Friday, May 7, 2010

Basketball Shooter - Team G

Team Members:
Charles Hartman - Programming and Wiring

Joe Heyman - Programming and Communications
Florentin Marty - Mechanical Fabrication and Design
Kenan Isik - Mechanical Fabrication and Design

I. Introduction

At first glance, it looks like a pretty simple task: throw a small ball into a closely positioned hoop. Even young children in primary school can accomplish this task with larger basketballs and over longer distances. Thus, building a system that has those skills should not be overly difficult. However, everything looks easier than it is. We quickly figured out that those primary school kids have a complex and integrated system that allows them to accomplish this task.

Coming from the idea of building a human-like throwing mechanism, we rapidly changed our design to a pinball spring and plunger inspired breech. The idea behind this system is that we use the energy stored in compressed springs to accelerate the basketball.  The energy stored in the spring is quadratically proportional to the compression (equation 1), and the kinetic energy of the ball also has a quadratic relationship to the speed (equation 2).  As we show in equation 3, this results in a linear relationship between the initial velocity of the ball and the spring compression distance (having neglected friction, drag and other nonlinearities).

E=\frac{1}{2}  k (\Delta x)^{2}         (1)

 m v^{2}               (2)
combining (1) and (2)

v=\sqrt{\frac{k}{m}}\Delta x            (3)

Figure 1: OAM system overview

Figures 1 is a general overview of OAM.  Here we present a general overview of the system and the basic function of each subsystem.  The breech motor, springs, and lead screw shown in figure 1 are the main components of the breech subsystem.  The breech is the firing mechanism responsible from transferring the energy of the motor through the springs and into shooting the balls.

Compared to the breech, the other subsystems are less complex. The reloader shown in figure 1 contains just two rails that guide the balls and a swipe mechanism that allows to load one ball after the other into the breech.

Another important part for the accuracy of the system is the turret. We designed the system so that the whole shooting mechanism is mounted on a rotating table. In order to achieve a high precision shot, it is crucial that the angle error is small. The lateral error for an error of \Delta \alpha is

error_{lat} = L *\Delta \alpha  

where L is the distance to the basket.  For example, with an angle error of 2° and a distance to the hoop of three feet, we have a lateral error of 1.25''. Therefore, an exact encoder for the detemination of the angle is very important. 

The gripper subsystem is driven by a small servo motor and is responsible for holding onto the ball carrier while the breech motor compresses the springs.  Once the desired compression length is reached, the gripper releases allowing the ball to shoot towards the hoop.

The ultrasonic sensor subsystem and the wireless router subsystem allow OAM to determine its position on the court.  The ultrasonic subsystem measures the longitudal distance to the backboard, and the user enters the lateral distance through a control GUI that communicates to OAM over the wireless router LAN.

The power supply subsystem consists of an 18V battery for powering the breech motor and a modified computer power supply to power the electronics.

a. Mechanical Schematics

In order to pull back the ball, we needed a system that connects to the ball holder and pulls it back. In the end and after several designs, we figured out a very simple one (Figure 2). A L shaped piece with a special designed end attaches to a counter piece from the ball holder, grips it and then is able to pull back the ball.

Figure 2: The ball gripper.

Ball holder
One of our coolness factors was able to shoot different sized balls. Thus, the ball holder must be able to hold different balls. Theoretically, one can describe a position of a 3 dimensional object with three points, hence, three shafts could have been sufficent to create a suitable ball holder. However, if we designed the holder like this, it could have led to a more unstable system or to other problems, due to unsymmetric balls- Therefore, we designed a ring (Figure 3) in which different sized ball would perfectly fit in and would also accelerate the ball more equally around its center of mass. 

Figure 3: The ball holder.

Ball holder base
Our primary issue was to decrease the friction between the linear guide rails and the ball holder base. In the early designs, we thought about using different materials, accept the higher friction but have a simpler and cheaper system. Nevertheless, we rethought our design and came to the conclusion that we needed bearings, because the ball holder base will be rapidly accelerated. Therefore, the friction would be high if we used two materials sliding on each other. To overcome this, we decided to buy bearings that allowed us to design a piece which could roll on the shafts with almost no friction.

Figure 4: The ball holder.
The reloader (Figure 4) seems to be a very simply design. However, as much as it looks simple and obvious to build it like that, it was not that clear in the beginning of the design process. Ideas such as a revolver-like reloading system and hopper type system were considered and discarded.  In the end, we used the simple idea of the swing based reloader and rails.

Figure 5: The turret turntable.

Figure 5 shows the turret system. This subsystem rotates the breech to aim towards the basket. In this system, instead of having a rotating shaft, we have a rotating motor which is attached to the base of the turret. When the motor's shaft starts turning, instead of rotating the gear on the turret, the motor starts to rotate around the gear.   This is due to the gear being fixed onto the turret and the base, on which the motor is attached, can turn along with the turntable underneath it. Basically it is like moving back when trying to push a well fixed barrier.

At the end of the shaft of the turret, we have an encoder to have feedback of the angular position of the base. Similar to the motor-gear system, when the base starts turning, the encoder turns around its shaft.

The base also holds the breech on it with two legs, which holds the upper side of the breech, and a rim which holds the down side of the breech.

b. Electronic Schematics

Figure 6: Breakout Boards Schematics.

A Video of our robot:

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