# Two Motor Indepedent Drive System



## Element One Dan (May 30, 2009)

Hey all,

Has anyone ever attempted a two motor independent drive system. I would be curious to hear about your algorithm to solve turning radius and overall traction control.


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## yarross (Jan 7, 2009)

Element One Dan said:


> Hey all,
> 
> Has anyone ever attempted a two motor independent drive system. I would be curious to hear about your algorithm to solve turning radius and overall traction control.


What do you mean by "turning radius"? It's not controlled by traction motor(s).
If you mean torque split control by "traction control", if you use DC motors, putting them in series gives you setup that mimics differential - equal torques regardless of speeds.
Using AC propulsion means the same torque command for both motors to achieve differential action.


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## Element One Dan (May 30, 2009)

Since the vehicle is turning around a pivot point, the radius of the inner tire is smaller than the outer tire. Therefore, the tire is moving faster. At the same time, weight distribution from going around a corner favors the outer tire and means that the inner one will slip under the max torque for the outer tire.


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## Anaerin (Feb 4, 2009)

Element One Dan said:


> Since the vehicle is turning around a pivot point, the radius of the inner tire is smaller than the outer tire. Therefore, the tire is moving faster. At the same time, weight distribution from going around a corner favors the outer tire and means that the inner one will slip under the max torque for the outer tire.


Given the same power through two motors, one with less strain against it will turn faster. Therefore, the outside wheel, which is taking the longer path, will be turned faster than the inside wheel, which will apply more torque.


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## nheistand (Jun 2, 2009)

The easy way to do a two independent motor setup is to put a pair of micro switches on the steering so as soon as you turn right or left the motor on the inside of the turn is switched off. You could do this by interrupting the throttle input for that motor. 

A more complex scheme would reduce the throttle input for the inside motor using a mechanical or elelectric arrangement. I like to envision two throttle cables pulled by a bar on a center pivot, then a pair of springs from the steering linkage pulling on one side or the other of the bar to bias the throttles.


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## Atarijedi (Apr 1, 2009)

Adding on to nheistand's idea, if your controller was based on a 0-5V input, you could have a potentiometer attached to the steering wheel which would modify the output of the throttle.

So if your throttle puts out 2.5V and you are turning right, the pot on the steering wheel would lower that voltage even more and slow down that right wheel?

Maybe use logarithmic pot's so that a small initial change wouldn't cause the wheels to slow down.


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## judebert (Apr 16, 2008)

Is all of this really even necessary?

In a gasser, the wheels are connected directly to the motor, so something needs to be done to accommodate the different wheel speeds, or a shaft is going to break. 

In an EV, the two motors would be "road-coupled". There are no shafts to break, just a magnetic field and a rubber/asphalt interface. If the rubber/asphalt grips the road more strongly than the motor torque, the motor will slip -- no biggie. If the motor torque is stronger than the rubber/asphalt interface, the wheel will spin -- only a little problem, since the driver will just back off on the throttle a bit. No permanent damage in either case.

The best solution, I think, would be to implement traction control on each motor. This would simply reduce torque whenever slippage was detected. It would not only fix this differential problem, but also account for minor differences in motors under demanding conditions, such as heavy acceleration. (I'm betting this is what ACPropulsion does with its 4-wheel traction control.)

It's the same problem as a road-coupled hybrid, and that's been around for a while. In short, I believe we're overthinking this.


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## grayballs (Aug 27, 2008)

judebert said:


> Is all of this really even necessary?
> 
> 
> It's the same problem as a road-coupled hybrid, and that's been around for a while. In short, I believe we're overthinking this.


 
I agree.... Too many bells and whistles,,, I believe the motors will slip enough that this is not a problem..... (At least I'm planning on that being the case) I'm in the process of just this application,, 2 motors, (one to each front axel) 2 controllers, etc. redundancy except for main pack,, trying not to 'over tech' it. I think it is too easy to get off track in the planning (dreaming) process.


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## Jokerzwild (Jun 11, 2009)

Is there a safety issue off the start, in drag settings? If one motor is pushing harder then the other then wouldn’t there be a problem. If I am wrong then this is the way to go because it gets rid of a lot of components that can brake.


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## grayballs (Aug 27, 2008)

Jokerzwild said:


> Is there a safety issue off the start, in drag settings? If one motor is pushing harder then the other then wouldn’t there be a problem. If I am wrong then this is the way to go because it gets rid of a lot of components that can brake.


I'm not sure what you mean by "drag settings". I'm reasonably certain there will be some minor torque steer issues. I'm working on a stereo (twin) resistor, for the controllers at this time. Hopefully I'll be able to tune it to eliminate most of the issues there. 
I'm not so concerned about breaking components,,, other than the usual 'let the smoke out' stuff. I'm just building a commuter to haul my old *** to town and back, not planning on a performance vehicle,,,, plus it puts the toys to work


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## Coley (Jul 26, 2007)

Quote:
"if you use DC motors, putting them in series gives you setup that mimics differential - equal torques regardless of speeds."

I think this went over their heads.

They want to make it much more complicated than it is.

ie: one controller, 2 motors wired in series = differential action already there!!

No torque steer....either....


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## Jokerzwild (Jun 11, 2009)

I would like 2 here how it works, everything goes over my head!


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## Coley (Jul 26, 2007)

Read it again......it is self explanitory......each motor will give same power, but at different speed, WHEN in a turn.....WAHLAA....differential action!!

No need to complicate it at all...............


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## Jokerzwild (Jun 11, 2009)

So you do not think the higher the torque is on the motor the difrent responce you will get?


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## Tesseract (Sep 27, 2008)

Torque in a series DC motor is proportional to current. If you connect two identical series dc motors in series then the current through each must be the same and so the torque will be the same. It's as simple as that, theoretically speaking.

Practically speaking, though, whether torque per amp for two separate series motors is the same depends on the qualifier _identical_. I am not sure how much variation there is among production motors, but I imagine there is some. I don't know of anyone that has built a vehicle like this, mainly because the commonly available motors are quite long so there probably isn't enough room on most cars to put two motors back to back and have enough room for the CV half shafts.


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## grayballs (Aug 27, 2008)

We are building a "transmission/direct drive" to replace the original transmission in the front drive. A quick line sketch should give you an idea


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## major (Apr 4, 2008)

Coley said:


> each motor will give same power, but at different speed,


Hi Coley,

You were correct the first time. The two motors when wired in series have equal torque, not power. If they have equal torque and different speeds, then the powers must be different.



> WHEN in a turn.....WAHLAA....differential action!!


Yes. And like the mechanical differential, the greased wheel gets the juice. The torque on each wheel is equal to each other and also equal to the lower of the two loads. So if you lose traction on one wheel it essentially goes to zero torque, you get zero torque on the other wheel and it stalls. Then the free wheel goes at twice the RPM.

The same is true of the electrical differential, 2 series motors in series with each other. If you lose traction on one motor it then sees twice the voltage and zero (or close to it) load. The result is rapid overspeed of that motor.

The series motors in series differential will work well but means have to be employed to protect from this overspeed problem. Unless you have an application where you can absolutely guarantee you will never lose traction on one of the drive wheels.

Regards,

major


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## Jokerzwild (Jun 11, 2009)

Awesome then this is what I am doing I never said it wouldnt work I am just being a pain to create discussion. This will make sure that your idea is sound. 

Do you think this would work the same for rear wheel drive? It sounds like it should!


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## sunworksco (Sep 8, 2008)

Just use 2 ac motors with each having its own electronic controller and you will have limited slip differential control.You can use the same on the rear end as well.


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## Jokerzwild (Jun 11, 2009)

Why does it have to be AC?


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## Batterypoweredtoad (Feb 5, 2008)

Major,
Doesn't the Tango use two separate motors and one controller? If they are independently driven but jointly powered then anyone who owns one could provide a real world test. Someone needs to email the guy with all the Tango Youtube videos and ask him to do a one wheel on grass one wheel on pavement burnout test and videotape it.


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## major (Apr 4, 2008)

Batterypoweredtoad said:


> Major,
> Doesn't the Tango use two separate motors and one controller?


Hi toad,

http://www.commutercars.com/specs.html 

Says two series motors with one Zilla 2k. Don't know if they run the motors in series or parallel or switch between the two modes.



> If they are independently driven but jointly powered then anyone who owns one could provide a real world test.


You mean like George Clooney? Yeah, I'll be sure to ask him I think there are just 2 or 3 of these built.

Regards,

major


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## Batterypoweredtoad (Feb 5, 2008)

Major-it bugs me that you make me do all the hard work! Getting ahold of Clooney is easy, just ask Julia Roberts for the number. 

On a serious note, there is someone posting hooligan behavior in a Tango on Youtube-Busting through snowdrifts, etc... If anyone truly is interested I bet we could get that guy to do the one wheel peel.


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## John (Sep 11, 2007)

Batterypoweredtoad said:


> Major,
> Doesn't the Tango use two separate motors and one controller? If they are independently driven but jointly powered then anyone who owns one could provide a real world test. Someone needs to email the guy with all the Tango Youtube videos and ask him to do a one wheel on grass one wheel on pavement burnout test and videotape it.


The Zilla controller is built to be able to run two motors in series (and parallel) and has over speed protection built into the controller (hairball?) as long as you fit the speed sensors to the motors. It's the reason I wanted a Zilla even though I'm only using one motor. The over speed protection makes a car a bit more idiot proof eg. if you step on the accelerator with it out of gear for instance.


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## Tesseract (Sep 27, 2008)

John said:


> ...over speed protection makes a car a bit more idiot proof eg. if you step on the accelerator with it out of gear for instance.


Yep - we learned that testing our controller on the dyno the other day. Didn't have the load turned up enough and when I cranked the throttle up the motor oversped enough to shoot out a cloud of black dust. So, adding the tach sensor code to the controller suddenly moved up in priority


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