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1 - The chassis
Submitted by Webbot on January 1, 2009 - 6:34pm.
We will build the chassis out of HDPE as it is easy to cut and put self-tapping screws into. We will make the base as a circle with the wheels inside the circumference - as this makes a shape that is hard to get snagged on chair legs etc.
Here are the basic materials we will need:-
First we will use our set square to make sure that we have a good 90 degree corner:-
Then take an old CD (you no longer need - as the pen may corrupt the CD. Use one of those free CDs you get in the post!) and a CD marker pen. These pens make a better marking on the board compared with pencil, biro or felt tip:-
Trace around the outer edge of the CD to make a circle. Then use the set-square to draw a box around the circle. Then draw the diagonals from each corner - the intersection is the center of the circle.
Then we need to cut some holes for the wheels to go through. By placing the servos flush with the base we can measure the distance from the base to the axle of the servo. Knowing the diameter of the wheels we are using, and Pythagoras' theorem then we can calculate the requried hole size in the chassis to accomodate the wheel. Obviously you need to increase these dimensions slightly to leave a gap around the wheel. I calculated mine, using Solarbotics wheels, as about a 6cm hole. The width of the hole should be wider than the wheel. So mark-up the wheel slot for the base:-
Use the jigsaw to cut around the circle and then to cut out the slots for the wheels. I have then mounted my Hitec servos using Universal Servo Mounting Brackets so that the wheels fit snuggly into the slots we have cut. The last step was to add some Velcro to the top of the servo housings (the big white squares on top of the servos show below).
Next: cut out a rough rectangle (no need to be at all accurate) whose width will go across the velcro on the two servos (to add a bit of strength) and goes to the back of the chassis and stick it on:-
I have then velcro'd a small ball caster unit to the top of this board. You could probably also use a Tamiya Ball Caster Unit or any other unit you prefer. The only caveat is that the net result is that the robot should stand upright.
The wheels and ball caster are available here (UK prices)
This additional 'shelf' also acts as somewhere to store the 9v battery to supply the $50 robot board. So let's add some velcro to the 9v battery:-
By adding some Velcro to the shelf we can now attach the 9v battery:-
So much for the bottom of the robot. Now let's see it from the top:-
Here you can see how the 9v battery is on the 'bottom shelf'. Above it, on top of the base, we have got the 4xAA battery box for driving the servos. This box has also been Velocro'd to the base.This battery box, and the 9v battery, are purposely placed towards the back of the robot so that the center of gravity rests upon the ball caster. Otherwise - when your robot goes from forward to backwards movement then the chassis will do a nose dive at the front.
Resting, just above it, is a $50 robot board. Yours may well look different to mine - but mine still uses the matrix board described in the tutuorial so the dimensions will be the same. The $50 board has not been mounted using the usual spacers. Instead: I have found some plastic alternatives (at my local hardware store) which means I can cut them to whatever length I need - which is cool.
You will also see that I have screwed on some micro-switches to the front of the robot. These are just to act as 'bumpers'. These are just on-off switches with a long arm on top. These need to be attached very close to the diameter of the chassis.
Here's another photo from the front:-
This shows the micro-switches as well as another HS311 servo (unmodified) upon which we can mount a sonar, etc, which could swing left/right. This servo has also been mounted using Velcro so that we can remove it or re-position it as needed.