Low-cost magnetic torque sensor principle

The torque is measured in a contactless technique without special treatment of the shaft, using the magneto-elastic effect of ferromagnetic materials. Strain or stress due to moments lead to a change of magnetic properties of the material (hysteresis deformation, permeability changes). Magnetoelastic sensors are distinguished by robustness and good reproducibility. However, inhomogeneities of the material of the shaft and remaining mechanical stresses cause the operating point on the hysteresis curve to change. With a new approach, the shaft undergoes cyclic complete magnetization and re-magnetization to effectively erase its magnetic and thermal "history". The material is premagnetized well into saturation by electromagnets to gain defined operating points. The structure of the new sensor is described and its virtues, like good linearity and independence of the air gap between the shaft and the coil are demonstrated for static measurements. The applied torque is determined by different characteristic changes of the hysteresis (changes of its area, gradient in the zero-crossing, saturation level, gradients of the branches). By polynomial fitting of the rising and falling branches of the hysteresis, accuracies of less than 2% are achieved with small variance.