Aren't actuators mechanical devices, and so aren't they part of the mechanical system?
Sensors and actuators connect the electronic control system to the mechanical system. Actuators convert electrical signals to mechanical force, motion etc. They are electromechanical devices. They convert electrical energy to mechanical energy, but they may convey information as well.
The diagram really makes the distinction. There are really four parts to the system in the diagram, the mechanical system, the electronic control system, the activating interface and the feedback interface.
The word &quot;actuator&quot; does not limit them to electrical input, but the context of this question and particularly the figure showing the input coming from the &quot;microprocessor system for control&quot; strongly implies that in inputs are electrical. If the input is air, hydraulic or something other than an electrical signal or signal and power combined, there would need to be electrical to &quot;other&quot; interface components that are part of the control unit. Historically, that would not be unusual, but in this context, it seems very unlikely.