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

3.3 Characterization of the Tire Forces 51even if the three wheels are not touching the ground. Also, from those data is still unknownwhy and how the tire can release after its stretching overcome the static friction force. Inorder to , more data were acquired from the force sensor by fixing a single wheel to thesensor and dragging it up and downwards upon a piece of wood, as shown in Figure 3.8(a).The wood surface was also tilted by different angles so that the wheel can feel differentnormal reaction forces to the surface. Thereby, the static friction force either increases ordecreases while dragging the wheel upon the piece of wood, depending on its slope.(a)(b)Figure 3.8: (a) Tire lateral force measurement by dragging a single wheel upon a piece of wood; (b) Tirevertical force measurement by dragging a single wheel upon a piece of wood.First, the wheel was dragged on the wood surface after fixing it to the sensor in such a wayit could not move in any direction, then it was fixed so that it could move along the directionperpendicular to the surface. Also, in both cases, the wheel was first pushed downwards,dragging it on the wood surface, and then was slowly pulled upwards until the measuredforce went to zero because of the tire releasing. As a consequence, by measuring the slopeof the last ramp, the tire elasticity coefficient can be accurately identify. Some examplegraphs of the two experiments are depicted in Figure 3.9.The graphs in Figure 3.9 show that the tire can release after stretching only when thewheel has the capability to move perpendicularly to the surface. Furthermore, by lookingat the last part of the graphs, it can be noticed that the tire-spring dynamics is non-linear,