PLC external timer

Hello - I am retrofitting a WWII era bottle filler with a PLC. I'm wondering how to easily use an EXTERNAL timer to specify fill time. That is, I cant hard-code a timer value - it needs to be user input. Something very simple, like a pot with an A/D converter. I know that there are timers for PLC use for industrial applications, but these seem overly complex and expensive. Any help would be appreciated.

-Alex B.

alehander,

You said that you had a PLC (Programmable Logic Controller). If you have a PLC, then for God's sake, man, set up your timer as an INTERNAL timer in the PLC. That is one function that PLC's do very well. Now, if your problem is that you need to be able to change the Timer Preset Value from outside the PLC program (the number it is timing up to), then there are serveral input methods. The simplest, cheapest way to input a number is probably with a BCD-coded thumbwheel switch. I am looking at a 10-position BCD switch in the Newark Electronics catalog for $12 dollars US. You can buy thumbwheel switches with 1, 2, 3, 4, or more digits. Each advance of the thumbwheel moves the switch 1 number up. The output, however, will be a binary-coded decimal number sent as a group of 1 or 0 inputs to your PLC. To have a range 0 - 32, you need 5 inputs. Six inputs will give you 0 - 64, and so on. Then inside your PLC logic, you set up rungs to translate the input back to a decimal number (or Octal if your PLC requires that). Then MOVE this number into your internal PLC timer Preset Value. That's it, you are done!

Alex,
There are alot of ways to acomplish your goal a lot depends on the PLC you have chosen. Currently text displays are very reasonable take a look at AutomationDirect.com you could purchase an inexpensive text display and have access to other parameters or other timers setpoints. Another option is a bcd thumbwheel which you can attach to discrete inputs this would depend on the range you want to set and the amount of discretes you can spare. A 5kohm pot with a 1kohm pot in series attached to 24Vdc can be used to generate a 4-20mA (actually 4-24mA) signal to an analog input card. It all depends on how much or little you want to spend.

...Why not use just a simple electronic timer (about 15 of our sterling pounds!). Use 1 O/P to initiate the Timer 'coil', and 1 I/P to acknowledge the Timer done.

Most Timers now come with universal supply voltage and almost limetless range settings.

Hope this helps.

Hi

If you feel that you MUST use an external timer, check out these timers from Red-Lion

http://www.redlion-controls.com/products/timers_control.html


We use the model "LIBT", (on a machine that is not PLC controlled)as it is easy for the operators to set the time period, and it is rated at IP65.

As mentioned above, an output from the PLC could initate the timing cycle, and the the timer could give an input back to the PLC when it is done.

(A PLC timer would be cheaper )

I cant hard-code a timer value - it needs to be user input.

Click to expand...

What kind of range does the user input have?
If you've got 1 extra input, and 4 extra outputs you can toggle through an array of times.
eg. 10, 15, 20, 25. then use the outputs to display the timer value with pilot lights.
Of course if you need fine adjustment then the Thumbwheel is best.
I'd keep the timing in the PLC.
Good Luck!

Yet another option...

How about taking your potentiometer directly into the PLC via a 0-10VDC analog input and using its value as the preset for and INTERNAL timer?

I use this technique all the time and the only additional cost I can think of (assuming a 0-10 VDC analog input is already available on your PLC) is possibly the cost of a 10VDC source. My guess is that your old machine probably already has a source like this.

Steve

time is running out

if plc has analog input well use it with a potmeter.
better to use coarse inside plc and fine with potmeter.

Another way is to use two inputs one for up and one for down. a counter in the plc to remember the time
this way you can adjust the timer internal with just two inputs.
i hope you are not german as it is ww2 machine.

Another way.....
Use only two discrete inputs! One for increase and one for decrease time. Setup a timer, use the inputs to 'add' or subtract' a small incremental number. (ex. push the increase button, add .05 seconds to a register used as the timer preset)
Just another thought...

I asked the same question ages ago but the answers here are a better response. In the mean time, I used a Zelio smart relay (not a real PLC but a vey simple version of one). You just write the ladder on your PC with easy free software, set a password and load it to zelio. The timers can be viewed on the lcd display and altered with mini keys beside lcd. If some timers need protection from operator's fingers, you can select p'word protection for that timer before loading. Get one from Schnieder.

Thanks for the replies!!!

Hello everone, and thank you for all the interesting ideas - I could not have imagined there were this many solutions. Well, after considering all you suggestions I'd have to say that the A/D idea is out - I simply cant spare that many inputs. This approch also gives very coarse adjustment and limited range. The "kitchen" timer, however, seems to be just fine - 1 output to start timer, 1 input to receive "end of timer". The increment/decrement idea also seems to work well - 1 input to increase the (internal) timer, 1 to decrease. Here, however, I'd have to use several outputs to display the internal timer value itself for the user... seems less efficient. I haven't bought the PLC yet, though I'm thinking a MicroLogix 1000 16- or 32- model. Only the more expensive ones have analog input - that would be by far the simplest timer input scheme. If you have any more ideas or see any flaws with my thinking here, please let me know. Thanks again, -Alex

If you havent bought the PLC and are considering a Micrologix take a look at the Automation Direct DL06 it is much less expensive and it has an LCD display that attaches to the PLC for around $60 which will alow an operator to adjust time set points.

History:
In 1980 Simatic 110A timers was external timer-units where ware 4 or 8 time-relay inside. F.e. Ouput 4.0 energizise relay and equal Input 4.0 told time have done. So external timer is common solution as you need.

Hello again... the $60 LCD seems overkill and I'm afraid it'll fry in our "industrial" conditions. I've found a few cheap timers, though - simple and bombproof. Now, for the PLC - I'm wondering about the development software - does any brand have freeware for developing ladder diagrams and programming the PLC? Thanks again, -Alex

P.S. I'm posting the general PLC question as a new thread.