Abstract:
A marking device for encoding metallic workpieces with two-dimensional matrix codes includes a striking tool for forming the code recesses, driven by an electromagnetic device. The driving movement is performed against the force of a return device. A positioning device displaceable on two axes (x, y) of a plane perpendicular to the striking direction (z) is used for positioning the striking tool in the desired code positions. An electronic control device for controlling movement of the striking tool includes means for presetting a higher current for the electromagnet device during a first acceleration phase of the striking tool and a lower current during a subsequent moving phase until the workpiece is impinged. In this manner, the precision of the code recesses in the workpiece can be exactly set or maintained, so that readability of the coding is substantially improved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a National Phase Application (35 USC 371) of PCT/EP2003/012409 and claims priority of German Application No. 102 57 532.0 filed Dec. 10, 2002. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a marking device for encoding metallic workpieces with two-dimensional matrix codes in which the information is present in the form of recessed embossed dots in a square or rectangular arrangement. The presence or lack of these embossed dots at the respective grid points represents the binary encoded information. 
   2. The Prior Art 
   To read back the information without error, the precision in placing the embossed dots is of high importance. The precise shape, size and depth of the dots are critical quality features. This is directly connected to the type of reading technology for such embossed or punched encodings, respectively, by means of CCD cameras. Illumination from the top or the side must create a contrast between light and dark from the respective recess by means of corresponding reflections, which is much more difficult than with printed black and white surfaces located on one level, for which the code was originally developed. A deviating shape or size of the individual recesses can easily cause (or undesirably not cause) a reflection which can lead to an undesired distortion of information. In the aerospace industry, requirements are even stricter for critical components under high load; these requirements aim at avoiding the reduction of mechanical stability due to the “notch effect”. 
   In order to achieve the required precision, the striking tool, normally embodied as a hard metal needle, must strike the metallic workpiece, on the one hand, very rapidly, but on the other hand, with precisely defined and reproducible energy. Many conditions must be taken into account as counteracting the desired precision. In case of an electric drive, for instance, the temperature of the copper coil of the electromagnet can increase during operation, reducing current flow and thus the power consumption of the electromagnet. During longer standstill periods of the marking device, the striking tool which is formed as a magnet keeper, or connected to or operatively connected with a magnet keeper, sticks so that the impact energy at the first dot is reduced. In principle, a striking movement which is too slow causes an oval distortion of the recess when the impact unit moves on during encoding. On the other hand, an impact speed which is too fast leads to a great variation in impact depth, since even minimum differences, e.g. due to overlaid mechanical oscillations in the striking mechanism, lead to slightly different energy outputs of the impact system during the formation of the recess. Furthermore, the material properties of the workpiece also influence the formation of the recess. Finally, mechanical tolerances also lead to errors, if they cause the movement of the magnet keeper to exceed the magnetically substantially linear range. 
   In known arrangements, the current is only intended to be switched on and off for the electromagnet. Clamping diodes or other overvoltage protection equipment are used for protection against overvoltage, when the electromagnet is switched off, as an inductive load. Bias resistors before the electromagnet for inducing a faster rise or drop of current in the magnet coil by increasing the time constant are also known. In these simple systems, in addition to one-time dimensioning, only the time of disconnecting can be varied after the current is switched on, whereas the entire time course of the working movement results exclusively from dimensioning and the prevailing boundary conditions. With such systems, the required precision cannot be attained. 
   In controlling solenoid valves, on the one hand, it is well-known to switch back to a lower holding current after the high turn-on current, which is first required for a fast movement. This switchover, however, does not take place until after switching of the valve, i.e. after the movement of the valve member, and is intended first to save energy and secondly to reduce heating of the solenoid valve. 
   SUMMARY OF THE INVENTION 
   The invention has as an object the improving of the movement of a striking tool driven by an electromagnet arrangement such that markings in the form of recesses can be formed with substantially higher precision. 
   Accordingly, the present invention provides a marking device for encoding a metallic workpiece with a two-dimensional matrix code which includes a striking tool; an electromagnetic device for driving the striking tool, with a working movement, to form the two-dimensional matrix code, as plural indentations, in the metallic workpiece; a return device for generating a force in opposition to the working movement; and a positioning device, displaceable in two dimensions within a plane perpendicular to the direction of the working movement, for positioning the striking tool in a desired encoding position. The marking device of the present invention further includes an electronic control unit for controlling the working movement of the striking tool, said electronic control unit setting a first current I 1  for the electromagnetic device during a first, acceleration phase of the working movement and setting a second current I 2 , lower than the first current, during a second, moving phase of the working movement, the second, moving phase extending from the first, acceleration phase until impingement of the striking tool on the metallic workpiece. 
   Advantageously, according to the invention, the current flow through the electromagnet can be set differently for the acceleration phase and the subsequent moving phase of the striking tool. On the one hand, this results in a fast acceleration, with the striking tool being moved against the workpiece in a defined manner after switchover to the lower current. This results in high regularity and reproducibility of the recess formed. Due to the substantially uniform movement because of the fact that the current is lower during the moving phase, a larger tolerance for the marking device&#39;s distance to the workpiece is permissible. With the known devices, a distance which becomes larger causes a deeper recess due to the longer acceleration phase. Also, because the current is lower during the moving phase, an uncontrollable, merely ballistic phase of “free flight” of the striking tool until it impinges on the workpiece surface is avoided, which would otherwise occur if the current were switched off before the tool impinges the workpiece; which, in turn, would be associated with larger tolerances of the markings. 
   In a simple embodiment, current switchover from the higher to the lower value in one or more steps, or continuously, takes place by means of a time control. Alternatively, this switchover can also take place in dependence on the position, with a position measuring device for controlling switchover being provided in at least one preset position. In the simplest case, this position measuring device can be a simple position sensor in a specific position or an end position sensor which responds after a certain distance traveled during the striking movement. 
   Advantageously, position measurement can also be employed to measure the length of the entire moving distance of the striking tool, i.e. for measuring the distance to the workpiece. The corresponding measured value can then also be used as a working parameter for defining the current intensities and times or positions, respectively. 
   For switching off the current exactly after the striking tool has impinged on the workpiece, preferably means for switching off the current when the impinging position is reached can be provided. In a particularly simple manner, the current increase of the supply current for the electromagnet arrangement can be detected with a current sensor, with this current increase taking place when the movement of the magnet keeper, i.e. the striking tool, has been stopped and there is no longer any change in inductivity in the coil of the electromagnet. 
   After the striking tool has impinged on the workpiece, the current is switched off so that the striking tool is returned to the rest position by the force of the reset device, such as e.g. a spring. Now, for avoiding rebound or need for absorption of the kinetic energy of the striking tool upon return to the rest position by absorption and/or rebounding, advantageously braking means for creating a brake current before the rest position is reached during the return motion of the striking tool can be provided. These means can be controlled in dependence on the time and/or the position, and the current value is selected such that the striking tool is braked, preferably, to a zero speed when the rest position is reached. In this manner, a very fast working cycle can be ensured. 
   The control equipment advantageously contains a microcomputer with a storage unit in which the working parameters are stored, especially current intensities, times, distance parameters, workpiece properties, temperatures, and the like. The working parameters are suitably contained in the form of tables and can be selected and/or altered in dependence on the respective marking process. Whereas some parameters have to be entered which take into account, e.g., the workpiece properties of the workpiece to be marked, other parameters, such as the temperature, can be detected by sensors, and again others are measured in the manner already indicated, e.g. the position of the striking tool along the entire distance of movement. 
   Advantageously, the control equipment in the form of a separate module is interposed between a main controller for the marking device and the electromagnet and can be retrofitted. 
   The various current values can be controlled in open-loop or closed-loop control, dependent on position or time, over the entire moving distance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are shown in the figures and explained in detail in the subsequent description. 
       FIG. 1  is a schematic view of the marking device for encoding metallic workpieces with two-dimensional matrix codes, 
       FIG. 2  is a schematic view of a first embodiment with a position-dependent control for the driving movement of the striking tool, and 
       FIG. 3  is a schematic diagram of a second embodiment with time-dependent control for the driving movement of the striking tool. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The marking head  10  which is schematically shown in a pictorial schematic in  FIG. 1  is equipped with an electromagnet coil  11  adapted for generating the striking movement of a striking tool  12  which, in this embodiment, is exemplified by a hard metal needle. The striking tool  12  is connected to a magnet keeper  9  which can be moved towards a workpiece  14  against the force of a return spring. Of course, a different well-known return device can also be envisaged, e.g. a return device with pneumatic, hydraulic or electromagnetic action. 
   The marking head  10  is adjustable, by means of a positioning device (not shown), in the x- and y-directions of a plane arranged in parallel with the plane of the workpiece  14 . In this manner, the marking head  10  can reach any position of the workpiece  14 . The marking head  10  is used to emboss coding dots in the form of recesses (indentations) in the metallic workpiece  14 . These coding dots form a two-dimensional matrix code representing binary encoded information. After the desired grid point has been reached, the striking tool  12  is moved against the workpiece  14  to create the desired code indentation. 
   Basic control of the marking head  10  is performed by a main controller  15  which controls the position of the marking head  10 , by means of the positioning device (not shown), and the triggering of the movement of the striking tool  12 . 
   Between the main controller  15  and the electromagnet coil  11 , a control unit  16  is interposed by means of which the exact movement of the striking tool  12  is controlled. A first embodiment of this control unit  16  is shown in  FIG. 2  and a second embodiment in  FIG. 3 . In the embodiment shown in  FIG. 2 , a current control stage  17 , which can be triggered from the main controller  15 , controls the electromagnet coil  11  of the marking head  10  via an amplifier unit  18 . The position signal S of a position detecting device  20  is fed into a position presetting stage  19  for detecting the current position of the striking tool  12 . This position detecting device is e.g. an inductive path-measuring system which is arranged outside the electromagnet coil  11  in  FIG. 1  but which can also be integral with the magnet drive. In the position presetting stage  19 , this position signal S is compared during the striking movement with a stored switchover value S 0 , and if the same is reached, a switchover is made from an initially high current value I 1  to a lower current value I 2 . The initially high current value I 1  is used for fast acceleration of the striking tool  12  during an acceleration phase, wherein the lower current value I 2  is selected such that after this acceleration phase, the striking tool can be guided to the workpiece with as uniform a speed as possible. Naturally, the return to the lower current value I 2  can also take place in several steps. When the striking tool  12  impinges on the workpiece  14 , the supply current for the electromagnet coil  11  rises, since when the movement of the magnet keeper  9  is finished, no change in inductivity in the electromagnet coil  11  any longer takes place. This rise in current is detected by a current sensor  21  and fed into an evaluation stage  22  for the rise in current, which evaluation stage  22  can contain e.g. a differentiation stage. When this rise in current is detected, the current for the electromagnet coil  11  is switched off by means of a reset signal R. 
   After the current has been switched off, the striking tool  12  and the magnet keeper  9 , are moved back into the rest position shown in  FIG. 1  by the force of the return spring  13 . If during the return motion, a position S 1  is detected before the rest position is reached, the current is switched on again by means of the current control stage  17  and then serves as a braking current. During this process, the position S 1  and the current intensity are selected such that the striking tool  12  is braked to a speed which is as close to zero as possible when the rest position is reached. For this purpose, either one of the currents I 1  or I 2  or a different current value can be set. 
   In a storage unit  23 , the working parameters for setting the positions and currents are stored. Such working parameters are e.g. current intensities, times, distance parameters, workpiece properties, temperatures and the like are stored in the form of tables. By means of these tables, the current intensities I 1  and I 2  as well as the positions S 0  and S 1  are then preset, e.g. calculated. These are parameters influencing the movement of the striking tool  12 . For instance, the temperature of the marking head  10  or the electromagnet coil  11 , respectively, can be measured in a manner which is not described in detail. Other working parameters, such as the material properties of the workpiece  14 , can be stored by means of an input device which is not shown. Another important parameter is the working stroke, i.e. the distance of the working movement until the tool impinges the workpiece  14 . By means of a measuring movement of the striking tool  12 , which takes place before the actual marking process, the distance can be measured by the position detecting device  20 . The measurement takes place until the tool impinges on the workpiece  14  which is signaled by the evaluation stage  22 . 
   Based on this measured value, the control parameters to be currently used for the respective workpieces  14  are then respectively altered, individually, in such a way that the striking energy effective for marking again corresponds to the desired value. 
   In another embodiment, this distance measurement can be applied to the position of the workpiece surface to be marked in relation to the assembly height of the marking head  10 . To this purpose, the height of the marking head  10  is set adjustably on a third NC axis. Now the striking tool  12  is completely extended with a current set by the current control stage  17 , sufficient to overcome the restoring force, and then the marking head  10  is driven against the workpiece surface from a known higher position. As soon as the striking tool  12  strikes the surface, it is retracted until the proximity sensor  20  in the marking head  10  emits a signal. Since the distance from the completely extended striking tool  12  to the switchpoint of the sensor is known, the position of the workpiece surface can be precisely determined from the entire traveling distance and used for precisely setting the desired distance of the striking tool  12  from the workpiece  14 . This procedure as well helps to eliminate the negative effects of workpiece tolerances. 
   After a certain standstill period, the magnet keeper  9  sticks more firmly (adheres) in its rest position than during the stroke movements of the marking process. For this reason, the control unit can increase the acceleration current I 1  for the first stroke movement. This increase can be set by reference to stored tables as well. 
   The current control stage  17  can control the current values I 1  and I 2  or other current values simply by open-loop control, or it can be adapted as a stage for closed-loop current control. 
   As a variation of the embodiment explained above, a simple position sensor can also be provided instead of the position measuring device  20 ; this sensor would only emit a switchover signal in case a fixed predetermined position S 0  or S 1 , respectively, is reached. It can be e.g. an end position sensor which emits a signal when the rest position has been distanced by a certain distance S 0  or when the magnet keeper  9  has come closer by a certain distance S 1  during the return motion. 
   The control unit  16  shown in  FIG. 2  is, for example, a microcomputer or microcontroller. The storage unit  23  will then be a non-volatile working memory of the microcontroller. 
   In  FIG. 3 , a modified control unit  16   a  is shown. Same or similarly working modules or elements are labeled with identical reference numbers and not again described in detail. 
   In the second embodiment, a time presetting stage  24  replaces the position presetting stage  19 . The time presetting stage  24  is triggered by a signal of the main controller  15 . After a certain time t 0 , switchover from the higher current value I 1  for the acceleration phase to the lower current value I 2  for the movement phase takes place. Correspondingly, the braking current is switched on during the return motion of the striking tool  12  after a time t 1 . The storage unit  23  contains the stored values t 0  and t 1  which are preset in the working parameter tables according to the first embodiment. 
   For open-loop and/or closed-loop control of the current, combinations of the two embodiments can also be implemented, i.e. the setting or control of the currents, respectively, take place partly depending on time and partly depending on the position.