[dtrip]

Hello guys,



It just occured to me a very simple setup for an electric car =


Rear Wheel Drive. Each rear wheel has its own motor
and is driven via a sprocket (fixed ratio) and a chain directly to the motor.

Like a go kart power system, but on both rear wheels independently.



There is no differential and/or gearbox. Just throttle and brake.



The differential is simulated by the controller, which takes into account
the turning wheel state and accelerates the wheels differently.



Turning without throttle should not pose a problem since the wheels
are all independent (hence, no differential required --> I suppose ?)



I want to know if this would be reasonably safe (for a RWD) and worth
mentioning, or does it have some serious flaw ?



Thanks for your time reading this,

Dimitris

 

[Emyr]

The differential is simulated by the controller, which takes into account
the turning wheel state and accelerates the wheels differently.

Turning without throttle should not pose a problem since the wheels
are all independent (hence, no differential required --> I suppose ?)

I want to know if this would be reasonably safe (for a RWD) and worth
mentioning, or does it have some serious flaw ?

Your wording suggests you'd be using ACIM or PMAC motors, with a separate power stage for each side; For DC motors, you can do it with one controller.

The formulas linking steering angle, wheelbase, track, turning circle and ideal speed for each wheel are all pretty basic geometry.

The problem with implementing a "software diff" is that your throttle is normally configured as a torque request rather than a speed control, but you'll want to reduce torque to the inside wheel if the weight transfer causes it to lose traction. You'll need a clever algorithm to tell the difference between acceleration and wheelspin.

A basic system would be to use steering angle to determine which is the outer wheel, apply torque control to that wheel (as in a single-motor system), then derive the inner wheel's target speed from the measured outer wheel speed.

 

[Hollie Maea]

We (EVDrive) did this with our UTV that we made a few years ago. We actually have motors on all front wheels (we used actual gear reductions rather than chain drives) and developed some torque vectoring software to do software differential and more. It's doable but not trivial.

 

[dtrip]

Thanks guys,

Emyr exactly this is what I mean.

As for the inner wheel spin due to weight shift, the car should provide some kind of TCS (traction control) anyway, according to a basic algorithm like =

1. Read front wheels (which are un-powered) for a reliable wheel rotation speed (Rs)

2. If rear wheel rotation > Rs then this means it is spinning. If it is powered, reduce the power given to it. If it is not powered, well, blink an error red LED because this should not be happening.

So some basic algorithm like this should cover the spinning in any case, be it weight shift, or stepping on ice from one side, etc...... right ?

Hollie,
thanks, your reply encourages me. I do not mind software complications, I am a computer programmer for 25 years now and have programmed Arduino and other controllers in the past.

 

[Karter2]

No need to bother with the agorithm complexity,..just power the motors as suggested with a common controller for dC drive or separate for AC motors.
The. " inside wheel traction ". concern is no different than any normal differential drive situation....if it hits mud, gravel or ice , it spins !
BUT,..unlike a differential drive, you will still have normal full torque being applied to the wheel with traction
And if you are smart and convert a modern car with existing traction control built in, that commonly works by using the independent brakes to limit the spil/slip of any low traction wheels.....quite separate from the drive controls.