Servo Motor Control in Resistance Welding Machine
With global environmental crisis becoming intensified, energy saving and emission reduction has become one target of the automobile industry. Therefore, all materials used by automobiles change correspondingly and light quality and high strength steel materials are preferred. Resistance welding and car body manufacturing is inseparable. Depending on its characteristics like wide speed regulation ratio, big rotational torque and high accuracy, resistance welding machines pressed by servo motor plays an important role in welding. Servo welding machine adopts servo motor driving, which is beneficial to soft contact of motor pressure, improves pressing speed and solves problems of traditional pressing methods better.
1. Characteristics of servo motor
Spot welding machine controlled by servo motor can improve control over welding force and realize accurate control over depth of fusion of welding force and welding points. PLC can control servo motor to program welding pressure conveniently so as to make forging force before and during welding be set in advance. Compared with traditional cylinder pressure, accuracy of pressure control can be greatly improved. In addition, servo motor itself has favorable dynamics and it can improve speed from zero to rated speed in several seconds. With its advantages, servo motor has a successful application in resistance welding machine.
2. Application of servo motor control in resistance welding machine
In welding of bottom cover of shock absorber for automobiles, dedicated seam welder controlled by servo motor is fully applied. In welding, work pieces are in the middle while welding wheels are in the both sides, and work pieces are in the state of dynamic clamping. Two servo motors are used to position work pieces at the same time and servo compression spring will press work pieces so as to obtain table welding pressure. Meanwhile, pressure feedback system equipped in terminals can be used to supervise pressure to guarantee accuracy of pressure control. Then, two servo motors can be used to control speed of welding wheel. Different wear degree of edges of welding wheels will cause different diameters of two wheels, thus causing the same running speed and different wire speed. On the basis of this, laser rangefinder can be used to measure diameter of welding wheels and semi-diameter of welding wheels can be obtained by taking centre distance of sensors and welding wheels as the parameters and make them minus distance measuring value of sensors to head faces of welding wheels. Through PLC real-time calculation, control and pressing servo motor will move to the specific location to obtain stable welding pressure. The equipment can guarantee stability of welding quality and improve automatic level and production efficiency. 
A typical example of application of servo motor in aviation enterprises is that bellow welders exclusive to aviation enterprises is to use resistance welding to weld bellows. To make up big differences of diameters of upper and below welding wheels, servo motors cannot only make welding speed be in accord but also guarantee quality of welding, thus safeguarding aviation safety.
Servo motor is also applied in aspect of flash welding technology. For preheating of work pieces, continuous flash need to realize welding of work pieces in the shortest time with the fastest speed and bigger pressure. Since electric arc will be faster, it requires its speed change with changes of position. Application of servo motor can better solve the problem and make welding quality tend to be stable gradually. In addition, in application of welding machine, former can welding needs another cam only if parameters change. Thus, it is not uneconomic and troublesome for medium and small lot production. However, servo motor drives can change welding parameters conveniently, which make application of flash butt welding machine play a great role.
On the basis of simplified control system structure, reinforcement and application of prominent advantages of servo motor control can make welding technology improve continuously in practice and finish resistance welding more quickly and accurately.
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