IRVINE,
Calif., – Off-highway
tractors slog through mud, muck, and other rough terrain
on a daily basis. Achieving high reliability and performance
in spite of these harsh conditions requires technology
at its best. To improve fuel efficiency and reliability
while decreasing manufacturing costs, the Challenger® track
tractor went through a major redesign from the ground up.
 An
integral design change to achieving these objectives was
switching from a hydraulic steering system to a steer-by-wire
(electronic) steering system. Incorporating an electronic
system would eliminate the many mechanical parts inherent
to a typical hydraulic steering system, thereby reducing
the overall weight of the tractor and automatically increasing
fuel efficiency. This design change would also improve
product reliability simply by eliminating the number of
parts potentially subject to failure. Fewer mechanical
parts also meant a significant reduction in manufacturing
costs. These long-term benefits overcame any short-term
costs required to develop the electronic steering. To design
the steer-by-wire system, the manufacturer turned to BEI
Duncan Electronics.
“The
new system required a custom electronic design as well
as mechanical packaging specific to the requirements of
the tractor,” explains Diana Westerlund, Account
Manager for BEI Duncan’s key Off-Highway Equipment
customers.
With an
electronic steering system, all sense of control is lost
by the operator unless torque is designed into the system
to simulate that feel. With hydraulics, the operator feels
the “drag,” or torque of the hydraulic steering
when he turns the steering wheel. With steer-by-wire, there
is no inherent torque because there is no mechanical connection;
everything is electronically controlled. When the steering
wheel is turned so many degrees, the sensors send an output
signal to the CPU − whether its voltage, resistance,
or duty cycle − to turn the wheels (or tracks) to
the appropriate angle. While the ability to have the feel
of operator control wasn’t critical to the actual
operation of the tractor, it was a definite product benefit
and key specification by the manufacturer.
 “Creating
just the right feel of steering control was subjective,” explains
Westerlund. “Since there was no standard to go by,
this proved to be Duncan ’s biggest challenge of
the project.” After a substantial amount of development
and field testing, Duncan designed a mechanical package
with precise spring and stop mechanism adjustments in the
actual sensor enclosure to ergonomically create the feel
of hydraulic steering.
A second
design challenge was developing a gear train reduction
into the steering system to enable a longer mechanical
movement over a shorter electrical angle. The gear train
reduction was needed to reduce the potential amount of
backlash created by the fact that the steering wheel can
move over 360 degrees, but a sensor cannot. With Duncan ’s
extensive experience designing memory seats for automotive
applications, they were well-equipped to meet this challenge.
The mechanical backlash was eliminated with the design
of a mechanical enclosure around the sensors. The resulting
gear train assembly minimized the movement to only one
degree and facilitated instantaneous and absolute position
feedback motion control.
Ensuring
safety was another important element in the new system
design. Because there is no mechanical linkage in a steer-by-wire
system, the tractor is completely dependent on the output
signals from the sensor for steering control – the
sensors in effect are the tractor’s steering.
To ensure a high margin of safety, Duncan designed and
developed the system with a triple-redundant sensor pack
with three completely independent sensor outputs, all driven
from a common steering shaft. The three sensors are always
working during vehicle operation. If one sensor fails,
a fault code alerts the operator for service, and the other
two sensors allow continued operation of the vehicle until
service is available.
Duncan ’s
design of the individual sensors and triple redundant sensor
assembly steer-by-wire system has been incorporated into
AGCO’s current Challenger MT700B Series of track
tractors.
For further
information, contact BEI Sensors, 7230 Hollister Ave, Goleta, CA, 93117, Tel: (805) 968-0782 or (800) 350-2727; Fax:
(805) 968-3154, E-mail: contact@beisensors.com |
