Logic Problem

Haven't you ever heard of an "IF/AND" statement or "IF/THEN" statement. This is really pretty easy and you should be able to solve it with simple Boolean operators.

I understand to use those statments but i cant get my head around it.

For example how am i going to shuffle the order of the ac's whilst saying that if temp is x amount then turn on ac1 when ac's1 order of operation will change weekly.

Also having the ac turn on 1 @ 35, 1,2 @37.5, 1,2,3 @ 40 is also confusing me, if all i had to do was turn on all 3 at a given temp would be much easier.

would it work something like this,

If temp is 35-37 turn ac 1 on end

If temp is 37.5-39.5 turn ac 1,2 on end

If temp is 39.5-40 turn ac 1,2,3 on end

Thats how im thinkin but dont thinks that will operate properly.

Use a thermostat and a sequence control panel. It is simple. Or three individual thermostats with sequence panel.

I would of or even a ddc but we were requested to use plc, i believe the decision had something to do with communication between units but that is not a concern of yet.

What software?

Allen bradley picosoft pro.

You are describing a classic "triplex alternator" control system. People like Time Mark make dedicated triplex controllers for pump systems, there is no reason why it could not work for AC units. The only trick is, they will alternate every cycle, not every week. I don't know why you would not want to allow it to operate that way though, it's the most reliable way to maintain even wear and tear on the equipment.

Hmmm, what you says make sense because the whole reason for the weekly cycle is to even out wear and tear. But its too late they have requested plc.

Write sub routine assigning the different ac to a variable. Changing them over a three week rotation. You may have to write the clock to keep time if the PLC does not have one or acquire software for one. The assign the variable to the temperature you wish them to come on.

Thanks this helps me out a bit. Can you explaine more?

Make state equation and use J-K flipflop

The pattern will repeat after every change of tempeature

This would be handy but the pattern has to repeat ever week, cheers for the suggestion.

First draw a flow chart for the events to happen the right way.

PLC capable of various configurations I/O including clock function for weekly change over are available from the manufacturers, you should be able to write a program in high level computer language to make the PLC device function appropriately.

If you are not in depth familiar with PLC, best advise to you is to send your requirements to any PLC system vendor, they will do it for a charge.

Hi,

i was in the middle of wirting the flow chart and this was the part doing my head in.

I have a bit of experience with plc obviously not enough but i dont have a choice and have to complete the project.

Simple enough: set up three internal relays for temp 1 = bit 1; temp 2 =bit 2; temp 3 = bit3.

then use week 1 "and" temp1 to ac 1 "or"

week 2 "and" temp 2 to ac1 "or"

week 3 "and" temp 3 to ac 1..

new rung:

week 1 "and" temp 2 to ac 2"or"

week 2 "and" temp3 to ac 2 "or"

week 3 "and" temp 1 to ac2.

new rung:

week 1 "and" temp 3 to ac 3 "or"

week 2 "and" temp 1 to ac 3 "or"

week 3 "and" temp 2 to ac 3.

done.

Hi thanks for the reply i am sought of following.

By reading what u have written i gaver that everyweek the ac swap at which temp to operate i require that....temp1 turns on one unit

temp2 turns on two units

temp 3 turns on Three units

Thus would wat u propose look more like this?

week 1 "and" temp1 to ac 1 "or"

week 2 "and" temp 2 to ac1 , ac2 "or"

week 3 "and" temp 3 to ac 1, ac2, ac3

new rung

week 1 "and" temp 2 to ac1, ac 2"or"

week 2 "and" temp3 to ac 2, ac3, ac1 "or"

week 3 "and" temp 1 to ac2.

sorry if i am clueless.

Dear sir

I have done similar assignment for Remote telephone exchanges with 2 nos ACs to work alternatively every 24 hours.

For you similar logic can be developed.

Please send your email id and 2 days time to send you the ladder diagram for any PLC.

Ladder diagram can be modified for your dedicated PLC.

regards

ramesh

Wong,

I would program the PLC as if it were two separate problems.

The 1st Program deals with temp and how many units to turn on and when (use m bits instead of outputs at this point).

2nd Program: Rotate the units to be turned on and in which order. If the same order over and over again is OK (i.e., 123, 231, 312), that will make it easier. Give each set a recipe name (PlanA, PlanB, PlanC). (this makes it easier for humans to understand.)

Now we select which "Plan" we want, based on which one was selected for each of the last two times (use holding registers and compare). Use the m bits set for the temp to turn on outputs for the AC units.

Sorry, my brain is VERY simple, but I am really pretty good at PLC programming. I just make it simple!

Cheers for the response, i am beginning to understand.

This sequencing is simple to do in ladder logic, but it will take several lines of code since you basically must build the "truth table" into your code. I used a timer preset to 168 hours (24x7) assuming that you have a low-end pico PLC which has no internal clock (though many pico PLCs do have clock/calendar capability).

This image defines all of the variables and presets that you stated:

This image shows the ladder logic for AC1:

All you have to do is move the "greater-than / less-than" instructions around to satisfy the conditions for AC2 and AC3. (Yes, you can use XIO instructions instead of less thans in the Unlatch rung.)

You should also think about what will happen the moment that a week ends. You may want to write some extra logic to keep AC1 and AC2 from switching jobs instantaneously when the presets change. So if AC1 is already running then let AC1's cycle finish before enabling AC2.

Oops - the counter should be preset to 4.

Also need to add "NOT" instructions to make only one of 3 conditions true at one time.

Thanks mate i can see this being i big help.

Hope it helps.

BTW: You did not specify how you wanted to write your logic. With a 1760-AWA you can display your code in 3 different formats. I use the ANSI/CSA format.

lol i dont even know what that means.

all i know is that it will be ladder logic.

The 3 standard forms of ladder logic that you can pick from when writing code for an Allen Bradley PicoPLC are:

1) Device Display - ladder logic that does not use graphical symbols that look like contacts and coils used for electrical wiring diagrams. Instead simple symbols like brackets and absolute value bars are used.

2) DIN IEC Format - ladder logic that with rungs running from top to bottom instead of left to right, so inputs are at the top and outputs are at the bottom.

3) ANSI/CSA Format - ladder logic uses graphical symbols that look like contacts and coils used for electrical wiring diagrams.

Step 1)

Set up "Counter" to increment by one every 7 days type integer range 1 to 3

Counter resets to 1 when incrementing from 3

Counter output = N

Step 2)

if temp > temp1 then turn on A/C N

If temp > temp2 then turn on A/C N+1

If temp > temp3 then turn on A/C N+2