disturbances vs variations?in control system?

Question

I am trying to understand feedback control systems from a text I have attached a snapshot and highlighted the confusing points, what is the difference between them? especially what is difference between variations and disturbances in a control system?

Answer 1

especially what is difference between \$\color{red}{\text{variations}}\$ and disturbances in a control system?

• For \$\color{red}{\text{variations}}\$, the quoted text in the question say "plant variations" and that is what I assume i.e. purely the plant (and not the load).
• For disturbances I assume this to mean load variations rather than plant variations.

Does my interpretation make it clearer?

A control system tries to minimize these separate problems.

Plant and load (connected to output) distinctions: -

Picture from here.

Answer 2

1.) e.g. a valve. It gets older, friction rises. but you want it always to move with same speed, open to same position... the controller then detects the deviation from the intended behaviour and changes the output signal, so the deviation gets zero. Or you build the same system 100times but all valves have minimal differences due the manufacturing process. Or due the assembly process... guess a worker is tightening skews to hard, so you have a tension on the housing,... Lets say these variations are constant disturbances (or changing over a very long period of time)

3.) e.g. environment temperature. You want a fluid in a tank to have temperature X no matter if the environment temperature is +50°C in Summer or -25°C in Winter, or 55°C in midday, and 10°C in night. So you use a controller to heat/cool the fluid until it reaches temperature X. This, as we already know is done by detecting the deviation from the intended temperature X and variation of the output, so the deviation gets zero. Disturbances can be quite fast like you have two machines next to each other, when turning on one of the machines it's generated magnetic field influences the other...