A Novel Method of Axle Torque Measurement for Off-Road Vehicles 2012-01-0310

Most vehicles designed primarily for off-road use - whether for the SUV, military, agricultural or earthmoving industries - employ all wheel drive systems. For off-road conditions where the traction is limited by the deformable nature of the ground, for example, loose track, soil or sand, providing a drive torque to all the wheels is the obvious design solution for maximising the total tractive effort. For military or commercial vehicles, this results in optimum mobility in difficult terrain, whereas for agricultural or earthmoving vehicles it often results in optimum work rates.

In order to analyse the performance of off-road vehicles, it is necessary to understand the torque and power flows through the driveline system to each axle or wheel. The research presented in this paper focuses on the use of novel, non-contact torque sensors to measure the driveline torque distribution. The example vehicle is a four wheel drive (4WD) agricultural tractor - but it is proposed that the same measurement techniques could be applied to any off-road vehicle.

The torque sensor is a non-contact, bi-directional transducer based on the magnetostrictive principle. The shaft to be measured has a chevron pattern plated on to its surface in copper - and then around this is placed a sensor housing with sets of primary and secondary windings. When a torque is applied to the shaft, the change in magnetic flux of the copper chevrons is sensed in the windings. The benefits of this system include simplicity, reliability, linearity compactness and ease of fitting. These features enabled torque measurements to be made at the input to the main gearbox (engine output torque) and the front and rear driveshafts of the tractor. The driveline measurement system in practice demonstrated substantial improvements over previous systems described in the literature, all of which appear to have been based on strain-gauge type devices.

The paper describes the experiences of using this system in field measurements with the tractor operating in high draught force conditions - as is normal for many agricultural tasks. Example results are used to show that the torque and power flows through the driveline are not as straightforward as previous research has suggested - the tractive force vs wheelslip characteristics at the front and rear tyres can cause interactions which result in unanticipated front and rear axle torque distributions.