Spot Weld Quality Monitoring System

This invention relates to in-process monitoring of welding conditions from which weld quality evaluations are automatically made and utilized to eject defective articles in production.

The sensing of electrical conditions and evaluation of associated parameters for determining the quality or integrity of welds is generally known as disclosed for example in U.S. Pat. No. 3,582,967 to Beckman owned by the assignee of the present application. According to the Beckman patent, the total energy delivered to a spot weld during a welding cycle through a standard welding machine is monitored by measurement of the input voltage across the welding electrodes. However, there are no heretofore known standards for the electrical conditions that occur during a welding operation to produce a weld of acceptable quality. In particular, there were no such standards available for in-process determination of quality for the spot welds formed along the seam of a can being welded by rotating electrode wheels during movement of the can along a conveyor path of a production line, so that cans with defective welds could be ejected at some location downstream of the welding station.

It is therefore an important object of the present invention to provide a method and apparatus for monitoring more meaningful conditions and evaluating measurements of such conditions during a welding operation for making rapid and accurate judgements as to weld quality.

Another object in combination with the foregoing object is to provide a method and means for utilizing the weld quality judgements to control ejection of articles with defective welds from the production line and interrupt the supply of welding energy to prevent equipment damage.

An important feature of the present invention is the discovery of certain reliable weld quality criteria from the measurement of variable electrical characteristics of the energy actually consumed in the weld nugget during the welding operation. Utilizing such discovery, voltage measurements are made of the voltage drops across the welding transformer and the circuit resistance in series between the transformer and welding electrodes to obtain a differential voltage signal, by a zero offset method. By adjustments made during such voltage measurements without any weld seam between the welding electrodes, the differential voltage signal obtained during the welding operation accurately corresponds to the energy actually consumed in heating the weld nugget. This differential voltage signal is evaluated by integration with respect to time for each half cycle of the voltage alternating frequency to produce a variable factor discovered to be an accurate quality criterion when compared with adjusted level limits for a given welding installation. When such variable factor exceeds the adjusted limits for more than one-half cycle of the AC voltage input, a fail signal is produced for an associated detection cycle corresponding to the half cycle of the AC voltage. Such fail signal is used to control ejection of the defective article at a station downstream of the welding station, mark the defective weld location and turn off heat during the welding operation to prevent damage from excessive voltage spikes.

It has been further discovered that the development of a fail signal during two consecutive half cycles signifies a bad weld. Appropriate gating circuitry is accordingly provided to transmit a reject signal to a register under the foregoing conditions, the gating circuitry being enabled by detection of a voltage spike in the differential voltage signal being evaluated. The reject signal loads a register and controls heat turn off and operation of a weld marker. The register when loaded timely operates an ejector for removal of the defective article from the production line.

These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

FIG. 1 is a schematic block diagram showing a typical installational arrangement for the present invention.

FIG. 2 is an electrical circuit diagram illustrating the system of the present invention.

FIG. 3 is a circuit diagram showing the reject logic of FIG. 2 in greater detail.

FIG. 4 is a front elevational view of a typical control panel of the monitor associated with the present invention.

FIG. 5 is a graphical illustration of certain operating characteristics associated with the present invention.

Referring now to the drawings in detail FIG. 1 schematically illustrates a welding arrangement for products such as cans 10 traveling along a conveyor path 12 in sequence through a welding station at which a welding apparatus 14 is located and an ejection station at which ejector 16 is located. The welder 14 is of a conventional seam welding type producing spot welds along a seam of the can, and is associated with a marker 18 adapted to mark spot weld locations along the seam. The quality of the spot welds are monitored and evaluated during the welding process by a monitor 20 in accordance with the present invention as will be explained hereinafter in detail. Monitor 20 recognizes the formation of a defective spot weld to operate the marker 18 and subsequently activate the ejector 16 for displacement of the can from the conveyor path. A can sensor 22 detects the arrival of the can at the ejection station in operative alignment with the ejector 16 so as to enable operation thereof in response to a reject signal previously generated in the monitor 20. An indicator 24 is operated to signify ejection of a can.

Referring now to FIG. 2, the welder 14 is shown as including rotatable wheel electrodes 26 and 28 connected across the terminals of the secondary winding 30 of a welding transformer 32 in series with circuit resistance 34. The primary winding of transformer 32 is connected to a suitable welding power supply 36 having a power control 38 associated therewith for interrupting the otherwise continuous supply of electrical energy having an AC voltage characteristic. As depicted in FIG. 5, a sinusoidal voltage curve 40 represents the product of welding current and resistance associated with the usual spot welding operation of the welder 14 which corresponds to a volt drop of 1.56 V across the transformer winding 30 without any can seam between the electrodes and a volt drop of 1.4 V with a can seam between the electrodes. The actual energy consumed in the weld nugget during a welding operation is found to be a small fraction of the energy supplied as represented by voltage curve 42 corresponding to a volt drop of 0.09 V. The areas 44 and 46 enclosed by curves 40 and 42 during a half cycle of the AC voltage represent the energy consumed with area 44 being the total energy supplied covering circuit losses in the buss bars and wire connections. Such circuit losses are cancelled out in accordance with the present invention leaving area 46 under curve 42 which provides a more sensitive basis for monitoring weld quality.

With continued reference to FIG. 2, the energy supplied to the welder is monitored by a voltage measuring circuit generally referred to by reference numeral 48 which includes a pair of amplifiers 50 and 52 having signal outputs respectively connected by rectifiers 54 and 56 to the inputs to a zero offset differential amplifier 58. The volt drop across the transformer secondary 30 constitutes the analog input to amplifier 50 while the volt drop across circuit resistance 34 constitutes the analog input to amplifier 52. The rectified outputs of amplifier 50 and 52 are applied to differential amplifier 58 in order to produce a differential voltage signal characterizing the energy used in the formation of each weld nugget as depicted by curve 42 in FIG. 5. A zero adjustment device 60 is associated with amplifier 58 to adjust its output to a substantially zero level while circuit 48 is measuring voltage with no can seam between the electrodes 26 and 28. The output of zero adjusted amplifier 58 during the movement of a can seam between the electrodes, accordingly provides a sensitive voltage measurement corresponding to the actual energy consumed in the formation of a spot weld each half cycle of the AC voltage.

The differential voltage output of amplifier 58 is applied through a voltage controlled oscillator (VCO) device 62 to an input pin of a signal evaluating microprocessor chip 63 functioning as a display and comparator circuit to which reset and latch control signals are applied from control logic 64. The control logic receives inputs from differential amplifier 58, a zero crossing detector 66 connected to the output of amplifier 50, and a signal generator 68 connected across the windings of the welding transformer 32 to monitor its transformation ratio. The microprocessor 63 is programmed to count pulse signals from the voltage controlled oscillator 62 for a predetermined period of time, to integrate the differential voltage input received from amplifier 58 with respect to time during each half cycle and compare the level of such integrated differential voltage with upper and lower limits set by limit adjusting devices 70 and 72. The integrated differential voltage is converted into a digital output registered on display 74. A bank of indicators 76, 78, 80 and 82 connected to another output pin of chip 63 signifies the magnitude in terms of low, pass, high and overange values. When the integrated differential voltage level, representing the energy consumed during a detection cycle in the formation of a spot weld, exceeds the adjusted limits set through devices 70 and 72, a fail signal appears at yet another output pin of chip 63 connected to signal line 84 to signify a defective weld.

The fail signal in line 84 is applied to a reject logic system 86 through which operation of the ejector 16 is controlled as diagrammed in FIG. 2. The differential voltage output of amplifier 58 in line 88 and the output of signal generator 68 in line 90 are also fed to the reject logic system from which a heat turn-off signal is obtained and applied through line 92 to the power control 38 for interrupting the supply of energy to the welder as aforementioned, in order to prevent damage from excessive sparking or flashing.

The reject logic system 86 shown in greater detail in FIG. 3, includes a two consecutive signal gating component 94 and a four consecutive signal gating component 90 connected in parallel to the fail signal line 84. Thus, the presence of a reject signal in line 84 during two consecutive half cycles and four consecutive half cycles causes gating components 94 and 96 to respectively apply inputs to NOR gate 98. A spike voltage detector 100, such as a conventional peak detector, connected to line 88 will sense the peak voltage in the differential voltage output being evaluated to apply another input to NOR gate 98 to produce a timely reject signal in line 102 in response to fail signals in line 84 during two consecutive cycles or a spike voltage. The reject signal is stored in a register 104 for subsequent read out. The reject signal is also optionally applied through switch 106 to one input of NOR gate 108, having a second input connected to the spike voltage detector 100. Accordingly, the reject signal is alternatively effective through NOR gate 108 to produce a heat turn-off signal in line 92 which otherwise occurs when a voltage peak is present, reflecting an abnormally high resistance in the seam being welded, caused for example by lacquer or paint. The heat turnoff signal in line 92 operates the marker 18 so as to identify the defective spot weld location on the seam.

The register 104 is enabled by an operating signal in line 90 to energize a reject relay 110 following receipt of a reject signal in line 102. Such energization of relay 10 occurs when the register is triggered by a signal input from sensor 22 to operate the ejector device 16, which may be of the air discharge type. The relay is also operative to illuminate the ejection indicator 24.

The spike voltage detector 100 has a reject inhibit switch 112 associated therewith to prevent a heat turn-off signal in line 92 disenabling the detector. The voltage level at which detection occurs is set by adjust control 114.

All of the indicator lamps and adjustment controls hereinbefore described are mounted on a control panel 116 of the monitor 20 as shown in FIG. 4. Additionally, digital displays 118 and 120 may be provided to indicate the limit settings of the adjustment controls 70 and 72. Indicators 122 and 124 furthermore signify the upper or lower limits being exceeded while an indicator 126 is provided for registering detection of a voltage spike by detector 100. For testing purposes a fail signal may be artificially injected through line 84 by actuation of a signal inject switch 128 on the control panel.

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.