# What is meant by "plant" in control system?

What is meant by "plant" in control system? Does it also means systems like DC motor or it only includes large systems like chemical (fertilizer or cement) plant?

Indeed, as Marko writes, it can be just a DC motor. In general it's "the thing being controlled".

In fact the word "plant" in plain English can mean

• "factory", as the OP points out (Cement factory)

and closer to the field of control theory it can also mean

• "machines", without the factory building

For instance "the heating plant for a home" or "nuclear plant for a submarine"; https://www.dictionary.com/browse/plant

I'll let the linguists battle it out, but it does origin from the word for "shrub", which rooted in soil, with branches, leaves etc.. and if you look at a power plant or heating plant, they look like that too: mounted to the ground, with protruding tubes, boxes etc.... so maybe plant -the machine- came before plant -the factory.

See also the comment by Ross, who found the etymology back to the 18th century: "at first with reference to the machinery, tools, apparatus, etc., later also the building"

There's something that connects the non-mobile machine, installed at a factory, with feed pipes, exhaust pipes etc..., with the rooted, and thus obviously non-mobile, plant.

And according to Andrew (see comments), in British English it's quite common to refer to heavy and mobile machinery like bulldozers, graders, and road rollers as "heavy plant"

• etymonline.com/search?q=plant ... Most extended usages are from the verb, on the notion of "something planted;" such as "construction for an industrial process," 1789, at first with reference to the machinery, tools, apparatus, etc., later also the building; also slang meaning "a spy" (1812). Many of these follow similar developments in the French form of the word. Sep 23, 2020 at 15:41
• And there's also "heavy plant" such as bulldozers, graders, and road rollers, leading the slightly amusing road warning sign "Heavy plant crossing". Sep 23, 2020 at 15:53
• @P2000 sorry, no, I don't have an answer for that. Sep 23, 2020 at 17:04
• Another example -- In HVAC the set of chillers or boilers / heat pumps and pumps, valves, control devices etc. is commonly referred to as a "plant". These devices are generally controlled as a system. Sep 23, 2020 at 18:22
• And most of the devices @UuDdLrLrSs refers to, along with others, are likely to be found in a "plant room". A lift (elevator) may also have its own plant room at the top of the shaft (or on the roof) Sep 23, 2020 at 18:54

It could be anything that has an input and gives an output. In control theory is depicted as a box with a relation Output VS. Input, we call this transfer function. It may have also multiple inputs and/or multiple outputs.

The physical size is not important, so yes your DC motor could also be a plant.