Off-highway tractors slog through mud, muck, and other rough terrain on a daily basis. Getting the job done in spite of these harsh conditions requires technology at its best.
For one heavy-equipment maker, the requirements were particularly high. In fact, AGCO Corp., Duluth, Ga., recently redesigned its Challenger track tractor, pushing the limits to improve fuel efficiency and reliability, and decrease manufacturing costs.
One of the design changes involved switching from hydraulic steering to an electronic steer-by-wire system. The goal was to eliminate the mechanical parts typical of hydraulics, thereby reducing tractor weight and increasing fuel efficiency. Fewer mechanical parts would also mean reduced manufacturing costs and less chance of product failure. To design the system, AGCO called on engineers at BE Duncan Electronics, Irvine, Calif.
With electronic steering, all sense of operator control is lost unless torque is projected back to the steering wheel. Operators are used to feeling the “drag,” or torque, on the wheel and actually use it as a sort of feedback mechanism. With steer-by-wire, there is no inherent torque because there is no mechanical connection; when the steering wheel is turned, sensors (presenting no mechanical resistance) sense the amount and send a proportional output signal to the CPU — voltage, resistance, or duty cycle — which instructs an actuator to turn the wheels (or tracks) to the appropriate angle.
Although the feel of operator control wasn't critical to operation, it was one of AGCO's key specifications. Creating the right steering feel proved to be Duncan's biggest challenge. After substantial testing, Duncan designed a mechanical package with precise “spring and stop” adjustments in the sensor enclosure to simulate the feel of conventional hydraulic steering.
A second challenge was to build a gear train reduction into the steering system to enable a longer mechanical movement over a shorter electrical angle. The design needed to reduce the amount of backlash created by the fact that the steering wheel can rotate past 360 degrees, but the (absolute) sensor cannot. Backlash was eliminated using a mechanical enclosure around the sensors. The resulting gear train minimized the movement to only one degree and helped optimize position feedback.
Ensuring safety was another important issue. Because there's no mechanical linkage in a steer-by-wire system, the tractor depends on output signals from the sensor for control; in effect, the sensors are the tractor's steering. To ensure safety, Duncan developed the system with a triple-redundant sensor pack with three independent outputs, all driven from a common steering shaft. If one sensor fails, a fault code alerts the operator while the other two sensors allow continued vehicle operation. For more information, visit beiduncan.com.