The press brake is a widely used bending processing equipment. Its main working principle is to use simple general molds, which can easily bend various complex parts through up and down sliding or reciprocating linear motion.
The technical level and performance indicators of the press brake control system will directly affect the processing accuracy of the product manufacturing process and the production efficiency of the product, thereby directly affecting the processing quality and production cost of sheet metal products, and has always occupied an important dominant position in various sheet metal component manufacturing processes.
The press brake is a widely used bending processing equipment, which mainly operates by using simple and general molds to easily bend various complex parts through up and down sliding or reciprocating linear motion.
The technical level and performance indicators of the press brake control system directly affect the processing accuracy of products and production efficiency, thereby directly affecting the processing quality and production cost of sheet metal products. It has always occupied an important dominant position in various sheet metal component manufacturing processes.
I. Introduction to Press Brake
The metal forming processing mold of the press brake is widely used due to its powerful multifunctionality and simple operation process. It has a wide range of applications in processing elastic metals and other engineering chemical materials, such as in industrial machinery, construction, household appliances, and other manufacturing industries, which use a large amount of sheet bending processing.
In order to effectively reduce production costs, improve the industrial labor production environment, reduce labor intensity, and improve the quality of industrial products, the importance of industrial automation and intelligent quality control of the press brake is becoming higher and higher.
Based on this, CNC technology for press brakes is continuously developing. This paper will focus on improving product quality through research on CNC press brakes.
In order to effectively compensate for the upward deformation of the compensating cylinder caused by the upward deformation of the beam matrix below the hydraulic pipe bender and hydraulic pipe rolling machine, a certain number of automatic compensating hydraulic cylinders with sufficient strength need to be installed in the lower beam matrix to automatically generate the deflection and deformation of the cylinder body towards the upward direction, and automatically compensate for the strength of the upward deformation of the compensating cylinder.
This design method inevitably requires the mechanical operator to provide an increase in compensation cylinder pressure for the compensating air cylinder. Since the increase in compensation cylinder pressure directly affects the processing quality of the bent engineering products, it is a very important measurement parameter in various product bending processes.
Currently, the various methods used to provide an increase in compensation cylinder pressure for the compensating air cylinder are either inaccurate or time-consuming.
There is still a big technological gap in how to achieve automatic intelligent control of hydraulic processing equipment parameters.
The new folding and bending machines abroad have successfully realized automatic hydraulics, and now widely employ hydraulic electro-hydraulic proportional drive and servo motor control transmission technology to precisely and automatically control the synchronous movement, position, speed, and pressure of the two hydraulic cylinders.
Compared with the mechanical-linked hydraulic servo system technology of hydraulic-mechanical-linked servo valve, the proportional control system technology of electro-hydraulic hydraulic servo valve is not only structurally adjustable and highly accurate in control but also easy to achieve interconnection between the two systems, due to the simple structure of the mechanical system and the mutual integration of the hydraulic servo system, as well as the simple and convenient interconnection with the CNC control system, the workload of product manufacturing, assembly, debugging, and daily maintenance of the folding and bending machine is greatly reduced.
This modern folding and bending machine design embodies advanced electronic process control and highly flexible automation control technology.
II. Development of CNC System for Press Brake
By introducing electronic systems into CNC press brakes, they become easier to operate manually, more efficient, and faster in response speed. The use of computer automatic control electronic systems not only directly replaces most of the work tasks previously performed by mechanical operators but also can be directly interconnected with three software programs of PLC’s CAD and C/CAM.
One of the highly developed application areas in the manufacturing industry of CNC folding and bending machines is to more effectively control the mechanical punching and bending functions through the use of automatic CNC bending systems and pressure sensor automatic feedback systems.
Nowadays, a digital encoder is usually installed on the slide block, which can monitor its moving position in real-time and transmit the encoding information through feedback to the high-speed hydraulic servo system controlled by the computer automatically.
Currently, there are many varieties and specifications of bending pipe machine tools. When the demand for this type of product specification is relatively high, some production enterprises that do not have market conditions will generally choose imported CNC small bending machines.
Usually, the product price of domestic imported CNC bending machines is about four times that of ordinary CNC bending machines of the same variety and specification, while the product price of domestic imported CNC bending machines is more than 10 times higher.
At present, in actual industrial applications, computers based on large industrial motion controllers in China use mixed motion control system methods such as fuzzy PID to drive servo motors directly to achieve synchronized control of various sliders for automatic bending machines.
The synchronization control accuracy is high and can fully meet the requirements of different production lines, but the control parameters of the fuzzy motion regulation algorithm are relatively complex, or domestic use of multi-mode synchronous operation position automatic control slider templates, using proportional valves and power amplifiers, using fuzzy PD control algorithms to achieve multiple slider synchronous control, synchronization operation position automatic control slider templates, etc., can use two assembly forms of single-mode computer automatic assembly and fuzzy STD bus in design, with better control motion effects and lower costs, but at the time, there were large errors and poor control motion effects.
1. Characteristics and Development Status of Electro-Hydraulic Servo Control Technology
Electro-hydraulic servo motor control is a new integrated automatic control system technology that combines various technological applications such as industrial automation, hydraulics, electronics, and computers into one.
It has a history of more than 50 years of extensive application development and innovation, and has been widely used in various fields of new industrial automation control technologies as an accurate and effective industrial automation control system design method.
One of the core components is the manual mechanical electro-hydraulic servo valve, and the dynamic and static motor control system performance of the electro-hydraulic servo valve will directly affect the normal operation performance of the entire system. The technology of electro-hydraulic servo motor control for manual mechanical systems as a new professional technical discipline was formed in the 1960s.
Compared with the performance of the electro-mechanical hydraulic servo motor system itself, it has many technical advantages such as small system volume, light weight, large driving force, torque and directional driving system power, easy direct driving to achieve relative linear and rotational motion in all directions, dynamic control of speed, displacement and directional driving torque, fast response speed, and high control accuracy, and also has the following main technical features respectively:
1) It also has high automation and static control accuracy and fast motor response speed;
2) It also has certain automation and anti-interference ability.
Currently, the development traces of electro-hydraulic servo control technology can be seen clearly in almost all engineering machinery and equipment, many of which have become the main methods of mechanical transmission and power control operations.
Electro-hydraulic servo control technology can be widely used in various fields such as environmental protection, medical treatment, metallurgy, textile, agriculture, forestry, fishery machinery, water conservancy and hydropower, engineering machinery, etc.
For example, large-scale electrical hydraulic automatic servo systems are required for large tunnel gates, various types of large winches, and ship lifts in the Three Gorges Project. New underground tunneling equipment with high power, self-braking ability, low vibration and noise, and low dispersed dust is also required for construction operations such as large-scale underground tunnel construction, drilling, blasting, and slag removal.
The rapid development of robotics for automatic walking engineering machinery also requires the use of electro-hydraulic servo automatic control technology systems.
Similarly, the automatic loading control system for press brakes also requires the use of a large number of new electro-hydraulic servo automatic control system technology. The rapid development of electro-hydraulic servo automatic control system technology will make it a complete engineering automation control technology with richer technical connotations.
Combined with modern computer control technology, it greatly improves control accuracy and work reliability and can realize complex intelligent control algorithms on computers.
III. Research on CNC System for Press Brake
The CNC press brake control axis generally consists of two or more axes, with the most important being the back gauge (X-axis) and the ram (Y-axis). The control precision of the X and Y axes should be above 0.1mm and 0.01mm, respectively.
Huazhong University of Science and Technology has been devoted to the development of CNC press brake systems since 1985. The earliest product used an Apple computer as the controller and a braking motor as the actuator, later replaced by an STD Bus industrial computer and a DC motor, and recently switched to an AC permanent magnet synchronous motor to achieve full digital control.
The X-axis is directly driven by a ball screw, while the Y-axis is driven by a belt pulley. To eliminate backlash, encoders are installed on the shaft of the back gauge, forming a closed-loop control system. The system has adjustment, calibration, and restart functions.
The input of processing parameters uses human-machine dialogue, with high-resolution CRT display, angle programming, and position programming, and logic control of switch quantities is completed through external PC. The system diagram is shown in Figure 1.
Currently, the development of CNC systems mainly reflects in the following aspects:
1. Multi-axis Control
In order to greatly improve the control precision and facilitate processing operations, the system has added movable axes such as wheels.
For example, in order to improve its induction to the product line, cam compensation control axes are added to the lower die and worktable.
In order to ensure accurate positioning of the material processing equipment, feeding control axes that can rotate up and down and left and right are added.
2. AC Servo Motor Replacing DC Servo Motor
The AC high-speed servo drive motor is a self-cooling, stepless automatic printing AC transmission mechanical structure, with various high reliability, high torque, high efficiency, and various power and high working accuracy output characteristics of a dynamic high-speed AC servo motor table.
It has low rotational inertia and acceleration torque when the speed is greater than one constant torque rotation, extremely low speed and low torque response time. Currently, the HD series AC synchronous servo motors and their driving devices have been produced in bulk.
The digital servo unit consists of an 8096 single-chip microcomputer control system and a power transistor voltage inverter power amplification and driving circuit.
It is mainly based on the theoretical concept of digital vector intelligent control system, it is a digitized intelligent vector control processing unit, which automatically receives the vector commands and control data sent by the upper computer through the system window to complete the motion of various axes.
Foreign press brakes have already realized hydraulic control, and now widely adopt electro-hydraulic proportional servo control technology, accurately controlling the synchronization, position, speed, and pressure of the two cylinders.
Compared with the hydraulic servo valve of the mechanical hydraulic system, electro-hydraulic servo proportional control technology is not only easy to adjust, with high control accuracy, easy to realize linkage between the two, but also has a simple mechanical structure, integrated hydraulic system, and simple connection with CNC system, thereby reducing the manufacturing, assembly, debugging, and maintenance workload of the press brake.
Manufacturers integrate electronic systems into their products in various ways to make these machines easier to operate, more efficient, and faster. Computer control systems can not only replace most tasks previously completed by operators but also directly connect with CAD and CAM software.
One of the highly developed areas in the bending machine industry is to use CNC systems and sensor feedback to better control stamping functions.
Today, digital encoders are generally installed on the ram to monitor its position and feedback the information to the computer-controlled hydraulic servo system.