The invention relates to a miniature motor, comprising a stator, a rotor, a bearing for bearing the rotor in a rotary manner, a rotor axis which is coupled to said rotor, and electric connecting means for electrically connecting the 5 stator to a power supply, the diameter of the rotor being in the order of magnitude of maximum 1 mm.

In recent years much research has been carried out on the miniaturization of mechanic systems, leading to a new field,

10 i.e. micromechanics. Some examples of miniaturized mechanical systems are pressure or current sensors and actuators like micro resonators and miniature motors. In manufacturing such systems sophisticated thin-film techniques are applied. A known miniature motor comprises a number of electrodes consti-

15 tuting a stator and cooperating with a rotor formed by a dielectric, the couple being produced by generating an elec¬ tric field. Nevertheless, this electrostatic miniature motor has a rather short life, that is to say some hundreds of revolutions, due to wear of the motor's thin-film bearings

20 caused by rotary friction. Moreover, the maximum number of revolutions per minute of this miniature motor is small, and its output power is limited as well.

The purpose of the present invention is to provide an improved 25 miniature motor which is not too complicated to manufacture, either.

, To this end, the invention provides a miniature motor as mentioned in the preamble, characterized in that the rotor

, 30 substantially consists of a magnetic material for forming a plurality of magnet poles that are arranged around the rotor axis, and in that the stator is constituted by a plurality of stator windings arranged around and spaced from the rotor.

Surprisingly, it appears that a standard operation of the miniature motor according to the invention allows correspon¬ ding electric current densities in the order of magnitude of some hundreds of ampere per square millimeter, particularly 200 A/mm ² or more. Such electric current densities cannot be reached by using the above-mentioned thin-film techniques and do well exceed the electric current densities that are consi¬ dered permissible so far for the conventional type of motors that are operative on the basis of a generated magnetic field. A possible explanation for the unexpected permissibility of said high values of electric current densities is that, al¬ though a very considerable dissipation may occur, the stator windings arranged around the rotor may lead to a current effi¬ ciency within reasonable limits.

It is observed that French patent 1.404.480 granted on May 24, 1965 describes a synchronous motor of a miniature type. The motor comprises an elongated, multipolar rotor made of a material having a low density such as ferrite. However, no dimensions of the different parts of the motor are specified. The use of the magnetic relatively weak ferrite indicates that - in any event - the dimensions of the motor are considerably larger than those of the motor according to the invention.

Additional characteristics and advantages of the miniature motor will become clear after the following description of an embodiment of the miniature motor according to the invention, including references to the enclosed drawing, wherein:

fig. 1 shows a schematic longitudinal cross-section of a preferred embodiment of the miniature motor according to the invention;

fig. 2 shows a side view of the miniature motor according to fig. 1 along line II-II;

fig. 3 shows a cross-sectional view of the miniature motor according to fig. 1 along line III-III;

fig. 4 schematically shows a stator winding of the miniature motor according to fig. 1; and

fig. 5 shows an electric scheme of the mutual connection of the stator windings and the connection to the connecting pins of the miniature motor according to fig. 1.

As is shown in a longitudinal cross-section of fig. 1, the embodiment of the miniature motor according to the invention comprises a rotor 1 mounted on a rotor axis 2 that is rotata- bly supported in bearings 3. The rotor 1 is accomodated in a cylindrically shaped stator casing 4, to the inner wall of which - in this case - four stator windings 5 have been ap¬ plied, for example by using an adhesive. The stator casing 4 may be made, for example, of a light-magnetic or non-magnetic material, such as annealed stainless steel. It is preferred to use non-magnetic material that is also electrically non-con¬ ductive, such as a suitable plastic material, thus avoiding an interfering of possible residual magnetism of the casing and an induction of the eddy currents in the casing.

At one end of the stator casing 4 the rotor axis 2 projects, so as to be coupled to a - non-illustrated - load, while at the other end of the stator casing 4 an electric connecting means 6 is provided which comprises three connecting pins 7a, 7b and 7c, which are electrically connected to connecting ends 8 of the stator windings 5. The connecting pins 7 are mounted on a circular plastic plate 9, which is illustrated further in fig. 2 showing a side view of the motor of fig. 1 along line II-II.

Fig. 3 shows a cross-sectional view along the line III-III of fig. 1, illustrating a thickening 10 of the rotor axis 2, around which two magnet poles 11 have been arranged. The rotor axis 2 is made of steel, while the magnet poles 11 are made of a permanent-magnet material. The magnet poles of the magnet may be formed integrally by means of a plastic material. The thickening 10 may be formed integrally with the rotor axis but also, for example, like a sleeve of a light-ferromagnetic material that fits over the rotor axis for the conduction of the magnetic flux. The bearings 3 may be of the type as used in the watch-manufacturing industry and are made of e.g. corundum. Nevertheless, due to the favourable ratio of the motor's mass to the specific bearing load, helical groove bearings are preferred. The bearings 3 are clamped by the stator casing 4.

Fig. 4 provides a detailed illustration of stator winding 5. The stator winding 5 comprises a number of - in this case eight - helically-wound windings 12 of insulated windings 12 of insulated conducting wire, such as varnished copper wire, having a substantially rectangular cross-section, e.g. 20 x 50

(μm) ² . As the conducting wire is wound in such a way that the short sides are lying in the plane of drawing, the resulting winding can easily be adjusted to the curvature of the inner wall of the stator casing 4 and the circular outer circumfe¬ rence of the rotor 1. At the connecting ends 8 the conducting wire is twisted through 90° in order to be easily fed along one of the bearings 3 to the electric connecting means 6 via slots 13 (see figure 3) provided at one end of the stator casing 4. In this way, the bearings 3 can be identical, and the connecting ends 8 of the stator windings can be easily bent around the plate 9 to effect an interconnection as well as a connection to the connecting pins 7. Moreover, the outer diameter of the miniature motor can be reduced partly because of that.

A proper way to obtain conducting wire being rectangular in cross-section is by sheet-rolling a conducting wire being circular in cross-section by means of a rolling machine for at least one time. A diameter of 36 μm appears to be a suit- able dimension for obtaining a rectangular cross-section of 20 X 50 (μm) ² .

A stator winding 5 may also be constituted by current conduc¬ tor circuits pressed onto a non-illustrated support using deposition or etching techniques. The pattern of the current conductor circuits may, for example, be substantially rectan¬ gular and have other lines and/or one or more circuits as broad as planes. Preferably, the support is made of a flexible material or properly preformed to be easily adjusted to the curved inner wall of the stator casing 4. However, self-sup¬ porting windings may be used as well. Good results have been obtained by using the so-called 'flexprint' techniques or the 'LIGA technique.

This design of stator and rotor make the miniature motor particularly suitable for being used as a synchronous motor. If properly fed, the miniature motor according to the inventi¬ on may also be used as a stepper motor.

Fig. 5 represents an electric diagram showing the connections between the four stator windings 5 and the connecting pins 7. The winding direction of each of the windings 5 is indicated by a dot. Pairs of opposed stator windings 5 are connected in series in order to provide a two-phase, bipolar stator.

It will be obvious that, depending on e.g. the number of magnet poles of the rotor and/or on the power supply fed to the miniature motor, a variable number of stator windings may

be applied, and that the stator windings may be interconnected in various ways. Thus, the above-described stator having four stator windings 5 can be easily adapted to form a four-pole stator by selecting the same current/winding direction for each of the four windings. Naturally, in that case the rotor should have four magnet poles to obtain a four-pole miniature motor.

In this context, a two-phase voltage or current system is an adequate power supply for the miniature motor. The power supply can easily be electrically connected to the motor via the connecting pins 7. This is essential for a proper operati¬ on of the miniature motor without the risk of damage to e.g. the connecting ends 8 of the stator windings 5.

The types of voltage or current provided by the power supply and/or the types of stator windings are preferably so selected as to limit the number of pulses in the electromagnetic couple of the motor. This is significant, as the ratio of the elec- tromagnetic couple to the moment of inertia of the motor as well as to the load is considerably large as opposed to that of motors of comparatively large dimensions, so that pulses in the couple may more easily lead to changes in the angular speed of the rotor axis.

A synchronous miniature motor produced according to the inven¬ tion shows i.a. the following specific features:

length stator casing : 2.3 mm diameter stator casing : 1 mm diameter rotor: : 0.7 mm diameter rotor axis: : 0.2 mm outer dimensions stator windings: 0.6 x 1.5 mm cross-section conducting wire : 20 x 50 (μm) ²

The miniature motor produced in this way has been tested at room temperature, the phase current provided by the power supply gradually being increased.

Surprisingly, an electric current density of 300 A/mm ² corres¬ ponding to an effective phase current of 300 mA appears to be permissible. Such an electric current density exceeds the maximum permissible electric current density for magnetic motors to a great extent. The related number of revolutions per minute of the motor measures circa 6 000, and the motor performs continuously for several weeks. A deliberate cooling- treatment, for example by producing an air flow or by placing the miniature motor in a cooling-liquid, is not necessary. The miniature motor's working area ranges from 0 to 200 000 revo- lutions per minute. The motor couple is in the order of magni¬ tude of 10" ⁶ - 10 ^"7 Nm.

Furthermore, measurements of miniature motors according to the invention have been taken, each of the rotors of which happen to have diameters of less than 0.7 mm. Even in the case of a rotor diameter of a mere 0.3 mm a secure operation seems to be possible, provided that the effective phase current exceeds the above-mentioned value. Given the latter rotor diameter, the stator casing diameter measured 0.7 mm.

The construction of the above-described miniature motor is simple and very compact, and the effects of the tolerances of the parts described are limited. Due to this, a reproducible result can be obtained when manufacturing the miniature motor in series.

It is observed that the space between the stator and the rotor can contain a liquid without any problem, which may be advan¬ tageous to certain applications.

It will be obvious to those skilled in the art that many variants of the above-described embodiment of the miniature motor are possible within the scope of the invention.