Torque sensors are now a key component of the BoP process in WEC, both in the Hypercar and LMGT3 classes. By providing a real-time, accurate reading of driveshaft torque, it is possible for the regulators to precisely monitor compliance with the BoP-imposed power curves for each manufacturer powertrain.
The sensors are supplied by USA-based MagCanica, and rather than using traditional strain gauging methods for torque measurement, they rely on the principle of magneto-elasticity, where the magnetic field of a ferrous material changes when subject to a twisting or bending force.
Magneto-elastic sensors produce signals that are a function of torsional stress, not strain. As a result, they are generally much stiffer mechanically than conventional elastic torque sensors; they also offer a far higher frequency response, typically of the order of 2-4kHz.
Measuring surface stress by magneto-elastic methods also provides a non-contacting system for measuring torque in a more compact construction than that required for either the twist angle or surface strain elastic methods.
Magneto-elastic sensors can be broken down into two distinct groups that measure magnetic quantities related to the surface shear stress in different ways. The first method measures magnetic permeability changes in the shaft surface caused by the stress-induced magnetic anisotropy, which affects the permeance of a magnetic flux path and uses a magnetizing source and a pick-up (sensing) coil. The second method relies on the principal that stress-induced magnetic anisotropy causes a permanently magnetized magneto-elastically active member to generate a measurable magnetic flux.
One advantage of a permeability-based sensor over traditional sensing methods inherent wireless transduction – removing the need for physical contacts with the rotating member – combined with mechanically robust construction. However, despite their various benefits, permeability-based magneto-elastic torque sensors suffer from a number of disadvantages that limit their use in a racing environment.
These problems derive from the fact that the variable being measured – permeability – does not depend solely on the applied torque. In any one material composition, even in a controlled environment, permeability can vary with temperature and magnetization. The result is that in many real-world environments, the changes due to these factors can exceed the changes in permeability that are a function of torque, making the measurements useless.
Sensors for racing
The second type of sensors are much more suitable for use in racing, and are the technology MagCanica uses in its sensors. They have many of the benefits of permeability-based sensors and overcome most of the problems. The sensors can be constructed either with a thin ring of magneto-elastically active material rigidly attached to a shaft, or by using a portion of the shaft itself as the magneto-elastically active element.
In response to the magneto-elastic energy associated with the bi-axial principal stresses by which torque is transmitted along the shaft, each moment will rotate toward the nearest positive principal stress direction and away from the nearest negative principal direction.
This reorientation of the originally circular magnetization results in a net axial magnetization component. The divergence of this component at the edges of the polarised bands is the source of a magnetic field in the space around the shaft which can be readily measured with one or more magnetic field sensors. This gets around the problems associated with measuring the permeability, enabling very accurate and repeatable readings to be taken. Also, the polarized bands can be easily incorporated into the driveshaft of a vehicle.
We will delve into the implications of toque sensors from a powertrain calibration perspective in a later article.
Further details on the specifics of MagCanica’s sensors can be found here
