LVDT reading on older vs newer PLC

[bill4807]

A recent post i just read about an LVDT triggered this question.

I've put these small LVDT's on cylinders to measure part length. They too had about 1.5" stroke/readable distance. I placed them on small 2" cylinders.

My part lengths varied within 1 to 35 mm.

I am curious how to calculate the actual resolution of the LVDT.
I just scaled it according to what i needed to complete my own readings.

But i had to do this on different machines some with newer PLC s some with older PLCs. Greater than 15 years old.
They are not allen bradley, siemens or mitsubishi. They are a machine control internal PLC.
But using different PLCs i did notice numbering feedback from the sensor before scaling were different.
For instance one plc went 0 - 32767 once it hit that it rolled over to a negative 32767 and counted up and back and forth.
Another plc went 0 - in the millions. (0 - 21,333,333) example.
I have them both set up to read/register the same number of bytes. (4) for this analog voltage input. 0-10v
Does this mean each version of plc read in a different format? I dont even know how to acurratly phrase the question i want to ask in programming terms.

Can anyone explain this to me?
Thanks,

 

[parky]

Many different PLC's (and Analogue Cards) use different resolutions for example Siemens S5 used the least significant bits for diagnostic bits like wire break etc. so the data was 4000-20000 for 4-20ma but in reality it was 0-2048 12 bit resolution. So it would be best to look at the specs on the cards. yes in S5 you could use the whole word but if one of the diagnostic bits was on it would change the value although only small. I'm a little sceptical about the millions value why would you want that type of resolution. most are 12-16 bit resolution and don't forget that for -10 to +10v will go negative as the MSB is usually the sign bit.

 

[bill4807]

Thanks for the reply.

I was mistaken about the setup. I did not mention the diagnostic bits.
I used 3 input bytes and one output byte.
This is how it tells me to set it up in the PLC programming manual.
All 4 of these machines use the same analog card, the same configuration and the same LVDT.
The only difference is the older or newer processor types.

So maybe each processor type processes using a different integer size?

 

[parky]

Maybe if you tell us what PLC's they are and the card types some others who have used them may be able to give you the answers.