Abstract: Flight simulation testing is an essential step in the development of modern flight control systems. The three-axis simulation turntable is the most critical device in a hardware-in-the-loop flight simulation system, used to realistically reproduce the angular motion of an aircraft during flight. Currently, existing three-axis simulation turntables do not meet the specific requirements of new UAV simulation. This paper introduces the basic composition, overall design, and key technology research of a three-axis simulation turntable control system suitable for UAV simulation experiments. Keywords: ISA bus, three-axis turntable, computer control, control system 1. Introduction In the development and use of modern aircraft flight control systems, flight simulation testing is an essential step. The attitude simulation turntable is the most critical device in a hardware-in-the-loop flight simulation system, used to realistically reproduce the angular motion of an aircraft during flight. Due to the rapid development of UAVs, existing three-axis simulation turntables cannot fully meet the needs of attitude simulation for new UAVs. To meet practical needs, we designed a new UAV attitude simulation turntable. Based on this platform, pure mathematical simulation and hardware-in-the-loop flight simulation can be achieved at a high level. 2. System Overall Design The three-axis simulation turntable is an electromechanical bearing omnidirectional rotation turntable. It is required to fully meet the requirements of simulating the aerial attitude of the ×× type UAV in the laboratory, and to detect the performance indicators of the flight control computer of this type of UAV. 2.1 System Overall Requirements (1) Speed: For real-time attitude simulation, all state variables of the system must be calculated once per unit time. For this reason, the system model must be reasonably optimized and solutions must be sought in the simulation algorithm. (2) Real-time: In order to understand the performance of the UAV, the state equation of the UAV must be solved in less than the sampling time using a real-time algorithm. (3) Synchronization: In order for the three-axis simulation turntable to achieve speed and real-time performance, the data transmission between the various modules of the entire system must be kept synchronous or have a small delay. The delay time must be much smaller than the sampling time of the system and the unit time for solving the state equation. (4) Anti-interference: In order for the system to work normally under various conditions, the system must have good anti-interference. (5) Interactivity: In order for the system to work better, the system must have a good interactive interface to meet the user's requirements when designing the entire system. 2.2 Overall System Structure The entire three-axis simulation turntable system can be broadly divided into two parts: the turntable body and the control system. The turntable body is designed with different structures depending on the specific requirements of the UAV. The turntable body consists of three parts: a pitch loop (outer ring), an azimuth loop (middle ring), and a roll loop (inner ring). Precision machining and assembly ensure the accuracy, perpendicularity, and intersection of the turntable's mechanical frame and axis system. All three axes are driven by stepper motors to control the turntable's position, speed, and acceleration. Simultaneously, the turntable body design aims to achieve good flexibility and decoupling through reasonable structural design, which will not be detailed here. The control system is the core of the entire system. Based on the functional characteristics of each part, it can be further divided into: an angle measurement subsystem, a servo subsystem, a rate subsystem, and a control computer. The control computer is the core of the control system. The specific system structure diagram is as follows: [align=center] Figure 1 System Composition Block Diagram (taking a single degree of freedom as an example)[/align] Due to actual needs and specific requirements, we adopted an industrial control computer in the design. The communication between various subsystems uses the ISA bus, which increases the flexibility of the entire control system design. At the same time, when selecting control methods, we can verify some newer, composite control methods and some intelligent control methods. By testing these control algorithms, we improved the response speed of the entire system, improved the dynamic performance and accuracy of the system, and also broadened the system's bandwidth. 3. System Hardware Design Since this three-axis simulation turntable system is used to accurately reproduce the free movement of an aircraft in the air in the laboratory, the performance of the control system will directly affect the accuracy of the simulation and the reliability of the simulation results. In order to achieve the performance indicators, the design of the subsystems must meet the accuracy requirements of the entire system. Since this turntable system is mainly used for attitude simulation of UAVs in the laboratory, and due to the functional requirements and performance characteristics of the UAV itself, this system does not have high requirements for the accuracy of the angle measurement system, but has high requirements for the rotational degree of freedom and flexibility of the turntable. Based on this, the main hardware design of this system adopts the following functional components: 3.1 Stepper Motor The servo mechanism of this system uses an MSMA042A1G stepper motor. The rotation angle of the stepper motor is controlled by sending pulse square waves, and the speed of the stepper motor is controlled by the frequency of the pulses. It features simple operation and high control precision. 3.2 Stepper Motor Controller and Control Circuit The stepper motor control card uses an 8254 timer/counter to generate pulse square waves for stepper motor operation and counts the output pulses. Counter 0 of the 8254 is set to square wave generator mode, and counter 2 is set to counter mode. Counter 0 controls the pulse output, and counter 2 counts the output pulses. The number of pulses set in counter 2 controls the stepper motor's rotation angle; counter 0 controls the stepper motor's rotation speed. 3.3 Angle Measurement Elements The angle measurement elements use a vertical gyroscope and a magnetic heading instrument. The vertical gyroscope and magnetic heading instrument sense the three azimuth angles of the three-axis turntable and feed back the corresponding voltage. The computer calculates the voltage to make the three-axis turntable rotate to the required position. The three-axis simulation turntable control system designed based on the above functional devices can well meet the performance indicators of the whole system. Moreover, by adding other methods in the software part, the accuracy and response speed of the system can be greatly improved. 4. System Software Design The control methods and strategies of the entire control system are mainly reflected in the software part of the system. The software part of the control system can be divided into two parts: the interactive interface and the control method. The interactive interface is mainly to meet the needs of users and facilitate use and maintenance. 4.1 Control Method The control method is the core part of the control system, mainly including: (1) System mathematical model. The system model is the foundation and object of the control system. The mathematical model established based on the actual simulated object is an approximation of the actual system. A good mathematical model is a model that is as simple as possible based on meeting the characteristics and performance requirements of the actual object. The mathematical model established in this system is based on the ×× type UAV and can meet the basic characteristics of this type of UAV and the performance requirements of the control system. (2) Control law and control algorithm. Control law and control algorithm are control strategies and methods adopted based on the established mathematical model and the functions to be achieved. Commonly used control laws and methods mainly include classical control methods and modern control methods. The control law adopted in this system is mainly determined according to this model of UAV, and a control strategy combining classical and modern control methods is used. 4.2 Software Flow The software flow is written according to actual needs. Since this system is mainly used for laboratory simulation, the specific flowchart is as follows: [align=center] Figure 2 System Software Flowchart[/align] 5. Key Issues and Solutions Large simulation equipment is subject to the adverse effects of mechanical resonance due to mechanical design and processing, which greatly affects the system performance. To solve this problem, we adopted optimal control and used a mixed-signal approach with a band-stop filter, which effectively solved the problem. Due to the existence of friction torque, the turntable is prone to low-speed crawling and speed fluctuations when running at low speeds. To achieve the desired static and low-speed performance, the friction torque of the system must be compensated. In terms of control, increasing the proportional gain and improving the integral action are used to improve the low-speed performance of the system. Due to the increased integral action and proportional gain required for low-speed performance, and the influence of mechanical resonance on dynamic performance, a single control method cannot meet the requirements. Therefore, several composite control methods were adopted to give the system different adaptability under different states and performance requirements. 6. Summary Through the overall system design, hardware design, and software design described above, this turntable system can effectively meet the requirements for attitude simulation of the ×× type UAV in the laboratory. Simultaneously, by modifying the system model parameters in the system software design, it can simultaneously meet the needs of laboratory simulation of various other UAVs, verifying the performance and reliability of the UAV's hardware parameters and software. Furthermore, this simulation turntable can also meet the testing requirements of some low-precision gyroscopes and inertial navigation systems. References: [1] Yu Deyi et al. Flight simulation system of NH-96 unmanned aircraft. Journal of Nanjing University of Aeronautics and Astronautics, 1998, 30: 1 [2] Research on the criterion for overcoming low-speed lag on simulation turntable. Systems Engineering and Electronics, 2002, 22: 7 [3] Wen Chuanyuan. Modern flight control system. Beijing University of Aeronautics and Astronautics Press, 1992