Best current solutions 
 
Two motorized wheels with Maxon motors-gears-encoders 
and one or 2 additional, passive, omnidirectional wheels (one of them on suspension with damper in the second case). 
 
Omnidirectional platform with four Mechanum wheels is a solution which provides 3 degrees of freedom and better stability but which requires more resources (4 Maxon motors and Indel AG controller, necessary suspension with elastic and damping features) 
 
 
Trajectories are typically defined as follows : 
- Select key points (x,y) on a map,  
- Interconnect them with linear sections 
- Resample the trajectory at regularly-spaced intervals  
- Smooth the trajectory, iteratively until some vehicle-related, maximum curvature constraint is met (reference for curvilinear smoothing) 
- Define local orientation (tangent angle) for each sample (x, y, theta) 
 
Trajectories are typically tracked as follows: 
 
- match vehicle position and orientation in terms of main structure 
- define relative position and orientation (transformation) of each wheel or limb (thereby superposing gait) 
- interpolation is made directly at individual wheel or limb level between so-defined successive positions along trajectory 
 
(reference for general trajectory definition and tracking): 
 
Past solutions 
 
- Two-wheels with stepper motors with proprietary sequencer and amplifier 
 
- Four wheels with DC-motors, or permanent magnet motors, or asynchronous motors, on articulated platform driven by differential speed of wheels 
 
- Six wheels with 12 DC-motors for Fido-type structure