Force / torque sensor with serpentine or helical deformation beam and overload beam

Abstract

A force / torque sensor (110, 130, 140, 150) includes a plurality of serpentine (116, 132, 142) or spiral (152) deformable beams connecting a TAP (112) and a MAP (114). These types of shapes increase the overall length of the deformable beams (116, 132, 142, 152), thereby reducing their stiffness. In addition to the deformable beams (116, 132, 142, 152), there are a plurality of straight overload beams (118), each of which is connected to one of the TAP (112) and MAP (114) at a first end and is spaced apart from the other of the TAP (112) and MAP (114) at a second end by an overload gap (120) of a predetermined width. In a first range of forces and torques, strain gauges (134) on the deformable beams (116, 132, 142, 152) convert compressive and tensile strains into electrical signals that are processed to resolve forces and torques. In a second range of forces and torques greater than the first range, the overload beam (118) closes the overload gap (120) to establish rigid contact with both the TAP (112) and the MAP (114). The stiffness of the sensor (110, 130, 140, 150) in the second range of forces and torques is greater than in the first range.

Description

[0001] Related applications

[0002] This application claims priority to U.S. Provisional Patent Application Serial No. 62 / 816,191, filed on March 10, 2019, and U.S. Patent Application Serial No. 16 / 682,475, filed on November 13, 2019, which are hereby incorporated by reference in their entireties. Technical Field

[0003] The present invention relates generally to force / torque sensors and, more particularly, to compact sensors that exhibit a high ratio of stiffness after overload activation to stiffness before overload activation by using a serpentine or helical deformable beam and an overload beam. Background Art

[0004] Robotics is a growing and increasingly important field in industrial, medical, aerospace, scientific, and other applications. In many cases, a robotic arm or its attached tool contacts a surface or object (such as a workpiece), and the applied force and / or torque must be closely monitored. Therefore, force / torque sensors are essential components of many robotic systems.

[0005] Force / torque sensors for robotic applications are known in the art. For example, U.S. Patent Nos. 10,422,707 and 10,067,019 describe robotic tool force / torque sensors. Figure 1 and 2 are included in this article as an introduction to the concept of strain gauge based force / torque sensors. Many variations of this type of sensor are known in the art, and Figure 1 The specific design depicted in FIG2 is not meant to be representative or limiting.

[0006] As described in the above patents, and with reference to Figure 1, one type of force / torque sensor 10 uses strain gauges 1-6 to measure the deformation of small beams 16a-c connecting two mechanical parts - one connected to a robotic arm and the other connected to a robotic tool (or a mechanical coupling to a tool) or other end effector. For example, a central "hub" called a tool adapter plate (TAP) 12 in the art can be connected to the tool. Another body arranged annularly around the TAP and spaced apart from the TAP, called a mounting adapter plate (MAP) 14 in the art, can be connected to the robotic arm (or vice versa). The TAP 12 and MAP 14 are interconnected by a plurality of relatively thin (and therefore mechanically deformable) beams 16a, 16b, 16c, which are arranged radially around the TAP 12 - in this design, similar to the spokes of a wheel. The relative force or torque between the objects attached to the TAP 12 and MAP 14, respectively, attempts to move the MAP 14 relative to the TAP 12, causing at least some of the beams 16 to deform or bend slightly, as shown in Figure 2 (not to scale). The deformation is enhanced by connecting each beam 16 to a very thin curved portion 17 of the MAP 14 .

[0007] Strain gauges 1-6 are fixed to one or more surfaces of each beam 16. The strain gauges 1-6 convert the strain at the beam surface caused by the mechanical deformation of the beam into an electrical signal. For example, the strain gauges 1-6 can be strain gauges that respond to some combination of tensile, compressive and shear strains by changing their resistance in response to elongation or contraction. Once calibrated, the signals from the strain gauges 1-6 on all beams 16 are processed together to resolve the magnitude and direction of the relative force and / or torque between the robot arm and the attached tool. As used herein, the term "strain gauge" refers to any sensor or transducer (or combination of sensors or transducers) that responds predictably to mechanical deformation of a material, and the response can be detected, quantified and used to resolve the force or torque causing the deformation. The patents cited above provide more details about specific aspects of the particular force / torque sensor 10 for which inventive improvements are described.

[0008] This general type of force / torque sensor works well within a limited range of force and / or torque values. Typical sensors are made of rigid materials, such as aluminum or steel, and they do not deform at all when very low forces are applied. Within this limited force / torque range, a beam or other deformable member deforms slightly, with the resulting tensile and compressive forces detected by strain gauges and quantified by the measurement electronics. However, as the magnitude of the applied force or torque increases, because the deformable member is quite short and very rigid, it can barely move further before suffering from metal fatigue or yielding—reducing or terminating the sensor's service life, respectively. Force / torque sensors are typically manufactured in various sizes and are specified for operation only within their specified range of force / torque values. However, not all robotic applications enjoy controlled environments. For example, robots deployed in real-world settings such as law enforcement, the military, and space exploration may encounter temporary or even sustained forces and torques exceeding their design ratings.

[0009] One way to prevent damage to a force / torque sensor in an overload situation is to provide an overload feature. One type of overload feature is referred to herein as an "overload beam" (as distinguished from a "deformable beam," which can be instrumented). Figure 1 An example is depicted as Figure 3 Included in this article. In this sensor, a deformable beam 2 connects TAP 1 to MAP 4 in the + / - x and y directions. The deformable beams 2 are equipped with strain gauges R17-R20, R21-R24, R25-R28 and R29-R32 on opposite sides of each beam 2. A set of overload beams 3 are radially distributed between the deformable beams 2 at a 45-degree angle. The overload beams 3 are not equipped with strain gauges. The overload beams 3 are connected to TAP 1 at one end, but are separated from the contact with MAP 4 at the other end by a narrow gap, such as a few thousandths of an inch. The width of this gap is designed to allow the overload beam 3 to move freely within the normal deformation range of the deformable beam 2 - that is, it allows the deformable beam 2 connecting TAP 1 and MAP 4 to move (deform) within its designed range of motion, allowing slight relative movement between TAP 1 and MAP 4. However, when the applied force or torque reaches the sensor's design maximum, and the deformable beam 2 deforms its maximum design amount, one or more overload beams 3 close the gap and make hard contact with the MAP 4. The overload beams 3 are stiffer / stronger than the deformable beams 2; therefore, further applied force / torque will not further deform the deformable beam 2 because the overload beams 3 absorb the excess force and prevent any further relative movement between the TAP 1 and the MAP 4.

[0010] FIG4 , from published Chinese application CN1283985, illustrates a different form of overload feature. A deformable beam 2 connects TAP 6 to MAP 9. At the deformation limit of deformable beam 2, relative horizontal movement of TAP 6 and MAP 9 is constrained by a pin 5 rigidly attached to MAP 9 and disposed within a hole 8 in TAP 6. The radius of hole 8 is slightly larger than the radius of pin 5 by δ1. Relative vertical movement of TAP 6 and MAP 9 is constrained by their spacing—TAP 6 and MAP 9 are separated by a gap of width δ2. Other configurations of overload features are known in the art.

[0011] Traditional overload features, such as Figure 3 The gap depicted in Figure 4 requires maintaining very tight tolerances between the mating overload features. This can increase the cost of the force / torque sensor. Furthermore, achieving very tight tolerances may require machining / alignment of the components after assembly into the sensor, which risks damaging the sensor and can increase lead times for machined components due to the addition of a separate step in the manufacturing process. Furthermore, if tight tolerances exist and the sensor deviates slightly from its intended zero-load position, the range that the sensor can accurately measure will not be centered around the zero-load position, potentially leading to premature sensor failure or the sensor's inability to accurately measure across its entire specified range.

[0012] Components that include overload features (i.e. Figure 3 The stiffness of the overload feature (the overload beam 3 of FIG. 4 or the pin 5 of FIG. 4 ) is typically slightly greater than the stiffness of the deformable member in the sensor, and the deformation experienced by the deformable member is sensed, measured, and converted into force and torque. Ideally, once the overload feature is activated—for example, once the gap is eliminated and the overload feature makes hard contact between the TAP and MAP—all further deformation in the deformable member (measured as strain) should cease to increase. However, in practice, if the stiffness of the overload feature is similar to the stiffness of the deformable member, applying more load to the sensor will continue to increase the stress measured by the strain gauge. This fact places a limit on how much force or torque can be applied to the sensor after the overload activation point and before the sensor structure yields—despite the overload feature.

[0013] The Background section of this document is provided to place the embodiments of the present invention in a technical and operational context to help those skilled in the art understand their scope and utility. The approaches described in the Background section may be explored, but are not necessarily approaches that have been previously conceived or explored. Unless expressly indicated otherwise, no statement herein is deemed to be prior art solely by virtue of its inclusion in the Background section. Summary of the Invention

[0014] A brief summary of the present disclosure is provided below to provide a basic understanding for those skilled in the art. This summary is not an extensive overview of the present disclosure and is not intended to identify key / essential elements of embodiments of the present invention or to delineate the scope of the present invention. The sole purpose of this summary is to present some of the concepts disclosed herein in a simplified form as a prelude to a more detailed description that will be presented later.

[0015] According to one or more embodiments described and claimed herein, a force / torque sensor includes a plurality of serpentine or spiral deformable beams connecting a TAP and a MAP. For each serpentine deformable beam, at least one directional path of the centerline of the serpentine deformable beam from the attachment point of the TAP to the attachment point of the MAP (or vice versa) deviates from the instantaneous straight line at least once to the left and at least once to the right. For each spiral deformable beam, at least one directional path of the centerline of the spiral deformable beam from the attachment point of the TAP to the attachment point of the MAP (or vice versa) deviates from the instantaneous straight line either to the left or to the right, and wherein the cumulative sum of such deviations is greater than 90 degrees. These types of shapes increase the overall length of the serpentine or spiral deformable beam, thereby reducing its stiffness. A plurality of overload beams, which may be straight, are interspersed between the serpentine or spiral deformable beams. Each overload beam is connected to one of the TAP and MAP at a first end and is separated from the other of the TAP and MAP by an overload gap of a predetermined width at a second end. During a first range of forces and torques, the serpentine or helical deformable beam deforms under relative motion between the TAP and MAP, and the strain gauges thereon convert the strain into electrical signals that are processed to resolve the forces and torques. Within a second range of forces and torques greater than the first range, one or more overload beams close the overload gap, establishing rigid contact with both the TAP and MAP, nominally stopping further relative motion between the TAP and MAP. The stiffness of the sensor within the second range of forces and torques is greater than that within the first range. This high stiffness ratio produces a sharp turn in the strain / force curve at the overload actuation point, with a low slope as the applied load increases. This increases the number of cycles the sensor can enter an overload state before metal fatigue occurs, and increases the maximum force / torque the sensor can withstand without yielding or breaking.

[0016] One embodiment relates to a force / torque sensor. The sensor includes a TAP operable to connect to a first object and a MAP operable to connect to a second object. The sensor also includes one or more serpentine deformable beams connecting the TAP to the MAP, wherein at least one directional path of the centerline of the serpentine deformable beam from the attachment point of the TAP to the attachment point of the MAP deviates from the instantaneous straight line at least once to the left and at least once to the right. Strain gauges are fixed to one or more surfaces of at least some of the serpentine deformable beams. The strain gauges are operable to convert strain in the serpentine deformable beam caused by beam deformation into electrical signals. The measurement circuit is operable to measure the direction and magnitude of the force and torque between the first object and the second object in response to the electrical signals from all the strain gauges.

[0017] Another embodiment relates to a force / torque sensor. The sensor includes a TAP operable to connect to a first object and a MAP operable to connect to a second object. The sensor also includes one or more helically deformable beams connecting the TAP to the MAP, wherein a centerline of the helically deformable beam deviates from an instantaneous straight line one to the left or right from at least one directional path from an attachment point of the TAP to an attachment point of the MAP, and wherein the cumulative sum of such deviations is greater than 90 degrees. Strain gauges are fixed to one or more surfaces of at least some of the helically deformable beams. The strain gauges are operable to convert strain in the helically deformable beams caused by deformation of the beams into electrical signals. The measurement circuit is operable to measure the direction and magnitude of the force and torque between the first object and the second object in response to the electrical signals from all of the strain gauges. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be described more fully below with reference to the accompanying drawings, in which embodiments of the invention are shown. However, the present invention should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided to make this disclosure thorough and complete and to fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

[0019] Figure 1 is a plan view of a prior art force / torque sensor (US 10,422,707 and US 10,067,019 Figure 1 ).

[0020] Figure 2 is Figure 1 A magnified view of the sensor showing the deformation of the beam (Fig. 2 of US 10,422,707).

[0021] Figure 3 is a plan view of a prior art force / torque sensor with an overload beam ( Figure 1 ).

[0022] FIG4 is a cross-sectional view of a prior art force / torque sensor with an overload pin ( Figure 1 ).

[0023] Figure 5 is a graph of strain versus load for a sensor that has a low stiffness ratio before and after overload activation.

[0024] Figure 6 is a graph of strain versus load for a sensor that has a high stiffness ratio before and after overload activation.

[0025] Figure 7 depicts the diagram and equations for the static analysis of a cantilever beam.

[0026] Figure 8 is a plan view of a force / torque sensor having a serpentine deformable beam according to one embodiment.

[0027] Figure 9 is a plan view of a force / torque sensor having a serpentine deformable beam according to another embodiment.

[0028] Figure 10 yes Figure 9 A perspective view of a serpentine deformable beam of a force / torque sensor.

[0029] Figure 11 is a plan view of a force / torque sensor having a serpentine deformable beam according to another embodiment.

[0030] Figure 12a-12c Various serpentine deformable beam shapes are depicted.

[0031] Figure 13a-Figure 13c Various helical deformable beam shapes are depicted.

[0032] Figure 14 is a plan view of a force / torque sensor having a helically deformable beam according to one embodiment. DETAILED DESCRIPTION

[0033] For simplicity and illustrative purposes, the present invention will be described primarily with reference to its exemplary embodiments. In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be readily apparent to those skilled in the art that the present invention may be practiced without limitation to these specific details. In this description, well-known methods and structures are not described in detail to avoid unnecessarily obscuring the present invention.

[0034] Although the stiffness of the overload feature in a force / torque sensor is typically somewhat greater than the stiffness of the deformable member measuring strain, it is the ratio of the sensor stiffness after the overload feature is activated to the sensor stiffness within its normal operating range that is important in assessing the effectiveness of the overload feature. A higher ratio reduces the rate at which stress increases on the sensor after the overload feature is activated, further increasing the maximum force that can be applied to the sensor before failure.

[0035] As mentioned above, ideally, all sensor motion (i.e., relative motion between the TAP and MAP) would cease once the overload feature activates—that is, once the gap closes and the overload beam or other member makes hard contact with the relevant opposing surfaces. In reality, however, as the applied force or torque increases, the sensor components continue to move, just not as much as before the overload feature activation because the contact between the two sides of the gap is not infinitely stiff. Therefore, the ratio of the sensor stiffness after the overload feature activation to the sensor stiffness before it is activated is a key metric.

[0036] Figure 5 and Figure 6 These are graphs for two different sensors, depicting the load (force or torque) applied on the x-axis and the maximum stress observed by the sensor on the y-axis. Note that the units of force are not important, as they scale with the size of the sensor. In the first sensor ( Figure 5 ), the stiffness after the overload feature is activated is not much greater than before activation - that is, k_overload / k_sensor ~= 1. In the second sensor, the stiffness after the overload feature is activated is much greater than before such activation - that is, k_overload / k_sensor ~= 10.

[0037] Pass inspection Figure 5 and Figure 6 , it can be seen that both sensors respond linearly within their design range—for applied loads up to 5, both sensors report corresponding stresses up to 5. Above an applied force of 5, the point at which the overload feature activates, both graphs change slope. Figure 5 In the first sensor depicted in , the sensor is not much stiffer after overload activation than before, and as the load continues to increase, the stress output increases almost as fast as before the overload activation.

[0038] In contrast, Figure 6 The second sensor depicted in is much stiffer after the overload activation than before, and as the load continues to increase, the stress output increases much more slowly—that is, the graph after the overload signature activation point has a much lower slope.

[0039] One goal in designing effective overload characteristics—particularly for force / torque sensors deployed in uncontrolled environments—is to be able to apply large forces / torques to the sensor before it experiences metal fatigue, yielding, fracture, or shattering. Figure 6 ) than the first sensor ( Figure 5 ) meets this goal to a greater extent. For example, if both sensors would break at a stress level of 7, the second sensor could withstand a load of up to 27 before breaking, while the first sensor would break at a load of only 9.

[0040] Therefore, a sensor whose stiffness after the overload feature is activated is much greater than the sensor stiffness within its normal operating range will perform better under overload conditions. In other words, making a force / torque sensor with an overload feature less stiff within its operating range will increase the maximum force it can withstand before failure.

[0041] One way to make a deformable beam-type force / torque sensor less stiff within its operating range is to reduce the stiffness of the deformable beam (while maintaining high stiffness in the overload beam or other overload features). A straightforward way to reduce beam stiffness without increasing beam stress is to increase its length. The deflection of a beam is proportional to the cube of its length; however, the stress in the beam is only proportional to its length. Figure 7 depicts the equations and graphs related to shear V and moment M for a cantilever beam of length L.

[0042] In many sensor designs, it is not possible to directly increase the length of the deformable beam, e.g. Figure 1 - those depicted in the figure, their total diameter is limited. In addition, this necessarily increases Figure 3 The length of the overload beams is reduced, thereby reducing their stiffness and failing to achieve the desired increase in the ratio of stiffness after activation of the overload feature to the stiffness before activation.

[0043] According to some embodiments of the present invention, as in Figures 8-11As depicted in , the length of a deformable beam - but not an overload beam - is increased by forming a serpentine deformable beam. As used herein, the term "serpentine" refers to a shape that deviates from a straight line by alternately bending or flexing to one side and then to the other side. In other words, the centerline of the serpentine deformable beam deviates from a straight line along the directional path of its length from the attachment point of the TAP to the attachment point of the MAP (or vice versa) by turning, bending or skewing to the left (or right) at least once, and then deviates further from the straight line by turning, bending or skewing to the right (or left) at least once. Of course, the serpentine deformable beam may deviate to the left and right multiple times along its length from the attachment point of the TAP to the MAP. The alternating deviations do not have to be continuous - that is, the serpentine deformable beam may make multiple turns in the same direction and then turn in the opposite direction. The deviation from a straight line may be in the form of an acute angle or a gradual curve. Note that a directional path that deviates from a straight line only undergoes one turn, i.e., to the left or to the right, but not to both (as in Figure 1 The deformable beam 16) is not included in the meaning of the term "serpentine" as used in this article.

[0044] In one embodiment, the serpentine deformable beam can include multiple straight beam segments connected at various angles, and some of these segments can extend parallel to each other to achieve a greater total deformable beam length while confining the serpentine deformable beam to a small space. In some embodiments, a segment or portion of the serpentine deformable beam can "fold back," or extend in the opposite direction from the previous segment or portion of the beam.

[0045] Figure 8 A force / torque sensor 110 is depicted, comprising a tool adapter plate (TAP) 112 that can be connected to a tool and a mounting adapter plate (MAP) 114 that can be connected to a robot arm (or vice versa). The MAP 114 is arranged generally annularly around the TAP 112. A plurality of serpentine deformable beams 116a, 116b, 116c (each comprising a plurality of deformable beam segments connected at an angle) connect the TAP 112 to the MAP 114. Figure 8 In the embodiment depicted in FIG, each serpentine deformable beam 116 includes a first portion connected to the TAP 112. The first portion is then connected to a second portion via a "T" connection. The serpentine deformable beam then "folds" at each end of the second portion in a serpentine fashion. Each serpentine portion is then connected to the MAP 114. Thus, in this embodiment, each serpentine deformable beam 116 includes two separate directed paths from the attachment point of the TAP 112 to two different attachment points of the MAP 114. Each directed path defines a serpentine shape, as that term is defined and used herein.

[0046] Due to their extended overall length, the serpentine deformable beams 116 allow slight relative motion between the TAP 112 and the MAP 114 in the xy plane and in the z direction (off the page) with relatively low stiffness. Figure 1-Figure 3 The sensor 110 has a greater degree of "looseness" or "play" within its operating range than comparable sized sensors of the prior art designs depicted in FIG. 1 having straight or T-shaped deformable beams. The serpentine deformable beam 116 is equipped with strain gauges (not shown) on one or more sides that convert compressive and tensile forces at the surface of the serpentine deformable beam 116 into electrical signals. The strain gauges may be wired in full, half, or quarter Wheatstone bridge configurations, as is known in the art. A data acquisition and processing system (not shown) processes the strain gauge outputs to resolve, for example, the six forces and torques (Fx, Fy, Fz, Tx, Ty, Tz) acting between the TAP 112 and the MAP 114, as is known in the art.

[0047] The force / torque sensor 110 also includes a plurality of overload beams 118a, 118b, 118c that extend from the TAP 112 (at a first end) to near, but not contacting, the MAP 114 (at a second end) (or vice versa). The overload beams 118 are radially interspersed between the serpentine deformable beams 116. Narrow overload gaps 120a, 120b, 120c, for example, ranging from tens of thousandths of an inch to a few thousandths of an inch, separate each respective overload beam 118a, 118b, 118c from the MAP 114. In effect, the overload gaps 120 define the second (non-connected) end of each overload beam 118. In some embodiments, the TAP 112, the serpentine deformable beams 118, the overload beams 118, and the MAP 114 are machined from a single piece of metal, thereby eliminating tolerance build-up in the manufacture of the overload features.

[0048] In one embodiment, each overload gap 120 is substantially circular. As shown, for three overload beams, the circular gap 120 must extend greater than 270 degrees of the circumference so that it will contact in sufficient directions to ensure that the TAP 112 does not travel with different gap distances in either direction. A uniform gap distance, or gap distances specifically offset in different directions to allow for different activation distances in Fxy / Tz, such as for four overload beams, is a driving factor in how the overload beam 118 contacts the MAP 114. The exact path followed by the gap 120—circular, elliptical, etc.—determines the local contact stress when the overload beam 118 contacts the MAP 114. In one embodiment, the overload gap 120 can be formed using wire electrical discharge machining (EDM), which allows for easy machining of the gap 120 to tight tolerances. In contrast to the serpentine deformable beam 116, the overload beam 118 is straight, without any bends or angles, and is shorter and thicker than the serpentine deformable beam 116. Consequently, they exhibit greater stiffness.

[0049] The overload gap 120 between the overload beam 118 and the MAP 114 provides overload activation or stops for the forces (Fxy) and torques (Tz) that move the TAP 112 relative to the MAP 114 in the xy plane. To provide overload stops in the z direction (off the page), flat plates are attached above and below the area where each overload beam 118 meets the MAP 114 - that is, above and below the overload gap 120 - with a small gap width defined by feeler gauges (shim stock). Alternatively, the plate covering this area may have a precise step machined into it. In this way, all overload stops are created with small gaps and tight tolerances using readily available technology that does not threaten to damage the sensor 110 and does not significantly increase the manufacturing process. An alternative embodiment of the overload stop feature in the z direction is to have both a flat surface and a taper machined into the plate above and below the sensing element. The plane can be placed close to the center of the transducer, and the taper extends from the plane so that both pure force overloads and torque overloads have a large contact area during the overload event, thereby reducing contact stresses and again improving fatigue life / strength.

[0050] Figure 9 A force / torque sensor 130 according to another embodiment is depicted. In this embodiment, serpentine deformable beams 132a, 132b, 132c are connected between the TAP 112 and the MAP 114. Each serpentine deformable beam 132 in this embodiment is connected to each of the TAP 112 and the MAP 114 at only one point. The serpentine deformable beams 132 are "folded," similar to Figure 8 embodiment to provide an extended overall length but in a small space. The MAP 114 body may occupy the space provided by Figure 8 The other "half" of the serpentine deformable beam 116 of the embodiment occupies a portion of the space, thereby further contributing to the stiffness of the entire sensor 130 after overload activation. The overload beam 118 and the overload gap 120 are constructed as described above.

[0051] Figure 10 Show Figure 9 Detail of one embodiment of a serpentine deformable beam 132 with a strain gauge 134 attached. The strain gauge attachment may be similar to that in Figure 8 116 of the embodiment. For clarity, the wiring of the strain gauges is not shown. Although shown on one surface, the strain gauges can be attached to the serpentine deformable beams 116, 132 on multiple surfaces (e.g., in pairs on opposing surfaces) and in any position or orientation.

[0052] although Figures 8-10 The serpentine deformable beams 116, 132 depicted in Figure 1 depict parallel runs of segment lengths of the beams 116, 132 connected to the next consecutive segment at opposite ends, thereby forming a "fan-folded" shape, but this shape is merely exemplary and not limiting.

[0053] Figure 11 Still another embodiment of a force / torque sensor 140 is depicted having serpentine deformable beams 142. In this case, the serpentine deformable beams 142a, 142b, 142c each bend only slightly to the left after their attachment point on the TAP 112, and then turn right several times before attaching to the MAP 114. However, because each beam 142 deviates from a straight line by alternately bending or flexing to one side (the left side) and then to the other side (repeatedly to the right), they meet the definition of a "serpentine" beam as used herein.

[0054] FIG. 12 depicts various deformable beam shapes, all of which are "serpentine" as the term is defined herein.

[0055] The deformable beam shape 12(a) is similar to Figure 9 and Figure 10 (or Figure 8 The deformable beam shape depicted in FIG. 1 is a diagram of a deformable beam having a serpentine shape (one side of an embodiment of the present invention). A directed line from the TAP attachment point to the MAP attachment point travels in a straight line and then deviates from the straight line by turning right 90 degrees. The beam then performs six 180-degree direction changes, each consisting of two consecutive 90-degree same-side turns. This is an example of a serpentine deformable beam that deviates from a straight line in both directions, but not continuously. As used herein, the deviation of a directed line from the centerline of a deformable beam is the deviation from the instantaneous centerline as the point under consideration moves along the beam—the deviation is not necessarily to either side of the initial centerline.

[0056] Shape 12(b) depicts another example of a serpentine path for a deformable beam. Starting from the TAP attachment point, the beam travels in a straight line, then turns left less than 90 degrees, turns right more than 90 degrees, turns left again more than 90 degrees, then turns right again less than 90 degrees but more than 90 degrees, and then continues in a straight line to the MAP attachment point. The bends in a serpentine deformable beam can be arcs, right angles, or obtuse angles with respect to the directed line of the beam's centerline.

[0057] Shape 12(c) depicts another example of a serpentine path for a deformable beam. This shape has no hard corners. However, the directional path along the centerline of the beam undergoes a 90-degree curve to the right, then a left curve, a right curve, and a left curve, each deflecting a total of 180 degrees, before bending 90 degrees to the right to reach the attachment point of the MAP. While this beam has no hard corners, its directional path deviates from a straight line in two directions, thus meeting the definition of a serpentine as used herein.

[0058] According to other embodiments of the present invention, the length of the deformable beam, rather than the overloaded beam, is increased by forming the deformable beam into a spiral shape. As used herein, the term "spiral" refers to a shape that deviates from a straight line by repeatedly or continuously bending or kinking to the same side, the cumulative sum being greater than 90 degrees. In other words, the directed path of the centerline of the spiral deformable beam along its length from the attachment point of the TAP to the attachment point of the MAP deviates from a straight line by repeatedly or continuously turning, bending, or skewing in one direction, i.e., to the left or to the right, the cumulative sum being greater than 90 degrees. Note that a directed path (e.g., a beam that undergoes a deviation from a straight line in one direction, the cumulative sum being equal to or less than 90 degrees) is not a straight line. Figure 1 The deformable beam 16) is not included within the meaning of the term "helix" as used herein.

[0059] Figure 14 An embodiment of a force / torque sensor 150 is depicted having a helically deformable beam 152. In this case, the helically deformable beams 152a, 152b, and 152c each turn only to one side—to the left—for a total of 180 degrees. This exceeds the cumulative minimum sum of deviations greater than 90 degrees to one side, according to the definition of a helically deformable beam used herein.

[0060] FIG. 13 depicts several examples of “helical” deformable beam shapes, as that term is used herein.

[0061] Deformable beam shape 13(a) includes a series of eight 90-degree turns, all to the right. The cumulative rightward deviation of the directed line from the TAP's attachment point to the MAP's attachment point is 720 degrees. In this embodiment, at least a portion of the MAP must lie out of the plane of the helical deformable beam in order to attach to it.

[0062] Shape 13(b) depicts another embodiment of a spiral path for a deformable beam. Starting from the TAP attachment point, the beam extends in a straight line and then continuously bends to the left. Before attaching to the MAP, the beam goes around 1 3 / 4 turn. The total directional deviation - all to the same side (left in this case) - is 630 degrees. This embodiment also requires that the MAP (or an arm or other member connected to it) be located out of the plane of the helical deformable beam.

[0063] Shape 13(c) depicts yet another embodiment of a spiral path for a deformable beam. In this embodiment, the beam extends from the TAP attachment point, through a series of left turns, to the MAP attachment point. Because the first and last segments of this composite beam are not parallel, the total angular deviation from the line leaving the TAP to the left is slightly greater than 180 degrees, and therefore clearly falls within this definition of a deviation greater than 90 degrees. In this embodiment, the TAP and MAP can be formed in the same plane as the spirally deformable beam.

[0064] In an embodiment of the present invention, the length of the serpentine deformable beam 116, 132, 142 or the helical deformable beam 152 is increased to reduce its stiffness; however, because the serpentine deformable beam 116, 132, 142 or the helical deformable beam 152 is "folded" within the space between the TAP 112 and the MAP 114, the overall size of the force / torque sensor 110, 130, 140, 150 is not increased. The lower stiffness of these serpentine deformable beams 116, 132, 142 or the helical deformable beam 152 increases the ratio of the sensor stiffness after overload activation to the stiffness of the applied load within the operating range, while producing proportional strain readings. This configuration allows for the use of an easily manufactured overload feature.

[0065] Throughout the design or rated operating range of the force / torque sensors 110, 130, 140, 150, the strain gauges on the serpentine deformable beams 116, 132, 142 or the helical deformable beam 152 detect compressive and tensile forces in these members caused by forces and / or torques applied between the TAP 112 and the MAP 114. Due to their large overall length, the serpentine deformable beams 116, 132, 142 or the helical deformable beam 152 can undergo bending or deformation, thereby allowing slight relative movement between the TAP 112 and the MAP 114.

[0066] When the maximum design force and / or torque is applied to the sensor 110, 130, 140, 150 and one or more of the serpentine deformable beams 116, 132, 142 or the helical deformable beam 152 has deformed or bent to allow the maximum design relative movement between the TAP 112 and the MAP 114, the one or more overload beams 118 will close the associated gap 120 and directly contact the MAP 114 or the plate above or below the sensor 110, 130, 140, 150. At this point, further relative movement nominally ceases. In practice, very slight movement will continue because the overload beam 118 and the MAP 114 are not infinitely stiff. However, any such movement will be detected and output as further strain on the serpentine deformable beam 116, 132, 142 or the helical deformable beam 152, which increases at a rate less than that experienced within the operating range of the applied force / torque. That is, the strain / force output curve will be similar to Figure 6 instead of Figure 5 curve.

[0067] Embodiments of the present invention present numerous advantages over prior art force / torque sensors. Due to the greater overall length of the serpentine deformable beams 116, 132, 142 or the helical deformable beams 152, the stiffness of these components is reduced without affecting their strain. This provides a significant difference in the stiffness of the overload feature 118, increasing the ratio of sensor stiffness after overload activation to that before overload activation (i.e., within the normal operating range). This high stiffness ratio creates a sharp inflection point in the strain / force curve at the overload activation point, with a low slope as the applied load increases. This increases the number of cycles that the sensor 110, 130, 140, 150 can undergo into an overload state before metal fatigue occurs, and increases the maximum force / torque that the sensor 110, 130, 140, 150 can withstand without yielding or fracturing. Increasing the overall length of the deformable beams 116, 132, 142, 152 by forming them in a serpentine or helical shape results in efficient space utilization without increasing the overall sensor size. Defining the non-connected end of the overload beam 118 by forming the overload gap 120 in the MAP 114 relaxes tolerances on the overload feature 118 compared to prior art methods and eliminates the need to machine the component after assembly.

[0068] Of course, the present invention may be implemented in other ways than those specifically set forth herein without departing from the essential characteristics of the present invention. These embodiments are to be considered in all respects as illustrative and non-restrictive, and all changes within the meaning and equivalent scope of the appended claims are intended to be included therein.

Claims

1. A force / torque sensor (110, 130, 140, 150) comprising a central tool adapter plate TAP (112) operable to connect to a first object and a mounting adapter plate MAP (114) operable to connect to a second object and arranged annularly around TAP (112), the force / torque sensor (110, 130, 140, 150) being characterized by: one or more serpentine deformable beams (116, 132, 142), each radially disposed about the TAP (112) and connecting the TAP (112) to the MAP (114), wherein at least one directed path along a centerline of the serpentine deformable beams (116, 132, 142) from an attachment point of the TAP (112) to an attachment point of the MAP (114) successively deviates from an instantaneous straight line at least once to the left and at least once to the right; a strain gauge (134) affixed to one or more surfaces of at least some of the serpentine deformable beams (116, 132, 142), the strain gauge (134) being operable to convert strain in the serpentine deformable beams (116, 132, 142) caused by deformation of the beams (116, 132, 142) into a signal; one or more overload beams (118), each radially arranged around the TAP (112) between the serpentine deformable beams (116, 132, 142) and connected to one of the TAP (112) and MAP (114) at a first end and spaced apart from the other of the TAP (112) and MAP (114) at a second end by an overload gap (120) of predetermined width, each overload beam (118) exhibiting greater stiffness than the serpentine deformable beams (116, 132, 142); and Measurement circuitry is operable to measure the direction and magnitude of forces and torques between the first object and the second object in response to signals from all of the strain gauges (134).

2. The force / torque sensor (110, 130, 140, 150) of claim 1, wherein the overload gap (120) is formed by a component that is different from the TAP (112) or MAP (114), the component being rigidly attached to one of the TAP (112) or MAP (114) and forming the overload gap (120) with the other of the TAP (112) or MAP (114).

3. The force / torque sensor (110, 130, 140, 150) according to claim 1, wherein each serpentine deformable beam (116, 132, 142) includes a plurality of parallel deformable beam segments, and the deformable beam segments are connected between the TAP (112) and the MAP (114) in a serpentine configuration.

4. The force / torque sensor (110, 130, 140, 150) of claim 1, wherein each serpentine deformable beam (116, 132, 142) comprises: Paragraph 1; first and second serpentine regions, respectively connected between the first segment and a separation point of one of the MAP (114) and the TAP (112); and A second segment is connected between the first segment and the other of the MAP (114) and the TAP (112).

5. The force / torque sensor (110, 130, 140, 150) of claim 1, wherein each of the plurality of overload beams (118) is radially interspersed between each of the plurality of serpentine deformable beams (116, 132, 142).

6. The force / torque sensor (110, 130, 140, 150) of claim 1, wherein each overload gap (120) is substantially circular.

7. The force / torque sensor (110, 130, 140, 150) of claim 1, further comprising a plate fixed above and below the MAP (114) or TAP (112) for each overload beam (118), the plate being spaced apart from the overload beam (118) by an overload gap, the plate defining a z-direction gap, the z-direction gap limiting movement of the overload beam (118) perpendicular to a common plane of the TAP (112), MAP (114) and the overload beam (118).

8. The force / torque sensor (110, 130, 140, 150) according to claim 7, further comprising a feeler gauge defining the z-direction gap.

9. The force / torque sensor (110, 130, 140, 150) of claim 7, wherein at least one of the MAP (114), the serpentine deformable beam (116, 132, 142), the overload beam (118) and the TAP (112) and the plate are machined from the same metal piece.

10. The force / torque sensor (110, 130, 140, 150) according to claim 1, wherein: Within a first range of forces or torques applied between the TAP (112) and MAP (114), the serpentine deformable beam (116, 132, 142) is operable to deform and the second end of the overload beam (118) does not contact the TAP (112) or MAP (114) due to the overload gap (120); and Within a second range of force or torque applied between the TAP (112) and MAP (114) that is greater than the first range, a second end of at least one overload beam (118) closes the overload gap (120) and contacts the TAP (112) or MAP (114), preventing further movement between the TAP (112) and MAP (114) and thereby nominally preventing further deformation of the serpentine deformable beam (116, 132, 142).

11. A force / torque sensor (110, 130, 140, 150) according to claim 10, wherein the sensor (110, 130, 140, 150) exhibits a first stiffness or resistance to motion under an applied force or torque within the first range of force or torque, and exhibits a second stiffness within the second range of force or torque, wherein the second stiffness is greater than the first stiffness.

12. The force / torque sensor (110, 130, 140, 150) of claim 11, wherein a ratio of the second stiffness to the first stiffness is 5 or greater.

13. The force / torque sensor (110, 130, 140, 150) of claim 12, wherein the ratio of the second stiffness to the first stiffness is 10 or greater.

14. A force / torque sensor (110, 130, 140, 150) comprising a central tool adapter plate TAP (112) operable to connect to a first object and a mounting adapter plate MAP (114) operable to connect to a second object and arranged annularly around the TAP (112), the force / torque sensor (110, 130, 140, 150) being characterized by: one or more helically deformable beams (152), each radially disposed about the TAP (112) and connecting the TAP (112) to the MAP (114), wherein at least one directed path along a centerline of the helically deformable beam (152) from an attachment point of the TAP (112) to an attachment point of the MAP (114) deviates from an instantaneous straight line one of leftward or rightward, and wherein a cumulative sum of the deviations is greater than 90 degrees; a strain gauge (134) affixed to one or more surfaces of at least some of the helically deformable beams (152), the strain gauge (134) being operable to convert strain in the helically deformable beams (152) caused by deformation of the beams (152) into a signal; one or more overload beams (118), each radially arranged about the TAP (112) between the helical deformable beams (152) and connected at a first end to one of the TAP (112) and the MAP (114) and spaced apart at a second end from the other of the TAP (112) and the MAP (114) by an overload gap (120) of predetermined width, each overload beam (118) exhibiting greater stiffness than the helical deformable beam (152); and Measurement circuitry is operable to measure the direction and magnitude of forces and torques between the first object and the second object in response to signals from all of the strain gauges (134).

15. The force / torque sensor (110, 130, 140, 150) of claim 14, wherein the overload gap (120) is formed by a component that is different from the TAP (112) or MAP (114), the component being rigidly attached to one of the TAP (112) or MAP (114) and forming the overload gap (120) with the other of the TAP (112) or MAP (114).

16. The force / torque sensor (110, 130, 140, 150) of claim 14, wherein each of the plurality of overload beams (118) is radially interspersed between each of the plurality of helically deformable beams (152).

17. The force / torque sensor (110, 130, 140, 150) of claim 14, wherein each overload gap (120) is substantially circular.

18. The force / torque sensor (110, 130, 140, 150) of claim 14, further comprising a plate fixed above and below the MAP (114) or TAP (112) for each overload beam (118), the plate being spaced apart from the overload beam (118) by an overload gap, the plate defining a z-direction gap, the z-direction gap limiting movement of the overload beam (118) perpendicular to a common plane of the TAP (112), the MAP (114), and the overload beam (118).

19. The force / torque sensor (110, 130, 140, 150) of claim 18, further comprising a feeler gauge defining the z-direction gap.

20. The force / torque sensor (110, 130, 140, 150) of claim 18, wherein at least one of the MAP (114), the helically deformable beam (152), the overload beam (118), and the TAP (112) and the plate are machined from the same metal piece.

21. The force / torque sensor (110, 130, 140, 150) of claim 14, wherein: The helically deformable beam (152) is operable to deform within a first range of forces or torques applied between the TAP (112) and MAP (114), and wherein the second end of the overload beam (118) does not contact the TAP (112) or MAP (114) due to the overload gap (120); and Within a second range of force or torque applied between the TAP (112) and MAP (114) that is greater than the first range, a second end of at least one overload beam (118) closes the overload gap (120) and contacts the TAP (112) or MAP (114), preventing further movement between the TAP (112) and MAP (114) and thereby nominally preventing further deformation of the helically deformable beam (152).

22. A force / torque sensor (110, 130, 140, 150) according to claim 21, wherein the sensor (110, 130, 140, 150) exhibits a first stiffness or resistance to motion under an applied force or torque within the first range of force or torque, and exhibits a second stiffness within the second range of force or torque, wherein the second stiffness is greater than the first stiffness.

23. The force / torque sensor (110, 130, 140, 150) of claim 22, wherein a ratio of the second stiffness to the first stiffness is 5 or greater.

24. The force / torque sensor (110, 130, 140, 150) of claim 23, wherein a ratio of the second stiffness to the first stiffness is 10 or greater.

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