Seam tracking sensor for welding robots

The invention relates to a seam tracking sensor for welding robots for tracking a machining path on workpieces with a sensor tip that is in contact with the workpiece and with measuring means for recording the deviation from the machining path.

Due to the increasing degree of automation in production companies, robot-assisted welding systems are becoming more and more important. For this, however, it is necessary for the robot head to be guided along the weld seam in welding technology. The advantages of such systems are their high economic efficiency and very good reproducibility. However, this reproducibility can only be achieved if the dimensions of the components or the clamping devices are adhered to and thus the difference between the component geometry and the programmed path is almost zero. Modern welding processes, such as laser hybrid welding or laser soldering, are particularly dependent on high dimensional accuracy, since the welding parameters, such as the distance to the component and lateral offset, must be precisely adhered to.

In order to still meet the high quality requirements of the operators of such systems, there are various sensor systems that record deviations from the weld seam online.

A very cheap and efficient method of seam tracking is to use the welding arc as a sensor. Changes in the welding current are measured and used for position control. The currents measured are indirectly proportional to the arc length, which means that the distance to the weld seam contour can be calculated. The lateral position of the weld seam is determined by a pendulum movement of the robot. This movement of the welding head produces an oscillating welding current signal with which the lateral position of the weld seam can be inferred. The distance to the workpiece is expressed as an offset in the signal curve. The biggest problem with this sensor system is the correct evaluation of the signals. As the sheet thickness decreases, it becomes more and more difficult to determine the lateral position, as the difference between the measurement signal and the noise becomes smaller and smaller.

Optical systems are also known for seam tracking, in which case a camera is arranged to record the weld seam and the recorded image is evaluated. The interfering influences of the welding process have a problem with this optical measuring system. In particular, rising smoke, weld spatter and of course the bright light of the arc make it difficult to correctly identify the weld seam. Another problem with optical systems is the large amount of data. Up to the present time, modern computers have had difficulties evaluating a 14 Mbyte / s signal, such as is provided, for example, in the systems with cameras known from the prior art, in real time, which results in high hardware costs.

Furthermore, tactile sensors are used for seam tracking. These work mainly on the principle of electrical scanning, in that a scanning pin is drawn over the weld joint and a signal is emitted that corresponds to the deflection. Due to their contact measurement principle, they are very immune to interference and therefore work very reliably even under difficult environmental conditions. In principle, tactile sensors are suitable for every welding process. As with all other seam tracking sensors, electric sensing fingers cannot track all seam geometries. Especially butt joints with a gap width of less than 1 mm can hardly be detected mechanically.

The tactile sensors for seam tracking known from the prior art work with differential converters. A special mechanism is used to transmit the movement of the feeler finger to an iron core, which moves within coils, the output signal of which is changed according to the position of the iron core. In the case of such tactile sensors, the mechanics of the sensor are relatively complex, which has a negative effect on the structural size, resulting in a large distance from the welding point.

A method and a device for tracking tools by means of edge tracking on a workpiece to be machined is known from DE 196 15 069 A1. The tool is moved by a handling device or a robot along a machining path on the workpiece. A measuring probe for recording the actual position of the edge is guided along the edge of the workpiece at a certain distance from the machining point of the tool, leading the machining parts. A control device that is connected to the probe and the measurement data of the probe with the specified data of the machining2

AT 41 2 456 B track compares and evaluates, generates control signals which are fed to a drive device. The drive device tracks the tool perpendicular (Z-axis) to the surface of the workpiece and / or transversely (Y-axis) to the machining direction of the tool from the recorded actual position of the edge of the workpiece.

Another method and a device for monitoring the welding process is known from US Pat. No. 5,877,960 A, in which optical sensors detect the weld seam before and after the welding head and a corresponding correction is made if the weld seam deviates from the desired path.

JP 10-180446 A shows a welding device with a detector that detects deviations of the welding torch from the desired weld seam. There is no information about the construction of the detector.

One way of detecting the deformation of a rod is known from US Pat. No. 6,531,861 B1, with several insulation measuring strips being arranged on the circumference of a rod clamped on one side, which can detect a deformation of the rod due to a force acting on its free end.

The object of the present invention is to provide a seam tracking sensor of the specified type, with a very simple and compact structure.

The objects of the invention are achieved in that a per se known strain gauge transducer for at least one measuring element is connected to the sensor tip to form a bending beam, so that an elastic deformation of the bending beam can be recorded with the at least one measuring element, the bending beam being resiliently supported and / or or is designed to be resilient in itself. The contact of the sensor tip with the workpiece creates mechanical stresses in the sensor tip and subsequently in the strain gauge transducer connected to the sensor tip, which are detected by the at least one measuring element. This means that deflections of the bending beam formed by the sensor tip and the strain gauge transducer can be determined directly. Another advantage is that this creates a very small size of the sensor. The resilient mounting of the bending beam and / or the resilient design of the bending beam increases the measuring path. The at least one measuring element is advantageously arranged in a housing.

However, it is also advantageous to have a design in which the measuring element is formed by a strain gauge, since this enables a very simple evaluation of the bending movement and a cost-effective structure is created.

An embodiment in which the strain gauge transducer is connected to the sensor tip via a predetermined breaking point is advantageous. Due to the electrically insulating design of the predetermined breaking point, the base body or the housing of the seam tracking sensor can be galvanically separated from the sensor tip, which is connected to the current-carrying workpiece. This means that no interference currents can affect the measuring elements or the evaluation elements.

As an alternative to this, the strain gauge transducer can be connected to the sensor tip via a spiral spring, which enables the sensor tip to deflect via the spiral spring.

A spring element, in particular a compression spring, can be arranged in the housing, which presses the transducer against a support ring so that the strain gauge transducer or the bending beam is greater than the spring force of the spring element, in particular the compression spring, when a normal force is applied to the sensor tip. stands out from the support ring. As a result, a significantly larger bending path can be achieved and damage to the seam tracking sensor can be avoided if the force exerted on the sensor tip is too great.

According to a further feature of the invention, the measuring elements are connected to evaluation electronics via lines. If the evaluation electronics are arranged within the housing of the seam tracking sensor, a compact structure is created.

Alternatively, the evaluation electronics can of course also be arranged externally.

If a union nut together with an adjusting disk is arranged on the housing, the preload of the spring element, in particular the compression spring, can be adjusted and the sensitivity of the seam tracking sensor can be set in a simple manner.

An embodiment is advantageous in which the adjustment of the preload of the spring element or the compression spring is carried out in such a way that the union nut is connected to the housing via a thread, so that the spring element,

AT 41 2 456 Β in particular the compression spring, can be compressed. This results in a very simple and inexpensive structure.

Advantageously, the sensor tip, the strain gauge transducer and possibly the predetermined breaking point or the spiral spring, the adjusting disk and the union nut have a bore through which a welding wire or soldering wire can pass. As a result, the seam tracking sensor can be used directly for feeding the welding wire and it can thus be positioned very close to the machining process, in particular the soldering process.

Further advantages to the individual claims can be found in the description.

The present invention is explained in more detail with reference to the accompanying drawings, which show exemplary embodiments of the seam tracking sensor.

Show in it:

1 shows a schematic perspective illustration of a seam tracking sensor;

2 shows a sectional illustration of the seam tracking sensor in a simplified, schematic illustration;

3 shows a further exemplary embodiment of a seam tracking sensor in a simplified, sectioned, schematic illustration; and

4 shows another exemplary embodiment of a seam tracking sensor in a simplified, sectioned, schematic illustration.

In Figs. 1 and 2, a seam tracking sensor 1 is shown, which consists essentially of the

Parts sensor tip 2, predetermined breaking point 3, housing 4, strain gauge transducer 5, union nut 6, compression spring 7, adjusting disk 8, support ring 9 and a rubber sleeve 10 consists. Furthermore, measuring elements 11 for detecting a bending movement are attached to the strain gauge transducer 5. It is of course possible for the support ring 9 to be formed directly by the housing 4. It is also possible for the adjusting disk 8 to be realized by the union nut 6.

Due to a physical contact between the sensing finger or sensor tip 2 and the component or workpiece (not shown), mechanical stresses arise in the sensor tip 2 and thus also on the strain gauge transducer 5. Depending on the deflection of the sensor tip 2, the contact pressure changes and, as a result, the mechanical stresses. A simple and inexpensive way to measure stresses or strains in components are strain gauges 12, as shown schematically in Fig. 2 is drawn. These consist of a base film onto which meander-shaped conductor tracks are vapor-deposited. The strain gauges 12 have the property that they change the resistance with minimal compression or expansion. If the strain gauges 12 are glued to a feeler finger, they are able to recognize minimal elastic deformations in the feeler finger and to output them as a change in resistance.

In the exemplary embodiment shown, the strain gauges 12 are used as measuring elements 11 and are used to obtain a Z signal and an X signal. In this case, the strain gauges 12 are fastened or glued onto the strain gauge transducer 5 in a simple form.

In the exemplary embodiments, four measuring elements 11 or strain gauges 12 are preferably used, which are connected via lines 13 to evaluation electronics 14, as indicated schematically in the case of a strain gauge 12. The evaluation electronics 14 can be arranged within the housing 4 of the seam tracking sensor 1 or also externally. It is of course possible to use fewer or more strain gauges 12.

As can be seen from the exemplary embodiment, a very compact structure with few parts is created, whereby the size of the Nahlverfolungsser.sors 1 can be kept very small. It is thereby achieved that the seam tracking sensor 1 can be positioned very close to the processing point, for example the welding process, and thus very good and exact position detection and seam tracking of a welding head (not shown) is made possible.

Furthermore, the strain gauge transducer 5 together with the predetermined breaking point 3 and the sensor tip 2 forms a bending beam 15. This is pressed against the support ring 9 via a spring element, for example via compression springs 7. If a normal force now acts on the sensor tip 2, the strain gauge transducer 5 will deform elastically like a bending beam 15. If the normal force is greater than the spring force of the compression springs 7, the strain gauge transducer 5 lifts off the support ring 9. This measure increases the measuring path significantly.

The seam tracking sensor 1 is used in such a way that the sensor tip 2 runs along it

AT 412 456 B of a workpiece, in particular on the edges of the edges of the workpiece to be welded or processed. For this purpose, the sensor tip 2 is pressed lightly against the workpiece and a specific signal is generated by the measuring elements 11. The sensor tip 2 then follows the seam, and if the sensor tip 2 deviates from the seam, a correction signal from the seam tracking sensor 1 or the evaluation electronics 14 is determined and thus a correction of the machining head or the welding head can be made. The workpiece is preferably processed by a robot (not shown), in which a corresponding processing path is programmed and this is monitored via the seam tracking sensor 1 and, if necessary, corrected. The seam tracking sensor 1 is preferably attached to the welding torch or to the welding head (not shown).

The union nut 6 together with the adjusting disk 8 is used to adjust the pretensioning of the compression springs 7. As a result, the force at which the strain gauge transducer 5 lifts off the support ring 9 can be adjusted. Thus, in the embodiment shown, the deflection force of the bending bar 15 formed by the parts of the sensor tip 2, predetermined breaking point 3 and strain gage sensor can be adjusted via the union nut 6, whereby the sensitivity, i.e. the response behavior, of the seam tracking sensor 1 can be adjusted. The adjustment of the preload of the compression spring 7 takes place in such a way that the union nut 6 is connected to the housing 4 via a thread 16, whereby the compression spring 7 is compressed by a corresponding rotation of the union nut 6 and thus a higher force is exerted on the bending beam 15. Of course, it is possible that the seam tracking sensor 1 can also be formed without such an adjustment option, with only the union nut 6 not being adjustable on the housing 4 for this purpose.

The resilient suspension of the bending beam 15 has the advantage that the sensor behavior can be adapted to the respective conditions. A change in the pretension leads to an increase in the linear range, that is to say that the force is increased until the seam tracking sensor 1 responds. The slope of the signal can also be changed. The higher the spring constant of the compression spring 7, the higher the slope and thus the accuracy of the seam tracking sensor 1.

In order to protect the strain gauges 12 from overstretching, a predetermined breaking point 3 can be arranged between the strain gauge transducer 5 and the sensor tip 2. If the predetermined breaking point 3 is made of plastic, a thermal and electrical connection from the sensor tip 2 to components connected to it is prevented.

In the evaluation electronics 14, a Wheatstone bridge circuit can be used to measure the minimal changes in resistance of the strain gauges 12. Since a bending stress is measured in the seam tracking sensor 1, a half-bridge circuit or full-bridge circuit is preferred. Superimposed normal strains can be compensated for. Since rectified changes in resistance cancel each other out, thermal expansions are also compensated very well. Since the measured bridge signal is then used to correct the position, the signal supplied is preferably amplified to 0 to 10 V, whereby a two-stage amplifier can be used to amplify the voltage difference of the measuring bridge to the desired output voltage. In order to filter out high-frequency interference, a low-pass filter is integrated in the second amplifier stage. In principle, the electronic structure is already known from the prior art, so that it will not be discussed in more detail.

In principle, it can therefore be said that when the force changes at the sensor tip 2, an elastic deformation is transmitted to the strain gauge transducer 5, which is picked up by the strain gauges 12. These form an electrical signal that is used to generate a correction signal in the evaluation electronics 13. In this way, for example, a position correction can be carried out on the robot using compensating axes or the robot path can be corrected directly.

FIG. 3 shows a further exemplary embodiment of a seam tracking sensor 1 in which a spiral spring 17 has been installed instead of the predetermined breaking point 3. This measure also increases the measuring path. The spiral spring 17 takes over the function of the compression spring 7, which is why this can be omitted. However, it is possible to use the compression spring 7 or another spring element in addition to the spiral spring 17, as shown.

AT 41 2 456 B

A further exemplary embodiment is shown in FIG. 4, the structure of the seam tracking sensor 1 basically corresponding to that according to FIGS. 1 and 2. In this embodiment, the sensor tip 2, the predetermined breaking parts 3, the DMS transducer 5 as well as the adjusting disk 8 and the union nut 6 have a bore 18 through which a welding wire 19, as shown schematically in FIG. 5, is guided. This exemplary embodiment is used specifically in a laser soldering process in which the welding wire 19 is brought up to the workpiece and melted using the laser.

In order to be able to carry out a hot wire soldering process, the welding wire 19 must be preheated. This is done in such a way that an electrical energy source (not shown) is connected between the workpiece and the welding wire 19, a potential difference being generated between the workpiece and the welding wire 19. The energy source is connected to the sensor tip 2, so that contact is made with the welding wire 19 via the sensor tip 2. Since the predetermined breaking point 3 is made of plastic, the sensor tip 2 is isolated from the subsequent components of the seam tracking sensor 1.

With such a design, the force acting on the seam tracking sensor 1 is now absorbed via the welding wire or a soldering wire.