Abstract:
A multidirectional motor system for transmitting motion to a moveable element in at least two directions that are not collinear comprising a first motor that transmits motion to the moveable element along a direction determined by the orientation of the first motor and a second motor operable to change the orientation of said first motor.

Description:
RELATED APPLICATIONS 
     The present application is a U.S. national application of PCT/IL98/00521, filed Oct. 26, 1998. 
     FIELD OF THE INVENTION 
     The invention relates to motors for providing motion to a moveable element and in particular to providing motion of a moveable element in more than one direction using motors, for example, piezoelectric motors. 
     INTRODUCTION 
     Piezoelectric motors use vibrators made of piezoelectric materials to convert electrical energy into mechanical motion. The motors are used in many and varied applications and have been designed to impart motion to moveable elements in, among other things, automotive fuel injectors, videocassette recorders, automatic cameras, computer disc drives, and precision microscope stages. 
     Most piezoelectric motors generally impart motion to moveable elements to which they are coupled back and forth along a single straight line, hereinafter referred to as an “axis of motion”. To impart motion to a moveable element along more than one axis of motion, generally a different piezoelectric motor is coupled to the moveable element for each different axis of motion desired. 
     Modem devices often comprise many small intricate parts that move with different forms of motion and in many different directions with respect to each other. These devices often have limited room available for a plurality of motors to effect these motions. It would be advantageous to have a piezoelectric motor that can by itself impart motion to a moveable element to which it is coupled along different axes of motion. 
     SUMMARY OF THE INVENTION 
     Aspects of preferred embodiments of the present invention relate to providing a piezoelectric motor, hereinafter referred to as a “multidirectional motor”, that can impart motion to a moveable element along a plurality of axes of motion. 
     In a preferred embodiment of the present invention a multidirectional motor comprises first and second piezoelectric motors. The first motor, hereinafter referred to as a “driving motor”, is coupled to a moveable element and imparts motion to the moveable element back and forth along an axis of motion. The direction of the axis of motion is determined by the orientation of the driving motor with respect to the moveable element. The second motor, hereinafter referred to as a “steering motor”, is coupled to the first motor. Activation of the steering motor changes the orientation of the first motor with respect to the moveable element and thereby the axis of motion along which the first motor imparts motion to the moveable element. 
     Whereas the driving motor and the steering motor are described as being piezoelectric motors it should be realized that the steering motor can be any suitable motor or actuator, such for example an electromagnetic motor, a gas driven motor or a solenoid, appropriately coupled to control the orientation of the driving motor. Furthermore, the driving motor can be any motor that is friction coupled to the moveable element so as to impart motion to the moveable element. 
     There is therefore provided in accordance with a preferred embodiment of the present invention a multidirectional motor system for transmitting motion to a moveable element in at least two directions that are not collinear, comprising: a first motor that is coupled to the moveable element and transmits motion to the moveable element along a direction determined by the orientation of the first motor; a second motor operable to change the orientation of the first motor. Preferably, the first motor is friction coupled to the moveable element and presses on a surface region of the moveable element. 
     Preferably, the second motor is operable to rotate the first motor around an axis through a point on the surface region of the moveable element on which the first motor presses. 
     Preferably, the multidirectional motor system comprises a frame in which the first motor is mounted and the second motor is operable to rotate the frame about the axis, which frame comprises at least one support that prevents the first motor from rotating with respect to the frame about the axis. 
     The frame preferably comprises a circularly cylindrical surface having an axis of revolution that coincides substantially with the axis and wherein the second motor presses on the cylindrical surface and is operable to rotate the cylindrical surface. 
     In some preferred embodiments of the present invention the cylindrical surface has an azimuthal extent about the axis of rotation that is substantially equal to 180°. In some preferred embodiments of the present invention, the cylindrical surface has an azimuthal extent substantially equal to 360°. 
     In some preferred embodiments of the present invention the at least one support is connected to the cylindrical surface. 
     In some preferred embodiments of the present invention the frame comprises a planar mounting plate having two parallel planar surfaces that are perpendicular to the axis of rotation and the cylindrical surface is fixed to a planar surface on one side of the mounting plate and the first motor is fixed to the frame on the other side of the mounting plate. In some preferred embodiments of the present invention the cylindrical surface is convex. In other preferred embodiments of the present invention the cylindrical surface is concave. 
     In some preferred embodiments of the present invention the frame comprises a planar mounting plate having two parallel planar surfaces, the axis passes through the mounting plate and is perpendicular to the planar surfaces and the second motor presses on one of the planar surfaces and is operable to rotate the mounting plate around the axis. 
     Additionally or alternatively, the first motor comprises a piezoelectric motor. Additionally or alternatively, the second motor comprises a piezoelectric motor. 
     There is further provided in accordance with a preferred embodiment of the present invention a method of transmitting motion to a moveable element along a plurality of directions comprising: friction coupling a first motor to the moveable element by pressing a first motor to a surface region of the moveable element, which first motor transmits motion to the moveable element along a direction that is determined by the orientation the first motor; and changing the orientation of the first motor. 
     Preferably, changing the orientation of the first motor comprises using a second motor to change the orientation of the first motor. Using the second motor preferably comprises using the second motor to rotate the first motor about an axis substantially perpendicular to the surface region. 
     Additionally or alternatively, the first motor is a piezoelectric motor. Additionally or alternatively, the second motor is a piezoelectric motor. 
     The invention will be more clearly understood by reference to the following description of preferred embodiments thereof read in conjunction with the figures attached hereto. In the figures identical structures, elements or parts which appear in more than one figure are labeled with the same numeral in all the figures in which they appear. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     FIGS  1 A- 1 C schematically show a multidirectional motor oriented to impart motion to a moveable element along different axes of motion, in accordance with a preferred embodiment of the present invention. 
     FIGS. 2A-2B schematically show the multidirectional motor shown in FIGS. 1A-1C rotating a sphere about different axes of rotation, in accordance with a preferred embodiment of the present invention; 
     FIGS. 3A-3B schematically show the multidirectional motor of FIGS. 1A-1C moving a motion stage along different axes of motion, in accordance with a preferred embodiment of the present invention; 
     FIG. 4 schematically shows another multidirectional motor in accordance with a preferred embodiment of the present invention; and 
     FIG. 5 schematically shows another multidirectional motor in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1A-1C schematically show a multidirectional motor  20  oriented to impart motion to a moveable element along different axes of motion, in accordance with a preferred embodiment of the present invention. Sizes of elements and components of multidirectional motor  20  are not necessarily to scale and the relative dimensions of the elements and components have been chosen for clarity of presentation. 
     Referring to FIG. 1A, multidirectional motor  20  comprises a driving piezoelectric motor  22  and a steering piezoelectric motor  24 . Driving motor  22  is used to impart motion to a moveable element to which multidirectional motor  20  is coupled and steering motor  24  is used to determine the direction of an axis of motion along which the motion is imparted. 
     Driving motor  22  preferably comprises a thin rectangular plate  26  formed from an appropriate piezoelectric material having two relatively large planar face surfaces  28  (only one of which is shown in FIG.  1 A). Piezoelectric plate  26  has long side edge surfaces  30  (a portion of one of side edge surfaces  30  and only an edge of the other is shown in FIG. 1A) and short top and bottom edge surfaces  32  and  34  respectively (only an edge of bottom edge surface  34  is shown in FIG.  1 ). Preferably, a friction nub  36  is located on top edge surface  32  for coupling vibratory motion of piezoelectric plate  26  to a moveable element. Friction nub  36  is preferably formed from a wear resistant material such as alumina. Preferably, driving motor  22  is a motor of a type described in U.S. Pat. No. 5,453,653, to Zumeris et al, or in European Publication EP 0 755 054, which are incorporated herein by reference. Piezoelectric plate  26  is controlled to vibrate by applying AC voltages to electrodes (not shown) located on face surfaces  28  of piezoelectric plate  26  as described in the cited references. 
     Vibrations in piezoelectric plate  26  cause friction nub  36  to vibrate with a motion, generally elliptical, that lies substantially in a plane parallel to face surfaces  28 . As a result, driving motor  22  is controllable to impart motion to a moveable element to which friction nub  36  is pressed substantially only along either of the two directions indicated by a double arrow line  40 . Double arrow line  40  passes through the region of contact between friction nub  36  and the moveable element and is parallel to the planes of face surfaces  28 . The direction of double arrow line  40  defines an axis of motion of multidirectional motor  20 . Double arrow line  40  is hereinafter referred to as “motion line  40 ” of multidirectional motor  20 . 
     Driving motor  22  is preferably mounted inside a rotation frame  42 . Rotation frame  42  has an axis of rotation  44  and preferably comprises a rotation collar  46  having a contact surface  48 . Preferably, rotation frame  42  comprises a pair of opposing upper U brackets  50  located above rotation collar  46  and a pair of opposing lower U brackets  52  below rotation collar  46 . Each upper U bracket  50  is preferably located on an upper bracket support  54  that extends upwardly from rotation collar  46 . Each lower bracket  52  is preferably located on a lower bracket support  56  that extends downwardly from rotation collar  46 . A base plate  60  preferably connects the ends of lower bracket supports  56 . 
     Driving motor  22  is positioned inside rotation frame  42  so that upper and lower U brackets  50  and  52  firmly “grasp” piezoelectric plate  26  along opposite side edge surfaces  30  so as to prevent piezoelectric plate  26  from rotating with respect to rotation frame  42 . Parts of upper U brackets  50  preferably press on regions of opposite side edge surfaces  30  that are substantially directly opposite each other. Similarly, parts of lower U brackets  52  preferably press on regions of opposite side edge surfaces  30  that are substantially directly opposite each other. Preferably, the regions of side edge surfaces  30  on which U brackets  50  and  52  press are located at or in the near vicinity of nodal points of a resonant vibration of piezoelectric plate  26 , namely at ⅓ and ⅔ of the length of the plate. 
     Preferably, the lower bracket  50  and the upper bracket  52  that press on one of the side edge surfaces  30  are designed using methods known in the art so that parts of the brackets exert resilient forces on the side edge surface. These forces urge piezoelectric plate  26  towards the opposing upper and lower U brackets  50  and  52  respectively along the other of side edge surfaces  30 , which opposing brackets preferably support piezoelectric plate  26  substantially rigidly. 
     Bottom edge surface  34  of piezoelectric plate  26  is preferably positioned near to base plate  60  with, preferably, a resilient biasing means  62  sandwiched between them. Resilient biasing means  62 , which can be a leaf or coil spring, a layer of resilient material, or other appropriate means known in the art, resiliently urges bottom surface  34  away from base plate  60 . Whereas U brackets  50  and  52  firmly grasp piezoelectric plate  26  so as to prevent rotation of driving motor  22  with respect to rotation frame  42 , they are designed to enable motion of driving motor  22  parallel to axis  44 . This enables biasing means  62  to resiliently maintain separation between bottom edge  34  and base plate  60 . In FIG. 1A biasing means  62  is shown as a leaf spring by way of example. 
     Steering motor  24  is preferably similar in construction to driving motor  22 . Steering motor  24  comprises a thin rectangular piezoelectric plate  64  and has top and bottom edge short surfaces  66  and  68  respectively. A friction nub  70  is preferably located on top edge surface  66 . Steering motor  24  is secured in a position, using methods known in the art, so that a resilient force  72  acting on bottom surface  68  presses friction nub  70  to a region of contact surface  48  of rotation collar  46 . Preferably, three bearings  74 ,  76  and  78  also press on rotation collar  46 . Preferably, bearings  74 ,  76  and  78  press on contact surface  48  at azimuth angles about axis  44  that are separated by 90°. Bearing  76  preferably presses on a region of contact surface  48  of rotation collar  46  that is directly opposite the region of contact surface  48  on which friction nub  70  presses. A bearing  80  preferably presses on the center of base plate  60 . Steering motor  24 , bearings  74 ,  76 ,  78  and  80  are appropriately mounted in a suitable support frame (not shown) using methods known in the art so that their relative positions are accurately fixed with respect to axis  44  and so that axis  44  is fixed with respect to the support frame. 
     The location of bearings  74 ,  76 ,  78  and  80  enable steering motor  24  to accurately rotate rotation frame  42  about axis  44  and fix the orientation of the plane of driving motor  22  so that motion line  40  points along any azimuth angle about axis  44 . Steering motor  24  thereby determines the directions along which driving motor  22  imparts motion to a moveable element to which friction nub  36  is pressed. 
     Electrical contact to electrodes of driving motor  22  that are electrified to excite vibrations of piezoelectric plate  26  are made through appropriate sliding contacts situated on rotation frame  42  using methods known in the art. These contacts assure that electrical contact with the electrodes exists for all orientations of rotation frame  42 . 
     When steering motor  24  is dormant, frictional forces between friction nub  70  and contact surface  48  prevent rotation collar  46  from moving and as a result, the orientation of the plane of driving motor  22  and motion line  40  is fixed. When steering motor  24  is activated, it is controllable so that friction nub  70  exerts force on rotation collar  46  along either one of the two directions indicated by double arrow  75 . The force generates a torque that rotates rotation frame  42  so that the plane of driving motor  22  and motion line  40  can be rotated in either the rotation of rotation frame  42  are indicated by double arrow line  76 . 
     FIGS. 1B and 1C show rotation frame  42  driving motor  22  and axis of motion  44  rotated with respect to the positions of rotation frame  42  driving motor  22  and axis of motion  44  respectively shown in FIG.  1 A. 
     Multidirectional motor  20  is coupled to a moveable element by positioning multidirectional motor so that friction nub  44  is resiliently pressed to an appropriate surface region of the moveable element by biasing means  62 . In order to rotate rotation frame  42 , torque generated by steering motor  24  must be sufficient to overcome frictional forces between friction nub  36  and the region to which friction nub  36  is pressed. These frictional forces generate torque that opposes torque generated by steering motor  24 . It should be recognized that this requirement is relatively easily met. Force applied to rotation collar  46  by steering motor  24  acts on a lever arm about axis  44  that is much larger than any lever arm about axis  44  on which frictional forces between friction nub  36  and the moveable element operate. 
     FIGS. 2A-2B schematically show a sphere  100  held between a ring bearing  102  and multidirectional motor  20  shown in FIGS. 1A-1C so that multidirectional motor  20  is controllable to rotate sphere  100  in different directions, in accordance with a preferred embodiment of the present invention. 
     Referring to FIG. 2A, ring bearing  102  has an axis of rotation  104  that passes through the center of sphere  100 . Multidirectional motor  20  is positioned so that axis of rotation  44  (shown in FIGS,  1 A- 1 C) of rotation collar  46  is coincident with axis of rotation  104  of ring bearing  102  and so that friction nub  36  is resiliently pressed to the surface of sphere  100  by biasing means  62 . Preferably, force that friction nub  36  exerts on sphere  100  presses the surface of sphere  100  to appropriate bearings (not shown) on the inside surface of ring bearing  102 . The bearings enable sphere  100  to rotate around any axis through the center of the sphere. Multidirectional motor  20  and ring bearing  102  are preferably mounted in a suitable structure so as to maintain their relative positions. 
     When driving motor  22  is activated, sphere  100  rotates around an axis through the center of sphere  100  that is perpendicular to axis  104  and motion line  40  of driving motor  22 . In FIG. 2A axis  106  is perpendicular to axis  104  and motion line  40 . When activated, driving motor  22  rotates sphere  100  clockwise or counterclockwise around axis  106  and the surface of sphere  100  moves along one of the corresponding directions indicated by double arrow line  108 . 
     Steering motor  24  is controllable to rotate motion line  40  to any azimuth about axis of rotation  104 . Multidirectional motor  20  is thereby controllable to rotate sphere  100  about any axis through the center of sphere  100  that is perpendicular to axis of rotation  104 . FIG. 2A shows multidirectional motor  20  after driving motor  24  has rotated rotation frame  42  clockwise 45° about axis  104  (or 315° counterclockwise about axis  104 ). Now when driving motor  22  is activated sphere  100  rotates around axis  110  and the surface of sphere  100  moves in one of the directions indicated by double arrow line  112 . It should be realized that when steering motor  24  rotates driving motor  22  sphere  100  will tend to rotate in the direction that driving motor rotates. If necessary this can be prevented by incorporating an appropriate braking mechanism that prevents sphere  100  from moving when steering motor  24  rotates driving motor  22 . 
     It should be note that whereas sphere  100  is shown being held by one ring bearing  102  to which sphere  100  is pressed by force exerted by friction nub  36 , other configurations for holding sphere  100  and coupling it to a multidirectional motor, in accordance with a preferred embodiment of the present invention, are possible and advantageous. For example, it is possible to maintain the position of sphere  100  with respect to friction nub  36  using three bearings in place of ring bearing  102 . A different orientation for ring bearing  102  other than that shown in FIGS. 2A and 2B is also possible in certain circumstances. For example, ring bearing  102  may be placed under the sphere if the weight of sphere  100  is large enough, and the use to which the sphere is applied always permits the weight of the sphere to be supported by ring bearing  102 . In other configurations, sphere  100  can be held between two ring bearings. Other possibilities will occur to persons of the art. 
     FIGS. 3A-3B schematically show multidirectional motor  20  coupled to a motion stage  120  having a surface  122  so as to move motion stage  120  in any direction parallel to surface  122 . Motion stage  120  is appropriately supported by any of the many methods known in the art so that it is movable in any direction parallel to surface  122 . As in the case with sphere  100 , in the case of motion stage  120 , multidirectional motor  20  is positioned with friction nub  36  in contact with surface  122  so that biasing means  62  is compressed and maintains friction nub  36  resiliently pressed to surface  122 . Driving motor  22  is controllable to move motion stage  120  backwards or forwards along motion line  40  and steering motor  24  rotates motion line  40  to any desired azimuth about axis  44 . FIGS. 1A and 1B show multidirectional motor  20  oriented to move motion stage  120  backwards or forwards along two different orientations of motion line  40 . Whereas in general a motor coupled to move a motion stage is coupled to the motion stage on a bottom surface of the motion stage, multidirectional motor  20  is shown coupled to motion stage  120  on a top surface, surface  122 , for clarity of presentation. 
     It should be realized that different variations of driving motor  20  are possible and advantageous and will occur to persons of the art. For example it is possible to support rotation collar  46  with two bearings instead of three. Other configurations for mounting piezoelectric plate  26  to rotation frame  42  are possible. For example, piezoelectric plate  26  can be prevented from rotating with respect to rotation frame  42  using U brackets that do not press on side edges  30  of piezoelectric plate  26 . Lateral stabilization of the position of piezoelectric plate  26  parallel to face surfaces  28  in this case can be achieved with two additional pressure supports on side edges  30  that press on opposite side edges  30 . Furthermore rotation frame  42  and piezoelectric plate  26  can be designed so that there is no resilient biasing means between bottom surface  34  of piezoelectric plate  26  and base plate  60  of rotation frame  42 . To obtain resilient contact between friction nub  36  and a moveable element, bearing  80  can be spring loaded or a frame to which the elements of multidirectional motor  20  are mounted can be resiliently pressed to the moveable element. 
     FIG. 4 shows another multidirectional motor  140  in accordance with a preferred embodiment of the present invention. Multidirectional motor  140  preferably comprises a ac driving motor  142 , a rotation plate  144  and a steering motor  146 . Driving motor  142  and steering motor  146  are preferably of the same type as shown in multidirectional motor  20  shown in FIGS. 1A-1C. Driving motor  142  has face surfaces  148 , side edges  150  and top and bottom edge surfaces  152  and  154  respectively. Top edge surface  152  preferably has a friction nub  156  for coupling vibratory motion of driving motor  142  to a movable element. Similarly, steering motor  146  has face surfaces  160  (only one of which is shown) and preferably a friction nub  162  mounted on a short top edge surface  164  of steering motor  146 . Friction nubs  156  and  162  are preferably made of wear resistant material such as alumina. Rotation plate  144  is preferably circular and has an axis of rotation  166 , a top surface  168 , a bottom surface  170  and an edge surface  172 . 
     Driving motor  142  is preferably mounted to rotation plate  144  between two constraining plates  174  that are fixed to top surface  168  of rotation plate  144  so that driving motor  142  does not rotate with respect to rotation plate  144 . (Constraining plate  174  that lies behind driving motor  142  in the perspective of FIG. 4 is shown in dashed lines.) Driving motor  142  is secured against lateral motion parallel to rotation plate  144  by two rigid supports  176  and two resilient supports  178  that urge driving motor  142  towards rigid supports  176 . In some versions of multidirectional motor  140 , bottom surface  154  of driving motor  142  is contiguous with top surface  168  of rotation plate  144 . Preferably however, a biasing means is sandwiched between bottom surface  154  of driving motor  142  and top surface  168  of rotation plate  144 . In variations with a biasing means, rigid and resilient supports  176  and  178  are designed to enable motion of driving motor  142  perpendicular to surface  168  of rotation plate  144  but not parallel to the plate. 
     Driving motor  142  is positioned so that the center of friction nub  156  is located on axis  166 . A motion line  180  parallel to face surfaces  148  of driving motor  142  indicates the axis of motion along which driving motor  142  imparts motion to a moveable element to which friction nub  156  is resiliently pressed. 
     Steering motor  146  and rotation plate  144  are preferably mounted to an appropriate mounting frame (not shown), using methods known in the art, so that rotation plate  144  is free to rotate about axis  166  and so that friction nub  162  of steering motor  146  is resiliently pressed to bottom surface  170  of rotation plate  144 . Preferably, friction nub  162  is pressed to bottom surface  170  close to edge  172  of rotation plate  144 . Face surfaces  148  of steering motor  142  are preferably parallel to the tangent to edge  172  at the point on edge  172  that is closest to the point at which friction nub  162  contacts bottom surface  170 . Preferably, bottom surface  170  is clad in a wear resistant coating in those regions where friction nub  162  contacts bottom surface  170 . In variations of multidirectional motor  140 , rotation plate  144  is made sufficiently large so that steering motor  146  and driving motor  142  can be positioned on the same side of rotation plate  144  and friction nub  162  is pressed to top surface  168 . 
     The orientation of motion line  180  is controlled by steering motor  146  which is controllable to rotate rotation plate  144  about axis  166 , preferably, in either of the directions indicated by double arrow line  182 . 
     Multidirectional motor  140  is coupled to a moveable element by resiliently pressing the mounting frame that holds driving motor  142  to the moveable element so that friction nub  156  resiliently presses on an appropriate surface of the moveable element. In variations of multidirectional motor  140  in which a resilient biasing means is sandwiched between top surface  168  of rotation plate  144  and bottom edge surface  154  of driving motor  142 , the frame of multidirectional motor  140  may be positioned rigidly with respect to the moveable element. In this case the biasing means serves to resiliently press friction nub  156  to the moveable element. 
     FIG. 5 shows another multidirectional motor  190  in accordance with a preferred embodiment of the present invention. Multidirectional motor  190  comprises a driving motor  192 , a steering motor  194  and a rotation platform  196 . Rotation platform  196  comprises a mounting plate  198  having a top surface  200  and a bottom surface  202 . Components and features of multidimensional motor  190  and parts of components and features of multidimensional motor  190  that are covered by mounting plate  198  in the perspective of FIG. 5 are shown in dashed lines. 
     A thin circularly cylindrical plate  204 , hereinafter referred to as a “coupling apron  204 ” extends downward from bottom surface  202  of mounting plate  198 . Coupling apron  204  has an inner contact surface  206  having a radius of curvature and an axis of rotation  208  perpendicular to and passing through a point of mounting plate  198 . Preferably, coupling apron  204  has an azimuthal extent about axis  208  substantially equal to 180°. 
     Driving motor  192  and steering motor  194  are preferably similar in construction to driving motor  22  and steering motor  24  comprised in multidirectional motor  20  shown in FIGS. 1A-1C. Driving motor  192  has face surfaces  210  and preferably comprises a friction nub  212  mounted on a short top edge surface  211 . Steering motor  194  has face surfaces  214  long edges surfaces  216  and preferably comprises a friction nub  218  on a short top edge surface  219 . 
     Driving motor  192  is mounted to rotation platform  196  on upper surface  200  of rotation platform  196  with face surfaces  210  perpendicular to upper surface  200  and with the center of friction nub  208  located on axis  208 . Driving motor  192  is mounted so that it does not rotate about axis  208  with respect to mounting platform  196  preferably using methods described above, variations thereof or other methods known in the art. A motion line  220  parallel to face surfaces  210  and edge surface  211  of driving motor  192  indicates the axis of motion along which driving motor  192  imparts motion to a moveable element to which friction nub  220  is pressed. 
     Steering motor  194  is held by a mounting collar  222  that is attached to bottom surface  202  of mounting plate  198  by a “lazy Suzan” bearing  224 . Lazy Suzan bearing  224  enables mounting collar  222  to rotate freely about axis  208  with respect to rotation platform  196 . Face surfaces  214  of steering motor  194  are held accurately parallel to mounting plate  198  by lazy Suzan bearing  224  and mounting collar  222 . Mounting collar  222  is designed so that steering motor  194  can move linearly relative to mounting collar  222  in a direction parallel to the long edges of edge surfaces  216  but is substantially prevented from rotating with respect to mounting collar  222  about axis  208 . A radius of rotation of coupling surface  206  that passes through the center of friction nub  218  passes through or very close to axis  208 . Steering motor  192  is resiliently urged towards contact surface  206  along a direction parallel to the long edges of edge surfaces  216  using methods known in the art so that friction nub  218  presses on contact surface  206  Contact surface  206  is preferably clad with a wear resistant material. 
     Multidirectional motor  190  is coupled to a moveable element by resiliently urging mounting collar  222  along axis  208  towards an appropriate surface of the moveable element so as to press friction nub  212  to the surface. An appropriate frame (not shown) that accurately maintains the orientation of mounting collar  222  and axis  208  fixed with respect to the moveable element supports mounting collar  222 . 
     Driving motor  190  is controllable to impart motion to the moveable element in the two directions indicated by motion line  220 . Steering motor  194 , when activated, imparts motion to coupling apron  204  so as to rotate rotation platform  196  clockwise or counter clockwise about axis  208  and thereby to rotate motion line  220 . The azimuthal range over which steering motor  194  rotates motion line  220  is determined by the azimuthal extent of coupling apron  206 . Since driving motor  192  is controllable to impart motion to the moveable element along either of the two directions indicated by motion line  220 , multidirectional motor  190  is controllable to impart motion to the moveable element over a range of azimuths that is twice as large as the azimuthal extent of coupling apron  206 . 
     In accordance with a preferred embodiment of the present invention, coupling apron  206  has an azimuthal extent greater than 180°. As a result, steering motor  194  can rotate motion line  220  through a 180° angle. Driving motor  192  can therefore impart motion to the moveable element in any azimuthal direction in a 360° range about axis  208 , i.e. in any direction about axis  208 . (It should be noticed that in order to obtain a full 180° range of rotation for motion line  220 , generally a diagonal length of face surfaces  214  of steering motor  192  must be less than twice the radius of rotation of coupling apron  204 .) 
     By positioning steering motor  194  under driving motor  192  so that it presses on inner surface  206  of coupling apron  204 , a particularly compact embodiment of the present invention is achieved. It is of course possible to couple driving motor  194  to the outside surface of coupling apron  204  and this can be advantageous. 
     In the claims and specification of the present application, each of the verbs, “comprise” and “include”, and conjugates thereof, are used to convey that the object or objects of the verb are not necessarily a listing of all the components, elements or parts of the subject or subjects of the verb. 
     The present invention has been described using a non-limiting detailed description of a preferred embodiment thereof. Variations of the embodiment described will occur to persons of the art. The detailed description is provided by way of example and is not meant to limit the scope of the invention, which is limited only by the following claims. 
     As used in the claims, the words “comprise” or “include” or their conjugates means “including, but not necessarily limited to.”