20, 1998 Design News
An electric challenge to hydraulic
Microelectronics and software
make an old concept newly competitive
Geoff Nairn, Contributing Editor
Electric power-assisted steering (EPAS) has tempted
automotive engineers since the 1950s. Even so, the promise
of smaller, lighter, and more efficient systems never
quite matched the low cost and performance of hydraulic
Advances in microelectronics, however, have rekindled
interest in EPAS. Motor drive stages, electronic control
units, and torque sensors can now be manufactured relatively
cheaply. These components, coupled with complex control
algorithms implemented in software, can rival or better
the performance and functionality of conventional hydraulic
steering systems. In addition, software can be fine-tuned
to deliver the desired mix of stability, robustness,
and steering "feel."
So says Tony Burton, control engineer with automotive
component supplier LucasVarity plc (Solihull, England).
The company is presently working on an EPAS system that
offers a lower component count than HYPAS, is three
to five kg lighter than a comparable HYPAS system, and
consumes 4 to 5% less fuel. Attached to the steering
column, the self-contained unit is said to be easy to
install and particularly cost-effective on smaller vehicles.
"There are often considerable difficulties fitting
HYPAS to a small car," Burton explains. The LucasVarity
system, he estimates, could be installed in four minutes
on a production line.
Control algorithms, implemented in software,
help EPAS challenge HYPAS in performance and
How it works. Operation of the LucasVarity
EPAS is straightforward. An optoelectronic sensor measures
driver torque applied to the steering wheel. The electronic
control unit takes this measurement and, through the
control software, drives the motor.
The brushless dc motor generates an additional torque
which acts through a reversible gearbox to the steering
mechanism, assisting the driver. This assistance torque
varies from about 15 Nm in a small "city"
car, to 75 Nm in a larger car such as a Ford Taurus,
and diminishes with road speed.
EPAS optical torque sensor meets reliability,
fault tolerance, and electromagnetic compatibility
requirements while low enough in cost to mass
A frequent complaint made of HYPAS systems is their
tendency to "over assist" the driver at higher
speeds. In poorly designed systems, this can result
in the vehicle oscillating or "yawing" around
the center line as the driver attempts to correct the
In the new EPAS system, software allows precise control
over steering behavior. Algorithms programmed into the
system define speed sensitivity, yaw damping, and steering
self-centering. Added algorithms can give steering a
"sports" feel or offer light load settings.
A safety relay incorporated into the design improves
fault tolerance, while the electronic control unit includes
diagnostic functions for fault detection and management.
If the system fails, its "fail stop" design
cuts all torque assistance and returns the driver to
For a torque sensor, LucasVarity incorporates a dual-channel
optical device. Its non-contacting design and mechanical
simplicity provide system reliability, while the use
of optics offers immunity to EM interference.
To operate, two patterned disks mount on either end
of the torsion bar separating the steering wheel and
steering column. Torque applied to the steering wheel
creates a relative movement between the two discs. Light
intensity reaching the photodetectors varies in proportion
Because either detector can be used to measure light
intensity and thus torque, the system is redundant.
Offset patterns on the two discs, furthermore, allow
the software to calculate the steering wheel's relative
position and velocity by comparing the two sensor signals.
Motor design. Achieving a smooth,
progressive feel at the steering wheel requires a motor
with low levels of ripple and cogging torque. LucasVarity,
therefore, uses a three-phase inverter to control motor
phase currents, and hence torque. An array of power
MOSFETS make up the circuitry; pulse width modulation
(PWM) regulates switching time and sequence for the
Since the power-switching stage encompasses the most
complex dynamics of the whole EPAS system, optimization
requires computer simulation. Lucas uses the Saber simulator
program from U.S. software house Analogy Inc. to analyze
alternative PWM strategies, and to model the complex
patterns of secondary currents induced when the MOSFET
stages are switched.
Low-cost components mean EPAS systems can
be engineered to cost the same as HYPAS while
offering additional advantages.
The aim, Burton explains, is to ensure smooth control
of the switching stages and also reduce the ripple currents
fed into the battery harness. "These currents have
to be filtered out to protect the electronics in the
EPAS control unit. By minimizing the ripple current,
we are able to use filter components with lower ripple
specifications and, therefore, lower cost."
LucasVarity's Electric Power Assisted Steering system
is targeted primarily at the European market where smaller
cars dominate and the advantages of EPAS offer the greatest
potential. "But there is no reason why the system
cannot be scaled up to larger cars," Burton adds.
The company claims to have negotiated a development
contract with a leading European carmaker which could
see EPAS installed on production models in the year
EPAS prospects, pros and cons
by Anna Kochan, UK
EPAS technology is new to Europe but has been available
on small (600 to 800 cc) Japanese cars for more than
10 years. First systems to arrive in Europe were adaptations
of Japanese park-assist units, such as the one incorporated
in the 1996 Renault Twingo.
Developed in collaboration with the Japanese company
Koyo, the Twingo EPAS provides assisted steering up
to a speed of 65 km/hr. "EPAS know-how is now being
transferred from Japan to France where production of
EPAS systems will start within six months," claims
Yannick Lacour, director of strategic planning at SMI,
the Koyo subsidiary in Irigny (near Lyon, France).
Another Japanese manufacturer bringing EPAS into Europe
is NSK. Its systems have been fitted into 1-l Opel Corsas
since 1997. NSK EPAS is also likely to be introduced
on the 1.2-l version of the Corsa, and is offered as
an option on the new Fiat Seicento.
For more powerful vehicles, some manufacturers are
starting to develop a hybrid approach to EPAS technology.
These are electro-hydraulic steering systems, which
use an electric motor to drive the hydraulic pump, rather
than driving it off the engine.
As a result, the hydraulic pump runs independently
from the engine, and needs only run when power support
is required at the steering wheel. It gives a fuel savings
of 0.2 litre per 100 km, claims Karl Mauer, Opel spokesman,
whose company is adopting a hybrid system on the new
Astra being released this year. The suppliers are TRW
and Delphi Saginaw.
Many vehicle manufacturers are, however, not satisfied
with current EPAS performance. "Right now they
don't give the comfort and safety we want to provide
our customers with," remarks Marc Boucque, R&D
spokesman at PSA. He talks of a "sticking"
phenomenon, meaning that the steering wheel does not
go naturally back to its original position after it
has been turned. However, Boucque expects EPAS performance
and cost to reach an acceptable level by 2000 when PSA
will incorporate it widely into their models.
Ford, like PSA, is not satisfied with current EPAS
performance. "We are not dismissing the technology,
but we have evaluated 12 of the systems that are currently
being developed or in production and none provided a
satisfactory dynamics performance," reports Ulrich
Eichhorn, manager, vehicle dynamics, advanced vehicle
technology and small and medium car vehicle center,
Ford Automotive Operations.
"They feel artificial because they suppress feedback
from the road. Vehicle response is not what we would
like it to be," he adds. Ford will probably use
electro-hydraulic technology as a first step towards
EPAS because it works with existing hydraulic systems,