Back in February 2002 we showed you how to modify the weight of a
speed-sensitive power steering system
(Modifying Speed-Sensitive Power Steering). In summary, we used a heavy duty pot to reduce the average
current flowing through the solenoid valve that controls power steering weight.
With reduced current flowing through it, the valve opened less which in turn
made the steering heavier. The guinea pig car was a 1998 Lexus LS400.
And what were the benefits of changing steering weight? Better feedback, more
‘meat’ in the steering inputs, better stability on freeway sweepers. In short,
if you haven’t experienced the ability to alter the weight of the steering, you
honestly don’t know what you’re missing out on.
The driving difference can be HUGE.
However the ‘pot’ technique had some disadvantages. The change to the
steering weight couldn’t be varied at different speeds – the curve provided by
the factory was retained, it was just moved downwards a bit. This resulted in
the fact that when the pot was set to give good steering weight at speed, the
steering was noticeably heavier than normal when parking.
Using the pot was an effective but blunt instrument.
The pot technique could also only be used in cars where a reduction in duty
cycle resulted in an increase in steering weight – in a car with a system that
worked the other way around, the pot couldn’t be used.
But as you may be guessing from this preface, we now have available a more
sophisticated technique to altering power steering weight that results in vastly
better adjustability. It uses the Silicon Chip Digital Pulse Adjuster
interceptor kit - electronics engineer John Clarke did the design and
development. We’ve covered the kit in two detailed articles –
Part 1
and
Part 2. The kit – and the book in which it is featured – are available
from the AutoSpeed Shop and Jaycar stores. Kit cost is just AUD$79.95, with the
Digital Hand Controller another AUD$59.95.
Variable Steering Weight
So what’s this steering stuff all about, anyway? Most recent cars have power
steering that varies in assistance with road speed.
(Older cars use a system that vary in assistance with engine speed,
where as revs rise, the power steering pump output alters to reduce assistance.
These systems are usually purely hydraulic, with no electronics able to be
modified.)
In the case of the Lexus, a solenoid valve is used to vary the flow of
hydraulic fluid to a reaction chamber - a fluid force that actually resists the
power assistance. If a lot of fluid is allowed to flow to the reaction chamber,
the steering effort is higher. If little fluid flows to the reaction chamber,
then the steering effort is lower.
The current to the solenoid is varied by means of pulse width modulation -
the current is pulsed on and off quickly. If it is on for only half of the time
(ie it has a duty cycle of 50 per cent) the coil will ‘see’ only half battery
voltage, and so will not close fully. If the duty cycle is reduced to, say, 30
per cent, then the valve will open a little more.
The pulses being sent to the solenoid valve look very much like an injector
duty cycle signal – they’re a square wave where the ’on’ time varies. But note
that unlike an injector, the frequency of the pulsing is so quick that the valve
doesn’t open and shut to the individual pulses - instead the plunger hovers at
mid-points.
Variable duty cycle pulses are exactly what the Pulse Width Adjuster is
designed to intercept, so the device can be used in this application.
Digital Pulse Adjuster
As we’ve already covered in detail in the earlier articles, the Digital Pulse Adjuster
takes in the signal from the ECU, alters it as the user wants, then spits the
revised signal out to the solenoid. In fact, it completely takes over the driving of the
solenoid.
In this application the variable duty cycle signal that normally controls the
power steer flow control solenoid is diverted into the Pulse Width Adjuster
(DPA), and the power steer solenoid is wired to the DPA’s output. Without making
changes on the DPA’s hand controller, the power steer solenoid behaves exactly
as it did in its unmodified state.
But the beauty of the DPA is that it can makes changes to the output,
based on the duty cycle coming in. So for example, all 50 per cent duty cycle
signals can be altered so that they are now 40 per cent. All 60 per cent signals
can become 55 per cent. And so on. But you really don’t need to understand all
of that – with the DPA working, all that you need to do is to press the up or
down buttons on the hand controller until the steering weight at different
speeds is as you want it.
Wiring-in the DPA
The DPA controls the solenoid by switching it to ground. In other words, it
expects that one side of the solenoid is always connected to 12V, and the other
side is earthed through the action of the DPA whenever the solenoid is turned
on. The DPA is set up like this because that’s the way most solenoids (including
injectors) are organised in a car. Unfortunately, in the case of the Lexus, the
solenoid wasn’t wired like this. However, a simple wiring change got around this
problem.
This is how the electronics of the Lexus power steering system are arranged.
As you can see, it’s pretty simple with the ECU having only one input (road
speed) and one output (solenoid duty cycle).
Here you can see how the duty cycle signal from the ECU is now fed to the
DPA. The DPA then drives the solenoid which has both of its wires completely
disconnected from the ECU. Because the DPA drives the solenoid by earthing it,
12V now needs to get fed to the other side of the solenoid.
Note that a change to a link on the PCB needs to be made when adopting this
wiring configuration – see our stories on the DPA for more on this.
A Fake Load?
In many cases a load will need to be provided to the ECU to replace the
solenoid, otherwise the ECU will register a fault code or may not even have an
output. This is achieved by placing a resistor across the wires that once went
to the solenoid. The resistor will normally be a 10-watt design of the same
resistance as the solenoid coil (which is easily found by measuring it with a
multimeter). Check that the resistor doesn’t become hot in normal use – if it
does, double the resistance value and wire two resistors in parallel. In some
cases the resistor(s) will need to be mounted on a heatsink in airflow eg under
the bonnet in a cool spot.
A single resistor (red) is shown in place here. However, in this case it
wasn’t needed as the power steer ECU in the Lexus was happy without it.
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Tuning
Unlike say an interceptor being used to tune mixtures, the tuning of the
power steering weight is nearly entirely based just on personal preference. But
what’s the actual process used?
The DPA’s LCD has two lines, one showing the input value (called ‘load
points’) and the other showing by how much the output has been altered at each
point. The load points correspond to the different duty cycles coming in, but
again that’s not something that you really need to worry about.
In the case of the Lexus, the input load number when the car was stationary
was 40, and when the output at this number was increased, the steering became
lighter. Driving the car showed that the input numbers covered the range from 40
to about 90, and so changes could be made over this range, with most being
reductions so as to make the steering heavier. (In cars with systems that work
the other way, the duty cycles of the valve could have been increased. However,
if increasing duty cycles, feel the temp of the solenoid to make sure that it
isn’t becoming too hot with the extra current passing through it. Normally,
there won’t be a problem.)
This graph shows the changes that were made on the Lexus. Low load numbers
(left-hand side of the graph) relate to low speeds and high load numbers
(right-hand side of the graph) to higher speeds. (See the breakout box below for
more on why this is the case.)
Load sites around 33-44 are for when the car is stopped or only just moving.
As you can see, at these points we increased the valve duty cycle, making the
steering lighter than factory. However, once the car is moving (around 45 - 53)
we add in steering weight by reducing duty cycle. Load sites 68-73 correspond to
cornering at about 70 - 90 km/h and you can see at across these speeds we made
the adjustments similar. Load sites 73-78 are at about 100-110 km/h, while from
load site 80 onwards we made the steering much firmer. In this land of speed
limits, I am rarely driving for long periods above 110 km/h, so the steering
could be made heavier at speeds higher than this to give better stability during
high-speed passing manoeuvres and so on.
Note that the highest load number seen was about 92 but the rest of the map
(up to load site 113) was completed in case a circumstance ever develops where
these numbers are reached.
Load Number vs Duty Cycle?
The DPA assigns input load numbers from 0-128 to the various duty cycles
covering the range from 0-100 per cent.
Therefore, 0 on the input on the hand controller = 0 per cent duty cycle, 32
on the input = 25 per cent duty cycle, 64 on the input = 50 per cent duty cycle,
96 on the input = 75 per cent duty cycle, and 128 on the input = 100 per cent
duty cycle.
Since in the case of the Lexus, input load numbers covered the range from
about 40 to 92, the valve duty cycle was actually varying from 31 to 72 per
cent.
As discussed in the main text, in the Lexus the load number shown on the hand
controller (ie valve duty cycle) rose almost in proportion with road speed. This
indicates that the factory set-up has the steering being lightened as
speed increases!
No wonder the standard car always felt nervous at high speed....
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With the map of tuning values stored by the DPA, the hand controller could be
disconnected, Link 3 on the board removed (this locks in place the frequency of
the solenoid valve operation) and the box mounted under the dash.
The Results
It is difficult to describe the magnitude of the difference that has been
made to the car. Using the pot technique, I’d already improved steering weight
in medium speed corners – say around 80 km/h. But until I’d used the DPA and
realised how light the factory control system was making the steering at higher
speeds, I simply hadn’t realised the potential gains in this area.
Now the steering weight is perfectly-weighted everywhere. Of course it is – I
have modified it to exactly suit my personal preferences!
For example, I need to do a three-point turn to get into my garage. Now it’s
a doddle – the steering is super light when the car is moving at slower than a
walking pace.
For example, most of the corners on my country road home are taken at around
80-90 km/h – the steering weight now gives a firm feedback that allows the
placing of the car more accurately, without the steering being overly heavy.
For example, I have always been unhappy with the stability of the car in
crosswinds at 100-110 km/h on the freeway. You could get into a little juggling
act with the wheel as you inadvertently over-corrected when the car moved around
in gusty crosswinds. In comparison, it now feels rock-solid.
For example, at very high speeds the road always seemed to narrow more than
it should. Now it doesn’t.
I could go on and on. If you think that a change in wheel alignment can make
a difference to how a car steers, try modifying the weight of the power
steering. In this car at least, the difference is quite remarkable.
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