The present invention concerns an actuator with interchangeable
bending-vibrating foils, based on a Langevin-type transducer.
More in detail, the present invention refers to a piezoelectric
actuator able to excite a bending motion in a steel foil; the overall system has
a high efficiency, low losses and high mechanicals stiffness. The applications of
the device are mainly in aesthetics and dermatology.
The typology of piezoelectric actuators able to excite
a bending motion in a steel foil is known.
The applications of the device are in dermatological aesthetics
and in particular it can be used for:
- Exfoliation of the most superficial horny layer (for example dead cells, sebum);
- Bacterial sterilisation (causing spots and acne);
- Increase of the blood sprinkling;
- Stimulation of the cell turnover;
- Elimination of the non-pathological brown blots of the superficial layers (for
- Rebalancing of the local metabolism;
- Removal of the wrinkles;
- Removal of the black points.
Efficiency of the treatment is obtained by making the steel
foil vibrating at the frequency of around 25 kHz.
To make the foil vibrating, the current state of the art
provides for the use of the device shown in figure
1. The device is constituted by the following parts:
- Vibrating steel foil 1';
- Four piezoelectric ceramics 2'; and
- Steel base 3'.
The motion is induced by exploiting the radial mode of
the piezoelectric ceramics 2': the ceramics placed on the upper surface of the base
3' are excited by a sinusoidal signal which is in counterphase with respect to those
placed on the lower surface, thus forcing the base itself to bend; and this motion
is therefore transferred to the vibrating foil 1'.
The excitation of a transversal mode in the piezoelectric
ceramics, i.e. a mode in which the direction of the motion is perpendicular to that
of the applied electrical field, is exactly one of the problems of the actuator
shown in figure 1: the transversal modes
show indeed a transduction efficiency lower than that of longitudinal ones, that
is those in which the motion direction is parallel to that of the applied electrical
Another problem is represented by the way by which the
foil 1' is bound to the base 3': in the current device the joint is realised by
brazing; in the actuator functioning exactly this one revealed as the weakest part:
it has been observed indeed a yielding, followed by a detachment of the foil 1'
of the base 3'; this phenomenon considerably reduces the useful life of the device.
Further, the stress undergone by the device of the prior
art can lead to the depolarisation of the piezoelectric ceramics, as well as their
detachment (they are only glued to the base).
Finally, the fact that the transducer and the foil are
a unique element excludes the possibility to change the foil with respect to the
zones of the body to be treated.
It is object of the present invention to provide a piezoelectric
actuator able to excite a bending motion in a steel foil which solves partially
or totally the problems of the prior art and avoids the above described disadvantages.
It is subject-matter of the present invention an actuator
with bending-vibrating foils, comprising:
- a transducer of the "Langevin" type, connected to a
- foil with a longitudinal principal extension direction, characterised in that:
- the transducer is such to excite longitudinal modes, i.e. modes in which the
direction of the produced motion is equal to that of the applied electrical field;
- foil is placed in such a way that its longitudinal principal extension direction
is substantially perpendicular to the direction of the motion produced by said transducer.
According to the invention, the connection between the
transducer and a foil is realised by means of a support that has a superficial dimensional
substantially equal to the cross-section of the end of the transducer which connects
to the foil and the other superficial dimensions equal to the width of the foil
at the connection zone. According to the invention, said support shows a threaded
aperture and the end of the transducer which connects to the foil presents a pin
for realising such a connection together with the bolt placed on the support, so
as to lower the stresses on such joint and allow the simple substitution by the
user of the whole constituted by foil and support.
Preferably according to the invention, the foil and the
support are formed as an only part.
Preferably according to the invention, said transducer
has an extended form and comprises a head and a tail between which piezoelectric
elements are placed.
Preferably according to the invention, said head and said
tail are made of steel, and said piezoelectric elements are made of ceramics.
Preferably according to the invention, said transducer
further comprises a concentrator placed between the foil and the head of the transducer,
said concentrator having cross-section smaller than the cross-section of said head.
Preferably according to the invention, the total length
of the transducer is substantially equal to one half of the wavelength &lgr; in
the material, calculated at the working frequency.
Preferably according to the invention, the thickness of
said support is comprised between 1 mm and 5 mm.
Preferably according to the invention, the head is provided
with a pin by which it connects to the piezoelectric elements and to the tail which
acts as bolt.
Preferably according to the invention, the head and the
concentrator are formed as an only piece.
Preferably according to the invention, between the piezoelectric
elements thin rings of copper are placed by which the electrical connections between
the actuator and an external excitation circuit are realised.
Preferably according to the invention, the electrical contacts
are further fixed between themselves and to the head and tail by means of an epoxy
Preferably according to the invention, the piezoelectric
elements are four.
Preferably according to the invention, the piezoelectric
elements are electrically connected between themselves in parallel and in such a
way that the head and the tail are at null potential.
The invention will be now described by way of illustration
but not by way of limitation, with particular reference to the figures of the enclosed
- Figure 1 shows a scheme of a traditional
piezoelectric actuator, cross-section (a) and plant (b);
- Figure 2 shows the distribution of
stresses intensity in the traditional device according to figure
1, calculated by simulation at working frequency (23.6 kHz);
- Figure 3 shows a scheme of an embodiment
of the piezoelectric actuator according to the invention;
- Figure 4 shows the distribution of
the displacements in the piezoelectric actuator according to figure
3, calculated by simulation at the frequency of 26,1 kHz;
- Figure 5 shows the distribution of
the stresses in the piezoelectric actuator according to figure
3, calculated by simulation at working frequency (26,1 kHz).
The research of a new solution of piezoelectric actuator
with bending-vibrating foils has been preceded by researches on the mechanical stability
of the prior art solution.
In order to verify whether the fragility of the prior art
device (figure 1) is due to structural
problems or to a bad realisation thereof, its functioning has been simulated by
means of ANSYS code which implements a finite-elements analysis method (FEM).
The results obtained by means of the FEM analysis are shown
in figure 2 and highlighted the maximum
of the mechanical stress exactly at the joint between the base 3' and foil 1'. The
stress, in this zone, calculated by imposing a power supply voltage with an amplitude
equal to 250 V (typical for those applications), varies, depending on the excitation
frequency, between 120 and 150 MPa and is of the same magnitude order of the yield
point, of around 500 MPa for the steel of which base and foil are constituted.
Finally, the fact that the joint is realised by brazing
further increases the risk of breaking; in such a case indeed the yield point is
very below of that of the material.
One can therefore conclude that the reason of the short
life of the prior art device is implied in its configuration and therefore is of
In figure 2 regions
can be observed as well, at the boundary between the ceramics and the base, which
are also interested by a high stress, between 1 and 20 MPa; the causes are to be
mainly attributed to a series of natural sub-modes, which superpose to the interest
principal mode, and so that the ceramics undergo light lateral torsions which result
in higher stress on the same. The negative consequence of this high value of the
stress is an excessive stress of the glue by which the ceramics are bound to the
base, with following detachment of the ceramics themselves, a phenomenon which occurred
on some samples of the actuator.
Finally, always in figure
2, in the middle of the ceramics farthest from the foil, one notices a
stress that can reach even 37 MPa, that is to exceed the limit value (30 MPa) beyond
which the ceramics depolarise and therefore loose the piezoelectric properties,
thus making the device as ineffective.
The device according to the present invention exploits
the action of the piezoelectric ceramics in a way completely different with respect
to the current device.
In particular, the bending motion of the foil is induced
by exciting the ceramics in the thickness mode, which, being a longitudinal mode,
allows to obtain a better transduction efficiency.
The proposed device is shown in figure
3; is constituted by the following products:
- Vibrating steel foil 1;
- Steel supports 2 of the foil;
- Step concentrator 3 of the Langevin actuator, made of steel;
- Tail 4 of the Langevin actuator, made of steel;
- Head 5 of the Langevin actuator, made of steel;
- Piezoelectric ceramics 6.
The parts 3, 4, 5 and 6 constitute a transducer or piezoelectric
actuator of the Langevin type, complete of concentrator 3. With the term "actuator"
is meant also the whole of the illustrated device.
The support 2 is necessary in order to transfer the motion
from transducer 20 to foil 1; for this reason its width is equal to that of the
foil 1, whilst the length is equal to the diameter of the concentrator 3.
The design parameters of the actuator according to the
invention are the longitudinal dimension of the body of the transducer (tail 4,
head 5 and piezoelectric ceramics 6), the ratio between the length of the tail 4
and of the head 5, the ratio between the radius of the head 5 and that of the concentrator
3. The length of the body of the transducer determines its resonance frequency,
the number and the dimensions of the piezoelectric ceramics, the efficiency of transduction,
whilst the ratio between the dimensions of the head and of the tail shifts the zone
in which the displacements in the longitudinal direction are maximal. Finally the
concentrator has the aim of the amplifying the displacements of the end connected
to the foil; the amplification factor is equal to the ratio between the radius of
the head and that of the concentrator.
The length of the transducer body (parts 4, 5 and 6) and
of the concentrator (part 3) are in general dimensioned so as to be, altogether
(and therefore the overall length of the transducer 20), equal to the wavelength
(&lgr;) in the material, calculated at working frequency: 25 kHz in the prototype
realised by the applicant.
The behaviour of the device has been studied by means of
the use of the ANSYS code and it has been verified that if the total length is equal
to the half of the wavelength (&lgr;/2), calculated at the working frequency,
the amplitude of the displacements of the vibrating foil 1 is larger than that obtained
by designing the transducer so that its dimension is equal to &lgr;. By using
the &lgr;/2 structure, the actuator behaves better even from an electrical viewpoint,
in the sense that the electrical resonance is more pronounced, and therefore one
expects a higher efficiency for that solution.
These results allow reducing the longitudinal overall dimensions
of the device.
Finally, by means of ANSYS, it has been possible to dimension
the thickness of the support 2 of the steel foil so as to transfer in optimised
way the motion of the transducer to the foil: for example, with a foil 1 of the
thickness of 0,5 mm, the thickness of the support 2 must be of 3 mm. Different dimensions
of the support 2 have been verified and the chosen solution allows obtaining a uniform
bending motion of the foil.
In figure 4 the
field of the displacements is shown, calculated by means of ANSYS at a frequency
of 26,1 kHz; as already stressed, the "Langevin" transducer has a length equal to
&lgr;/2 at that frequency. As one can see, the foil 1 moves uniformly by bending
and the displacements have their maximum amplitude exactly at his free end. Further
the displacements in the "Langevin" type actuator are negligible with respect to
those of the foil, and therefore one expects a good stiffness of the whole structure.
In order to verify whether actually the proposed structure
is stiffer than that currently used, also the stresses inside it had been calculated
by exciting it at 26,1 kHz, with a sinusoidal voltage of amplitude equal to 250
V. In figure 5 the field of stresses along
the structure is shown.
As one can see, only in a small part, in the middle of
the contact zone between the foil 1 and a support 2, the stress reaches a value
of about 100 MPa, whilst in the remaining part it is below 76 MPa and even sinks
below 13 MPa in the external zones. With respect to the traditional solution, the
improvement is therefore remarkable.
Even the stress on the piezoelectric ceramics is very small,
below 15 MPa; this avoids the depolarisation of the ceramics. Finally, one has the
maximum of the stress on the foil, however it doesn't exceed 140 MPa and therefore
it is very below the stress limit of the steel.
Concerning the possibility of using vibrating foil with
form and dimension different from that shown in figure
3 and in the ANSYS simulations (figures
4 and 5), the solution is very
simple, it is sufficient to simply substitute the foil 1 and support 2 with the
new foil complete with its own support.
In order to describe even in more detail the practical
realisation of the device according to the invention, one makes reference to
figure 3, where different parts constituting
it are highlighted.
The assembling ways are the following: the vibrating foil
1 and the support 2 are preferably realised in a unique block of harmonic steel
and the support has in the middle a hole; further, the upper end of the step concentrator
3 of the actuator has a threaded pin which crosses the support 2 and allows to fix
the two parts by means of a bolt placed on the support.
This system to fix the foil to the actuator has the advantage
to lower the stresses on the joint, indeed here the foil is not brazed laterally
on the support, as in the current device, but superposed to it; it further offers
to the user the possibility of easily substituting foil and support, so as to use
the foil most suitable to the zone to be treated.
The step concentrator 3 and the head 5 of the actuator
realised in a sole part made of steel; as above mentioned, the upper end of the
concentrator 3 is provided with a pin in order to be able to fix on it the foil
1 with the support 2, the lower end of the concentrator 3 is also provided with
a threaded pin which is screwed on to the tail 4 of the actuator, which in turn
acts as a tightening bolt. The lower pin has also the aim of tighten the piezoelectric
ceramics 6, which are rings, between the head and the tail of the " Langevin" actuator
and of allowing therefore to realise a single device.
Between the ceramics 6, thin rings of copper are placed,
with the same surface of the ceramics, by which the electrical connections between
the actuator and the excitation circuit are realised.
By means of the mechanical torque by which the head and
tail of the " Langevin" are mutually tightened, one sets the pre-stress of the piezoceramics;
the pre-stress serves to improve the performances of the whole device.
Finally, the ceramics and the electrical contacts are further
bound mutually and to the parts 5 and 4 by means of an epoxy resin, which guarantees
a perfect tightening of the structure, avoids possible loosenings due to vibrations,
but most of all avoids the presence of air between the various parts of the actuator,
what would have dreadful effects on the propagation of the elastic waves in the
Concerning the piezoelectric ceramics, one has chosen to
use four ones; as said, this choice influences the efficiency of the actuator, in
the sense that the larger is the amount of the ceramics the greater will be efficiency,
on the other hand an excessive number of ceramics reduces too much the electrical
capacity of the device, therefore it increases its input impedance and make it difficult
to supply it from an electrical viewpoint.
Exactly to limit the input electrical impedance of the
device, the piezoceramics are connected between them in parallel and in such a way
that the parts 4 and 5 in figure 3 are
at null voltage; the last arrangement allows to avoid risks of exposure to electrical
discharges in the use of the device.
The preferred embodiments of the invention and some of
its variations have been hereinabove described, but it should be understood that
those skilled in the art can make modifications or changes therein without departing
from the scope of this invention as defined by the following claims.