I am currently studying Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering, seventh edition, by William Bolton. Chapter 1 Introducing Mechatronics has the following figure:

enter image description here

I am a bit confused about this. Aren't actuators mechanical devices, and so aren't they part of the mechanical system, rather than separate from it? Or am I misunderstanding what qualifies as a "mechanical device"?

I would greatly appreciate it if people would please take the time to clarify this.

  • \$\begingroup\$ actuators can be hydraulic or electrical for example so it depends what you are looking at. \$\endgroup\$
    – Solar Mike
    Commented Jul 7, 2020 at 18:05
  • 2
    \$\begingroup\$ It's more like the boundary between the mechanical system and electrical system (or I would like to call it "information system"). Some books do combine it into the "mechanical system" block as "subject under control". \$\endgroup\$ Commented Jul 7, 2020 at 18:06
  • \$\begingroup\$ This is a mechanical system centered perspective. That little box called "microprocessor system for control" could be replaced (germane to this group) with "electronics system for control" that may include a micro, or not. Then all the little boxes are really just "transducers." \$\endgroup\$
    – jonk
    Commented Jul 7, 2020 at 18:06
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    \$\begingroup\$ Would you call an electric motor electrical or mechanical? Electro-magnet? Servo coil? \$\endgroup\$
    – Transistor
    Commented Jul 7, 2020 at 18:07
  • 1
    \$\begingroup\$ No, they aren't purely mechanical, but e.g. electro-mechanical. This division allows you to reason about the purely mechanical system and what's driving it independently. \$\endgroup\$
    – user16324
    Commented Jul 7, 2020 at 18:28

2 Answers 2


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 "actuator" does not limit them to electrical input, but the context of this question and particularly the figure showing the input coming from the "microprocessor system for control" 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 "other" interface components that are part of the control unit. Historically, that would not be unusual, but in this context, it seems very unlikely.


"Actuators" are mechanical motion-controlled devices, but do not define the input motive force or excitation.

The input can be air, hydraulic (oil or water), magnetic, light, electrical, heat, pH, and moisture changes or any electric controllable force or resulting position for a servo system.

A digital actuator converts the command from D to A to force or position or some analog value like resistance.

It could be a digital controlled hydraulic brake with a wheel rotor encoder feedback for example for speed control used in modern cars. The choice to have sensors , exciters and actuators integrated or separate depends on the application needs for integrity, size and cost.


In my 1st SCADA design with robotics in 1977, the Sr. ME designed the robot with pneumatic actuator valves for the high torque/ volume ratio using 1/8" air hoses. Old cars use the same with intake manifold vacuum pressure into a reservoir tank to torque the wipers motor. It walked upside down in a ceiling tubes with retractable legs and used collets to grip the legs inside the tube so it could rotate a step to the next hole. There are about 2000 holes or 1000 U tubes in a CANDU secondary heat exchanger (radiator) that required a robot to walk and insert 2 Eddy current probes to be driven in long adjacent tubes.

  • Linear Actuators are usually Linear Motors and can vary in size from micro to x Hp. The original HDD's used these are looked like "Voice Coils" hence their alias name.
  • Rotary Actuators in HDD's now also magnetic using rectangular formed coils over long flat magnets. Since the magnetic force can vary at each end from the middle and reduces with rising temp, an auto-calibration is often mandatory.

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