Apparatus and method for performing a hammer test

Abstract

Apparatus for performing a hammer test, comprising: -an actuator (2) that produces at least one mechanical pulse intended to strike a structure to be examined; -a measuring means (3) for measuring the mechanical response offered 5 by the structure to be examined in response to said mechanical pulse; -a control unit (4); -a positioning means (5) for positioning the actuator of at least one mechanical pulse in a predetermined portion of the structure to be examined; said positioning means (5) being controlled on the basis of at 10 least one item of information coming from said control unit (4); said actuator (2) being supported by the positioning means (5).

Description

[0001] DESCRIPTION

[0002] APPARATUS AND METHOD FOR PERFORMING A HAMMER TEST

[0003] Technical field

[0004] The present invention relates to an apparatus and a method for performing hammer tests. In particular, the invention allows the automatic implementation of what in the technical field is the already well-known hammer test. This in order to perform an experimental modal analysis of a structure. For example, this structure can be a mechanical component (or an assembly of components) used in plants, machines, etc.

[0005] Prior art

[0006] It is known to manually apply stresses in predetermined points on the structure with a hammer and measure the response of the structure. The response of the structure to the pulse is measured using intrusive (accelerometers, strain gauges, etc.) or non-intrusive (laser vibrometer, proximity sensors, etc.) measuring systems. The collected data can be analysed to identify natural frequencies, ways of vibrating, and damping factors of the structure. Depending on the response of the structure and the intensity of the force, it is therefore possible to obtain information relating to the experimental dynamic response of the structure.

[0007] The known solutions described above enable various advantages. In particular:

[0008] - ease and speed: it is relatively simple to perform and does not require expensive or complicated equipment;

[0009] - portability: the necessary tools are portable, which allows performing onsite tests on a wide range of structures in the field of mechanical, aerospace and automotive engineering;

[0010] - localization of the measurements: allows measurements to be made at specific points by providing detailed data on the local response of the structure;

[0011] - it constitutes a compromise between contact and non-contact exciters: the hammer comes into contact with the structure for a very short time,simulating an impulsive excitation that allows not to significantly alter the dynamic properties of the analysed component.

[0012] However, since the operation is performed manually, the quality of the exciting pulse depends on the skill of the operator. In practice, the following limitations and problems are systematically encountered:

[0013] • application of impulsive energy concentrated only at the point of impact: this can cause not only a non-linear response, but also damage to the structure in the event of high excitation levels, even if limited in time;

[0014] • variable impact duration: this depends on the mass and stiffness of the hammer, as well as the skill of the operator;

[0015] • occurrence of "multiple blows": a common problem is the unintentional application of multiple and not single hammer strikes, or very close pulses. This leads to a low quality of energy transmission from the hammer to the structure; consequently also the identification of the frequency response function is not optimal as the input is more than one and not just one as it should be. From the point of view of the frequency content of the blow, a strongly distorted excitation frequency spectrum is obtained, implying a very different excitation amplitude for different frequencies;

[0016] • inaccuracy in the application of the pulse: it is not possible to determine with certainty the direction of application of the pulse, which is assumed to be orthogonal to the surface of the component at the point of impact;

[0017] • poor repeatability of the hammer strikes: it is not possible to apply two consecutive identical hammer strikes due to the poor accuracy in hitting the target point, the different force amplitude and the direction of the pulse relative to the surface of the component at the measurement point;

[0018] • limited access to some areas of the structure: the operator may encounter difficulties in exciting points that are difficult to have access to, especially when it is necessary to change the excitation point while keeping the measurement point fixed.

[0019] Solutions are also known in which there is a device equipped with an actuator for the hammer; the positioning is often managed through anarticulated system that must be manually configured by the operator. This process may result in inaccuracies in the positioning of the hammer, affecting the direction and point of application of the pulse. In addition, if it is necessary to change the point of impact, the articulated system must be disassembled and reassembled, an operation that takes time and can lead to errors in positioning. Even the application of repeated pulses on the same point can alter the configuration of the articulated system, making the successive hammer strikes different from one another.

[0020] Aim of the invention

[0021] In this context, the technical task underpinning the present invention is to provide an apparatus and a method for implementing a hammer test which obviates the drawbacks of the prior art cited above.

[0022] In particular, it is an object of the present invention to provide an apparatus and a method for performing hammer tests that allow the automatic and repeatable positioning of the hammer relative to the target point to be impacted, in addition to allowing a precise adjustment of the intensity of the pulse. The technical task mentioned and the objects stated are substantially achieved by an apparatus and a method for performing hammer tests comprising the technical features set out in one or more of the appended claims.

[0023] Brief description of the drawings

[0024] Further features and advantages of the present invention will become more apparent from the indicative, and therefore non-limiting, description of a preferred but not exclusive embodiment of an apparatus and a method for performing hammer tests as illustrated in the accompanying drawings in which:

[0025] - figure 1 shows a perspective view of an apparatus for performing hammer tests according to the present invention;

[0026] - figures 2 and 3 show two schematic views of the apparatus of figure 1 during operation.

[0027] Detailed description of preferred embodiments of the inventionIn the appended figures, reference number 1 indicates an apparatus for performing hammer tests (this type of test is well known if performed with traditional apparatuses and methodologies). In particular, the apparatus 1 allows the automatic performance of hammer tests.

[0028] The apparatus 1 comprises an actuator 2 of at least one mechanical pulse intended to strike a structure to be examined. The actuator 2 can also be defined as an “instrumented hammer".

[0029] Suitably the actuator 2 is an electromagnetic actuator; it suitably comprises a striker; it suitably comprises a solenoid driving the striker. Preferentially, the force exerted by the striker on the structure is directly proportional to the acceleration of the moving part of the solenoid. The electromagnetic actuator is suitably a linear electromagnetic actuator (meaning that the striker is driven by the actuator 2 to move along a straight line in order to impact the structure to be examined).

[0030] The apparatus 1 comprises a force transducer 22 which measures the intensity of the force exerted by the actuator. For example, said transducer 22 comprises / is a load cell. It can also be called a force sensor. The transducer 22 is located for example on the tip of the actuator 2 intended to come into contact with the structure.

[0031] Suitably, the voltage supplied to the solenoid and the duration of the passage of current in the solenoid allow the intensity of the applied force to be adjusted. The duration of the passage of current in the solenoid is adjusted by a transistor switch.

[0032] In fact, the voltage is supplied to the solenoid by a direct current generator. In particular, the generator supplies current to the circuit of which the solenoid is the user. This circuit is open by default. When the operator / control unit decides to activate the solenoid, the circuit is closed by a switch (for example the transistor switch) so that the current flows through the solenoid, which is then reopened an instant later.

[0033] Suitably, the apparatus 1 comprises elastic means that recalls the actuator 2 in a rest position (after it has released the mechanical pulse).Suitably, the apparatus 1 comprises a containment casing 23 containing at least in part the actuator 2 (and suitably also the elastic return means). The apparatus 1 also comprises a control system 24 of the actuator 2. The control system 24 may comprise an electronic board (e.g. Arduino or Raspberry) onto which the software for controlling the solenoid (or more generally the actuator 2) has been loaded. The control system 24 is external to the casing 23. In particular, the control system 24 is remote or otherwise spaced from the actuator 2 and / or positioning means 5 (defined below). The control system 24 can be integrated into the control unit 4 (introduced below).

[0034] The apparatus 1 also comprises a measuring means 3 for measuring the mechanical response offered by the structure to be examined in response to said mechanical pulse.

[0035] The measuring means 3 can be of various kinds. For example, it may include / be:

[0036] - accelerometers or strain gauges or other intrusive measuring systems; or - laser vibrometer or proximity sensors or other non-intrusive measuring systems.

[0037] As mentioned above, the apparatus 1 also comprises a control unit 4.

[0038] The apparatus 1 comprises positioning means 5 for positioning the actuator 2 in a predetermined portion of the structure to be examined (target point to be hit). The positioning means 5 can position the actuator 2, automatically, in any point of a portion of the surface of the structure to be examined. The actuator 2 is removably connected to the positioning means 5 to allow the replacement thereof. This allows to extend the operating range. Suitably, when the actuator 2 performs its mechanical action on the structure to be tested, the positioning means 5 remains stationary.

[0039] The positioning means 5 is controlled on the basis of at least one item of information coming from the control unit 4. The actuator 2 is at least partly supported by the positioning means 5. The control unit 4 may comprise adedicated software or App.

[0040] The positioning means 5 comprises an articulated arm 50 supporting (in particular at one end) the actuator 2. The arm 50 is a robotic arm. Suitably it comprises one or more joints connecting successive segments of the arm 50. The acquisition means 6 for acquiring at least one image is at least in part supported by said articulated arm 50, preferably at the actuator 2. The means 6 is therefore placed at one end of the arm 50. The apparatus 1 comprises acquisition means 6 for acquiring at least one image of the structure to be examined in operative communication with the control unit 4. For example, the means 6 comprises / is a camera.

[0041] The positioning means 5 is controlled on the basis of at least one item of information coming from the acquisition means 6.

[0042] In addition or alternatively said item of information may not come from the acquisition means 6. For example, it could come from an input provided by the user to the control unit 4.

[0043] The control unit 4 is remote relative to the actuator 2. Suitably the control unit 4 is remote relative to the positioning means 5, in particular it is remote relative to the arm 50. The control unit 4 and the positioning means 5 can be connected wirelessly or by cables and / or connectors.

[0044] The apparatus 1 may comprise a user interface 25 that allows the user to provide input to the control unit 4 and / or the control system 24. The user interface 25 may comprise for example a PC or a tablet or a smartphone. In fact, the user interface 25 can for example be separated and connected wirelessly to the control unit 4 and / or to the control system 24 or by means of a physical connection. In addition or alternatively the user interface 25 could be present in a casing of the control unit 4 and / or the control system 24.

[0045] An object of the present invention is also a method for implementing hammer tests; suitably this is aimed at an experimental modal analysis. In particular, this method provides for the automatic performance of the hammer test.Such a method is advantageously implemented by an apparatus 1 having one or more of the features described previously.

[0046] The method comprises the step of preparing a structure 100 to be examined. For example, this structure 100 can be a mechanical component or an assembly of components used in plants, machines, etc. The method also comprises a step of associating with the structure 100 to be examined a measuring means 3 for measuring a mechanical response offered by the structure 100 to be examined in response to a mechanical pulse.

[0047] For example, an accelerometer or strain gauge or other intrusive measuring systems may be constrained to the structure. In addition or alternatively a laser vibrometer or proximity sensors or other non-intrusive measuring systems may be placed in proximity to the structure 100.

[0048] Advantageously, but not necessarily, the method comprises the step of applying on the structure 100 a visual reference 101 in at least one point to be subjected to stress (see for example the visual reference 101 in figures 2 or 3). Advantageously more visual references will be applied to structure 100. The visual reference is of known shape, for example of circular or triangular shape. The visual reference 101 is applied through a mould or a positioning mask, so that the geometric centre of gravity of the visual reference 101 coincides with the point to be subjected to stress. The visual reference 101 can be pasted or drawn on the frame 100.

[0049] The method may include applying the actuator 2 near the structure to be analysed so that it can operate on the entire working area of the structure 100. In this regard, preliminary analyses may be necessary to put the characteristics of the robotic arm (number of axes, dimensions, etc.) in relation with those of the component to be analysed (geometry, dimensions, etc.).

[0050] Suitably the method may provide for the recognition of the visual reference 101 (or of the visual references 101) and the storage of the corresponding coordinates in the space or in the plane. During this step, the robotic armmoves autonomously along the surface of the structure 100, without touching it, to allow the apparatus 1 to process the images of the visual reference 101 , identifying and storing the position of their centre of gravity. In the case where the reference 101 is a circle or an equilateral triangle this could appear respectively as an ellipse or an isosceles or scalene triangle due to the not perfectly orthogonal perspective of the acquisition means 6 relative to the surface of the structure 100. Absolute precision is not necessary at this stage; the exact position can possibly be determined later. Once the visual reference 101 has been identified, the algorithm for the control of the positioning means 5 will suggest the optimal configuration to reach them, i.e. the most stable one, using optimization algorithms based on machine learning.

[0051] The method also comprises the step of positioning, by means of a positioning means 5 controlled by a control unit 4, an actuator 2 that produces a mechanical pulse in a predetermined portion of the structure 100 to be examined (target point to be impacted). The positioning means 5 comprises an articulated arm 50 that can assume the configuration that allows the correct positioning of the actuator 2 at the desired point.

[0052] This step provides in particular to move, controlled by the control unit 4, a moving element (typically an articulated arm) that supports the actuator 2 and possibly also the acquisition means 6. Advantageously, but not necessarily this may be facilitated by knowing the approximate position of the visual reference 101.

[0053] The step of positioning the actuator 2 comprises the step of adjusting the orthogonality of the actuator 2 relative to a portion (target) of the structure 100.

[0054] The step of adjusting the orthogonality comprises the steps of:

[0055] - acquiring, by means of acquisition means 6, at least one image of the visual reference 101 ;

[0056] - comparing information associated with said image of the visual reference 101 acquired by the acquisition means 6 with information associated witha predefined target image (for example, the visual reference 101 could be an equilateral triangle, but if the actuator 2 and the image acquisition means 6 are not correctly oriented or positioned relative to the visual reference, a different figure could be displayed, for example an isosceles triangle; in this regard, note the different image displayed by the user interface 25 in Figure 2 and Figure 3 that correspond to different inclinations of the acquisition means 6);

[0057] - correcting the orientation or position of the positioning means 5 until the comparison between the information associated with said image of the visual reference 101 and the information associated with said predefined target image signals the reaching of a position that falls within a predefined range of tolerance with respect to that of the predefined target image. For example if a circular visual reference 101 appears as an ellipse, the position of the positioning means 5 will be corrected until a circular shape is obtained. Similarly, if a visual reference 101 shaped as an equilateral triangle is detected by the means 6 as a scalene or isosceles triangle, the correction will take place until the image of an equilateral triangle is obtained.

[0058] By way of non-limiting example, use can be made of different Python libraries for image processing, identification of specific geometric patterns, data management and compatibility between the various components of the system. An exemplary list of usable libraries is shown below:

[0059] •Library for performing mathematical operations and matrix management: Library / algorithm name: NumPy.

[0060] Used in data science for statistical analysis, in machine learning for data pre-processing and in scientific calculation for numerical simulations.

[0061] Source: Installed via the Python package manager, pip (PyPI archive). •Image processing

[0062] Library / algorithm name: OpenCV-Python and OpenCV-Contrib-Python. Widely used in fields such as robotics for object detection and tracking, medical imaging for diagnostic image analysis, and automotive for thedevelopment of autonomous driving systems.

[0063] Source: Installed via the Python package manager, pip (PyPI archive). •Data manipulation and analysis:

[0064] Library / algorithm name: Pandas.

[0065] Used in finance for time series analysis, in data science for data cleansing and exploration, and in economics for managing large datasets.

[0066] Source: Installed via the Python package manager, pip (PyPI archive). •Image file management and processing:

[0067] Library / algorithm name: Pillow.

[0068] Used in web development for image scaling and compression, graphic design for basic image manipulation, and scientific research for image file uploading and processing.

[0069] Source: Installed via the Python package manager, pip (PyPI archive). Before applying the mechanical pulse, the method may provide for an automatic calibration of the force intensity. After an initial pulse, the force transducer 22 measures the amplitude of the pulse, the intensity of which, if necessary, is corrected by a control algorithm. The control can act on two parameters: the voltage supplied to the solenoid and the duration of the passage of current in the solenoid, adjusted by a transistor switch. The method therefore provides for applying said mechanical pulse to the structure 100 to be examined. This happens after positioning the actuator 2, by means of the positioning means 5, in front of a predefined point. In particular, it also happens after correctly orienting the actuator 2 relative to the surface of the structure in the point to be impacted (target). In fact, as better explained below, the actuator 2 should strike the predefined point orthogonally to the structure 100. The automation of the positioning means 5 involves the use of control algorithms to automatically position the robotic arm relative to the structure to be analysed.

[0070] The method also comprises the step of determining, by means of the measuring means 3, the mechanical response offered by the structure 100 to be examined following the mechanical pulse.Suitably, the method may comprise the step of correcting the intensity of the force applied by the actuator if the mechanical response offered by the structure 100 to be examined following the mechanical pulse is outside pre-established ranges.

[0071] After each blow, it is checked whether the configuration of the positioning means 5 (in particular of the robotic arm 50) or the position of the structure 100 has been altered. It is advisable to check the configuration of the robotic arm after each blow to ensure proper alignment before the next blow.

[0072] Possibly the positioning process, the application of the mechanical pulse and the detection of the mechanical response also takes place for the further visual references 101.

[0073] In a particular and non-limiting example solution, apparatus 1 may comprise:

[0074] • Servomotors

[0075] The servomotors for robotic arms are precision motors that control the position, speed, and torque of the arm joints. They offer accurate and adjustable movements to perform tasks with high precision and reliability in robotics.

[0076] Manufacturer example: Farnell (Avnet company);

[0077] Model example: MONKMAKES SKU00051;

[0078] Number of units; 8;

[0079] • 6-axis robotic arm

[0080] The robotic arm is used to autonomously position the automatic hammer near the target points to be impacted;

[0081] Manufacturer example: Universal Robots;

[0082] Model example: UR3e;

[0083] Number of units; 1 ;

[0084] A smaller test arm can also be used to train the control algorithm.

[0085] • Solenoids.

[0086] It is the part of the system for actuating the hammer strike, on the tip ofwhich a force transducer is applied;

[0087] Manufacturer example: Electroimanes NAFSA S.L.;

[0088] Model example: ER series, ERD series;

[0089] Number of units; 2 per model.

[0090] The present invention achieves important advantages.

[0091] First of all, it allows precise positioning. In fact, the system allows the automatic hammer to be accurately positioned relative to the structure 100. In a particular solution there is also an algorithm that identifies the image of the visual reference 101 corresponding to the position of the point of impact. This allows to optimize both the distance of the tip of the device from the component and the perpendicularity relative to the surface in the point of impact.

[0092] Another important advantage is that the force pulse is controlled; in fact, the system allows a force pulse of predetermined amplitude to be applied, defined by the operator or automatically adapted based on the mass and stiffness characteristics of the component to be analysed.

[0093] It is therefore also possible to perform repeatable hammer strikes while maintaining the same relative position between the automatic hammer and the structure 100, thanks to a control that is activated at the end of each hammer strike to correct any perturbations of the initial relative position. The apparatus 1 also allows the positioning of the actuator 2 in areas of the component that are not accessible to the operator or to the larger commercial automatic devices.

[0094] Another advantage is the possibility of operating with time savings. The method according to the present invention provides for performing several Hammer tests, changing the excitation point while keeping the response measurement point fixed. Compared to the roving response approach, which requires fixing the excitation point and varying the position of the measurement point, this method offers several advantages. For example, in the roving response approach, changing the position of invasive motion transducers such as accelerometers can alter the mass distribution on thestructure, thereby affecting the natural frequencies and modes of vibrating of the component. However, the roving excitation method (while keeping the response measurement points fixed and changing the excitation point) usually has disadvantages, such as the long duration of the tests and the difficulty of exciting complex geometry structures. The proposed system overcomes both these problems, positioning itself autonomously and precisely close to the target points, even in areas that are difficult to have access to, and completing the entire experimental modal analysis session without the intervention of the operator.

[0095] The invention thus conceived is susceptible of numerous modifications and variants, all falling within the scope of the inventive concept that characterises it. Moreover, all the details may be replaced by other technically equivalent elements. All the materials used, as well the dimensions, may in practice be any whatsoever according to needs.

Claims

CLAIMS1. An apparatus for performing a hammer test, comprising:-an actuator (2) that produces at least one mechanical pulse intended to strike a structure to be examined;-a measuring means (3) for measuring the mechanical response offered by the structure to be examined in response to said mechanical pulse; -a control unit (4);-a positioning means (5) for positioning the actuator of at least one mechanical pulse in a predetermined portion of the structure to be examined; said positioning means (5) being controlled on the basis of at least one item of information coming from said control unit (4); said actuator (2) being supported by the positioning means (5).

2. The apparatus according to claim 1, characterised in that it comprises an acquisition means (6) for acquiring at least one image of the structure to be examined in operative communication with the control unit (4); said positioning means (5) being controlled on the basis of at least one item of information coming from the acquisition means (6).

3. The apparatus according to claim 2, characterised in that the positioning means (5) comprises an articulated arm (50), at one end of which said actuator (2) is placed; said acquisition means (6) for acquiring at least one image is least in part placed on said articulated arm (50), at said actuator (2).

4. The apparatus according to any one of the preceding claims, characterised in that said control unit (4) is remote relative to the positioning means (5) and actuator (2).

5. The apparatus according to any one of the preceding claims, characterised in that it comprises a force transducer (22), placed at said actuator (2), which measures the intensity of the force exerted by the actuator (2).

6. The apparatus according to any one of the preceding claims, characterised in that said actuator (2) of at least one pulse is removablyconnected to the positioning means (5) to allow the replacement thereof.

7. A method for performing a hammer test, comprising the following steps: - preparing a structure (100) to be examined;- associating with the structure (100) to be examined a measuring means (3) for measuring a mechanical response offered by the structure (100) to be examined in response to a mechanical pulse;- positioning, by means of a positioning means (5) controlled by a control unit (4), an actuator (2) that produces the mechanical pulse in a predetermined portion of the structure (100) to be examined;- applying said mechanical pulse to the structure (100) to be examined; - determining, by means of the measuring means (3), the mechanical response offered by the structure (100) to be examined following the mechanical pulse.

8. The method according to claim 7, characterised in that it comprises a step of applying on the structure (100) a visual reference (101) in at least one point to be subjected to stress.

9. The method according to claim 7 or 8, characterised in that the step of positioning the actuator (2) of the mechanical pulse comprises a step of adjusting the orthogonality of the actuator (2) of at least one pulse relative to a portion of the structure (100); said method comprising the sub-steps of:- acquiring, by means of acquisition means (6), at least one image of the visual reference (101);- comparing information associated with said image of the visual reference acquired by the acquisition means (6) with information associated with a predefined target image;- correcting the orientation and / or position of the positioning means (5) until the comparison between the information associated with said image of the visual reference and the information associated with said predefined target image signals the reaching of a position that falls within a predefined range of tolerance with respect to that of the predefined targetimage.

10. The method according to claim 7 or 8 or 9, characterised in that it corrects the intensity of the force applied by the actuator (2) of at least one pulse if the mechanical response offered by the structure (100) to be examined following the mechanical pulse is outside pre-established ranges.

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