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• Sajiv Shah

FRC 2021 Infinite Recharge @ Home Strategy Part 5: Analyzing Shot Trajectories

Updated: Mar 14

In the last post, we set up some graphs to show our shot trajectories from multiple heights and different shooter locations to analyze what the variability and parameters of our shooter may be.

The two trajectories here represent two different shots that our robot may take to get into the different goals. As seen, the second one (further) has not been adjusted to get through the goal, but with a change in the initial velocity and angle, it will be able to get through. Let's analyze what these trajectories tell us about three major shooter variables.

Angle: Changing our angle is essentially a must. Without changing our shooter angle, the graphs here display that we simply cannot make a shot from the two major shooting locations we identified, and anything in between. Because of this, it is fairly important that we are able to change the angle of our shooter to any location, at at the minimum two fixed locations. There are many methods of doing so, which will need to be prototyped.

Height: Changing the initial height of the shooter gives a couple of advantages. 1) it could possibly help shoot balls over defense and 2) it could achieve new trajectories. Unfortunately, in this case, none of those advantages are achieved. We will look at the defense in a later post, but here with a variable angle and variable exit speed, a change in height is not necessary at all. Furthermore, it does not help to change the height and hold another variable constant, since we do not achieve the same range of variability.

Ball exit speed: In inches/second, the exit velocities of these shots were fairly high. For the near shot, it was close to 800 in/s, and for some far shots it reached up to 1300 in/s. Now, this was an area where we noticed that something in the calculated trajectories must be incorrect. In our last season's prototype, we used a single wheel shooter that only spun the ball up to around 800 in/s max, and yet the ball could cover a much greater distance. Going any faster would be very dangerous, and the shooter was already pretty loud. So why are the numbers in our trajectory so much greater than the actual values? We finally identified that the failure to include air resistance and the lift force that is an effect of backspin threw our trajectories way off.

As seen in the image above, there are multiple forces that act on an object spinning through the air. Our trajectory only takes gravity into account, but drag and lift are very significant and can throw off any trajectory. In another post, we will analyze the significance of the Magnus effect and how much it really impacts a ball trajectory.

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