Speed control of electricmotor

Thread Starter

Macnerd

Joined May 22, 2014
67
My question is about controlling the speed of an electric motor in a car such as the Tesla.

The car has 2 motors up front. Each motor drives a separate wheel. Whenever the car makes a turn, the outside wheel will make more revolutions than the inside wheel. How is the speed controlled since both motors will turn at the same speed? If there's only one motor & there's a mechanical differential, then there's no problem. There has to be some kind of controller that "knows" that the car is turning & changes the speed of the motor turning the outside wheel. Actually, I suppose, that both motors have to be controlled individually. So, how is it accomplished?
 

Dodgydave

Joined Jun 22, 2012
8,677
Usually each motor is pwm controlled, and when turning a corner the inside motor is reduced in speed so it turns in that direction.
 

MaxHeadRoom

Joined Jul 18, 2013
19,697
In servo control this is done by something called Electronic gearing, each motor has an encoder, one geared off of the other, and one is gradually reduced in rpm as the radius of the turn increases/decreases.
Max.
 
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Thread Starter

Macnerd

Joined May 22, 2014
67
Usually each motor is pwm controlled, and when turning a corner the inside motor is reduced in speed so it turns in that direction.
But how does the inside motor "know" that the car is making a turn? Likewise, the outside motor needs to "know" that the car is making a turn so it speeds up. I would think that both motors would need to "know" that the car is turning. I would think that the inside motor speed would slow down & the outside motor speed would speed up. The controller would need to control the speed of both motors so that the tires wouldn't spin. How is that accomplished?
 

mcgyvr

Joined Oct 15, 2009
5,394
Possibly just another sensor on the steering wheel or linkage..
I thought its all drive by wire for steering,etc.. so that would be easy..
 

Dodgydave

Joined Jun 22, 2012
8,677
But how does the inside motor "know" that the car is making a turn? Likewise, the outside motor needs to "know" that the car is making a turn so it speeds up. I would think that both motors would need to "know" that the car is turning. I would think that the inside motor speed would slow down & the outside motor speed would speed up. The controller would need to control the speed of both motors so that the tires wouldn't spin. How is that accomplished?

The receiver controls the pwm from the user who controls the remote control, or a sensor on the steering rack.
 

Thread Starter

Macnerd

Joined May 22, 2014
67
In servo control this is done by something called Electronic gearing, each motor has an encoder, one geared off of the other, and one is gradually reduced in rpm as the radius of the turn increases/decreases.
Max.
Does that mean that there's an encoder on the steering wheel column that tells the controller whether the car is going straight or turning?
 

mcgyvr

Joined Oct 15, 2009
5,394
Does that mean that there's an encoder on the steering wheel column that tells the controller whether the car is going straight or turning?
Possibly..
I'm not sure Tesla has disclosed the complete control/sensor scheme to the public..
There are sensors all over cars now and even more so in sophisticated electric cars..
 

MaxHeadRoom

Joined Jul 18, 2013
19,697
Does that mean that there's an encoder on the steering wheel column that tells the controller whether the car is going straight or turning?
There is usually an encoder of some kind on each wheel, Automobiles have these now for different reasons ABS etc, and Locomotives have them on each wheel to detect wheel slip.
In the servo world, one encoder is slaved off of the other for electronic gearing, so two motors could each be slaved off of a wheel column encoder that indicates the turn radius.
Max.
 

shortbus

Joined Sep 30, 2009
7,342
My question is about controlling the speed of an electric motor in a car such as the Tesla.

The car has 2 motors up front. Each motor drives a separate wheel. Whenever the car makes a turn, the outside wheel will make more revolutions than the inside wheel. How is the speed controlled since both motors will turn at the same speed? If there's only one motor & there's a mechanical differential, then there's no problem. There has to be some kind of controller that "knows" that the car is turning & changes the speed of the motor turning the outside wheel. Actually, I suppose, that both motors have to be controlled individually. So, how is it accomplished?
Why do you think the Tesla S has two motors on the front? It does have two motors, one for the rear and one for the front. Both with a differential, that allows them to make the turns, just like an ICE car.
 

bwilliams60

Joined Nov 18, 2012
1,246
Many years ago, ABS systems were developed and wheel speed sensors, usually of the permanent magnet or pulse generator type, were utilized along with a reluctor or tone wheel with a certain number of teeth on it. Together they could send voltage signals back to the ABS module indicating the degree of wheel rotation. As hybrid and electric vehicles became more prominent, motor controllers were developed to monitor these signals on a data bus and adjust wheel speed accordingly. These motors on decel also serve to generate electricity back onto the system and provide braking capabilities as well. If wheel slip occurs, power is transferred to another wheel to get traction. Hope this helps.
 

shortbus

Joined Sep 30, 2009
7,342
@bwilliams60 , don't know of any electric auto that doesn't use motor per wheel drive. Can you link to one? All of them I know of use a differential and gears to drive the wheels. A electric motor has too high of an RPM for direct drive, doesn't it?

The Tesla the op was asking about has two motors, but not like he envisions them. It uses one for front drive and one for rear drive.
 

bwilliams60

Joined Nov 18, 2012
1,246
Shortbus, you are correct in stating that the majority of EV's are using differentials but there is a new breed of EV's spawning in the industry. Have a look:
http://cleantechnica.com/2013/08/01/in-wheel-ev-motor-from-evans-electric-unveiled-in-australia/
I have spent some time with electric vehicles and Hybrid vehicles and am looking forward to seeing how this technology impacts my industry. Looking forward to seeing what they can do with the performance curves on these units.
 

shortbus

Joined Sep 30, 2009
7,342
tom s & bwilliams, thanks for those. Was aware of the concept but didn't know it was in production. Th ts asked about Tesla and they use front and rear motor/transaxles.
 

Picbuster

Joined Dec 2, 2013
990
But how does the inside motor "know" that the car is making a turn? Likewise, the outside motor needs to "know" that the car is making a turn so it speeds up. I would think that both motors would need to "know" that the car is turning. I would think that the inside motor speed would slow down & the outside motor speed would speed up. The controller would need to control the speed of both motors so that the tires wouldn't spin. How is that accomplished?
It looks difficult but: put and measure (current) torque and sync of two both ( left and right) wheels. When a turn arrives the motors will try to force strait on resulting in L & R difference in sync and torque use this information to chance the rpm's . you need a MPU to do the job avoiding unwanted corrections.
 

tcmtech

Joined Nov 4, 2013
2,868
If the electric motors are being operated in a constant torque mode rather than constant RPM mode then slight differences in speed between the motors are of no concern to the system.

That's how it has operated on large industrial electric drive systems that have independent motors for each wheel or axle for around a century now.
 

GopherT

Joined Nov 23, 2012
8,012
The new Accura Super-Handling All wheel drive goes beyond differentials, it uses Torque Vectoring.


Acura, for instance, has offered its Super Handling All-Wheel-Drive (SH-AWD) system for several years. It monitors vehicle speed, wheel speed, gear position, steering angle, yaw rate, lateral G forces and other inputs, while automatically adding torque to the outside rear wheel in corners to make the car turn quicker. A set of electromagnetic clutches in the rear differential passes the torque from side to side. The system, which normally distributes torque 90 percent up front and 10 percent in the rear, quickly changes to a 50/50 split during acceleration or hard cornering. The system can then send some or all of that 50 percent going to the rear axle directly to the outside tire to make the vehicle bend into a corner more sharply. Mitsubishi, a torque vectoring pioneer, has used a similar system called Active Yaw Control in the rear axle of its high-performance Evolution sport sedan since the late '90s.
Some have dual power (hybrid systems) that supplement combustion engine for additional 75 HP - who said the electric motor was for efficiency. It's for more power!

Check out the 2016 Acura NSX turbo-hybrid electric for all the fun you can have with your racing leather on.
http://www.caranddriver.com/features/2016-acura-nsx-dissected-powertrain-chassis-and-more-feature
 

#12

Joined Nov 30, 2010
18,170
The Ford Explorer I recently bought has steering wheel angle detection and a lateral G force detector.
It also has failures in every module except the ECU and the transmission controller, and I'm not sure about them because I only drove it home from the purchase.:eek:

(Chant)
Must install more microprocessors!
Must install more microprocessors!
 
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