While our project's genesis started from the Make Compressed Air Rocket design, I have a couple of design changes in mind. The launcher needs to be miniaturized -- both because I want to use it indoors at Maker Faires and other events and also because I want it to fit on a robot. I also want the launcher to be automatable, so the robot can autonomously hunt its prey -- er, I mean so it can safely launch the rocket from a distance. Automating the launch will also help us 'tell the story' -- with countdown lights, warning buzzers, etc.
I decided to start from the FIRST 2012 Pneumatic Base Kit -- primarily because this would be my first pneumatic design and I wasn't really sure what would be safe. I figure I can rely on the people who set up the rules & equipment for FIRST to have done their safety homework. When I used the Make compressed air kit with my scout troop, I was constantly on edge because it was manually inflated and had no over-pressure release safety built in. They boys continually wanted to ramp up the power, pumping up to and beyond the recommended limits. The FIRST kit comes with a couple of safety features, a relief valve factory set to 120 PSI that limits the maximum pressure accumulation, a dump valve which can be used to manually depressurize the system, and a digital pressure switch which signals when a target pressure of 115 PSI has been reached. I used the relief and dump valved and replaced the digital 115 PSI pressure switch with an analog 100 PSI pressure sensor from Freescale Semiconductor so I can precisely measure and vary the launch pressure.
I like the fact that FIRST teaches students how to build using professional grade equipment. Since I'm building this project with Joshua, my 16 year old son who is looking to start an early career as a machinist at a local community college, I want him to get experience working with good components. The FIRST equipment uses nice push-to-connect fittings for the tubing so we can experiment and change designs without damaging the parts. When using PVC piping which is glued together, any design change or mistake and you have to start with a fresh set of parts.
Our first assembled prototype:
The initial build went well. We were able to manually activate the Festo valve and tested a few rockets inside the house at 20 and 40 PSI. 20 PSI gives a respectable launch, about as fast as you'd throw the rocket. 40 PSI is just at the edge of putting a dent in the plasterboard of the walls or ceiling.
The design goal is to fit the launch system on top of a robot. Our current robotic platform of choice at work is the Parallax Eddie which has an 18" round disk as its platform. I'm designing our platform so it can be either mounted on top of the Eddie or carried onsite. It also needs some simple waterproofing since here in the Pacific Northwest we can't always wait for a sunny day for our outdoor scouting events. Next steps will be to add the Arduino control system and the 18" base plates.
- Parts used out of the FIRST 2012 Pneumatic Base Kit, $281
- Festo 12 volt Valve Kit, $112
- 101 PSI Analog Pressure Sensor, $14
- Brass Tube, Compression Fitting, Larger tubing from local hardware store, $7
If I was going to rebuild from scratch again, I'd consider a few changes, notably:
- Save money by ordering just the parts used ($190) instead of the full kit ($281) from AndyMark.Com.
- Look for a potentially better compressor. The VIAIR is a bit weak and noisy. Were this system to be used for evil, say hunting cubicle coworkers, the noise of the air compressor firing up would give it away.
- The Festo valve is quite pricey and is more difficult to use than expected (more on this later). I'd look for an equivalently small & light valve that is simpler to use -- something like a small version of the sprinkler valve from the Make kit. One reason I went with the Festo valve is it has a fast activation speed (7 milliseconds) and I'm hoping to limit air output by using the Arduino to give very short bursts of air, so any replacement would need to be equivalently fast.