An actuator is basically a motor used to control or generate motion. The energy required for controlling or generating motion is obtained from an electric source or from pressurized air. While a conventional motor produces rotary motion, an actuator is used to produce constrained motion, like produces motion along a straight line. The technological advancement in actuators has led to the advent of high resolution, precise motion, very sensitive and fast responding actuators. There are several different types of actuators available today, prime amongst them are Mechanical, Hydraulic, Pneumatic, Piezoelectric, Electro-mechanical and electro-hydrostatic actuators. Now, let us discuss each of these types in detail.
• Mechanical servo actuators: These actuators use a mechanical arrangement like the lead screw, screw jack, ball screw, roller screw, wheel, and axle or Cam to convert rotary motion into constrained motion. They are cheap, independent and do not require any external power. However, automation is not possible with such actuators, and they can be used only manually.
• Hydraulic actuators: These actuators use a hollow cylinder filled with a liquid and having a piston inserted in it. When an unbalanced pressure is attached to the piston, it generates a force that can move external objects. As liquids are incompressible, these actuators can produce precise displacement. Also, displacement is possible only along the axis of the piston.
• Pneumatic actuators: These actuators are similar to the hydraulic actuators, with the only difference that compressed gas is used to generate force instead of liquids. As the air in the only input source required in these actuators, they can be used in any place.
• Piezoelectric actuators: The piezoelectric effect is the property of certain materials to produce the electric voltage when mechanical pressure is applied to them. Such materials expand when a voltage is applied to them. Such materials are called piezoelectric materials and are used in actuators to produce very precise motion. However, they require very high voltage to operate, and can be used for a very short range of motion.
• Electro-hydrostatic actuators: They are also known as electro-hydraulic actuators. They use electro-hydraulic automation technology for inlet guide vane actuation, bleed heat valve control and remote subsea actuation. They have a back and forth rotating pump, to draw liquid from a pressurized reservoir through a set of valves. The precise movement of rotating pump in these actuators is obtained by using digital positioners for feedback and a servo controller for controlling the pump. These actuators are compact and provide precise electric actuation for turbine fuel and steam control.
Alexander Albert is an expert automation and control engineer having an overall industry experience of over 15 years. Here, he provides an introduction to the different types of actuators available today, including the very advanced electro-hydraulic actuators, which use digital positioners for feedback and servo controller for control.