Many common sensors require power to generate a signal. These include strain gauges and RTDs. How to provide external voltage or current excitation signals?
Signal conditioning products also provide excitation for some transducers. For example:strain gauges, thermistors and RTDs, require external voltage or current excitation signals.
Transducer excitation provides this power so sensors do not require external power sources.
A good question. To be honest, I struggle with this topic every time I work with a customer. Every site prefers to do it differently.
In short, yes, you will need power on each I/O somehow. Where the power comes from can differ. One thing to look at is what your controls system you are using. I.e. most controls systems provide the power for RTDs...I cant think of any that require external power. So for an RTD, you just need to connect up your 3 (or 4) wires to the Control module/pack, and it should work.
I am not sure about strain gauges. Power may come from the controls as well. If you are looking at vibrations, systems like Mark VIE and Bentley 3500 provide the power.
Digital inputs could require external power. Most PLCs require external wetting, some provide the power themselves.
Digital outputs mostly require external power. Systems like Mark Vie can source the power, but it depends on the induction loading.
If you know what controls systems you are working with, check the manuals. Their I/O modules usually come with some wiring drawings that are needed.