Stuck

Need help with a problem, designing a PLC ladder program, here it goes. A temperature control system consists of four thermostats controlling three heating units. The thermostat contacts are set to close at 50, 60, 70, & 80 degrees Fahrenheit, respectively. The PLC ladder logic program is to be designed so that, at a temperature below 50 dgrs F, 3 heaters are to be ON. Between 50 dgrs to 60 dgrsF, two heaters are to be ON. For 60 to 70 dgrsF, one heater is to be ON. Above 80 dgrs F, there is safety shut-off for all three heaters in case one stays on due to malfunction. A master switch is to be used to turn the system ON and OFF. Prepare a typical PLC program for this control progarm.

Thanks, I appreciate it!
Maybe this will help me understand what I'm reading.

Please refer to the responses to jenny's thread.

You've defined a system with five inputs and four outputs. The inputs are the four temperature levels and the master switch. The outputs are the three heaters and the safety shutoff. You've also defined four output conditions, one heater on, two heaters on, three heaters on, master shutoff actuated.

Your task is to match the state of the inputs to the output conditions.

Hint: Below 50 degrees = all inputs off

Watch out for:
When you say "one heater on", does it matter which one? When you say "two heaters on", does it matter which two?

Extra credit:
How many possible combinations of the four inputs exist? Which of those combinations represent impossible physical conditions? What should your system do if it detects one of those impossible conditions?

When you say "Four thermostats" I assume you have inputs assigned to these on your input table. I am also assuming that you do not care what heater/s come on when the conditions are met. Try this: you need to wire your emergency shut off thermostat directly to a control relay that is capable of supplying power to all three heater contactors or you can hook that thermostat up to a relay that controls three separate (motor) contactors. Then in your logic program you need to install some logic that will sound an alarm or somehow alert the operators that the heaters have been disabled due to a preset condition. Simply taking a sample from the thermostat or from the relay supplying the motor contactors can do this. I urge you to use a motor contactor because of the safety features built into that type of contactor vs. a simple high power contactor. Then from here on it is simple all you have to do is take your thermostat inputs and either run subroutines or build ladder logic in one file to perform your heater conditions. i.e. Thermostat 1 <= 50 deg: Thermostat 2 <= 60 deg Thermostat 3 <= 70 deg and Thermostat 4 <=80 deg. Now condition 1 the temperature is less than 50 degrees and all the thermostats contacts are open. So we need to start all the heaters. Condition 2 between 50 and 60 degrees thermostat 1 contacts will be closed so we will have a 1 at that thermostat’s input. So now we need to write logic that tells two heaters to turn on. And so on.

PLC for Dummies

Hello good people,

Is there such a thing as PLC study guide for dummies. I'm taking a course @ M.C.C. & I am having trouble with understanding the basic concept of PLC... ladder diagrams, how to prepare typical PLC programs, ...etc.!

Thanks for all your help!

Phil has a book and a study guide that teaches the basics of PLC's http://www.plcs.net/contents.shtml

NOW my little contribution to understanding the basics...PLC's were created to replace RELAY logic. Relays were electrical devices that were turned on or off depending on conditions (ie inputs).

A standard relay logic schematic:

       Stop PB       Start PB

                       __|__

1-------0__|__0-------0     0------------------(relay1)----2

                  |            |

                  |---|     |--|

                   relay1 auxillary contact

.
Note: Number 1 is power and Number 2 is common connection for power..ok? Lets assume conventional flow for power..ie from positive (1) to negative (2) in this case. Stop is a normally closed pushbutton so power flows thru it to the Start pushbutton(a normally OPEN pushbutton) and one side of Relay 1 auxillary contact. When you PUSH on the Start PB then power flows to the relay which closes/energizes it...which in turn "latches/holds" relay1 closed via the auxillary contacts.

With a PLC things are a little different because they offer versatility but for the basics they follow the same format.

-----|input1|--------------------(relay1)

  |           |

  |--|relay1|-|

.
In this case all you need is power on or off from an input, depending on if you use XIO or XIC to turn on and latch the relay. Its all done in software.

Others will offer more and better assistance but hopefully this will assist you in the basics. Post more if you need more.

NOTE: I noticed the fix isnt in on problem with text going off screen after posting ladder..ie still have to add extra character.

???

Hello,
I'm still in the fog, I was wondering if someone can draw this out so I can see what I don't understand.

Thanks for all the input!

One of the most important aspects of any endevor is to become knowlegable enough to "Ask the right questions".

This can come only after a particular someone puts in some time to figure out what reasonable questions are!

Ya gotta put some effort in!

YOU have to read and try. Then YOU have to tell us what you don't understand!

Otherwise, I'll be forced to explain "Why we are here!" Then, I'll have to kill you... it's a secret, don't you know?


Basically, I think you are a wimp, a whiner, a baby that wants the world handed to him.

I challenge you to get serious and put some effort in. If you can't steer your way clear to do so, then you might consider doing wrist-exercises, as in flippin' burgers! "Do ya want fries with that M'am?"

I'm worried someone is trying to teach you PLCs without you having any knowledge of ladder logic or electrical schematics. If you understand these you can easily pick up just about any book on PLCs and have it make enough sense to struggle through. If you don't have the background, you shouldn't be learning to program them yet. I'm worried that PLC programming is being taught to non-engineers/electricians in an effort to make them job ready, but without an electrical education they won't understand the basic concepts enough to keep a job they do get. Let me know your background and I can help out a little more.

Vetteboy

You state that you are stuck. I believe that you can not be stuck because you have to start something before you get stuck at it. You, obviously, have not started--i.e. you have exerted absolutely no effort, except to ask for a solution.