Chris Umbel

Raspberry Pi Pig Tank

This is my first post about the Raspberry Pi Pig-Tank -- a project my son and I have been working on for a little while. The project is still incomplete, but very functional. When it finally feels "1.0" we'll make a YouTube video that details the project further, but in the meantime I thought it'd be potentially useful to others to see our progress thus far.

Eventually I'll also get a post together that outlines measurements, part numbers and has detailed photos, but that's a ways off.

Motivation and Goals

See, I've been fiddling with Raspberry Pi for several years now, but have largely used them simply as small, lower-power computers. Anything involving more interesting hardware-wise would have me resorting to a microcontroller. I was interested in using a Pi as a control system just for the heck of it, but lacked the inspiration for an interesting project.

After building a simple RC tank kit from Popular Mechanics my son mentioned the idea of mounting a camera on it. The kit itself lacked any real intelligence. It was pretty much just a chassis with treads, motors, motor drivers, IR receiver (this kit is technically not RC because it's not *Radio* controlled), battery box... standard stuff. If we were going to get a camera involved we were going to need to get some proper computing power on there. Considering I have more Raspberry Pi and camera boards than I can shake a stick at it seemed perfect for the project.

As our conversations continued it was clear that we'd have the opportunity to extend the range of the vehicle. A Pi can easily interface with various wireless communication systems: XBee, WiFi, GSM... To keep things simple to start we agreed that WiFi made sense so we could at least control the vehicle around the house.

Also, it seemed reasonable that we should ditch specialized remote controls and go with something web-based. That would allow us to use our laptops, tablets and phones to control the vehicle. Not only are they easy for dealing with the video output of cameras but it also increases the cool factor a bit.

We agreed that our end goal should be to pilot the vehicle 0.6 miles from our house to the local grocery store and back just as an arbitrary measure of awesomeness (we'd obviously have to go beyond simple WiFi to get it done). If anything I pushed for that goal just to get it in his brain that remote control can actually be *very* remote. Maybe it would give him some additional appreciation for the wonderful work that's been done on massively-frickin-ridiculously-remote-controlled vehicles (MFRRCVs) like the great work NASA has done on the mars rovers. If we can take this thing from tens of feet to hundreds to miles the possibilities are endless!

Materials

So I set about getting materials together to build the bloody thing.

The plan was simple for the structure and drive. Use the chassis, treads and motors from the kit for the structure and drive system. From there holes could be drilled and components fastened.

The control system will be a Raspberry Pi with a USB WiFi dongle. Due to favorable power consumption characteristics a Model A was chosen (the Model A runs sub-200 mA idle while the B runs > 450 mA).

We went with these 7.4V (both 1000 mAh and 2200 mAh versions fit in my case) LiPo batteries I had laying around. Although the Pi and motors were all fine with 5V these batteries would give us some extra voltage for other additional systems in the future (maybe an amplifier for some crazy sound or something). The drawback of that extra 2.4V was that I had to employ a voltage regulator and waste some space to heatsink it, but that didn't seem unreasonable.

The standard Raspberry Pi camera module seemed to be small and light enough to fit the bill, plus I had a few stashed away. There are also relatively easy to use programs (raspystill, raspyvid) that provided a great starting point.

We also eventually decided to add some red LED eyes and use a 10mm white, ultra-bright LED as a headlight so I grabbed some from the parts bin.

Now to drive DC motors in both directions something like an H-bridge would be required. Sure, I could have monkeyed around with some MOSFETs to get one in place, but I decided to go with a little driver board from Pololu instead (uses a Toshiba TB6612FNG dual motor driver). I've used different driver boards of theirs in 3D printing with good results so I figured it would likely save me some headaches.

Since there was no way all of our madness was going to fit into the body from the tank kit it was clear that we were going to have to mount a larger enclosure on the kit's chassis. We selected an appropriately-sized project box from Radio Shack to serve this need. Sure, it would end up looking like a rolling box, but who cares.

Physical Construction

The first step was to get the electronics together to prove that all of the ideas would work. After breadboarding everything out I put a board together from prototyping PCB with 8-pin female headers for the motor driver; 4-pin headers for ground, 7.4V and 5V power rails and the regulator with its associated capacitors/rigamarole.

To get the basic structure in place we started chassis from the tank kit and sawed off the connector pieces that the body attached to. From there we drilled some holes through both the base of the project box and the chassis for mounting hardware and motor wires. Machine screws fastened the bottom of the box to the chassis and long M2.5x20mm-ish screws were inserted to ultimately mount the Pi and the custom board mentioned above.

We mounted the Pi and the custom board on the skinny screws using nylon spacers to keep everything apart. Everything was (and still is) wired up using jumper wires. 5V goes to the Pi's 5V in, the Pi's GPIO pins go to the motor driver's inputs and pretty much everything works as expected.

After toggling GPIO pins from test code to move the motors it was clear that we were on the right track. But, you know, the boy wanted more. He wanted LED eyes. Feature-creep happens at home too, I guess. It seemed that if we were going to go as far as to add ornamental LEDs a headlamp would be a useful addition. Since the Pi's GPIO pins don't source enough to get the 10mm ultra-bright LED to burn your retinas I put a board together with PNP transistors to trip it.

With the connection of the camera and mounting of the wheels/tracks we were in good shape. Everything worked with some test code so it was time to turn my attention to putting proper control software together.

Software

Now we came to the point where I'm actually somewhat professionally qualified (I'm a software developer, I don't hack apart toys by trade). It was time to develop the control software.

Since we were sticking with a vanilla Raspbian install on the Pi our options were open. It seemed the path of least resistance was to hack in python and use the "RPi.GPIO" package to control the peripherals. As I stated initially we wanted the system to be web-based. Our needs were simple and small so I chose CherryPy for the job.

To start we kept the video simple. Rather than shoot and stream video we just used raspystill to shoot stills a few times per second. Our web interface would then periodically refresh to get a current view of the situation. If this proves cumbersome we may instead choose to stream video.

In time I'll get the software more organized and get it up on Github, but I'm just not there yet.

The Result

Well, here it is in its current form doing my bidding.

Fri Mar 07 2014 00:00:00 GMT+0000 (UTC)

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