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CLOSED LOOP CONTROL 63

–0.95 0.95

–0.95 –0.9 0.90.95 ω –1.0 –0.9 0.91.0 ω
sin(2pi*0.9*t) sampled at 1.9 Hz

1

0.5

0

–0.5

–1
0 2 4 6 8 10 12 14 16 18 20
FIGURE 2.14: Beat phenomenon and its explanation in the frequency and time domain.
Notice that the sampling theorem is not violated. The sampling frequency is very close to the
minimum requirement.

2.3 OPEN LOOP CONTROL VERSUS CLOSED
LOOP CONTROL

Open loop control means control decisions are made without making use of any measure-
ment of the actual response of the system. An open loop control decision does not need a
sensor. Closed loop control means control decisions are made based on the measurements
of the actual system response. The actual response is fed back to the controller, and the
control decision is made based on this feedback signal and the desired response. In compar-
ison, we will take the real-world issues into consideration, namely, disturbances, changes
in process dynamics, and sensor noise. These are common real-world problems faced to
different degrees of significance in every control system. Consider a general dynamic pro-
cess nominally modeled by G(s), controlled by an open loop and a closed loop controller
(Figure 2.15).

w(s) ΔG(s)
Disturbance
w(s) ΔG(s) r(s) + D(s) + G(s) y(s)
–
r(s) + y(s)
D(s) G(s)
1.0
+
Sensor
noise
v(s)
(a) (b)
FIGURE 2.15: Open loop versus closed loop control system comparison.