WELDING TORCH POSITION DETECTION SYSTEM AND METHOD

Abstract: A system and method for detecting the position of a welding torch means (20) with respect to a workpiece (14) by varying the electrical power to the arc, measuring the effect of such variation on the arc current and or arc voltage and processing the detected effect to produce welding torch means position related information. Also, a welding system and method which comprises utilizing this detection system and method along with means (30, 32, 34) to control the position of the welding torch means (20) with respect to the workpiece (14).

Description: This invention relates to a welding torch means position detecting system. Such a position detecting system is already known to include means for directing consumable electrode metal through contact means in a welding torch means to a workpiece. A welding arc between the consumable electrode and the workpiece is derived from an electrical power source; and so-called "through-the-arc" electrical measurement techniques provide measurements of welding parameters which can be used to influence distance-controlling means for controlling the arc length. Typical of such through-the-arc techniques is the disclosure in US Patent Specification No. 3961154. Consumable electrode arc welding processes usually involve various means for regulating arc voltage to indirectly control arc length. While the use of an automatic feedback control to vary the welding arc voltage also may control the arc length to some degree, there is no known practical method for sensing the arc voltage between the point at which the electrode material becomes molten or the point from which the arc emanates and the base metal or part to be welded. The closest practical measurement of arc voltage is usually made between the contact tip of the workpiece. Controlling this measured voltage provides limited control over the actual arc voltage and an indirect control of the arc length, provided that the distance between the contact tip and the base metal does not vary over too wide a range. Assuming the contact potential (i.e. - the potential across the point of current transfer between the electrode and the contact tip) to be negligible, the voltage being regulated between the contact tip and the base metal is equal to the voltage drop across the electrode stick-out (i.e. - current (I) times electrode stick-out resistance (Re)) plus the voltage drop across the actual arc. Both of these voltage drops are functions of length: increased arc length and/or stick-out yield higher voltage. Stickout is defined as the length of electrode material or the distance from the end of the contact tip to the point on the electrode material from which the arc emanates. The distance from the end of the contact tip to the base metal equals the stick-out length plus the actual arc length. Therefore, any variation in the distance between the end of the contact tip and the base metal causes a variation in the stick-out and the actual arc length, which variation causes corresponding variations in the actual arc voltage. In actual practice, other undesirable effects may occur due to the variation of the distance between the contact tip and base metal. If the torch to base metal distance increases, a long electrode stick-out will result in greater electrode preheating because more power is imparted to the electrode due to greater IRe voltage drop. This, in turn, causes the arc current to decrease. Also, the extent and variation of electrode cast affects the exact positioning of the arc. Therefore, there is a need to maintain a constant electrode stick-out length or a constant contact tip to base metal proximity. There are a number of present methods for controlling the height or proximity of the welding torch with respect to the base metal. Existing systems are based on contacting mechanical followers, contacting electro-mechanical sensors, non-contacting eddy current transducers, non-contacting optical sensors, as well as through-the-arc current sensing techniques. All of the present techniques suffer various disadvantages. For example, published European Patent Application No. 0111110 discloses a relatively complicated method of maintaining the distance between the torch and the workpiece, including perturbing the electrical welding parameters by varying the position of the welding torch in order to create an error signal which may be processed to provide torch alignment information. According to the present invention, there is provided a welding torch means position detecting system comprising means for directing consumable electrode metal through contact means in a welding torch means to a workpiece, means for providing electrical power to establish a welding arc between the consumable electrode metal and workpiece, means for perturbing the electrical welding parameters, and distance-controlling means for controlling the distance between the contact means and the workpiece using measurements of the welding parameters; characterised in that the means for perturbing the electrical welding parameters is effective to vary either the arc voltage or the arc current by a predetermined amount, and the distance-controlling means uses said predetermined amount in conjunction with only one welding parameter measurement namely the arc voltage in the case where the arc current variation is predetermined or the arc current in the case where the arc voltage variation is predetermined. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:- FIG. 1 is a block diagram of a generalized torch proximity/weldment geometry detector and torch proximity controller; FIG. 2 is a block diagram for a specific implementation of an automatic torch proximity control using voltage detection means and the existing characteristics of commercially available pulsed power sources; and FIG. 3 is a sketch of the arc current and arc voltage waveforms produced by the system shown in FIG. 2, wherein the current pulse waveshape has been idealized. The methods of the invention for through-the-arc detection of welding torch position (i.e., proximity to base metal and/or weldment geometry) are particularly useful in consumable electrode arc welding processes. The term "through-the-arc" is defined here as pertaining to the information derived from the welding arc itself. Briefly, these techniques include directly measuring the distance related electrical impedance between the contact tip and the workpiece, detecting and processing distance related components of the contact tip to work piece voltage, or detecting and processing distance related components of arc current. Referring initially to FIG. 1, there is illustrated a block diagram of a generalized torch proximity and weldment geometry detector 10 and position controller system 11 according to the invention. The detector 10 requires apparatus for feeding a consumable electrode material 13 through a welding torch which has a contact tip or element 20 and then to a base metal, weldment, workpiece or part to be welded 14, where a welding arc 22 is established. The contact element 20 provides the means for transferring electrical power from a welding power source 19 to the electrode material and ultimately to the welding arc 22. Leads F and G connect the welding power source 19 to the contact tip 20 and weldment 14, respectively. The apparatus for feeding the consumable electrode material comprises a feed rate controller 15, a feed drive motor 18, "pinch" drive rolls 16, and a spool of consumable electrode material 12. The feed rate controller is used to control or regulate the rate at which the consumable electrode is fed into the arc. All of the elements described up to this point are common to any apparatus for arc welding with consumable electrode material. The additional elements which may be required specifically for detector 10 are means for varying the electrical power supplied to the arc (shown here as part of element 19), an arc current sensor 21, a contact tip to workpiece voltage sensor 25, and impedance computing means 26. It should be noted that the means for varying the electrical power supplied to the arc need not reside within the power source. Externally connected electrical means could be used. For example, an a.c. signal could be transformer coupled in series with or across the power source output. Also, the means for varying the electrical power could be generated in more subtle ways, such as perturbing the feed rate of the consumable material or even perturbing the contact tip to work piece distance. The power variation may also take on many forms. For purpose of illustrating a detection method in the following detailed description, it will be assumed that a small sine wave of current has been superimposed on the average welding current. Depending on the method of implementation, some of the elements or blocks comprising the detector 10 may not be required. If the power varying means produces a well defined current or voltage variation for specific sets of weld parameters, the impedance computation means 26, and either the current sensor 21 or the voltage sensor 25, respectively might not be needed. For example, the system of FIG. 2 illustrates a system which only requires voltage sensing means. In these situations, the voltage or current sensor elements could also contain the elements required to process the sensed signals into distance related information which would then be sent directly to the negative input of summing element 30. If needed by the impedance computing means, the voltage sensor, or the current sensor, an optional synchronizing signal could be developed within the power varying means, or derived directly from the voltage or current signals within the voltage or the current sensor. FIG. 2. illustrates the case where the synchronizing track and hold signals needed for sensed voltage processing are developed with the power source. Referring again to FIG. 1, a detection technique for the case where the method of varying the power supplied to the arc by superimposing a small sine wave of current on the average welding current is described hereinbelow. In this case, current sensor 21 may contain a narrow passband filter tuned to the frequency of the sine wave and a peak-to-peak amplitude detector which produces an output E representative of the peak-to-peak current of the sine wave component. Voltage sensor 25 senses the voltage between the contact tip and the workpiece. The voltage sensor 25 may also use a passband filter and peak-to-peak amplitude detector to produce an output D representative of the peak-to-peak amplitude of the sine wave component of voltage e c . The impedance computing means then produces an output B by calculating the ratio of D to E, which is therefore representative of the impedance between the contact tip and the workpiece. The approach is not limited to the use of peak-to-peak detectors, because other types of detectors such as absolute value, rms, etc., could be used to obtain the needed voltage and current information. Assuming the impedance between the contact tip and electrode material is relatively small, the total impedance between the contact tip and the work piece contains two major components: the stick-out impedance and dynamic impedance of the arc at the frequency of the sine wave. The stick-out impedance is highly dependent on stick-out length, and, particularly at higher measurement frequencies, the arc impedance is relatively independent of arc length. Even so, output B provides a good indication of stick-out length and indirectly provides an indication of torch to work piece distance. For a given set of welding parameters, the torch proximity can be controlled by methods which cause output B to correspond with a desired or commanded value. The torch proximity/weldment geometry detector 10 provides the key element needed in controlling the relative positions of the torch and workpiece. The detector can be directly used with a controller to control the torch to workpiece proximity by moving either the workpiece or the torch in the manner described hereinbelow. Also, by combining the detector with mechanisms which cause oscillating or weaving movement of the torch and/or workpiece across the weld joint and then combining output of the detector with the weave pattern information, information can be obtained relating to the workpiece geometry and/or the relative position and location of the torch and the work piece. In this manner, the edge or edges of a prepared weld joint can be detected for use in automatic edge or seam tracking. Referring again to FIG. 1, a detailed description of the torch proximity control system 11 is illustrated. In addition to all of the elements of the detector 10, this system 11 also contains a command or reference input A, summing means 30, torch proximity motor controller means 32, and a torch proximity motor and mechanism 34 for adjusting the weld torch to base metal distance. When configured as an impedance controller, these elements work together to indirectly control the proximity of the torch to workpiece. Summing means 30 compares the commanded impedance A with the actual distance related impedance B at the output of impedance computing means 26 (output of the detector) and generates a signal C which is a function of the difference between signals A and B. The torch proximity motor controller 32 uses this differential signal C to adjust the distance between the torch and base metal. When the measured impedance is greater than the commanded impedance, the distance is reduced and when the measured impedance is less than the commanded impedance, the distance is increased with the result that the measured impedance is brought into correspondence with the commanded value. Holding the impedance constant indirectly holds the torch to workpiece distance constant at the desired or commanded value. The torch proximity controller can be implemented in various ways such as, "bang bang" or proportional control. Averaging or other processing means can be incorporated within the controller to make the system less sensitive to noise and to allow optimization of the system response to variations in proximity. Referring to FIG. 2, there is illustrated a block diagram for a specific implementation of an automatic torch proximity control using voltage detection means and the existing characteristics of commercially available pulsed power sources. The proximity detector 40 contains an apparatus for feeding a consumable electrode material 13 through a welding torch which has a contact tip or element 20 and on to the base metal, weldment, workpiece, or part to be welded 14, where the arc 22 is established. The contact element 20 provides the means for transferring electrical power from the pulsed welding power source 24 to the electrode material and ultimately to the welding arc. Leads F and G connect the welding power source to the contact tip and weldment, respectively. The apparatus for feeding the consumable electrode material comprises a feed rate controller 15, a feed drive motor 18, "pinch" drive rolls 16, and a spool of consumable electrode material 12. The feed rate controller is used to control or regulate the rate at which the consumable electrode is fed into the arc. All of the elements described up to this point are common to any apparatus for pulsed arc welding with consumable electrode material. The additional elements specifically required for detector 40 are differential amplifier 28, E h track and hold 42, E l track and hold 44, and summing means 46. Detector 40 functions in the following manner. The pulsed power source produces current pulses shown as i a in FIG. 3, resulting in corresponding voltage waveform e c . Differential amplifier 28 is used to sense voltage e c and remove any common mode errors. The output J of amplifier 28 is connected to the inputs of track and hold elements 42 and 44. Synchronizing signals K and L, developed within the power source, are used by the track and hold elements 42,44 to obtain voltages E h and E l as defined in FIG. 3. Summing means 46 develops an output B which is a function of the difference E h minus E l . This difference is related to the impedance between the contact tip and the work piece, and the impedance is related to the torch to work piece distance as previously discussed. In addition to the elements of detector 40, automatic proximity control system 41 also contains a command or reference input A, summing means 30, torch proximity motor controller means 32, and a torch proximity motor and mechanism 34 for adjusting the weld head to base metal distance. When configured as a peak-to-peak voltage controller, these elements work together to indirectly control the proximity of the torch to work piece. Summing means 30 compares the commanded p-p voltage A with the actual distance related p-p voltage B at the output of detector 40 and generates a signal C which is a function of the difference between signals A and B. The torch proximity motor controller 32 uses the differential signal C to adjust the distance between the torch and base metal. When the measured voltage is greater than the commanded voltage, the distance is reduced and when the measured voltage is less than the commanded voltage, the distance is increased with the result that the measured voltage is brought into correspondence with the commanded value. Holding the peak-peak voltage constant indirectly holds the torch to work piece distance constant at the desired value. If synchronizing signals K and L are not readily available within the power source, they can be developed directly from current or voltage measurements. The pulsed power source could include multiple power sources such as those described in U.S. Patent 4,409,465 to Yamamoto et al. or U.S. Patent 4,438,317 to Ueguri et al. or a single power source such as that described in U.S. Patent 4,301,355 to Kimbrough et al. These patents are expressly incorporated herein by reference. At first glance, it might seem that the methods of FIG. 1 and FIG. 2 might seriously interact with other controllers currently used to regulate arc voltage or current. However, these controllers generally regulate either average, pulse, or background voltage, whereas the systems of FIG. 1 and FIG. 2 may be configured to control peak-to-peak voltage or current, thereby minimizing such interaction. In fact, experience has shown that a pulsed power source which regulates arc voltage combined with the system of FIG. 2 work hand-in-hand to control both proximity and the average welding voltage. It is conceivable that a proximity controller could be used in a pulsed power system to regulate the voltage (or current) during either the pulse interval or low current interval in combination with power source regulating means which regulate the voltage (or current) during either the low current interval or the pulse interval, respectively. The combination of the two regulators would then achieve regulation of the peak-to-peak voltage, thereby regulating torch proximity. The methods disclosed are based on through-the-arc sensing. Through-the-arc sensing offers the advantages of being non-contacting (not influenced by irregularities in the surface of contact) and sensing directly at the arc location. With the possible exception of optical sensing, the other techniques require the sensing mechanism to be offset from the location of the arc and the molten puddle. This offset can cause problems when welding irregular shapes and contours. Optical sensing and processing can be used, but this type of sensing tends to be much more complicated and expensive than through-the-arc sensing. Present current sensing techniques suffer from a very poor signal to noise ratio and require substantial processing and filtering. The latter severely limits the response time of the control process, which in some applications limits the technique to abnormally and unacceptably slow welding rates. The system of FIG. 2 has been found to have an excellent signal to noise ratio and works well at higher welding rates. While analog control methods are shown, digital controllers can just as easily be used with the reference input(s) instead being numeric in value.

Claims: A welding torch means position detection system comprising means (16, 18, 15) for directing consumable electrode metal (13) through contact means (20) in a welding torch means to a workpiece (14), means (19) (24) for providing electrical power to establish a welding arc (22) between the consumable electrode metal (13) and workpiece (14), means (19) (24) for perturbing the electrical welding parameters, and distance-controlling means (21/25, 26, 30, 32, 34) (28, 42, 44, 46, 30, 32, 34) for controlling the distance between the contact means (20) and the workpiece (14) using measurements of the welding parameters; characterised in that the means (19) (24) for perturbing the electrical welding parameters is effective to vary either the arc voltage or the arc current by a predetermined amount, and the distance-controlling means (21/25, 26, 30, 32, 34) (28, 42, 44, 46, 30, 32, 34) uses said predetermined amount in conjunction with only one welding parameter measurement namely the arc voltage in the case where the arc current variation is predetermined or the arc current in the case where the arc voltage variation is predetermined. The system as claimed in claim 1; characterised in that the means for perturbing the electrical welding parameters is associated with or incorporated in the said means (19) (24) for providing electrical power. The system as claimed in claim 1 or 2; characterised in that the distance-controlling means (21/25, 26, 30, 32, 34) includes passband filter means (21/25) tuned to the frequency of perturbation of the electrical welding parameters. The system as claimed in any one of the preceding claims; characterised in that the distance-controlling means (21/25, 26, 30, 32, 34) includes impedance-computing means (26) connected to respond to said predetermined amount and to said one welding parameter measurement to compute a value representative of electrical impedance between the contact means (20) and the workpiece (14). The system as claimed in claim 1 or 2; characterised in that the means for perturbing the electrical welding parameters is provided by virtue of said means (24) for providing electrical power being a pulsed power source. The system as claimed in claim 5; characterised in that the distance-controlling means (28, 42, 44, 46, 30, 32, 34) includes sampling (tracking) means (28, 42, 44, 46) for producing a voltage-difference signal (B) related to the impedance between the contact means (20) and the workpiece (14). The system as claimed in claim 6; characterised in that the sampling (tracking) means (28, 42, 44, 46) comprises a differential amplifier (28) connected to sense voltage between the contact means (20) and workpiece (14) and providing an output (J) connected to two sample (track)-and-hold devices (42, 44) responsive to synchronising signals (K, L) associated with the frequency of said pulsed power source (24). The system according to claim 1; characterised in that the means for perturbing the electrical welding parameters is effective by moving either the contact means (20) or the workpiece (14).

Inventors: ROTHERMEL, RONALD R.
Applicants: DIMETRICS,INC.
Application year: 1985
Main classification: B23K9
IPC codes: B23K9