Modern Control System Theory and Design, 2nd Edition

1.2.  OPEN-LOOP CONTROL SYSTEMS

Open-loop control systems represent the simplest form of controlling devices. Their concept and functioning are illustrated by several simple examples in this section.

Figure 1.1 illustrates a simple tank-level control system. We wish to hold the tank level h within reasonable acceptable limites even though the outlet flow through valve V₁ is varied. This can be achieved by irregular manual adjustment of the inlet flow rate by valve V₂. This system is not a precision system, as it does not have the capability of accurately measuring the output flow rate through valve V₁, the input flow rate through valve V₂, or the tank level. Figure 1.2 shows the simple relationship that exists in this system between the input (the desired tank level) and the output (the actual tank level). This signal-flow representation of the physical system is called a block diagram. Arrows are used to show the input entering and the output leaving the control system. This control system does not have any feedback comparison, and the term open loop is used to describe this absence.

The angular position of a missile launcher being controlled from a remote source is illustrated in Figure 1.3. Commands from a potentiometer located at a remote location activate the positioning of the missile launcher. The control signal is amplified and drives a motor which is geared to the launcher. The block diagram corresponding to Figure 1.3 is illustrated in Figure 1.4. The input is the desired ...