Design Solution
In order to have the wheel assembly successfully pivot, (with the least amount of resistance), the wheel must be directly centered under the pivot axis. Alignment offsets, and wheel width increase resistance, therefore increase the amount of force required to turn the assembly. Fortunately a simple combination of Liftarms gives both the motor, wheel, and pivot turntable the right spatial relationship. Pending sufficient drive from the motor this configuration can also accommodate various diameter wheels with varying offset.

Steering Drive Train
A single pivoting wheel assembly is not much use by its self, but when you have four, all sorts of maneuvers are possible. The key to Rover movement is the coordination of movement between all wheel assemblies. This task is coordinated by the Rovers electronic brain (the RCX in our case) and implemented through gearing and motors located within each wheel assembly.

For simple forward or reverse travel, all wheels are parallel, and are driven in the same direction – quite straight forward (no pun intended).

However when the Rover wishes to turn, or change direction a basic series of steering events take place. In sequential order these are:

Forward (or reverse) movement is resumed.

Design Solution
Steering is rather complicated but it makes for some rather compulsive viewing. Interestingly the key to the LEGO® implementation lies in the fact that:

Wheel assemblies are controlled in alternating pairs.

As illustrated earlier each wheel has a drive assembly that holds a motor. The motor requires power to run, and cables have to be routed from the RXC to the motor through the steering assembly (large Technic turntable). Because the wheel assembly will only ever be traveling through 45° it is more than safe to route the cables through the turntable to the motor below without a danger of them getting twisted or tangled.

Because the timing and coordination of pivoting the wheel assemblies is critical, each wheel assembly is mechanically linked by a drive train. The drive train is configured to link diagonally opposite wheel assemblies, rotating them (opposite pairs) in unison. A 24t Crown gear drives the outer ring of the steering assembly turntable, while drive is supplied by 2x linked LEGO® 9V Geared motors.

The RXC uses a rotation sensor to monitor the turning process, and control the mechanically linked motors. The gearing of the steering drive train is such that it takes approximately 2 seconds to move the wheel assemblies through 45°.

YogiCub’s chassis is square, with each wheel assembly placed at a corner. This means the pivot point of each wheel assembly turns on a tangent to a common circle. This means when the Rover rotates through a steering sequence there is no scrubbing of the wheels – reducing resistance.

Both steering motors are centrally located within the Rover keeping it balanced when on the move. The steering drive train and complete wheel assemblies work in unison to provide consistent, and controllable steering.