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Old 13th March 2012, 13:45   #1
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Default How does Electric power steering work?

Hi,
Im trying to understand how electric power steering works.

I own a zen, i see a motor below the steering column and a box.

Can someone tell me how it works?
i would like to know what type of motor is used, sensors used,..etc.

I observed 4 wires connecting the motor, 2 are thick red/black wires and the other 2 are thin wires.
Does ECU play a role other than feeding speed information?

Thanks in advance.
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Old 13th March 2012, 17:42   #2
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Default Re: How does Electric power steering work?

Quote:
Electric power steering (EPS or EPAS) uses an electric motor to assist the driver of a vehicle. Sensors detect the position and torque of the steering column, and a computer module applies assistive torque via the motor, which connects to either the steering gear or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. Engineers can therefore tailor steering-gear response to variable-rate and variable-damping suspension systems, optimizing ride, handling, and steering for each vehicle.[15] On Fiat group cars the amount of assistance can be regulated using a button named "CITY" that switches between two different assist curves, while most other EPS systems have variable assist. These give more assistance as the vehicle slows down, and less at faster speeds. In the event of component failure that fails to provide assistance, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems.
Electric systems have an advantage in fuel efficiency because there is no belt-driven hydraulic pump constantly running, whether assistance is required or not, and this is a major reason for their introduction. Another major advantage is the elimination of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing and maintenance. By incorporating electronic stability control electric power steering systems can instantly vary torque assist levels to aid the driver in corrective maneuvers.[16]
The first electric power steering system appeared on the Suzuki Cervo in 1988. Today a number of manufacturers use electric power steering.
Power steering - Wikipedia, the free encyclopedia
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Old 13th March 2012, 22:02   #3
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Default Re: How does Electric power steering work?

Wanted to say that I used to like hydraulic PS but the new recent EPS is much smoother and less headaches. Only Gripe there is no serviceable part for EPS only replacement. In my wagon R it was good for 1.5 lac kms
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Old 15th March 2012, 10:40   #4
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Default Re: How does Electric power steering work?

I don't have tons of experience with this, so what i say might not be entirely complete or correct :

Quote:
Originally Posted by T1000 View Post
I own a zen, i see a motor below the steering column and a box.
There are two places the electric motor can be mounted :

Column mounted : The motor is placed on the steering column, and exerts additional force onto the column itself, like on your zen.

Rack mounted : The motor directly drives the steering rack (part of the "rack and pinion" steering system).

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Quote:
Originally Posted by T1000 View Post
Can someone tell me how it works?
1) There is a sensor somewhere along the steering column that senses torque (ie the rotational force you exert on the steering wheel).
2) Once the torque crosses a particular limit, the electric motor begins to aid the turning in the direction that you're exerting the force.
3) As soon as the wheels turn the desired amount, your steering input lessens, reducing the torque sensed by the sensor, which in turn stops the motor.
4) The amount of "help" given by the motor is controlled based on the speed of the vehicle. (More help at lower speeds).

Quote:
Originally Posted by T1000 View Post
i would like to know what type of motor is used, sensors used,..etc.
The torque sensor *might* be a potentiometer that is driven by some sort of clock-spring. Not sure. Would love to see pictures.

Another (newer?) option is a magnetic sensor that senses position differences between inner and outer shafts on the steering column.

Motor is probably a stepper motor. Once again, my guess, since stepper motors can rotate at various speeds (the EPS motor definitely has a gearbox to aid it). Position feedback probably comes in from another sensor somewhere.

Quote:
Originally Posted by T1000 View Post
I observed 4 wires connecting the motor, 2 are thick red/black wires and the other 2 are thin wires.
If you need to control a motor accurately (ie control speed and position), you need more than 2 wires.

The thicker red and black wires supply the power to the motor, whereas the other two wires could be either :
1) Supplying data to motor control circuitry within the motor housing
2) Relaying back data of the motor's exact position
3) Stepper motors need more than 2 wires to run, due to the way they are constructed

Also, remember that automotive systems need a certain amount of redundancy and checks built in.

Quote:
Originally Posted by T1000 View Post
Does ECU play a role other than feeding speed information?
I'm not sure. It might differ from car to car?

If the car has a dedicated EPS control module which includes the logic, then the ECU probably only supplies speed data. If not, then it might actually pass the instructions on to the EPS module, which would still be required to drive the motors.

Quote:
Originally Posted by Catalyst_delhi View Post
...less headaches.
Yep!

- No horsepower sapped from the engine
- No pump running when the car is going straight (like in most hydraulic systems)
- No hydraulic fuel required (top ups, leakages, etc)

Add to that other advantages :
- Speed sensitive "help"
- Customizable amount of "help" (though i've not seen any car implement this yet. Maybe its built in to cars like the M5?)
- The electric motor can turn the wheels on its own (eg. auto-parking feature on the Audis)
- The possibility for a direct and more solid connection between the steering and the rack (rack mounted EPS), for much more road feedback.

cya
R

Last edited by Rehaan : 15th March 2012 at 10:59.
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Old 15th March 2012, 12:09   #5
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Default Re: How does Electric power steering work?

^^ just to add to what R has mentioned , there is only one disadvantage in the column mounted EPS - the rack and pinion could wear out fast.

In the conventional hydraulic set up it is the tie rod ends which go kaput first ; the rack and pinion remains intact.
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Old 15th March 2012, 12:43   #6
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Post Re: How does Electric power steering work?

Quote:
Originally Posted by T1000 View Post
Hi, I'm trying to understand how electric power steering works. I own a zen, i see a motor below the steering column and a box.
Can someone tell me how it works ? I would like to know what type of motor is used, sensors used,..etc. Does ECU play a role other than feeding speed information ?
A very good link on EPS systems. This web-site has numerous DIY articles that are extremely informative.
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Old 15th March 2012, 13:04   #7
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Default Re: How does Electric power steering work?

Quote:
Originally Posted by Rehaan View Post
Add to that other advantages :
- Customizable amount of "help" (though i've not seen any car implement this yet. Maybe its built in to cars like the M5?)

cya
R
Customisable amout of help is available with Fiat in Europe, a feature they call the "city" button (which some people derisively refer to as the "girly" button!)
Pressing this button increases the amount of assistance given by the electric power steering motor.
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Old 20th March 2012, 17:46   #8
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HI KiloAlpha,im_srini,Rehan ,vigsom,Catalyst delhi,cranky
Thanks for the information. Can anyone get more specific details about the type of motor and those circuits functionality?

Quote:
Originally Posted by Rehaan View Post
I don't have tons of experience with this, so what i say might not be entirely complete or correct :

The torque sensor *might* be a potentiometer that is driven by some sort of clock-spring. Not sure. Would love to see pictures.
I guess the torque sensor must be a strain gauge.

Quote:
Originally Posted by Rehaan View Post
The thicker red and black wires supply the power to the motor, whereas the other two wires could be either :
1) Supplying data to motor control circuitry within the motor housing
2) Relaying back data of the motor's exact position
3) Stepper motors need more than 2 wires to run, due to the way they are constructed
You are right, i got some more information, the two wires are data wires.

But im not able to identify the functionality of the wires.
A servo motor has 2 power lines and one wire to control its position
Stepper motor takes 5 wires , but the motor has only 4 wires.

Im too curious to dismantle the system and explore it! :-).
A new power steering system costs around 5K + labour .

Quote:
Originally Posted by Rehaan View Post
If the car has a dedicated EPS control module which includes the logic, then the ECU probably only supplies speed data. If not, then it might actually pass the instructions on to the EPS module, which would still be required to drive the motors.
Sometime back the speedometer cable snapped in my car, but the power steering was working fine. i wonder how?

Quote:
Originally Posted by im_srini View Post
A very good link on EPS systems. This web-site has numerous DIY articles that are extremely informative.
Hi Srini, This link is great, Thanks!,

Last edited by Rehaan : 20th March 2012 at 20:05. Reason: Merging consecutive posts.
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Old 20th March 2012, 20:12   #9
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Default Re: How does Electric power steering work?

Quote:
Originally Posted by T1000 View Post
I guess the torque sensor must be a strain gauge.
More info from the www :

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Quote:
Originally Posted by T1000 View Post
You are right, i got some more information, the two wires are data wires.

But im not able to identify the functionality of the wires.
A servo motor has 2 power lines and one wire to control its position
Stepper motor takes 5 wires , but the motor has only 4 wires.
There are so many different types i'm not sure! Its possible that the 4th wire is connected to some other sensor within the motor (remember that auto systems = lots of self-checks / redundancy)

Quote:
Originally Posted by T1000 View Post
Sometime back the speedometer cable snapped in my car, but the power steering was working fine. i wonder how?
Was it a cable or a wire?
Either way, that cable/wire is only relaying speed info FROM the ECU TO the instrument cluster. The link between the speed sensor (on the wheel / axle) and the ECU must have been fine.

^ I wonder if the speed sensor dies, will the electronic power steering default to a fixed (non-variable) assistance setting? Or would it just throw an error?

cya
R

Last edited by Rehaan : 20th March 2012 at 20:13.
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Old 20th March 2012, 22:56   #10
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Default Re: How does Electric power steering work?

Hi,
It will be a cheap (=low grade, most probably sensorless) servo motor. Possibly a brushless PMDC motor. Almost certainly not a stepper.

The torque measurement problem is interesting. Somehow, I think it will a derived value taken from say a reaction arm, rather than from the steering shaft. Don't know, so would like to know for certain!

Regards
Sutripta
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Old 21st March 2012, 02:05   #11
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Default Re: How does Electric power steering work?

Quote:
Originally Posted by Rehaan View Post

^ I wonder if the speed sensor dies, will the electronic power steering default to a fixed (non-variable) assistance setting? Or would it just throw an error?
If the speed sensor dies then there will be no steering assist from the system, however vehicle homologation rules ask for the steering control to be designed in such a way that even when the steering assistance system fails, the vehicle can still be controlled. The maximum steering effort required to control the vehicle is also clearly quoted (I dont remember the value)
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Old 21st March 2012, 10:40   #12
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Default Re: How does Electric power steering work?

Quote:
Originally Posted by Rehaan View Post
More info from the www :
Was it a cable or a wire?
Either way, that cable/wire is only relaying speed info FROM the ECU TO the instrument cluster. The link between the speed sensor (on the wheel / axle) and the ECU must have been fine.

^ I wonder if the speed sensor dies, will the electronic power steering default to a fixed (non-variable) assistance setting? Or would it just throw an error?
cya
R
It was the cable that comes from the gearbox, it snapped while driving and the ignition warnning lamp came on. i didnt see any change in steering effort.

i had to change the cable and reset the ECU to disable the warning lamp.
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Old 21st March 2012, 10:49   #13
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Default Re: How does Electric power steering work?

Almost all manufacturers have moved to EPS systems despite its inherent disadvantages (no or low steering feel & feedback) - all in the interests of fuel efficiency.

Any idea what is the average percentage fuel saved by shifting to EPS from hydraulic steering unit in a normal sized car?
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Old 21st March 2012, 12:20   #14
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Post Re: How does Electric power steering work?

Quote:
Originally Posted by T1000 View Post
Hi,
I'm trying to understand how electric power steering works. I own a zen, i see a motor below the steering column and a box. Can someone tell me how it works ? I observed 4 wires connecting the motor, 2 are thick red/black wires and the other 2 are thin wires.
Are the 2 thin wires black ? I tried looking up the EPS section in the Alto's service manual, it says that the motor has a 4-pin connector with 1 red & 1 black wire for the motor & 2 black wires for a clutch (I think something similar to the A/C clutch). Here are some pics...

General description of parts in the Alto's EPS...

How does Electric power steering work?-alto_eps_0.jpg

System & connection diagram...

How does Electric power steering work?-alto_eps_1.jpg

Quote:
Originally Posted by T1000 View Post
Can anyone get more specific details about the type of motor and those circuits functionality ? But im not able to identify the functionality of the wires. A servo motor has 2 power lines and one wire to control its position. Stepper motor takes 5 wires, but this motor has only 4 wires.
Does the connector on the motor look like the one marked X01 in the 2nd pic ?

T1000, of course, the assumption is that the EPS in the Zen would be very close or even identical to the one in the Zen. Looking at the Swift's EPS, it appears to be a more sophisticated unit. In the Swift the EPS motor is on the steering rack rather than on the column & as far as I can see, it does not have a clutch. The EPS control module in the Swift also has many more diagnostic codes. Will put up the pics if its of interest to anybody...

Last edited by im_srini : 21st March 2012 at 12:42.
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Old 21st March 2012, 14:15   #15
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Default Re: How does Electric power steering work?

Quote:
Originally Posted by smartcat View Post
Almost all manufacturers have moved to EPS systems despite its inherent disadvantages (no or low steering feel & feedback)...
I think this is a misconception.

EPSs are capable of providing just as much (or more) feel and feedback than a hydraulic unit - it just depends on how the system has been implemented and tuned.

Quote:
Originally Posted by smartcat View Post
Any idea what is the average percentage fuel saved by shifting to EPS from hydraulic steering unit in a normal sized car?
I remember for the 2011 Fiesta Ford said : "EPS provides a fuel efficiency gain of up to 5% vis--vis a hydraulic unit.". (As always, don't miss the word "upto").

cya
R
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