I am working in an industrial environment with lots of PLCs running various machinery. Whereever I am, there is a PLC nearby and most of them have analog input modules with unused terminals - nothing is wired to these terminals.
On the other hand from time to time I have to use a sensor (for tasks unrelated to the purposes of the surrounding PLCs). The sensor outputs 0-10V signal, which I would like to continuosly monitor for a few hours or even a few days at, say, the rate of ~1 sample/sec. It is tempting to piggyback on the existing PLCs for the task of monitoring the sensor rather than installing a new data acquisition setup. So I contemplate hooking the sensor to one of the unused analog inputs, adding some code somewhere, and retrieving the data for plotting into some GUI running on a remote machine in my office (the PLCs are all connected to a LAN, which I have access to).
Is this a reasonable idea? Do I need to modify the code that is resident in PLC CPU or just the SCADA code? Is it likely to interefere with the operation of the PLCs?
The examples of PLCs present at my work place: Omron Sysmac CJ2M, Siemens S7-300.
Just to be clear I do not consider doing any of this clandestinly. I am however completely new to PLC, although I have experience with DAQ systems (such as made by National Instruments).

 

Is there an existing "data acquisition setup" that you're not inclined to use? Why? The owners of the PLCs will ask the same question.

The AI's-on-the PLC approach is technically doable, if the PLC 'owners' are willing to let you alter the PLC program(s). If your PC is outside the 'operational' area that might be security concern.

You'll need some HMI/SCADA software to collect the data and give you a means of looking at it. Is NI software available?

 

why don't you make your own small plc in a box, low cost, which you can take anywhere, and use as you like.
this plc can do some good logic and save some data as well.

 

There's a whole category of data acquisition instruments known as 'data loggers' that are self-contained units with analog inputs, some storage and some means of exporting data, sometimes as a .csv file. Google can find them for you.

 

For a standalone logger, I've had good results from Onset's Hobo units. The ones I used were already in service back in 2010, so they were older models but they worked well and were very easy to set up and get the data off of later. I would have no problem getting one of these (with appropriate accessories):
https://www.onsetcomp.com/products/data-loggers/mx1105

 

This will work fine if all of your PLC's are networked
Create a separate subroutine and data table in the PLC's you want to monitor.
Use an OPC server to scan the PLC's at your desired interval(s) and scale / concentrate the data.

 

why don't you make your own small plc in a box, low cost, which you can take anywhere, and use as you like.
this plc can do some good logic and save some data as well.

With so many lightly used PLCs around, adding another one goes against my understanding of sustainability.

 

There are plenty of data loggers on the market for trouble shooting, PLC's are mainly used for control, inless the data logging feature is part of it menu choices. Drop-in programming would need to be documented and confirmed as not interfering with the control algorithms oftem used for safe operation of the process being controlled.