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This study addresses the problem of efficiently navigating a differential drive mobile robot to a target pose in a region with obstacles, without explicitly generating a trajectory. The robot is assumed to be equipped with an omnidirectional range sensor, while the region may or may not be a priori known. Given the known obstacles in each iteration of the controller, the shortest path connecting the robot and the target point provides a raw desired movement direction. Considering the unobstructed area in that direction, the size of the robot and the obstacle contours in its visibility range, the reference direction is determined. Finally, respecting the velocity and acceleration constraints of the robot, the angular velocity is properly selected to rotate the robot towards the reference direction, while the linear velocity is chosen to efficiently minimise the distance to the final target, as well as to avoid collisions. After the robot has reached the target, the controller switches to orientation mode in order to fix the orientation. Experimental studies demonstrate the effectiveness of the algorithm.
A non-invasive technique for condition monitoring of brushless DC motor drives is proposed in this study for Hall-effect position sensor fault diagnosis. Position sensor faults affect rotor position feedback, resulting in faulty transitions, which in turn cause current fluctuations and mechanical oscillations, derating system performance and threatening life expectancy. The main concept of the proposed technique is to detect the faults using vibration signals, acquired by low-cost piezoelectric sensors. With this aim, the frequency spectrum of the piezoelectric sensor output signal is analyzed both under the healthy and faulty operating conditions to highlight the fault signature. Therefore, the second harmonic component of the vibration signal spectrum is evaluated as a reliable signature for the detection of misalignment faults, while the fourth harmonic component is investigated for the position sensor breakdown fault, considering both single and double sensor faults. As the fault signature is localized at these harmonic components, the Goertzel algorithm is promoted as an efficient tool for the harmonic analysis in a narrow frequency band. Simulation results of the system operation, under healthy and faulty conditions, are presented along with the experimental results, verifying the proposed technique performance in detecting the position sensor faults in a non-invasive manner.
Path planning under uncertainty in an unknown environment is an arduous task as the resulting map has inaccuracies and a safe path cannot always be found. A path planning method is proposed in unknown environments towards a known target position and under pose uncertainty. A limited range and limited field of view range sensor is considered and the robot pose can be inferred within certain bounds. Based on the sensor measurements a modified map is created to be used for the exploration and path planning processes, taking into account the uncertainty via the calculation of the guaranteed visibility and guaranteed sensed area, where safe navigation can be ensured regardless of the pose-error. A switching navigation function is used to initially explore the space towards the target position, and afterwards, when the target is discovered to navigate the robot towards it. Simulation results highlighting the efficiency of the proposed scheme are presented.
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