Abstract:
The present invention to provides a force measuring device suitable for use in robotic applications, having a high degree of sensitivity, stiffness, compact size, and reasonable cost. The invention includes an inner plate, surrounded by an outer plate. The inner and outer plates are integrally connected by a plurality of strain sensing flexures, generally in the form of strain rings, on which strain sensing devices are mounted. Local loads measured by the strain rings are converted to global loads, which describe forces present between the inner and outer plates.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Force measuring transducers are well known to those in the art. Generally, they are composed of an inner component surrounded by an outer component. Strain measuring flexures, usually deformable beams, link the inner and outer components and deflect under forces being transmitted from the two components. Strain sensing devices are placed on the deformable flexures to measure the deflection of the beams under the force loads. Most of the prior art devices suitable for measuring torque and forces in automobile wheels and other industrial applications, for example, are relatively large and bulky, and lack the sensitivity to measure smaller force loads. Generally, the aforementioned devices are too large and bulky to find use in miniaturized robotics applications.  
         [0002]     In the field of robotics, the measurement of forces in the robot appendages is becoming an essential requirement. Bipedal robots, such as those manufactured by the Honda Corporation (marketed under the name of Asimo), or Sony Corporation&#39;s SDR-4x, need very sensitive force measurement transducers incorporated into the wrists and feet to aid in balance and touch functions. Such force measuring devices require a compact size, high sensitivity, and high stiffness. These are requirements that are not met by the conventional force measuring technology of the prior art.  
         [0003]     Thus, it would be desirable to provide a force measurement device suitable for use in robotic applications, having a high degree of sensitivity, stiffness, compact size, and reasonable cost.  
       SUMMARY OF THE INVENTION  
       [0004]     It is an object of the present invention to provide a force measuring device suitable for use in robotic applications, having a high degree of sensitivity, stiffness, compact size, and reasonable cost. In one embodiment of the present invention, a force measuring device having three integral components includes a first component comprising a rigid inner plate, a second component comprising a rigid outer plate surrounding the inner plate and oriented generally parallel to the inner plate, and a third component comprising a plurality of flexure links disposed between the inner plate and the outer plate, integrally connecting the inner plate to the outer plate, the flexure links comprising strain rings operative to measure forces between the inner plate and the outer plate. In another embodiment of the present invention, a force measuring device having three integral components includes a first component comprising a rigid inner plate, a second component comprising a rigid outer plate surrounding the inner plate and oriented generally parallel to the inner plate, and a third component comprising a plurality of strain rings disposed between the inner plate and the outer plate, integrally connecting the inner plate to the outer plate, and operative to measure forces between the inner plate and the outer plate. The strain rings include a generally annular shaped top surface, a generally annular shaped bottom surface, and an inner surface extending between the top and the bottom surfaces, the inner surface defining a right circular cylinder. An outer surface extends between the top and bottom surfaces, the outer surface having a generally cylindrical shape. The strain rings further include a first interconnecting segment extending over a first portion of the outer surface, integrally connecting said strain ring with the inner plate, and a second interconnecting segment extending over a second portion of said outer surface, integrally connecting the strain ring with the outer plate.  
         [0005]     In another embodiment of the present invention, a force measuring device having three integral components includes a first component comprising a rigid inner plate, a second component comprising a rigid outer plate surrounding the inner plate and oriented generally parallel to the inner plate, and a third component comprising a plurality of strain rings disposed between the inner plate and the outer plate, integrally connecting the inner plate to the outer plate, and operative to measure forces between the inner plate and the outer plate. The strain rings include a generally annular shaped top surface, a generally annular shaped bottom surface, and an inner surface extending between the top and the bottom surfaces, the inner surface defining a right circular cylinder. An outer surface extends between the top and bottom surfaces, the outer surface having a generally cylindrical shape. The strain rings further include a first interconnecting segment extending over a first portion of the outer surface, integrally connecting said strain ring with the inner plate, and a second interconnecting segment extending over a second portion of said outer surface, integrally connecting the strain ring with the outer plate. At least one strain measuring device is attached to the inner surface.  
         [0006]     In another embodiment of the present invention, a force measuring device having three integral components includes a first component comprising a rigid inner plate, a second component comprising a rigid outer plate surrounding the inner plate and oriented generally parallel to the inner plate, and a third component comprising a plurality of strain rings disposed between the inner plate and the outer plate, integrally connecting the inner plate to the outer plate, and operative to measure forces between the inner plate and the outer plate. The strain rings include a generally annular shaped top surface, a generally annular shaped bottom surface, and an inner surface extending between the top and the bottom surfaces, the inner surface defining a right circular cylinder. An outer surface extends between the top and bottom surfaces, the outer surface having a generally cylindrical shape. The strain rings further include a first interconnecting segment extending over a first portion of the outer surface, integrally connecting said strain ring with the inner plate, and a second interconnecting segment extending over a second portion of said outer surface, integrally connecting the strain ring with the outer plate. At least one strain measuring device is attached to the inner surface and to the top surface.  
         [0007]     In another embodiment of the present invention, a force measuring device having three integral components includes a first component comprising a rigid inner plate, a second component comprising a rigid outer plate surrounding the inner plate and oriented generally parallel to the inner plate, and a third component comprising a plurality of strain rings disposed between the inner plate and the outer plate, integrally connecting the inner plate to the outer plate, and operative to measure forces between the inner plate and the outer plate. The strain rings include a generally annular shaped top surface, a generally annular shaped bottom surface, and an inner surface extending between the top and the bottom surfaces, the inner surface defining a right circular cylinder. An outer surface extends between the top and bottom surfaces, the outer surface having a generally cylindrical shape. The strain rings further include a first interconnecting segment extending over a first portion of the outer surface, integrally connecting said strain ring with the inner plate, and a second interconnecting segment extending over a second portion of said outer surface, integrally connecting the strain ring with the outer plate. At least one strain measuring device is attached to the inner surface and to the bottom surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will be better understood when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:  
         [0009]      FIG. 1A  is a plan view of a force sensing device having four equally spaced strain rings in a circular symmetry according to an embodiment of the present invention;  
         [0010]      FIG. 1B  is a perspective view of the embodiment of  FIG. 1A ;  
         [0011]      FIG. 2A  is a plan view of a force sensing device having three equally spaced strain rings in a circular symmetry according to an embodiment of the present invention;  
         [0012]      FIG. 2B  is a perspective view of the embodiment of  FIG. 2A ;  
         [0013]      FIG. 3A  is a plan view of a force sensing device having four equally spaced strain rings in a square symmetry according to an embodiment of the present invention;  
         [0014]      FIG. 3B  is a perspective view of the embodiment of  FIG. 3A ;  
         [0015]      FIG. 4A  is a plan view of a force sensing device for robot applications according to an embodiment of the present invention;  
         [0016]      FIG. 4B  is a perspective view of the embodiment of  FIG. 4A ;  
         [0017]      FIG. 5  is a plan view of a force sensing device relative to external forces applied to the sensing device according to an embodiment of the present invention;  
         [0018]      FIG. 6  is a partial section view of a strain ring according to an embodiment of the present invention; and,  
         [0019]      FIG. 7  is a partial section view of a strain ring showing the positions of sensing elements according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     In the following description, the term force sensing device will signify the complete assembly, which generally includes three main components: an inner plate, an outer plate, and a plurality of flexures connecting the inner and outer plates. In the embodiments described in the present invention, the flexures are generally strain rings, onto whose surfaces a plurality of strain sensing electronic devices are placed. The strain rings provide a force sensing flexure element of high stiffness and high sensitivity, while allowing a compact design. It is generally desirable to have all of the three basic components (inner plate, outer plate, and strain rings) fabricated from a single piece of starting material, which can reduce fabrication costs and assure defect free operation. It is possible, however, to join the three main components with fasteners, adhesives, solder or welding, assuming that the joining technique does not compromise the force measurements being made. If such techniques are used, the bond created between the components must be much stronger than the forces being measured, so that no additional deflection is introduced into the structure between the bonded parts.  
         [0021]      FIG. 1A  is a plan view of a force sensing device  10  having four equally spaced strain rings  14  in a circular symmetry according to an embodiment of the present invention. Force sensing device  10  is composed of three main components: an outer circular shaped plate  12 , an inner circular plate  16 , and strain rings  14   a - d  integral to and connecting inner and outer plates  12  and  16 . Plates  12  and  16  are generally aligned to be parallel with each other. Strain sensing devices (not shown) are mounted on surfaces of the strain rings  14   a - d . External attachment to the outer plate  12  may be made via holes  13   a - d . External attachment to the inner plate  16  may be made via holes  17   a - d . Attachment may be made through any convenient attachment means (such as bolts, screws, pins, inserts, etc) know to those skilled in the art. The embodiment  10  may be used, for example, in a robot application to measure forces between an end effector (or mechanical “hand”) and a wrist. The wrist may be attached to the outer plate  12 , and the end effector to the inner plate  16 . Or vise versa. Forces transmitted from the hand to the wrist, or wrist to the hand, can be measured by device  10  connecting them. This function can be of paramount importance where a robotic system interacts with a human being, since the application of too much force can be disastrous.  
         [0022]      FIG. 1B  is a perspective view of the embodiment of  FIG. 1A .  
         [0023]      FIG. 2A  is a plan view of a force sensing device having three equally spaced strain rings in a circular symmetry according to an embodiment of the present invention. Force sensing device  20  is composed of three main components: an outer circular shaped plate  22 , an inner circular plate  26 , and strain rings  24   a - c  integral to and connecting inner and outer plates  22  and  26 . Plates  22  and  26  are generally aligned to be parallel with each other. Strain sensing devices (not shown) are mounted on surfaces of the strain rings  24   a - c . External attachment to the outer plate  22  may be made via holes  23   a - f . External attachment to the inner plate  26  may be made via holes  27   a - f . Attachment may be made through any convenient attachment means (such as bolts, screws, pins, inserts, etc) know to those skilled in the art. This embodiment  20  may be used in applications similar to that described above for force sensing device  10 .  FIG. 2B  is a perspective view of the embodiment of  FIG. 2A . Both force sensing devices  10  and  20  can be utilized to measure all force loads of interest. Embodiments with three strain rings (such as device  20 ) have a lower cost due to having fewer strain sensing electronic devices. Embodiments with four strain rings (such as device  10 ) have the advantage of separating orthogonal forces loads due to orientation, thus making data evaluation more direct and improving accuracy. In some geometries, such as the foot geometry described below in FIGS.  4 A/ 4 B, four strain rings fit the structure better in that the loads through each strain ring are better balanced.  
         [0024]      FIG. 3A  is a plan view of a force sensing device  30  having four equally spaced strain rings  34   a - d , in a rectangular symmetry according to an embodiment of the present invention. Force sensing device  30  is composed of three main components: an outer rectangular shaped plate  32 , an inner rectangular plate  36 , and strain rings  34   a - d  integral to and connecting the inner and outer plates  32  and  36 . Plates  32  and  36  are generally aligned to be parallel with each other. Strain sensing devices (not shown) are mounted on surfaces of the strain rings  34   a - d . External attachment to the outer plate  32  may be made via holes  33   a - d . External attachment to the inner plate  36  may be made via holes  37   a - d . Attachment may be made through any convenient attachment means (such as bolts, screws, pins, inserts, etc) know to those skilled in the art.  FIG. 3B  is a perspective view of the embodiment of  FIG. 3A .  
         [0025]      FIG. 4A  is a plan view of a force sensing device  40  for robot applications according to an embodiment of the present invention. The specific shape illustrated may be used, for example, in a robot foot application. Force sensing device  40  is composed of three main components: an outer plate  42 , an inner plate  46 , and strain rings  44   a - d  integral to and connecting the inner and outer plates  42  and  46 . Plates  42  and  46  are generally aligned to be parallel with each other. Strain sensing devices (not shown) are mounted on surfaces of the strain rings  44   a - d . External attachment to the outer plate  42  may be made via holes  43   a - f . External attachment to the inner plate  46  may be made via holes  47   a - f . Attachment may be made through any convenient attachment means (such as bolts, screws, pins, inserts, etc) know to those skilled in the art.  FIG. 4B  is a perspective view of the embodiment of  FIG. 4A .  
         [0026]      FIG. 5  is a plan view of a force sensing device  50  relative to external forces applied to the sensing device according to an embodiment of the present invention. The force sensing device  50  is capable of measuring forces being transmitted from inner plate  52  through to outer plate  51  (and vise versa). These forces are generally referred to as global forces, which can be represented by Cartesian coordinate system Fx (ref  62 ), Fy (ref  60 ) and Fz (extending out of the plane of the paper, not shown). Global forces may also include moments Mx, My, and Mz (not shown). Global forces are measured indirectly by the strain rings  54   a - d , which are designed so that the global forces Fx, Fy, Fz, Mx, My, and Mz can be derived from local loads measured directly by the strain rings. These local loads are generally a horizontal load H (ref  56 ), a vertical (shear) load oriented  6   ut  of the plane of the paper and centered within the strain ring  54 , and a twisting load oriented along an axis connecting the two strain ring attachment points connecting the strain rings  54   a - d  to the inner plate  52  to the outer plate  51 . Strain sensing gauges are placed on strain rings  54   a - d  in locations to minimize (or eliminate) the impact of a compressive force P (ref  58 ), which is directed along the axis connecting the two strain ring attachment points. The global forces are computed from the geometry of the various strain ring locations combined with the local forces measured at each strain ring, using techniques known by those skilled in the art. For example, a simple Global force Fx (positive to the right) would result in a negative H force at strain ring  54   a , an equal positive H force at strain ring  54   c , and no forces measured at strain rings  54   b  and  54   d . In another example, a global force Mz (a moment force around the Z axis) would result in equal local H forces of the same magnitude and sign at each of the four strain rings  54   a - d . In a third example, a global moment force My would produce local twisting loads, of opposite sign, at strain rings  54   a  and  54   c , and vertical shear loads, of opposite sign, at strain rings  54   b  and  54   d . In a fourth example, a global force Fz would produce equal local vertical (shear) load forces at all four strain rings oriented in the same direction as Fz. In a similar manner, global forces can be resolved from local forces for the three strain ring embodiment shown in  FIGS. 2A and 2B .  
         [0027]      FIG. 6  is a partial section view of a strain ring according to an embodiment of the present invention. The strain ring is generally the shape of a hollow cylinder, having an outer diameter  63 , and an inner diameter  61 . Preferably, the strain ring is a hollow right circular cylinder. The top surface  76  is generally of an annular shape, as is the bottom surface (not shown). Inner surface  72  is generally cylindrical in shape, preferably defining a right circular cylinder of diameter  61 . Inner surface  72  extends from top surface  76  to the bottom surface. The height of the strain ring, which is the distance between the top and bottom surfaces, is generally less than or equal to the thickness of the outer plate  60  or inner plate  62 . Preferably, the height of the strain ring is less than or equal to the thinner of the two plates  60  and  62 . A first interconnecting segment  66  extends from the outer diameter of the strain ring at dashed line  70 , merging into the structure of inner plate  62 , integrally connecting the strain ring to the inner plate  62 . A second interconnecting segment  64  extends from the outer diameter of the strain ring at dashed line  68 , merging into the structure of outer plate  60 , integrally connecting the strain ring to the outer plate  60 . An outer surface  74  is generally cylindrical in shape, and extends from interconnecting regions  64  an  66  at an outer diameter  63 , between top and bottom surfaces.  
         [0028]      FIG. 7  is a partial section view of a strain ring showing the positions of sensing elements according to an embodiment of the present invention. Strain sensing devices, preferably strain gauges, are mounted on various surfaces of the strain ring to measure local loads. As previously mentioned, the global loads are then computed from the local load information provided by all the strain rings connecting the inner and outer plates of the force measuring device. The location of the strain gauges may be clarified by reference to a set of orthogonal axis  80  and  82 , whose intersection is located at the center of circle defined by the inner diameter  61 . For measuring the H load  102 , at least one strain gauge is placed on the inner surface  72  at the locations indicated by ref  98  or ref  100 . Preferably, bending type guages at both locations  98  and  100  are used. Alternatively, four bending type gauges can be used, one orthogonally oriented pair at position  98  and one orthogonally oriented pair at position  100 . Since bending gauges have a preferred direction of maximum sensitivity, an orthogaonally oriented pair means two bending gauges mounted in close proximity to each other, but having their preferred directions of highest sensitivity at right angles to each other. The orthogonally oriented pairs are also known as Poisson&#39;s gauges and can improve the strength of the measurement by about 30%. Gauge positions  98  and  100  are located at an angular position alpha  84 , measured from horizontal axis  82 . Alpha is between 40 to 50 degrees, preferably 45 degrees. Strain gauges located at  98  and  100  have a minimal sensitivity to P load  104 , which can be easily calibrated out in use. For measuring load V (not shown), shear type strain gauges may be located at positions  94  and  96  on the inner surface  72  of the strain ring. At least one gauge is required, but preferably two are used. Alternatively, four gauges in a Poisson&#39;s configuration may also be utilized.  
         [0029]     In another embodiment of the present invention, the V load is measured by placing bending gauges on the top surface  76  at locations  90  and  92 . Alternatively, the gauges may be placed on the bottom surface at corresponding positions (not shown). Between 1 and four gauges may be used, as described above. Gauge positions  90  and  92  are located at an angular position alpha  84 , measured from horizontal axis  82 . Alpha is between 40 to 50 degrees, preferably 45 degrees. The H load is measured by placing gauges at positions  98  and  100 , as explained in detail above.  
         [0030]     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.