Describes how to "build one of these" yourself. Pretty much is a step-by-step guide to build your own track drive robot.

It is pretty simple and relatively cheap and actually kind of a fun way to become familiar with Robots and Robotics in general.

It isn`t long after getting your first robot, you start to wish that little wheeled bot could climb over things in a relentless forward motion - - Like a Tank!

Even though the old timers told me I didn`t want the hassles of a track-drive robot, I knew it wasn`t true. I wanted one! I wanted it simple and cheap and programmable for fun!

Get the Tamiya Tracked Vehicle Chassis Kit to start with. It's a kit, so it will make for good practice. If you can build it you end up with a little bot that moves backwards, forwards and stops at the flick of a switch. That may be all you want to do. You may decide you want a tracked robot by just playing around, then go out and get yourself a fancy commercial unit.

I, on the other hand, didn't want to invest hundreds of dollars, but going forwards, backwards, and stopping wasnt' enough for me! No!!!

I wanted to use dual differential steering. In other words, TWO motors. With two motors I can move both motors and in either direction. That means the trakBot can make spinning turns as well as going forward and backwards and stopping!

Moving from one motor to two motors meant a slight upgrade. Another purchase...

Tamiya Twin Motor Drive installation is easy and putting the motor together is easy too. There are instructions for assembling the motor here on CwhatIcanDo. Don't forget the grease! You will have learned to put grease in all the right places having already assembled one gearbox to build the base kit.

You'll have to make new holes in the wood base of the trakBot to mount the wider twin drives. But, you'll see - They work great. The nice thing is the small motors don't draw too much current so the H-Bridge circuit built in works well, even when you use variable speeds.

The picAxe Motor Controller is ideal for controlling the trakBot. Things stay just a little hot to the touch running the motors at full on, full off type operation.

For trakBot, I chose the lowest gearing in my twin motor gearbox, so top speed is still pretty slow, but it will climb things like crazy.

When you buy your motor control, you may only get a straight 8 bit picAxe chip free with the board. That means, you'll have to buy another chip, because only the 8M variant has interrupts and we use interrupts in this design.

You wouldn't have to use interrupts, you could constantly scan the signal from the IR detector. But using an interrupt works better and makes the bot run in interesting ways.

Add an IR detector so your bot can identify objects. At least if they are within 30-40 cm from the bot. That's why you want to put the IR detector on the front of the bot slanted to the side. With the cone shaped output of the detector, you will be able to pretty much cover the whole front end of the trakBot, so it will be able to recognize objects more effectively.

Basically using an IR detector instead of building your own, takes advantage of the optical system built in. This IR detector is not sensitive to sunlight, and, it "sees" black plastic which is very difficult for IR eyes anyway.

So spend the money, the good IR detector will pay off in the end.

Simplicity is the key. Use the motor mounts themselves to hold the motor control pc board, and to hold the IR detector.

I used solder tabs for that job... Just bend the solder tab, mount it against the twin motor drive by removing a screw, then putting the screw back in to hold the pc board in place. Yes, I only used 1 screw.

To mount the IR detector, so I could lean it back from the front of the bot to allow the cone shaped sensitive area to cover the whole front of the bot. This way, you can use only one detector where normally you would mount one on each side so the output of the IR "cone" covers the other.

Too expensive to buy another sensor, so I just leaned it back using a metal jumper from a large electrical connection. These things are handy, but any easily bendable bracket would work. On the top of the bracket, mount another solder lug as shown in the picture.

This is a sort of block diagram with schematic wiring in it. You can easily see how to hook up each motor, and where the output of the IR detector (and power and ground) hookup.

I mounted an on/off toggle to the platform, and I use a toggle switch when I'm programming, just so I don't have to keep moving the jumper around.

Programming is done with an optional cable (you can build one yourself). All the instructions for programming and wiring are available for free from vendors who sell the chips.

The picAxe chip has a tiny BASIC interpreter built into the chip. That means you don't have all the instruction space in memory because of the space the BASIC interpreter takes up. But for the 8 pin chips, which are doing specialized work (like driving motors), you can't beat the simplicity and ease of reprogramming of a picAxe.

So it's a great choice for the trakBot!

Overview:

The free BASIC Software that is available for free from picAxe vendors allows you to program the 08m to control the motors via the Dual H-bridge supplied on the PC board. The BASIC interpreter uses a slightly "laid-back" version of BASIC, so it won't do super-exciting math or anything like that.

But BASIC does allow you to follow a process from beginning to end. For example - The robot, before moving forward by enabling the H-Bridge might check to see if there are any obstacles in it's way, if not, it might move forward. Since this process is sequential, it means if you move the bot a foot at a time it might not see an object put in front of it out of it's current range. Because it's only checking every foot of bot movement. That won't prove practical.

So you could just loop around, enable the motor, check for obstacles, enable again, etc. Now the bot is "pulsing" it's way around. Motor 1 on, Motor 1 off, check obstacles. Then if no obstacle, do that for Motor 2, and so on. The problem is the time loop has to be short or else the bot will wiggle. One motor on, relatively (to the bot) long wait to check and see if it is going to crash.

Not a bad way to run a robot, if the loop is fast enough, it will work well. But hey! There is a much better and simpler way to run a robot.

Using Interrupts:

The M8 let's you monitor one of it's pins for a low or high (you decide) and then forces the software program to jump to a certain point, perform a subroutine, the jump right back into the program again right where it left off even if mid instruction when the interrupt occurs.

Look in the listing below to see how the INTERRUPT subroutine is operated. This is the subroutine where the bot decides to evade or continue. It happens whenever the IR detector hits the threshold voltage (a low, logic 0). It does this when an object appears in front of it.

Thus the computer doesnt' have to loop around it's sensors all day. An interrupt, interrupts the logical flow to make that decision to avoid an obstacle. It turns out that even if the computer is executing a PAUSE (basically twiddling it's thumbs for a length of time you specify) it is subject to an interrupt.

Thus, in the code below, the MAIN routine just PAUSEs for nearly a minute, then jumps back and PAUSEs once again forever. Unless an object comes into the IR detector's view.

The interrupt routine in the example is not very sophisticated, it just stops backs up a little and turns right or left (depends on how you decided to wire the bot) then tries to move forward again. Except for a random number generator, the bot would simply be a simpler "wall follower" pattern. But it proves to be stimulating and is actually fun to watch.

Wall Follower Listing...


'-------------------------------------------------------
'
' picAxe MOTOR Control for 08m chip
' interrupts drive evasive tactics
' SOUNDS: Using 3-level A/D Converter output
' Created by: Jim Huffman
' Rev 0.1 November 11, 2007
' Rev 1.0 December 02, 2007
' Using the AXE motor drive combo board
'
' Rev 2.0 December 31, 2007
'-------------------------------------------------------
'
' Initialize System Wide Values
'
' Directives
' Select and initialize chipset
#picaxe 08m ' this is: picaxe08 in the setup that
' comes with the board
'#terminal 4800
'
' System resources
symbol m1 = 0 '(leg 7)
symbol m1en = 1 '(leg 6)ADC1
symbol m2en = 2 '(leg 5)ADC2,PWMout2
symbol IRsense = 3 '(leg 4)
symbol m2 = 4 '(leg 3)ADC4
' (leg 1 Vdd, leg 8 = Vss, leg 2 = Serial In)
'
' Motor Control constants
' speed control

symbol R = w0
symbol y = w1
symbol z = w2
symbol alrmCnt = w3

'
'--------------------------------------------------------
' Code Blocks:
'
init:
' set pins 0,1,2, 4 as outputs
let dirs = %00010111
let pins = %00000000 'set outputs low initially

random R
pause 1000

' Stop Motor
gosub move_stop 'don't go anywhere yet
pause 1000
'

setint %00001000,%00001000

'
'
main:
gosub move_forward
random R
pause 100

goto main
'
'
'--------------------------------------------------------
' Move this baby OUTPUT 2
move_forward:
let pins = %00010001
return

move_backward:
let pins = %00000110
return


move_stop:
let pins = %00000000 'kill everything!
return

turn_right:
let pins = %00010010 '
return

turn_left:
let pins = %00000101 '
return


interrupt:
' sertxd("The value of R is ",#R,13,10)
gosub move_stop
gosub move_backward
pause 300
gosub turn_right
pause 30
pause b1 'add a random spin around time
gosub move_stop

setint %00001000,%00001000

return

What's next?

The program I've included here is very simple. If you want more complexity, there is a picAxe 18x motor controller available, with the H-Bridge and a set of four high-power output lines. I've used one of those to drive the speaker. It's quite loud enough for a bot.

The 8M is a dandy processor, but you can do so much more with them. And every experiment is just a simple download and test away.

Like controlling the speed of the motor by using PWM (pulse width modulation) so you can have your version of the bot slow down when it sees an obstacle. Then you can use your own avoidance routine to make it go...

Let me know how building this bot works out for you. I have had a lot of fun with this bot, and there has been only an occasionally track loss or any of the other horror stories I've heard about tracked robots. Don't believe 'em, a tracked robot is a great starting place for a robot hacker like me, and YOU...