Tether Tension Control Research Articles

Spin deployment of Hub-Spoke tethered satellite formation with sliding mode tether tension control

This work develops a tension control strategy for deploying an underactuated spin-stable tethered satellite formation in the hub-spoke configuration. First, the Lagrange equation is used to model the spin-deployment dynamics of the tethered satellite formation. The central spacecraft is modeled as a rigid body, and the tethered subsatellites are simplified as lumped masses. Second, a pure tension controller has been proposed to suppress the tether libration motion in the deployment without thrusting at the subsatellites. A nonlinear sliding mode control is introduced in the tension controller for the underactuated system to suppress the periodic gravitational perturbations caused by the spinning hub-spoke tethered satellite formation. The unknown upper bounds of the perturbations are estimated by adaptive control law. The bounded stability of the closed-loop tension controller has been proved by the Lyapunov theory. Finally, numerical simulations validate the effectiveness and robustness of the proposed controller, i.e., tethers are fully deployed stably to the desired hub-spoke configuration.

Dynamic Behavior Analysis and Stability Control of Tethered Satellite Formation Deployment.

In recent years, Tethered Space Systems (TSSs) have received significant attention in aerospace research as a result of their significant advantages: dexterousness, long life cycles and fuel-less engines. However, configurational conversion processes of tethered satellite formation systems in a complex space environment are essentially unstable. Due to their structural peculiarities and the special environment in outer space, TSS vibrations are easily produced. These types of vibrations are extremely harmful to spacecraft. Hence, the nonlinear dynamic behavior of systems based on a simplified rigid-rod tether model is analyzed in this paper. Two stability control laws for tether release rate and tether tension are proposed in order to control tether length variation. In addition, periodic stability of time-varying control systems after deployment is analyzed by using Floquet theory, and small parameter domains of systems in asymptotically stable states are obtained. Numerical simulations show that proposed tether tension controls can suppress in-plane and out-of-plane librations of rigid tethered satellites, while spacecraft and tether stability control goals can be achieved. Most importantly, this paper provides tether release rate and tether tension control laws for suppressing wide-ranging TSS vibrations that are valuable for improving TSS attitude control accuracy and performance, specifically for TSSs that are operating in low-eccentricity orbits.

Analytical and Experimental Investigation of Stabilizing Rotating Uncooperative Target by Tethered Space Tug

Attitude stabilization of a massive rotating uncooperative target by a tethered space tug is studied for orbit maneuvering in the postcapture operation analytically and experimentally. The perturbed orbital propagation of the tethered space system is described with nonsingular orbital elements and the relative equations of motion of the tethered space system, including the tether libration and attitudes of target and tug, are established in the orbital frame. The tether tension control, the tug's attitude control, and the combination control of them are designed to suppress the attitude and libration motions of the target/tug and the tethered space system with bounded stability. The control law's effectiveness is demonstrated by simulation and then validated experimentally by a microgravity testbed with two tethered free-floating air-bearing satellite simulators.

Tether tension control law design during orbital transfer via small-gain theorem

Issues of controller design via small gain theorem for tethered satellite system (TSS) during orbit maneuvering under transversal thrust are considered. First, dynamic equations of tethered satellite system in orbital maneuvering are established based on a dumbbell model. It is proved that the in-plane system is weak minimum phase by selecting a proper Lyapunov candidate function, and there are some difficulties for controller designing based on nonlinear model directly. Next, a controller is designed by utilizing Small-gain Theorem based on a control oriented model. An input-to-state stable (ISS) based tether rate control law is derived as the reference signal for the tether length system. A tether tension control law is proposed such that the uncertainties caused by linearization will be suppressed by properly selected control parameters and the closed loop system will be forced to a small neighborhood of origin as time approaches infinity. Finally, numerical simulation results demonstrate the validity of the proposed algorithm.

Stability and control of tethered satellite with chemical propulsion in orbital plane

The tethered satellite with chemical propulsion has broad application prospects in the disposal of abandoned satellites, the orbital rescue of spacecrafts, and the transportation of space supplies, which is completely different from the traditional applications of tethered satellites. Therefore, new research on its dynamics, stability, and control becomes useful and interesting. In this article, based on a dumbbell model of tethered satellite, the dynamics equations of tethered system in orbital maneuvering are established. Furthermore, according to the definitions of transversal and radial propulsive coefficients, analytical solutions of the equilibrium position for librational angle are derived during maneuvering in orbital plane; meanwhile, the effects of propulsive coefficients on librational stability are analyzed, which provides a basis for a selection of expected attitude trajectory. Then, a method of hierarchical sliding-mode tension control is presented to track the expected in-plane angle. This method can address the underactuated problem of tethered systems without either complex coordinate transformation for the system state model or constraint equation restrictions. During orbital flight, in-plane and out-of-plane angles are decoupled, so the tether tension control cannot be conducted to inhibit the out-of-plane angle. To solve this problem, the binormal component of thrust acceleration normal to the orbital plane is adopted as a control variable, and a feedback linearization-based thrust controller is designed to damp out the out-of-plane angle. Afterwards, orbital transfer cases between two circular orbits are studied to demonstrate the effectiveness of the tethered satellite with chemical propulsion. Numerical simulation results indicate that the stability of librational angles has a close relation to propulsive coefficients, and distributions of stable centers and unstable saddle points are totally different on both sides of bifurcation point. In addition, tracking control requirements for tethered satellite are guaranteed by designed controllers, which ensure flight safety in orbital maneuvering.

New Sensor and Actuator for Space Tether Control Technology

Tether technology is highly significant for the realization of large space flexible structures. It is possible to decrease the weight of a structure by employing tether on the system. In addition, a tether system can be employed to increase the stiffness of space structure systems and also to suppress the vibration of flexible space structures. When tether is used in a mission, the measurement of the tether's dynamic behavior, as both tension and speed, is necessary, in order to control the vibration of flexible space structures. The range of tension should approach micro level at 10-2 N in micro power, to control the structure in space. In this paper, the effectiveness of vibration control is examined by employing a tether on a flexible space structures such as a SSPS. The vibration control is analyzed numerically to simulate the behavior of a flexible space structure in a flexible panel-tether system and the effectiveness is shown for the tether tension control. A contactless measurement method is also studied by measuring the tether tension and the speed without contacting tether. This method allows the tether tension to be measured without any disturbance.

Dynamics of tethered payloads with deployment rate control

Introduction D of deployment of a tethered payload from an orbiting spacecraft has been studied by both lumped mass and continuum models. Although both models have advantages and disadvantages, the lumped-mass model is believed to be more general in that it describes small curvature and large elastic displacement of the tether. On the issue of control, both tether tension control and tether deployment rate control' have been proposed, though deployment rate control is definitely easier to mechanize. A theory for deployment rate control with a lumped-mass model of the tether has not been published in the literature, to the best of the author's knowledge. The present Note seeks to fill this void.

Effect of tether flexibility on the tethered Shuttle subsatellite stability and control

This paper investigates the effect of tether flexibility on the (in-plane) stability regions as a function of tether tension control parameters during stationkeeping. It is found that the size of the stability regions for the flexible tether is reduced considerably as compared with that for the rigid massive tether for some control parameters. An alternate optimal control law, which includes additional feedback of the first vibrational mode and its rate, is introduced; the results of stationkeeping simulations show that the transient responses of both the in-plane swing angle and vibrations are improved as compared with previously developed control laws. For retrieval a typical nonlinear control law, which includes the nonlinear feedback of the tether length, in-plane and out-of-plane swing angles and their rates, is developed. Simulation results show that the amplitudes of the in-plane and out-of-plane swing angles could be reduced significantly; also, it is demonstrated that the amplitudes of the tether vibrational modes should be considered for the selection of the control gains.

On the control of the space shuttle based tethered systems

Spatial dynamics of the Space Shuttle based tethered satellite system is investigated using a nonlinear model that accounts for the aerodynamic drag in a rotating oblate atmosphere. Results show that the normally unstable retrieval manoeuver can be stabilized satisfactorily using a nonlinear tether tension control strategy which depends on the tether length, its variation with time and pitch rate. Effectiveness of the control is illustrated through an example involving a 100 km tether supporting a proposed satellite for charting the earth's magnetic field.