Imagine if you spun the wheels of a 4wd vehicle with four independently controlled motors, instead of any differentials. Based on the angle of the steering wheel, each wheel would turn at the correct proportional rate for its position going around the curve. Even if the vehicle was floating in the air, the wheels would all move correctly, and there would be no need to apply any other kind of correction. I’m thinking servomotor and microprocessor based control here.
If I understood correctly, the OP is describing a system where the 4 wheels are not only independently powered, but also controlled to specific speeds. So if one wheel were to hit a patch of ice, it wouldn’t slip and spin. It would continue to turn at a speed that it WOULD turn if it had traction.
I don’t think the vehicles mentioned above have such a system. The Mars rovers might.
Then again, I’m not sure if this is different from, or better than, an independent drive system with traction control.
Also, in many modern cars with regular differentials, the computers will apply brakes to the wheels on one side to further slow them down. Not as elegant as independent motors, but it works…
NASA’s crawler transporter would count too. Electric drive at four corners.
The interesting question with such a scheme is in controlling not the desired rotational speed, but the torque needed. For maximum control you are managing the forces between the tyre and the road surface. The tyre is not just a surface, the entire thing is a mix of flexible components, from the carcase to the individual tread blocks. They flex, and the tread blocks actually walk. The best torque on a wheel is a complicated thing. If you lose grip you will want to speed the wheel up to match the road surface again to allow grip to be re-established. Unless you are on a loose surface. In principle completely independent drive at each corner could be as good as it gets.
There are some very advanced all wheel drive vehicles that achieve this and more with differentials that are intelligently controlled. Cars like the Mitsubishi Evo, the Subaru WRX and the Nissan GTR for example.
Top Gear tested an Evo against a WRX a number of years ago. To demonstrate the smart differentials, Jeremy ran one car around a skid pad as fast as it could go with the smart feature turned off. I forget the exact speed, but it was about 30 mph. It could go no faster, just scrubbed the tires and slowly drifted off the line.
Then he turned it back on. Holy crap. The car sailed past the previous speed with ease and continued accelerating with no drama, no smoking tires, no drifting wide of the line. At about 50 mph, while the car was still accelerating, Jeremy was forced by the G loads to stop the car, exit and vomit. It could turn faster then he could withstand.
Those cars do this on real world roads, not just skid pads. On tarmac, dirt, snow, what have you. Of course the tuned versions are world beating rally cars.
Some 4wd cars/SUV’s have this for the secondary axle (whether its front or back is not the point.)
Basically they have the standard two wheel drive system, but then independent thrust on the other two. Its not only 5% power, but its for control of the vehicle in slippery conditions.
Anti-skid braking is the same idea in the opposite direction.
As you say, the implementation is not so much top-down analytical as bottom-up sensor-driven.
My equipment has independent 8-wheel braking. With a fairly intricate cross-wheel comparator mesh that effectively “votes” on which wheel(s) are losing speed sync with the others and are assumed to be skidding vice gripping.
Certainly the same general approach could be applied to a group of powered wheels for acceleration management.
Heavy trucks are usually fitted with a manually operated differential lock.If there are two driven axles, there will be three differentials. Operating the lock means that the axles (and therefore the wheels) on both sides of the differential rotate at the same speed, regardless of traction.
There is always an override that prevents it from working at more than some slow speed (5kph?) This is to avoid damage to the tyres and because it is difficult to turn.
Some are describing vehicles with separate motors on each wheel, or some separate control element (like braking), such that each wheel can be moved arbitrarily (more or less). That isn’t all that’s required in what I’m describing, though. There also has to be a control system calculating the proper speed for each wheel, which depends on the instantaneous turn radius and speed. Do any of these examples cited do that?
I did that with the “Really Big Truck” link you gave, which describes the BelAz 75710. This uses a Siemens “Integrated Drive Systems (IDS)” control system. There is independent control of each of the four wheels, but they don’t calculate ideal speed. Instead, it is an elaborate slip detection and correction system that controls torque and backs off when there’s slippage.
I didn’t mean to snub people’s suggestions – sorry I made it sound that way! What I meant to ask was whether people actually knew if the links they gave were to systems that work this way, or if they were just trying some possibilities.