Abstract: The three-axis electric drive swing table is a high-precision physical simulation test system integrating mechanics, electronics, instrumentation and computer. Its control is mainly achieved through three closed loops of position, speed and current. This paper focuses on the speed loop of the dual closed loop speed regulation system by introducing differential negative feedback and conducting Simulink simulation. The results show that it can significantly improve the system performance and thus improve the control accuracy of the entire system. Keywords: swing table; double closed loop; differential negative feedback [b][align=center]Modeling and Simulation of Double Closed Loop System for 3-axis electric sway test platform Li Guang-wei, Han Ru-cheng, Pan feng, Wang Yan-xia[/align][/b] Abstract: The simulation sway test system is a high-accurate physical simulation system, which contains machinery, electricity, instrumentation, and computer technology and achieves its purpose mainly by controlling the position loop, velocity loop, and current loop. By adding differential negative feedback to the velocity loop of the traditional double closed loop, the simulation results show that the system's performance is significantly improved, thus enhancing the accuracy of the entire swing table. Key words: swing table; double closed loop; differential negative feedback; 1. Introduction The three-axis electric drive swing table (also known as a turntable) is a high-precision physical simulation test system integrating machinery, electricity, instrumentation, and computer technology. It can not only simulate the motion attitude of ships in the longitudinal, transverse and bow three degrees of freedom under different sea conditions, and conduct technical performance test under simulated swaying conditions, but also provide the "true value" of the attitude angle of the reference plane of the swaying table. It can be used to analyze the static performance of the tested equipment under various attitude angle combinations and the dynamic performance under various swaying conditions, and can also be used to evaluate the technical performance of the tested equipment under swaying conditions [1]. The three-axis electric drive swaying table is mainly composed of mechanical system, control system, attitude angle measurement and computer data acquisition and processing system and auxiliary systems such as safety protection. 2. Principle of speed and current double closed-loop speed regulation system The position follow-up system of the three-axis electric drive swaying table is composed of speed, current and position three closed-loop control. Among them, in order to obtain position tracking, the current and speed double closed-loop system must be designed first. 2.1 Composition of speed and current double closed-loop DC speed regulation system The speed and current double closed-loop control DC speed regulation system is the DC speed regulation system with good performance and the most widely used. In order to achieve the separate action of speed and current negative feedback, two regulators can be set in the system to regulate speed and current respectively, that is, speed negative feedback and current negative feedback are introduced respectively. The two are nested (or cascaded) connected. As shown in Figure 1. Among them, ASR is the speed regulator, ACR is the current regulator, TG is the tachogenerator, TA is the current transformer, UPE is the power electronic converter, Un* is the speed set voltage, Un is the speed feedback voltage, Ui* is the current set voltage, and Ui is the current feedback voltage. The output of the speed regulator is used as the input of the current regulator, and the output of the current regulator is used to control the power electronic converter UPE. From the perspective of the closed-loop structure, the current loop is inside, called the inner loop; the speed loop is outside, called the outer loop. This forms a speed and current dual closed-loop speed regulation system [2]. [align=center]Figure 1 Structure of a Dual-Loop DC Speed ​​Control System Based on Speed ​​and Current[/align] 2.2 Engineering Design of a Traditional Dual-Loop DC Speed ​​Control System Based on Speed ​​and Current A dual-loop speed control system based on speed and current is a type of dual-loop system. The general method for designing a dual-loop control system is to start from the inner loop and gradually expand outwards, designing one loop at a time. Therefore, for a dual-loop speed control system, one should start with the current loop, first determining the structure and parameters of the current regulator, then treating the entire current loop as a component within the speed loop, and using it together with other components as the control object of the speed loop, before determining the structure and parameters of the speed regulator. Based on the structure diagram of the dual-loop DC speed control system based on speed and current, and following the engineering design method for designing a dual-loop DC speed control system based on speed and current, the dynamic structure block diagram of the dual-loop DC speed control system based on speed and current can be obtained as shown in Figure 2. In the block diagram, filtering components (including current filtering, speed filtering, and two given filtering components) are designed. Since the feedback signal from the current detection unit often contains AC components, a low-pass filter is required. Toi is the current feedback filter time constant, and its size is selected as needed. The filter can filter out the AC components in the current feedback signal, but at the same time, it delays the feedback signal. In order to balance this delay, an inertial element with the same time constant is added to the given signal channel, which is called the "given signal filter" element. Its significance is: to make the given signal and the feedback signal undergo the same delay, so that the two are properly matched in time. The speed feedback signal obtained from the tachogenerator contains the commutation ripple of the motor, which also needs to be filtered. The speed feedback filter time constant Ton is also determined according to the specific situation. A filter element with a time constant of Ton is also introduced in the speed given channel [3]. [align=center] Figure 2 Dynamic structure block diagram of speed and current dual closed-loop DC speed regulation system[/align] 2.3 Improved dual closed-loop speed regulation system According to the automatic control system design theory, the dual closed-loop speed regulation system using PI regulator has good steady-state and dynamic performance, simple structure, reliable operation, and convenient design. However, its dynamic performance is lacking in speed overshoot and poor anti-disturbance performance. Therefore, speed differential negative feedback is introduced into the traditional speed converter, which can suppress speed overshoot until it is eliminated, and at the same time greatly reduce the dynamic speed drop. Its dynamic structure block diagram is shown in Figure 3. Among them, Todn is the speed differential filter time constant. [align=center] Figure 3 Dynamic structure block diagram of improved speed and current dual closed loop DC speed regulation system[/align] 3. Modeling of speed, current and position dual closed loop speed regulation system based on Simulink 3.1 System simulation model Since the permanent magnet DC torque motor uses special magnetic materials, it has a larger torque coefficient under the same rotor outer diameter and armature current, so the torque generated is also larger, which significantly improves the acceleration performance and response characteristics of the motor, and can output a larger torque at low speed [4]. For this reason, we adopted a split-mounted torque motor (that is, the motor rotor is directly mounted on the shaft of the platform), which eliminates the reduction gear and eliminates the backlash effect, thus improving the rigidity of the whole system, the stability of the low-speed design and the tracking accuracy. The motor parameters are as follows: peak stall voltage 200V, peak stall current 13A, maximum no-load speed 30r/min, armature resistance 15.2Ω, continuous stall torque ≥300N·m, continuous stall voltage 92.3V, continuous stall current 6A, rotor moment of inertia 4.5㎏·m2, armature inductance 170mH, torque fluctuation coefficient ≤1%. According to the engineering design method of the speed and current dual closed-loop DC speed regulation system, based on the motor parameters mentioned above, the Simulink simulation model of the dual closed-loop speed regulation system was established using MATLAB software [5], as shown in Figure 4, its unit step response curve is shown in Figure 5, and its unit step load disturbance curve is shown in Figure 6. [align=center] Figure 4 Simulation model of the dual-closed-loop speed control system[/align] [align=center] Figure 5 Unit step response curve of the dual-closed-loop speed control system[/align] [align=center] Figure 6 Unit step load disturbance response curve of the dual-closed-loop speed control system[/align] 3.2 Improved system simulation model As can be seen from Figures 5 and 6, the overshoot of the speed control response is relatively large, and the system's resistance to load disturbances is relatively weak. To reduce its overshoot, we designed a differential negative feedback for its speed loop, as shown in Figure 7. Its unit step response curve is shown in Figure 8, and its unit step load disturbance curve is shown in Figure 9. [align=center] Figure 7 Simulation model of the speed differential negative feedback dual-closed-loop speed control system Figure 8 Unit step response curve of the speed differential negative feedback dual-closed-loop speed control system Figure 9 Unit step load disturbance response curve of the speed differential negative feedback dual-closed-loop speed control system[/align] 3.3 Simulation result analysis From the comparison of Figures 5 and 6 and Figures 8 and 9, it can be seen that after adopting speed differential negative feedback, the overshoot of the system is significantly reduced, and the effect of speed differential negative feedback is fully demonstrated. Its load disturbance is much smaller than the dynamic drop of the traditional dual-closed-loop system, thus improving the control accuracy of the turntable system. If the differential negative feedback gain is further increased, it is possible to achieve virtually no overshoot, but the maximum dynamic drop time and recovery time of the system will be longer. 4. Conclusion The three-degree-of-freedom turntable control system is an advanced control system based on position, speed, and current three-closed-loop control. The design of the speed and current dual-closed-loop is a prerequisite for the design of the position loop. By adding speed differential negative feedback to the speed loop, the dynamic and static performance of the system can be significantly improved, the steady-state accuracy of the system can be enhanced, and the ideal control effect can be achieved. References: [1] Zhou Li, Zhang Lin. Control system of swaying shaft test bench [J]. Engineering Machinery, 2003, (05): 29-31. [2] Chen Boshi, Ruan Yi, Chen Weijun, et al. 3rd edition. Automatic control system of electric drive - motion control system [M]. Beijing: Machinery Industry Press, 2003. [3] Yang Geng, Luo Yingli, et al. Motor and motion control system [M]. Beijing: Tsinghua University Press, 2006. [4] Cong Shuang, Li Zexiang. Practical motion control technology [M]. Beijing: Electronic Industry Press, 2006. [5] Huang Zhonglin, Zhou Xiangming. MATLAB calculation and simulation training of control system [M]. Beijing: National Defense Industry Press, 2006.