GB2256730A - Control of tig welding gap - Google Patents

Abstract

The welding gap in a TIG welding process is controlled by monitoring the arc voltage and comparing the measured value VI with a desired value VR and with upper and lower band limits VU and VL on opposite sides of the desired value. The welding gap is corrected at a faster rate if the measured value is outside the band limits and at a slower rate if it is inside the band. The welding head is therefore able to follow a sloping weld line without hunting. The differences between the measured voltage and each preset voltage VU, VR and VL are generated as binary digits OU, OR, and OL in comparators 18, 16 and 20. Controller 14 determines the value of the binary word comprising the three digits in the order OU, OR, OL and applies the appropriate correction rate. <IMAGE>

Description

WELDING CONTROL This invention relates to an apparatus for and a method of arc welding. It is particularly concerned with TIG welding, but can be utilized for other welding techniques.

In TIG welding it is desirable to keep the gap between the torch electrode and the workpiece constant irrespective of the workpiece profile. Arc voltage control (AVC) systems are used for this purpose on the premise that the arc voltage is proportional to the arc gap.

Typically, the welding apparatus has drive means for relative movement between the welding torch and the workpiece in the direction of the weld, transverse to the axis of the torch electrode, and the voltage control signal actuates an adjustment drive which controls the torch position axially of the electrode. The AVC system monitors the voltage between the workpiece and the power source for the welding arc to generate a control signal dependent on the deviation of the voltage monitored from a reference voltage, which is indicative of a changing arc gap. The adjustment drive thereupon changes the distance between torch and workpiece in order to return the arc voltage to the reference voltage level.

Because of noise in the signal circuit, primarily from the DC power source it employs, the system will tend to cause the arc gap to fluctuate. In order to overcome that problem most AVC systems incorporate a dead band about the reference voltage in which the system does not attempt to correct deviation. A correcting movement is initiated only when the measured voltage is either above the reference level plus an upper dead band margin or below the reference level minus a lower dead band margin.

Although these known systems are generally satisfactory in operation, a problem can arise when the torch is moving over a workpiece surface which slopes relative to the direction of movement of the torch. Because of the dead band in the control system, when the surface of the workpiece is converging towards or diverging away from the current traverse direction of the torch, the adjustment will tend to lag relative to the actual electrode-to-workpiece gap. That is to say, the electrode-to-workpiece gap tends to become larger when the torch is being traversed down a slope and smaller when the torch is being traversed up a slope.

The more rapid the traverse, the greater the lag will be. Consequently, the speed of traverse of the torch up or down steeply sloping surfaces must be reduced, relative to surfaces having no slope, in order to prevent deterioration of weld quality due to the electrode getting too close to the surface, or too far away from it. This is undesirable because it reduces productivity while at the same time increasing the overall heat input to the workpiece.

It is an object of the present invention to provide an AVC system which is better able to maintain a desired electrode-to-workpiece arc gap while traversing slopes on workpieces.

According to one aspect of the invention there is provided a method of controlling the movement of a welding electrode relative to a workpiece in an arc welding process, wherein the voltage of the arc is monitored and the distance between the electrode and the workpiece is adjusted so as to counter changes of said arc voltage from a desired value, the control being effected in a plurality of states comprising a first state operable within a predetermined tolerance range for said voltage and a second state outside said range, the electrode position being adjusted at a slower rate in the first control state than the second said state.

According to another aspect of the invention, apparatus for controlling the movement of a welding electrode relative to a workpiece in an arc welding process comprises means for monitoring the arc voltage, means for relative displacement between the electrode and the workpiece along the workpiece for effecting the welding process therealong and means for adjustment of the distance between the electrode and the workpiece for determining the arc voltage, control means for said adjustment means comprising means for sensing arc voltages respectively within and outside a predetermined voltage range, the indication of voltages within said range by said sensing means being arranged to actuate the operation of said adjustment means at a slower rate than is actuated by the indication by the sensing means of voltages outside said range.

Conveniently said sensing means comprises comparator devices for comparing the actual arc voltage with a respective preset voltage values. In a preferred arrangement, two said comparator devices compare the actual arc voltage with voltage values at the top and bottom respectively of said predetermined range, and a third comparator device compares the actual voltage with a voltage value intermediate said top and bottom levels, in particular, the desired value.

A combined control signal can be derived from said comparison of voltage values in the respective devices, preferably the comparisons each producing a binary signal indicating whether the measured value is greater or lesser than the preset value, and said signals are combined as ordered digits of a plural-digit binary number the value of which can then determine the rate of said adjustment.

With such an arrangement, it is possible to produce reliable control signals with relatively simple circuitry which does not require sensitive or low tolerance components.

By way of example, an embodiment of the invention will be described in more detail with reference to the accompanying schematic drawings, in which: Fig. 1 illustrates welding control apparatus according to the invention for a TIG installation, and Fig. 2 is a logic diagram of the control sequence.

In Fig. 1, a welding torch 2 comprising an electrode 4 is held in guide means (not shown) to be displaced by conventional drive means (also not shown) along a horizontal path (arrow X) over a workpiece W. A constant current power source 12 is connected across the torch electrode 4 and the workpiece W to provide electrical power to maintain the welding arc between them but the main components of the TIG system itself are not shown because they can be entirely conventional.

The torch 2 is vertically displaceable (arrow Y) in a mounting 6 by a vertical lead screw 8 driven by a motor 10, towards and away from the workpiece to regulate the welding gap, typically about 2mm. An electronic controller 14 actuates the motor 10 to correct deviations from a desired welding gap between the electrode and the workpiece by moving the torch 2 towards and away from the workpiece. It does this by monitoring the arc voltage, which is typically about 12V, and adjusting the gap to keep this value constant.

The Iworkpiece W illustrated in Fig. 1 slopes relative to the horizontal travel direction X of the welding torch. It is therefore required to adjust the torch vertically as welding proceeds and the controller 14 acts on the motor 10 to maintain the arc gap constant within close limits.

The controller 14 comprises a store for the parameters for the welding operation, including the voltage difference between the electrode and the workpiece that is characteristic of the desired welding gap, and the dead band limits intended to prevent hunting of the motor 10 eg. on account of the noise in the circuit sensing the voltage difference.

The desired electrode voltage VR and the voltages representing upper and lower limits of the selected dead band, Vu and #L respectively, are output as analog reference signals to respective comparators 16,18,20.

Each of the comparators 16,18,20 receives as a second input the instantaneous electrode voltage difference VI at the welding gap and produces outputs OR 0U OL respectively which are fed to the controller 14. Each output is produced as a binary 0 if the instantaneous input V1 is less than the reference input, or a binary 1 if it is greater than the reference input.The comparator outputs are combined and evaluated in the controller as a decimal value with the possible results shown in the following table:

COMPARATOR OUTPUTS 0L0R0U COMBINEDDECIMAL VALUE 0 0 0 0 1 1 6 4 1 1 0 6 1 1 1 7 Other decimal values have no reality and would be ignored by the central processing unit of the controller 14.

The possible decimal values indicate not only whether the welding gap is greater or lower than the desired value but also whether or not it is within the preset dead band. Decimal 0 indicates a gap less than the desired value to the extent that it is below the inner dead band level, and similarly decimal 7 indicates a gap so large as to be outside the outer dead band level. Decimal 4 shows the gap to be within the dead band but below the desired value and decimal 6 shows a voltage value also within the dead band but above the desired value.

The controller 14 is so arranged that when it senses a decimal 0 or 7 it drives the motor 10 at a fast speed to tend to restore the desired voltage by bringing the electrode to the desired distance from the workpiece. In response to decimal 4 or 6 it also actuates a correcting movement, but drives the motor 10 at a lower speed. Because of this stepped response, the correcting movements are not influenced by noise in the voltage sensing circuit, while the relatively slow speed of movement within the dead band is able to control hunting.

The control sequence is illustrated in more detail in Fig. 2. The required parameters, including VR VL Vu are downloaded from a host computer (not shown) into the controller. If welding is taking place the comparators produce their binary outputs from the analogue inputs and the decimal value code output by the comparators is sampled. If the code read is not 0, 4, 6 or 7 the sampling is repeated since an error has been indicated.

The valid codes when sensed trigger the electrode displacements described inwards or outwards at a faster or a slower speed, as appropriate.

The control arrangement described is able to reduce very considerably the lagging effect that will otherwise occur when moving over sloping surfaces at a very wide range of angles relative to the direction of electrode movement. This result is achieved without requiring greater precision in measuring and evaluating the voltage signals, since it is necessary only to discriminate between four incremental signal levels. It will be understood that the arrangement is able to respond also to changes of level of the workpiece welding zone other than slopes.

It may also be noted that the control system described can be applied to existing welding installations which will usually have a gap-adjusting motor already.

Claims (14)

Claims:

1. Apparatus for controlling the movement of a welding electrode relative to a workpiece in an arc welding process, comprising means for monitoring the arc voltage, means for relative displacement between the electrode and the workpiece along the workpiece for effecting the welding process therealong and means for adjustment of the distance between the electrode and the workpiece for determining the arc voltage, control means for said adjustment means comprising means for sensing arc voltages respectively within and outside a predetermined voltage range, the indication of voltages within said range by said sensing means being arranged to actuate the operation of said adjustment means at a slower rate than is actuated by the indication by the sensing means of voltages outside said range.

2. Apparatus according to claim 1 wherein said sensing means comprises comparator devices for comparing the actual arc voltage with respective preset voltage values.

3. Apparatus according to claim 2 comprising two said comparator devices for comparison of the actual arc voltage with voltage values at the top and bottom respectively of said predetermined range, and a third comparator device for comparison with a voltage value intermediate said top and bottom levels.

4. Apparatus according to claim 3 wherein said intermediate level is a voltage value indicating the welding electrode is at a desired distance from the workpiece.

5. Apparatus according to any one of claims 2 to 4 wherein means are provided for combining the outputs of said comparator devices to determine whether an adjustment of the arc gap is made at the slower rate or a faster rate.

6. Apparatus according to claim 5 wherein the comparator devices have binary outputs, each indicating whether the measured value is greater or lesser than the respective preset value of the device, the respective outputs forming ordered digits of a plural-digit binary number the value of which determines the rate of said adjustment.

7. A method of controlling the movement of a welding electrode relative to a workpiece in an arc welding process, wherein the voltage of the arc is monitored and the distance between the electrode and the workpiece is adjusted so as to counter changes of said arc voltage from a desired value, the control being effected in a plurality of states comprising a first state operable within a predetermined tolerance range for said voltage desired value and a second state outside said range, the electrode position being adjusted at a slower rate in the first control state than the second said state.

8. Method according to claim 7 wherein the actual voltage measured is compared with a plurality of different preset values to determine the control state.

9. Method according to claim 8 wherein three preset values are used, representing the limiting values of said range and an intermediate value.

10. Method according to claim 9 wherein said desired value is chosen as the intermediate value.

11. Method according to any one of claims 8 to 10 wherein a combined control signal is produced from said comparison of voltage values.

12. Method according to claim 11 wherein the comparisons produce binary signals indicating whether the measured value is greater or lesser than the preset value, and said signals are combined as ordered digits of a plural-digit binary number the value of which determines the rate of said adjustment.

13. Apparatus for controlling the movement of a welding electrode constructed and arranged for use and operation substantially as described herein with reference to the accompanying drawings.

14. A method of controlling the movement of a welding electrode substantially as described herein.