A hardness sensor based on a polymer optical fiber knot sensitive structure and a silica gel substrate

Abstract

The present invention discloses a hardness sensor based on a polymer optical fiber knot sensitive structure and a silica gel substrate. It includes a polymer optical fiber, a silica gel layer, a probe and a rigid housing. A knot is formed on the polymer optical fiber to form a polymer optical fiber knot, and the polymer optical fiber knot is embedded in the silica gel layer and wrapped by the silica gel layer. Both the silica gel layer and the polymer optical fiber are placed in the rigid housing; the polymer optical fiber knot takes the optical fiber kink as the pressure sensitive point, and the pressure sensitive point is arranged close to the probe. The present invention has the characteristics of small size, simple production, low cost, anti-electromagnetic interference and corrosion resistance.

Description

Technical Field

[0001] The present invention relates to a hardness sensor, and more particularly to a hardness sensor based on a polymer optical fiber knot sensitive structure and a silicone substrate. Background Art

[0002] Hardness is an inherent physical property of an object. In the fields of mechanical engineering and robotics, hardness refers to the ability of a local surface of an object to resist penetration when it is squeezed by a harder object. According to the measured parameters, it can be divided into two categories: one measures the magnitude of the elastic deformation generated by the object under test when it bears a certain pressure; the other measures the ability of the object under test to resist plastic deformation when it is squeezed by a harder object. For touch, the hardness of an object generally refers to the feedback force from the object's resistance to deformation when a human hand touches the object.

[0003] In the field of materials, there are three most commonly used types of hardness measurements: scratch hardness, indentation hardness, and rebound hardness. Each type of hardness has its own measurement criteria and methods, and the mechanical meanings of various hardness standards are different, so they cannot be converted to each other, but the experimental results can be compared.

[0004] Indentation hardness refers to the ability of an object under test to resist intrusion when it is squeezed by a harder and sharper object. The measurement method is to place an indenter of a specified size on the object under test and apply a certain pressure. The magnitude of the plastic deformation generated on the material surface is equivalent to the hardness of the material under test. The most commonly used Brinell hardness, Rockwell hardness, Vickers hardness, etc. all belong to the category of indentation hardness, and their main classification basis is factors such as the type of indenter and the magnitude of the load used during measurement.

[0005] However, in the field of humanoid tactile perception, such as in robotic tactile sensors, it is difficult to directly apply the measurement method of indentation hardness, and a hardness sensor that is simple to use and does not damage the object under test needs to be developed. Summary of the Invention

[0006] In order to solve the problems existing in the background art, the purpose of the present invention is to provide a hardness sensor based on a polymer optical fiber knot sensitive structure and a silicone substrate, which has the characteristics of small size, simple production, low cost, anti-electromagnetic interference, and corrosion resistance.

[0007] The technical solution of the present invention is as follows:

[0008] I. A hardness sensor based on a polymer optical fiber knot sensitive structure and a silicone substrate:

[0009] It includes a polymer optical fiber, a silica gel layer, a probe, and a rigid housing. A knot is formed on the polymer optical fiber to form a polymer optical fiber knot, and the polymer optical fiber knot is embedded in the silica gel layer and wrapped by the silica gel layer. The silica gel layer and the polymer optical fiber are both placed in the rigid housing.

[0010] The described rigid housing includes a probe adapter, an upper cover, and a base; the upper cover and the base are connected to form the housing, a cavity is provided inside the housing, and the probe, the silica gel layer, and the polymer optical fiber wrapped by the silica gel layer are arranged in the cavity. The probe is installed in the cavity through the probe adapter.

[0011] The polymer optical fiber knot uses the kink of the optical fiber as the pressure-sensitive point, and the pressure-sensitive point is arranged close to the probe.

[0012] The two end pigtails of the polymer optical fiber extend out from the silica gel layer, pass through the holes in the base, and extend outwards to be respectively connected to a light source and a photodetector.

[0013] One end of the probe is a flat end face as the tail, which contacts the flat end face of the silica gel layer in the cavity, and the other end passes through the hole of the probe adapter and exposes as the head for contacting the surface of the object to be measured.

[0014] The polymer optical fiber knot is made by first making a loose knot in the polymer optical fiber without tightening it, then passing a metal rod through the loop in the loose knot and tightening it, and then removing the metal rod.

[0015] II. Manufacturing method applied to the hardness sensor:

[0016] The method is to first arrange the polymer optical fiber knot wound around the metal rod on the base, use the metal rod to define the winding diameter of the polymer optical fiber knot, then set a retaining wall above the peripheral edge of the base. A pool is formed in the middle of the retaining wall. After injecting liquid silica gel into the pool and curing it, the main part of the silica gel layer is made. Finally, after removing the metal rod, a hollow core is formed at the original position of the metal rod, and liquid silica gel is injected into the hollow core and cured to make the remaining part of the silica gel layer.

[0017] Take a section of polymer optical fiber and make a loose knot without tightening it. Pass the end pigtails of the polymer optical fiber through the holes in the base. Take a metal rod and pass it through the loop in the knot. Tighten the polymer optical fiber so that the loose knot binds the metal rod tightly, and embed the knot into the positioning groove in the base. Infiltrate some adhesive into the holes where the end pigtails of the polymer optical fiber pass through the base to make the knot not loose.

[0018] III. Hardness detection method applied to the hardness sensor:

[0019] Step 1: Connect one end of the polymer optical fiber in the hardness sensor to a light-emitting diode and the other end to a photodiode. Provide current to the light-emitting diode through a data acquisition board in real time to make the light-emitting diode emit light. The light emitted by the light-emitting diode is conducted through the polymer optical fiber to the photodiode and received. Measure the photocurrent received by the photodiode in real time and convert it into voltage V.

[0020] Step 2: Fix the hardness sensor on the displacement stage at the initial position so that the probe is perpendicular to the surface of the object to be measured.

[0021] Step 3: Move the hardness sensor at a constant speed so that the probe contacts the surface of the object to be measured, and then continue to move the hardness sensor so that the probe is completely pressed into the object to be measured until the upper cover of the hard shell contacts the surface of the object to be measured. Record the voltage V at this time, and then move the hardness sensor upward to the initial position.

[0022] Step 4: Calculate the hardness H = f(V) according to the voltage V and the hardness conversion fitting formula obtained by pre-calibration, where f() represents the function expression with V as the independent variable.

[0023] In specific implementation, H = -28.301V + 62.127. The usage range is 0.4 < V < 1.65, and the measurement range is Shore hardness type A 16 - 48.

[0024] The core sensing element of the present invention is made by knotting a polymer optical fiber. The polymer optical fiber knot is wrapped by silica gel, and the optical fiber kink region is the pressure-sensitive point. When the sensor probe is pressed into objects with different hardnesses, the probe tail presses the silica gel to stimulate the pressure-sensitive point for detection.

[0025] The polymer optical fiber is a common optical fiber made of materials such as polymethyl methacrylate. When the polymer optical fiber is knotted, an external force is applied at the knotting position to change the bending radius, resulting in a significant change in the light transmittance of the optical fiber and a significant change in the light intensity at the output end of the optical fiber.

[0026] Silica gel is used to protect and fix the polymer optical fiber knot, and at the same time adjust the sensitivity and range of the sensor. When the sensor probe is pressed into the surface of the object, the probe tail presses the silica gel to stimulate the pressure-sensitive point, weakening the light intensity at the end of the optical fiber. The higher the hardness of the object to be measured, the weaker the light intensity at the end of the optical fiber. The present invention cleverly makes a hardness sensor using this principle.

[0027] For a general hardness sensor, if a polymer optical fiber with a polymer optical fiber knot is set, it is difficult to fix the knot type at a specific position, form a specific orientation or a specific diameter. Secondly, if only an optical fiber is used, due to the certain hardness and plasticity of the material, the elasticity and load-bearing capacity of the knot type are limited, and it is difficult to flexibly control its sensitivity and range to force, resulting in limitations in the usage scenarios of the sensor.

[0028] In the present invention, innovatively, the polymer optical fiber knot is wrapped by a silica gel layer and then contacts with the probe for hardness detection. By coating the polymer optical fiber with the polymer optical fiber knot with the silica gel layer, that is, using the superior elastic mechanical properties and coating ability of the silica gel to solve the above technical problems in the manufacturing process and measurement, thereby reducing the manufacturing difficulty, improving the yield rate, and meeting the wide range requirements for sensitivity and range in various usage scenarios.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] The polymer optical fiber and silica gel of the present invention are common items with low prices. Therefore, the present invention does not need to purchase or synthesize special sensitive materials, nor does it require special chemical synthesis or precision micro-nano processing equipment;

[0031] The range and sensitivity of the hardness sensor of the present invention can be adjusted by parameters such as the optical fiber material, optical fiber diameter, polymer optical fiber knot diameter, silica gel hardness, probe diameter, and probe height;

[0032] The present invention can resist strong electromagnetic interference and tolerate humid and corrosive environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a cross-sectional schematic diagram of the hardness sensor of the present invention;

[0034] Figure 2 is an external schematic diagram of the present invention;

[0035] Figure 3 is an exploded schematic diagram of the present invention;

[0036] Figure 4 is a schematic diagram of the working principle of the present invention;

[0037] Figure 5 is a schematic diagram of making a polymer optical fiber knot with a specific size by means of a metal rod in the present invention;

[0038] Figure 6 is a schematic diagram of arranging the polymer optical fiber knot in the present invention;

[0039] Figure 7 is a schematic diagram of making the main body of the silica gel layer in the present invention;

[0040] Figure 8 is a schematic diagram of filling the hollow core of the silica gel layer in the present invention.

[0041] In the figure: 1—polymer optical fiber with a knot, 101—loose polymer optical fiber knot, 2—pressure-sensitive point on the knot, 3—silicone layer, 31—main body of the silicone layer, 4—rigid probe, 5—rigid housing, 51—probe adapter, 52—upper cover, 53—base, 6—object to be measured, 7—metal rod, 8—enclosure used to make the main body of the silicone layer, 9—pool formed by the enclosure and the base, 10—hollow core of the silicone layer. Detailed implementation mode

[0042] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0043] As Figure 1 shown, the specifically implemented sensor includes a polymer optical fiber 1, a silicone layer 3, a probe 4 and a rigid housing 5. An annular knot is formed on the polymer optical fiber 1 to form a polymer optical fiber knot, and the polymer optical fiber knot is embedded in the silicone layer 3 and wrapped by the silicone layer 3. Both the silicone layer 3 and the polymer optical fiber 1 are placed in the rigid housing 5.

[0044] The rigid housing 5 includes a probe adapter 51, an upper cover 52 and a base 53; the upper cover 52 and the base 53 are connected to form a housing, and the upper cover and the base are fixed by two screws. A cavity is provided inside the housing, and the probe 4, the silicone layer 3 and the polymer optical fiber 1 wrapped by the silicone layer 3 are arranged in the cavity. The probe 4 is installed in the cavity through the probe adapter 51. The polymer optical fiber knot wrapped by silicone is placed on the base, and the upper part is covered by the upper cover.

[0045] The rigid probe 4, the probe adapter 51, the upper cover 52 and the base 53 are all made of metal materials, such as aluminum alloy, or can also be common non-metal materials for 3D printing such as acrylonitrile-butadiene-styrene copolymer ABS.

[0046] The polymer optical fiber is a type of optical fiber with a core layer material made of highly transparent polymers such as polymethyl methacrylate PMMA, polystyrene PS, and polycarbonate PC, and a cladding layer material made of fluoropolymer or PMMA with a lower refractive index.

[0047] The polymer optical fiber knot uses the kink or overlap of the optical fiber as the pressure-sensitive point, and the pressure-sensitive point is arranged close to the probe 4, that is, the thickness of the silicone layer 3 between the polymer optical fiber knot at the pressure-sensitive point and the probe 4 is the smallest.

[0048] The two end pigtails of the polymer optical fiber 1 extend out of the silicone layer 3 and pass through the holes in the base 53 and extend outwards to be respectively connected to a light source and a photodetector, so that light is incident from one end of the polymer optical fiber 1 and exits from the other end.

[0049] The probe adapter 51 separates the upper cover 52 from the probe 4 and is located in the center of the upper cover 52.

[0050] One end of the probe 4 is a flat end face as the tail, which contacts the flat end face of the silica gel layer 3 in the cavity, and the other end passes through the hole of the probe adapter 51 and exposes as a hemispherical head for contacting the surface of the object to be measured 6.

[0051] When the head of the probe 4 is pressed into the surface of the object, the tail of the probe 4 will press the silica gel layer 3 and stimulate the pressure-sensitive point.

[0052] In this way, after the probe 4 is completely pressed into the probe adapter 51, since the upper cover 52 begins to contact the object to be measured 6, the probe 4 no longer continues to press the silica gel layer 3 inward, and the pressure reaches the maximum value and remains stable. At this time, the transmittance of the polymer optical fiber knot reaches the lowest value, and the light intensity at the end of the optical fiber is also the lowest value and remains stable.

[0053] The hardness of the object to be measured 6 affects the depth of the probe pressed into it, thereby affecting the pressure of the probe pressing the silica gel layer. The higher the hardness of the object to be measured 6, the weaker the light intensity at the end of the optical fiber.

[0054] As Figure 5 shown, the polymer optical fiber knot is made by first tying a loose and non-tightened knot 101 with the polymer optical fiber 1, then passing the metal rod 7 through the loop in the loose knot 101 and tightening it, and then taking out the metal rod 7.

[0055] The solution of the present invention adjusts and controls the sensitivity and range of the sensor by adjusting the elastic coefficient of the material used for the polymer optical fiber knot, the winding diameter of the polymer optical fiber knot, the hardness, thickness of the silica gel layer, the diameter and height of the probe, etc.

[0056] In specific implementation, the two tail fibers of the polymer optical fiber 1 pass through two holes on the base 53 and are fixed to the hole wall surface by an adhesive. The diameter of the hole is 270 microns, slightly larger than the optical fiber diameter of 250 microns. The distance between the two holes is equal to the diameter of the polymer optical fiber knot. In the present invention, the diameter of the polymer optical fiber knot refers to the inner diameter of the polymer optical fiber knot. The outer diameter of the polymer optical fiber knot differs from the inner diameter by twice the optical fiber diameter, i.e., 500 microns. The diameter of the polymer optical fiber knot shall not be less than 2 mm, otherwise the polymer optical fiber knot will have serious plastic deformation and lose pressure sensitivity.

[0057] The silica gel layer 3 wraps the polymer optical fiber knot inside and is tightly combined with the base 53. The top surface of the silica gel layer 3 is higher than the pressure-sensitive point 2 by a height difference of 1 mm. The height difference should be greater than zero to ensure that the pressure-sensitive point 2 is not worn. The greater the height difference, the higher both the detection lower limit and upper limit of the present invention, and the lower the sensitivity. The hardness of the silica gel layer can be Shore 00 type 10 - 50 or Shore A type 10 - 60. The harder the silica gel, the larger the range of the present invention and the lower the sensitivity.

[0058] As Figure 4As shown in the figure, after the hard probe 4 comes into contact with the object 6 to be measured, the head of the probe 4 presses into the surface of the object 6 to be measured, and the tail of the probe 4 squeezes the silica gel layer 3 inside the hard shell 5. The increase in the internal pressure is measured by the pressure-sensitive point 2 of the polymer optical fiber knot 1, which is manifested as a decrease in the optical fiber transmittance and a decrease in the light intensity at the end of the optical fiber. When the hard shell 5 comes into contact with the object 6 to be measured, the hard probe 4 no longer squeezes the internal silica gel layer 3, and the light intensity at the end of the optical fiber reaches equilibrium. Since the hardness of the object 6 to be measured determines the depth that the hard probe 4 can press into it, and thus determines the pressure applied to the silica gel layer 3, there is a clear functional relationship between the light intensity at the end of the optical fiber and the hardness of the object 6 to be measured. By calibrating the sensor of the present invention using an object 6 to be measured with a known hardness and clarifying this functional relationship, this sensor can be used for hardness measurement.

[0059] The manufacturing process of the sensor of the present invention is as follows:

[0060] As Figures 6 - 8 shown in the figure, the method is to first arrange a polymer optical fiber knot wound around a metal rod 7 on a base 53, use the metal rod 7 to define the winding diameter of the polymer optical fiber knot, then set up a retaining wall 8 above the peripheral edge of the base 53. A pool 9 is formed in the middle of the retaining wall 8. After injecting liquid silica gel into the pool 9 and curing it, the main part of the silica gel layer 3 is made. Finally, after removing the metal rod 7, a hollow core 10 is formed at the original position of the metal rod 7. After injecting liquid silica gel into the hollow core 10 and curing it, the remaining part of the silica gel layer 3 is made. The silica gel injected twice is tightly combined at the interface to form an integral body without delamination.

[0061] In specific implementation, the polymer optical fiber 1 first makes a loose knot but does not tighten it, and then the metal rod 7 is passed through the loop of the loose knot and tightened to form a polymer optical fiber knot wound around the metal rod 7.

[0062] As Figure 6 shown in the figure, take a section of polymer optical fiber 1 to make a loose knot but do not tighten it. Pass the pigtails at both ends of the polymer optical fiber 1 through the holes of the base 53. Take a metal rod 7 and pass it through the loop in the knot. Tighten the polymer optical fiber 1 so that the loose knot binds the metal rod 7 tightly, and embed the knot into the positioning groove pre-opened in the base 53. The width and depth of the positioning groove are the same as or slightly larger than the diameter of the optical fiber. Immerse some adhesive in the holes where the pigtails of the polymer optical fiber 1 pass through the base 53 to make the knot not loose.

[0063] As Figures 7 - 8As shown, then set the enclosure 8 on the base 53, and inject liquid silicone into the pool 9 formed by the enclosure 8 and the base 53 to wrap the upper half of the polymer optical fiber knot 1 and the metal rod 7, forming the main body 31 of the silicone layer. After the main body 31 of the silicone layer is cured, the metal rod 7 is spirally taken out from it (while pushing or pulling the metal rod, rotate the metal rod) to form the hollow core 10. Inject the same kind of liquid silicone into the hollow core 10 to wrap the remaining part of the polymer optical fiber knot 1. The silicone injected twice is tightly combined at the interface without delamination. After the silicone is cured, the silicone layer 3 is formed.

[0064] The hardness detection process of the sensor of the present invention is as follows:

[0065] Step 1:

[0066] Connect one end of the polymer optical fiber 1 in the hardness sensor to the light-emitting diode, and the other end to the photodiode; in real time, provide current to the light-emitting diode through the data acquisition board to make the light-emitting diode emit light. The light emitted by the light-emitting diode is conducted through the polymer optical fiber 1 to the photodiode and received. Measure the photocurrent received by the photodiode in real time and convert it into the voltage V;

[0067] Step 2: Fix the hardness sensor on the displacement table at the initial position so that the probe 4 is perpendicular to the surface of the object to be measured 6;

[0068] Step 3: Slowly move the hardness sensor at a constant speed so that the probe 4 contacts the surface of the object to be measured 6, and then continue to move the hardness sensor so that the probe 4 is completely pressed into the object to be measured 6 until the upper cover 52 of the hard shell 5 contacts the surface of the object to be measured 6. Record the stable voltage V at this time, and then slowly move the hardness sensor up to the initial position;

[0069] Step 4: Calculate the hardness H = f(V) according to the voltage V combined with the hardness conversion fitting formula obtained by pre-calibration, where f() represents the function expression with V as the independent variable. In the present invention, H = -28.301V + 62.127. The usage range is 0.4 < V < 1.65, and the measurement range is Shore hardness type A 16 - 48.

[0070] Calibrate in advance according to the objects to be measured with different known hardnesses in Steps 1 - 3 of the usage steps to determine the hardness conversion fitting formula H = f(V). The hardness of the object to be measured is measured by a hardness meter.

[0071] If silicone with a Shore 00 type hardness of 30 and a polymer optical fiber knot with a diameter of 3.5 mm is used, the hardness conversion fitting formula obtained by calibration can refer to H = -28.3V + 62.1, and the effective range is 0.4

Claims

1. A hardness sensor based on a polymer optical fiber knot sensitive structure and a silica gel substrate, characterized in that: It includes a polymer optical fiber (1), a silica gel layer (3), a probe (4) and a rigid housing (5). A knot is formed on the polymer optical fiber (1) to form a polymer optical fiber knot, and the polymer optical fiber knot is embedded in the silica gel layer (3) and wrapped by the silica gel layer (3). Both the silica gel layer (3) and the polymer optical fiber (1) are placed in the rigid housing (5). The rigid housing (5) includes a probe adapter (51), an upper cover (52) and a base (53). The upper cover (52) and the base (53) are connected to form a housing. A cavity is provided inside the housing, and the probe (4), the silica gel layer (3) and the polymer optical fiber (1) wrapped by the silica gel layer (3) are arranged in the cavity. The probe (4) is installed in the cavity through the probe adapter (51). The polymer optical fiber knot takes the fiber kink as the pressure sensitive point, and the pressure sensitive point is arranged close to the probe (4).

2. The hardness sensor based on a polymer optical fiber knot sensitive structure and a silica gel substrate according to claim 1, characterized in that: Both ends of the polymer optical fiber (1) extend out of the silica gel layer (3), pass through the holes in the base (53) and extend outwards to be respectively connected to a light source and a photodetector.

3. The hardness sensor based on a polymer optical fiber knot sensitive structure and a silica gel substrate according to claim 1, characterized in that: One end of the probe (4) is a flat end face as the tail, which contacts the flat end face of the silica gel layer (3) in the cavity, and the other end passes through the hole of the probe adapter (51) and exposes as the head for contacting the surface of the object to be measured (6).

4. The hardness sensor based on a polymer optical fiber knot sensitive structure and a silica gel substrate according to claim 1, characterized in that: The polymer optical fiber knot is made by first tying a loose knot in the polymer optical fiber (1) without tightening, then passing a metal rod (7) through the loop in the loose knot and tightening it, and then taking out the metal rod (7).

5. A hardness detection method applied to the hardness sensor according to any one of claims 1-4, characterized in that: Step 1: Connect one end of the polymer optical fiber (1) in the hardness sensor to a light-emitting diode and the other end to a photodiode. Provide current to the light-emitting diode through a data acquisition board in real time to make the light-emitting diode emit light. The light emitted by the light-emitting diode is conducted through the polymer optical fiber (1) to the photodiode and received. Measure the photocurrent received by the photodiode in real time and convert it into a voltage V. Step 2: Fix the hardness sensor on the displacement stage at the initial position so that the probe (4) is perpendicular to the surface of the object to be measured (6). Step 3: Move the hardness sensor at a constant speed so that the probe (4) contacts the surface of the object to be measured (6), and then continue to move the hardness sensor so that the probe (4) is completely pressed into the object to be measured (6) until the upper cover (52) of the rigid housing (5) contacts the surface of the object to be measured (6). Record the voltage V at this time, and then move the hardness sensor upward to the initial position. Step 4: Calculate the hardness H = f(V) according to the voltage V in combination with the hardness conversion fitting formula obtained by pre-calibration, where f() represents the function expression with V as the independent variable.

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