Navigation for an Intelligent Mobile Robot - IEEE Xplore

CROWLEY: NAVIGATION FOR AN INTELLIGENT ROBOT 37The correspondence types are used to extend thecomposite local model segment during the update process.4) Is the segment LM the longest found so far?The correspondence function provides both the index of thecorresponding segment and the type of correspondence.D. The Update Process-- Local Model SegmentSensor Model SegmentThe update process is the mechanism by which linesegments from the sensor model, global model, and contact Fig. 7. Orientation error given by average difference in angle betweensensor enter and refine the composite local model. During sensor model segments and corresponding local model segments.each sonar scan, segments from the sensor model are matchedto the current composite local model, and the result of F. Updating the Vertex Positions and Segment StatesThe main functions of the update process are as follows.1) Increase the confidence state of transient segments forwhich there is a corresponding segment in the sensormodel.2) Decay the confidence of segments that should be visible,but for which there is not a corresponding segment the inmost recent sensor model.3) Add newly observed sensor model segments and segmentsrecalled from the global model to the compositelocal model.4) Refine the vertex position of segments which are“reinforced” by the sensor model.E. Marking the Visible Segments in the Composite LocalModelSegments in the composite local model for which there is nocorrespondence are only modified under two conditions.The segment was marked as visible during constructionof the sonar model.The nearest point on the line segment is more than a givendistance from the IMP’s current position.The second condition is a simple mechanism by whichsegments “fall off the end of the world” as the IMP movesaway from them. The actual distance is relatively unimportantas long as it is beyond the sonar range and the current area oflocal navigation. Of course, the larger this distance, the more“extra” segments the system has to consider on eachcalculation.The first condition establishes a “visible horizon” for theIMP. As each point is added to the Sensor Model, the functionVISIBLE is called, for a point at the direction of the beam and therange of the sonar, to determine which segment in thecomposite local model should be visible. If a segment is found,the difference in angle between the beam and that segment iscomputed. If this difference in angle is not small ( c 15 ”) thenthat composite local model segment is marked as visible.Segments for which the angle of incidence of the sonar beam isvery small are not detected reliably by the sonar and are thusnot marked as visible.Unconnected vertices should be extended when there is acorrespondence of types 1, 2, or 3.The position of connected vertices has precedence overthe position of an unconnected vertex.A segment is more stable when both its vertices areconnected.The actual rules for state updates are implemented as casestatements based on the current state and then ’on thecorrespondence type.After the vertex positions and states of the segments in thecomposite local model have been updated, segments from thesensor model for which there was no correspondence in thecomposite local model are added to the composite local modelin the lowest confidence state (state 1). A relabeling process isthen used to connect adjacent segments for which the verticesare very close.VI. CORRECTING THE ESTIMATED POSITIONLocal path execution and learning and updating the compositelocal model all depend critically on maintaining an accurateestimate of the IMP’s current position. An instantaneousestimate of the IMP’s position is maintained from the rotaryposition encoders on the IMP’s wheels. This estimatedposition is monitored and corrected by a process based oncomparing the sensor model to the composite local model.Before the composite local model is updated from the sensormodel, the correspondence between the sensor model and thecomposite local model is used to detect and correct anysystematic error in the estimated position of the IMP.As each sensor model line segment is obtained, thecorrespondence is found to the most likely line segment in thecomposite local model. The difference in angle between thesesegments is then computed. When a sensor model has beenconstructed from a complete scan of the rotating depth sensor,the average error in angle is computed. This average error iscomputed from the difference in orientation between SensorModel segments and the corresponding composite local modelsegments, as illustrated by Fig. 7. The sensor model is thenrotated around the position of the IMP by this average error in