# Dr Ranjan Vepa

*BTech(IITMadras) MASc(Wat) PhD(Stan)*

## Research Overview

Simulation, control engineering, aeroelasticity, smart structures, flow control, biomimetic robotics, biomedical control systems

### Interests

The research is dedicated to the study of dynamics and its control of robotics and biomedical systems with applications in mobile robots, aerospace vehicles and UAVs, and energy systems. The following projects are currently being addressed:

Modelling and Active Control of a Spacecraft Thrusters Dynamics: In this project, the possibility of feedback servo control of a spacecraft thruster’s specific impulse by a boundary feedback system is theoretically considered. The motivation to introduce feedback control is two-fold. The first is to stabilize any inherent plasma instabilities and the second is regulate the output specific impulse of the thruster. Two cases are considered: Electro-thermal thrusters and Electro-dynamic thrusters. A paper entitled "Feedback Control of a Spacecraft Electro-dynamic Thruster" has been accepted for publication in Acta Astronautica. The methodology is also being currently extended to other electrodynamic thrusters, Hall current thrusters and magnetic nozzles to facilitate the complete regulation and control of the specific impulse and the thrust of a spacecraft thruster.

Stability and Control of Orbiting Space Manipulators and Planetary vehicles: In this project the Gravity Gradient Stability of a satellite and stability and control of the attached manipulator system is investigated to assess the satellite’s overall stability in all of the equilibrium configurations of the manipulator. Also being considered is the stability and control of planetary rovers which are also supporting large manipulators. The navigation of such rovers and the application of SLAM is also of interest. A book on Dynamics and Control of Autonomous Space Vehicles and Robotics was published by Cambridge University Press in May, 2019.

Joint Position Localisation of Spacecraft and Debris for Autonomous Space Navigation: Applications In this project the nonlinear relative motion equations of an orbiting body relatively close to an elliptic reference orbit are derived directly in terms of the additional perturbation forces acting on the body and in terms of four of the classical osculating orbital elements. The relative motion dynamics of the satellite and a debris body, both of which are modelled relative to the same reference elliptic orbit, include perturbations. The dynamic responses are filtered to remove the fast and secular motions of the orbiting body. The dynamics of the debris and the satellite and the unscented Kalman filter (UKF) are employed to estimate the position of the debris body relative to the spacecraft in an elliptic Earth orbit. Further applications of this work to orbiting satellites in various configurations are also being considered. While several papers related to this topic have already been published in the Journal of Navigation, a paper relating directly to the current work is under review.

Optimal trajectories for asteroid rendezvous: Near-Earth objects (NEOs) moving in resonant, Earth-orbit like orbits are potentially important, as there is always the possibility of a few of them colliding with the Earth at some point in the future. The appearance of potentially hazardous NEOs has led to the concept of asteroid deflection. In this project the problem of synthesizing an optimal trajectory to a NEO such as an asteroid is considered. A particular strategy involving the optimization of a co-planar trajectory segment that permits the satellite to approach and fly alongside the asteroid is chosen. Two different state space representations of the Hill Clohessy Wiltshire (HCW) linearized equations of relative motion are used to obtain optimal trajectories for a spacecraft approaching an asteroid. It is shown that by using a state space representation of HCW equations where the secular states are explicitly represented, the optimal trajectories are not only synthesized rapidly but also result in lower magnitudes of control inputs which must be applied continuously over extended periods of time. Thus the solutions obtained are particularly suitable for low thrust control of the satellites orbit which can be realized by electric thrusters. This work has been submitted for publication and is under review. It is also being extended to deal with asteroids in complex orbits.

Optimal Design and Simulation of All Electric Aircraft (Phd Project): The recent upheavals in weather patterns have led several scientist to believe that we should reduce our dependence on fossil fuels. Several designs of all electric aircraft have been proposed. These designs offer tremendous scope for the optimization of the aerodynamic shape and design, the structural design of the aircraft, the propulsion motors and the energy storage systems or batteries and the flight path that has to be tracked as the aircraft traverses between two airports. In this project the following aspects are under consideration: i) The Optimal Control (for max thrust) of Ducted Fans for Distributed Propulsion ii) Modelling and Estimation of Batteries all-electric aircraft & vehicles iii) All Electric A/C: Flight Path Optimisation A book entitled Electric Aircraft Dynamics: A Systems Engineering Approach, is due to be published by CRC press in May 2020, highlighting the current problems hurdles in the development of an all-electric aircraft.

Transonic Unsteady Potential Flow over Very Large Aspect Ratio Wings (Funded Post Doctoral Work): In this project, the prediction of the unsteady flow field over typical large aspect ratio (AR) wings in the transonic flow regime but below the sonic Mach number is of interest. In particular, the prediction of the velocity potential, the pressure field and the lift and the induced drag of high AR wings is successfully undertaken. The methodology adopted is a computational approach based on the transonic small disturbance unsteady potential equation. The method was first validated by applying it to NASA's common research model (CRM) wing, the AGARD 445.6 wing model and the ONERA M6 wing. In order to avoid any difficulties due to the inherent instabilities and uniqueness of the solutions in the flow regime of interest, the flow field and its features are first computed for a typical high AR wing, which is of a much lower AR than a similar planform as the wing under consideration. The wing is gradually ‘morphed’ increasing its AR in small increments, so in the end the AR of the planform and its shape are that of the wing under consideration. It is shown that the higher AR wings generally have a higher lift coefficient as well as a higher lift to drag ratio. With NASA's common research model (CRM) wing, there is an increase in maximum lift with increasing AR while the induced drag is almost the same. There is also an optimum sweep angle, which is different for each angle of attack so that variable sweep lifting surfaces may be designed to provide optimum solutions. The computed flutter speeds indicate an expected reduction with increasing AR. This work has been submitted for publication and is under review. It is planned to extend the methodology to non-conventional wings and configurations.

Active Flutter Suppression System in the Transonic Domain using a Computational Model (Funded Post Doctoral Work): In this project the problem of synthesising an active flutter suppression system for a fluttering wing in the transonic domain using a computational model, such as the transonic, small disturbance, non-linear potential analysis method, is addressed. Given a feedback control law, it is possible in principle, to assess its capacity to actively suppress flutter in the transonic flow domain. Thus it is essential that a set of feasible control laws are first constructed. In this paper this is done by applying the doublet-lattice program and extracting the low-frequency quasi-steady loads. Only the aerodynamic stiffness and the aerodynamic damping, obtained from the low frequency asymptote, are used. Thus there is no need for any augmented states. It must be said that ignoring the augmented states is completely equivalent to assuming that their dynamics is relatively fast, which justifies the simplification, especially for establishing an initial set of optimal control laws. The aerodynamic loads are computed for all of the available structural modes and no attempt is made to reduce the order of the model. It was decided at the outset not to introduce any reduced order modelling, in order to ensure that there was no increase in the flutter speed. Once a set of control laws are available, they are evaluated by using a complete nonlinear computational model in the transonic flow regime and the most suitable set of control laws are selected, based 0n the closed loop performance and the desired stability margins. The methodology is applied to several benchmarking wing models and it is shown that the design of control laws for active flutter suppression is feasible.

Stability and Active control of wind turbine and Permanent Magnet Synchronous Generator: A common strategy in controlling a permanent magnet synchronous generator (PMSG) driven by a wind turbine is the maximization of output power of the wind turbine itself. A control strategy must be adopted, is to deliver a desired reduced amount of power whenever it is required. In order to realize the direct control of wind turbine output power across a wide range of wind speeds, a linearized parameter varying dynamic model of the nonlinear wind turbine system including wind disturbances is developed and used in this project. The stability of the wind turbine system is analysed and a blade pitch controller is designed, based on the linearized, parameter-varying, model-predictive control and is validated. Thus, the wind turbine is regulated in a way that the generator delivers the demanded power output to the load. Moreover, the blade pitch control system also performs the key function of augmenting the stability of the wind turbine, for the right choice of the gains. An active stall controller is currently being developed to control the wind turbine's power output by operating the wind turbine with blade angle of attack in the near stall region.

Control and Navigation of Capsule robots and Prosthetic limbs (PhD Projects): The use of robotic instruments capable of assisting staff while they perform interventional or diagnostic procedures is constantly on the increase. One of these is the capsule robot which has seen many rapid strides in development. Robotic systems are being continuously developed which use magnetically enabled components both inside and outside the human body for navigating a capsule robot by controlling an external magnetic field. Capsule robots are currently being extensively researched for applications both in cardiology and gastroenterology. A typical application in cardiology involves magnetic steering of endocardial catheters for a range of interventional uses. In the case of gastroenterology use of magnetic navigation for steering a capsule both for exploratory studies and for targeting specific points in the body, is being investigated extensively. A capsule robot is expected to be minimally disruptive to the standard daily operations of the gastrointestinal tract and be able to safely operate within this environment, while also being sufficiently sensitive, responsive and controllable while in use. It is expected that the capsule robot would be magnetically guided while ensuring that the capsule stably controlled as it maintains both its position and orientation as desired by an operator. A similar controller was developed for prosthetic limbs in a previously completed doctoral study.