Evaluation of measurement signals from a vacuum leak detector

Abstract

A method for evaluating a measurement signal of a vacuum leak detector is provided. [Solution] [0010] TIFF2024526892000032.tif81154

Description

Technical Field

[0001] The present invention relates to a method and an apparatus for evaluating a measurement signal of a vacuum leak detector.

Background Art

[0002] A vacuum leak detector is used to detect a leak in a test object. For this purpose, the vacuum leak detector includes a test chamber for placing the test object, a vacuum pump connected to the test chamber for evacuating the test chamber, and a gas detector connected to the test chamber and configured to analyze the gas drawn from the test chamber. Specifically, the gas detector is configured to detect the test gas that exists inside the test object, enters the test chamber through the leak of the test object, and is supplied from the test chamber to the gas detector.

[0003] The suction capacity of the vacuum leak detector for the test gas used (usually helium) is determined by the vacuum pump used, as well as the cross-sectional area and length of the connection to the test chamber (i.e., the volume of the gas conduction path between the test chamber and the gas detector). This effective suction capacity of the test gas determines the system time constant of the vacuum leak detector with respect to the volume of the test object. The system time constant is the time of the system-related increase of the measurement signal due to the reaction of the gas sensor to the gas component, or the time of the system-related decrease of the measurement signal due to the disappearance of the gas component in the gas sensor. At this time, the gas component in the gas sensor is typically the component of the test gas that leaks from the leak and becomes the detection target for leak detection. Generally speaking, the system time constant is considered to be the elapsed time until the measurement signal drops to a ratio of 1 / e when the test gas is sprayed on the leak or the leak is opened, or when the spraying on the leak is stopped or the leak is closed, or the elapsed time until the measurement signal rises to a ratio of (1 - 1 / e). Simply put, in some cases, the vacuum time constant of the vacuum leak detector (the volume of the test object or the test chamber divided by the effective suction capacity of the vacuum leak detector) may be regarded as the system time constant. [Overview of the project] [Problems that the invention aims to solve]

[0004] Suction capacity can be increased by increasing the size of the pump or by increasing its volume or cross-sectional area, which improves capacity / performance and response time. However, the volume, cross-sectional area, and dimensions of the pump cannot be freely increased. Moreover, increasing the volume reduces the amplitude of the leak signal, making leak detection more difficult.

[0005] One of the objectives of the present invention is to provide a method and apparatus that can shorten response time and improve signal evaluation. [Means for solving the problem]

[0006] The method according to the present invention is determined by the configuration of claim 1. The apparatus according to the present invention is determined by the configuration of claim 16.

[0007] In the method of the present invention, first, the system time constant τ of the vacuum leak detector is determined. The system time constant τ represents the system's response time to changes in the gas state at the gas detector. That is, the system time constant τ corresponds to the time of system-related rise in the measurement signal due to the detection of gas leakage from a leak in the test object, and / or the time of system-related fall in the measurement signal due to the discontinuation of detection of gas components from the gas leak. In the simplest case, the system time constant τ corresponds, for example, to the elapsed time until the measurement signal decays to a ratio of 1 / e (where e is Euler's number) of the initial signal.

[0008] After determining the system time constant, the test specimen is placed in the test chamber, and the chamber is evacuated using a vacuum pump to perform actual leak detection. A measurement signal I(t) of the gas drawn in from the test chamber at the current time t is generated by the gas detector. The current time t may be, for example, the starting point of a new measurement or a new series of measurements. In a series of measurements, measurements may be performed at regular intervals t0.

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[0012] In this invention, whether or not there is a leak in the test object is determined based on the difference calculated in this way, i.e., for example, I(t+t0)-C2×I(t+t0). For example, if this difference is extremely large, it means that the gas detector has detected an additional gas component that was not previously present, which may indicate a leak. On the other hand, if the difference is extremely small, it may indicate that the conditions at the detector have not changed, i.e., that the test object is dense. For this purpose, the difference can be compared with, for example, a threshold. In this way, by utilizing knowledge and the system time constant, it is possible to speed up leak testing and leak detection.

[0013] Preferably, the difference between the predicted measurement signal and the measurement signal actually measured at time t+t0 is multiplied by a constant C1 for numerically adjusting the difference to match the actual leakage amount. C1 and / or C2 are real numbers. Preferably, C2 is between 0 and 1, and C1 is greater than 1.

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[0015] For example, when measuring the partial pressure of a leaking gas (for example, helium as the leaking gas), if the measurement signal rises due to the presence of the leaking gas, it can be determined that a leak exists if the difference exceeds a predetermined threshold. This threshold may be 0. If the measurement signal drops due to the presence of the leaking gas, it can be determined that a leak exists in the test object if the difference falls below a predetermined threshold (for example, 0).

[0016] Preferably, when comparing the difference with a threshold, it can be confirmed whether the difference is a) greater than the threshold and / or b) within the range from zero to the threshold and / or c) less than zero (i.e., negative). In case a), it can be determined that there is a leak in the test specimen. In case b), it can be determined that there is no leak in the test specimen. In case c), it can be determined that there is an error, such as an inaccurate system time constant.

[0017] The threshold can serve to take into account background signals, such as background noise in the measurement signal after the system time constant has elapsed. The background signal may be due to noise and / or offset signals in the signal. The offset signal may be due to gas components internally released from the walls of the leak detection system, for example. The threshold can be set, for example, to 5 to 10 times the value of the background noise or background signal averaged over a predetermined period.

[0018] When comparing the difference with a threshold, it is even more advantageous to use the difference averaged over a predetermined period in order to ignore temporary outliers in the measurement signal.

[0019] The system time constant τ may be determined using a test leak (e.g., a spray leak, etc.), or may be calculated from the volume of the test chamber and the suction capacity of the vacuum pump.

[0020] The system time constant τ is determined from the rate of increase or decrease of the measurement signal (i.e., the measured leak rate signal) when the leak is connected or disconnected (i.e., when the leak is sprayed or when the spraying of the leak is finished).

[0021] At this time, the system time constant τ can be directly determined from the elapsed time until the measured signal drops to 1 / e of the measured signal, for example, caused by a test leak or the like. The system time constant τ can be directly determined from the elapsed time until the measured leak rate signal rises to (1 - 1 / e) times the measurement signal of the test leak.

[0022] Alternatively or in addition, the system time constant τ may be determined by the formula: τ = t / (Ln(I(t = 0)) - Ln(I(t = t))) (where I(t = 0) is the measurement signal at the stop, removal, or power - off of the test leak, and I(t = t) is the measurement signal at any time t after the stop of the test leak). At this time, the system time constant can be determined by taking I(t = 0) as the measurement signal obtained as the difference between the measurement signals before the start of the test leak and at the stop time of the test leak, and taking I(t = t) as the difference between the measurement signals before opening the test leak and at time t after the stop of the test leak.

[0023] The basic idea is to substantially speed up the signal. The system time constant of this system is a certain value that can be technically measured, and it is possible to start or stop an appropriate test leak and measure it with a leak detector. By knowing the time behavior, it is possible to predict how the signal will behave at any point when the spraying of the test gas is not currently being performed. This situation can be mathematically described by the following relational expression:

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[0027] 1) Determine the system time constant for leak rate measurement using an appropriate method (internal test leak, external test leak, spray leak, input of volume and suction capacity). 2) Optionally, the leakage signal is filtered to match the system time constant. 3) The above formula is used to speed up the signal. 4) Optionally, adjust the desired system time constant (boost coefficient) to match the test gas injection time, or adjust the injection time to match the set system time constant. result: The signal is transformed as shown in the following equation:

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[0029] (In the formula, I(t) is the leak rate signal at time t, I(t-τ / n) is the leak rate signal at time t-τ / n, F is the leak rate suppression coefficient (0.9...0.999), n is the boost coefficient (the magnitude of the virtual suction capacity compared to the actual suction capacity), and τ is the system time constant, which can be obtained from, for example, the effective suction capacity and volume (the system must be measured in advance).)

[0030] As a result, the measurement signal becomes faster. The system time constant of the system needs to be determined in advance, for example, using a built-in test leak.

[0031] (Approach and basic philosophy) In a vacuum leak detector, the system time constant, i.e., the maximum speed at which the signal can rise or fall, is determined by the relationship between the effective suction capacity and the chamber volume. Physically speaking, it is not possible to increase the speed of the signal's rise or fall; it only slows down due to factors such as gas release or penetration from the surface, or vacuum evacuation of unconsidered volumetric space. If we ignore all the effects of slowing down, the signal behavior can always be described by a simple e-function.

[0032] For an ascending signal, the function is as follows:

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[0034] For a descending signal, it would be as follows:

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[0036] Since it is an exponential function, the maximum possible change in the signal I(t) can always be determined (by differentiation). The maximum possible derivative (up or down) can always be determined from the aforementioned leakage signal.

[0037] The basic idea is to treat the signal as if it were in decline at any point within the signal. In other words, the largest possible physical change is always subtracted from the signal. Since this becomes an e-function, it is appropriate to describe it in units of the system time constant.

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[0041] Due to the properties of exponential functions, the above equation can be greatly simplified:

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[0043] This will allow us to verify how this new signal behaves under various conditions.

[0044] (In the case of an increasing signal)

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[0045] In other words, the result obtained is, ideally, a time-independent signal with reduced amplitude. Time independence is always achieved when both the signal at time t and the signal at time t-τ are exactly on the rising edge of the signal. The signal before that is rising exponentially. What does this mean? When helium is blown, after a given time (τ / n), the target amplitude has already been reached. The signal is still rising, but the filtered signal is already outputting the final value. To reach approximately the final value, two system time constants are usually required. A value of n=1 effectively halves the effective system time constant; that is, the suction capacity doubles. However, it is preferable to consider this as a virtual suction capacity. Values ​​significantly greater than 1 may also be used. In this way, this filter makes it easy to set the virtual suction capacity of the vacuum leak detector to 200 l / s. As n increases, the measured signal becomes even smaller:

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[0048] (In the case of a descending signal)

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[0049] Basically, subtracting the expected signal results in zero. However, the signal is zero only when both the signal at time t and the signal at time t-τ / n are constantly on the falling edge. When helium is sprayed, the signal becomes visible for almost exactly the duration of time τ / n.

[0050] Naturally, a value of zero is completely useless for a leak detector. Even a slight error in determining the system time constant τ can result in a negative leak rate, indicating that leakage is causing suction. Ideally, a small but detectable leak signal should be obtained by applying a slight multiplication correction using the parameter F.

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[0052] With F=0.9, the displayed leak amount becomes one order of magnitude smaller, and with F=0.99, the leak amount becomes two orders of magnitude smaller.

[0053] (In the case of a stable signal)

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[0055] (Summary of this filter) To avoid negative leakage values ​​and display accurate leakage levels, convert the signal to a filtered signal as follows:

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[0057] (In the formula, I(t) is the leak rate signal at time t, I(t-τ / n) is the leak rate signal at time t-τ / n, F is the leak rate suppression coefficient (0.9...0.999), n is the boost coefficient (the magnitude of the virtual suction capacity compared to the actual suction capacity), and τ is the system time constant, which can be obtained from, for example, the effective suction capacity and volume (the system must be measured in advance).)

[0058] (characteristic) This filter is basically configured to accelerate the signal of spray leak (i.e., all signals that vary depending on the effective suction capacity and volume).

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[0060] (Prerequisite) This filter assumes that the system time constant is known with sufficient accuracy. Achieving this with an unknown system requires measurement, which is possible with a vacuum leak detector equipped with an internal or external test leak. For this purpose, the test leak must be opened and waited until the signal is sufficiently stable. Naturally, the test leak must be connected to the chamber. The system time constant can then be determined by turning off the test leak and measuring its decay time (e.g., the time it takes to reach 1 / e of the initial signal). Of course, this can take up to 1 minute if the volume is large and the effective suction capacity is small. On the other hand, this also provides the user with information about the system time constant, i.e., the required leak blowing time. Because the measurement time is remarkably short, the time spent determining the system time constant is quickly compensated for by a few measurement points.

[0061] (noise) The filtered signal exhibits significantly more noise than the original signal from the leak detector. This is because at least two signal values ​​are required each time, and the useful signal becomes even smaller as n increases. However, when the volume is large and the spraying time is short, the signal becomes small, so the frequency of the leak amount displayed by the leak detector needs to be extremely small. However, this filter compensates for this as well. Therefore, even if the background noise is large, it does not have a significant inhibitory effect.

[0062] The embodiments will be described below with reference to the drawings. [Brief explanation of the drawing]

[0063] [Figure 1] This is a diagram of the signal path obtained in the first embodiment. [Figure 2] This is a schematic diagram of a vacuum leak detector. [Figure 3]This diagram shows how the system time constant of a system is determined. [Figure 4] Another diagram showing how the system time constant is determined. [Figure 5] Another diagram showing how the system time constant is determined. [Figure 6] This figure illustrates an example of filtering and explains accelerated signals and system time constants. [Modes for carrying out the invention]

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[0065] Figure 2 is a schematic diagram of a vacuum leak detector comprising a test chamber 12, a gas detector 14, and a vacuum pump 16. The gas detector 14 is connected to the test chamber 12 via a gas conduit 18, allowing gas to flow through it. The test chamber houses a test object 20 filled with a test gas. The gas detector 14 is electronically connected to an evaluation unit 22 that receives and processes electronic measurement signals generated by the gas detector.

[0066] In another embodiment, a UL1000 equipped with a 50L barrel was used. A limp valve (Limpventil) was used with the TI4-6 as a controllable test leak. For data input to this filter, the leak amount signal was combined with a fixed filter to avoid the effects of differences in filtering time and to enable measurement of noise amplification.

[0067] Various tests were conducted by opening and closing the aforementioned limp valve to examine signals at various stages of high-speed operation. Specifically, questions were investigated regarding signal drop and rise, as well as the accuracy and degree of discrepancies in the leakage rate predictions made by this filter. The scope of this filter was 1 × 10⁻⁶. -3 ~1 × 10 -9 It was mbar × l / second.

[0068] 1) This filter works well. 2) The signal can be substantially accelerated with a coefficient of 32, which allows for a virtual suction capacity of nearly 1000 l / second in a typical leak detector. 3) The built-in test leak allows for the determination of the system time constant required for this filter, but this takes 15-20 seconds. 4) Compared to the input signal, the accelerated signal shows a noise increase of a factor of (1.4), which is consistent with the theoretical hypothesis. 5) However, for short-duration spray pulses (short relative to the system time constant), the increase in the noise coefficient by a factor of (1.4) is assumed to be not excessively high, provided that the speed factor is not too high. Noise is proportional to the spraying time. 8) The leakage rate prediction is good, and this filter exhibits very little signal load overload or underload. 9) By smoothing the input signal to match the actual system time constant, noise is significantly improved without greatly degrading the system time constant. 10) For a volume of 50 liters, the waiting time for spraying after a large leak can be reduced to approximately 1 minute.

[0069] Figure 3 shows an example of determining the system time constant of a leak detection system, where the rise or fall of a logarithmic measurement signal is the target of measurement.

[0070] Figure 4 shows an example of directly determining the system time constant, where the elapsed time until the measured signal drops to a ratio of 1 / e is the measurement target.

[0071] Figure 5 shows an example of directly determining the system time constant, where the elapsed time until the measured signal rises to a ratio of (1-1 / e) is measured.

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Claims

【Request Item 1】 【Number 1】

2. A method according to claim 1, characterized in that when the difference exceeds a threshold, it is determined that a leak has been detected in the test object.

3. A method according to claim 1, characterized in that the test subject is determined to be dense when the difference is 0 or greater and less than or equal to a threshold.

4. A method according to claim 1, characterized in that an error is determined when the difference is less than 0.

5. The method according to claim 2, characterized in that the threshold is r times the value of the background signal of the measurement signal, and r is a rational number greater than 0. [Request Item 6] [Number 2]

7. The method according to claim 6, characterized in that the second constant C2 is a positive real number less than 1.

8. The method according to claim 6, characterized in that the first constant C1 is a real number greater than 1.

9. The method according to claim 6, characterized in that the second constant C2 includes the nth root of Euler's number e.

10. The method according to claim 6, characterized in that the second constant C2 is multiplied by a coefficient F, and F is a rational number less than 1. [Request Item 11] [Number 3]

12. The method according to claim 1, characterized in that the system time constant τ is determined using a test leak, the test leak may be a test leak built into the vacuum leak detector or an external test leak that can be connected to the test chamber (12).

13. The method according to claim 1, characterized in that the vacuum time constant of the vacuum leak detector is calculated as the system time constant τ from the volume of the test chamber (12) and the piping line connecting the test chamber (12) to the gas detector (14), and the suction capacity of the vacuum pump (16).

14. The method according to claim 1, characterized in that the system time constant τ is determined from the elapsed time from the time the power supply of the test leak is turned off, stopped or removed from the vacuum leak detector until the measurement signal decays to a predetermined value, or from the elapsed time from the time the power supply of the test leak is turned on, started or added to the vacuum leak detector until the measurement signal rises to a predetermined value.

15. The method according to claim 14, characterized in that the elapsed time corresponds to the time during which the measurement signal is 1 / e times the steady state of the test leak measurement signal.

16. A vacuum leak detector that carries out the method according to claim 1, Vacuum pump (16) and A test chamber (12) connected to the vacuum pump (16), A gas detector (14) connected to the test chamber (12), In a vacuum leak detector equipped with, An evaluation unit (22) for evaluating the measurement signal of the gas detector (14), configured to perform the method described in any one of claims 1 to 15, A vacuum leak detector characterized by having the following features.

17. A vacuum leak detector according to claim 16, wherein the evaluation unit (22) includes a memory in which processing procedures are stored.

18. A vacuum leak detector according to claim 16, characterized in that the evaluation unit (22) includes a microcontroller configured to automatically perform the method.

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