Design & Development
The idea for a electronically controlled four-wheel drive system emerged at BorgWarner in 1985. BorgWarner’s original design called for using both a software controlled electromagnetic multi-disc (also called multi-plate) clutch pack and a planetary or bevel geared center differential together. The first prototype system was crude, and its original clutch pack controller was a rheostat.
The geared center differential would be used to allow the front and rear drive shafts to turn at different rotational speeds so as to eliminate any "drivetrain binding" or "torque windup" while the system was being used on pavement. The intelligent multi-disc clutch would be used to progressively transfer torque back-to-front and front-to-back between the front and rear drive shafts when needed and would also lockup providing a permanently locked front 50:50 rear torque distribution.
Within a year, the controller had become larger and more complex. A breadboard electronic version complete with sensor amplifiers and large control modules consumed the entire back end of a station wagon. Input data came from variable reluctance sensors installed at the front drive shaft, rear drive shaft, throttle, brakes, and steering. The goal was to control clutch actuation by controlling current.
However, as the development process continued, engineers at BorgWarner discovered that, with clever software programming, they could control the variable electromagnetic multi-disc clutch to a point where it would allow the front and rear drive shafts to turn at different rotational speeds on its own, without the aid of a planetary or bevel geared center differential. In Auto mode the new software algorithms allowed the multi-disc clutch pack's friction and clutch discs to slip as needed, simulating a planetary or bevel geared center differential, making a geared differential redundant. Thus, the geared differential was abandoned and therefore never included on the final production version of the four-wheel drive system.
| "We asked ourselves: Why do we need a differential? There was all of that iron, all that weight, all that cost.
And here, we saw that we could control the clutch pack very precisely without it." - Ronald A. Schoenbach |
The design team gained confidence in their concept in 1989, when they made a breakthrough in the multi-disc clutch's control system. Development of closed-loop control smoothed its operation. Using it, the multi-disc clutch made smaller adjustments, but did it more frequently. The microprocessor would review input from sensors every 20 milliseconds and decide if the front axle needed more torque. Using this technique, the unit's computer watched for drive wheel slip. If it sensed as little as half an rpm difference between the front and rear axles, it sent a power signal to the multi-disc clutch. The clutch engaged, diverting torque to the front axle in 10 percent increments, until it alleviated the drive wheel slip. As a result, the system could control runaway drive wheel speed in as little as a third of a wheel revolution.
Read more about this topic: Control Trac
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