| Module forging items are applied broadly in automobile manufacturing, national defence industry and other industrial fields, and the higher and higher precision requirement for module forging items urges module technology to develop in the direction of precision molding. Screw press is commonly regarded as the optimal equipment for precision forging because of its favorable energetic character. In terms of driving mode, there are four types including friction, hydraulic, clutch and electrical screw press, and among which, electrical screw press adopts the technique driven directly with AC motors instead of those driven with fluid transmission or traditional mechanical drive. Electrical screw press works with short transmission chain and it is easy to be maintained. In recent years, as numerical control technique has been applied, the newly electrical screw press with numerical control has emerged. This type of electrical screw press has advantage of keeping energy under accurate control and it works with high efficiency. The technology of electrical screw press develops rapidly and has replaced other types of screw press abroad, but in China, this type of electrical screw press is still mainly imported. Thereby, it is important to do some research in this technique and put it into practice to break up the foreign monopoly on this technology.As to numerical control electrical screw press, there still exists some problems with key techniques to solve as the following at present: (1)The choice of the power of a motor.(2)The selection of the most effective speed regulation technology.(3)The way to solve the problem in output torque and temperature rising of a motor at start state under load.(4)The design of the total control system.(5)The control method of exact energy and back stroke position. Based on the key techniques of electrical screw press mentioned above and the advanced experiences introduced from abroad, many experiments and researches have been carried out.At the beginning, the main energetic parameters of electrical screw press have been analyzed. Then, the calculation of motor power has been given out according to the performance items. Based on the work procedures, the key technologies of numerical control electrical screw press have been explained more clearly.During the work process of electrical screw press, motors loaded with large inertia must alternate between positive and negative rotation frequently, and it is necessary to realize a quick and steady torque output and to avoid an abnormally high temperature rising. These depend on the application of advanced speed regulation technology. Therefore, speed regulation technologies including Transvector Control and Direct Self Control have been analyzed. The start current and start torque of a motor have been emulated and compared applying different speed regulation methods through Matlab. The result of emulation has proved that the application of Transvector Control and Direct Self Control can not only minish the start current but realize more output torque.An experimental test with water dynamometer has been set up to simulate the working condition of electrical screw press. Every time the water flux is changed, the load is altered. During the period of varying load, the torque overload ability and the temperature rising of asynchronous motor and homemade servo motor were observed. It has been demonstrated that less start current and lower temperature in the same overload condition can be realized with DSC method.As the working environment is wicked, programmable logical controller (SIMATIC S7-200 or S7-300) has been used as a controlling core for a convenient and credible control system: (1)In this system, DSC of speed regulation technology has been applied to drive motors and touch screen has been adopted as user interface. Gear transmission with high efficiency is applied to the system. During back stroke process, the brake ability of the motors themselves is utilized to decelerate and stop the slider without special hydraulic system. (2)To reduce eccentric load, the mode driven by double motors at the same standard are applied, which means two motors running or reversing at the same time via transducer are fixed at the top of the body with a 180 angular interval, driving the flywheel equably. After analyzing different control projects theoretically, the author adopted the full coordination method, which practically verifies that it can keep well synchronization between the two motors. The optimal performance can be realized by PID controller.After the control system has been set up, Asynchronous motor, servo motor and variable frequency motor have been selected respectively to test the system by striking at a top power. The result proves that the torque output abilities of the three types of motors at start state are almost equal, but the temperature rising of asynchronous motor is most inconspicuous while working continually with the same time span. The scheme that both asynchronous motors are driven by ACS800 inverter has been eventually confirmed. The result also confirms that DSC of speed regulation technology realizes the totally constant output torque at start state with large-inertia load. Both motors can be driven running at a certain overloading ration in an intermittent working condition. The motor's overload ability is exerted and the stable temperature rising of the motor is realized, which fulfills the demand of numerical control electrical screw press.To fulfill exact control on strike energy and block position, a high performance numerical control software system is essentially needed. Numerical control electrical screw press software system is designed and realized with module program method. The modularization structure makes it possible for each subprogram to work separately and the whole program to expend easily. In order to keep exact strike energy, the control manner alternating between speed mode and torque mode is applied. Thus, the stoke energy tolerance is below 2% and the phenomenon of over-current which always occurs when the motor reverses sharply is avoided. To improve the efficiency, it is demanded to accelerate the back stroke speed of the slider. But if its speed is too high, the slider may exceed the distance limit under large inertia load because of the shortage of the motor's own brake power. However, a speed layout method has been designed. And the process of this method has been done just after setting parameters, so there is no use of applying too many math operations which may take much executing time of PLC. The position error of back stroke is ultimately kept within±2mm.The research results mentioned above have been applied into the study of numerical control electrical screw press and many favorable achievements have been achieved. The author has participated in the development of 2.5MN-16MN numerical control electrical screw press, Which has solved the technical difficulties in the torque response and serious heat of the motors during the frequent start process, and the energy and back stroke position can be exactly controlled. The technology of making middle and small tonnage electrical screw press is no longer lacked in China. |
PDF Full Text Request