Total Newbie Here

The company I work for asked me to program a simple PLC (Apparently since I'm a computer programmer, it should be easy for me). Anyways, I usually learn pretty fast but I'd love some help to get started. The program goes like this :

There's an input that comes from the cylinder exit. When that cylinder exit is clear, it triggers the output. When it's not, the output stays closed.

There's another input that comes from the conveyer traffic with a timer. If the cylinder exit is clear, it triggers the output, if not, it waits 10 secs and tries again.

I *hope* that was clear enough. I'll be programming this on a Logo!, but if I can have a base idea of what the ladder looks like I can switch it to the Logo!.


Thank you so much for all your help !!!

Let's see if I understand...

It sounds (to me) like you have a conveyor with a solenoid operated air cylinder which pushes product off the conveyor (maybe to another conveyor?). There is a sensor located BEFORE of the cylinder that senses when there is product ready to be pushed. There is also a sensor located AFTER the cylinder to confirm that the exit is clear.

What is the state of each sensor when product is detected? ON when product is present, or OFF when product is present? Similar question for the cylinder. I assume the output should turn ON to push the product?



-Eric

Tell your boss that lines of code and ladder logic are worlds apart, especially since you don't seem to have the electrical knowledge that you need to figure out circuits.

That being said,
Myself, I wouldnt use a PLC for your situation, it would probably be less work to use a couple relays and a timer. This diagram I made up only uses 2 relays and a timer.

cyl exit input

-----|/|----------------------------(output1)---

 

conveyor input    output1    timer

-----| |------------| |-------|/|---(output2)---

            | output 1 

            ------|/|----------------(timer)---

                 timer |

------------------| |--

Andy, I think that there is only one Output, probably a Air Cylinder Solenoid Valve as Eric wrote. Even though your relays could be used to do the job, I think the goal is to teach Melp how to write PLC programs.

Who makes the Logo? EDIT: I see it is a Siemens small "logic module" PLC. Is there a free download available? I could not determine that at the Siemens Logo site.

I got a little bit more information.

There's a box that gets out of a machine, if there's no traffic, it gets out. (If I1 = 0 and I2 = 1 : O=0) If there's traffic (ie another box passing on the conveyor), it cuts the output for x secs (So if I1 = 0 and I2 = 0 : O=1 for 10 secs).

I hope I make sense.

And yes Lancie1, Logo is made by Siemens. I think you can find a demo here : http://support.automation.siemens.c...tandard&viewreg=WW&objid=10805253&treeLang=en

Thanks !!!!

. . .if there's no traffic, it gets out. (If I1 = 0 and I2 = 1 : O=0) If there's traffic (ie another box passing on the conveyor), it cuts the output for x secs (So if I1 = 0 and I2 = 0 : O=1 for 10 secs).

Click to expand...

You description says that for a box to "get out", the output must be OFF and for a box to be inhibited, the Output must be ON. Before I write some logic, is this correct (it is backward from the typical case - unless it is a gate or a brake)?

Also, being a computer programmer, you may not fully realize that PLC programs are very "platform-dependent". A program that I can write for you in Siemens MicroWin S7-200 or Rockwell RSLogix will not even be close to running on your Siemens LOGO (and vice-versa). That is why I asked about the software. Best I can gather, there are no free software downloads available, so I cannot write the logic for you in LOGO.

I'll try to be clearer even more. We have a conveyor that takes all boxes from 6 different machines and brings them to a box closer. We added a few more machines but on the other side of the box closer. So we have one conveyer coming from the left, that reaches the end, before switching to the other conveyor. So if there's a box, the box must be pushed to the other conveyor if there's no other box coming. If there's a box, it must wait 10 secs before trying to push the box again.

I really hope this helps

As for the logo, as I can see, it can take ladder programs and change them into blocks.

Thanks once again !!

So if there's a box, the box must be pushed to the other conveyor if there's no other box coming. If there's a box, it must wait 10 secs before trying to push the box again.

Click to expand...

EDIT: This is important to making the logic correct: if your LOGO PLC "Output" is OFF, then the Pusher is switched to ON?

Remember, it is the PLC Outputs that do the work, peform the action, and carry the loads. The PLC inputs are only sensors and switches that are the eyes, ears, and fingers for the PLC. The outputs are the muscles and the physical actions of the outputs must be well-known before logic can be written.

If there's a box coming (meaning that an eye detects it) the pusher must wait 10secs before re-trying to push it.

I am going to do you a favor and AssUMe that the pusher operates like most do, and when your PLC Pusher Output is ON, the Pusher operates, and when your PLC Output is OFF, the Pusher does not operate. Note that this is OPPOSITE of what you said earlier. I can't see that any responsible electrical designer would knowingly design a solenoid valve to be ON 99% of the time and only go OFF when a box needs pushing. I will write you some rung logic based on that ASSUMPTION.

Yeah that's it ! Sorry if I'm confused (and confusing)

Here is a LogixPro Simulator version for Rockwell RSLogix programming software. Your LOGO version should be similar, except that it will probably NOT have the built-in (TT) Timer Timing bit. I added the relay B3:0/0 to create the Timer Timing function (a bit that is ON only when the timer is RUNNING, not when it reaches its time preset value).

Thanks for all your help. I'm getting closer, but the timer isn't working. I'll try to figure it out. Thank you !

Melp said:

Thanks for all your help. I'm getting closer, but the timer isn't working. I'll try to figure it out. Thank you !

Click to expand...

Logo version of Lancies program
(sorry no comments)

Thanks, Moggie!

Melp, this logic is ONLY as good as your original description. I think you know how good that was. It could be that I1 and I2 are reversed. Your description of exactly which input sensor does WHAT was a little confusing.

It may be that you do not need the I1 bit on the first rung. Depending on how the pusher operates, it could even stop the pusher before it completes the pushing job (if the exiting box turns the pusher off before it is fully out). PLC programming is a little different from computer programming in that you really need to be familiar with the exact characteristices of the physical devices in order to write a good PLC program.

Note that your conveyor sensors can be selected and wired to have Normally Open (NO) logic or Normally Closed (NC) logic. In other words, when the sensor is blocked, its output to your Logo input can be either ON or OFF. You really need to know that for EACH sensor in order to get your program to work. I AssUMed that both sensors are N.O. and produce an ON signal when blocked.

Another important question is whether this Pusher Output has to be ON to keep the pusher operating (some conveyor box pushers have built-in control functions) and if so, does the box stay in front of the I2 sensor long enough for the pusher to finish? If not, then another timer may be needed to keep the pusher ON for whatever time it needs to push the box on to the other conveyor.