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A system for closed-loop temperature control of high-frequency welding systems has been sought for over 20 years; however, until recently the available instrumentation for temperature measurement was not capable of operating in the conditions prevailing on a typical tube or pipe mill. The major problems are: 1. A very small available target size. 2. The positional instability of the target due to changes in incoming material dimensions and mill parameters. 3. The presence of large quantities of cooling water in the immediate weld area with attendant smoke and steam. 4. The possibility of an occasional momentary arc occurring in the weld area under certain mill operating conditions. 5. The very small ratio of the heated material to the power available for heating, leading to the possibility of extremely rapid temperature changes. 6. The presence of a small but significant amount of radio frequency (RF) radiation in the immediate vicinity of the weld.
A combination of the latest developments in color ratio pyrometry, in modern digital control techniques, and in coolant control in the weld area has led to the development of a closed-loop control system which will hold the mean weld temperature constant while rejecting the momentary disturbances to the temperature signal which are inevitable in a normal production process.
high frequency, resistance, welding, temperature, control
Director of Research, Thermatool Corp., Stamford, CT