**I. Overview** In configuring an automation system, ensuring its advanced nature, reliability, and rationality while avoiding redundant functions and reducing investment costs is the primary consideration during system upgrades. Northeast Light Alloy Co., Ltd., facing the problem of high electrical system failure rates and poor reliability in its 1560mm bending and straightening machine train, while demanding extremely high product quality and dimensional accuracy, decided to upgrade the original thyristor control system with a new digital system. The new system utilizes a SIEMENS programmable controller SIEMATICs7-300, a 1:3 window control center WINCC, an ABB all-digital speed control system DCS500, and frequency converters ACS60O. Based on PROFIBUS bus technology, communication with ABB's DCS500 and ACS60O is achieved. The SIEMATICs7 PLC serves as the main control device, and the host computer manages production and displays the process. After the upgrade, the system is successfully running, with significantly improved reliability, avoiding serious production bottlenecks such as sudden belt breakage and overspeeding. II. Original Control System Configuration The tension straightening machine train is shown in Figure 1, consisting of four DC motors: one for uncoiling, one for trimming, one for straightening, and one for winding. The original thyristor control system was a three-phase bridge circuit with an analog regulator, set and interlocked by a SYSTEMV industrial computer and PLC. Using plug-in electronic circuit boards and discrete thyristor components, the core of the equipment was a tension straightening device composed of two S-rollers and three straightening work rolls. The inlet and outlet tension rolls were driven by a DC motor, while a small-power AC motor drove a planetary gear system to adjust the speed difference between the two tension rolls. Two photoelectric encoders measured the speed of each tension roll, and the computer system calculated the elongation based on this, comparing it with the set value. If an error existed, the planetary gear system was adjusted via the AC motor, thus forming a closed-loop control. The position of the straightening work rolls could be arbitrarily set to achieve different straightening effects. The winding and uncoiling tensions could be set according to processing requirements. Under tension, the strip was stretched and straightened by the three work rolls. **III. Modification of the Application Bus System** The original system power distribution is basically maintained, but the disc shear drive is changed to dual-side motor AC speed regulation. Other DC motors and their power remain unchanged. The basic part of the control system mainly consists of a SIEMENS PIEMASTER 7-300, a remote station ET2Z0M, an operator workstation WINCC, a touch screen TP, a D (S50o) fully digital DC speed regulation system, and an ACS600 frequency converter. The fully digital DC speed regulation system and frequency converter are connected to the PIESIIMATIC 7 via the PROFIBUS-DP protocol. Processing parameters and process automation are set through the HOST computer, and the speed and tension current of the constant tension bending straightener are allocated by the PIE system. The system structure is shown in Figure 2. The main station is a PIEMASTER 7-300 with a CPU of S7315-2DP. The main control system contains programs that perform real-time control and logic control of the system, completing elongation control and work roll position adjustment. One remote station ET2Z0M is used as the input of the operation signal, and the other is used as the input and output of the logic signal. After the main control system starts, it first reads the hard hotspots, then calls the application program, outputs the results, and returns to the program starting point when it encounters END, running in a loop. The elongation control and work roll position adjustment still use the original control process, but the speed and position measurement signals are directly input to the S7-300, and the given data and control results are set and displayed through the HOST computer, improving accuracy and eliminating runaway phenomena. The disc shear machine was changed to a dual-motor drive based on mechanical structure considerations. The key to dual-motor, dual-feed power is equal rotational speed. A PROFIBUS bus is used, with the two frequency converters connected and communicating through the same node, and the slave motors follow the given motor speed for synchronization. The HOST computer uses a P4 CPU, Windows NT operating system, and a CP5611 network card for bus connection. Siemens WINCC monitoring software is used for configuration, enabling graphical and screen display and alarm functions. The main functions include setting production process parameters, monitoring process data such as current, tension, roll diameter, machine speed, elongation, and work roll status and position, displaying and storing various curves, mainly including tension curves, speed curves, and elongation curves, and generating production data statistics reports, storage, and printing. System maintenance is performed by professionals using the interface to adjust and modify system data. IV. Advantages of the Modified Control System The open fieldbus standard PROFIBUS-DP is characterized by high efficiency, low cost, and plug-and-play functionality, and is commonly used in factory automation. PROFIBUS uses a bus topology with termination at both ends, ensuring that connecting and disconnecting one or more stations during operation does not affect other stations. The physical layer is PS485 for PROFIBUS-DP, using asynchronous NRZ transmission encoding, a baud rate of 12 Mbit/s, and shielded twisted-pair cable. PROFIBUS-DP is suitable for replacing expensive cables between HE/PC and I/O. It is fast, requiring <2ms to transmit 1KB of I/O data. Powerful tools reduce configuration and maintenance costs; it is supported by all major PLC manufacturers, with a wide range of products available, allowing for both periodic and non-periodic data transmission. This modification uses shielded twisted-pair cable PROFIBUS-DP, with 10 node addresses defining the connected devices, replacing a large number of original cables and wires, making installation simple, easy, and reliable. Accordingly, speed control devices and components from Siemens and ABB, compatible with PROFIBUS-DP, were selected to facilitate configuration and reduce programming workload. The DCS500 fully digital DC speed control system is versatile; when used in specific speed control schemes, system parameters and control methods can be set as required. Torque setting can be selected in three modes: speed, tension-unwinding, and tension-winding. In this modification, the drives of the straightener, unwinder, and winding machine were set according to these three modes, and the structural principle is shown in Figure 3. Due to the formulaic nature of the fully digital speed control system, parameter setting has shifted from adjusting the capacitors and potentiometers of the analog system to digital input and selection of status codes, resulting in extremely high debugging efficiency, comprehensive protection measures, and good installation and operational reliability. The failure rate of the modified tension-bending straightener train control system has been significantly reduced, and all electrical and process indicators have met the design requirements, fully utilizing the equipment's performance and realizing the networking of the control system. The retrofit of the thyristor analog speed control system chose a fully digital control system based on fieldbus technology, not only because of the application of the latest technology, but also because of its good practicality and scalability for networked management of the production process. [ Click to download: Retrofitting a Thyristor Control System Based on Fieldbus Technology Editor: Chen Dong]