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A linear actuator is an actuator that creates linear motion (i.e., in a straight line), in contrast to the circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in computer such as disk drives and printers, in and dampers, and in many other places where linear motion is required. Hydraulics or Pneumatics cylinders inherently produce linear motion. Many other mechanisms are used to generate linear motion from a rotating motor.
Some mechanical linear actuators only pull, such as hoists, chain drive and belt drives. Others only push (such as a cam actuator). Pneumatic and hydraulic cylinders, or lead screws can be designed to generate force in both directions.
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 jackscrew or 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 , , , goniometers and other positioning instruments. For accurate and repeatable positioning, index marks may be used on control knobs. Some actuators include an encoder and digital position readout. These are similar to the adjustment knobs used on micrometers except their purpose is position adjustment rather than position measurement.
There are many types of motors that can be used in a linear actuator system. These include dc brush, dc brushless, stepper, or in some cases, induction motors. It all depends on the application requirements and the loads the actuator is designed to move. For example, a linear actuator using an integral horsepower AC induction motor driving a lead screw can be used to operate a large valve in a refinery. In this case, accuracy and high movement resolution aren't needed, but high force and speed are. For electromechanical linear actuators used in laboratory instrumentation robotics, optical and laser equipment, or X-Y tables, fine resolution in the micron range and high accuracy may require the use of a fractional horsepower stepper motor linear actuator with a fine pitch lead screw. There are many variations in the electromechanical linear actuator system. It is critical to understand the design requirements and application constraints to know which one would be best.
Compact linear actuators use specially designed motors that try to fit the motor and actuator into the smallest possible shape.
The speed and force of an actuator depend on its gearbox. The amount of force depends on the actuator’s speed. Lower speeds supply greater force because motor speed and force are constant.
One of the basic differences between actuators is their stroke, which is defined by the length of the screw and shaft. Speed depends on the gears that connect the motor to the screw.
The mechanism to stop the stroke of an actuator is a limit or micro switch, which can be seen in the image below. Microswitches are located at the top and bottom of the shaft and are triggered by the up and down movement of the screw.
Most electro-mechanical designs incorporate a lead screw and lead nut. Some use a ball screw and 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 gear is common as this allows a smaller built-in dimension still allowing great travel length.
A traveling-nut linear actuator has a motor that stays attached to one end of the lead screw (perhaps indirectly through a gear box), the motor spins the lead screw, and the lead nut is restrained from spinning so it travels up and down the lead screw.
A traveling-screw linear actuator has a lead screw that passes entirely through the motor. In a traveling-screw linear actuator, the motor "crawls" up and down a lead screw that is restrained from spinning. The only spinning parts are inside the motor, and may not be visible from the outside.
Some lead screws have multiple "starts". This means 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.
The braking force of the actuator varies with the angular pitch of the screw threads and the specific design of the threads. have a very high static load capacity, while have an extremely low load capacity and can be nearly free-floating.
Generally it is not possible to vary the static load capacity of screw actuators without additional technology. The screw thread pitch and drive nut design defines a specific load capacity that cannot be dynamically adjusted.
In some cases, high viscosity grease can be added to linear screw actuators to increase the static load. Some manufacturers use this to alter the load for specific needs.
Static load capacity can be added to a linear screw actuator using an electromagnetic brake system, which applies friction to the spinning drive nut. For example, a spring may be used to apply brake pads to the drive nut, holding it in position when power is turned off. When the actuator needs to be moved, an electromagnet counteracts the spring and releases the braking force on the drive nut.
Similarly an electromagnetic ratchet mechanism can be used with a linear screw actuator so that the drive system lifting a load will lock in position when power to the actuator is turned off. To lower the actuator, an electromagnet is used to counteract the spring force and unlock the ratchet.
Since the motor moves in a linear fashion, 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 due to a reliance solely on magnetic attraction and repulsion forces. Most linear motors have a low load capacity compared to other types of linear actuators. Linear motors have an advantage in outdoor or dirty environments in that the two halves do not need to contact each other, and so the electromagnetic drive coils can be waterproofed and sealed against moisture and corrosion, allowing for a very long service life. Linear motors are being used extensively in high performance positioning systems for applications which require various combinations of high velocity, high precision and high force.
A common form is made of concentric tubes of approximately equal length that extend and retract like sleeves, one inside the other, such as the telescopic cylinder.
Other more specialized telescoping actuators use actuating members that act as rigid linear shafts when extended, but break that line by folding, separating into pieces and/or uncoiling when retracted. Examples of telescoping linear actuators include:
| Mechanical | Cheap. Repeatable. No power source required. Self-contained. Identical behavior 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 . Position feedback possible. | Many moving parts prone to wear. |
| Linear motor | Simple design. Minimum of moving parts. High speeds possible. Self-contained. Identical behavior extending or retracting. | Low to medium force. |
| Very small motions possible at high speeds. Consumes barely any power. | Short travel unless amplified mechanically. High voltages required, typically 24V or more. Expensive and fragile. Good in compression only, not in tension. Typically used for Fuel injection. | |
| light and inexpensive | Low efficiency and High temperature range required | |
| Very high forces possible. Relatively high power to size ratio (or power density). | Can leak. Requires position feedback for repeatability. External hydraulic pump required. Some designs good in compression only. | |
| Strong, light, simple, fast. | Precise position control impossible except at full stops | |
| 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: Moving iron controllable actuator | High force and controllable. Higher force and less losses than moving coils. Losses easy to dissipate. Electronic driver easy to design and set up. | Stroke limited to several millimeters, less linearity than moving coils. |
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