3MEMS3: Launcher Introduction and Energy Source

During my robot blogs posts, I have discussed the obstacle course which the robot must navigate. There will be a dedicated post on the building of this course however one substantial part which I must design is a ping pong ball launcher.

there are quite a lot of rules which this launcher must comply with, this has directed my design considerably and rules out many effective options for launching a ping pong ball. The rules state some form of mechanical potential energy source, the intent behind the brief is really a pneumatic actuation.

The project brief states:

• “In order to complete the obstacle course, you must design and manufacture a device that can transfer ping pong balls to the robot as shown in the figure below — note the size constraints. The device should use an energy storage device that converts potential energy into kinetic energy when released (the recommended design will use pneumatic force from an inflated balloon). Elements for the device can be 3D printed as needed using printers in the Parsons. The forces and energy stored by the device should be low, and not present any risk of harm.”
• Only one ball can be fired at a time.
• New balls cannot be loaded manually

This diagram is included in the brief to outline the layout and requirements of the launcher. My interpretation of this diagram is that the highest point of the system must be no taller than 0.9x. ‘x’ being the height of a standard glass coke bottle is 23.5cm. This means ‘a’ is 23.5 x 0.9 = 21.15cm.

Living in a small house with limited tools and no workshop space I had a bit of a task on my hands. Despite not having an air compressor I was determined to stick with pneumatics.

I decided to make a pressurised gas canister to power the launcher. I had a 700ml steel bottle to repurpose (a single wall bottle so no vacuum insulation to worry about).

In order to pressurise the canister, I used the valve from an old bicycle inner tube. I used a Presta valve as the stem of the valve is threaded so it stays in place whether it is pressurised or not. I drilled into the flask and fed the valve through the hole from the inside then tightened the nut of the Presta valve and finished the seal with some adhesive. The use of this valve also easily allows me to use a bike tire pump to add pressure into the system before use.

The pressure of the canister is released out of some small plumbing tap that I found. This is secured by an M10 nut off an old bike on the inside meaning the total cost of the canister comes to €0!

After I applied some initial sealant (Gorilla wood glue I found) to the canister leaks would occur after a certain pressure. I thought I would have to make a bigger canister to provide that volume of gas needed for multiple launches. However using scrap wood I made a simple jig to hold the canister in place, after removing much of the sealant, I layered up multiple thin layers over the course of a few days. This has allowed for a much higher pressure to be reached without leaks!

It’s not the neatest thing I’ve ever made but it works well and should be good for five launches (maybe more). I’m yet to see how the pressure drops off and how that might affect the range of the launcher but at full pressure, it seems to be very strong.

All in all, I was surprised I was able to make this with items from my shed and cupboards, certainly had some luck on my side finding parts that fit so well, but this is just one half of the launcher.

I have designed and 3d printed a funnel to distribute the pressurised gas around a ping pong ball, the design process of this will be outlined in a future blog post. The hopper will also feed directly into this funnel for a continuous supply of balls.