pic: Steering mechanism

[jskene]

Yes, this type of steering mechanism is referred to as Ackerman steering. Each wheel turns around its own turning axle, in our case both the front and rear wheels turn, so we get a very small steering radius.

Due to the sharp turning angle we have (45 degrees), the fact that we turn all 4 wheels, and the extreme grip of our tires on carpet (coefficient of friction is about 1.2!), we cannot tolerate any slip angle (difference between the angle the wheel is actually turned to and the angle it should be turned to to remain tangent to the turning circle), so we had to make the inside and outside wheels turn different amounts, so they remain tangent to the circumference of the turning circle they are supposed to follow.

It makes for some interesting mechanical design, as well as some neat programming.

Here is a photo of the steering shafts (note that we also have 4 wheel drive in addition to 4 wheel steering):

The steering arms of the left wheels are connected together, so their steering angle is identical. The same is done on the right wheels. The left and right sides are driven by independent steering motors, each with its own steering angle sensor. When the driver pushes the joystick to the left, say 20°, the left wheels become the inside wheels, and the left steering motor turns the wheels to the desired 20° steering angle, as measured by a potentiometer connected to the steering axle.

So, referring to the diagram below, we can calculate θo as follows:

tanθi = W/Xi
So Xi = W/Tanθi
Tanθo = W/(Xi+T)
θo = ArcTan(W/(Xi+T)
= ArcTan(W/(W/Tanθi +T)

We are also using an electronic differential, by setting the relative speed of the inside and outside motors to be proportional to their respective turn radii.

We also plan to add speed-sensitive steering, so at high speed the ratio between joystick angle and turn angle is reduced. Hopefully that will minimize the chance of flipping over with our 100" arm extended.