Machine Design

Measuring torque without contact

Measuring torque in real time has stymied engineers for years. Strain gages provide a partial solution, but they are too delicate for hostile environments. A new approach, Embedded Magnetic Domain (EMD) from Fast Technology, Livonia, Mich., uses magnetic induction and high-speed signal processing to instantaneously and accurately measure torque in static or rotating shafts.

The low-cost and noncontact sensor method works like this: The portion of the shaft being measured is given a specific magnetic pattern. Torque on the shaft changes the shaft's magnetic field. A patented circuit detects the change and determines the amount of torque in the shaft. The sensor can be mounted up to 10 mm from the shaft and works through dirt, fluids, and nonmagnetic materials. EMD technology measures torque at any shaft speed and with minimal hysteresis.

There are a few limits to the technology. For example, the shaft must be able to be magnetized (i.e., it must be ferrous and contain some nickel), and not reach its Curie point, the temperature at which a material's magnetic properties change. The shaft also can't be damaged or bent, and the sensors must be shielded from other strong magnetic fields. Sensors can be made to accommodate a wide range of shaft sizes. The system can measure torque that falls between 0.0184 to 737.56 lb-ft (0.025 to 1,000 Nm (typically with accuracies of 0.5%. The stable, low-power sensors output high signal levels.

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