Resistance weld control with line impedance compensation
First Claim
1. A digital weld control system for a resistance welder of the type connecting and disconnecting through a welding contactor, a welding transformer load from an alternating current weld power source to control timing periods of a welding sequence and a level of heat intensity in the weld to ensure quality welds, the digital weld control system comprising:
- A. a phase reference clock;
B. a voltmeter;
C. a current meter;
D. a firing control;
E. an impedance estimator for estimating load impedance magnitude and power factor based on data from previous welds;
F. a power source estimator for estimating a source power based on a lumped parameter model of the weld power source; and
G. control means responsive to the lumped parameter model and an estimated voltage drop caused by the presence of line impedance in the weld power source for adjusting firing points of the welding contactors in a next preceding line voltage cycle and for maintaining welding current constant and independent of line voltage variations and independent of actual weld current drawn by the welding transformer load.
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Accused Products
Abstract
A phase controlled weld controller system uses an internal model of weld line and load impedance and of open circuit weld input voltage to develop a nominal firing angle sequence to maintain a desired weld current sequence. The weld controller computes a nominal firing angle sequence based on estimated models of line impedance, open circuit line voltage, and an estimated relation between the load current and conduction angle and the mathematical relation between firing angle, conduction angle and load circuit power that will achieve a desired weld sequence if the models are accurate. The weld controller subsequently uses measured values received in real time while the weld sequence is being executed to modify the nominal firing angle to better achieve the desired objective. This approach allows the weld controller to quickly achieve the desired weld sequence with the actual load, while permitting low feedback gains, resulting in a system that is quick and accurate in response without being overly sensitive.
19 Citations
17 Claims
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1. A digital weld control system for a resistance welder of the type connecting and disconnecting through a welding contactor, a welding transformer load from an alternating current weld power source to control timing periods of a welding sequence and a level of heat intensity in the weld to ensure quality welds, the digital weld control system comprising:
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A. a phase reference clock; B. a voltmeter; C. a current meter; D. a firing control; E. an impedance estimator for estimating load impedance magnitude and power factor based on data from previous welds; F. a power source estimator for estimating a source power based on a lumped parameter model of the weld power source; and G. control means responsive to the lumped parameter model and an estimated voltage drop caused by the presence of line impedance in the weld power source for adjusting firing points of the welding contactors in a next preceding line voltage cycle and for maintaining welding current constant and independent of line voltage variations and independent of actual weld current drawn by the welding transformer load. - View Dependent Claims (2, 3)
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4. A method of estimating a value of an ideal voltage source and line impedance for use in resistance welder control applications comprising:
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a. measuring a line voltage and a resulting load current on a first cycle of line voltage; b. measuring the line voltage and resulting load current on a second cycle of line voltage in which the load current is different than the first cycle measured load current; c. computing a voltage difference between the line voltage of the first cycle and that of the second measured cycle; d. computing a current difference between the first cycle measured load current and second cycle measured load current; e. dividing the voltage difference by the current difference to obtain the estimate of line impedance; and f. computing the estimated ideal voltage source value by multiplying the estimated line impedance value by the weld current on one of the line cycles and adding a resulting value to the measured voltage of that line cycle. - View Dependent Claims (5, 6)
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7. A method of estimating an ideal open circuit voltage and line impedance of a lumped parameter model of an estimated power source for use in resistance welder control applications comprising:
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a. measuring a line voltage on a first subset of half cycles of line voltage in which weld current has not been commanded; b. applying a digital filter to the first subset of half cycles of line voltage to create a single value representing an estimate of the voltage of an ideal voltage source; c. measuring a line voltage and weld current on a second subset of half cycles of line voltage and weld current on cycles in which weld current has been commanded to flow; d. applying a digital filter to a second subset of half cycles of line voltage to create a single value representing an estimate of the voltage of the subset of the weld pulse; e. applying a digital filter to the second subset of half cycles of weld current to create a single value representing an estimate of the weld current of the subset of the weld pulse; f. computing a difference between the estimate of line voltage of the first subset of half cycles and the estimate of the second subset of half cycles; and g. dividing the voltage estimate difference by the estimate of weld current of the second subset of half cycles to obtain an estimate of power system source impedance. - View Dependent Claims (8, 9, 10)
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11. A digital weld control system for generating from a power source, a weld cycle having a sequence of nominal firing angles to achieve a sequence of target half cycles of weld current applied to a welding load, the digital weld control system comprising:
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A. means for determining a lumped parameter model of the power source including an ideal voltage source and serially connected line impedance; B. means for creating an internally maintained lumped parameter model of the welding load based on information gathered from previous welding cycles; and C. means for using the lumped parameter models of the power source and welding load to achieve a power source compensated target weld current sequence. - View Dependent Claims (12, 13, 14, 15, 16, 17)
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Specification