CN114235230A - Flexible six-dimensional force sensor based on mortise and tenon joint structure - Google Patents
Flexible six-dimensional force sensor based on mortise and tenon joint structure Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/18—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material
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Abstract
The invention discloses a flexible six-dimensional force sensor based on a mortise and tenon structure. The flexible printed circuit board comprises a flexible boss, a PTFE film, an FPCB flexible printed circuit board and a flexible base which are arranged from top to bottom in sequence; the flexible boss is of a quadrangular frustum pyramid structure, square convex blocks are arranged at four corner positions of the bottom surface, a cross structure matched with the square convex blocks of the flexible boss is arranged on the top surface of the flexible base, and the bottom of the flexible boss and the top of the flexible base are in concave-convex interlocking to form a mortise-tenon structure; the PTFE film and the FPCB flexible circuit board are of a laminated folding structure embedded with the bottom surface of the flexible boss or the top surface of the flexible base, the sensing units are reasonably arranged on the flexible circuit board through the base, and the flexible six-dimensional force sensor can generate unique signal response under various deformations through the encapsulation of the PTFE film. Under different external stimuli, the independent deformation mechanism of the interlocking structure enables the sensor to decouple the translation force and the torsion moment in the x direction, the y direction and the z direction.
Description
Technical Field
The invention belongs to the technical field of sensors, and particularly relates to a flexible six-dimensional force sensor based on a mortise and tenon structure.
Background
In recent years, flexible sensors have gradually replaced traditional sensors as important modules of intelligent wearable electronic devices, and become hot spots for research of artificial intelligence and wearable physical equipment. In order to better realize human-computer interaction, flexible wearable sensors, such as pressure sensors, strain sensors, temperature sensors, optical sensors, and the like, have been widely developed and applied to the fields of medical diagnosis, human-computer interaction, motion detection, and bionic robots. Although these sensors have achieved excellent effects in terms of sensitivity, detection limit and detection range, and their performance is also significantly improved, for force stimuli in different directions or disturbances of external variables, common pressure sensors cannot accurately identify the strain direction because they can only monitor external forces caused by pressing, and they can only detect unidirectional stimuli, limiting the application of strain detection or bending deformation detection in multi-axis motion.
Most external stimuli in the environment are six-dimensional, including forces and moments in the x, y, and z directions. The ideal wearable physical device has the capability of recognizing the motion with multiple degrees of freedom, and can be suitable for complex environments with different loading conditions. For a multi-dimensional sensor, such as a six-dimensional force sensor, six-dimensional force and moment information of a three-dimensional space can be sensed simultaneously: force components along the three x, y, z coordinate axes and three moment components about the coordinate axes. The sensor can not only accurately assess the magnitude and location of the external load, but also distinguish the type and direction of the stimulus. The multi-dimensional sensor is capable of recognizing multiple external stimuli simultaneously, indicating respective directions without being disturbed by external variables.
Therefore, the flexible six-dimensional sensor technology can be applied to various complex environments, and feasibility of the flexible six-dimensional sensor technology is imperative.
Disclosure of Invention
In order to solve the problems in the background art, the invention provides a flexible six-dimensional force sensor based on a mortise and tenon structure, which solves the limitation of unilateral stimulation response, and realizes the identification of strain direction and the differentiation of stimulation types by the six-dimensional force sensor; the independent deformation mechanisms of the interlocking structures of the present invention enable the present sensor to decouple translational forces and torsional moments in the x, y, and z directions under different external stimuli. .
The technical scheme adopted by the invention is as follows:
the flexible six-dimensional force sensor based on the mortise and tenon structure comprises a flexible boss, a PTFE film, an FPCB flexible circuit board and a flexible base which are sequentially arranged from top to bottom; the flexible boss is of a quadrangular frustum pyramid structure, square convex blocks are arranged at four corner positions of the bottom surface, a cross structure matched with the square convex blocks of the flexible boss is arranged on the top surface of the flexible base, and the bottom of the flexible boss and the top of the flexible base are in concave-convex interlocking to form a mortise-tenon structure; the FPCB flexible printed circuit board is of a layered folding structure embedded with the top surface of the flexible base, the middle of the layered folding structure is hollowed to form four inverted U-shaped bulges, twelve flexible sensing units are arranged on the upper surface of the FPCB flexible printed circuit board and are respectively adhered to the top surfaces and the two side surfaces of the four bulges, and the twelve flexible piezoresistive sensing units are packaged through a PTFE (polytetrafluoroethylene) film attached to the upper surface of the FPCB flexible printed circuit board; the PTFE film is provided with the foam-rubber cushion with the same thickness as the flexible sensing unit at four corners of the upper surface corresponding to the square convex block of the flexible boss, so that gaps are prevented from being generated when the flexible boss and the flexible base are assembled up and down.
The flexible boss and the flexible base are made of flexible silica gel materials. The preparation method of the flexible boss 1 and the flexible base 6 comprises the following steps: manually stirring and mixing Polydimethylsiloxane (PDMS) and a curing agent (10: 1) for 10 minutes by a glass rod, then putting the mixture into a vacuum freeze dryer for vacuumizing operation, wherein the vacuumizing operation is to remove bubbles in the solution and prevent the vacuum phenomenon in the cured silica gel, then filling the vacuumized solvent into an external tool mold coated with a surface treatment agent, putting the external tool mold into an oven, heating and curing at the temperature of 70 ℃, and demolding.
The PTFE film, the flexible piezoresistive sensing unit and the FPCB flexible circuit board form a force sensing layer, and the force sensing bottom is bonded on the flexible base through an acrylic adhesive tape.
Interpolation electrodes are plated at the positions of the FPCB flexible circuit boards corresponding to the flexible piezoresistive sensing units, one lead of each interpolation electrode is integrated at the interface of the front face of the flexible piezoresistive sensing unit, and the other lead of each interpolation electrode is integrated at the interface of the back face of the flexible piezoresistive sensing unit.
When the flexible six-dimensional force sensor is acted by six-dimensional force in space, the flexible boss generates pressure to the piezoresistive sensing unit after being deformed, and the piezoresistive sensing unit transmits a voltage signal to an upper computer.
The six-dimensional force is specifically as follows: a three-dimensional coordinate system is constructed by taking the center of the top surface of the flexible boss as an origin, six-dimensional forces are respectively Fx, Fy, Fz, Mx, My and Mz, the Fx and the Fy are respectively transverse forces borne by the flexible boss in the directions of an x axis and a y axis, the Fz is a positive pressure borne by the top surface of the flexible boss in the direction of the z axis, the My and the Mz are transverse torque forces borne by the flexible boss in the directions of the x axis and the y axis, and the Mx is a longitudinal torque force borne by the flexible boss in the direction of the z axis.
The flexible six-dimensional force sensor is applied to intelligent external force detection equipment such as a robot fingertip and a wearable medical device and other complex environment detection stimulation occasions.
The invention has the beneficial effects that:
when the flexible six-dimensional force sensor is used, due to the fact that the flexible six-dimensional force sensor is small in size, light and portable, the lower surface of the flexible six-dimensional force sensor is attached to an object to be detected, and when the upper surface of the flexible six-dimensional force sensor is pressed or twisted and loaded from the outside, twelve piezoresistive sensing units in the sensor are stimulated, so that the sensor generates resistance change.
The flexible six-dimensional force sensor can be applied to intelligent external force detection equipment such as a machine finger tip, a wearable medical device and the like and other complex environment stimulation detection occasions according to the characteristics of the flexible six-dimensional force sensor. For example, when the robot needs to perform complex operations of object movement and rotation, such as screwing a bottle cap, solving a magic cube or assembling parts, the fingertips and joints of the robot will be subjected to forces and moments in three directions, and force sensing information during the operation just needs a six-dimensional force sensor for detection and decoupling.
Drawings
FIG. 1 is an exploded view of a flexible six-dimensional force sensor according to the present invention;
FIG. 2 is a schematic view of a flexible silica gel boss on a flexible six-dimensional force sensor in accordance with the present invention;
fig. 3 is a schematic view illustrating a folding structure of the FPCB flexible printed circuit board according to the present invention;
FIG. 4 is a schematic diagram of a flexible piezoresistive sensing unit of the flexible six-dimensional force sensor of the present invention;
FIG. 5 is a schematic view of a flexible six-dimensional force sensor foam pad of the present invention;
FIG. 6 is a schematic view of a flexible silicone base of the flexible six-dimensional force sensor of the present invention;
FIG. 7 is a schematic front and back sides of an FPCB flexible printed circuit board according to the present invention;
FIG. 8 is a schematic diagram showing the location distribution of twelve flexible piezoresistive sensing units according to the present invention;
FIG. 9 is a schematic structural diagram of a flexible six-dimensional force sensor according to the present invention;
FIG. 10 is a data diagram of the flexible six-dimensional force sensor of the present invention detecting spatial six-dimensional forces (Fx, Fy, Fz, Mx, My, Mz).
In the figure: the flexible piezoresistive pressure sensing device comprises a flexible boss 1, a PTFE film 2, a flexible piezoresistive sensing unit 3, a spongy cushion 4, an FPCB flexible printed circuit board 5 and a flexible base 6.
Detailed Description
The invention is explained in detail below with reference to the figures and examples.
As shown in fig. 1 and 9, the present invention includes a flexible boss 1 having a quadrangular frustum pyramid structure, a PTFE film 2 having a space-folding structure and capable of fitting the quadrangular frustum pyramid, a flexible piezoresistive sensing unit 3, an FPCB flexible printed circuit board 5 having a space-folding structure and capable of fitting the quadrangular frustum pyramid, and a flexible base 6 having a cross mechanism. The flexible boss and the cross-shaped flexible base 6 are inspired by the mortise and tenon structure of the traditional ancient Chinese building, and the upper boss and the lower base of the sensor are in concave-convex interlocking. The boss with the quadrangular frustum pyramid shape is formed by pouring flexible silica gel in a customized mould and demoulding, the whole body is soft, external load with six degrees of freedom can be realized, and the PTFE film 2 is used for receiving signals of the piezoresistive sensing units and is embedded in an interlocking structure and needs to be folded into a shape of a space structure; the flexible sensing units are cut into a square shape, twelve flexible piezoresistive sensing units are required for the sensor, and the flexible piezoresistive sensing units are uniformly distributed on the FPCB flexible printed circuit board and packaged by a PTFE film to obtain a force sensing layer; the FPCB flexible printed circuit board is a layer which has the same structure with the PTFE film and can be ensured to be embedded into the flexible silica gel base with a cross structure, and the front surface and the back surface of the FPCB flexible printed circuit board are respectively provided with a lead. Due to the existence of the flexible sensing unit, a gap is generated when the flexible interlocking structure is assembled, and the spongy cushion 4 with the same thickness as that of the piezoresistive sensing unit is used for filling the gap and is respectively attached to four top corners of the upper surface of the force sensing layer, so that the sensor is ensured to be balanced and stable in work.
The flexible boss 1 and the flexible base 6 are both formed by manually stirring and mixing Polydimethylsiloxane (PDMS) and a curing agent (10: 1) for 10 minutes by a glass rod, then putting the glass rod into a vacuum freeze dryer for vacuumizing operation, wherein the vacuumizing operation is to remove bubbles in a solution to prevent the vacuum phenomenon in the cured silica gel, then filling a vacuumized solvent into an external tool mold coated with a surface treatment agent, putting the external tool mold into an oven, heating and curing the solvent at the temperature of 70 ℃, and then demolding; the electrode layer is formed by plating electrodes on a magnetron sputtering instrument;
as shown in fig. 7, the flexible piezoresistive sensing units are purchased from commercial sensing units, twelve sensing units are divided into 1mm × 1mm × 0.1mm, PDMS curing agent (10: 1) is used for stirring, mixing and vacuuming solvent to be used as adhesive, and the twelve sensing units are respectively adhered to the upper surface of the FPCB flexible printed circuit board and two side surfaces of four ends of the cross structure.
When the flexible six-dimensional force sensor is used, due to the excellent characteristics of small volume (7mm multiplied by 7mm) and light weight and portability and high resolution (0.05N and 0.2 N.mm), the lower surface is attached to an object to be detected, and when the upper surface is pressed or twisted and loaded from the outside, twelve piezoresistive sensing units in the sensor are stimulated, so that the resistance of the sensor is changed. The flexible six-dimensional force sensor can be applied to intelligent external force detection equipment such as a machine finger tip and a wearable medical device and other occasions where complex environments detect stimulation according to the characteristics of the flexible six-dimensional force sensor.
The composite dielectric layer of the flexible six-dimensional force sensor integrates the PTFE film, the FPCB flexible printed circuit board and the twelve sensing units into a whole, and the base surface is formed by a plane which is continuously bent by 90 degrees, so that the stability of the structure is kept while the matrix undergoes certain deformation, the planar design thinking of the traditional flexible touch sensor is broken through, and the flexible six-dimensional force sensor is the core of the invention. The electrode layer is obtained by external processing according to the special customization of overall structure, and the purpose is that the signal change of the twelve units of convenient detection also easily decouples simultaneously.
External stimuli of six-dimensional forces (Fx, Fy, Fz, Mx, My, Mz) in space can be detected by four deformations of the upper flexible silicone protrusions and the flexible base interlocking structures. And drawing a calibration curve according to the data obtained by the calibrated data acquisition unit, wherein the detection range of the positive pressure Fz of the sensor is 0.1N-3N, the detection ranges of the lateral forces Fx and Fy are-1N, the detection ranges of the torques Mz and Mx are-4 N.mm, and the detection range of the My is 0.4 N.mm-4 N.mm. When the force exceeds 20% of the detection range, the upper bulge and the lower base of the sensor are separated, and the voltage change of the sensing unit is too small to identify the pressure change. As known from the measurement data of the attached drawings, the flexible six-dimensional force sensor not only can effectively solve the problem of unilateral stimulation response limitation, but also can identify the strain direction generated by the spatial six-dimensional force and distinguish different stimulation types.
As shown in fig. 8, twelve flexible piezoresistive sensing units 3 are arranged between the PTFE membrane 2 and the FPCB flexible circuit board 5, and each sensing unit can feed back signals independently due to reasonable spatial arrangement; four piezoresistive sensing units (R11, R12, R13 and R14) are arranged at corresponding positions of four end parts of the cross structure, two piezoresistive sensing units are respectively arranged on two sides of the four end parts, R21 and R22 are arranged on two sides of the R11, R31 and R32 are arranged on two sides of the R12, R41 and R42 are arranged on two sides of the R13, and R51 and R52 are arranged on two sides of the R14.
The method is characterized in that six-dimensional calibration force is applied to the flexible six-dimensional force sensor for calibration, four loading methods are designed, and the sensor is statically calibrated by positive pressure, transverse force, longitudinal torque and transverse torque. To make the loading operation more convenient and accurate, a mechanical test system (INSTRON LEGEND2345) is used to load the pressure and a data collector (DAQVANTECH USB _ HRF4028) is connected to the sensors to collect the voltage signals. The positive pressure calibration method is simple, the positive pressure is calibrated after pressure is directly applied to the upper surface of the sensor, the four pressure sensing units on the top surface of the flexible base are stressed to send data signal changes, and the data acquisition unit stops when the data acquisition unit cannot identify the voltage signal changes; the transverse force is obtained by applying a load through horizontal feeding, then the sensor is calibrated by the transverse force (Fx, Fy), after the horizontal force is applied, the stress of two pressure sensing units on one side of one beam of the flexible base cross structure can generate data signal change, and the sensor is stopped when a data acquisition unit cannot identify the voltage signal change; the calibration principle of the transverse torques (Mx and My) is calibrated by generating torques through force arms, four pressure sensing units on the single side surface and the upper surface of one beam in the cross structure of the flexible base are stressed to generate data signal changes after the torques are applied, and the calibration is stopped when a data acquisition unit cannot identify the voltage signal changes; the calibration mode of the longitudinal torque (Mz) is similar to that described above, the longitudinal torque Mz of the sensor is calibrated by using the z-axis torque calibration device, when (reverse) forward rotation is applied to the sensor, data signal changes occur in the four sensing units corresponding to the side surface of the flexible base, and the calibration is stopped when the data acquisition unit cannot identify the voltage signal changes.
Carrying out stress detection on the flexible six-dimensional force sensor: and calculating the six-dimensional calibration force and the calibration voltage signal by an orthogonal parallel six-dimensional force sensor static calibration algorithm to obtain a mapping relation matrix between the six-dimensional force and the calibration voltage signal, and calculating to obtain the acting force applied to the six-dimensional force sensor according to the mapping relation matrix and the voltage signal output by the flexible six-dimensional force sensor during detection. Or the flexible six-dimensional force sensor is calibrated for multiple times, applied six-dimensional force and corresponding calibration voltage signals are used as a sample set, the sample set is divided into a training set and a testing set and input into the DNN deep neural network for training, voltage signals output when the flexible six-dimensional force sensor detects are input into the DNN deep neural network after training, and the acting force applied to the six-dimensional force sensor is output.
According to the stress loading condition of the flexible six-dimensional force sensor, firstly, a coordinate system is defined, an upper flexible substrate of the flexible six-dimensional force sensor is used as a force measuring body, a coordinate axis is established on the upper surface of the flexible six-dimensional force sensor, the axis which is vertical to the center of the surface and is upward is a Z axis, the axis which is positioned on the upper surface of the substrate, is vertical to the Z axis and points to the right front side is an X axis, the axis which is positioned on the upper surface of the substrate, is vertical to the Z axis and points to the left front side is a Y axis, and the origin of the coordinate system is established in the center of the upper surface of the sensor as shown in figure 1; analyzing the sensing units clamped in the middle layer of the sensor as shown in fig. 8, when the sensor is subjected to external normal (Z-axis) pressure, the sensor is equivalent to extruding the whole sensor from top to bottom, the electrode layer changes, and four piezoresistive sensing units (R11, R12, R13 and R14) for detecting positive pressure output four changed signal values to the outside; when the flexible six-dimensional force sensor is subjected to horizontal tangential (X-axis or Y-axis) external force, two piezoresistive sensing units respectively receive signals when the flexible six-dimensional force sensor is loaded in different directions, for example, when the flexible six-dimensional force sensor is subjected to horizontal force along the X-axis forward direction, the two piezoresistive sensing units (R21 and R41) generate signal changes correspondingly, and if the flexible six-dimensional force sensor is subjected to opposite horizontal force along the X-axis, the flexible six-dimensional force sensor generates signals R22 and R42; when the sensor is subjected to a horizontal force along the positive Y-axis direction, the two piezoresistive sensing units (R32 and R52) generate signal changes, and when the sensor is subjected to a horizontal force along the negative Y-axis direction, the signals of R31 and R51 change; when the flexible six-dimensional force sensor is subjected to external loading torsion force (Mx, My or Mz), the deformation of the sensor under the action of the torque is more complicated than that under the action of the force, the flexible base body can rotate along the X, Y, Z three-axis torque, when the sensor is subjected to the forward torsion of the Z axis, the lower flexible base body is kept stable, the upper flexible base body rotates around the Z axis, the piezoresistive sensing units are stimulated by the extrusion deformation caused by the rotation so as to output changed signal values to the outside, when the applied torque is positive rotation force (clockwise rotation), the four piezoresistive sensing units of R21, R31, R42 and R52 can generate signals, and when the applied torque is opposite to counterclockwise rotation force, the signals are generated by the R22, R32, R41 and R51; when the sensor is subjected to a torque when the sensor is subjected to a tangential (X-axis or Y-axis) the flexible matrix deforms similarly to the deformation under tangential forces, since forces and moments in this direction will move the upper flexible matrix to one side, except that a transverse torque will cause a rotation, resulting in more response of the sensing unit.
Taking the coordinate axes shown in fig. 8 as an example, when clockwise torque is applied to the Y-axis, signals of R11, R21, R13 and R41 change, and similarly, when clockwise torque is applied to the X-axis, signal values of R51, R14, R31 and R12 occur, and accordingly, signal values of four piezoresistive sensing unit cells also occur under counterclockwise transverse torque.
Fig. 10 shows a variation curve of the measured force value and the standard load force value of the sensor in each direction. Wherein (a) the output value for each dimension is when a force in the x-direction is loaded; (b) when the output value for each dimension is loaded with a force in the y-direction; (c) the output value for each dimension is when a force in the z direction is loaded; (d) when the output value for each dimension is loaded with a moment in the x-direction; (e) when the output value for each dimension is loaded with a moment in the y-direction; (f) the output value for each dimension is when the moment in the z direction is loaded.
Claims (7)
1. A flexible six-dimensional force sensor based on a mortise and tenon structure is characterized by comprising a flexible boss (1), a PTFE film (2), an FPCB flexible circuit board (5) and a flexible base (6) which are sequentially arranged from top to bottom;
the flexible boss (1) is of a quadrangular frustum pyramid structure, square bumps are arranged at four corner positions of the bottom surface of the flexible boss (1), a cross structure matched with the square bumps of the flexible boss (1) is arranged on the top surface of the flexible base (6), and the bottom of the flexible boss (1) and the top of the flexible base (6) are in concave-convex interlocking to form a mortise and tenon structure;
the FPCB flexible printed circuit board (5) is of a layered folding structure embedded with the top surface of the flexible base (6), the middle of the layered folding structure is hollowed to form four inverted U-shaped bulges, twelve flexible sensing units (3) are arranged on the upper surface of the FPCB flexible printed circuit board (5), the twelve flexible sensing units are respectively adhered to the top surfaces and two side surfaces of the four bulges, and the twelve flexible piezoresistive sensing units (3) are packaged through PTFE (polytetrafluoroethylene) films (2) attached to the upper surface of the FPCB flexible printed circuit board (5);
and sponge pads (4) with the thickness consistent with that of the flexible sensing units are arranged at four corners of the upper surface of the PTFE film (2).
2. The flexible six-dimensional force sensor based on the mortise and tenon structure is characterized in that the flexible boss (1) and the flexible base (6) are made of flexible silica gel materials.
3. The flexible six-dimensional force sensor based on the mortise and tenon structure is characterized in that a PTFE film (2), a flexible piezoresistive sensing unit (3) and a FPCB flexible circuit board (5) form a force sensing layer, the force sensing layer is embedded between a flexible boss (1) and a flexible base (6), and the bottom of the force sensing layer is bonded on the flexible base (6) through an acrylic adhesive tape.
4. The flexible six-dimensional force sensor based on the mortise and tenon structure is characterized in that interpolation electrodes are plated at the position of the FPCB flexible circuit board (5) corresponding to each flexible piezoresistive sensing unit, one lead of all the interpolation electrodes is integrated at the interface of the front face of the flexible piezoresistive sensing unit, and the other lead of all the interpolation electrodes is integrated at the interface of the back face of the flexible piezoresistive sensing unit.
5. The flexible six-dimensional force sensor based on the mortise and tenon structure is characterized in that when the flexible six-dimensional force sensor is subjected to six-dimensional force in space, the flexible boss (1) deforms and then generates pressure on the piezoresistive sensing unit, and the piezoresistive sensing unit transmits a voltage signal to an upper computer.
6. The flexible six-dimensional force sensor based on the mortise and tenon structure of claim 5, wherein the six-dimensional force is specifically: a three-dimensional coordinate system is constructed by taking the center of the top surface of the flexible boss as an origin, six-dimensional forces are respectively Fx, Fy, Fz, Mx, My and Mz, the Fx and the Fy are respectively transverse forces borne by the flexible boss in the directions of an x axis and a y axis, the Fz is a positive pressure borne by the top surface of the flexible boss in the direction of the z axis, the My and the Mz are transverse torque forces borne by the flexible boss in the directions of the x axis and the y axis, and the Mx is a longitudinal torque force borne by the flexible boss in the direction of the z axis.
7. The flexible six-dimensional force sensor based on the mortise and tenon structure as claimed in claim 1, wherein the flexible six-dimensional force sensor is applied to a mechanical fingertip and a wearable medical device.
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN117249936A (en) * | 2023-09-27 | 2023-12-19 | 北京科技大学 | Flexible six-dimensional force sensor based on flexible optical waveguide |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
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| CN117249936A (en) * | 2023-09-27 | 2023-12-19 | 北京科技大学 | Flexible six-dimensional force sensor based on flexible optical waveguide |
| CN117249936B (en) * | 2023-09-27 | 2024-05-07 | 北京科技大学 | Flexible six-dimensional force sensor based on flexible optical waveguide |
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