1. Field of the Invention
The present invention relates to a method of controlling a welding current through a feedback control loop, and an inverter-controlled DC resistance welding apparatus.
2. Description of the Related Art
Conventional inverter-controlled DC resistance welding apparatus convert a supplied DC voltage into a high-frequency AC voltage with an inverter which comprises switching elements, supplies the high-frequency AC voltage to the primary winding of a welding transformer, rectifies a high-frequency pulse voltage generated across the secondary winding of the welding transformer by way of, for example, full-wave rectification with a rectifier, and thereafter applies the voltage to a workpiece, which is to be welded, through a welding electrode. The inverter-controlled DC resistance welding apparatus of this type are advantageous in that the welding transformer is small in size and they utilize electric energy from the power supply with high efficiency.
Because of the small size, the welding transformer can be assembled in the tip end of a welding arm of a welding robot, with the result that the length of the arm from the output terminals of the secondary winding of the welding transformer to the welding electrode is relatively small. In applications which require a welding current of about 15000 A or more to weld steel sheets for automobile bodies, the relatively small arm length from the output terminals of the secondary winding of the welding transformer to the welding electrode is effective to reduce a voltage drop across the arm and the amount of heat generated thereby, resulting in an increase in the efficiency with which electric energy from the power supply is utilized.
The inverter-controlled DC resistance welding apparatus may employ a constant secondary current control process for controlling the welding current to be supplied to the workpiece, i.e., the current from the secondary winding of the welding transformer, at a preset constant reference value. According to the constant secondary current control process, the inverter is energized in a pulse-width-modulation (PWM) mode, and the secondary current from the welding transformer is detected by a current sensor such as a toroidal coil that comprises a toroidal core and a plurality of turns of an insulated wire such as an enameled wire wound around the toroidal core. The detected secondary current is compared with the constant reference value, and the pulse duration of the PWM mode is varied depending on the deviation of the detected secondary current from the reference value, so that the secondary current of the welding transformer will be brought into conformity with the constant reference value through a feedback control loop.
In the constant secondary current control process based on the feedback control loop, since the welding current is directly fed back for control, it can be controlled with an accuracy of about .+-.150 A (about .+-.1% if the welding current is 15000 A), for example, for thereby uniformizing the welding quality of the workpiece.
If the primary current is controlled using a feedback control loop, then since the welding current is indirectly controlled, it is controlled with an accuracy of about .+-.3%.
With the inverter-controlled DC resistance welding apparatus, as shown in Japanese patent publication No. 8-4943, for example, when the welding current (the secondary current) abruptly decreases owing to a change in the manner in which the welding electrode contacts the workpiece, the pulse duration and peak value of the primary current increase in order to make up for the reduction in the welding current. The increased energy of the primary current then tends to break the switching elements of the inverter. Japanese patent publication No. 8-4943 discloses a technique for preventing the switching elements from being broken. According to the disclosed technique, the primary current is also detected, and a primary current value corresponding to the preset reference value for the secondary current is calculated, with a limit level being established which is several % higher than the calculated primary current value. If the detected primary current exceeds the limit level while the secondary current is being supplied, i.e., while the workpiece is being welded with the secondary current, then the pulse duration of the primary current is limited to protect the switching elements against breakdown.
The inventors have found a phenomenon in which while the secondary current is being controlled using a feedback control loop, only the detected primary current increases continuously or stepwise to a certain level though the secondary current detected by the current sensor is of the preset reference value, i.e., a normal value.
In an effort to analyze the phenomenon, the inventors detached the toroidal coil, which is the current sensor for detecting the secondary current, from the welding apparatus and measured the impedance of the toroidal coil. The inventors have revealed that when a physical shock is applied to the current sensor during the measurement of the impedance thereof, the impedance (inductance) is reduced. It has also been found that there is a secondary current sensor whose impedance (inductance)is reduced even when no physical shock is applied to the current sensor during the measurement of the impedance thereof. Another finding is that when a brand-new toroidal coil is attached to the welding apparatus and the secondary current is controlled using a feedback control loop again, the detected secondary current and the detected primary current are free of variations.
It has been presumed from these findings that the efficiency with which the secondary current sensor converts a current into a voltage is lowered. If the secondary current sensor is degraded, then the above phenomenon, i.e., the phenomenon that only the detected primary current increases though the detected secondary current is of the preset reference value while the secondary current is being controlled using a feedback control loop, can be explained.
When the inventors disassembled and inspected the degraded current sensor, i.e., the degraded toroidal coil, they discovered a contact between the core and insulated wire of the toroidal coil or a rare short or a short circuit between adjacent ones of the turns of the insulated wire.