Self-sensing SMA bionic hand mechanism with temperature and position feedback

Abstract

The application discloses a kind of temperature, location feedback self-sensing SMA bionic hand mechanical mechanism, including arm docking mechanism, the first pipe connection fixing member is clamped and positioned at one end position of the arm docking mechanism, the second pipe connection fixing member is clamped and positioned at the other end position of the first pipe connection fixing member, SMA wire fixing frame is clamped and positioned between the first pipe connection fixing member and the second pipe connection fixing member.The application discloses a kind of temperature, location feedback self-sensing SMA bionic hand mechanical mechanism, the characteristics of SMA wire heat drive are exerted, without installing position feedback sensor, realize wire material shrinkage control finger coordinated movement, realize the action of gripping various material articles, and the material quality of the article is grasped according to the self-adjusting of pinch force, without additional auxiliary sensor, the feeling of grasping and the realization of grasping position control can be obtained by the feedback temperature of wire material for user, beneficial to action control.

Description

Technical Field

[0001] This invention relates to the field of bionic hands, and particularly to a bionic hand mechanical mechanism with temperature and position feedback self-sensing SMA. Background Technology

[0002] A bionic hand mechanical mechanism is a support device that simulates the hand. With the development of medical and other technologies, some disabled people with amputated arms can assemble mechanical hands to replace human hands for operation. The development of mechanical hands is also becoming more and more rapid. With the continuous development of technology, people's requirements for the manufacturing process of bionic hand mechanical mechanisms are also getting higher and higher.

[0003] Existing bionic hand mechanisms have certain drawbacks in use. Most traditional prostheses only have forward control functions, requiring extensive training and adaptation over a long period. Adding sensory functions to the prosthesis, mimicking native human senses such as touch and temperature, provides users with realistic perception, allowing them to quickly adapt to the prosthesis's actuation characteristics and improve control stability. For example, adding force feedback significantly improves the success rate and efficiency of grasping compared to when it's not force feedback. Bandage-type pressure feedback devices, for instance, provide feedback to the user on the tension and relaxation of the bandage to improve grasping success. However, these feedback devices also suffer from drawbacks such as large size and weight, making installation and wearing difficult and negatively impacting user experience. Therefore, we propose a self-sensing SMA bionic hand mechanism with temperature and position feedback. This mechanism eliminates the need for additional sensors, feeding back temperature changes during the actuation process to the user's skin and position information to the control system, achieving simple and efficient bionic hand movements. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a bionic hand mechanism using SMA (Silk Magnetite) with temperature and position feedback self-sensing. Leveraging the thermal actuation characteristics of SMA silk, it achieves coordinated finger movement by controlling silk contraction without the need for position feedback sensors. This enables the grasping of objects of various materials, with the pinching force automatically adjusting based on the material of the grasped object. No additional auxiliary sensors are required; the user receives the gripping sensation and position control solely through the temperature feedback from the silk, facilitating motion control and effectively solving the problems in the prior art.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a bionic SMA hand mechanism with temperature and position feedback self-sensing, including an arm docking mechanism. A first tube connecting fastener is engaged and positioned at one end of the arm docking mechanism, and a second tube connecting fastener is engaged and positioned at the other end of the first tube connecting fastener. An SMA wire fastener is engaged and positioned between the first and second tube connecting fasteners. A finger seat is engaged and positioned on the outer side of the first tube connecting fastener, connecting to the fingertip. A fisheye connection structure connects the finger seat and the fingertip. A heat-conducting material mounting hole is provided on the inner side of the end of the SMA wire fastener near the second tube connecting fastener. A through groove is provided inside the SMA wire fastener, and SMA1 and SMA2 wires are arranged inside the through groove. The fisheye connection structure has two types of movable drive configurations.

[0006] As a preferred technical solution of this application, the SMA1 filament is made of a material with shape memory effect, the heat of the filament is fed back to the user's skin, and the change in the resistance of the filament is fed back to the control system as position feedback. The SMA2 filament is made of a material with superelastic effect.

[0007] As a preferred technical solution of this application, a PCB board docking mechanism is fixed at the rear end of the second tube connecting fastener. A locking screw is positioned on the inner side of the second tube connecting fastener and the PCB board docking mechanism. The locking screw is made of copper and serves as both a drive output connection and a position information feedback connection. The locking screw tightens the positions of the SMA1 wire and the SMA2 wire.

[0008] As a preferred technical solution of this application, in the first structure of the active drive of the fisheye connection structure, the upper end of the fisheye connection structure is connected to the position of the SMA1 wire, the lower end of the fisheye connection structure is connected to the position of the SMA2 wire, the SMA1 wire drives the fisheye connection structure to straighten, and the SMA2 wire drives the fisheye connection structure to bend.

[0009] As a preferred technical solution of this application, in the second structure of the active drive of the fisheye connection structure, a torque spring is provided in the middle of the fisheye connection structure, and an SMA2 wire is connected to the bottom of the fisheye connection structure. The SMA2 wire drives the fisheye connection structure to bend, and the torque spring drives the fisheye connection structure to straighten.

[0010] As a preferred technical solution of this application, the finger seat, the first tube connecting fastener, and the SMA wire fixing bracket are positioned by a snap-fit ​​and can be disassembled. The through groove is provided in four sets, and the SMA1 wire and SMA2 wire are respectively provided with a meandering structure inside two sets of through grooves. The fisheye connection structure is an inner and outer circular structure.

[0011] As a preferred technical solution of this application, the resistance change of the SMA1 wire is related to the length change of the wire, and the rotation of the drive mechanism is directly related to the SMA1 wire, so that the resistance change of the wire is related to the position.

[0012] As a preferred technical solution of this application, the drive system of the SMA1 wire connection controls the shape memory effect caused by the heating phase change of the wire to drive the mechanism, and the resistance change during the process can reflect the action position of the mechanism. The drive system can detect the resistance value to know the actual execution position and realize self-sensing position perception control.

[0013] Compared with existing technologies, this invention provides a bionic hand mechanism with temperature and position feedback self-sensing SMA fibers, which has the following advantages: This bionic hand mechanism with temperature and position feedback self-sensing SMA fibers leverages the thermal actuation characteristics of SMA fibers. It achieves coordinated finger movement by controlling the contraction of the fibers without the need for a position feedback sensor, enabling the grasping of objects of various materials. The pinching force is self-adjusted according to the material of the grasped object. No additional auxiliary sensors are required; the user can obtain a grasping sensation and achieve grasping position control through the feedback temperature of the fibers, facilitating motion control. This is achieved through the thermal actuation process of the SMA fibers. The shrinkage of the filament generates heat, driving thermal characteristics and resistance changes, enabling a self-feedback system for the driving process. This improves the system's control stability and demonstrates the bionic advantages of self-sensing. The resistance-position feedback self-sensing SMA bionic hand mechanism eliminates the need for additional sensors, feeding position information back to the control system. This simple and efficient mechanism enables the bionic hand to move. The control system uses EEG signals to provide control that more closely approximates actual needs, integrating force and thermal feedback information to achieve a closed-loop control system. The design has been optimized for appearance and ergonomics. The entire bionic hand mechanism is simple in structure, easy to operate, and performs better than traditional methods. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the overall structure of a bionic SMA hand mechanism with temperature and position feedback self-sensing according to the present invention.

[0015] Figure 2 This is a schematic diagram of the SMA wire fixing frame in a self-sensing SMA bionic hand mechanical mechanism with temperature and position feedback according to the present invention.

[0016] Figure 3 This is a schematic diagram of the structure of the SMA wire fixing frame passing through two sets of SMA wires in a self-sensing SMA bionic hand mechanical mechanism with temperature and position feedback according to the present invention.

[0017] Figure 4This is a schematic diagram of the first driving method of the fisheye connection structure in the SMA bionic hand mechanical mechanism with temperature and position feedback self-sensing according to the present invention.

[0018] Figure 5 This is a schematic diagram of the second driving method of the fisheye connection structure in the SMA bionic hand mechanical mechanism with temperature and position feedback self-sensing according to the present invention.

[0019] Figure 6 This is a schematic diagram of the second tube connection and fixing component in a self-sensing SMA bionic hand mechanical mechanism with temperature and position feedback according to the present invention.

[0020] Figure 7 This is a schematic diagram of the principle of a bionic SMA hand mechanical mechanism with temperature and position feedback self-sensing according to the present invention.

[0021] Figure 8 This is a schematic diagram illustrating the current-temperature change relationship in the driving process of a self-sensing SMA bionic hand mechanical mechanism with temperature and position feedback according to the present invention.

[0022] Figure 9 This is a schematic diagram illustrating the current-force relationship in the driving process of a self-sensing SMA bionic hand mechanical mechanism with temperature and position feedback according to the present invention.

[0023] Figure 10 This is a schematic diagram illustrating the strain (position)-resistance change relationship of the filament during the driving process in a self-sensing SMA bionic hand mechanical mechanism with temperature and position feedback according to the present invention.

[0024] In the diagram: 1. Arm docking mechanism; 2. First tube connection fastener; 3. Finger seat; 4. Fisheye connection structure; 5. Fingertip; 6. SMA wire fixing bracket; 7. Heat-conducting material mounting hole; 8. Second tube connection fastener; 9. PCB board docking mechanism; 10. Locking screw; 11. SMA1 wire; 12. SMA2 wire or spring; 13. Torque spring; 14. Through slot. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0026] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0027] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances. Example

[0028] like Figure 1-10As shown, a bionic SMA hand mechanism with temperature and position feedback self-sensing includes an arm docking mechanism 1. A first tube connection fastener 2 is engaged and positioned at one end of the arm docking mechanism 1, and a second tube connection fastener 8 is engaged and positioned at the other end of the first tube connection fastener 2. An SMA wire fastener 6 is engaged and positioned between the first tube connection fastener 2 and the second tube connection fastener 8. A finger seat 3 is engaged and positioned on the outer side of the first tube connection fastener 2, connecting to a fingertip 5. A fisheye connection structure 4 connects the finger seat 3 and the fingertip 5. A heat-conducting material mounting hole 7 is provided on the inner side of the end of the SMA wire fastener 6 near the second tube connection fastener 8. A through groove 14 is provided inside the SMA wire fastener 6, and a... There are SMA1 wire 11 and SMA2 wire or spring 12. The fisheye connection structure 4 has two types of active drive settings. It has five sets of finger seats 3 and fingertips 5. All five sets of fingertips 5 can bend and straighten. The thumb bends upward and forms an enclosing structure with the other fingers. Each set of fingertips 5 is connected by a rigid SMA wire fixing frame 6. Each SMA wire fixing frame 6 has four holes. Two of the holes pass through the SMA1 wire 11 and are not energized, but only serve as springs. The other two holes pass through the SMA2 wire or spring 12 and can be energized to retract. Normally, the fingertips 5 are straightened by the action of the SMA1 wire 11. When gripping, the SMA2 wire or spring 12 moves and the fingertips 5 bend. The entire drive mechanism is installed in a cavity with a hand docking shape. The shape of the cavity is customized according to the shape of the user's arm.

[0029] Furthermore, the SMA1 wire 11 is made of a material with shape memory effect. The heat from the wire is fed back to the user's skin, and the change in the wire's resistance serves as position feedback to the control system. The SMA2 wire or spring 12 is made of a material with hyperelastic effect or a material with the elastic recovery effect of a spring. The SMA1 wire 11, SMA2 wire, or spring 12 adopt a circuitous structure, with pulleys at the circuitous points to conduct force with the rigid wire. The rigid wire has the characteristic of not deforming under tension, which facilitates the electrical control of the SMA2 wire or spring 12. In this way, the control end can be located at the far end away from the hand. By adjusting the structure, the SMA1 wire 11 can operate in a hyperelastic state, with the hyperelastic range being 7-8% of the total length, acting as an elastic element. By adjusting the structure, the SMA2 wire or spring 12 can operate in a shape memory state. The shape memory effect can be controlled by controlling the heating state with current, thereby controlling the stroke.

[0030] Furthermore, a PCB board docking mechanism 9 is fixed at the rear end of the second pipe connecting fastener 8. A locking screw 10 is positioned on the inner side of the second pipe connecting fastener 8 and the PCB board docking mechanism 9. The locking screw 10 is made of copper, which facilitates the connection of drive and feedback signals. It can reliably contact the copper in the corresponding locking hole on the PCB and form an electrical circuit with the circuit on the PCB, which facilitates signal control and detection.

[0031] Furthermore, the active drive setting of the fisheye connection structure 4 has two structures. In the first structure, the upper end of the fisheye connection structure 4 is connected to the position of SMA1 wire 11, and the lower end of the fisheye connection structure 4 is connected to the position of SMA2 wire or spring 12. SMA1 wire 11 drives the fisheye connection structure 4 to straighten, and SMA2 wire or spring 12 drives the fisheye connection structure 4 to bend. In the second structure, a torque spring 13 is set in the middle of the fisheye connection structure 4, and SMA2 wire or spring 12 is connected to the bottom of the fisheye connection structure 4. SMA2 wire or spring 12 drives the fisheye connection structure 4 to bend, and torque spring 13 drives the fisheye connection structure 4 to straighten.

[0032] Furthermore, the finger seat 3, the first tube connecting fastener 2, and the SMA wire fixing bracket 6 are positioned by a snap-fit ​​mechanism and can be disassembled. The through groove 14 is provided with four sets, and the SMA1 wire 11, SMA2 wire, or spring 12 are respectively provided with a meandering structure inside two sets of through grooves 14. The fisheye connection structure 4 is an inner and outer circular structure. Example

[0033] like Figure 1-10 As shown, a bionic SMA hand mechanism with temperature and position feedback self-sensing includes an arm docking mechanism 1. A first tube connecting fastener 2 is engaged and positioned at one end of the arm docking mechanism 1, and a second tube connecting fastener 8 is engaged and positioned at the other end of the first tube connecting fastener 2. An SMA wire fixing bracket 6 is engaged and positioned between the first tube connecting fastener 2 and the second tube connecting fastener 8. A finger seat 3 is engaged and positioned on the outer side of the first tube connecting fastener 2. The finger seat 3 is connected to the fingertip 5, and a fisheye connection structure 4 connects the finger seat 3 and the fingertip 5. A heat-conducting material mounting hole 7 is opened on the inner side of the end of the SMA wire fixing bracket 6 near the second tube connecting fastener 8. A through groove 14 is opened in the inner position of the SMA wire fixing bracket 6, and an SMA1 wire 11 and an SMA2 wire or a spring 12 are arranged in the inner position of the through groove 14. The movement drive of the fisheye connection structure 4 has two structures.

[0034] like Figure 7-10 As shown, where Figure 7 This is a schematic diagram of the entire process. Figure 8 For example, consider the current-temperature relationship during the driving process. Figure 9 For example, the current-force relationship during the driving process. Figure 10 Taking the strain (position)-resistance change relationship of the wire in the driving process as an example, the resistance change of SMA1 wire 11 is related to the length change of the wire. Since the rotation of the driving mechanism is directly related to SMA1 wire 11, the resistance change of the wire is related to the position. Therefore, the driving system controls the wire to heat up and undergo phase change to induce shape memory effect to drive the mechanism. The resistance change during the process can reflect the action position of the mechanism. The driving system can detect the resistance value to know the actual execution position and realize self-sensing position perception control.

[0035] Working principle: This invention includes an arm docking mechanism 1, a first tube connection fixing component 2, finger seats 3, a fisheye connection structure 4, fingertips 5, an SMA wire fixing bracket 6, heat-conducting material mounting holes 7, a second tube connection fixing component 8, a PCB board docking mechanism 9, locking screws 10, SMA1 wire 11, SMA2 wire or spring 12, a torque spring 13, and a through groove 14. In use, five sets of finger seats 3 and fingertips 5 are provided. All five sets of fingertips 5 can bend and straighten, with the thumb bending upwards to form an enclosing structure with the other fingers. Each set of fingertips 5 is connected to a rigid SMA wire fixing bracket 6. Each SMA wire fixing bracket 6 has four holes, two of which pass through the SMA1 wire 11 and are not electrically connected; they only serve to return the fingertips to the base. The other two holes, through which the SMA2 wire or spring 12 passes, allow for electrical contraction. Normally, the fingertip 5 is extended under the influence of the SMA1 wire 11. When gripping, the SMA2 wire or spring 12 activates, causing the fingertip 5 to bend. The entire drive mechanism is housed in a cavity with a hand-like shape, customized to fit the user's arm. The SMA1 wire 11, SMA2 wire, or spring 12 employ a circuitous structure, with pulleys at the circuitous points cooperating with the rigid wire to transmit force. The rigid wire is designed to resist deformation under tension, facilitating electrical control of the SMA2 wire or spring 12. This allows the control end to be located at a distance from the hand, enabling the SMA1 wire 11 to operate in a hyperelastic state, with a hyperelastic range of 7-8% of its total length. It acts as an elastic element, controlling the SMA2 wire or spring 12 to operate in a shape memory state. The shape memory effect can be controlled by adjusting the heating state with current, thus controlling the stroke. The finger joint connects the fingertip 5 and finger seat 3 via the fisheye connection structure 4. The finger joint is controlled by the pulling of the SMA1 wire 11, SMA2 wire, or spring 12, controlling the rotation of the fingertip 5 to form a gripping action. The fisheye connection structure 4 is fixed to the finger seat 3 by an axle and convex mechanism, and mates with the fingertip 5 through a concave-convex mechanism. The rotating locking device tightens and fixes the SMA1 wire 11 and SMA2 wire or spring 12. After the finger part is properly aligned, the tightness of the SMA1 wire 11, SMA2 wire, or spring 12 can be adjusted. The locking screw 10 is made of copper. The copper in the corresponding locking holes on the PCB can reliably contact and form an electrical circuit with the circuitry on the PCB, facilitating signal control and detection. The SMA wire holder 6 has mounting holes for thermally conductive material near its end on its inner side, and an insulating thermally conductive rubber ring is installed inside. This allows the small amount of heat generated when the SMA wire contracts during power-on to be fed back to the skin in contact. The amount of heat generated is related to the tightness of the wire, reflecting the current force applied by the finger and providing feedback to the user on the actual force exerted. The resistance change of SMA1 wire 11 is directly related to the change in wire length. Since the rotation of the drive mechanism is directly connected to SMA1 wire 11, the resistance change of this wire is related to its position. Therefore, the drive system controls the shape memory effect caused by the phase change of the wire's heating.The change in resistance during the process reflects the movement position of the mechanism. By detecting this resistance value, the drive system can determine the actual execution position, thus achieving self-sensing position perception control.

[0036] It should be noted that, in this document, relational terms such as first and second (number one, number two), etc., are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0037] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A temperature, position feedback self-sensing SMA bionic hand mechanism, comprising an arm docking mechanism (1), characterized in that: The arm docking mechanism (1) is provided with a first pipe connecting fixing part (2) at one end, a second pipe connecting fixing part (8) at the other end, an SMA wire fixing frame (6) between the first pipe connecting fixing part (2) and the second pipe connecting fixing part (8), a finger seat (3) outside the first pipe connecting fixing part (2), a fish-eye connecting structure (4) between the finger seat (3) and the fingertip (5), a heat-conducting material mounting hole (7) inside the end of the SMA wire fixing frame (6) close to the second pipe connecting fixing part (8), a through slot (14) inside the SMA wire fixing frame (6), and SMA1 wire (11) and SMA2 wire (12) inside the through slot (14). The SMA1 wire (11) and the SMA2 wire (12) are respectively provided with a detour structure inside the two groups of through slots (14), and the detour place is provided with a pulley to conduct force with a hard wire. The fish-eye connecting structure (4) is provided with two structures for driving, and is used for connecting the finger seat (3) and the fingertip (5), converting the expansion and contraction movement of the SMA1 wire (11) and the SMA2 wire (12) into the rotary movement of the fingertip (5), so as to realize the bending and straightening of the fingers. The SMA1 wire (11) is made of a material with shape memory effect, and the wire heating feedback is given to the skin of the user. The resistance change of the wire is used as position feedback to the control system. The SMA2 wire (12) is made of a material with super elastic effect. The resistance change of the SMA1 wire (11) has a certain relationship with the length change of the wire, and the driving mechanism rotation is directly related to the SMA1 wire (11). The resistance change of the wire has a certain relationship with the position. The driving system connected with the SMA1 wire (11) controls the shape memory effect driving mechanism caused by the wire heating phase change, and the resistance change in the process can reflect the action position of the mechanism. The actual execution position can be known by detecting the resistance value of the driving system, so as to realize the self-sensing position sensing control.

2. A temperature, position feedback self-sensing SMA bionic hand mechanism according to claim 1, characterized in that: The rear end of the second pipe connecting fixing part (8) is fixed with a PCB board docking mechanism (9), the inner side of the second pipe connecting fixing part (8) and the PCB board docking mechanism (9) is provided with a locking screw (10), and the locking screw (10) is made of copper material as driving output connection and position information feedback connection. The locking screw (10) tightens the positions of the SMA1 wire (11) and the SMA2 wire (12).

3. A temperature, position feedback self-sensing SMA bionic hand mechanical mechanism according to claim 1, characterized in that: In the first structure of the fish-eye connecting structure (4), the upper end of the fish-eye connecting structure (4) is connected with the SMA1 wire (11), the lower end of the fish-eye connecting structure (4) is connected with the SMA2 wire (12), the SMA1 wire (11) drives the fish-eye connecting structure (4) to straighten, and the SMA2 wire (12) drives the fish-eye connecting structure (4) to bend.

4. A temperature, position feedback self-sensing SMA bionic hand mechanical mechanism according to claim 1, characterized in that: In the second structure of the movable driving of the fish-eye connecting structure (4), the middle part of the fish-eye connecting structure (4) is provided with a torque spring (13), the bottom of the fish-eye connecting structure (4) is connected with an SMA2 wire (12), the SMA2 wire (12) drives the fish-eye connecting structure (4) to bend, and the torque spring (13) drives the fish-eye connecting structure (4) to straighten.

5. A temperature, position feedback self-sensing SMA bionic hand mechanical mechanism according to claim 1, characterized in that: The finger seat (3), the first pipe connecting fixing part (2) and the SMA wire fixing frame (6) are positioned through buckling and can be disassembled, four groups of through grooves (14) are arranged, and the fish-eye connecting structure (4) is an inner-outer circular structure.

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