Torque sensor using coupled loads and fewer strain gauges

Abstract

To provide a sensor which is optimized for sensing the axial torque and utilizes a coupled off-axis load to reduce the minimum number of strain gauges required to resolve the axial torque.SOLUTION: A torque sensor includes a transducer plate having a central region and a peripheral part connected by a plurality of spokes and instrumentation beams. The transducer plate exhibits mechanical compliance under the axial torque, but stiffness under off-axis loads. Strain gauges mounted on the instrumentation beam detect the deformation caused by the axial torque.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates generally to force and torque sensors, and more particularly to sensors optimized for sensing axial torque that utilize coupled off-axis loads to reduce the minimum number of strain gauges required to resolve the axial torque. [Background technology]

[0002] Robots are essential for manufacturing, testing, assembling, and packaging products, assisted and remote surgery, space exploration, operation in hazardous environments, and many other applications. Many robots and robotic applications require quantification of applied or experienced forces, such as material removal (grinding, polishing, etc.), part assembly, remote excavation, or other manipulation of the environment.

[0003] Industrial robots typically comprise a versatile actuator, or "arm," with multiple segments connected by electromechanical joints that move and rotate in various axes and planes, providing multiple degrees of freedom. Six-degree-of-freedom (6-DOF) robotic arms are commonly used in industrial manufacturing, including tasks such as welding, material handling, material removal, and painting. The 6-DOF design provides movement in the x, y, and z planes, as well as flexibility, strength, and reach for many tasks. It can perform roll, pitch, and yaw movements of the robot tool, or "end effector," that interacts with the workpiece.

[0004] In many applications, it is necessary or desirable to monitor the forces between the end effector and the workpiece. For example, a "force control" task requires that the end effector be controlled to apply a predetermined force (or within a predetermined range of forces), and that the contact force and / or torque be measured and fed back to the robot control system. To measure the forces and torques at the end of a 6-DOF robot, two conventional approaches exist: placing a 6-axis force / torque sensor between the robot and the end effector, or measuring the torque at each of the robot's multiple joints and calculating the resulting forces and torques at the end.

[0005] U.S. Pat. No. 10,422,707, assigned to the assignee of the present application and incorporated herein by reference in its entirety, describes a compact six-axis force / torque sensor. The sensor is based on a conventional design with a hub (connected to the tool) connected to an annular ring (connected to the robot) by multiple deformable beams, which include flexures to increase the beam's deformation under load. Strain gauges attached to the deformable beams measure tension and compression at the surface of the beams as they deform under an applied load, and the output of the strain gauge circuitry is decoded and mapped to six forces (Fx, Fy, Fz) and six torques (Tx, Ty, Tz) via a decoupling matrix developed during a calibration procedure. A minimum of six strain gauges is generally required, and many designs utilize more (e.g., mounted on opposing sides of each deformable beam or on all four sides). As described in the '707 patent, one or more non-stress strain gauges can also be used to provide a baseline for temperature compensation and reduce errors introduced by thermal drift. Due to the large number of strain gauges and other instrumentation electronics, conventional 6-axis force / torque sensors are expensive.

[0006] Robot joint torques can be estimated from motor currents. However, these results tend to be noisy. Alternatively, joint torque sensors can be installed at each of several joints. Joint torque sensors tend to have higher accuracy and a higher signal-to-noise ratio. Based on the requirement that only one torque (Tz) must be measured, conventional joint torque sensors can follow the six-axis force / torque sensor design described above by adjusting the sensor geometry and / or strain gauge placement.

[0007] Known joint torque sensors have a number of deficiencies. They are typically sensitive to off-axis loads. That is, forces and torques other than the desired Tz cause beam deformation and generate strain gauge outputs, which can appear as errors in the axial torque Tz measurement. Known joint torque sensor designs are also sensitive to torque ripple, which is often seen when measuring torque close to strain-wave gearboxes, also known as harmonic drives. Torque ripple is a periodic variation in torque measurement that is difficult to compensate for. Even after modifying a six-axis force / torque sensor design to measure only the axial torque Tz, sensors with precisely machined thin features (e.g., flexures) and numerous strain gauges are still expensive to manufacture. Because several of them are required to instrument a six-degree-of-freedom robotic arm, this approach remains costly for many applications.

[0008] The Background of this specification is provided to place embodiments of the present invention in a technical and operational context and to assist those skilled in the art in understanding their scope and usefulness. The approaches described in the Background are approaches that could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless expressly identified as such, no statement in this specification is admitted to be prior art merely by virtue of its inclusion in the Background. Summary of the Invention

[0009] The following presents a simplified summary of the present disclosure in order to provide a basic understanding to those skilled in the art. This summary is not an extensive overview of the disclosure and is not intended to identify key / critical elements of embodiments of the present invention or to delineate the scope of the present invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0010] According to one or more embodiments described and claimed herein, a torque sensor includes a transducer plate having a central region and a periphery connected by a plurality of spokes and instrumentation beams. The spokes and instrumentation beams of the transducer plate are designed to allow significant mechanical compliance only when an axial torque is applied. One embodiment employs a single-surface strain gage arrangement on the instrumentation beam to detect deformation caused by axial torque, while other embodiments employ a side-beam strain gage arrangement. The spokes and instrumentation beams directly connect the central region and the periphery without an intervening flexure. The instrumentation beams may be asymmetric, allowing for gage placement in regions of high sensitivity to axial torque and low sensitivity to off-axis loads without placing the strain gages on the neutral axis. The strain gage responses from some off-axis loads are designed to be coupled or linearly dependent on the strain gage responses of other off-axis loads. This reduces the number of strain gages required to at least partially resolve all loads. The spokes are cost-effectively formed by removing adjacent transducer plate material with simple shapes such as through-holes and / or arc slots. The instrumentation beam is similarly formed by removing transducer plate material within the radial slots. The straight edges of the radial slots adjacent to the convex arc edges of the holes or arc slots result in an asymmetric beam shape. The strain gages can be connected in various configurations, such as Wheatstone quarter, half, or full bridge topologies.

[0011] One embodiment relates to a torque sensor configured to measure axial torque between a first object and a second object. The sensor includes a generally circular, generally planar transducer plate having a central region having a vertical central axis z and a periphery. One or more first mounting holes are proximate the central region and configured to be attached to the first object. One or more second mounting holes are proximate the periphery and configured to be attached to the second object. A plurality of voids are formed through the transducer plate between the central region and the periphery, with adjacent voids defining spokes connecting the central region and the periphery. One or more radial slots are formed through the transducer plate between the central region and the periphery, each adjacent to at least one void. The transducer plate between the radial slots and voids defines an instrumentation beam connecting the central region and the periphery. Strain gauges are attached to the instrumentation beam. A strain gauge circuit is connected to the strain gauge and configured to output a voltage that depends on the strain-dependent resistance of the strain gauge.

[0012] Another embodiment relates to a torque sensor comprising: a transducer plate having an axis and configured to be mounted between a first object and a second object; and four strain gauges mounted to the transducer plate, each strain gauge connected to a strain gauge circuit; the transducer plate configuration, strain gauge arrangement, and strain gauge circuit are configured such that strain gauge responses to two different pairs of forces and torques are linearly dependent, respectively, such that the strain gauge response under each pair of forces differs by a factor from the strain gauge response under that pair of torques; and the four strain gauge circuit outputs comprise a 4x4 linear system from which at least a torque about the axis can be determined directly from the strain gauge responses and a decoupling matrix.

[0013] Yet another embodiment relates to a method of manufacturing a torque sensor. A generally circular, generally planar transducer plate is provided having a central region having a vertical central axis z and a peripheral region. One or more first mounting holes are formed proximate the central region, the first mounting holes configured to be attached to a first object. One or more second mounting holes are formed proximate the peripheral region, the second mounting holes configured to be attached to a second object. A first plurality of voids are formed through the transducer plate between the central region and the peripheral region, adjacent voids defining spokes connecting the central region and the peripheral region. One or more radial slots are formed through the transducer plate between the central region and the peripheral region, each radial slot adjacent to at least one void, and the transducer plate between the radial slots and voids defines an instrumentation beam connecting the central region and the peripheral region. Strain gauges are attached to the instrumentation beams. The strain gauge is connected to a strain gauge circuit that outputs a voltage that depends on the strain dependent resistance of the strain gauge. [Brief explanation of the drawings]

[0014] The present invention will now be described in more detail with reference to the accompanying drawings, which show embodiments of the invention. However, the invention should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference characters refer to like elements throughout.

[0015] [Figure 1] FIG. 1 is a plan view of a torque sensor according to one embodiment.

[0016] [Figure 2A] , [Figure 2B] , [Figure 2C] , [Figure 2D] 2A-2D are simplified plan views of torque sensors according to various embodiments.

[0017] [Figure 3] Figure 3 is a close-up view of the instrumented beam, showing the tensile strain under axial torque Tz.

[0018] [Figure 4A] FIG. 4A is a diagram and close-up of the axial torque Tz of two instrumented beams, showing tensile and compressive strains.

[0019] [Figure 4B] FIG. 4B is a diagram and enlarged view of the bending torque Ty of two instrumented beams, showing tensile and compressive strains.

[0020] [Figure 5] FIG. 5 is a plan view of the torque sensor showing the arrangement of the single-sided strain gauges.

[0021] [Figure 6] FIG. 6 is a partial perspective view of the torque sensor showing a neutral axis strain gauge disposed on the inner surface of the radial slot.

[0022] [Figure 7] FIG. 7 is a partial perspective view of a torque sensor showing non-neutral axis strain gauges positioned on the inner surface of the radial slot.

[0023] [Figure 8A] FIG. 8A is an electrical schematic diagram of a four Wheatstone quarter-bridge strain gage circuit.

[0024] [Figure 8B] FIG. 8B is an electrical schematic diagram of two Wheatstone half-bridge strain gauge circuits.

[0025] [Figure 8C] FIG. 8C is an electrical schematic diagram of a Wheatstone full bridge strain gauge circuit.

[0026] [Figure 9] FIG. 9 is a flow diagram of steps in a method of manufacturing a torque sensor. DETAILED DESCRIPTION OF THE INVENTION

[0027] For simplicity and exemplary purposes, the present invention will be described primarily by reference to exemplary embodiments thereof. In the following description, numerous specific details are set forth in order 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 being limited to these specific details. Well-known methods and structures are not described in detail herein in order to avoid unnecessarily obscuring the present invention.

[0028] While embodiments of the present invention are particularly suited for deployment as robotic joint torque sensors, those skilled in the art will readily recognize that their applications are largely independent of the design and instrumentation of the joint torque sensor. Accordingly, the embodiments of the present invention described herein refer to a "torque sensor" configured to measure an axial torque Tz between two objects. In a robotic joint torque sensor application, the objects connected to the torque sensor may be a first segment of a 6DOF robotic arm (or any object rigidly attached thereto) and an actuator, such as a motor, that rotationally moves a second segment of the robotic arm relative to the first segment.

[0029] The use of strain gauges in force and torque sensors is well known. Strain gauges, particularly silicon strain gauges, exhibit strain-dependent resistance due to their change in length when attached to a surface undergoing expansion or compression. Some configurations of strain gauge circuits, such as Wheatstone bridge circuits, output a voltage that can be used to quantify the resistance of the strain gauge. As used herein, "strain gauge response" refers to the difference in the output voltage of a strain gauge circuit between an unloaded state and a state in which a mechanical load is applied. As known in the art, comparing the responses of multiple strain gauges placed at various positions and orientations on the sensor body can provide information about the load applied to the sensor. The mathematical construct used to remove such ambiguity is referred to herein as a decoupling matrix. The decoupling matrix is ​​typically determined during a calibration procedure.

[0030] A known problem in the prior art of torque sensor design is decoupling the strain gauge response due to axial torque from the strain gauge response due to other forces and torques, collectively referred to herein as “off-axis loads.” Such decoupling has been attempted in the prior art primarily through gauge placement, the use of flexures, circuit design, and software.

[0031] One known approach is to position strain gauges that are insensitive, or at least less sensitive, to off-axis loads. This is typically achieved by positioning the gauges on the neutral axis of the deformable beam. In most prior art torque sensors, the strain gauges are positioned on the lateral (side) surfaces of the deformable beam, maximizing the surface tension and compression of the beam under axial torque loads. In sensors using single-surface gauges (where all strain gauges are on the same surface of their respective deformable beams (such as the above-incorporated '707 patent)), the strain gauges are generally sensitive to strain from all forces and torques applied to the sensor. However, single-surface gauges are attractive because they can reduce the number of strain gauges required, are much easier to position and mount, and dramatically lower manufacturing costs.

[0032] Another approach is to use a series flexure. A flexure is a structural member that is much less stiff than the rest of the sensor, which allows compliance in a specific direction and therefore reduces unwanted strain on the gauge. In a torque sensor, the series flexure is typically placed between the deformable beam (itself a relatively stiff flexure) that carries (mounts) the strain gauge and the outer hub of the sensor. Sensors with series flexures tend to be expensive to manufacture due to the thin width of the series flexure and the need for precise machining.

[0033] In some torque sensors, strain gauges that independently sense off-axis loads are canceled or at least mitigated by pairing them with other gauges in a Wheatstone half-bridge or full-bridge topology. Finally, for multiple force- and torque-sensing strain gauges, the effects of off-axis loads are mitigated or eliminated by mapping the strain gauge signals to force and torque measurements using a decoupling matrix.

[0034] FIG. 1 illustrates a torque sensor 10 according to one embodiment of the present invention. Torque sensor 10 utilizes a unique mechanical design with no series bends, a single surface gauge, strategic gauge placement, a quarter-bridge circuit topology, and a decoupling matrix to measure axial torque Tz while mitigating the effects of off-axis loads. Torque sensor 10 includes a generally circular and generally planar transducer plate 12. For compactness and cost-effectiveness, in one embodiment, it is milled or otherwise formed as a single plate of metal.

[0035] The transducer plate 12 includes a central region 14, which in the illustrated embodiment includes a central hole that allows for the passage of mechanical, electrical, pneumatic, and other utilities between the robot arm segments. In other embodiments, the central region 14 may be solid or include only a single mounting hole. A central axis z, perpendicular to the plane of the transducer plate 12, passes through the center. The transducer plate 12 also includes a peripheral portion 16. A first mounting hole 18 is formed in or near the central region 14 and configured to mount the torque sensor 10 to a first object. A second mounting hole 20 is formed in or adjacent to the peripheral portion 16 and configured to mount the torque sensor 10 to a second object. The torque sensor 10 is optimized to measure axial torque Tz between a first object and a second object while minimizing the adverse effects of off-axis loads.

[0036] A plurality of spokes 22 connect the central region 14 to the periphery 16. In the embodiment shown in FIG. 1, the spokes 22 are formed by forming a plurality of voids 24 such that the material of the transducer plate 12 between adjacent voids 24 forms the spokes 22. In the embodiment shown in FIG. 1, the voids 24 are through-holes drilled through the transducer plate 12, which can be made very easily and inexpensively during manufacturing, although in general the voids 24 are not limited to through-holes. In one embodiment, each spoke 22 comprises two concave circular sides.

[0037] Two radial slots 28 are also formed through the transducer plate 12 between the central region 14 and the periphery 16. Each radial slot 28 is adjacent to one or more voids 24, such that the material of the transducer plate 12 between the radial slot 28 and the adjacent void 24 forms an instrumentation beam 26. In the embodiment of FIG. 1 in which the voids 24 are through holes, each instrumentation beam 26 has a straight edge on one side and a concave arc edge on the other side, although this configuration is not a limitation of this embodiment. The instrumentation beams 26 do not include thin features, simplifying and reducing the manufacturing cost of the torque sensor 10.

[0038] As shown in Figure 5, a strain gage 30 is attached to each instrumentation beam 26. In the embodiment shown in Figure 1, the radial slots 28 have generally straight sides (radially) and the adjacent voids 24 have curved sides, so that the instrumentation beam 26 is asymmetric and strain from the applied load is concentrated or non-uniform radially along the instrumentation beam 26. As explained more fully herein, the strain gages 30 are attached to the instrumentation beam 26 in areas that experience significant stress from the axial torque Tz but little stress from the off-axis torques Tx, Ty, mitigating the adverse effects of off-axis loads on the off-axis torque Tz measurement.

[0039] The transducer plate 12, having spokes 22 and instrumentation beams 26 connecting the central region 14 and the periphery 16, exhibits little mechanical compliance or deformation under an axial torque Tz. However, the transducer plate 12 exhibits a much higher relative stiffness under off-axis loads. As such, the transducer plate 12 resists deformation out of its plane (e.g., under off-axis torques Tx, Ty) or attempts to move the central region 14 toward or away from the periphery 16 in any radial direction (e.g., off-axis forces Fx, Fy) or to move them axially (Fz).

[0040] As one skilled in the art will readily appreciate, torque sensor 10 can include additional features. For example, Figure 1 shows electronics mounting holes 32 that provide protected space for electronic components such as fixed resistors, analog-to-digital converters, microprocessors, memory, etc. Additionally, Figure 1 shows alignment dowel holes 34 that receive corresponding dowels to ensure proper positioning and alignment of torque sensor 10 when assembled to, for example, a robotic arm.

[0041] The formation of spokes 22 by drilling adjacent voids 24 and the formation of instrumentation beams 26 by forming radial slots 28 adjacent to the voids 24 results in a compact transducer plate 12 that is simple and inexpensive to manufacture, yet has the desired target compliance and stiffness described above. However, those skilled in the art will readily recognize that embodiments of the torque sensor 10 are not limited to the particular transducer plate 12 shown in FIG. 1 .

[0042] 2A-2D illustrate alternative designs for the transducer plate 12 for various embodiments of the torque sensor 10. For simplicity, FIGS. 2A-2D show only the voids 24 forming the spokes 22 and the radial slots 28, which, together with adjacent voids 24, define the instrumentation beam 26. Mounting holes 18, 20, and other features are omitted. The voids 24 may be simple through-holes, as shown in FIGS. 1, 2A, 2C, and 2D, arc slots, as shown in FIG. 2B, or any other shape. Similarly, the radial slots 28 may be rectangular with parallel radial sides, as shown in FIGS. 1 and 2B. Alternatively, the radial sides may be non-parallel (e.g., aligned with the actual radius), as shown in FIG. 2A. Note that FIG. 2C illustrates both types of radial slots 28. Other shapes are possible within the broad scope of the present invention. For example, in FIG. 2D, radial slot 28 is a through hole and instrumentation beam 26 includes concave arcuate edges on both sides.

[0043] The spokes 22 formed between the air gaps 24 are most flexible under axial torque Tz. The spokes 22 are most flexible under off-axis loads (i.e., forces Fx, Fy, Fz and torque Tx , Ty ) which reduces the effect of off-axis loads on the measurement of the axial torque Tz. The spokes 22 are shown in FIGS. 1, 2A, 2C, and 2D as being formed with voids 24 in the form of through-holes. This contributes to cost reduction and ease of manufacturing. However, embodiments of the present invention are not limited to this shape. As shown in FIG. 2B, arc slots provide additional design flexibility without significantly increasing manufacturing costs. Furthermore, the absence of thin features contributes to cost reduction and ease of manufacturing. While the torque sensor 10 functions with zero spokes 22, i.e., no voids 24 formed in the transducer plate 12, approximately 4 to 10 spokes provide desirable sensitivity under the axial torque Tz and the off-axis loads Fx, Fy, and Fy. Fz , Tx, and Ty.

[0044] FIG. 3 is an enlarged view of the instrumentation beam 26. While the instrumentation beam 26 shown herein generally has one straight side (the side of the radial slot 28) and one arcuate side (the side of the through-hole void 24), this is not a limitation of this embodiment. The void 24 and the radial slot 28 preferably form an asymmetric instrumentation beam 26. As FIG. 3 shows, the asymmetry of the instrumentation beam 26 in the radial direction results in a strain concentration (shown by the diagonal lines) in response to an applied axial torque Tz. A strain gauge 30 is attached to the instrumentation beam 26 at the location of maximum strain resulting from the axial torque Tz.

[0045] 4A shows the placement of strain gauges 30 and the distribution of strain within instrumentation beam 26 when torque sensor 10 is subjected to axial torque Tz. Areas shaded with small dots are in tension and areas shaded with large dots are in compression, with the strains reversing under an axial torque in the opposite direction. Strain gauges 30 are positioned along instrumentation beam 26 in areas that experience maximum strain under axial torque Tz.

[0046] As can be seen in Figure 4B, the strain distribution is significantly different under the plane bending torque Ty. Again, the areas shaded with small dots are in tension, while the areas shaded with large dots are in compression relative to the particular direction Ty depicted. The placement of the strain gauges 30 ensures that they experience little to no strain and therefore produce minimal output for this off-axis load.

[0047] Thus, in some embodiments, the strain gauges 30 are mounted on the instrumentation beam 26 at locations that are the sites of maximum strain under Tz and simultaneously minimum strain under Tx and Ty. This is important because in a single-surface gage arrangement, all strain gauges tend to generate a response to all applied loads. In these embodiments, the design of the transducer plate 12 and the placement of the strain gauges 30 contribute significantly to mitigating the adverse effects of off-axis loads when measuring the axial torque Tz. In the illustrated embodiment, the mounting locations of the strain gauges 30 are along the straight edges of the radial slots 28, which also facilitates manufacturing by providing a reference for the orientation of the strain gauges 30. The absence of thin series bends further aids manufacturability and cost reduction.

[0048] FIG. 5 illustrates the placement of four strain gauges 30 on a torque sensor 10 according to one embodiment. As in FIGS. 4A and 4B, in this embodiment, strain gauges 30-0, 30-1, 30-2, and 30-3 are each mounted on the same side (top surface) of a different instrumentation beam 26 at a location that experiences maximum strain under axial torque Tz but minimum strain under bending torques Tx and Ty. Mounting all four strain gauges 30 on the top surface greatly facilitates manufacturing. As explained more fully herein, the design of the transducer plate 12 and the placement of the strain gauges 30 as shown in FIG. 5 results in a linear dependence between off-axis loads Fx and Ty, as well as between Fy and Tx. This is because Tz The number of strain gauges 30 required to measure σ is reduced from six required for a single surface mount in the prior art to only four, which further reduces cost and manufacturing complexity.

[0049] However, the present invention is not limited to mounting strain gauges 30 on a single surface. FIG. 6 shows strain gauges 30-0 and 30-3 mounted on the side of the instrumentation beam 26 that forms the inside (inner side) of the radial slot 28 (corresponding strain gauges 30-1 and 30-2 are similarly mounted on the inside (inner surface) of the radial slot 28, not visible in FIG. 6). In this embodiment, each strain gauge 30-0, 30-1, 30-2, and 30-3 is mounted on a different instrumentation beam 26 and is mounted along the neutral axis of that surface of the instrumentation beam 26. The neutral axis of the instrumentation beam 26 is the line that experiences no strain when one side of the surface is in tension and the other side is in compression. In this position, the strain gauges 30 respond primarily to strains aligned longitudinally along the surface of the instrumentation beam 26. Thus, in this embodiment, the torque sensor 10 is less sensitive to off-axis loads (i.e., Fz, Tx, Ty) than the embodiments shown in Figures 4A, 4B, and 5. However, mounting strain gauges 30 at these locations is more difficult and therefore increases manufacturing costs.

[0050] In yet another embodiment shown in Figure 7, two strain gauges 30 are mounted on the inner surface of each of two instrumentation beams 26 spaced apart from the neutral axis. In this arrangement, the strain gauges 30 generate outputs for applied loads that impose different strains on the inner surface of the radial slot 28. Like the embodiment of Figure 6, in this embodiment, the torque sensor 10 is less sensitive to off-axis loads (i.e., Fz, Tx, Ty) than the embodiments shown in Figures 4A, 4B, and 5. However, mounting strain gauges 30 at these locations is also more difficult, thus increasing manufacturing costs.

[0051] Generally, a functional torque sensor 10 can be obtained with as few as one strain gage 30, but there is no way to compensate for strain gage 30 output due to off-axis loads. Adding more strain gages 30 generally increases the accuracy and resolution of the torque sensor 10 and, in conjunction with consideration of the transducer plate 12 design and strain gage placement, can eliminate or mitigate off-axis loads from measuring the axial torque Tz. For example, more than four strain gages 30 can provide improved sensitivity, but additional strain gages 30 increase costs. As described in the above-incorporated '707 patent, strain gages 30 can be attached to unstressed members of the torque sensor 10, and their outputs can be used to compensate for variations in the strain gage 30 output with temperature changes.

[0052] One novel concept of the present invention, which is not limited to torque sensors or any particular embodiment described herein, is strain gauge 30 response coupling. As used herein, response "coupling" refers to the design of a force / torque sensor, including the mechanical features of the sensor body, the placement of strain gauges on the sensor body, and the strain gauge circuitry from which the gauge response is derived, such that the strain gauge response under one force or torque is linearly dependent on the strain gauge response from another torque or force. Two strain gauge responses are coupled when one strain gauge response vector is a multiple of the other strain gauge response vector. Strain gauge responses are linearly dependent when one strain gauge response vector is a multiple of one or more different strain gauge response vectors.

[0053] In embodiments of the present invention, the response coupling of the strain gauges 30 reduces the number of strain gauges 30 required to independently measure the axial torque Tz from the conventional requirement of six strain gauges 30 to only four, yet the four strain gauges 30 are sensitive to all six forces and torques (e.g., as in a single surface arrangement).

[0054] Table 1 shows representative examples of the outputs of strain gages 30-0 through 30-3 under all six applied forces and torques. The gage signal is the output voltage of the strain gage 30 circuit. The gage response is the change in the gage signal between a no-load condition and a load of a specified magnitude applied to a specified axis. [Table 1] Table 1: Strain gauge response of typical loads

[0055] For example, the Fx gauge response vector <1,1,-1,-1> means that the output voltage of the gauge circuit of strain gauges 30-0 and 30-1 increases by 1 volt when a force of 1 N is applied along the x-axis from an unloaded state, and the output voltage of the gauge circuit of strain gauges 30-2 and 30-3 decreases by 1 volt for the same change in load on sensor 10 (with no apparent change in temperature).

[0056] In this example, the gauge responses of Fy and Tx are coupled, or linearly dependent. The gauge response for Tx is five times the gauge response for Fy for each strain gage 30. For example, for a force vector <5,-5,-5,5>, without information about the applied load, it is not possible to distinguish whether a 5 N force was applied at Fy or a 1 Nm torque was applied at Tx. The force Fy and torque Tx are linearly dependent, or coupled, in that one gauge response vector is a multiple of the other gauge response vector. In this representative example, the gauge responses of Fx and Ty are also coupled; they are -3 Note that the values ​​differ by a multiple of .

[0057] Because the two force-torque pairs are coupled, they can each be combined, and the gage responses from all applied loads can be represented as a 4x4 linear system: four force / torque loads (individual or combined) result in four unique strain gage response vectors. This system can be inverted to map the strain gage response vectors to forces and torques. Table 2 shows the four unique gage responses and the direct or combined forces and torques that give rise to them. [Table 2] Table 2: Strain gauge response using combined force / torque pairs

[0058] For two pairs of coupled forces / torques, the strain gauge 30 responses provide a full-rank linear system, and the response vectors of the four strain gauges 30 uniquely identify and at least partially quantify the four mechanical loads: Tz, Fz, FyTx coupled, and FxTy coupled. In torque sensor 10, the axial force and coupled force / torque decompositions are ignored, and the desired axial torque Tz is directly decomposed by the decoupling matrix. In other applications where information about the Fz, FyTx coupled, and / or FxTy coupled loads may be useful, these can also be directly decomposed by the decoupling matrix. In the case of coupled loads, the potential force or torque is identified, but the magnitude cannot be quantified. In some cases, positional information about the robot arm segments can be used to decouple the coupled loads, allowing for more complete resolution. Even without such positional information, the axial force Fz is directly and completely resolved, so torque sensor 10 is essentially a force / torque sensor for the axial (z) axis.

[0059] FIG. 8A shows four strain gages 30-0, 30-1, 30-2, and 30-3 in a Wheatstone quarter-bridge circuit. Strain gage 30 is connected in series with a fixed resistor between a source voltage and ground, and the gage circuit output voltage is taken at the center node. As described above, the gage response measured in this manner can be independently resolved into Tz, Fz, FyTx coupling, and FxTy coupling. As described in the above-incorporated '707 patent, a stress-free strain gage can be included as a reference for gauge output variation with temperature. A commonly assigned U.S. patent application filed concurrently with this application, entitled "Quarter-Bridge Hardware Temperature Compensation for Force / Torque Sensor," which is incorporated herein by reference in its entirety, describes a system and method for linearizing the operation of strain gages in a quarter-bridge circuit by adding and adjusting the values ​​of trimming resistors across the load-bearing and stress-free strain gages, thereby providing a substantially flat strain gage response over a predetermined temperature range. The quarter-bridge circuit also allows for pre-load compensation: for example, if an off-axis load is applied to the torque sensor before the axial torque is measured, the output of the quarter-bridge gauge circuit can resolve both the pre-load and the axial torque, increasing the accuracy of the latter measurement.

[0060] The quarter-bridge configuration is not the only circuit topology useful in embodiments of the present invention. Figure 8B shows strain gages 30-0, 30-1, 30-2, and 30-3 in a Wheatstone half-bridge circuit. This circuit is more temperature stable than the quarter-bridge circuit of Figure 8A. However, it cannot simultaneously and independently resolve Tz and Fz and is sensitive to gain changes caused by sensor preload.

[0061] Figure 8C shows strain gages 30-0, 30-1, 30-2, and 30-3 in a Wheatstone full-bridge circuit. This circuit is also more temperature stable than the quarter-bridge circuit of Figure 8A. However, it cannot simultaneously and independently resolve Tz and Fz and is sensitive to gain changes caused by sensor preloading.

[0062] FIG. 9 illustrates steps in a method 100 of manufacturing a torque sensor 10 according to an embodiment of the present invention. A generally circular, generally planar transducer plate 12 is provided (block 102). The transducer plate 12 has a central region 14 having a vertical central axis z and a peripheral region 16. One or more first mounting holes 18 are formed proximate the central region 14 (block 104). The first mounting holes 18 are configured to be attached to a first object (e.g., a first segment of a robot arm). One or more second mounting holes 20 are formed proximate the peripheral region 16 (block 106). The second mounting holes are configured to be attached to a second object (e.g., a motor controlling motion between a first segment of a robot arm and a second segment of the robot arm). A plurality of voids 24 are formed through the transducer plate 12 between the central region and the peripheral region (block 108). Adjacent voids 24 define spokes 22 connecting the central region 14 and the periphery 16. One or more radial slots 28 are formed through the transducer plate 12 between the central region 14 and the periphery 16 (block 110). Each radial slot 28 is adjacent to at least one void 24. The transducer plate 12 between the radial slots 28 and the voids 24 defines an instrumentation beam 26 connecting the central region 14 and the periphery 16. Strain gauges 30 are attached to the instrumentation beams 24 (block 112). The strain gauges 30 are connected to a strain gauge circuit that outputs a voltage that depends on the strain-dependent resistance of the strain gauges 30 (block 114).

[0063] Embodiments of the present invention offer numerous advantages over the prior art. The design of the transducer plate 12 enables the torque sensor 10 to comply with axial torque but exhibit high relative stiffness to off-axis loads, making the sensor 10 much less sensitive to the deleterious effects of off-axis loads. In one embodiment, strain gauges 30 are attached only to the top surface of the transducer plate 12, resulting in low manufacturing costs. The transducer plate 12 employs simple geometries and easily formed air gaps 24 and axial slots 28 for ease of manufacturing. The resulting spokes 22 and instrumentation beam 26 do not employ complex, expensive, thin features. The asymmetric nature of the instrumentation beam 26 creates a location that is insensitive to strains caused by off-axis loads, enabling top-surface gauging that is largely insensitive to bending torques Tx and Ty. The transducer plate 12 also allows for a large central through-hole for the passage of utilities or mechanical connections between robot arm segments. The design of the transducer plate 12, the arrangement of the strain gauges 30, and, in one embodiment, the quarter-bridge circuit topology, allow direct decomposition of the axial torque Tz (and, if desired, Fz, the FyTx coupling, and the FxTy coupling) using only four strain gauges 30.

[0064] In general, all terms used herein should be interpreted according to their ordinary meaning in the relevant technical field unless a different meaning is expressly given and / or is implied from the context in which it is used. All references to a / an / the element, apparatus, component, means, step, etc. should be openly interpreted as referring to at least one instance of the element, apparatus, component, means, step, etc., unless expressly stated otherwise. The steps of any method disclosed herein need not be performed in the exact order disclosed, unless a step is expressly described as following or preceding another step and / or it is implicit that a step must follow or precede another step. Any feature of any of the embodiments disclosed herein may be applied to any other embodiment, whenever appropriate. Similarly, any advantage of any embodiment may be applied to any other embodiment, and vice versa.

[0065] As used herein, the term "configured to" means set, organized, adapted, or arranged to operate in a particular manner, and this term is synonymous with "designed to." As used herein, the term "substantially" refers to nearly or essentially, but not necessarily entirely, mechanical or component value tolerances, measurement errors, random variations, and similar sources of inaccuracy. As used herein, the term "generally circular" refers to a shape having a central region and a perimeter approximately equidistant from the center, but is not necessarily limited to a geometric circle. For example, hexagons, octagons, etc. are polygons that are "generally circular." As used herein, the term "generally planar" refers to a shape having opposing planar surfaces, but does not exclude non-planar features such as rims, lips, flanges, etc.

[0066] Of course, the present invention may be practiced otherwise than as specifically described herein without departing from its essential characteristics. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes which come within the meaning and range of equivalency of the appended claims are intended to be embraced therein.

Claims

1. 1. A torque sensor configured to measure an axial torque between a first object and a second object, comprising: a circular, planar transducer plate having a central region with a vertical central axis z and a periphery; one or more first attachment holes adjacent the central region and configured to be attached to the first object; one or more second mounting holes adjacent the periphery and configured to be attached to the second object; a plurality of voids extending through the transducer plate between the central region and the periphery, adjacent voids defining spokes connecting the central region and the periphery; one or more radial slots extending through the transducer plate between the central region and the periphery, each adjacent to at least one void, the transducer plate between the radial slots and the voids defining instrumentation beams connecting the central region and the periphery, each instrumentation beam being generally radially oriented along its entire length and having the same thickness as the transducer plate along its entire length; a strain gauge attached to the instrumentation beam; a strain gauge circuit connected to the strain gauge and configured to output a voltage dependent on the strain-dependent resistance of the strain gauge; A torque sensor comprising:

2. The sensor of claim 1 , wherein the at least one void is a through-hole or an arc slot, and the at least one radial slot is a through-hole.

3. The sensor of claim 1 , wherein two sides of at least one radial slot are parallel to the radial direction.

4. The sensor of claim 1 , wherein two sides of at least one radial slot are non-parallel to the radial direction.

5. The sensor of claim 1 , wherein the transducer plate exhibits greater mechanical compliance under a torque Tz about the axis z than under other forces Fx, Fy, Fz, or torques Tx, Ty.

6. The sensor of claim 1 , wherein the sensor comprises at least four strain gauges, each mounted on a separate instrumentation beam.

7. The sensor of claim 6 , wherein all of the strain gauges are mounted on the same side of each instrumentation beam.

8. The sensor of claim 6 , wherein each strain gauge is mounted on a side of a respective instrumentation beam that is inside a radial slot.

9. 9. The sensor of claim 8, wherein each strain gauge is mounted on a respective instrumentation beam along the inner neutral axis of the radial slot or offset from the inner neutral axis of the radial slot.

10. 7. The sensor of claim 6, wherein a first two forces or torques are combined such that a strain gauge response under one of the first two forces or torques is a multiple of the strain gauge response under the other of the first two forces or torques.

11. 11. The sensor of claim 10, wherein two different second forces or torques are combined such that the strain gauge response under one of the second two forces or torques is a multiple of the strain gauge response under the other of the second two forces or torques.

12. 12. The sensor of claim 11, wherein the four strain gage responses and individual and combined loads comprise a 4x4 linear system, and wherein the torque about axis z, Tz, is determined directly from the four strain gage responses and a decoupling matrix.

13. The sensor of claim 12 , wherein a force Fz parallel to the axis z is also determined directly from the four strain gauge responses and the decoupling matrix.

14. 2. The sensor of claim 1, wherein the torque sensor is a robot joint torque sensor, the first object is an actuator of a joint between two segments of a robot arm, and the second object is attached to one of the segments.

15. 1. A method of manufacturing a torque sensor, comprising: providing a circular, planar transducer plate having a central region with a vertical central axis z and a periphery; forming one or more first attachment holes proximate the central region, the first attachment holes configured to be attached to a first object; forming one or more second mounting holes proximate the perimeter, the second mounting holes configured to be attached to a second object; and forming a plurality of voids through the transducer plate between the central region and the periphery, adjacent voids defining spokes connecting the central region and the periphery; forming one or more radial slots through the transducer plate between the central region and the periphery, each adjacent at least one void, the transducer plate between the radial slots and the voids defining instrumentation beams connecting the central region and the periphery, each instrumentation beam being generally radially oriented along its entire length and having the same thickness as the transducer plate along its entire length; attaching strain gauges to an instrumentation beam; connecting the strain gauge to a strain gauge circuit that outputs a voltage that depends on the strain dependent resistance of the strain gauge; A method comprising:

16. The method of claim 15 , wherein forming a first plurality of voids through the transducer plate comprises drilling a first plurality of through holes.

17. The method of claim 15 , wherein mounting strain gauges to the instrumentation beam comprises mounting four strain gauges, each on the same surface of a different instrumentation beam.

18. 18. The method of claim 17, wherein the locations of the air gaps and radial slots, the arrangement of the strain gauges, and the configuration of the strain gauge circuitry are such that strain gauge responses to two different pairs of forces and torques are linearly dependent, whereby the strain gauge response under the force of each pair differs by a factor from the strain gauge response under the torque of that pair, and the four strain gauge responses comprise a 4x4 linear system from which at least torque about the central axis z can be determined directly from the strain gauge responses and a decoupling matrix.

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