What does a controller react to in control systems?

A controller in control systems is designed to maintain a specific process variable at a desired setpoint by reacting to changes in that variable. In this context, the typical parameters that controllers monitor and respond to include pressure, temperature, and humidity. These variables are critical in various applications, such as HVAC systems or industrial processes, where maintaining specific conditions is essential for efficiency, safety, and product quality.

The reason pressure, temperature, and humidity are the most relevant for a controller's function is that they directly impact the overall performance of the system being controlled. For example, in a heating system, if the temperature decreases below a setpoint, the controller will activate the heating element to bring the temperature back up to the desired level. Similarly, in a pressure control system, if the pressure drops, the controller will respond by adjusting valves or pumps to restore the appropriate pressure.

In comparison, while voltage and current, humidity and chemicals, and air flow and resistance can be monitored in certain systems, they do not encompass the broad applicability and critical role that pressure, temperature, and humidity play in control system dynamics.