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electro mechanical actuators

Electro-mechanical actuators

 

Electro-mechanical actuators are similar to mechanical actuators except that the control knob or handle is replaced with an electric motor. Rotary motion of the motor is converted to linear displacement of the actuator. There are many designs of modern linear actuators and every company that manufactures them tends to have their own proprietary method. The following is a generalized description of a very simple electro-mechanical linear actuator.

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Typically, a rotary driver (e.g. electric motor) is mechanically connected to a lead screw so that the rotation of the electric motor will make the lead screw rotate. A lead screw has a continuous helical thread machined on its circumference running along the length (similar to the thread on a bolt). Threaded onto the lead screw is a lead nut with corresponding helical threads. The nut is prevented from rotating with the lead screw (typically the nut interlocks with a non-rotating part of the actuator body). Therefore, when the lead screw is rotated, the nut will be driven along the threads. The direction of motion of the nut will depend on the direction of rotation of the lead screw. By connecting linkages to the nut, the motion can be converted to usable linear displacement. Most current actuators are built either for high speed, high force, or a compromise between the two. When considering an actuator for a particular application, the most important specifications are typically travel, speed, force, and lifetime.

Principles

In the majority of linear actuator designs, the basic principle of operation is that of an inclined plane. The threads of a lead screw act as a continuous ramp that allows a small rotational force to be used over a long distance to accomplish movement of a large load over a short distance.

Linear motors

A linear motor is essentially a rotary electric motor laid down on flat surface. Since the motor moves in a linear fashion to begin with, no lead screw is needed to convert rotary motion to linear. While high capacity is possible, the material and/or motor limitations on most designs are surpassed relatively quickly. Most linear motors have a low load capacity compared to other types of linear actuators.

Advantages and Disadvantages of electric actuators

Actuator Type Advantages Disadvantages
Mechanical Cheap. Repeatable. No power source required. Self contained. Identical behaviour extending or retracting. Manual operation only. No automation.
Electro-mechanical Cheap. Repeatable. Operation can be automated. Self contained. Identical behaviour extending or retracting. DC or Stepping motors. Position feedback possible. Many moving parts prone to wear.
Linear motor Simple design. Minimum of moving parts. High speeds possible. Self contained. Identical behaviour extending or retracting. Low force.
Piezoelectric Very small motions possible. Requires position feedback to be repeatable. Short travel. Low speed. High voltages required. Expensive. Good in compression only. Not good in tension.
Hydraulic Very high forces possible. Can leak. Requires position feedback for repeatability. External hydraulics pump required. Some designs good in compression only.
Wax motor Smooth operation. Not as reliable as other methods.
Segmented spindle Very compact. Range of motion greater than length of actuator. Both linear and rotary motion.
Moving coil Force, position and speed are controllable and repeatable. Capable of high speeds and precise positioning. Linear, rotary, and linear + rotary actions possible. Requires position feedback to be repeatable.
MICA High Force and controllable. Higher force and less losses than moving coils [2]. Losses easy to dissipate. Electronic driver easy to design and set up. Stroke limited to several millimeters, less linearity than moving coils