A method for measuring the thickness of double-layer oil films using positron time-of-flight

Abstract

The present invention provides a method for measuring the thickness of a double-layer oil film using positron time-of-flight, comprising: measuring the time difference between each pair of photons arriving at the detector, and obtaining the location of the annihilation event through the known distance of the detection crystal pair and the speed of light; due to the limited time resolution of the detection crystal, the calculated annihilation point is not a point but an interval, and the interval conforms to the Gaussian distribution. The annihilation point position obtained according to each response line is classified by interval and the response lines in the overlapping interval are removed. In order to eliminate the influence of the continuous decay of activity over time on the number of response lines, after classification, the number of response lines of the double-layer oil film is respectively compared with the number of response lines of the reference part annihilation point position in the same interval or the interval symmetrical with the LOR center, and the thickness of the double-layer oil film in the inner cavity of the industrial part is obtained. The present invention realizes the non-invasive and non-destructive in-situ or online measurement of the thickness of the double-layer oil film in the inner cavity of the closed industrial part, and the operation is flexible and simple, which is feasible.

Description

Technical Field

[0001] The invention belongs to the technical field of positron annihilation, and in particular relates to a method for measuring the thickness of a double-layer oil film by utilizing the positron flight time. Background Art

[0002] A lubricating oil film is a crucial feature for protecting mechanical components and reducing frictional losses. Its primary purpose is to prevent direct contact between metal parts, especially for components with relative motion, such as plungers and cylinders. By forming a lubricating oil film on the friction surfaces, friction and wear between parts are reduced, extending their service life. Oil film thickness, typically measured in microns, is a key indicator of the lubrication status of mechanical components. Excessively thick film can lead to oil leakage and hinder the extrusion action. Excessively thin gaps can result in direct contact between metal parts, causing dry friction wear between the friction pairs.

[0003] For example, the plunger pump, a core component of hydraulic equipment and known as the "heart" of the hydraulic system, is widely used in fields such as aerospace. The trend toward higher pressures and higher speeds in hydraulic systems places new demands on the design and manufacture of plunger pumps. Currently, the primary cause of performance failure in plunger hydraulic pumps is lubrication and failure of the critical friction pair. In particular, wear and burns often occur in the plunger pair, which comprises the piston and cylinder body. This accounts for over 60% of reported plunger pump failures. The piston pair experiences complex motion and forces within the cylinder, and is subjected to long-term service under high temperatures, pressures, and loads. This compresses the lubricating oil film within the piston pair, easily disrupting its full lubrication state and causing dry friction wear between the piston pairs. Oil film thickness is a key indicator of the lubrication performance of mechanical components and a crucial parameter for assessing the mechanical efficiency, reliability, and service life of equipment. Therefore, developing effective methods to accurately measure and evaluate the oil film thickness within the internal cavities of complex industrial components under demanding operating conditions is of great practical and engineering significance.

[0004] Common methods for measuring oil film thickness include electrical, optical, and ultrasonic methods. The resistance method measures the voltage across the oil film and the current flowing through it, calculates the resistance of the oil film, and uses the corresponding relationship between resistance and film thickness to determine the measured film thickness. However, due to the inherent electrical instability of the oil, the error cannot be guaranteed to be within the industrially acceptable range, making it unsuitable for measuring oil film thickness. The capacitance method substitutes the measured capacitance into the capacitance formula to calculate the oil film thickness. However, capacitance is determined by the overall distance between the surfaces and cannot determine the minimum oil film thickness, making it applicable only to applications with uniform film thickness. The optical interferometry method utilizes the principle of optical interference and the relationship between the interference conditions of the reflected light and the optical path difference to infer the oil film thickness from the interference fringes. The optical method requires that the components on both sides of the oil film must be transparent, making it unsuitable for use with enclosed and complex industrial parts. Ultrasonic measurement determines oil film thickness based on the time difference of interface reflection signals. However, because bubbles are common in lubricants and alter the speed of sound, it is difficult to isolate their influence from the overall reflection signal. Furthermore, current temperature compensation has limitations, which can affect measurement results. These methods are difficult to implement for measuring oil film within complex industrial components under harsh conditions. Summary of the Invention

[0005] Purpose of the invention: In order to solve the problem of measuring the thickness of double-layer oil films in the inner cavity of complex industrial parts under extreme working conditions, the purpose of the embodiments of the present application is to propose a method for measuring the thickness of double-layer oil films using positron time of flight, based on the characteristic that γ photons can penetrate dense metals to detect the oil film in the inner cavity of complex industrial parts such as plunger pumps, and obtain the distance range of the annihilation point through the time of flight information. The classification method is determined according to the relationship between the spacing of the double-layer oil films, that is, the inner diameter of the industrial cavity and 6σ, and the obtained response line data is classified. Finally, the number of response lines of the double-layer oil film is compared with the number of response lines of the reference part to eliminate the influence of elements such as activity on the number of response lines, and the oil film thickness of each layer of the double-layer oil film is obtained.

[0006] To achieve the double-layer oil film thickness measurement of industrial parts, the present invention includes the following steps:

[0007] Step 1: inject a volatile organic solvent into the test piece to clean the residual lubricating oil and oil stains;

[0008] Step 2, fully mixing the positron nuclide and lubricating oil, stirring evenly, to obtain a mixed oil solution containing the positron nuclide;

[0009] Step 3: Inject the mixed oil solution into the test piece and the reference piece. The test piece includes a cylindrical cylinder and a plunger, and the reference piece is a metal combination with the same structure in the detection area and a known single-layer oil film thickness. (Because the activity of the oil film is difficult to obtain during the detection process and the activity changes over time, the reference piece uses a ratio method to eliminate the effect of the activity on the number of response lines.)

[0010] Step 4: Allow the test piece and the reference piece to operate simultaneously for a period of time (usually 5 to 10 minutes) so that a layer of mixed oil solution is evenly adhered to the inner walls of the test piece and the reference piece;

[0011] Step 5: Select two pairs of detection crystals and perform homogenization calibration. Place the test piece and the reference piece in the middle of the γ-photon detection crystal pair, and make the line connecting the detection crystal pair perpendicular to the oil film surface.

[0012] Step 6: The two pairs of detection crystals simultaneously sample the test piece and the reference piece to obtain response line data with flight time;

[0013] Step 7: For the response line data obtained by detecting the device under test, the response line data is classified according to the difference in time between the gamma photons reaching the detection crystal pair obtained by time of flight (TOF) and the distance between the two layers of oil film, thereby obtaining the response line data of each layer in the two layers of oil film;

[0014] Step 8: By comparing the number of response lines of the single-layer oil film of the reference part with the number of response lines of each layer of the double-layer oil film, the thickness of each double-layer oil film in the inner cavity of the industrial part is calculated.

[0015] In step 2, the positron nuclide is 18 F(half-life T 1 / 2 =109.8min), and the activity depends on the measurement accuracy. The number of response lines required to be detected is generally between several thousand and tens of thousands, which mainly depends on the measurement accuracy requirements. While controlling the cost, it can also reduce random errors and ensure the measurement accuracy.

[0016] In step 5, the test piece and the reference piece are placed in the middle of the γ-photon detection crystal pair respectively, so that the detection crystal pair is symmetrically located on both sides of the test piece and the reference piece, and the position of the oil film detection point of the reference piece relative to the detection crystal pair is consistent with the position of the detection point of one layer of the double oil film of the test piece relative to the crystal pair.

[0017] Step 7 includes: using dt to represent the time resolution, and obtaining the uncertainty length dx representing the location of the event:

[0018]

[0019] Where c is the speed of light;

[0020] Positrons and electrons in the mixed oil solution annihilate to produce a pair of gamma photons with the same energy and opposite directions. The detection of the pair of gamma photons with the same energy and opposite directions by a pair of detection crystals is recorded as a coincidence event, also known as a line of response (LOR). Suppose the detection crystal pair consisting of the first detection crystal A and the second detection crystal A' detects a pair of coincidence events, t A and t A’ The time when the first detection crystal A detects the γ photon and the time when the second detection crystal A' detects the γ photon are respectively, the time difference Δt A for:

[0021] Δt A =t A -t A' (2)

[0022] The position where the annihilation event occurs on the line of response LOR is:

[0023]

[0024] Here, x is the distance between the annihilation point and the center of the response line (LOR). The time difference indicates the position of the annihilation point relative to the midpoint of the line connecting the detection crystal pair. Therefore, when a mechanical component, such as a plunger and pump body, is in operation, a gap oil film forms between the plunger and the cylinder. During detection, the detection crystal pair detects a double layer of oil film on either side of the plunger, with the distance between the oil films approximately equal to the plunger diameter. Because the distance between the two oil films exists, the time difference between the annihilation points of the two oil films varies. Therefore, the response line data for the two oil films can be separated based on the calculated annihilation point locations, yielding the response line data for each of the two oil films.

[0025] The annihilation point position calculated from the detected response line is a Gaussian distribution centered on the true annihilation point position. The half-width height of the Gaussian distribution is the uncertainty length, and the probability density function f(y) is:

[0026]

[0027] Among them, y is the specific value of the random variable; μ represents the position of the true annihilation point; σ is the standard deviation, σ = dx / 2.36; e is the natural constant. By using the time-of-flight technology to measure the time difference between the arrival of photon pairs at the detector, since the distance and the speed of light of the detection crystal are known, the position where the annihilation event occurs can be determined. Because the time resolution of the detection crystal is limited, the annihilation point obtained through the time of flight is not a single point but a region, and the calculated annihilation point position distribution conforms to the Gaussian distribution. The uncertain length is related to the half-width at half maximum of the Gaussian distribution and the time resolution. The standard deviation σ of the Gaussian distribution is the uncertain length dx / 2.36. For the double-layer oil film, the distance between the oil films is usually the inner cavity diameter of the industrial part cylinder. Applying the TOF technology to the measurement of the double-layer oil film thickness, the approximate position of the oil film is calculated through the TOF information, and the response line data is determined to belong to which layer of the oil film according to the different intervals where the calculated annihilation event occurs.

[0028] Due to the limited time resolution caused by the performance problems of the detector and the electronics, the position of the annihilation point obtained through the time difference of γ photons arriving at the detection crystal pair can only be determined within a certain distance range, and the calculated annihilation point position distribution conforms to the Gaussian distribution. The higher the time resolution, the narrower the half-width at half maximum of the Gaussian distribution, and the narrower the corresponding range of the true annihilation point position.

[0029] Step 7 also includes: A pair of detection crystals detect the double-layer oil film. The distance between the detection crystal and the center of the LOR is L. On the left and right sides of the detected double-layer oil film point are the first oil film 1 and the second oil film 2 respectively. The positions of the first oil film 1 and the second oil film 2 are μ1 and μ2 respectively, and the thicknesses of the first oil film 1 and the second oil film 2 are ξ1 and ξ2 respectively; When the interval between the first oil film 1 and the second oil film 2 is greater than 6σ, the probabilities P(-L < x1 < 0) on the left side of the LOR center and P(0 < x2 < L) on the right side of the calculated annihilation point positions of the first oil film 1 and the second oil film 2 are respectively:

[0030]

[0031] Among them, x1 is the annihilation point position calculated according to the time of flight TOF of the response line data of the first oil film 1, x2 is the annihilation point position calculated according to the time of flight TOF of the response line data of the second oil film 2, and d is the integral symbol;

[0032] The number N of response lines detected by the detection crystal pair at time t is:

[0033] N = αRB q (t) = αRB q (0)e -λt (7)

[0034] Among them, α is the response line coefficient; B q(0), B q (t) represents the activity of a single radionuclide at time 0 and time t, respectively; λ is the decay constant; and R is the counting rate of the detection crystal pair.

[0035] Step 7 also includes: if the two oil films use the same positron nuclide mixed oil solution, the radioactivity ratio of the first oil film 1 and the second oil film 2 is the same, and the oil film density of the first oil film 1 and the second oil film 2 is also the same, then the activity B of the first oil film 1 at time t is q1 (t) and the activity B of the second oil film 2 q2 (t) are:

[0036] B q1 (t)=ρξ1SC0e -λt (8)

[0037] B q2 (t)=ρξ2SC0e -λt (9)

[0038] Where ρ is the density of the mixed oil solution prepared in step 2, S is the scanning cross-sectional area of ​​the detection crystal pair, and C0 is the radioactivity ratio of the oil film at the initial moment;

[0039] Assuming that the detection crystal pairs are sampling from time t1 to time t2, the ratio of the number of response lines N1 of the first oil film 1 to the number of response lines N2 of the second oil film 2 is:

[0040]

[0041] Wherein, α1 and α2 are the response line coefficients of the first oil film 1 and the second oil film 2 respectively, and R1 is the counting rate of the first detection crystal A and the second detection crystal A′.

[0042] Step 7 also includes: the first oil film 1 and the second oil film 2 have the same structure and material, and the positions of the first oil film 1 and the second oil film 2 are symmetrical about the LOR center, so α1=α2, and the final ratio of the thickness of the first oil film 1 and the second oil film 2 to the number of response lines is:

[0043]

[0044] If the distance between the two oil films is greater than or equal to 6σ, the annihilation points calculated for the two oil films are determined to be concentrated in two areas, and the two areas do not overlap. The two-segment method is used to distinguish the response line data. The response line data is determined to be from the first oil film 1 or the second oil film 2 based on the annihilation point's location to the left or right of the LOR center.

[0045] When the double-layer oil film spacing is greater than the uncertainty length and less than 6σ, the three-segment method is used to distinguish these response line data. Let l be the spacing between the double-layer oil films. According to the 3σ rule of the Gaussian distribution, the occurrence positions of the annihilation events calculated for the first oil film 1 are concentrated in the interval (μ1 - 3σ, μ1 + 3σ), and the occurrence positions of the annihilation events calculated for the second oil film 2 are concentrated in the interval (μ2 - 3σ, μ2 + 3σ). Then, the overlapping C interval of the occurrence positions of the annihilation events calculated for the first oil film 1 and the second oil film 2 is (-3σ + 0.5l, 3σ - 0.5l). The overlapping interval is directly removed. At this time, the calculated annihilation point position in the A interval (-L, -3σ + 0.5l) is the response line data of the first oil film 1, and the calculated annihilation point position in the B interval (3σ - 0.5l, L) is the response line data of the second oil film 2.

[0046] Step 7 also includes: The probabilities P1(-L < x1 < -3σ + 0.5l) that the calculated annihilation point positions of the first oil film 1 are in the A interval and P2(3σ - 0.5l < x2 < L) that the calculated annihilation point positions of the second oil film 2 are in the B interval are respectively:

[0047]

[0048] Step 7 also includes: Setting the detection crystal pair to sample from time t1 to time t2, and the ratio of the number of annihilation events occurring in the A interval to the number of annihilation events occurring in the B interval is:

[0049]

[0050] where N1' is the number of annihilation events occurring in the A interval, that is, the number of response line annihilation events in the A interval in oil film 1, and N2' is the number of annihilation events occurring in the B interval, that is, the number of response line annihilation events in the B interval in oil film 2.

[0051] Step 8 includes: Setting the oil film in the reference piece as the reference oil film 3, the thickness of the reference oil film 3 is known, and the reference oil film 3 is located in the right-side gap in a symmetric metal structure such as a piston pump. Let the thickness of the reference oil film 3 be ξ3. The reference piece is detected by the detection crystal pair B, B'. Then, when the double-layer oil film spacing is greater than or equal to 6σ, the probability P(0 < x3 < L) that the calculated annihilation point position of the single-layer oil film of the reference piece is on the right side of the LOR center, that is, in the interval (0, L), is:

[0052]

[0053] where x3 is the annihilation point position calculated according to the time of flight TOF of the response line data of the reference oil film 3;

[0054] The relationship between the number of response lines N1 of the first oil film 1, the number of response lines N2 of the second oil film 2, and the number of response lines N3 of the reference oil film 3 is as follows:

[0055] N1:N2:N3 = R1ξ1:R1ξ2:R2ξ3 (16)

[0056] Among them, R2 is the counting rate of the detection crystal for B and B';

[0057] When the double-layer oil film spacing l is greater than the uncertainty length and less than 6σ, that is, dx < l < 6σ, the probability P3(3σ - 0.5l < x3 < L) that the annihilation point position calculated by the single-layer oil film of the reference piece is in the B interval, that is, (3σ - 0.5l, L), is:

[0058]

[0059] Among them, μ3 is the true annihilation point position of the reference oil film 3;

[0060] The ratio of the number of response lines N1' of the first oil film 1 in the interval A, the number of response lines N2' of the second oil film 2 in the interval B, and the number of response lines N3' of the reference oil film 3 in the interval B is:

[0061] N1':N2':N3' = R1ξ1:R1ξ2:R2ξ3 (18)

[0062] The thicknesses of each layer of the double-layer oil film are obtained by using the ratio relationship between the thickness of the measured double-layer oil film and the reference oil film.

[0063] Beneficial effects:

[0064] The present invention proposes a non-destructive testing method using positron annihilation technology and TOF technology to measure the thickness of a double-layer oil film in a hermetically sealed space surrounded by metal. This method designs a reference piece with the same detection crystal detection area structure, eliminates the influence of the continuously decaying activity on the number of response lines by using the ratio method, and classifies the double-layer oil film response line data according to the TOF information to realize the thickness measurement of the double-layer oil film in the inner cavity of complex industrial parts in a harsh environment. The present invention realizes the non-invasive and non-destructive in-situ or online measurement of the thickness of the double-layer oil film in the inner cavity of the hermetically sealed industrial parts, and the operation is flexible and simple, with feasibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] The following further specifically describes the present invention in conjunction with the drawings and specific embodiments, and the above and / or other advantages of the present invention will become clearer.

[0066] Figure 1 It is a flow diagram of the present invention.

[0067] Figure 2Schematic diagram of the TOF technology in an embodiment of the present invention.

[0068] Figure 3 Schematic diagram of the detector installation position in an embodiment of the present invention.

[0069] Figure 4 Schematic diagram of the case where the distance between the two oil films is greater than or equal to 6σ in an embodiment of the present invention.

[0070] Figure 5 Schematic diagram of the case where the distance between the two oil films is less than 6σ in an embodiment of the present invention.

[0071] Figure 6 Schematic diagram of a reference experiment in an embodiment of the present invention. DETAILED DESCRIPTION

[0072] The present invention provides a method for measuring the thickness of a double-layer oil film using positron time-of-flight, comprising the following steps:

[0073] Step 1: Inject a volatile organic solvent into the test piece to clean the residual lubricating oil, oil stains and other impurities;

[0074] Step 2: Fully mix the positron nuclide and lubricating oil and stir them evenly to obtain a mixed oil solution containing the positron nuclide;

[0075] Step 3: Inject the mixed oil solution into the test piece and the reference piece. The main structure of the test piece is a cylindrical cylinder and a plunger. The reference piece is a metal combination with the same structure in the detection area and a known single-layer oil film thickness.

[0076] Step 4: Allow the DUT and reference parts to operate simultaneously for a period of time (5 to 10 minutes) to allow a layer of mixed oil solution to evenly adhere to the inner surfaces of the DUT and reference parts.

[0077] Step 5: Select two pairs of detection crystals and perform homogenization calibration. Place the test piece and the reference piece in the middle of the γ photon detection crystal pair, and the line connecting the detection crystal pairs is perpendicular to the oil film surface.

[0078] Step 6: The two pairs of detection crystals simultaneously sample the DUT and the reference component to obtain response line data with time of flight.

[0079] Step 7: For the response line data obtained by detecting the device under test, the response line data is classified according to the difference in time between the gamma photons reaching the detection crystal pair obtained by the time of flight and the distance between the two layers of oil film, thereby obtaining the response line data of each layer in the two layers of oil film;

[0080] Step 8: By comparing the number of response lines of the single-layer oil film of the reference part with the number of response lines of each layer of the double-layer oil film, the thickness of each double-layer oil film in the inner cavity of the industrial part is calculated.

[0081] In this embodiment, in step 1, a volatile organic solvent is used to clean the test piece to prevent unmarked lubricating oil from being present in the oil film attached to the inner wall of the cylinder body during subsequent measurements.

[0082] In step 2, the following scheme is adopted:

[0083] The positron emission nuclides used here are 18 F, this is because 18 F(T 1 / 2 =109.8min) has a longer half-life, which gives it more time to complete the labeling reaction and 18 F is also easier to obtain than other nuclides and is the most commonly used positron-emitting nuclide in positron emission computed tomography (PET). For oil film measurements, if the number of detected response lines is too small, it will lead to a decrease in accuracy due to random errors. Therefore, the number of detected response lines must be as large as possible while ensuring that the nuclide activity is within a certain range due to the dead time effect. The dead time effect refers to the situation when two photons are continuously incident on the same detector within a very short time interval. Because the detection crystal requires a certain amount of time (called dead time) to receive and process the photon signals, when this processing time is greater than the difference in the incidence time of the consecutive photons, the detector will only receive and record the first photon event and will not have time to process the second photon event, thus ignoring the event without processing. The positron-emitting nuclides in the mixed oil solution used in this scheme require the number of detected response lines to be between several thousand and tens of thousands.

[0084] In this embodiment, step 5 is implemented using the following solution:

[0085] Ideally, when scanning a uniform point source, the response line counts obtained by each of the two selected pairs of detector crystals are identical. However, due to factors such as the non-uniform response of the crystals, inconsistencies in the gain of the photomultiplier tubes, the effects of the coupling agents between the crystals and the light guide, and between the light guide and the PMT on light output, and differences in the electronic circuit channels, the counts between the detector crystals in the PET detector are uneven, that is, the count rates vary. Therefore, before the experiment, it is necessary to determine the count rate of the detector crystal pair used. Before the experiment, the thickness of a uniform standard oil film is simultaneously detected using two pairs of detector crystals to obtain the corresponding number of response lines. Because the density, thickness, and activity of the oil film are completely consistent, any discrepancies in the number of response lines obtained are simply due to differences in the count rates between the different detector crystal pairs. The ratio of the corresponding number of response lines is the count rate ratio.

[0086] Because the position of the detection crystal and the measured object affects some parameters of the oil film measurement method, it is required that the line connecting the detection crystal pair is perpendicular to the oil film surface, and the measured object and the detection crystal pair are symmetrical about the same axis, such as Figure 3 shown.

[0087] In this embodiment, step 7 is implemented using the following solution:

[0088] Due to the limited time resolution caused by the performance of the detector and electronics, we can only estimate the approximate location of the annihilation event on the LOR. Figure 4 shown.

[0089] Using dt to represent the time resolution, we can get the uncertainty length dx representing the location of the event:

[0090]

[0091] Where c is the speed of light.

[0092] Suppose a pair of coincidence events detected by detection crystals A and A', t A and t A’ The time when crystal AA' detects the γ photon is detected respectively, and their time difference Δt A for:

[0093] Δt A =t A -t A' (2)

[0094] The approximate location of this annihilation event on the LOR is:

[0095]

[0096] Where x is the distance between the approximate location of the annihilation and the center of the LOR.

[0097] Due to the uncertainty length, the calculated annihilation point position is a range. The calculated annihilation point position is a Gaussian distribution centered on the true annihilation point position, and its half-width height is the uncertainty length. Its probability density function is:

[0098]

[0099] Where μ represents the actual annihilation point position. Here σ = dx / 2.36.

[0100] The annihilation point locations of a double oil film calculated using TOF information exhibit a Gaussian distribution due to limited temporal resolution. Based on the 3σ rule of Gaussian distribution, which states that the probability of a value being distributed in the range (μ - 3σ, μ + 3σ) is 0.9974, when the distance between the two oil films is greater than or equal to 6σ, the calculated annihilation points are concentrated in two regions, with no overlap. However, when the distance between the two oil films is less than 6σ, the distribution of the annihilation point locations calculated using TOF information exhibits cross-mixing. Therefore, the cross-mixing, difficult-to-distinguish response line data is removed, and classification is performed based on the remaining calculated annihilation point locations.

[0101] A pair of detection crystals are used to detect the double oil film. The distance between the detection crystal and the LOR center is L. The left and right sides of the double oil film point detected are oil film 1 and oil film 2 respectively. The oil film positions are μ1 and μ2 respectively. The oil film thicknesses are ξ1 and ξ2 respectively. Figure 5 When the interval between oil film 1 and oil film 2 is greater than 6σ, the probabilities of the annihilation points of oil film 1 and oil film 2 being on the left and right sides of the LOR center are:

[0102]

[0103] Among them, x1 is the annihilation point position calculated based on the response line data of the first oil film 1 according to the flight time TOF, and x2 is the annihilation point position calculated based on the response line data of the second oil film 2 according to the flight time TOF. Because L>6σ, the annihilation point positions calculated from the response line data of the left oil film are basically on the left side of the LOR center, and the annihilation point positions calculated from the response line data of the right oil film are basically on the right side of the LOR center.

[0104] The number of response lines N detected by the detection crystal at time t is:

[0105] N=αRB q (t) = αRB q (0)e -λt (7)

[0106] Among them, α is the response line coefficient, which is related to the material, size, wall thickness, and position of the object being measured. Since the response line generated in the positron annihilation process will produce a certain angle offset when passing through the metal wall, and the probability of gamma photons generating scattering events when passing through metals of different materials and thicknesses is also different, resulting in different numbers of true coincidence events, it is necessary to unify them through the response line coefficient; B q (0), B q (t) is the activity of a single radionuclide at time 0 and time t, respectively; λ is the decay constant related to the half-life; R is the counting rate of the detection crystal pair.

[0107] Because the two oil films use the same positron nuclide mixed oil solution, the radioactivity ratio C (C = B q / g, the ratio of radioactivity to its mass is called radioactivity ratio) is the same, and the oil film density of oil film 1 and oil film 2 is also the same. Then the activity B of oil film 1 and oil film 2 at time t is q1 (t), B q2 (t) are:

[0108] B q1 (t)=ρξ1SC0e -λt (8)

[0109] B q2 (t)=ρξ2SC0e -λt (9)

[0110] Where ρ is the density of the configured mixed oil solution, S is the scanning cross-sectional area of ​​the detection crystal pair, and C0 is the radioactivity ratio of the oil film at the initial moment.

[0111] Assume that the detection crystal pairs are sampled from time t1 to time t2. The ratio of the number of response lines of oil film 1 to the number of response lines of oil film 2 is:

[0112]

[0113] Among them, α1 and α2 are the response line coefficients of oil film 1 and oil film 2 respectively, and R1 is the counting rate of detector A-A'.

[0114] Because the internal oil film structure of industrial components, such as plunger pumps, is often symmetrical, oil film 1 and oil film 2 have the same structure and material, and the positions of the two oil films are symmetrical about the LOR center, so α1 = α2. The final ratio of the thickness of oil film 1 and oil film 2 to the number of response lines is:

[0115]

[0116] From this, we can see that for a double oil film, if the distance between the two films is large (i.e., greater than or equal to 6σ), the two-segment method can be used to distinguish the response line data. In this case, the annihilation point locations calculated from the TOF information of the two films are somewhat distant, making them easy to distinguish. Here, the annihilation point's location to the left or right of the LOR center is used to determine whether the response line data originates from film 1 or film 2.

[0117] When the distance between the two oil films is greater than the uncertainty length and less than 6σ, the three-segment method is used to distinguish these response line data. The principle is as follows: Figure 6 As shown. Let l be the distance between the two oil films. According to the 3σ rule of Gaussian distribution, the calculated annihilation event locations for oil film 1 are concentrated in (μ1-3σ, μ1+3σ), and the calculated annihilation event locations for oil film 2 are concentrated in (μ2-3σ, μ2+3σ). The interval C where the concentrated distribution of the two oil film locations overlaps is (-3σ+0.5l, 3σ-0.5l). Therefore, the overlapping intervals are directly removed and not considered. At this time, the calculated annihilation point locations in interval A (-L, -3σ+0.5l) are the response line data of oil film 1, and in interval B (3σ-0.5l, L) are the response line data of oil film 2.

[0118] The probability that the annihilation point positions calculated for oil film 1 and oil film 2 are in interval A or interval B respectively is:

[0119]

[0120] Wherein, x1 is the annihilation point position calculated based on the response line data of oil film 1 according to the TOF information, and x2 is the annihilation point position calculated based on the response line data of oil film 2 according to the TOF information.

[0121] Because the cylinder body of this type of industrial part is a symmetrical structure and is placed at the center or central axis of the detector, the two oil films are also symmetrical with the center, that is, μ1 = -μ2, so P1 = P2.

[0122] Assume that the detection crystal pairs are sampled from time t1 to time t2. The ratio of the number of annihilation events occurring in interval A to that in interval B is:

[0123]

[0124] Wherein, N1' is the number of annihilation events occurring in interval A, i.e., the number of response line annihilation events in oil film 1 occurring in interval A, and N2' is the number of annihilation events occurring in interval B, i.e., the number of response line annihilation events in oil film 2 occurring in interval B. N1 is the number of response lines occurring in oil film 1, and N2 is the number of response lines occurring in oil film 2.

[0125] In this embodiment, the above step 8 is implemented using the following solution:

[0126] According to equation (7), the activity decays continuously with time. Since it is difficult to obtain the activity of the oil film during the detection process, a reference oil film model is designed. In the detection crystal pair, the structures of the measured part and the reference part in the detection area are the same, and the ratio method is used to eliminate the influence of the activity on the number of response lines.

[0127] For the reference oil film 3, the oil film is located in the gap on the right side in the symmetric metal structure. Let the thickness of the oil film be ξ3, and the detection crystal pairs B and B’ detect the reference part. Then when the distance between the double-layer oil films is greater than or equal to 6σ, the probability that the annihilation point position calculated from the single-layer oil film of the reference part is on the right side of the LOR center, that is, in the interval (0, L), is:

[0128]

[0129] Among them, x3 is the annihilation point position calculated from the response line data of the reference oil film 3 according to the TOF information.

[0130] The relationship between the number of response lines among the oil film 1, the oil film 2 and the reference oil film 3 is:

[0131] N1:N2:N3=R1ξ1:R1ξ2:R2ξ3 (16)

[0132] Among them, R2 is the counting rate of the detector B - B’ used for detecting the reference oil film. [[ID=The relationship between the number of response lines among the oil film 1, the oil film 2 and the reference oil film 3 is:

[0133] Similarly, for the double-layer oil film with the distance l greater than the uncertainty length and less than 6σ, that is, dx < l < 6σ, the probability that the annihilation point position calculated from the single-layer oil film of the reference part is in the B interval, that is, (3σ - 0.5l, L), is:

[0134]

[0135] Among them, μ3 is the true annihilation point position of the reference oil film 3. Since the structures of the reference part and the part to be measured are the same, the materials are the same, and the placement positions of the detection crystal pairs are also the same, so μ3 = μ2, that is, P1 = P2 = P..

[0136] The ratio of the number of response lines of the oil film 1 in the interval A and the oil film 2 and the reference oil film 3 in the interval B is:

[0137] N1':N2':N3'=R1ξ1:R1ξ2:R2ξ3 (18)

[0138] Because the thickness of the reference oil film, the number of response lines of the oil film, and the counting rate of the detection crystal pair are all known, the thickness of each layer of the double-layer oil film can be obtained by using the ratio relationship between the thickness of the measured double-layer oil film and the reference oil film. [[ID=The relationship between the number of response lines among the oil film 1, the oil film 2 and the reference oil film 3 is:

[0139] The present invention utilizes TOF technology to effectively classify the originally collected double-layer oil film response line data that is difficult to distinguish, and then uses the ratio method to obtain the thickness of each layer of the double-layer oil film based on the linear relationship between the number of response lines and the oil film thickness. Compared with the commonly used electrical method with unstable electrical properties of oil and large errors, the optical method that requires transparent components, and the ultrasonic method whose accuracy is easily affected by the environment, the positron annihilation technology is more suitable for measuring the thickness of the oil film inside a closed metal due to the strong penetration of gamma photons and the fact that it is not affected by electric fields, magnetic fields, and temperature changes. The present invention only uses two pairs of detection crystal pairs relative to a detector array composed of multiple detection crystal pairs, which greatly reduces the cost of equipment manufacturing; by controlling the activity of the lubricating oil nuclides and the sampling time of the detector, a sufficient number of response lines are obtained, which reduces random errors and improves the accuracy of oil film thickness measurement. The present invention is simple in calculation and flexible in operation, and can achieve non-destructive measurement of the oil film thickness in the inner cavity of complex industrial parts such as plunger pumps when the oil film is in operation.

[0140] The present invention provides a method for measuring the thickness of a double-layer oil film using positron time-of-flight. There are numerous methods and approaches for implementing this technical solution. The above is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications are also within the scope of protection of the present invention. Any components not specified in this embodiment may be implemented using existing technologies.

Claims

1. A method for measuring the thickness of a double-layer oil film using positron time of flight, characterized in that: The steps include the following: Step 1: Inject a volatile organic solvent into the test piece to clean the residual lubricating oil and oil stains. Step 2: Thoroughly mix and stir a positron nuclide with the lubricating oil to obtain a mixed oil solution with the positron nuclide. Step 3: Inject the mixed oil solution into the test piece and the reference piece. The test piece includes a cylindrical cylinder block and a plunger, and the reference piece is a metal combination with the same structure in the detection area and the single-layer oil film thickness is known. Step 4: Operate the test piece and the reference piece simultaneously for a period of time so that a layer of mixed oil solution uniformly adheres to the inner walls of the test piece and the reference piece. Step 5: Select two pairs of detection crystal pairs and perform normalization calibration. Place the test piece and the reference piece respectively in the middle of the γ-photon detection crystal pairs, and the connection line of the detection crystal pairs is perpendicular to the oil film surface. Step 6: The two pairs of detection crystal pairs simultaneously sample the test piece and the reference piece respectively to obtain response line data with flight time. Step 7: For the response line data obtained by detecting the test piece, classify the response line data according to the different time differences of the γ-photons reaching the detection crystal pairs obtained from the flight time TOF and the distance between the double-layer oil films, so as to obtain the response line data of each layer in the double-layer oil film. Step 8: Calculate the thicknesses of the two layers of the double-layer oil film in the inner cavity of the industrial part by comparing the number of response lines of the single-layer oil film of the reference piece with the number of response lines of each layer of the double-layer oil film.

2. The method according to claim 1, characterized in that In step 2, the positron nuclide is 18 F.

3. The method according to claim 2, characterized in that In Step 5, place the test piece and the reference piece respectively in the middle of the γ-photon detection crystal pairs, so that the detection crystal pairs are symmetrically located on both sides of the test piece and the reference piece respectively, and the position of the oil film detection point of the reference piece relative to the detection crystal pair is the same as the position of the oil film detection point of a certain layer of the double-layer oil film of the test piece relative to the crystal pair.

4. The method according to claim 3, characterized in that Step 7 includes: Use dt to represent the time resolution and obtain the uncertain length dx representing the event occurrence position: where c is the speed of light; Positrons and electrons in the mixed solution annihilate to produce a pair of gamma photons with opposite directions and the same energy. The pair of gamma photons with opposite directions and the same energy are detected by a pair of detection crystals respectively, which is recorded as a coincidence event, also known as a line of response LOR. Suppose the detection crystal pair consisting of the first detection crystal A and the second detection crystal A' detects a pair of coincidence events, t A and t A’ The time when the first detection crystal A detects the γ photon and the time when the second detection crystal A' detects the γ photon are respectively, the time difference Δt A for: Δt A =t A -t A' (2) The position where the annihilation event occurs on the response line LOR is: where x is the distance between the annihilation position and the center of the response line LOR; The position of the annihilation point calculated from the detected response line is a Gaussian distribution centered on the true annihilation point position. The half-width at the height of the Gaussian distribution is the uncertain length, and the probability density function f(y) is: where y is the specific value of the random variable; μ represents the true annihilation point position; σ is the standard deviation, σ = dx / 2.36; e is the natural constant.

5. The method according to claim 4, characterized in that Step 7 also includes: A pair of detection crystal pairs detect the double-layer oil film. The distance between the detection crystal and the center of the LOR is L. The left and right sides of the detected double-layer oil film points are the first oil film 1 and the second oil film 2 respectively. The positions of the first oil film 1 and the second oil film 2 are μ1 and μ2 respectively, and the thicknesses of the first oil film 1 and the second oil film 2 are ξ1 and ξ2 respectively. When the interval between the first oil film 1 and the second oil film 2 is greater than 6σ, the probabilities P(-L<x1<0) on the left side of the LOR center and P(0<x2<L) on the right side of the annihilation point positions calculated for the first oil film 1 and the second oil film 2 are: Among them, x1 is the annihilation point position calculated from the response line data of the first oil film 1 according to the flight time, x2 is the annihilation point position calculated from the response line data of the second oil film 2 according to the flight time TOF, and d is the integral symbol; The number N of response lines detected by the detection crystal pair at time t is: N=αRB q (t)=αRB q (0) and -λt (7) Among them, α is the response line coefficient; B q (0), B q (t) represents the activity of a single radionuclide at time 0 and time t, respectively; λ is the decay constant; and R is the counting rate of the detection crystal pair.

6. The method according to claim 5, characterized in that Step 7 also includes: if the two oil films use the same positron nuclide mixed oil solution, the radioactivity ratio of the first oil film 1 and the second oil film 2 is the same, and the oil film density of the first oil film 1 and the second oil film 2 is also the same, then the activity B of the first oil film 1 at time t is q1 (t) and the activity B of the second oil film 2 q2 (t) are: B q1 (t)=ρξ1SC0e -λt (8) B q2 (t)=ρξ2SC0e -λt (9) Among them, ρ is the density of the mixed oil solution configured in step 2, S is the scanning cross-sectional area of the detection crystal pair, and C0 is the specific radioactivity at the initial time of the oil film; It is set that the detection crystal pair samples from time t1 to time t2, and the ratio of the number N1 of response lines of the first oil film 1 to the number N2 of response lines of the second oil film 2 is: Among them, α1 and α2 are the response line coefficients of the first oil film 1 and the second oil film 2 respectively, and R1 is the counting rate of the first detection crystal A and the second detection crystal A'.

7. The method according to claim 6, characterized in that Step 7 also includes: The structures and materials of the first oil film 1 and the second oil film 2 are the same, and the positions of the first oil film 1 and the second oil film 2 are symmetric about the LOR center. Therefore, α1 = α2. Finally, the ratio of the thickness of the first oil film 1 and the second oil film 2 to the number of response lines is: If the distance between the double-layer oil films is greater than or equal to 6σ, it is determined that the annihilation points calculated from the double-layer oil films are concentrated in two regions, and there is no overlap between the two regions. The response line data is distinguished by the two-segment method, and it is determined whether the response line data comes from the first oil film 1 or the second oil film 2 according to the left and right positions of the annihilation point at the LOR center; For the double-layer oil film spacing greater than the uncertainty length and less than 6σ, the three-segment method is used to distinguish the response line data. Let l be the spacing between the double-layer oil films. According to the 3σ rule of the Gaussian distribution, the positions where the annihilation events calculated for the first oil film 1 are concentrated in (μ1 - 3σ, μ1 + 3σ), and the positions where the annihilation events calculated for the second oil film 2 are concentrated in (μ2 - 3σ, μ2 + 3σ). Then the C interval where the positions where the annihilation events calculated for the first oil film 1 and the second oil film 2 are concentrated overlap is (-3σ + 0.5l, 3σ - 0.5l). The overlapping interval of the distribution is directly removed. At this time, the calculated annihilation point position in the A interval (-L, -3σ + 0.5l) is the response line data of the first oil film 1, and the calculated annihilation point position in the B interval (3σ - 0.5l, L) is the response line data of the second oil film 2.

8. The method according to claim 7, characterized in that Step 7 also includes: The probabilities P1(-L < x1 < -3σ + 0.5l) that the annihilation point position calculated for the first oil film 1 is in the A interval and P2(3σ - 0.5l < x2 < L) that the annihilation point position calculated for the second oil film 2 is in the B interval are respectively:

9. The method according to claim 8, characterized in that Step ​ 10. The method according to claim 9, characterized in that Step 8 includes: setting the oil film in the reference piece as the reference oil film 3, the thickness of the reference oil film 3 is known, the reference oil film 3 is located in the gap on the right side in the symmetric metal structure, setting the thickness of the reference oil film 3 as ξ3, and detecting the reference piece by the detection crystals B and B'. Then when the distance between the double-layer oil films is greater than or equal to 6σ, the probability P(0 < x3 < L) that the annihilation point position calculated from the single-layer oil film of the reference piece is on the right side of the LOR center, that is, in the interval (0, L) is: Where, x3 is the annihilation point position calculated according to the time of flight TOF from the response line data of the reference oil film 3; The relationship between the number of response lines N1 of the first oil film 1, the number of response lines N2 of the second oil film 2 and the number of response lines N3 of the reference oil film 3 is: N1:N2:N3 = R1ξ1:R1ξ2:R2ξ3 (16) Where, R2 is the counting rate of the detection crystals B and B'; When the distance l between the double-layer oil films is greater than the uncertain length and less than 6σ, that is, dx < l < 6σ, the probability P3(3σ - 0.5l < x3 < L) that the annihilation point position calculated from the single-layer oil film of the reference piece is in the B interval, that is, (3σ - 0.5l, L) is: Where, μ3 is the true annihilation point position of the reference oil film 3; The ratio of the number of response lines N1' of the first oil film 1 in the interval A, the number of response lines N2' of the second oil film 2 in the interval B, and the number of response lines N3' of the reference oil film 3 in the interval B is: N1':N2':N3' = R1ξ1:Rzξ2:R2ξ3 (18) Use the ratio relationship of the thicknesses of the measured double-layer oil film and the reference oil film to obtain the thicknesses of each layer of the double-layer oil film.

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