Gimbal System Research Articles

Neuro-observer based control of double gimbal control moment gyro systems

In this paper, a new third-order nonlinear dynamics solving the mismatched problem is proposed for double gimbal control moment gyros (DGCMGs) affected by friction and coupling torques, unmodeled dynamics, or parameter uncertainties; the new nonlinear dynamics with the control inputs and the disturbances into the same equations should not be linearized and any control method handling multivariate systems with cross-coupling between channels can be then used to design a controller. Secondly, a feed-forward neural network based observer with disturbance rejection ability is proposed to estimate both the exogenous/endogenous disturbances and the states of the system, the number of necessary sensors being decreased. Thirdly, in order to control the rotations of the inner/outer gimbals, compensate the disturbances, enhance the robustness of the control system, and handle the channel interferences, we design, evaluate, and compare two novel control architectures using the Lyapunov theory, the backstepping method or the dynamic inversion control technique. Each of the two control architectures uses two interconnected controllers (an inner gimbal controller and an outer gimbal controller), a neuro-observer, and two reference models to generate the necessary fictive control signals. The simulation results show that the proposed control approaches provide very good angular rate precision of the DGCMG system and successfully handle a wide range of disturbances, parameter uncertainties, and channel interferences. The smaller fluctuations of the rotation angles, angular rates, and angular accelerations in the gimbal channels indicate that the backstepping control provides better ability to reject disturbances, smaller overshoot and convergence time than the dynamic inversion control technique.

Analysis of Transmission Loss and Boresight Error of a Curved FSS Radome-Enclosed Antenna

In this paper, we numerically study a curved frequency selective surface (FSS) radome, which encloses a waveguide slot array antenna operating at 10GHz, using the ray tracing technique and flat model. The transmission loss (TL) and boresight error (BSE) are calculated for various degrees of freedom, such as elevation and azimuth angular scanning or gimbal systems of array antennas to examine the electromagnetic properties of the curved FSS radome. Our calculations of radiation patterns, TL, and BSE for a multi-layered dielectric radome are compared with those of a commercial EM solver for validation. Importantly, we quantify phase distortions, incurred by the curved FSS radome, using the insertion phase delay (IPD) of transmitted fields on the radome surface. Thereby, we demonstrate that the BSEs strongly depend on (1) the spatial distribution of phase distortions on the radome surface and (2) their average level. The present method is highly suited for analyzing radomes with arbitrary surface patterns inserted.

Design of Robust Fault-Tolerant Control for Target Tracking System With Input Delay and Sensor Fault

This paper presents a robust fault-tolerant control scheme that provides reliable tracking and effective disturbance rejection for a low-cost tracking system. As the controlled apparatus, a 2-axis gimbaled mechanism is involved in a networked control system, and an unreliable sensor brings sensor drift into the system. Besides, external disturbances heavily affect the system in practical applications. Representation of the faulty gimbal system with its constraints and disturbances is firstly introduced. Then, the design of the fault-tolerant controller is detailed, which consists of two components: (1) an unknown input observer that estimates the fault and effect of disturbances and network delay simultaneously, and (2) a robust control law, using the observer estimations, designed based on the combination of the super-twisting algorithm, backstepping procedure, and integral sliding mode control technique. Subsequently, simulations and experiments are conducted, in which the performance of the proposed control system is compared to those from the previous studies. The results show the superiority and reliability of the proposed control system.

비행체 탑재 김발시스템의 위치제어 성능 향상을 위한 경로 생성 알고리즘 설계

비행체 탑재 김발시스템의 위치제어 성능 향상을 위한 경로 생성 알고리즘 설계

Control law for line of sight stabilization in two-axis gimbal system

A two-axis gimbaled stabilization system in air vehicles must stabilize the line of sight of the payload toward a target against the external motion induced by air vehicle maneuvering and aerodynamic forces. The target tracking and pointing performances of the air vehicles are largely affected by air vehicle motion decoupling capability. In this work, the controller design is carried out for a two-axis gimbal system. The plant model is generated using experimental frequency response data and mathematical formulation of the system. The mixed sensitivity problem is posed and weighting functions are selected so that they not only fulfill all the design goals but also accommodate the modeling uncertainties. The stabilization loop is designed and implemented in digital signal processor-based hardware for only one axis (in azimuth).

Control of double gimbal control moment gyro systems using the backstepping control method and a nonlinear disturbance observer

To cancel the negative effects of the friction torques, unmodeled dynamics, and coupling torques, affecting the gimbal precision of a double gimbal control moment gyro (DGCMG) system, this paper proposes a new control architecture to handle these exogenous/endogenous disturbances and to control the two gimbals of the system. It combines a backstepping controller with a nonlinear disturbance observer, which preserve the advantages of the two entities. Firstly, the state space equations of the DGCMG system are established with the friction torques considered exogenous disturbances and unmodeled dynamics regarded as endogenous disturbances. Secondly, a nonlinear observer is used for the estimation of the disturbances. Thirdly, using the observer estimation errors, a backstepping controller is designed to enhance the robustness of the control system, to handle the channel interferences, and to solve the mismatched problem without decoupling between the inner/outer gimbal channels. The obtained control architecture is software implemented and validated; the results show that the proposed control approach not only enhances the robustness in relation to the exogenous/endogenous disturbances, but also improves the angular velocity precision of the two gimbals of the DGCMG system.

A Robust Double Active Control System Design for Disturbance Suppression of a Two-Axis Gimbal System

This paper presents a robust controller design with disturbance decoupling and rejection of a two-degree-of-freedom (2-DOF) Inertially Stabilized Platform (ISP). The objective of these mechanisms is to stabilize the line of sight (LOS) of imaging sensors pointing towards a specific target. There is currently tremendous interest in ISP applications in marine systems. Such a harsh environment subjects the imaging sensors to multiple disturbances, which requires the design of robust control strategies to enhance the performances of ISP systems. The controller designed in this study is a double active controller composed of an inner compensator, and a feedback controller designed based on the H∞ framework. The main advantage of the proposed controller is that it can be implemented in real time, with lower computational complexity and good performance. In this paper, a comparative experimental study was conducted between the designed controller and an integral sliding-mode controller (ISMC). The comparison was achieved through two major tasks of ISP systems: motion tracking and target tracking.

Real-Time Visual Tracking of Moving Targets Using a Low-Cost Unmanned Aerial Vehicle with a 3-Axis Stabilized Gimbal System

Unmanned Aerial Vehicles (UAVs) have recently shown great performance collecting visual data through autonomous exploration and mapping, which are widely used in reconnaissance, surveillance, and target acquisition (RSTA) applications. In this paper, we present an onboard vision-based system for low-cost UAVs to autonomously track a moving target. Real-time visual tracking is achieved by using an object detection algorithm based on the Kernelized Correlation Filter (KCF) tracker. A 3-axis gimbaled camera with separate Inertial Measurement Unit (IMU) is used to aim at the selected target during flights. The flight control algorithm for tracking tasks is implemented on a customized quadrotor equipped with an onboard computer and a microcontroller. The proposed system is experimentally validated by successfully chasing a ground and aerial target in an outdoor environment, which has proven its reliability and efficiency.

Air-LUSI: Development of a pointing and tracking control system for lunar spectral measurements

The airborne lunar spectral irradiance mission is an inter-agency partnership between the US National Aeronautics and Space Administration and the US National Institute of Standards and Technology that aims to make SI-traceable measurements of lunar spectral irradiance at visible to near-infrared wavelengths with unprecedented accuracy. This information is vital to using the Moon as a calibration source for Earth observing satellites. To minimize uncertainty, the lunar measurements are made above 90% of the Earth’s atmosphere from an Earth Resources 2 aircraft, a civilian descendant of the U-2 spy plane. Situated in a large wing pod, a custom-designed telescope automatically tracks the Moon and the measurements are fed into a spectrometer. This information is being used to develop an extremely accurate model that can be used to calibrate satellites. An Engineering Flight Campaign was completed in August 2018 and a Demonstration Flight Campaign in November 2019, which demonstrated autonomous lunar acquisition and tracking as well as measurements of the Moon’s spectral irradiance from an altitude of approximately 21 km. This article presents the simplified double gimbal control system design that was used to manipulate the telescope, and was capable of targeting the Moon with a root mean squared tracking error of about 0.1∘.

Automatic Kinematic Calibration of Multiaxis Gimbal Systems

This article presents an automatic and efficient calibration method for multiaxis gimbal systems. The calibrated kinematic parameters consist of the axis orientation, the joint encoder nonlinearity map, and the endpoint gyro bias. The calibration of these parameters is conducted simultaneously by sequentially and independently moving one gimbal joint back and forth. With such straightforward intuition, the algorithm is formulated based on the product-of-exponential formula with an analytically derived error model. The key and unique contribution of this algorithm is that it produces a closed-form optimal solution, which is computation efficient and easy for embedded implementation. Both numerical and experimental tests are performed to validate the effectiveness of the proposed algorithm in calibrating the kinematic parameters of gimbal systems and compensating the gyro bias.

Two-axis Gimbal System Stabilization Using Adaptive Feedback Linearization

Background: This paper is devoted to adaptive feedback linearization based stabilization of a two-axis gimbal platform. Methods: A Lyapunov based adaptation mechanism is proposed for stabilization of a two axes gimbal system through a feedback linearized control structure. The uncertainty of the products of inertia is incorporated in the design procedure. Results: The performed computer simulations demonstrate the robust performance of the proposed two-axis gimbal stabilizer considering the cross-coupling effects between inner and outer gimbals. Conclusion: The proposed control structure could be utilized in guided missiles, spacecraft, and navigation systems.

Stabilization of A Two-DOF Gimbal System Using Direct Self-Tuning Regulator

Stabilization of A Two-DOF Gimbal System Using Direct Self-Tuning Regulator

Calibration, Compensation and Accuracy Analysis of Circular Grating Used in Single Gimbal Control Moment Gyroscope.

The accuracy of the circular grating is the key point for control precision of the single gimbal control moment gyroscope servo system used in civilian micro-agile satellites. Instead of using the multi reading heads to eliminate eccentricity errors, an algorithm compensation method based on a calibration experiment using a single reading head was proposed to realize a low-cost and high accuracy angular position measurement. Moreover, the traditional hardware compensation method using double reading heads was also developed for comparison. Firstly, the single gimbal control moment gyroscope system of satellites was introduced. Then, the errors caused by the installation of the reading head were studied and the mathematic models of these errors were developed. In order to construct the compensation function, a calibration experiment using the autocollimator and 24-sided prism was performed. Comparison of angle error compensation using the algorithm and hardware method was presented, and results showed that the algorithm compensation method proposed by this paper achieved the same accuracy level as the hardware method. Finally, the proposed method was further verified through a control system simulation.

Lightweight Integrated Solution for a UAV-Borne Hyperspectral Imaging System

The rapid development of unmanned aerial vehicles (UAVs), miniature hyperspectral imagers, and relevant instruments has facilitated the transition of UAV-borne hyperspectral imaging systems from concept to reality. Given the merits and demerits of existing similar UAV hyperspectral systems, we presented a lightweight, integrated solution for hyperspectral imaging systems including a data acquisition and processing unit. A pushbroom hyperspectral imager was selected owing to its superior radiometric performance. The imager was combined with a stabilizing gimbal and global-positioning system combined with an inertial measurement unit (GPS/IMU) system to form the image acquisition system. The postprocessing software included the radiance transform, surface reflectance computation, geometric referencing, and mosaic functions. The geometric distortion of the image was further significantly decreased by a postgeometric referencing software unit; this used an improved method suitable for UAV pushbroom images and showed more robust performance when compared with current methods. Two typical experiments, one of which included the case in which the stabilizing gimbal failed to function, demonstrated the stable performance of the acquisition system and data processing system. The result shows that the relative georectification accuracy of images between the adjacent flight lines was on the order of 0.7–1.5 m and 2.7–13.1 m for cases with spatial resolutions of 5.5 cm and 32.4 cm, respectively.

Downstream interference effect of low-Scruton-number high-rise buildings under turbulent boundary layer flow

This paper investigates the variation of root-mean-square values of acceleration responses and corresponding aerodynamic dampings of two square cross-sectional high-rise buildings with low Scruton numbers due to interference effects caused by neighboring identical buildings. Aero-elastic vibration test is adopted for displacement and acceleration measurements of a rigid, base-pivoted aero-elastic principal building model by a two-directional sway gimbal system. The principal building is manufactured in the shape of a square prism model with an aspect ratio of eight. Twenty close interference locations are of interest, with a special focus on the across-wind responses. Results show that the variations of interfered acceleration responses and estimated damping ratios exhibit different tendencies for different interference locations at different reduced velocities. For the typical upstream interfered effect, the interfering building located at the oblique-upwind locations in this study, the acceleration response with lower Scruton number is suppressed at lower reduced velocities and is amplified at higher reduced velocities. On the other hand, for the secondary downstream interference effect, i.e., the interfering building located at downwind locations, the amplification or reduction in acceleration response is less sensitive to Scruton number but is related to the gap between two buildings. The corresponding response spectra and resultant damping ratios are then calculated to enhance the understanding of different interference mechanisms. Finally, an idealized two-dimensional CFD simulation technique is adopted to further explain the differences between the upstream and downstream interference mechanisms from the viewpoint of the vortex movement between two buildings.

Model predictive control of three-axis gimbal system mounted on UAV for real-time target tracking under external disturbances

The fact that Unmanned Aerial Vehicles (UAVs) move in a specific path and that the camera in the gimbal system mounted on the UAV adhere to the right target attracts the attention of many researchers. The effective control of the gimbal system directly affects the performance of the UAV which is tracking a predetermined moving target, following a specified path. The contribution of this study is not only modelling three-axis gimbal system mounted on mobile platform based on nonlinear Hammerstein block structure to control effectively using model predictive controller (MPC) but also improving real time target tracking performance under external disturbances. A novel Hammerstein model based MPC controller is successfully proposed for real time target tracking of three-axis gimbal system applying flight scenarios of UAV to be robust under external disturbances. In this study, firstly, the mathematical model of three-axis gimbal system mounted on UAV is developed based on traditional Newton–Euler method. Secondly, linear and nonlinear modeling based on the input and output data of the three-axis gimbal system mounted on UAV moving autonomously for target tracking is emphasized. The linear output error (OE) and nonlinear block structure Hammerstein models of the three-axis gimbal system are identified under the external disturbance effect, respectively. Then, the identified Hammerstein model is embedded in the flight control card, which included the three-axis gimbal system control to realize real time target tracking of the UAV. Afterwards, the MPC of three-axis gimbal system is realized under the external disturbance with linear and nonlinear models. Also, the performance of proposed MPC controller with Hammerstein model is evaluated comparing with conventional PID controller in terms of robustness and quantitative study of error analysis. Finally, the stability and robustness of the three-axis gimbal system controlled with the MPC algorithm has been investigated by the test results carried out in different scenarios. The simulation and experimental results show that the proposed MPC algorithm with Hammerstein model in this paper can ensure that the UAV exactly tracking the target while maintaining stability, even with external disturbances.

Anti-Disturbance Control Based on Cascade ESO and Sliding Mode Control for Gimbal System of Double Gimbal CMG

Disturbance torque and unmodeled dynamics are the main important factors which affect the angular velocity precision of a gimbal system in double gimbal control moment gyro (DGCMG). In this paper, a compound control method combining reduced-order cascade extended state observer (RCESO) and integral sliding mode control (SMC) is proposed, aiming at effectively eliminating the influence of exogenous disturbances and unmodeled dynamics, and achieving high precision angular velocity control of the gimbal system. Firstly, the disturbance torque is observed by the RCESO, and the parameter tuning method of RCESO is studied. Secondly, to enhance the robustness of the whole control system and take into account disturbance estimation errors, an integral sliding mode control is introduced. Simulation and experimental results show that the proposed control method not only improves the compensation performance of the gimbal system with external disturbances, but also enhances the robustness and improves the angular velocity precision of the gimbal system.

Coupled Dynamic Modeling and Analysis of the Single Gimbal Control Moment Gyroscope Driven by Ultrasonic Motor

A control moment gyroscope (CMG) is a key actuator for spacecraft's attitude and stability control. But there exist micro-vibrations from high-speed flywheel, which will affect the performance of the spacecraft greatly. Due to the unbalanced mass and the supporting structure, the high-speed rotating flywheel could generate disturbance torque, which will be passed to the gimbal to deteriorate the gimbal servo system. At the same time, the rotation of the gimbal will increase the disturbance torque for gyroscopic effect. According · to the bearing theory, the stiffness of the bearing is affected by the load that applied on it, which results the influence on the flywheel vibration and the generated disturbance torque. This means that there is a coupling effect in the CMG. In this article, we focused on this coupling relationship between the flywheel vibration and the gimbal rotation in a single gimbal control moment gyroscope (SGCMG) system. Firstly, a vibration model of the flywheel with variable stiffness supporting is established. And the rotation model of the gimbal is also developed. The flywheel model and the gimbal model are coupled with each other through the variable stiffness supporting and gyroscope effect. Based on the numerical simulations the interactions among the bearing stiffness, the flywheel vibration and the gimbal rotation are analyzed. Finally, experiments are carried out on the prototype. The experimental results have verified the theoretical analysis and the simulations, and which would support the design of high-performance control system.

Determination of uncertain parameters of a two-axis gimbal and motion tracking via Fuzzy logic control approach

This paper proposes a new method to improve fuzzy control performance accuracy in the stabilization of the two-axis gimbal system. To this end, due to the fact that the knowledge of the accurate behavior of the system plays a substantial role in fuzzy control performance, all the uncertain parameters of the dynamic model such as friction, mass imbalance and moments of inertia are estimated prior to the controller design and without imposing any computational burden on the control scheme. To estimate the uncertainties and disturbances which exist in the dynamic equations, an identification process formulated as an inverse problem is utilized, and the Gauss– Newton method is adopted for the optimization process. Regarding the severe sensitivity of inverse problems to measurement errors, this undesirable effect is reduced by using a proper smoothing technique. In order to increase the accuracy of the final results, a novel procedure for calculation of the sensitivity coefficients of the inverse problem is proposed. This procedure is based on the direct differentiation of the governing equations with respect to the unknown parameters. At the end, simulation results are presented to confirm the effectiveness of the proposed scheme.

The numerical and experimental investigations of a gimballed pendulum energy harvester

A simple or single-degree-of-freedom (1-DOF) pendulum system is a typical mechanism that has been used for vibration energy harvesting. Generally, a 1-DOF pendulum system is constrained to move on a single plane which limits the system when excited by multi-directional disturbances. In order to overcome this limitation, this paper considers the application of a 2-axis gimbal pendulum system. The gimballed pendulum system consists of two perpendicular horizontal pivots which allow the pendulum to achieve spherical or orbital motion around the pivots. Theoretical and experimental investigations of a gimballed pendulum energy harvesting system for multi-directional excitation are described in this paper. A mathematical model of the mechanical arrangement is derived based on the Lagrange equation and a simplified assumption that the perpendicular pivots are treated as two coupled rotating references with geometric relationships. This derived model is able to realistically reflect and identify the asymmetric properties between the coupled references created by a gimbal mechanism. The theoretical and experimental investigations of 1-DOF and coupled-DOF (2-DOF) pendulum dynamics are presented using a constant excitation amplitude over a range of frequencies with different heading angles representing the directions of the external disturbance. The numerical and experimental results show generally good correlation and the dominant coupling effect of the coupled rotating references that modulates the pendulum responses is theoretically and practically captured. Interestingly, the measured simultaneous electrical power production generated by the coupled dynamics of the gimballed pendulum achieves less power compared to when it dynamically performs as a 1-DOF system at the same disturbing condition.