- Linear actuator
__TOC__A linear actuator is a device that develops force and motion, from an available energy source, in a
linearmanner, as opposed to rotationally like an electric motor. There are various methods of achieving this linear motion. Several different examples are listed below.
Types of Linear Actuators
Mechanical actuators typically convert rotary motion of a control knob or handle into linear displacement via screws and/or gears to which the knob or handle is attached. A
jackscrewor car jack is a familiar mechanical actuator. Another family of actuators are based on the segmented spindle. Rotation of the jack handle is converted mechanically into the linear motion of the jack head. Mechanical actuators are also frequently used in the field of lasers and optics to manipulate the position of linear stages, rotary stages, mirror mounts, goniometers and other positioning instruments. For accurate and repeatable positioning, index marks may be used on control knobs. Some actuators even include an encoder and digital position readout. [ [http://www.teach-ict.com/gcse/software/control/miniweb/pg9.htm# Teach ICT - GCSE ICT computer control ] ] These are similar to the adjustment knobs used on micrometersexcept that their purpose is position adjustment rather than position measurement.
Hydraulic actuators or
hydraulic cylinders typically involve a hollow cylinder having a piston inserted in it. The two sides of the piston are alternately pressurized/de-pressurized to achieve controlled precise linear displacement of the piston and in turn the entity connected to the piston. The physical linear displacement is only along the axis of the piston/cylinder. This design is based on the principles of hydraulics. A familiar example of a manually operated hydraulic actuator is a hydraulic car jack. Typically though, the term "hydraulic actuator" refers to a device controlled by a hydraulic pump.
The piezoelectric effect is a property of certain materials in which application of a voltage to the material causes it to expand. Very high voltages correspond to only tiny expansions. As a result, piezoelectric actuators can achieve extremely fine positioning resolution, but also have a very short range of motion. In addition, piezoelectric materials exhibit
hysteresiswhich makes it difficult to control their expansion in a repeatable manner.
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.
Typically, a rotary driver (e.g.
electric motor) is mechanically connected to a lead screwso 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.
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.
Many variations on the basic design have been created. Most focus on providing general improvements such as a higher mechanical efficiency, speed, or load capacity. There is also a large engineering movement towards actuator miniaturization.
Most electro-mechanical designs incorporate a
lead screwand lead nut. Some use a ball screwand ball nut. In either case the screw may be connected to a motor or manual control knob either directly or through a series of gears. Gears are typically used to allow a smaller (and weaker) motor spinning at a higher rpm to be geared down to provide the torque necessary to spin the screw under a heavier load than the motor would otherwise be capable of driving directly. Effectively this sacrifices actuator speed in favor of increased actuator thrust. In some applications the use of worm gearis common as this allow a smaller built in dimension still allowing great travel length.
Some lead screws have multiple "starts". This means that they have multiple threads alternating on the same shaft. One way of visualizing this is in comparison to the multiple color stripes on a candy cane. This allows for more adjustment between thread pitch and nut/screw thread contact area, which determines the extension speed and load carrying capacity (of the threads), respectively.
linear motoris 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 relatively low load capacity compared to other types of linear actuators.
wax motortypically uses an electric current to heat a block of wax causing it to expand. A plunger that bears on the wax is thus forced to move in a linear fashion.
Segmented actuators consist of discrete chain elements which are interlinked to form a rod (the technology is known as the
segmented spindle) thus making the actuator extremely compact.
Advantages and Disadvantages
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. Many moving parts prone to wear. Linear motor Simple design. Minimum of moving parts. High speeds possible. Self contained. Identical behaviour extending or retracting. Relatively 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, positionand speedare programmableand repeatable. Capable of high speeds and precise positioning. Linear, rotary, and linear + rotary actions possible. Requires position feedbackto be repeatable.
* [http://www.engineeringtalk.com/guides/linear-actuator.html Engineeringtalk]
* [http://staff.science.uva.nl/~leo/lego/piston.html Leo Dorst's Lego linear actuator]
* [http://www.youtube.com/watch?v=TEfkJSA5CmE The principle of the KATAKA segmented spindle Linear Actuator]
* [http://www.linak.com Electric Linear Actuators from LINAK]
* [http://www.kataka.dk Linear Electromechanical Actuators based on the Segmented Spindle]
* [http://www.artektron.se Linear Actuators from ARTEKTRON]
* [http://www.smac-mca.com Moving coil linear and rotary technology - SMAC]
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