Friction-based tactile sensors for measuring clamping safety

Abstract

A system for assessing clamping safety, the system comprising: a contact surface having at least a first contact surface region and a second contact surface region, the first contact surface region being configured to resist slippage less than the second contact surface region; and a sensor for detecting slippage in the first contact surface region.

Description

[0001] This invention application is a divisional application of Chinese Patent Application No. 201880059924.5, filed on August 14, 2018 (PCT application No.: PCT / AU2018 / 050859, priority: AU2017903239, priority date: August 14, 2017; priority: AU2018901816, priority date: May 24, 2018), entitled "A Friction-Based Tactile Sensor for Measuring Clamping Safety". Technical Field

[0002] This disclosure relates to apparatus and methods for measuring clamping safety, and to apparatus and methods for improving clamping safety. Background Technology

[0003] Holding and lifting objects using robotic grippers is a challenging task. Robotic grippers lack the ability of a human hand to detect valuable information about objects and contact interfaces. In most cases, grippers offer no tactile feedback. Those grippers that do provide tactile feedback typically measure a characteristic, such as gripping force, or detect slippage of the object relative to the gripping point. Despite continuous advancements in robotic and prosthetic gripper designs attempting to mimic the dexterity of the human hand, comparable performance remains elusive.

[0004] Tactile sensing is a hot field and aims to fill this gap; however, most existing tactile sensors focus on determining the normal and tangential forces at the interface. While these quantities are important, other tactile parameters are also crucial for dexterity. Two such parameters are the coefficient of static friction (μ). s (and the occurrence and extent of initial slip.)

[0005] The static friction coefficient of the contact interface helps determine the minimum clamping force (normal force) required to hold an object of a specific weight (tangential force). In certain clamping positions, if the static friction coefficient is accurately estimated and the tangential force can be measured, the clamping force (normal force) can be adjusted to securely hold the object.

[0006] Numerous tactile sensors for measuring the coefficient of static friction and the occurrence and extent of initial slip have been reported in the literature; however, many of these sensors have one or more of the following limitations:

[0007] (i) Before attempting to clamp an object, it is necessary to probe the object first.

[0008] (ii) Significant slippage occurs during operation before a static friction coefficient measurement is obtained.

[0009] (iii) Continuous measurements of the static friction coefficient cannot be provided under varying friction conditions, and

[0010] (iv) Continuous monitoring of normal and tangential forces is required.

[0011] Within this disclosure, some of these problems can be resolved or mitigated, or at least one alternative can be provided.

[0012] An alternative to measuring the static friction coefficient is to detect initial slip and adjust the clamping force after such an event occurs. Initial slip is defined as the relative displacement occurring in a localized area of ​​the contact interface, while total slip involves the relative displacement across the entire contact interface. However, despite numerous slip sensors reported in the literature, there is currently no dominant, mature technology for manually sensing slip.

[0013] Therefore, it would be advantageous to the field if the device could accurately detect the initial slip while the clamping is still secure, thereby enabling force modulation before the clamping is completely lost.

[0014] MIT researchers have developed a device called GelSight for detecting initial slippage. This device uses a transparent silicone material and a camera to measure slippage over a contact area by tracking the movement of a dot pattern etched onto the silicone. However, the device's ability to detect initial slippage is limited by its use of a flat and continuous surface. The flat surface limits the establishment of a pressure differential at the contact interface, thus limiting the traction difference. The continuous nature of the elastomer-sensing material hinders the independence of movement between different local areas of the sensing interface, further impeding the occurrence of initial slippage. The GelSight sensor is also limited to sensing relatively low-frequency tactile events because it relies on image processing of a video stream to detect movement of the silicone at the contact interface.

[0015] Another advantage of the existing technology is that the tactile sensor device can reliably detect and signal impending slippage and estimate friction force regardless of the material it is in contact with.

[0016] It should be understood that if any prior art is cited in this document, such citation does not acknowledge that the prior art has become part of the general knowledge of the field in Australia or any other country. Summary of the Invention

[0017] A system for evaluating clamping safety is disclosed, the system comprising: a contact surface having at least a first contact surface region and a second contact surface region, the first contact surface region being configured to resist slippage less than the second contact surface region. The disclosed system further includes a sensor for detecting slippage in the first contact surface region. In some forms, the contact surface is deformable.

[0018] The detection of slippage can be used to assess clamping safety and provide feedback within the system, thereby increasing clamping strength to enhance clamping safety.

[0019] In some forms, the contact surface includes a plurality of protrusions extending from the substrate surface. In some forms, the protrusions are compressible. In some forms, the protrusions are in the form of elongated struts. In some embodiments, the distance by which the protrusions in the first contact region extend away from the substrate surface may be less than the distance by which the protrusions in the second contact region extend away from the substrate surface. In some embodiments, the protrusions may be positioned to form an array. A method for evaluating clamping security is also disclosed, the method comprising detecting slippage at a contact surface having at least a first contact surface region and a second contact surface region, the first contact surface region being configured to resist slippage less than the second contact surface region. In some forms, the method comprises utilizing a sensor.

[0020] Variations in the height or distance of the protrusion from the substrate surface can be advantageous because, in some applications, this allows the device to detect initial slippage. When the protrusions are compressed to the same final height, the difference in protrusion height causes the protrusions to experience a normal force, which depends on the distance the protrusion extends from the substrate surface. When a tangential force is also applied, assuming the protrusions do not significantly bend, all protrusions with the same cross-sectional area experience the same tangential force. Therefore, the ratio of tangential force to normal force experienced by each protrusion varies with the protrusion height. If we further assume that the sensor surface maintains a constant coefficient of static friction, then as the tangential force increases, the protrusion at the lowest normal force (i.e., the shortest protrusion when the device is removed) will slip first when the ratio of tangential force to normal force is greater than the coefficient of static friction. As the tangential force further increases, the next shortest protrusion will slip, and so on, until the highest protrusion has slipped. In this way, each initial slip event serves as a warning that the clamping force / normal force should be increased to maintain a stable grip on the object.

[0021] In another embodiment of this disclosure, the movement of each protrusion can be independent of each other. Independent movement of at least two pillars is advantageous because it makes it possible to measure relative movement on the contact surface, which can occur at different levels only across the entire contact surface.

[0022] In another embodiment of this disclosure, the uncompressed height of the protrusion in the first contact region from the substrate surface is less than the uncompressed height of the protrusion in the second contact region from the substrate surface.

[0023] In another embodiment of this disclosure, the normal force experienced by the protrusion in the first contact area is less than the normal force experienced by the protrusion in the second contact area.

[0024] In another embodiment of this disclosure, the protrusion is a support.

[0025] In another embodiment of this disclosure, the first end of the protrusion is connected to the substrate surface, and the second opposite end of the protrusion forms a circular or spherical or other uneven tip.

[0026] In another embodiment of this disclosure, the contact surface is primarily made of silicone.

[0027] In some embodiments of this disclosure, the base surface may be planar, while in other embodiments, the base surface may be non-planar. When the surface of the object being held is planar, the relative compression of each protrusion against the common base surface can be easily determined even if the base surface is not planar. Three-dimensional forces can be measured regardless of the surface shape or whether the base surface is non-planar.

[0028] The system also includes a sensor or sensor system adapted to measure slippage at the first contact area to detect initial slippage. The sensor can be of various forms.

[0029] In some configurations, the sensor is positioned behind the substrate surface, adjacent to each cavity.

[0030] In some forms, the sensor is adapted to detect initial slip.

[0031] In some forms, the sensor is adapted to estimate frictional forces.

[0032] In some configurations, an orifice with a diameter smaller than that of the cavity is located between the sensor and the cavity.

[0033] In some forms, the sensor includes a quadrant photodiode configured to detect light emitted from an LED located on the cavity side of the substrate surface within a given protrusion, reflected from a reflector located at or near the distal end of the cavity, and traveling through the aperture to the sensor.

[0034] In some forms, the contact surface includes a plurality of protrusions extending from the substrate surface, the protrusions having internal cavities, and wherein the sensor includes a CCD array, the system being configured such that the CCD array detects light emitted from the substrate surface into the cavity, reflected from a reflector located at a distal end of the cavity, and propagating through a hole in the substrate surface to reach the sensor.

[0035] In some forms, compression of the protrusion along the z-axis normal to the substrate surface leads to an increase in the diameter of the detected spot.

[0036] In some forms, the contact surface includes a plurality of protrusions extending from the substrate surface, the protrusions having internal cavities, and wherein the sensor includes a CMOS photosensitive array, the system being configured such that the CMOS photosensitive array detects light emitted from the substrate surface into the cavity, reflected from a reflector located at a distal end of the cavity, and propagating through a hole in the substrate surface to reach the sensor.

[0037] In some embodiments of this disclosure, the rate at which the sensor signals an initial slip warning, the rate at which a slip event is detected, or the number of warnings can be used to indicate the urgency of the need for corrective action or the magnitude of the force required to perform corrective action.

[0038] In some forms, a method for estimating frictional forces at a contact surface is disclosed, the method comprising: providing a plurality of protrusions extending from a substrate surface to a tip; measuring the displacement of the tip in three spatial dimensions; and estimating the forces applied to the tip in the three spatial dimensions.

[0039] In some forms, slip is detected when one or more protrusions no longer move at the same speed as other protrusions in the array. In some forms, slip is detected by vibration. In some forms, after slip is detected, the system checks the ratio of the tangential force to the normal force at the instant of slip to allow estimation of the coefficient of friction.

[0040] In some forms, a method for detecting torque normal to a contact surface is disclosed, the method comprising: providing a plurality of protrusions extending from a substrate surface to a tip; measuring the displacement of the tip in three dimensions; estimating the forces applied to the tip in the three dimensions; and measuring the deflection about an axis normal to the substrate surface at the point of extension of the protrusions.

[0041] In some forms, a method for analyzing slip movement or texture at a contact surface is disclosed, the method comprising: providing a plurality of protrusions extending along an axis from a substrate surface to a tip; measuring the displacement of the tip in three dimensions at high resolution to enable measurement of vibration of the tip; and using the vibration measurements to estimate texture or detect slip. Attached Figure Description

[0042] Embodiments will now be described by way of example only with reference to the accompanying drawings, in which:

[0043] Figure 1A This is a cross-sectional view of an embodiment of the contact surface of this disclosure;

[0044] Figure 1B This is a cross-sectional view of the embodiment in Figure 1 under compression.

[0045] Figure 1C This is a cross-sectional view of the embodiment in Figure 1 under compression.

[0046] Figure 2 This is a perspective view of the contact surface disclosed herein;

[0047] Figure 3 This is a cross-sectional perspective view of the contact surface of this disclosure;

[0048] Figure 4 This is a cross-sectional view of an embodiment of a contact surface integrated with a sensor system according to the present disclosure;

[0049] Figure 5 The fabrication of a prototype of one embodiment of this disclosure is shown, illustrating the mold and the resulting silicon profile;

[0050] Figure 6 A test bench for testing at least one embodiment of the present disclosure is shown;

[0051] Figure 7 A single video frame captured during the testing process is shown;

[0052] Figure 8 A to Figure 8 C provides a graphical view of the ratio of tangential force to normal force for slippage of each support at each normal force level. Figure 8 A shows a high-friction surface. Figure 8 B shows the basic friction surface, while Figure 8 C indicates a low-friction surface;

[0053] Figures 9A to 9C A graphical representation of displacement and force detection using an embodiment of this disclosure is provided. Figure 9AProvides a reference strut tip displacement. Figure 9C It provides reference force data, and Figure 9B The optical sensor data output is shown;

[0054] Figure 10 The detection of vibration response using embodiments of this disclosure is illustrated. Detailed Implementation

[0055] In the following detailed description, reference is made to the accompanying drawings, which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings, and defined in the claims are not intended to be limiting. Other embodiments may be utilized and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that various different configurations can be arranged, replaced, combined, separated, and designed as aspects of this disclosure generally described herein and shown in the accompanying drawings, all of which are covered in this disclosure.

[0056] Figure 1 discloses a system for evaluating clamping safety, the system including a contact surface 10 having at least a first contact surface region 12 and a second contact surface region 14. In the illustrated form, the contact surface is in the form of a plurality of protrusions 16 extending from a base surface. Only two protrusions 16 are shown in Figure 1; however, those skilled in the art will understand that the contact surface may include a plurality of protrusions 16 extending from a base surface 18. The protrusions 16 are in the form of elongated struts having attachment ends 19 that engage or are integral with the base surface 18, and extending to a tip 20 having a hemispherical end profile in some embodiments. In the illustrated form, the elongated struts have similar cross-sectional dimensions and varying lengths extending from the base surface. In this illustrated form, the variation in the distance the protrusions extend from the base surface defines the first contact surface region 12 and the second contact surface region 14. Specifically, the first contact surface region is configured to have less resistance to slippage than the second contact surface region. In the illustrated form, only two elongated protrusions with different heights relative to each other are shown.

[0057] Figure 1 is a simplified model of a possible embodiment, wherein a height l C The longer central protrusion or pillar is surrounded by eight shorter outer pillars, each of which has a height of l. O Alternative embodiments include multiple pillars of varying heights.

[0058] Figure 1A Two uncompressed protrusions 16 are shown, each protrusion being in the form of a strut and having an equal diameter D, but with different heights l. C and l O .

[0059] exist Figure 1B In the figure, device 10 is shown in contact with a flat surface 24, such as the surface of an object being held. In this figure, the two supports are subjected to a total normal force F. N Compressed to the same final height, this results in different normal forces (compressive forces) on each support.

[0060] Figure 1C The diagram illustrates the addition of a tangential force, which acts through the flat surface 24 contacted by the shear sensor, causing each of the protrusions 16 to also experience a tangential force. When no protrusion slips against the surface, and assuming the support cannot be significantly bent, both the surface and the protrusions will experience the same tangential force, and the sum of these tangential forces equals the total tangential force F. T .

[0061] In some embodiments of this disclosure, the movement of each protrusion 16 can be independent of each other. Independent or partially independent movement of at least two protrusions in the form of struts 16 is advantageous because it makes it possible to measure relative movement on the contact surface, which can occur at different levels only across the entire contact surface 10.

[0062] In some embodiments of this disclosure, the contact surface 10 is primarily made of silicone.

[0063] like Figure 1C As shown, the normal force on the protrusion 16 of the first surface contact region 12 is smaller compared to the longer protrusion 16' of the second surface contact region 14. In some forms, it is assumed that each protrusion has the same coefficient of static friction; Figure 2 In the embodiment shown, the outer protrusion of the first contact surface region 12 will slide with a smaller tangential force than the central protrusion 16' of the second contact surface region.

[0064] If the spring constant (k) and diameter (D) of a single protrusion are known, and the tangential and normal forces at the instant the shorter protrusion slips are measured, the ratio of the total tangential to normal force at which the longer protrusion will slip can be predicted. Therefore, the static friction coefficient is an estimate of the ratio of the tangential to normal force at the instant the longer protrusion slips. In some forms, assuming the material is linearly elastic, the total force on the measuring device is measured and distributed among the protrusions. In some forms not shown, the force can be measured individually for multiple protrusions or groups of protrusions.

[0065] Ideally, since the protrusion does not slip, it should deflect at the same speed as the surface it presses against. Conversely, when the protrusion slips, the deflection speed should tend to 0 mm / s. In practice, due to the bending of the protrusion or support, the moment of slippage is defined as the instant when the deflection speed of the shorter protrusion decreases to 20% of the deflection speed of the longest protrusion, provided that the deflection speed of the longest protrusion is sufficiently large.

[0066] With more protrusions and greater height differences, a wider range of frictional and normal forces can be accommodated, and with increased tangential forces, more warnings may be provided to prevent object loss. The rate at which warnings are signaled and the frequency of warnings can indicate the urgency of corrective action required. Furthermore, for each warning, more information about the contact interface can be obtained.

[0067] Regardless of whether continuous normal and tangential force monitoring is present, the disclosed device can be advantageously used to improve the dexterity of machine grippers and prosthetic grippers. In the absence of continuous monitoring, the rate and frequency of warning signals can still indicate the urgency and magnitude of corrective action required. With continuous force monitoring, the static friction coefficient can be determined, and further information about gripping correction can be provided. Regardless of the type of monitoring, a warning can be issued upon detection of initial slippage.

[0068] In some embodiments of the device, the contact surface 12 is planar, making it possible to determine the relative compression of each protrusion.

[0069] In some forms, the system also includes a sensor or sensor system adapted to measure slippage at the first contact area 12 to detect initial slippage. The sensor can take various forms.

[0070] like Figure 2 As shown, the contact surface may include a base surface 18 having a plurality of protrusions extending therefrom. In some forms, the contact surface also includes a lower support surface 41 and an upper support surface 40. In some forms, the upper support surface includes a plurality of holes through which the protrusions may extend. In some forms, the lower surface 41 may include a support or cavity for supporting a sensor or other system component.

[0071] In some embodiments of the device, such as in Figure 3 and Figure 4As shown, the illumination reflector 31, aperture 32, and light sensor 33 form a pinhole camera configuration 30, thereby allowing measurement of the three-dimensional deflection of the illumination reflector 31. This deflection is related to the three-dimensional deflection of the tip of the protrusion 16. The pinhole camera configuration 30 is a configuration in which an inverted image of the light source, traveling from the light source 34 through the aperture 32, is projected onto a screen or sensor 33 below. The light source 34 illuminates the cavity 35 inside each protrusion 16 so that it is reflected from the reflector 31 at the distal end of the cavity 35 and returned through the pinhole camera 32 to the sensor 33, which is mounted at a suitable distance behind the base surface 18 and at the pinhole camera 32.

[0072] In some embodiments of the device, the light source 34 originates from a small disk from which a circular spot of light is projected. By monitoring the position of the spot, the sensor 33 is able to detect the three-dimensional position of the light source 34 relative to the aperture 32. In some embodiments, the spot expands when the protrusion is compressed, meaning that deformation along an axis normal to the substrate can be measured.

[0073] The device also allows for the measurement of changes or deformations in three dimensions: the x and y axes of a cross section on the substrate surface and the z axis, which extends normally to the surface at a point where a given protrusion extends from the substrate surface.

[0074] Visual representations of changes or deformations on all three axes allow for the measurement of three-dimensional displacements applied to the protrusion or its tip. This three-dimensional visual representation of displacement includes a visual representation of the movement of the light spot to demonstrate the angular movement of the protrusion in the xy-plane tangent to the substrate surface, and a visual representation of the movement of the protrusion along the z-axis normal to the substrate surface through variations in the size of the light spot. This visual representation allows for the determination of forces applied to the protrusion in all three dimensions. This measurement of three-dimensional forces also allows for the estimation of frictional forces in the event of slippage.

[0075] In some forms, slip is detected when one or more protrusions no longer move at the same speed as other protrusions in the array. In some forms, slip is detected by vibration. In some forms, after slip is detected, the system examines the ratio of the tangential force to the normal force at the instant of slip to estimate the coefficient of friction.

[0076] Furthermore, because xy-axis deflection or the curl of force about the z-axis can be sensed, measuring force in three dimensions allows the protrusion array to estimate torque. This torque estimation allows for increased clamping safety, as the clamping force can be increased as needed to account for the increased torque.

[0077] Measuring three-dimensional displacement with high bandwidth and very high spatial resolution allows for the sensing of vibrations in protrusions. Sensing vibrations can provide alerts related to slippage events. Alternatively, sensing vibrations provides a means of sensing texture and the ability to distinguish textures. Furthermore, the sensing of vibrations can lead to the transduction of speech or music, or other sounds from the vibrating surface.

[0078] In some embodiments of the device, the cavity 35 inside the protrusion may include a reflector disk 31 and a hole 32 illuminated by an LED 34.

[0079] In some embodiments of the device, the cavity 35 has a conical shape.

[0080] In some embodiments of the device, a sensor 33 in the form of a quadrant photodiode located below aperture 32 can detect the position and / or size of the projected light spot reflected from reflector 31. In such embodiments, the calculation of the spot position is related to the position of the tip of the protrusion in the xy plane tangent to the substrate surface. This calculation uses a relatively simple formula, where each axis is calculated by subtracting the difference between the two halves of the sensor and normalizing for the total received light. Similarly, the spot size is related to the position of the tip of the protrusion along the z-axis normal to the substrate surface, which is related to compression or release along the z-axis normal to the substrate surface. The calculation of the tip's position along the z-axis can be easily performed by measuring the intensity of the light falling on the photodiode. The simplicity of these calculations makes the design suitable for microprocessors, even with large sensor arrays.

[0081] In some embodiments of the device, the pinhole camera configuration 30 is capable of measuring the direction and magnitude of the deflection of the protrusion 16 in two dimensions by examining the relative proportions of light illuminating each of the four quadrants of the photodiode sensor 33; when the protrusion 16 deflects, the direction of the light beam illuminating the pinhole 32 will change. For smaller-scale embodiments of the device, a CCD or CMOS photosensitive array may be used instead of the photodiode sensor 33. Variations and modifications may be made to the foregoing description without departing from the spirit or scope of this disclosure.

[0082] Example

[0083] In some of the illustrated embodiments, the forces on the protrusions or supports can be estimated as follows. It is assumed that the material behaves linearly elastic according to Hooke's Law. Note that the total normal force F... N It is the sum of the normal forces acting on each support.

[0084] When a single central pillar is surrounded by eight external pillars, this means:

[0085] F NC =kΔlC ,

[0086] (1a)

[0087] F No =kΔl O =k(Δl) C -d)=F NC -kd, and (1b)

[0088] F N =F NC +8F NO =9kΔl c -8kd, (1c)

[0089] Where k is the spring constant of the elastic material of the support. Therefore:

[0090] And (2a)

[0091]

[0092] When a tangential force is also applied to the sensor through the shear contact surface, each of the supports will also experience a tangential force (see [link]). Figure 1C When all supports are engaged with the surface (without slippage) and assuming (i) the compressive strain on the supports is small relative to their height, (ii) the height difference between the longer and shorter supports is also small relative to their height, and (iii) the supports do not bend significantly relative to their height, then the supports all experience approximately the same tangential force, and the sum of these tangential forces equals the total tangential force F. T In the case where a single central pillar is surrounded by eight external pillars, this means:

[0093] F TC =F TO , and (3a)

[0094] F T =F TC +8F TO =9F TO (3b)

[0095] Where F TC It is the tangential force on the central support, and F TO It is the tangential force on one of the external supports.

[0096] Because the normal force on the external support is small, and μ s This is the same for each support, so the outer support will slip under a smaller total tangential force compared to the central support. The outer support will begin to slip under the following conditions:

[0097] FTO >μ s F NO (4)

[0098] This occurs when the total tangential force is as follows:

[0099]

[0100] When the outer support slips while the central support remains stuck, due to the coefficient of dynamic friction (μ) k Therefore, the external support contributes a limited amount of tangential force to the total tangential force.

[0101] F TO =μ k F NO ,

[0102] (6a)

[0103] And the central support will begin to slip under the following conditions:

[0104] F TC >μ s F NC (6b)

[0105] This occurs when the total tangential force is as follows:

[0106]

[0107] Now, μ s Always greater than or equal to μ k However, if we assume μ s =μ k Then equation (7) can be simplified to:

[0108]

[0109] Combining equations (5) and (8), we get:

[0110]

[0111] This means that if k and d are known, and the total tangential and normal forces at the instant of slippage of the outer support are measured (respectively...), and If this is the case, then the ratio of the total tangential force to the normal force that will cause the central support to slip can be predicted; that is, the static friction coefficient can be estimated by sensing when the outer support slips and then checking the force at that time.

[0112] In some forms, the sensor base (from which the struts originate) has a diameter Dtotal = 80 mm and a thickness of 3 mm, and each of the cylindrical struts has a diameter D = 10 mm and a hemispherical end, with the center-to-center spacing approximately 15 mm. The hemispherical end is chosen because a flat end with sharp edges would result in a large compressive force at the edge of the strut contact area before the strut slips. In this embodiment of the sensor (similar to the simple model above), eight outer struts surround a single central strut arranged in a 3x3 grid. The height of the central strut is l. C =15mm, while the height of the external support is l O =14mm; that is, the height difference between the central support and the outer support is d=1mm. To manufacture this prototype, silicone was cast into a 3D-printed ABS plastic mold.

[0113] The mold was printed using a 3D printer on thermoplastic material. To smooth the 3D printing lines on the surface of the mold, it was suspended in an acetone vapor bath at room temperature for 3 hours. Figure 5 A and Figure 5 B shows the mold before and after the acetone vapor bath.

[0114] A skin-safe two-component silicone with low viscosity, easy flowability, and a short curing time was used as the material for the prototype. According to the manufacturer's instructions, the two components were mixed in equal parts and casting was performed in a single pour. No degassing process was required; however, the silicone was poured from a certain height to allow for better control of the pouring flow, and the mold was gently shaken to remove any air bubbles present in the silicone. The silicone was demolded after curing (see [link]). Figure 5 C to Figure 5 E). Figure 5 C shows a top view of the silicone prototype; Figure 5 D shows a prototype with mounting supports; Figure 5 E shows a side view of the silicone prototype.

[0115] To verify the prototype's operation, multiple experiments were conducted to apply normal and tangential forces to the prototype. To perform these tests, a test bench was used, comprising an XYZ stage, a 3D force / torque sensor, the prototype, a transparent acrylic surface, and a camera. The test bench and testing procedures are described below.

[0116] Figure 6 An exemplary test bench is shown for testing prototypes of at least one embodiment of this disclosure. The figure shows an XYZ stage labeled A, an acrylic surface labeled B, a prototype labeled C, a 3D force / torque sensor labeled D, a video capture platform labeled E, and a support frame labeled F.

[0117] Figure 6 The XYZ stage (M-605.1DD, Physik Instrumente (PI) GmbH & Co. KG, Karlsruhe, Germany), consisting of three translational stages, is used to bring a transparent acrylic surface into contact with a prototype, and then shear the acrylic surface across the prototype's surface. Each stage has a travel range of 25 mm and a maximum speed of 50 mm / s. -1 The accuracy is 0.1 μm, with step sizes as low as 0.3 μm. Compression generates a normal force acting on the strut, and shearing of the surface upon contact with the object to be clamped generates a tangential force on each of the struts.

[0118] A 3D force / torque sensor (Mini40, SI-80-4, ATI Industrial Automation, Eppendorf, NATO, USA) is mounted between the prototype and the support frame (see [link]). Figure 6 D). Forces and torques acting on the prototype were sampled at 1 kHz using a PowerLab 16 / 35 data acquisition unit (AD Instruments, Bella Vista, New South Wales, Australia).

[0119] Video of the prototype in contact with a clear acrylic surface was captured using the native video recording app on a 16GB iPhone 6 (model A1586). The iPhone was placed on the platform so that the pillar was viewed from directly below through the clear acrylic surface (approximately 100mm), with the central pillar positioned in the center of the image. The iPhone was connected to a MacBook Pro running QuickTimePlayer 10.4 to achieve a frame rate of 59.97fps. -1 The iPhone screen was recorded in .MOV format at a resolution of 1334×750 pixels. Camera calibration was performed using the MATLAB (R2014b, Mathworks, Natick, MA) camera calibration application. Lens distortion coefficients (radial and tangential) were calculated, and at the edges of the sensor (beyond the maximum deflection of either strut), the distortion was no greater than 1.1 pixels, corresponding to approximately 0.12 mm. In comparison, the diameter of the tracking point on the strut was approximately 5 pixels. Because this distortion has the effect of biasing the measured strut deflection, its effect on the results is only to change the point in time at which the strut slips relative to the table; however, determining the timing of this event depends more on the slip detection rules used.

[0120] A small hole is created at the center point of the selected support using a pin, and this hole is filled with black ink to form a reliable mark for tracking during video analysis. Additionally, a black and white checkerboard pattern consisting of three 10mm squares in a row is attached to the acrylic surface to provide reference points for tracking the surface's position, as well as a reference for spatial unit conversion (pixels to mm), which is reasonable given the negligible lens distortion.

[0121] The XYZ stage is programmed to operate at 2.5mm.s -1 The XYZ stage moves vertically toward the prototype at a predetermined position, generating the required normal force (0.5 N for measurement in μs, while 5 N, 7.5 N, 10 N, 12.5 N, and 15 N for analyzing the strut slip behavior – see below). The XYZ stage holds this position for 1.5 s, then moves at a speed of 2.5 mm. -1 The machine moves laterally a total of 15mm at a certain speed. Then, the worktable moves vertically back to the starting height from the prototype, thus relieving the force, and then moves laterally to return to the starting position.

[0122] By recording the XYZ stage positions that generate the required normal forces (5N, 7.5N, 10N, 12.5N, and 15N), the spring constant can be calculated using Hooke's Law. Since all nine sensor supports are pressed against the acrylic surface at these normal force levels, the spring constant k is equal to 1 / 9 of the gradient of the line defined by the stage position relative to the normal force.

[0123] To ensure that the three surfaces used for testing have different μs values ​​and that the μs of each surface remains consistent throughout the test, μs needs to be measured. μs is measured by performing the above protocol under a normal force of 0.5 N, under which only the central pillar is in contact with the surface. For each combination of friction conditions and test normal force, the frictional force is measured before and after the behavior of the test pillar (see below).

[0124] The sensor behavior was tested at five different normal force levels: 5N, 7.5N, 10N, 12.5N, and 15N. The XYZ stage was programmed to apply normal and tangential forces as described above. Simultaneously, force / torque signals from the ATI sensor were recorded, and video of the strut in contact with the acrylic surface was captured.

[0125] Three surfaces with different frictional properties were used: (i) acrylate cleaned with ethanol (basic friction conditions), (ii) acrylate coated with olive oil (low friction conditions), and (iii) acrylate coated with a thin layer of soap that had been allowed to dry (high friction conditions).

[0126] For each combination of normal force (5N, 7.5N, 10N, 12.5N, and 15N) and surface (oiled acrylate, alcohol-cleaned acrylate, soap-coated acrylate), the following tests were performed: friction was tested at 0.5N (once), pillar behavior was tested at the desired normal force (five times), and friction was tested at 0.5N (once).

[0127] To eliminate any high-frequency noise in the force signal, a second-order low-pass Butterworth filter with a cutoff frequency of 10 Hz is applied.

[0128] The recorded video was used to monitor the deflection of the prototype's central pillar and one of eight outer pillars during lateral movement across the acrylic surface. The Kanade-Lucas-Tomasi algorithm was used to perform point tracking in MATLAB (Mathworks, Natick, MA, USA). Three points were tracked throughout the video recording: (i) the center of the central pillar, (ii) the center of one of the outer pillars—selected as the pillar directly in front of the central pillar in the shear direction—and (iii) a point on a reference mesh (used to monitor the position on the acrylic surface).

[0129] The point tracking results give the deflection of the central and outer pillars relative to the acrylic surface, followed by the deflection relative to the undeflected position of each pillar. Figure 4 A single frame showing video tracking and pillar deflection is presented. A 5Hz second-order low-pass Butterworth filter is then applied to eliminate tracking jitter in the deflection data.

[0130] Because the raw data was recorded on two different devices, the filtered force / torque and deflection signals needed to be synchronized. At the end of each stimulus, the XYZ stage was accelerated along the negative Z direction (normal to the strut) to retract the acrylic surface away from the sensor and remove the normal force. This sudden removal of the tangential bending force resulted in a significant acceleration of the measured normal force and the calculated central strut deflection. Large negative peaks in the second derivatives of the time relative to the filtered normal force and central strut deflection were used to synchronize the force and deflection data.

[0131] Ideally, because the worktable is at 2.5mm.s -1 The movement is at a certain speed, so if the support gets stuck (does not slip), the support should also move at a speed of 2.5mm at its tip. -1 The velocity deflection, and during the strut slippage, the deflection velocity strain is 0 mm.s -1However, this is not the case in reality, and due to the bending of the strut, the strut initially appears to move at the same speed as the table; however, this speed gradually decreases as the strut oscillates at the point of contact. Therefore, the instant of slippage is heuristically determined as the strut's deflection speed (i.e., the first derivative of the deflection position with respect to time) decreasing to 5% of the table speed (i.e., the strut's deflection speed first drops below 0.125 mm / s). -1 The threshold is the instantaneous point at which the inter-frame noise level is close to zero but higher than the strut deflection velocity. In this work demonstrating the operational principle of the prototype, this threshold is sufficient to identify the instantaneous slip; however, in other practical situations, the detection algorithm will certainly be more complex / robust. In some forms, each protrusion can be internally instrumented using the aforementioned light and pinhole methods to measure the deflection and vibration of the protrusion. In some forms, this may mean that the slip event will be explicitly identified.

[0132] The ratio of the tangential force to the normal force at the instant of slippage of the central support (the only support in contact under a normal force of 0.5 N) is taken as μ. s The estimated value.

[0133] The sensor operates on the principle that the outer (shorter) support should slide under a smaller tangential force compared to the central (taller) support. To determine if this requirement is met, the tangential and normal forces at the instants of sliding for both the outer and central supports were determined and compared. The prediction (Equation (9)) is also based on the tangential and normal forces measured when the external support slips (respectively...). and )Calculated, and compared with the measured Comparison, It was measured at some point later, when the central support finally slipped.

[0134] result

[0135] The spring constant k of the sensor struts is calculated using the table position (mm) for each of the normal forces (N). The spring constant k is the gradient of the best-fit line divided by the number of struts: k = 1.174 N / mm.

[0136] It has been observed that the normal force decreases when the acrylic plate begins to shear (at approximately 2.4 s). This is expected because the XYZ platform is programmed to remain at the same height while shearing the acrylic surface, and the sensor's struts bend, meaning the effective height of the sensor will decrease slightly. (Take μ) s The ratio of tangential force to normal force at the moment of slippage of the central support.

[0137] refer to Figure 7The image shows a single video frame captured during the testing process. The red cross indicates the original position of the marker; the blue cross indicates the current position the marker has moved to along with the acrylic plate. The values ​​highlighted in yellow are the marker deflection (mm) (top), the center post deflection (mm) (middle), and the outer post deflection (mm) to the left of the center post (bottom).

[0138] refer to Figure 8 The figure shows a graphical view of the ratio of tangential force to normal force for slippage at each normal force level for A) high-friction surface, B) base-friction surface, and C) low-friction surface. The labels are mean values, and the error bars extend to ±SD. The dashed horizontal line indicates μs measured at a normal force of 0.5N.

[0139] like Figures 9A to 9C As shown, the optical sensor is capable of measuring the displacement of the protrusion and the force exerted on it. In the graphical representation, Figure 9A A reference displacement is provided. The XY coordinates are obtained by tracking a colored point on the tip of a support using a video camera from above. A transparent, thick sheet of plexiglass is brought into contact with the tip of the support using a robotic stage, and the plexiglass is moved around to generate XYZ displacement. The robotic stage then provides the Z coordinate.

[0140] Figure 9C The reference force is shown. This was obtained using a commercially available 3-axis force sensor.

[0141] Figure 9B The diagram shows the displacement and force measurements using four photodiodes after some simple preprocessing. As shown, if there are four photodiodes arranged in a quadrant pattern...

[0142] PQ

[0143] RS

[0144] The light intensity sensed by the photodiode can then be preprocessed as follows to obtain an intermediate curve.

[0145] Z = P + Q + R + S (i.e., the sum of all S's)

[0146] X = [(P+R)-(Q+S)] / Z (i.e., subtract the right from the left, normalize)

[0147] Y = [(P+Q)-(R+S)] / Z (i.e., top minus bottom, normalized)

[0148] Ultimately, two mapping functions are learned to measure... Figure 9B The displacement and force values ​​shown are mapped to: Figure 9A The value in the table shows the reference displacement in three dimensions; or Figure 9CThe value in the figure shows the reference strength in three dimensions.

[0149] like Figure 10 As shown, the sensor can detect vibration. In this example, the protruding tip contacts a shaker vibrating along a single axis. The attached image shows the test results, where a 10-micrometer (0.01 mm) vibration at 330 Hz (i.e., compression of the strut) is applied along the Z-axis of the strut. The red dotted trace indicates the displacement of the shaker, while the blue trace indicates the response of the photodiode sensor; this sensor response value, shown by the blue trace, is obtained using the calculation Z = P + Q + R + S, and is expressed in volts.

[0150] In the appended claims and the preceding description of the invention, unless the context otherwise requires it due to the language of expression or necessary implication, the word “comprise” or variations such as “comprises / comprising” are used in an inclusive sense; that is, in various embodiments of the invention, the presence of the stated feature is specified, but the presence or addition of other features is not excluded.

Claims

1. A system for estimating frictional force, characterized in that, The system includes: A contact surface includes a plurality of protrusions extending from a substrate surface, the contact surface including a first contact surface region and a second contact surface region, the first contact surface region being configured to have less resistance to slippage than the second contact surface region; and The sensor arrangement is configured to detect the displacement of at least three of the plurality of protrusions in three different dimensions to measure the three-dimensional force applied to at least three of the plurality of protrusions at the moment of slippage in the first contact surface region.

2. The system as described in claim 1, characterized in that, The displacement of the plurality of protrusions is the result of the translation or rotation of the clamped object.

3. The system as described in claim 1, characterized in that, The system is further configured to estimate the torque applied to at least three of the plurality of protrusions.

4. The system as described in claim 1, characterized in that, The system is further configured to sense vibrations of at least three of the plurality of protrusions associated with a slippage event.

5. The system as described in claim 1, characterized in that, The system is further configured to sense vibrations of at least three of the plurality of protrusions on different surface textures.

6. The system as described in claim 1, characterized in that, The plurality of protrusions are deformable.

7. The system as described in claim 1, characterized in that, The plurality of protrusions are longitudinal axes extending from the base surface to the tip, and the contact surface is located at or near the tip.

8. The system as described in claim 1, characterized in that, The plurality of protrusions are positioned to form an array.

9. The system as described in claim 1, characterized in that, The plurality of protrusions are configured to move independently of each other.

10. The system as claimed in claim 1, characterized in that, The plurality of protrusions are configured such that, when used under a consistent clamping pressure, the normal force on the protrusions in the first contact surface region is less than the normal force on the protrusions in the second contact surface region.

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