Help! Final PLC Project

mooner · Mar 2, 2009

I am working on my Final PLC project. We have to design a Traffic Light System. Red is on for 7.5 Seconds, Green 5 secs and Yellow 2.5. There are only two sets of lights needed. North South and East West.

I have the light sequence working, the problem I am having is with the interrupt. We need to have an interrupt for predestrian crossing. Then when pushed it switches the light sequence.

Thanks for the help

mooner · Mar 2, 2009

This is what I have so far

Pages 3 and 4

Steve Bailey · Mar 2, 2009

I am working on my Final PLC project.

Good for you.

We have to design a Traffic Light System. Red is on for 7.5 Seconds, Green 5 secs and Yellow 2.5. There are only two sets of lights needed. North South and East West.

That sounds pretty simple.

I have the light sequence working

Congratulations!

the problem I am having is with the interrupt.

Uh-Oh.

We need to have an interrupt for predestrian crossing. Then when pushed it switches the light sequence.

Modify your existing program to include it.

Thanks for the help

You're welcome.

mooner · Mar 2, 2009

I am asking for help here, and yet all you do is tell me to modify my existing program

I figured you would be able to help me with that rather then be a ****ing dick about it

Steve Bailey · Mar 2, 2009

You claim you've got the normal sequence working, but I have my doubts. Have you actually run this on a real PLC? What is the "Traffic_Light" symbol?

mooner · Mar 2, 2009

Yes I have run it on an actually PLC, thew Traffic_light symbol is my power switch to start the actually sequence of the traffic lights

mooner · Mar 2, 2009

I know for a fact the light sequence is working, I have tested in on the Siemens PLC we are using. I have tried several ways of adding in an interrupt but none have worked. I have tried adding in a reset to the circuit to resest the sequence but that is only causing the timers to reset. The traffic lights are still staying the same.

Steve Bailey · Mar 2, 2009

So after one cycle you have to turn the Traffic_Light switch off and then back on to start another cycle? That's not quite the way the traffic lights I'm familiar with operate.

Terry Woods · Mar 2, 2009

Be the computer!

Mooner,

Consider this very carefully...
What would YOU do if YOU were the computer?
What would YOU do if YOU had to control the lights manually?

Make a list, step by step, of how YOU would handle the interrupt.

bernie_carlton · Mar 2, 2009

Steve is being cranky because we get so many of these. At least you have shown some work while many show absolutely nothing and expect the whole thing to be handed to them.

Post the following:

Exactly how will the pedestrian crossing affect the normal sequence.

Show a 'non-pedestrian' timing diagram for the complete cycle.

Then show a 'pedestrian-interrupted' timing diagram.

Actually, creating both of these will probably give you the answer without our help.

bernie_carlton · Mar 2, 2009

Steve - there's a timer reset at the fourth rung. That appears to be the cycle repeat method.

Steve Bailey · Mar 2, 2009

Bernie,

You're right. I missed that.

Mooner,

What lights are supposed to be on during the pedestrian step? Red in both directions?

I'm going to try to give you a hint without too obviously handing you a solution. Have you considered the possibility of adding an additional step to your sequence as well as using a variable as a timer's preset instead of a constant?

jvdcande · Mar 2, 2009

There used to be a thread about a traffic light program where an SFC was posted. It even had three interrupts. I remember one of the replies implied we have really, really big monitors over here in our very little country. Ah, if there only was a search button available on this site

.

Longhorn · Mar 2, 2009

I have completed this task before but using Logixpro, my appoach was to set a retentive timer to a reasonable large value ( enough to cover a full cycle) then using "Limits" allow each light an allocated slot of time. When a pedestrian crossing button is pressed, it requested an "ok to cross" latch which i only allowed whilst the red light was active.
The best way is to plan out the steps as the others say and then think about how and when you can slot in the pedestrian crossing part, try a flow chart.

Hope this helps

LH

Stationmaster · Mar 2, 2009

"Dick" is okay but "a m b i e n" gets censored????? mmmmmKaaaayyyyyYYYY....

Okay, I'm not trying to be a "dick" here, but if you only have two sets of lights, (N/S and E/W) why do you need a "pedestrian" function? What exactly does it do? Shouldn't the pedestrian just WAIT and cross when light is green in his or her direction?

Do you REALLY want to stop traffic in all directions at a busy intersection so one person can saunter across the street? (The right-turn-on-red traffic will likely get 'em anyway.)

At ANY rate, you have to have a better description than a "pedestrian interrupt" to define the function well enough to start programming. (A metaphor for "you have to listen to your customer well enough to know what he REALLY wants").

Once you figure out, STEP by STEP, whats going to happen when the button is pushed, your first step might be to latch on a relay that only UN-latches when the "pedestrian function" has been executed (step by step).

Stationmaster

Help! Final PLC Project

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