The invention relates to a device for contact-free data gathering from a thermal machining system with at least one machining tool and one workpiece.
A workpiece such as a metal plate or the like can be cut into the desired shape with a cutting torch or a laser cutting head by means of which a cutting beam is aimed at the surface and it then cuts through the workpiece.
In general, the cutting torch is mounted in a mobile holder so that the cutting torch can move along the workpiece in the desired pattern. The distance between the torch nozzle and the workpiece must be constant so that an optimal cutting of the workpiece is achieved and maintained. If the distance between the torch nozzle and the workpiece is too small, then uneven spots in the workpiece can cause contact with the tip of the torch, thereby extinguishing the flame and interrupting the cutting of the workpiece. If the distance is too great, this can impair the cutting quality and the cut will be interrupted. In the case of laser cutting, great distance changes cause a shift in the position of the focal point in the metal and it is no longer possible to cut.
It is known that distances to workpieces are measured with capacitive measuring methods and that the holder of the cutting torch is kept at a constant height above the surface of the workpiece via a closed control circuit (West German patent no. DE 26 41 851 A1, East German patent no. DD 225 651 A1).
In the case of capacitive distance measurement, sensor elements are used in the vicinity of the torch nozzle, and these sensor elements are designed in the shape of a ring, horseshoe or plate. Since they are located in the hot or spraying area of the cutting beam, they only have a short service life and must be replaced at regular intervals.
In order to obtain an adequately strong measuring signal with the known models, as a rule, annular sensor elements with large diameters are used, which are disadvantageous whenever the end edge of the workpiece runs close by or whenever workpieces have already been cut out of the workpiece plate in the immediate vicinity, so that the sensor element is no longer positioned completely above the surface of the workpiece. This leads to erroneous measuring signals.
In the case of devices with an oscillator circuit and a frequency-determining capacity with a subsequent PLL circuit or the like, LC combinations are also necessary in the vicinity of the sensor elements. In actual practice, this calls for a positioning of electronic components in the hot area of the cutting process and a large diversity of parts for the different machines and torch models. Moreover, due to the high-voltage ignition systems used, high field strengths and severe electromagnetic interferences occur. The interference contours at the torch are increased. Thus, although the known devices fulfill the required functions in all kinds of different systems, the mechanical, electrical and electronic complexity is very great. Individual mechanical and electrical design elements of the thermal machining system lead to changeable parasitic capacities which enter into the distance measurements; these elements include factors such as the size of the torch carriage, the type, size and shape of the cutting torch, the type of the nozzles and nozzle caps, type of gas, marking tools, torch holders, distance to adjacent torches when the height of the individual torch shifts as well as, in the case of multiple torch aggregates, additional mounted elements in the vicinity of the torch and process fluctuations typical in actual practice with electric arcs as well as with autogenic and laser processes as well as the cable lengths used, whose capacity and plug-and-socket connections in the measuring circuit and transition resistances, for example, in slip rings with infinitely rotatable aggregates.
Moreover, it is also known that thermal influences have an impact on electronic components in the vicinity of the torch as well as on the capacity of cables and plug-and-socket connections and, especially in the case of frequency-based devices, these influences can lead to drifting during operation. The dynamic range of such devices is currently only a distance of a few millimeters from the nozzle to the sheet (workpiece) and calls for a complex and precise adjustment of the sensor elements.
An arrangement to compensate for interfering emissions of electromagnetic high-frequency vibrations with contact-free sensing devices is described in German patent no. DE 30 42 781 A1.
Moreover, a device for contact-free determination of the position and/or the dielectric properties of objects is known for use in an annealing furnace; this device sensitively detects any interference in an electrical alternating field. It consists of an arrangement of three electrical conductors, two of which are operated as field-generated transmission electrodes with a low-frequency, opposite-phase alternating voltage, whereas the third conductor serves as a current-sensitive measuring electrode. The results of frequency-sensitive and phase-positive processing of the measuring signals is a high measuring sensitivity and a great insensitivity to electrical interference effects (European patent no. EP 0 038 551 B1).
In this context, the direction-dependence of the two transmission electrodes is detrimental during cutting processes since such processes also involve cutting in a plane in any desired direction at any time. The distorting effect of cut joints on the measured value is especially great when the cut is made in the direction of the sensor electrodes. Furthermore, installing two transmission electrodes in the vicinity of the torch entails great mechanical difficulties.
Moreover, an inductive device is known which consists of at least one receiving coil and at least one primary excitation coil connected to a HF generator for inducing voltage values which can be predetermined in the undisturbed state and which have opposite polarity in the receiving coil. In order to compensate for errors and deviations in the induction in the receiving coil, there is at least one additional excitation coil which generates a compensation field or compensation voltages in the receiving coil (European patent no. EP 0 300 974 A1).
In addition to the physically differing signal stabilities and interference voltage distances that exist for the various measuring methods, the state of the art makes it possible to achieve the necessary functions in the different systems, but the influences stemming from the occurrence of plasma and thus of differing conductivities and parasitic capacities in the area of the sensor elements or of the measuring electrodes lead to complicated subsequent electronic correction measures of the distance signal. The actual measurement distance between the sensor elements and the workpiece is affected. The autogenic flame, the plasma light arc or the laser beam, etc., influence and distort the measured values of capacitive sensor systems in various ways, depending on the process and on the process state.
The result is that there are large differences between the positions of the tools to the sheet metal which can be achieved with and without plasma. The effect is negative for the cutting process as well as for the automation of the machine.
In most cases, during the individual cutting phases, an incorrect distance measurement which is detrimental to the cutting quality and consequently an incorrect height of the nozzle above the sheet has to be accepted as a compromise. In order to avoid cutting interruptions during autogenic welding as a result of flashbacks, for example, excessive distances often have to be maintained during the cutting operation, with the result that the quality of the cut deteriorates and the effectiveness of the process decreases or else the cutting speed is too low. Conversely, if the distances are too great, an inadequate power density can interrupt the cutting process.
Furthermore, a state of the art is known in which the capacitive annular distance sensor is arranged concentrically around the torch nozzle, and its distance to the workpiece is greater than that of the torch nozzle. In addition to detecting the distance, this sensor ring also serves to detect a collision of the sensor ring and the workpiece if the sensor comes into lateral contact with the workpiece in the case of a horizontal movement path. An interactive electronic system evaluates the contact and activates the subsequent control steps in order to move the torch with the torch nozzle out of the danger zone.
The lower torch nozzle is not protected by the state of the art, especially in the case of a vertical movement path.