Keywords: Synchronous motion control, stamping automation
Stamping is one of the four major processes in automobile manufacturing. The most important equipment in stamping is the press, along with other auxiliary equipment such as uncoiling lines and turning machines. Presses can be loaded and unloaded manually or automatically. Automated loading and unloading equipment is more specialized and expensive. With the increasing demands for production efficiency in modern factories and the continuous improvement of electrical control technology, stamping automation technology has developed rapidly in the past decade or so. From the early sequential control between presses and robotic arms for loading and unloading, it has evolved to the master-slave control between synchronous presses and rapid feeding mechanisms, thus significantly improving production efficiency. In stamping automation equipment, due to the high requirements for control performance and fast dynamic response, servo technology with multi-axis motion control is mostly used. Controllers generally include both logic control and motion control functions; some integrate them into one controller, while others have separate controllers for logic control and motion control. In terms of transmission structure, servo motors are mostly used through synchronous belts or lead screws, with reciprocating positioning accuracy within 0.5mm. After years of development and improvement, stamping automation equipment now generally includes the following types:
1. Robotic arm loading and unloading
An automated robotic loading and unloading mechanism (see Figure 1) generally includes a destacking device, a centering device, a loading robot, an unloading robot, and a shuttle carriage. The loading/unloading robots and the shuttle carriage typically have 2-3 motion axes driven by servo motors. Manufacturers of robotic arms worldwide include ISI (USA), Oyabe (Japan), ABB (Sweden), and Kingmax (Taiwan).
2. Robot loading and unloading
Robotic loading and unloading mechanisms typically include depalletizing robots, centering devices, and loading/unloading robots. The main difference between robotic arms and dedicated robotic arms is that they utilize general-purpose industrial robots for loading and unloading operations, typically large industrial robots with a load capacity of 60-100 kg. Major manufacturers worldwide capable of producing stamping robots include FANUC and Yaskawa of Japan, ABB of Sweden, and KUKA of Germany.
3. Fast feeding mechanism for loading and unloading
Rapid loading and unloading mechanisms evolved from robotic arms, offering greater speed and representing a more specialized form of automated stamping equipment. This structure is not limited to the two-dimensional coordinate system of a robotic arm; its unique mechanism is specifically designed for sheet metal conveying between presses. A single conveying mechanism between presses can contain up to a dozen servo motor shafts (see Figure 2). Manufacturers capable of producing rapid loading and unloading systems are primarily professional stamping equipment manufacturers, such as Schuler of Germany and Komatsu of Japan.
Robotic arm loading and unloading methods appeared earlier, but due to a lack of significant advantages, orders for them are decreasing. As industrial robots have matured and are easy to maintain, robotic loading and unloading are increasingly becoming the preferred choice for manufacturers with limited budgets. High-speed feeding, on the other hand, has emerged gradually in the last decade. While its technology is not yet fully mature, its significant speed advantage and ability to integrate seamlessly with high-end presses ensure it will play a crucial role in the future stamping industry. A comparison of the characteristics of the three loading and unloading mechanisms is shown in the table below.
Synchronous motion control technology
Synchronous motion control evolved from servo drive technology. A servo driver drives a servo motor and collects feedback signals to the driver's core control unit, achieving high dynamic response motion control. In CNC machining centers and robots, multiple servo axes rely on interpolation calculations by the control unit to achieve interpolation motion trajectory curves for two or more axes. However, in some equipment with higher motion control requirements, a master-slave synchronization relationship is required between the various motion axes, leading to the further development of synchronous motion control technology. Synchronous motion control can enable cooperative motion between two robots. In automated stamping equipment, synchronous control can achieve synchronization between presses and between the press and the rapid feeding mechanism, maximizing the production efficiency of the entire stamping production line (see Figure 3).
Currently, many industrial control integrators have developed synchronous motion control functions, with Siemens' SIMOTION motion control system and Rexroth's INDRAWORKS motion control system being the most widely used. The former has extensive applications in food processing, printing, and automotive manufacturing, while the latter is more commonly used in automotive manufacturing and tire manufacturing. In the stamping automation industry, companies providing synchronous control solutions include Siemens (Germany), Rexroth, and Reynolds (Japan). Internationally, manufacturers capable of using synchronous control integration technology on high-end stamping production lines include Schuler-Vangardon (Germany), Komatsu (Japan), and GUDEL (Switzerland). These equipment manufacturers have manufactured numerous synchronous high-speed stamping lines already in operation, primarily concentrated in Japan and Europe. In China, some lines are also in operation or undergoing installation and commissioning. Representative examples include the Schuler stamping line used by FAW Car, the Vangardon stamping line by Shanghai GM, the GUDEL stamping line by Beijing Hyundai, and the GUDEL stamping line by Kia Motors. These are all stamping lines that utilize synchronous technology and have been put into operation in recent years. From an electrical control perspective, both Schuler-Vangardon and Goodyear use an industrial computer with synchronous control functionality as the control core, Rexroth servo drives as the drive elements, and Rexroth servo motors as the actuators to achieve motion control. How then does the press act as the main shaft? The signal comes from an encoder mounted on the press cam. In the entire synchronous control process, this encoder acts as the main shaft, and the other slave shafts move along with this main shaft according to a pre-programmed synchronous curve.
As shown in Figure 4, RM represents the press spindle, which is a real shaft. VM is a virtual spindle; when running synchronously with the press, RM and VM are coupled; when the feeding mechanism runs independently, RM and VM are decoupled. VX , VY , VZ, etc., represent virtual axes of the feeding servo mechanism in the directions of feeding, lifting, etc. When running automatically or in a single cycle, VX and VM run synchronously according to a pre-programmed cam curve to achieve continuous movement of the feeding mechanism; when manually feeding the various axes of the mechanism, VX , etc., are decoupled from VM . X1 , X2 , etc., always maintain linear synchronization with VX to ensure the synchronization of multiple motors in the same direction of movement.
Figures 5 to 7 show motion curves created using software. The horizontal axis represents the main axis angle, and the vertical axis represents the slave axis position. Different synchronization curves were used to achieve programmable motion control of the feeding mechanism in various directions and the press. Figure 5 shows a synchronization curve where the slave axis does not follow the main axis, and Figure 6 shows a fifth-power motion curve where the slave axis follows the main axis, first accelerating, then moving at a constant speed, and then decelerating. The programmable combination of Figures 5 and 6 achieves the complete synchronization motion shown in Figure 7. This curve is the curve required for the synchronization motion between VX , VOY, etc., and VM . The synchronization curves created for VX , VOY , VZ , VA , and VB are shown in Figure 8.
Conclusion
The application of synchronous motion technology on stamping lines has significantly improved production efficiency, triggering a revolution in the stamping industry. The overall speed of large stamping lines has increased from 6-8 pieces per minute to 10-15 pieces per minute, maximizing the high efficiency of automated production lines. While the cost of this high-speed feeding line is 30-40% higher than the original robotic automated lines, its production efficiency is nearly doubled. This is bad news for stamping equipment manufacturers, as it means users will no longer need as many presses. However, increasing productivity is an inevitable trend in modern industry, and other stamping equipment manufacturers are also developing their own synchronous high-speed stamping lines. It is believed that with the continuous development of control technology, synchronous motion control will be more widely used.
