Quick and dirty interface with a PLC

Projects General Guidance
1

Hello from Finland,

We're in the process of building and testing a fairly interesting wood gasifier (well, three actually) all to a pilot project with open ended goals.

Lots of gusto, very little money. There's an Unitronix 1040, which I though to cobble up with an arduino or two as a PC and a data logger for testing purposes while we figure out actual parameters for the final logic (which will just be a VFC and a few timed contactors, data logging will be separate on site).
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Out automation guy is very good, but retired and works only when feels like it, so it falls on me to come up with something not industrial in robustness nor price.

I've got about 14 digital in pins with down 0-5v and up 13-28v available, but I need half a dozen temerature readings from ambient to 1000 C and the same in pressure delta at about 10 kPa. Since this is non critical, I though of aggregating the signals to an arduino, and then serializing them to the PLC. Price cutoff point is well below the cost of an I/O extension. Analog in is expensive as hell.

I'm not sure how to approach this. I would't want to try to implement i2c on ladder logic, and i have RS232 and CANBUS available. Both shields exist for arduino, but then again..

What about just stepping up the TTL signal to 24 volts and then parallelise the data. I'm good with 1 Hz frequency, if that's what it takes. Just have maybe four bits for sensor address, and ten for value. Scan and repeat.

Would you user the serial, canbus, or a bunch of wires from arduino to PLC and make your own very dumb protocol?

Rush and no skill. It's been years since I've been playing with this low level stuff. Got the blinking leds, temp readings and some other basic stuff done as a warmup to arduino, but that's about it.

This is the simple power plant. The others are already pushing the limits of our chosen PLC, and if I can manage to work something out here, I would offload some of the less mission critical sensors from the PLC to one of these nifty microcontrollers.

2

My feeling is that good quality analog circuitry has its price. Finally you are going to design a product for reliable use, not a toy. At least you should know enough about building reliable temperature and pressure input modules, or you risk to kill much more expensive machines or people.

A parallel communication is acceptable with one Arduino, otherwise an (industry grade) serial bus is a much better solution.

3

to get started, I would look at therm-couples as the inputs. there are a few that will work for your higher temperature ranges and there is lots of support for the Arudino.

I would break your project into segments, you probably can get a completely stand alone temperature monitor / logger up and running with all work being mostly done by others. even with a digital readout.

Once that is running, your next step would be to feed it to the PLC.

One step at a time.