A linear actuator is an actuator that creates motion 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
peripherals such as disk drives and printers, in valves and dampers, and in
many other places where linear motion is required. Hydraulic or pneumatic
cylinders inherently produce linear motion. Many other mechanisms are used
to generate linear motion from a rotating motor.
Types = Mechanical actuators =
Mechanical linear actuators typically operate by conversion of rotary motion
into linear motion. Conversion is commonly made via a few simple types of
mechanism: Screw: leadscrew, screw jack, ball screw
and roller screw actuators all operate on the principle of the simple machine
known as the screw. By rotating the actuator's nut, the screw shaft moves in
a line. Wheel and axle: Hoist, winch, rack and
pinion, chain drive, belt drive, rigid chain and rigid belt actuators operate
on the principle of the wheel and axle. A rotating wheel moves a cable, rack,
chain or belt to produce linear motion. Cam: Cam actuators function on a
principle similar to that of the wedge, but provide relatively limited travel.
As a wheel-like cam rotates, its eccentric shape provides thrust at the
base of a shaft. Some mechanical linear actuators only
pull, such as hoists, chain drive and belt drives. Others only push. 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 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 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.
= Hydraulic actuators = Hydraulic actuators or hydraulic
cylinders typically involve a hollow cylinder having a piston inserted in it.
An unbalanced pressure applied to the piston generates force that can move an
external object. Since liquids are nearly incompressible, a hydraulic
cylinder can provide controlled precise linear displacement of the piston. The
displacement is only along the axis of the piston. 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.
= Pneumatic actuators = Pneumatic actuators, or pneumatic
cylinders, are similar to hydraulic actuators except they use compressed gas
to generate force instead of a liquid. They work similarly to a piston in which
air is pumped inside a chamber and pushed out of the other side of the
chamber. Air actuators are not necessarily used for heavy duty
machinery and instances where large amounts of weight are present. One of
the reasons pneumatic linear actuators are preferred to other types is the fact
that the power source is simply an air compressor. Because air is the input
source, pneumatic actuators are able to be used in many places of mechanical
activity. The downside is, most air compressors are large, bulky, and loud.
They are hard to transport to other areas once installed. Pneumatic linear
actuators are likely to leak and this makes them less efficient than
mechanical linear actuators. = Piezoelectric actuators =
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
hysteresis which makes it difficult to control their expansion in a repeatable
manner. = 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. There are many designs of
modern linear actuators and every company that manufactures them tends to
have a proprietary method. The following is a generalized description of a very
simple electro-mechanical linear actuator.
Simplified design Typically, an electric motor is
mechanically connected to rotate a lead screw. A lead screw has a continuous
helical thread machined on its circumference running along the length.
Threaded onto the lead screw is a lead nut or ball nut with corresponding
helical threads. The nut is prevented from rotating with the lead screw.
Therefore, when the lead screw is rotated, the nut will be driven along
the threads. The direction of motion of the nut depends 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 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, accuracy, and lifetime.
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, even
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. Standard vs compact construction
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 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, 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, 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.
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 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. 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. 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. = Linear motors =
A linear motor is functionally the same as a rotary electric motor with the
rotor and stator circular magnetic field components laid out in a straight line.
Where a rotary motor would spin around and re-use the same magnetic pole faces
again, the magnetic field structures of a linear motor are physically repeated
across the length of the actuator. 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.
= Telescoping linear actuator = Telescoping linear actuators are
specialized linear actuators used where space restrictions exist. Their range of
motion is many times greater than the unextended length of the actuating
member. 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: Helical band actuator
Rigid belt actuator Rigid chain actuator
Segmented spindle Advantages and disadvantages
See also Hoist
References External links
Leo Dorst's Lego linear actuator