Abstract:
A three-dimensional force and torque converter unit for measuring an external force or torque applied to the unit and converting it into a signal, whereby the signal may be used to control a system or device incorporating the converter unit. The converter unit includes a controller formed with four spaced apart arms having six or more degrees of constraint. A force or torque may be applied to the tip portions of each of the arms via a gripping means. Sensors are used to measure the deflection of the arms under an applied loading or torque and an output signal is generated.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is a 35 U.S.C. §371 national phase conversion of PCT/AU2003/001420 filed 28 Oct. 2003, which claims priority to Australian Application No. 2002952290 filed on 28 Oct. 2002. 
   The PCT International Application was published in the English language. 
   FIELD OF THE INVENTION 
   The present invention relates to control devices and more particularly is concerned with such devices which respond to input forces or torques in three dimensions and permit a control signal to be derived for controlling a machine such as a computer controlled system or the like. However, devices embodying the invention may be applied to other uses. 
   BACKGROUND OF THE INVENTION 
   The present inventor is also an inventor of inventions in this field relating to three dimensional force and torque sensing devices which are the subject of U.S. Pat. Nos. 4,811,608; 5,222,400; 5,706,027 and 5,798,748. 
   A further prior published proposal in the field is U.S. Pat. No. 4,589,810 Heindl et al. 
   In recognising this other prior published material, the inventor does not admit that any of these other proposals are necessarily known to persons working in the field or of that of common general knowledge in any particular country. 
   The inventors prior U.S. Pat. No. 4,811,608 discloses a six arm device where the arms are orthogonally arranged and responses in the arms to force or torque with respect to any axis in three dimensions are monitored using sensors. 
   The inventor has now appreciated that new and useful alternatives to his own prior art and other prior art items disclosed above would be highly advantageous and the present invention is concerned with such alternatives. 
   SUMMARY OF THE INVENTION 
   In summary the invention may be described as a controller having four and only four arms extending from a body portion which is adapted to support the device, the arms being spaced from one another in three dimensions and the device having six or more degrees of constraint, tip portions of each of the arms engaging in connection means providing restricted relative motion, the connection means being attached to a gripping means which can apply force and/or torque in a three dimensional sense, the device including response detection means for monitoring responses in at least three of the four arms to provide an output signal representative of force and/or torque applied through the gripping means. 
   In some embodiments the device is arranged to control a system with the signal. 
   The arms may be arranged in a tetrahedron shaped envelope and optionally are almost equally spaced from one another in a symmetrical sense with included angles of approximately 109°. However a small degree of non-symmetry is advantageous to ensure there is some preloading mechanically which addresses friction issues yet provides a device in which the computer based system can rapidly perform the relevant calculations that derive an accurate output signal. 
   Most usefully the arms are constrained such that the device has eight degrees of constraint. 
   This may be achieved by the tip of each arm having a ball element which is slidable along a cylindrical bore associated with the connection means and rotatable within reasonable limits inside the bore. Thus each such connection has freedom to engage in translational movement along the axis of the bore and limited freedom to rotate. The ball joint is thus constrained in two directions defining a plane at right angles to the axis of the bore and there are four dimensions of freedom in total and two constraints at each joint. 
   Optionally, the sensors for monitoring response in the arms are disposed around a circular path in a plane. The sensors may advantageously be an optically based system. 
   The optical system can detect very accurately extremely small deflections in the arms responsive to the applied force or torque. 
   Another embodiment is one in which six sensors are provided in an array so that displacements in an X-Y set of directions for each of the four arms is achieved giving eight readings which can be resolved to give the required output signal. 
   Another advantageous embodiment of the present invention includes a plurality of optical sensors as component parts of the response detection means. These optical sensors are concentric and disposed on the same plane. 
   The six sensors may optionally be configured in pairs around three of the four arms. 
   The present invention, embodiments of which have been described above, may be usefully arranged as a component of a computer system whether incorporated as an external facility or as an integral sub-system. 
   By way of technical background, an explanation of principles which may further explain the invention or some of its embodiments will be given, but the applicant is not to be bound by the completeness or correctness of this explanation. Further features of a preferred embodiments will also be explained. 
   The constraint relationship between two bodies can be determined by summing the constraints of the joint or joints between the two bodies excluding mechanisms which have special geometric alignments. A perfectly constrained device would have exactly six degrees of constraint. Perfectly constrained designs require high joint tolerances to avoid a rattling due to the joint clearances or to avoid excessive friction of the joints due to interference. In practice a slight interference renders the product unusable so perfectly constrained designs tend to exhibit a small amount of rattle due to the clearances in the joints. It is also desirable to provide a small amount of damping through some friction of the joints. 
   When a control device having a displaceable grip is designed, it is useful to recognise that when the grip is released damping avoids vibration issues and avoids the requirements of a very lightweight grip, as is the case with purely spring-based designs. The friction of a perfectly constrained design, when the grip is released, is only dependent upon the weight of the grip and the frictional properties of the materials and hence is not adjustable in a typical design. 
   Overconstrained designs can be easily preloaded by slightly offsetting either side of a joint. Optionally only a small overconstraint is used to avoid tolerancing issues. A preferred embodiment of the present invention is slightly overconstrained with eight degrees of constraint. This allows the arms of the tube protrusions to be offset slightly relative to the connection means such as the cylindrical bores to introduce a slight preload when the device is at rest. 
   Durability of a design is impacted heavily by the wear characteristics of a joint. In perfectly constrained designs with point contact a small amount of wear increases the slop of the joint resulting in increased rattle of the device. The present preferred embodiments have line contact joints that wear much more slowly than point contact. In conjunction with a small preload the device does not exhibit slop. 
   The preferred embodiment has a central body and arms moulded as a single unit to form a four-armed, generally star-shaped body which for convenience in this specification will be known as a “tetra-star” to provide rigid mounting of the arms of the body and to reduce cost. A complex tool is required to mould the central star part and each arm is formed by three sections of the tool. The preferred embodiment has spherical tips that engage with bores in an outer ball or shell which forms the grip. The mould has three parting lines. To avoid any flash from affecting the operation of the ball-in-hole joints, the ideal spherical surface is optionally cut back along the parting lines with a cylindrical surface so the flash will not touch the surface of the cylindrical bore associated with the outer ball. 
   In the preferred embodiment, there is an inner ball structure for mounting the tetra-star and comprising a lower and an upper section. Four holes in the inner ball are provided for the cylindrically bored extensions from the outer ball to pass through and engage the tetra-star&#39;s arms. These holes also limit the range of motion of the extensions and prevent the arms from being overstressed. Impact loads are passed directly from the extensions to the inner ball structure thereby avoiding damage of the tetra-star&#39;s arms so that a robust design is achieved. 
   Preferred embodiments use infrared LEDs and photodiodes to detect the tetra-star&#39;s arm displacements. Only six sets of sensors are required for the full 3D force and 3D torque computation. These are optionally arranged as three pairs with one arm having no sensors. Two pairs on two arms and the other two arms with a single sensor is also possible but less desirable. Similarly eight sets of sensors could be used with a pair for each arm. Each arm would optionally have the optical axes perpendicular to each other. 
   In the preferred embodiment a shadow mask technology is used for sensing the displacement using an infrared LED and an infrared photodiode. The use of infrared provides greater immunity from ambient light affecting the measurement. Light falling on the photodiode from the LED generates a small current. As the arm deflects, the amount of light varies and in turn the amount of current varies. Greater linearity is achieved by keeping the voltage across the photodiode constant using an appropriate circuit. Each LED/photodiode pair has a characteristic loss factor measured as the ratio of the LED drive current vs. the photodiode output current with no shadow. This is typically around 200:1. For good accuracy the drive circuitry and/or computation needs to compensate for the variation in loss factor. 
   The preferred embodiment has ball-in-hole joints being 2 degree-of-constraint joints. These have line contact between the spherical ball-tip surface and the whole surface. 

   
     DESCRIPTION OF THE FIGURES 
     For exemplification only the invention will be described with reference to the following illustrative drawings: 
       FIG. 1  is a schematic three dimensional representation of a base unit of a three dimensional control device, eg: for controlling computers; 
       FIG. 2  is a schematic vertical cross section through the device and having a generally spherical gripping cap for manual manipulation to operate the device; 
       FIG. 3  is a schematic three dimensional view of a tetra-star component used in the device; 
       FIG. 4  is a schematic three dimensional representation from the interior of one of the segments of the cap of the device and used for gripping purposes; and 
       FIG. 5  is a three dimensional exploded view of the device of  FIGS. 1-4  in the form of a practical embodiment. 
       FIG. 6  is a schematic view similar to the view of  FIG. 3 , however the tetra-star component is viewed along the axis of one of the arms. This view also shows one of the optics sub-assemblies. The tip of the arm has been excluded to provide a better view of the optics sub-assemblies. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The principal components of the device comprise a tetra-star body  10  base, an inner bowl shaped cap  12  and an outer cap  13  formed from segments, one of which is shown in  FIG. 4 . 
   The tetra-star  10  has four arms  14  extending along respective axes from central body  15 , the axes, being substantially uniformly geometrically disposed relative to one another. Each arm  14  has an elongated reduced cross-section cylindrical portion  14 A extending from a tapered base  16  and leading to a tip  17  having an enlarged head with, the surface profile including substantially a spherical portion  18  with a flattened end face  19 . A series of structural webs  20  are individually formed on the tetra-star body. 
   The inner cap  12  has apertures  30  for accommodating tubular retainers  24  associated with the outer cap  13  and thereby limited displacement of the cap  13  (which acts as a grip). 
   As most clearly seen in  FIG. 2  one of the arms extends substantially vertically upwards and, as described above, a preferred embodiment has optical sensing for detecting flexing in the arms.  FIG. 2  shows schematically a photo detector unit  21  having a light omitting diode (LED)  22  and photo detector  23 . Each of the arms  14  is constrained with line contact in a respective tubular retainer  24  which is integrally formed with and projects inwardly from the respective cap segments of  13  to engage the tips  17 . 
   Referring now to  FIGS. 5 and 6  small cut-outs  31  in each of the three lower printed circuit boards (PCB)  32  provides clearance for the assembly of the photo detector unit  21  into the inner cap  12 . Each of the three lower PCBs  32  mounts a printed circuit board interface  34 . There are three triplets of optics subassembly supports  35  protruding from the structural webs  20  to easily and accurately mount respective optics subassemblies  36  which include the PCB  32  and two photo detector units  21 . 
   The PCB interface  34  provides interconnections for the optics subassemblies  36  and mounts interface electronics (not shown). A ribbon cable (not shown) is soldered to the PCB interface  34  and runs inside a stem  11  for connection to external electronics (not shown). 
   The top and bottom sections of the inner cap  12  respectively include three pairs of clips  37  and three pairs of clip apertures  38  for inter-engagement. The stem  11  has three screw bosses (not shown) for mounting the device to a base (not shown), a ribbon cable exit slot and a keying slot to ensure the device is mounted correctly. The edges of the segments of the outer cap  13  have interlocking tabs  39  for mutual attachment and assembly. These tabs  39  require all four parts of the outer cap  13  to be assembled at the same time. The interlock design of the tabs  39  require a simple two-part moulding tool for manufacture. Although the segments of the outer cap  13  mechanically engage, the segments are glued for strength. 
     FIG. 6  clearly shows how the arms  14  are offset from the light omitting diodes  22  and corresponding photos detectors  23  such that the variation in light due to the deflection of the arms  14  can be easily measured. 
   The tetra-star  10  is designed for plastic injection moulding. Notably, the spherical portion  18  of each arm  14  needs to be accurate and has sections profiled to keep any moulding flash below the spherical portion  18 . The type of plastic needs to have a good fatigue life to handle the repetitive bending stresses imposed on the arms  14  and it should have low friction with the outer cap material. Delrin® is a suitable material for the tetra-star  10 . 
   The arrangement is such that the application of force or torque through the outer cap  13  with respect to any axes is detected by a characterising flexing in the arms. This flexing can be detected and computation determines the appropriate signal to be directed to a device such as a computer. 
   As the outer cap  13  is moved, the four tubular retainers  24  push on the four arms  14  deflecting them so they oppose the displacement of the outer cap  13 . Ignoring the very small and hence insignificant frictional components, each arm tip  17  force vector can be considered as a 2D force vector lying in a plane normal to the corresponding axis of the tubular retainer  24 . A simplifying assumption is made that each plane remains stationary as the outer cap  13  moves. The very small errors due to this assumption are insignificant. The deflection of each arm tip  17  is proportional and in the same direction as the 2D force vector. Using standard engineering mathematics, each 2D force vector acting through a arm tip  17  can be transformed into a 3D force vector and a 3D torque vector acting through the centre of the device. The 3D force vector and 3D torque vector acting on the outer cap  13  is then calculated by summing the four 3D force vectors and summing the four 3D torque vectors respectively. 
   The force vector  13  acting on an arm tip  17  is proportional to the deflection measured by the photo detector unit  21  (or sensor  21 ) located part way down the length of the arm  14 . The ratio of the force on the arm tip  17  to the measured deflection is constant and can be measured experimentally or calculated from an arm&#39;s spring constant combined with geometric calculations of the shape of a deflected arm  14 . Given the constant ratio, the force is easily calculated from the deflection by multiplication. 
   From engineering theory a minimum of six single value sensors are required to measure a simultaneous 3D force vector and 3D torque vector. Clearly, a device with four pairs of sensors, a pair for each arm, is functional. A device with three pairs of sensors can be used if the fourth 2D force vector can be calculated from the other three. Consider the device of  FIG. 5  where the lower three arms  14  have sensors  21  but the top arm  14  does not. Using each of the three measured 2D force vectors the force component tangential to a circle, centred on the centre of the device and passing through the centre of the top arm tip  17 , is calculated. These three force vector components are then mathematically rotated so as to act through the centre of the top arm&#39;s tip  17 . These three force vectors are then summed to calculate the 2D force vector associated with the fourth arm  14 . 
   It is helpful to consider the simple situation where the outer cap  13  is pushed downwards by a force acting through the centre of the device. The top arm  14  does not deflect but the lower three arms  14  deflect downwards sharing the load equally. The required tangential components happen to be the same as their respective 2D force vectors. Rotating these force vectors so that they act through the centre of the top arm  14  results in three equal force vectors acting 120° to each other and therefore adding to zero as expected. 
   It is also theoretically possible to have a device with two pairs of sensors  21  on two arms  14  and two single sensors  21 , appropriately oriented, on the other two arms  14 . 
   In this specification, the word “comprising” and its variations, such as “comprises”, has a meaning such that the word does not preclude additional or unrecited elements, substances or method steps, in addition to those specifically recited. Thus, the described apparatus, substance or method may have other elements, substances or steps in various embodiments of the invention. The purpose of the claims is to define the features which make up the invention and not necessarily all features which a working embodiment of the apparatus, substance or method, to which the invention defines, may have. The apparatus, substance or method defined in the claims may therefore include other elements, steps or substances as well as the inventive elements, steps or substances which make up the invention and which are specifically recited in the claims.