What component is attached to a control valve and compares the stem motion of the valve to the controller signal?

An actuator is a crucial component in a control valve system because it converts the control signal from the automation system into mechanical motion. In this context, the actuator is responsible for manipulating the valve stem's movement in response to the signal received from the controller. This comparison between the stem motion and the controller’s signal is essential for achieving precise control over flow rates, pressure, or other process parameters.

Actuators can be powered by different sources such as pneumatic, electric, or hydraulic systems, depending on the application requirements. By accurately adjusting the valve position to align with the signal, the actuator helps maintain the desired conditions in the process. This feedback mechanism is a fundamental aspect of process control, ensuring that the system responds dynamically to changes in operational conditions.

In contrast, the other options serve different functions: sensors primarily measure process variables, regulators maintain set parameters, and transmitters send signals to a control system. Although these components are integral to the overall process control system, they do not perform the direct comparison between stem motion and the control signal like an actuator does.