“The traditional steering system is powered by direct
connection to the engine, so at idle, you are giving up power to
the systems, but when you drive at high revs, you over-power,”
said Director of ITRL Peter Georén, an expert on transport
innovation. “The design trend is to use the engine more for
propulsion than for powering the subsystem.”
Another limitation of the conventional steering rack is that
only the front wheels turn and do so with limited degrees of
movement. This, for example, makes squeezing into a tight
parking space challenging and requires the widely dreaded
operation known as parallel parking. Despite the fact that some
newer vehicles can do this for us, it would still be much more
efficient if a car could enter a parking spot perpendicularly.
Apart from parking, having multiple steering actuators enables
control of the car to make the ride more pleasant, safer with
reduced rolling and curve resistant.
Although not currently a part of the steering system,
adjustment of the camber, the vertical alignment of the wheel in
relation to the car, is a precision operation that must be done
with special equipment whenever a pothole or other trauma
throws it off. If this adjustment could be done more easily –
even automatically – it could save time and money, make for a
smoother drive with less tire wear, and when needed, increase
the lateral grip to improve safety.
The steer-by-wire initiatives are part of a broader set of
environmental and sustainable transport challenges that KTH
and ITRL are working on. ITRL conducts its testing on a 400
kg research concept vehicle (RCV), which can carry two
passengers and reach a speed of 70 km/h (Figure 1).
On each wheel of the RCV is a steering apparatus, which
is controlled by a linear actuator. Traditionally, pressurised
fluid delivered via hydraulic tubing provides the steering force,
whereas in the ITRL’s RCV, electric wires replace hydraulic
tubing and activate the small motor built into to the actuator
housing. Drivers control the electrical signals using a steering
wheel, but this works more like the controller of a video game
than the conventional post and tubing system.
To equip the system for testing over-actuated technology, the
ITRL team determined that the RCV would need eight actuators:
one on each wheel for controlling the steering and another on
each wheel for controlling the camber. Finding the right linear
actuator took some time. They submitted the specifications to
a number of vendors, most of whom could meet the specs at
a higher voltage or on AC. However, only Thomson Industries
could meet all of the specifications exactly as required. The
company supplied four high-speed Max Jac MX24-B8M10E0
actuators to perform the steering functions and four Pro Series
actuators to change the wheel camber inclination.
As expected, the new electric linear actuators were more
responsive than the hydraulic actuators because of the speed
with which electricity travels across a wire. Moreover, the
digital control combined with the specially engineered steering
architecture provided a level of flexibility not possible with
conventional actuation. It is this capability, for example, that
could replace parallel parking with perpendicular parking or
someday enable automatic realignment during driving to reduce
rolling resistance or adjust after hitting a pothole.
Figure 2. b) The modified MaxJac actuator
installed in the RCV. Photo courtesy of ITRL.
Figure 2. a) The Max Jac. linear
actuator in a typical application.