What is the main characteristic of a linear control system?

The main characteristic of a linear control system is that the output is directly proportional to the input throughout the operating range. This means that for any given change in input, the resulting change in output is consistent and predictable.

Linear control systems are governed by the principle of superposition, meaning that if multiple inputs are applied, the responses to these inputs can be added together to determine the overall output. This linearity ensures that the system behaves predictably, making it easier to model and control, which is a fundamental requirement in control engineering.

In contrast, the other characteristics described in the options point to non-linear behaviors or undesired system traits. For instance, unpredictability beyond certain input levels indicates non-linearity, oscillatory behavior without damping suggests instability, and output constancy despite input changes indicates a lack of responsiveness typical of a non-linear system or a control issue. Thus, the ability of a linear control system to maintain a proportional relationship across its entire range is what defines its fundamental behavior and differentiates it from other types of control systems.