The dual axis inertial testing turntable is the core equipment for performance testing of inertial navigation systems and attitude control systems. By simulating the angular motion of the carrier in two-dimensional space, it provides accurate attitude reference and motion excitation for inertial devices (such as gyroscopes, accelerometers) and inertial systems. The technical performance of the turntable directly determines the accuracy and reliability of inertial testing, and its core relies on high-precision motion control principles and high rigidity, low interference structural design. This article will elaborate in detail on the core logic, key technologies, and structural design of motion control, as well as the key components and design points, revealing the inherent mechanism of achieving high-precision angular motion simulation.

1、 The motion control principle of dual axis inertial testing turntable

The core goal of the motion control of the dual axis inertial testing turntable is to achieve independent or coordinated angular motion of two orthogonal axes (usually the azimuth axis and the elevation axis), meeting the posture simulation requirements in different testing scenarios, such as constant speed rotation, angular position positioning, sinusoidal oscillation, etc. Its control principle is centered around closed-loop control of "instruction generation signal feedback error correction", integrating key technologies such as kinematic solution, servo drive, and high-precision detection to ensure the accuracy and dynamic response performance of output angular motion.

（1） Core control logic: closed-loop control architecture

The measurement and control system is an important component of the turntable, and its main functions can be summarized as: implementing the servo control strategy of the system, completing the technical performance and functions of the system, and ensuring the normal, safe, and reliable operation of the system.

1. principleThe control of the turntable is based on error control theory, which means that the difference between the command value and the feedback value is the error, and the ideal goal of control is to make the error zero. The error is processed through PID algorithm, feedforward correction algorithm, friction compensation algorithm, and other algorithms to generate a voltage value. Then, the voltage value is output through an industrial general-purpose D/A board as the input of the motor driver. The motor driver drives the motor according to the given voltage to achieve control of the motor. The motor drives the turntable frame to rotate, and the angle of rotation is collected by an angle encoder. After passing through the angle measurement module and data acquisition card, the feedback value is fed back to the control program, which is then compared with the command value. This loop control continues until the error is zero.

The system adopts a subordinate structure control method composed of analog current loop and digital position loop control circuit. By using a D/A conversion card to control the input of the motor driver, the motor driver drives the motor to achieve control of the motor. The position signals of the two shafts are fed back to the control program through an angle encoder, an angle measurement module, and a data acquisition card. The control system then uses PID control algorithm and advanced robust control algorithm to control the turntable, thus forming the position loop of the system. The position loop is the main feedback loop of the system, which is used to ensure the control accuracy and dynamic requirements of the system. The current loop of the system is implemented internally through the driver, which forms the armature current negative feedback to reduce the impact of power supply voltage fluctuations, improve the linearity of control torque, and prevent overcurrent in the power conversion circuit and motor.

2. control softwareThe turntable control software is divided into upper layer (comprehensive management level) and lower layer (direct control level). The upper and lower layers communicate through shared memory and are implemented on one computer. The upper layer forms the centralized monitoring and comprehensive management level of the two-dimensional turntable, mainly realizing the online comprehensive management, performance testing, security protection settings, and monitoring functions of the system's non real time processes. The lower level of the software is the direct control level of the two-dimensional turntable control system, which is used to form independent servo control circuits.

The Central Monitoring System (CMS) is a specialized hardware device for control systems, which communicates directly with control software through interfaces to achieve control of the working status, data detection, and monitoring alarm management of various channel servo systems. The monitoring system has security protection and logical control functions for the entire device.

3. Servo control schemeThe control system has two independent digital servo control channels and adopts a digital servo control system with a microcomputer controlled driver torque motor direct drive framework. The digital angular position feedback loop is composed of high-precision feedback components and digital conversion devices, which can meet the accuracy and performance requirements of the system. Using an industrial control computer as the main control computer for the servo system can ensure the implementation of system performance and also effectively implement system control strategies, ensuring the full guarantee of system performance.

The entire controller consists of four components: a classical PID controller, a zero phase difference feedforward controller based on zero point pre compensation, an adaptive friction compensator, and a robust controller based on disturbance observer.

The position loop adopts a composite control structure, which combines feedforward control and feedback control. Its advantage is that the tracking performance of the system can be considered separately from the stability of the system. Feedforward control is used to improve system tracking performance without affecting system stability, while closed-loop control is used to ensure system stability, robustness to external disturbances, and parameter changes.

In position closed-loop control, a robust control method based on disturbance observer is adopted, where the disturbance observer is used to suppress torque disturbances and linearize the system. The basic idea is to equate the differences between the actual object and the nominal model output caused by external torque interference and model parameter changes to the control input, that is, to observe the equivalent interference, and introduce equal compensation in the control to suppress the interference and enhance the robustness of the control system. The design of position closed-loop mainly considers system stability and static position errors, and adopts effective logical filtering measures for position feedback to remove the influence of errors and codes. The position closed-loop controller adopts composite control to ensure smooth operation of the closed-loop system and no overshoot in response. Its parameters can be adaptively adjusted to adapt to different loads and enhance the robustness of the control system to parameter changes.

（2） Key technology: high-precision detection and error compensation

The accuracy of closed-loop control depends on high-precision feedback detection and effective error compensation, which is the core technical support for the motion control of the dual axis turntable

1. High precision angular position/angular velocity detectionUsing high-precision detection components to real-time collect the motion status of the turntable frame, providing reliable basis for error correction. Common detection components include photoelectric encoders, rotary transformers, circular induction synchronizers, etc. Among them, the circular induction synchronizer has the characteristics of high precision, high stability, and strong anti-interference ability, and is widely used in high-precision turntables; Optoelectronic encoders have the advantages of fast response speed and high resolution, making them suitable for scenes with high dynamic performance requirements. To further improve detection accuracy, multi reading head subdivision technology is commonly used, which reduces the impact of marking and installation errors on detection components by superimposing and subdividing signals from multiple reading heads.

2. Error compensation technologyCompensating for system errors and random errors during the movement of the turntable through a combination of software and hardware is the key to improving control accuracy. System errors mainly include mechanical transmission errors, geometric errors of the frame (such as two axis orthogonality errors, radial and end face circular runout of the shaft system), and dead zone errors of the motor; Random errors mainly include load disturbances, temperature drift, external vibrations, etc. The compensation strategy includes: firstly, offline calibration compensation, which calibrates the system error through high-precision measurement equipment such as laser interferometers, establishes an error model, and calls the model in real-time for error cancellation during the control process; The second is online adaptive compensation, which uses adaptive control algorithms to identify random errors such as load disturbances and temperature drift in real time, dynamically adjust control parameters, and improve the system's anti-interference ability.

2、 Structural Design of Dual axis Inertial Testing Turntable

The structural design of the dual axis inertial testing turntable needs to meet the core requirements of "high precision, high rigidity, low interference, and lightweight", ensuring that the mechanical structure can accurately transmit motion while reducing the impact of its own interference on testing accuracy. The core of its structure consists of a turntable frame, shaft components, transmission mechanism, support structure, and protective device. The design of each part directly determines the mechanical performance and testing accuracy of the turntable.

（1） Core structure composition

1. Turntable frameAs the core component for carrying test pieces and achieving angular motion, it is divided into an inner frame (pitch axis frame) and an outer frame (azimuth axis frame), which are orthogonally connected by axis components. The framework design needs to balance rigidity and lightweight: insufficient rigidity can cause deformation during motion, affecting attitude accuracy; Excessive weight will increase the motor load and reduce dynamic response performance. High strength aluminum alloy is usually used as the frame material, and the frame structure is optimized through finite element analysis. Strengthening ribs are set in key areas to enhance structural rigidity while reducing weight.

2. Shaft components: is the core component that ensures high-precision angular motion of the turntable, directly determining the rotational accuracy and stability of the shaft system. The shaft system components mainly consist of the main shaft, bearings, bearing seats, and locking mechanisms. To improve the rotation accuracy, high-precision rolling bearings (such as angular contact ball bearings, tapered roller bearings) or hydrostatic bearings (gas hydrostatic bearings, liquid hydrostatic bearings) are usually used: rolling bearings have the advantages of simple structure, low cost, and fast response, and are suitable for medium to high-precision turntables; Static pressure bearings support the spindle by forming an oil/gas film through high-pressure gas or liquid, and have the characteristics of no friction, low wear, and high rotational accuracy, making them suitable for ultra high precision turntables. During the assembly process of the shaft system, it is necessary to strictly control the pre tightening force of the bearings, reduce the radial and end face circular runout of the main shaft, and reduce the impact of temperature changes on the accuracy of the shaft system through temperature compensation design.

3. transmission mechanismResponsible for transmitting the motion of the motor to the turntable frame, and its transmission accuracy directly affects the motion control accuracy of the turntable. Common transmission methods include direct drive and indirect drive: Direct drive (DD drive) is the preferred transmission method for high-precision turntables, which directly connects the motor rotor to the turntable frame, eliminates the intermediate transmission link, and has the advantages of high transmission accuracy, fast response, and no transmission clearance; Indirect drive transmits motion through transmission components such as gears, synchronous belts, and lead screws, making it suitable for scenarios with high loads. However, precise machining and assembly are required to control the transmission clearance and reduce transmission errors.

4. Supporting structure and protective deviceThe supporting structure includes a base and a bracket, which are used to fix the various components of the turntable. It needs to have sufficient rigidity and stability to avoid the influence of external vibrations on the movement of the turntable. Usually cast iron or granite is used as the base material. Granite has good seismic resistance and stability, which can effectively absorb vibration and improve the static accuracy of the turntable. The protective device is mainly used to protect the internal components of the turntable, prevent dust, water vapor, etc. from entering the shaft system and transmission mechanism, and avoid safety accidents during the testing process. It usually uses protective components such as sealing covers and safety gratings.

（2） Key points of structural design

1. Two axis orthogonality designThe orthogonality error of two axes is a key geometric error that affects the accuracy of dual axis linkage, and it needs to be ensured through precision design and assembly. During the structural design phase, optimize the installation position of the shaft components through 3D modeling to ensure that the centerlines of the two shafts are strictly orthogonal; During the assembly process, a laser interferometer is used for real-time measurement. By adjusting the installation accuracy of the bearing seat, the orthogonality error is controlled within a few seconds.

2. Lightweight and Dynamic Balance DesignUneven weight distribution between the turntable frame and the load can result in centrifugal force during motion, causing vibration and affecting dynamic accuracy. Therefore, it is necessary to carry out lightweight design of the turntable frame, and eliminate eccentric mass through dynamic balance testing and calibration. Dynamic balance correction usually uses weighting or de weighting methods to control the unbalance of the turntable within a very small range, ensuring the stability of the turntable during high-speed rotation.

3. Interference suppression designThe mechanical interference of the turntable itself (such as bearing friction, transmission clearance) and external interference (such as vibration, temperature changes) can seriously affect the testing accuracy, and it needs to be suppressed through structural design. One is to adopt vibration isolation design, setting up vibration isolation pads or platforms between the base and the ground to absorb external vibrations; Secondly, temperature control design is adopted, with heating/cooling devices and temperature sensors installed inside the turntable to control the working temperature of the turntable in real time, reducing the impact of temperature changes on the accuracy and material properties of the shaft system; Thirdly, optimize the wiring and pipeline design to avoid pulling and friction between cables and pipelines during the movement of the turntable, and reduce interference torque.

4. Test piece installation and interface designThe installation accuracy of the test piece directly affects the reliability of the test results, and high-precision installation interfaces and positioning benchmarks need to be designed. Usually, positioning pins, end face flanges, and other positioning methods are used to ensure that the installation center of the test piece coincides with the rotation center of the turntable; At the same time, necessary signal and power interfaces should be reserved to facilitate the connection between the test piece and the external testing system, and the interface design should avoid affecting the motion range and accuracy of the turntable.

3、 Conclusion

The motion control principle and structural design of the dual axis inertial testing turntable are an organic whole. The high-precision requirements of motion control depend on the high rigidity and low interference of the structural design, and the optimization of the structural design provides a good foundation for the implementation of motion control algorithms. With the development of inertial navigation technology towards high precision and miniaturization, the performance requirements for dual axis inertial testing turntables are constantly increasing. In the future, advanced control algorithms (such as intelligent control and robust control) and high-precision structural design technologies (such as additive manufacturing and precision assembly) need to be further integrated to continuously improve the testing accuracy, dynamic response performance, and reliability of turntables, providing strong support for the development of inertial technology.