Position measurement for a welding gun

Abstract

A device and a method for position measurement for welding guns, wherein based on the inductance of the coil, which changes depending on the insertion depth of the welding stud into a solenoid coil, it is inferred how far the welding stud is inserted into the solenoid in order to allow conclusions to be drawn concerning a proper arrangement of the welding stud relative to the workpiece.

Claims

1. A welding method that is designed to lift a welding element, in particular a welding stud (B), by an amount from a workpiece (K) at the start of a welding operation by means of an electromagnetic coil (S) in order to generate an arc between the welding element (B) and the workpiece (K), wherein the method may be carried out in multiple consecutive process runs (V.sub.1, V.sub.2 . . . ), and in a process run (V.sub.1, V.sub.2 . . . V.sub.i, . . . ) comprises at least the following method steps: (b) determining a variable (F, D, D.sub.F) that represents the inductance (L) of the coil (S), (c) automatically controlling the welding operation based on a comparison of the variable (F, D, D.sub.F) that represents the inductance (L) of the coil (S) to a predefined setpoint value (W).

2. The method according to claim 1, characterized in that prior to method step (b) the following method step is carried out: (a) applying a voltage (U(t)) to the coil (S) at a starting point in time (to) and measuring the current (I(t)) flowing through the coil as a function of time (t), wherein the variable (F, D, D.sub.F) that represents the inductance (L) of the coil (S) is formed in method step (b) using at least the voltage (U(t)), the current (I(t)), and an electric coil resistance (R).

3. The method according to claim 2, characterized in that as the variable (F, D, D.sub.F) that represents the inductance (L) of the coil (S) in method step (b) of claim 1, at least one time-dependent value (F(t)) is determined at at least one first point in time (t1) as a first value (F(t1)), wherein a) the first value (F(t1)) is compared to a predefined setpoint value (W) that is associated with this first value, or b) in addition to the first value (F(t1)), for each of the various further points in time (t2, t3, t4 . . . ) at least one further associated value (F(t2), F(t3), F(t4) . . . ) is determined, and either a difference quotient $D=\left(\frac{F\left(t_2\right)-F\left(t_1\right)}{t_2-t_1}\right)$ is determined, based on two of these values (F(t2), F (t3), F(t4) . . . ), or the slope (D.sub.F) of a regression line for the curve of F(t) is determined, based on more than two values (F(t1), F(t2), F(t3) . . . ) and the associated points in time (t1, t2, t3 . . . ), and the difference quotient (D) or the slope (D.sub.F) or a comparative value derived therefrom is compared to a setpoint value (W) that is predefined in this regard, wherein based on the comparison, measures for further carrying out the method are derived.

4. The method according to claim 3, characterized in that the time-dependent value ((F(t)) is determined based on the formula $F\left(t\right):=\mathrm{Ln}\left(\frac{U\left(t\right)}{U\left(t\right)-I\left(t\right).Math.R}\right).Math.\frac{1}{R}$

5. The method according to claim 2, characterized in that for a process run (V.sub.i), the electric coil resistance (R) is determined according to the formula $R=U\left(t\right)/I\left(t\right),$ wherein the values from a preceding, preferably immediately preceding, process run (V.sub.i-1) are used for the voltage (U(t)) and the current ((I(t)).

6. The method according to claim 5, characterized in that the values of the voltage (U(t)) or of the current (I(t)) that are predefined or measured after a predefinable waiting period (T.sub.R) elapses, beginning at the starting point in time (t.sub.0), are used for calculating the electric coil resistance (R).

7. The method according to claim 1, comprising the following automatic method steps in a process run (V.sub.i): (a) applying a voltage (U(t)) to the coil (S) beginning at a starting point in time (t.sub.0) and measuring the current (I(t)) flowing through the coil as a function of time (t); (b) determining a time-dependent value ((F(t)) that represents the inductance (L) of the coil (S), based on the formula $F\left(t\right):=\mathrm{Ln}\left(\frac{U}{U-I\left(t\right).Math.R}\right).Math.\frac{1}{R}$ at at least two various points in time (t1, t2, t3 . . . ) and forming the difference quotient (D) $D=\left(\frac{F\left(t_2\right)-F\left(t_1\right)}{t_2-t_1}\right)$ or forming the slope (D.sub.F) of a regression line that is formed using multiple values (F(t1), F(t2), F(t3) . . . ), b.sub.1) wherein the points in time (t1, t2, t3 . . . ) used to form the difference quotient (D) or the slope (D.sub.F) each lie within a predefinable measuring period (T.sub.M), beginning at the starting point in time (t.sub.0), and b.sub.2) wherein the value which, in a process run (V.sub.i-1) that precedes, preferably immediately precedes, the process run (V.sub.i), results from the values for the voltage U(t) and the current I(t) that are detected therein after a predefinable waiting period (T.sub.R) elapses, beginning at the starting point in time (t.sub.0), is used as the electrical resistance value (R) according to the formula $R=\frac{U\left(t\right)}{I\left(t\right)}$ (c) comparing the difference quotient (D) or the slope (D.sub.F) or a comparative value derived therefrom to a predefinable setpoint value (W); (d) continuing the welding method with electromagnetic lifting of the welding stud from the workpiece (K) and generating an arc, or ending the welding method without generating an arc, based on the comparison result.

8. The method according to claim 7, wherein the following apply: waiting period (T.sub.R)>10 ms, and/or measuring period (T.sub.M)<5 ms.

9. The method according to claim 1, characterized in that a measure for the insertion depth of a coil core (Q), coupled to the welding element, in the coil (S) is derived from the variable (F) that represents the inductance (L) of the coil (S) in order to store this measure or a value formed with same and/or display it to the operator and/or compare it to a setpoint value (W) for further method control.

10. A device for carrying out a welding method according to one of the preceding method claims, comprising a welding gun (P) with a support tube (H) and a welding element (B) that is held within the support tube (H) by spring tension, an electric coil (S) with a coil core (Q) that is movable therein and coupleable to the welding element (B), a controller (G) that is designed to act on the coil with a voltage (U) at a starting point in time (to) in order to generate a coil current (I(t)) and an electromagnetic force that acts on the welding stud (B) by use of the coil core (Q), characterized in that the controller (G) is designed to measure the coil current (I(t)) and determine therefrom the electrical resistance (R) of the coil (S) and a variable (F) that represents the inductance (L) of the coil (S), in particular a time-dependent value F(t) or a difference quotient (D) or a slope (D.sub.F) defined according to claim 3 in order to continue or terminate the welding method as a function of the value (F(t), D, D.sub.F) thus determined.

11. The device according to claim 10, wherein the controller (G) is designed to determine the electrical resistance (R) after a predefinable waiting period (T.sub.R) elapses, beginning at the starting point in time (t.sub.0) and/or the time-dependent value (F(t)) or a difference quotient (D) or a slope (D.sub.F) therefrom before a predefinable measuring period (T.sub.M) elapses, beginning at the starting point in time (t.sub.0), wherein the following preferably are to apply: 10 ms<waiting period (t.sub.R)<30 ms, and/or starting point in time (t.sub.0)<measuring period (t.sub.L)<5 ms.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Several aspects of the method according to the invention are explained in greater detail below based on examples in the figures, which show the following:

[0062] FIG. 1 shows a simplified illustration of the problem underlying the invention,

[0063] FIG. 2 shows several components for carrying out the method according to the invention, and

[0064] FIG. 3 shows a diagram for illustrating the time-dependent value F(t).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] FIG. 1 shows a simplified schematic illustration of the problem underlying the invention. A welding gun, illustrated only in part, includes a support tube H, which at the start of a welding operation is to be placed with its lower open end on a surface of a workpiece K. A welding stud B that is to be welded to the workpiece K is guided centrally within the support tube. The welding stud B is initially held by a stud holder N. A coil core Q coupled to the stud holder N protrudes into an electric coil S, not illustrated in FIG. 1, that is part of the welding gun, and that is preferably immovably mounted relative to the support tube H. A magnetic force is exerted on the coil core Q, and thus on the stud holder N, by applying a coil voltage U(t) to the coil S, which causes the stud holder together with its welding stud B to lift from the workpiece K, opposite the force of a spring, not shown. An arc is generated between the lower tip of the welding stud B and the workpiece K which melts the workpiece K and/or the welding stud B, thus forming a weld pool. The welding stud B may subsequently be lowered in the direction of the workpiece K, i.e., into the weld pool, for example by switching off the coil voltage U(t) by means of the elastic force, so that after cooling, a solid weld joint is formed between the welding stud B, which is then detached from the stud holder N, and the workpiece K.

[0066] A support tube H that is correctly (orthogonally) placed on the workpiece at the start of the welding operation is illustrated in the left part of FIG. 1, the welding stud B with its lower tip lying in the plane that delimits the support tube with respect to the workpiece K or that is formed by the surface of the workpiece K. In this case, the coil core Q that is coupled to the welding stud B protrudes into the coil S by an amount z.sub.1. The subsequent automatic lift of the stud B from the workpiece K may then take place at a predefined height as intended, with formation of the arc or weld pool that is also a function of the height, and welds the stud perpendicularly to the workpiece K.

[0067] In contrast, the case of a support tube H that is obliquely placed on the surface of the workpiece K is illustrated in the right part of FIG. 1. In this case, the welding stud protrudes beyond the front delimiting plane of the support tube H, as a result of which the coil core Q protrudes into the coil by a smaller amount z.sub.2. After the coil voltage is applied, the lift of the welding stud B is undefined or too great, so that the correct formation of the arc is impaired, and the welding stud B, if a suitable weld pool is even formed at all, is obliquely welded to the workpiece K. Such a low-quality weld result should be prevented.

[0068] FIG. 2 shows a simplified illustration of several components of a device for carrying out a welding method according to the invention. The device includes a stud holder N with a welding stud B temporarily inserted therein, wherein a coil core Q coupled to the stud holder N protrudes into an electric coil S, analogously to the example according to FIG. 1. The inductance of the coil S changes as a function of how far the coil core Q is inserted into the coil S. With reference to the example from FIG. 1, it is apparent that, based on the insertion depth of the coil core Q in the coil S, a statement may be made concerning the particular inductance L of the coil S.

[0069] For this purpose, a controller G is provided which is designed to apply a predefinable coil voltage U to the coil S in a process run V.sub.i at a starting point in time t.sub.0, for example by means of a transistor or field effect transistor, not denoted in greater detail. The controller preferably includes a microcontroller for data processing. In addition, the controller G is designed to measure the coil current I(t) as a function of time t. The controller G advantageously also includes an electronic memory M for storing or reading out data. These data may include various predefined or measured physical variables, in particular the coil voltage U(t), the coil current I(t), the electric coil resistance R, predefined or determined reference values, etc.

[0070] Furthermore, according to the invention the controller G is designed to determine a variable that represents the inductance L of the coil S, based on the physical variables that are measured during a process run or predefined. By comparing the value that represents the inductance L of the coil to a predefined reference value or setpoint value range W.sub.a-b, it may be established whether the coil core at the start of the welding process is inserted far enough into the coil, i.e., the welding stud has been positioned perpendicularly with respect to the workpiece surface. On this basis the controller G may automatically control the further course of the welding operation, in particular disconnecting the coil S from the coil voltage U if a value that represents the inductance L of the coil is outside a setpoint value range, so that a welding gun that is placed obliquely on the workpiece may be assumed.

[0071] The controller illustrated in FIG. 2 is designed to form a time-dependent value F(t) that represents the inductance L of the coil according to formula (2) in the above description. This value may preferably be formed at at least two different points in time t.sub.1 and t.sub.2 in order to form therefrom a gradient D of the time-dependent value F(t), which for further method control within the controller may be compared to a setpoint value range W.sub.a-b. The two points in time t.sub.1 and t.sub.2 lie within a predefinable measuring period T.sub.M of less than 5 milliseconds, beginning at the starting point in time t.sub.0.

[0072] FIG. 3 shows an example of the curve of a time-dependent value F(t) that represents the inductance of the coil S as a function of time t for three different cases, beginning at 0.5 milliseconds after the application of the coil voltage U at point in time t.sub.0, up to the elapse at approximately 3 milliseconds. All three curves show an essentially linear profile. The uppermost, dotted-line curve represents the case of a welding gun that is not placed on the workpiece at all. In this case, as is readily inferable from the example in FIG. 1, the coil core Q protrudes even farther from the support tube H than shown in the right portion of FIG. 1. The inductance L of the coil S is correspondingly (too) low or the slope of the associated dotted-line curve is correspondingly large, for example approximately 7.0 kohm.Math.ms.sup.1. In the curve for the value F(t), an obliquely placed welding gun according to the example of the middle curve in FIG. 3 results in a slope D.sub.F of approximately 6.5 kohm.Math.ms.sup.1, for example. The welding process is to be carried out only for a correctly placed welding gun, for which a curve F(t) according to the lowermost, dashed-line curve having a slope D.sub.F of 6.0 kohm.Math.ms.sup.1, for example, results. For this purpose, a setpoint value W of 6.1 kohm.Math.ms.sup.1 obtained from empirical determinations, for example, could have been stored in the controller, so that after the curve of F(t) is determined, the controller G then continues the welding process with stud lift-off and arc generation only when the measured value within a tolerance window, determined by the setpoint value, has this desired slope.

[0073] Of course, depending on the welding process and the boundary conditions to be considered (coil voltage, coil type, desired stud lift-off, etc.), different setpoint values may be specified, and depending on the process, for example stored in the memory M, ready for retrieval.

[0074] Lastly, the controller G according to FIG. 3 is also designed to determine the electric coil resistance R according to the equation R=U/I in order to take this coil resistance into account in calculating the time-dependent value F(t). In a process run V.sub.i for calculating F(t), the value of the coil resistance R that results from the values of the coil voltage U(t) and the coil current I(t) in the directly preceding process run V.sub.i-1 is preferably used. The value of the coil current I(t) that was measured by the controller G after a predefinable waiting period T.sub.R elapses, beginning at the starting point in time t.sub.0, is advantageously used. The waiting period is preferably greater than 10 milliseconds, most preferably greater than 30 milliseconds, since after such a waiting period elapses, the inductance L of the coil no longer has an appreciable influence on the coil current, and a sufficiently reliable value for the coil resistance may thus be determined.

LIST OR REFERENCE NUMERALS

[0075] B welding element, welding stud [0076] D difference quotient [0077] D.sub.F slope of a regression line [0078] F, F(t) variable that represents the inductance L of the coil S [0079] G controller [0080] H support tube [0081] I, I(t) current through the coil S [0082] K workpiece [0083] L inductance of the coil S [0084] M memory [0085] N stud holder [0086] P welding gun [0087] Q coil core [0088] R electrical resistance of the coil S [0089] S coil [0090] t time [0091] t.sub.0 starting point in time [0092] t.sub.1, t.sub.2 predefinable points in time [0093] T.sub.M predefinable measuring period [0094] T.sub.R predefinable waiting period [0095] U, U(t) voltage present at the coil S [0096] V.sub.i ith process run [0097] V.sub.i-1 process run directly preceding the ith process run [0098] W setpoint value [0099] z.sub.1, z.sub.2 insertion depth