I am at a lose

I have a slight problem whie doing a programming on RSLogix500. I have an input that comes from a selector switch, "auto" or "manual", to the PLC.
I have 2 outputs from the PLC, wired to 2 relays, one of them outputs to a motorised speed adjustment (for auto) and another to a hand-tuned potentiometer (for manual). This speed adjustment is to regulate the speed of the running engine.

The problem now is that when my engine is running, I need to turn it to the other selection, no matter if my selection now is Auto or Manual. When I changed the selection, it will experience a momentum of an open circuit, thus putting an emergency stop the the engine, and I will have an over-speed fault. So I need a timer to hold the relay that was energised first, as well as to energise the selected relay. This way I will have the effect of a "make-before-break", and thus not endangering my running engine.

I added the follow ladder to my program. I tried it the previous time with latches, but I can't get the result I need. Someone have a look at the following and help me comment, please?
I have been fixing many glitches in this engine control panel since morning 10am. I'm still working now at 9pm and I'm beat. x_x

 I:4/8     T4:49/DN      B3:4/1

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

 B3:4/1                  T4:48 TOF

---||----------------------[ ] 50 M.SEC

 T4:48/DN  T4:49/DN      O:7/3

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

 T4:49/DN  T4:48/DN      O:8/14

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

 I:4/8                   B3:4/2

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

 T4:48/DN |

|---||----|

 B3:4/2                  T4:49 TOF

---||----------------------[ ] 50 M.SEC

Thank you very much!
regards
ST

Here's a quick stab at it, some else may have a better solution.
I have not tested, or tried it. However, it may get you pointed in the right direction.

You will have to change the addresses to fit your application. I used these just to show the logic.

Ken

I need time to ingest this. Please tolerate my questioning.
I wanna ask: If my speed adjust is on Manual now, and I changed it to Auto, it will have a "make-before-break" contact effect, and then my Auto relay will be energised, and Manual relay will de-energised in 50 m.sec.
So with that logic you mentioned, it does not need using memory bits?

Thank you.

regards
ST

Like I mentioned earlier, I have not tested my logic, but I believe it will work.

When the desired state is switched, the timer timing bit will hold the last state until the timer stops. The timer logic needs to be placed in the order I have it, timers before the outputs.
You will have to determine the correct timer values. I do not believe any internal bits are required.


Ken

Ken

Thank you! Over the night, I have come up with a few variations. I'll try them all out later today, and see the results. hehehe...

Thanks again!

regards
ST

up thread again

I just wanna bring up this thread again. I am thankful for the tips I learned from this forum, and am glad to say my function acceptance test on the Engine Control Panel has been a success today!
I can look forward to cleaning up the mess I created and shipping this project to the off-shore platform.

regards
ST

My suggestion is the following. It will give one scan-cycle overlap.


Auto
On = Auto
I:4/8
---||-------------|----------(O:7/3) Auto-Mode
O:7/3..... O:7/4
|---||------|/|---|




Manual
Off = Manual
I:4/8
---|/|----------|-----------(O:7/4) Manual-Output
O:7/4.....O:7/3
|---||----|/|---|

Hi Ken,
How do you copy the ladder and paste on the forum.
Thanks

See this thread: http://www.plctalk.net/qanda/showthread.php?s=&threadid=2062&highlight=paste

follow up

Follow up:
I loaded the example logic I posted earlier into a micrologic and the make before break function works fine. However, at processor start up, assuming the auto desired switch is false, you will get both relay outputs until the manual timer times out.
You need to add an initialization routine that prevents that.

Ken
Yes. The make before break function is what I need to prevent the engine from over speeding. I've set the timing to 30ms and now the engine doesn't burn no more.
I got my panel wraped up yesterday and the movers will get them loaded up by monday.

regards
ST