阅读说明：本技术 基于比率荧光技术的发光光弹性涂层应变测量系统及方法 (System and method for measuring strain of luminous photoelastic coating based on ratio fluorescence technology ) 是由 花晓彬 花世群 于 2020-12-16 设计创作，主要内容包括：本发明公开了一种基于比率荧光技术的发光光弹性涂层应变测量系统及方法,包括激发光波发生系统、第一二色分光镜、待测构件、检偏镜、第二二色分光镜、第一发射光采集系统、第二发射光采集系统和数据处理器；所述待测构件表面设有发光光弹性涂层,所述发光光弹性涂层中设有能够被同一波长激发的两种荧光材料；所述激发光波发生系统产生激发光波射向发光光弹性涂层,两种荧光材料受激产生发射光波,发射光波经第二二色分光镜分离形成反射光束O-Ⅰ和透射光束O-Ⅱ,第一发射光采集系统和第二发射光采集系统分别采集反射光束O-Ⅰ和透射光束O-Ⅱ的参数,同时传输至数据处理器。有益效果：本发明能够消除外部环境因素对测量结果的影响,提高测量精度。(The invention discloses a system and a method for measuring the strain of a luminous photoelastic coating based on a ratio fluorescence technology, which comprises an excitation light wave generating system, a first dichroic spectroscope, a component to be measured, an analyzer, a second dichroic spectroscope, a first emitted light collecting system, a second emitted light collecting system and a data processor, wherein the excitation light wave generating system is connected with the first dichroic spectroscope; the surface of the component to be tested is provided with a luminous photoelastic coating, and two fluorescent materials which can be excited by the same wavelength are arranged in the luminous photoelastic coating; the excitation light wave generating system generates excitation light waves to irradiate the luminescent photoelastic coating, the two fluorescent materials are excited to generate emission light waves, and the emission light waves are separated by the second dichroic beam splitter to form a reflected light beam O Ⅰ And a transmitted beam O Ⅱ The first emission light collection system and the second emission light collection system respectively collect the reflected light beam O Ⅰ And a transmitted beam O Ⅱ And simultaneously transmitting the parameters to the data processor. Has the advantages that: the invention can eliminate the influence of external environmental factors on the measurement result and improve the measurement precision.)

1. A luminous photoelastic coating strain measurement system based on ratio fluorescence technology is characterized in that: the device comprises an excitation light wave generation system (1), a first dichroic spectroscope (2), a component to be detected (3), an analyzer (4), a second dichroic spectroscope (5), a first emission light collection system (6), a second emission light collection system (7) and a data processor (8); a luminous photoelastic coating (31) is arranged on the surface of the component to be measured (3), and a first fluorescent material and a second fluorescent material which can be excited by the same wavelength are arranged in the luminous photoelastic coating (31);

the excitation light wave generation system (1) generates excitation light waves to exciteThe light wave passes through the first dichroic beam splitter (2) and vertically irradiates to a luminous photoelastic coating (31) of a component to be detected (3), the first fluorescent material and the second fluorescent material are excited to generate an emission light wave, the emission light wave is reflected by the first dichroic beam splitter (2), passes through the analyzer (4) and irradiates to the second dichroic beam splitter (5), and the emission light wave is separated by the second dichroic beam splitter (5) to form a reflected light beam O_IAnd a transmitted beam O_IIThe first emission light collection system (6) collects the reflected light beam O_IThe second emission light collection system (7) collects the transmitted light beam O_IIThe parameters acquired by the first emission light acquisition system (6) and the second emission light acquisition system (7) are simultaneously transmitted to the data processor (8).

2. A luminescent photoelastic coating strain measurement system based on ratiometric fluorescence techniques of claim 1, characterized in that: the excitation light wave generation system (1) comprises a light source (11), a light filter (12), a polarizer (13) and a quarter wave plate (14), wherein light waves emitted by the light source (11) generate circularly polarized light through the light filter (12), the polarizer (13) and the quarter wave plate (14) in sequence.

3. A luminescent photoelastic coating strain measurement system based on ratiometric fluorescence techniques of claim 1 or claim 2, characterized in that: the luminous photoelastic coating (3) adopts a double-coating structure consisting of a luminous bottom layer and a photoelastic outer layer of luminous materials.

4. A luminescent photoelastic coating strain measurement system based on ratiometric fluorescence techniques of claim 1 or claim 2, characterized in that: the luminous photoelastic coating (3) adopts a single-coating structure with double functions of light emission and photoelasticity.

5. A method for measuring strain of a luminescent photoelastic coating based on ratiometric fluorescence techniques, as claimed in claim 1 or claim 2, comprising the steps of:

step one, surface treatment of a component (3) to be detected: treating the surface of the component (3) to be detected, and then preparing a luminous photoelastic coating (31);

step two, loading the component (3) to be tested: after the luminous photoelastic coating (31) is cured, loading the component (3) to be tested to enable the component to be tested to be in a plane stress state;

step three, exciting the luminescent photoelastic coating (31): the excitation light wave generating system (1) generates excitation light waves, the excitation light waves pass through the first dichroic beam splitter (2) and vertically irradiate to a luminous photoelastic coating (31) of the component to be detected (3), and the first fluorescent material and the second fluorescent material are excited to generate emission light waves;

step four, separating the emitted light waves: the emitted light wave is reflected by the first dichroic beam splitter (2), passes through the analyzer (4) and emits to the second dichroic beam splitter (5), and the emitted light wave is separated by the second dichroic beam splitter (5) to form a reflected light beam O_IAnd a transmitted beam O_II；

Step five, collecting the reflected light beam O at the same time_IAnd a transmitted beam O_IIThe parameters of (2): the first emission light collection system (6) collects the reflected light beam O_IThe second emission light collection system (7) collects the transmitted light beam O_IIThe parameters acquired by the first emission light acquisition system (6) and the second emission light acquisition system (7) are simultaneously transmitted to the data processor (8);

step six, calculating the maximum shear strain: after the acquisition of the required parameters is completed, the data processor (8) collects the reflected light beam O by the first light collection system (6) on the same target area on the luminous photoelastic coating (31)_IAnd the second emission light collection system (7) collects the transmitted light beam O_IISubsequent ratio calculations are performed to obtain the maximum shear strain in the coating plane at the selected target area.

6. The method for measuring strain of a luminescent photoelastic coating based on ratiometric fluorescence technique of claim 5, wherein: the excitation light wave generation system (1) in the third step generates the central wavelength of lambda_exThe excitation light wave of (1); the peak emission wavelength of the emission light wave generated by the first fluorescent material is lambda₁The peak emission wavelength of the emission light wave generated by the second fluorescent material is lambda₂。

7. The method for measuring strain of a luminescent photoelastic coating based on ratiometric fluorescence technique of claim 5, wherein: in the fifth step: the first emission light collection system (6) collects the reflected light beam O_ILight intensity of I₁And a second emission light collection system (7) collects the transmitted light beam O_IILight intensity of I₂Are respectively as

I₁＝I_avg-1+I_R1F₁(γ,h₁)sin2(α-θ) (1A)

I₂＝I_avg-2+I_R2F₂(γ,h₂)sin2(α-θ) (1B)

Wherein: i is_avg-1And I_avg-2The average value of the signal intensity measured by the first emission light acquisition system (6) and the second emission light acquisition system (7) respectively in the process of rotating the analyzer (4) by 180 degrees; i is_R1And I_R2In addition to the parameters relating to the intensity of the excitation light and the coating, the parameters relating to the quantum yield of the first and second fluorescent materials, respectively, are also related, and the ratio C ═ I_R2/I_R1Is a constant determined by the relative quantum yield of the second fluorescent material to the first fluorescent material; f₁(γ,h₁) Is the maximum in-plane shear strain gamma of the first fluorescent material and the luminescent photoelastic coating (31) and the photoelastic thickness h corresponding to the first fluorescent material₁A function of interest, and h₁＝λ_exλ₁/[2πk(λ_ex+λ₁)]；F₂(γ,h₂) Is the maximum in-plane shear strain gamma of the second fluorescent material and the luminescent photoelastic coating (31) and the photoelastic thickness h corresponding to the second fluorescent material₂A function of interest, and h₂＝λ_exλ₂/[2πk(λ_ex+λ₂)](ii) a k is the optical sensitivity of the coating; alpha is an included angle between the light passing direction of the analyzer (4) and the light passing direction of the polarizer (13); theta is an included angle between the first main strain direction in the luminous photoelastic coating (31) and the light transmission direction of the polarizer (13);

selecting a first emission light collection system (6) and a second emission light collection system(7) The number N of the fluorescence signal intensity images to be collected is an integer, the analyzer (4) is rotated to enable the value of the included angle alpha between the analyzer and the polarizer (13) in the light transmission direction to be (N-1) pi/N respectively, wherein: n is 1, 2, … …, N, and reflects the beam O at each alpha angular position_IAnd a transmitted beam O_IIAre acquired simultaneously by means of a first emission light acquisition system (6) and a second emission light acquisition system (7), respectively.

8. The method for measuring strain of a luminescent photoelastic coating based on ratiometric fluorescence technique of claim 7, wherein: after the required image acquisition is completed in the sixth step, the data processor (8) acquires a reflected light beam O from the same target area on the luminous photoelastic coating (31) by the first luminous light acquisition system (6)_IStrength I of₁Collects the transmitted light beam O with a second emission light collecting system (7)_III of (A)₂And finally, solving the numerical solution of the maximum shear strain gamma in the coating surface at the target area according to the strength ratio in the formula (2):

Technical Field

The invention relates to a coating strain measurement system and method, in particular to a system and method for measuring full-field strain distribution of a member surface by applying a ratio fluorescence technology to a luminous photoelastic coating, and belongs to the technical field of experimental strain analysis.

Background

The traditional photoelastic coating surface mount method is an experimental strain analysis method which comprises the steps of firstly adhering a layer of strain sensitive transparent thin sheet made of epoxy resin and the like on the surface of a component to be measured, observing and measuring an isodyne line (representing a main strain difference) and an isodip line (representing a main strain direction) through a photoelastic meter, and obtaining the full-field strain distribution of the surface of the component. The system has the characteristics of intuition, image, real-time performance, full field and the like; meanwhile, the strain field under various conditions such as elasticity, plasticity, elastic-plasticity, fatigue fracture and the like can be measured, so that the strain field can be adopted in various fields.

In order to overcome the limitations of time-consuming, substrate-strengthening effect, complicated data processing and the like in practical application of the conventional photoelastic coating patch Method, a research group led by doctor of James Paul Hubner, University of Florida (University of Florida) has proposed a new Method of luminescent photoelastic coating for experimental strain analysis and their patent technologies, US20040066503(A1) (Method and apparatus for measuring strain using a luminescent photoelastic coating), and US 20092601177 (A1) (measuring Method of strain of single-layer luminescent photoelastic coating without thickness dependence). When excited by incident circular polarized light waves, the fluorescent material added into the luminous photoelastic coating is used for acquiring and processing fluorescence signal images emitted by excitation, and measuring the full-field strain distribution of the surface of the component. In the above patent technology, the strain distribution analysis of the member surface is a single-wavelength fluorescence analysis method, that is, fluorescence signal intensity images of the fluorescent material in the luminescent photoelastic coating at a single wavelength are collected by a scientific grade CCD, the optical strain response of the coating is obtained, and then the maximum shear strain in the coating surface is obtained from the coating light intensity polarization coefficient measured by experiments. The light intensity polarization coefficient of the coating is an experimental parameter related to multiple factors such as the reflection of the surface of the substrate, the intensity and the incident angle of the exciting light wave, a measuring instrument, the environment and the like, and the measurement result of the experiment is directly influenced by the temporal instability and the spatial nonuniformity of the light intensity of the exciting light wave, so that the precision of the strain analysis result is reduced.

Disclosure of Invention

The purpose of the invention is as follows: aiming at the defects in the prior art, the invention provides a system and a method for measuring the strain of a luminous photoelastic coating based on a ratio fluorescence technology; according to the invention, two different fluorescent materials are added into the luminous photoelastic coating, the influence of the external environment on the two fluorescent materials is the same, the intensity ratio of the two materials is adopted to weaken or even eliminate the influence of the change of external environment factors on the measurement result, and the measurement precision is improved.

The technical scheme is as follows: a luminescent photoelastic coating strain measurement system based on ratio fluorescence technology comprises an excitation light wave generation system, a first dichroic spectroscope, a component to be measured, an analyzer, a second dichroic spectroscope, a first emitted light acquisition system, a second emitted light acquisition system and a data processor; the surface of the component to be tested is provided with a luminous photoelastic coating, and a first fluorescent material and a second fluorescent material which can be excited by the same wavelength are arranged in the luminous photoelastic coating;

the excitation light wave generating system generates excitation light waves, the excitation light waves pass through the luminous photoelastic coating of the component to be detected, which is vertically irradiated by the first dichroic beam splitter, the first fluorescent material and the second fluorescent material are excited to generate emission light waves, the emission light waves are reflected by the first dichroic beam splitter and then pass through the analyzer to irradiate the second dichroic beam splitter, and the emission light waves are separated by the second dichroic beam splitter to form reflected light beams O_IAnd a transmitted beam O_IIThe first emission light collection system collects the reflected light beam O_IThe second emission light collection system collects the transmitted light beam O_IIThe parameters collected by the first emission light collection system and the second emission light collection system are simultaneously transmitted to the data processor.

According to the invention, two different fluorescent materials are added into the luminous photoelastic coating, the same influence of the external environment on the two fluorescent materials is utilized, and the intensity ratio of the two materials is adopted to weaken or even eliminate the influence of the change of external environment factors on the measurement result, so that the measurement precision is improved;

preferably, in order to generate circularly polarized light, the excitation light wave generation system includes a light source, a filter, a polarizer and a quarter-wave plate, and the light wave emitted by the light source sequentially passes through the filter, the polarizer and the quarter-wave plate to generate circularly polarized light.

Preferably, the luminous photoelastic coating is a double-coating structure consisting of a luminous bottom layer and a photoelastic outer layer of a luminous material.

Preferably, the luminous photoelastic coating adopts a single-coating structure with double functions of luminescence and photoelasticity.

A method for measuring the strain of a luminous photoelastic coating based on a ratio fluorescence technology comprises the following steps:

step one, surface treatment of a component to be detected: treating the surface of the component to be detected, and then preparing a luminous photoelastic coating;

step two, loading the component to be tested: after the luminous elastic coating is cured, loading the component to be tested to enable the component to be tested to be in a plane stress state;

step three, exciting the luminous photoelastic coating: the excitation light wave generating system generates excitation light waves, the excitation light waves pass through the luminous photoelastic coating of the component to be detected, which is vertically irradiated by the first dichroic beam splitter, and the first fluorescent material and the second fluorescent material are excited to generate emission light waves;

step four, separating the emitted light waves: the emitted light wave is reflected by the first dichroic beam splitter, passes through the analyzer and emits to the second dichroic beam splitter, and the emitted light wave is separated by the second dichroic beam splitter to form a reflected light beam O_IAnd a transmitted beam O_II；

Step five, collecting the reflected light beam O at the same time_IAnd a transmitted beam O_IIThe parameters of (2): the first emission light collection system collects the reflected light beam O_IThe second emission light collection system collects the transmitted light beam O_IIThe parameters collected by the first emission light collection system and the second emission light collection system are simultaneously transmitted to the data processor;

step six, calculating the maximum shear strain: after the required parameters are collected, the data processor collects the reflected light beam O by the first light collecting system for the same target area on the luminous photoelastic coating_IAnd the second emission light collection system collects the transmission light beam O_IISubsequent ratio calculations are performed to obtain the maximum shear strain in the coating plane at the selected target area.

Preferably, the excitation light wave generation system in the third step generates a central wavelength λ_exThe excitation light wave of (1); peak emission of emitted light waves generated by the excitation of the first fluorescent materialWavelength of λ₁The peak emission wavelength of the emission light wave generated by the second fluorescent material is lambda₂。

Preferably, in the fifth step: the first emission light collection system collects the reflected light beam O_ILight intensity of I₁And a second emission light collection system collects the transmitted light beam O_IILight intensity of I₂Are respectively as

I₁＝I_avg-1+I_R1F₁(γ,h₁)sin2(α-θ) (1A)

I₂＝I_avg-2+I_R2F₂(γ,h₂)sin2(α-θ) (1B)

Wherein: i is_avg-1And I_avg-2The average value of the signal intensity measured by the first emission light collecting system and the second emission light collecting system respectively in the process of rotating the analyzer by 180 degrees; i is_R1And I_R2In addition to the parameters relating to the intensity of the excitation light and the coating, the parameters relating to the quantum yield of the first and second fluorescent materials, respectively, are also related, and the ratio C ═ I_R2/I_R1Is a constant determined by the relative quantum yield of the second fluorescent material to the first fluorescent material; f₁(γ,h₁) Is the maximum in-plane shear strain gamma of the first fluorescent material and the luminescent photoelastic coating and the photoelastic thickness h corresponding to the first fluorescent material₁A function of interest, and h₁＝λ_exλ₁/[2πk(λ_ex+λ₁)]；F₂(γ,h₂) Is the maximum in-plane shear strain gamma of the second fluorescent material and the luminescent photoelastic coating and the photoelastic thickness h corresponding to the second fluorescent material₂A function of interest, and h₂＝λ_exλ₂/[2πk(λ_ex+λ₂)](ii) a k is the optical sensitivity of the coating; alpha is an included angle between the light passing direction of the analyzer and the light passing direction of the polarizer; theta is an included angle between a first main strain direction in the luminous photoelastic coating and a light transmission direction of the polarizer;

selecting the intensity of fluorescence signals to be collected by the first emission light collection system and the second emission light collection systemThe number N of the degree images is an integer, the analyzer is rotated to make the values of the included angles alpha between the analyzer and the polarizer in the light transmission direction respectively be (N-1) pi/N, wherein: n is 1, 2, … …, N, and reflects the beam O at each alpha angular position_IAnd a transmitted beam O_IIAre acquired simultaneously by the first and second emission light acquisition systems, respectively.

Preferably, after the required image acquisition is completed in the sixth step, the data processor acquires the reflected light beam O from the first light acquisition system for the same target area on the luminous photoelastic coating_IStrength I of₁Collecting the transmitted light beam O with a second emission light collecting system_III of (A)₂And finally, solving the numerical solution of the maximum shear strain gamma in the coating surface at the target area according to the strength ratio in the formula:

has the advantages that: according to the invention, two different fluorescent materials are added into the luminous photoelastic coating, the same influence of the external environment on the two fluorescent materials is utilized, and the intensity ratio of the two materials is adopted to weaken or even eliminate the influence of the change of external environment factors on the measurement result, so that the measurement precision is improved; the fluorescence excited and emitted by two different fluorescent materials in the luminous photoelastic coating is separated by using a dichroic beam splitter and an interference filter, so that two CCD (charge coupled device) devices are conveniently used for simultaneously collecting two beams of fluorescence signal intensity images with different emission wavelengths for ratio fluorescence calculation; the luminous photoelastic coating method based on the ratio fluorescence technology solves the problem that strain analysis can be carried out based on the fluorescence polarization coefficient measured in advance in the existing luminous photoelastic coating method; the method not only reduces the error introduction link of the measurement result, but also reduces the requirements of the measurement system on the stability of the light intensity of the exciting light wave on time and the uniformity of the light intensity on space, and can greatly reduce the equipment investment in the system.

Drawings

FIG. 1 is a schematic diagram of the optical path of the present invention.

Detailed Description

The invention will be further described with reference to the following figures and specific examples, but the scope of the invention is not limited thereto.

As shown in FIG. 1, a black primer layer 32 is coated on the surface of the component 3 to be tested after decontamination treatment, and a luminescent photoelastic coating 31 is sprayed or brushed on the surface of the black primer layer 32, that is, the photoelastic coating is added with a common excitation wavelength λ_exTwo fluorescent materials: a first fluorescent material and a second fluorescent material; the light source 11 may be a laser or an LED light source, and light waves emitted from the light source 11 pass through a band-pass interference filter 12 (with a central wavelength λ)_ex) Then, the light enters the polarizer 13; the linearly polarized light passing through the polarizer 13 is incident on the quarter-wave plate 14 (the fast axis direction of the quarter-wave plate forms an included angle of 45 degrees with the polarization direction of the polarizer 13); the circularly polarized light transmitted from the quarter-wave plate 14 is used as an excitation light wave, passes through the first dichroic beam splitter 2, and vertically enters the surface of the luminescent photoelastic coating 31; the fluorescence of stimulated emission of the two fluorescent materials in the luminescent photoelastic coating 31 (the peak emission wavelength of the first fluorescent material is λ₁The peak emission wavelength of the second fluorescent material is lambda₂) As the emitted light wave, after being reflected by the first dichroic beam splitter 2, the emitted light wave vertically enters the analyzer 4; the emitted light wave passing through the analyzer 4 is divided into a reflected light beam O by a second dichroic beam splitter 5_I(fluorescence of the first fluorescent material in the emitted light wave) and the transmitted light beam O_II(the second fluorescent material in the emitted light wave fluoresces) two measuring lights; o is_IThrough a first bandpass interference filter 61 (with a central wavelength of λ)₁) Then, the image is collected by the scientific grade CCD camera 62 through the focusing lens: o is_IIPassing through a second band-pass interference filter 71 (with a central wavelength of λ)₂) Then, the data is collected by a scientific grade CCD camera 72 through a focusing lens; the CCD camera 62 and the CCD camera 72 perform fluorescence O on the two fluorescent materials_IAnd O_IIThe synchronous acquisition and the subsequent image processing of (2) are accomplished by software on the data processor 8.

The intensity images captured by CCD camera 62 and CCD camera 72 are processed in an image correlation process to ensure that the ratio is divided when the ratio is calculatedNot from the reflected beam O_IAnd a transmitted beam O_IIThe two fluorescence intensity values of the image correspond to the same target area on the luminescent photoelastic coating 31.

The component 3 to be measured is in a plane stress state under the action of external load. When the luminescent photoelastic coating 31 is subjected to a wavelength λ_exWhen the vertically incident circularly polarized light is excited, the fluorescence emitted by the first fluorescent material and the second fluorescent material in the luminescent photoelastic coating 31 is divided into O as an emission light wave by the second dichroic beam splitter 5_IAnd O_IITwo beams of measurement light, O collected by a scientific grade CCD camera 62_ILight and O collected by CCD camera 72_IIThe light intensity can be expressed as:

I₁＝I_avg-1+I_R1F₁(γ,h₁)sin2(α-θ) (1A)

I₂＝I_avg-2+I_R2F₂(γ,h₂) sin2(α - θ) (1B) wherein: i is_avg-1And I_avg-2The average value of the signal intensity measured by the CCD camera 62 and the CCD camera 72 respectively in the process of rotating the analyzer 4 by 180 degrees; i is_R1And I_R2Is a parameter which is related to the quantum yield of the first fluorescent material and the second fluorescent material respectively besides the excitation light wave intensity and the coating, and the ratio C-I of the first fluorescent material to the second fluorescent material_R2/I_R1Is a constant determined by the relative quantum yield of the second fluorescent material to the first fluorescent material; f₁(γ,h₁) Is the maximum in-plane shear strain gamma of the first fluorescent material and the luminescent photoelastic coating 31 and the photoelastic thickness h corresponding to the first fluorescent material₁A function of interest, and h₁＝λ_exλ₁/[2πk(λ_ex+λ₁)]；F₂(γ,h₂) Is the maximum in-plane shear strain gamma of the second fluorescent material and the luminescent photoelastic coating 31 and the photoelastic thickness h corresponding to the second fluorescent material₂A function of interest, and h₂＝λ_exλ₂/[2πk(λ_ex+λ₂)](ii) a k is the optical sensitivity of the coating; alpha is between the light passing direction of the analyzer 4 and the light passing direction of the polarizer 13An included angle; θ is the angle between the first principal strain direction in the luminescent photoelastic coating 31 and the light transmission direction of the polarizer 13.

When the maximum in-plane shear strain γ of the luminescent photoelastic coating 31 is measured, based on the preselected number N of fluorescence signal intensity images to be collected by the CCD camera, the analyzer 4 is rotated, and the angle α between the analyzer and the light-transmitting direction of the polarizer 13 is changed, so that the values of α are (N-1) pi/N (where N is 1, 2, … …, N), respectively. The CCD camera 62 and the CCD camera 72, which are previously subjected to the alignment operation, are controlled by the data processor 8 to be respectively aligned with the pair O_IAnd O_IIThe light simultaneously collects the signal image corresponding to each alpha angle, and then all the collected O_IAnd O_IILight pattern respectively determining the same target area on the surface of the luminous photoelastic coating 31 at O_IIntensity I on the image₁And O_IIIntensity I on the image₂Finally, the numerical solution of γ is found according to the intensity ratio in equation (2).

The present invention is not limited to the above-described embodiments, and any obvious improvements, substitutions or modifications can be made by those skilled in the art without departing from the spirit of the present invention.