S. Vougioukas

A Mission Planner for an Autonomous Tractor

In this article, a mission planner of field coverage operations for an autonomous agricultural tractor is presented. Missions for a particular autonomous tractor are defined using an XML (extendible markup language) formatted file that can be uploaded to the tractor through the user interface. Using the tree hierarchy of the mission file, several actions are determined, including the sequence of points the tractor has to follow, the type of motion between successive points (e.g., straight motion or maneuvering), the type of predefined turning routine used in maneuvering, and the actions that should be taken once the tractor reaches the desired point (e.g., raising or lowering the attached tool, turning on or turning off the power take-off). In order to automatically create the XML mission files, a program was developed using the MATLAB technical programming language. The program uses data regarding the field (geometry, dimensions, field sub-regions, working direction, initial and final desired locations of the tractor), the operating width, and the operation type (mowing, spraying) as inputs. The planning method is based on an algorithmic approach where field coverage planning is transformed and formulated, via semantic representations, as a vehicle routing problem (VRP). By using this approach, the total non-working distance can be reduced by up to 50% compared to the conventional non-optimized method. Three sets of experiments are presented. In the first set, three fields were separately covered; in the second set, three neighboring fields were covered as part of a single tractor mission; and in the third set of experiments, a single field was covered during a hypothetical spraying operation for two different locations of the refilling facility.

A yield mapping system for hand-harvested fruits based on RFID and GPS location technologies: field testing

It is proposed that radio frequency identification (RFID) technology be used to overcome the limitations of existing yield mapping systems for manual fresh fruit harvesting. Two methods are proposed for matching bins—containing harvested fruits—with corresponding pairs of trees. In the first method, a long-range RFID reader and a DGPS are mounted on an orchard tractor and passive low-cost RFID tags are attached to the bins. In the second method, the DGPS is not used and RFID tags are attached to individual trees as well as bins. An experimental evaluation of the accuracy and reliability of both methods was performed in an orchard. The first method failed in half of the trials because the tree canopies interfered with the GPS signal. The RFID reader miss ratio for the detection of the bins was 0.32% for both methods. However, the attachment of RFID tags on suitable tree branches (to achieve 100% detection), in the second method, is not a well-defined procedure; some trial is demanded to determine the best positions and orientations of the tree tags in order for the RFID reader to successfully detect them. The first method seems more promising if robust tractor location under foliage can be achieved.

Reactive Trajectory Tracking for Mobile Robots based on Non Linear Model Predictive Control

In this paper, a nonlinear model predictive tracking (NMPT) controller for mobile robots is presented. The basic idea is to use a motion model for the vehicle and compute in real-time an optimal M-step-ahead control sequence, which minimizes the total M+l step tracking error of the projected motion. In the presence of obstacles, the controller deviates from the reference trajectory by incorporating into the optimization obstacle-distance information from range sensors (e.g., laser scanner, ultrasound). Numerous simulations were performed and the NMPT consistently converged to the desired trajectories and followed them accurately, despite large initial errors and discontinuities in the desired velocities and orientations. The controllers performance depended strongly on parameters such as the optimization horizon M, and the cost-weights assigned to the various tracking errors. The optimization horizon regulates a trade-off between timely obstacle avoidance and tracking quality (large M) vs. consistently fast convergence (small M). The cost-weights affect tracking quality and also the shape of the path, by regulating trade-offs among position, orientation, and velocity errors. Overall, NMPT seems to offer a promising approach for advanced precision guidance applications, and deserves further investigation.

A two-stage optimal motion planner for autonomous agricultural vehicles

This paper presents a two-stage motion planning algorithm which can compute low-cost motions for autonomous agricultural vehicles, for a given cost function defined over the entire path (e.g., shortest path, maximum clearance, etc.). In the first stage, the algorithm utilizes randomized motion planning to explore the space of possible motions and computes a feasible sub-optimal trajectory. In the second stage, the optimization of the stage-1 motion is formulated within the optimal control framework and function-space gradient descent is used to minimize the cost of the entire motion. The numerical results suggest that the two-stage motion planner can compute optimal or quasi-optimal motions in free space very quickly. In the presence of obstacles however, the execution time increases significantly. Furthermore, kino-dynamic, or dynamic motion models seem to be necessary in order to produce smooth motion trajectories.

A system for basic-level network fault management based on the GSM short message service (SMS)

This work describes the design and implementation of a distributed system which uses the wireless GSM short message service (SMS) data transfer technology to give network managers access to services on remote network equipment via a cellular phone. Existing work has been restricted to simple network monitoring, i.e. one way transmission of alert messages from network monitoring application to designated mobile phones, through a GSM modem connected to the computer hosting the monitoring application. In our work, we develop a multi-user, multi-session, two-way network management system via SMS. First, we present the SMS mechanism and discuss its features and restrictions. Then, we analyze the requirements of our system, and present a design of distributed architecture for it. Next we present in detail our implementation of the proposed system, based on Java servlet technology. Finally, we report our experiences with using it, and propose directions for future work.

Noninvasive condition assessment and event timing for power circuit breakers

Noninvasive diagnostics, involving short-time spectra, short-time energy, and automatic timing techniques, were used to assess the mechanical condition of oil and puffer circuit breakers and to determine the timing of events occurring during operation. Analysis of the vibration signatures leads to (1) the characterization of a circuit breaker through a single index, the resolution ratio, and (2) automatic timing algorithms. The accuracy of the automatic timing algorithms permits both the timing of events and the detection of shifts in the timing associated with changes in the condition of a unit. Application of these methods to several oil and SF/sub 6/ puffer circuit breakers is described. Results are reported for circuit breaker operation under unenergized and energized-loaded conditions. >

Route planning for orchard operations

Mission and route planning for an agricultural robot.Orchards is a well-suited operational environment for the application of deterministic behaviour robotic systems.Modelling of inter- and intra-row orchards operations.Reduction in the non-working time ranged between 10.7% and 32.4%. Orchard operations are considered a promising area for the implementation of robotic systems because of the inherent structured operational environment that arises from time-independent spatial tree configurations. In this paper, a route planning approach is developed and tested using a deterministic behaviour robot (named AMS - autonomous mechanisation system). The core of the planning method is the generation of routing plans for intra- and inter-row orchard operations, based on the adaptation of an optimal area coverage method developed for arable farming operations (B-patterns). Experiments have verified that operational efficiencies can be improved significantly compared with the conventional, non-optimised method of executing orchard operations. Specifically, the experimental results showed that the non-working time reduction ranged between 10.7% and 32.4% and that the reduction in the non-working distance ranged between 17.5% and 40.2% resulting to savings in the total travelled distance ranged between 2.2% and 6.4%.

Optimization of Robot Paths Computed by Randomized Planners

Randomized path planners have been successfully used to compute feasible paths for difficult planning problems. Such paths are typically computed without taking into account any optimality criteria and may contain many “jagged” segments because of the randomness involved in their generation. This paper presents a two-phase path planning algorithm, which uses a randomized planner to compute low-cost paths, and gradient descent to locally optimize these paths by minimizing a Hamiltonian function. The algorithm is tested on motion planning for a non-holonomic car-like robot. The results indicate that the two-phase approach is practical; however, gradient descent seems to be inefficient for the optimization of long paths.

Compliance synthesis for force guided assembly

In robotic assembly operations the robot comes in contact with the environment and some form of compliance is typically used to prevent excessive contact forces. These contact forces provide information about the contact geometry and can be used to guide the assembly operation. Thus, compliance can be thought of as a task-dependent mapping from sensed forces to corrective motions which bring the robot closer to its goal. In this work we describe a methodology for the automated synthesis and verification of this mapping. The possible erroneous configurations of the robot are computed via Monte-Carlo simulation of the task execution. The static contact force at each erroneous contact configuration is computed and a corrective motion is computed and mapped to the force. The validity of the mapping is also checked to ensure that the sensed forces will provide unambiguous information during the actual execution of the planned compliant motion. Simulation and experimental results are also presented.

A METHOD TO DEFINE AGRICULTURAL ROBOT BEHAVIOURS

A new method is described that can be used to decompose human controlled agricultural operations into an autonomous tractor. Four main levels of subsumption have been identified: Operation, Task, Optimisation and Primitive Actions where each level is subsumed by the level above. Tasks were classified into two distinctive roles, deterministic tasks that can be planned and optimised before the operation begins and reactive tasks and their associated behaviours that deal with unknown conditions whilst in the field. The tasks and optimisations can be further decomposed into primitive actions, which in turn are converted into the tractor directrix. Examples of this method are given for exploring an unknown area and ploughing a field.