conveyors and sootblower motor

I have not receive my plc tutorial yet (ccnow order number 13-3860) and I am quite new to PLC..so I have two pressing question for our panel of experts.

1. You are asked to put the controls for a conveyor onto a PLC. The operator will control the conveyor with a Run/Stop position switch and the operator will be given a lamp indication of the motor running or stopped with a green/red lamp respectively. The Run/Stop switch must go through the stop position before the conveyor will start. The operator will also be given a lamp indicating that there has been an E-stop, which the operator can cancel with the stop switch position. The conveyor will have 4 emergency stop cords along its' sides. Write a ladder logic program (I have access to square-D or telemecanique plc only).

2. Write the ladder logic to control a sootblower motor. The requirements are that a lance will be out of the boiler, as indicated by a "home" limit switch. When requested to go in, the sootblower must leave the home limit switch within 5 seconds, or the insert request is to be cancelled. When in the boiler, the lance must reach its' end-of-travel limit-switch within 30 second, or an automatic retract should be initiated. When the lance reaches its' end-of-travel, the reverse motor starter should be closed. There should be an operator-retract available to permit a "retract request" of the lance at any time. In any case of the sootblower motor being reversed (withdrawing of the lance), there should be a 3-second delay between the forward contact opening and the reverse contact closing. Local emergency stop will be hardwired locally, and need not go into the PLC.

Well, Dean, we aren't going to do your homework for you. But because you are buying Phil's book we know your heart is in the right place, so I'll give you a couple of hints to get you started:

1) Make a list of all of the inputs and outputs to the PLC, including the channel designation for the PLC I/O. This is called a point list, and should be one of the first things you create for any project.

2) Convert your text description for one of the projects to "pseudo code". That is, create an outline type list in English for each step or function the program should have.

3) Don't try to develop the program in your head completely before you start. Instead, create one rung at a time, for one function at a time. Start with something simple, like having a contact from the run input on your moor turn on a Run pilot light. That way you can get the feel of the language and build your confidence level incrementally as you go along.

Well, I must say that homework questions are getting better - maybe there's a higher quality of profs in Canada than at NYP.

1. You are asked to put the controls for a conveyor onto a PLC.

First I'd ask what their budget was. I get paid for programming, not for answering questions on the net (which is why this is my lunch break).

Seriously, though, start this like any other programming project.

1. Make a list of all inputs and outputs. Pay attention to details. For example, is that "Run/Stop Switch" two inputs (one for Run, one for Stop, with a spring return to center, or a straight two-position selector switch, with only one PLC input
2. Write a detailed sequence. Avoid the word 'it'. The sequence should look at the world from the PLC's point of view, not the Operator's. Not "The Operator presses the 'Start' button to start the motor....", but "When the 'Start' button is pressed, the motor is started....". Doing it this way starts to expose "gotcha"s (like what happens if the 'Start' button is pressed (by the operator) when the E-stop is tripped?). Rememer, the PLC doesn't 'know' that the sootblower has left the home limit switch, only that the home limit switch signal dropped out after the sootblower was started. If you mention anything that isn't included in the previous step, REDO.
3. Focus on each output. Ask yourself - when does the PLC energize the output? When does it stop energizing it? If the answer isn't in the previous step, REDO. Try to use the words 'IF' 'AND', 'OR', and 'NOT'. Be careful of 'WHILE' and 'UNTIL'. Avoid 'BECAUSE' and 'SHOULD'
4. You should be able to do a direct translation of the previous step into code. If not, post again, and we should be able to clarify your thinking.

Hi,

I teach PLC's at Lane Community College-after spending several years in industry designing systems. What I learned during those years and always pass on to my classes is a distillation of how I do all my programs. Basically the steps go like this:

1)Define ALL real world Inputs and Outputs-name them and address them right now. Try to also define any timers or counters you may need, although this may come later. Include in the definition what condition is considered "ON" and what is considered "OFF" from the perspective of the PLC.

2)Make a little chart of all Output coils-including any known Tmr/ctr's. There should be three columns-What turns coil ON, what turns coil OFF, and does it need to latch? Be very detailed-include every single Input or other coil that affects this one Output. Try to use statements like "AND", "OR", "NOT".

3)In doing step 2, you will probably have defined several internal "soft" coils that are needed for intermediate logic. Repeat steps 1 and 2 with these.

4)You should have a fairly clear English language version of each rung at this point. Convert to ladder logic.

5)Download and troubleshoot one rung at a time.

I've found that I can get my programs about 80-90% correct at first writing by using this method. Good luck!

Fiora Starchild
Electronics Instructor
Lane Community College
Eugene, OR

Tom Jenkins said:

Well, Dean, we aren't going to do your homework for you. But because you are buying Phil's book we know your heart is in the right place, so I'll give you a couple of hints to get you started:

1) Make a list of all of the inputs and outputs to the PLC, including the channel designation for the PLC I/O. This is called a point list, and should be one of the first things you create for any project.

2) Convert your text description for one of the projects to "pseudo code". That is, create an outline type list in English for each step or function the program should have.

3) Don't try to develop the program in your head completely before you start. Instead, create one rung at a time, for one function at a time. Start with something simple, like having a contact from the run input on your moor turn on a Run pilot light. That way you can get the feel of the language and build your confidence level incrementally as you go along.

Click to expand...

sir,
i have bought the DVD from Mr Phil,
kindly help me so that i can write simple programms to understand the funktions of the mitsubishi fx which i have.
thankyou
God bless you.

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