New to PLCs, looking for tips/advice on conveyor exercise.

Hello, I'm a recent graduate who is starting a new job working with PLCs next week (primarily Allen-Bradley). My employer gave me a PLC simulation program (LogixPro) and a few exercies I could work through to gain some familiarity with PLCs priot to starting. My educational background is mostly microcontrollers and software, so it's taking me a bit of time to get used to programming with ladder logic. Ron Beaufort's videos were extremely helpful. I was hoping some of the experts here could offer their thoughts on some of my work/understanding thus far.


Below is one of the exercises I've worked through. The conveyor/vat setup shown in the attached picture was used for this exercise:

Completely design and de-bug a ladder control circuit which will automatically position and fill the boxes which are continuously sequenced along the conveyor. Ensure that the following details are also met:

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• The sequence can be stopped and re-started at any time using the panel mounted Stop and Start switches.
• The RUN light will remain energized as long as the system is operating automatically.
• The RUN light, Conveyor Motor and Solenoid will de-energize whenever the system is halted via the STOP switch.
• The FILL light will be energized while the box is filling.
• The FULL light will energize when the box is full and will remain that way until the box has moved clear of the prox-sensor

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My ladder logic program is shown in the other image.


*Rung 0: Start logic.
The seal-in to start the process when the start button is pressed (the run lamp is used as a “run” flag)


*Rung 1: Motor logic.
If the system is in run-mode the conveyor will advance, but only if 1.) the proximity sensor hasn’t been tripped OR 2.) the full-lamp is not energized. This ensures that a full box will not stop the conveyor, even if the proximity sensor is tripped.


*Rung 2: Filling logic.
If the system is in run-mode and the proximity sensor is tripped, but the box is not full (both level sensor and full-lamp are not engaged)the fill valve will open and the filling lamp will light.


*Rung 3: Full/advance logic.
If the proximity sensor is engaged and the level sensor trips, the full lamp energizes and remains energized until the proximity sensor is cleared.


When I run the simulation it works as intended, just looking for any tips/advice.


Thanks,
Zazoo

I wouldn't change anything. Your well on your way. Good luck at your new Job.

I am new to PLC as well but here is my thoughts after a couple months of PLC work. That does look like the correct run logic. This will run great in a test setting. Now to advance your understanding of the code and process you should have some error checking. Examples conveyor didn't stop in time, filling for an excess amount of time and no fill indicator. basically you want to check for things that can go wrong and compensate. Also, should start looking at using internal tags or bits to add things like state indicators, program control and make it so you don't have to change all of your code if you have to use a different IO point.

You look like your going great.

Zazoo,

Welcome to the PLC forum!

When I run the simulation it works as intended, just looking for any tips/advice.

Click to expand...

Great job. There are a few things that can be done to decrease the spills that will occur if you run at "100% Scan Speed" (a simulation of speeding up the PLC scan rate and also the conveyor speed). If you run at 100% for a long time (at least 5 minutes, sometimes 10), or if you stop and restart rapidly, you may see some spilled product building up on the floor. If it gets deep enough there will be a fire and bad results!

There are several problems that duplicate what you might see in a real silo feeder and conveyor. The solenoid is not 100% accurate and not real fast at closing, at times filling more than the box can hold. The level sensor is not real fast, at times not showing Full when the box is already full. When the feeder solenoid valve closes, there is some product below the solenoid, but it has not yet fallen into the box. If the solenoid valve is not deenergized BEFORE the sensor shows FULL, then it can be overfilled by a small amount (look at the chute distance from the valve to the top of the box). These are all real-world problems that occur with silo feeders.

If you are interesed, I can suggest some rung changes to completely eliminate the spills even at 100% speed. These suggested changes are relevant and mimic the way an actual hopper feeder works.

This version of the Silo Exercise 1 has one addition that will allow closing the solenoid BEFORE the remaining material falls into the box. If you want verfication, set the speed to minimum 1%, and watch Rung 001. You will see that the Box Full Indicator opens slightly before the Box Full Level Sensor, allowing space in the box for the falling material. The Box Level Sensor can be deleted from Rung 001, but I left it in to show how the new additon to the Box Full Indicator can prevent overfilling.

There is also another useful method that can be added to improve it even more.

Wow, thanks for all this great info.
I'll take a look at this updated exercise when I get home tonight.

Thanks again!
Zazoo

Thanks for the feedback.

Note that with the new level counter, you can adjust the GEQ Source A (currently set at "93") to work with the speed of your own computer. Because computers have different capabilities and LogixPro runs at different speeds on each computer, you may need to change the Rung 004 GEQ Source A from 93 to a lower number, if you are getting spills after 5 minutes, or to a higher number if you get no spills and can afford to fill the box slightly fuller.

Here is a picture of the rungs for those without the LogixPro software.

Correction: I meant Source B of the GEQ instruction can be changed from 93. The GEQ parallels and works with the Box Level Sensor. If either fails, the other takes up the load. Using the Box Level Sensor only results in a few grams being spilled for each box.

No big deal - except to the owner interested in his profits, and the guy who will eventually have to sweep it up and haul it back somewhere for recycling or disposal, and for the long-term survival of the company.