HK1120935B - Drive module comprising mems micromotor, process for the production of this module and timepiece fitted with this module - Google Patents

Description

Drive module for a micro-electromechanical system micromotor, production process and timepiece comprising such a drive module

Technical Field

The present invention relates to a drive module for a clock movement and a timepiece such as a wristwatch equipped with such a drive module.

More particularly, the invention relates to a drive module intended to be engaged with a clock wheel, having a plate made of crystalline or amorphous material, comprising a lower layer forming a substrate, and an upper layer into which a micro-motor of MEMS type is etched, wherein the micro-motor has at least one actuator rotatably driving a rotor.

Background

Silicon chips are typically rectangular in shape to maximize the number of components per wafer. This is well suited to the arrangement of the electronic component system functional blocks, since they are also rectangular. In the case of MEMS-type micromotors provided in silicon plates and using comb-type interdigital electrostatic actuators or "comb drives", the actuators generally have a rectangular shape, but since they must generate high forces, they occupy a large area and therefore they cannot be separated into several pieces to be distributed optimally on a rectangular chip. Since the rotor of the micromotor is circular, it is even more difficult to optimize the occupied surface on the silicon plate and on the wafer supporting the micromotor, which consequently results in a large unused area of the silicon plate. The optimization becomes more complex if the goal is to maximize the efficiency of the micromotor by optimizing the arrangement of the actuator or actuators.

Disclosure of Invention

The present invention aims to solve these problems, while obtaining a micromotor with improved efficiency, by proposing a drive module that allows optimization of all these parameters, in particular by optimizing the area of the module that produces the necessary silicon plates.

In this object, the invention proposes a drive module of the aforementioned type, characterized in that a pinion arranged coaxially with the rotor is rotatably connected to the rotor and is arranged above the rotor, said pinion being provided to engage with the clock wheel in an engagement zone located in the vicinity of the outer peripheral edge of the plate, the rotor being arranged on the plate so as to minimize the distance between the outer peripheral edge of the rotor and the outer peripheral edge of the plate corresponding to the engagement zone, and the diameter of the pinion being greater than the diameter of the rotor so as to project into the engagement zone with respect to the plate.

According to other features of the invention:

the actuator has a stylus which is movable in a direction parallel to the plane of the plate, the stylus being fitted on its free end with a pawl which is provided to cooperate with saw gear teeth provided on the outer peripheral edge of the rotor to rotatably drive it in sequence, and the angular position of the interlocking region of the pawl and rotor being slightly offset at an angle relative to the engagement region;

the stylus extends in a direction that divides the actuator into two fully symmetrical parts;

providing two actuators, each having a movable stylus, the free ends of which are provided with pawls, one for pushing and the other for pulling, to cooperate with the teeth of the rotor on each side of the engagement region;

the actuators describe between them an angle in the general range between 80 and 140 degrees, wherein the bisector of the angle passes through the joining area and through the axis of rotation of the rotor, so that the plates generally have a "V" shape defined by the outer contour of the plates;

a plate having a central portion supporting the rotor and two lateral portions, the plate having a profile generally orthogonal to each other and the two rectangles forming the two lateral portions corresponding to the intersection of a transverse rectangle forming the central portion, the transverse rectangle describing a 45 degree angle with respect to each of the two other rectangles, a majority of each lateral portion surface being occupied by the actuator and a majority of the central portion surface being occupied by the rotor, and an engagement area being provided proximate to one of the peripheral edges of the central portion;

the plate has terminals for connecting the actuator to the electronic module, and the terminals are arranged on the central portion on the side opposite to the engagement zone with respect to the rotor axis;

a drive module provided in a case having a lower plate provided to be fastened to a timepiece member such as a core plate and a cover fastened to the lower plate;

the cover has an open indentation in one of its outer peripheral edges and the pinion is received in the indentation;

the invention also proposes a process for producing a drive module, characterized in that it comprises a step of etching several plates in a sheet of crystalline or amorphous material, such as a silicon wafer, in which the plates are staggered in rows in a herringbone manner, the plates of two adjacent rows being oriented in opposite directions.

The invention further proposes a timepiece having a movement rotatably driven by a drive module according to any of the above features.

Drawings

Other characteristics and advantages of the invention will become better apparent from reading the following detailed description, with reference to the attached drawings, given by way of non-limiting example, wherein:

FIG. 1 is a cross-sectional view schematically showing a timepiece constructed in accordance with the teachings of the present invention;

fig. 2 is a perspective view, partially in section, showing the movement of the timepiece of fig. 1 fitted with a drive module having a micro-motor of the MEMS type;

FIG. 3 is a top view schematically illustrating the drive module of FIG. 2;

fig. 4 is an exploded perspective view showing the driving module of fig. 2 and a housing enclosing the driving module;

FIG. 5 is an enlarged view in axial cross-section taken along line 5-5 schematically showing part of the drive module and illustrating the pinion and rotor of the micromotor rotatably mounted about the shaft;

FIG. 6 is a schematic view in axial section taken along line X' X illustrating pin-wise actuation of a pinion by a rotor;

FIG. 7 is a schematic top view illustrating the pin-wise actuation of the pinion by the rotor;

FIG. 8 is a schematic view of an axial section taken along X' X illustrating a variation of the assembly of the shaft relative to the rotor;

fig. 9 is a plan view schematically showing the arrangement of the assembly according to fig. 8, an elastic fastening structure provided in the plate for clamping and centering the shaft;

figure 10 is a top view that schematically represents a silicon wafer and illustrates an example of an assembly of several micromotors on the wafer.

Detailed Description

Fig. 1 schematically shows a timepiece 10 in the form of a wristwatch equipped with a drive module 13 according to the teachings of the invention, wherein the drive module 13 is here provided within a casing 12.

In this case, timepiece 10 includes a case 14 closed by a glass cover 16, a dial 18 and an analog display device in the form of a hand 20. A pointer 20 is provided to be rotatably driven by the drive module 13 according to the invention through a movement 22 comprising, for example, a scale element. The drive module 13 is supplied with electric power by a battery 24. In this case, case 12, drive module 13, movement 22 and battery 24 are mounted on plate 26 and together form movement mechanism 27 of timepiece 10, movement mechanism 27 being fastened to the inside of case 14. The movement mechanism 27 clearly comprises other elements (not shown), in particular an electronic module with integrated circuit, a time base with quartz crystal, a printed circuit board, etc.

Fig. 2 shows parts of a movement mechanism 27 of the timepiece 10, in particular a plate 26 to which the case 12 and the movement 22 are mounted.

The drive module 13 is intended to engage with a clock wheel, i.e. the input wheel 28 of the movement 22.

The different elements of the drive module 13 according to the invention are shown in more detail in figures 3 to 7.

The drive module 13 has a plate 30 made of crystalline or amorphous material, for example silicon, with a lower layer forming a substrate 32 and an upper layer 34, into which a micro-motor 36 of MEMS (micro-electro-mechanical system) type is etched. In this case, the micromotor 36 is formed by two actuators 38, 40 that rotatably drive a rotor 42 etched into the upper layer 34.

Each actuator 38, 40 has a stylus 44, 46 movable in a direction a1, a2 parallel to the plane of the plate 30. Each stylus 44, 46 is fitted at its free end with a pawl 48, 50 provided to cooperate with saw gear teeth 52 provided on the outer peripheral edge of the rotor 42 to rotatably drive it in sequence.

Each stylus 44, 46 preferably extends in a direction a1, a2 that divides the associated actuator 38, 40 into two fully symmetrical parts. The first actuator 38 preferably includes a push pawl 48 and the second actuator 40 preferably includes a pull pawl 50.

In this case, each actuator 38, 40 is a comb-type interdigital electrostatic actuator and is formed by etching into the silicon plate 30. Here, plate 30 is a silicon-on-insulator (SOI) type plate and has a thick lower silicon substrate layer 32, an intermediate layer 54 of silicon oxide, and a silicon upper layer 34 having a thickness less than that of substrate 32.

The fixed part of each actuator 38, 40 has a supply terminal 56, 58 for electrical connection to the electronic module, and the movable part of each actuator 38, 40 has a contact terminal 57, 59 which applies a determined potential to these movable parts, in this case zero volts.

Micromotors with electrostatic actuators built in a silicon plate are described and shown, for example, in patent document WO2004/081695, which is incorporated herein by reference. In this document, the motor is formed in the silicon layer by etching. It has a toothed driving wheel and a driving pin cooperating with the toothing of the wheel to make it rotate. Each drive pin is displaceably coupled to a movable comb that is displaced relative to the fixed comb depending on a voltage applied to the fixed comb.

An embodiment using an SOI plate is described in the aforementioned document with reference to fig. 7A to 7D.

According to an advantageous embodiment, each actuator 38, 40 is connected to a passive pawl 49, 51, the locking area of which is located between the engagement area 70 and the locking area of the associated pawl. These passive pawls 49, 51 remain in resilient engagement with the rotor 42 to ensure accurate angular positioning, particularly during the drive phase when the other pawls 48, 50 are displaced.

According to the embodiment shown in fig. 3 to 7, the rotor 42 is guided by a centrally integrated or guided slide bearing 60, which is formed simultaneously with the pawls 48, 50 and has a bearing clearance in an amount of between 4 and 10 microns, with an approximate lower limit corresponding to a 80 micron thick silicon layer. The pawls 48, 50 will work well if they act on a tangential path significantly larger than this clearance, i.e. typically between 20 and 100 microns. This advantageously corresponds to the range of possible routes for which the stylus 44, 46 is guided by a deflection spring (not shown).

The torque of the rotor 42 is transmitted to the pinion gear 62 through a system similar to a crankshaft. A pinion 62 located directly above the rotor 42 is coaxial therewith and guided by a central shaft 64. The pinion gear 62 is provided with a pin 66 that fits into a slot 68 of the rotor 42. As illustrated in the diagram of fig. 7, operating clearances j _ goup, j _ rot, j _ pi are provided between the different elements of the rotor 42 and the pinion 62. Thus, the rotor 42 and pinion 62 are coupled at an angle but laterally independent: the clearance in the plane xy is taken up by the bearing 60 of the rotor 42 and the shaft 64 of the pinion 62. Thus, the lateral reaction forces caused by the load are not absorbed at the rotor 42 by the bearing 60, but are absorbed by the guidance of the shaft 64 by the pinion 62. Thus, the micro-fabricated elements of the micromotor 36 are protected from the high forces exerted by the watch element, for example in the event of an impact.

Pinion 62 is provided to engage input wheel 28 of movement 22 in an engagement region 70 located near an outer peripheral edge 72 of plate 30.

According to an advantageous feature, the rotor 42 is arranged on the plate 30 so as to minimize the distance D between the teeth 52 of the rotor 42 and the outer peripheral edge 72 of the plate 30 corresponding to the engagement zone 70. Furthermore, the outer diameter of the pinion 62 is slightly larger than the diameter of the rotor 42 so that it protrudes into the engagement region 70 relative to the plate 30.

To simplify the drawing in fig. 7, the rotor 42 is shown with only four slots 68 and the pinion 62 with only four pins 66. According to an advantageous embodiment, as shown in particular in fig. 3 and 4, eight slots 68 and eight pins 66 are provided.

According to a preferred embodiment, the angular position of each pawl 48, 50 and the interlocking area of the rotor 42 is offset at an angle relative to the engagement area 70. The locking region of each pawl 48, 50 forms an angle β with the axis X' X. α denotes the angle between the radius of the pin 66 passing through the edge of the slot 68 that abuts it when engaged and the axis X' X at a given moment (fig. 7).

Thus, for a given radius of the circle of the pinion 62, the rotor 42 and the pin 66, a suitable choice of all the parameters (α, β, j _ rot, j _ pi, j _ goup) will contribute to the efficiency of the transmission of mechanical power from the rotor 42 to the pinion 62. Thus, in the particular case of the invention, where β is 45 °, if the clearance is properly adjusted, the efficiency of the four pin system is close to 85%, which improves efficiency compared to the case where the rotor 42 and pinion 62 are bonded to each other. In fact, in the latter case, all loads are found in silicon-silicon friction form laterally at the bearing 60 and vertically between the periphery of the rotor 42 and the substrate 32 due to the tilting torque. Here, the silicon-silicon friction is very unfavorable, and the static dry coefficient is close to 0.4.

This transmission scheme further allows for variation in the diameter of the pinion gear 62 to accommodate torque and speed depending on load. Furthermore, if the pinion 62 is sufficiently large and protrudes beyond the peripheral edge 72 of the plate 30, the engagement by the tabs will be simplified and the driving module 13 can be assembled in modular form on the plate 26 of the timepiece 10, i.e. without dismantling/reattaching the driving wheel 28.

According to different variants:

the rotor 42 is microfabricated in situ and on the same substrate 32 as the actuators 38, 40 to ensure mating with the bearings 60 and detents;

another variant includes separately fabricating the rotor 42 on the same wafer or on another wafer, and the rotor 42 is then assembled on the plate 30 or stator. This allows reducing the radial clearance if desired, in case the rotor is guided by the bearing 60;

the group of related variants includes the rotor 42 and/or pinion 62 microfabricated by a process other than DRIE machining (laser cutting, EDM, LIGA, microinjection, etc.) and then assembled on the plate 30 or stator;

another set of related variations includes pins 66 formed by a second level of lithography in the pinion gear 62 and/or in the rotor 42.

The drive module 13 according to the present invention allows for increased modularity to accommodate loads by allowing the use of pinions 62 of different diameters without modifying the rest of the module 13. In this way, thanks to the presence of pinion 62, integrated into drive module 13 and rotatably connected to micromotor 36, a mechanical interface for connecting watch movement 22 is already present, also obtaining an improved modularity of the assembly.

Pinion gear 62 may be made of a metal, such as brass, with associated pin 66 also being formed of metal. The pinion 62 can also be made of plastic material by moulding in one piece with the pin 66. Embodiments of the pinion gear 62 made of plastic material with a molded-on metal pin 66 are also possible.

According to the embodiment shown in particular in fig. 4 and 5, the axis of rotation of the pinion 62 is formed by a stepped shaft 64 made of cut-out metal, which is inserted into the plate 30 through a first hole 74 formed in the base sheet 32 and which is pushed into a second hole 76 formed in a plate 106 of the casing 12. In this embodiment, the radial force applied to the shaft 64 is absorbed by the plate 106.

The shaft 64 has a lower end block 78 that defines with a lower intermediate block 80 an upwardly directed first shoulder surface 82 that axially abuts a lower face of the plate 30. The lower intermediate block 80 has a diameter substantially equal to the diameter of the first bore 74 and extends into the bore 74. The shaft has an upper intermediate piece 84 which is slightly smaller in diameter than the adjacent lower intermediate piece 80 and extends into a through hole 86 of the pinion 62 to rotatably guide it. The upper intermediate block 84 and the upper end block 88 together define a second shoulder surface 90 against which a retaining ring 92 advanced onto the upper end block 88 is axially retained in abutment.

Since the rotatable guidance of the rotor 42 is provided by the bearing 60, which is formed by a photolithographic etching process in the same way as the first hole 74 determining the centering of the shaft 64 with respect to the bearing 60, a very advantageous centering of the shaft 64, the pinion 62, the bearing 60 and the rotor 42 is obtained.

Furthermore, the lower face of the pinion facing the bearing has a projection 94, which prevents the pinion 62 from abutting axially against the rotor 42, in particular in the case of tilting, and this prevents damage to the rotor 42.

Fig. 8 and 9 show another advantageous embodiment in which the shaft 64 is mounted in the plate 30 in a push-in arrangement by means of a resilient fastening structure 96 provided in the substrate 32 around the first hole 74. In this embodiment, radial forces applied to the shaft 64 are absorbed by the substrate 32 and thus the resilient fastening structure 96.

The resilient fastening structure 96 is here formed by a flexible sheet 98 lithographically into the rear face of the plate 30. Since the front lithography of the upper layer 34, including the pawls 48, 50 and the rotor 42, is also very accurately aligned and centered (with an error of less than 1 micron) with respect to the rear lithography, the guiding and centering results in it being more accurate than with an axis formed monolithically from the plate 30, since the radial clearance can also be reduced to 1 micron.

Due to this precise alignment and centering, the bearing 60 can be omitted, so that the rotor 42 is directly rotatably guided by the shaft 64. Thus, the shaft 64 can rotatably guide the rotor 42 and the pinion gear 62. Since the shaft 64 is formed by cutting, it is possible to reach very limited production tolerances, achieving a very precise assembly, which in particular ensures reliable operation of the actuators 38, 40. The rotor 42 is then guided by the outer axial wall of the shaft 64.

Shaft 64 can be finally secured in plate 30 by additional fastening methods, such as welding to base plate 32 by weld 99 shown in fig. 8 or also by gluing.

By applying a solid film to the outer axial wall of the shaft 64, friction between the components can be reduced and the problem of friction of the shaft 64 can be solved.

The resilient fastening structure 96 can be chosen, in particular from the example described and shown in patent document CH695395, or from other structures capable of ensuring the precise centering and clamping of the shaft 64 on the plate 30, for example in the form of flexible tabs with free ends.

Advantageously, considering in particular fig. 3, the actuators 38, 40 describe between them an angle of approximately 90 degrees, the bisector of which passes through the engagement zone 70 and through the rotation axis z' z of the rotor 42, so that the drive module 13 has a general "V" shape defined by the outer contour of the plate 30, this contour being optimized.

The plate 30 has a central portion 100 supporting the rotor 42 and two lateral portions 102, 104. The outer contour of the plate 30 generally corresponds to two rectangles orthogonal to each other and forming the two lateral portions 102, 104 intersecting a transverse rectangle forming the central portion 100, the transverse rectangle describing an angle of 45 degrees with respect to each of the two other rectangles. A major portion of the surface of each lateral portion 102, 104 is occupied by the actuator 38, 40, while a major portion of the surface of the central portion 100 is occupied by the rotor 42. The engagement region 70 is disposed proximate one of the peripheral edges 72 of the central portion 100.

The terminals 56, 57, 58, 59 are preferably arranged on the central portion 100 on the opposite side of the engagement zone 70 with respect to the axis z' z of the rotor 42.

It should be noted that the "V" shape of the drive module 13 has the advantage of being able to optimize the efficiency of the micromotor 36 with respect to the surface of the plate 30 used, and the surface of the crystalline or amorphous material used to form the micromotor 36 and the drive module 13. Thus, when the plate 30 is made of a silicon wafer 101, as schematically shown in fig. 10, the "V" shape allows the replication of the staggered arrangement of the plates 30 on the wafer surface to maximize the number of micromotors 36 obtained from a given surface of the silicon. In particular, according to the example shown in fig. 10, the plates 30 can be arranged in parallel columns on the wafer in a herringbone manner, two adjacent columns Cn, Cn +1 being oriented in opposite directions. Furthermore, the lateral portions 102 of two adjacent plates 30 of two adjacent columns Cn, Cn +1 are adjacent and aligned.

The angle described by the two actuators 38, 40 preferably ranges between 90 and 140 degrees. The larger the angle, the more optimal the staggering of the plates 30 on the wafer 101, but the significant angle requires that the styli 44, 46 of the actuators 38, 40 are out of alignment with respect to their respective axes of symmetry a1, a2, which is detrimental to the mechanical efficiency of the actuators 38, 40.

According to the embodiment shown in the figures, the case 12 comprising the drive module 13 has a lower plate 106 provided to be fastened to an element of the timepiece 10, here to the plate 26 of the movement, and the plate 30 of the drive module 13 is mounted on the lower plate 106. The housing 12 has a protective cover 108 which covers the drive module 13 fastened to the lower plate 106, in this case by means of screws 109, and which holds the drive module 13 against the lower plate 106.

In this case, the upper face of the lower plate 106 has a recess or receptacle 110 into which the plate 30 of the drive module 13 is received in a substantially complementary manner.

The cover 108 has an open indentation 112 in one of its outer peripheral edges, and the pinion 62 is received in the indentation 112 after the cover 108 is mounted to the base 106.

Advantageously, a printed circuit 114 is interposed between lower plate 106 and cover 108 to electrically connect micromotor 36 to the electronic module of timepiece 10 via its terminals 56, 57, 58, 59.

According to an exemplary embodiment (not shown), drive module 13 can be mounted directly on plate 26, which allows to dispense with housing 12, in particular to minimize the number of components to facilitate assembly of movement mechanism 27 and to minimize the space requirements of the drive device. A protection element may be provided on the drive module 13 to protect its components.

Claims (13)

1. A drive module (13) intended to be engaged with a clock wheel (28), having a plate (30) made of crystalline or amorphous material, said plate (30) comprising a lower layer forming a substrate (32), and an upper layer (34) into which a MEMS-type micromotor (36) is etched, wherein the micromotor (36) has at least one actuator (38, 40) rotatably driving a rotor (42), characterized in that: a pinion (62) arranged coaxially with the rotor is rotatably connected to and disposed above the rotor, the pinion engaging the clock wheel at an engagement region (70) located near an outer peripheral edge (72) of the plate (30), the rotor (42) being disposed on the plate to minimize a distance between the outer peripheral edge of the rotor (42) and the outer peripheral edge (72) of the plate (30) corresponding to the engagement region (70), and the pinion (62) having a diameter greater than a diameter of the rotor (42) so as to protrude into the engagement region (70) relative to the plate (30).

2. The drive module (13) according to claim 1, characterized in that: the actuator (38, 40) has a stylus (44, 46) movable in a direction parallel to the plane of the plate (30), the stylus (44, 46) being fitted at its free end with a pawl (48, 50) cooperating with saw gear teeth (52) provided on the outer peripheral edge of the rotor (42) to rotatably drive it in sequence, and the angular position of the interlocking regions of the pawl (48, 50) and the rotor (42) being slightly offset at an angle relative to the engagement region (70).

3. The drive module (13) according to claim 2, characterized in that: the contact pins (44, 46) extend in a direction that divides the actuator (38, 40) into two completely symmetrical parts.

4. The drive module (13) according to claim 2 or 3, characterized in that: two actuators (38, 40) are provided, each having a movable stylus (44, 46) whose free end is provided with a pawl (48, 50), one for pushing and the other for pulling, to cooperate with the gear teeth (52) of the rotor (42) on either side of an engagement region (70).

5. The drive module (13) according to claim 4, characterized in that: the actuators (38, 40) make an angle between them in the general range of 80 to 140 degrees, wherein the bisector of the angle passes through the engagement region (70) and through the axis of rotation of the rotor (42) such that the plate (30) has a "V" shape defined by the outer profile of the plate (30).

6. The drive module (13) according to claim 5, characterized in that: the plate (30) has a central portion (100) supporting the rotor (42) and two lateral portions (102, 104), the plate (30) having a profile generally corresponding to two rectangles orthogonal to each other and forming the two lateral portions (102, 104) intersecting a transverse rectangle forming the central portion (100) at an angle of 45 degrees with respect to each of the two other rectangles, a major portion of the surface of each lateral portion (102, 104) being occupied by the actuator (38, 40) and a major portion of the surface of the central portion (100) being occupied by the rotor (42), and the engagement region (70) being disposed near one of the outer peripheral edges (72) of the central portion (100).

7. The drive module (13) according to claim 6, characterized in that: the plate (30) has terminals (56, 57, 58, 59) to connect the actuators (38, 40) to the electronic module, and the terminals (56, 57, 58, 59) are arranged on the central portion (100) on the opposite side of the engagement area (70) with respect to the axis of the rotor (42).

8. The drive module (13) according to claim 1, characterized in that: the drive module is provided in a case (12) having a lower plate (106) and a cover (108) fastened to the lower plate (106), the lower plate being fastened to the timepiece element (10).

9. The drive module (13) according to claim 8, characterized in that: the cover (108) has an open indentation (112) at one of its outer peripheral edges, and the pinion (62) is received in the indentation (112).

10. The drive module (13) according to claim 1, characterized in that: the drive module is disposed within a housing (12) having a lower plate (106) secured to the core plate (26) and a cover (108) secured to the lower plate (106).

11. Process for producing a drive module (13) according to claim 5, characterized in that: the process comprises a step of etching several plates (30) in a sheet of crystalline or amorphous material, wherein the plates (30) are staggered in several columns (Cn) in a herringbone manner, the plates (30) of two adjacent columns (Cn) being oriented in opposite directions.

12. The process of claim 11, wherein: the sheet of crystalline or amorphous material is a silicon wafer (101).

13. Timepiece (10) with a movement (22) rotatably driven by a drive module (13), the drive module (13) being intended to engage with a clock wheel (28), the drive module having a plate (30) made of crystalline or amorphous material comprising a lower layer forming a substrate (32), and an upper layer (34) into which a micro-motor (36) of the MEMS type is etched, wherein the micro-motor (36) has at least one actuator (38, 40) rotatably driving a rotor (42), a pinion (62) arranged coaxially with the rotor being rotatably connected to the rotor and provided above the rotor, said pinion being provided to engage with the clock wheel in an engagement zone (70) located in the vicinity of an outer peripheral edge (72) of the plate (30), the rotor (42) being provided on the plate so as to minimize the distance between the outer peripheral edge of the rotor (42) and an outer peripheral edge (72) of the plate (30) corresponding to the engagement zone (70), and the pinion (62) has a diameter greater than that of the rotor (42) so as to project into the engagement region (70) relative to the plate (30).