EP2430434A1

EP2430434A1 - Capture of thermal images of an object

Info

Publication number: EP2430434A1
Application number: EP10718539A
Authority: EP
Inventors: Max Rothenfusser; Christian Homma
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2009-05-14
Filing date: 2010-04-26
Publication date: 2012-03-21
Also published as: US20120062751A1; WO2010130558A1; DE102009021233A1

Abstract

The invention relates to an apparatus and a method for capturing thermal images of an object (1). The apparatus has: an excitation unit (2) for mechanically exciting the object (1) with a periodic excitation signal, a camera (3) for capturing the thermal images of the object, wherein a thermal image has a plurality of pixels, wherein a pixel is respectively intended to represent a heat signal (8) acquired from the object (1), and means (4) for matching the capture of the thermal images of the object (1) and the periodic excitation signal in such a manner that thermal images captured in a plurality of periods of the periodic excitation signal can be used to determine information relating to the heat signals (8) respectively represented by the pixels during a period.

Description

description

Capturing thermal images of an object

The invention relates to a device and a method for detecting thermal images of an object. Such a device has an excitation unit for the mechanical excitation of the object with a periodic excitation signal. Furthermore, the device has a camera for acquiring the thermal images of the object.

The acquisition of thermal images is also referred to as thermography. Thermography is an imaging process that makes infrared radiation visible. The infrared radiation emitted by an object can be interpreted as a temperature distribution. For this purpose, a thermal imaging camera converts the heat radiation (infrared light) of the object, invisible to the human eye, into electrical signals with the help of special sensors, which can be easily processed. The term thermography is usually used as a synonym for the term infrared thermography. A problem in the detection of thermal images of an object excited by a periodic excitation signal is that thermal imaging cameras available today have only a limited frame rate, typically in the range between 50 Hz to 1000 Hz. This limits the applicability of this technique.

EP 1 582 867 A2 discloses an error detection system which evaluates thermal images of a structure excited by sonic or ultrasonic energy. The system includes a transducer for coupling a sound signal into the structure. The sound signal causes the heating of errors in the structure. A thermal imaging camera captures an image of the structure warmed by the sound signal.

The invention has for its object to improve the detection of thermal images of an object. This object is achieved by a device for acquiring thermal images of an object with an excitation unit for the mechanical excitation of the object with a periodic see excitation signal, with a camera for detecting the thermal images of the object, wherein a thermal image having a plurality of pixels, and wherein Pixel is provided in each case for representing a thermal signal detected by the object, and with means for adjusting the detection of the thermal images of the object and the periodic excitation signal such that in a plurality of periods of the periodic excitation signal detected thermal images information with respect to each of the pixels shown heat signals can be determined during a period.

This object is achieved by a method for acquiring thermal images of an object, with the following steps:

Mechanical excitation of the object with a periodic excitation signal, - capturing the thermal images of the object, wherein a thermal image comprises a plurality of pixels, wherein each pixel represents a heat signal detected by the object, wherein the detection of the thermal images of the object and the mechanical excitation with the periodic Excitation signals are tuned such that from the thermal images detected in a plurality of periods of the periodic excitation signal information can be determined with respect to each of the pixels represented heat signals during a period.

The invention is based on the idea of exploiting the periodicity of the mechanical excitation of an object in a novel manner in order to compensate for the limited frame rate of a camera for acquiring thermal images. The advantage of the mechanical excitation with a periodic excitation signal is that the thermal responses of the object excited by the mechanical excitation repeats substantially during each period of the periodic excitation signal. In order to obtain information relating to the heat signals respectively represented by the pixels of a thermal image, which are emitted by the object during a period, it is sufficient to detect and evaluate the corresponding heat signal during a plurality of periods of the periodic excitation signal respectively at respectively selected times. In order to make this possible, according to the invention the detection of the thermal images of the object and the periodic excitation signal are coordinated with one another. This allows significantly higher frequencies of the periodic excitation signal, thus opening up completely new fields of application of such a method.

According to an advantageous embodiment of the invention, the information includes a course of the heat signal during a period of the periodic excitation signal. This offers the advantage that a course of the thermal signals can be determined, although the frame rate of the camera is low compared to the frequency of the periodic excitation signal.

According to a further advantageous embodiment of the invention, the information includes an amplitude and a phase of the heat signals. This offers particular advantages in applications in which the exact course of a heat signal is not important, but information about the amplitude or the phase of the heat signal sufficient.

Advantageously, the camera is provided for detecting a sequence of the thermal images of the object. In particular, such a sequence comprises a sufficient number of thermal images in order to be able to determine the information relating to the respective heat signals represented by the pixels during a period of the periodic excitation signal.

In accordance with a further advantageous embodiment of the invention, evaluation means are provided for determining the information relating to the heat signal respectively represented by the pixels. nale during a period. Such evaluation means include, for example, computers.

The means for adjusting the detection of the thermal images of the object and the periodic excitation signal are advantageously designed such that they have a pulse generator unit for generating sampling pulses, wherein the camera and the pulse generator unit are coupled to each other such that the detection of one of the thermal images by one of the sampling pulses is triggerable. This enables the time-accurate triggering of the camera to capture the thermal images.

This is exploited according to a further advantageous embodiment of the invention, in particular, that for each subsequent period of the excitation signal, a sampling pulse with a comparison to a generated for each earlier period of the excitation signal sampling pulse continuously increasing delay can be generated. The effect of this delayed sampling is that so over the period of several periods of the excitation signal different portions of the respective heat signal can be detected and evaluated. In this case, this sampling pulse can be generated after one or in each case a multiple of periods of the excitation signal, depending on the embodiment of the invention.

In order to be able to make further statements regarding the object, the evaluation means for determining frequency components of the heat signals are provided according to a further advantageous embodiment of the invention. In particular, this makes it possible to determine fundamental frequency components and components of higher harmonics.

The advantages of the invention come according to a further advantageous embodiment of the same in particular to bear, when the periodic excitation signal a frequency between 2 kHz and 200 kHz, in particular a frequency in the ultrasonic range has.

The method according to the invention can be used in particular for error detection, for measuring mechanical stresses and / or for fatigue analysis of an object.

The invention will be described in more detail below with reference to the exemplary embodiments illustrated in the figures and explained.

Show it:

1 shows a device for recording thermal images of an object in a schematic representation,

2 shows a periodic excitation signal as well as heat signals caused thereby,

3 shows a stress-strain diagram,

4 shows the detection of a heat signal over several periods,

5 shows a parameter determination by means of lock-in technology,

6 shows the detection of a heat signal, and

7 shows a resulting signal when using the lock-in technique.

The thermographic examination of a periodically excited object provides a lot of different information about the respective object. The advancement of technology depends mainly on the availability of fast and sensitive thermal imaging cameras, especially infrared cameras. Typical applications are error detection with acoustic thermography, the measurement of voltage divisions as well as the fatigue analysis of an object. For each of these applications, different measurement systems are required according to the prior art. The frame rate of commercially available cameras is typically in the range between 50 Hz and 1000 Hz (full frame). This limits the applicability of the technique. There are no satisfactory solutions for the methods of investigation described below.

For non-destructive testing by means of acoustic thermography, it must be ensured that the maximum mechanical load-bearing capacity of an object is not exceeded at any point in the object. However, it is not yet possible to measure and display the voltage distribution for excitations in the upper audio range (a few kHz) down to the ultrasound range with infrared cameras.

Both the size and the morphology of cracks determine their findability. In particular, for closed cracks, the signal-to-noise ratio in acoustic thermography is often too small, so that these errors are not found with high probability by a check. A lock-in technique based on the excitation frequency (eg 20 kHz) would considerably improve the detectability. However, the available frame rates of commercially available infrared cameras are not suitable for resolving the heat signal in the ultrasonic range.

The thermographic lifetime prediction based on periodic loading is also limited by the available frame rate of the infrared camera. A maximum excitation frequency of about 30 Hz is known so far. Furthermore, this technique can not be easily applied to all test objects, but is available only with appropriately designed test objects.

The core problem is thus the limited frame rate of infrared cameras. The currently achievable maximum frequency for high-speed cameras in full-screen mode is approximately 1000 Hz. For example, if each period of the thermal signal is to be resolved with N = 10 samples per period, the maximum mechanical excitation frequency is 100 Hz.

The following describes the required components and techniques for frequency resolved acoustic thermography application. A schematic system structure of a device for detecting thermal images of an object 1 is shown in FIG. The device has an excitation unit 2 for the mechanical excitation of the object 1 with a periodic excitation signal. A camera 3 is for detecting the thermal images of the object 1. A thermal image has a plurality of pixels. One pixel is used for each

Representation of a thermal signal 8 detected by the object 1. The device has means 4 for tuning the detection of the thermal images of the object 1 and the periodic excitation signal. The means 4 have a pulse generator unit 6 for generating sampling pulses, also referred to below as stroboscopic signals. There are different ways to generate the stroboscopic signal:

A microcontroller is programmed to provide sampling pulses with increasing delay. - A specially developed electronic circuit with

Hardware components (counters, timer with delay function).

Commercially available plug-in circuits for computers, with counter, time function and delay function.

According to the exemplary embodiment of the invention shown in FIG. 1, the excitation signal output by the excitation unit 2 to the object 1 is generated in a generator 7. Any known type of excitation in the acoustic or ultrasonic range can be used (eg electromagnetic excitation, piezo oscillators, ultrasonic cleaning baths, electromagnetic ultrasonic transducers (EMAT = Electro Magnetic Acoustic Transducer), etc.). The acquisition of the thermal images of the object 1 and the mechanical excitation with the periodic excitation signal are adjusted such that information about the thermal signals 8 respectively represented by the pixels during a period can be determined from thermal images acquired in a plurality of periods of the periodic excitation signal. By means of evaluation means 5, the information relating to the respective thermal signals 8 represented by the pixels is determined during a period.

For the recorded thermal signals 8, post-processing is performed for each individual camera pixel. Both the Fourier analysis and the lock-in technique are methods for determining the contribution (amplitude and phase) of both the fundamental frequency and the higher harmonics as well as each other relevant frequency component. The required post-processing depends on the respective application.

FIG. 2 shows the time profile of a periodic excitation signal 20. Furthermore, the thermoelastic portion 21 of a heat signal, which is emitted by an object excited by the excitation signal 20, and the thermoplastic portion 22 of the heat signal are shown schematically. The thermal signals radiated by the object represent the thermal responses to the periodic excitation. The scaling of the horizontal time axes is always the same in order to facilitate a comparison of the signals shown at certain times.

The frequency component with the frequency f _s is based on the so-called thermoelastic effect and is suitable for evaluating the local voltage. The thermoelastic effect is a reversal of the known thermal expansion and causes the periodic heating and cooling of the object.

3 shows a typical stress-strain diagram for a cyclically excited object. The mechanical clamping σ (ordinate 30) over the strain ε (abscissa 31) applied. The reference numeral 32 denotes a linearly elastic stress-strain curve, the reference numeral 33 a schematic elastic-plastic stress-strain curve. The elastic-plastic stress-strain curve 33 has a hysteresis.

All higher harmonics of the heat signal are based on the non-linear mechanical behavior of the test object, which manifests itself in a hysteresis of the stress-strain relationship. This thermoplastic effect is irreversible and causes a temperature increase in both the pressure and the voltage phase of the excitation. Therefore, the heat signal has a frequency component with twice the fundamental frequency f _s , in addition higher harmonics and a temperature increase progressing over time.

For metallic objects, this effect - unlike plastics - is typically very small, but can still be used for certain materials, e.g. As steel, be found. It also provides a quantitative measure of the fatigue state of certain materials.

In the case of a crack, both the higher harmonics and the progressive increase in temperature are a local phenomenon caused by heat-radiating effects on the crack front or crack tip (friction, damping, plastic deformation).

FIG. 4 shows a periodic heat signal 42 to be detected, whose magnitude (ordinate 40) is plotted over time (abscissa 41). One period of the heat signal 42 is designated by the reference numeral 45. The cyclic detection of the heat signal 42 takes place with a period 44. The reference symbol 43 denotes the integration time t _{x of} the camera detection (also referred to below as the image acquisition time t _x ). The respectively detected by the camera area of the heat signal 42 is marked with circles 46. For each subsequent period of the excitation signal with a frequency f _s , a trigger pulse for the camera is generated with a continuously increasing delay Δt. If the maximum frame rate of the camera is not high enough, this triggering pulse can also be generated in each case for a multiple of periods of the excitation signal. Thus, for example, at an ultrasonic frequency of 20 kHz and a frame rate of the camera of 1 kHz, a trigger for the camera is triggered only for every twentieth ultrasonic period. Depending on the required temporal resolution, a number N of these sampling pulses is needed to sample and reconstruct a complete period of the infrared signal. The stroboscopic sampling pulses are synchronized with the excitation signal. The parameters describing the stroboscopic signal are excitation frequency f _s and number N of sampling intervals for one period and the corresponding resultant increasing time delay Δt. The image acquisition time t _x must be adjusted accordingly.

5 shows the measurement of amplitude and phase of a heat signal by means of the so-called stroboscopic lock-in technique. The signal strength (ordinate 50) of a thermal signal 52 is plotted over time (abscissa 51). Reference numeral 53 denotes the integration time t _{x of} the camera. The period duration t _{a of} the detection is denoted by the reference numeral 54, the period t _{s of} the signal by the reference numeral 55. According to the embodiment shown in FIG 5, the heat signal 52 during the time sections 57, 58, 59 and 60 is detected. The captured, over the

Integral time 53 is the integrated value of the heat signal 52 during the time interval 57. Accordingly, the value integrated during the time segment 58 should be referred to as B, during the time period 59 as C and during the time period 60 as D. The delay of the detection with respect to the heat signal is denoted by the reference numeral 56 and has the value .DELTA.t. For Δt = t _s / 4 the following results: Amplitude = (1/4) * (A - C) Phase = 0

For any value Δt the following relations result:

Amplitude = (1/4) * SQRT ((A - C) ² + (B - D) ² ) Phase = arctane ((B - D) / (A - C))

6 shows the resulting signal 66, which approximates the course of the heat signal 65 during a period of the periodic excitation signal.

7 shows the resulting signal 70, which occurs when detecting a heat signal by means of the lock-in technique described in connection with FIG.

The described embodiments of the invention provide new possibilities of acoustic thermography, in which the detection and the processing of the frequency component of the heat signal for frequencies up to the ultrasound range take place. In order to resolve such a signal, a stroboscopic technique is proposed. Subsequent analysis provides the frequency components of the signal for each pixel of the image sequence.

The fundamental frequency as well as the higher harmonics are used for different applications:

1. Determination of local voltages with periodic excitation.

It is possible to visualize and quantify the stress distribution of a cyclically exposed test object for frequencies up to the ultrasound range. The frequency component used for this is the fundamental frequency f _s . 2. Error detection

This application is based on the higher harmonics (2f _s , 3f _s , etc.) - Depending on the set parameters, the signal-to-noise ratio and thus the probability of error detection improve compared to known techniques. The higher the frequencies used, the lower the fuzziness of thermal diffusion error detection. This allows more accurate localization and sizing of defects.

3. Thermographic lifetime prediction

Again, the higher harmonics (2f _s , 3f _s , etc.) are used. The proposed method avoids costly fixed setups and the need for specially shaped test objects. Thus, real test objects can be examined by coupling an excitation signal. Furthermore, the examination time is significantly reduced because the excitation frequency used can be significantly higher.

For the three applications described, only a single experimental setup is necessary (see FIG. 1). The proposed method provides a complete record of the thermal response of a periodically loaded test object. Simply by selecting the respective frequency component, it is decided which application is selected.

Claims

claims

1. Apparatus for detecting thermal images of an object (1), - with an excitation unit (2) for the mechanical excitation of the object (1) with a periodic excitation signal,

- With a camera (3) for detecting the thermal images of the object, wherein a thermal image having a plurality of pixels, wherein a pixel is provided in each case for representing a detected by the object (1) heat signal (8) and

with means (4) for tuning the acquisition of the thermal images of the object (1) and the periodic excitation signal in such a way that from thermal images acquired in a plurality of periods of the periodic excitation signal, information regarding the respective thermal signals (8) represented by the pixels a period can be determined.

2. Apparatus according to claim 1, characterized in that the information includes a course of the heat signals (8) during a period of the periodic excitation signal.

3. Apparatus according to claim 1 or 2, characterized in that the information includes an amplitude and a phase of the heat signals (8).

4. Device according to one of the preceding claims, characterized in that the camera (3) for detecting a sequence of thermal images of the object (1) is provided.

5. Device according to one of the preceding claims, characterized in that evaluation means (5) are provided for determining the information relating to the respective heat signals (8) represented by the pixels during a period.

6. Device according to one of the preceding claims, characterized in that the means (4) for tuning the Detecting the thermal images of the object (1) and the periodic excitation signal, a pulse generator unit (6) for generating sampling pulses, wherein the camera (3) and the pulse generator unit (6) are coupled together such that the detection of one of the thermal images by one of the sampling pulses is triggerable.

7. Apparatus according to claim 6, characterized in that for each subsequent period of the excitation signal, a sampling pulse can be generated with a delay which is continuously increasing in comparison with a sampling pulse generated for a respective earlier period of the excitation signal.

8. Device according to one of the preceding claims, character- ized in that the evaluation means (5) for determining frequency components of the heat signals (8) are provided.

9. Device according to one of the preceding claims, character- ized in that the periodic excitation signal has a frequency between 2 kHz and 200 kHz, in particular a frequency in the ultrasonic range.

10. A method for acquiring thermal images of an object (1), comprising the following steps:

mechanical excitation of the object (1) with a periodic excitation signal,

Detecting the thermal images of the object (1), wherein a thermal image has a multiplicity of pixels, one pixel representing in each case a heat signal (8) detected by the object (1), the detection of the thermal images of the object (1) and the mechanical excitation with the periodic excitation signal are tuned such that from the heat images detected in a plurality of periods of the periodic excitation signal information with respect to each of the pixels heat signals (8) during a period can be determined.

11. The method according to claim 10, characterized in that the information includes a course of the heat signals (8) during a period of the periodic excitation signal and / or an amplitude and a phase of the heat signals (8).

12. The method according to claim 10 or 11, characterized in that the detection of one of the thermal images is triggered by sampling pulses.

13. Application of the method according to one of claims 10 to 12, for error detection, for measuring mechanical stresses and / or for fatigue analysis of the object (1).