Designing a PLC controlled conveyor system

[snehjoe03]

Need help with designing and test a PLC-controlled pneumatic system that conveys manufactured parts and loads them on a packing machine. I am attaching a file with all the requirement for this project.

Detailed system operation is as follows:
• Initially, both cylinders are retracted and the RUN lamp is off.
• Depressing a START pushbutton causes the RUN lamp to turn on.
• When a part arrives on the lift cylinder, it activates photoelectric switch PE1. This
causes the lift cylinder to extend and raise the part, as shown in Figure 11-1 (b).
As the lifted part moves away from photoelectric switch PE1, the switch
deactivates but this does not affect the operation of the system.
• When the lift cylinder is fully extended, it stops, while the load cylinder extends
and pushes the part onto the packing machine, as shown in figure 11-1 (c).
• Both cylinders retract when the load cylinder becomes fully extended.
• When 5 parts have been pushed onto the packing machine, the system stops,
while the PACK PARTS lamp turns on to inform the operator that the parts are
ready to be packed. The system can then be restarted by depressing the RESET
pushbutton, which turns off the PACK PARTS lamp, and then depressing the
START pushbutton.
• Depressing the STOP pushbutton at any step in the process causes both
cylinders to retract. This also causes the RUN lamp to turn off. The next time the
START pushbutton is depressed, the counting of the loaded parts will restart from
the value attained when the system was stopped.

 

[Ronnie Sullivan]

Do you mean - please do my project for me and I'll take all the credit?
Or do you mean 'help'
If so, what have you done so far and what are you having trouble with?

 

[parky]

Well first of all nobody would design a system like this in reality. From the description it would require some operations to be done based on time, and are the solenoids that drive the cylinders single acting or double acting (single acting will drive the cylinder forward and when de-energised drive it back double acting will drive it forward and if de-energised will stay put until the second solenoid is energised to drive it back) not a good explanation on the tutors side, Ideally, the cylinders would have proximities or limit switches at the end of travel to determine the positions i.e. retracted/extended. in the case of no sensors for cylinder position then you will have to use timers set at time values to allow for the movement to position.

If possible, I would ask the question are there sensors for the cylinders and are they single or double acting.
Unfortunately, many exercises for colleges are poorly explained.
But there are two basic ways of controlling a sequence in strait ladder logic and these are steps using memory bits or words using numbers.
First of all write out the conditions for each step.
example:
Input 1 = Start PB
Input 2 = Stop PB
Input 3 = Part in place PE
Input 4 = Raise up Sensor
Input 5 = Raise down Sensor
Input 6 = Push cylinder forward sensor
Input 7 = Push cylinder back Sensor
Output 1 = Run Lamp
Output 2 = Raise cylinder up solenoid
Output 3 = Raise Cylinder down solenoid
Output 4 = Push cylinder forward solenoid
Output 5 = Push cylinder back solenoid
Step 1. Start bit on and raise cylinder down and push cylinder back and a part in place PE. energise push up cylinder.
Step 2. Start bit on and raise cylinder up and not part in place PE. energise Push cylinder fwd.
Step 3 start bit on and raise cylinder up and push cylinder forward energise push cylinder back and Raise cylinder down solenoids.
You could use internal bits as the step flags, these are enabled for each step and reset when the next step condition is true.
The other way is to do the same thing using an integer variable and move a value into it then compare the value with the step numbers.
in this case
Start with an integer of 0
Step 1. Start bit on and raise cylinder down and push cylinder back and a part in place PE. then put 1 into the step number, use a compare to drive the solenoid and so on.
There are some problems you need to overcome depending on the type of solenoids for example if the solenoids are single acting you need to ensure that the raise cylinder stays energised on step 1 & 2 but released on step 3.
Give it a try then post your code most here will not do your assignment but will help if you get stuck. Also, tell us what PLC simulator you are using it helps.
First of all you need to write down the I/O and create your symbols so you know what each contact or coil does.
Put comments on the rungs it helps others understand what is going on and helps you when trying to de-bug it.

 

[VAN]

• Depressing a START pushbutton causes the RUN lamp to turn on.
• When 5 parts have been pushed onto the packing machine, the system stops,
while the PACK PARTS lamp turns on to inform the operator that the parts are
ready to be packed. The system can then be restarted by depressing the RESET
pushbutton, which turns off the PACK PARTS lamp, and then depressing the
START pushbutton.
• Depressing the STOP pushbutton at any step in the process causes both
cylinders to retract. This also causes the RUN lamp to turn off. The next time the
START pushbutton is depressed, the counting of the loaded parts will restart from
the value attained when the system was stopped.

I'm not really here to help but more shame whoever came up with your assignment.

The person that made this assignment is like many professors with zero industry experience. Hell when I was doing my degree the head of the engineering department never had a real job. Just school and then professor, the other professors use to make fun of it. BUT I think its a real problem with education.

Good luck on the assignment, best chance at getting help is making an attempt first.

Ask the professor if in the real world a person could hold the start button forever and its acceptable the system doesn't start... or hell someone could hold the stop button forever and it never stops. WHAT IS THE POINT!!!

 

[jaden]

You will also need to make some definitions as to pneumatic valve types, ie; Single solenoid-spring return, double solenoid, etc. I would recommend you make an I/O list of all devices needed.

 

[jaden]

Oops; Didn't read for detail.

 

[daba]

Operators are gonna love having to press 2 buttons to make it work again ...

"When the lift cylinder is fully extended...." how do you know that has happened ? There could be low air, so it never extends, so a timer is useless.....

Like so many others, I question the sanity of the teaching staff.. How can they expect to train someone to take a step into the real world with stupid exercises like this.

One of my training pieces of kit is a "dumb waiter", with a reversible lift motor, complete with travel limits (reed switches actually), top and bottom doors with door closed detection (again reed switches), and illuminated call buttons at both floors.

I give it to students, and say I have "lost" the I/O assignments, and they have to discover them. When they have discovered the I/O, they have to come up with a program to control it.

I also stipulate that the maximum operator actions should be to open and close the door if the lift is at their floor, or a single press of the "call" button to bring it to their floor.

Obviously in the real world I would have locks on the doors to prevent them being opened when the waiter compartment is not there and stationary, and there would be interlocks to prevent the lift from moving if a door is not shut.

I end the exercise with a discussion of everything I left out of the model, and why it should be in ... !!

 

[daba]

Too many "training" exercises miss so much vital detail, the sort of detail that can save life and limb etc.

rant over ....

 

[parky]

If it was me I would challenge the lecturer on all those points already mentioned and his attention to detail, but then again his marking of your work may suffer lol, teachers/lecturers who have never been in industry are very highly strung in general well in my experience. I could tell a story of taking a lecturer to a major food plant and what happened but it is a bit long winded and nobody would be that interested lol.