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BBB

the three-legged walker
A project in the MIP-course

Contents of page

General Description
Hardware
Software
Machineware
The Story
In Action
Citizen
 [robot picture]

Aim

The aim for this project was to build a three-legged walker that should walk a special path consisting of: The first goal was to walk that path "as fast as possible", but ended with Publicums Vote. There where some constraints put on the machine. It should have an IntelTM 80C196KB microcontroller, be autonomous and manage the track mentioned above.

A General Description

The robot consist of a fuselage and two feet which are movable forward/backward respectively up/down separately by four R/C-servos. The feet are so big so it can stand on one foot at the time. To be able to walk at all there are cut-out "toes". To get rid of a slipping robot the feet are padded with an old carpet for mouses. It has a display onboard mainly for debugging purpose. It also has the microcontrollers built-in PWM output connected so we are able to play samplings and tones.

Hardware(ie electronics)

 The hardware consists roughly of an IntelTM 80C196KB microcontroller, 32K ROM, 32K RAM and a PAL.
[A picture of the schematic diagram]
Schematic diagram of the controller board The schematic in PostScriptTM
The PAL was used for addressdecoding and to divide CLKOUT from the microcontroller by two.

The divided signal is then rerouted back to the microcontroller to run Timer2. Timer2 is used for controlling the PWM-signal to the servos so we can generate four PWM-outputs with the HSO and without any CPU-overhead.

Addressdecoding was simple since we made the EPROM the occupy the lower part and the RAM occupy the upper part of the 64K address space. On the running prototype we patched the RAM space to consist of an EPROM with samplings to be played during boot up. We used the internal RAM instead. To split the linear address space in two parts was really unnecessary since the processor start execute code att 0x2000. The other groups have moved addresspin A13 of the EPROM be connected to addresspin A14 on the microcontroller. But they have to move the code when they burn their EPROM's.

The eight AD-channels are connected to a 10-pin IDC-connector so the construction be extended easily.

Software

The software was coded in C and compiled by an IAR C-compiler for the IntelTM 80C196. The reason for choosing that processor was orders from the institution, DAMEK. They are commited to that processor and have docs, compilers and emulators for it.

By programming the HSO correctly we can generate (in princip) four independent PWM outputs, which we used for controlling the servos. It's described in IntelTM "16-bit Embedded Controllers" pp.4-48--4-49.

LCD.

The code was split in small separate parts for easy maintenance. The parts became relatively small, but it simplified debugging a lot anyhow. The source code is available underneath, including a short description on each file.
wait.h Defintionfile for waitfunctions
wait.c Contains one function that wait a number of milliseconds. Based on two stupid forloops. Ie not so exact, but OK for our purpose.
servo.h Definitionfile for servocontrolling functions
servo.c Functions to reprogram the HSO for pulsewidth generation to the servos described above.
lcd.h Defintionfile for handling the LCD-display attached to BBB
lcd.c Several functions to initiate, print text and numbers to the LCD-display
main.c Includes the "main" function plus some extra interrupthandling

Machineware

Introduction

The robot has two feet that it walks with. The feet are so big the robot can stand stable all the time. As engine the robot uses regular radio control servos. The mechanics in the robot are designed to be as simple as possible, but at the same time light weight and robust. For that reason we chose to build the robot mainly in plastic, brass and glass fiber epoxy.

Mechanical data

Weight : 1640 g
Length : 20 cm
Width : 21 cm
Height : 20 cm
Velocity on flat surface : approx. 1.3 cm/s
Current consumption (flat surface) : approx. 1A mean with Î = 4A
Current supply : 7.2V Ni-Cd accumulator battery, 750 mAh
Maximum climbing angle : ±30°
Mechanical principle : Two large feet. Statically stable.

Choice of material

The first prototype had a chassis of 1 mm thick bent steel plate. We came to the conclusion that it was too heavy and were forced to substitute it with 1.6 mm glass fiber epoxy with aluminum angles. For the legs we selected 6 mm brass poles. Brass poles were also used for the sliding rails to the leg holders. The leg holder were made of PEHM which is a nylon similar plastic. The feet are made of 4 mm thick aluminum sheet so it could be light weight and also strong. The feet are also wrapped with rubber to prevent sliding on slippery surfaces.

Engine element

At first we had a lot of suggestions for engines for the robot. The first was solenoids of some kind. They are very fast, but we came to the conclusion - after some experiments - that they are too heavy relative to the force they can deliver. Solenoids can only deliver force in their innermost position. We also thought of using stepper motors, but they were also too heavy and too slow forour application. Another solution was to use a regular motor with a eccentric placed pin and by that accomplish the up/down going movement. This solution was rejected when we realized that we wouldn't have to use the micro processor for anything. So finally we came to the conclusion to use servo motors for controlling the robot. They are easy to control,quite strong in relation to their weight and not so expensive.

Function

The function of the robot is simple. The aim is that the robot all the time should be statically stabile, it would always have is center of gravity inside it's supporting surface. This implicit that the robot got large feet with several toes that goes into each other. Each foot is attached to a leg that consists of two parallel brass poles. The leg is attached to the chassis via the leg holder. The leg holder can slide on the parallel brass poles that is attached to the chassis. On each of the leg holder there is a servo that controls the up/down movement of the feet and on the chassis there is two other servos that controls the back/forward movement on the leg holder. In this way the robot can start with lifting one of its foot, move it forward, put it down and then move the whole body a little bit forward. And then the next leg and so on. The servos are not so strong that the robot can move the body forward when it stands on just one leg.

The Story

 In 1997:

In action:

 We were invited to the morningshow of TV4. Here are two pictures of the event.
[Robotbuilders at TV4]
Two happy robotbuilders

[The happy team at TV4]
The striketeam at the logotype of TV4.

Citizen:
[Picture of Jacob] [Picture of Stefan]
Jacob Rengman Stefan Petersen
[Disclaimer]
This page is maintained by Stefan Petersen and Jacob Rengman