Renuganth Varatharajoo

SPHERES interact—Human–machine interaction aboard the International Space Station

The deployment of space robots for servicing and maintenance operations that are teleoperated from the ground is a valuable addition to existing autonomous systems, because it will provide flexibility and robustness in mission operations. In this connection, not only robotic manipulators are of great use, but also free-flying inspector satellites supporting the operations through additional feedback to the ground operator. The manual control of such an inspector satellite at a remote location is challenging, because navigation in three-dimensional space is unfamiliar and large time delays can occur in the communication channel. This paper shows a series of robotic experiments, in which free flyers are controlled by astronauts aboard the International Space Station (ISS). The Synchronized Position Hold Engage Reorient Experimental Satellites (SPHERES) were utilized to study several aspects of a remotely controlled inspector satellite. The focus in this case study is investigating different approaches to human–spacecraft interaction with varying levels of autonomy under zero-gravity conditions.

Operation for the combined energy and attitude control system

Purpose – An architecture is proposed based on the torque mode operation for the combined energy and attitude control system (CEACS). The CEACS energy storage and attitude control performances are demonstrated.Design/methodology/approach – All the relevant system equations are established. The torque mode‐based architecture is designed. The architecture is evaluated through numerical treatments.Findings – The proposed CEACS architecture can simultaneously manage the energy storage and attitude control tasks.Research limitations/implications – This research work is exclusively for small satellites in the LEO missions. However, the proposed CEACS architecture is applicable for other missions.Practical implications – The torque mode operation will allow an ironless motor/generator design for CEACS.Originality/value – The results demonstrate the CEACS operation in the torque mode.

A review of conventional and synergistic systems for small satellites

Purpose – To compare the conventional reaction wheel and battery systems with the combined energy and attitude control system. The system mass, volume and power requirements are revealed corresponding to the small satellite missions.Design/methodology/approach – All the relevant system parametric equations are established. The system mass, volume and power are estimated accordingly for the conventional and the combined systems. Then, both systems are compared with respect to the typical small satellite missions.Findings – The combined system outperforms the conventional system in most small satellite missions. However, there are some small satellite missions where the conventional systems are better in terms of the mass and volume budgets.Research limitations/implications – This research work is exclusively for small satellites in the LEO orbits.Practical implications – A reasonable information for sizing the combined energy and attitude control system is established. The system mass, volume and power bud...

Evaluation of a spacecraft attitude and rate estimation algorithm

Purpose – This paper aims to present the development and performance evaluation of an attitude and rate estimation algorithm using an extended Kalman filter structure based on a body‐referenced representation of the state.Design/methodology/approach – The algorithm requires only geomagnetic field data and can be used as a low‐cost alternative or as a back‐up estimator in the case of attitude sensor failures. The satellite rate is estimated as a part of the filter state and thus no gyroscope is necessary. The assessment of the algorithm performance is realized through a Monte Carlo simulation using a low‐Earth orbit, nadir‐pointing satellite.Findings – Given some attitude and rate error requirements, the range of admissible initial errors on the filter state and the effect of un‐modelled disturbance torque are determined, along with the achievable attitude and rate accuracies.Practical implications – Because the simulation set‐up is clearly stated, the results of this evaluation can be used as a benchmark ...

Approach for Combining Spacecraft Attitude and Thermal Control Systems

Coupling of existing spacecraft subsystems could be an alternative to reduce the emerging cost, decrease the system volume/mass and complexity, increase the reliability, and enhance the overall performance of future spacecraft. Therefore, a new concept to combine the attitude and thermal control systems is presented. The concept is based on electric conducting e uids used in a closed loop for the attitude and thermal control systems. The potential coupling is discussed for two alternative setups incorporating electromagnetic pumping and thermoelectric phenomena, respectively. The corresponding equations are derived and the end-to-end system demonstration is performed. The results demonstrate that the new concept could replace the existing subsystems using current available technology, especially the thermoelectric option, when used as a combined attitude actuator and thermal radiator.

A novel design of spacecraft combined attitude & sun tracking system using a versatile fuzzy controller

Purpose – The purpose of this paper is to develop a theoretical design for the alternative attitude control of the rotation about the pitch axis for the nadir-pointing spacecraft in the event of inertial actuator faults. Design/methodology/approach – This paper presents a novel and viable solution to that problem using the combined attitude and sun tracking system (CASTS) that was conceived from an engineering problem-solving toolkit called TRIZ. Linear and fuzzy controllers are used to test the spacecraft CASTS architecture. All the relevant governing equations of the control system and disturbance rejection methods are developed. Findings – The performance of the proposed CASTS control strategy is tested through numerical simulations. The results strongly suggest that the novel proposed control scheme is effective and promising for controlling the satellite attitude and sun tracking simultaneously in the presence of disturbance torques. Research limitations/implications – This work is mainly focused on ...

Thermal Buckling and Post-Buckling Improvements of Laminated Composite Plates Using Finite Element Method

Thermal buckling and thermal post-buckling behaviours of laminated composite plates are improved by embedding shape memory alloy wires within laminates of composite plates. The procedure is to use the recovery stress that is induced when the reverse transformation of the shape memory alloy from martensite to austenite phases is constrained. For aerospace applications where the source of the shape memory alloy heating is the high temperature environment itself, a study is conducted to see the effect of shape memory alloy in improving the thermal buckling and post-buckling of composite plates. Due to the temperature dependent nature of the composite matrix and the shape memory alloy, the finite element formulation developed here is in the incremental form. Solving this non-linear model using the developed in-house source code, critical loads are determined and the post-buckling paths of the shape memory alloy composite plates are traced. This study shows that by embedding the shape memory alloy within composite plates, the thermal buckling and post-buckling behaviours of composite plates can be improved substantially.

Enhanced attitude control structure for small satellites with reaction wheels

Purpose – This paper aims to describe a design enhancement for the satellite attitude control system using reaction wheels, and the wheel momentum unloading using magnetorquers. Design/methodology/approach – The proportional – integral–derivative-controller and active force control (AFC) schemes are developed together with their governing equations for closed loop system of attitude control. Four numerical simulations were carried out using the Matlab – Simulink™ software and results were compared. Findings – From the results, it is evident that the attitude accuracies for roll–pitch–yaw axes have improved significantly through the proportional – derivative (PD) – AFC controller for the attitude control and the wheel momentum can be well maintained during the momentum unloading scheme. The results show that the AFC has a high potential to be implemented in the satellite attitude control system. Practical implications – Using AFC, the actual disturbance torque is considered totally rejected by the system w...

Analytic solutions for Tethered Satellite System (TSS) subject to internal and external torques undergoing a spin-up and spin-down maneuvers

Purpose – The purpose of this paper is to develop analytic solutions for a tethered satellite system (TSS) subjected to internal tether tension moment and external aerodynamic torque for spin-up and spin-down manoeuvres. Design/methodology/approach – Analytic solutions for TSS based on the approximation of Euler’s equations of motion via Fresnel integrals and sine and cosine integrals. Test simulation was performed for two cases (spin-up and spin-down manoeuvres). The conclusion is based on graphical interpretation. Findings – The effects of angular velocities on X, Y and Z axes of the TSS under the influence of combined torques from internal tether tension and external aerodynamic drag influenced during spinning manoeuvres are shown graphically. Research limitations/implications – This research focuses only on a circular orbit, which is one of the simplest orbits without many variables taken into account such as flight path angle and true anomaly. It could get quite complex for other orbit types like ell...

Sliding Mode Control Techniques for Combined Energy and Attitude Control System

Combined Energy and Attitude Control System (CEACS) is an optimization approach that combines the energy storage system and the attitude control system. With a double counter rotating flywheel simultaneously serving as energy storage device and as attitude control actuator, CEACS requires an accurate control strategy to obtain the mission requirements. In addition, it is important to design the control law to be invariant to uncertainties and disturbances, and guarantee robustness as CEACS inherits these in-orbit uncertainties. This paper presents a nonlinear control employing sliding mode to enhance the CEACS attitude control capability. The mathematical model for the conventional and boundary layer sliding mode controls are developed herein for CEACS. The controller provides enhancement in pointing accuracies, reasonable transient responses and a robustness against uncertainties and in-orbit disturbances.