Process control question... To PLC or not to PLC?
I'm looking for a system to log data and control motors/heaters/etc on a small pilot plant setup. I plan on using a series of pressure sensors (4-20 mA), thermocouples/RTDs to log the data. I need to find something (like a PLC, maybe), that can both read and log analogue and digital inputs and also control the devices by sending signals back to the motors, etc. I'd like all the comms to be wireless, and in that respect I am way out of my depth. Anybody had experience with this stuff?
Posted Apr 21, 2010, 11:58 PM UTC
15
Well, I have no actual experience using a PLC (only in classes), but it sounds like you really should use a PLC for this, especially if its in an industrial setting and you already have 4-20mA stuff.
Some Googling found this site: http://www.plcdev.com/cheap_plcs
As for wireless communication. Do you want Controller/PC wireless comm. or Controller/Device's Wireless? Or Both?
Lot's of PLC's I remember seeing communicate over ethernet. Which means you might be able to find a wireless hub and hook up that to the PLC and log data to a PC that way.
tl;dr 1) Find a PLC that logs data, 2) find a way to get it to communicate to a PC, (probably ethernet).
Thanks mate!
You might know the answer to another question I have... At the present, I've only ever seen pressure sensors and thermocouple/RTD units that give a 4-20 mA analogue signal. If I use these I will eat up all my analogue inputs on my PLC very quickly, as they seem to always have a heap of digital I/O, but hardly any analogue I/O. Is it possible to buy a sensor with an inbuilt D/A converter so that I can use the digital ins/outs instead of the analogue ones? Or is there a converter I can buy and put between the sensors and PLC?
I think it would be really hard to find sensors that include a digital bus. Just because different applications will want different bit-widths.
I think you have 2 realistic options: Make up your own ADC, find an ADC, or look into some other controllers that have everything you need.
Heres a thread on making a 4-20mA sensor work with an ADC on a microcontroller.
This is a design assistant to make a controller that fits what you need. I'd try this first, see what you need and how much it will cost. At the very worst it will give you an idea of what things you need to buy.
Here's another similar product that might be something you want to use. (Looks pretty simple to use).
I hope that helps.
Thanks very much!