Achieving digitization and automation in sheet metal processing is necessary for the transformation and upgrading of sheet metal industry, and is also an important sign of the advancement of CNC press brake technology.
Through independently developed sheet metal robots, CNC press brake loading and unloading and unmanned operation can be achieved, and seamless integration and connection between robots and CNC press brakes can be carried out, providing basic technical support for digital workshops and intelligent manufacturing in the sheet metal processing industry.
1. Brief Introduction to Hydraulic Servo System
There are two commonly used hydraulic servo systems: servo valve control system and servo direct-drive pump control system. The servo valve control system controls the flow of the hydraulic cylinder by using a servo valve, which has the advantages of fast response speed and good dynamic characteristics.
However, it has disadvantages such as high requirements for hydraulic oil quality, high temperature rise, low efficiency, and large power loss. The servo direct-drive pump control system changes the output flow of the pump by changing the speed of the servo motor, thereby changing the movement of the hydraulic cylinder and achieving speed control.
The servo direct-drive pump control system has the advantages of compact structure, fewer components, fewer failure points, low requirements for oil quality, small system power loss, high efficiency, wide speed control range, and easy achievement of high-precision control.
Therefore, it is an area where traditional valve-controlled press brake systems cannot breakthrough.
Factors such as oil temperature, oil volume elasticity modulus, viscosity, and system pressure all affect the performance of the hydraulic servo direct-drive system.
In order to achieve reliable and accurate control of the hydraulic servo direct-drive pump control system, a mathematical model of the system must be established.
In system identification modeling, the commonly used dynamic characteristic measurement methods include time-domain step response determination and frequency-domain frequency determination method.
The former only needs to add a step input signal to the measured object and measure the response curve of the output at the output end, from which the transfer function of the object under study can be obtained through analysis.
This measurement method has simple equipment and low workload, but its test accuracy is not high.
The frequency-domain determination method applies the same amplitude but different frequency sine signals to the object under study, measures the amplitude ratio between the input and output signals, and obtains the amplitude-frequency characteristics of the tested system.
2. Electrical Control System
The electrical control system uses the ESMotion N35 motion controller and Omron CP1H-XA40DR PLC connected via the MODBUS communication protocol, supplemented with high-resolution displacement sensors, high-response pressure sensors, and an upper computer HMI.
The ESMotion controller is used as a closed-loop control sub-module to perform the task of high-precision position control during the work process. The control system diagram is shown in Figure 2.
When the oil press is in the process that does not require precise positioning control such as fast descent, pressure release, and return, the closed-loop position function is turned off.
The motion controller accepts speed and pressure commands from the PLC through Modbus communication and only serves as an intermediate transition to control the servo driver.
During the working stage, high-precision closed-loop control is initiated, and the motion controller directly takes the signals from the displacement sensor and pressure sensor for closed-loop operation, outputs the calculated speed command, and controls the servo motor’s speed to achieve high-precision positioning control.
To avoid the problem of position deviation caused by the delay of cutting off the electromagnetic valve when the position control is in place, the control of the pressurized solenoid valve is specially treated, and solid-state relays are used to control the on and off of the solenoid valve.
After the motion controller judges the position-in-place signal, it directly controls the action of the solid-state relay.
In addition, the motion controller is externally connected with a safety check loop to detect the communication status with the PLC in real-time.
Once a communication or system error is detected, the safety alarm mechanism is triggered to ensure the safety of the machine and personnel.
2.1 Motion Controller
The ESMotion series products are multi-functional motion controllers designed for complex motion control and CNC machine tool applications. The system adopts modular design and has rich external interface, including analog, serial port, Ethernet, encoder, etc.
It supports the VTB integrated development environment, embedded with a large number of algorithms and function functions; supports various interpolation methods, multi-axis motion control, and supports Modbus protocol, CANOpen protocol, EtherCAT protocol, etc.
The NG35 series is the highest-end controller for multi-axis control, which uses a 150MHz, 32-bit microcontroller to achieve fast and accurate interpolation control of the axis. It can be expanded with up to 8 I/O modules, and supports Ethernet program download and debugging.
Especially, it can draw the expected motion curve, capture the actual motion curve, compare the two and adjust the corresponding parameters to make the actual sliding block trajectory approach the ideal trajectory. This function greatly simplifies the adjustment and diagnosis process of the entire motion system.
2.2 Communication Protocol
Modbus protocol is a universal language used on electronic controllers, and has become a common industrial standard. Through this protocol, communication between controllers, networks, and other devices can be realized.
There are two transmission modes in the Modbus protocol: ASCII and RTU. Since the main advantage of the RTU mode is that its character density is higher than the ASCII mode under the same baud rate, the RTU transmission mode is used to connect the motion controller and PLC in this electrical control system.
All devices on the Modbus bus should have the same communication mode and serial communication parameters.
First, the serial port settings need to be set on the PLC side, mainly involving parameters such as baud rate, format, mode, and the number of received bytes. The corresponding parameters of the ESMotin motion controller also need to be set accordingly to complete the communication settings of the hardware part.
Since the Omron PLC stores data in words as the smallest unit, while the controller communicates in bytes, attention should be paid to the corresponding conversion between words and bytes during data transmission and verification.
3. Applications Related to Servo Pump Press Brake
The digitization and automation of sheet metal processing are necessary for the transformation and upgrading of China’s sheet metal industry, and it is also an important sign of the improvement of CNC press brake technology.
By independently developing sheet metal robots, the loading and unloading of CNC press brakes can be automated, and seamless system integration between robots and CNC press brakes can be achieved, providing basic technical support for realizing digital workshops and intelligent manufacturing in the sheet metal processing industry.
The bending unit based on robot operation has been widely used in industries such as high and low voltage switch cabinets, elevator door panels, and anti-theft door manufacturing.
Based on robot technology, the plate bending unit integrates CNC press brakes, robots, material transportation, material sorting, and material storage mechanisms to achieve fully automatic production from material feeding, bending to stacking.
The bending unit can be equipped with universal robots from companies such as Kuka, ABB, Yaskawa, Estun, etc., and sheet metal robots from companies such as Yangzhou Hengjia and Tianshui Forging.
Various robot companies have developed process packages and software packages suitable for robot bending to meet the linkage between press brakes and robots.
Estun has developed Estun Smart Robot Bending, a dedicated offline programming software for bending robots, which can automatically generate robot bending programs by importing the bending workpiece drawings into the software, eliminating the need for manual teaching and solving the problem of bending follow-up. It shortens the programming time and adapts to mass production.
The bending software package supports two communication methods: I/O and Ethernet communication with the press brake system, which can read press brake signals, mold parameters, bending speed, and directly use the teaching box to call the press brake instructions and control the press brake to start.
4. Conclusion
In conclusion, this precision finishing machine is designed and developed to reduce the amount of post-grinding processing for powder metallurgy parts.
The entire control system breaks through the conventional method and adopts servo pump control instead of servo valve throttling control, which not only avoids the rapid rise of oil temperature caused by a large amount of overflow but also simplifies the pipeline connection and reduces the risk of joint oil leakage.
The master-slave module control method is used in the electrical control system, fully utilizing the characteristics of the motion controller with fast computing power, high processing speed, and strong PLC logic control.
The motion controller and PLC are effectively combined through the Modbus communication protocol, closely linked to each other and play their respective advantages to achieve high-precision positioning control of this machine tool.
The repeatability precision can reach ±0.01mm, and the running efficiency is 6 pieces/minute, which meets the customer’s requirements.
