A dual-axis inertial test rate table is a core piece of equipment for performance testing of inertial navigation systems and attitude control systems. By simulating the angular motion of a carrier in two-dimensional space, it provides precise attitude references and motion excitations for inertial devices (such as gyroscopes and accelerometers) and the inertial system . The rate table 's technical performance directly determines the accuracy and reliability of inertial testing, and its core relies on high-precision motion control principles and a high-rigidity, low-interference structural design. This article will elaborate on the core logic of motion control, key technologies, core components of structural design, and design considerations, revealing the intrinsic mechanism by which it achieves high-precision angular motion simulation.

I. Motion Control Principle of a Dual-Axis Inertial Test Rate Table

The core objective of motion control for a dual-axis inertial test rate table is to achieve independent or linked angular motion on two orthogonal axis (typically azimuth and pitch axis) to meet attitude simulation requirements in various testing scenarios, such as constant-speed rotation, angular position positioning, and sinusoidal oscillation . Its control principle is based on a closed-loop control system of "command generation - signal feedback - error correction," integrating key technologies such as kinematic calculation, servo drive, and high-precision detection to ensure the accuracy of the output angular motion and dynamic response performance.

(I) Core control logic: Closed-loop control structure

The measurement and control system is an important component of the rate table . Its main functions can be summarized as: implementing the system's servo control strategy, fulfilling the system's technical performance and functions, and ensuring the system's normal, safe, and reliable operation. 

1. Principle : The rate table control is based on error control theory, where the difference between the command value and the feedback value is the error, and the ideal control goal is to make the error zero. This error is processed by PID algorithms, feedforward correction algorithms, friction compensation algorithms, etc., to generate a voltage value. This voltage value is then output through an industrial standard D/A board as the input to the motor driver. The motor driver drives the motor according to the given voltage to control the motor. The motor drives the rate table frame to rotate, and the rotation angle is acquired by an angle encoder, fed back to the control program (i.e., the feedback value) through an angle measurement module and data acquisition card. This feedback value is then compared with the command value, and this cycle of control continues until the error is zero.

The system employs a subordinate control structure consisting of an analog current loop and a digital position loop. The input to the motor driver is controlled via a D/A converter card, and the motor driver drives the motor to achieve motor control. The two shafts transmit shaft position signals via angle encoders, which are then fed back to the control program through an angle measurement module and a data acquisition card. The control system then uses PID control algorithms and advanced robust control algorithms to control the turntable, thus forming the system's position loop. The position loop is the system's main feedback loop, ensuring the system's control accuracy and dynamic requirements. The system's current loop is implemented internally by the driver. This current loop forms armature current negative feedback to reduce the impact of power supply voltage fluctuations, improve the linearity of the control torque, and prevent overcurrent in the power conversion circuit and motor.

2. Control Software : The rate table control software is divided into an upper layer (integrated management level) and a lower layer (direct control level). The upper and lower layers communicate via shared memory and are implemented on a single computer. The upper layer forms the centralized monitoring and integrated management level of the two-dimensional rate table, mainly realizing the online integrated management of non-real-time processes, performance testing, safety protection settings, and monitoring functions. The lower layer of the software is the direct control level of the two-dimensional rate table control system, used to form various independent servo control loops.

A Central Monitoring System (CMS) is a dedicated hardware device within a control system. It communicates directly with the control software via an interface to control the operational status of each channel's servo system, detect data, and manage alarms. The CMS also provides security protection and logical control functions for the entire system.

3. Servo Control Scheme : The 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. A digital angular position feedback loop, composed of high-precision feedback elements and a digital converter, meets the system's accuracy and performance requirements. Using an industrial control computer as the main control computer for the servo system ensures the realization of system performance and effectively implements the system control strategy, thus fully guaranteeing system performance.

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

The position loop employs a composite control structure, combining feedforward and feedback control. Its advantage lies in separating the system's tracking performance from its stability. Feedforward control improves tracking performance without affecting stability, while closed-loop control ensures system stability and robustness against external disturbances and parameter variations.

In position closed-loop control, a robust control method based on a disturbance observer is employed. 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 disturbances and changes in model parameters to the control input, i.e., to observe the equivalent disturbance and introduce an equivalent compensation in the control to suppress the disturbance and enhance the robustness of the control system. The design of the position closed loop mainly considers system stability and static position error, employing effective logic filtering measures for the position feedback to remove the influence of bit errors and misinterpretations. The position closed-loop controller uses composite control to ensure smooth operation of the closed-loop system with no overshoot. Its parameters can be adaptively adjusted to adapt to different loads, enhancing the robustness of the control system to parameter changes.

(II) Key Technologies: High-precision detection and error compensation

The accuracy of closed-loop control relies on high-precision feedback detection and effective error compensation, which are the core technological supports for the motion control of a dual-axis rate table.

1. High-precision angular position/angular velocity detection : High-precision detection elements are used to acquire the motion state of the rate table frame in real time, providing a reliable basis for error correction. Commonly used detection elements include photoelectric encoders, rotary transformers, and circular induction synchronizers. Among them, circular induction synchronizers are widely used in high-precision rate tables due to their high precision, high stability, and strong anti-interference capabilities; photoelectric encoders, on the other hand, have the advantages of fast response speed and high resolution, making them suitable for scenarios with high dynamic performance requirements. To further improve detection accuracy, multi-readhead subdivision technology is typically used. By superimposing and subdividing the signals from multiple readheads, the influence of marking errors and installation errors of the detection elements is reduced.

2. Error compensation technology : This technology, combining software and hardware, compensates for systematic and random errors present during rate table movement, and is crucial for improving control accuracy. Systematic errors mainly include mechanical transmission errors, frame geometric errors (such as orthogonality errors between two axes, radial and axial runout of the shaft system), and motor dead zone errors. Random errors mainly include load disturbances, temperature drift, and external vibrations. Compensation strategies include: first, offline calibration compensation, which uses high-precision measuring equipment such as laser interferometers to calibrate systematic errors, establish an error model, and call the model in real time during control to cancel errors; second, 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 capability.

II. Structural Design of a Dual-Axis Inertial Test Rate Table

The structural design of a dual-axis inertial test rate table must meet the core requirements of "high precision, high rigidity, low interference, and lightweight." It must ensure that the mechanical structure can accurately transmit motion while minimizing the impact of its own interference on the testing accuracy. Its core structure consists of the rate table frame, shaft system assembly, transmission mechanism, support structure, and protective devices. The design of each part directly determines the mechanical performance and testing accuracy of the rate table.

(I) Core Structure Composition

1. Table frame : As the core component for supporting the test specimen and realizing angular motion, it consists of an inner frame (pitch axis frame) and an outer frame (azimuth axis frame), which are orthogonally connected by an axis system assembly. The frame design must balance rigidity and lightweight: insufficient rigidity will cause deformation during motion, affecting attitude accuracy; excessive weight will increase the motor load and reduce dynamic response performance. High-strength aluminum alloy is typically used as the frame material. Finite element analysis is used to optimize the frame structure, and reinforcing ribs are added to key areas to improve structural rigidity while reducing weight.

2. Shaft system assembly : This is the core component ensuring the high-precision angular motion of the rate table, directly determining the rotational accuracy and stability of the shaft system. The shaft system assembly mainly consists of the spindle, bearings, bearing housings, and locking mechanisms. To improve rotational accuracy, high-precision rolling bearings (such as angular contact ball bearings and tapered roller bearings) or hydrostatic bearings (gas hydrostatic bearings and liquid hydrostatic bearings) are typically used. Rolling bearings have the advantages of simple structure, low cost, and fast response, making them suitable for medium- to high-precision rate tables. Hydrostatic bearings support the spindle through an oil/gas film formed by high-pressure gas or liquid, featuring frictionless operation, low wear, and high rotational accuracy, making them suitable for ultra-high-precision rate tables. During shaft system assembly, the bearing preload must be strictly controlled to reduce the radial and axial runout of the spindle. Simultaneously, temperature compensation design is used to reduce the impact of temperature changes on shaft system accuracy.

3. Transmission mechanism : Responsible for transmitting the motor's motion to the rate table frame, its transmission accuracy directly affects the rate table's motion control accuracy. Common transmission methods include direct drive and indirect drive: Direct drive (DD drive) connects the motor rotor directly to the rate table frame, eliminating intermediate transmission links. It has the advantages of high transmission accuracy, fast response, and no transmission backlash, making it the preferred transmission method for high-precision rate tables. Indirect drive transmits motion through transmission components such as gears, synchronous belts, and lead screws. It is suitable for scenarios with heavy loads, but requires precision machining and assembly to control transmission backlash and reduce transmission errors.

4. Support Structure and Protective Devices : The support structure, including the base and brackets, is used to fix the various components of the rate table. It must have sufficient rigidity and stability to prevent external vibrations from affecting the rate table's movement. Cast iron or granite is typically used as the base material. Granite has good shock resistance and stability, effectively absorbing vibrations and improving the rate table's static accuracy. Protective devices are mainly used to protect the internal components of the rate table, preventing dust, moisture, etc., from entering the shaft system and transmission mechanism, while also preventing safety accidents during testing. These typically include sealing covers and safety gratings.

(II) Key Points of Structural Design

1. Two-axis orthogonality design : The orthogonality error between the two axes is a key geometric error affecting the accuracy of dual-axis linkage, and must be ensured through precise design and assembly. During the structural design phase, the installation position of the shaft system components is optimized through 3D modeling to ensure that the centerlines of the two axes are strictly orthogonal. During the assembly process, a laser interferometer is used for real-time measurement, and the orthogonality error is controlled within a few seconds by adjusting the installation accuracy of the bearing housing.

2. Lightweight and Dynamic Balancing Design : Uneven weight distribution between the rate table frame and the load can generate centrifugal force during movement, causing vibration and affecting dynamic accuracy. Therefore, a lightweight design for the rate table frame is necessary, along with dynamic balancing testing and correction to eliminate eccentric mass. Dynamic balancing correction typically involves adding or removing weights to control the rate table's imbalance within a minimal range, ensuring stability during high-speed rotation.

3. Interference Suppression Design : Mechanical interference from the rate table itself (such as bearing friction and transmission clearance) and external interference (such as vibration and temperature changes) can severely affect testing accuracy, and must be suppressed through structural design. First, a vibration isolation design is adopted, placing vibration isolation pads or platforms between the base and the ground to absorb external vibrations. Second, a temperature control design is adopted, installing heating/cooling devices and temperature sensors inside the rate table to control the rate table's operating temperature in real time, reducing the impact of temperature changes on shaft accuracy and material properties. Third, the wiring and conduit design is optimized to avoid tension and friction between cables and conduits during rate table movement, reducing interference torque.

4. Test piece installation and interface design : The installation accuracy of the test piece directly affects the reliability of the test results, requiring the design of a high-precision installation interface and positioning reference. Positioning methods such as locating pins and end flanges are typically used to ensure that the installation center of the test piece coincides with the rotation center of the rate table. Simultaneously, necessary signal and power interfaces should be reserved to facilitate connection between the test piece and external testing systems, and the interface design must avoid affecting the rate table's range of motion and accuracy.

III. Conclusion

The motion control principle and structural design of a dual-axis inertial testing rate table form an organic whole. The high precision requirement of motion control depends on the high rigidity and low interference of the structural design, while the optimization of the structural design provides a solid foundation for the implementation of motion control algorithms. As inertial navigation technology develops towards higher precision and miniaturization, the performance requirements for dual-axis inertial testing rate tables are also constantly increasing. In the future, it is necessary to further integrate advanced control algorithms (such as intelligent control and robust control) with high-precision structural design technologies (such as additive manufacturing and precision assembly) to continuously improve the testing accuracy, dynamic response performance, and reliability of the rate table, providing strong support for the development of inertial technology.