Continuing last month’s fundamentals-based theme, I’d like to address the devices that make closed-loop control work. As discussed, control loop algorithms are implemented by software within a BAS controller. However, the input and output signals associated with the loops come from the sensors and and go to the actuators that are mounted on/near the controlled equipment. These are the most important parts of what are typically referred to as the “field devices” portion of a BAS.
Sensors provide the “controlled variable” signal input to the algorithm. These devices always provide a varying signal (i.e., they are not on/off devices) and are mostly for temperature or pressure measurement. Airflow and CO2 sensors are becoming more prevalent, and humidity, level, etc., are also used in some applications.