# Synchronizing wheel speeds...



## RobSmith (Aug 13, 2010)

Jongscx said:


> Ok, so here's my situation. I'm planning on converting a RWD go kart/atv into a hybrid EV. It's large enough and street legal, so I'll be using it to commute to and from class.
> 
> The setup is that it has a 150cc 4stroke motor in the back running a CVT to an LSD (Limited slip differential).
> 
> ...


Hi,
I do not know if this idea is suitable for your application but this is what I have previously done..
I made a small computer buggy that I needed to get to drive in a straight line. What I did to get the motors to rotate at exactly the same rate was to position the two motors so the shafts pointed towrds each other. On the end of each motor shaft I made up a 'T' shaped piece with a magnet in each end. When the motor speeds were quite close the magnets would attract the opposite pole on the opposite motor and the motor speeds would be forced to match. If one motor had its power removed the magnets would not be able to hold that force and the buggy would turn.
The motor to wheel gearing was very low geared so when a motor was stopped it did not need any brake to turn around one wheel. 

Rob


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## Jongscx (Aug 18, 2010)

Unfortunately, because I am using the hub motors in the wheels themselves, I don't think I could do this. Maybe if I used onboard motors and CV joints, maybe...

Also, this doesn't fix my problem of synchronizing with the gas engine either for when I want to run it in a hybrid mode...


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## RobSmith (Aug 13, 2010)

Jongscx said:


> Unfortunately, because I am using the hub motors in the wheels themselves, I don't think I could do this. Maybe if I used onboard motors and CV joints, maybe...
> 
> Also, this doesn't fix my problem of synchronizing with the gas engine either for when I want to run it in a hybrid mode...


Does it need to syncronise or can you do it with a proportion of power delivery. The electric motors could be quite easily monitored for their power input and you could maybe strain guage a suspension link to give you a thrust loading from the wheels driven from the IC engine.
I suspect the biggest issue in that would be smoothing out the signal spikes from bumps and holes in the road.

Edit: you could maybe monitor the engine power roughly by using an air mass meter (if it is a petrol not diesel) as there would be a rough correlation between power and the air sucked in. The Bosch air mass meter on my car has a 0-5v output which might be a reasonably easy way to feed into some circuitry.

Just a few ideas.
Rob


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## Jongscx (Aug 18, 2010)

My choice of synchronization was two-fold.

The first is because I need a synchronization method anyway. This is not an EV only vehicle. Voltage input would only supply a rough wheel speed control. Because of variations in the motor, speed controls, and even the battery condition (during the ride), the wheel speed will not be consistent. Even if it's off by 1-2 rpms, that will increase tire wear between the EV motor driven wheels and the ICE wheels.

The second is because in the rear differential I already have a method of power splitting. By synchronizing the front wheel speed to those, it assures the proper ratio during turns, allowing for proper tire speeds around corners, etc.


I know very small PM electric motors (like the 12V Power wheels motors) can be hooked up in series and act like they're an open differential, where the motor with less resistance will take more of the voltage and spin faster... but I don't think that's viable when we're talking about higher voltages and speeds.


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## Jongscx (Aug 18, 2010)

RobSmith said:


> Edit: you could maybe monitor the engine power roughly by using an air mass meter (if it is a petrol not diesel) as there would be a rough correlation between power and the air sucked in. The Bosch air mass meter on my car has a 0-5v output which might be a reasonably easy way to feed into some circuitry.
> 
> Just a few ideas.
> Rob


Yeah, again that would be monitoring the inputs into the engine, not the outputs. While they are related, there are lots of variables at play that doesn't make it that simple. (I wish it did though >_<)


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## RobSmith (Aug 13, 2010)

Jongscx said:


> My choice of synchronization was two-fold.
> 
> The first is because I need a synchronization method anyway. This is not an EV only vehicle. Voltage input would only supply a rough wheel speed control. Because of variations in the motor, speed controls, and even the battery condition (during the ride), the wheel speed will not be consistent. Even if it's off by 1-2 rpms, that will increase tire wear between the EV motor driven wheels and the ICE wheels.
> 
> ...


Ah, excelent. You answered my question I posted elesewhere on the forum.
Edit: 11pm here so off to bed
Rob


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## Woodsmith (Jun 5, 2008)

Jongscx said:


> I can assume that the wheels on one side will be spinning at the same rate during turns but even when moving forward, I'd need them all to spin at the same rate.


Actually the wheels will never really spin at the same rate.

Going around a corner, unless you have four wheel steering with a system better then Akermann to control the steering, all four wheels will turn at a different speed.

In a straight line even a bump or different tyre wear will cause the wheels to turn at different speeds.

If you really wanted the wheels to turn at the same speed then you need to have them physically joined at the axle and front to back but that would make it, at best, a pain to drive and, at worse, undrivable.

Depending on what your ultimate goal is, if you want four wheel drive you may be better off with a transfer box set up to divide all the available power to both axles, with a suitable torque split in the centre differential if needed, and then drive it with a motor and an ICE connected to each end of another differential that drives the transfer box.

The relative speeds of the motor and the ICE can be resolved in the diff between them and if you have differing torques from then you could have a torque biased diff.
the out put of that diff would be from the crown wheel, or diff carrier, part and that would then drive the rest of the transmission.

It gets complicated the more you want from it but it depends on what you want from it.


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## Evilsizer (Jan 25, 2010)

why not just got all electric on it? the amount of weight from adding batteries+ ICE, it seems is going to make it rather slow. not sure how much power that ICE engine has. if you wanted you could go two EV motors, one for the front and one for the back. could be something like this
http://www.marselectricllc.com/me0909.html
light weight, 6.5HP contentious and 20HP for 30 secs. or go with a single larger motor for the AWD setup. maybe this motor?
http://www.marselectricllc.com/me1002.html


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## Duncan (Dec 8, 2008)

Hi Jongscx

You don't need to worry about synchonisation - the road will do it for you
you could simply power one wheel and it would be drivable!

A normal car diff is an open system - it is effectively balanced by the road

If one wheel "pushes" a bit harder than the other your steering balances it - just like road camber!

You don't think your tires are exactly the same size do you?


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## Jongscx (Aug 18, 2010)

Duncan said:


> Hi Jongscx
> *1*
> You don't need to worry about synchonisation - the road will do it for you
> you could simply power one wheel and it would be drivable!
> ...


1)Yes, I've ridden a go kart where only one wheel was powered and had the brake on it... It had a bad tendency to swerve into a ditch when accelerating... and braking made me go into the other lane. >.< Scale that up to normal roads and you see the problem there...


2)A normal car has a diff... this one won't. Is that the problem we're running into here? I guess that's my fault that I haven't explained it well enough.
I'm using two small electric motors to run the left and the right wheels. Yes, I can just hook them both up to a linked throttle and go... but that's the same as putting a solid axle on a car and trying to go around a corner.

3)Yes, with one wheel pushing harder than the other, I CAN just apply countersteer... but that's just reacting to the problem. I'm in the design phase right now, I'm thinking it's better to fix the problem now instead of in the build phase... or worse, the rebuild.

I don't see what road camber has to do with this? 

4)And yes. Unless I severely over or under inflated my tires, they should be the same size within a certain margin of error.

@EvilSizer -- Yes, I COULD go all electric, but it would be a big hassle for me right now. I live in a dorm and probably an apartment in the near future... I also live in Georgia, which will probably be one of the later adopters of road-side charging spots... so a full plug-in electric is more or less out of the question. I'm going more for electric assist with a removable battery pack I can roll off the car or something. 
In any case, that's not really the point of my post, but thanks for asking.



Woodsmith said:


> Depending on what your ultimate goal is, if you want four wheel drive you may be better off with a transfer box set up to divide all the available power to both axles, with a suitable torque split in the centre differential if needed, and then drive it with a motor and an ICE connected to each end of another differential that drives the transfer box.
> 
> The relative speeds of the motor and the ICE can be resolved in the diff between them and if you have differing torques from then you could have a torque biased diff.
> the out put of that diff would be from the crown wheel, or diff carrier, part and that would then drive the rest of the transmission.
> ...


That's exactly the kind of complication that I'm avoiding. If I wanted to do that, I'd just connect the ICE to one leg of the diff, one motor to the other, and another to the last one which connects to the driveshaft... and I'd have a ghetto Prius PSD... but alsa.

It's all possible to do that mechanically, you just explained it to me. Now how do I do it electrically? I can measure wheel speed with a slotted disk, or some magnets and a hall sensor. From there, I can extrapolate torque, etc. It's just that middle part that's left. I guess, an electronic differential of sorts...


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## Jongscx (Aug 18, 2010)

Oh my god... it took all that to come up with the search term: "Eletronic differential"...

and here's one right now. 
http://www.4qd.co.uk/prod/index.html

I'm going to smack my forehead and read over this information now. thanks for the discussion guys.


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## Woodsmith (Jun 5, 2008)

I'm glad you found an answer.

Sometimes it is a little difficult when you have a clear idea of what you want but we don't understand the context of the question. We could easily be suggesting things you have already considered and rejected.

Unfortunately I am a 'mechanical' persion. I see solutions in moving physical bits of 'stuff' around. Haven't got my head around 'little black boxes' that do that sort of thing at a quantum level by moving electrons around, can't visualise it.


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## Jongscx (Aug 18, 2010)

Woodsmith said:


> I'm glad you found an answer.
> 
> Sometimes it is a little difficult when you have a clear idea of what you want but we don't understand the context of the question. We could easily be suggesting things you have already considered and rejected.
> 
> Unfortunately I am a 'mechanical' persion. I see solutions in moving physical bits of 'stuff' around. Haven't got my head around 'little black boxes' that do that sort of thing at a quantum level by moving electrons around, can't visualise it.


Seems like I'm onto something here... nobody's perfected this yet >_< There are a bunch of technical papers on people who've researched the topic, but they all rely on stuff like steering angle and lateral g-forces on the car... thus, things like road conditions and road camber (now I gotcha!) throw the systems off... 

Yeah, I'm a mechanical person too... electrical is ok, so long as it's Wire A to Battery Tab B... But it's the coding and the control systems and the feedback loops and the microcontrollers.... that I'm clueless over. Why am I majoring in Mechatronics Engineering again?


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## Duncan (Dec 8, 2008)

Hi Jongscx

You don't understand - driving one wheel only is an extreme case
all vehicles operate in an assymetric manner

*What I am telling you is that you are finding a problem that does not exist*
*Just hook up a linked throttle and go*

No its not the same as a solid axle!

I have just realized what is happenning - 
The controler/throttle *does not set the motor speed* - such a controler would make the vehicle undrivable

Early diesel trucks had such a controler - it was OK with a low power-weight but when in a car situation it was deadly
When the driver increased throttle the (speed) governor would see too low a speed and go to full fuel until the speeds matched -nasty!

The controler/throttle effectively controls the torque curve - push harder you move to a different torque curve - this then controls the vehicles speed
More "throttle" moves to a different torque curve the vehicle accelerates until the required torque equals the available torque

Effectively your pedal controls the available torque 

As you corner the outside wheel will accelerate and the inside wheel slow - both motors will move along their torque curves with the control input constant *no worries!*


- the tolerance on tire size is not small - try measuring a few! - in fact if you are Karting you should be doing this for tire selection for different tracks

The road camber was an example of the lack of symetry in the real world -

Your linked throttles will not be identical - and will have slightly different responces - so does your diff! - welcome to the real world

Your machine will give enough real problems for you to solve - you don't need to worry about a non problem


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## Jongscx (Aug 18, 2010)

@duncan

Thank you. That makes sense, as I apparently had no idea how ESCs works. I'd assumed by the name that they... controlled speed. Ok, thanks for clearing that up.

So what you're saying is that by setting the "speed controls" to the same level, they will deliver a corresponding amount of power to the motors. Very high initially at startup, ramping down as the vehicle gets up to speed, and maintaining constant once a certain velocity is reached because at that speed, the power required to move the car is constant? 

Turns would cause the following effect. As the car is turning, the outside tire has to move faster than the inside tire. This is a similar situation to if the outside tire had less resistance to the inside, and so for the same amount of power, results in different amounts of travel? 

Am I getting that right?


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## Duncan (Dec 8, 2008)

Hi Jongscx

I should not have said that all controllers work that way - but any that do work as "pure speed controllers" will give an un-drivable car.

We have all driven IC cars and they have the basic characteristics you mention (without the lovely high torque at low speeds) 

If you are using an industrial controller then you may have to alter its settings to get a usable torque curve - but even a full PID speed controller - (Proportional, Integral, Differential) can be set to react the way we want it 

I have driven an IC car with two engines - one driving the rear wheels - one the front wheels
The two engines had different gearing, engine capacity and state of tune

It worked fine!
The throttle cable for the rear engine could be tugged on to give a bit extra to unsettle the rear at the start of a corner!

Stick your motors on couple the throttles and go for it!


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## Anaerin (Feb 4, 2009)

As I understand it:

If you connect your motors in series, you will get an effective "Open differential". The faster moving wheel will have a lower resistance than the slower-moving one, and so will get more power and turn at a higher speed, just like a regular mechanical differential will do. In other words, when you corner, the vehicle will try and corner tighter, and if you have a wheel lifted (virtually) all power will go to the lifted wheel.

If you connect your motors in parallel, you will get an effective "limited slip differential". The slower moving wheel will push harder to try and regain equilibrium, which will cause the vehicle to constantly try and straighten. If you have a wheel lifted, the power will be split between the wheels so you will still get forward momentum.

Go-Karts typically have a solid axle, and rely on the wheels slipping to be the differential. This is impractical (and dangerous) in regular cars, as it lessens control (Effectively putting at least one wheel into a skid) when cornering, and can cause transmission wind-up, which applies torque strain to the axle and can cause it to snap.


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