An autonomous roving vehicle Part 1 of n

Introduction

Back when dinosaurs roamed the earrh I started my career as a computer programmer for a company that gave me the opportunity to work in many different areas of interest. One area that particularly interested me was the up and comming field of robotics and some of the projects we worked on for the automotive industry afforded me the opportuity to develop firmware for various machines that had a bit of intelligence build into them. At this time I also became interested in electronics and asked some of the electrical engineers I worked with if they would tutor me in electronics and in return I would tutor them in programming. Well it worked out very well and I learned a lot but when I left the company I took a job in Florida and the new job did not afford me the opportunity to pursue my new found interests so it laid dormant for many years. But a few years ago I retired and the interest in electronics and robotics has risen to the surface and I have decided to once again pursure this interest.

My plan is to create an autonomous roving vehicle and have given the creation the name of Robbie after the lovable robot from the old TV series Lost in space, you know the one...danger Will Robinson... Along the way I intend to document the effort in a series of articles of which this is the first. I will discuss the basic chassis design, design decisions and parts used to bring Robbie to life, where to find them and cost. The design and parts I use in this project are just the way I did it and should only be taken as a point of reference.

In the beginning

I started my research about a month prior to doing any design and then another couple of weeks before I started ordering parts. I started by purchasing a book that I would highly recommend before doing anything else, Robot Builders Bonanza by Gordon McComb now in it's 4th edition, it is a must have reference for every roboteer that covers every facet of the robot building process. The book helped me greatly when it cam time to order the parts I needed such as; the type motor, correct gear ratio, what material to use for the chassis, etc.. Get the book you won't regret it!

So now I've read the book and ordered the parts and I'm ready to start designing the chassis. Years ago I used a CAD program called TurboCAD for a couple of personel projects and so I knew a little about how to use it. I still had an old version (TurboCAD 6.0 they are now up to Version 20.0) but it wouldn't run on windows 7 and I didn't want to shell out another $100 for the newest version so I searched ebay and found an older verion 18.0 for around $16 with shipping. For the price difference and what I wanted to do with it I decided the older version would work just fine. If you don't know CAD or don't want to shell out the money the design can be done any way that suits you including on a napkin if that's all you've got, it just doesn't matter. The images below are what I came up with.

The anatomy

I reworked the drawings several times before I got what I think will work, but the plastic could only be bought in 24"X48" sheets so I've got plenty of material to work with if I find it doesn't suit my needs.

The Brains

I had originally planned on using only one Microcontroller for this project, an ATMega2560 running at 16MHz but I found a Dagu motor controller that handles 4 DC motors with encoding for $22 and ordered it because the controllers that I had made myself only handeled 2 motors without encoding so at the very least I would have had to put together another board and it wouldn't have been nearly as compact and didn't include the encoding. Since the encoders are so CPU intensive I decided to dedicate an ATMega28P to the task and will use a modified form of SPI to communicate between the uControllers. I worked on the SPI communications while I was waiting for the parts to come and just about got it finished but not tested so will include it in the next series download.

A short diversion to explain encoding and why it's useful may be in order here. Encoding is comprised of two parts; a moving component with a set number of slots, holes or special reflective material and a sensor to detect them. Each time one of the slots are detected 2 interrupts occur; one when it detects the beginning of the slot and the other when the opening is no longer detected thus producing a waveform similar to "Encoder Input A" and "Encoder Input B" shown in the following image taken from the Dagu manual. The wheel decoders I bought have two encoders for each wheel that are approximately 90 degrees out of phase and the logic on the board mixes the two waeforms to produce one waveform as the image illustrates. The reason that the sensors are put out of phase is so that the direction of the device may be determined from the encoder information. Although encoding is a great way to help determine distance there are inherent problems such as when the wheels are turning and the rover is not moving such as when driving on ice or if the rovers movement is restricted somehow.

Locomotion

There are two main ways that can be used to move your rover around; tracks or wheels and both have there pros and cons but I chose wheels because I thought it would be a simpler setup and figured the 4 wheel drive system would provide more power and be easier to operate on different surfaces.

Deciding what type of motor to use is next step in our design stage and there are several types to chose from; continuous servo, DC or stepper.

• Continuous servo - is just a special type of servo that doesn't have the constraint of limited rotation. A servo is just a geared DC motor with feedback circuitry in a sealed package. They are controlled by a 50Hz signal and varying the pulse width to determine the angular position.
• DC motor - A geared DC motor is a continuous rotating device without any feedback and when you apply power it moves, apply power in reverse and the motor rotates the opposite way. If you want to find out more about DC and Stepper motors check out my article Motor Primer and the L293D Quad Half-H Driver.
• Stepper motor - also a contnuous rotating device but when you apply power it moves only a few degrees then stops, to keep it rotating you pulse power to the motor. They don't provide the torque that the other two motors do and are heavier.

Of these I chose a geared DC motor with a 100:1 gear ratio which under load provides 320RPM and to relate this to how fast the rober will travel we can use the following calculation;

speed = Wheel Circumference * RPM

42mm = 1.65354 inches
Wheel Circumference = 5.19

speed = 5.19 * 320 RPM = 1661.48 Inches/min. or 27 inches/sec

At first glance this seems a little to fast but I will be controlling the speed of the motor using Pulse Width Modulation (PWM) which uses a variable duty cycle to control the power going to the device. To learn more about PWM I found a very good video that describes it in just a couple of minutes. find it here.

Bill of materials

I've included a list of the major parts that includes where I ordered them from and the price as close as I can remember;

Robbie BOM

Price	Where	Description
~$20.00	Lightake.com	ATMega2560 Got it cheap because it was Ver 1)
$12.00	Adafruit.com	ATMega328P Bareboanes AVR dev. board
$21.95	Jameco.com	Dagu Motor Controller, 4 channel 4.5A 4.5-12V with encoder
4@$15.95 ea.	pololu.com	100:1 Micro gearmotor, high power
$39.95	pololu.ocm	Set of 42X19mm wheels with encoders
$6.98	pololu.com	Set of 42X19mm wheels
$44.01	BatterySpace.com	7.2V Nattery pack with charger
$39.00	USPlastics.com	.200" Rigid expanded PVC
$247.69

So I got all the parts that I needed for the basic rover for right at $250, some of the parts I had in stock and some I had to order but this should be just about all I need to order as most of the other parts that I'll be needing I have stock. There are cheaper and easier ways to build a working autonomous vehicle and to find out how refer to the Robot Builders Bonanza book.

Most of the parts don't need an explanation, if you've done any development with the AVR line of uConrollers you will have at least a basic knowledge of the inner workings of these marvelous devices and if not this probably wouldn't be a good beginners project. The one part that needs more discussion is the Dagu controller board, the manual is very sparse and there doesn't seem to be a lot about it when you google. I haven't worked a lot with it yet but I will say it seems to be a pretty nice controller and I think it will do a good job driving all 4 of my motors. I've written a C++ class that provides most of the basic functionality that the board is capable of and provided it for download here. It probably won't be very useful as it is written for the ATMega328P but it can be used as a reference or modified for use with other uControllers.

One other thing I will mention here that is of importance is the use of 2 power supplies, one for the logic and the other for the motors. This separation of power is necessary as the motors are very power hungry and generate a lot of electrical noise and the logic needs a stable noise free signal or as noise frree as possible. For the logic power I took the battery out of an old phone that I had and removed the connectors from the phones main board and mounted them on a home made board. This was a litte tricky as the connector was surface mount, looks a little kludgy but it works just fine.

Conclusion

Right now I'm still working with the motor controller getting the firmware tested and adding another layer on top of the basic functionality to coordinate movement of the 4 wheels in a more intelligent manner. In the next installment in this series I plan on documenting my efforts to get everything mounted and the rover moving around on it's own. At that point it will be like Mr. Magoo, for those old enough to remember blind as a bat and no intelligence but moving on it's own none the less.

References