Patent Publication Number: US-2021173540-A1

Title: Artificial finger tip sliding touch sensor

Description:
CROSS REFERENCE TO THE RELATED APPLICATIONS 
     This application is the national phase entry of International Application No. PCT/CN2018/088078, filed on May 23, 2018, which is based upon and claims priority to Chinese Patent Application No. 201711235136.0, filed on Nov. 30, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the technical field of sensors, and more particularly, to an artificial fingertip sliding tactile sensor. 
     BACKGROUND 
     Tactile sensing is an essential component of dexterous hand systems such as bionic prosthetic hands and manipulators for directly interacting with the external environment, and provides an important way to perceive environmental information. Robot tactile information can be divided into kinesthetic information from muscles and sliding sensory information. The sensing and control of manipulation can be performed orderly only when the two kinds of sensory information are combined. Among them, the kinesthetic information is mainly based on force sensory information, and reflects the position information of an end effector by detecting the force sensory information of the end effector; and the sliding sensory information includes information on shape, measurement, humidity, texture and others to achieve accurate recognition of objects. In order to accurately obtain the sliding sensory information and the kinesthetic information, it is imperative to develop an artificial fingertip sliding tactile sensor capable of accurately detecting the force applied on the top end of the sensor and the texture information of objects via a polyvinylidene fluoride (PVDF) film and a hydraulic sensor, and measuring the kinesthetic information on contact status via strain gauges, thus achieving the recognition, classification and manipulation of the objects, which improves the intelligent level and manipulation level of the dexterous hand system. 
     SUMMARY 
     In order to achieve accurate recognition and classification of objects, the present invention provides an artificial fingertip sliding tactile sensor capable of accurately detecting the force applied on the top end of the sensor and the texture information of objects via a PVDF film and a hydraulic sensor, and measuring the kinesthetic information on contact status via strain gauges, thus achieving the recognition, classification and manipulation of the objects, which improves the intelligent level and manipulation level of the dexterous hand system. In order to achieve the above-mentioned objective, the present invention provides an artificial fingertip sliding tactile sensor. The artificial fingertip sliding tactile sensor is installed at the end of a bionic prosthetic hand or a manipulator, and includes a PVDF film, a rubber fingertip, a filling liquid, a sealing plug, a hydraulic sensor, a housing, an inner framework, strain gauges, a detection circuit and connecting bolts. The rubber fingertip is a hemispherical cavity, and is filled with the filling liquid. The outer surface of the rubber fingertip is engraved with concentric circular textures. The PVDF film is attached to the outside of the rubber fingertip. A sealing plug is provided on one side of the rubber fingertip and configured to seal the rubber fingertip. The hydraulic sensor is installed at the bottom of the sealing plug. The main body of the housing is a rigid cylindrical structure. The periphery and the top of the housing are thin-walled. The top of the housing is provided with a circular opening. The bottom of the housing is a flange-like structure, and four circular smooth through holes are uniformly distributed on the flange-like structure. The rubber fingertip is arranged on the circular opening of the housing. The inner framework includes a cylindrical head, a vertical strain rod and a base. The cylindrical head is provided at the top end of the vertical strain rod, and the bottom of the vertical strain rod is connected to the base. The detection circuit is provided inside the base. The strain gauges are respectively attached on four sides of the vertical strain rod and adjacent to the base. Four circular threaded through holes are uniformly distributed on the base. The connecting bolts pass through the circular smooth through holes at the bottom of the housing and are screwed into the circular threaded through hole on the base to connect the inner framework to the housing. 
     Preferably, the PVDF film is a hemispherical flexible film. The hemispherical flexible film includes a piezoelectric film, a metalized electrode, a protective film and a lead wire. During a sliding movement, a micro-unit of the PVDF film is compressed by fine texture particles on the surface of the object to generate an induced charge, so that the surface texture characteristics of the object in contact with the PVDF film can be judged. 
     Preferably, the rubber fingertip is a hemispherical cavity, and concentric circular textured protrusions are arranged on the outer surface of the rubber fingertip. The bottom of the rubber fingertip is provided with a cylindrical recess. The cylindrical recess has a diameter larger than the diameter of the cylindrical head of the inner framework and is used for connection and fixing. One side of the rubber fingertip is provided with a circular liquid injection hole. The sealing plug covers the circular liquid injection hole for transmitting external pressure. 
     Preferably, the front end of the sealing plug is a rigid conduit, and the bottom of the sealing plug is provided with a bottom piston. The bottom piston is a flexible rubber disc. The bottom piston and the rigid conduit are connected by rubber. The hydraulic sensor is installed at the connection between the bottom of the rigid conduit and the bottom piston and configured to measure a change of the liquid pressure caused by the external pressure. 
     Preferably, the centers of the cylindrical head, the vertical strain rod and the base of the inner framework are linearly arranged in a straight line. The strain rod is an elongated rigid straight rod with a square cross section, and the base is a flat cylinder. 
     Preferably, the detection circuit is in the shape of a disc. The diameter of the detection circuit is less than the inner diameter of the housing. A square through hole is formed at the center of the detection circuit. The side length of the square through hole is greater than the side length of the cross section of the strain rod. Measurement circuits required by various sensing elements are arranged on the detection circuit. 
     Preferably, the filling liquid is a non-conductive and non-corrosive liquid that has strong fluidity and is less likely to be volatilized and solidified. 
     Preferably, the PVDF film and the rubber fingertip are closely attached by an adhesive. The cylindrical recess and the embedded cylindrical head are closely attached by the adhesive. The rubber fingertip and the housing are closely attached by the adhesive. 
     Preferably, one side of the housing is provided with a rectangular hole allowing the lead wire of the sensor to pass therethrough, wherein the rectangular hole is adjacent to the bottom of the housing. 
     Preferably, the strain gauges are fixed on the four sides of the strain rod by an adhesive, respectively, wherein the strain gauges are adjacent to the base and located at the same height. Two strain gauges on opposite sides form a half-bridge circuit for measuring the force applied on the end of the strain rod. 
     The present invention provides an artificial fingertip sliding tactile sensor. The PVDF film, the hydraulic sensor and the strain gauges are innovatively combined to extend the original use functions of the sensor, thus realizing the measurement of sliding sensory information and kinesthetic information. The sensor exhibits improved sensitivity by the design of bionic fingerprint texture structure, and thus can detect small texture changes, thereby improving the measurement sensitivity. The force applied on the sensor is converted into a change in liquid pressure. The liquid has certain compressibility and fluidity, which is similar to the deformation of the fingertip of the human body. The artificial fingertip sliding tactile sensor can accurately detect the force applied on the fingertip and can obtain force signals from different positions and different attitudes. The present invention has the advantages of simple structure and easy installation, debugging, and maintenance as well as good stability, good linearity, good repeatability, and high measurement accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the overall structure of the sensor of the present invention. 
         FIG. 2  is a cross-sectional view of the overall structure of the sensor of the present invention. 
         FIG. 3  is a schematic diagram of the PVDF film of the present invention. 
         FIG. 4  is a cross-sectional view of the rubber fingertip of the present invention. 
         FIG. 5  is a schematic diagram of the housing of the present invention. 
         FIG. 6  is a schematic diagram of the inner framework of the present invention. 
         FIG. 7  is a schematic diagram of the detection circuit of the present invention. 
         FIG. 8  is a cross-sectional view of the sealing plug of the present invention. 
     
    
    
     In the figures:  1 -PVDF film,  2 -rubber fingertip,  21 -liquid injection hole,  22 -cylindrical recess,  3 -sealing plug,  31 -rigid conduit,  32 -hydraulic sensor,  33 -bottom piston,  4 -housing,  41 -circular opening,  42 -circular smooth through hole,  43 -housing square hole,  5 -inner framework,  51 -cylindrical head,  52 -vertical strain rod,  53 -base,  54 -circular threaded through hole,  6 -strain gauge,  7 -detection circuit,  8 -connecting bolt,  9 -filling liquid. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be further described in detail hereinafter with reference to the drawings and the specific embodiments. 
     The present invention provides an artificial fingertip sliding tactile sensor capable of accurately detecting the force applied on the top end of the sensor and the texture information of objects via a PVDF film and a hydraulic sensor, and measuring the kinesthetic information on contact status via strain gauges, thus achieving the recognition, classification and manipulation of the objects, which improves the intelligent level and manipulation level of the dexterous hand system. 
     In a specific embodiment of the present invention, the present invention provides an artificial fingertip sliding tactile sensor. The artificial fingertip sliding tactile sensor is installed at the end of a bionic prosthetic hand or a manipulator and configured to obtain information on texture, material and shape of the object and information on contact status between the finger and the contact object, so as to realize accurate recognition, identification and manipulation of the object. The artificial fingertip sliding tactile sensor includes the PVDF film  1 , the rubber fingertip  2 , the filling liquid  9 , the sealing plug  3 , the hydraulic sensor  32 , the housing  4 , the inner framework  5 , the strain gauges  6 , the detection circuit  7  and the connecting bolts  8 . The rubber fingertip  2  is a hemispherical cavity, is filled with the filling liquid. The outer surface of the rubber fingertip  2  is engraved with concentric circular textures. The PVDF film  1  is attached to the outside of the rubber fingertip and configured to perform texture detection on the object. The sealing plug  3  seals the rubber fingertip. The hydraulic sensor  32  is installed at the bottom of the sealing plug and configured to measure the force applied on the rubber fingertip. The main body of the housing  4  is a rigid cylindrical structure. The top of the housing  4  is provided with a circular opening. The bottom of the housing  4  is a flange-like structure. Four circular through holes are uniformly distributed on the flange-like structure and used for connection and fixing. The inner framework  5  includes a cylindrical head, a vertical strain rod and a base. The strain gauges are respectively attached on four sides of the vertical strain rod and adjacent to the base. Four circular threaded through holes are uniformly distributed on the base. The connecting bolts  8  pass through the through holes at the bottom of the housing and are screwed into the threaded through hole on the base to connect the inner framework to the housing. 
     The PVDF film  1  of the present invention is a hemispherical flexible film including a piezoelectric film, a metalized electrode, a protective film, a lead wire and others. During a sliding movement, a micro-unit of the PVDF film is compressed by fine texture particles on the surface of the object to generate an induced charge, so that the surface texture characteristics of the object in contact with the PVDF film can be judged. 
     The rubber fingertip  2  of the present invention is a hemispherical cavity. Concentric circular textured protrusions are arranged on the outer surface of the rubber fingertip  2 . The cylindrical recess  22  at the bottom of the rubber fingertip  2  has a diameter slightly larger than the diameter of the cylindrical head  51  of the inner framework and is used for connection and fixing. The cavity is filled with the filling liquid  9  through the circular liquid injection hole  21  on one side of the rubber fingertip  2  for transmitting external pressure. 
     The front end of the sealing plug  3  of the present invention is the rigid conduit  31 . The bottom piston  33  is a flexible rubber disc. The piston and the conduit are connected by rubber. The hydraulic sensor  32  is installed at the connection between the bottom of the conduit and the bottom disc and configured to measure the change of liquid pressure caused by external pressure. 
     The main body of the housing  4  of the present invention is a rigid cylindrical structure. The periphery and the top of the housing  4  are thin-walled. The top of the housing  4  is provided with the circular opening  41 . The bottom of the housing  4  is a flange-like structure. The four circular smooth through holes  42  are uniformly distributed on the edge of the bottom of the housing  4  for connection and fixing. 
     The inner framework  5  of the present invention includes the cylindrical head  51 , the vertical strain rod  52  and the base  53 . The centers of the cylindrical head  51 , the vertical strain rod  52  and the base  53  are linearly arranged in a straight line. The strain rod  52  is an elongated rigid straight rod with a square cross section. The strain gauges  6  are respectively attached on four sides of the strain rod  52  and adjacent to the base. The base  53  is a flat cylinder. The four circular threaded through holes  54  are uniformly distributed on the edge of the base  53  for connection and fixing. 
     The detection circuit  7  of the present invention is in the shape of a disc. The diameter of the detection circuit  7  is slightly less than the inner diameter of the housing  4 . A square through hole is formed in the center of the detection circuit  7 . The side length of the square through hole is slightly greater than the side length of the cross section of the strain rod  52 . Measurement circuits required by various sensing elements are arranged on the detection circuit  7 . 
     The head of the connecting bolt  8  of the present invention has a diameter larger than the diameter of both the threaded through hole  54  and the smooth through hole  42 . The specification of the external thread on the shank is consistent with the specification of the internal thread in the threaded through hole  54 . The inner framework  5  is fixedly connected to the housing  4  by the connecting bolts  8 . 
     The filling liquid  9  of the present invention is a non-conductive and non-corrosive liquid that has strong fluidity and is less likely to be volatilized and solidified. 
     The PVDF film  1  of the present invention is closely attached to the rubber fingertip  2  by an adhesive. The circular textured protrusions on the surface of the rubber fingertip  2  can still be sensed through the PVDF film  1 . 
     The diameter of the circular opening  41  at the top of the housing of the present invention is slightly larger than the diameter of the cylindrical head  51  of the inner framework, so that the inner framework does not directly contact the top of the housing. 
     The diameter of the cylindrical recess  22  at the bottom of the rubber fingertip of the present invention is slightly larger than the diameter of the cylindrical head  51  of the inner framework, but the cylindrical recess  22  and the cylindrical head  51  are of equal height, so that the cylindrical head  51  can be embedded in the cylindrical recess  22 . The cylindrical recess  22  and the cylindrical head  51  are fixedly connected by an adhesive. Also, the bottom of the rubber fingertip  2  and the top of the housing  4  are fixedly connected by the adhesive. 
     The diameter of the main body of the housing  4  of the present invention is equal to the maximum diameter of the rubber fingertip  2 . The centers of the housing  4  and the rubber fingertip  2  are linearly arranged in a straight line. 
     The flange-like structure at the bottom of the housing  4  of the present invention is equal to the diameter of the main body of the housing  4  and equal to the outer diameter of the base  53  of the inner framework. The four through holes on the base  53  also have the same position, but the diameter of the circular smooth through holes  42  is slightly larger than the diameter of the circular threaded through hole to facilitate the connection via the connecting bolts  8 . 
     One side of the housing  4  is provided with the rectangular hole  43  allowing the lead wire of the sensor to pass therethrough, wherein the rectangular hole  43  is adjacent to the bottom of the housing  4 . 
     The detection circuit  7  of the present invention is located inside the housing. The strain rod  52  passes through the square through hole in the center of the detection circuit  7 . The electrical signal of the detection circuit  7  is connected to an external device through the hole  43  at the bottom of the housing  4  via the lead wire. 
     The strain gauges  6  of the present invention are fixed on four sides of the strain rod  52  by an adhesive, wherein the strain gauges  6  are adjacent to the base and located at the same height. Two strain gauges  6  on opposite sides form a half-bridge circuit for measuring the force applied on the end of the strain rod  52 . 
     Referring to  FIGS. 1 and 2 , the artificial fingertip sliding tactile sensor provided in the present invention is installed at the end of a manipulator or a bionic prosthetic hand and configured to obtain information on texture, material and shape of the object and information on contact status between the finger and the contact object, so as to realize accurate recognition, identification and manipulation of the object. The artificial fingertip sliding tactile sensor includes the PVDF film  1 , the rubber fingertip  2 , the filling liquid  9 , the sealing plug  3 , the hydraulic sensor  32 , the housing  4 , the inner framework  5 , the strain gauges  6 , the detection circuit  7  and the connecting bolts  8 . 
     Referring to  FIG. 3 , the PVDF film  1  is a hemispherical flexible film including a piezoelectric film, a metalized electrode, a protective film, a lead wire and others. The bottom edge of the PVDF film  1  has an outer diameter of  18  mm and a thickness of 1 mm. The electrical signal of the PVDF film  1  is connected to the detection circuit  7  via the rectangular hole  43  of the housing through the lead wire. 
     Referring to  FIG. 4 , the rubber fingertip  4  is a hemispherical cavity, and is made of rubber and has certain flexibility. The rubber fingertip has a maximum diameter of 16 mm and a wall thickness of 1 mm. Concentric circular textured protrusions are uniformly distributed on the outer surface of the rubber fingertip, wherein the height of the protrusion is 0.2 mm, and the difference in diameter between the concentric circular textured protrusions is 1 mm. The cylindrical recess  22  with a diameter of 5 mm and a height of 2.5 mm is arranged at the bottom in the center of the rubber fingertip and used for connection and fixing. The circular liquid injection hole  21  on one side of the rubber fingertip has a diameter of 5 mm. The cavity is filled with the filling liquid  9  for transmitting external pressure. 
     Referring to  FIG. 5 , the main body of the housing  4  is a metal cylindrical structure with a diameter of 16 mm and a height of 30 mm. The periphery and the top of the housing  4  has a thin wall with a thickness of 1 mm. The circular opening  41  with a diameter of 18 mm is formed at the center of the top of the housing  4 . The bottom of the housing  4  is a flange-like structure with a diameter of 24 mm and a thickness of 2 mm. Four circular smooth through holes  42  are uniformly distributed on the edge of the bottom of the housing  4 , and have a diameter of 3.3 mm. The distance between the center of the through hole and the center of the flange-like structure is 10 mm. The rectangular hole  43  with a size of 5 mm*10 mm is formed on one side of the housing  4  at the height of 5 mm from the flange-like structure and used for allowing the lead wire to pass therethrough. 
     Referring to  FIG. 6 , the inner framework  5  includes the cylindrical head  51 , the vertical strain rod  52 , and the base  53 . The centers of the cylindrical head  51 , the vertical strain rod  52 , and the base  53  are linearly arranged in a straight line. The cylindrical head  51 , the vertical strain rod  52 , and the base  53  are made of metal materials. The cylindrical head has a diameter of 4.8 mm and a height of 2.5 mm. The strain rod has a square cross section with a side length of 1.5 mm and has a height of 30 mm. The strain gauges  6  of the strain rod are attached on the four sides of the strain rod and adjacent to the base. The base is a flat cylinder with a diameter of 24 mm and a height of 3 mm. The four circular threaded through holes  54  provided with internal threads with a standard size of M3 are uniformly distributed on the edge of the base. The distance from the center of the circular threaded through hole to the center of the base is 10 mm. 
     Referring to  FIG. 7 , the detection circuit  7  is in the shape of a disc, is made of a printed circuit board (PCB), and has a diameter of 12 mm. A square through hole is formed in the center of the detection circuit  7  and has a side length of 2 mm. Measurement circuits required by various sensing elements are arranged on the detection circuit  7 . 
     Referring to  FIG. 2 , the connecting bolt  8  has a head with a diameter of 6 mm and a shank provided with an external thread with a standard size of M3. The inner framework and the housing are fixedly connected by the connecting bolts. 
     Referring to  FIG. 2 , the filling liquid  9  is pure water, and is injected into the rubber fingertip by a needle syringe through the liquid injection hole  21 . The PVDF film  1  is closely attached to the rubber fingertip  2  by an adhesive. The circular textured protrusions on the surface of the rubber fingertip can still be sensed through the PVDF film. 
     Referring to  FIG. 2 , the cylindrical head  51  can be embedded in the cylindrical recess  22 . The cylindrical head  51  and the cylindrical recess  22  are fixedly connected by an adhesive. The bottom surface of the rubber fingertip  2  and the top surface of the housing  3  are also fixedly connected by the adhesive. 
     Referring to  FIG. 6 , the strain gauges  6  are fixed on the four sides of the vertical strain rod  52  by an adhesive, and are 5 mm away from the base. Two strain gauges on the opposite sides form a half-bridge circuit for measuring the force applied on the end of the strain rod. 
     Referring to  FIG. 8 , the front end of the sealing plug  3  is a rigid conduit  31  with a length of 10 mm and an outer diameter of 5 mm. The bottom piston  33  is a flexible rubber disc with a diameter of 20 mm and a thickness of 3 mm. The piston and the conduit are connected through a rubber transition. The hydraulic sensor  32  is installed at the connection between the bottom of the conduit and the bottom disc, and configured to convert the change of liquid pressure into a change of voltage to measure the change of liquid pressure caused by external pressure. The piston and the liquid injection hole are closely connected to prevent liquid from leakage. 
     The above-mentioned description is only the preferred embodiments of the present invention, and not intended to limit the present invention in any other form. Any modifications or equivalent replacements made based on the technical essence of the present invention shall fall within the scope of protection claimed in the present invention.