Method and Apparatus for Automatic Welding Control

Abstract: A method and an apparatus for automatically controlling the welding condition or the welding torch position in arc welding which makes use of a welding power source of constant-voltage or constant-current type. The apparatus has an oscillator (6) for superposing an AC-signal of a predetermined frequency to the arc and a detector (7-10) for detecting the arc sound pressure level corresponding to the frequency of the superposed signal. This apparatus is capable of accurately detecting the arc length or a horizontal position of the welding torch by a simple method without being substantially affected by the noises produced by the arc. Control of the torch position, as well as welding conditions such as welding current, arc voltage and so on, can be precisely performed by making use of the detection of the arc length.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to an apparatus for and a method of detecting arc length, and an apparatus for and a method of controlling welding condition, suitable for use in automatic welding which makes use of a welding power supply having constant-voltage or constant-current characteristics. DESCRIPTION OF THE RELATED ART In the field of arc welding, methods have been proposed for enabling automatic control of the distance between a welding torch and a base metal member (referred to as "torch height" hereinafter). For instance, Japanese Patent Unexamined Publication No. 51-67242 discloses two types of method: namely, a so-called mechanical method in which the torch height is detected mechanically by means of a probe or a contactor secured to the welding torch, and a so-called electrical method in which the torch height is controlled by lifting and lowering relays which are energized in a controlled manner in accordance with a change in the welding current. The mechanical method, however, is not preferred because it requires that a complicated arrangement including a probe to be attached to the torch or other part near the torch, resulting in a lower efficiency of the welding work. In addition, the mechanical method tends to be adversely affected by welding conditions such as spatter, heat and so forth, impairing the torch height controlling performance. The electrical method is generally free from the problems encountered with the mechanical method, because it makes use of welding phenomena. In general, however, welding phenomena include complicated disturbance factors, and transfer of particles vary according to the welding conditions. In order to realize a reliable automatic torch height control, a complicated electric circuit is required to raise the cost of the apparatus to an impractical level. Methods also have been proposed for controlling welding conditions such as welding current and arc voltage. For instance, Japanese Patent Unexamined Publication No. 61-14083 discloses, with the knowledge that the sound (referred to as "arc sound") generated at the weld portion is closely related to the welding phenomena, a method in which the vibration of the arc column is detected by, for example, a microphone and the shielding condition of the weld portion is checked in accordance with the result of the detection. This method is suitable for detecting extreme change in the arc sound attributable to a disturbance of the shield gas stream, but is not suitable for detecting slight change in the welding current or voltage caused by any reason because of difficulty encountered with the evaluation of the welding condition caused by many noise components of the arc and vibration of the arc itself. Ordinary welding power supply having drooping characteristics, except for a power supply having a complete constant-voltage characteristics, generally experiences variation in the welding current and arc voltage as a result of a change in the arc length. In case of a power supply used in TIG welding, the welding current is maintained constant by virtue of the constant voltage characteristics, but the arc voltage is changed undesirable. The arc voltage is substantially proportional to arc length so that the welding conditions can be controlled if the arc length can be suitable controlled. In addition, the detection of the arc length enables the torch height to be adequately controlled. Prior art document WO-A1-81/01676 discloses a method of detecting the distance between a welding torch and a welding work piece. The distance is deteced from the phase difference between the A.C. signal superposed on the welding current and the arc sound picked up by a microphone. It is the object of the present invention to provide a method and an apparatus for detecting the arc length, controlling the torch heights and controlling the welding condition on the basis of the arc sound. This object is solved in accordance with the features of the independent claims. Dependent claims are directed on preferred embodiments of the present invention. According to the present invention, an overlap signal is superposed on an arc and the detection and/or control of the arc length and/or torch height is effected on the basis of the arc sound pressure level corresponding to the frequency of the superposed signal. Definition of terms will be given in regard to the positional relationship between the torch according to the present invention and the base metal members as the welding object. In some cases, the base metal members are a pair of flat tabular sheets which are held in horizontal posture or a pair of flat tabular sheets abutting together at an angle to each other. Thus, the term base metal members is used to mean welding objects of a variety of shapes and positions with which the welding by the present invention can be carried out. One of the base metal members is always on a vertical line passing through the torch, i.e., on a line which passes the torch and extends in the direction of the gravity. Thus, the term "torch height" is used to mean the distance between the welding object and any desired portion of the torch as measured in the vertical direction or in the direction of gravity. A predetermined length of a welding wire always projects from the end of the torch or a tip which is the wire attaching portion on the end of the torch, so that fixed positional relationships exist between the torch, tip and the welding wire. Thus, the control of the torch height can be treated as the control of the distance between the wire end and the base metal member or the distance between the torch end and the base metal member or the distance between a desired portion of the torch body and the base metal member. The term "horizontal torch position" or the "lateral torch position" is used to mean the distance between the welding line and the point where a vertical line which extends downward from a desired portion of the torch body towards the base metal member intersects the upper surface of the member when a portion of the base metal members exists at a lateral side of the torch or the welding wire. When the positional relationship or the shape of the base metal members portion on the lateral side of the torch or of the welding wire with respect to the welding line is constant along the welding line, the horizontal distance between a desired portion of the torch and the above-mentioned lateral portion of the base metal members or, alternatively, the distance between the above-mentioned crossing point of the vertical line and the lateral portion of the base metal members is used as a reference for determining the lateral or horizontal position. The invention according to Claim 1 provides an arc length detection apparatus comprising: oscillation means superposing a predetermined signal on a welding arc; arc sound detecting means detecting the welding arc sound generated by the arc with the predetermined signal superposed thereon; sound pressure level extraction means extracting, from the output of the arc sound detecting means, the arc sound pressure level signal having a frequency corresponding to the frequency of the predetermined signal; and arc length determination means for determining the value of the length of the welding arc from a previously stored relationship between the arc length and the arc sound pressure level of a frequency corresponding to the frequency of said predetermined signal, in accordance with the arc sound pressure level signal corresponding to the frequency of said predetermined signal output from the sound pressure level extraction means. The invention according to Claim 2 also provides an arc length detection method comprising: superposing a predetermined signal to a welding arc; detecting the welding arc sound generated by the arc, determining arc sound pressure level of a frequency corresponding to the frequency of the predetermined signal from the sound received from the welding arc; and determining the length of the welding arc from a predetermined relationship between the arc sound pressure level and the arc length, in accordance with the determined arc sound pressure level. The invention according to Claims 3 and 4 also provides a torch height automatic welding control apparatus comprising: an arc length detection apparatus according to Claim 1; comparator means conducting comparison between an electrical signal output from the arc length detection apparatus and corresponding to the arc length and a set arc length signal derived from a reference arc length setting device; welding condition control means for controlling the welding condition in such a manner as to negate an offset signal obtained as a result of comparison performed by said comparator means; and driving means driving the welding torch up and down in such a manner as to negate an offset signal from the comparator means, thereby controlling the welding torch at a constant height. Another form of this torch height automatic welding control apparatus according to Claim 5 comprises torch height control means for controlling the torch height in terms of the distance between the torch and a base metal member from a previously stored relationship between the arc sound pressure level and the torch height, in accordance with the arc sound pressure level signal output from the sound pressure level extraction means. The invention according to Claims 6 and 7 also provides a torch height control method comprising: comparing an electrical signal corresponding to the arc length determined by the above-mentioned arc length detection method with a set signal derived from a reference arc length setting device; controlling the welding condition in such a manner as to negate an offset signal obtained as a result of the comparison; and driving the welding torch up and down in such a manner as to negate the output offset signal obtained as a result of the comparison. A constant-voltage type power source or a constant-current type power source is used as the power source in the automatic welder of the present invention. An A.C. signal is preferably used as the predetermined signal to be superposed. Such an A.C. signal can have various waveforms such as triangular waveforms and rectangular waveforms. It is also to be noted that the amplitude or frequency of the signal to be superposed may be varied at an intermediate portion of the application period. The memory means stores the relationship between the arc length or the torch height and the arc sound pressure level corresponding to the arc length or the torch height. The arc sound detecting means preferably has an internal or an external deciding means to decide whether a power circuit is shorted or not and preferably picks up the arc sound only when the arc is normally maintained. For instance, when automatic welding is executed using a power source having a constant voltage characteristic (in general, slightly drooping characteristics), the arc length varies according to welding conditions such as the torch height, wire feed fluctuation and welding condition. It is also known from experiments that the arc sound itself contains many pulsating components. Therefore, the detection of the torch height relying upon the arc sound itself has a small S/N ratio and, hence, has a small reliability. The arc sound, however, is closely related to the welding phenomena, so that it has been considered and discussed as being one of important factors to be detected for the purpose of automatic welding control. The invention is based upon a knowledge which has been confirmed through experiment: namely, the fact that the S/N ratio can be raised by externally adding a superposition signal to the arc and the sound pressure level corresponding to the frequency of the superposition signal corresponds to a change in the arc length. Thus, according to the present invention, the arc length can be determined in accordance with the sound level at the frequency of the superposition signal and the control of the torch height or the welding condition, as well as horizontal torch position, can be adequately conducted on the basis of the thus determined arc length. The present invention thus enables, through a simple method which is free from the influence of noises, a highly precise detection of the arc length and control of the torch position and welding condition. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of an arc length detection apparatus in accordance with the present invention; Fig. 2 is an enlarged view of a portion of the apparatus of Fig. 1 near a welding torch; Fig. 3 is a chart showing time-sound pressure level characteristics of the apparatus shown in Fig. 2; Figs. 4A and 4B are enlarged views of the portion of the apparatus of Fig. 1 near the welding torch; Fig. 5 is a chart showing time-sound pressure level characteristics of the apparatus shown in Fig. 2; Fig. 6 is a chart showing the time-sound pressure level characteristic of the apparatus shown in Fig. 1; Fig. 7 is a chart showing the arc length-sound pressure level characteristic of the apparatus of Fig. 1; Figs. 8 and 9 are block diagrams of torch height control apparatus embodying the present invention; Fig. 10 is a characteristic chart showing sound pressure level-torch height characteristic of the apparatus shown in Fig. 9; DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the accompanying drawings. Referring to Fig. 1 which is a block diagram of an arc length detection apparatus embodying the present invention, a torch 2 carries a welding wire 4 which opposes base metal members 3. A welding power source 1 is electrically connected to the wire 4 and the base metal members 3 so that an arc is generated at the end of the wire 4. A symbol ℓ represents the arc length. An oscillator 6 is connected to the welding power source 1. A microphone 7 is attached to the torch 2. An amplifier 8, a band-pass filter 9, a rectifier 10 and an arc length extraction 11 are connected in the mentioned order to the output terminal of the microphone 7. An arc length/sound pressure level table 12 also is connected to the input side of the arc length extraction 11. The amplifier 8, band-pass filter 9, rectifier 10, arc length extraction 11, arc length/sound level table 12, oscillator 6 and the microphone 7 in combination provide an arc length detection apparatus which is denoted by 13. The welding power source 1 supplies electrical power to the torch 2 and the base metal members 3 so that an arc 5 of a length ℓ is generated between the wire 4 and the base metal members 3. The illustrated manner of connection of the oscillator 6 is not exclusive. Namely, the oscillator 6 may be connected in any suitable manner provided that it can oscillate and superpose an A.C. signal of a predetermined frequency, e.g., 1 KHz, on the arc 5. The electrical signal corresponding to the sound picked up by the microphone 7 is amplified by the amplifier 7 the output of which is delivered to the band-pass filter 9. The band-pass filter 9 is designed to pass only the electrical signal of the frequency set by the oscillator 6. The signal which has passed through the band-pass filter is delivered to the arc length extraction 11 through the rectifier 10. The arc length extraction 11 produces an electrical signal ℓ s corresponding to the arc length ℓ, in accordance with the received electrical signal representing the sound pressure level, by making reference to the arc length/sound pressure level table 12. A description will be given of the principle of operation of the arc length detection apparatus of Fig. 1, with specific reference to Figs. 2 to 7. Fig. 2 illustrates the state of generation of an arc as obtained when no superposition signal is applied to the arc. Usually, the arc has a fluctuating portion as denoted by hatching 14. The arc sound measured by the microphone 7 varies in relation to time in a manner shown in Fig. 3 in which T represents the time while P represents the sound pressure level. It will be seen that the arc sound pressure level fluctuates within a range represented by ΔPA. This fluctuation appears as noises which hampers precise detection of the arc sound pressure level which represents the torch height. Figs. 4A and 4B schematically show the states of arc generated when an A.C. signal such as a sine wave signal of a predetermined frequency is applied to the arc by means of the oscillator 6. More specifically, Fig. 4A shows the state in which the arc length is greater than a set length, while Fig. 4B shows the state in which the arc length is shorter than the set arc length. As will be seen from broken-line curves in these Figures, the arc 5 oscillates in synchronization with the superposition signal added thereto. Pa and Pb represent the arc sound pressure levels, while ℓa and ℓb represent the arc lengths. Fig. 5 shows the manner in which the sound pressure level varies in accordance with the oscillation of the arc. A curve Pa shows the sound pressure level as obtained when the arc length ℓ is ℓa, while a curve Pb shows the sound pressure level as obtained when the length ℓ is ℓb. As will be seen from Fig. 5, there is a tendency that the sound pressure level P becomes higher as the arc length becomes greater. This is attributable to a fact that the spread of the arc becomes greater as a result of elongation of the arc with the result that the level of the sound pressure propagated through the air is increased. Fig. 6 shows the frequency distribution as obtained when the superposition signal is added to the arc. In this Figure, a symbol f s represents the frequency of the superposition signal, while Pfs represents the sound pressure level of the same frequency as the superposition signal. From this Figure, it will be understood that peaks of sound pressure appear at the same frequency as the superposition signal, so that the noise components (fPa) corresponding to the fluctuation ΔP a shown in Fig. 3 can be neglected. Fig. 7 shows the relationship between the arc length ℓ and the sound pressure level Pfs which corresponds to the frequency fs of the superposition signal. A symbol ℓ₀ represents the initial set value of the arc length, while P₀ represents the sound pressure level corresponding to the frequency of the superposition signal. The relationship as shown in Fig. 7 is stored in the form of a table, i.e., the arc length/sound pressure table 12 of Fig. 1, so that the arc length can be determined by making reference to the table in accordance with the detected sound pressure level. Fig. 8 is a block diagram of an embodiment of a torch height control apparatus which makes use of the arc length detection apparatus of Fig. 1. As stated before, the arc length detection apparatus 13 delivers an electrical signal ℓ s corresponding to the detected arc length. This electrical signal ℓ s is delivered to a comparator 14 which also receives an electrical signal ℓ 0s derived from a reference arc length setting device 15. The comparator 14 compares these signals with each other and produces an offset signal "e" corresponding to the difference between the signals ℓ s and ℓ 0s . A motor control circuit 16 controls a motor 17 such as to drive the torch up or down thereby to negate the offset signal "e", whereby the torch height Ex is controlled in such a manner that the actual arc length ℓ coincides with the set arc length ℓ₀. In thiscase, the torch height Ex means, for example, the distance between the end surface of a tip formed on the end extremity of the torch member and the upper surface of the base metal member 3 as measured in the vertical direction, i.e., in the direction of force of gravity, as illustrated in Fig. 8. Although not shown, a control apparatus of the present invention may be constructed such as to control the welding current, arc voltage or other factors besides the torch height, in such a manner as to nullify the arc length offset signal "e". Namely, fluctuation in the arc length may be caused also by factors other than fluctuation in the torch height, e.g., a variation in the wire feed rate, welding current or the arc voltage. According to the present invention, therefore, such factors can be controlled by detecting the arc length in the manner described and operating a suitable controller in place of the motor control circuit 16 of Fig. 8 in accordance with the result of detection of the arc length. Although a constant-voltage type power source is used as the welding power source of the present invention, the described control can equally be executed with a constant-current type welding power source. It is also to be noted that the A.C. signal to be superposed on the arc can have various waveforms such as a sine waveform, a rectangular waveform or a triangular wave-form, provided that it can produce distinctive peaks of the arc sound pressure level as shown in Fig. 6. Although the described embodiment employs a separate oscillator designed to superpose an A.C. signal on the arc, the separate provision of the oscillator is not exclusive. Namely, the external oscillator may be omitted if a power source having an oscillator therein, e.g., a pulse welding power source, is used as the welding power source. The superposition of the A.C. signal need not be executed continuously. In other words, the period of time through which the A.C. signal is added to the arc may be determined as desired. Thus, the A.C. signal to be superposed to the arc may be a so-called intermittent signal. According to another embodiment of the present invention which is not shown, a short-circuit arc detection circuit is added to the circuit shown in Fig. 8, in order to eliminate influences of noises generated at the time of short-circuiting of the arc and at the time of recovery of the arc. With such an arrangement, it is possible to detect the arc sound only when the arc is normally maintained. When a welding robot is used for the purpose of driving the welding torch, the motor control circuit 16 of the arrangement shown in Fig. 8 may be substituted by a robot control circuit. Referring now to Fig. 9 showing a different embodiment, a reference arc sound pressure level setting device 18 supplies a comparator 14 with a reference arc sound pressure level signal S₀ which corresponds to the set value of the distance between the torch and the base metal member. The comparator 14 also receives an arc sound pressure level signal S a and compares the signals S₀ and S a with each other. Then, a control which is substantially the same as that explained in connection with Fig. 8 is executed in accordance with the result of the comparison. Fig. 10 is a characteristic chart showing the relationship between the sound pressure level (reference value) and the distance of the torch from the base metal member. In this embodiment, therefore, the welding condition is automatically conducted directly in accordance with the sound pressure level, without requiring the detection of the arc length. The preferred embodiments described hereinbefore have a common feature that an A.C. signal of a predetermined frequency is superposed to the welding arc and only the component of the arc sound pressure level corresponding to this frequency is picked up. In a specific form of the invention, means are provided for enabling discrimination between a state in which short-circuiting of arc is taking place and a state in which the arc is normally maintained, so that the extraction of the arc sound pressure level is executed only when the arc is normally maintained. The A.C. signal superposed on the arc has an oscillation amplitude which is greater than the oscillation of ordinary arc column, and only the arc sound pressure level component of the frequency corresponding to the A.C. signal is extracted, so that the arc sound pressure level corresponding to the arc length or torch position can be precisely detected without being influenced by noise components, thus ensuring a high degree of precision in the automatic welding control.

Claims: An arc length detection apparatus comprising: oscillation means (6) superposing a predetermined signal on a welding arc (5); arc sound detecting means (7) detecting the welding arc sound generated by said arc (5) with said predetermined signal superposed thereon; sound pressure level extraction means (8-10) extracting, from the output of said arc sound detecting means (7), the arc sound pressure level signal having a frequency corresponding to the frequency of said predetermined signal; and characterized in further comprising: arc length determination (11) means determining the value (ℓ) of the length of said welding arc from a previously stored relationship between the arc length and the arc sound pressure level of a frequency corresponding to the frequency of said predetermined signal, in accordance with the arc sound pressure level signal corresponding to the frequency of said predetermined signal output from said sound pressure level extraction means. An arc length detection method comprising: superposing a predetermined signal to a welding arc; detecting the welding arc sound generated by the arc, determining arc sound pressure level of a frequency corresponding to the frequency of said predetermined signal from the sound received from said welding arc; and characterized in further comprising the steps of: determining the value (ℓ) of the length of said welding arc from a previously stored relationship between the arc length and the arc sound pressure level of a frequency corresponding to the frequency of said predetermined signal, in accordance with the arc sound pressure level signal corresponding to the frequency of said predetermined signal. An automatic welding control apparatus comprising the arc length detection apparatus of claim 1, further comprising: comparator means (14) conducting comparison between an electrical signal (ℓ s ) output from said arc length detection apparatus (13) and corresponding to the arc length and a set arc length signal (ℓ 0s ) derived from a reference arc length setting device (15); and welding condition control means controlling the welding condition in such a manner as to negate an offset signal obtained as a result of comparison performed by said comparator means. The control apparatus according to claim 3, wherein the welding condition control means comprise: driving means (16, 17) driving the welding torch up and down in such a manner as to negate an offset signal from said comparator means, thereby controlling said welding torch (2) at a constant height (Fig. 8). The automatic welding control apparatus according to claim 4, wherein the driving means comprise: torch height control means controlling the torch height in terms of the distance between the torch and a base metal member from a previously stored relationship between the arc sound pressure level and the torch height, in accordance with the arc sound pressure level signal ouput from said sound pressure level extraction means. An automatic welding control method comprising the arc length detection method of claim 2, further comprising the steps of: comparing an electrical signal corresponding to the determined arc length with a set signal derived from a reference arc length setting device; and controlling the welding condition in such a manner as to negate an offset signal obtained as a result of the comparison. The control method according to claim 6, wherein the control of the welding condition comprises: driving the welding torch up and down in such a manner as to negate the output offset signal obtained as a result of the comparison.

Inventors: ARAYA TAKESHI, HIOKI SUSUMU, NAKAJIMA JUN, ARAYA, TAKESHI, HIOKI, SUSUMU, NAKAJIMA, JUN
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: B23K9
IPC codes: B23K9