Ladder logic Problem

We are asked to program a conveyor to carry two boxes along it and seperate the long from the short by the means of a kicker at end of conveyor.There is one sensor at start of conveyor x0 another sensor x1 after it . And a third sensor x2 at kicker to tell kicker to kick.The boxes are same width and height but the length varies.I can sepertae the boxes if ther is a gap between. But the problem is were asked to seperate the boxes with them all touching and in different orders meaning long box short box long box or two short boxes 1 long box. We are only to kick long boxes. The way the two sensors are positioned at start of conveyor is when x0 and x1 are true long box detected but when short box passes through the two sensors at one stage the two inputs will be off meaning short box detected. But if two short boxes pass touching the the plc thinks its a long box because x0 and x1 are on the gap between the sensors never takes place . We are only allowed to solve the problem using the sensors cant use timers or shift registers. Does any one have any ideas would greatly be

What is length of boxes?
Are the short and long box lengths constant?
Reason for question is
is length of two short boxes greater or less than length of one long box.

The boxes touching criteria is what I think is the major problem. If you could separate them it would be much simpler.

If box lengths are not constant it gets worse.

Need more data

Dan Bentler

Sounds like homework to me.

I would have a slower conveyor feeding a faster conveyor to introduce a gap. However that does not use ladder logic

Liam 28,

After reading your post I'm reminded of a junior college class I took during the early 60's. This was an electrician's class and the instuctor had just finished a class on how "three way" switches work in a house hallway. As a brain teaser he had us try to figure out how to wire up a light bulb upstairs up so that it could be turned on and off from downstairs as well as upstairs. But the problem was you were only allowed two wires inbetween the upstairs and the downstairs.

It was a week before I would see him again. This problem occupied my mind all weekend. I figured a way to do it with diodes, SPST switches and a AC supply. I presented an actual working example to him. I was very proud of myself.

He went to the blackboard and drew his solution to the problem.
The solution was to wire a light bulb base with two wires to the fuse box. The kicker was to use a base with a pullcord switch on it and extend the string out the window to the other floor window. This way you could pull the string from each floor and turn the light on and off.

Second Problem

You actually have 2 problems as I see it. IF the boxes are pushed to gether and do not have a gap then how will the kicker sensor detect which box to kick out?

As someone has already suggested you really need to seperate the boxes.

cjd1965 said:

Sounds like homework to me.

I would have a slower conveyor feeding a faster conveyor to introduce a gap. However that does not use ladder logic

Click to expand...

Agree on both

Dan Bentler

leitmotif said:

What is length of boxes?
Are the short and long box lengths constant?
Reason for question is
is length of two short boxes greater or less than length of one long box.

The boxes touching criteria is what I think is the major problem. If you could separate them it would be much simpler.

If box lengths are not constant it gets worse.

Need more data

Dan Bentler

Click to expand...

Hey there
yes the box lengths are constant.Two short boxes are longer than one long box . Were not allowed slow the conveyor or put a space between the boxes.We can only use the two sensors to discrimnate x0 and x1.Iv been at this for 3 weeks now still no joy . I can discrimnate signal boxes but when there touching i run into problem that twoo short boxes touching look like one long box.Any suggestions would be great.

Sorry guys there is a small gap when the boxes are touching.So when two small boxes pass there will a time when x0 goes on then off then x0 and x1 off and the x0 off and x1 on thats one short box after passing two sensors.

I'm getting a headach reading your problem description.

Post the original question as it was from your instructor and tell us what work have you done so far.

there is a small gap when the boxes are touching

Click to expand...

So are they touching, or is there a small gap?

Use 2 guides to funnel the boxes into a straight line in the center of the conveyor.

Now close the 2 guides so they lightly touch both sides of each box as it exits the funnel.

Now decrease the distance between the guides until the boxes slow down but do not stop.

Now you have a gap.

Now use a binary and or ladder to tell the kicker which box is coming next.

Move that result into a register.

Repeat that using enough registers/memory locations to until the first box gets to the kicker.

The kicker logic looks at the first memory location and when you get the "box at kicker" input and "box is long" then kick else go next. AND incriment to the next memory location by setting a bit after each box.

SO slow down create gap and read either one switch or two move result into mem location =long or short box in position one.

Repeat for each box using a new location for each until the first box gets to the kicker, assume they are all short for distance so you do not have an empty spot.

If conveyor speed is constant no problem.

Gap will be constant.

When the kicker switchs shows a box present, compare the first memory location to the short or long bit and kick or dont...........and theeeeeen set a bit so the next time you get the box at kicker input, the kicker looks at the second location.


When the first location has been set as read by the kicker, it should be available for the input side to refill with box data. You might need an extra location or two for overlap.

I did this to get away from my current issue not help someone cheat at school, I see the date puts us at a safe distance from that concern, and I like to troubleshoot.

But in "the real world" you rarely have someone telling you that you can't use an extra piece, and noone will tell you not to use a timer etc.

You WILL have situations where you do not have extra memory or hardware though, so it is good practice.

There must be a gap between boxes to allow sensors PLC to actuate and for blow off to do what it is supposed to do.

IF a gap two inches six inches is not allowed
THEN
1. Tell your management you cannot do it within their restrictions. Slowing the boxes then speeding to allow a gap is not going to kill them. Commit blasphemy and no longer revere the Production God.
2. Tell your instructor to get real and assign real world solvable problems.

Dan Bentler

Any way you look at it you need a transion on your sensor. If all of the boxes are touching it will look like one big box period. All the programing in the world is not going to fix that.

If you can not change conveyor speed to seperate your boxes you are going to have to change how your sensors "see" the boxes so it can detect the gap. With almost no gap present your probably are going to have to use a narrow beam retro reflective sensor. Possible problem here will be if the distance the box is from the sensor changes alot or if there is writing on the side of the box that could create a false signal.

Bottom line as far as I can see: Get your sensors seeing boxes.
consistantly. The programing after that is pretty simple.

Liam 28 said:

We are asked to program a conveyor to carry two boxes along it and seperate the long from the short

We are only allowed to solve the problem using the sensors cant use timers or shift registers.

Click to expand...

Liam 28,

Your ability to think is being tested. When a magician performs an act he gets the croud to focus on something that is not important to the outcome.

FIFO Rejects

I have used the attached to reject parts from conveyor. You could modify it to use the length sensors.