In the past we have looked at wheel design and the kinematics of skid steer and mecanum wheels. In this post we will take a quick look at different types of mobility types (ie. wheels, tracks, and how to connect them) for drive selection.
I know the items in this list are not direct comparisons. For example listing rocker bogie is not necessarily exclusive to other types listed. Also I know that you can design a vehicle to get around the pros and cons that I list below. The list below is based on the common mechanical configurations.
For the purpose of this post:
Holonomic motion is where a vehicle can go in all X and Y directions and is not constrained.
non-holonomic motion is where a vehicle is constrained to only certain motions. An example is a car that is not capable to drive sideways (with its ackerman steering). (Don’t yell at me about the parallel parking magical cars. They are not the norm.)
|Tracks (Differential Drive)||Good mobility for rough terrain, low ground pressure||Complex mechanism, high power usage when turning|
|Skid Steer 4 wheel||Very simple, high weight capacity||High power usage, requires skidding/slipping|
|Differential drive 2 wheel + Passive Caster(s)||Easy||Lower weight designs, less precise controls. bad for obstacles/bumps|
|2 wheel + 1 Powered Steering Caster||Easy mechanical, powered steering wheel for control||Lower weight designs, potentially a lot of weight on steering wheel.|
|2 wheel (Segway Drive)||Easy mechanical, fun!||Not dynamically stable|
|Ackerman Steering||Wheels do not need to slip to turn. Fixed rear wheels makes control geometry easier.||Increased motor count|
|Rear wheel forklift steering||Tight maneuverability, good if weight is on front wheels||Less responsive than ackerman, unstable at “high” speeds|
|Independent all wheel steering||Lot of flexibility for motion||Complexity of mechanism and large number of motors. More coordination needed for turns.|
|Rocker Bogie / Body Averaging||Helps body average the chassis to minimize pitch. Allows all wheels to maintain ground contact while distributing the weight.||Mechanical complexity|
|Omni||Non-holonomic, light, simple, Best performance when traveling diagonal, number of wheels can be varied||Motion can be bumpy, sensitive to nonsmooth terrain, low torque for pushing|
|Mecanum O*||Skid steer is possible, holonomic motions, can support heavy loads||Motion can be bumpy, very sensitive to nonsmooth terrain, can be noisy, lots of parts|
|Crab Steer||Allows 4 wheel drive vehicle to translate at an angles/horizontally (holonomic motion). Often wheels can all turn in for point turns.||All 4 wheels need to be actuated to spin and turn (often with 8 motors, but can be done with less). Can be unstable on hills.|
|Legs (2,4,6,etc..)||Ease of steeping over difficult terrain||Hard, complex, high power|
|Single Ball (Ballbot)||Small size base, highly maneuverable, can change robots orientation||Hard controls, not dynamically stable, good for taller robot and not wider robots|
|Inchworming** (Peristaltic Motion)||Increased drawbar and slope climbing due to utilizing static wheels/friction||Requires extra actuation, slower than typical motion, can be high power consumption|
*Mecanum X vs O configuration is based on how the wheels are mounted. With mecanum wheels the left and right side wheels are physically different. Looking from the bottom you want an O configuration. The X configuration does not allow rotation (without fighting the rollers).
** Basic description of inchworming. Inchworming moves one set of wheels at a time to utilize static friction for increased mobility.
For more examples of unusual locomotion menthods visit unusuallocomotion.com. Main image also comes from that site.
Are there other mobility options that I should have included? Leave it in the comments below.