Canadian researchers have proposed a design for a new radio telescope consisting of an array of very large ree ectors. A largemultitethered aerostat will be used to support each ree ector’ s receiver. The length of each tether can be adjusted by ground-based winches, and active control is used to overcome the effects of wind turbulence on the aerostat. To aid in the system’ s mechanical and controller design, a computer model has been developed, including models of the tethers and the aerostat. Two types of aerostat have been considered: spherical and streamlined. A wind turbulence model is incorporated to provide disturbance to the system. Controllers are used to adjust the tether lengths based on feedback of the receiver’ s position. The model is used to optimize the control gains. We e nd that, with 50 kW available at each winch, it is possible to maintain the receiver’ s position within 85 cm of its desired location, even in the worst-case cone guration.

Dynamics/Control of a Radio Telescope Receiver Supported by a Tethered Aerostat

This paper considers the problems of dynamically modeling and automatically controlling the motion of a small maneuverable flight vehicle that is being towed by a much larger one. Mathematical models of the components of the system, including the towing aircraft, tow cable reel mechanism, tow cable, and vehicle aerodynamics, are described. The development of an autopilot for stability augmentation and maneuvering of the towed vehicle is discussed. An overview of the implementation of the models in a digital computer simulation program is provided, and some typical results obtained using the program are presented. A comparison of theoretical and experimental results is also made. ROM the early days of powered flight, there has been an interest in towing objects from aircraft. Objects that have been towed include antennas, banners, gliders, and targets. Because of the interesting, and sometimes troublesome, characteristics of the motions of some towed objects and the cables, or towlines, the dynamics of the cables and the towed objects have been studied by several investigators at various times during the last 60 years.1'10 The problem of describing the motion of a towed cable/ object system is far from trivial. The equations of motion are, in general, coupled, ordinary, and partial differential equations (i.e., hybrid equations). Various approaches have been taken in studying such systems. Glauert1 and Phillips2 did some pioneering work on the stability of the motion of bodies towed from aircraft. Phillips3 also analyzed the stability of the motion of a cable used to tow a nonlifting body. He used a continuum model for the cable and studied the effects of towing speed on the stability of lateral oscillations of the cable and concluded that the oscillations would be damped out if the speed of flight were less than the speed of propagation of vibrational waves along the cable.

Dynamics and Control of Maneuverable Towed Flight Vehicles

This paper presents a dynamics analysis of a streamlined aerostat tethered to the ground by a single tether. A nonlinear dynamics model of this system is e rst assembled. The tether is modeled using a lumped mass approach, and the viscoelastic properties of the tether are included. The aerostat is modeled using a component breakdown approach. The dynamics equations of the cable and aerostat are then assembled into a single system of nonlinear differential equations. A linearization of this system is then performed using a e nite difference approach. The resulting linear equations of motions are decoupled into longitudinal and lateral subsets. The stability properties of each subset is then studied as a function of wind speed. The effect of varying tether length is also investigated.

Stability Analysis of a Tethered Aerostat

When the towpoint of an aerial cable system moves in a tight circular path, the drogue at the cable tip moves towards the center of the circle, and its altitude decreases relative to its equilibrium position in forward flight. Such a system has both military and civilian applications, including remote pickup and delivery of payloads. This work studies the transitional dynamics of such a system as the aircraft changes from straight flight to circular flight. The system dynamics are modeled using a discretized cable model, allowing the cable to take on zero tension values. Numerical simulation results show that the cable becomes slack during the transition if the aircraft turns too rapidly. Parametric studies of the towpath are performed for both tow-in and tow-out maneuvers. Tension waves can be reduced by appropriate control of the towpoint. Simulated annealing is used to optimize some parameters used to specify the tow-in maneuver. Alternatively, a deployment controller is developed using fuzzy logic that avoids some of these problems by deploying the cable while I the aircraft orbits. Instability of deployment for certain combinations of cable length and length rate are observed.

Dynamics of Circularly Towed Aerial Cable Systems, Part 2: Transitional Flight and Deployment Control

This paper presents a strategy for optimal estimation of parameters for towed cable systems using moving horizon estimation (MHE). The main contributions of the work include a novel formulation of MHE using a dead-band that explicitly rejects measurement noise, real-time implementation results, and the investigation of time-varying stochastic disturbances as well as unknown yet constant disturbances. Further analysis is conducted on the observability and sensitivity of key parameters to determine which parameters can be estimated by the proposed approach using real-time streaming data from experiments. In addition to the real-time results, an offline multiobjective optimization is conducted to reveal the tradeoff between the computational burden and estimation variation. A Pareto frontier is used to demonstrate a series of optimal experimental configurations obtained by adjusting the model complexity (number of cable links and horizon steps). Finally, a multivariate parameter estimation is performed to explore the applicability of the proposed approach in simultaneously estimating multiple parameters.

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Parameter estimation for towed cable systems using moving horizon estimation

A mn-linear dynamic simulation quter program for a tethered aerostat has been developed ad applied to the study of aerostat response to turbulence and other disturbances. The theoretical &el includes a dynamic tether, dynamic rmtion of the ballonet air and six degrees of freedm for the aerostat. Static and dynamic coefficients measured in a rotating-arm tow tank are employed and the equations of mtion are written for a reference other than the mss center. A traveling-wave turbulence model has also been developed. In a program verification study the flight of an instnntr=nted amstat is compared with simulated flight with input winds reconstructed fm msweplwts on the amstat.

J.A. Krausman

Papers

Nonlinear Dynamic Simulation of a Tethered Aerostat

Nonlinear dynamic simulation of a tethered aerostat