There are two basic groups of electric linear actuators: Direct Drive and Transmission-based (“Converting”). The direct drives, also known as linear motors, generate a linear motion and force directly from the electric energy. On the other hand, the converting ones in the first step generate a rotational motion, which is subsequently converted into linear by a suitable mechanical device.
The majority of direct operating electric linear actuators utilize electromagnetic effect. In common use are the solenoid types and the various designs of linear motors. Every rotational motor such as brush type DC, brushless DC (Electronically Commutated), AC induction, reluctance, stepper type etc. can be used in a form of a linear motor. Linear motors offer the high linear speed, fairly high accuracy and repeatability, high stiffness and a simple reliable mechanical design. Despite these advantages, linear motors are not widely used because the comparably low linear forces they generate – the low force density (force/volume), the strong attraction of metallic and nonmetallic contaminants and last but not least because of the high costs involved. The applications are mostly in clean, laboratory type environments. The solenoid type electric linear actuators utilize the electromagnetic attraction between coils, coil and permanent magnet or coil and ferromagnetic material. These actuators usually feature a rod type design (moving rod). Inside a stationary coil set, there is a set of magnets or coils attached to a plunger or a rod which is movable in a linear direction. The linear forces achievable with this type of actuator are in the low to middle range, The linear speeds can be substantial at low loads, and fairly high frequency oscillating linear motion can be achieved. The positioning accuracy is usually somewhat limited, unless a sophisticated, expensive coil set design with LVDT feedback and closed loop control is selected. Applications for the solenoid type electric linear actuators are the high cycle rate limited stroke automation tasks or as electromechanical oscillators and vibrators.