Preliminary Study on System Reform of Import Processing Center

Abstract: MC340/TWIN is a double-spindle machining center produced by STAMA GmbH in Germany. Due to the severe burn, the original system cannot be restored. The technical renovation work adopts the German SIEMENS-840D control system to replace the Japanese FANUC-15M system of the original machine tool configuration, involving a large number of hard and software technologies. The article briefly introduces the relevant transformation methods and technologies.

The MC340/TWIN twin-spindle machining center was a CNC machine tool imported from STAMA GmbH in Germany in 1994 and was almost scrapped due to accidents. Apart from repairing damaged mechanical components, we have also updated the control system of the damaged components. 1 CNC eight-axis control of the original eight-axis control The original control system is Japan's FANUC-15M system. The CNC part directly controls two spindles (one motor drives two spindles, the C-axis) and X, Y, Z, and A. Axis, and control rotary table (C2 axis), magazine axis (Q axis) and linear motion type robot axis (V axis) through PLC and STAMA company's own interface. The PLC uses a total of 288/192 I/O points. The C2 axis (rotary workbench) is divided into two work areas. While working in one work area, the other work area can load and unload workpieces. Two work areas can process eight work pieces once per cycle. In addition, the tool change of the original machine tool has its own particularity, and the tool change operation is completed by the three axes, Q axis (magazine axis), V axis (linear robot axis) and Z axis. The process is: When there is a tool change instruction, the Q axis goes to the correct tool position, the V axis takes the robot to the tool magazine to take the tool, and then moves to the spindle end, and the other two claws of the robot grab the two cutters on the spindle. Then, the tool is pulled out by Z-axis movement. After the robot turns 180°, the Z-axis is lowered again and the new tool is inserted into the spindle. The V-axis is then returned to the tool magazine and the replaced tool is inserted into the original position of the tool magazine. Therefore, the entire process of tool change involves the movement of three axes and various actions of the robot, as well as a large number of solenoid valves and position detection elements, and the entire motion is more complicated.

figure 1

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Eight-axis control is now the FANUC-15M system is the product of China's embargo, after research and demonstration, we have chosen four-axis linkage SIEMENS-840D CNC system. The spindle and the X, Y, Z, and A axes are directly controlled by the CNC and can fully meet the original specifications. The original PLC controlled C2, Q, V three axes, also changed to CNC direct control. Because the selected Siemens standard operating panel does not occupy external I/O points, and there are dedicated position encoders inside the servo motor, a lot of I/O points are saved, so the I/O points occupied by PLC control are reduced to 192/160 points. . The PLC function of the original machine tool was realized, the PLC program was optimized, and the function of the original machine tool was improved. The specific method is as follows. Q axis control method The original Q axis control block diagram shown in Figure 1. When the CNC issues a tool change command, the digital command and control signals are sent through the PLC, converted into analog commands through the interface circuit, and then the Siemens drive drives the motor to rotate. With the tool magazine synchronous rotation, and corresponding 56 knife absolute encoder ENC2 feedback the rough position of the tool number to the PLC, to achieve the purpose of coarse positioning, and then by the precision potentiometer: to accurately locate, and then carry out the subsequent tool change work . In this mechanical transmission chain, the above two position sensors are not installed in the motor. Mechanical transmission chain diagram shown in Figure 2. After multi-stage gears and worm and worm gear transmission, the transmission ratio of the motor and the absolute encoder (ENC2) is 157.357/1, and the transmission ratio of the motor and precision potentiometer (R) is 2.81/1, which is not an integer. An incremental encoder is installed in the servomotor associated with the system, but it is not suitable for determining the position of the tool because it does not have a one-to-one correspondence. After analyzing the characteristics of the entire magazine drive mechanism and encoder, we used an external incremental encoder, installed in the position of the potentiometer R, that is, the transmission ratio of the encoder to the magazine is 56:1. Like the X, Y, Z, and A axes, it is convenient to return to the reference tool position (defined as No. 1 tool), and each tool position has a one-to-one correspondence with the encoder. This corresponding accuracy is also 128 times that of the CNC. The role of frequency is better than the positioning of the original precision potentiometer, thus achieving two goals of coarse positioning, and the function of the magazine axis is realized. V-axis control method The original control block diagram is similar to the Q-axis control chart, except that there is no potentiometer fine-positioning. The V axis position is detected by the absolute encoder (ENC2). The control of the V-axis in this transformation is also directly controlled by the CNC. Since the V axis is hidden in the spindle column cavity, it is inconvenient to return to the reference point and must have a position memory. Therefore, we chose the absolute position encoder that is matched with the 840D to cover the full stroke of the V axis, and together with the built-in position encoder of Siemens motors The precise positioning of the V-axis is completed and the control accuracy achieved is higher than the original.

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Control method of C2 axis The original electrical control accuracy of C2 axis is not high, and its accuracy is ensured by the meshing of the gears. The worktable rotates back and forth between 0° and 180°. The control block diagram is shown in Figure 3. Its position is detected by two limit switches at the 0° and 180° points. The principle is: After the CNC issues a work area exchange instruction, the PLC decodes and generates a drive instruction to raise the work platform. The interface board and the driver drive motor are then rotated in a predetermined direction, and a plurality of switches provide signals for the motor to accelerate, decelerate, and position the table. After the workbench is in place, the motor is stopped and the workbench is lowered to complete a conversion process. In this modification, the axis was also changed to a Siemens digital servo motor directly controlled by the CNC. The specific operation process is similar to the original one. The differences are as follows: 1Because the CNC servo shaft is used, and the motor is equipped with a built-in incremental encoder, it must be turned back to the reference point for normal operation. In order not to add new components, The PLC program was specially compiled so that the computer automatically sets the coordinate values ​​according to the switch status after the start of the computer, and the simulated zero return operation under the condition that the worktable is not operating is completed. 2 Although it is a rotary motion, due to the limitation of the mechanical structure, it can only rotate between 0° and 180°, similar to the motion characteristics of a linear axis. Therefore, designating it as a linear axis can reliably limit its rotation between 0° and 180°, avoiding possible 360° rotation of the rotating shaft, and ensuring the reliability of the worktable. Some specialities of the PLC control axis to the CNC control axis The original PLC control axis Q, V, and C2 have large gaps in the transmission mechanism and do not require high position accuracy. Therefore, they are mainly controlled by the speed loop. After changing to CNC control, both speed loops, position loops, and real-time monitoring are easy to generate position deviations and speed deviation alarms, which increases the difficulty of debugging. In the renovation, we strengthen the rigidity of the mechanical transmission chain on the one hand, and increase the clearance compensation on the other hand, increase the dynamic and static position detection tolerances, appropriately increase the gain and arrival inspection time, reduce the acceleration and maximum speed and other parameter settings. Ways to overcome.

Figure 4

The V-axis structure has not only a large gap but also a large elastic deformation, and the schematic structure thereof is shown in FIG. 4 . Its mechanical arm is approximately 640mm long, so the elastic deformation of the V-axis during operation affects the position detection at any time through the screw, making the shaft easily oscillating during rapid movement. According to this feature, we must properly reduce the gain in the debugging, reduce the speed, increase the detection time in place, in order to ensure the stability of the system. Figure 4 In the tool change process, Q, V, Z triaxial movements are involved. The problem of coordination between the V and Z axes is not large. However, the Q and V axes have certain problems in coordination. This is because the machine tool has two spindles. Each time the robot manipulates two knives from the magazine, three blades are separated by two. Knife, coupled with the elastic deformation of Q and V axes, so when the robot picks up the knife from the magazine, it easily interferes and alarms. In addition to the above debugging, it also sets the segmented shifting motion in the PLC program to overcome the two axes. The interference ensures the reliability of tool change. 2 The transformation effect has been redesigned and rebuilt by the mechanical overhaul and the above control system, and all the functions of the original machine tool have been restored. All the technical indicators have met the requirements of STAMA's factory: If the X, Y, Z rapid movement speed reaches 22m/min, The feed speed reaches 1～10000mm/min, the spindle speed is 6000r/min, and it can also be positioned at any angle. The positioning accuracy of X, Y, and Z is 6.546 μm, 8.254 μm, and 9.621 μm (factory specification is ≤12 μm), and the repeatability of positioning is 3.15 μm, 3.533 μm, and 3.25 μm (factory specification is ≤ 8 μm). The system control is stable and reliable, and the machine tool operation is safe and convenient.

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