ABSTRACT This paper introduces the indirect Uncoiler indirect tension control system, analyzes the principles of the tension control in this system, the roll diameter calculation and other major areas, also introduces the Siemens T400 Technology Board and the application in the tension control of the Uncoiler.
KEYWORDS Indirect tension control; Uncoiler; T400.
1 Introduction
In modern continuous strip steel processing lines, the strip steel must operate under stable tension. Constant tension ensures accurate strip alignment and guarantees the process requirements of the production line. Large tension fluctuations not only affect equipment operation but also lead to frequent strip breakage accidents, disrupting normal production. In actual control systems, PLC user-programmed control is typically used. However, due to process demands, increasingly higher requirements are being placed on tension control, and the traditional PLC control has limitations in terms of accuracy and stability. This paper addresses this issue by proposing a tension control scheme for the uncoiler based on the T400. Considering process requirements and economic costs, the uncoiler does not have tension detection equipment; therefore, an indirect tension control method is adopted.
2. Control Principles
Indirect tension control means that only a tension setpoint is used, without collecting the actual tension value using a tension detector. Tension control does not form a closed loop; instead, it employs uncoiler torque and current control, meaning the system is essentially operating only in the current loop. After the uncoiler unwinds the coil, the coil diameter continuously changes, causing tension variations.
During operation, the inlet pinch roll pulls the strip steel, and the uncoiler applies a torque opposite to the uncoiling direction, so that the strip steel between the pinch roll and the uncoiler has tension. The magnitude of the tension is proportional to the magnitude of the electromagnetic torque of the uncoiler. During the uncoiling process of the strip steel at a constant speed, as the diameter of the steel coil decreases, the torque generated by the motor also decreases. However, if constant tension is to be maintained in the inlet section, the motor torque and the strip steel tension torque must always remain equal.
The tension torque remains constant.
It can be seen that below the base speed, since it remains constant, constant tension control can be achieved as long as the uncoiler current changes proportionally with the roll diameter.
Above the base velocity, it is the stage of weak magnetic velocity generation, and the back electromotive force E remains unchanged.
n: angular velocity of the uncoiler;
D: Uncoiler roll diameter
At this point, as long as the armature current of the uncoiler changes in a direct proportion to the speed of the strip, constant tension control can be achieved.
3. Indirect tension control
3.1 Introduction to T400 Process Board
The Siemens T400 board is the process board for SIMOVERT main drive frequency converters. It can be extended to the system unit functions of related drive processes to complete complex open-loop and closed-loop control with high dynamic response, and the sampling time can reach 100.
The T400 technology modules can be freely configured via CFC, a tool that defines the functions of different modules, and the generated software is downloaded into the T400 program memory. Communication between the process board and the main drive is achieved through dual-port RAM, and can also communicate with the host computer via PROFIBUS DP and USS.
Its relevant parameters are shown in Table 1:
Table 1
The operating environment of the T400 is shown in Figure 1. It connects the communication module CBP and the basic electronic unit CU board. All three modules are installed in the main drive unit slot and interconnected via a local bus adapter (LBA). The CU board is the control board for the Siemens drive, and the CBP board is responsible for communication with PROFIBUS. Parameter transfer between the T400 process board and the main drive is achieved through a parallel dual-port RAM interface.
Figure 1
4.2 Control Principle of T400 Board
Figure 2 Control principle diagram
The T400 control mode adopts indirect tension control, as shown in Figure 2. This system has two control loops: a speed control loop and a current control loop. The control principle is as follows: When establishing tension, the signal from the host computer or operating console sets the tension T, which is the input of the tension potentiometer. From I=DT/2, it can be seen that the torque current is directly proportional to the product of tension T and winding diameter D. After compensation, DT becomes the output limit value of the speed regulator, which is the input value of the electromagnetic torque current regulator. With the tension setpoint unchanged, from the tension torque formula =DT/2, it is known that the tension torque is directly proportional to the winding diameter; the larger the winding diameter, the larger the torque, and the higher the current regulator input value, resulting in a larger motor current I. Correspondingly, in = , the torque is also larger. After unwinding, D continuously decreases. From T=I/D, it can be seen that, as I also decreases, the torque also decreases, thus maintaining a constant T.
(3) Calculation of T400 roll diameter
In indirect tension control, D directly affects the control of tension and is a very important calculation parameter. Real-time calculation of coil diameter is also an important process function of the T400 functional version. When the steel coil rotates once, the initial diameter begins to decrease by twice the average thickness of the strip. The diameter is obtained by subtracting this decrease from the initial diameter.
Where: : Calculated roll diameter; : Number of mandrel rotations in each measurement cycle; hs: Current strip thickness;
Due to external interference factors such as roller slippage and coil eccentricity, coil calculations require at least one additional measurement method to correct the calculated amount. A common approach is to use the encoder value on a guide roller near the uncoiler for synchronous correction.
When a tape breakage or other malfunction occurs during operation, the current roll diameter can be maintained. After production resumes, the calculation starts from the maintained current roll diameter, avoiding situations such as loose rolls. This is the roll diameter maintenance function of the T400.
(4) Tension control is achieved
In actual control, the total output torque of the uncoiling motor includes strip tension torque, bending torque compensation, inertia compensation, friction torque compensation, strip thickness compensation, etc.
Each compensation torque has a detailed calculation formula, which will not be repeated here. The functional block in the process plate calculates the tension torque based on the actual coil diameter of the strip and the tension set value, and adds additional tension compensation to finally obtain the tension torque set value. The torque set value from the tension controller is sent to the frequency converter. The frequency converter calculates the given current of the drum motor, completes the frequency conversion speed regulation control, and finally realizes the tension control.
4. Conclusion
Due to its fast processing speed and high precision, the T400 is ideally suited for high-speed, high-precision control applications. Compared to PLC controllers, the process board is directly mounted within the transmission equipment, allowing direct data exchange with the transmission device, significantly reducing data transmission time and response speed. In addition to its integrated software package for controlling special operating conditions, the T400 includes numerous customizable function blocks that can be selected to achieve the user's control objectives. Its integrated anomaly alarm and fault diagnosis functions also play a significant role in maintenance. The application of the T400 on the uncoiler of Wuhan Iron and Steel's No. 3 galvanizing line has demonstrated excellent performance, with more stable inlet tension control, meeting production requirements and improving work efficiency.
