Diary of a Student Engineer Part 6: Time to Launch

It’s been a long time coming but I have finally come round to writing a blog post on the design of my Ping-Pong Ball Launcher for the upcoming obstacle course challenge. The design problem that was assigned is:

A device is needed to project a ping-pong ball (or multiple balls) into a stationary container.

A few added restrictions for this assignment include:

  • The launcher must store the required potential energy to launch the ping-pong ball.
  • The launcher must have an automated reloading system (i.e. you can’t use your hands).
  • The ping pong ball must clear an obstacle, therefore requiring the ball to have a positive elliptical trajectory.

To satisfy these specifications, the first major decision to be made was the form of potential energy I would use as the basis for the design. To decide, I took a systematic approach where I named all the possible solutions, such as balloons, elastic bands and springs, and then bench-marked them using headings like: Ease of Manufacturing, Accessibility to Materials & Suitability to Task.

Before this process, I initially felt a balloon with compressed air would work well for this task, however through the bench-marking process I soon realised it would be quite difficult to maintain a consistent force for each launch. This would result in each launch having a different trajectory, becoming quite unpredictable — not ideal. When I ultimately scored each potential solution under each heading, it became clear that elastic potential energy was the best way forward for the task in hand. A launcher based on elastic potential energy is easy to make ( I had a prototype made in 20 mins), it doesn’t require any non-regular household materials, and most importantly the elastic bands adhere to Hooke’s Law (kind of) where the potential force is proportionate to the elastic’s displacement. This means that I could control the variables that determines the distance travelled by the ping-pong ball.

Hooke’s Law

Next, I got straight to the prototyping stage. Since we are dealing with relatively unknown parameters such as the aerodynamics of a ping pong ball and the elasticity of household elastic bands, I think that taking an iterative, hands-on approach is a far quicker and efficient way of designing an appropriate launcher. Calculations and simulations are often useful, and very economical for engineers before investing in design solutions but when you can afford to build multiple design solutions, the ensuing real-world results tell you a lot more about your design than even the most advanced engineering models and equations. The following design is what I eventually came up with. It is now the underlying design of my final ping pong ball launcher.

Initial Launcher Idea
Further Details & Prototype

Building on this concept of a cardboard tube (aka a toilet roll) launcher I also had to incorporate a reloading mechanism and establish a suitable launch angle for the projectile. This gave rise to my final design which is a lot more elaborate and had a few of the following design tweaks.

As it turns out, a scaled replica does not always give an accurate idea of how the materials will behave under relatively higher loads and stresses. For the prototype, I only needed a proof of concept and so the launcher projected the ball about half the distance it needed to travel for the task in hand. So upon building the final product, it dawned on me that as I increased the elastic potential energy — by pulling the elastic bands further back, generating more tension — the cardboard structure began to fail under the increased compressive stress placed on the structure by the elastic bands.

So how did I overcome this? Well, clearly cardboard does not have the stiffness properties required to resist the compressive forces caused by storing a relatively large amount of potential elastic energy. So, it was time to incorporate a stronger material that could handle this load. In this case, I used a plastic chopping board from the kitchen as a base for the launcher and then used a drawstring and paper clip to transfer the trigger device to the board itself. So, it would look like this:

Design Changes for Final Design

Next, it was time to add the reloading mechanism to the launcher. Given the limited materials available to me, I decided to use the most ubiquitous force (and resource) available to me for the basis of this design: gravity. Simplicity was key here so I decided to incorporate an easy ‘ramp’ mechanism to store the ping pong balls before launch. The difficult aspect of this was then choosing the method for transferring each ball from the storage device to the launcher. Initially, I considered incorporating a servo motor that would ‘flick’ each ball into the launcher on demand. But there was a more simple, lower tech solution — a ‘sea-saw’ design — as shown here:

Reloading Mechanism

And with that, it just came a case of going from paper to reality. Here’s the final result!

End, Elevation & Plan View of Final Launcher

Note: I haven’t included the cylinder on which the base would balance to reload the device. But you’ll see it in action in the next post for the obstacle course challenge!

As you can see, I added an extra Nespresso box to the main launch cannon as well as excessive duct tape to ensure the rigidity of the structure is fit for purpose. The launching mechanism itself is actually inside this structure which you can see in the plan view. As discussed, it consists of two toilet rolls, the smaller one inside the larger roll and attached using paper clips and elastic bands. The pencil which is attached to the smaller cylinder (and acts as the slingshot for the ping pong ball) is in turn attached to the drawstring which can be pulled back and clipped to the paper clip at the base. This acts as the trigger and allows for the rigid chopping board to absorbs the increased stress imposed by the stretched elastic bands.

Next up was to test the launcher. A nice design feature of this launcher is; since the tensile stress imposed on the elastic bands can be varied, so can the resulting distance for the ping pong balls (Hooke’s Law). This worked very well for determining where to position the robot to catch the balls at the end of the obstacle course.

So that’s all I have to say specific to the launcher. However be sure to tune in to the next blog to see it in action and how I got on with the obstacle course challenge!