Abstract:
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 p 1  and a second set of teeth distributed at a second pitch p 2 . The stator comprises N stator contacts comprising a plurality a of teeth distributed at the pitch p 1 , distributed at a third pitch equal to p 1 (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 p 2 , distributed at a fourth pitch equal to p 1 (b+ 1 /N), and able to cooperate with the teeth of the second set of the first rotor.

Description:
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 p 1 , and a second set of teeth distributed at a second pitch p 2 , and   a 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 p 1 , where a is an integer, the N stator contacts being distributed on the stator at a third pitch equal to p 1 (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 p 1 /N,
 
the N rotor contacts comprising a plurality b of teeth distributed at the pitch p 2 , where b is an integer, the N rotor contacts being distributed on the second rotor at a fourth pitch equal to p 2 (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 p 2 /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 p 1 , 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 p 1 , 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 p 1  and aligned between the various rings, an internal surface of each ring comprising teeth distributed at the pitch p 2  and 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 p 2  and 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 p 1 , 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and further advantages will become apparent from reading the following description which is given with reference to the attached drawings, in which: 
         FIG. 1  shows, in the form of a simplified block diagram, a first example of a stepping motor according to the invention; 
         FIGS. 2 to 6  illustrate the operation of the stepping motor from  FIG. 1  during various power supply phases; 
         FIG. 7  shows a second example of a stepping motor according to the invention; 
         FIGS. 8 and 9  show a perspective view and a sectional view, respectively, of a stator of the stepping motor from  FIG. 7 ; 
         FIG. 10  shows a part of an intermediate rotor of the stepping motor from  FIG. 7 ; 
         FIG. 11  shows a part of a central rotor of the stepping motor from  FIG. 7 ; 
         FIG. 12  illustrates a longitudinal sectional view of the operation of the stepping motor from  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows, 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 motor  10  shown in  FIG. 1  comprises a stator  11 , a first rotor  12  and a second rotor  13  having a permanent magnet  14 . The stator  11  has four stator contacts  111  to  114 . Each stator contact  111 - 114  comprises two teeth  115  which are spaced apart from one another by a pitch p 1 , and a coil  116  that can be supplied with an electric current in order to create an electromagnetic field. The stator contacts  111 - 114  are distributed on the stator  11  at a pitch p 1 (2+1/N). More specifically, the stator contacts are disposed such that one of the teeth  115  of a stator contact  111 - 114  is located at a distance p′ 1  from a contiguous stator contact. The pitch p′ 1  is determined as a function of the pitch p 1  and the number N of stator contacts. It is equal to (1+1/N).p 1  or, in the example of  FIG. 1 , (1+1/4).p 1 . The first rotor  12 , also known as the intermediate rotor, is in sliding connection with respect to the stator  11  along 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 rotor  12  comprises a first set of teeth  121  distributed at the pitch p 1  and positioned opposite the teeth  115  of the stator contacts  111 - 114 . On account of the difference between the pitches p 1  and p′ 1 , it is not possible for the teeth  115  of all of the stator contacts  111 - 114  to be aligned simultaneously with the teeth  121  of the intermediate rotor  12 . For each step of the motor, two teeth  121  are aligned with the teeth  115  of one of the stator contacts  111 - 114 . The intermediate rotor  12  also comprises a second set of teeth  122  distributed at a pitch p 2  which is different from the pitch p 1 . The second rotor  13 , also known as the central rotor, is in sliding connection with respect to the intermediate rotor  12  along 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 stator  11 . The central rotor  13  comprises four sets  131  to  134  of teeth  135 , known as rotor contacts, by analogy with the stator contacts  111 - 114 . Generally, the central rotor  13  comprises N rotor contacts, or as many rotor contacts as there are stator contacts. Each rotor contact  131 - 134  comprises two teeth  135  that are spaced apart from one another by the pitch p 2 . The rotor contacts  131 - 134  are distributed at a pitch p 2 (2+1/N). More specifically, the rotor contacts are disposed such that one of the teeth  135  of a rotor contact  131 - 134  is located at a distance p′ 2  from a contiguous rotor contact. The pitch p′ 2  is determined as a function of the pitch p 2  and of the number N of rotor contacts and stator contacts. It is equal to (1+1/N).p 2  or, in the example of  FIG. 1 , (1+1/4).p 2 . On account of the difference between the pitches p 2  and p′ 2 , it is not possible for the teeth  135  of all of the rotor contacts  131 - 134  to be aligned simultaneously with the teeth  122  of the intermediate rotor  12 . For each step of the motor, two teeth  122  are aligned with the teeth  135  of one of the rotor contacts  131 - 134 . The permanent magnet  14  is attached to the central rotor  13  so as to create or increase the flow of the magnetic current between the stator contacts  111 - 114  and the rotor contacts  131 - 134 . 
       FIGS. 2 to 6  illustrate the operation of the stepping motor  10  schematically shown in  FIG. 1  during the successive power supply phases thereof. A phase corresponds to a period of time during which a coil  116  of one of the stator contacts  111 - 114  is supplied with power. The coil itself may also be known as a “phase”. During each phase, the intermediate rotor  12  and the central rotor  13  are positioned so as to minimize the reluctance between one of the stator contacts  111 - 114  and the corresponding rotor contact  131 - 134 .  FIG. 2  shows the respective positions of the stator  11 , the intermediate rotor  12  and the central rotor  13  during a first phase, specifically when the coil  116  of the stator contact  111  is supplied with power. In order to minimize the reluctance between the stator contact  111  and the rotor contact  131 , two teeth  121  of the intermediate rotor  12  are aligned with the teeth  115  of the stator contact  111 , and two teeth  122  of the intermediate rotor  12  are aligned with the teeth  135  of the rotor contact  131 . The magnetic field  20  established between the stator contact  111  and the rotor contact  131  is thus at a maximum. 
       FIG. 3  shows the stepping motor  10  during the second phase, that is to say when the coil  116  of the second stator contact  112  is supplied with power. The positions of the intermediate rotor  12  and of the central rotor  13  during the first phase are shown by dashed lines. In order to minimize the reluctance between the stator contact  112  and the rotor contact  132 , two teeth  121  of the intermediate rotor  12  are aligned with the teeth  115  of the stator contact  112 , and two teeth  122  of the intermediate rotor  12  are aligned with the teeth  135  of the rotor contact  132 . Conventionally, the passage from the first alignment between the teeth  115  of the stator contact  111  and the teeth  121  of the intermediate rotor  12 , to the second alignment between the teeth  115  of the stator contact  112  and the teeth  121  of the intermediate rotor  12  causes the intermediate rotor  12  to move with respect to the stator  11  by a distance d 1  equal to p 1 /N, or in this case p 1 /4. Analogously, the passage from the first alignment between the teeth  122  of the intermediate rotor  12  and the teeth  135  of the rotor contact  131 , to the second alignment between the teeth  122  of the intermediate rotor  12  and the teeth  135  of the rotor contact  132  causes the central rotor  13  to move with respect to the intermediate rotor  12  by a distance d 2  equal to p 2 /N, or in this case p 2 /4. In as much as the difference between the pitches p 1  and p 2  is relatively small, the intermediate rotor  12  is driven in a first direction S 1  and the central rotor  13  is driven in a second direction S 2 , opposite to the first direction. Thus, the resultant movement of the central rotor  13  with respect to the stator  11  is less than each of the two relative movements. The distance d 3  covered by the central rotor  13  with respect to the stator  11  is equal to the distance (d 2 −d 1 ), that is to say (p 2 −p 1 )/N. It follows that the distance d 3  may be chosen to be as small as desired by choosing appropriate values of the pitches p 1  and p 2 . 
       FIG. 4  shows the stepping motor  10  during the third phase, that is to say when the coil  116  of the third stator contact  113  is supplied with power. The intermediate rotor  12  and the central rotor  13  are again shown by way of dashed lines in the positions which they occupied during the first phase. During this third phase, it is the teeth  115  of the stator contact  113  which are aligned with teeth  121  of the intermediate rotor  12 , and it is the teeth  135  of the rotor contact  133  which are aligned with teeth  122  of the intermediate rotor  12 . The passage from the alignments of the second phase to the alignments of the third phase causes the intermediate rotor  12  to move again with respect to the stator  11  by the distance d 1  and in the direction S 1 , and the central rotor  13  to move again with respect to the intermediate rotor  12  by the distance d 2  and in the direction S 2 . The central rotor  13  has thus undergone a movement equal to 2.(d 2 −d 1 ) since the first phase. 
       FIG. 5  shows the stepping motor  10  during the fourth phase, that is to say when the coil  116  of the fourth stator contact  114  is supplied with power. In this phase, the teeth  115  of the stator contact  114  are aligned with teeth  121  of the intermediate rotor  12 , and the teeth  135  of the rotor contact  134  are aligned with teeth  122  of the intermediate rotor  12 . The passage from the alignments of the third phase to the alignments of the fourth phase causes the intermediate rotor  12  to move again with respect to the stator  11  by the distance d 1  and in the direction S 1 , and the central rotor  13  to move again with respect to the intermediate rotor  12  by the distance d 2  and in the direction S 2 . The central rotor  13  has thus undergone a movement equal to 3.(d 2 −d 1 ) since the first phase. 
       FIG. 6  shows the stepping motor  10  during the fifth phase. This phase corresponds in fact to the first phase, in which the coil of the first stator contact  111  is 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 rotor  12  to move with respect to the stator  11  by the distance p 1 —or 4.d 1 —and in the direction S 1 , and the central rotor  13  to move with respect to the intermediate rotor  12  by the distance p 2 —or 4.d 2 —and in the direction S 2 . Consequently, the movement of the central rotor  13  with respect to the stator  11  is equal to p 2 −p 1 . 
     The exemplary embodiment of the stepping motor in  FIG. 1  may 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 p 1 (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 p 2 (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 p 1  or p 2 , 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′ 1  from 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′ 2  from one of the teeth of a consecutive rotor contact. The pitches p′ 1  and p′ 2  were indicated as being equal to (1+1/N).p 1  and (1+1/N).p 2 , 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 p 1 /N and p 2 /N, respectively. The teeth of the stator, of the intermediate rotor and of the central rotor were shown schematically in the form of triangles in  FIGS. 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 p 2  may be equal to 1.1 times the pitch p 1 . The difference between the pitches p 1  and p 2  may be adapted depending on the desired angular resolution between the stator and the central rotor. 
       FIG. 7  shows 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 motor  30  comprises a stator  31 , an intermediate rotor  32  and a central rotor  33  having a permanent magnet  34 . The permanent magnet  34  is secured to the central rotor  33 . The rotors  32  and  33  are in pivoting connection with respect to the stator  31  about an axis Y. 
       FIGS. 8 and 9  show the stator  31  of the motor  30  from  FIG. 7  in a perspective view and in a sectional view along the axis Y, respectively. The stator  31  comprises four stator contacts  311 ,  312 ,  313  and  314 . Each stator contact  311 - 314  has four ring portions  311 A- 311 D,  312 A- 312 D,  313 A- 313 D and  314 A- 314 D, respectively. These ring portions are generically denoted  31 A- 31 D. Each ring portion  31 C is offset in translation along the axis Y from the corresponding ring portion  31 A. The ring portions  31 B and  31 D are positioned facing each ring portion  31 A and  31 C, respectively. Each ring portion is toothed at one and the same pitch p 1 . The teeth of the ring portion  312 A are angularly offset from the teeth of the ring portion  311 A by a pitch p′ 1 . The pitch p′ 1  is equal to 1/4.p 1 . More generally, the angular offset is equal to 1/N.p 1 , where N is the number of stator contacts. Similarly, the teeth of the ring portions  313 A and  314 A are angularly offset from the teeth of the ring portions  312 A and  313 A, respectively, by the pitch p′ 1 . The same goes for the ring portions  311 B- 314 B,  311 C- 314 C, and  311 D- 314 D. The teeth of the ring portions  31 A are aligned with the teeth of the respective ring portions  31 C, and the teeth of the ring portions  31 B are aligned with the teeth of the ring portions  31 D. The stator also comprises eight coils  316  which are supplied with power in pairs. A first coil  316  makes it possible to supply power to the ring portions  311 A and  311 C. A second coil  316  makes it possible to supply power to the ring portions  311 B and  311 D. Analogously, the six other coils make it possible to supply power individually to the ring portions  312 A and  312 C,  312 B and  312 D,  313 A and  313 C,  313 B and  313 D,  314 A and  314 C, and  314 B and  314 D. 
       FIG. 10  shows a perspective view of a part  32 A of the intermediate rotor  32 . The part  32 A comprises four concentric rings, known as stages  321  to  324 , distributed along the axis Y and rotating as one about the axis Y. The number of stages of the part  32 A is equal to the number N of stator contacts. The stages  321 - 324  are electromagnetically isolated from one another by spacers  325 . The external surface of each ring  321 - 324  carries a set of teeth  326  distributed at the pitch p 1 . The teeth  326  of each stage  321 - 324  are aligned with those of the other stages. The internal surface of each ring  321 - 324  carries a set of teeth  327  distributed at the pitch p 2 . The teeth  327  of each stage  321 - 324  are aligned with those of the other stages. The intermediate rotor  32  comprises two parts  32 A and  32 B which rotate as one about the axis Y. The part  32 B, not shown, is identical to the part  32 A. The part  32 A is aligned with the ring portions  31 A and  31 B, and the part  32 B is aligned with the ring portions  31 C and  31 D. More specifically, the stages  321 - 324  of the part  32 A are respectively positioned opposite the ring portions  311 A and  311 B,  312 A and  312 B,  313 A and  313 B, and  314 A and  314 B. The stages  321 - 324  of the part  32 B are respectively positioned opposite the ring portions  311 C and  311 D,  312 C and  312 D,  313 C and  313 D, and  314 C and  314 D. The intermediate rotor  32  is dimensioned such that the teeth  326  can cooperate with the teeth of the ring portions  31 A- 31 D. 
       FIG. 11  shows a perspective view of a part  33 A of the central rotor  33 . The part  33 A comprises four concentric rings, known as stages  331  to  334 , distributed along the axis Y and rotating as one about the axis Y. More generally, the part  33 A comprises as many stages as the number N of stator contacts. The stages  331 - 334  are electromagnetically isolated from one another by spacers  335 . The external surface of each ring  331 - 334  carries a set of teeth  336  distributed at the pitch p 2 . The teeth  336  of each stage  331 - 334  are offset by a pitch p′ 2 , equal to 1/4.p 2  or, more generally, 1/N.p 2 . The central rotor comprises two parts  33 A and  33 B that rotate as one about the axis Y. The part  33 B, not shown, is identical to the part  33 A. The part  33 A is aligned with the part  32 A of the intermediate rotor  32 , and the part  33 B is aligned with the part  32 B of the intermediate rotor  32 . The central rotor  33  is dimensioned such that the teeth  336  can cooperate with the teeth  327  of the intermediate rotor  32 . The stepping motor  30  thus operates analogously to the stepping motor  10  illustrated in  FIGS. 1 to 6 . 
       FIG. 12  illustrates a longitudinal sectional view along the axis Y of the operation of the stepping motor  30  in a third power supply phase. In this phase, the teeth of the ring portions  313 A,  313 B,  313 C and  313 D are aligned with the teeth  326  of the rings  323  of the two parts  32 A and  32 B of the intermediate rotor  32 . Moreover, the teeth  327  of these same rings  323  are aligned with the teeth  336  of the rings  333  of the two parts  33 A and  33 B of the central rotor  33 . Field lines  41  and  42  can thus flow between the stator  31 , the intermediate rotor  32 , the central rotor  33  and the permanent magnet  34 . 
     By studying  FIG. 12 , it will be understood that the stepping motor  30  could be modified without departing from the scope of the invention. For example, the intermediate rotor  32  and the central rotor  33  may have only a single part of N stages, and the stator  31  may have only the eight ring portions  31 A and  31 B. The field lines are thus established between the ring portions  31 A and  31 B. By contrast, it is possible for the stepping motor only to have the ring portions  31 A and  31 C, or  31 B and  31 D. The two intermediate rotor  32  and central rotor  33  parts 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 in  FIGS. 7 to 11 .