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Electro-mechanical actuators

A linear actuator using standard motors will commonly have the motor as a separate cylinder attached to the side of the actuator, either parallel with the actuator or perpendicular to the actuator. The motor may be attached to the end of the actuator. The drive motor is of typical construction with a solid drive shaft that is geared to the drive nut or drive screw of the actuator.

Compact linear actuators use specially designed motors that try to fit the motor and actuator into the smallest possible shape.

  • The inner diameter of the motor shaft can be enlarged, so that the drive shaft can be hollow. The drive screw and nut can therefore occupy the center of the motor, with no need for additional gearing between the motor and the drive screw.
  • Similarly the motor can be made to have a very small outside diameter, but instead the pole faces are stretched lengthwise so the motor can still have very high torque while fitting in a small diameter space.

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.

Variations

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 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 allow 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.

Static load capacity

Linear screw actuators can have a static loading capacity, meaning that when the motor stops the actuator essentially locks in place and can support a load that is either pulling or pushing on the actuator. This static load capacity increases mobility and speed.

The braking force of the actuator varies with the angular pitch of the screw threads and the specific design of the threads. Acme threads have a very high static load capacity, while ball screws 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.[5]

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.

Dynamic load capacity

Dynamic load capacity is typically referred to as the amount of force the linear actuator is capable of providing during operation. This force will vary with screw type (amount of friction restricting movement) and the motor driving the movement. Dynamic load is the figure which most actuators are classified by, and is a good indication of what applications it would suit best.[6]

Speed Control

In most cases when using an electro-mechanical actuator, it is preferred to have some type of speed control. Such controllers vary the voltage supplied to the motor, which in turn changes the speed at which the lead screw turns. Adjusting the gear ratio is another way to adjust speed. Some actuators are available with several different gearing options.[7]

Duty Cycle

The duty cycle of a motor refers to the amount of time the actuator can be run before it needs to cool down. Staying within this guideline when operating an actuator is key to its longevity and performance. If the duty cycle rating is exceeded, then overheating, loss of power, and eventual burning of the motor is risked.