## 4 Experiments
### Performance Tests  
Before testing the networking behaviors of the mobile sensor nodes, we need to investigate their basic motion performance first. In the path planning of the mobile nodes, we use only two basic motion components, i.e. _go-straight_ and _in-situ turning_. We have to measure the cumulative error of each basic motion respectively. Three mobile nodes are randomly selected from the group to perform the test. Each node is programmed to move on a flat table-top surface and perform only one basic motion at a time. Then the average errors are calculated to represent the accuracy of localization. Figures 9 and 10 show the cumulative error test results for mobile node localization. As shown in Figure 9, when the node performs in-situ turning, the calculated position is very close to the true position, but a steady deviation from the control position still exists. As shown in Figure 10, the node is programmed to move straight along the x-axis from the starting point (0, 0). After running for predefined steps it stops at the end point (2250, 105) and the calculated position of the end point is (2220, 50). The final trace of the mobile node shows that the positioning error increases faster in the y-direction. Those results were to a great extent caused by the errors of wheel diameter and track width. Although we can make compensation for the positioning errors according to the test results of each node, a more practical solution is to improve the accuracy of mechanical processing and assembly. Figure 8: Testbed setup for the experiments on the mobile sensor network. Figures 11 and 12 show the positioning error changes during the motion tests. According to the error curves in Figure 11, there is little change in the positioning errors during in-situ turning. According to the error curves in Figure 12, all three kinds of error distances increase with the travelling distance. Due to the size limitations of the testbed, we currently can only let the mobile node go a distance of no more than 3 meters. If the mobile node takes a longer route, we can deduce from the tests that the cumulative positioning errors will be too big to be acceptable for practical applications. Although we can make compensation or add more sensors to decrease the errors when the traveling distance is too long, we do not encourage others to do so. The primary design consideration of this kind of platform is that it should be as small and simple as possible. We do not expect it to be as powerful as the conventional mobile robots. It in fact serves as a small slave robot for nearby bigger robots or human operators. Therefore we should avoid forcing it to travel too far alone. We can use a big carrier robot to carry the mobile nodes to approach the target to the best of its abilities before the carrier robot can not move forward any more. And that will to a great extent decrease the distances the mobile nodes have to go. Figure 10: Cumulative errors in localization when the node is instructed to go straight. Figure 9: Cumulative errors in localization when the node performs in-situ turning. In the above tests, we command the nodes to move at about eighty percent of its full speed. Since the speed of the two DC motors is regulated by Pulse Width Modulating, we directly use the PWM value to represent the motion speed. When we set the PWM value to its maximum value, the mobile node will reach a speed of 0.15m/s. The following tests aim to investigate what the performance of the node will be when varying the moving speed. Figure 11: Error analysis of the in-situ turning test. E(T-C) denotes the error between true position and control position. E(E-C) denotes the error between calculated position and control position. E(E-T) denotes the error between calculated position and true position. Figure 12: Error analysis of the straight-going test. Figure 13 shows the positioning error changes when the node moves in different speed. It can be seen that the positioning errors are relatively big when the speed is lower than 700 PWM value.