Many automobile components suppliers exhibited hub units with built-in tire force sensors at the 40th Tokyo Motor Show.
A hub unit quickly detects the forces of road surfaces applied on a vehicle. The acquired information is used to control the movement of the vehicle body.
Conventionally, because the forces were detected indirectly by yaw rate sensors or acceleration sensors, changes of vehicle bodies due to the applied forces were not detected. But the tire force sensors integrated in hub units are more responsive because the forces applied on tires are detected from the difference of displacements between the outer ring and the inner ring of the bearing or the bearing itself inside the hub unit. This allows more refined control of the vehicle body movement.
Nihon Seiko Co Ltd (NSK) uses a wheel element and a magnetized encoder to detect the displacement inside the bearing. Multiple V-shaped magnetized slits are formed along the circumference (so that corners of Vs are overlapped) of the ring-shaped encoder, which is mounted on the inner ring.
On the outer ring, three pairs of wheel elements are located at an interval of 120° (the pairs of wheel elements are positioned symmetry with the center line that connects corners of Vs). If a lateral force is applied on the tire, the inner ring is displaced from the outer ring toward the direction parallel to the bearing, while the inner ring is displaced from the outer ring toward the direction vertical to the bearing.
The displacements are detected by the six wheel elements. If explained in more detail, it detects the phase difference in pulse signals output from each wheel element that changes responding to revolution of the tire.
The displacements are analyzed as directional components of lateral, vertical and longitudinal directions, and then the displacements are converted to forces based on the stiffness characteristics of the bearing. The responsiveness is about 10 times faster than existing methods, which incorporate yaw rate sensors. Tire forces can be detected in 10msec.
NSK plans to commercialize a hub unit that can detect lateral forces as a first step. The company intends to release the unit in 2010. This is because vertical and longitudinal forces cause less displacement of the inner ring from the outer ring compared with lateral forces, when the same load is applied, and also because the SN ratio (signal-to-noise ratio) needs to be improved. The company exhibits a system consisting of a hub unit, steering-by-wire function, etc.
NTN uses a strain gauge instead of wheel elements and magnetized encoders. The strain gauge positioned on the outer side of the bearing detects tire forces through deformation of the bearing.
The system of NSK that employs wheel elements needs the tire to revolve for it to operate, but the system that employs a strain gauge operates without revolution of the tire. The responsiveness is so high that it can detect in less than 1msec (depending on the performance of signal-processing micro computer).
However, "Load cells and displacement gauges are delicate devices to be used in a laboratory," NSK says. "We don't want to use them in a vehicle that has to endure the travel distance of 200,000-300,000km."
Its reliability remains unclear. NTN has manufactured a hub unit capable of detecting lateral forces and is currently testing its resistance to mud, water, stones, etc. The commercialization is slated for 2010 or later.
JTEKT Corporation also exhibited a similar unit, but will not announce the details. But judging from the information that the unit measures displacements of the inner and the outer rings, it is seems to be more similar to the NSK's unit.
"Our unit is different from NSK's in the number and layout of sensors, which improved reliability, for example," a spokesperson of JTEKT said.
The company says it can not disclose the sensor used in the unit either. It expects to commercialize the unit in two years.