Torque and Force Transducers

Abstract

A six-axis force torque transducer (FTT) includes a hub and at least one bending beam disposed on the hub and extending outward from the hub. Each of the at least one bending beam includes a U-beam having a substantially U-shaped cross section and at least one beam plate attached to the U-beam at a portion of the U-beam remote from the hub. A first strain gauge support including at least one strain gauge is mounted on an outer surface of the at least one U-beam. A second strain gauge support including at least one strain gauge is mounted on an outer surface of the at least one beam plate. A connecting element electrically connects the strain gauge of the first strain gauge support and the strain gauge of the second strain gauge support in a bridge configuration.

Description

[Technical Field]

[0001] (Related application data) This application claims the benefit of U.S. Provisional Application No. 63 / 031,774, filed May 29, 2020, the entire disclosure of which is incorporated herein by reference.

[0002] The disclosed implementations relate to the field of measuring forces using strain gauges attached to deformable structures, and more particularly to force torque transducers (FTTs) and measurement methods used to measure three orthogonal forces and three orthogonal torques. [Background technology]

[0003] A conventional six-axis FTT converts applied forces and torques into voltage, resistance, capacitance, or current. A deformable mechanical structure is used, and when a load is applied to the structure, the structure's surface deforms. Strain gauge sensors are used to convert the deformation into changes in resistance, voltage, capacitance, or current. The changes in this signal are then measured by electronics (known as "A / D converters") that digitize the analog signal for processing by a computer in a known manner. Such FTTs can be used, for example, in various robotics applications. To extract the three orthogonal forces and three orthogonal torques applied to the sensor, a minimum of six strain measurements must be obtained from different locations on the FTT's deformable structure. Each strain measurement is typically obtained by a resistive strain sensor, also known as a "resistive strain gauge" (SG). The configuration of the mechanical structure's SGs and the geometry of the mechanical structure directly affect the manufacturing cost and manufacturing quality of the FTT.

[0004] The sensing element of a 6-axis FTT typically consists of two stiff hubs that support the applied load and are connected by one or more radially and equally spaced rectangular bending beams. A compliant section exists between the bending beams and the stiff hub or hubs to mechanically decouple the applied force. Strain gauges are attached to the surface of the bending beams.

[0005] To measure all six components of force and torque, the most common FTTs have SGs mounted not on the same tangent plane but on surfaces that are typically offset 90 degrees from each other relative to an axis. These SGs are linked by electrical connections, such as wires, to form a half- or full-Wheatstone bridge ("bridge"). The most common configuration is a rectangular cross-section beam with SGs fixed on all four surfaces. The beam can be hollow to provide greater mechanical flexibility. At least three of these beams are required to generate a sufficient number of signals (minimum six) to analyze three orthogonal forces and three orthogonal torques. This approach allows for the design of very compact FTTs, since all available surfaces on the bending beam can be used for strain measurements. However, accessing the surfaces to attach and solder wires to the SGs in such designs is difficult. Therefore, manual labor and custom tooling are typically required, making them expensive to manufacture. Typically, installing strain gauges is a time-consuming process that requires manual labor, special tools and equipment, and highly skilled personnel.

[0006] In some cases, only one or two surface sections on the same plane or slightly inclined planes are sufficient to generate an appropriate signal sensitive enough to analyze two perpendicular forces. In this case, strain gauges are combined into quarter-bridge and / or half-bridge configurations to analyze the two necessary components of each beam. This design facilitates access to the SG connection pads for soldering, further facilitating automation of FTT manufacturing and significantly reducing costs. In some cases, instead of soldering wires, PCBs are used to connect the SGs appropriately by simply contacting the SG terminal pads. However, when strain gauges are used to measure perpendicular strain, signal decoupling is proportional to the distance from the neutral axis of the beam. Therefore, to achieve the accuracy and sensitivity levels of the sensor in the first example above, the beams would need to be wider, resulting in a larger FTT diameter for the same SG.

[0007] Furthermore, temperature compensation can only be achieved by attaching an external sensor to the surface where no pressure is applied. The drawback of this external sensor is that heat conduction between the two surfaces is relatively slow. Therefore, the temperatures of these two surfaces are not synchronized, and drift becomes an issue. Furthermore, heat dissipation is not symmetrical and is highly affected by the external boundary temperature conditions.

[0008] In another example, the strain measurement may be partly vertical and partly in shear. In such a case, the strain gauges are all located on top of one surface, which also makes manufacturing easier. However, this makes the mechanical structure more complex and therefore more expensive.

[0009] Generally speaking, the instrumentation of FTTs by bonding SGs is one of the most significant parts of the costs associated with their manufacture. FTTs are expensive because they require customized equipment to manufacture. Furthermore, special algorithms are developed to compensate for temperature effects, and the quality control of the manufactured sensors becomes difficult when many wires are soldered to a small device. Summary of the Invention

[0010] Disclosed implementations include an FTT that converts three orthogonal force components and three orthogonal torque components applied to the FTT into six or more electrical signals. This implementation enables the design and manufacture of a compact six-axis FTT without the need for specialized equipment and tooling. For example, the FTT can be used for force feedback in robotics for fine fingertip sensing or large force manipulation tasks where force sensitivity and force control are essential.

[0011] The core components of the present invention are a rigid tool mounting hub, one or more rigid mounting fixtures, and a mechanically deformable structure (referred to as a "bending beam"). The bending beam consists of a substantially U-shaped beam in cross section attached to two upright plate beams. The electrical circuitry for a full-bridge strain gauge group capable of measuring shear strain is located on the outer surface of the U-shape, and two interconnected half-bridge strain gauge groups capable of measuring shear strain are located on the upright plate beams. A printed circuit board (PCB) is provided and contains any necessary electronic circuitry and / or connections.

[0012] The above components can be configured as follows: A rigid hub is connected to a rigid mounting fixture by three radially arranged bending beams. These components form a single, integrated structure that can be manufactured by milling or other techniques. Full-bridge strain gauges are fixed on the U-shaped beams of the bending beam. In one implementation, half-bridge shear strain gauges are used instead of a full bridge, with the remaining bridge completed by resistors. Two half-bridge strain gauges are fixed on the two upright beam plates of the bending beam to form a full bridge. In one implementation, a pair of single shear strain gauges is fixed on the upright beam plates to form a half bridge, with the remaining bridge completed by resistors. The strain gauges can be electrically connected by a PCB to form the above-mentioned half and full bridges. The outer surface of the bending beam, on which the SGs are attached, is used to measure the shear strain generated by the two applied normal forces. These surfaces are easy to access, and therefore strain gauges can be attached by hand without the need for special equipment to access the surfaces. Furthermore, the surfaces are used for the compliance of the sensors, which is required to mechanically decouple the forces applied to the FTT's reference frame. Thus, all available, accessible surfaces can be used in the most efficient way, making manufacturing easy and inexpensive, and resulting in a very compact FTT.

[0013] The strain gauges for each of the two forces are electrically connected in a manner that compensates for temperature variations in the structure. Specifically, the SGs are symmetrically placed on the same bending beam and are in close proximity to each other, so they share the same temperature with little delay / variation. Additionally, the strain gauges are temperature compensated for the material to which they are attached.

[0014] A calibration process using ground truth measurements on a selected transducer reference frame uses six signal outputs to reproduce the three applied forces and three applied torques. Using the disclosed implementations, ultra-compact, high-performance six-axis force-torque transducers can be fabricated using off-the-shelf strain gauges that are compatible with current industrial robotic systems. Furthermore, it should be noted that the disclosed implementations do not require special assembly skills or tooling, resulting in low manufacturing costs.

[0015] The disclosed implementation is a six-axis force torque transducer (FTT) comprising: a hub; at least one bending beam disposed on the hub and extending outward from the hub, each of the at least one bending beam including a U-beam having a substantially U-shaped cross section and at least one beam plate attached to the U-beam at a portion of the U-beam remote from the hub; a first strain gauge support including at least one strain gauge and mounted on an outer surface of the at least one U-beam; a second strain gauge support including at least one strain gauge and mounted on an outer surface of the at least one beam plate; and a connection element for electrically connecting the strain gauge of the first strain gauge support and the strain gauge of the second strain gauge support in a bridge configuration.

[0016] There can be three U-beams radiating from the hub, and each U-beam can have two beam plates attached to it. A plurality of mounting fixtures can each connect a distal end of one of the beam plates to a distal end of a corresponding beam plate attached to an adjacent one of the U-beams.

[0017] Each of the at least one U-shaped beam may include two substantially parallel plates and an orthogonal connecting plate. The first strain gauge support may be mounted on the orthogonal connecting plate. The second strain gauge support may be disposed in a plane that intersects at a right angle with a plane defined by the outer surfaces of the at least one beam plate.

[0018] The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a perspective view of a six-axis FTT with three bending beams, their stiff hubs, SG configuration, and PCB, and a Cartesian reference coordinate system, according to a disclosed implementation. [Figure 2] FIG. 2 is a detailed perspective view of the bending beam of FIG. 1 according to a disclosed implementation. [Figure 3a] FIG. 3 is a top view of the bending beam of FIG. 2. [Figure 3b] 10A is a cross-sectional view of a U-shaped portion of a bending beam with an SG on top, according to a disclosed implementation. [Figure 4a] FIG. 1 is a top view of a six-axis FTT, including a Cartesian reference coordinate system, according to a disclosed implementation. [Figure 4b] FIG. 4b is a side view of the FTT of FIG. 4a, including a Cartesian reference frame. [Figure 5a] 1 is a schematic diagram of an example of a combination of SGs according to a disclosed implementation. [Figure 5b] 1 is a schematic diagram of an example of a combination of SGs according to a disclosed implementation. [Figure 5c] 1 is a schematic diagram of an example of a combination of SGs according to a disclosed implementation. [Figure 5d] 1 is a schematic diagram of an example of a combination of SGs according to a disclosed implementation. [Figure 6] 1A-1C are schematic top views of several configurations of SGs on supports that can be used in accordance with the disclosed implementations. DETAILED DESCRIPTION OF THE INVENTION

[0020] Various disclosed implementations will be described with reference to the following details, and the various implementations will be illustrated by the accompanying drawings. The following description and drawings are illustrative of the invention and should not be construed as limiting the invention. Numerous specific details are set forth to provide a thorough understanding of the various disclosed implementations. However, in some cases, well-known or conventional details are not set forth in order to present a concise description of the disclosed implementations. Positional terms such as "top," "bottom," and "side" are used herein as relative descriptors, and the implementations may be positioned in any orientation and / or reference coordinate system.

[0021] FIG. 1 is a perspective view of a FFT 100 according to a disclosed implementation. The FFT 100 includes a rigid tool mounting hub 109, three rigid mounting fixtures 110, and at least one mechanically deformable metallic or non-metallic structure 200 (referred to herein as "bending beam 200"), only one of which is labeled in FIG. 1. The bending beam 200 is described in more detail below with reference to FIG. 2. A compliant portion 107 of the bending beam 200 is designed to mechanically decouple applied forces and is defined at a portion of the bending beam 200 that couples to the hub 109. Shear-type SG strain gauge sets 101a, 101b are attached to each surface of the bending beam 200 and electrically connected, in a known manner, to a PCB 103 that includes appropriate electrical components defining a half-bridge or full-bridge connection.

[0022] FIG. 2 shows one of the bending beams of FIG. 1 in more detail. The bending beam 200 consists of a U-beam 108 with a U-shaped cross section (see FIG. 3a) rigidly attached to two upright beam plates 107 by elements 106. Four full-bridge strain gauges (111, 112, 113, 114) are attached to the top surface of the U-beam 108 via supports 102 in a manner configured to measure shear strain in the U-beam 108. Two half-bridge strain gauge supports 101a and 101b are attached in a manner that allows shear strain on the upright beam plates 107 to be measured using SGs 115 and 116. The SGs are electrically connected to the PCB 103 via electrical connections 104 and 105, which may include wires. The hub 109 is connected to a rigid mounting fixture 110 by three radially arranged bending beams 107, respectively. These elements therefore form one integral structure that can be manufactured by conventional techniques such as milling and casting techniques.

[0023] Figure 3a shows a top view of a bending beam 200. Figure 3b shows a cross section of a U-beam 108. Note that the U-beam 108 has a substantially U-shaped cross section. As used herein, the phrase "substantially U-shaped" refers to a structure having two substantially parallel elements connected by a substantially orthogonal element. This can include flat elements connected at a 90-degree angle, as shown in Figure 3b, or connected somewhat curved elements that approximate a U-shaped cross-sectional shape.

[0024] The support 102 is fixed to the U-beam 108 of the bending beam 200. As shown in Figures 3a and 3b, the support 102 is glued to the outer upper surface of the U-beam 108. The shear strain field on the surface to which the support 102 is attached is approximately homogeneous. When a shear force F1 (see Figure 2) is applied to the FTT at 45 degrees from the principal deformation direction of the strain gauges, a homogeneous shear strain field is generated. The SGs are aligned in the same direction as the principal strain. Each of the four SGs in the attached full-bridge set is subjected to the same strain magnitude. Two of the four SGs are compressed, and the other two are tensioned. Figure 5a shows the electrical connections of a Wheatstone bridge that can generate an electrical signal when force F1 is applied to the bending beam.

[0025] In another implementation, the support 102 can be attached to the bottom surface of the U-shaped U-beam. In another implementation, as shown in FIG. 5c, only two strain gauges are used to form a half-bridge on the support 102. This configuration can generate an electrical signal from the applied force F1. In this implementation, a SG with the same sensitivity would generate a signal half as large. Using even more sensitive strain gauges can result in a more compact FTT, which is advantageous. FIG. 5b shows another combination: one half-bridge used on the U-beam to measure force F1 and two half-bridges attached to a rectangular beam to measure force F2.

[0026] In another implementation, the two half-bridge SG supports can be attached to opposite faces of the U-beam 108. This embodiment is advantageous because it eliminates the coupling that occurs when torque M1 is applied, and the beam behaves solely as a single-axis load cell that can generate a signal only when force F1 is applied, eliminating torque M1.

[0027] The strain gauges can be temperature matched and temperature compensated for the bending beam material in known manner. Furthermore, if the strain gauges are mounted in the same area of ​​each sensor, i.e., physically close to each other, the temperature difference is generally negligible. Thus, each of the U-beam signals across the FTT is temperature compensated by the means by which the strain gauges are positioned and electrically connected.

[0028] With reference to FIG. 3, an upright beam plate 107 is used to mount the strain gauges necessary to measure the normal force F2 (121). Due to the compliance required by the FTT, the upright beam plate 107 is used to decouple all applied force and torque components, particularly the Fx, Fy, and Mz components. This combination means that any available surface can be used to mount the strain gauges, resulting in a very compact six-axis FTT. The SG supports 101a and 101b (see FIG. 1) mounted on these surfaces are also shear-type and generate signals when normal forces are applied to the bending beams. Each support can support two strain gauges 115 and 116, forming a half-bridge shear strain gauge set, aligned in the same direction as the force-induced principal strain, typically at 45 degrees from the horizontal 120 as shown in FIG. 2 (see FIG. 2). The strain gauges can be aligned at 90 degrees from each other. When force F2 is applied, two strain gauges are compressed and the other two are pulled, in the same manner as strain gauge set 102 described in the previous paragraph. The difference is that these SG sets are electrically connected to each other via PCB 103. The strain gauges are temperature compensated to match the material to which they are attached and are placed between the stiffness parts 109 and 110 of the FTT. The strain gauges are placed symmetrically so that any temperature difference from one surface to the other is negligible. Furthermore, the strain gauges are electrically connected to eliminate strain induced from temperature fluctuations in the bending beam.

[0029] In another implementation, supports 101a and 101b may each contain only one strain gauge, aligned 45 degrees from horizontal 120 or oriented transverse to the force-induced principal strains. These strain gauges can be connected in a half-bridge configuration using PCB 103, as shown in Figure 5d.

[0030] Figure 4a shows a top view of the implementation described above with three bending beams 108 connected to form a complete 6-axis FTT. The FTT may use a rigid mounting fixture 110 mounted on a rigid housing or adapter 109 on the force application side. In this example, the three force and three torque reference frames 117 are chosen to be on the top surface of the housing or adapter 109. Figure 4b is a side view of the implementation of Figure 4a, with the x- and z-axes of the reference frames indicated.

[0031] Figure 5 shows an expanded view of another embodiment of the invention using the possible combinations of SGs used in a bending beam, as already mentioned. It can be seen that there are more than five possible combinations for measuring the shear strain induced from two normal forces applied on a bending beam.

[0032] FIG. 6 shows diagrams of several types of SG supports and their SGs that can be used in other embodiments of the present invention. The SGs can be positioned at 90 degrees to each other and aligned in the same direction as the principal strain induced from the applied shear stress. There can be one, two, three, or even four strain gauges on a single strain gauge support. The strain gauges can be arranged in any of the patterns shown in FIG. 6. Known metal foil or semiconductor foil resistive SGs can be used to convert strain into changes in electrical resistance. Metal foil SGs are typically cheaper, capable of measuring larger strains, and more robust, but have a smaller gain factor. Semiconductor SGs can be expensive, fragile, and difficult to handle, but have a high gain factor and are therefore preferred when sensitivity is a key concern. Because of their higher sensitivity, electronic circuitry can be minimized, resulting in a very compact FTT.

[0033] In a known manner, multiple strain gauges can be connected via a PCB to form the half-bridge and full-bridge configurations mentioned above. SGs can be attached to the outer surfaces of the bending beam to measure the shear strain generated by two applied normal forces. These surfaces are easily accessible for mounting and gluing strain gauges manually, without the need for special equipment. The beam also exploits the compliance of the sensor, which is needed to mechanically decouple the applied forces from the FTT reference frame. Thus, all surfaces are utilized in the most efficient way, and fabrication is simple, resulting in a cost-effective and compact FTT.

[0034] The SGs can be electrically connected to compensate for temperature variations in the structure. The SGs are symmetrically positioned on the same beam, allowing them to share the same temperature with negligible delay due to their physical proximity. Furthermore, the strain gauges used in this invention are temperature compensated for the material to which they are attached. A calibration process using ground calibration on the transducer's reference frame 117, selected using the least-squares method, allows six signal outputs to be used to reproduce three applied forces and three applied torques. This implementation can be used to fabricate an ultra-compact, high-performance six-axis force-torque transducer using off-the-shelf strain gauges that is compatible with current industrial robotic systems.

[0035] The present invention has been described with respect to various implementations. Those skilled in the art will recognize that various modifications can be made without departing from the scope of the invention, which is defined in the appended claims.

Claims

1. Hub and at least one bending beam disposed on the hub and extending outwardly from the hub, each of the at least one bending beam having a U-shaped beam having an orthogonal element and two substantially parallel elements extending downwardly from a bottom surface of the orthogonal element to define a substantially U-shaped cross section, and at least one beam plate attached to the U-shaped beam at a portion of the U-shaped beam remote from the hub; a first strain gauge support comprising at least a first strain gauge and a second strain gauge mounted on an outer surface of the orthogonal element of the U-beam, the first strain gauge support being arranged to have a principal deformation direction such that a shear force F1 applied to the hub in a direction perpendicular to the longitudinal axis of the orthogonal element subjects the first strain gauge and the second strain gauge to strains of the same magnitude, causing the first strain gauge to be compressed and the second strain gauge to be elongated; a second strain gauge support mounted on an outer surface of the at least one beam plate, the second strain gauge support including at least two strain gauges disposed on the at least one beam plate, the second strain gauge support having a principal deformation direction aligned with a principal strain induced by a shear force F2 applied to the hub in a direction 90 degrees from the shear force F1 and normal to a surface of the first strain gauge support; a connecting element for electrically connecting the first strain gauge and the second strain gauge of the first strain gauge support and the at least two strain gauges of the second strain gauge support in a bridge configuration; A six-axis force torque transducer (FTT) having:

2. 2. The FTT of claim 1, wherein the FTT has three bending beams, each of the U-shaped beams having two beam plates, each beam plate having a proximal end attached to the U-shaped beam and a distal end opposite the proximal end, and the FTT further has a plurality of mounting fixtures, each of which connects the distal end of each beam plate to the distal end of a corresponding one of the two beam plates attached to an adjacent one of the U-shaped beams.

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