A device and method for measuring small force based on a variable stiffness flexible pivot structure

Abstract

The present invention discloses a device and method for measuring micro-force based on a variable stiffness flexible pivot structure, the device includes a cylindrical capacitor, a Z-axis precision displacement stage, a first XY horizontal displacement stage, a flexible pivot structure, a differential laser interferometer, a center of gravity adjustment block, a tray, a tension spring, and a Z-axis lifting platform. The flexible pivot structure can generate vertical motion around the rotation center, including an upper rod, a middle rod, a bottom rod, and a plurality of leaf-shaped flexible hinges; the differential laser interferometer measures the relative displacement between the outer electrode of the cylinder and the inner electrode of the cylinder; the center of gravity adjustment block adjusts the center of gravity of the flexible pivot structure; the tray is used to carry the loaded force value, and the lower end of the middle rod is connected to the inner electrode of the cylinder; one end of the tension spring is fixed on the Z-axis lifting platform, and the tension force is variable. The present invention can realize the adjustment of the vertical stiffness of the flexible pivot structure, and meet the force resolution and measurement accuracy requirements of micro-force values ​​within the range of 10μN to 1mN.

Description

Technical Field

[0001] The invention belongs to the field of tiny force value measurement, and in particular relates to a tiny force value measurement device and method based on a variable stiffness flexible pivot structure. Background Art

[0002] Modern space exploration and micro-nano manufacturing research fields require high resolution and high accuracy in the measurement of micro forces. At present, the application of micro forces is mainly concentrated in the range of 10μN to 1mN, and the measurement uncertainty of micro forces needs to be better than 10 -5 Magnitude.

[0003] The measurement results of tiny forces are limited by the accuracy of physical weights in the milligram (mg) range, and the existing measurement system can only achieve 10 -3 The measurement uncertainty of the order of magnitude cannot meet the demand for small force measurement in the future high-quality industrial development represented by miniaturization, intelligence and digitalization. With the announcement of the use of Planck's constant ( h ) redefines the unit of mass, so that the tiny force value that originally relied on the physical mass benchmark can be directly traced back to the Planck constant, getting rid of the dependence on physical weights, and reducing its measurement uncertainty from the existing 10 -3 Breakthrough to 10 -4 At present, the electrostatic force balance device based on the electrostatic force method is mainly used internationally to achieve the purpose of tracing the tiny force value to the Planck constant. The electrostatic force generated by the capacitor under the action of the electric field is used to balance the force value to be measured, and the force value to be measured is associated with the quantized basic units such as time, charge and length, so as to complete the direct tracing of the tiny force value.

[0004] As a prior art, CN204101217U discloses a micro-nano force value standard device based on the principle of electrostatic force, which uses a cylindrical capacitor, in which the cylindrical outer electrode is fixed on the base, and the inner electrode is suspended in the electric field of the outer electrode through an elliptical hinge. The device reduces the stiffness of the inner electrode in the direction of movement through an elliptical hinge with self-weight balance. Due to the low processing accuracy of the integrated elliptical hinge, the stiffness in the vertical direction can only be reduced to about 10N / m, and the stiffness cannot be adjusted after the processing is completed.

[0005] However, in order to reduce the measurement uncertainty of small force values ​​to 10 within the range of 10μN to 1mN, -5The force resolution should be better than 0.1nN at 10μN and better than 10nN at 1mN. The corresponding stiffness range of the flexible structure in the vertical direction of the electrostatic force balance device should be 0.01N / m to 1N / m. However, the flexible structure of the prior art usually adopts an integrated elliptical flexible hinge. Since the stiffness of the elliptical flexible hinge is affected by the processing accuracy and cannot be adjusted after forming, the stiffness value of the electrostatic force balance is unique and cannot be adjusted over a large range, resulting in a narrow measurement range. The force resolution is not enough to meet the measurement uncertainty requirements of small force values ​​in the range of 10μN to 1mN. Summary of the invention

[0006] The purpose of the present invention is to provide a small force measurement device and method based on a variable stiffness flexible pivot structure to address the problem that the stiffness of the integrated flexible elliptical hinge in the existing electrostatic force balance device cannot be adjusted, resulting in a narrow range of small force measurements and excessive measurement uncertainty. The device and method can adjust the vertical stiffness of the flexible pivot structure to meet the requirements of force resolution and measurement accuracy within different measurement ranges.

[0007] One aspect of the present invention provides a small force measurement device based on a variable stiffness flexible pivot structure, comprising a cylindrical capacitor, a Z-axis precision translation stage, a first XY horizontal translation stage, a flexible pivot structure, a differential laser interferometer, a center of gravity adjustment block, a tray, a tension spring, and a Z-axis lifting platform.

[0008] The cylindrical capacitor comprises a cylindrical outer electrode and a cylindrical inner electrode, the cylindrical outer electrode is fixed on a Z-direction precision displacement stage, the Z-direction precision displacement stage is fixed on a first XY horizontal displacement stage, the first XY horizontal displacement stage is used to adjust the relative position of the cylindrical outer electrode and the cylindrical inner electrode in the horizontal direction to ensure that the cylindrical outer electrode and the cylindrical inner electrode are coaxially installed, and the Z-direction precision displacement stage is used to drive the cylindrical outer electrode to reciprocate in the vertical direction;

[0009] The flexible pivot structure can generate vertical movement around the rotation center, and includes an upper rod, a middle rod, a bottom rod, a fixed support and a plurality of leaf spring-type flexible hinges. The upper rod, the middle rod and the bottom rod form a centrally symmetrical mechanical structure. One end of the upper rod and the bottom rod are respectively connected to the fixed support, and the other ends of the upper rod and the bottom rod are respectively connected to the two ends of the middle rod. The upper rod, the middle rod, the bottom rod and the fixed support are fixedly connected by the plurality of leaf spring-type flexible hinges. The end of the cylindrical inner electrode is connected to the flexible pivot structure. The vertical movement of the flexible pivot structure around the rotation center can drive the cylindrical inner electrode to generate relative displacement in the cylindrical outer electrode.

[0010] The differential laser interferometer is used to inject a measuring beam into the bottom of the cylindrical outer electrode and the cylindrical inner electrode to measure the relative displacement between the cylindrical outer electrode and the cylindrical inner electrode;

[0011] The center of gravity adjustment block is connected to the upper rod of the flexible pivot structure and is used to adjust the center of gravity of the flexible pivot structure so that it coincides with the rotation center of the flexible pivot structure;

[0012] The tray is connected to the upper end of the middle rod to bear the loaded force, the lower end of the middle rod is connected to the cylindrical inner electrode, and the central symmetry axis of the tray and the cylindrical inner electrode coincides;

[0013] One end of the tension spring is fixed to the symmetrical center position of one end of the upper rod, and the other end is fixed to the Z-axis lifting platform. The tensioning force of the tension spring can change with the movement of the Z-axis lifting platform along the Z-axis.

[0014] Preferably, the overall stiffness of the flexible pivot structure in the vertical direction is:

[0015] ,

[0016] in, k 1 is the initial positive stiffness of the flexible pivot structure, k 2 is the elastic coefficient of the extension spring, l 1 is the length from the end of the flexible pivot structure to the rotation center, l 0 is the original length of the extension spring, l 2 is the length of the tension spring after stretching.

[0017] Preferably, the device further comprises an extension rod, a damping aluminum sheet, a permanent magnet and a second XY horizontal displacement stage.

[0018] The extension rod extends along one end of the bottom rod, and the damping aluminum sheet is connected to the end of the extension rod, and can generate a damping force in the magnetic field formed by the permanent magnet, so as to reduce the convergence time of the flexible pivot structure after the force value is loaded. The permanent magnet is fixed on the second XY horizontal translation stage, and the second XY horizontal translation stage is used to adjust the position of the permanent magnet to ensure that the damping aluminum sheet is always in the middle position of the permanent magnet.

[0019] Preferably, the transfer function of the flexible pivot structure is:

[0020] ,

[0021] in, J M is the moment of inertia of the flexible pivot structure, F is the small force to be measured, c The damping force generated by the damping aluminum sheet.

[0022] Preferably, the upper rod, the middle rod and the bottom rod are sheet structures, and there are four fixed supports, two of which are connected to the two corners of one end of the upper rod, and the other two fixed supports are connected to the two corners of one end of the middle rod; there are eight spring-type flexible hinges, which are respectively located on the eight corners connected between the upper rod, the middle rod, the bottom rod and the fixed supports.

[0023] Another aspect of the present invention provides a method for measuring a small force value based on a variable stiffness flexible pivot structure, using the above device to measure a small force value, comprising:

[0024] Step S1: According to the force value to be measured F m The size of the Z-axis lifting platform installed with the tension spring is adjusted so that the tension of the tension spring is adjusted to a predetermined value, thereby making the overall stiffness of the flexible pivot structure in the vertical direction within a predetermined range;

[0025] Step S2: The outer cylindrical electrode is driven to reciprocate in the vertical direction by the Z-axis precision translation stage to measure the capacitance value of the cylindrical capacitor. At the same time, the relative displacement value between the inner cylindrical electrode and the outer cylindrical electrode is measured by the differential laser interferometer. The capacitance gradient d of the cylindrical capacitor is calculated based on the measurement results. C / d z ;

[0026] Step S3: by applying voltage to both ends of the cylindrical capacitor, the cylindrical inner electrode and the cylindrical outer electrode reach an initial equilibrium position. At this time, the voltage applied to both ends of the cylindrical capacitor is V U The electrostatic force generated is F e1 ;

[0027] Step S4: The force value to be measured F m When the load is applied to the tray in the vertical direction, the inner electrode of the cylinder moves in the vertical direction under the guidance of the flexible pivot structure. The relative displacement between the inner electrode of the cylinder and the outer electrode of the cylinder increases, and the potential difference applied to the two ends of the inner electrode of the cylinder and the outer electrode of the cylinder is changed, so that the relative displacement between the inner electrode of the cylinder and the outer electrode of the cylinder in the vertical direction is restored to the initial equilibrium position. The voltage applied to the two ends of the cylindrical capacitor at this time is V L The electrostatic force generated is F e2 ;

[0028] Step S5: Calculate the force value to be measured as follows:

[0029] .

[0030] The device and method for measuring small forces based on a variable stiffness flexible pivot structure according to the above aspects of the present invention can adjust the vertical stiffness of the flexible pivot structure to meet the requirements of force resolution and measurement accuracy within different measurement ranges. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings used in the description of the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work:

[0032] Figure 1 It is a structural schematic diagram of a small force value measuring device based on a variable stiffness flexible pivot structure according to an embodiment of the present invention;

[0033] Figure 2 is a schematic structural diagram of a flexible pivot structure according to an embodiment of the present invention;

[0034] Figure 3 It is a schematic diagram of the stiffness adjustment principle of the flexible pivot structure of an embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] An embodiment of the present invention provides a small force value measuring device based on a variable stiffness flexible pivot structure. Figure 1 FIG. 1 is a schematic diagram of a structure of a small force value measuring device based on a variable stiffness flexible pivot structure according to an embodiment of the present invention. Figure 1 As shown, the small force measuring device based on the variable stiffness flexible pivot structure of the embodiment of the present invention includes a cylindrical capacitor 1, a Z-axis precision translation table 2, a first XY horizontal translation table 3, a flexible pivot structure 4, a differential laser interferometer 5, a center of gravity adjustment block 7, a tray 8, a tension spring 9, and a Z-axis lifting platform 10.

[0037] The cylindrical capacitor 1 includes a cylindrical outer electrode 11 and a cylindrical inner electrode 12. The cylindrical outer electrode 11 is fixed on a Z-axis precision translation stage 2. The Z-axis precision translation stage 2 is fixed on a first XY horizontal translation stage 3. The first XY horizontal translation stage 3 is used to adjust the relative position of the cylindrical outer electrode 11 and the cylindrical inner electrode 12 in the XY horizontal direction to ensure that the cylindrical outer electrode 11 and the cylindrical inner electrode 12 are coaxially installed. The Z-axis precision translation stage 2 is used to drive the cylindrical outer electrode 11 to reciprocate in the vertical direction.

[0038] The end of the cylindrical inner electrode 12 is connected to the extended end of the flexible pivot structure 4. When the flexible pivot structure 4 moves in the vertical direction around the rotation center, it can drive the cylindrical inner electrode 12 to generate the same relative displacement in the cylindrical outer electrode 11. The differential laser interferometer 5 injects the measuring beam into the bottom of the cylindrical outer electrode 11 and the cylindrical inner electrode 12 through the reflector group 6. When the cylindrical outer electrode 11 and the cylindrical inner electrode 12 generate relative movement, the differential laser interferometer 5 can measure the relative displacement.

[0039] Figure 2 Schematic diagram of the structure of a flexible pivot structure of an embodiment of the present invention, wherein (a) is a three-dimensional view, (b) is a partial enlarged view, and (c) is a partial top view. Figure 2 As shown, in this embodiment, the flexible pivot structure 4 includes an upper rod 41, a middle rod 42, a bottom rod 43, a fixed support 45 and a plurality of leaf spring-type flexible hinges 44. The three independent rods of the upper rod 41, the middle rod 42 and the bottom rod 43 form a centrally symmetrical mechanical structure. One end of the upper rod 41 and the bottom rod 43 are respectively connected to the fixed support 45, and the other end is respectively connected to the two ends of the middle rod 42. The fixed support 45 can be fixedly connected to other supporting mechanisms. The upper rod 41, the middle rod 42, the bottom rod 43 and the fixed support 45 are fixedly connected by the plurality of leaf spring-type flexible hinges. Figure 2 In the illustrated embodiment, the upper rod 41, the middle rod 42 and the bottom rod 43 are sheet structures, there are four fixed supports 45, two of which are connected to the two corners at one end of the upper rod 41, and the other two are connected to the two corners at one end of the middle rod 42; there are eight spring-type flexible hinges, which are respectively located at the eight corners connected between the upper rod 41 and the middle rod 42, and the bottom rod 43 and the fixed supports 45.

[0040] The length from the rotation center to the end of the flexible pivot structure 4 is l 1. The rods are fixedly connected by reed-type flexible hinges to reduce the vertical stiffness of the flexible pivot structure 4. The vertical stiffness of the flexible pivot structure 4 composed of 8 reed-type flexible hinges is k1. The upper rod 41 of the flexible pivot structure 4 is connected to a gravity center adjustment block 7, which is used to adjust the gravity center of the entire flexible pivot structure 4 so that it coincides with the rotation center of the flexible pivot structure 4. The distance between the gravity center adjustment block 7 and the rotation center of the flexible pivot structure 4 is h， The mass of the center of gravity adjustment block is m The upper end of the middle rod 42 is connected to a tray 8 for bearing the loaded force, and the lower end is connected to the cylindrical inner electrode 12. The central symmetry axis of the tray 8 and the cylindrical inner electrode 12 coincide with each other, ensuring that the movement direction of the cylindrical inner electrode 12 caused by the force loading coincides with the direction of gravity.

[0041] The elastic modulus is k The tension spring 9 of 2 is used to generate negative stiffness. One end of the tension spring 9 is fixed at the symmetrical center position of one end of the upper rod 41, so that the tension spring 9 passes through the central symmetry line of the two leaf spring type flexible hinges of the one end of the upper rod 41 in the vertical direction, and the other end is fixed on a Z-axis lifting platform 10 moving in the Z-axis direction. The tensioning force of the tension spring 9 can change with the movement of the Z-axis lifting platform 10 in the Z-axis direction. The original length of the tension spring 9 is l 0, the length after stretching is l 2.

[0042] exist Figure 1 In the embodiment of the present invention, the micro-force value measuring device further includes a damping aluminum sheet 20, a permanent magnet 21, a second XY horizontal translation stage 22 and an extension rod 23. The extension rod 23 extends along one end of the bottom rod 43. In order to form a symmetrical structure, the extension length is also l 1. A damping aluminum sheet 20 is connected to the end of the extension rod 23, which can generate a damping force in the fixed magnetic field formed by the permanent magnet 21, and is used to reduce the convergence time of the flexible pivot structure 4 after being loaded. The volume of the damping aluminum sheet 20 in the magnetic field can be adjusted to generate damping forces of different sizes. The permanent magnet 21 is fixed on the second XY horizontal displacement stage 22, and the position of the permanent magnet 21 is adjusted by the second XY horizontal displacement stage 22 to ensure that the damping aluminum sheet 20 is always in the middle position of the permanent magnet 21.

[0043] In order to trace the tiny force directly to the Planck constant and improve the measurement uncertainty of the tiny force, the change of the force needs to be converted into the change of the displacement in the vertical direction in the electrostatic force balance. Therefore, the flexible pivot structure in the electrostatic force balance is required to be close to a zero stiffness system in the loading direction of the measured force, and the stiffness in the other five degrees of freedom must be much greater than the loading direction of the measured force, to ensure that the loading direction of the measured force is the same as the movement direction of the flexible pivot structure and coincides with the gravity direction. That is, the flexible pivot structure is a single degree of freedom system, and displacement is only allowed along the gravity direction after the force is loaded. Considering the bearing capacity and freedom limitation of the flexible pivot structure, it is impossible to use other hinge combinations with lower torsional stiffness. The reed-type flexible hinge has the smallest stiffness among them, with a nominal stiffness of 0.0013N / m rad. Therefore, on the basis of using the reed-type flexible hinge, it is necessary to further reduce the stiffness of the entire flexible pivot structure. At the same time, as the stiffness of the flexible pivot structure decreases, external disturbances such as loading and unloading of the load and environmental disturbances will act as excitation sources to affect the stability of the system, causing the entire system to be in a weakly damped oscillation state, and the electrostatic force output cannot be converged to the set value through the control method. Therefore, it is necessary to flexibly adjust the system damping ratio according to the stiffness. In this regard, the embodiment of the present invention compensates for the positive stiffness of the flexible pivot structure by increasing the negative stiffness, reduces the overall stiffness of the flexible pivot structure in the vertical direction, improves the force value resolution of the electrostatic force balance, and adds a passive magnetic damping system with an adjustable damping ratio to achieve stable control of the flexible pivot system under different stiffness conditions.

[0044] Specifically, the micro-force value measuring device based on the variable stiffness flexible pivot structure of the embodiment of the present invention compensates for the positive stiffness of the flexible pivot structure by increasing the negative stiffness, thereby reducing the overall stiffness of the flexible pivot structure in the vertical direction. The stiffness adjustment principle of the flexible pivot structure is as follows: Figure 3 According to the momentum theorem, the dynamic equation of the flexible pivot system about the rotation center O is:

[0045] (1)

[0046] in, J M is the moment of inertia of the flexible pivot structure, F is the small force to be measured applied at the end A of the flexible pivot, c is the magnetic damping force applied at the end B of the flexible pivot. Considering that the flexible pivot system makes a small swing near the equilibrium position, formula (1) is locally linearized, that is, sin θ (t)= θ (t), cos θ (t)=1, then formula (1) can be expressed as follows:

[0047] (2)

[0048] The initial conditions of the system are: F (0)=0, θ (0)=0, the input of the flexible pivot system is the force at the end point A F , the output is the rod around O Corner of point θ . Figure 3 middle, O is the rotation center of the flexible pivot structure. O’ is the center of rotation of the tension spring.

[0049] By performing Laplace transform on formula (2), we can obtain:

[0050] (3)

[0051] Among them s represents the complex variable in the Laplace transformation, indicating the frequency response of the system. The Laplace transformation is mainly performed to transform the function in the time domain shown in formula (2) into the function in the complex frequency domain shown in formula (3) so as to analyze the control characteristics of the flexible pivot system.

[0052] Transforming formula (3) yields the transfer function of the flexible pivot structure:

[0053] (4)

[0054] Among them, the overall stiffness of the flexible pivot structure is:

[0055] (5)

[0056] in, k 1 is the initial positive stiffness of the flexible pivot structure. The initial positive stiffness is the stiffness of the flexible pivot structure without adding a tension spring. At this time, the stiffness is only determined by the reed-type flexible hinge. The overall positive stiffness is the stiffness of the flexible pivot structure after adding a tension spring. The stiffness is calculated by formula (5). From formula (5), it can be seen that the elastic coefficient is k 2 tension spring can effectively control the magnitude of the negative stiffness in the flexible pivot system and reduce the vertical stiffness of the flexible pivot structure in formula (5). c represents the magnetic damping force. The damping force can be adjusted by controlling the volume of the damping aluminum sheet in the fixed magnetic field formed by the permanent magnet. Therefore, the overall damping ratio of the flexible pivot structure can be adjusted, thereby improving the stability of the flexible pivot structure for different vertical stiffnesses.

[0057] In one embodiment, multiple springs may be used instead of a single spring to control the magnitude of the negative stiffness, thereby compensating for the positive stiffness of the flexible pivot structure and reducing the overall stiffness of the structure in the vertical direction.

[0058] An embodiment of the present invention further provides a method for measuring a small force value based on a variable stiffness flexible pivot structure, and the small force value measurement is performed using the small force value measurement device based on a variable stiffness flexible pivot structure of the above embodiment, comprising the following steps:

[0059] Step S1: According to the force value to be measured F m The size of the Z-axis lifting platform installed with the tension spring is adjusted to ensure that the tension of the tension spring is at a suitable predetermined value, so that the overall stiffness of the flexible pivot structure in the vertical direction is within a reasonable predetermined range;

[0060] Step S2: The outer cylindrical electrode is driven to reciprocate in the vertical direction by the Z-axis precision translation stage to measure the capacitance value of the cylindrical capacitor. At the same time, the relative displacement value between the inner cylindrical electrode and the outer cylindrical electrode is measured by the differential laser interferometer. The capacitance gradient d of the cylindrical capacitor is calculated based on the measurement results. C / d z ;

[0061] Step S3: by applying voltage to both ends of the cylindrical capacitor, the cylindrical inner electrode and the cylindrical outer electrode reach an initial equilibrium position. At this time, the voltage applied to both ends of the cylindrical capacitor is V U The electrostatic force generated is F e1 ;

[0062] Step S4: The force value to be measured F m Loaded to the tray in the vertical direction, under the guidance of the flexible pivot structure, the inner electrode of the cylinder moves in the vertical direction, and the relative displacement between the inner electrode and the outer electrode of the cylinder increases. The potential difference loaded to the ends of the inner electrode and the outer electrode of the cylinder is changed through the PID program, so that the vertical relative displacement between the inner electrode and the outer electrode of the cylinder is restored to the initial equilibrium position. The voltage loaded on both ends of the cylindrical capacitor at this time is V L The electrostatic force generated is F e2 ;

[0063] Step S5: Through the above process, the force value to be measured can be converted into electrostatic force. The electrostatic force can be traced back to Planck's constant. The magnitude of the electrostatic force can be expressed by formula (6):

[0064] (6)

[0065] Force value to be measured F m The relationship with Planck's constant is:

[0066] (7)

[0067] in, is the product of the proportional factor and the numerical constant. The force to be measured can be traced back to the Planck constant ( h ), the electron charge constant ( e ), speed of light( c ), hyperfine level transition constant ( ), the traceability path is relatively clear. Therefore, the micro-force value measurement device and method based on the electrostatic force principle of the embodiment of the present invention can trace the measured force value to Planck's constant by converting the measured micro-force value into the electrostatic force along the axis of the cylindrical capacitor, and can realize the flat traceability of the micro-newton force value, which is not limited by the uncertainty of the mass physical reference, and has the ability to increase the measurement accuracy of the micro-force value in the range of 10μN to 1mN from 10μN to 10μN. -3 The level is increased to 10 -5 Magnitude of ability.

[0068] In summary, the micro-force measurement device and method based on the variable stiffness flexible pivot structure of the embodiment of the present invention can flexibly adjust the stiffness value of the flexible pivot structure according to the magnitude of the micro-force to be measured, so as to meet the requirements of force resolution and measurement accuracy in different measurement ranges. The micro-force measurement device based on the variable stiffness flexible pivot structure of the embodiment of the present invention uses a reed-type flexible hinge to connect three independent rods to form a centrally symmetrical flexible pivot structure, and uses the negative stiffness provided by the tension spring to offset the positive stiffness introduced by the reed-type flexible hinge, thereby achieving precise control of the overall stiffness of the flexible pivot structure, reducing the vertical stiffness of the flexible pivot structure, and adjusting the tension of the tension spring to adjust the size of the vertical stiffness within the range of 0.01N / m-0.6N / m.

[0069] The above description is only by way of illustration of certain exemplary embodiments of the present invention. It is undoubted that those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A small force measurement device based on a variable stiffness flexible pivot structure, characterized in that: It includes a cylindrical capacitor, a Z-axis precision translation stage, a first XY horizontal translation stage, a flexible pivot structure, a differential laser interferometer, a center of gravity adjustment block, a tray, a tension spring, and a Z-axis lifting platform. The cylindrical capacitor comprises a cylindrical outer electrode and a cylindrical inner electrode, the cylindrical outer electrode is fixed on a Z-direction precision displacement stage, the Z-direction precision displacement stage is fixed on a first XY horizontal displacement stage, the first XY horizontal displacement stage is used to adjust the relative position of the cylindrical outer electrode and the cylindrical inner electrode in the horizontal direction to ensure that the cylindrical outer electrode and the cylindrical inner electrode are coaxially installed, and the Z-direction precision displacement stage is used to drive the cylindrical outer electrode to reciprocate in the vertical direction; The flexible pivot structure can generate vertical movement around the rotation center, and includes an upper rod, a middle rod, a bottom rod, a fixed support and a plurality of leaf spring-type flexible hinges. The upper rod, the middle rod and the bottom rod form a centrally symmetrical mechanical structure. One end of the upper rod and the bottom rod are respectively connected to the fixed support, and the other ends of the upper rod and the bottom rod are respectively connected to the two ends of the middle rod. The upper rod, the middle rod, the bottom rod and the fixed support are fixedly connected by the plurality of leaf spring-type flexible hinges. The end of the cylindrical inner electrode is connected to the flexible pivot structure. The vertical movement of the flexible pivot structure around the rotation center can drive the cylindrical inner electrode to generate relative displacement in the cylindrical outer electrode. The differential laser interferometer is used to inject a measuring beam into the bottom of the cylindrical outer electrode and the cylindrical inner electrode to measure the relative displacement between the cylindrical outer electrode and the cylindrical inner electrode; The center of gravity adjustment block is connected to the upper rod of the flexible pivot structure and is used to adjust the center of gravity of the flexible pivot structure so that it coincides with the rotation center of the flexible pivot structure; The tray is connected to the upper end of the middle rod to bear the loaded force, the lower end of the middle rod is connected to the cylindrical inner electrode, and the central symmetry axis of the tray and the cylindrical inner electrode coincides; One end of the tension spring is fixed to the symmetrical center position of one end of the upper rod, and the other end is fixed to the Z-axis lifting platform, and the tensioning force of the tension spring can change with the movement of the Z-axis lifting platform along the Z-axis; The overall stiffness of the flexible pivot structure in the vertical direction is: Wherein, k1 is the initial positive stiffness of the flexible pivot structure, k2 is the elastic coefficient of the tension spring, l1 is the length from the end of the flexible pivot structure to the rotation center, l0 is the original length of the tension spring, and l2 is the length of the tension spring after stretching.

2. The device according to claim 1, characterized in that It also includes an extension rod, a damping aluminum sheet, a permanent magnet and a second XY horizontal translation stage. The extension rod extends along one end of the bottom rod, and the damping aluminum sheet is connected to the end of the extension rod, and can generate a damping force in the magnetic field formed by the permanent magnet, so as to reduce the convergence time of the flexible pivot structure after the force value is loaded. The permanent magnet is fixed on the second XY horizontal translation stage, and the second XY horizontal translation stage is used to adjust the position of the permanent magnet to ensure that the damping aluminum sheet is always in the middle position of the permanent magnet.

3. The device according to claim 2, characterized in that The transfer function of the flexible pivot structure is: Among them, J M is the moment of inertia of the flexible pivot structure, F is the small force to be measured, and c is the damping force generated by the damping aluminum sheet.

4. The device according to any one of claims 1 to 3, characterized in that The upper rod, the middle rod and the bottom rod are sheet structures, and there are four fixed supports, two of which are connected to the two corners of one end of the upper rod, and the other two are connected to the two corners of one end of the middle rod; there are eight spring-type flexible hinges, which are respectively located on the eight corners connected between the upper rod, the middle rod, the bottom rod and the fixed supports.

5. A method for measuring small forces based on a variable stiffness flexible pivot structure, characterized in that: The device according to any one of claims 1 to 4 is used to measure a small force value, comprising: Step S1: According to the force value F to be measured m The size of the Z-axis lifting platform installed with the tension spring is adjusted so that the tension of the tension spring is adjusted to a predetermined value, thereby making the overall stiffness of the flexible pivot structure in the vertical direction within a predetermined range; Step S2: driving the cylindrical outer electrode to reciprocate in the vertical direction by the Z-axis precision translation stage to measure the capacitance value of the cylindrical capacitor, and at the same time using a differential laser interferometer to measure the relative displacement value between the cylindrical inner electrode and the cylindrical outer electrode, and calculating the capacitance gradient dC / dz of the cylindrical capacitor based on the measurement results; Step S3: by applying voltage to both ends of the cylindrical capacitor, the inner electrode and the outer electrode of the cylinder reach an initial equilibrium position. At this time, the voltage applied to both ends of the cylindrical capacitor is V U , the electrostatic force generated is F e1 ; Step S4: The force value F to be measured m When the load is loaded to the tray in the vertical direction, the inner electrode of the cylinder moves in the vertical direction under the guidance of the flexible pivot structure, and the relative displacement between the inner electrode of the cylinder and the outer electrode of the cylinder increases, which changes the potential difference loaded to both ends of the inner electrode of the cylinder and the outer electrode of the cylinder, so that the relative displacement between the inner electrode of the cylinder and the outer electrode of the cylinder in the vertical direction is restored to the initial equilibrium position. Suppose the voltage loaded on both ends of the cylindrical capacitor is V L , the electrostatic force generated is F e2 ; Step S5: Calculate the force value to be measured as follows:

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