AutoSpeed - Modifying Electric Power Steering

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Issue: 571 Section: DIY Tech Features 4 May, 2010

Modifying Electric PowerSteering

Dirt cheap modification gives control over

electric power steering weight

by Julian Edgar

Click on pics to view larger images

At a glance...

Electric power steering assistance

modification

Very cheap and easy technique

Full adjustment over assistance

Email a friend Print article

This article was first published in 2005.

More and more cars are now being fitted with fully

electric power steering. In these systems, hoses,

pumps and reservoirs are dispensed with – instead,

an electric motor attached to the steering column

does all the work. The advantages to the

manufacturer include lower servicing and assembly

costs, and to the consumer, less likelihood of

failure through power steering fluid leaks. Fuel

consumption is also improved.

But there’s another advantage to the modifier, one

which so far has been completely overlooked.

Because it’s an electronically-controlled system, it’s

easy to alter the characteristics of electric power steering to suit individual preferences. Specifically,

you can alter the steering weight to radically improve steering feel and high speed stability.

And you want the good news? You can give your electric power steer car user–adjustable control over

the steering assistance for under ten bucks.

The modification covered in this story was carried out on a Toyota. However, we’d expect that

very similar changes would be possible on any car with electric power steering that uses a

torque-based measuring system to determine the amount of electric assist.

Electric Power Steering Systems

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oSpeed - Modifying Electric Power Steering http://autospeed.com/cms/ti tle_Modifying-Electric-Power-Steering/A_...

6 2011/02/04 09:58 P



It’s now over four years since we covered the

basics of electric power steering (see Electric Power

Steering) but what do most electric power steering

systems look like today? Well, some are simpler

than covered in that story.

Typically, an electric power steering system

consists of:

- a powerful electric motor geared to the steering

shaft

- torque sensor(s) that detect how much effort isbeing put into the steering

- an electric power steering Electronic Control Unit

(ECU)

- a road speed input to the ECU

The ECU looks at the steering torque and steering direction being applied by the driver, and at the road

speed, and directs the electric motor to provide the required amount of assistance in the correct

direction.

Torque refers to the strength of twist being applied to a shaft. The higher the twisting force, the

higher the torque.

Since the key ingredient in modification is the torque sensor, let’s take a closer look at it.

As with some conventional hydraulic power-assisted steering systems, a torsion bar is used measure

the relationship between the torque being applied to the steering wheel by the driver and the

resistance being posed by the tyres.

It’s important to realize that this measured torque

is a two-way process – if the front wheels are on

wet grass they’ll turn very easily, so despite the

driver turning the steering wheel hard, not much

torque will need to be applied to alter the steering

angle of the tyres. However, if the front tyres are

on coarse bitumen, they will resist turning and so

the amount of torque needing to be applied by the

driver will be much higher to get the tyres to turn.

In other words, the torque sensor indicates boththe driver’s input of torque and the torque reaction

of the tyres.

The use of the torsion bar therefore takes into

account the real steering effort needing to be

applied – irrespective of road surfaces, tyre inflation pressures, and road speed.

So how does this torsion bar system work? The

torsion bar forms part of the steering column – it

twists when subjected both to high input torque and

high tyre reaction torque. Two sensors are used.

Each measures the amount of twist and outputs a

voltage that is proportional to this. When no twist is

occurring, the voltage output of each sensor is in

the middle of its range. So, with sensors with anoutput range of 0-5V, each sensor reads close to

2.5V when there’s no steering torque being input.

However, when subjected to torsion, the sensors’

output voltages change. When there’s increasing

left-turn steering torque being applied, one sensor

increases in its output voltage while the other sensor decreases in its output. The opposite occurs on

right-hand corners.


6 2011/02/04 09:58 P



Therefore, the larger the difference between

the output voltages of the two sensors, the

more steering effort that is occurring.

This graph shows the output of the two torque

sensors and how this relates to the amount of

steering effort. It can be seen that at the point

where there is no steering torque being applied,

the sensor voltage output lines cross.

One of the hard points to grasp about these torque-measuring systems is that the output

difference between the two sensors is not proportional to the amount of steering lock applied. This

is because the tyres mostly resist being turned when they are being turned – once a certain

amount of tyre angle has been adopted, the effort required to maintain that steering lock is much

less than the effort required to first gain it. Instead, the greatest difference between the two

sensor outputs occurs when steering input is being rapidly applied on a grippy surface at low

speed... which is fine, because that’s when you most need the assistance!

Because of the way the output signals of the torque sensors are configured, the ECU knows both the

direction that the torque is being applied in and how great it is. The ECU then instructs the electric

motor to assist appropriately, and as a result, the required steering torque effort by the driver

decreases, resulting in a lower difference in the output voltages of the torque sensors. The assistance

provided by the motor is therefore reduced.

Modifying the System

So to summarise the above paras for those just

skipping along: the greater the difference in the

output voltages of the two torque sensors, the

greater the amount of steering torque that the ECU

knows is being applied to the steering.

In nearly all cases, the desired outcome of

modified electric power steering will be more

steering feel – or in other words, you want lesspower assist. At higher speeds this results in better

turn-in cornering feel, better straightline stability

and a far more secure on-road feel. Sure, there wil l

be slightly heavier steering when parking, but

unless you’re very frail, that’s unlikely to be a

problem.

So to achieve the outcome of less power assistance, the ECU needs to be fooled into thinking that there

is less steering input effort than is really occurring. To achieve this, all that we need to do is reduce

the difference between the voltages of the torque sensors.

This can be achieved very simply by the use of just two multi-turn potentiometers (pots). Even

including the cost of a box to mount the pots in, the total bill will be under 10 bucks.

So, how do you know if your late model car has electric power steer? The easiest way is to lookfor the underbonnet presence of a hydraulic power steering fluid reservoir. If the car has power

steering and there’s no reservoir, it must be electric – or the car uses a combined hydraulic

system that powers the steering and brakes.

How to Do It

The first step with any electric power steering

system is to disable the system and go for a drive.

Usually, switching off the system is just a case of

pulling the electric power steering fuse or relay. Of

course the steering will be much heavier when

moving slowly, but the car will still be driveable.

What you are looking for is the change in steering

weight at speed – say 80 km/h.

Is it much heavier, or the same as usual?

If it’s the same as usual, the amount of power

assistance being applied at this speed must

normally be zero. (In that case, you’re not going to be able to improve steering weight by modifying


6 2011/02/04 09:58 P



the system!) However, i f you notice a firmer, meatier steering weight, you can be sure that there’s too

much assistance normally being given at this speed – and so there’s room to make improvements.

The next step is to find some of the functions of the power

steering ECU pins. At a pinch you can get away without a

workshop manual but it’s always best to have one. Earth

one lead of a multimeter and then use the other to

backprobe the plugged-in power steering ECU. Have the

car running and use an assistant to waggle the steering

while you’re taking the measurements.

On the Toyota Prius on which this modification wasperformed, the following important voltages were found:

Torque sensor #1 – 2.5V output with no torque input,

varying downwards with left-hand torque and upwards

with right-hand torque

Torque sensor #2 - 2.5V output with no torque input,

varying upwards with left-hand torque and downwards

with right-hand torque

5V regulated output

Either of the two sensors can be intercepted – the ECU is just looking for the difference between the

output voltages. So how is the modification done? The following diagram shows just how easy it is.

Let’s take it step by step. Pot 1 is placed across the

5V-to-earth connections. If this pot is set to itsmiddle position, 2.5V will be available on its wiper.

Pot 2 is wired with one end connecting to this 2.5V

supply and the other to the sensor output. This

pot’s wiper goes to the ECU.

If the wiper of Pot 2 is placed closer towards Pot 1,

the signal the ECU sees will be held more and more

at 2.5V – that is, no torque change. On the other hand, if the wiper of Pot 2 is placed closer to i ts other

end, the ECU will see more and more of the unaltered signal.

So with Pot 1 set to provide 2.5V on its output, by adjusting Pot 1 you can alter the signal from being

always held at 2.5V at one extreme, to being dead standard at the other extreme. Set Pot 2 to

‘in-between’ positions and you can get ‘in-between’ values.

The two pots used are 10 kilo-ohm multi-turn designs. If you use small trimpots these are very cheap,or if you use full-size multi-turn units, more expensive. We set the system up with the latter, simply

because we had them already on the shelf. (Always use multi-turn – eg 10-turn - pots as this makes

the setting-up much easier.)

Install Pot 1 - it goes between the 5V regulated

supply and earth. Use a multimeter to measure the

voltage on the central wiper terminal (the meter

connected with one probe to the wiper and the

other to earth) and then adjust the pot so that its

output voltage is the same as the ‘at rest’ sensor

output voltage. In this case, that was 2.5V.

Then cut the signal wire between the sensor and

the ECU. Connect the sensor end of this wire to oneend of Pot 2, and connect the other end of Pot 2 to

the wiper of Pot 1. The wire to the ECU then

connects to the wiper of Pot 2.

When doing the wiring it’s easiest to ignore the

description and simply look at the diagram.

Adjust Pot 2 so that its wiper fully at the end closest

to the signal input. Start the car and drive it – it should drive normally. If it doesn’t, check your wiring.

Then adjust Pot 2 so that the wiper starts to move towards the other end. The steering should now get

heavier. If you go too far, it’s likely that you’ll trigger a fault condition – when setting this pot, drive

the car lots to make sure that (a) the weight is good across a variety of driving situation, and (b) no

fault condition is triggered.

Results


6 2011/02/04 09:58 P



Using a Fluke 123 Scopemeter to data-log both the

input signal from the sensor and the modified

output shows the changes that have been made.

As can be seen, the input trace (bottom) and the

output trace (top) appear to have the same shape.

However, close inspection shows that the upper

trace always moves less distance from the

midpoint of about 2.5V. In fact the recorded

minima (circled) show that the output dropped only

as low as 1.853V, compared with 1.102V for the input. Other data (not shown here) indicates that the

maximum voltage recorded on this drive from the sensor was 3.673V, versus 3.029V on the modified

output.

In other words, the output voltage holds closer to the ‘no-torque’ value of about 2.5V, telling the ECU

that there was less steering torque being input than there really was.

The result is less power assist and so greater road feel.

Because the ECU has a road speed input, the steering still alters in weight with speed. In this car, the

parking weight is slightly increased – which is neither here nor there – but from about 60+ km/h there

is noticeably better road feel than standard. Turning into a high speed corner gives far more reassuring

feedback as to what the front tyres are doing, in addition to giving weight against which the steering is

worked – allowing more precise inputs of lock.

As we said with the last car where we modified the steering weight (Modifying Speed-Sensitive Power

Steering), when you have the ability to alter this characteristic, you suddenly realise with startlingclarity that the amount of steering weight makes a huge and instant difference to how the car feels on

the road.

Thanks to Silicon Chip magazine’s John Clarke for technical help during the development of this

modification.

Wiring–Up Pots

A potentiometer (pot) is a simple electronic component. Most pots are rotary designs - like the

volume control on an older radio, as you turn the shaft, the internal wiper moves along a

resistance track.

There are only three terminals – shown here as A, B and

W. Most pots have clearly laid-out terminals but if the potyou are using is confusing, a simple check with a

multimeter (set to resistance) will show you what’s what.

Between terminals A and B should be the full value of the

pot, eg with a 10 kilo-ohm pot, around 10K resistance.

Measuring between either A and B and W (the wiper) will

give a resistance that alters as you adjust the pot.

In the application shown in this story, the A and B

terminals of the pot can be connected either way around –

this will just alter the direction that you turn the pot to go

up or down in signal.

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