A ring magnet pressed onto a shaft is the active element in a magnetoelastic torque-sensing system. Pick-up electronics, mounted nearby, convert the ring's magnetic field to an electric signal proportional to torque. The method works on shafts of any size and over a wide range of torque loads from a few ounce-inches to thousands of pound-feet.

If there's one thing that separates today's machines from their predecessors it's their ability to respond to constantly changing conditions. This lifelike quality derives from electronic intelligence, usually a microprocessor, and an array of sensors that measure what the machine is trying to do in addition to any outside influences that might affect what the machine has to do next.

Unfortunately, one of the most important machine variables, torque, has also been one of the toughest to measure. For lack of a practical sensor, some machines approximate torque from related variables, such as motor current, while other machines ignore it altogether.

This may change, however, as researchers learn more about the effect of mechanical torque on certain magnetic materials. Stress-sensitive magnetoelastic elements embedded in or fastened to shafts and other structural components may one day produce all the torque signals any machine will ever need.

Weighing the options

Producing and controlling rotary motion is one of the most common machine functions. Rotation – whether you're talking about a lathe, washing machine, or a steering system on a car – is usually defined in terms of speed and torque. While today's technology is more than adequate for measuring rotary speed, the same hasn't been true for torque.

Commercial torque sensors fall into one of several categories based on the nature of the interaction between the shaft and sensor. The most common variety, strain gages, measure the local strain on a shaft due to applied torque. Besides being expensive, strain gages require an electrical or telemetric connection to the rotating shaft, and frequent correction for environmental variables.

Another group of detectors, elastic sensors, key on the mechanical twisting that results from torque. Here, a dedicated sensing shaft, or "torsion bar," accumulates an angular displacement between opposite ends. This displacement may be measured optically or by using a pair of magnetic encoders or resolvers. The compliance of the bar, its length, and disproportionately large radii at the ends present functional and packaging problems in most applications.

Surface acoustic wave (SAW) methods are also used to measure torque. SAW sensing systems look for strain-induced changes in the propagation speed of ultrasonic sound within the shaft. The catch is getting the vibration energy into the shaft. It requires putting transducers on a rotating component, and energizing them using radio waves. Making the transducers is also a challenge, as they require extremely tight manufacturing tolerances.

Some torque sensors, early magnetostrictive types, work by measuring stress-induced changes in the magnetic permeability of a special alloy, either attached to or comprising the shaft. This method requires two coils; one to induce a magnetic field in the active area and another to pick it up. Drawbacks arise primarily from the potential for inaccuracies caused by the many other variables that influence permeability, as well as substantial power consumption.

Attractive alternative

Another torque-sensing method that harnesses magnetic interactions is the magnetoelastic type. The key element here is a ferromagnetic ring or shaft section that's circumferentially magnetized. Normally, under no load, the magnetic flux remains inside the ring and, as a result, if you measured the external field, you'd get a reading of zero.

Torsional stress, however, realigns the magnetic domains in the ring, forcing the flux outside. This produces an external field whose polarity and strength correspond to the direction and magnitude of the torque applied to the shaft.

Perhaps the best way to understand how magnetoelastic sensing works is to start with the magnetic sensing ring itself. The ring is made from a highly permeable, magnetostrictive material formed to fit the host shaft. In its unconditioned state, it's magnetically isotropic, meaning the magnetic properties are the same in any direction because individual magnetic domains, or moments, are randomly aligned.

In order to work, the ring must be pressed onto a tapered shaft made from a low-permeability material. The resulting strain induces an circumferential "hoop" stress that clamps the ring to the shaft, while creating a preferred axis of magnetization. Because the magnetic domains may point in either circular direction, the net magnetization in the sensing ring is still zero.

Once the ring is firmly in place, it can be magnetized in a preferred direction, along the circumference, by spinning the shaft-ring assembly in a strong field. Another way to magnetize the ring would be to launch a highcurrent pulse down the shaft. Depending on shaft dimensions, a 1,000-A pulse held for about 10 msec should be more than sufficient to align the magnetic domains.

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