Fanjita's Rants (Entries tagged as raspberry pi)

Raspberry Rover part 2 : The Hardware

nospam@example.com (Fanjita) — Tue, 11 Sep 2012 21:41:34 +0100

The new, improved, but uglier Raspberry Rover!

A few months back, I posted about my prototype "Raspberry Rover" - and then I had to stop work on it for a while, because things were busy at work, and my part-time work on iFringe and FringeGuru were reaching their busiest time of the year with the Edinburgh Fringe Festival coming up.

Recently I was able to pick it back up again, and fix a few of the snags that I'd come up against. It's now in a state where I feel able to share some of the lessons learnt. It's far from perfect, but it is functional with some clear ways forwards.

This posting will deal with the hardware design, and I'll talk about software and further plans for the future later.
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Running a Raspberry Pi from batteries

nospam@example.com (Fanjita) — Mon, 02 Jul 2012 22:22:10 +0100

One of the essential properties of a mobile, Raspberry Pi-based robot is that it needs to run on battery power - trailing a power cord around is not much use.

The problem is that the Pi takes an appreciable amount of current (say 500mA, depending on activity and attached peripherals), and needs a pretty narrow input voltage range (5V +/- 0.25V, or so). Because battery voltage varies pretty wildly depending on the current charge level, running directly from a battery is not really sensible.

So, I set about looking into various options for converting standard battery voltages into something suitable for the Pi.

Using a linear regulator

The traditional approach, back when I was first tinkering with electronics about 30 years ago, would be to put together enough batteries to get a significantly higher voltage than 5V (say, 4x nonrechargeable AA to get 6V, or 6x rechargeable AA for 7.2V), and then run that through a linear regulator (e.g. 7805-series IC) to get a steady 5V.

There are 2 main problems with this approach.

Linear regulators are inefficient, and effectively burn off the excess voltage as heat. That means that you're just wasting battery life, and also probably have to deal with dissipating that heat with a heatsink.
The Pi draws quite a lot of current, so it would need quite a large regulator, along with a large heatsink.

Fortunately, there are much better approaches nowadays, in the form of switched-mode regulators, which are much much more efficient, even at high currents.
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The Raspberry Rover

nospam@example.com (Fanjita) — Mon, 11 Jun 2012 23:50:10 +0100

Here's a little video of my work-in-progress robot rover. It uses a Raspberry Pi, running OpenCV with a webcam, to see and react to its environment.

The current program is just a proof-of-concept prototype - it looks in the field of view for any strongly green objects. If it can see one, it turns to face it, then advances until it is close. If it can't see any, then it turns on the spot until it can.

The current platform was built very cheaply - parts from a 3 in 1 All Terrain Robot from I Want One Of Those, a Slice of Pi prototyping board from Ciseco, a cheap Hue HD webcam, plus a 2-channel H-Bridge motor control IC from Hobbytronics, and of course the Raspberry Pi. Total cost is probably about £60.
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Capturing webcam video with OpenCV on Raspberry Pi / Arch Linux

nospam@example.com (Fanjita) — Wed, 06 Jun 2012 22:31:54 +0100

Self-portrait of a Raspberry Pi

Although installing OpenCV on the Raspberry Pi was made very easy by Arch Linux (see previous blog post), persuading OpenCV to capture images from my webcam was substantially harder. But, eventually I managed to get it working. Here's how.

My first attempt followed a sample program found on Stack Overflow. But what I found was that I was repeatedly receiving select timeout errors on the cv.QueryFrame(...) call. I also received 3 VIDIOC_QUERYMENU: Invalid argument errors every time I called cv.CaptureFromCAM(...), and Xlib: extension "RANDR" missing on display "localhost:10.0" the first time that I called cv.NamedWindow(...). Following those errors, the captured frame was displaying as a black screen, so I assumed that one of them was fatal.
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Running OpenCV headlessly in Arch Linux

nospam@example.com (Fanjita) — Tue, 05 Jun 2012 20:33:41 +0100

I've been experimenting with trying to get a usable OpenCV environment up and running on my Raspberry Pi, so that I can experiment with using it for machine vision. The catch is that I don't have a decent monitor and keyboard hooked up to the Pi, and I'm using Arch Linux - so most available tutorials aren't quite applicable to me. This blog post describes what I've done so far to get up and running using remoted X-Windows to get the job done.

Since I have an iMac as my main development machine, I already have a decent X server built in. I've happily used that in the past to remote VirtualBox virtual machine desktops from Debian, and it's a convenient and efficient way of running graphical apps on a headless server. So, that seems the logical way to go here.

The first step was to get a basic X client up and running on the Pi. This was probably the hardest part, and involved a lot of searching around, via both pacman and Google. Below are what should be the minimal set of steps to get this working, to save future readers from the trouble...
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Initial Raspberry Pi adventures : Setting up a webcam

nospam@example.com (Fanjita) — Sat, 02 Jun 2012 22:32:59 +0100

I recently got my Raspberry Pi, and although I haven't had a lot of time to play with it, I've been able to do a little bit of mucking around.

I initially started with the Debian OS distribution, since I already run several Debian servers (such as this one) and it, or Ubuntu, are usually my distro of choice. But the experience with Debian on the Pi seems pretty flaky - no sshd by default (which matters to me since I only plan to run my Pi without a screen), and lots of people seem to have trouble getting hardware up and running (especially webcams), so in this case I fairly quickly moved over to Arch Linux - if only for the experience of playing around with another distro.
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