Force Sensor

Abstract

A tactile sensor consisting of a flexible main body with embedded magnets, soft magnetized cilia on the main body and magnetic sensing elements embedded in a substrate layer. The magnetic sensing elements generate signals in response to changes in the magnetic field. Tactile interaction with the main body and cilia displaces the magnetic material and changes the magnetic field. The cilia are sensitive to force magnitudes less than mN, while the main body is sensitive to a larger range of force magnitudes. Multiple sensing elements, magnets and cilia may be distributed over a large and curved surface. Three-axis forces may be measured using the sensor.

Description

[Technical field]

[0001] Field The present invention relates to a force sensor, and more particularly to a force sensor capable of sensing a contact force. [Background technology]

[0002] background Applications that require interaction between physical bodies benefit from tactile contact information between these bodies. Some examples of such applications are robotic manipulation, prosthetics, and rehabilitation. Tactile information may be used to detect contact, regulate forces between physical bodies, prevent extreme collisions, or obtain information about the touching bodies.

[0003] This information can be provided by different types of tactile sensors. Desirable characteristics of tactile sensors include flexibility, sensitivity, and ease of manufacture. Sensor flexibility is desirable in many applications as it improves the safety of the sensor and other objects in the environment and increases the robustness of object grasping. Sensitivity determines the range of tasks for which the sensor can be used. Ease of manufacture allows for efficient sensor production.

[0004] Patent Document 1 describes a flexible and easy-to-manufacture tactile sensor that is manufactured by embedding a magnet in a flexible material and placing a magnetoresistance element and an inductor at the bottom.

[0005] US Patent No. 5,399,633 describes another tactile sensor that achieves flexibility by placing a flexible magnetic layer over a non-magnetic layer and an induction coil. The sensor contains a layer of low magnetic permeability to reduce the effects of magnetic noise.

[0006] US Patent No. 5,399,633 describes another flexible tactile sensor formed using magnetic rubber that increases its conductivity under pressure. The magnetic rubber contains a pair of electrodes insulated by the rubber, which is coated with a resin body.

[0007] Non-Patent Document 3 describes a Hall-effect based miniature sensor that can measure three-axis forces. The mentioned sensor consists of a soft silicon body, a magnet and a Hall-effect sensor embedded in the body. The sensor is easy to manufacture and miniaturize. The force range of the sensor is reported to be approximately 10 mN to 11 N. Non-Patent Document 1 also describes a distributed tactile sensor that works on the same principle. In this work, a number of magnets and Hall-effect sensors are distributed on a flexible printed circuit board (PCB). The sensor is then placed on a curved robot fingertip.

[0008] Non-Patent Document 2 describes another tactile sensor that uses hair-like cylindrical structures made of magnetized soft material and giant magnetoresistance (GMR) sensors to detect forces smaller than mN. In this document, the cylindrical structures are called "cilia". The cilia are fabricated by mixing NdFeB magnetic beads with polydimethylsiloxane (PDMS). The mentioned study reports that the minimum force detected was 333 μN.

[0009] Generally, for flexible magnetic sensors, the stiffness of the sensor body limits the sensitivity to small forces. The sensitivity of a flexible sensor can be increased by using softer materials. However, the use of softer materials reduces the range of maximum measurable forces. Thus, determining the flexibility of the sensor body is a trade-off between sensitivity and maximum measurable force. Summary of the Invention [Problem to be solved by the invention]

[0010] It is an object of the present invention to provide a flexible tactile sensor that is sensitive to both small and large forces. [Means for solving the problem]

[0011] According to the present invention there is provided a force sensor comprising: having a magnetic sensing element; having a flexible body; a magnet connected to the flexible body; and The flexible body has a plurality of flexible protrusions attached thereto, the plurality of flexible protrusions having magnetic material embedded therein.

[0012] According to the present invention there is also provided a force sensor system comprising a plurality of the above-described force sensors mounted on a substrate, preferably a flexible printed circuit board.

[0013] Sensors according to embodiments of the present invention can be reliably used for a wide range of applications. They can have a simple structure that is easy to manufacture. The present invention achieves this solution by combining different approaches. [Brief description of the drawings]

[0014] For a more complete understanding of the present invention, reference is made to the following description and accompanying schematic drawings.

[0015] [Figure 1] 1(a) and (b) are cross-sectional views of variations of a hemispherical embodiment having different cilia types. [Diagram 2] 2(a) and (b) depict the behavior of a hemispherical embodiment under an external force applied to a point on the sensor. [Diagram 3] 3(a) and (b) are perspective views of two embodiments. [Figure 4] 4(a)-(c) show a cross-section and two plan views of alternative planar embodiments with different cilium topologies. [Diagram 5] 5(a) and (b) depict an embodiment having multiple distributed tactile sensors on a curved surface. [Figure 6] FIG. 6 is a graph of the average readings of the sensitivity tests for the ciliated and non-ciliated prototypes. [Figure 7] FIG. 7 is a graph of the three-axis (x, y, z) readings of the sensor prototype with cilia during sensitivity testing. [Figure 8] FIG. 8 is a graph of the three-axis (x, y, z) readings of the reference sensor prototype without cilia during sensitivity testing.

[0016] In the various drawings, like parts have like reference numerals. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS It is proposed to combine flexible magnetic sensors and soft magnetized cilia to sense both very small (less than mN) and large range forces. An embodiment consists of a flexible main body embedding a magnet, soft magnetized cilia attached to the main body and a magnetic sensing element on a substrate layer. The sensing element generates a signal according to the magnetic field created by the magnetic material in the sensor body. As a result, the sensor generates a signal when the sensor body interacts with a foreign object that disturbs the shape and position of the magnetic element.

[0018] The term "cilia" is used herein to refer to soft, elongated bodies (e.g., made of elastomer, preferably soft silicone). The cilia may have embedded magnets or may have magnetized powder. Preferably, the magnetized powder is evenly distributed in the soft body. The cilia cause changes in the magnetic field when they are deformed by an external force. In embodiments, the cilia are used to sense forces smaller than mN and to obtain local surface information about interacting objects. The count, size, topology and position of the cilia may be different for different applications. The shape and material of the cilia may also be different. The magnets or magnetized materials in the cilia may differ in size and material composition. The cilia may be cylindrical for ease of manufacture and to have an isotropic response. The term "cilia" is used in other contexts to mean "hair-like", but it is not intended that the cilia of the embodiments have dimensions similar to hairs.

[0019] In an embodiment, the main body of the sensor consists of a flexible material (preferably soft silicon) and a magnet embedded in the body. The body deforms in response to an external force and as a result the magnet is displaced. Thus, the magnetic field changes as a result of the external force. The main body can be used to sense a large range of forces. The relationship between the magnetic field and the external force depends on the properties of both the flexible body and the magnet. The elasticity, size and shape of the flexible material may be tailored for different applications. The elasticity affects the sensitivity and the maximum force range of the sensor. Different shapes and sizes may be suitable according to the surface on which the sensor is mounted, the object with which the sensor interacts and the role of the sensor. As an example, the sensor may have a shape of the body part depending on its role. The size, position and material of the magnet may also be different to suit the requirements of the task. The magnet is preferably permanent. The magnet is preferably centered on the sensing element such that the magnetic field is symmetric about its main axis.

[0020] The change in the magnetic field is sensed using a magnetic sensing element on the substrate layer. The sensing element may be, for example, a Hall effect sensor or a magnetoresistive sensor such as GMR. Preferably, the sensing element is capable of sensing three-axis changes in the magnetic field. The sensing element may be a combination of multiple magnetoresistive elements.

[0021] The sensor components can be miniaturized and distributed over a surface. There are commercially available magnet and Hall effect sensor chips in the mm scale. These can be arranged on a PCB to create a distributed tactile sensor with a spatial density of less than cm. It is possible to fabricate physically even smaller magnet, cilia and Hall effect sensor chips. Distributed sensors may be placed on curved surfaces using flexible PCBs as substrate layers. The PCB may also support other components such as digital-to-analog converters (if the magnetic sensing elements do not have digital outputs) and communication components (e.g. 12C or SMBus buses or multiplexers).

[0022] The soft body used in the embodiment can be manufactured either by 3D printing or by molding and curing liquid elastomer. The magnets can be embedded in predefined holes in the soft body. Alternatively, the cilia can be manufactured by molding and curing a mixture of elastomer and magnetic particles, followed by magnetizing the particles under a magnetic field. Thus, the present invention is suitable for mass production.

[0023] The embodiments may be used in a variety of different tactile sensing applications. They may be attached to robots to detect contact and prevent hard collisions. They may provide tactile feedback to improve the quality of robotic grasps and the safety of interactions. The high sensitivity of cilia is beneficial for handling fragile or soft objects such as test tubes or strawberries. The embodiments may be used in various robotics applications such as agricultural, medical and manufacturing robotics to obtain information about the properties of objects in the environment. The distributed sensing capabilities are useful in classifying objects and understanding their surface features. The invention may also be used in prosthetic and wearable devices to provide feedback to users. Other applications will occur to those of skill in the art upon reading this description.

[0024] Figures 1(a) and (b) show two hemispherical tactile sensors A and B in schematic form. In each of Figures 1(a) and (b) a cross-sectional side view of the embodiment is shown. These tactile sensors consist of a soft body 1, preferably of approximately hemispherical shape with a hollow center, a substrate layer 2 (on which the magnetic sensing element 3 and the soft body 1 are arranged), a magnet 4 embedded in the soft body and a number of soft cilia 5a, 5b attached to the soft body. These sensors differ according to the type of cilia they contain. The cilia may for example be made of a soft material with a magnet embedded inside it 5a or made of a soft material with magnetized particles 5b.

[0025] In Figures 2(a) and (b) the behaviour of tactile sensor B under external forces is shown. The resting configuration of the components is shown in dashed lines. When a foreign object approaches the sensor, one or more of the cilia will first come into contact, as shown in Figure 2(a). The soft cilia are easily deformed by the external force, and the displaced magnetic material causes a fluctuation in the magnetic field. As a result, the signal generated by the sensing element 3 changes. As the cilia 5 are farther away and magnetically weaker than the magnet 4, they produce a smaller change in the magnetic field.

[0026] If the foreign object moves closer to the sensor, it will contact the main body 1, as shown in Fig. 2(b). An external force will deform the main body 1, and the magnet 4 will be displaced according to the magnitude and direction of the external force, causing a fluctuation in the magnetic field. As a result, the sensing element 3 will generate a different signal. The movement of the main magnet 4 will create a larger change in the magnetic field than the movement of the cilia 5, because it is larger and closer to the sensing element.

[0027] In Fig. 3(a) and (b) are shown perspective views of two hemispherical embodiments of the tactile sensor C, E. As depicted, there is a central cilium and two rings of five cilia arranged such that the cilia are radially aligned. More or fewer cilia may be provided (e.g., 1-50 cilia per sensor). Other arrangements (e.g., evenly spaced cilia) may be used. The cilia may be irregularly arranged. Sensor C has a flat top and sensor E has a convex top (e.g., dome-shaped).

[0028] Figures 4(a)-(c) show two planar embodiments of tactile sensors C and D in a schematic way. Figure 4(a) shows a cross-sectional side view of planar embodiment C, and Figure 4(b) shows a top-down view of planar embodiment C. The main body 1 has a generally cylindrical shape with a hollow center. The cilia 5 are only attached to the top surface of the main body that is expected to come into contact with other objects. The cilia are arranged in a radial fashion, as shown in Figure 3. Figure 4(c) presents a top-down view of embodiment D, which differs in cilia count and topology. Sensor D has a rectangular prism shape (e.g., generally square) and has nine cilia arranged in a 3x3 grid. Again, more or fewer cilia may be provided. Other arrangements (e.g., cilia evenly spaced) may be used. The cilia may also be irregularly arranged.

[0029] In Fig. 5(a) an embodiment F of a distributed tactile sensor is shown. There are a number of sensing elements 3 on a curved substrate layer 2. There are a number of magnets 4 embedded in the soft body 1 and a number of cilia 5 distributed on the soft body 1. Each magnet and cilia is located in the vicinity of a magnetic sensing element. There may be more cilia 5 than sensing elements 3. Desirably there is a 1:1 relationship between the magnets 4 and the sensing elements 3. In another arrangement, the flexible body has the form of a sheet (with a flexible protrusion on one side and a magnet embedded therein). The sheet is spaced apart from the substrate on which the magnetic sensing elements are mounted. In yet a further arrangement shown in Fig. 5(b), a distributed tactile sensor G has a number of separate soft bodies 1 with spaces between them mounted on a curved substrate 2. As shown, the soft bodies are flat-topped and have generally vertical sidewalls, but any convenient shape may be used. The arrangement shown in FIG. 5(b) with separated protrusions would exhibit less cross-talk between the sensing elements for better localization of sensing.

[0030] Soft silicone (DragonSkin TM A prototype of embodiment C was produced by molding and curing a mold of a Hall effect sensor (MLX90393 by Melexis NV). The mold contained sockets for magnets in the main body and on the tip of the cilium 5. The prototype contained one magnet in the main body and eleven smaller magnets on the cilium. The substrate layer was a plastic box containing a Hall effect sensor (MLX90393 by Melexis NV). We compared this prototype with a reference sensor (identical to embodiment C except that it does not have a cilium, i.e. it only has one magnet 4 in the main body). The sensitivity of both prototypes was tested by placing two different plastic cubes on the sensor. The cubes had different weights: 7g and 40g. The cube remained on the sensor for 2 seconds, after which the cube was left and the sensor was left stationary for 2 seconds. This procedure was repeated 9 times for each pair of sensor and cube. Then, for each repetition, two readings r立方体 ,r 静止 is sampled, and r 立方体 is the reading after 1 second of cube placement, and r 静止 is the reading when the sensor is at rest. The reading at time t is the 2-norm of the 3D x,y,z measurements:

[0031]

number

[0032] Figure 8 shows the average readings for each sensor-cube pair. The values ​​are grounded, i.e., r 接地 =r 立方体 -r 静止 The bars show the average of nine readings, and the light lines are the standard deviation. The ciliated prototype produces higher values ​​for each cube. We also see that the readings increase more for the ciliated sensor when we increase the weight of the cube. The average ratio of readings for the heavy and light cubes is 4.716 for the ciliated prototype and 3.334 for the reference prototype. These observations indicate that the ciliated prototype has a higher sensitivity and that it can detect smaller contact forces.

[0033] In Figure 9, the x,y,z readings of the prototype with cilia are shown during sensitivity testing. The sensor reads residual forces x,y because the cilia bend in these directions. The spikes in the readings are due to the momentary impact of placing the cube on the sensor. We ignore these spikes in our analysis by waiting 1 second before sampling the readings. In some applications, the spikes may provide useful information (e.g., regarding the timing of the contact).

[0034] Figure 10 shows the x, y, z readings of the prototype without cilia during sensitivity testing. Since the force is applied along the gravity direction, the x, y readings can be neglected. Momentary spikes in the readings are ignored in our analysis as described above.

[0035] It will be appreciated that the output of the magnetic sensing elements will need to be processed to obtain useful information. The exact form of such processing will depend on the particular application, the structure of the device, and the information to be obtained. In general, smaller forces affecting only the cilia (flexible protrusions) will result in a higher frequency output than larger processes affecting the flexible body.

[0036] Although specific embodiments of the invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The above description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

[0037] References Patent Document 1: JP 2011-153826 A Patent Document 2: International Publication No. 2019 / 049888 Patent Document 3: JP 2013-232293 A Non-patent document 1: Tomo, Tito Pradhono, et al. “Covering a robot fingertip with uSkin: A soft electronic skin with distributed 3-axis force sensitive elements for robot hands.” IEEE Robotics and Automation Letters 3.1 (2017): 124-131. Table 2:Ribeiro, Pedro, et al. “Bioinspired ciliary force sensor for robotic platforms.” IEEE Robotics and Automation Letters 2.2 (2017): 971-976. Table 3:Volume, Tito Pradhono, et al. “Design and Characterization of a Three-Axis Hall Effect-Based Soft Skin Sensor”. Sensors 16.4 (2016):491.

Claims

1. A force sensor, and said force sensor is: Having a magnetic sensing element; Having a flexible body; It has a magnet connected to the aforementioned flexible body; and, The flexible body has a plurality of flexible protrusions attached thereto, and the plurality of flexible protrusions have a magnetic material embedded therein. The aforementioned force sensor.

2. The force sensor according to claim 1, wherein the magnet is embedded in the flexible body.

3. The force sensor according to claim 1 or 2, wherein the magnet is a permanent magnet.

4. The force sensor according to claim 1 or 2, wherein the magnetic material has magnetic bodies embedded in each flexible protrusion.

5. The force sensor according to claim 4, wherein the magnetic material is a magnetic powder dispersed on the flexible protrusion.

6. The force sensor according to claim 1 or 2, wherein the magnetic sensing element has a Hall effect sensor or a magnetoresistive sensor.

7. The force sensor according to claim 1 or 2, further comprising a substrate on which the magnetic sensing element and the flexible body are mounted.

8. The flexible body is configured such that a space exists between the flexible body and the magnetic sensing element. The force sensor according to claim 7.

9. The force sensor according to claim 8, wherein the flexible body has a dome-like shape.

10. The force sensor according to claim 8, wherein the flexible body has a generally flat outer surface.

11. The force sensor according to claim 1 or 2, wherein the ratio of flexible protrusions to magnetic sensors is in the range of 1:1 to 50:

1.

12. The force sensor according to claim 1 or 2, wherein the flexible projection is substantially cylindrical.

13. A force sensor system having a plurality of force sensors according to claim 1 or 2 mounted on a single substrate.

14. The force sensor system according to claim 13, wherein the substrate is a flexible printed circuit board.

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