Magnetised shafts measure torque by field
Tom Shelley reports on a truly non-contact method for measuring torque.
A non-contact measuring system sensitive enough to detect torque applied by hand pressure that has become established in Formula 1 motorsport, is beginning to find additional applications in industrial condition monitoring.
Developed by NCT Engineering in Unterhaching near Munich, the sensor was invented by Lutz May, and uses Pulse Current Modulated Encoding (PCME) technology to magnetise drive shafts.The sizes of the shafts can range from dental drills to drive shafts in ships and even oil drill shafts. Thus far, most experience with the sensor has been gained in gearbox shaft and power tool drive shaft applications, where reliable and repeatable measurements have to be taken under extreme conditions and at very low costs. That said, the sensor has been used for R&D in everything from power supply applications to the testing of consumer goods.
The unit depends on the inverse magnetostrictive, or Villari, effect in which there is a change in domain magnetisation when a stress is applied to a material. This can be positive (magnetisation increased by tension in iron) or negative (magnetisation decreased by tension in nickel). It responds only to force and does not require the shaft to distort.
Best response is achieved when the nickel content of the alloy used to make the shaft is greater than 1.6% and chromium is also present. In addition, where the PCME sensing technology is applied should be dimensionally symmetric and hardened as a non-hardened sensing region produces large measurement hysteresis. The minimum axial width that can be encoded is 25mm, although 50mm is preferred. The minimum radial spacing between the shaft surface and the sensing coil unit is 5mm, although 10mm or more is preferred.
One of its advantages, according NCTE's sales manager Joachim Heckler, is that it can be used at high temperatures – nearly 485ºC in the case of the shaft. The pickup coil or 'secondary sensor unit' (the encoded shaft is the 'primary' sensor) is able to cope with temperatures of up to 210ºC, or 250ºC in the short term. The signal condition and signal processing electronic unit is about the size of a matchbox and can be incorporated into other equipment.
Heckler describes the system as 'very robust and very stable', being able to withstand vibration levels of more than 1000g in all three axes. The magnetisation of the shaft is very low, meaning it does not pick up debris. Signal bandwidth is 30kHz, which accommodates shaft rotations of more than 200,000 rpm. Signal resolution is 0.01% and repeatability is 0.05% for many applications.
The applications have included testing helicopter gas turbines and gearboxes, studying breakaway torque forces in an impact power tool bit holder, measuring drilling shaft bending and torque forces down oil boreholes, measuring torque forces in avionic flap control, and measuring torque forces in F1 KERS systems. However, the business is now moving into more routine industrial applications.
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