The present invention relates to an ultrasonic sensor system for controlling a resistance spot welding process according to the general class of the independent claim.
The essence of the method described in European Patent Application EP-A-653 061 is to investigate the intended weld region by ultrasonic transmission during the welding operation using shear and transversal waves by situating an ultrasonic transmitter and an ultrasonic receiver for shear waves on each of the external electrode adapters of the two diametrically opposed welding electrodes. Starting at the ultrasonic transmitter on one welding electrode, the ultrasonic signal passes through the weld material—two or more sheets to be welded—and the other welding electrode until it reaches the ultrasonic receiver. Said ultrasonic receiver converts said ultrasonic signal to a measurable electrical signal U, the temporal course of which can be depicted using the equation U=UO·sin ωt. In this equation, ω is the angular frequency of the ultrasonic wave, and t is the time. The through-transmission signal is detected online, and its amplitude UO is used as the control variable for amplitude and the shape of the welding current curve over time. The transversal wave is selected because the influence of fluid formation in the weld nugget on the dampening of a through-transmitted wave is very strong with this type of wave. The amplitude UO of the transversal wave—which changes markedly and in characteristic fashion over the course of the welding process—permits a reliable determination of the formation and size of the weld nugget and can therefore be used as a manipulated variable for a control process.
The basic feasibility of the method and the reliability of the examination findings are crucially dependent on the ultrasonic sensors used, their location relative to the welding electrodes, and the sound propagation inside the welding electrodes. In the realization according to EP-A-653 061, an arrangement of ultrasonic sensors is selected in which the ultrasonic transmitter and ultrasonic receiver are mounted on the external electrode adapters or on the electrode holders, which are not shown in the drawing. Shear waves, transversal waves, or torsional waves having a frequency of less than 1 MHz are generated. It is stated that it is particularly advantageous to generate horizontally polarized transversal waves, since they have a low tendency to undergo undesired mode changes when reflections occur inside the sound-directing electroder holder.
Transversal or shear waves propagate only in solid bodies, and not in fluids. In these types of waves, the particles or atoms oscillate perpendicular to the propagation direction of the wave. The direction of oscillation of the particles or atoms is also referred to as the polarization direction or, within an imagined coordinate system, as the polarization vector.
Transversal waves that propagate in the longitudinal direction inside a longitudinally-extending, laterally-limited solid body, e.g., a plate or a hollow cylinder, are said to be “horizontally polarized” when the polarization vector of the sound wave, i.e., the direction of oscillation of the particles or atoms, is parallel to one of the lateral limiting surfaces. If, for example, a transversal wave is coupled into part of the end surface of a hollow cylinder, which said transversal wave propagates in the axial direction of the cylinder, it is horizontally polarized if its polarization vector points in a tangential direction of the cylinder.
The ultrasonic transmitters and receivers are “shear wave test heads”. They contain flat and, usually, round piezoelectric plates having a diameter ranging from a few mm to a few cm, and that execute a shearing motion when excited with electric voltage or, conversely, when they receive, they react to a received shear wave with a reception voltage. Since, when a shear wave test head of this type is mounted directly on the external electrode adapter, the main emission direction of the sound would not be directed in the direction of the weld material, but rather at the center of the electrode, wedge-shaped attachments are preferably used, that are installed between the test heads and the welding electrodes and permit the main emission direction of the test head to be oriented toward the weld material at an angle that is markedly less than 90°, e.g., approximately 45°. This is the only way to bundle an adequate portion of the sound energy toward the welding spot with this sensor arrangement.
German Patent Application DE-A-199 37 479, which was published at a later date, describes an ultrasonic sensor system that is improved in this regard. With said ultrasonic sensor system, the piezoelectric shear wave plate or the complete shear wave test head is installed in a recess inside the electrode adapter for transmitting and/or receiving. In fact, said piezoelectric shear wave plate or the complete shear wave test head is installed in such a manner that the piezoelectric plate is oriented nearly perpendicular to the electrode adapter, and the main emission direction of the transmitter and the main reception direction of the receiver are therefore parallel to the electrode adapter and are directed exactly at each other. This allows such a level of ultrasonic intensity to be produced in the welding spot and, during reception, it allows a received signal to be generated that is so great that an adequate wanted-to-unwanted signal ratio exists with regard for the further evaluation for controlling the welding process. Rectangular piezoelectric shear wave plates are used in this case. Basically speaking, however, they can have another geometric form (e.g., round, semicircular, or rhombic) as well.
Very generally speaking, if material areas to be examined are investigated by ultrasonic transmission using a separate ultrasonic shear wave transmitter and a separate ultrasonic shear wave receiver, there is always the difficulty that the transmitter and receiver must be directed at each other exactly with regard for the polarization direction of the shear wave produced. To provide the user with a rough orientation, the particular polarization directions are therefore always marked on the housing when shear wave test heads are used. In a transmitter-receiver arrangement, the polarization directions of the transmitter and receiver must match, because the two ultrasonic shear wave test heads behave, in terms of the amplitude of the electrical received signal, like two optical polarization filters in terms of the passage of light: if the two shear wave test heads are in exact parallel alignment and the maximum reception voltage is UO, the reception voltage is U(α), depending on the angle α at which the two polarization directions are rotated relative to each other:U(α)=UO·cos(α)·sin(ωt)(ω=angular frequency, t=time)
When α=90°, the amplitude UO·cos (α) of the reception voltage U(α) is theoretically zero. Due to diffraction and refraction phenomena, and the natural sound field characteristics of a piezoelectric disk, however, a finite value is still usually measured for U(α) when α=90°. Said value is so small, however, (1 to 10% of UO), that the received signal can no longer be reliably evaluated.
These facts also affect the sensor systems described hereinabove for monitoring a resistance spot welding process, in particular: the polarization directions of the ultrasonic shear wave transmitter and receiver installed on the electrode adapters or integrated in the electrode adapters must be directed toward each other and mounted in such a manner that their polarization directions are parallel to each other. If not, the through-transmission amplitude is too low. When the shear wave sensors are mounted on the electrode adapters, or the electrode adapters are installed in the electrode holders when the sensors are integrated in the adapters, an adjustment step must be carried out. To do this, the sensors and/or the electrode adapters with the preinstalled sensors are turned, in a first rough step, until one can see that the markings of the polarization directions of the transmitter and receiver are parallel with each other. A fine adjustment is then carried out, again by turning the sensors or the electrode adapters. To do this, the reception voltage is observed and brought to a maximum value. This procedure is complicated, time-intensive, and fraught with error if it is not carried out with the proper level of care. It must be carried out by trained technicians, because the ultrasonic signal must be observed and interpreted as well, for control purposes. If the sensors or electrode adapters are worn, they cannot be simply replaced by untrained personnel. When the sensors are replaced, one must also put up with an undesirably long period of downtime of the welding machine because of the adjustment that must be carried out.
The object of the present invention is to provide a sensor system for shear waves that functions without having to set up the transmitter and receiver as described hereinabove, enabling the sensors to be replaced easily during initial installation or when they are worn, when they are used for controlling a resistance spot welding process.
The ultrasonic sensor system, in particular for controlling a resistance spot welding process, has at least one receiver that detects the ultrasonic signals from the area to be examined, whereby at least two piezoelectric sensors are used as a receiver that are arranged in such a way that their polarization direction vectors indicate various directions, said vectors being projected in a plane perpendicular to the propagation direction of an ultrasonic wave to be detected. This insures that at least one of the piezoelectric sensors detects a signal—that is different from zero—independently of the polarization direction of the wave to be detected. In particular, it is independent of how the receiver is positioned relative to the transmitter. As a result, complex adjustment procedures can be eliminated. The downtimes of resistance spot welding systems can therefore be greatly reduced.
In an advantageous further development, it is provided that the output variables of the at least two sensors be coupled in a signal processing unit accordingly in order to detect a measure of the amplitude of the ultrasonic wave. This coupling increases the sensitivity of the system. Using the types of coupling named in the further dependent claims, it can be insured that the output signal does not fall below a certain minimum level. This increases the reliability of the evaluation and, therefore, the quality of the controlling of the resistance spot welding process.
In an advantageous further development it is provided that the piezoelectric plates have a stacked configuration. This results in the absence of lateral misalignment, in particular, so that the sound field is absorbed by both piezoelectric plates at the same point. This makes the arrangement particularly suited for use with any spacially inhomogeneous ultrasonic wave field. The signal processing unit can make appropriate corrections to easily compensate for the phase displacement that occurs in terms of the sound propagation time.
The present invention provides that, rather than using a single piezoelectric shear wave receiver, a plurality of identical shear wave receivers are used, the polarization directions of which are located in a common plane, but that have various directions within the plane, so that a shear wave that is propagating at a right angle to this plane always delivers a received signal that is different from zero to at least one of the receivers, independently of its polarization direction in this plane, and that the reception voltages of the individual shear wave receivers are transmitted to an electronic circuit device that generates an output signal by suitably coupling the individual reception voltages, which said output signal is different from zero and is proportional to the amplitude of the shear wave to be received given any position of the polarization direction.
In terms the application for controlling a resistance spot welding process, the present invention is based, in particular, on the knowledge that a low-frequency (<1 MHz) shear wave that is introduced into the welding electrode—which is cylindrical and hollow inside in order to accommodate the cooling water—propagates more or less homogeneously through the entire cross section of the welding electrode on its way to the receiver on the other welding electrode. This is due to the fact that, with typical propagation speeds of 3000 m/s, the wavelength of the shear wave in the cylindrical shaft of the welding electrode ranges from a few millimeters to a few centimeters. Welding electrodes typically have an outer diameter of 15–30 mm, and their walls are typically 4–8 mm thick. The magnitude of the cross section of the electrode adapter is therefore equal to or smaller than that of the wavelength. The cross section of the welding electrode itself is already such a small aperture opening for the propagating ultrasonic wave that a nearly undirectional propagation of sound takes place, and the sound wave fills the entire cross section of the electrode adapter after just a short path of travel.