As mentioned in the previous post LLRF control system, there are two types of control: open-loop and closed-loop.

Open loop

A simply open loop control system is a sprinkler system. I know what I want the system to do. and I can’t monitor what the system is actually dong.


The freature of a feedback loop:

  • Reactive / Error-driven Automatically compensates for disturbances (controller acts on error)
  • Automatically follows change in desired state (set point can change)
  • Can improve undesirable properties of system/plant Can be very simple


Control element responds to change in command or measured disturbance in a pre-defined way.

Based on prediction of plant behavior (requires model)

Can react before error actually occurs:

  • Overcome sluggish dynamics and delays
  • Does not jeopardize stability

Can compensate for known plant dynamics, delays (before you get errors), so may not generalize to other conditions. It will not be accurate if the system changees.


Feedforward and feedback are often used together

 Feedforward component provides rapid response

 Feedback component fills in the rest of the response accurately, compensating for errors in the model Reference link

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