Ok, How many
people saw the Robot move on TV? Anyone? Well the fact is that there was a man in that robot suit and almost all the time
he was pulled by strings. There were no motors in the treads driving him/it along.
And really
the treads are a bad idea.
Look, the design of the B9 tread section sucks. No Robot Engineer worth
his direct deposit pay check would ever design a tread section just like we saw on TV: It just would not ever work well. Ever.
Sure you could go forwards and backwards but turns? Not easy, Not fun. There are way too many:Tread section problems
So instead
of repeating 40 year old mistakes I set out to make a tread section that looks much like the TV one but works like we think
we saw on TV but without the problems.
The design
I came up with is a compromise between treads and wheels. Under most conditions it acts like a wheeled system and other conditions
it acts like treads. Below is a sketch made during development:
It uses commercially available belts as treads, a pair of 2" wide synchronous belts. And instead of normal B9 wheels there are a total
of 12 (6 per side) wheels made up of commercially available wheels with machined aluminum side plates. Below is a typical
wheel:
Here is some data on the parts used:
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Part
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Mfg/Supplier
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Part Number
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Any motor of this size will do.
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ANSI 40, 1/2" pitch, 2W093
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Gates 2000-8MGT-50 Powergrip
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Structurally the tread section is just a box bolted together at the edges to blocks of alumium. The only real
exception to this is at the front and rear bottom edges. Here there are 3” dia aluminum rounds (see above) that not
only act to hold the box together they also support wheels and form important curves.
And if you
look (up on the sketch picture) close on the right side (the second from the right) you will see a wheel that does not support
or contact the tread. This is the steering wheel(s). They are mounted on a hub that is turned by a small gear motor to steer.
The steering
wheel hub uses 2 wheels to reduce the torque required to turn (steer) the wheels. If a single wheel was used it would take
a large amount of torque to turn the wheel on the steering axis. With 2 wheels things are very different, much easier.
Take a look at any modern baby carriage, they have steerable front wheels like this.
The hub is
located on top by a shoulder bolt mounted in a flanged oil-lite bearing. Positioning the hub axially (up and down) is a needle
thrust bearing with hardened thrust washers. If it is necessary to make an adjustment to keep the robot vertical It is possible
to adjust the position of the hub by adding spacer washers here.
Three roller
cam followers locate the hub in the radial direction. You can see one of them in the lower picture.
 
The wheel(s)/tread
on the far left contacts the ground and normally drives the robot. Under
normal conditions, and normal means a flat and smooth surface, only these tread/wheel over on the left (really the front)
and the steering wheel(s) contact the ground, effectively, a tricycle system. Very nice, all three points will always be in
contact with the ground. Many "real" robots (Roomba and others) use such a wheeled
system.
The treads,
other then at the front wheels, are located so that they are always about 1/4" above the ground under normal conditions. No
one, but an ant, should notice. But if an obstacle is encountered, say a door sill, the treads should contact the object and
provide adequate traction to carry the robot till it has cleared the obstacle. Of course while going over such an object the
steering wheel(s) may not be effective.
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