Abstract: Automation of welding process is one of the most urgent and important problems in modern　industries, the first task for solving it is to realize the automatic seam tracking and the key　point of realizing seam tracking is to develop suitable sensors. RWTH Aachen developed a new　seam tracking system, the special feature of which is the using of arc itself as the sensor, and　therefore it possesses superiority in many respects over all existing seam tracking systems.　However it has also a radical shortcoming, I. E. sensitivity to short-circuit transfer which makes　it useless in such cases. Recently observations and study on this system have further been made　in RWTH Aachen, and the problem was successfully solved. As a result of this work, this new　system can be used not only for submerged arc welding, spray transfer MIG-MAG welding, but　also for short-circuit transfer welding process.This paper describes the principle, mechanism, electrical block diagram of the system and the　results of the latest study on it.