Electrostatic microactuators, active three-dimensional...

Details Electrostatic microactuators, active three-dimensional... Electrostatic microactuators, active three-dimensional...

2000-07-28

2002-10-08

Huson, Gregory L. (Department: 3751)

Surgery

Means for introducing or removing material from body for...

Treating material introduced into or removed from body...

C310S020000, C310S0400MM, C310S309000, C318S135000, C604S021000

Reexamination Certificate

active

06461337

ABSTRACT:

The present invention relates to the field of electromechanical microactuators, that is to say the field of microsystems adapted for delivering a controlled mechanical force in response to an electrical excitation.
The document U.S. Pat. No. 5235225 describes a structure in which the basic actuator consists of two parallel stators each furnished with a plurality of electrodes and with a flexible rotor, which is at least partially electrically conductive and disposed between the two aforesaid stators. The displacement of the rotor is effected through sequential control, progressing in space, from an electric voltage applied to the electrodes.
More precisely still, the present invention relates to the field of electrostatic microactuators known as “Scratch Drive Actuators” or “SDAs”. Specifically, the present invention relates to an electrostatic microactuator based on distributed elementary SDAs.
A description of these actuators will be found in the documents [1], [2] and [3].
These actuators proposed some years ago, are most particularly intended for the direct driving of micromachines of micrometer dimensions. They have the particular feature of associating a mechanism for transferring mechanical energy by friction with the conventional implementation of an electrostatic force field.
The aforesaid documents may usefully be referred to for a proper understanding of the general structure and the operation of these actuators.
The latter are shown diagrammatically in
FIGS. 1A
to
1
D attached.
In essence, an SDA comprises a plate or beam (
10
), made for example of polysilicon, furnished at one end with a projecting strut or pad
12
, directed toward a substrate
20
, made for example of silicon, covered with an insulating layer
22
, made for example of Si
3
N
4
. A generator
30
is adapted for applying voltage pulses between the plate
10
and the substrate
20
.
As may be seen in
FIG. 1B
, on a rising pulse edge, the plate
10
is drawn toward the substrate
20
by the electrostatic force generated between the latter. The bearing of the strut
12
on the layer
22
imposes a static flexion on the plate
10
, which in turn gives rise to an offset of the strut
12
.
On the falling edge of the pulse, as may be seen in
FIG. 1C
, the plate
10
tends to revert to its rest geometry, by virtue of the elastic energy stored up in the plate
10
, and has therefore been shifted by a flexion of the plate
10
, which in turn gives rise to an offset of the strut
12
.
On the falling edge of the pulse, as may be seen in
FIG. 1C
, the plate
10
tends to revert to its rest geometry, by virtue of the elastic energy stored up in the plate
10
, and has therefore been shifted by an amplitude dx with respect to its former position, by reason of the bearing defined between the strut
12
and the layer
22
.
Thus, these systems make it possible to convert mechanical oscillations of very small amplitude originating in the static flexion of the thin plate
10
, into a rigid body motion of this same plate.
The lower the height of the strut
12
situated under the flexing plate
10
, the stronger the electrostatic forces produced at the plate
10
/substrate
20
interface, for a given excitation voltage emanating from the generator
30
. The height of the strut
12
typically of the order of a micrometer, moreover introduces a high gearing reduction in the energy conversion mechanism at the plate
10
/substrate
20
interface. The gearing reduction intrinsic in the very small mechanical deformations involved in energy conversion by friction, contributes to a dual increase in the driving forces generated during the displacement of the plate. SDAs thus have the particular feature of developing sizeable useful forces at low speed, in the absence of any auxiliary speed decrease.
The length of the displacement step depends on the height of the strut
12
, on the stiffness of the plate
10
and on the control voltage applied. The displacement step is typically of the order of 25 nanometers for a plate
10
exhibiting a width of the order of 50 micrometers, a thickness of the order of 1 micrometer and a length of the order of 60 micrometers.
The repeating of such cycles makes it possible to accumulate displacement steps and consequently allows a sizeable relative displacement between the plate
10
and the substrate
20
.
However, although they are showing themselves to be very promising, to the knowledge of the inventors, SDAs have hitherto remained on the laboratory scale and have not enjoyed industrial development.
This seems to be due in particular to the fact that the force generated by the known SDAs remains limited even if it is sizeable on the micrometer scale. This force, typically of the order of from 50 to 100 micro newtons for an SDA energized at a peak excitation voltage of the order of 100 V, can satisfy only a limited number of applications reserved exclusively for the micromachine scale.
Also, attempts to appreciably intensify this force by increasing the size of the SDAs have not been crowned with success hitherto.
Indeed, on the one hand the electrostatic forces involved in the actuation decrease very rapidly with increasing dimensions of the SDAs. On the other hand, the production processes involved in fabricating SDAs prevent the production of devices having a thickness greater than a few microns, which constitutes an intrinsic limitation to the increasing of the other dimensions of the SDA.
The object of the present invention is now to propose novel means making it possible to implement SDAs industrially.
Contrary to current attempts tending to increase the size of an SDA so as to obtain an acceptable output force, within the framework of the present invention it is proposed to retain SDAs of small size, but to increase their number and to associate them under suitable conditions so as to allow the addition of the forces generated by each of these SDAs, namely by using means adapted for, on the one hand, applying to said SDAs an external mechanical prestress able to allow a superposition of the forces generated by the various SDAs and, on the other hand, communicating to an external load the entire driving force emanating from the collective behavior of these same SDAs.
This mechanical prestress of the SDAs is advantageously obtained with the aid of a bias voltage applied at rest to the set of SDAs.
To allow the entire driving force emanating from the collective behavior of the SDAs to be communicated to an external load, according to an advantageous characteristic of the present invention, the sheet carrying the SDAs is placed in a mechanical clearance at the interface of two solid bodies articulated together.
The inventors have in fact demonstrated that such a prestress, associated with means guaranteeing the communication of the driving force, to the external load, was indispensable for allowing aggregation of the forces generated by the various SDAs.
The cooperation of microactuators has already been utilized in the field of the motorization of micromachines but, to the knowledge of the inventors, only via the development of conveyors of objects by friction in the horizontal plane, so as to profit from the gravity of the displaced object. With such devices, the driving forces communicated to the movable element depend only on the mass of the displaced object, as well as on the coefficient of friction at the object/actuators interface (in accordance with Coulomb's laws of solid friction) . In this case the driving forces communicated to the displaced object are unaffected both by the number and by the driving characteristics of the actuators participating in the motorization. Moreover, these same driving forces depend on the configuration of the machine (or conveyor) in space and in particular on the horizontality of the object transfer plane. It is clear, consequently, that increasing the number of actuators participating in the motorization of one and the same load does not necessarily lead to a matching increase in the useful forces involved

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