Advance Modeling of a Skid-Steering Mobile Robot for Remote ...

3.3 Characterization of the Tire Forces 44first pulled backwards and sidewards by tying one side of a rope to the robot platform andthe other side to the force sensor on the testing system machine. As the testing machinemoves only vertically while the robot must be pulled horizontally, the rope passed through apulley in between the robot and the sensor. It is worth noticing that, since the pulley has verylow friction, the mass of the rope is relatively low and the angle between the sensor and thepulley is small, we can consider with good approximation that the vertical force measuredby the sensor coincides with the horizontal force pulling the robot. Figure 3.3(a) shows themeasurement setup previously described.(a)(b)Figure 3.3: Tire lateral force measurement setup: (a) Sidewards pull using a rope with four wheels touching onconcrete; (b) Sidewards pull using a steel cable with the wheels on four oiled plastic pads.To measure the tire longitudinal force, the robot’s platform was pulled backwards at differentconstant velocities (v x ), first with the wheels stopped, and then by spinning the wheelsat different angular velocities until the motors stalled. Figure 3.4 depicts a plot of the tirelongitudinal force measured by the force sensor. The first part, i.e. approximatively fromzero to five centimeters, corresponds to the static force measured with the wheels stopped,and the second part, i.e. nearly from five centimeters to the end, corresponds to the kineticforce measured with the wheel spinning. From the first part of the graph in Figure 3.4, it canbe noticed that the force presents a saw-tooth shape, as if an elastic material was pulled andthen suddenly released. By looking at the wheels while performing the test, we noticed that,