What is not a basic part of a closed control loop?

In a closed control loop, the primary components include a controller, a controlled device (also known as the process), and a variable element, which typically refers to the measurement used to assess the current state of the system being controlled.

The controller is responsible for receiving the feedback from the process and making decisions based on the difference between the desired set point and the actual output. The controlled device is the mechanism that carries out the control actions to affect change in the process. The process itself is where the actual operation takes place, such as a heating system or pressure regulation.

While the variable element might refer to specific metrics being monitored within a system (such as temperature or pressure), it is not categorized as a fundamental component like the controller, controlled device, or the process. Instead, variable elements can be considered as indicators or measurements that provide feedback to the controller. Thus, in the context of identifying what is not a basic part of a closed control loop, the variable element fits this definition.