Summary
The seventh week of the lab involved programming and testing the AEV to run on a straight track. The purpose of this lab was to verify that the AEV wheel count sensor was working properly, as well as collect data from a trial run to calculate the propeller force and friction force for the vehicle. This knowledge and data collection helped the team figure out how efficient the parts are and how much friction there is. This is important to know because it will affect how the AEV runs when trying to complete the assigned task as efficiently as possible. The team was also able to see how they currently compare to other groups. This ranking will determine how successful the AEV will be in the competition.
First, the AEV was weighed on a scale. Next, a simple code was written so the AEV would run at a certain motor speed along the straight track. The code was uploaded to the AEV, the AEV was set on the track, the initial position was noted, and the ‘on’ switch was pressed. Once the AEV had stopped moving and was stationary for fifteen seconds, the final position was noted and the AEV was taken off the track. Data was loaded from the AEV to the computer. The team used an Excel spreadsheet to put in the weight and position traveled by the AEV as well as the motor speed that the AEV ran on. The team also viewed the time it took the motors to stop, the marks count when the motors had stopped, the time it took the AEV to stop, and the marks when the AEV had stopped from the downloaded data. The team later input these values into the Excel spreadsheet. These values were used to see how well the wheel sensor was working as well as to calculate the propellor and friction force for the AEV.
Results and Analysis
In this lab, data about the forces that act on the AEV during its test run was quantified. By testing in a straight track, with known acceleration and then coasting over a known distance, all of the forces acting on the AEV could be calculated.
Figure 1: AEV Velocity vs. Time
From plotting the velocity of the AEV during the run, important information could be drawn about the acceleration and forces acting on the vehicle. First, by an understanding of calculus and physics, acceleration can be found to be the time derivative, or the slope, of the velocity. Since the velocity of the AEV is a straight line, it has constant slope, and therefore the acceleration of the AEV is either constantly positive, before t=4, or constantly negative, after t=4 and before the AEV stops moving. By calculating the slope of the line segment, the magnitude of the acceleration can be found. Secondly, using Newton’s laws, the acceleration must be equal to the summation of the forces acting on the AEV. Given that the only forces acting on the AEV are the propeller force, the frictional force from the wheels, and a negligible resistive force from the air, the magnitude of each force can be found. In the first portion, where acceleration is positive, the net force is the summation of the propeller and frictional force and is equal to the acceleration times the mass. In the second part of the run, where acceleration is negative, the only force acting on the AEV is friction, which is equal to the mass of the AEV multiplied by the acceleration. The frictional force can then be solved for and, from that, the difference of the net and frictional forces is equal to the propeller force.
Once these calculations have been made, the data could be compared to the data collected by the class as a whole in order to judge how the team’s vehicle is performing in relation to the other groups. Out of the 17 groups in the class, Team L was one of five groups to use a motor speed of 25, meaning that those AEVs were light, having a difference in the forces that enabled our AEV to go beyond the length of the straight track. However, this difference from the other groups also means the data from the other 12 groups, having different motor speeds, should not be interpreted in the same manner that the data from the other 4 groups with the same motor speed should be. Compared to the class, Team L’s vehicle mass was above the average by 0.8 grams. The frictional force on the team’s AEV was 0.8 gram-force, or 17 percent, lower than the class average. Because of the mass’s relationship to the frictional force, the weight of the AEV being slightly above average is not important because of the large difference in the frictional force. The propeller force of the group’s AEV was about 9 percent lower than the average of the class making the net force of the team’s AEV about four percent lower than that of the class average. On first glance, this data does not seem positive, but when taking into account that our propeller was operating at 5 percent less power than most of the class, the relative performance of the AEV is above average.