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What is meant by "plant" in control system?

This above question gives some detail about the term "plant" in context of control systems.

But it mainly mentions "DC motor" as example plant.

But what can be the other common examples of plant?

RC circuit or RLC circuit can also be considered as a plant?

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  • \$\begingroup\$ maybe also an actual plant (biological entity) ... since its growth is being controlled \$\endgroup\$
    – jsotola
    Sep 24 at 16:30
  • \$\begingroup\$ Site rule: This question is likely to be answered with opinions rather than facts and citations. It should be updated so it will lead to fact-based answers. \$\endgroup\$
    – Andy aka
    Sep 24 at 18:12
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    \$\begingroup\$ @Andyaka I don't see how this question would be considered as opinion based. Although the question is not well worded, the answer is that pretty much anything can be a plant, but not all systems can be approximated well by a linear system and used with classical control. The examples can be definitely come from facts and citations too. \$\endgroup\$
    – jDAQ
    Sep 24 at 21:02
  • \$\begingroup\$ @jDAQ read the title of the question please. Read the 4th line of the question please. The question is soliciting opinions. The question is not something that would be considered as opinion based <-- I mean how can it <-- it's soliciting opinions. If you have an answer that circumvents this issue then that's really good. Please post it. \$\endgroup\$
    – Andy aka
    Sep 24 at 21:35
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    \$\begingroup\$ The question is asking for examples that can be obtained from facts and come from proper references, many of which being question here on EESE on "how to control X plant". I could agree it needs more focus, and for that it might be closed, but not that it is opinion based, as in "[a] belief or conclusion held with confidence but not substantiated by positive knowledge or proof". We probably disagree on how opinion-based examples for this question will be, to me they are less opinion-based than most debugging question on this site. \$\endgroup\$
    – jDAQ
    Sep 24 at 22:19

6 Answers 6

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Plant, in the context of a control system, means the thing being controlled towards a goal. The only limit to that is your imagination.

One of the earliest examples of a servo mechanism was the speed control of a steam engine, via spinning weights that controlled a steam valve through centrifugal force.

Another example might be the control of temperature in a vat of reacting chemicals, where the control system flows cold water through submerged pipes to keep it cool.

You might call the attitude control of an aircraft plant, when the angle of an elevator is being adjusted to control it.

A DC motor is a good example as a) it could be used to control just about anything mechanical and b) it is a 'difficult' plant to control. If you adjust the voltage, the current and hence torque will lag behind due to inductance, torque produces acceleration, which gives velocity, and if you want position, then you already have two integrations and a lag in the loop. This is guaranteed to be unstable unless you take active steps to stabilize it by design.

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"Plant" refers to the system that we are trying to control the performance characteristics of. "Plant" could refer to any system from a dc motor to a nuclear power station.

Control engineering is all about forcing a system (or plant) to perform in the way we want it to rather than how it naturally wants to, that is to say how it would perform if we tried to control it in an open loop way.

So by enclosing the system (or plant) in a closed loop control system (and a PID control system would perhaps be the most common example) we can force the system (or plant) to perform in the way we want it to.

For example, by enclosing the system (or plant) in a closed loop control system we could speed up the response of the system (or plant) to a demand at the input. The system (or plant) would reach its required output faster than if it was operated open loop and allowed to perform as it naturally wanted to.

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But what can be the other common examples of plant?

  • Boiler.
  • Engine.
  • Reactor.
  • Refrigeration.
  • Air handling system / temperature / humidity control.
  • Steering system - ship / road vehicle / aircraft.
  • Speed control / cruise control.
  • Autopilot.
  • Position control - balance system (Segway).
  • Industrial process control.
  • Camera auto-focus.

The list is endless.

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    \$\begingroup\$ Most of your examples are control systems, Not plants. For example, What is the plant for a speed control system a plant could be a car. \$\endgroup\$
    – RussellH
    Sep 24 at 17:41
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Anything you want to control. The voltage output of a power supply is one I encounter a lot. Motors are just an easy example because they're easy to conceptualize. It could be the current through an LED, or the brightness of a CCFL. Outside of electrical systems, it could be the supply of oxygen to a blast furnace, or the reaction rate of a nuclear reactor.

Basically, if you can think of something you want to control, you can consider it as the plant in a control systems problem.

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In control systems, the plant is not controlled. Properly, a behavior or action of the plant is what is controlled. Saying that a motor is controlled begs the question, "What about the motor is being controlled?" There are several choices.

  1. Angular position
  2. Angular velocity
  3. Torque

The motor is the plant. The actions are controlled.

For example: The speed of an electric fan is the controlled variable of the plant. The plant has several components

  1. The air that is flowing through the fan
  2. The fan itself
  3. The motor armature
  4. The motor transducer
  5. The motor electric coils
  6. a MOSFET driver that modulates the supply voltage
  7. The PWM circuitry

schematic

simulate this circuit – Schematic created using CircuitLab

The plant can be made of many items that manipulate or transform signals. Be careful not to confuse the plant with the control system. this example the speed of the fan is controlled. The air velocity could be the controlled variable instead.

Some common examples of plants

  1. Heater
  2. Vehicle -Speed and direction control
  3. Light bulb
  4. Jet engine-Exhaust gas velocity and volume control
  5. Airplane

RC circuit or RLC circuit can also be considered as a plant?

Yes it can. But it can also be considered as a feedback control system .

Consider the following:

schematic

simulate this circuit

Figure 1 is an RC lowpass filter. The dashed box encloses the plant. The contains the plant as shown in Figure 2.

The filter can be represented as a closed loop system as shown in figure 2. In this case the input to the plant is \$(V_{C}-V_{i})\$. In both cases R and C represent the plant but in different ways.

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Plant in the context of control systems means something which is being controlled such as part of a factory which has lot of actuators and sensors which typically control something or read data. Usually, a plant is a sub system of a factory or may be the whole factory in a manufacturing environment. Actuators control something like motors, belts and drives, fans, boilers, heaters etc while sensors read something like temperature,speed,pressure,humidity etc. The list of sensors and actuators can be small or large depending on size of the plant. There may be instrumentation equipment which are also part of the plant.

Typically, the plant is monitored and controlled with special software called SCADA which is copyrighted by a company which makes control system software. A typical usecase of SCADA is that a user of the software can monitor all parts of a fully or partially automated plant in periodic intervals and change something at any time. The entire plant can be viewed on a display in a GUI. There are many such software similar to SCADA and they are usually custom made for each plant. Such control system is always on as it is never shut down when the plant is running.

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