Abstract:
An actuation system for at least two mobile elements with dynamically compensated and opposite relative motions, without disturbance of the elements fixed in the same rigid structure as it, and resistant to exterior loadings, in the case of a translational motion of the mobile elements, includes, in a rigid structure, at least one linear actuator linked to a motion transmission device with four rigid arms, articulated at their ends and forming a lozenge, of which each of two first opposite vertices is linked to a corresponding mobile element, and whose other two opposite vertices have a single translational degree of freedom.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claim priority to foreign French patent application No. FR 10 00585, filed on Feb. 12, 2010, the disclosure of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention pertains to an actuation system for at least two mobile elements with dynamically compensated and opposite relative motions, without disturbance of the environment and resistant to exterior loadings. 
     BACKGROUND OF THE INVENTION 
     One of the main applications of the actuation systems to which the present invention pertains relates to mechanisms of “Phase Modulation” type for optical instruments in space, intended to perform rotational or translational cycles of a “modulator plate” with position increments (four-phase modulation). 
     In an optical instrument, of interferometer type, this mechanism is situated between the splitter plate and one of the two stepped mirrors. The angle of “tilt” (case of rotation) or the linear displacement (case of translation) of the “modulator plate(s)” makes it possible to modify the length of an optical path. 
     In this type of mechanism, the technical problem resides mainly in several points:
         The loads and moments must be balanced and must remain internal to the mechanism, so as not to disturb the remainder of the instrument.   In the case of several “modulator plates”, their respective motions must be perfectly synchronized, while limiting the number of actuators (in the case of a single optical plate, a “counterweight” element is used).   The assembly, when it is aboard a spacecraft, during the launch phase, must be “integral”. This means that, despite the absence of any specific lashing system, it must not degrade under the effect of the accelerations generated by the launcher.       

     Shown diagrammatically in  FIG. 1  is an autonomous and non-disturbing actuation system  1  for controlling opposite and synchronized rotational motions of two elements, an optical plate  11 , and a counterweight frame  10  which are supported in rotation about a common axis “O” with the aid of a bearing comprising the flexible elements  12 ,  13  and which is similar to the bearing  34  represented in  FIG. 5 , the assembly forming part of an optical space instrument. This system essentially comprises: a support frame  9 , two actuators  14   a ,  14   b  disposed so as to exert a couple between the elements  10  and  11 , so as to orient them one with respect of an angle α. The actuators are of piezo-electric type. The element  10  comprises at its ends inertia pieces  10   a ,  10   b  intended to limit the mass of the assembly. The limits of the path of the optical beam received by the device  1  have been delimited by two dashed lines T 1 , T 2 . 
     The system represented in  FIG. 2 , of the type with linear motions, comprises: a carrier rigid structure  9  supporting two compensating prismatic optical plates  10  and  11  by way of identical flexible metallic guidance platelets  12   a ,  13   a , these two compensating prismatic optical plates  10  and  11  being propelled by two specific actuators  14   a  and  14   b . A plate  13   a  is represented in the magnified detail view in the right part of  FIG. 2 . 
     During the operation of a linear mechanism of this type, in order to limit the forces tending to disturb the instrument, each setting of an element into motion must be compensated by an equivalent load in the opposite direction and along an axis passing through the centres of gravity of the elements in motion. The same holds for rotary mechanisms where the centres of gravity of the elements in motion must preferably be situated on a single axis of rotation. 
     The system presented in  FIG. 1  proposes a specific actuator for motorizing each element, thereby multiplying the number of components. Synchronization of the motions is obtained by complex electronic circuits. The non-convergence of the thrust vectors of the actuators and the desynchronization of the motions generate dispersions towards the instrument. 
     The linear system presented in  FIG. 2  also proposes a specific actuator for motorizing each of the two elements. The non-convergence of the two thrust vectors of the actuators and the electronic synchronization of the motions generate dispersions towards the instrument. 
     SUMMARY OF THE INVENTION 
     The subject of the present invention is an actuation system for at least two mobile elements with dynamically compensated and opposite relative motions, without disturbance of the elements fixed in the same rigid structure as it, and resistant to exterior loadings. Such a system must be compact and free of play, it must afford “automatic” synchronization of the motion of these mobile elements (without needing any additional synchronization mechanism) by being self-lashed (without needing any specific lashing mechanism), and very precise (precision such as required for optical instruments) and having a long lifetime (for example more than 15 years). 
     The first embodiment of the actuation system in accordance with the invention is an actuation system for at least two mobile elements with relative translational motions, and it is characterized in that it comprises, in a rigid structure, at least one linear actuator linked to a motion transmission device with four rigid arms, articulated at their ends and forming a lozenge of which each of two first opposite vertices is linked to a corresponding mobile element and whose other two opposite vertices have a single translational degree of freedom and are linked to the actuator, each of the two mobile elements being linked solely to one of the said first vertices. 
     When there are 2n mobile elements (n&gt;1), the latter are considered pairwise. 
     The second embodiment of the actuation system in accordance with the invention is an actuation system for at least two mobile elements with dynamically compensated and opposite relative rotational motions, and it is characterized in that it comprises, in a rigid structure, at least one linear actuator linked to a motion transmission device with four rigid arms, articulated at their ends and forming a lozenge, each arm of each pair of opposite arms being linked on one side to a mobile element and on the other side to a mobile element, the two opposite vertices of the lozenge comprising the link to the actuator(s) having a single translational degree of freedom, and it is characterized in that the mobile elements are each articulated to at least one bearing. 
     When there are 2n mobile elements (n&gt;1), the latter are considered pairwise. 
     Thus, the system of the invention uses a combination of articulated levers making it possible to obtain, on the basis of a linear input motion, two perfectly synchronized alternate motions in opposite directions (in rotation or in translation). The devising of these levers is such that it makes it possible to eliminate the forces or couples exported from the mechanism to the elements fixed in the same rigid structure as it (elements which go to make up an optical bench, when the application of this system relates to optical instruments). The triangulation of these levers is such that it also makes it possible to convey the vibratory loads of the launch to the actuators which initiate the motion and thus to avoid the use of a specific lashing mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing in which: 
         FIGS. 1 and 2 , already described hereinabove, are simplified diagrams of prior art actuation devices, and 
         FIGS. 3 to 7  are simplified diagrams, equivalent functional diagrams, viewed from above, and detail views of actuation devices in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is described hereinbelow with reference to optical instruments for spacecraft, but of course it is not limited to this application alone, and may be implemented in various applications, onboard or otherwise, in which it is necessary to impress precise translational or rotational motions, oscillatory or otherwise (these motions being synchronized, that is to say symmetric with respect to a nominal or rest position of relatively low amplitude, for example of a few tenths of a millimeter or degree) on mobile elements without disturbing their support devices, these elements being, if appropriate, protected from the abrupt accelerations that their supports might undergo. 
     Shown diagrammatically in  FIG. 3  is an exemplary embodiment of the device of the invention for two different states: in the left part of the figure, this device is represented in the “rest” state, and in the right part of the figure, it is in the activated state of the actuator. Represented at the bottom in each of these two parts, left and right, of  FIG. 3 , is an exemplary device embodiment and at the top its equivalent functional diagram. 
     The device of the invention is supported by a rigid structure, which is here a framework similar to that represented in  FIG. 2 . Of this framework, only the uprights  15  which vertically bracket (perpendicularly to the plane of the drawing) the optical plates  16 ,  17 , have been shown diagrammatically. The uprights  15  of the framework support the two substantially mutually parallel optical plates  16 ,  17 , by way of flexible metallic platelets  18  allowing the optical plates translational motions parallel to their own plane. It will be noted that because these platelets  18  deform by rotating slightly, the distance between their ends decreases slightly, therefore the distance between the plates  16 ,  17  and the axis  19 D also varies slightly. However, this variation does not have any harmful effect because it is sought to vary the path of the optical beam by making it pass through a different plate thickness, in fact the sum of the thicknesses of the two optical plates. However, if this were to pose a problem, opposed mechanical plates would be used (as is the case for the transverse guidance with the two flexible plates  22  and  24 ), or else compensated guidance plates of known type. 
     The motions of the optical plates are controlled by a linear actuator  19 , for example of the piezo-electric type with cylindrical body. From each end of the body  19 A of this actuator there projects an actuation arm,  19 B,  19 C respectively, these arms being coaxial with the axis  19 D of the cylindrical body. The platelets  18  are, in the rest state, perpendicular to the axis  19 D of the body of the actuator  19 . The ends of the arms  19 B,  19 C of the actuator  19  are each linked, in a fixed or articulated manner (pivots  20 ,  21 ), respectively to each optical plate  16 ,  17 : the end of the arm  19 B is linked to the plate  16 , while the end of the arm  19 C is linked to the plate  17 . Thus, this actuator is mounted floating between the two mobile optical plates. 
     Ties  22 ,  23  exhibiting “transverse flexibility” are fixed, parallel to the axis  19 D, between the uprights  15  of the framework taken pairwise on either side of the axis  19 D. This transverse flexibility signifies here that the middle of each of these ties exhibits a single translational degree of freedom in a direction perpendicular to the axis  19 D. According to the embodiment represented, each of these ties is parallel, in the rest state, to the axis  19 D and consists for example of two mutually parallel flexible metallic platelets disposed a small distance apart (a few millimetres, for example, however, the bigger the distance, the better the guidance will be). However, of course these ties may be realized differently, on condition that the said “transverse flexibility” is adhered to. 
     A pivot  24 ,  25  is fixed respectively in the middle of the length of each of the ties  22 ,  23 . Identical rigid rods  26 ,  27 ,  28  and  29  respectively link the pairs of pivots  20 - 24 ,  24 - 21 ,  21 - 25  and  25 - 20 , and thus form the sides of a regular lozenge. Given that, as specified hereinabove, the platelets  18  are realized in such a way as to allow the plates to support only translational motions parallel to the axis  19 D, the pivots  20  and  21  can only move along the axis  19 D. 
     The views of  FIG. 3  being views from above, it is of course understood that only the elements  18  and  20  to  29  fixed at the upper part of the plates  16  and  17  have been represented, and that elements identical to the elements  18  and  20  to  29  are advantageously disposed at the lower part of the plates  16  and  17 . 
     As a variant, the device described hereinabove may be composed of a single specific synchronization system if the mass to be set into motion can be symmetrized about the actuator. 
     The motions permitted to the pivots  20 ,  24 ,  21  and  25  are translational motions with a single degree of freedom (neglecting the slight variation in distance between the optical plates and the axis  19 D, as noted hereinabove), this being symbolized in the top part of  FIG. 3  by “glideways” G in which the corresponding pivots move. Thus, when the actuator  19  is controlled in such a way as to make its arms  19 B,  19 C extend out from the body  19 A, the centres of the pivots  20  and  21  move apart, substantially along the axis  19 D, driving the plates  16  and  17  in opposite directions in a synchronous manner (of course, it is assumed here that the opposite motions of the arms of the actuator are mutually synchronous). The lozenge formed by the rods  26  to  29  flattens (right part of  FIG. 3 ), that is to say the pivots  24  and  25  approach one another, and their centres move on the perpendicular bisector of the straight segment delimited by the centres of the pivots  20  and  21 . It follows from this that the platelets of the ties  22  and  23  curve towards the axis  19 D and that the pivots  24  and  25  move along a straight line perpendicular to the axis  19 D, and therefore have a translational motion with a single degree of freedom. 
     Because the motions of the plates  16  and  17  are “controlled” by the deformations of the lozenge formed by the rigid rods  26  to  29 , any acceleration undergone by the whole of the system of the invention does not modify the relative positions of these two plates (if, of course, the arms of the actuator  19  are locked in their position). Conversely, the motions of the two optical plates are not transmitted to the framework, and therefore to the other instruments secured to this framework because these motions balance each other by virtue of the same lozenge. 
     Shown diagrammatically in  FIG. 4  is a variant of the system of  FIG. 3 . In this  FIG. 4 , the elements similar to those of  FIG. 3  are assigned the same numerical references. The essential difference between these two systems is that the actuator  19  is mounted between the pivots  24  and  25  (these pivots  24  and  25 , as well as the pivots  20  and  21  are modified accordingly, if necessary). Because in this embodiment of  FIG. 4  the same fixings by platelets  18  of the optical plates  16 ,  17  and the same lozenge formed by the rigid rods  26  to  29  are retained, the advantages cited hereinabove of the system of  FIG. 3  are retained. 
     In  FIGS. 5 and 6  has been represented the system of the invention in the case where it is necessary to impress a rotational motion on optical plates. This system is supported by a structure (not represented) which is advantageously similar to that shown diagrammatically in  FIGS. 3 and 4 . 
       FIG. 5  illustrates the principle of the rotational motion through a simplified diagram showing a single plate in two different positions, as well as a perspective detail view of a guide bearing that may be used in the system of the invention, while  FIG. 6  represents the equivalent functional diagrams of the system of the invention with two optical plates in two different positions, and a partial schematic view of a detail of this system. 
     To simplify the drawing, only a single optical plate  30  is represented in  FIG. 5 , in two different positions: position  30 A is the “rest” position (actuator not activated), and position  30 B is the position of this same plate after a rotation of a few degrees clockwise impressed by the actuator  31 . The actuator  31 , which may be the same as the actuator  19 , as illustrated by the drawing, is represented solely in the rest position, for which its two arms  31 A,  31 B are retracted to the maximum. The ends of the arms  31 A,  31 B each comprise a pivot ( 35 A,  36 A respectively) linked by a rigid rod  32 ,  33  respectively, to a pivot ( 30 C,  30 D, respectively) fixed to a corresponding end of the “horizontal” upright of the frame of the optical plate  30 . These rods are also represented for the two positions of the plate  30 : positions  32 A,  33 A for the rest position and positions  32 B,  33 B after the said rotation. The bearing  34  supporting the rotation of the plates of the system of the invention may be advantageously such as that represented in the detail view of  FIG. 5 . This bearing has been described in French patent application No. 07 53521 filed on Feb. 27, 2007, and will therefore not be described here. The ends of the arms  31 A,  31 B are guided so as to have only a single translational degree of freedom (along a straight line coincident with the longitudinal axis of the actuator  31 ), this being symbolized in  FIGS. 5 and 6  by “glideways”  35  and  36 . 
     The complete system with two rotary optical plates or a plate and a counterweight element has been shown diagrammatically in  FIG. 6 , the left view corresponding to the “rest” position of the two mobile elements (substantially mutually parallel elements), and the right view corresponding to a position after a small rotation of the two plates in contrary directions. This system comprises, in addition to the optical plate  30 , a second optical plate  37 , these two plates each being articulated on a bearing such as the bearing  34 . Each optical plate is actuated by the actuator  31  by way of rigid rods  38 ,  39  in the same manner in which the plate  30  is actuated. The rods  32 ,  33 ,  38  and  39  are disposed so as to form the sides of a regular lozenge. The elements fulfilling the function of the glideways  35  and  36  are elements similar to the ties  22  and  23  of  FIGS. 3 and 4 . 
     The details of the device affording the function of the glideway  36  have been represented in the partial detail view of  FIG. 7 . This device comprises a flexible tie  40  consisting, like the tie  23 , of two flexible metallic platelets in the middle of the length of which has been fixed a pivot  41  which is linked to the end of the arm  31 B of the actuator. One of the ends of each of the rods  33  and  38  is articulated on a pivot  41 . The platelets of the tie  40  are, for example, as represented in  FIG. 7 , curved towards the optical plates when the arms of the actuator  31  are retracted, and rectilinear (or curved in the contrary direction) when these arms are extended out from the body of the actuator to the maximum. The other elements of this system of  FIGS. 5 and 6  (framework, pivots, “glideways” relating to the arms of the actuator) may be the same as those of the system of  FIG. 3 .