[ausev] 2 motors - serial vs. parallel

[Gil Dawson]

At 12:20 A -0600 5/31/08, evehicle4me at hischopsticks.org wrote:
>What I am talking about is putting one separate electric motor in 
>the hub of, for example, the rear wheel on the right side and then 
>one separate electric motor in the hub of the rear wheel on the left 
>side.  There may be a separate controller for each wheel or there 
>may be one controller that can control both of them.  If each motor 
>has its individual controller maybe there is a computer that can 
>coordinate them.  That is what my post is all about.  There is no 
>mechanical differential in this model.  There is no mechanical 
>transmission and set of gears with this model. There needs to be 
>some way (at least I think) to slow the wheel on the inside of the 
>turn down or to speed up the wheel on the outside of the turn.  This 
>is what I am calling and "electronic differential".  This should not 
>be confused with a mechanical differential that has electronic 
>components.  There is no mechanical device with this at all.

Oh.  Now I get it.  Yes, the wheels do need to turn at different 
rates when you go around a corner, and you're wondering how to 
include this in your design.

One possibility is to ignore it: the different torques the two motors 
feel will cause them to spin at sightly different rates anyway.  The 
tires may still skid a bit, and your motors may heat up a little 
extra from the higher torque, but that might not be significant. 
Your car will go.

>If each motor has its individual controller maybe there is a 
>computer that can coordinate them.

Now this design is much more interesting.  Suppose we had such a 
computer: what would we ask it to do?  Suppose there existed a gadget 
called an "electronic differential" that governed your two 
controllers.  What would we expect it to do?

Perhaps we could ask it to ensure that the power being sent to the 
two wheels is always the same, regardless of the individual motors' 
torque and rotation rates.  You can probably imagine how such a 
device might be built, even without a computer.  This simple rule 
might work very well ordinarily, but it might also have some 
interesting effects on the car's handling when one wheel encounters a 
patch of oil on the road.

Perhaps we could ask it to also detect when one wheel is skidding and 
then shift power to the other wheel.

As it's watching the wheels turn anyway, we could ask it to tell us 
if it seems that one tire's pressure seems to be getting lower than 
the other's.

You are noticing, I hope, that the computer that coordinates your two 
controllers will affect the handling and performance of your car.

If I understand correctly, you're looking for information from people 
who have been through this and worked out some of these rules.  I 
think the phrase you should Google is "wheel motor", if you haven't 
already.  There's a discussion on Wikipedia...

     http://en.wikipedia.org/wiki/User:A-Hrafn/Wheel_motor

...which addresses some of these rules.  It also addresses a few more 
related to the specific issues (like "unsprung weight") of putting 
the motors inside the wheels.  These may not apply to your design, 
but you may find others that do.

--Gil