Open loop control vs. closed loop (feedback) control


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The next two figures present the structures of open loop and feedback control systems.
Figure 1 shows an open loop control system whose function is to cause the output y(t) to follow the
reference r(t).
Feedback control system design for zero steady state error
In general, to follow a reference R(s) with zero steady-state error, the path between the
reference and the system output should contain a term like R(s). For instance, to follow a ramp
(velocity) reference R(s) =1/ s2 one requires at least two integrators in the path from the reference to
the output. That is, for zero steady-state error in response to a given reference, the control system
should contain a model of the desired reference trajectory.
Also, to reject the steady state component of a disturbance of nonzero mean the path between
the reference and the disturbance input should contain a term like D(s).
These two rules express the internal model principle.

The advantages of feedback control

- a well designed controller can cancel the steady state error such that the output of the system
will follow, after the transient response is finished, the desired specified output i.e. the
reference, even in the presence of uncertainties relative to the model of the system or the
appearance of exogenous disturbances.