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Industrial Hardware
Few types of hardware, I had occasion to puts hands on, while working in steelworks. Some devices like pneumatic valves, current - pressure ITP converters, to servers and various types of PLCs.

ITP - Current to Pressure

It basically converts current signal from PLC like GEM80s, Simens, HPCi, to respective pressure. The pressure can be then applied to pneumatic valves, or other hardware. 
ITP with valves are typically controlled via PID controllers.

Kaytola Flow Meters

These devices measure how much fluid (i.e. oil) flows through them and returns electric signal back to electrical counter devices. Such can be PLCs. Interface linking were typically much more complex and required interfacing through modbus protocols. 
As an example of the use case, are hydrolic pumps, for controlling hydrolic pistons and press.

Pneumatic Valve

This one is quite aged pneumatic valve. It uses air pressure from for example ITP, to control position of the valve. This valve controls the flow of fluids like for example water. 

Older valves are much harder to control with PID, as they introduce mechanical friction and responsiveness of the valve can be less predictable. Depends on condition and the valve type, it may be quite apparent effect of sticky valve, which doesn't react directly to the changing pressure. Instead it may start responding after some pressure builds up, causing it to jump into position. With IPD, it may move to overshoot from the desired position.

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