EP1356273A2

EP1356273A2 - Ultrasonic sensor system for horizontally polarised transversal waves

Info

Publication number: EP1356273A2
Application number: EP02708201A
Authority: EP
Inventors: Volker Arndt; Michael Lach; Michael Platte; Heinz-Ullrich Mueller
Original assignee: Karl Deutsch Pruef- und Messgeratebau & Co KG GmbH; Robert Bosch GmbH
Current assignee: Karl Deutsch Pruef- und Messgeratebau & Co KG GmbH; Robert Bosch GmbH
Priority date: 2001-02-02
Filing date: 2002-02-04
Publication date: 2003-10-29
Also published as: US7055388B2; WO2002061414A2; DE10104610A1; WO2002061414A3; US20040083813A1; DE10104610B4

Abstract

The invention relates to an ultrasonic sensor system, especially for controlling a resistance welding process. Said system comprises at least one receiver which is used to detect the ultrasonic signals from the area to be examined. At least two piezoelectric sensors (31, 32) are used as a receiver and are arranged in such a way that their polarisation direction vectors indicate various directions, said vectors being projected in a plane perpendicular in relation to the propagation direction of an ultrasonic wave to be detected.

Description

Ultrasonic sensor arrangement for horizontally polarized transverse waves

State of the art

The invention relates to an ultrasonic sensor arrangement for performing a process control in resistance spot welding according to the preamble of the independent claim.

The method described in EP-A-653 061 essentially consists of penetrating the intended welding area with shear or transverse waves during the welding process by arranging an ultrasonic transmitter and an ultrasonic receiver for shear waves on the outer electrode shafts of the two opposite welding electrodes , Starting from the ultrasonic transmitter at one welding electrode, the ultrasonic signal passes through the weld metal - two or more sheets to be welded - as well as the other welding electrode to the ultrasonic receiver, from which it is converted into a measurable electrical signal U, the course of which is determined by U = U ₀ « sinωt can be represented, where ω is the angular frequency of the ultrasonic wave and t the time is. The transmission signal is recorded online and its amplitude U _{Q is used} as a control variable for the amplitude and time profile of the welding current. The transverse wave is chosen because the influence of the liquid formation in the welding lens on the damping of a transmitted wave is very large with this type of wave. The amplitude Uo of the transverse wave, which changes significantly and characteristically in the course of the welding process, allows a reliable conclusion to be drawn about the design and size of the welding lens and can therefore be used as a control variable for a control process.

The basic feasibility of the method and the reliability of the test result depend crucially on the ultrasonic sensors used, their arrangement with regard to the welding electrode and the sound propagation within the welding electrodes. In the implementation according to EP-A-653 061, an ultrasound sensor arrangement is selected in which ultrasound transmitters and ultrasound receivers are attached to the outer electrode shafts or to the electrode holders (not shown here). Shear or transverse waves or torsion wave waves with a frequency of less than 1 MHz are generated. It is stated to be particularly advantageous to generate horizontally polarized transverse waves since these have a low tendency to undesired mode conversions in the case of reflections within the sound-conducting electrode holder.

Transversal or shear waves propagate only in solid bodies, but not in liquids. The particles or atoms vibrate perpendicular to the direction of propagation of the wave. The direction of oscillation of the particles or atoms is also referred to as the direction of polarization or, within an imaginary coordinate system, also as the polarization vector. Transversal waves that propagate in the longitudinal direction within an elongated and laterally delimited solid, for example a plate or a hollow cylinder, are referred to as horizontally polarized if ^" the polarization vector of the sound wave, ie the direction of vibration of the particles or atoms, is parallel to If, for example, a transverse wave is coupled into part of the end face of a hollow cylinder and propagates in the axial direction of the cylinder, then this is horizontally polarized if its polarization vector points in a tangential direction of the cylinder.

The ultrasonic transmitters and receivers are so-called shear wave probes. They contain flat and usually round piezoplates of a few mm to a few cm in diameter which, when excited with an electrical voltage, perform a shear movement or, conversely, react to a received shear wave with a received voltage when received. Since the main emission direction of the sound would not be directed towards the weld metal but towards the center of the electrode when such a shear wave test head was attached directly to the outer electrode shaft, preference is given to using wedges which are attached between the test heads and the welding electrodes and an alignment of the main emission direction effect the test head to the weld metal at an angle that is clearly below 90 °, e.g. approx. 45 °. This is the only way to bundle a sufficient fraction of the sound energy towards the welding spot with this sensor arrangement.

The post-published DE-A-199 37 479 describes an ultrasound sensor arrangement which has been improved in this regard and in which the piezoelectric shear wave plate or the complete one in the case of transmission and / or reception Shear wave test head is mounted in a recess within the electrode shaft, in such a way that the piezoelectric plate is oriented approximately perpendicular to the electrode shaft and thus the main emission direction of the transmitter and the main reception direction of the receiver are parallel to the electrode shaft and are directed exactly towards one another. In this way, such an ultrasound intensity can be generated in the welding point and, in the case of reception, such a large reception signal that there is a sufficient useful-interference ratio with regard to the further evaluation for process control of the welding process. Rectangular piezoelectric shear wave plates are used here. In principle, however, these can also have any other geometric shape (for example round, semicircular or diamond-shaped).

If - in general terms - material areas to be examined are transmitted with a separate ultrasonic shear wave transmitter and an ultrasonic shear wave receiver, the problem always arises that the transmitter and receiver must be exactly aligned with one another with regard to the polarization direction of the generated shear wave. For rough orientation of the operator, the respective polarization directions are always marked on the housing for shear wave test heads. In a transmitter-receiver arrangement, the polarization directions of the transmitter and receiver must match because the two ultrasound shear wave probes behave like two optical polarization filters with regard to the passage of light with regard to the amplitude of the received electrical signal: Is with exactly parallel alignment of both shear waves -Probes the maximum receive voltage U ₀ , the receive voltage is U (α) depending on the angle α, by which the two polarization directions are rotated against each other:

U (α) = Uo «cos (α)» sin (ωt) (ω = angular frequency, t = time) For α = 90 ° the amplitude (U ₀ »cos (α)) of the received voltage U (α) is the theory to zero. Due to the diffraction and refraction phenomena as well as the natural sound field characteristics of a piezoelectric disk, a finite value for ü () is usually measured even at α = 90 °. However, this is so small (1 to 10% of U ₀ ) that reliable evaluation of the received signal is no longer possible.

This applies in particular to the sensor arrangements for process monitoring during resistance spot welding described above: The polarization directions of the ultrascr-all-Scr-ex-wave transmitters and receivers that are attached to the electrode shafts or integrated in the electrodes must be aligned and aligned with one another be mounted so that their polarization directions are parallel to each other. Otherwise the falls

Transmittance amplitude too low. When installing the shear wave sensors on the electrode shafts or the electrode shafts on the electrode holders, if the sensors are integrated in the shafts, an adjustment step is absolutely necessary. In a first rough step, the sensors and / or the electrode shafts with the preassembled sensors are first rotated until the markings of the polarization directions of the transmitter and receiver are apparently parallel to one another. A fine adjustment is then made again by turning the sensors or the electrode shafts by observing the received voltage and bringing it to the maximum value. This procedure is cumbersome, time-consuming and error-prone if it is not done with the necessary care is performed. It can only be carried out by trained specialist personnel, since the ultrasound signal must also be observed and interpreted for control purposes. In the event of wear of the sensors or the electrode shafts, simple replacement by untrained personnel is not possible. Due to the required adjustment, an undesirably long downtime of the welding machine must also be accepted when changing the sensor.

The object of the present invention is to provide a sensor arrangement for shear waves which works without aligning the transmitter and receiver in such a way that the sensors can be easily replaced both during initial assembly and in the event of wear in relation to use for process control in resistance spot welding. This object is solved by the features of the independent claim.

Advantages of the invention

The ultrasound sensor arrangement according to the invention, in particular for process control in resistance welding, comprises at least one receiver which detects the ultrasound signals coming from the area to be examined, at least two piezoelectric sensors being used as receivers, which are arranged such that they are in a plane perpendicular to the Direction of propagation of a directional vector of the polarization projected to be detected has different directions. This ensures that at least one of the piezoelectric sensors detects a signal other than zero, regardless of the polarization direction of the wave to be detected. It is particularly independent of how the receiver is relative to the transmitter is arranged. As a result, complex adjustment processes can be avoided. The service life of resistance welding systems can be reduced considerably.

In an expedient development, it is provided that the output variables of the at least two sensors are correspondingly combined in a signal processing in order to detect a measure of the amplitude of the ultrasonic wave. This link increases the sensitivity of the arrangement. The types of linkage mentioned in the further dependent claims can ensure that the output signal does not fall below a certain minimum amount. This increases the evaluation reliability and thus the quality of the process control in resistance welding.

In an expedient development, it is provided that the piezoelectric plates are designed using stacking technology. In particular, no lateral offset is achieved here, so that the sound field is recorded by both piezo plates at one and the same location. The arrangement is therefore particularly suitable for any spatially inhomogeneous ultrasonic wave field. The signal processing can easily compensate for the phase shift that occurs with respect to the sound propagation time by appropriate corrections.

Further expedient further developments result from further dependent claims and from the description.

The invention consists of using several identical ones instead of a single piezoelectric shear wave receiver, the polarization directions of which lie in a common plane, but have different directions within the plane, so that one another Shear wave propagating perpendicular to this plane, regardless of its polarization direction in this plane, always delivers a reception signal different from zero to at least one of the receivers, and that the reception voltages of the individual shear wave receivers are fed to an electronic circuit device which generates an output signal by suitable combination of the individual reception voltages, which differs from zero for any position of the polarization direction of the shear wave to be received and is proportional to the amplitude of the shear wave to be received.

With regard to the application for process control in resistance spot welding, the invention is based, in particular, on the knowledge that a low-frequency (<1 MHz) shear wave, which is introduced into the cylindrical welding electrode, which is hollow to hold the cooling water, approaches on the way to the receiver the other welding electrode spreads more or less homogeneously over the entire cross section of the welding electrode. This is because at typical propagation speeds of 3000m / s, the wavelength of the shear wave in the cylindrical shaft of the welding electrode is a few millimeters to a few centimeters. Welding electrodes typically have an outer diameter of 15 - 30 mm and a wall thickness of 4 - 8 mm. The cross section of the electrode shaft is therefore of the same or smaller order of magnitude as the wavelength. The cross section of the welding electrode itself therefore already represents such a small aperture for the propagating ultrasound wave that the sound propagation is almost undirected and the sound wave fills the entire cross section of the electrode shaft after a short travel distance. drawing

Exemplary embodiments are shown in the drawing and are described in more detail below.

1 shows the principle of operation of the sensor arrangement using the example of two piezoelectric disks which are arranged at an angle of 90 ° to one another,

FIG. 2 shows the sensor arrangement in connection with a circuit device,

FIG. 3 shows a further sensor arrangement outside the welding electrode,

FIG. 4 shows a sensor arrangement integrated in the welding electrode,

and FIG. 5 shows a further sensor arrangement using stack technology.

FIG. 1 shows the principle of operation of the sensor arrangement according to the invention using the simplest example of two piezoelectric disks 31 and 32, which are arranged at an angle (α1-α2) = 90 ° to one another. The two piezoelectric disks 31 and 32 designed as shear wave vibrators are positioned as receivers in a shear wave field which may be homogeneous within the area surrounded by line 33. The direction of propagation ^'of the shear wave is perpendicular to. Paper plane. Pl and P2 are the polarization vectors (or directions of polarization) of the two piezo disks 31 and 32. P3 is the polarization vector of the shear wave passing through the paper plane, αl is that between the polarization vector P3 of the shear wave and the Polarization vector Pl of the first piezo disk 31 existing

Angle. α2 is that between the polarization direction P3

Shear wave and the polarization vector of the second

Piezo disk 32 existing angles. The receive voltages Ul and U2 of the shear wave sensors 31 and 32 are then:

Ul = Uo «cos (αl)« sin (ωt) = Al »sin (ωt)

U2 = Uo »cos (α2) • sin (ωt) = Uo« cos (αl - 90 °) »sin (ωt) =

Uo »sin (αl)» sin (cot) = A2 «sin (ωt) (ω = angular frequency of the ultrasonic wave, t = time) From the amplitudes AI = Uo» cos (αl) and A2 = Uo »sin (αl) the Received signals U1 and U2 of the two shear wave sensors or piezo disks (31, 32) are therefore always at least one different from zero. According to FIG. 2, the received signals U1 and U2 of the two shear wave sensors or piezo disks 41, 42 can now be fed to a circuit device 44 which can link the individual receive voltages in a variety of ways in such a way that a single output signal Ug (eg Ug = Ag » sin (ωt)), results, whose amplitude Ag is more or less independent of the polarization direction of the shear wave to be detected.

The same considerations apply mutatis mutandis when the piezoelectric shear wave plates used for reception in FIG. 1 are arranged inclined with respect to the paper plane and form an angle γ with the incident wavefront that is different from zero. In this case, the same considerations apply to the vectors of the polarization directions of the shear wave test heads or of the shear wave piezo plates projected into the plane perpendicular to the direction of propagation of the shear wave to be detected.

The following examples of linking the received signals in the sense of the invention are both In terms of circuitry, it can be easily carried out using analog integrated circuits (ICs) as well as digitizing the ultrasound signals and subsequent arithmetic operations:

a) The signals Ul and U2 of the shear wave sensors or piezo plates are first squared and then added.

Ug = ul ² + U2 ² = Uo ² »cos ² (αl)» sin ² (ωt) + Uo ² «sin ² (αl)» sin ² (cot) = Uo ² »sin ² (ωt)

Since cos ² (αl) + sin ² (αl) is always 1, the resulting amplitude of the received signal is completely independent of the angle αl always Uo ² , the square of the received voltage amplitude of a single shear wave receiver with its polarization direction parallel to the polarization direction of the shear wave to be detected.

b) Starting from case a), the result is still etched:

Ug = = Uo «| sin (ötf) |

In this case, the received signal Ug always corresponds exactly to the received signal of an individual shear wave receiver, regardless of the angle α1, with its polarization direction parallel to the polarization direction of the shear wave to be detected.

C) The absolute values (amounts) are formed and added from both receive voltages:

Ug = = üo »| sin (örf) |» (| cos (orl) | + | sin (αl) |)

This result is also with a fixed assignment of the polarization directions of the transmitter and receiver sensors to each other as it is used in

Resistance spot welding systems after mounting the sensors in the welding guns is already quite sufficient: the amplitude of Ug in this case would be Uo »(| cos (αl) j + | sin (c) |) and would depend on the mounting of the sensors or the angle (αl) is always between 1 and v2, but never 0. Thus, regardless of the angle αl, there would always be a sufficient reception voltage without the

Polarization directions of transmitter and receiver would have to be aligned with each other.

d) The absolute values (amounts) can also be formed from U1 and U2, the two amounts can be compared and only the larger of the two values can be switched as output signal Ug. In this case, the amplitude of Ug would always be between 1 and (2) / 2:

Ug = Max (| -7l |, ) = Uo »| sin (όtf) | «Max (| cos (orl) |, | sin (αl) |)

The arrangement of the two or more reception sensors, the reception voltages of which are further processed according to the invention in a suitable manner - for example as described above - can be implemented in a wide variety of ways. If a sensor arrangement according to EP-A-653 061 with shear wave test heads attached to the side electrode shaft is selected for process control in resistance spot welding, then a second identical shear wave test head 52 on the electrode shaft can easily be shown on the receiver side in addition to the first shear wave test head 51 5.1 attach. The installation location is chosen so that the position with respect to the longitudinal axis 55 of the electrode shaft is identical and there is only an angular offset by, for example, 90 ° (FIG. 3) in the plane perpendicular to the longitudinal axis 55 of the welding electrodes. In a sensor arrangement according to DE-A-199 37 479 with sensors integrated in the electrode shafts, two shear wave sensors with a polarization direction offset by 90 ° can be located on the receiving side within a cross section 66 of the electrode shaft 6.1, which lies in a plane 66 perpendicular to the central axis 65 of the electrode shaft can be introduced, for example by positioning two otherwise identical piezoelectric shear wave plates 61 and 62 exactly offset by 90 °. (Fig. 4).

Instead of arranging two or more shear wave reception sensors next to one another in the ultrasonic shear wave field, there is in principle also the possibility of positioning shear wave sensors with an offset of the polarization directions according to the invention at one and the same location in the sound field. 5 shows this using the simple example of only two reception sensors used with a polarization direction offset by 90 °: Here, the fact is used that piezo transducers can also be designed and manufactured using stacking technology. In FIG. 5, accordingly, two identical shear wave piezo plates 71, 72 with polarization directions P71, P 72 offset by 90 ° are stacked one above the other in alignment. The two shear wave piezoplates 71, 72 are e.g. acoustically conductively connected to one another by extensive gluing or soldering. Electrical supply lines 78, 79 are attached to the surfaces of the piezo disks in such a way that the receiving voltages U71, U72 of the two piezo plates 71, 72 can be tapped separately on them. Further details of the converter structure, such as protective layers or damping bodies, are designed in accordance with the prior art. They are omitted in FIG. 5 since they are not the subject of the invention and are no longer required to explain the mode of operation of the invention.

5 are generally more complex to produce in practice than conventional probes with only one layer of piezo elements. In the present case, however, there is the fundamental advantage that there is no lateral offset between the individual piezo plates or sensors, but the shear wave sound field is recorded by both piezo plates except for an offset in the direction of sound propagation at one and the same location. Therefore, this embodiment of the invention can also be used with any spatially inhomogeneous shear wave field. The offset in the direction of sound propagation only affects the reception voltages U71 and U72 as a small phase shift with respect to the sound propagation time, which can be electronically or computationally compensated or neglected in the further processing of the reception voltages according to the invention in a signal processing unit.

The invention is not restricted to the use of a horizontally polarized transverse wave, which is always bound to a lateral transmission medium (rod, plate, electrode shaft). The invention works for any transverse wave, regardless of whether it spreads in a r'-; limited or unlimited medium.

Claims

Expectations

1. Ultrasound sensor arrangement, in particular for transverse or shear waves, with at least one receiver that detects the ultrasound signals coming from the area to be examined, characterized in that at least two piezoelectric sensors (31, 32) are used as receivers, which are arranged in this way are that their direction vectors of polarization (Pl, P2) projected into a plane perpendicular to the direction of propagation of an ultrasonic wave to be detected have different directions.

2. Device according to claim 1, characterized in that the sensors (31, 32) are arranged such that the direction vectors of the polarization (Pl, P2) are preferably offset by 90 °.

3. Device according to one of the preceding claims, characterized in that the output variables of the at least two sensors (31, 32) of a signal processing (44) are supplied, which are dependent on the

Output variables of the sensors (31, 32) an provides what is a measure of the amplitude of the ultrasonic wave to be detected.

4. Device according to one of the preceding claims, characterized in that the sensors (31, 32) are preferably arranged next to one another in one plane.

5. Device according to one of the preceding claims, characterized in that the sensors (31, 32) consist of piezoelectric plates (71, 72) which are stacked on top of one another in alignment.

6. Device according to one of the preceding claims, characterized in that the sensors (31, 32) are integrated in the shaft of an electrode (6.1) for resistance welding.

7. Device according to one of the preceding claims, characterized in that the combination of the output variables of the sensors (31, 32) consists in adding the squared individual signals and / or forming the root from the summand.

8. Device according to one of the preceding claims, characterized in that the combination of the output variables of the sensors (31, 32) consists in adding the amounts of the individual signals.

9. Device according to one of the preceding claims, characterized in that the combination of the output variables of the sensors (31, 32) consists in feeding only the larger of the individual signals for further processing.

0.Device according to one of the preceding claims, characterized by the use for process control or process monitoring in resistance welding.