Hello

Deke

Member
Join Date
Sep 2002
Posts
15
Hi everyone,

Just wanted to say, that I have been looking for a good forum for PLC's, and I think I have found it! I will soon be bothering all you guru's with lots of questions about AB ML1000 w/Analog PLC coupled to a Microview.

Okay so I will start now!

First question:
I'm going to hook up a linear transducer to the Analog in and I am wondering will I need to do something like IIM or JMP's to have the PLC keep up with the input from the transducer?

TIA
Deke
 
The PLC will sample the analog input once per scan. Check the specs on the analog input for that model to find out the conversion frequency of the ADC. That will tell you if you have fresh data every scan or not. If the ADC can't convert any faster than the PLC scan rate, there is no point to doing anything in ladder logic to try to update any faster.

On the other hand, if the ADC is faster than the PLC scan rate, it becomes a judgement call whether to use mid-scan I/O updates to get the freshest data. The downside to updating the I/O in the middle of the program is that doing so adds to the scan time, so it's easy to reach a point of diminishing returns.
 
There are a couple of 'quirks' regarding the Micrologix platform and 'fast' analog inputs that we recently discovered on a system in which we were trying to use an analog position sensor for high speed programmable limit switching (to replace mechanical limit switches).

Don't enable any more channels than you actually need. The more channels you use, the slower the update for the entire module.

Don't use the built-in software filter! Set the configuration to 'no filter'. It seems that if you have more than one channel configured with a filter, the update time for the whole module becomes very slow.
This was determined by moving the input value to a spare analog output channel and observing the output on an oscilloscope. You could see very large (200-250msec) steps in the output as the position transducer was operated thru its stroke. This was with a dummy test program with no other logic running i.e. simple move of input word to output word with no other rungs at all!

If you need to filter one of the other channels, consider writing your own filter routine in ladder.

I'm not sure about the 1000, but the 1500 doesn't support Immediate Input reads for analog input modules. It does support IIM's for the local inputs. I don't know if this is an issue due to local I/O vs expansion I/O or what.
 
Thanks a lot for your input guys, really appreciated. :) I have made my ladder program so that I'm using jumps from the first couple of rungs to just reading the analog input. Scan time is less then 50 microsec's. Thanks for the filter tips, I am only going to be using just the one channel of input, and I have it set at the default settings of 60Hz on the filter and 8ms.

I have yet to hook it up, because I'm waiting for the Balluff transducer to ship. :(

Will let ya know how goes if your interested.

Thanks again!
Deke :)
 
You can always hook up a potentiometer to simulate the voltage output of your Balluff sensor for testing. If you have +24vdc or +/15vdc supply (which I assume you do since you need something for your Balluff) just add a resistor in series to limit your pot voltage to +5v or +10v.
 

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