A new fabrication technique solves the problem of an external magnetic field’s adverse effects on a magnetostrictive torque sensor.
Radu Andreescu and Mool C. Gupta, Old Dominion University
Accurate sensing and measurement of torque is one of the primary objectives in the automotive industry, and the Villari effect can be used to satisfy this requirement. The operating principle of a magnetostrictive torque sensor (U.S. Patent #5351555) is shown in Figure 1 in the form of a noncontact sensor for use with rotating shafts.
A magnetostrictive material coating is rigidly attached to the shaft. An easy axis of magnetization is created in the tangential direction by mechanical stresses. The coating is then magnetized by passing a pulsed current through the shaft. Transducer operation is based on the reorientation of the circumferentially directed remanent magnetization in the coating.
The remanent magnetization, the amount of magnetization that remains in a material after an externally applied field has been removed, is initially oriented in the tangential direction, and the magnetic field created by the shaft is zero. When torque is applied to the shaft, the remanent magnetization reorients and becomes increasingly helical as the torque value increases. This reorientation produces a magnetic field, proportional to the torque, to be detected by a nearby magnetic-field sensing device. The output signal from this device is conditioned in associated electronic circuitry to provide a signal that can be used in a control unit. The drawback is that the generated magnetic fields are weak and the orientation of the magnetization in the coating can be affected by an external axial magnetic field-Earth's, for instance.
One way to overcome this problem is to replace the single circularly polarized coating with a coating divided into two oppositely polarized circumferential regions (U.S. Patent #5520059). The coating is magnetized using two identical permanent magnets brought close to the shaft while the shaft is rotating slowly.
The most important requirements for the sensor are:
By minimizing the energy (u/ = 0), the equilibrium orientation of the magnetization under different values of the applied torque can be obtained and the axial magnetic field created can be calculated.
Figure 3 shows the calculated value of B with torque for a nickel coating with a residual strain of 1200 µstrain, magnetized by pulsed current as in described in Figure 1A.
The observation point is at the center of the magnetized section and at 0.35 mm from the coating surface. The parameters used in the calculation are D0 = 25.1 mm, Di = 24.1 mm and L = 28 mm.
Figure 4 shows the measured values of B and induced strain for the nickel coating, with torque cycling between -8 and 8 N·m (the units for magnetic field are arbitrary).
It can be seen that both magnetic field and transferred strain (which is proportional to the applied torque) are highly linear under cycling torque loading.
In conclusion, by using a magnetostrictive material, torque values can be measured in a reliable, cost-effective manner. Magnetostriction is an inherent material property that will not degrade with time.
Dr. Radu Andreescu is a Postdoctoral Fellow, Applied Research Center, Old Dominion University, Newport News, VA.
Dr. Mool C. Gupta is Director, Applied Research Center, Old Dominion University, Newport News, VA; email@example.com.