Double rotor stepping motor

In precision mechanical actuators, wherein a high precision stepping motor can notably be used in the actuating mechanisms of artificial satellites, a stepping motor comprises two rotors that rotate in opposite directions to generate a low-amplitude movement between one of the rotors and a stator. The first rotor comprises a first set of teeth distributed at a first pitch p1 and a second set of teeth distributed at a second pitch p2. The stator comprises N stator contacts comprising a plurality a of teeth distributed at the pitch p1, distributed at a third pitch equal to p1(a+1/N), and able to cooperate with the teeth of the first set. The second rotor comprises N rotor contacts comprising a plurality b of teeth distributed at the pitch p2, distributed at a fourth pitch equal to p1(b+1/N), and able to cooperate with the teeth of the second set of the first rotor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to foreign French patent application No. FR 1203295, filed on Dec. 5, 2012, the disclosure of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention lies in the field of precision mechanical actuators. It relates to a high precision stepping motor and can notably be used in the actuating mechanisms of artificial satellites.

BACKGROUND

Artificial satellites generally require numerous actuating devices. These devices may notably serve to deploy panels from a storage configuration to a deployed configuration, to orient pointing mechanisms in various directions, or to actuate elements of optical instruments such as mirrors. Generally, the context of space imposes constraints in terms of power consumption, reliability, weight and size. In addition, actuating devices often have to have high precision, that is to say a low angular resolution in the case of rotary motors. Stepping motors are commonly used as mechanical actuators for aerospace applications. Specifically, this type of motor has a number of advantages, such as low friction, a possibility of holding position without consuming power, and simplicity of control. In particular, no automatic control is necessary to hold a particular position. Stepping motors also have low angular resolution, which can reach several tenths of a degree. However, a decrease in the angular resolution is accompanied by an increase in the size and the mass of the motor. In addition, finer angular resolutions may be necessary. One solution consists in adding a mechanical reducing gear at the output of the stepping motor. However, the introduction of a reducing gear involves a decrease in the energy efficiency on account of the friction which it entails, and an increase in the weight and size. Another solution consists in the microstep control of the stepping motor. This solution requires more expensive electronics and does not make it possible to maintain a holding torque without a power supply.

SUMMARY OF THE INVENTION

One aim of the invention is notably to remedy all or some of the abovementioned drawbacks by proposing a stepping motor that affords a very low angular resolution while having a simple mechanical design and simple electronic control, a limited size, and a possibility of holding position without consuming power. To this end, the subject of the invention is a double rotor stepping motor having a differential movement. More specifically, the subject of the invention is a stepping motor comprising:a stator comprising N stator contacts, where N is an integer greater than or equal to three,a first rotor which is able to move with respect to the stator about an axis, the first rotor comprising a first set of teeth distributed at a first pitch p1, and a second set of teeth distributed at a second pitch p2, anda second rotor which is able to move with respect to the first rotor about the axis, the second rotor comprising N rotor contacts,
the N stator contacts comprising a plurality a of teeth distributed at the pitch p1, where a is an integer, the N stator contacts being distributed on the stator at a third pitch equal to p1(a+1/N), the teeth of the first set being able to be aligned individually with one of the stator contacts, the passage from one alignment to a consecutive alignment causing the first rotor to move in a first direction with respect to the stator by the pitch p1/N,
the N rotor contacts comprising a plurality b of teeth distributed at the pitch p2, where b is an integer, the N rotor contacts being distributed on the second rotor at a fourth pitch equal to p2(b+1/N) and being able to be aligned individually with one of the teeth of the second set, the passage from one alignment to a consecutive alignment causing the second rotor to move in a second direction, opposite to the first direction, with respect to the first rotor by the pitch p2/N.

According to one particular embodiment, the movements between the stator, the first rotor and the second rotor are rotational movements about the axis.

Each stator contact may comprise a first ring portion, an internal surface of which is toothed with the pitch p1, the teeth of the ring portion being able to be aligned with teeth of the first set of the first rotor. Each stator contact may also comprise a second ring portion, an internal surface of which is toothed with the pitch p1, the second ring portion being disposed symmetrically about the axis with respect to the first ring portion, the teeth of the second ring portion being able to be aligned with teeth of the first set of the first rotor. The first rotor and the second rotor may thus each have N concentric rings distributed along the axis and electromagnetically isolated from one another, the first and second ring portions of each stator contact being aligned with one of the rings of the first rotor and with one of the rings of the second rotor so as to allow a magnetic field to flow between the first ring portion and the second ring portion.

According to one particular embodiment, the first rotor comprises two parts that rotate as one about the axis, each part having N concentric rings distributed along the axis and electromagnetically isolated from one another, an external surface of each ring comprising teeth distributed at the pitch p1and aligned between the various rings, an internal surface of each ring comprising teeth distributed at the pitch p2and aligned between the various rings, the second rotor comprising two parts that rotate as one about the axis, each part of the second rotor having N concentric rings distributed along the axis and electromagnetically isolated from one another, an external surface of each ring comprising teeth distributed at the pitch p2and offset with respect to the teeth of the other rings by the pitch p′2, each ring of the first rotor being aligned with one of the rings of the second rotor.

Moreover, each stator contact may comprise four concentric ring portions, each ring portion being toothed with the pitch p1, for each stator contact, a first ring portion and a second ring portion being disposed symmetrically about the axis and cooperating with one of the rings of the first part of the first rotor and with one of the rings of the first part of the second rotor, a third ring portion and a fourth ring portion being disposed symmetrically about the axis and cooperating with one of the rings of the second part of the first rotor and with one of the rings of the second part of the second rotor.

The invention has notably the advantage that it allows full pitch control of the stepping motor, with said stepping motor having a very small angular movement between the second rotor and the stator between two successive power supply phases.

DETAILED DESCRIPTION

FIG. 1shows, in the form of a simplified block diagram, a first example of a stepping motor according to the invention. The motor is shown here in the form of a linear motor. However, it may also be a rotary motor in a flat development. The stepping motor10shown inFIG. 1comprises a stator11, a first rotor12and a second rotor13having a permanent magnet14. The stator11has four stator contacts111to114. Each stator contact111-114comprises two teeth115which are spaced apart from one another by a pitch p1, and a coil116that can be supplied with an electric current in order to create an electromagnetic field. The stator contacts111-114are distributed on the stator11at a pitch p1(2+1/N). More specifically, the stator contacts are disposed such that one of the teeth115of a stator contact111-114is located at a distance p′1from a contiguous stator contact. The pitch p′1is determined as a function of the pitch p1and the number N of stator contacts. It is equal to (1+1/N).p1or, in the example ofFIG. 1, (1+1/4).p1. The first rotor12, also known as the intermediate rotor, is in sliding connection with respect to the stator11along an axis X (or in pivoting connection about an axis orthogonal to the axis X in the case of a rotary motor). The sliding connection should be understood broadly, that is to say that the connection must comprise at least one degree of freedom in translation along the axis X. The intermediate rotor12comprises a first set of teeth121distributed at the pitch p1and positioned opposite the teeth115of the stator contacts111-114. On account of the difference between the pitches p1and p′1, it is not possible for the teeth115of all of the stator contacts111-114to be aligned simultaneously with the teeth121of the intermediate rotor12. For each step of the motor, two teeth121are aligned with the teeth115of one of the stator contacts111-114. The intermediate rotor12also comprises a second set of teeth122distributed at a pitch p2which is different from the pitch p1. The second rotor13, also known as the central rotor, is in sliding connection with respect to the intermediate rotor12along the axis X (or in pivoting connection in the case of a rotary motor). It is thus also in sliding connection with respect to the stator11. The central rotor13comprises four sets131to134of teeth135, known as rotor contacts, by analogy with the stator contacts111-114. Generally, the central rotor13comprises N rotor contacts, or as many rotor contacts as there are stator contacts. Each rotor contact131-134comprises two teeth135that are spaced apart from one another by the pitch p2. The rotor contacts131-134are distributed at a pitch p2(2+1/N). More specifically, the rotor contacts are disposed such that one of the teeth135of a rotor contact131-134is located at a distance p′2from a contiguous rotor contact. The pitch p′2is determined as a function of the pitch p2and of the number N of rotor contacts and stator contacts. It is equal to (1+1/N).p2or, in the example ofFIG. 1, (1+1/4).p2. On account of the difference between the pitches p2and p′2, it is not possible for the teeth135of all of the rotor contacts131-134to be aligned simultaneously with the teeth122of the intermediate rotor12. For each step of the motor, two teeth122are aligned with the teeth135of one of the rotor contacts131-134. The permanent magnet14is attached to the central rotor13so as to create or increase the flow of the magnetic current between the stator contacts111-114and the rotor contacts131-134.

FIGS. 2 to 6illustrate the operation of the stepping motor10schematically shown inFIG. 1during the successive power supply phases thereof. A phase corresponds to a period of time during which a coil116of one of the stator contacts111-114is supplied with power. The coil itself may also be known as a “phase”. During each phase, the intermediate rotor12and the central rotor13are positioned so as to minimize the reluctance between one of the stator contacts111-114and the corresponding rotor contact131-134.FIG. 2shows the respective positions of the stator11, the intermediate rotor12and the central rotor13during a first phase, specifically when the coil116of the stator contact111is supplied with power. In order to minimize the reluctance between the stator contact111and the rotor contact131, two teeth121of the intermediate rotor12are aligned with the teeth115of the stator contact111, and two teeth122of the intermediate rotor12are aligned with the teeth135of the rotor contact131. The magnetic field20established between the stator contact111and the rotor contact131is thus at a maximum.

FIG. 3shows the stepping motor10during the second phase, that is to say when the coil116of the second stator contact112is supplied with power. The positions of the intermediate rotor12and of the central rotor13during the first phase are shown by dashed lines. In order to minimize the reluctance between the stator contact112and the rotor contact132, two teeth121of the intermediate rotor12are aligned with the teeth115of the stator contact112, and two teeth122of the intermediate rotor12are aligned with the teeth135of the rotor contact132. Conventionally, the passage from the first alignment between the teeth115of the stator contact111and the teeth121of the intermediate rotor12, to the second alignment between the teeth115of the stator contact112and the teeth121of the intermediate rotor12causes the intermediate rotor12to move with respect to the stator11by a distance d1equal to p1/N, or in this case p1/4. Analogously, the passage from the first alignment between the teeth122of the intermediate rotor12and the teeth135of the rotor contact131, to the second alignment between the teeth122of the intermediate rotor12and the teeth135of the rotor contact132causes the central rotor13to move with respect to the intermediate rotor12by a distance d2equal to p2/N, or in this case p2/4. In as much as the difference between the pitches p1and p2is relatively small, the intermediate rotor12is driven in a first direction S1and the central rotor13is driven in a second direction S2, opposite to the first direction. Thus, the resultant movement of the central rotor13with respect to the stator11is less than each of the two relative movements. The distance d3covered by the central rotor13with respect to the stator11is equal to the distance (d2−d1), that is to say (p2−p1)/N. It follows that the distance d3may be chosen to be as small as desired by choosing appropriate values of the pitches p1and p2.

FIG. 4shows the stepping motor10during the third phase, that is to say when the coil116of the third stator contact113is supplied with power. The intermediate rotor12and the central rotor13are again shown by way of dashed lines in the positions which they occupied during the first phase. During this third phase, it is the teeth115of the stator contact113which are aligned with teeth121of the intermediate rotor12, and it is the teeth135of the rotor contact133which are aligned with teeth122of the intermediate rotor12. The passage from the alignments of the second phase to the alignments of the third phase causes the intermediate rotor12to move again with respect to the stator11by the distance d1and in the direction S1, and the central rotor13to move again with respect to the intermediate rotor12by the distance d2and in the direction S2. The central rotor13has thus undergone a movement equal to 2.(d2−d1) since the first phase.

FIG. 5shows the stepping motor10during the fourth phase, that is to say when the coil116of the fourth stator contact114is supplied with power. In this phase, the teeth115of the stator contact114are aligned with teeth121of the intermediate rotor12, and the teeth135of the rotor contact134are aligned with teeth122of the intermediate rotor12. The passage from the alignments of the third phase to the alignments of the fourth phase causes the intermediate rotor12to move again with respect to the stator11by the distance d1and in the direction S1, and the central rotor13to move again with respect to the intermediate rotor12by the distance d2and in the direction S2. The central rotor13has thus undergone a movement equal to 3.(d2−d1) since the first phase.

FIG. 6shows the stepping motor10during the fifth phase. This phase corresponds in fact to the first phase, in which the coil of the first stator contact111is supplied with power. The same alignments as those of the first phase are obtained. The successive passages from the first to the fifth phase have thus caused the intermediate rotor12to move with respect to the stator11by the distance p1—or 4.d1—and in the direction S1, and the central rotor13to move with respect to the intermediate rotor12by the distance p2—or 4.d2—and in the direction S2. Consequently, the movement of the central rotor13with respect to the stator11is equal to p2−p1.

The exemplary embodiment of the stepping motor inFIG. 1may be generalized. In particular, as indicated above, the invention may be applied to rotary stepping motors. In such a case, the movements of the rotors are rotary movements, and the pitches in question are angular pitches. Furthermore, a number N of rotor contacts and stator contacts equal to 4 was considered. However, the number N may have any integer value greater than or equal to 3. Generally, each stator contact and each rotor contact may comprise one or more teeth. With a being an integer representing the number of teeth of each stator contact, the stator contacts are distributed on the stator at a pitch equal to p1(a+1/N). Similarly, with b being an integer representing the number of teeth of each rotor contact, the rotor contacts are distributed on the rotor at a pitch equal to p2(b+1/N). Preferably, the rotor contacts and stator contacts comprise the same number of teeth. When a contact comprises a plurality of teeth, these teeth are distributed at the pitch p1or p2, depending on whether it is a stator contact or rotor contact, respectively. Each tooth positioned at the end of the plurality of teeth of a stator contact must be at the distance p′1from one of the teeth of a consecutive stator contact. Similarly, each tooth positioned at the end of the plurality of teeth of a rotor contact must be at the distance p′2from one of the teeth of a consecutive rotor contact. The pitches p′1and p′2were indicated as being equal to (1+1/N).p1and (1+1/N).p2, respectively. However, on account of the periodicity of the teeth of the intermediate rotor and of the central rotor, these pitches may also be equal to p1/N and p2/N, respectively. The teeth of the stator, of the intermediate rotor and of the central rotor were shown schematically in the form of triangles inFIGS. 1 to 6. However, any other shape of tooth may be used within the scope of the invention. More generally, the teeth may be replaced by any means that is able to generate positions having a reluctance less than that of the other positions. In particular, materials having different electromagnetic properties may be used. By way of example, the pitch p2may be equal to 1.1 times the pitch p1. The difference between the pitches p1and p2may be adapted depending on the desired angular resolution between the stator and the central rotor.

FIG. 7shows a second exemplary embodiment of a stepping motor according to the invention. In this case, it is a rotary stepping motor having variable reluctance and staged rotors. The stepping motor30comprises a stator31, an intermediate rotor32and a central rotor33having a permanent magnet34. The permanent magnet34is secured to the central rotor33. The rotors32and33are in pivoting connection with respect to the stator31about an axis Y.

FIGS. 8 and 9show the stator31of the motor30fromFIG. 7in a perspective view and in a sectional view along the axis Y, respectively. The stator31comprises four stator contacts311,312,313and314. Each stator contact311-314has four ring portions311A-311D,312A-312D,313A-313D and314A-314D, respectively. These ring portions are generically denoted31A-31D. Each ring portion31C is offset in translation along the axis Y from the corresponding ring portion31A. The ring portions31B and31D are positioned facing each ring portion31A and31C, respectively. Each ring portion is toothed at one and the same pitch p1. The teeth of the ring portion312A are angularly offset from the teeth of the ring portion311A by a pitch p′1. The pitch p′1is equal to 1/4.p1. More generally, the angular offset is equal to 1/N.p1, where N is the number of stator contacts. Similarly, the teeth of the ring portions313A and314A are angularly offset from the teeth of the ring portions312A and313A, respectively, by the pitch p′1. The same goes for the ring portions311B-314B,311C-314C, and311D-314D. The teeth of the ring portions31A are aligned with the teeth of the respective ring portions31C, and the teeth of the ring portions31B are aligned with the teeth of the ring portions31D. The stator also comprises eight coils316which are supplied with power in pairs. A first coil316makes it possible to supply power to the ring portions311A and311C. A second coil316makes it possible to supply power to the ring portions311B and311D. Analogously, the six other coils make it possible to supply power individually to the ring portions312A and312C,312B and312D,313A and313C,313B and313D,314A and314C, and314B and314D.

FIG. 10shows a perspective view of a part32A of the intermediate rotor32. The part32A comprises four concentric rings, known as stages321to324, distributed along the axis Y and rotating as one about the axis Y. The number of stages of the part32A is equal to the number N of stator contacts. The stages321-324are electromagnetically isolated from one another by spacers325. The external surface of each ring321-324carries a set of teeth326distributed at the pitch p1. The teeth326of each stage321-324are aligned with those of the other stages. The internal surface of each ring321-324carries a set of teeth327distributed at the pitch p2. The teeth327of each stage321-324are aligned with those of the other stages. The intermediate rotor32comprises two parts32A and32B which rotate as one about the axis Y. The part32B, not shown, is identical to the part32A. The part32A is aligned with the ring portions31A and31B, and the part32B is aligned with the ring portions31C and31D. More specifically, the stages321-324of the part32A are respectively positioned opposite the ring portions311A and311B,312A and312B,313A and313B, and314A and314B. The stages321-324of the part32B are respectively positioned opposite the ring portions311C and311D,312C and312D,313C and313D, and314C and314D. The intermediate rotor32is dimensioned such that the teeth326can cooperate with the teeth of the ring portions31A-31D.

FIG. 11shows a perspective view of a part33A of the central rotor33. The part33A comprises four concentric rings, known as stages331to334, distributed along the axis Y and rotating as one about the axis Y. More generally, the part33A comprises as many stages as the number N of stator contacts. The stages331-334are electromagnetically isolated from one another by spacers335. The external surface of each ring331-334carries a set of teeth336distributed at the pitch p2. The teeth336of each stage331-334are offset by a pitch p′2, equal to 1/4.p2or, more generally, 1/N.p2. The central rotor comprises two parts33A and33B that rotate as one about the axis Y. The part33B, not shown, is identical to the part33A. The part33A is aligned with the part32A of the intermediate rotor32, and the part33B is aligned with the part32B of the intermediate rotor32. The central rotor33is dimensioned such that the teeth336can cooperate with the teeth327of the intermediate rotor32. The stepping motor30thus operates analogously to the stepping motor10illustrated inFIGS. 1 to 6.

FIG. 12illustrates a longitudinal sectional view along the axis Y of the operation of the stepping motor30in a third power supply phase. In this phase, the teeth of the ring portions313A,313B,313C and313D are aligned with the teeth326of the rings323of the two parts32A and32B of the intermediate rotor32. Moreover, the teeth327of these same rings323are aligned with the teeth336of the rings333of the two parts33A and33B of the central rotor33. Field lines41and42can thus flow between the stator31, the intermediate rotor32, the central rotor33and the permanent magnet34.

By studyingFIG. 12, it will be understood that the stepping motor30could be modified without departing from the scope of the invention. For example, the intermediate rotor32and the central rotor33may have only a single part of N stages, and the stator31may have only the eight ring portions31A and31B. The field lines are thus established between the ring portions31A and31B. By contrast, it is possible for the stepping motor only to have the ring portions31A and31C, or31B and31D. The two intermediate rotor32and central rotor33parts are thus necessary. Furthermore, the number N of stages and of stator contacts may have any integer value greater than or equal to 3. Moreover, the shapes of the teeth may differ from those shown inFIGS. 7 to 11.