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 AGCO 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 significantly reduces 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 extensive experience designing memory seats for automotive applications, Duncan eliminated the mechanical backlash 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 safety margin, 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.