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Traverse Unit Take-up System Enhances Extrusion Machines
Written by Administrator   

How a machine manufacturer added value to its product while at the same time controlled costs

Jennings extrusion machine
Fig. 1 Jennings extrusion machine with operator at the controls (below)
Jennings extrusion machine controls
Fig. 1a
In-line oven system
Fig. 2 In-line oven system vaporizes the lubricant from the PTFE extrudate

Jennings International, Norristown, PA, USA, is a manufacturer of semi-custom polytetrafluoroethylene (PTFE) fine powder paste extrusion equipment. The company's product line includes specialized machinery for extruding PTFE-insulated wire, PTFE hose and tubing and PTFE film and tape.

Adding value to the PTFE extrusion process
The chemical and physical properties of PTFE derive from the material's fluorocarbon structure. PTFE possesses virtually universal chemical inertness, insolubility in solvents below 300°C (572°F) and good thermal stability. PTFE's electrical insulation properties include low dielectric loss, low dielectric constant and high dielectric strength.

Using Teflon® brand precursors, Jennings patented its first extruder in 1953. The process has been modified over time to accommodate the more advanced PTFE material which enters the process as a fine powder resin. The PTFE resin is stored in a temperature-controlled room where it is mixed with a lubricating solvent. The resin and lubricant mixture is then solidified in a preform press.

The resulting PTFE preform is then loaded into the extruder. It is pressed through a die and extruded as a tube, tape or insulation over wire. An in-line oven system vaporizes the lubricant from the PTFE extrudate. A sintering oven is used to cure the product where temperatures up to 850°F (1562°C).

After cooling, the product is wound onto a take-up spool for distribution. The wire take-up procedure is where Jennings International saw an opportunity to add value and strengthen its CRM (customer relationship management) policies. To improve time-to-market capability for its customers, Jennings International began to design a linear traverse assembly into the extrusion line. The linear traverse assembly guides the PTFE-insulated wire directly onto take-up spools thus eliminating the need for the customer to design and fabricate their own take-up systems.

Most Jennings customers have selected this added-value option. Jennings had to develop a manufacturing plan to build in the traverse unit without adversely affecting selling price, or their own product development costs.

Linear traverse unit take-up system on Jennings extruding machine
Fig. 3 Linear traverse take-up system on Jennings extruding machine

Adding value without adding costs
PTFE extrusion lines can represent a significant capital investment depending on the size and stroke length of the extruder and auxiliary equipment. Naturally, production machinery carrying a large capital investment pays for itself only if the production process is operated at peak, or near-peak, efficiency rates - as close to non-stop as possible. That meant Jennings had to provide a traverse solution that was both reliable and essentially maintenance-free. Their choices were to build such a system or use a supplier capable of designing and fabricating the assembly. They chose the latter, selecting Amacoil/Uhing rolling ring linear motion systems.

"We knew we needed a simple design for the traverse unit," said John Porta, Jennings International VP. "Even though the traverse is a very minor part of the machine, if it breaks down or requires frequent maintenance, valuable production time is lost."

Should the traverse system fail, an even larger problem exists for Jennings customers. Porta explained that since the Jennings machine is used in a batch process operation, successful production requires complete consumption of any given batch. If the traverse system breaks down, it is feasible that the remaining batch would be lost resulting in a serious drop in production revenues.

The simplicity of the rolling ring operating principle met Jennings' needs. The type of rolling ring assembly Jennings International uses in its take-up system provides automatic reversal without clutches, cams or gears. It is not necessary to reverse the drive motor - the linear traverse direction is controlled by purely mechanical means.

Comparison of rolling ring and screw-based drive systems
Fig. 4 Comparison of rolling ring (left) and screw-based (right) drive systems

The rolling ring design enabled Jennings International to use a single motor in the traverse system, instead of one each to drive the take-up spool and the linear traverse unit. This is possible because rolling ring engineering permits the use of a simple pulley system to synchronize the rotational motion of the take-up spool with the traversing motion of the linear drive. Had a separate motor been required for the traverse unit, reversing the traverse system would have required the drive motor be slowed, stopped and reversed. The motor would have had to be a variable speed motor capable of rotating in two directions. By avoiding this, Jennings also eliminated the need for a traverse motor encoder and controls. The total savings for Jennings International is about US$1,000 per machine.

Said Porta, "The system can be free of complex electronic controls. This keeps our cost down so we can offer the added value of the traverse drive unit without adversely affecting the selling price of an extrusion line. Our customers like that we can offer a take-up solution with minimal operator training requirements because it further controls the costs to acquire the Jennings machine."

Also influencing Jennings' decision to use a rolling ring drive is the fact that the system runs on a smooth, unthreaded shaft. Thread clogging in the traverse assembly posed a problem for some customers. Designing and fabricating a shaft bellows assembly to keep the threads clean was a slight additional cost. The smooth shaft solution works better. According to Porta, "Rolling ring technology has proven itself as a reliable and maintenance-free solution."

Since the linear drive supplier is experienced in building wire winding traverse drive assemblies, Jennings encourages its customers to deal directly with Amacoil for any concerns with the traverse. "Our partnership with Amacoil has worked very well," said Porta. "When they have questions about the traverse assembly, our customers appreciate dealing directly with Amacoil."

Adding the traverse assembly option to its extruding machines has enabled Jennings International to significantly strengthen its customer relations. The rolling ring linear motion solution proved inexpensive and effective. It enabled Jennings to meet its objective of adding value without significantly increasing its product development costs.


Written by Administrator   
Monday, 20 December 2010 14:59

To meet the demand of controllable millimeter-stroke actuators, there are two possible starting points. One is to
consider improvement of moving coil actuators, the other is to consider improvement of moving iron actuators.
Following this approach and using its experience on the different types of magnetic actuators, Cedrat
Technologies has developed new specific Moving Iron Controllable Actuators, called MICA. This actuator
circumvents previous controllability limitations of standard Moving Iron actuators while keeping their high
forces capabilities. Compared with moving coils of the same force, the MICA are twice less in mass while
requiring 3 times less electric power. Another significant advantage of the MICA is a much better heat
dissipation and reliability as the MICA coil is fixed into the iron stator. These actuators have been successfully
tested in Active Control of Vibration and vibration generators. The paper aims at presenting the properties of the
MICA Moving Iron Controllable Actuators after introducing the Moving Coil Actuators, these being the
initiators and the competitors of MICA technologies.
Keywords: Magnetic Actuator, Moving Coil, Control
There is a strong demand of controllable actuators
for both traditional and new applications. A
controllable actuator should be able to accelerate,
break, inverse the motion of the load, all along the
stroke. It means the force produced by the actuator
should be proportional (at least roughly) to the
applied electric excitation, and in particular, the sign
of the actuation force could be changed all along the
As an example of traditional mass application of
magnetic actuators which would benefit of
controllability, there are the circuit breakers. They
would improve the life time of their electric contacts
by “soft landing” using controllability [1]. In circuit
breakers for AC current, there are also interests for
synchronization of the opening or the closing of the
circuit breaker with the 0 current in order to avoid
electric arcs [2]. In this application, the stroke of the
actuators is in the range of 1 to 10 millimeters and
the required force bandwidth is above 100Hz.
Many new applications requiring controllable
actuators are found in mechatronic or adaptronic
systems [3]. A typical application is the active
control of vibration (AVC). For this kind of
applications there are mainly two kinds of
controllable actuators: piezoelectric actuators and
moving coil actuators (also called Voice coil or
Lorentz actuators). Piezoelectric actuators offers
large forces (up to 1kN or more) but even with
amplified piezoelectric actuators (see for example
[4]), displacements are limited to 1mm. Moving
coil actuators offer large displacements (up to
10mm) but if acceptable actuator mass is lower than
1kg, forces are very low, typically less than 50N in
steady state. So there is a gap in performances
between both solutions. The fact is what is required
for several embedded AVC applications such as
meet in air&space or automotive is precisely into
this gap: Displacements in the range of 1 to 5mm
and force bandwidth of more than 100Hz, with
actuators mass less than 1kg. These requirements
are similar to previous one.
To meet these all requirements, there are two
possible starting points. One is to consider
improvement of moving coil actuators, the other is
to consider improvement of moving iron actuators.
The Moving Coil Actuators are based on the
Lorentz force which is strictly proportional to the
applied current.
The usual Moving Iron Actuators are more generally
called electromagnets. They use the magnetic
attraction force that exists between two soft
magnetic parts in presence of a magnetic field. It is
generally much higher than Lorentz force.
Typically, for a similar mass one can expect a factor
10. It is why Moving Iron Actuators are the most
popular magnetic actuator type. In principle, the
magnetic force is intrinsically quadratic meaning
that only attraction forces can be produced, so they
are not controllable. To get it back, a return spring is
added, leading to one fixed position at rest. Such an
actuator even with a return springs is generally not
able to perform fine control functions.
A new trend consists in trying to improve the
controllability of moving iron actuators, while
keeping their force density superiority. One new
approach used for circuit breakers consists in using
appropriate current laws. Although they prove their
effectiveness in the test conditions [1], these laws
cannot anticipate disturbances due to wear or
temperature and are very specific to the application.
Another approach consists in combining a moving
iron and a moving coil into one actuator. This
approach has been exploited in a new actuator
patented by Schneider Electric [5]. However this
provides only partial controllability as it adds a
reluctant attractive force to a voice coil, which
makes it unpractical as regard AVC applications for
example. Using its experience on moving coil
actuators [6], moving magnets actuators [7] and
moving iron actuators, Cedrat has developed new
specific Moving Iron Controllable Actuators, called
MICA [8]. This circumvents previous controllability
limitations of standard Moving Iron actuators while
keeping high forces capabilities.
This paper aims at presenting the properties of the
MICA Moving Iron Controllable Actuators after
introducing the Moving Coil Actuators, these being
the initiators and the competitors of MICA
Moving Coil Actuators
Moving Coil Actuators can be customized thanks to
following parameters: The magnetic force is
determined by the product of the coil current and the
magnetic field. This field is produced by a magnetic
circuit including a permanent magnet. Increasing
force leads to a trade-off between the coil electric
power and magnetic circuit mass. The heating of the
coil is the main force limitation. Its thermal behavior
results not only of the previous trade-off but also of
the heat exchange design. As the coil is not in
contact with iron, the heat drain is difficult
especially in vacuum application. In this case
thermal drains can be implemented. The guiding can
take benefit of the absence of transverse forces in a
moving coil to use an elastic guiding. This is
interesting to get a wear-free and hysteresis-free
As an example, a moving coil actuator for high
precision positioning and compatible with space
requirements, called VC-1 has been designed and
successfully tested by Cedrat [6]. General space
requirements are the use of no degassing material,
no lubrication, low mechanic time constant, low
electrical power, and thermal energy evacuation
through radiating and conducting exchanges. In
particular, as the electric power on board satellites is
very limited, their design is performed with a special
care of the force produced versus electric power.
These have been accounted in the design, the
realization and the test of the VC-1 prototype.
Thanks to a good design of thermal drains, the
actuator presents a rather high force capability.
The VC-1 (fig. 2 & 3) is a cylindrical actuator of
71mm in diameter and 49mm in length. Its total
mass is 500gr. The moving part is a central feed
through shaft. The stator is based on a NdFeB
hollow permanent magnet with a 1.3T
magnetization and a standard magnetic steel for the
magnetic circuit. Coil is guiding by flexural blades
and is drained by flexible thermal drains to reduced
Fig. 1: VC-1 Voice Coil Actuator
Fig. 2: FLUX FEM analysis of VC-1 and VC-2 cross
sections, accounting for axial symmetry
(z vertical axis)
The VC-1 stroke is 3 mm. The coil resistance is 0.11
Ohm. The force factor is 1.9N/A. It leads to a forceto-
power ratio of 5.5N/W½. After Thermal Vacuum
qualification, the nominal force in vacuum is fixed
to 13N for a continuous 5.5 W electrical
consumption. Max force can be increase according
to the duty cycle. Because of better exchanges, the
nominal force in air is 30N. It leads to a force-tomass
ratio of 60N/kg. The peak force could reach
100 N with a 5% duty cycle. Although sometimes
useful for transient applications, this large force is
not exploitable in AVC. The actuator has passed
successfully a life time test of 107 cycles. However
after this, thermal drains showed some fatigue signs.
Improvements of the forces are limited as there are
only few design parameters. This has been explored
by optimizing the magnetic circuit shape using
FLUX FEM software [9] to define a new geometry
VC2 and by implementing high performance
magnetic materials in second step, giving the VC2b
(see Fig.3). The VC-1 force is improved of 20%
with the VC-2 and of 40% with VC-2b.
Performances are in table 1 and details in [LDIA].
This work shows that Moving coils actuators can be
improved, but only in a limited amount. Coil heating
remains a strong force limitation. New technological
works for space applications are in progress in an
ESA TRP project “Moving Coil Motor”.
Moving Iron Controllable Actuators
Several Moving Iron Controllable Actuators
(MICA) actuators have been designed by Cedrat
Technologies aiming at a good controllability as a
moving coil with a higher force versus power and a
higher force per mass than a moving coil.
A MICA general concept is shown of fig 3. The
actuator is cylindrical with a z axis. A stator
containing the coil presents 2 poles. A moving shaft
presents 2 shifted poles. Permanent magnets (not
shown) produce a magnetic bias Hsa and Hsb of the
opposite air gaps having same direction. The coil is
used to create opposite dynamic magnetic fields Hda
and Hdb which can be reversed with the current.
The total field in the air gaps can be increased or
decreased. The shaft is attracted to the air gap
having the largest total field. This allows the shaft to
move in one direction or the opposite one. As will
be shown a good linearity is even obtained, leading
to an actuator competing with moving coil actuators.
Fig. 3: MICA general concept
MICA coil located in the stator provides two first
advantages: At first there is no moving coil,
avoiding fatigue of moving wires supplying the
moving coil. Secondly, the thermal drain of the coil
is performed by the stator iron, which is thermally
efficient and mechanically reliable.
Fig. 4: MICA40-3 prototype
The MICA 40 (fig.4) is one realization targeting
improvement of VC1-1 or VC-2 : a size a bit
smaller with same stroke of 3mm and a controllable
steady state force in the 40N range. Its length is
80mm and the side of the square section is 39mm.
Its weight is 0.358kg. Its coil is made of 282 turns,
leading to a resistance of 1.86 Ohms.
The forces are computed with FLUX for different
currents and different position along the 3mm stroke
(fig.5), accounting for non linearity of magnetic
materials. The model predicts the force is almost
proportional to the current and can be inverted
whatever the position, as a moving coil. According
to the model, a force of 18N is achieved with a
current of about 2A, with an electric power 3.7W.
The nominal force of 41N, giving a force-to-mass
ratio of 114N/kg, is achieved with a current of about
4A with a power of 15W. Thus, the force factor is
9N/A. It leads to a force-to-power ratio of
10.6N/W½. All these factors are well above those of
VC1, VC2 and VC2b.
Fig. 5: Force vs position of MICA40-3 for currents
varying between –5A and + 5A (FLUX result)
The force test consists in measuring the force
produced by the actuator versus the applied current
in any position along the possible stroke, using a
force sensor, a position sensor, a current sensor, a
current generator and a micro positioning screw to
position the actuator moving axis. The measured
forces versus applied current from –2A to +2A at
different positions are shown on figure 9. They are
closed to theoretical expectations. In spite of some
hysteresis, which does not exist with moving coil,
the controllability is demonstrated. The measured
force at 2A is about 25N in the central position,
which is higher than expected.
Fig. 6: MICA40-3 Force vs current at different positions
Fig. 7: MICA40-3 Force vs current at different
frequencies for the central position
The forces have been also measured at different
frequencies from 0.1Hz to 300Hz. The forces appear
rather independent of frequency. The force versus
current (fig. 11) has been measured to assess some
saturation effect. A force of 100N at 7.5A was
achieved without clear saturation. The thermal
behavior is presented on fig. 9, by the self heating of
the actuator when supplied with a DC current of 2A,
and its cooling when current is switched off. An
increase if 30°C is achieved in 5min.
Fig. 9: MICA40-3 self-heating when supplied at 2A and
its cooling when current is switched off
The table 1 compares the moving coils VC-1, VC-
2b, LA17-28 from BEI [9] to the moving iron
MICA40-3. LA17-28 has a larger stroke but it is not
guided. Such a stroke is not really useful in AVC
applications. Force per mass and force per power are
in favor of the MICA. For a similar force as VC-2b
it requires almost 3 times less power while being
Table 1: Comparison of Moving coil & moving iron
Several other MICAs have been developed for
offering forces up to 500N [8]. These actuators have
been successfully tested in AVC and vibration
generators. The fig.10 shows a typical AVC test: the
MICA 170 actuator is fixed to a mass and is excited
with large vibrations amplitudes produced by an
APA500L. Typically the amplitudes are 0.5mm
from 10 to 500Hz. When the MICA170 is operated
it reduces the vibrations on the mass by 15dB to
20dB according to the modes. It shows that the
MICA technology, even not strictly linear, is
controllable enough to perform control of large
vibration amplitudes.
Fig. 10: MICA170-3 Vibration test : Experiment & results
Moving coil actuators and new controllable moving
iron actuators are two types of controllable actuators
that have been studied and compared. Moving coil
actuators are hysteresis-free, but their coil heating
limits their force capability. New controllable
moving iron actuators offers higher force per power
and higher force to mass ratio. They are also more
robust. They offer a new solution for stringent
mechatronic applications such as anti vibration
The authors thank M.Csukai (OSEO) and M.Amiet
(DGA) for supporting these works.
[1] P.Pruvost, Control of magnetic actuators in electric
contactors by current shaping, Proc Actuator
2006, Pub Bremen Messe, 2006, pp 136-139
[2] N. Beyrard, Circuit breaker-contactor with a
piezo-electric controlled locking, WO2006111407
[3] H.Janocha., Adaptronics and Smart Struc-tures,
2nd edition, Pub Springer, Aug. 2007
[4] Piezo Actuator catalogue, Cedrat Technologies,
France, 2005, 99p
[5] C.Bataille, Electromagnetic actuator with movable
coil, EP 1655755 A1, 2006
[6] F. Bloch, Space compliant Moving Coil Actuator.
Proc ACTUATOR 2004 Conf, Pub. Messe
Bremen (G), June 2004, pp 661-664
[7] P. Meneroud, Bistable micro actuator for energy
saving Proc Actuator 2006, Pub Messe Bremen
(G), June 2006, pp 744-747
[8] F.Claeyssen, Moving coil or moving iron …,
LDIA2007, Lille 2007
[9] New Linear Magnetic Actuators, Cedrat,
Jan 2007, 18p
[10] Kimco Magnetics Div., BEI Technologies, Inc,

Pure Analog Throughput for Very High Brushless Motor Resolution
Written by Administrator   
Tuesday, 11 August 2009 13:22
The TA333 High Power Linear Drive is the newest addition to the Trust Automation line of high performance drives. The TA333 drive features a true Class-AB linear amplifier with pure analog throughput at currents up to 25A. Highly configurable, the drive can be used with a brushless motor using external sinusoidal commutation, with an internally commutated brushless motor with Hall Effect sensor feedback for smooth trapezoidal operation, a 2-phase stepper motor, or up to two voice coil motors. The drive is ideal for applications such as: "very high resolution" inspection systems, metrology instruments, and medical applications.

The TA333 is ideal for overcoming issues such as high inertia mismatched stages and low inductance motors. It is easily configured for three-phase DC brushless servo motors using Hall Effect sensors, three-phase AC brushless motors using external sinusoidal commutation, single-phase DC brushed servo motors in bridged mode, brushless linear motors and voice coil linear actuators. This flexibility allows motion system designers to easily integrate the latest developments in sinusoidal motor control and its benefits of zero cogging, no torque ripple, and smooth motion.

For applications requiring extremely low electrical noise the TA333 linear drive utilizes an external 24 VDC source of power for the internal logic. For applications, which are not as sensitive to electrical noise, an internal 24 VDC source can be used. The very low electrical noise of the TA333 linear drive makes it ideal for integration in or near systems that have noise sensitive circuitry, such as transducers and sensors. Also the audible noise problems associated with PWM Drives are eliminated and the typical hall "ticking" noise associated with Trapezoidal commutation is greatly reduced.

This powerful ±100 Volt, 25 Amp peak linear drive can be configured to interface with any motion controller with a ±10 VDC command output. High performance sinusoidal commutation systems require a motion controller with the capability to generate two ±10 VDC command signals. The third command signal is generated by the drive to maintain the highest level of precision. The TA333 can be set up to operate in trapezoidal mode using Hall Effect sensors as feedback. The drive also features DTS (Dynamic Transconductance Selection) control, which allows the transconductance (torque control) to be changed on-the-fly allowing very high resolution control without sacrificing power for resolution capability.

Compact, measuring just 14.9 in. (37.9 mm) X 7.7 (19.5 mm) X 4.7 (11.9 mm), the TA333 has integral thermally controlled variable speed forced air cooling, a housing designed to protect against operator injury, easily made connections using ribbon connectors, SMB coaxial connectors, and pluggable-terminal connectors, all demonstrating Trust Automation's commitment to operator safety, quiet environment, and ease of integration.
Linear actuator
Written by Administrator   
Monday, 20 December 2010 14:46
From Wikipedia, the free encyclopedia
Conceptual design of a basic linear actuator. Note that in this example the lead screw (gray) rotates while the lead nut (yellow) and tube (red) do not.

A linear actuator is an actuator that, when driven by a non-linear motion, creates linear motion (as opposed to rotary motion, e.g. of an electric motor). Mechanical and hydraulic actuation are the most common methods of achieving the linear motion.

Mechanical actuators

A mechanical linear actuator with digital readout.

Mechanical linear actuators operate by conversion of rotary motion into linear motion. Conversion is commonly made via a few simple types of mechanism:

Screw: 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. By rotating a wheel/axle (e.g. drum, gear, pulley or shaft) a linear member (e.g. cable, rack, chain or belt) moves.[1]
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 (e.g. hoist, chain drive and belt drive) and others only push (e.g. cam actuator).

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 even include an encoder and digital position readout.[2] These are similar to the adjustment knobs used on micrometers except that their purpose is position adjustment rather than position measurement.

[edit] Hydraulic actuators
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.

[edit] Pneumatic actuators
Pneumatic actuators, or pneumatic cylinders, are similar to hydraulic actuators except they use compressed gas to provide pressure instead of a liquid.

[edit] 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.

[edit] Electro-mechanical actuators


A miniature electro-mechanical linear actuator where the lead nut is part of the motor. The lead screw does not rotate, so as the lead nut is rotated by the motor, the lead screw is extended or retracted.
Typical compact cylindrical linear electric actuator
Typical linear or rotary + linear electric actuator
Moving coil linear, rotary and linear + rotary actuators at work in various applications

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.

[edit] Simplified design
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 or ball 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, 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 actuate a large valve in a refinery. In this case, accuracy and move resolution down to a thousandth isn't needed, but high force and speed is. For electromechanical linear actuators used in laboratory instrumentation robotics, optical and laser equipment, or X-Y tables, fine resolution into the micron region 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's critical to understand the design requirements and application constraints to know which one would be best.

[edit] 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.

[edit] 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.

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.

[edit] 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.

[edit] Wax motors
A wax motor typically 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.

[edit] Telescoping linear actuator
Telescoping linear actuators are specialized linear actuators used where space restrictions or other requirements require. 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
[edit] Advantages and disadvantages

Actuator TypeAdvantagesDisadvantages
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 in tension.
Hydraulic Very high forces possible. Can leak. Requires position feedback for repeatability. External hydraulic 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 (moving iron controllable actuator) High force and controllable. Higher force and less losses than moving coils [3]. Losses easy to dissipate. Electronic driver easy to design and set up. Stroke limited to several mi
Last Updated on Monday, 20 December 2010 14:56
Linear Drive Slowdown is Independent of Motor -- Simplifies Operation, Enhances Accuracy
Written by Administrator   
Tuesday, 11 August 2009 13:17
Linear Drive Slowdown is Independent of Motor -- Simplifies Operation, Enhances Accuracy

Uhing Company now offers an option for its Model RG rolling ring linear drives whereby the end stop assemblies are fitted with specially machined v-shaped cams to decrease the linear speed of the drive prior to reversal without using electronic controls or reducing motor speed. After reversal, the cams permit smooth ramp up of the drive speed eliminating possible whipping action of tools such as spray heads or cutting blades.

Dissipating momentum in this manner enhances processes where over-travel or abrupt movement of the tool head is undesirable and smooth reversal improves accuracy. Slowdown cam applications include applying coatings, slitting, spray painting and moving measuring probes. Because ramp down/up of linear speed is mechanically controlled, independent of the motor, operation of the drive is simple requiring only minimal operator training.

The slowdown cam option is available on new Uhing rolling ring drive assemblies and, in some cases, may be retrofitted in the field. The length of the cam arms is specific to the size of the Uhing drive on which the cam will be used. Once installed, the slowdown cams must be adjusted in relation to the position of the end stops to assure proper functioning and accurate location of the drive reversal points. Amacoil technical representatives can assist with this process and also in sizing and selecting Uhing drives for custom applications.
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