In the previous section we talked about systems which are constructed to accomplish tasks.
However, not always does a system behave like we expect it to do; the actual output is not the desired output (the steady-response is not what we want it to be). Reasons for this can be intrinsic system mechanics, disturbances, or computation delays. Even in systems which give the desired steady-state output, the transient response of the system might not be as fast as we would like it to be. And in some cases, the system might not be stable at all. Given these different scenarios (individually or a combination of them), we are inspired to design control systems and augment our systems with them. The control system will make sure the system performs what it is expected of it.
In general, there are two types of control systems: open-loop and closed-loop.
Open-loop control systems are very simple. Given a reference input, they only command a certain “non-changing” control input to the system and do not make any corrections if the system experiences any disturbances.
Closed-loop control systems use feedback to make sure the system is performing according to specifications. A closed-loop control system, senses the actual output, and through feedback, compares it to the reference input (desired output). According to the difference, which is called the error, the control system commands a corresponding control input to the system, in order to decrease the error. The control system repeats this process in real-time until it drives the error to zero.
In the next section we will dissect a system along with its closed-loop control system, to examine their different components.
