Patent Publication Number: US-2022214237-A1

Title: Device for measuring a torque and strain wave gearing comprising such a device

Description:
This application is the U.S. National Phase of PCT Appln. No. PCT/DE2020/100350 filed Apr. 29, 2020, which claims priority to DE 10 2019 112 146.9 filed May 9, 2019, the entire disclosures of which are incorporated by reference herein. 
     The present disclosure relates to a device for measuring a torque occurring in a strain wave gearing of a robot. In particular, the device is used in robot joints. The present disclosure further relates to a strain wave gearing. 
    
    
     BACKGROUND 
     A measuring device for determining a torque acting on an axis is known from DE 10 2010 029 186 A1, wherein the measuring device comprises a first and a second device. The devices are each designed to generate an analog electrical signal associated with the torque. Two independent torques are determined by means of downstream analog-to-digital converters and downstream digital evaluation devices. The devices are made up of strain gages applied to mechanical measuring bodies. 
     DE 10 2014 210 379 B4 describes a torque sensor and a method for measuring torques occurring at or in a joint of a jointed-arm robot. The sensor comprises a plurality of measuring spokes, which are designed in such a way that they deform under the action of a torque. The sensor also comprises strain gages which are arranged on the measuring spokes. 
     DE 10 2012 208 492 A1 describes a method for producing a strain gage arrangement on the surface of a machine element. A deformation-sensitive measurement layer with an overlying protective layer is applied to the surface. The protective layer is removed locally by laser processing and the exposed measurement layer is electrically contacted. Furthermore, it can be gathered from this publication that an insulation layer can be arranged between the surface of the machine element and the measurement layer. 
     DE 10 2014 219 737 A1 describes a device for measuring a torque applied to a rotatably mounted component. A carrier component is arranged on the component, on which a deformation-sensitive material is applied as a coating. The deformation-sensitive material forms a torque measurement arrangement. 
     A method and a device for determining an output torque of an electric motor are known from DE 103 17 304 A1. A gear with a ring gear is arranged downstream of the electric motor. A dynamic motor torque is measured by means of a torque sensor, which is supported in a fixed position on the ring gear. 
     DE 10 2013 204 924 A1 describes an arrangement for determining a torque acting on a shaft. In particular, the arrangement is part of a steering column of a vehicle. The arrangement comprises a first steering shaft section on the side of the steering wheel, a second steering shaft section on the side of the steering gear, and a torsion section connecting the steering shaft sections. Furthermore, the arrangement comprises a direct coating for torque measurement, which has a strain gage. 
     The prior art shows that for the measurement of a torque acting on a shaft, measuring arrangements are used in which strain gages are applied outside or on the shaft. 
     For robotic gearings, it is of great importance to accurately determine the torque transmitted by a strain wave gearing. For example, robot arms are used as prostheses for humans in medical technology, among other applications, where the robot arm must perform both precise mechanical and gross mechanical movements at different speeds and with different loads during operation. The same applies to industrial robots. 
     Strain wave gearings are used, among other applications, as axle drives in robots, motor vehicles, in machine tools and in drives for printing machines. Torque transmitting strain wave gearings are also known as harmonic drives or harmonic gearing. A strain wave gearing commonly includes an input shaft, an elliptical disc, a flexible spline, an outer ring, an input shaft, and a housing. The flexible spline is externally toothed and the outer ring is internally toothed, with the two components arranged coaxially to one another so that the teeth mesh with one another. 
     Devices for torque measurement in robot arms are known, which are mounted outside the gear housing of the robot arm. For example, deformation bodies with strain gages arranged on them are arranged in the area of the robot arms, in particular the robot joints. Using the strain gages, shear strains are recorded to determine the applied torque at the robot joint. 
     SUMMARY 
     Based on the prior art, an object of the present disclosure is to provide an improved torque measuring device which is designed to save space and which at the same time provides a high level of accuracy and robustness. 
     The device according to the present disclosure is used to measure a torque of a strain wave gearing. The torque measuring device comprises a component and a plurality of layers which are arranged one above the other on the component and which are part of a direct coating of strain gages. An electrically insulating insulation layer is arranged directly on the component. A deformation-sensitive measurement layer is arranged directly on the insulation layer. 
     The component is part of a robotics system, in particular the strain wave gearing. The component supporting the multiple layers is a flexible spline. 
     One advantage of the device according to the present disclosure is that it is designed to save space, since additional deformation bodies, which are only used to measure torque, are not required. Another advantage of the device is that it enables high accuracy and precision during operation and is very robust. 
     The component is preferably made of metal. Alternatively, the component is made of a semiconductor material. The flexible spline has a toothing on its outer radius. For example, the component may be a cylindrical steel sleeve that is flexible within the desired limits. 
     In a preferred embodiment, a protective layer is applied to the deformation-sensitive measurement layer, which protects the layers located below the protective layer from environmental influences. The protective layer is preferably made of an organic material. Alternatively, the protective layer is preferably made of an inorganic material. 
     The measurement layer is used to measure a strain or shear of the component, wherein a torque is measured. 
     The measurement layer preferably consists of metal or an alloy, in particular a nickel alloy. The nickel alloy is preferably a nickel-chromium alloy (NiCr). 
     The measurement layer preferably has a structuring. Particularly preferably, the measurement layer has a spatial structuring that forms a striped pattern. Different embodiments may, for example, have stripes in the angular range between 35° and 55° to the component longitudinal axis. Preferably, the structuring is created by means of a laser or by etching, wherein the structuring is created only after the measurement layer has been applied to the component. 
     The insulation layer preferably consists of one or multiple different oxides. Alternatively, the insulation layer consists of Diamond Like Carbon (DLC). The insulation layer can alternatively consist of one or more oxides and DLC. The insulation layer particularly preferably consists of Al 2 O 3  (aluminum oxide) and/or SiO 2  (wollastonite). 
     For example, the insulating layer may be produced by a physical vapor deposition process (PVD) or a chemical assisted physical vapor deposition process (PACVD). In one embodiment, the insulation layer is produced by a combination of the PVD and PACVD processes. 
     Preferably, the sequence of layers applied to the component, consisting of measurement layer, insulation layer and protective layer, has a total thickness of less than 200 μm. Particularly preferably, the sequence of layers comprising the measurement layer and the insulation layer has a total thickness of less than 20 μm. 
     Preferably, further elements can be arranged on the component. In one embodiment, electronic components for signal preamplification and/or for signal evaluation and/or for signal transmission are arranged on the component. 
     In one embodiment, electrically conductive contact layers that make contact at least in sections are formed between the stripe sections. 
     The strain wave gearing according to the present disclosure comprises a device for measuring a torque according to the device described above with all of its embodiments. Further, the strain wave gearing comprises a drive shaft, a wave generator which may be a rolling bearing with a non-circular, e.g., elliptical, inner ring and a deformable outer ring, a ring gear, and an elastic sleeve referred to as a flexible spline. The latter component of the device exhibits external toothing and the ring gear exhibits internal toothing. The flexible spline and ring gear are arranged coaxially to each other so that the gear teeth mesh with one another. The inner ring of the wave generator is positioned on the drive shaft so that it drives the component. 
     The strain wave gearing preferably also has a housing in which the aforementioned gearing components are at least partially arranged. 
     The strain wave gearing according to the present disclosure advantageously saves installation space, since the device, and with it the coating, is arranged within the housing and no additional deformation bodies are necessary. Due to the high precision that the device provides by accurately measuring a torque, the device and the strain wave gearing are applicable and particularly advantageous in the field of robotics. In particular, the device and the strain wave gearing are advantageous in protecting against collisions or in regulating force and stiffness. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
       Further advantages and details of the present disclosure arise from the following description of preferred embodiments with reference to the attached drawing. In the figures: 
         FIG. 1  shows a side view and a detailed view of a first embodiment of a device according to the present disclosure; 
         FIG. 2  shows a sectional view and a detailed view of the device shown in  FIG. 1 ; 
         FIG. 3  shows a plan view of a second embodiment of the device; 
         FIG. 4  shows a side view of the device shown in  FIG. 3 ; 
         FIG. 5  shows a sectional view and a detailed view of the side view of the device shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a side view and a detailed view of a first embodiment of a device according to the present disclosure. The device represents a flexible spline usable in a strain wave gearing, wherein the flexible spline consists of a disk  01  and a cylindrical component  02  axially adjoining the disk. Preferably, the flexible spline is made of steel. The cylindrical component  02  or sleeve is arranged on the inner diameter of the disk  01 . The cylindrical component  02  has an external toothing  03  on its section facing away from the disk  01 . A deformation-sensitive measurement layer  04  in the form of a strain gauge, in particular in the form of a Sensotect strain gauge, is arranged on the section of the outer circumference of the cylindrical component  02  facing the disk  01 . An insulating insulation layer  06  is formed between the base material of the cylindrical component  02  and the deformation-sensitive measurement layer  04 . A torque of the strain wave gearing is determined by means of the deformation-sensitive measurement layer  04 . The measurement layer preferably has a structuring which forms a striped pattern. 
     Furthermore, a detailed view of the deformation-sensitive measurement layer  04  is shown in  FIG. 1 . In the example shown, the formed structure of the measurement layer  04  runs in numerous meanders, the axes of the non-curved sections of the structure being inclined to the cylinder axis of the component  02 . 
     One of the advantages of the device according to the present disclosure is that it is designed to save installation space. 
       FIG. 2  shows a sectional view of the flexible spline shown in  FIG. 1  with the disk  01  and the cylindrical component  02 . In a detailed view of  FIG. 2 , the sequence of layers of the device is shown. On the cylindrical component  02 , which is made of steel, the insulation layer  06  is applied, on which the deformation-sensitive measurement layer  04  and a protective layer  07  arranged thereon are applied. The deformation-sensitive measurement layer  04  is a structured NiCr functional layer. 
       FIG. 3  shows a plan view of a further embodiment of the device. Differing from the device shown in  FIG. 1 , here the disk  01  has the deformation-sensitive measurement layer  04 . No deformation-sensitive measurement layer is formed on the outer circumference of the cylindrical component  02 . The individual components of the deformation-sensitive measurement layer  04  are circumferentially distributed on the disk  01 . The device is designed here as a collar sleeve. 
       FIG. 4  shows a side view of the collar sleeve shown in  FIG. 3 . Since the deformation-sensitive measurement layer  04  is formed on the disk  01 , the measurement layer on the outer circumference of the cylindrical component  02  is missing. In the area of the cylindrical component  02  facing away from the disk  01 , the toothing  03  is also formed on the outer circumference. 
       FIG. 5  shows a sectional view of the side view of the device shown in  FIG. 4 . Furthermore,  FIG. 5  shows a detailed view of the sequence of layers of the disk  01 . The insulation layer  06 , preferably consisting of Al 2 O 3 , is arranged on the steel disk  01 . The deformation-sensitive measurement layer  04  with a protective layer  07  located thereon is arranged on the insulation layer  06 . Contact layers  08  for making electrical contact are located between the individual deformation-sensitive measurement layers  04 . 
       FIG. 6  schematically shows a strain wave gearing  10  according to the present disclosure comprises a device  12  for measuring a torque according to the device described with respect to  FIG. 1 . Further, the strain wave gearing  10  comprises a drive shaft  14 , a wave generator  16  which may be a rolling bearing with a non-circular, e.g., elliptical, inner ring  18  and a deformable outer ring  20 , a ring gear  22 , and an elastic sleeve in the form of the flexible spline  01 ,  02 . The flexible spline  01 ,  02  exhibits external toothing  03  and the ring gear  22  exhibits internal toothing  22   a . The flexible spline  01 ,  02  and ring gear  22  are arranged coaxially to each other so that the gear teeth  03 ,  22   a  mesh with one another. The inner ring  18  of the wave generator  16  is positioned on the drive shaft so that it drives the component. Device  12  may be part of a robot arm or a robot arm joint  24  of a robotics system  26 . 
     LIST OF REFERENCE SYMBOLS 
     
         
           01  Disk 
           02  Cylindrical component 
           03  External toothing 
           04  Deformation-sensitive measurement layer 
           06  Insulation layer 
           07  Protective layer 
           08  Contact layer 
           10  Strain wave gearing 
           12  Device for measuring torque 
           14  Draft shaft 
           16  Wave generator 
           18  Inner ring 
           20  Outer ring 
           22  Ring gear 
           22   a  Internal toothing 
           24  Robot arm joint 
           26  Robotics system