Rotating Tabletop

Hello all,

First let me state that I am fairly new to PLCs.

I am attempting to create a rotating tabletop, which will manuver four cups under a liquid dispencer one at a time. Once the cup is located under the liquid dispencer, a combination of two to four liquids will be poured into the cup. The liquid combination will be determined by user controlled input switches.

I am using a 24V DC motor (yes, I know I probably should have used a servo) to rotate the tabletop and four magnets and a hall-effect sensor to determine the position of the cups.

I would like to automate this process using a PLC (AB SLC-5/02 using RSLogix).

I want the tabletop to spin ninety degrees after the liquids have been poured and after a ten second delay to minimize drippage. I tried using the Done Bit of a TON timer to latch the motor and the hall-effect sensor to unlatch it, but had no luck.

I would really like to use subroutines for each liquid combination, but I have had a hard time latching a JSR with the output of a timer.

Sorry if I'm rambling. Any suggestions with the ladder logic, especially the subroutines, would be greatly appreciated.


Thanks,
Zucker

You should post your ladder logic so people can see what might be going wrong. From your description of what you did, my guess is that you need a time delay to allow the magnet to clear the hall effect switch. I'm guessing that both the TON's DN bit that latches the motor on and the hall effect switch that unlatches it are true at the same time.

Hi Steve,

You are right, I forgot to mention that I used a two second time delay to allow the magnet to separate from the hall-effect sensor. It works fine for one liquid combination.

I really need help with subroutines. I cannot get my timers in my subroutines to start timing.

Thanks,
Zucker

Zucker,

I'm not completely clear on what you're doing but I can give you some tips on subroutines. In general I would not recommend making your JSR's conditional, i.e. having them switch on and off based on the state of other logic on the rung. There are times where this is necessary but it can often cause problems with output coils and timers in the subroutine. I usually put JSR's on a rung without any other logic, so that the subroutine runs every scan; this is functionally identical to just putting all the ladder in the main program (but a little more organized and easier to troubleshoot).
If you're looking to change the liquid combinations, this could be an application for recipes and you may be able to use indirect addressing or something similar.

-John

zucker said:

I tried using the Done Bit of a TON timer to latch the motor and the hall-effect sensor to unlatch it, but had no luck.

Click to expand...

Did you try driving the OTU and OTL with one-shots? If you use a bit to hold a latch, as long as it's on the other state will be locked out.