Contents 1. Introduction
We have been interested in autonomous control of an articulated steering type vehicle like a wheel loader(WL) [1]. During the operation at the construction site, a typical motion of this machine such as shovelling the material from the gravel pile and then unloading to the bed of dump truck, forms a v-shaped path as in Figure 2. If we take this path with a combination of line segments and circular arcs, WL has to experience the discontinuity of curvatures at the junction between line and arc segment which results in the step transition of steering angle(Figure 3) because curvature of the vehicle's trajectory and steering angle is proportional if the steering angle is small. However, due to the inertia of wheel loader, it hardly tracks the line-arc combination trajectory during the transition from B to SE or from SF to C In Figure 3 and tracking error may remain. On the other hand, it seems to be a good idea if curvature profile ought to be changed to a profile of A to D via B, CC and C. With profile from B to C via CC, WL will move on a symmetrical clothoid(Figure 2) [1] which has the initial position at B, and the trajectory curvature, i.e., steering angle will be proportionally increased from zero as function of the distance until the maximum value at CC. Then the steering angle will be linearly decreased as a function of distance to zero at position C in Figure 2 and 3. The same situation is applied to the course from A to gravel pile position as well. The symmetrical clothoid has better property than line-arc combinations because there is no discontinuity of the trajectory curvature [4], which means that angular velocity of the body of WL is continuous and its angular acceleration is also bounded. Usually, at the construction site, the suitable position for shovelling gravel at the gravel pile, and the direction, with which a wheel loader will leave it, as well as the position of dump truck, and the direction, with which the wheel loader will approach it, are initially specified. The problem discussed here is how to plan the v-shaped path with combinations of line segments and symmetrical clothoids under the initial and termination constraints given by and . This problem can be resolved into two parts:
Finding a location of summit of v-shape, and
Calculation of clothoid curve parameters.
Miniature Wheel Loader at Intelligent Robot Laboratory, University of Tsukuba