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AVR 2-Servo Walker

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flannelhead

Tue Feb 10 2009, 01:01PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

I decided to post my little non-HV side project, a walking robot.

I still don't have a website of my own, so this post is written in quite good detail. However, there's also a shorter description for those who don't want to read all the technical stuff.

Short explanation
The robot is controlled by a ATtiny2313 microcontroller, which can be programmed from a computer. ATtiny2313 seemed quite a good choice for the purpose because of its low price and a 16-bit timer with two outputs capable of doing a PWM signal -> just enough to control two servo motors. The robot moves forward by moving its legs as pairs (front legs, rear legs). The legs are moved by regular RC model servo motors. The controller sets the positions of the servos with a PWM signal.

Youtube video:

In detail

Electronics

The circuit board in the picture above is the brain of the bot. Notice that there are quite few components on the board. I like Atmel AVR microcontrollers quite a lot, because they don't need too many surrounding components. In fact, even the resistors could be left out. They're there to protect the parallel port from getting damaged in case something shorted out on the circuit board. Notice the black connector. It is the programming connector. I didn't want to include a big parallel port connector on the board, so I made this kind of a programming cable:

It connects the 5 pins needed for programming to the right pins of the parallel port. They can be seen in the schematic below.

There's nothing special about the schematic. The usual colour coding of servo motor cables is:
red: +
black: -
yellow or any other colour: PWM

Mechanics
The chassis of the robot is made out of plexiglass. I chose it mainly because it looks cool, but it also provides a solid base for mounting the servos. It was quite easy to machine. Bending was done by heating it with a hot air gun.

The legs are made out of coat hangers. My leg design is far from optimal, but works well enough for me. The legs are attached to the servo shafts like this:

Notice the blue heatshrink tubing in the tips of the legs in the first picture. It is needed to prevent the legs from slipping. They still slip a bit so perhaps I'll have to invent something better.

Programming interface
One advantage of the AVR chips is that they can be programmed while they're in their actual circuit. So no more need to take the chip out of its socket for programming. In this project I am using 'dapa', direct parallel port programming, which is quite outdated but still works well. I tried serial programming, but it ended up with some verification errors, maybe caused by my USB-to-serial programmer. There are a plenty of different interfaces for programming AVRs. Ladyada has some excellent articles about programmers and programming:

The whole AVR tutorial is also recommended reading.

Actual programming and firmware

So how do you actually program a microcontroller? First, you need to choose a programming language. If we are going to use open-source software, we are limited to C and asm. C suits well for this purpose because of its easily understandable syntax and effectiveness. We need a special compiler, called avr-gcc. Linux users can just install packages avrdude, binutils-avr, avr-libc and gcc-avr from the package repository. Windows users will have to download and install WinAVR which should contain everything you need.

What does real AVR code look like? Here's an example:

TCCR1A=(1<<COM1A1)|(1<<COM1B1)|(1<<WGM11);
TCCR1B=(1<<WGM13)|(1<<WGM12)|(1<<CS10);

This is what it's all about. Registers and bit manipulation. Don't be scared by the cryptic letter combinations, they will be all explained in the ATtiny2313 datasheet. It really isn't that complex at all.

]Download the source files[/file]

So how do you control a servo motor?
A servo motor needs a 50Hz PWM signal with pulsewidth varying from 1 to 2 milliseconds. 1.5ms is where the shaft is at its center position, 1 and 2 ms are the extreme positions. So all that the program has to do is to rotate the servos from side to side.

The sourcecode has to be first compiled with avr-gcc. Then it is converted to Intel HEX (ihex) format with avr-objcopy. Then the hex file, containing the firmware, is uploaded to the chip with avrdude. I've wrapped this procedure in a makefile, so anyone building this robot won't need to worry about that part.

Did you read this far? Congratulations! Now go to build your own robot, it's pretty easy and a great fun.

TODO

better grip for the feet
improve walking routine

I am currently developing light sensors for the robot, so it can be programmed to be afraid of light or seek light, you decide

Links - further reading
ATtiny2313 datasheet
Ladyada's AVR tutorial Also check out the projects section

If you have any questions, feedback, comments, ideas for improvements, feel free to ask! I'll do my best to make this article easy to understand.

3l3ctrici7y

Tue Feb 10 2009, 02:18PM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

That looks pretty cool :)

Pencil eraser refills might work as feet to reduce the slip. A spot of glue could be used to hold the eraser in place on the tip of the leg.

aonomus

Tue Feb 10 2009, 03:23PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Or alternatively just a big gob of hotmelt glue....

Nifty walker, do you have any other plans such as sensors, or is this just a tiny toy for kicks?

flannelhead

Tue Feb 10 2009, 03:31PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

aonomus wrote ...

Nifty walker, do you have any other plans such as sensors, or is this just a tiny toy for kicks?

I'm planning to equip him with light sensors, so it could be made to seek light or be afraid of light.

Tom540

Tue Feb 10 2009, 05:29PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

Hey that's cool, I used to be all about this kind of stuff. Pretty much were I started in electronics. Mine were mostly BEAM though.

I see you already got heat shrink on the feet. What I used to do is bend the ends of the legs backward so they are kind of like barbs and will dig in more when the bot pushes back but wont get stuck when moving the other direction. If you know what im trying to say. I can post a pic later.

-Tom

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