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   YogiCub  M.O.P Rover

FIDO Rover WheelsSteering Assembly
The last, and most important function of the Wheel Assembly is its ability to steer the Rover. Probably the most distinctive aspect of an exploration Rover is the way it steers and maneuvers across uneven and rocky terrain. Most Rovers have six wheels, of these four are used for steering. located on each corner of the Rover, the pivoting wheel assemblies are able to turn through 45° of movement. The design of the pivot assembly calls for some more large turntables, Liftarms and careful consideration of wheel placement, and width.



Wheel AlignmentDesign 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 Assembly Steering Assembly
Steering Assembly Steering Assembly



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).

Parallel wheels All wheels driven in the same direction
All wheels parallel All wheels driven in the same direction

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

    Movment StoppedForward (or reverse) movement is stopped.
     The outermost wheels are pivoted through 45°. Diagonally opposite wheels are rotated in the same direction.
     TurnWheels on either side turned in the opposing direction causing the Rover to pivot. The longer the wheels are left to turn the more the Rover pivots. A short duration results in a small change of direction. A long duration will see the Rover pivot through 360°.
     After turning to face the desired direction the outer wheels are rotated back to parallel

Movment ContiunedForward (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:

   Wheels assemblies only pivot through 45°.
(All that is required to enable the Rover to rotate a full 360°)

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.


Steering Motors Protective Cluch Gear
A 24t Clutch gear is mounted below the Worm gear to protect the motors from any mechanical jams along the steering drive train if they occur.



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°.

Turning RadiusYogiCub’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.



Complete Steering Assembley



Complete Steering Assembley



Complete Steering Assembley



From the first Rocky Rover through Sojourner (Top) to the FIDO Rover (Bottom) 4-Wheel independent steering has been used to steer and direct Rover movement.


FIDO Rover


Robert Stehlik’s Rover has 4 wheel drive and steering. Drive to each wheel is achieved through a 40t gear and a universal joint. All four wheels are tangent to a common circle to avid scrubbing while turning.

Check out Rob’s Articulated 4x4

4x4 Articulated Steering

Joe Nagata has a new MOC which uses 4 wheels, 3 motors and an RCX.

Check out the Amazing Car

Amazing Car
Download Code

See Sojourner in action on Mars! This movie contains a compilation of image sequences taken by the Mars Pathfinder Lander. It is a great illustration of the unique steering technique employed by Rovers.

Sojourner Turning

Windows Sojourner | .wmv | 7.8mb

  See more Sojourner movies


It is interesting to compare the 'turning styles' of Rovers. The following movie compares Sojourner, Rocky 7, MER and YogiCub's turing characteristics while performing a simple steering sequence.

MER Turning

Windows Steering Comparison
| .wmv | 3.8mb


YogiCub has a common mechanical linkage which turns all pivoting wheel assemblies.

This is driven by 2x mechanically linked 9V geared motors. The final gearing at the turntables is 56:1 (1 rotation for every 56 of the motor). This is figured out by the following:

Motor driving Worm gear

Worm gear driving a 24t Clutch gear

24t Crown gear driving a 56t turntable outer ring.

Overall gearing
168:3 or 56:1

RPM at Turntable
(assuming 220RMP at motor)
~ 3.929 RPM

Estimated time to travel 45°
= 1.91 sec

Total ° traveled in 60 seconds
1,414.29° = 3.929 x 360

Total ° traveled in 1 second
23.57 ° = 1,414.29/ 60

Time taken to travel 45°
1.91 = 1/23.57 x 45

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