It's all about going fast. If you are going to deliver packages a mile or more away, you need to move with the traffic.
If you are a little box bot, your tires are going to wear out, or you are going to get run over. If you are a Segway bot, the faster you go the closer you are to falling down. Heaven forbid if you are trying to run.
Like a mountain bike, no matter how fast you go, you can tolerate bumps and dips in the road. If you get bumped, a quick swerve of the steering and you are back up. With a Segway, if you clip a wheel, you are on your face in a jiffy.
The technical term is "high control authority". A bicycle, under way, is remarkably stable.
Segway style robots are really very clever. To accelerate forward, they need lean forward. To lean forward they first need to move the wheels backward a bit. This causes the body to fall forward. The servos then push the wheels back under you maintaining the lean angle as you accelerate.
To stop, they must put the wheels out in front to lean back. If the motors ever reach their maximum speed, it can not recover and it falls down. If you simply "slammed on the brakes" you would fall flat on your face.
Twills do not have this dynamic constraint. The long wheelbase means you can accelerate and brake as hard as you like. The technical term is "stiffness in the direction of travel". That is a fancy way of saying, when you slam on the brakes, you stop.
Three wheels are nice and simple, but have important limitations. This stance eliminates the ability to "lean into the turn" unless you add complex extra machinery. Without leaning, the turning speed is limited. In the "delta" configuration this especially a problem since a perfectly reasonable turn can suddenly turn to disaster if heavy braking is needed.
We are all amazed at the progress in walking robots. Unfortunately they are mechanically complex and would be difficult to maintain in real world situations. The latest from Boston Dynamics is incredible. It is no coincidence that they have added wheels because, "Wheels are much more efficient." A Twill is among the most efficient modes of transportation, second only to jet propulsion.
Segway style robots are remarkably robust when stopped. When well tuned, you can hardly see them moving. Others have shown resilience to full kicks and slams. When a Twill robot stops, it turns one of the wheels becoming "half of a Segway." This exactly how one balances on a "fixie bike." While standing they can be gently pushed out of the way and they will comply without complaining.
How a bicycle turns is subtle. You might think that to turn left you turn to the left or you lean to the left. Neither is true. In fact, to turn left, you turn briefly to the right. This pulls the wheels out from under you causing you to lean to the left. This sets you into the turn. To get out of the left turn, you turn more to the left to get the wheels under you again. Watch closely next time you see or ride a bicycle.
Wind load is a consideration for all types of transportation. Twills do well since they are free to "lean into the breeze. A pair of pressure sensors are used to measure this side force. This information is used 500 times per second to "feed forward" a correction. Gusts are handled as "disturbance influences."
The tall skinny stance of TwillBots allows them to drive on the side of the road. This keeps them from taking up a lane of traffic. They are so skinny and nimble they can share the sidewalk with foot traffic. They even fit in an elevator.
Twill configuration results in the simplest mechanics possible. There are no chains or belts or even gears in the system. With full-size motorcycle tires and bearings, we can achieve the longest possible service life with minimal servicing. Yes, the software is complicated. Compared to autonomy and navigation software that others have developed it is quite modest.
A three or four wheel vehicle is "statically stable". You turn off the power and it just stands there. But the side of the road is usually slanted significantly to the curb. So a tall three wheeler would be leaning to the right much of the time. To keep from falling over, it needs to be roughly as wide as it is tall. Twills do not have this constraint. Regardless of the "roll" of the rode, they stand vertically.
In the course of our research, we have perfected a compact inexpensive technology package. This version is available for academic research groups. We are discussions with several university groups.