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Huntsville, AL, United States

Fahimi F.,UAHuntsville
Vehicle System Dynamics | Year: 2013

Most of the controllers introduced for four-wheel-steer (4WS) vehicles are derived with the assumption that the longitudinal speed of the vehicle is constant. However, in real applications, the longitudinal speed varies, and the longitudinal, lateral, and yaw dynamics are coupled. In this paper, the longitudinal dynamics of the vehicle as well as its lateral and yaw motions are controlled simultaneously. This way, the effect of driving/braking forces of the tires on the lateral and yaw motions of the vehicle are automatically included in the control laws. To address the dynamic parameter uncertainty of the vehicle, a chatter-free variable structure controller is introduced. Elimination of chatter is achieved by introducing a dynamically adaptive boundary layer thickness. It is shown via simulations that the proposed control approach performs more robustly than the controllers developed based on dynamic models, in which longitudinal speed is assumed to be constant, and only lateral speed and yaw rate are used as system states. Furthermore, this approach supports all-wheel-drive vehicles. Front-wheel-drive or rear-wheel-drive vehicles are also supported as special cases of an all-wheel-drive vehicle. © 2013 Taylor and Francis Group, LLC. Source

Fahimi F.,UAHuntsville | Saffarian M.,Technical University of Delft
International Journal of Control | Year: 2011

An alternative approach for automatic trajectory-tracking control of small unmanned helicopters with fly-bar is proposed. This approach uses the spatial three-dimensional coordinates of a point on the helicopter's local coordinate frame other than its centre of gravity, called the control point, and the helicopter's yaw angle as four control outputs. The helicopter is assumed to have four independent control inputs. With this choice of control outputs, the helicopter's input-output model becomes a square control system, which opens the possibility of implementation of many robust nonlinear control methods that are suitable for such systems. The helicopter, which has six rigid body degrees of freedom (DOFs), has two underactuated DOFs (UA-DOFs). It is proved that the zero-dynamics of the UA-DOFs are inherently stable, leading to a stable control system. A sliding mode controller is designed for trajectory-tracking of the outputs. It is verified via simulations that the response of the control outputs and UA-DOFs are in fact stable. © 2011 Taylor & Francis. Source

Fahimi F.,UAHuntsville | Thakur K.,UAHuntsville
2013 International Conference on Unmanned Aircraft Systems, ICUAS 2013 - Conference Proceedings | Year: 2013

Unmanned Aircraft Systems (UAS) can significantly benefit from vision-based control when conventional sources of accurate position/orientation (pose) data (e.g. Global Positioning System/Inertial Measurement Unit) are not available. Here, a new paradigm to visual servoing is presented that is fundamentally different than the conventional paradigms; Position-Based Visual Servoing (PBVS) and Image-Based Visual Servoing (IBVS) approaches. With the new paradigm, measurement of the pose state variables of the UAS is not necessary. The image features are directly fedback to control the UAS's motion. However, these image features are directly related to the error of the pose of the UAS compared to a user-defined desired pose. The error in the pose of the UAS in Euclidean space is calculated in real-time. Then, a closed-loop model-based control law, which is aware of the UAS dynamics, uses these errors to control the UAS. So, unlike in IBVS methods, the presented approach does not generate undesirable motions for the vehicle in the Euclidean space. Also, the proposed method does not require numerical calculations. So, it is not as computationally expensive as the PBVS methods are. Using the proposed paradigm, an UAS can be controlled in real-time to move on any user-defined desired trajectory with respect to a fixed target by only using visual feedback. The approach is formulated in a general form for robotic vehicles with 6 Degrees-of-Freedom (DOFs). Then, as an example, it is simulated on for a quadrotor UAS. © 2013 IEEE. Source

Hill J.,UAHuntsville | Fahimi F.,UAHuntsville
Robotica | Year: 2015

A disturbance rejection controller is proposed based on the general dynamic model of 3D biped robots. For the first time, with this proposed approach, not only the Zero Moment Point (ZMP) location remains unchanged in presence of disturbances but also the longitudinal and lateral ground reaction forces and the vertical twist moment remain unchanged. This way, slipping as well as tipping is prevented by the controller. The swing phase of the robot's walking gait is considered. An integral sliding mode architecture is chosen for the disturbance rejection. The support forces and moments of the stance foot are the control outputs. The acceleration of the arm/body joints are chosen as the inputs. During the disturbance rejection, the leg joints remain at their desired trajectory. Since the leg joint trajectories are unaffected, the robot is still able to complete its step as planned, even when bounded disturbances are experienced. For simulations, the general method is applied to an 18-degree of freedom biped humanoid robot. Simulations show that the controller successfully mitigates bounded disturbances and maintains all of the support reactions extremely close to their desired values. Consequently, the shift in the position of the ZMP is negligible, and the robot foot does not slip. © 2014 Cambridge University Press. Source

Nolen C.,UAHuntsville | Fahimi F.,UAHuntsville
ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) | Year: 2013

A redundancy resolution scheme to maintain a desired support force on the stance foot of a redundant humanoid robot is considered. The proposed motion planner will mitigate the effect of unknown external disturbances on the robot. For the first time, the proposed approach considers not only the position of the Zero-Moment-Point (ZMP), but also the reaction forces in the plane of the foot and the reaction moment normal to the plane of the foot. So, not only possible tipping of the robot due to an external disturbance, but also possible sliding of the foot, is addressed. An acceleration level configuration control approach is used to find the optimum solution for joint accelerations of the robot to reproduce both the desired gait and support reactions as close as possible. These joint accelerations are then integrated to develop joint positions to be implemented by the robot. The motion planner is demonstrated by simulating a planar humanoid robot. It is shown the proposed approach is effective in mitigating external disturbances while maintaining the desired body trajectory. Copyright © 2013 by ASME. Source

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