Bidirectional power converter of energy storage battery pack

Abstract

The invention discloses a bidirectional power converter of an energy storage battery pack, relates to the technical field of power converters, and aims to solve the problem that the temperature of the battery pack rises due to the fact that the energy storage battery pack generally comprises a large number of single batteries which may be overcharged, overdischarged, short-circuited and other abnormal conditions in the charging and discharging process. If a battery pack or a connecting line is short-circuited, and a switching device needs to bear higher voltage and current stress, so that the switching device is damaged during working, circuit detection is carried out in a pre-contact mode, and the working efficiency is improved. In a switching-on process, circuit pre-detection can be realized through cooperation of components such as a motor, an elastic sheet, a limiting clamping block, a metal contact sheet and a reset spring, whether the circuit is abnormal can be judged within 5 seconds, and automatic circuit breaking can be realized through structures such as the elastic sheet and a fuse when the circuit is abnormal, so that accidental switching-on in a fault stage is avoided; and meanwhile, the temperature detection accuracy of the fuse is improved, and safe and stable operation of the circuit is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of power converters, and in particular to a bidirectional power converter for an energy storage battery pack. Background Art

[0002] With the continuous growth of energy demand and the increasing emphasis on renewable energy, energy storage technology plays a vital role in modern energy systems. As an efficient energy storage method, energy storage battery packs are widely used in electric vehicles, smart grids, distributed energy systems and other fields. However, energy storage battery packs face many challenges during use, the most important of which is safety.

[0003] In this regard, publication number CN105453403B discloses a power converter, which is designed to detect a ground fault and / or a short circuit of the first terminal. The ground fault and / or short circuit detection circuit comprises: a coil having a winding, the winding being arranged adjacent to the power supply, wherein the winding does not surround the power supply; and an estimation unit, which is used to estimate the voltage induced in the winding in order to detect a ground fault and a short circuit.

[0004] Since energy storage battery packs usually contain a large number of battery cells, these cells may experience abnormal conditions such as overcharging, over-discharging, and short circuit during the charging and discharging process, which may cause the battery pack temperature to rise, the pressure to increase, and even cause serious consequences such as fire or explosion. Therefore, in the bidirectional converter of the energy storage system, if the battery pack or the connecting line is short-circuited, the switching device needs to withstand higher voltage and current stress, resulting in damage during operation.

[0005] In view of the above problems, a bidirectional power converter for an energy storage battery pack is proposed. Summary of the invention

[0006] The object of the present invention is to provide a bidirectional power converter for an energy storage battery pack, which solves the problem of damage to a switch module caused by circuit abnormality in the background technology.

[0007] To achieve the above object, the present invention provides the following technical solution: a bidirectional power converter for an energy storage battery pack, comprising a mounting plate, a shielding cover is mounted on the top surface of the mounting plate, a fixing frame is wrapped on the outer surface of the shielding cover, heat dissipation holes are opened on the surface of the shielding cover, and a PCB board is mounted on the surface of the mounting plate;

[0008] An isolation component is embedded in the interior of the shielding cover, the isolation component includes a partition plate, a pad is installed on the side of the partition plate, the top surface of the pad is connected to the isolation plate, the top of the partition plate is connected to a heat dissipation fin, the top of the isolation plate is connected to a contact component, the contact component includes a track groove, a sliding bar is embedded in the track groove, a limit card block is installed on the upper surface of the sliding bar, the limit card block is set in an arc shape, a wire is connected to the left side of the limit card block, a metal contact is set on the right side of the limit card block, a reset spring is embedded in the inner side of the metal contact, the top of the isolation plate is connected to a connecting wire, and the top surface of the isolation plate is provided with a connecting block;

[0009] An emergency component is fixedly connected to the middle position of the track groove, and the emergency component includes a support column, an active gear rod is embedded in the inner side of the support column, the outer surface of the active gear rod is wrapped with an elastic sheet, the elastic sheet is arranged in an arc shape, a limiting sleeve is fixedly connected to the inner wall of the elastic sheet, a fuse is embedded in the interior of the limiting sleeve, a heat collecting plate is fixedly connected to the limiting sleeve, the outer surface of the heat collecting plate is wrapped with a winding, and a driven tooth is also embedded in the inner side of the support column, and the driven tooth is meshed with the active gear rod.

[0010] Preferably, the mounting plate is located between two sets of shielding covers, the shielding covers cover the front and back sides of the mounting plate, PCB boards are mounted on the front and back sides of the mounting plate, and the shielding covers are used to wrap the PCB boards.

[0011] Preferably, the partition plate is arranged in the middle of the shielding cover, and the partition plate divides the interior of the shielding cover into two independent left and right spaces. The isolation component, the contact component and the emergency component are arranged in the right space of the shielding cover.

[0012] Preferably, the sliding bar passes through the interior of the track groove and maintains a sliding connection with the track groove, and the limiting block is embedded in a sliding groove opened on the inner side of the track groove and passes through it.

[0013] Preferably, the connecting wire is embedded in the interior of the connecting block, the connecting block is made of copper, and a groove is provided on the connecting block for guiding the metal contact to slide left and right. When the metal contact is engaged with the connecting block, the connecting wire, the connecting block, the metal contact and the wire are electrically connected.

[0014] Preferably, the active gear rod is rotationally connected to the support column, the rotation of the active gear rod is driven by a motor, a driven tooth is arranged directly below the active gear rod, the driven tooth is rotationally connected to the support column, and only half of the driven tooth is provided with a latching tooth.

[0015] Preferably, the elastic sheets are arranged in two groups, two groups of limiting sleeves are installed on the inner side of each group of elastic sheets, and the fuse passes through four groups of limiting sleeves in sequence to form a ring structure.

[0016] Preferably, the limiting sleeve is composed of a ceramic tube and a silver wire, wherein the ceramic tube is wrapped around the surface of the silver wire, and the ceramic tube is used to wrap and seal the silver wire.

[0017] Preferably, the heat-resistant temperature of the silver wire is 200 degrees Celsius.

[0018] Preferably, two limiting sleeves on the inner side of the same group of elastic sheets pull the elastic sheets to form a curved shape.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. A bidirectional power converter for an energy storage battery pack provided by the present invention adopts a pre-contact method for circuit detection by setting structures such as a mounting plate, a shielding cover, an isolation component, a contact component and an emergency component. During the closing process, the circuit can be pre-detected by the cooperation of components such as a motor, an elastic sheet, a limit card block, a metal contact sheet and a reset spring. It can be determined within 5 seconds whether the circuit is abnormal. In the event of an abnormality, the circuit can be automatically disconnected by structures such as an elastic sheet and a fuse to avoid accidental closing during the fault stage. At the same time, the accuracy of the fuse in temperature detection is improved, ensuring the safety and stable operation of the circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0022] Figure 2 It is a schematic diagram of the internal structure of the present invention;

[0023] Figure 3 It is a structural schematic diagram of a PCB board of the present invention;

[0024] Figure 4 It is a structural schematic diagram of the mounting plate and the PCB board of the present invention;

[0025] Figure 5 It is a schematic diagram of the structure of the partition plate and the pad of the present invention;

[0026] Figure 6 It is a structural schematic diagram of the isolation plate of the present invention;

[0027] Figure 7 It is a schematic diagram of the structure of the metal contact sheet and the connecting wire of the present invention;

[0028] Figure 8 It is a structural schematic diagram of the emergency assembly of the present invention;

[0029] Fig. 9It is a schematic diagram of the structure of the metal contact sheet and the connecting wire of the present invention in a connected state;

[0030] Fig.10 It is a schematic diagram of the structure of the winding and the active gear rod of the present invention;

[0031] Fig.11 It is a schematic diagram of the structure of the active gear rod and the driven gear of the present invention;

[0032] Fig.12 It is a schematic diagram of the structure of the limit sleeve and the fuse of the present invention.

[0033] In the figure: 11, mounting plate; 12, shielding cover; 13, fixing frame; 14, heat dissipation hole; 15, PCB board; 2, isolation component; 21, partition plate; 22, pad; 23, isolation plate; 24, heat dissipation fin; 25, connecting line; 3, contact component; 31, track groove; 32, sliding bar; 33, metal contact piece; 34, limit card block; 35, wire; 36, reset spring; 37, connecting block; 4, emergency component; 41, support column; 42, elastic sheet; 43, collector plate; 44, winding; 45, active gear rod; 46, limit sleeve; 47, fuse; 48, driven gear. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] In order to further understand the content of the present invention, the present invention is described in detail in conjunction with the accompanying drawings.

[0036] Combination Figure 1-Figure 12 A bidirectional power converter for an energy storage battery pack of the present invention comprises a mounting plate 11, a shielding cover 12 is mounted on the top surface of the mounting plate 11, a fixing frame 13 is wrapped on the outer surface of the shielding cover 12, a heat dissipation hole 14 is opened on the surface of the shielding cover 12, a PCB board 15 is mounted on the surface of the mounting plate 11, the mounting plate 11 is between two groups of shielding covers 12, the shielding cover 12 covers the front and back sides of the mounting plate 11, the PCB board 15 is mounted on the front and back sides of the mounting plate 11, the shielding cover 12 is used to wrap the PCB board 15, and the wrapping of the shielding cover 12 increases the protection of the electrical components on the PCB board 15 to avoid damage, and at the same time isolates part of the dust and moisture, delaying the aging of the components.

[0037] An isolation component 2 is embedded in the interior of the shielding cover 12. The isolation component 2 includes a partition plate 21. The partition plate 21 is arranged in the middle position of the shielding cover 12. The partition plate 21 divides the interior of the shielding cover 12 into two independent spaces on the left and right. The isolation component 2, the contact component 3 and the emergency component 4 are arranged in the right space of the shielding cover 12. By dividing the internal space, the various structures are isolated from each other. In this way, electromagnetic interference can be reduced during power conversion and the conversion efficiency can be improved. A pad 22 is installed on the side of the partition plate 21. The top surface of the pad 22 is connected to the isolation plate 23. The top of the partition plate 21 is connected to the heat dissipation fin 24. The top of the isolation plate 23 is connected to the contact component 3. The contact component 3 includes a track groove 31. A sliding bar 32 is embedded in the track groove 31. The sliding bar 32 runs through the inside of the track groove 31 and maintains a sliding connection with the track groove 31. The limit block 34 is embedded in the track groove 31. The metal contact 33 is inserted into the slide groove provided on the inner side of the track groove 31 and passes through it. The sliding bar 32 slides inside the track groove 31, driving the metal contact 33 and the connecting block 37 to contact each other, thereby realizing the closing operation. A limit block 34 is installed on the upper surface of the sliding bar 32. The limit block 34 is set in an arc shape. A wire 35 is connected to the left side of the limit block 34. A metal contact 33 is set on the right side of the limit block 34. A return spring 36 is embedded in the inner side of the metal contact 33. A connecting wire 25 is connected to the top of the isolation plate 23. A connecting block 37 is set on the top surface of the isolation plate 23. The connecting wire 25 is embedded in the inside of the connecting block 37. The connecting block 37 is made of copper. A groove for guiding the metal contact 33 to slide left and right is provided on the connecting block 37. When the metal contact 33 is engaged with the connecting block 37, the connecting wire 25, the connecting block 37, the metal contact 33 and the wire 35 are electrically connected.

[0038] An emergency component 4 is fixedly connected to the middle position of the track groove 31, and the emergency component 4 includes a support column 41, an active gear rod 45 is embedded in the inner side of the support column 41, and an elastic sheet 42 is wrapped on the outer surface of the active gear rod 45. The elastic sheet 42 is arranged in an arc shape, and two limiting sleeves 46 on the inner side of the same group of elastic sheets 42 pull the elastic sheet 42 to form a curved shape, which can be better wrapped on the limiting block 34 through the curved state, thereby limiting the position of the limiting block 34, and the inner wall of the elastic sheet 42 is fixedly connected to the limiting sleeve 46, and the elastic sheet 42 is arranged in two groups, and two groups of limiting sleeves 46 are installed on the inner side of each group of elastic sheets 42, and the fuse 47 passes through the four groups of limiting sleeves 46 in sequence to form a ring structure, and the limiting sleeve 46 is composed of a ceramic tube and a silver wire, wherein the ceramic tube is wrapped on the surface of the silver wire, and the ceramic tube is used to wrap and seal the silver wire, and the heat-resistant temperature of the silver wire is 200°C. Celsius, a fuse 47 is embedded in the limiting sleeve 46, a heat collecting plate 43 is fixedly connected to the limiting sleeve 46, a winding 44 is wrapped on the outer surface of the heat collecting plate 43, a driven tooth 48 is also embedded in the inner side of the support column 41, the active gear rod 45 is rotatably connected to the support column 41, the rotation of the active gear rod 45 is driven by a motor, a driven tooth 48 is arranged directly below the active gear rod 45, the driven tooth 48 is rotatably connected to the support column 41, only half of the driven tooth 48 is provided with a latching tooth, the driven tooth 48 is meshed with the active gear rod 45, wherein the half-tooth arrangement of the driven tooth 48 can realize the separation between the driven tooth 48 and the active gear rod 45, after the separation, the activity of the elastic sheet 42 will not affect the driven tooth 48, wherein the active gear rod 45 is connected to the self-test circuit, and the self-test circuit is mainly composed of a power supply, a control chip, a detection element, an actuator and an indication element;

[0039] The power supply provides power to the entire circuit. The detection elements include a voltmeter and an ammeter for monitoring the circuit status. The control chip includes a timer chip. By setting the resistance and capacitance parameters, the capacitor starts to charge after the circuit is powered on. When the charging time reaches 5 seconds, the output state of the trigger chip changes. When the count reaches the time value corresponding to 5 seconds, the control signal is output. The executive element includes a relay, which acts after receiving the signal from the control chip to disconnect the circuit. The indicator element includes a light-emitting diode, which can be used to display the self-test results. In this way, when the self-test circuit is working, the control chip accurately times the time, and controls the executive element to disconnect the circuit after 5 seconds, thereby ensuring that the self-test is completed within the specified time and the circuit is disconnected in time when necessary, to ensure the safety and stable operation of the circuit.

[0040] When the power converter is used, the sliding bar 32 first slides inside the track groove 31, so that the sliding bar 32 drives the metal contact 33, and compresses the return spring 36 during the sliding process. When the metal contact 33 moves and contacts the connecting block 37, the elastic sheet 42 is engaged with the upper surface of the limit block 34 to limit the position of the limit block 34, thereby limiting the position of the metal contact 33. When the metal contact 33 contacts the connecting block 37, the connecting wire 25 is inserted into the connecting block 37. At this time, the connecting wire 25, the connecting block 37, the metal contact 33 and the wire 35 are connected, and the current is then transmitted to the PCB board 15 through the wire 35, and then transmitted to the connecting wire 25, and finally transmitted out again through the PCB board 15 to realize power conversion.

[0041] For bidirectional converters, there are buck mode and boost mode. In buck mode, by controlling the on and off of the switch tube, the high input DC voltage is reduced to a lower output voltage to achieve forward power conversion. The boost mode is the opposite, raising the low input DC voltage to a higher output voltage to complete the reverse power conversion.

[0042] In the conversion stage, in order to avoid abnormalities in the circuit, an isolation component 2, a contact component 3 and an emergency component 4 are set to avoid damage to the energy storage battery caused by abnormalities. The specific operations are as follows:

[0043] During the power conversion stage, the most obvious abnormal change is the temperature rise. Therefore, after connection, the circuit detection is carried out by adopting the pre-contact method. First, when the metal contact 33 and the connecting block 37 are in contact, the current can be transmitted to the PCB board 15 through the connecting line 25 through the connecting block 37, the metal contact 33, and the wire 35. When the switch is closed for the first time, the circuit of the motor is first connected. At this time, the motor starts to work. The motor controls the driven gear 48 to rotate. When the driven gear 48 rotates, it will drive the active gear rod 45 to rotate. When the active gear rod 45 rotates, it will drive the elastic sheet 42 to rotate. When the elastic sheet 42 rotates until it covers the limit block 34, the position of the limit block 34 is limited. At this time, the metal contact 33 and the connecting block 37 are locked together. At this time, the whole circuit is connected. The motor starts to rotate in the reverse direction when all the circuits are connected. When the motor rotates in the reverse direction, it drives the elastic sheet 42 to gradually move away from the metal contact sheet 33. As the elastic sheet 42 and the metal contact sheet 33 are completely separated, the metal contact sheet 33 loses the constraint of the elastic sheet 42. At this time, the reset spring 36 is reset, and after the reset, the metal contact sheet 33 and the connecting block 37 are pushed to separate. At this time, the circuit is in an open circuit state, thereby realizing the pre-detection of the circuit. In this stage, the reverse rotation of the motor drives the elastic sheet 42 until it separates from the metal contact sheet 33 in 5 seconds. In this connection process, the circuit is detected by an ammeter and a voltmeter to determine whether the circuit is abnormal. When the circuit is normal, the circuit is connected again. At this time, the motor stops working when controlling the elastic sheet 42 to engage with the metal contact sheet 33.

[0044] If an abnormality occurs, there are two situations, one is manual intervention, and the other is device judgment. Specifically, when the circuit is closed for the first time, the circuit abnormality is detected within 5 seconds. At this time, the staff will re-inspect the circuit. This is manual intervention. When the circuit is closed for the first time, the circuit abnormality causes the temperature to rise rapidly. At this time, the elastic sheet 42 will quickly straighten from the original arc state. At this time, the elastic sheet 42 will no longer have the function of limiting the metal contact 33, so that the metal contact 33 is forced to separate from the connecting block 37. This is device judgment;

[0045] The changes of the elastic sheet 42 are as follows:

[0046] When the circuit is closed for the first time, the abnormal temperature of the circuit rises rapidly. When the temperature rises above 200 degrees Celsius, the fuse 47 will be melted. After the fuse 47 is melted, the two ends of the elastic sheet 42 lose the traction of the fuse 47 and are directly straightened from being bent. At this time, the elastic sheet 42 can no longer limit the position of the metal contact 33. The metal contact 33 is separated from the connecting block 37 under the action of the reset spring 36. In the subsequent closing, the state of the elastic sheet 42 has changed, and the position of the metal contact 33 can no longer be restricted. At this time, the subsequent closing cannot be completed. In this way, the safety of operation is increased, and the accidental closing during the fault stage can be avoided. Through early intervention, the loss is reduced.

[0047] When no fault occurs when the switch is closed and the temperature remains normal, the fuse 47 will not break. At this time, the elastic sheet 42 can still ensure the limitation of the metal contact 33. In subsequent work, if an abnormal temperature rise occurs, the fuse 47 can be broken to achieve circuit breaking. In order to facilitate accurate control of the temperature, the heat collecting plate 43 and the winding 44 are provided to absorb heat, so that the heat can be accumulated on the heat collecting plate 43 and the winding 44. Because the fuse 47, the heat collecting plate 43 and the winding 44 are tightly connected together, the temperature changes of the heat collecting plate 43 and the winding 44 can be fed back to the fuse 47 in time, thereby improving the accuracy of the temperature detection of the fuse 47 and improving the sensitivity of the automatic circuit breaking due to abnormal faults.

[0048] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0049] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A bidirectional power converter for an energy storage battery pack, comprising a mounting plate (11), characterized in that: A shielding cover (12) is installed on the top surface of the mounting plate (11), a fixing frame (13) is wrapped around the outer surface of the shielding cover (12), a heat dissipation hole (14) is opened on the surface of the shielding cover (12), and a PCB board (15) is installed on the surface of the mounting plate (11); An isolation component (2) is embedded inside the shielding cover (12), the isolation component (2) comprises a partition plate (21), a backing plate (22) is installed on the side of the partition plate (21), the top surface of the backing plate (22) is connected to an isolation plate (23), the top end of the partition plate (21) is connected to a heat dissipation fin (24), the top end of the isolation plate (23) is connected to a contact component (3), the contact component (3) comprises a track groove (31), a sliding member is embedded inside the track groove (31), A limit block (34) is installed on the upper surface of the sliding bar (32), the limit block (34) is arranged in an arc shape, a wire (35) is connected to the left side of the limit block (34), a metal contact (33) is arranged on the right side of the limit block (34), a return spring (36) is embedded in the inner side of the metal contact (33), the top end of the isolation plate (23) is connected to a connecting wire (25), and a connecting block (37) is arranged on the top surface of the isolation plate (23); An emergency component (4) is fixedly connected to the middle position of the track groove (31), and the emergency component (4) comprises a support column (41), an active gear rod (45) is embedded in the inner side of the support column (41), and the outer surface of the active gear rod (45) is wrapped with an elastic sheet (42), and the elastic sheet (42) is arranged in an arc shape. The inner wall of the elastic sheet (42) is fixedly connected to a limiting sleeve (46), and a fuse (47) is embedded in the inner side of the limiting sleeve (46). A heat collecting plate (43) is fixedly connected to the limiting sleeve (46), and the outer surface of the heat collecting plate (43) is wrapped with a winding (44). A driven tooth (48) is also embedded in the inner side of the support column (41), and the driven tooth (48) is meshed with the active gear rod (45).

2. The energy storage battery bidirectional power converter according to claim 1, characterized in that: The mounting plate (11) is located between two sets of shielding covers (12), the shielding covers (12) cover the front and back sides of the mounting plate (11), a PCB board (15) is installed on the front and back sides of the mounting plate (11), and the shielding covers (12) are used to wrap the PCB board (15).

3. The energy storage battery bidirectional power converter according to claim 1, characterized in that: The partition plate (21) is arranged in the middle of the shielding cover (12), and the partition plate (21) divides the interior of the shielding cover (12) into two independent spaces on the left and right. The isolation component (2), the contact component (3) and the emergency component (4) are arranged in the space on the right side of the shielding cover (12).

4. The energy storage battery bidirectional power converter according to claim 1, characterized in that: The sliding bar (32) passes through the interior of the track groove (31) and maintains a sliding connection with the track groove (31), and the limiting block (34) is embedded in a sliding groove opened on the inner side of the track groove (31) and passes through it.

5. The energy storage battery bidirectional power converter according to claim 1, characterized in that: The connecting wire (25) is embedded in the interior of the connecting block (37); the connecting block (37) is made of copper; a groove for guiding the metal contact piece (33) to slide left and right is provided on the connecting block (37); when the metal contact piece (33) is engaged with the connecting block (37), the connecting wire (25), the connecting block 37, the metal contact piece (33) and the wire (35) are electrically connected.

6. The energy storage battery bidirectional power converter according to claim 1, characterized in that: The active gear rod (45) is rotatably connected to the support column (41), and the rotation of the active gear rod (45) is driven by a motor. A driven tooth (48) is arranged directly below the active gear rod (45), and the driven tooth (48) is rotatably connected to the support column (41). Only half of the driven tooth (48) is provided with a latching tooth.

7. The energy storage battery bidirectional power converter according to claim 1, characterized in that: The elastic sheets (42) are arranged in two groups, and two groups of limiting sleeves (46) are installed on the inner side of each group of the elastic sheets (42). The fuse (47) passes through the four groups of limiting sleeves (46) in sequence to form a ring structure.

8. The energy storage battery bidirectional power converter according to claim 1, characterized in that: The limiting sleeve (46) is composed of a ceramic tube and a silver wire, wherein the ceramic tube is wrapped around the surface of the silver wire, and the ceramic tube is used to wrap and seal the silver wire.

9. The energy storage battery bidirectional power converter according to claim 8, characterized in that: The heat-resistant temperature of the silver wire is 200 degrees Celsius.

10. The energy storage battery bidirectional power converter according to claim 7, characterized in that: Two limiting sleeves (46) on the inner side of the same group of elastic sheets (42) pull the elastic sheets (42) to form a curved shape.

Images