Open-Loop Systems

A generic open-loop system is shown in Figure 1.6(a). It starts with a subsystem
called an input transducer, which converts the form of the input to that used by the
controller. The controller drives a process or a plant. The input is sometimes called
the reference, while the output can be called the controlled variable. Other signals,
such as disturbances, are shown added to the controller and process outputs via
summing junctions, which yield the algebraic sum of their input signals using
associated signs. For example, the plant can be a furnace or air conditioning system,
where the output variable is temperature. The controller in a heating system consists
of fuel valves and the electrical system that operates the valves.
The distinguishing characteristic of an open-loop system is that it cannot
compensate for any disturbances that add to the controller's driving signal (Disturbance
1 in Figure 1.6(a)). For example, if the controller is an electronic amplifier and
Disturbance 1 is noise, then any additive amplifier noise at the first summing
junction will also drive the process, corrupting the output with the effect of the
noise. The output of an open-loop system is corrupted not only by signals that add to
the controller's commands but also by disturbances at the output (Disturbance 2 in
Figure 1.6(a)). The system cannot correct for these disturbances, either.
Open-loop systems, then, do not correct for disturbances and are simply
commanded by the input. For example, toasters are open-loop systems, as anyone
with burnt toast can attest. The controlled variable (output) of a toaster is the color
of the toast. The device is designed with the assumption that the toast will be darker
the longer it is subjected to heat. The toaster does not measure the color of the toast;
it does not correct for the fact that the toast is rye, white, or sourdough, nor does it
correct for the fact that toast comes in different thicknesses.
Other examples of open-loop systems are mechanical systems consisting of a
mass, spring, and damper with a constant force positioning the mass. The greater the
force, the greater the displacement. Again, the system position will change with a
disturbance, such as an additional force, and the system will not detect or correct for
the disturbance. Or assume that you calculate the amount of time you need to study

for an examination that covers three chapters in order to get an A. If the professor
adds a fourth chapter—a disturbance—you are an open-loop system if you do not
detect the disturbance and add study time to that previously calculated. The result of
this oversight would be a lower grade than you expected.