WO2014114884A1

WO2014114884A1 - Device suitable for the qualitative characterisation of the state of a surface

Info

Publication number: WO2014114884A1
Application number: PCT/FR2014/050128
Authority: WO
Inventors: Patrick Eisen; Pierre Jean MEAUSOONE
Original assignee: Universite De Lorraine
Priority date: 2013-01-25
Filing date: 2014-01-23
Publication date: 2014-07-31
Also published as: FR3001548A1; EP2948765A1; FR3001548B1

Abstract

A device suitable for the qualitative characterisation of the state of a surface. The invention concerns a device suitable for the qualitative characterisation of the state of a surface of an object, for example for the qualitative characterisation of a surface of a piece of wood, comprising: • a feeler (10), • a means for rotating the feeler, such that the feeler follows a circular trajectory, • an electroacoustic transducer (30), configured to produce an electrical signal representative of the sound waves transmitted by the feeler. Application to the qualitative characterisation of a surface, for example at the end of a production line producing parts. Application to all types of materials: woods, metals, plastics, fabrics, etc.

Description

Device adapted for the qualitative characterization of the state of a surface

Technical field and state of the art

The invention relates to a device adapted for the qualitative characterization of the state of a surface of a material. The invention has been developed for the characterization of the surface condition of pieces of raw wood, machined wood parts or pieces of wood-based materials such as particle boards, such as MDF panels (Medium Density Fibreboard ), PPSM (Melamine-surfaced Particleboard), OSB (Oriented Strand Board), etc. But the invention was then successfully tested on other materials such as composites, metals, plastics, fabrics, etc.

Standards define parameters characterizing quantitatively the surface condition of the machined parts and more particularly the roughness and the corrugations (ISO Standard 1562, 1996), in particular the distance between the hollows and the bumps of the streaks appearing on the surface, and tools such as roughness meters can be used to determine these parameters.

The document Dl = DE 29 15 229 proposes for its part to measure the roughness of a surface of a metal part by measuring the amplitude of the vibrations of a stylet displaced on the surface of the part to be tested according to linear oscillations and or angular while the part to be tested is rotated by a machining lathe.

However, all known devices are complex to implement and rather expensive; also in practice and for a quick check that, at least for specific applications such as small series of manufacturing or laboratory studies for example, the visio-tactile analysis remains the preferred technique. This technique involves finger probing the surface and looking at it to estimate its surface condition. This technique is obviously preferred for its simplicity of implementation. However, it remains very subjective, with results largely dependent on the experience of the person practicing it, an experience that rarely extends to more than one or two types of materials.

DESCRIPTION OF THE INVENTION The invention proposes a new device adapted for the qualitative characterization of the state of a surface, for example for the qualitative characterization of a surface of a piece of wood. More specifically, the invention relates to a device comprising:

• a probe,

A means for rotating the probe, so that the probe follows a circular path and performs a complete revolution,

An electroacoustic transducer, configured to produce an electrical signal representative of the sound waves emitted by the probe. The invention thus exploits the acoustic vibrations of the probe moving over the surface to be characterized, and the circular trajectory of the probe makes it possible to take into account the variations of the surface state in all directions. The amplitude of the sound waves gives information on the amplitude of the roughnesses on the characterized surface, and the frequency of the sound waves gives an indication of the resonance of the material and its hardness. In practice, to characterize the surface of a material, the device according to the invention is simply placed on the surface to be characterized and the drive means of the probe is activated.

According to one embodiment, the drive means is configured to rotate the probe with constant power. Thus, the time taken by the probe to take a turn is representative of the adhesive power of the surface of the tested material.

In one embodiment, the drive means includes an arm at a first end of which the probe is attached, and a motor configured to drive the arm in rotation so that the probe travels in a circle. The arm guides the probe in a perfect circle, which ensures a regular measurement in all directions.

Preferably, the motor is coupled to the arm via a belt. The use of a belt makes it possible not to transmit to the probe the vibrations of the engine, vibrations generating sound waves which disturb the measurement made by the transducer.

Preferably, the device according to the invention comprises means configured to press the probe on the surface to be characterized according to a constant force. This eliminates the risk of rebound of the probe on the surface to be characterized. Thus, the probe remains plated on the surface to be characterized and perfectly follows the variations, and the measurement of sound waves by the transducer is not parasitized by bouncing phenomena, echoes, etc.

The device according to the invention also comprises a housing capable of resting by at least three feet on the surface to be characterized; the drive means is positioned inside the housing, the probe and the transducer are positioned under the housing. The housing facilitates the manipulation of the device. The housing is preferably provided with at least three legs, each preferably having the smallest contact area possible. Thus, the device according to the invention is in contact with the surface to be characterized on the smallest possible areas, so as not to disturb the measurements.

The device according to the invention may further comprise a weighting means, configured to immobilize the device during the rotation of the probe. For this, the housing can be made in whole or part in a heavy material such as a metal (steel, etc.), or a heavy metal part can be positioned at the bottom of the housing. This to have a perfectly stable device, which does not move when the probe rotates.

The device according to the invention may further comprise a display means for receiving and displaying the electrical signal emitted by the transducer. The display means is for example a screen allowing an operator to view the received signal.

The device may further comprise a data processing means configured to receive the electrical signal emitted by the transducer, and to produce at least one parameter of the electrical signal, a characteristic parameter of a state of the surface to be characterized. The data processing means thus facilitates the task of the operator in the interpretation of the measurements made. The parameter (s) is / are for example:

• a duration of the measurement, corresponding to the time taken by the probe to turn 360 ° and to go around a circle, and / or

· An effective value of the signal, and / or

• an average value of the signal and / or

• a signal form factor, defined as the ratio of the rms value to the average value, and / or

• integration, and / or

• one or more characteristic frequencies, and / or

· FFT (Fast Fourier Transform) analysis, to calculate the main frequencies representative of the measured surface.

The parameter can also be a magnitude representative of the anisotropy of the surface state of the material. Such a parameter is particularly advantageous for characterizing the surface state of a piece of wood, whether or not it is raw. The surface quality measured in a direction substantially perpendicular to the wood fiber may be less good than in the direction substantially parallel to the wood fiber. More generally, such a parameter is particularly advantageous for estimating the importance of the anisotropy of the surface state of a material.

In one example, the magnitude representative of the anisotropy of the surface state of the material is defined by the ratio between the rms value of the electrical signal on the first and third quadrants and the rms value of the electrical signal on the second and the fourth quadrants. fourth quadrants, the first, second, third and fourth quadrants corresponding to a rotation of the probe at an angle of 0 to 90 °, 90 ° to 180 °, 180 ° to 270 ° and 270 to 360 °, respectively; angle 0 corresponding to a preferred direction of the material.

In a preferred embodiment, the data processing means is a computer means such as a computer, comprising in particular a wired or wireless receiving means of the electrical signal supplied by the transducer, a memory storing a computer program comprising instructions of a program configured for determining the one or more parameters above, means such as a microprocessor configured to execute the program instructions, display means configured to display the one or more parameters determined by the program instructions, and optionally means for storing the parameter (s) determined by the program instructions. The means for storing the parameter or parameters is, for example, a data memory storing in each line of a parameter database a designation of an object, the electrical signal produced by the transducer and / or one or more determined parameters. by the data processing means from the electrical signal.

The data processing means may further comprise comparison means, for comparing an electrical signal produced by the transducer and / or one or more parameters determined by the data processing means with the contents of the database. Thus, it is possible to determine the object under test, or to determine objects whose characteristics in terms of surface quality are close, etc. The device according to the invention has many other advantages.

The device is completely autonomous, just put it on the surface to be characterized. In particular, it does not require external drive means, such as for example a machining lathe.

It can be used at any time, especially outside of any machining phase of the surface to be characterized, and far from any machining machine. Indeed, during use of the device according to the invention, the surface to be characterized is totally immobile. Thus, the movements of the feeler are not influenced by parasitic vibrations, such as for example the vibrations of a machining machine carrying the part whose surface is to be characterized, and the electrical signal produced by the transducer is not influenced by noise such as 50Hz noise of a machine tool. The device according to the invention can be used on any type of material, and in particular on screened materials, ie materials having a large surface anisotropy, for example wood, fabric, etc.

The device according to the invention is small and light as will be seen better later in an example, so easily transportable from one surface to be characterized to another.

Brief description of the figures

The invention will be better understood, and other features and advantages of the invention will become apparent in light of the following description of embodiments of a device according to the invention. These examples are given in a non-limiting manner. The description is to be read in conjunction with the appended drawings in which:

FIGS. 1 and 2 are a front view and a view from above of a device according to the invention,

FIGS. 3 to 8 show results obtained by a device according to the invention for different materials.

Description of an embodiment of the invention

As said above, the invention relates to a device adapted for the qualitative characterization of the state of a surface of an object, for example for the qualitative characterization of a surface of a piece of wood. The device according to the invention essentially comprises a probe, a means for rotating the probe, so that the probe follows a circular path, and an electroacoustic transducer, configured to produce an electrical signal representative of the sound waves emitted by the probe.

The example shown is a prototype whose efficiency has been verified and which is used regularly in a machine shop. The probe 10 is a ball whose diameter, from 1 to 10 mm, is chosen according to the material to be characterized and the desired accuracy. The prototype made has a generally cylindrical shape, about 10 cm in diameter and 15 cm in height, for a weight of about 1200 g.

The means for rotating the probe comprises: An arm 21, at one end of which is fixed the probe,

A motor 22, whose axis of rotation is coupled via a drive belt to an axis of rotation 23 positioned at a second end of the arm.

The electroacoustic transducer 30 is positioned just above the probe, at the first end of the arm, in order to record the waves produced by the mechanical vibrations of the probe as it traverses the surface to be characterized.

When it is rotated, the probe travels a circle of diameter of the order of 20 to 80 mm. The diameter of the circle and the number of measuring points chosen along the circle depend on the desired accuracy.

A control means starts the motor, digitizes the electrical signal supplied by the transducer by sampling it at an acquisition frequency of the order of 8000 to 22050 Hertz and then stops the motor when the probe has taken a turn. The choice of the sampling frequency (ie the number of points recorded per second) depends in particular on the surface to be characterized and the desired accuracy. A sampling frequency of around 8000 Hertz gives good results for the study of pieces of wood.

The drive means, and more precisely the motor 22, is configured to rotate the probe with a constant power, of the order of 100 mW. The probe being rotated at constant power, the time taken by the probe to go through a complete revolution is variable, depending on the mechanical strength of the surface of the object to be characterized; this mechanical resistance depends in particular on the adhesion and the quality of the surface to be characterized. Thus, the probe takes about a second to make a turn on a non-adherent surface object, and can take up to 2 seconds to traverse a turn on an object with a sticky surface. For example, the probe takes much longer to travel one turn on the surface of an object made of an adhesive material such as silicone than to walk a turn on the surface of an object made of a non-adhesive material such as than teflon.

The device according to the invention may also comprise means configured to press the probe 10 on the surface to be characterized according to a constant force. By way of example, in FIG. 1, the horizontal arm 21 carries the probe 10 at one end and the arm 21 has a second end connected to the vertical axis 23 by an elastic return means positioned inside the arm 21 (thus not visible in Figure 1). This elastic return means is for example a spring arranged to push the arm 21 and the probe down with a constant restoring force. In another example, the plating means may be a ballast positioned at the free end of the arm 21, in the vicinity of the probe, which applies a constant force of gravity and thus plates the probe against the surface to be characterized.

The device shown also comprises a rotary optical encoder 50, comprising a wheel with notches, for accurately measuring the angle of rotation of the arm. The measurement of the rotation angle makes it possible to stop the rotation of the probe when it has made a turn.

The device also comprises a housing 60 in which are positioned in particular the drive means, the encoder, the control means. The axis of rotation 23 of the arm passes through the bottom of the housing, the arm carrying the probe and the transducer are positioned under the housing. In the example shown, the housing is ballasted by a bottom 61 made of thick metal and the housing walls are made of transparent plastic material. The housing comprises at least three feet 62. On the prototype made, two feet are identifiable by their color (eg black) relative to the third (eg red). To facilitate the identification of the measurement results on the display screen, the two black feet are aligned in a preferred direction of the surface to be characterized, for example in the direction of the fiber of a wood or in the direction of the weft of a fabric.

The device according to the invention is completed by a data processing means (of which only the display screen is shown in FIGS 3-8) connected to the control means in order to recover the sampled data produced by the control means. In practice, the data processing means is a computer storing software developed to process the received data.

The computer notably allows the display of the electrical signal produced by the transducer; it also makes it possible to determine parameters of this signal and in particular:

• the duration of a measurement, that is to say the time taken by the probe to travel a lap,

• the effective value of the signal,

• the average value of the signal

• the signal form factor

• signal integration

• one or more characteristic frequencies of the signal

• the anisotropy of the surface tested.

• FFT analysis, to calculate the main frequencies representative of the measured surface.

The effective value of the signal is calculated according to the relation: Vf = square root [(sum of i = 1 to n of (xi- 128) * (xi- 128) / n], where n is the number of samples measured on one turn, xi is the amplitude of the signal measured for sample i, ie, a mathematical representation:

RMS value =

The average value of the signal is calculated according to the relation: Vm = (sum of i = 1 to n of (xi- 128) / n, ie a mathematical representation:

Average value =

not

The form factor is defined by the ratio between the rms value Vf and the average value Vm; a mathematical representation: RMS value

Form Factor =

Average value

The integration of the signal is calculated according to the relation:

Integration = j, l _; ; - 128}

In one example, the anisotropy of the surface is defined by the ratio between the rms value of the electrical signal on the first and the third quadrants and the rms value of the electrical signal on the second and fourth quadrants, the first, second, third and fourth quadrants corresponding to a rotation of the probe, respectively from 0 to 90 °, from 90 ° to 180 °, from 180 ° to 270 ° and from 270 to 360 °.

Note that, in the above equations and in a known manner, the number 128 is used for a data acquisition made on 8 bits (from 0 to 255). 256 would be used for 9-bit acquisition, 512 for 10-bit acquisition, and so on.

To facilitate the digital processing of the measured data, the device according to the invention is positioned on the surface to be characterized so that the angle θ corresponds to a preferred direction of the surface of the material, for example a direction where the roughness is low, for example a direction where the surface condition seems of very good quality, as a direction parallel to the fiber when the material is wood. The device according to the invention can also be positioned in a random manner on the surface to be characterized, for example if the anisotropy of the surface is weak and if a palpation of the surface does not make it possible to determine a preferred direction.

The first and third quadrants are for example called perpendicular quadrants and the second and fourth quadrants are for example called parallel quadrants.

The anisotropy is thus calculated as follows: An = [square root [(sum of i = 1 to n / 4 of (xi * xi)) / (n / 4)] + square root [(sum of i = n / 2 to 3n / 4 of (xi * xi)) / (n / 4)]] divided by [square root [(sum of i = n / 4 to n / 2 of (xi * xi)) / (n / 4 )] + square root [(sum of i = 3n / 2 to n of (xi * xi)) / (n / 4)]. According to this calculation, an isotropic surface has an anisotropy equal to 1.

Rms value perpendicular quadrants

Anisotropy =

Rated value parallel quadrants

FIGS. 3 to 8 show examples of measurement results obtained from the prototype made for:

• the planed surface of a spruce object (Fig. 3),

• the surface of a varnished bamboo object (fig 4),

• the surface of a Teflon object (fig 5),

• the surface of a Jacquard fabric object (Fig. 6).

· The surface of an object in PPSM (Melamine Surfaced Particle Board) (Fig. 7).

• the surface of an object (adhering conveyor belt) made of silicone (Fig. 8). On each figure:

• the graph at the top left is a representation of the measured signal,

The graph at the bottom left is a representation of a Fourier transform of the measured signal and, under the graph, the characteristic frequencies determined by the Fourier transform are indicated,

"The table at the top right indicates the rms value, the average value, the integration and the signal form factor, on each of the quadrants respectively denoted S0 (probe rotation angle between 0 and 90 °), SI (angle rotation between 90 and 180 °), S2 (angle of rotation between 180 and 270 °) and S3 (angle of rotation between 270 and 360 °),

• In this first table is indicated the anisotropy and the time of travel (or time process).

The values of the integration characterize the surface quality of the materials:

• Teflon, a soft, isotropic and smooth surface material: low integration value (around 4600) and an anisotropy close to 1.

• Planed Spruce, wood material, anisotropic: average integration value (of the order of 61000) and an anisotropy of 1.55, characteristic of the direction of fiber direction.

• The PPSM, very hard surface material, isotropic and structured: high integration value (of the order of 141000) and an anisotropy close to 1.

Unsurprisingly, soft materials, such as Teflon, Jacquard or even spruce, have a surface with low characteristic frequencies, representative of rather serious sounds and damped acoustic waves. Conversely, a rather hard material, such as bamboo or PPSM, has a surface with higher characteristic frequencies, representative of rather acute sounds, less damped acoustic waves.

It is also verified that rather isotropic materials have an anisotropy close to 1 (0.96 for Teflon, 0.99 for PPSM) whereas rather anisotropic materials have anisotropy farther away from 1 (0.77 for Jacquard fabric, 1.55 for spruce) , 1.59 for bamboo).

It is also noted that the values of the travel time are very high for adherent materials (1833 ms for a silicone adherent conveyor belt and low for materials with a low coefficient of friction (955 ms for Teflon).

As seen in these examples, the device according to the invention gives above all qualitative information on the state of a tested surface.

Different applications are possible.

For example, it is possible by a plurality of measurements, corresponding to different materials and / or different surface finish states, to constitute a reference database; base that can be used later to characterize an unknown surface for example. In another example, it is possible to define ranges of acceptable parameters (tolerances) then to test, at the output of a working machine (cutting, polishing, etc.) pieces of wood (or any other material), if the area worked parts has acceptable parameters or not. In this case, the device according to the invention serves as a device for checking the quality of the parts produced.

Claims

1. Device adapted for the qualitative characterization of the state of a surface of an object, for example for the qualitative characterization of a surface of a piece of wood, comprising:

A probe (10),

An electroacoustic transducer (30), configured to produce an electrical signal representative of the sound waves emitted by the probe.

2. Device according to the preceding claim, wherein the drive means is configured to rotate the probe with a constant power.

3. Device according to one of the preceding claims, wherein the drive means comprises an arm (21) at one end of which is fixed the probe (10) and a motor (22) configured to drive the rotating arm of so that the probe runs through a circle.

4. Device according to claim 3, wherein the motor (22) is coupled to the arm (21) via a belt.

5. Device according to one of the preceding claims, also comprising a means configured to press the probe (10) on the surface to be characterized in a constant force.

6. Device according to one of the preceding claims, also comprising a housing (60) capable of resting by at least three feet (62) on the surface to be characterized, the drive means being positioned inside the housing, the probe (10) and the transducer (30) being positioned under the housing.

7. Device according to one of the preceding claims, also comprising a weighting means (61), configured to immobilize the device during the rotation of the probe (10).

8. Device according to one of the preceding claims, further comprising a data processing means configured to receive the electrical signal emitted by the transducer, and produce at least one parameter of the electrical signal, characteristic parameter of a state of the surface to characterize.

9. Device according to the preceding claim, wherein the one or more parameters is / are:

• a length,

• an effective value, and / or

• a mean value and / or

• a form factor, defined by the ratio of the rms value to the average value, and / or

• integration and / or

• one or more characteristic frequencies,

10. Device according to one of claims 8 or 9 wherein the parameter is a magnitude representative of the anisotropy of the surface state of the material.

11. Device according to claim 10, wherein the magnitude representative of the anisotropy of the material is defined by the ratio between the rms value of the electrical signal on the first and third quadrants and the rms value of the electrical signal on the second and the fourth quadrants. fourth quadrants, the first, second, third and fourth quadrants corresponding to a rotation of the probe respectively at an angle of 0 to 90 °, 90 ° to 180 °, 180 ° to 270 ° and 270 to 360 °, angle 0 corresponding to a preferred direction of the material.