Abstract: This paper analyzes the design concept and control principle of the AC servo control system for cold bending forming lines of shelving, based on the technical characteristics and product process requirements, combined with the practical application of Mitsubishi series servo drives and servo motors, to solve the problem of fixed-length control. The actual application achieves a control accuracy of approximately ±0.1mm and avoids cumulative errors, achieving the goal and requirements of high-precision position control. Keywords: AC servo system, fixed-length control, position control 1. Introduction 1.1 Introducing online pre-punching and online hydraulic stop-shear technology into the cold bending forming production line of shelving uprights expands the design range and manufacturing accuracy of the cross-sectional shape of the shelving uprights, meets the requirements of the design and assembly of the shelving steel structure system, and greatly optimizes the composition mechanism of the shelving steel structure. Especially with the rise and widespread development of automated warehouse (AS/RS) systems in China, higher requirements have been placed on the hole position accuracy and length control of shelving uprights. 1.2 This article analyzes and discusses the online pre-punching and online hydraulic stop-shear control devices implemented using AC servo control principles in imported cold bending forming production lines for shelving uprights. The aim is to achieve high positional accuracy control in many cold bending applications, and to share this with colleagues. 2. Working Principle of Shelving Cold Bending Forming Line 2.1 Basic Production Process and Equipment Composition of Shelving Cold Bending Forming Line: 2.1.1 The general production process of shelving components is: Uncoiling—Leveling—Online servo feeding punching—Forming rolling—Straightening—Fixed-length cutting (or servo tracking cutting)—Packaging—Post-coating treatment, etc.; 2.1.2 The corresponding equipment is: Uncoiling machine—Leveling machine—Servo feeding device—Press machine—Cold bending rolling mill (approximately twenty stations)—Straightening head—Hydraulic cutting device and hydraulic station—Packaging machine or other auxiliary equipment + electrical control system, etc. 2.2 Basic Principle of AC Servo Control System of Shelving Cold Bending Forming Line: As shown in Figure 1. (System Composition and Principle) 2.3 This system consists of five parts: a computer (PC), a servo drive control card, an AC servo speed control system (AC servo controller, servo motor, and corresponding control cables), sensor detection and feedback, and an auxiliary active motion execution system. The main control program is only a few hundred kilobytes long and runs under the DOS operating system. The main control microcomputer is connected to the servo drive control card through the printer port LP1 and sends position or speed commands via data lines. It performs online adaptive adjustment or sets PID adjustment parameters (the program is designed to be open for easy adjustment) as shown in the attached diagram. It also performs digital-to-analog (D/A) conversion and outputs ±10V analog signals through the corresponding control board. After being amplified by the AC servo amplifier, the signals drive the servo motor. The semi-closed-loop or closed-loop position control feedback system uses signals (A, B, IN pulses) provided by the incremental photoelectric encoder mounted on the motor shaft (or by setting a measuring roller and an incremental photoelectric encoder conversion device on the object being driven) to complete the position feedback of the position servo system. The sensing elements in the position feedback loop... - Incremental photoelectric encoders transmit the real-time displacement (or rotation angle) changes of moving components to the field control station (PC) via a long-distance transmission in the form of A and B phase differential pulses for encoder pulse counting to obtain digital position information. The main control microcomputer calculates the deviation between the given position and the actual position (i.e., the feedback position), and adopts a corresponding PID control strategy based on the deviation range. The digital control action is converted into analog control voltage through digital-to-analog conversion and output to the servo amplifier, ultimately adjusting the motor movement to complete the repeated closed-loop feedback positioning control of the desired value. In terms of control principle, this achieves high-precision positioning with small errors. Then, the main control program issues running commands to the auxiliary main action execution system to complete specific mechanical brake actions, online stamping movements of the press, hydraulic stop shear cutting movements, etc. 2.4 Main features of this unit: High initial investment cost, limited maximum AC servo power, but low subsequent operating cost, especially high first-time yield of rack components, high product precision, wide application range and high added value. 3. Analysis and working principle of automatic feeder and punching device 3.1 The automatic feeding device for the online pre-punching process of the cold-bending forming production line for shelf uprights consists of a pair of upper and lower φ75 guide rollers. Its main power comes from an AC servo motor, and it feeds the material using the friction between the sheet and the upper and lower guide rollers. The strip steel distribution holes for the shelf uprights are punched on a press. The main design is shown in Figure 2. This device was originally designed with a 3.7KW servo control system from BALDOR (USA). However, due to new product development, the working transmission load was increased. Based on the working principle shown in Figure 2, the position control between the power control section and the AC servo control is mainly achieved through ±10V analog... The signal is used to achieve this, and there is no power limitation on the AC servo system. In principle, it can be replaced with a 5KW AC servo controller and AC servo motor from the Mitsubishi MR-J2S series servo amplifier. Based on the production accuracy requirements of the corresponding shelf components and the determination of the servo control accuracy as ±0.1, the ratio of the circumference of the measuring roller to the measurement accuracy range is approximately 1178 (without considering the frequency division relationship of the rotary encoder). It is advisable to select a rotary encoder of 1200PPR or higher, and it can well achieve the position accuracy control requirements in the nearly four years of application. 3.2 The Mitsubishi MR-J2 servo system has the characteristics of good machine responsiveness, low-speed stability, and optimal state adjustment including the mechanical system; the speed frequency response reaches 550HZ or higher, which is very suitable for high-speed positioning applications. It is more suitable for equipment with a high load inertia ratio and poor toughness (this requirement is more important for our company's chain drive units). 3.3 The automatic feeding device mainly consists of the structure shown in Figure 3. (1) Photoelectric sensor 1# mainly provides feedback on the status of the steel strip entering the working area of ​​the press, such as: excess material, insufficient material, etc. (2) The servo motor transmits the conveying power to the downward guide roller through the gearbox. The transmission ratio i of the gearbox (such as i=11 in the design selection) and the speed of the motor determine the feeding and positioning speed of the system. (3) The rotary encoder measures the position signal transmitted by the upper guide roller (passive feeding and passive measurement point) through the movement between it and the sheet metal. (4) The mechanical brake realizes the position fixation after positioning. (5) Photoelectric sensor 2# realizes the transmission of position signals required by the working control of the press. (6) The upper and lower dies realize the online punching of holes. It requires the press tonnage to match, and the machine tool or die precision to match, etc. 3.4 The specific feed step distance for each die is determined by the corresponding counting pulse count or length conversion value comparison set on the PC, and coordinated with the passive measurement feedback of the angle encoder connected to the upper guide roller (set by the program). This achieves adjustable, high-precision, and error-free step-distance feeding stamping of the stamping sheet. The accumulated error is handled by the error compensation algorithm set in the program or by manual online correction, ensuring high-quality hole spacing of the rack uprights. Practice has proven its practicality. 3.5 The automatic feeding device in the equipment system overcomes the shortcomings of manual feeding of the pre-opened steel strip of the rack uprights. It features simple operation, reliable operation, and high control precision, which can greatly improve labor productivity. When combined with a high-speed, high-precision press, it can achieve a working frequency of 70 times/minute and a working pressure of over 2500KN. It can form an independent operating system. 4. Analysis and Working Principle of Shelf Cutting Device 4.1 The basic control principle is the same and uses a unified system. Its characteristics are: The number of holes on the shelf uprights is measured by a reflective photoelectric switch. This primarily determines the presence or absence of holes by generating a specific hole count signal. When a certain number of holes is reached, the internal main control program converts the hole counting mode to a length measurement mode. Similarly, it completes the position feedback and positioning control of the position servo system. After the main control microcomputer calculates the deviation between the given position and the actual position (i.e., the feedback position), it adjusts the AC servo motor movement online in a timely manner. After achieving the desired positioning, the main motion stops and the hydraulic cutting device controls the solenoid valve to generate the cutting sequence. (Note: The main power of the cold bending forming unit is still provided by the movement of this AC servo motor; therefore, a high-power AC servo control system and drive system should be selected. The system power selected for this equipment reaches 37KW or more.) 4.2 The main differences between hydraulic stop-shear control mode and flying shear control mode are: ① Hydraulic stop-shear has high control precision, with a maximum precision of approximately ±0.1mm and no cumulative error. This is mainly reflected in the precision and timing requirements of the passive incremental photoelectric encoder, resulting in a higher initial investment. However, it offers higher first-pass yield and material utilization. Flying shear control requires additional follow-up and reset devices, making the control system simpler. ② In terms of control principle, hydraulic stop-shear offers absolute control precision, eliminating speed difference errors. Flying shear offers relative control precision, representing the relative error between the shearing position and the workpiece movement. Uncertainties in speed operation patterns or fluctuations in machine resistance and workload can cause irregular errors. ③ The main motion speed of flying shear control is relatively constant, facilitating the setting and adjustment of operating parameters for supporting welding equipment. In contrast, the main motion curve of hydraulic stop-shear control is more complex, involving high and low speed transitions and motion stop states, sometimes resulting in longer calibration times. ④ Production efficiency differs significantly. Flying shears have higher production efficiency and are easier to control. ⑤ The requirements for equipment maintenance and operation control differ considerably. ⑥ The hydraulic stop-shear mode is more conducive to solving cutting defects such as cutting deformation and rebound of cold-bent profiles. In summary, it is necessary to formulate and select a reasonable equipment control operation mode according to the characteristics of the cold-bent products to obtain the maximum benefit. 5. Several main issues in control system design 5.1 Control accuracy of input signal: The ratio of the circumference of the measuring roller to the measurement accuracy range ultimately determines the production control accuracy of the product. Products with a larger ratio should be selected as much as possible, and appropriate measuring roller materials, contact damping and elastic coefficient between the measuring roller and the cold-bent parts should be selected to increase the friction coefficient and contact pressure to prevent the generation of material slippage error during the measurement process. 5.2 Control accuracy of output signal: Different PID control algorithms in the position loop determine the control accuracy and results obtained by PID control. For example, the solution method is the step response method, and three action characteristics are adopted according to the control characteristics: 1) only proportional control; 2) PI control; 3) PID control; and PID calculation is performed according to the velocity shape and the differential fractal operation formula of the measured value to execute the forward and reverse action calculation and control under the corresponding accuracy requirements. 5.3 PID System Parameter Tuning: The main control microcomputer sends PID parameters to the control card to check if the given parameters meet the requirements of the control system. This process needs to be implemented using online parameter tuning. The main task of parameter tuning is to determine K(P), A(I), B(D), and the sampling period Timer. Increasing the proportional coefficient K(P) makes the servo drive system more sensitive and responsive, but too large a value will cause oscillation and increase the settling time. Increasing the integral coefficient A(I) can eliminate the steady-state error of the system, but the stability decreases. The derivative control B(D) can improve the dynamic characteristics, reduce the overshoot, and shorten the settling time. The specific tuning process requires improving the control algorithm of the PID controller in the digital position loop and formulating the field-adaptive parameters and actual field adjustment settings based on the PID controller. Tuning should be done separately according to different products or load conditions; otherwise, oscillations in the position control process are likely to occur. An open adjustment is set in the design program as shown in the figure. 5.4 Within a certain error range, the mechanical precision control of the system can improve the electrical control precision (encoder pulses). Combined with a high-performance AC servo drive system, it can achieve high-precision position control requirements in many situations, and also improve the efficiency and accuracy of position positioning. 5.5 The main program is an AC servo control system based on a PC development platform. Its main functions include: human-machine interface adjustment of product production data, equipment parameter setting, and PID parameter tuning; data transmission and processing between the PC and various modules; position loop PID control algorithm; control of servo motor movement; and realization of actions of various related equipment. Other features, such as setting and adjusting the stamping step distance, adjusting the corresponding number of output pulses for a certain length value, setting and adjusting the press control precision, servo feeding precision, and servo feeding length value, are all open-ended designs. 5.6 The main program design incorporates fault warning segments for some equipment, greatly improving the operability of the equipment and the control of product production quality, and also reducing equipment fault inspection time to a certain extent. 6. Conclusion 6.1 Practical application shows that selecting a suitable AC servo system can meet the requirements of fast response speed, high speed accuracy, and strong robustness of the control system. The highest position control accuracy in practical applications is around ±0.1mm, and cumulative errors can be avoided. This control system can be applied to the production of high-precision open-type cold-formed steel products, especially products similar to shelf uprights, i.e., cold-formed steel uprights with high-precision hole position requirements on the online pre-punching production line. 6.2 The AC servo system applied to the shelf cold-formed production line can indeed achieve high position control accuracy; and the online pre-punching mode and hydraulic stop-shear mode can be used independently, such as the shelf beam production process which does not have an online pre-punching mode. References: 1. Guo Qingding, Wang Chengyuan. "AC Servo System". 2. Luo Hui, Yin Quan, et al. "Design of PC-based AC Drive Control Algorithm Development Platform". 3. Dong Qian, Xie Jianying. "PID Regulator Design in Motion Control System". 4. Wang Xianjin. "Cold-formed Steel Production and Application". 5. Zhang Weiguo. "A Brief Analysis of Cold-Formed Rolling Process of Shelf Components" (Welded Pipe). Author Biography: Zhang Weiguo, male, born in 1968, graduated from Jiangsu University with a bachelor's degree in Equipment Engineering and Management, holds the title of engineer, and is mainly engaged in equipment management and maintenance, product development, etc. in enterprises.