Prototyping our Tennis Ball Shooter

We’ve worked on one prototype for our tennis ball shooter.  The goal now is to refine it and focus on quickly creating an accurate prototype that we can easily evaluate the effectiveness.  First off we need to determine our goal and work out some specifics.

Prototype purpose

Our goal is to have our robot shoot a tennis ball from the 3 point line of a basketball and hit the basketball backboard.

tennisballpath

What do we know?

  1. Distance from 3 point line to rim is 19.75′ at the high school level (thanks Google).
  2. The rim is 10′ from the ground
  3. The backboard is 6′ wide x 42″ tall
  4. A Tennis ball is 2.483″ in diameter
  5. At 18 pounds of force a tennis ball will compress 0.25″ to 0.4″ ( http://www.tennisindustrymag.com/articles/2007/07/ball_testing.html )
  6. We have formulas to determine the calculations to determine the speed and angle we need. https://www.wired.com/2011/10/optimizing-a-basketball-shot/
  7. We will be using a Mini-cim motor with a gear ratio of 4:1 (this is simply because we have it on hand).

    mini-cim-motor-curve-20151207
    http://motors.vex.com/vexpro-motors/mini-cim-motor

img_20171112_1616109145874901423413748.jpg

What do we need to do?

  1. Calculate the distance we need for our shooter (the distance between the wheel and the wall)- ideally it will be the diameter of the tennis ball minus the compression amount we want to try.  We will build two prototypes for different compression rates.
  2. Run the calculations based on the height of our robot base.
  3. In a drawing program create a template that we can print out, glue to our plywood and drill the holes required
  4. Cut the threaded rod to size (note it has to be long enough for the 2 sheets of plywood, diameter of the tennis ball, two nuts and washers.
  5. Mount the gear box to the motor, we will use the distances from our climbing mechanism from Trillium to make it easy.
  6. Wire up so we can run the motor without programming.

Building the Prototype

tennisballshootertemplate

Here is the plan for version 1 of the prototype – take 2 pieces of plywood (ensure they are square) and screw them together.  Glue the template tennisballshootertemplate to the plywood (keep it square!)

Note: when printing make sure it does not resize to fit.

Drill with a small drill bit to ensure accuracy and then increase to the correct size bit.

Cut the threaded rod to size (measure twice before you cut and get someone to check your measurements, also wear gloves as it will be hot and potentially sharp) and secure the pieces the correct distance apart.  Cut the Lexan to size and drill holes for zip-ties to the threaded rod.

Update

When drilling holes always use a punch to ensure your bit is going in the exact centre.

We mounted the threaded rod and cut the lexan to size. The lexan is now drilled and installed.

We made aluminum hubs that press fit into our shooting wheels.  A hex bore was used so we can mount on hex shaft. We drilled a hole and used our tap and die set to thread it. A set screw was added so we can secure it to the shaft without shaft collars.

The video above shows a test we ran with a drill to ensyre the ball moves through it.

Next steps:

Mount a geared motor and secure the shaft for testing.

Make multiple prototypes to test different compression values.

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