- U turn: So far we have described how to do parallel passes that are straight and with negligible overlap; but, when we get to the headlands at the edge of the field we need to turn around and catch the next pass. We need to do a U turn. An optimum U turn would be a turn with the shortest distance in the least time. Manual U turns are anything but optimum. The operator goes deep into the headland in order to be given time to merge onto the desired course line. This is exacerbated by the distance the applicator drags behind the tractor. Also, if the passes are not precisely perpendicular to the headland, extra distance is required to make up for the wedge.
Executing a U turn in principle is the same as staying on course for a straight pass. The difference is that a U turn requires an arced or curved path, whereas the parallel pass is straight. The turning rate for a straight path is zero, whereas the turning rate for a U turn can be calculated, knowing the width of the implement and the speed of the tractor. Assuming that we want the applicator turned off during the turn, it is also necessary to know where the headland starts and the drag-distance of the applicator. Let’s list all the parameters that are required to define the desired course of a perfect U turn: width of the implement or applicator, the drag distance of the applicator, edge of the headland, angle of headland to parallel pass. And to execute the steering to the desired course of the headland we need to know the position of the tractor, the speed of the tractor, and the turning rate. By using all these parameters, it would be possible to automatically do a U turn, by steering the tractor automatically onto the desired predetermined U turn course. Yes, it is more complicated, with more input information required, but it is doable.
A U turn course is much more than a half a circle because some time is required for the tractor going straight, to actually turning to a rate that defines a half circle. We could call this the transition time to get into the turn. The same transition time is required to come out of the turn. The transition time is set by the time it takes to change the turning rate and it could be significant for a larger massive tractor. The transition, makes the calculation of a U turn much more arduous.
The application control is a part of the U turn. We want the applicator turned off in the headland; yet, we don’t want to miss anything. By knowing the delay of the applicator control, it would be possible to precisely turn the applicator on and off at the precise time.
There would be an advantage in doing the headlands at the very end. With manual driving, the headland marks where we are to turn. With auto-steering, there is no need to have a visual mark, as the system knows the location of the headland. In doing the headlands last, it would be possible to cover all the turn marks and ruts made with the parallel pass turns.
Knowing when to turn in a V trap is another interesting dilemma for auto-steering. The system must always be cognizant of having enough room to turn before being trapped; even though it might result in some misses.