FR2658961A1 - Electrostatic micromotor - Google Patents

Abstract

The micromotor (1) includes a rotor (3) fixed so as to rotate with respect to a stator (2) comprising coplanar electrodes intended to create an electric field between the stator (2) and the rotor (3). In order to enhance the useful torque which the motor (1) can deliver, the rotor (3) includes a peripheral ring (13) arranged facing electrodes (10) and linked to a hub by arms (14) which allow this ring (13) to tilt easily in response to the electric field created by the electrodes.

Description

ELECTROSTATIC MICROMOTOR
The present invention relates to an electrostatic micromotor comprising a rotor having a hub and a peripheral zone, a stator comprising coplanar electrodes arranged opposite said peripheral zone and intended to produce an electric field between said stator and said rotor, and means for connect said rotor hub to said stator in a rotary manner.

 All the micromotors which will be mentioned below, including the micromotor according to the invention, are manufactured in silicon wafers or of another material having similar characteristics using methods derived from those which are used to manufacture integrated circuits, and which are well known.

Examples of manufacturing procedures for these micromotors are described, in particular, in US Pat. No. 4,740,410, in the minutes of the conference entitled "IC-Processed Electrostatic Micromotors" made by MM. L.-S. Fan et al. at the Congress on Integrated Electrical Devices (IEEE Integrated Electrical Devices
Meeting) which was held from 11 to 14 December 1988 in San Francisco (USA) and in the minutes of the conference entitled "IC-Processed Micro-motors: Design, Technology and Testing" made by MM.

Y.-C. Tai et al. at the Congress on Electromechanical Microsystems (IEEE Micro-Electro-Mechanical Systems Meeting) held in Salt-Lake City (USA) from February 20 to 22, 1989.

 These methods will therefore not be described again here, even if they may differ slightly from one another depending on the particular constitution of these micromotors.

 The conference reports mentioned above describe micromotors comprising a flat rotor in the shape of a cross with four or eight branches joined by a central hub and a stator comprising six or twelve control electrodes arranged in a circle around this rotor and in the same plan as this.

 The means for fixing the rotor to the stator comprise a cylindrical opening pierced in the central hub and a pivot forming part of the stator and engaged in this opening.

 To rotate the rotor of this motor, each arm of the latter is subjected to a tangential force produced by an electric field created by a voltage applied to some of the control electrodes.

 But this electric field also subjects the arms of the rotor to radial forces. If the ends of these arms were always strictly at the same distance from the control electrodes, these radial forces would compensate two by two and their result on the rotor would be zero.

 However, it is obviously impossible to manufacture such an engine while respecting this condition, since, in particular, there must be a certain clearance between the hub of the rotor and the fixed pivot of the stator so that this rotor can rotate around this pivot.

 The distances separating the ends of two diametrically opposite arms of the control electrodes are therefore always different, and the radial forces which are exerted on these arms, and which are inversely proportional to the square of these distances, do not compensate for each other. The rotor is therefore also subjected to a resulting radial force which causes friction of the internal wall of the opening of its hub against the fixed pivot passing through this opening.

 This friction obviously reduces the useful torque that the motor can provide, and can even prevent any rotation of its rotor. Furthermore, the effect of this friction cannot be compensated by an increase in the voltage applied to the control electrodes, since such an increase would have the effect of increasing the radial force exerted on the rotor, and therefore also of increasing the friction mentioned above.

 Various types of motors which do not have this drawback are described in an article entitled "Harmonic Electrostatic Motors" written by MM. W. Trimmer and R. Jebens and published in number 20 (1989) of November 15, 1989 of the review "Sensors and Actuators" published by the Elsevier Sequoia Company (Netherlands).

 One of these motors, illustrated diagrammatically by FIGS. 4 and 5 of this article, comprises a flat stator provided with control electrodes and a rotor having the shape of a solid circular disc which, in the absence of voltage on the electrodes of the stator, is also flat and also parallel to the stator.

 The means for fixing this rotor to the stator comprise a pivot forming part of the rotor and certainly, although this is not shown, a bearing fixed to the stator and in which this pivot is intended to rotate.

 The stator electrodes are fed successively and cyclically so that the points on the periphery of the rotor touch one after the other points on the stator located on a circle whose diameter is less than the diameter of the rotor.

 As a result, the rotor turns around its axis in the same direction as that in which the point of contact between this rotor and the stator traverses the circle mentioned above.

 The electrostatic force which attracts one of the points on the periphery of the rotor of this motor against the stator is obviously substantially perpendicular to the plane of the latter and therefore has practically no radial component, which is an advantage compared to other electrostatic motors. described above.

 But this electrostatic force creates a tilting torque which tends to rotate the plane of the rotor around a straight line perpendicular to the axis of rotation of this rotor and to the right joining the center of the latter to the point of application of this strength.

 As this rotor necessarily has a certain rigidity, this tilting torque also causes an increase in friction between the pivot of the rotor and its bearing.

 The applicant has observed that, although the rotor of this motor is described as "flexible" in the article mentioned above, it exhibits, because of its shape of a solid disc, a stiffness such that this friction due to this torque tilting is important enough to significantly decrease the torque that can be supplied by this engine.

 The object of the present invention is to propose a micromotor of the same kind as the latter, but in which the friction between the mobile part and the fixed part of the means for fixing the rotor to the stator is significantly lower, the torque provided by the micromotor of this invention being so, all other things being equal, significantly more important.

 This object is achieved by the claimed micromotor, which comprises a rotor having a hub and a peripheral zone, a stator comprising coplanar electrodes arranged opposite said peripheral zone and intended to produce an electric field between said stator and said rotor, and means for rotating connection of said rotor hub to said stator, and which is characterized in that said peripheral zone is constituted by a ring substantially planar and parallel to the plane of said electrodes in the absence of said electric field and covering at least partially said electrodes in a plan view of said motor, and that said rotor further comprises means for mechanical connection of said ring to said hub which are elastically deformable in response to a force applied to said ring in a direction substantially perpendicular to said plane of the electrodes, said hub, said zone peripheral and said connecting means defining between them openings passing through said rotor.

 According to a particular embodiment, said mechanical connection means comprise a straight arm arranged along a radius of said rotor.

 According to another particular embodiment, said mechanical connection means comprise an arm comprising two portions arranged along a radius of said rotor and connected to each other by a plurality of portions alternately perpendicular and parallel to said radius.

 According to yet another particular embodiment, said mechanical connection means comprise a first arm having a first end connected to said peripheral zone and arranged along a first radius of said rotor, a second arm having a first end connected to said hub and arranged according to a second radius of said rotor, and a connecting element connected to the second end of said first arm and to the second end of said two arms.

 The micromotor of the present invention will be described in detail below using the appended drawing which shows, by way of example pte rcn | r rses embodiments of this micromotor.

a this dess n, where the same references designate elements ana
- Figure 1 is a schematic cross section of an embodiment of the micromotor according to the invention;
- Figure 2 is a schematic plan view of the stator of the micromotor according to the invention;
- Figures 3, 4 and 5 are schematic plan views of the rotor of various embodiments of the micromotor according to the invention; and
- Figure 6 is a schematic cross section of another embodiment of the micromotor according to the invention.

 It should be noted that the thicknesses of the various elements of the motors shown in FIGS. 1 and 6 have been greatly exaggerated compared to the other dimensions of these elements to facilitate understanding of these figures.

 The micromotor according to the invention shown in schematic section in FIG. 1 with the reference 1 comprises a stator 2 and a rotor 3.

 In the embodiment represented by this FIG. 1, the stator 2 is substantially flat, except at its center where a pivot 4 is arranged.

 This pivot 4 is in one piece with the stator 2 and comprises two stops 5 and 6 separated by a cylindrical portion 7 having an axis 7a, which is engaged in an opening 8, also cylindrical, which comprises the hub 9 of the rotor 3 (see Figures 3 to 5).

 The diameter of the opening 8 formed in the hub 9 is slightly greater than that of the cylindrical portion 7 and less than that of the stops 5 and 6.

 The rotor 3 can therefore rotate around the cylindrical portion 7 of the pivot 4, and it is retained axially by the stops 5 and 6.

 In another embodiment of the motor 1, shown diagrammatically in FIG. 6, the pivot 4 does not have a stop 6, and the rotor has a circular rib 20 which rests on the face 2a of the stator 2 and maintains the hub 9 at a certain distance from this face 2a.

 This rib 20, which has also been shown, in broken lines, in FIGS. 3, 4 and 5, could be replaced by bosses separated from each other resting on the face 2a of the stator 2 and fulfilling the same office as this rib 20. An embodiment comprising such bosses has not been shown.

 In the rest of this description, reference will only be made to the embodiment of the motor 1 which is shown in FIG. 1, but it will be easily seen that all the advantages.

 The stator 2 further comprises, in this example, sixteen control electrodes 10a to 10p arranged in a circle concentric with the cylindrical portion 7 of the pivot 4. These electrodes, which will be called electrodes 10 in the following description, are all visible in the plan view of the stator 2 shown in FIG. 2.

 It should be noted that, normally, these electrodes 10 are not completely visible in a plan view of the motor 1, since they are partially hidden by the rotor 3, as will be described later. FIG. 2 therefore represents the stator 2 of a motor 1 from which its rotor 3 has been removed.

 It should also be noted that the cross section of the stator 2 shown in Figure 1 is made along the axis AA in Figure 2, and that the cross section of the rotor 3 also shown in Figure 1 is made along the axis BB of Figure 3 which will be described later.

 The electrodes 10 are electrically isolated from each other and individually connected to a control circuit, examples of which will be described later, by conductive tracks 1 to 1.

 These electrodes 10 are covered with a thin insulating layer 12 which is planar, at least in the region of these electrodes 10, and whose face facing the rotor 3 constitutes the face 2a of the stator 2.

 As will be seen below, the rotor 3 is substantially flat and parallel to the face 2a of the stator 2 when the motor 1 is not running, that is to say when no voltage is applied to one or to the other of the electrodes 10. In addition, this rotor 3 is kept at a certain distance from this face 2a by the stop 6 of the pivot 4.

 Figures 3, 4 and 5 show various embodiments of the rotor 3. It should be noted that, normally, the hub 9, already mentioned, of this rotor 3 is not entirely visible in a plan view of the engine 1 , because the central part of this hub 9 and the opening 8 which is formed therein are hidden by the stop 5 located at the end of the pivot 4.

 In all of its embodiments, and in particular in those which are shown by way of nonlimiting examples in FIGS. 3, 4 and 5, the rotor 3 comprises a peripheral circular ring 13 concentric with the hub 9 and connected to the latter by connecting elements which will be described later. As will be seen below, these connecting means define with the ring 13 and the hub 9 openings passing through the thickness of the rotor 3 while, in the motor described by the article by MM. W. Trimmer and R. Jebens mentioned above, this rotor has the shape of a solid disc and has no opening.

 The peripheral ring 13 is arranged so that, in a plan view of the motor 1, it at least partially covers the electrodes 10. For a reason which will be made apparent from this description, this ring 13 is preferably even arranged so that it is entirely located opposite the electrodes 10, always in a plan view of the motor 1.

 For reasons which will also be made clear later on in this description, and also in all of the embodiments of the rotor 3, the ring 13 must be able to withstand all the forces applied to it during the operation of the engine without undergoing any notable deformation. In addition, the connecting elements of the ring 13 to the hub 9 must be shaped so as to easily deform when a force is applied at a point on the ring 13 in a direction substantially perpendicular to the plane of the electrodes 10, to that, in response to this force, the plane of the ring 13 can easily tilt around a straight line substantially perpendicular to the axis 7a of the cylindrical portion 7 of the pivot 4 and to the radius of this ring 13 passing through the point d application of this force.

 These connecting elements of the ring 13 to the hub 9 must however be rigid enough so that they do not deform appreciably under their own weight and under that of the ring 13, or at least so that this ring 13 does not touch the stator 2 when the motor 1 is not operating, that is to say when no electrical voltage is applied to the electrodes 10, and this regardless of the orientation of the motor 1 relative to the horizontal.

 In the embodiment of the rotor 3 shown in FIG. 3, these connecting elements comprise four rectilinear radial arms 14. Two of these arms 14 are visible, in section, in FIG. 1 which, as already mentioned, represents the motor 1 in a case where it is equipped with the rotor 3 of FIG. 3.

 It can be seen that the conditions mentioned above are fulfilled when the arms 14 have dimensions such that they have a low resistance to the bending forces which they undergo when the force mentioned above is applied to the place where they are connected to the ring 13, as well as a low resistance to the torsional forces which they undergo in response to this force when the direction of their length coincides with the straight line, mentioned above, around which the plane of the ring 13 must be able to tilt.

 In the embodiment of the rotor 3 represented by FIG. 4, the connecting elements of the ring 13 and of the hub 9 also comprise four arms, designated in this case by the reference 15.

 These arms 15 are not straight but each have two end sections 15a and 15b connected respectively to the hub 9 and to the ring 13 and aligned radially with one another, two intermediate sections 15c and 15d parallel to the sections 15a and 15b but arranged at a certain distance on either side of the latter, and three connecting sections 15e, 15f and 15g perpendicular to the preceding ones and connecting respectively the sections 15a and 15c, 15b and 15d, and 15c and 15d.

 We see again that the dimensions of the different sections 15a to 15g of the arms 15 can be easily chosen so that the latter have the characteristics mentioned above.

 It can also be seen that, all other things being equal, the shape of the arms 15 of the rotor 3 of FIG. 4 makes it possible to give these arms 15 resistance to the bending and torsional forces which they undergo in response to a force applied in a point on the ring 13 in a direction perpendicular to the plane of the rotor 3 which is weaker than that which can be given to the arms 14 of the rotor 3 of FIG. 3.

 But it can also be seen that this shape of the arms 15 of the rotor 3 of FIG. 4 has the consequence that their resistance to bending in the directions parallel to the plane of this rotor 3 is less than that of the arms 17 of the rotor 3 of FIG. 3.

 However, as will be made clear below, a low resistance of the elements for connecting the ring 13 with the hub 9 to the bending and torsional forces mentioned above is more important for the proper functioning of the engine 1 than high resistance to bending of these elements in directions parallel to this plane. The form of execution represented by FIG. 4 is therefore more favorable, from this point of view, than that which is represented by FIG. 3.

 In the embodiment of the rotor 3 represented by FIG. 5, the connecting elements of the peripheral ring 13 and of the hub 9 comprise an intermediate ring 16 concentric with the peripheral ring 13 and with the hub 9 and connected respectively to these the latter by a first pair of straight arms 17 and by a second pair of arms 18. The two arms 17 and the two arms 18 are arranged on a first and, respectively, on a second diameter of the rotor 3, these two diameters being perpendicular l to each other.

 It is easy to see that, also in this embodiment, it is easy to give the various components of these connecting elements dimensions such that these connecting elements have the characteristics mentioned above.

 It is obvious that there are very many other embodiments of the rotor 3 in which the elements for connecting the ring 13 to the hub 9 also have the desired characteristics. These other embodiments will not be described here because their embodiment follows easily from that of the embodiments of FIGS. 3, 4 and 5.

 Thus, for example, these connecting elements may have more or less than four arms similar to the arms 14 and 15 of Figures 3 and, respectively 4, and even have only one.

 Still for example, these connecting means may comprise four arms respectively similar to the arms 17 and to the arms 18 of FIG. 5 and, in place of the ring 16 of this same figure, two portions of symmetrical ring, one of the other relative to the center of the rotor 3 and each connecting one of the arms similar to the arms 17 to one of the arms similar to the arms 18. These connecting means could even have only two arms similar to the 'one of the arms 17 and the other to one of the arms 18, connected by a single portion of intermediate ring.

 With regard to the operation of the motor 1, all these embodiments of the rotor 3 are practically equivalent. Likewise, it is practically irrelevant for this operation that the hub 9 of the rotor 3 is kept at a certain distance from the face 2a of the stator 2 by the stop 6 in FIG. 1 or by the rib 20 in FIG. 6.

 In order not to unnecessarily complicate the description of this operation which will follow, no special reference will therefore be made to one or the other of these embodiments, but simply to the rotor 3, and this description will be limited to the case of figure 1.

 We will first consider the case where the rotor 3 of the motor 1 is made of an electrically conductive material, for example polycrystalline silicon, where the pivot 4 of the stator 2 is also made of an electrically conductive material, which can also be silicon polycrystalline, and where this pivot 4 is electrically connected, for example, by a connection embedded in the material of the stator 2, to a first terminal of the electrical supply source of the device formed by the motor 1 and its control circuit.

 In this case, the rotor, the hub 9 of which is always in contact with the pivot 4, is therefore also connected to this first terminal of this power source. If necessary, the external surface of the pivot 4 and / or of the rotor 3 can be covered with a thin layer of a material which is good conductor of electricity, such as a metal, to improve the electrical contact between the pivot 4 and the rotor 3.

 The control circuit of the motor 1 is arranged in this case so as to connect successively and selectively each of the electrodes 10 to the second terminal of the power source of the device when this motor 1 is to operate.

 Let us first assume that this control circuit connects the electrode 10a to this second terminal of the power source.

 In response to the control voltage thus applied between the rotor 3 and this electrode 10a, the latter and the portion of the ring 13 which faces it are subjected to an electrostatic force produced by the electric field created by this voltage and which tends in attracting this portion of the ring 13 against this electrode 10a.

 If the control voltage resulting from this force is sufficient, the latter subjects the ring 13 to a tilting torque which turns its plane around a straight line substantially perpendicular to the axis 7a of the cylindrical portion 7 of the pivot 4 and to the radius of this ring 13 passing through the point of application of this force. In response to this tilting torque, a point on the edge 13a of the ring 13 (FIG. 2) touches the face 2a of the stator 2 opposite the electrode 10a, thanks to the characteristics of the chosen connecting elements of this ring 13 at the hub 9 of the rotor 3 mentioned above.

 As the surface 2a of the stator 2 is constituted by the insulating layer 12, there is no electrical contact between the ring 13 and the electrode 10a.

 The surface of the mechanical contact zone between the ring 13 and the face 2a of the stator 2 obviously depends on the hardness of the materials present and the force created by the electric field, as well as on the flexibility of the connecting elements of the ring 13 to hub 9.

This surface is however small enough that it can be assimilated to a point which will be called contact point in the following description.

 If now the control circuit suppresses the voltage which it applied between the rotor 3 and the electrode 10a, the portion of the ring 13 which was pressed against the face 2a of the stator tends to return to its initial position due to the elasticity of the connecting elements of the ring 13 to the hub 9.

 If in addition, at the same time as it removes the voltage between the rotor 3 and the electrode 10a, the control circuit applies a voltage of the same value between the rotor 3 and the electrode 10b, the latter and the portion of the ring 13 which faces it are in turn subjected to an electrostatic force which causes the displacement of the point of contact between the edge 13a of the ring 13 and the face 2a of the stator 2 to a new position located opposite of electrode 10b.

 During this displacement of the contact point, the edge 13a of the ring 13 rolls on the face 2a of the stator 2, provided of course that the resistive torque applied to the rotor 3 is not too high, and that the control voltage is applied between the electrode 10b and the rotor 3 exactly at the instant when it is removed between the electrode 10a and this rotor 3, or even slightly before this instant.

 This process can obviously be continued, the motor control circuit 1 successively and individually applying a voltage between the rotor 3 and each of the electrodes 10c, 10d, etc.

 It follows from this successive application of the control voltage between the rotor 3 and the electrodes 10 that the edge 13a of the ring 13 rolls on the surface 2a of the stator 2 and that the point of contact between this edge 13a and this face 2a runs through a circle, which has been shown in broken lines in FIG. 2 with the reference 19. This circle 19 will be called rolling circle in the rest of this description.

 In the example which has just been described, the contact point describes this rolling circle 19 in the direction defined by the alphabetical order of the references of the electrodes 10. It is obvious that this direction of displacement of the contact point along of this rolling circle 19 can be reversed by changing the order in which the electrodes 10 are subjected to the control voltage.

où r est ce rayon du cercle 19, R est le rayon du rotor 3 et d est la distance entre l'anneau 13 et la face 2a du stator 2 en l'absence de tension sur les électrodes 10.It is easy to see that the radius of this rolling circle 19 is less than the radius of the rotor 3, since it is given, as a first approximation, by the relation

where r is this radius of the circle 19, R is the radius of the rotor 3 and d is the distance between the ring 13 and the face 2a of the stator 2 in the absence of voltage on the electrodes 10.

 It obviously follows that the circumference of the rolling circle 19 is shorter than that of the rotor 3 and that, each time that the contact point makes a complete turn on the face 2a of the stator 2, it makes less than one turn on the edge 13a of the ring 13.

 In other words, if one designates by P1 the point of the edge 13a which is in contact with the face 2a of the stator 2 when the control voltage is applied to a determined electrode 10, it is another point P2 of this edge 13a which is in contact with this face 2a when the same electrode is again subjected to this control voltage after all the other electrodes have been successively subjected to this voltage.

 The distance between these points P1 and P2, on the edge 13a, is equal to the difference between the length of the circumference of the rotor 3 and the length of the circumference of the circle 19.

 Relative to the direction of rotation of the contact point on the circle 19, the point P1 is in front of the point P2. The rotor 3 therefore rotates around the pivot 4 in the same direction as the contact point or, in other words in the same direction as that in which the electrodes 10 are successively subjected to the control voltage.

In addition, the axis of rotation of the rotor 3 is substantially coincident with the axis 7a of the cylindrical part 7 of the pivot 4.

It is easily seen that, whatever the angle traversed by the point of contact between the rotor 3 and the stator 2, the ratio K between this angle and the corresponding angle of rotation of the rotor 3 around the pivot 4 is given by the relationship
r
Rr in which r and R are respectively the radii of the rolling circle 19 and of the rotor 3 already defined above.

 As the distance d which separates the ring 13 and the surface 2a of the stator 2 in the absence of voltage on the electrodes 10 and which defines the radius r relative to the radius R as has been shown above is generally small screw with respect to this radius R, the ratio K can be very large.

 Thus, for example, the applicant of the present application has produced a motor such as that of FIGS. 1 and 2 in which the distance d is approximately two micrometers and the radius R of approximately 250 micrometers. In this engine, the ratio K is therefore approximately equal to 31,000.

 This means that the motor control circuit must perform approximately 31,000 times the successive supply of all the electrodes 10 so that the rotor 3 makes a revolution around the pivot 4.

 As already mentioned, the electrostatic force resulting from the application of a voltage between the rotor 3 and one of the electrodes 10 has no radial component, that is to say directed along a radius of the rotor 3, but creates a tilting torque which rotates the plane of the ring 13 around a straight line substantially perpendicular to the axis of rotation of the rotor 3 and to the radius of this ring 13 passing through the point of application of this strength.

 This tilting torque is equal, at least as a first approximation, to the product of the radius of the ring 13 by the electrostatic force which is necessary so that the edge 13a of this ring 13 just touches the face 2a of the stator 2. As this force is obviously the lower the more flexible the rotor, we see that, in the case of the motor according to the invention, this tilting torque, and therefore the friction between the hub 9 and the pivot 4 are much more low, and therefore the useful torque that can provide this engine is significantly greater, than in the case of the engine described in the article mentioned above.

 It will be noted that this useful torque which the motor can supply depends directly on the value of the control voltage applied between the electrodes 10, one after the other, and the rotor 3, but that the tilting torque mentioned above is practically independent of this control voltage, provided of course that the latter is greater than that which is necessary for the edge 13a of the ring 13 to just touch the face 2a of the stator 2.

 The maximum value of this useful torque therefore corresponds to the maximum voltage which can be applied between these electrodes 10 and this rotor 3, that is to say, theoretically, to the breakdown voltage of the insulation layer 12.

 In practice, this control voltage is of course lower than this breakdown voltage for obvious safety reasons.

 During the rotation of the rotor 3, the means of connection of its ring 13 to its hub 9 are obviously subjected to a bending force directed substantially in the plane of the rotor 3. But it is easy to determine the dimensions of the various components of these means so that they do not risk breaking in response to this bending force when the motor supplies its maximum torque.

 This bending force obviously causes a deformation of these connecting means, which causes an angular offset between the hub 9 and the ring 13. But, in most applications of a micromotor according to the invention, this offset does not play a great role, and that is why, as has already been mentioned, the means for connecting the ring 13 to the hub 9 need not necessarily have a high resistance to bending in the plane of the rotor 3.

 The engine control circuit 1, the operation of which has just been described, has not been represented by a figure since its production poses no problem to those skilled in the art. It will simply be noted that if the motor 1 is produced in a wafer of semiconductor material, this control circuit can easily be produced in the same wafer and at the same time as this motor.

 We will now consider the case where the rotor 3 of the motor 1 is also made of a conductive material, for example polycrystalline silicon, but where the pivot 4 of the stator 2 is made of an insulating material, or else is made of a conductive material but n ' is not connected to one of the terminals of the device's power source.

 In this case, the motor control circuit must be arranged so as to selectively and cyclically apply a voltage between two adjacent electrodes 10 when the motor 1 is to operate.

 Let us assume at the outset that these two adjacent electrodes are the electrodes 10a and 10b.

 The electric field produced by the control voltage applied between these terminals 10a and 10b extends in an area located on either side of the plane of these electrodes, and therefore in particular in the space separating the latter from the ring 13 rotor 3.

 This electric field in turn creates an electrostatic force which tends to attract the portion of the ring 13 located opposite the electrodes 10a and 10b against the latter.

 If the voltage applied to these electrodes 10a and 10b is sufficient, the ring 13 rocks as has been described above and, also as above, a point on its edge 13a comes to touch the face 2a of the stator 2. But in this case, the contact point is located between these electrodes 10a and 10b.

 If now the control circuit removes the voltage between the electrodes 10a and 10b and simultaneously applies a voltage of the same value between the electrodes 10b and 10c, the point of contact between the edge 13a and the face 2a of the stator 2 moves up to that it is between these electrodes 10b and 10c.

 As in the first case described above, the edge 13a rolls without sliding on the face 2a of the stator during this movement, provided of course that the resistive torque applied to the rotor is not too high, and that the control voltage is applied between the electrodes 10b and 10c exactly at the time when it is removed between the electrodes 10a and 10b, or even slightly before this time.

 This process can obviously be continued, the motor control circuit then applying the control voltage successively to each pair of adjacent electrodes 10c and 10d, 10d and 10e, etc., and the edge 13a of the rotor 3 rolling on the face 2a of stator 2 as described above.

 Also as above, this rolling of the edge 13a of the ring 13 on the face 2a of the stator 2 causes the rotor 3 to rotate about its axis in the direction of rotation of the point of contact of this edge 13a on this face 2a.

 Similarly, the order in which the pairs of adjacent electrodes are subjected to the control voltage can obviously be reversed to change the direction of rotation of the rotor 3.

 The control circuit making it possible to operate the motor 1 in the manner which has just been described has also not been shown since its production does not pose any problem to those skilled in the art either.

 It is obvious that all the considerations made above on the deformations undergone by the means of connection of the ring 13 to the hub 9 and on the advantages of the engine according to the invention compared to known engines apply without change to the case which has just been described.

 It will be noted however that when the control voltage is applied to two adjacent electrodes 10, as has just been described, the rotor 3 is brought, with respect to each of these electrodes 10, to a voltage substantially equal to half of this control voltage. As a result, the latter can be approximately twice as high as in the case where the motor 1 is controlled in the first manner described above, since it is only half of the control voltage which must be less than the breakdown voltage of the insulating layer 12. It also follows that, all other things being equal, the useful torque which the motor 1 can provide is significantly higher when it is controlled in the manner which has just been described.

Claims (4)

 1. Electrostatic micromotor (1) comprising a rotor (3) having a hub (9) and a peripheral zone (13), a stator (2) comprising electrodes (10) coplanar arranged opposite said peripheral zone (13) and intended to produce an electric field between said stator (2) and said rotor (3), and means (4) for rotating connection of said hub (9) of the rotor (3) to said stator (2), characterized in that said peripheral zone (13) consists of a ring (13) substantially planar and parallel to the plane of said electrodes (10) in the absence of said electric field and covering at least partially said electrodes (10) in a plan view of said motor ( 1), and that said rotor (3) further comprises mechanical connection means (14; 15; 16, 17, 18) of said ring (13) to said hub (9) which are elastically deformable in response to a force applied to said ring (13) in a direction substantially perpendicular to said plane of said electr odes (10), said hub (9), said peripheral zone (13) and said connecting means (14; 15; 16, 17, 18) defining between them openings passing through said rotor.  2. Micromotor according to claim 1, characterized in that said mechanical connection means (14; 15; 16, 17, 18) comprise a rectilinear arm (14) arranged along a radius of said rotor (3).  3. Micromotor according to claim 1, characterized in that said mechanical connection means (14; 15; 16, 17, 18) comprise an arm (15) comprising two portions (15a, 15b) arranged along a radius of said rotor ( 3) and connected to each other by a plurality of portions (15c, 15d, 15e, 15f, 15g) alternately perpendicular and parallel to said radius.  4. Micromotor according to claim 1, characterized in that said mechanical connection means (14; 15; 16, 17, 18) comprise a first arm (17) having a first end connected to said peripheral zone (13) and arranged along a first radius of said rotor (3), a second arm (18) having a first end connected to said hub (9) and arranged along a second radius of said rotor, and a connecting element (16) connecting to the second end of said first arm (17) and at the second end of said second arm (18).