

[SmokingGun]

By now, many of you know that the real problem is combining the various types of pushbutton switches to control the motors. All switches are not created equal, and for motor control, the momentary type is generally the best or preferred type. The maintained-contact type have one bad problem - they are hard to combine with other types and after a power failure, they can restart dangerous equipment unexpectedly.

So how do you solve the problem where your customer (your instructor in the is case) demands that you combine incompatible devices safely? HINT: Use OSRs to convert the maintained states of the maintained-contact and toggle (alternating) switches to momentary pulses, which are then compatible with the momentary switches and work well for motor controls.

The instructor said "They are interlocked switches. In other words, once one of them is working, the other one must be forbidden."

So I will work on a new rung in the morning and upload my revision then. I have an idea of what I need to do and I will see if I can figure it out. Again thanks for everyone's help

 

[krakenfan69]

If the photo switch on the door is nc then shouldn't it be programmed as an xic giving an input constantly that would stop the door if it is lost because something broke the beam OR if the dog chewed the wires off the low mounted switch? With the current programming losing the input ALLOWS the door to close. The same argument could be made for the limit switches to ensure fail safe operation.

Or am I confused again?

Kraken Fan #69

 

[SmokingGun]

He didn't teach us about one shots so I am not sure if he wanted us to use that or not. I just did another revision so here it is... I may have made a mess of it but it does look like it may work now it is crowded. Again Lancie I appreciate all of the help you have provided!!

 

[Lancie1]

The instructor said "They are interlocked switches. In other words, once one of them is working, the other one must be forbidden."

That is one way, but could leave you open for a liability or negligence lawsuit (in the real world). Juries are not too friendly with programmers and their employers, especially if they think there is a large insurance policy. I have been on serval cases as expert witness, and it is easy to get a $10 million judgement for bypassing STOP buttons. If it is labeled STOP but fails to stop a motor, then watch out - someone must be blamed and make sure it is not you - CYA.

Here is another method to handle the various switches without risky bypassing or disabling ANY stop switches, and also allows Close or Open buttons to operate at any time. It also handles the problem of the green light going on "while the door is opening" but not before it is high enough to allow the vehicle to pass under, using a timer. It is up to you to set the timer comparison statement (now at 7 seconds) to a time that allows the door to reach that level. Like I said before, the best solution would be to not turn on the green light until the door is fully open, but then that would mean your solution does not meet all the requirements. This version does. It is running on the LogixPro Simulator program (simulating a RSLogix program). The LogixPro uses different addresses than your exercise, so you will have to convert it to use the proper addresses and timer time bases (LogixPro only has the 0.1 time base, not the 1.0 or 0.01 as RSLogix has). You should have no trouble in converting because I have labeled each address according to your problem and also used rung comments to explain each rung. The ZIP file is the actual Logix RSL program.

NOTE: I cannot guarantee that I covered all bases or interpreted all requirements correctly. There may be unknown errors, as with any program. Check it carefully and use at your own risk!

 

[Lancie1]

If the photo switch on the door is nc then shouldn't it be programmed as an xic giving an input constantly that would stop the door if it is lost because something broke the beam OR if the dog chewed the wires off the low mounted switch?

Yes, that is correct.

 

[Lancie1]

He didn't teach us about one shots so I am not sure if he wanted us to use that or not.

Because you live in the US, you are free to learn about One-Shots without permission from your instructor. However, the International Telecommunications Union is working day and night through the United Nations to create new UN rules so that governments control and tax all information on the Internet. Only about 30 of the 193 UN governments oppose this takeover of free information, so its going to happen, unless something changes.

However, you could substitute 1-second timer and Timer/TT bits in place of the OSRs and OTE bits. Timers actually will work better to control real motor starters, which require some small 1-second time for the coil to energize and then close an auxiliary RUN feedback contact to the PLC (as opposed to PLC simulated devices which operate in one PLC scan).

 

[SmokingGun]

Because you live in the US, you are free to learn about One-Shots without permission from your instructor. However, the International Telecommunications Union is working day and night through the United Nations to create new UN rules so that governments control and tax all information on the Internet. Only about 30 of the 193 UN governments oppose this takeover of free information, so its going to happen, unless something changes.

However, you could substitute 1-second timer and Timer/TT bits in place of the OSRs and OTE bits. Timers actually will work better to control real motor starters, which require some small 1-second time for the coil to energize and then close an auxiliary RUN feedback contact to the PLC (as opposed to PLC simulated devices which operate in one PLC scan).

I appreciate all of the help. I made the suggested corrections and submitted my project. Lancie I really appreciate everything you have helped with. I look forward to learning more with you guys in the future!
-Chris

 

[Lancie1]

The dreaded PLC Double-Coil Error!

I hope that you did correct your previous "double-coil" error, as I warned you about in Post #16 . Otherwise your grade may be a lot less than you wished!

Here is a version using timers in place of the OSRs that are unknown to your PLC classs. I used 1.5 seconds for the Close Motor Stop button timers, but 1.0 seconds works just as well.