PLC timers

What PLC are you using? I would start a retentive timer when the first pushbutton is pressed and stop it when the second on was pressed
 
I have never programmed a PLC before. I wanted to make sure it was capable before buying. I think the Allen-Bradley Micro 830 is the one for the job. How accurately can a retentive timer time the difference between the two?
 
It depends what you consider "accurate". Basically, you have to consider scan time. If the ladder that include the "start timer" instruction is near the top of the scan and the button is pressed while the bottom of the program is scanned, the program will not detect the input until the next time the inputs are scanned. This may be only a millisecond or two later - it depends on your PLC type, the number & type of instructions in the program, etc. Same thing for detecting the second pushbutton.

Without familiarity with the 830 PLC, I can only assume that their timers are as good as the micrologix, which are generally sufficienct for most tasks.

If the 830 has a "FAST" input function, you can try using it. Some of the Micrologix do. Scan time will be a thing of the past.
 
The site says it has a 100 kHz speed HSC available for a 24V DC input. I just need it to scan the inputs every mS or so. Is there a way to store these times then communicate them with another device every 20s?
 
It would be easy to read the values with an HMI every 20 seconds. You could also set up a spreadsheet or database that's updated with values from the PLC every 20 seconds.
 
The high-speed counter is a special hardware circuit embedded in the controller that performs features like encoder pulse counting or frequency measurement. You wouldn't use it to measure the time between two ordinary inputs.

What exactly are the physical inputs to the controller, and what is the time resolution you are looking for ?

Are you a student at Texas A&M (I noticed you're in College Station) or is this a commercial application ?
 
rguimond:
Thank you very much that sounds like what I need. Now I need to figure out how to do it.

Ken Roach:
I am a student working for TTI (Texas Transportation Institute). Installed in the roads are induced coils which emit a voltage when a car passes over them. There are usualy two of them next to each other. By finding the time it takes a car to pass over both coils we can find the speed of the car. There are many cars that pass over these coils so we would need to store these values (rguimond said I could do this in a spreadsheet). The PLC will be wired to the Txdot servers to update the information every 20 seconds.
 
Neat project ! I always figured there were purpose-built controllers for this sort of thing, but it's certainly do-able with a logic controller.

How much precision are you looking for ?

By my back-of-the-napkin calculations, 3 milliseconds is the difference between 60.0 and 60.1 MPH.

Getting down into the millisecond range typically requires you to use interrupts in a PLC to get consistency.

But if all you needed was 1 MPH resolution, you would have 30 milliseconds difference. That's well within the scan rate of any small PLC.
 
Txdot does use purpose-built controllers for this task, however, they do not buy enough of them and they are worried that the manufacturer is going to stop producing them so they asked us to develop an independent one.

We only need resolution down to the MPH.

Where do you recommend I begin?
 
in reality this is totally impossible with a PLC do to the nature it works with its in/outputs. a normal unit has a scantime of 10 msec. way too much for this detection.
You will have to use an analog method. like a 555 (yes from TI)
the speed is both rising edges
the length is the rising and lowering pulse of the second one.
 
If you need the most precision then consider using Beckhoff with their XFC technology, using that each ultra-fast IO card stores an array of status along with a timestamp, therefore you can use your PLC to know the most exact time between changes without worrying about the timestamp.

Advantages of using PC based controls ;)
 
Ok, I am getting mixed signals here.

Shooter, we already have a mechanism to tell us (digitally) when a car is over the inducer. I just need to figure out a way to find the difference (in time) from the two coils.

Joan, this sounds like it would work perfectly, but it might be a bit overkill.

I need a PLC that will scan about every 1mS-3mS and a function that will let me find the difference in time between two inputs turning on.
 

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