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Monday, 20 December 2010 14:59

Abstract:
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
Introduction
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
stroke.
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
technologies.
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
actuator.
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
heating.
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
lighter.
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
Conclusion
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
control.
Acknowledgement
The authors thank M.Csukai (OSEO) and M.Amiet
(DGA) for supporting these works.
References
[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,

 
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