An actuator is an electromechanical device which converts energy into mechanical work (or motion). For robots, actuators are like muscles that perform work. The work can be either to induce motion, or to object motion; i.e. either to start a movement, or to stop it. There are different types of actuators available and most of them either create rotational motion, or linear motion. (Oscillatory motion is rarely used, and even if required can be created using a linear or a rotary actuator)
As the name says, linear actuator creates linear motion, i.e. it creates to and fro motion. These actuators can be driven by either linear or rotational motion.
To simplify things, let us take an example of a bicycle. When the cyclist pushes the pedal, it rotates the bottom bracket in a cycle which is connected to a roller chain. Now the rotational motion from the bracket creates a linear motion in the roller chain.
Rotational Actuator (Rotary Actuator)
Rotational actuator induces rotary or rotational motion. A simple DC motor is an example of rotational actuator. Similar to linear actuator, rotation actuators can be driven by either linear motion or rotational motion.
Let us continue with our bicycle example to understand how linear motion can be used to create rotary motion. The roller chain in a bicycle is connected to sprocket gear of the driving wheel (Normally back wheel). When a cyclist pedals, the roller chain (remember that roller chain is an example of linear motion) rotates the sprocket gear creating a rotational motion further rotates the wheel.
Most actuators can be mechanically designed to induce rotary motion or linear motion. A simple nut attached to a linear member can create a rotary motion. On the other hand, attaching a screw to a rotary actuator creates linear motion.
The below animations shows both linear motion and rotary motion. The bottom bracket rotates creating a linear motion in roller chain. Further the same linear motion of the chain creates rotary motion in sprocket gear.
Actuators require energy to create motion and the source of energy is usually electric current, pneumatic pressure or hydraulic fluid. In the next section, we will understand the different actuators available for robots, and their energy sources.