Two-axis Gimbal Research Articles

Stabilization of the Two-Axis Gimbal System Based on an Adaptive Fractional-Order Sliding-Mode Controller

ABSTRACTThis paper introduces an adaptive fractional-order sliding-mode controller for stabilization of a two-axis gimbal platform in the presence of the torque disturbance effects. To tend the angular velocities of the inner gimbal in the elevation and azimuth axes to zero, an adaptive fractional-order sliding-mode approach is utilized. To achieve this goal, fractional-order sliding surfaces in both azimuth and elevation axes and the corresponding adaptive controllers with adjustable parameters tuned according to Lyapunov-based adaptation mechanisms are employed. The optimal value of the fractional order is obtained through an integral square error performance index minimization. The moments products uncertainties are incorporated in the design procedure. The numerical simulations demonstrate that the proposed control approach is robust against the moments products variations.

Selection of scanners for use in lidar systems

We compare the parameters for three types of scanners used in terrain-measurement lidar systems to facilitate spacecraft landing safety on extraterrestrial objects. The following 2D scanner designs are discussed: a rotating-wedge scanner with a straight-through optical path (Risley scanner), a scanner with a 12-sided line-scan prism, a frame deflector with a uniaxial oscillating mirror, and a scanner with an oscillating mirror mounted in a two-axis gimbal. We calculated the scan trajectories and estimated the required software-based object-plane beam corrections in the scanner-system deflector angle coordinates.

Thrust vector control of upper stage with a gimbaled thruster during orbit transfer

In launching Multi-Satellite with One-Vehicle, the main thruster provided by the upper stage is mounted on a two-axis gimbal. During orbit transfer, the thrust vector of this gimbaled thruster (GT) should theoretically pass through the mass center of the upper stage and align with the command direction to provide orbit transfer impetus. However, it is hard to be implemented from the viewpoint of the engineering mission. The deviations of the thrust vector from the command direction would result in large velocity errors. Moreover, the deviations of the thrust vector from the upper stage mass center would produce large disturbance torques. This paper discusses the thrust vector control (TVC) of the upper stage during its orbit transfer. Firstly, the accurate nonlinear coupled kinematic and dynamic equations of the upper stage body, the two-axis gimbal and the GT are derived by taking the upper stage as a multi-body system. Then, a thrust vector control system consisting of the special attitude control of the upper stage and the gimbal rotation of the gimbaled thruster is proposed. The special attitude control defined by the desired attitude that draws the thrust vector to align with the command direction when the gimbal control makes the thrust vector passes through the upper stage mass center. Finally, the validity of the proposed method is verified through numerical simulations.

Coupling effect and control strategies of the maglev dual-stage inertially stabilization system based on frequency-domain analysis

Maglev dual-stage inertially stabilization (MDIS) system is a newly proposed system which combines a conventional two-axis gimbal assembly and a 5-DOF (degree of freedom) magnetic bearing with vernier tilting capacity to perform dual-stage stabilization for the LOS of the suspended optical instrument. Compared with traditional dual-stage system, maglev dual-stage system exhibits different characteristics due to the negative position stiffness of the magnetic forces, which introduces additional coupling in the dual stage control system. In this paper, the coupling effect on the system performance is addressed based on frequency-domain analysis, including disturbance rejection, fine stage saturation and coarse stage structural resonance suppression. The difference between various control strategies is also discussed, including pile-up(PU), stabilize-follow (SF) and stabilize-compensate (SC). A number of principles for the design of a maglev dual stage system are proposed. A general process is also suggested, which leads to a cost-effective design striking a balance between high performance and complexity. At last, a simulation example is presented to illustrate the arguments in the paper.

Inertially Stabilized Platform for Airborne Remote Sensing Using Magnetic Bearings

Multiaxis gimbal configurations are commonly used in the line of sight stabilization systems for airborne remote sensing. Although these configurations are cost-effective, system performances deteriorate in the case of large payload, due to nonlinear friction and bandwidth limit of the servo loops. In this paper, a novel maglev dual-stage inertially stabilized platform is described, which encompasses a two-axis gimbal assembly as the coarse stage and a 5-DOF (degree of freedom) magnetic bearing as the fine stage. The coarse stage is mounted on the aircraft consisting of a lateral gimbal and an elevation gimbal to isolate the large angular motion of the vehicle and provide coarse pointing. The payload instrument is suspended in the elevation gimbal utilizing an electromagnetic bearing with vernier gimbaling capability to compensate the residual angular motion of the gimbal. An actual prototype of the proposed platform was designed and fabricated to illustrate the framework. The performance of the magnetic suspension system is investigated through experiments. Detailed models are established for the magnetic suspension system and the gimbal stabilizing system to develop the control strategy of the dual-stage system. Finally, the effectiveness of the design is demonstrated through simulation studies, where a 3.5 times stabilization precision improvement is prefigured compared to a reference conventional gimbal system.

A Passive Parallel Master-Slave Mechanism for Magnetic Resonance Imaging-Guided Interventions.

A passive, parallel master-slave mechanism is presented for magnetic resonance imaging (MRI)-guided interventions in the pelvis. The mechanism allows a physician to stand outside the MRI scanner while manipulating a needle inside the bore and, unlike a powered robot, does not place actuators in proximity to the patient. The manipulator combines two parallel mechanisms based on the Delta robot architecture. The mechanism also includes a two-axis gimbal to allow for tool angulation, giving a total of five degrees of freedom so that the physician can insert and steer a needle using continuous natural arm and wrist movements, unlike simple needle guides. The need for access between the patient's legs and within the MRI scanner leads to an unusual asymmetric design in which the sliding prismatic joints form the vertices of an isosceles triangle. Kinematic analysis shows that the dexterity index of this design is improved over the desired workspace, as compared to an equilateral design. The analysis is extended to estimate the effect of friction and model the input:output force transmission. Prototypes, with final dimensions selected for transperineal prostate interventions, showed force transmission behavior as predicted by simulation, and easily withstood maximum forces required for tool insertion.

Sensor-driven area coverage for an autonomous fixed-wing unmanned aerial vehicle.

Area coverage with an onboard sensor is an important task for an unmanned aerial vehicle (UAV) with many applications. Autonomous fixed-wing UAVs are more appropriate for larger scale area surveying since they can cover ground more quickly. However, their non-holonomic dynamics and susceptibility to disturbances make sensor coverage a challenging task. Most previous approaches to area coverage planning are offline and assume that the UAV can follow the planned trajectory exactly. In this paper, this restriction is removed as the aircraft maintains a coverage map based on its actual pose trajectory and makes control decisions based on that map. The aircraft is able to plan paths in situ based on sensor data and an accurate model of the on-board camera used for coverage. An information theoretic approach is used that selects desired headings that maximize the expected information gain over the coverage map. In addition, the branch entropy concept previously developed for autonomous underwater vehicles is extended to UAVs and ensures that the vehicle is able to achieve its global coverage mission. The coverage map over the workspace uses the projective camera model and compares the expected area of the target on the ground and the actual area covered on the ground by each pixel in the image. The camera is mounted on a two-axis gimbal and can either be stabilized or optimized for maximal coverage. Hardware-in-the-loop simulation results and real hardware implementation on a fixed-wing UAV show the effectiveness of the approach. By including the already developed automatic takeoff and landing capabilities, we now have a fully automated and robust platform for performing aerial imagery surveys.

A Comprehensive Model of a Tracker System Based on Motion Equations of a Two Degree-of-Freedom Gimbal System

In this paper a comprehensive model of a tracker system including derive subsystem is presented. Since the main part of the under consideration system consists a two Degree-Of-Freedom (DOF) gimbal subsystem, at first, the model of a two-axis gimbal is considered. For this purpose, after introducing the coordinate systems and different transformation matrices, the related equations of pitch and yaw axes by considering the friction torques, mass unbalances and cable restraint torques are derived. Moreover, all disturbance terms and the methods for their reduction or elimination are investigated. Then, the derive system of the gimbal including a DC motor with a gear is modeled and governing overall equations of the whole system is obtained. In order to have a model with practical considerations, the model of rate gyro which is used for measuring the angular velocities is also included. Finally, the simulation results of the under consideration case study by using the derived comprehensive dynamical equations of this paper are provided. Analysis of these results are used to synthesis the behavior of the practical system and choosing the suitable control structure in order to achieve stabilization and tracking objectives.

Design and Implementation of Electronic Controllers and its Automated Vision System

In this paper a model of a two-axis gimbal is revised. Its performance, in front of the external perturbation, is presented for a PD controller, a fuzzy logic controller and a model reference adaptive controller. A robotized gimbal system is implemented in which real and practical experiments are performed. Its dynamic performance, in front of external perturbations, is analyzed, evaluated, and the results are presented.

The Upper Atmosphere and Plasma Imager/the Telescope of Visible Light (UPI/TVIS) onboard the Kaguya spacecraft

Abstract The Upper Atmosphere and Plasma Imager (UPI) was placed in a lunar orbit in order to study both the Moon and Earth. The UPI consists of two telescopes: a Telescope of Extreme Ultraviolet (TEX) and a Telescope of Visible Light (TVIS), which are both mounted on a two-axis gimbals system. The TVIS is equipped with fast catadioptric optics and a high-sensitivity CCD to image swift aurora and dark airglow in the terrestrial upper atmosphere. TVIS has a field-of-view equivalent to the Earth’s disk as seen from the Moon. The spatial resolution is about 30 km × 70 km on the Earth’s surface at auroral latitudes. The observation wavelengths can be changed by selecting different bandpass filters. Using the images of the northern and southern auroral ovals taken by TVIS, the intensities and shapes of the conjugate auroras will be quantitatively compared. Using the airglow imaging, medium- and large-scale ionospheric disturbances will be studied. In this paper, the instrumental design and performance of TVIS are presented.

Modeling and Consideration of AMB-Flywheel Supported by Two-axis Gimbals

A flywheel energy storage system using an active magnetic bearing (AMB) is an effective method for storing electric power and reducing energy consumption. The stabilization of a flywheel rotor system supported by magnetic bearings fixed on the ground has been done. Currently, an electrical vehicle with a flywheel energy storage system using a magnetic bearing and a two-degree-of freedom gimbal which means two-axis gimbal have been carrying out. In this paper, the mathematical model for both the flywheel and the gimbals has been derived. Also, a discussion of the gimbal design is presented. It was verified through experiments that the proposed model in this paper is very effective.

Target-acquisition performance in undersampled infrared imagers: static imagery to motion video

In this research we show that the target-acquisition performance of an undersampled imager improves with sensor or target motion. We provide an experiment designed to evaluate the improvement in observer performance as a function of target motion rate in the video. We created the target motion by mounting a thermal imager on a precision two-axis gimbal and varying the sensor motion rate from 0.25 to 1 instantaneous field of view per frame. A midwave thermal imager was used to permit short integration times and remove the effects of motion blur. It is shown that the human visual system performs a superresolution reconstruction that mitigates some aliasing and provides a higher (than static imagery) effective resolution. This process appears to be relatively independent of motion velocity. The results suggest that the benefits of superresolution reconstruction techniques as applied to imaging systems with motion may be limited.

Performance of active vibration control technology: the ACTEX flight experiments

This paper discusses the development and results of two intelligent structures space-flight experiments, each of which could affect architecture designs of future spacecraft. The first, the advanced controls technology experiment I (ACTEX I), is a variable stiffness tripod structure riding as a secondary payload on a classified spacecraft. It has been operating well past its expected life since becoming operational in 1996. Over 60 on-orbit experiments have been run on the ACTEX I flight experiment. These experiments form the basis for in-space controller design problems and for concluding lifetime/reliability data on the active control components. Transfer functions taken during the life of ACTEX I have shown consistent predictability and stability in structural behavior, including consistency with those measurements taken on the ground prior to a three year storage period and the launch event. ACTEX I can change its modal characteristics by employing its dynamic change mechanism that varies preloads in portions of its structure. Active control experiments have demonstrated maximum vibration reductions of 29 dB and 16 dB in the first two variable modes of the system, while operating over a remarkable on-orbit temperature range of -80 °C to 129 °C. The second experiment, ACTEX II, was successfully designed, ground-tested, and integrated on an experimental Department of Defense satellite prior to its loss during a launch vehicle failure in 1995. ACTEX II also had variable modal behavior by virtue of a two-axis gimbal and added challenges of structural flexibility by being a large deployable appendage. Although the loss of ACTEX II did not provide space environment experience, ground testing resulted in space qualifying the hardware and demonstrated 21 dB, 14 dB, and 8 dB reductions in amplitude of the first three primary structural modes. ACTEX II could use either active and/or passive techniques to affect vibration suppression. Both experiments trailblazed spacecraft bus smart structures by developing over 20 new technologies. As pathfinders, experience was gained in the implications of space system analyses, verification tests, and for ways to leverage this technology to meet new satellite performance requirements.

A high-gain trireflector antenna configuration for beam scanning

A trireflector antenna configuration is presented that allows for beam scanning with minimum reflector motion, no feed motion, and high primary aperture utilization for all scan directions. Such characteristics are particularly desirable for geostationary satellite applications. Omnidirectional beam scanning is achieved by rotational movement of a relatively small tertiary reflector about a fixed point such that a simple two-axis gimbal can be employed. The trireflector configuration was specifically designed for high-gain passive remote sensing applications, and therefore the emphasis is placed on high-gain beam scanning; however, the concepts developed are equally valid for low-gain applications and can find use in a broad range of applications that require rapid beam scanning and simple reflector motion.

Fine tuning of linear accelerator accessories for stereotactic radiotherapy

Purpose : Experience with the University of Wisconsin's stereotactic radiotherapy (SRT) accessory system was applied to build a new system, facilitate alignment of linac photon beams with a Brown-Roberts-Wells (BRW) stereotaxy, and increase the versatility and stability of the stereotaxy. Methods and Materials : High tensile strength stainless steel was used in the floor stand to increase the range of gantry rotation relative to ranges allowed by truss-mounted stands. The collimator assembly and floor stand were each fitted with two-axis gimbal and translation adjustments in addition to the floor stand's three-axis adjustments. The head ring positioning assembly was fitted with two braces to prevent the head ring from deforming with patient motion. Six MV linac photon beam characteristics were measured with a computer-controlled scanning system and a diode in water, at source to surface distances (SSD) of 80 and 100 cm, and for 13 divergent collimators ranging in diameter from 1–4 cm at 100 cm SSD. Quality assurance software was applied to screen data for questionable consistency or symmetry. Integrity of the stereotaxy was evaluated with target simulation films and repeated measurements which were part of the quality assurance of clinical treatments. A method was developed using a glass etched contact reticle to obtain average simulated target to beam center distances (δav) from target simulation films. Results and Conclusion : New aspects of the current system have improved the ability to fine tune and analyze stereotactic alignment. Beam characteristics met stringent output criteria and penumbral widths were the same or narrower than penumbral widths reported elsewhere. The precision of measuring δav was 0.1 mm, and δav averaged over 50 target simulation films was 0.7 ± 0.1 mm. Results suggest that it may be useful to determine δav from target simulation films with the method described here.

Design and fabrication of a miniaturized optical correlator

A miniature Vander Lugt optical correlator has been built to demonstrate this technology for guidance and navigation applications. Using a liquid crystal light valve as the input spatial light modulator, a cemented set of Porro prisms to fold the light path, and a combination holographic matched spatial filter and optical element, this rugged assembly is a prototype seeker head for an Army missile. The correlator is battery operated and mounts in a precision two-axis gimbal. The system correlates on aerial imagery and guides the missile to its preselected ground target. The correlator is 105 mm in diameter and 90 mm long. Including input optics, it weighs 2.3 kg.

Precision Stabilization Systems

This paper discusses the relationships that must be considered in analyzing the design and performance of typical precision stabilization systems. In particular, the disturbance inputs that are due to kinematic and geometrical coupling are examined in depth by considering systems implemented with either a conventional two-axis gimbal or a traditional half-angle mirror assembly. The disturbance coupling that is associated with the gimbal drive actuators and the effect of structural compliance on the stabilization control loops are also discussed in detail. Other aspects of stabilization system design that are considered include the potential improvements in performance that can be achieved by using appropriate multiaxis gimbal configurations and by utilizing the angular pointing characterisitics of beam expansion telescopes.

Precise Attitude Stabilization of a Large Orbiting Telescope Coupled to a Crew Compartment

Computer simulation results are presented for the planar equations of motion of an attitude-stabilized orbiting telescope passively coupled to a manned service module. This coupling is provided through a set of soft springs and a two-axis gimbal aligned with the telescope center of mass. Principal nonlinearities in the suspension system and the telescope control system are included. The simulation indicates preliminary nary feasibility of this operational mode.