WO1998034090A1

WO1998034090A1 - Method and device for measuring the pattern depth on a tyre

Info

Publication number: WO1998034090A1
Application number: PCT/EP1998/000558
Authority: WO
Inventors: Joachim Bürger; Stephan la Barré
Original assignee: Buerger Joachim; Barre Stephan
Priority date: 1997-02-03
Filing date: 1998-02-03
Publication date: 1998-08-06
Also published as: AU6294398A

Abstract

The present invention relates to a method and device for measuring the pattern depth of a tyre (10) mounted on a vehicle. The tyre (10) is installed on at least one rotating roll (14, 16). The pattern area (30) is submitted to a linear laser ray (28), which produces on said pattern a light spot. The light reflected by the tyre pattern is recorded by an image defining sensor (26), which observes the position or shape of the light spot. The signals from the image defining sensor are processed so as to obtain output data related to the pattern depth. During the measuring operation, the vehicle tyre (10) is rotated to permit pattern measurement in various places on the tyre.

Description

Method and device for measuring the tread depth of a vehicle tire

Technical field

The invention relates to a method for measuring the tread depth of a motor vehicle tire mounted on a motor vehicle, wherein

the tire tread of the motor vehicle tire is exposed to laser light,

the light reflected from the tire tread is detected by an image-resolving sensor and

the signals of the image-resolving sensor are processed to generate output data in accordance with the profile depth.

The invention further relates to a device for measuring the tread depth of a motor vehicle tire mounted on a motor vehicle, comprising

a laser for generating a laser beam which can be aligned on the tread surface of the motor vehicle tire to produce a light spot,

an image-resolving sensor through which the position of the light spot can be observed,

- Büdverarbeitr gsmittel, through which from position data of the image-resolving sensor

Measured values for the tread depth of the tire tread can be generated. The profile of automotive tires is very important for safety. Via the tread, rainwater can flow off to the side under the tire, so that the tire does not float and the grip is lost (aquaplaning). This is particularly important in modern, high-speed passenger cars. In many countries, therefore, a minimum depth of the profile is required by law (in Germany 1.6 mm). Even with a tread depth of less than 3.0 mm, water displacement in the rain is reduced to only 30% of the new tire value. The profile of motor vehicle tires is subject to heavy wear. This wear is difficult to see for the owner of the vehicle.

Underlying state of the art

DE 43 16 984 AI discloses a method and a device for automatically determining the tread depth of motor vehicle tires. A partially transparent measuring plate is arranged in the bottom of a measuring station. A measuring head is located below the measuring plate. The measuring head has a laser and an image-resolving sensor as a triangulation unit. To measure the tread depth of a motor vehicle tire, the measuring plate is rolled over by the tire or the tire is placed on the measuring plate. The laser then creates a light spot on the tread's surface. The position of the light spot is observed by the sensor. The sensor output signals are transferred to an evaluation unit, which determines the size of the tire tread. The measuring head is arranged on a chute and can be moved transversely to the rolling direction of the tire.

In the device described in DE 43 16 984 AI, the measurement of

Tire tread inevitably under load, the knobs of the tire tread being compressed in the radial direction of the tire in the area of the tire footprint. In order to avoid a falsified measurement result, the laser is aligned in such a way that the laser beam strikes the tire tread outside the contact area of the tire.

In order to be able to determine the tire profile even when the tires are dirty, an outlet nozzle for water is arranged in the area of the measuring plate and acts on the tire profile with water before or during the measurement. Furthermore, a self-cleaning system is provided, which by cleaning the

Clean the soiled measuring plate. With this automatic device, it cannot be guaranteed that an area of the tire representative of the tire tread will be scanned. In order to increase the probability of a representative measurement, several measuring plates and measuring heads are therefore provided one behind the other in the rolling direction of the tire.

DE-OS 1 809 459 describes a method and a device for automatically determining the tread depth of motor vehicle tires in flowing traffic. The measuring principle is based on the so-called light section method. A protective opening is provided in the street surface, below which the measuring arrangement is arranged in a pit. A photoelectric contact is actuated by the tire to be measured, which triggers an electron flash. A narrow band of light is generated by the electron flash. The light band is sharply imaged on the tire surface through the protective opening. The light band is reflected as a step line by the profile of the tire surface, the step height being proportional to the tread depth. The step-like light band image is magnified on a photo layer through a telescope and a camera lens. Part of the imaging light beam is reflected by a semi-transparent mirror onto a plate, which consists of a grid of photoelectric elements. An electronic evaluation device determines the step heights and thus the profile depth from the output signals of the photoelectric elements.

EP 0 469 948 AI also describes a device for automatically determining the tread depth of motor vehicle tires in flowing traffic. Here too, a laser creates a light spot on the tire surface and the light spot is observed by an image-resolving sensor. The measuring device is located under the road surface. An opening covered by a window is provided in the street surface. Several measuring units are provided one behind the other in the direction of travel.

WO 96/37754 discloses a method and a measuring device for measuring the tread depth of a motor vehicle tire, a laser measuring head being used which works according to the triangulation method. The laser measuring head is positioned with respect to the motor vehicle tire in such a way that a reference surface assumes a defined position with respect to the tire. The laser beam from the laser is guided through the reference surface at an angle to the base of the tire profile, so that a light spot is generated on the base of the tire profile. The position of the

Light spots are observed and a measure of the depth of the tire tread is obtained. The measuring device is designed as a mobile measuring device, the laser measuring head being arranged on a rod. The evaluation unit, battery and printer are in a separate housing arranged. The laser measuring head is manually brought up to the tires of parked motor vehicles by means of the rod. It is also mentioned that the measuring device can be stationary and can be used in a brake test system or car wash. For this purpose, the measuring head is let into the road and moved over the tire profile by means of stepper motors when a tire is above the measuring head.

The above-mentioned methods and devices for measuring the tread depth of a motor vehicle tire show various disadvantages:

In all known methods, only along a line of the tire tread transverse to

Direction of tire roll measured. It depends solely on chance whether the tread depth at this point of the tire is representative of the tread depth of the entire tire. It is also not possible to select a specific location on the tire tread surface at which the measurement is carried out, since it is completely arbitrary which location of the tread surface is currently above the measuring device.

Attempts have been made to avoid this disadvantage by providing several measuring units arranged one behind the other in the direction of travel. However, this is very complex and only leads to the fact that several lines of the profile surface are measured. This also cannot guarantee a representative measurement of the entire profile surface.

When measuring the tread pattern along a single line transverse to the rolling direction of the motor vehicle tire, the tread grooves are therefore only recorded along a line to the rolling direction of the tire. If the vehicle tire is heavily soiled at this point or contains small stones in the tread grooves, the measurement will be falsified. It is neither possible to recognize such distorted measured values nor to average them out.

Disclosure of the invention

The invention has for its object to increase the reliability of the profile measurement of a method or a device of the type mentioned.

In relation to the method, this object is achieved according to the invention by a method for

Measurement of the tread depth of a motor vehicle tire mounted on a motor vehicle solved in which (a) the motor vehicle tire is applied to at least one rotatable roller,

(b) the profile surface of the motor vehicle tire is struck by a linear laser beam,

(c) the light reflected from the profile surface is detected by an image-resolving sensor,

(d) the signals of the image-resolving sensor are processed to generate output data in accordance with the profile depth, and

(e) the motor vehicle tire is rotated during the measurement so that the measurement can be carried out at several locations on the tire tread.

In relation to the device, this object is achieved according to the invention by a

Device for measuring the tread depth of a motor vehicle tire mounted on a motor vehicle, comprising

(a) at least one rotatable roller to which the motor vehicle tire is applied,

(b) a laser and first optically imaging means, which can be aligned on the tread surface of the motor vehicle tire in order to produce a helien-shaped light spot,

(c) an image-resolving sensor, by means of which the position or the shape of the linear light spot can be observed,

(d) Image processing means by means of which measured values for the tread depth of the tire tread can be generated from position data of the image-resolving sensor.

By rotating the motor vehicle tire, the tread surface can be measured along several different lines transversely to the rolling direction of the tire, the measuring unit being able to be held stationary. It is therefore possible to measure larger areas of the profile surface and to obtain a representative value for the profile depth. Measured values which are falsified by local contamination of the profile surface or stones in the profile grooves can be averaged out because of the larger number of independent measured values or can be recognized as outliers. The motor vehicle tire is advantageously rotated by at least one full revolution during the measurement. The entire tread surface of the motor vehicle tire can be recorded during the measurement. The minimum profile depth or an average profile depth can then be determined from the measured values obtained.

The motor vehicle tire is preferably applied to two rotatable rollers, of which at least one can be drivable. If the automobile tire is attached to the rollers during the measurement, the automobile tire can be rotated by rotating one of the rollers.

The motor vehicle tire can also be rotated by the motor vehicle drive. The rollers can be braked when the tire is to leave the rollers.

The rollers can be parts of a Brernsprüfmeßvorrichtung. A brake tester generally includes two roles by which the motor vehicle tire e.g. during the brake test is rotated at a speed of 5 km / h. Underneath the roles is usually a pit. In this case, the laser and the image-resolving sensor can be arranged in this already existing pit. The profile measurement can take place before, during or after the brake test measurement.

When using rollers, the motor vehicle tire only touches the rollers. The rest of the tire tread surface is freely accessible for tread measurement, so that the tread measurement does not have to be carried out at a stressed point on the tire, even if the

Measured vertically from below.

When the motor vehicle tire rotates, centrifugal forces become effective, by which dirt and loose stones are removed from the tread surface. If the tread measurement only takes place when the motor vehicle tire has some

Made revolutions, this improves the measurement of the profile.

It is advantageous if the laser and the image-resolving sensor are protected from contamination. This can be done, for example, by means of a pivotable protective device which only releases the laser and the sensor during the measurement. But it can also be done in that the laser and the sensor itself are pivotable. The first optically imaging means (for example a cylindrical or rod lens) for producing a limeniform light spot on the tread surface of the motor vehicle tire are arranged between the laser and the tread surface and expand the laser beam in one direction.

The image of the linear laser beam on the profile surface can e.g. recorded and evaluated with a CCD camera. However, this type of recording leads to a large amount of data, since CCD cameras generally have 512 x 512 pixels, all of which must be digitized, saved and subsequently evaluated using software. When measuring the profile of a motor vehicle tire, it is often only the profile depth that is of interest and not the exact profile profile on the profile surface. In this case, it is advantageous if the laser light reflected from the profile surface is anamorphically imaged onto the image-resolving sensor by second optically imaging means (for example a cylindrical or rod lens), the image-resolving sensor then being able to consist of only a single sensor line.

(An "anamorphic image" is understood to mean an optical image in which the image scale is different in two directions perpendicular to one another. The image is contracted in relation to the dimensions of the object depicted in the direction of the largest angle of view. Such images can be Realize, for example, a cylindrical or rod lens.) The two-dimensional image of the laser high line on the profile surface is thus optically essentially reduced to a one-dimensional image, which optically reduces the data. The elimination of a two-dimensional detector element brings not only a reduction in costs but also the advantage that the readout process is simpler and the costs for the readout electronics can thus be reduced. Another advantage is the high availability of line sensors with a large number of individual elements, which is crucial for the resolution of the overall system. Furthermore, the measuring speed can be increased.

An embodiment of the invention is explained below with reference to the accompanying drawings.

Brief description of the drawings

Fig. 1 is a schematic representation and shows an embodiment of the device according to the invention. FIG. 2 is a schematic representation and shows the course of the laser beam on the profile surface when using the light section method with linear illumination.

Fig. 3 illustrates the image of the reflected from the profile surface

Laser light on a single sensor line when using the light section method of FIG. 2.

FIG. 4 is a flow chart and illustrates a possible sequence of measuring the tread depth of the motor vehicle tire.

FIG. 5 is a flow chart and illustrates a possibility of data evaluation when measuring the tread depth of the motor vehicle tire.

Preferred embodiment of the invention

In Fig. 1, 10 denotes a motor vehicle tire whose profile is to be measured. The axis of rotation of the motor vehicle tire 10 is designated by 12. The motor vehicle tire 10 bears on a first and a second roller 14 or 16 of a brake test stand 18 (not shown in detail). The rollers 14 and 16 are each about an axis of rotation

20 or 22 rotatably mounted.

A laser 24 and an image-resolving sensor 26 are arranged in a recess in the brake test bench 18 below the two rollers 14 and 16. The laser 24 and the image-resolving sensor 26 are combined as one unit to form a laser measuring head. The image-resolving sensor 26 is connected to image processing means (not shown) in the form of an electronic signal evaluation unit. Commercially available components are used as the laser, sensor and signal evaluation unit.

The laser 24 is arranged so that the laser beam 28 of the laser 24 forms an angle with the normal of the tread surface 30 of the motor vehicle tire 10.

FIG. 2 shows the course of the laser beam on the profile surface 30 when using linear illumination. Procedures which a

Using lighting is called light sectioning (see, for example, company publication

'' Beam shaping optics '' from Schäfter & Kirchhoff, Celsiusweg 15, 22761 Hamburg). The profile surface 30 is illuminated by a linear laser beam from the laser 24. The line-shaped laser beam extends transversely to the rolling direction of the Motor vehicle tire 10 and can either cover only part of the width or the entire width of the tread surface 30. 2, the linear laser beam detects three longitudinal grooves 32, 34 and 36 of the profile surface 30.

2 and 3, the line-shaped laser beam strikes the tread surface 30 at a certain angle to the radial direction of the tire 10 (i.e. to the normal of the tread surface 30). (However, the line-shaped laser beam can also be directed perpendicularly onto the tread surface 30.) This results in a stepped laser high line on the tire surface 30, each step height corresponding to a specific tread depth. The three longitudinal grooves 32, 34 and 36 of those shown in Figs. 2 and 3

Profile surface 30 have the same profile depth. The stepped laser guideline shown consists of four sections 38, 40, 42 and 44 corresponding to the profile depth zero and three sections 46, 48 and 50 corresponding to the profile depth of the longitudinal grooves 32, 34 and 36.

The graded laser line 38, 40, 42, 44, 46, 48, 50 can be evaluated by various methods. For example, the laser high line can be recorded by a CCD camera, which is positioned at a certain angle to the laser beam. Depending on the depth of the profile grooves 32, 34 and 36, the image of the sections 46, 48 and 50 will appear on the camera at different locations. The

Evaluation of the image gives the professional information of the area covered by the laser 24. This evaluation of the recorded image is carried out via triangulation in a known manner which is not described here in detail.

3 shows a second possibility of imaging the stepped laser high line 38, 40,

42, 44, 46, 48, 50 shown schematically. The stepped laser vertical line 38, 40, 42, 44, 46, 48, 50 is anamorphically imaged by a cylindrical lens 52 (represented by two crossed double arrows) onto a line detector 54 (e.g. diode line). As a result of this mapping, all laser high-line sections assigned to a specific profile depth are displayed on a single diode in the diode row 54, regardless of which profile groove the laser high is reflected on. In FIG. 3, the laser light line sections 38, 40, 42 and 44 are thus depicted on the diode 56 and the laser light line sections 46, 48, 50 on the diode 58 of the diode row 54. (This means that the information at which level of the profile surface a specific profile depth was measured is lost.) The distance between the two diodes 56 and 58 is proportional to that

Profile depth of the grooves 32, 34 and 36. Each diode in the diode row thus corresponds to a specific profile depth. If several diodes of the diode row 54 are now acted upon by the laser light, the intensity ratio of the laser light can affect the individual Diodes to the total intensity on the area proportion of the corresponding profile depth to the entire profile surface.

A possibility of the course of the measurement shown in FIG. 3 will now be described with reference to FIGS. 4 and 5. In this measurement described, the tire 10 is rotated continuously at a constant speed (e.g. 5 km / h).

First, the tire 10 is brought into position by the tire 10 on the two tubes 14 and 16 (FIG. 1) being brought into the position shown in FIG. 1. This is represented by block 60 in FIG. 4. Then the laser measuring head is brought into position with the tire 10

(Block 62). This is e.g. necessary if the laser measuring head is swung away between measurements or if the height is adjusted to the size of the tire 10. The measurement is started (block 64). This can be done by pressing a corresponding push button (not shown). The laser 24 (FIG. 2) is switched on by actuating the pushbutton. Then the measured values of the

Tire tread depth detected (block 66). This is done by the highly sensitive detectors (e.g. photodiodes) in row 54 (e.g. diode array with 128 diodes) converting the intensity of the light struck by the respective diode into a voltage proportional to the respective light intensity. These voltage values are read out serially at a certain clock frequency T and converted into digital values (8 bits) by an A / D converter. These digital values are saved. The measurement is ended after one or more revolutions of the tire 10 (block 68) and the measured values are evaluated. This is represented by block 70 and will be described later with reference to FIG. 5. The results of the evaluation are stored (block 72) and the measurement results are output (block 74). The measurement results can be output automatically or by actuating a pushbutton (not shown) which activates a printer (not shown).

The evaluation of the measured values represented by block 70 in FIG. 4 is described in more detail in FIG. 5. First, the line data stored with each clock signal is read

(Block 76). Each element of the data array corresponds to a diode of the diode array, i.e. a position value of the tire tread. The diodes located near a first end of the diode array 54 (eg diode 56 in FIG. 3) correspond to position values which are assigned to the surface of the tire tread, and the diodes located near a second end of the diode array 54 (eg diode 58 in

3) correspond to position values which are assigned to the greatest tread depth of the tire tread. The line data is added up (block 78). Starting from the end of the line assigned to the surface of the tire, the first is now pronounced Line maxima determined (block 80). This maximum corresponds to the diode 56 in the schematic representation of FIG. 3. This maximum is assigned to the surface of the tire, ie the tread depth zero. The measured intensity of the diodes of the diode array lying in front of this maximum (ie below the diode 56 in FIG. 3) comes from soiling on the profile surface and does not produce a pronounced maximum. The last pronounced maxima of the line is then determined (block 82). This maximum corresponds to the diode 58 in the schematic representation of FIG. 3. This maximum represents a reasonable measure for the tire tread depth to be determined. The distance between the two maximums is then proportional to the tire tread value.

It should be expressly mentioned that this method for determining the tire tread depth represents only one of many possibilities for evaluating the measurement data. The measurement data of the data array contain more information than is used in this method. For example, not only a single value of the tire tread depth can be determined, but several values or the entire tread depth profile of the tire. It is therefore possible to also obtain information about the depth and area proportions of tread grooves, the position values of which lie between the position values of the surface of the tire and the position values of the tread profile value determined using the method described above.

It should also be mentioned that regions of the profile surface which are far away from the center of the profile surface make a relatively small contribution to the intensity of the diodes. This is related to the angular distribution of both the reflected and the radiated laser light (Lambert's law of cosines). This fact affects the measurement in a favorable way. The profile surface of the tire is namely in the

Usually slightly bent, so that measurements far from the center lead to incorrect values. These measured values are therefore "optically attenuated".

Before using the device, it is advisable to allow the measuring time between two readings of the line detector so that none of the diodes enter the

Saturation works because otherwise the measurement result will be falsified.

Claims

claims

1. A method for measuring the tread depth of a motor vehicle tire (10) mounted on a motor vehicle, characterized in that

(a) the motor vehicle tire (10) is placed on at least one rotatable roller (14, 16),

(b) the profile surface (30) of the motor vehicle tire (10) is acted upon by a linear laser beam (28),

(c) the light reflected from the profile surface (30) is detected by an image-resolving sensor (26; 54),

(d) the signals of the image-resolving sensor (26; 54) are processed to generate output data in accordance with the profile depth, and

(e) the motor vehicle tire (10) is rotated during the measurement, so that the measurement can be carried out at several locations on the tire tread.

2. The method according to claim 1, characterized in that the motor vehicle tire (10) is rotated by the motor vehicle drive.

3. The method according to claim 1, characterized in that the motor vehicle tire (10) is rotated by driving the or one or more of the tubes (14, 16).

4. The method according to any one of claims 1 to 3, characterized in that the motor vehicle tire (10) is rotated during the measurement by at least one full revolution.

5. The method according to any one of claims 1 to 4, characterized in that the entire tread surface (30) of the motor vehicle tire (10) is detected during the measurement.

6. The method according to claim 5, characterized in that the laser light reflected from the profile surface (30) is anamorphically imaged on the image-resolving sensor by optically imaging means (52).

7. The method according to claim 6, characterized in that the laser light reflected from the profile surface (30) is imaged on a sensor line (54).

8. Device for measuring the tread depth of a motor vehicle tire mounted on a motor vehicle, characterized by

(a) at least one rotatable tube (14, 16) to which the motor vehicle tire (10) is applied,

(b) a laser (24) and first optically imaging means which can be aligned on the tread surface (30) of the motor vehicle tire (10) to produce a linear light spot (38, 40, 42, 44, 46, 48, 50),

(c) an image-resolving sensor (26; 54) by means of which the position or the shape of the hhien-shaped light spot (38, 40, 42, 44, 46, 48, 50) can be observed,

(d) Image processing means by means of which measured values for the tread depth of the tire tread can be generated from position data of the image-resolving sensor (26; 54).

9. The device according to claim 8, characterized by two rotatable tubes (14, 16).

10. The device according to claim 8 or 9, characterized in that the or at least one of the tubes (14, 16) is drivable.

11. The device according to claim 10, characterized in that the tubes (14, 16) are part of a Brernsprüfmeßvorrichtung.

12. Device according to one of claims 8 to 11, characterized by second optically imaging means (52), which

(a) are arranged between the tread surface (30) of the motor vehicle tire (10) and the image-resolving sensor (54) and

(b) the laser light reflected from the profile surface (30) is anamorphically imaged on the image-resolving sensor (54).

13. The apparatus according to claim 12, characterized in that the second optically setting means (52) contain a cylindrical or rod lens.