TIM-01: The Drive Train

Every thing has to start some ware. I decided to start at the bottom and work my way up.

I decided to go for the tripod approach, but I want to make it big enough to fetch me a cup of tea from the kitchen. (we all have our dreams)

Also any parts I buy to be as cheap as possible.

The largest and cheapest wheels (that look like a vehicle wheel) I could find on eBay was these:

I did not think three would look right, I thought they would look small just three of them, so I decided to have three pairs, six of them. (little did I know the problems this would cause at the time)

I want at some stage for my robot to be able to draw, like I made here: Tim's Draw Bot

So I have decided to drive it using Stepper Motors.

My First Drive Frame

You can see in the image I just have one axle for each pair of wheels.

I put an offset swivel on the rear pair so that it would turn like a castor, depending on the direction it was going.

(Oh, you will see in the following sketches that parts are a little different to this one, this one was too wide, it could not navigate past some of my furniture. )

I first noticed on the rear wheels, that when they rotate around the offset swivel the pair of wheel need to go in opposite directions.

I had to fit two separate axles, so that each wheel rotated independently.

This was still not enough, I found that, if the ground was not at 90 Deg. to the vertical swivel connection.

The wheel assembly would still not swivel.

The wheel will always try to position itself down hill, relative to the axis of the swivel connection.

So it would also do the same if the swivel connection was not vertical to the floor.

(This is why shopping trolley wheel wobble).

So I had to fit a second swivel joint 90 Deg. to the first.

This now allows the weight of the robot to be applied evenly to each wheel, while on a gradient, allowing the rear wheels to rotate freely in which ever direction my robot is traveling.

Here's a detailed view of the bearing layout

Now that I had sorted the rear wheels, I had the front ones to deal with.

As I have mentioned before, I want it to be able to draw, like the Draw Bot I made.

This will involve, being able to turn the robot precise amount of degrees.

This is just calculating the circumference of the wheels, to calculate how much they need to turn, to move around the circumference of a larger circle, with a diameter the distance between the right and left wheels.

Simple. Oh but there is a pair of wheels right and left.

When the robot turns the inner wheels will need to move slower, because they are following a smaller radius.

I have two options here; I could have one wheel free wheel and drive only one, or I could add differential gearing to both wheels.

As I feel I will need full traction from both wheels I have gone with differential gearing, if I am able to design and print small enough gears.

I was able to design and print a differential gear system for the front wheels, here is a sketch showing what I came up with. (removed a wheel and added colours to show more clearly)

All the front wheels now turn at there own pace.

But there is still another problem, I'm sure you have seen or come across the issue where, a car with a fixed rear drive axle has one rea wheel in mud and the other wheel on firm ground.

The car will just sit there with one rear wheel spinning.

Well now I have put differential gears in the front wheels, this can happen on uneven ground.

To fix this, I need to allow the pair of front wheels assembly to swivel like I did with the rear wheels. Allowing both wheels to be in contact with the ground all the time.

Luckily I have put the front wheels on arms, because I want to be able to rise and lower the robot.