Magnetic Method For Measuring Torque

If you are monitoring changes in spindle torque as an indicator of tool wear or breakage, you will be interested in a new line of sensors that uses Embedded Magnetic Domain (EMD) technology—a combination of magnetic induction and electronic processing techniques—to provide low-cost, non-contact measurement of torque and angle on any nickel-bearing ferrous shaft. The technology has proven successful in industrial power tool applications such as pneumatic and electric torque wrenches, nut runners and screwdrivers, and the developer is busily expanding into machine tool applications as well.

Here’s how EMD works. First, the nickel-bearing ferrous shaft is permanently magnetized. The magnetic field generated in the shaft is highly sensitive to and varies directly with changes in the amount of torque applied. A non-contact sensor, which can be positioned as much as 10 mm from the shaft, measures the variation and produces an output signal proportional to the applied torque.

The developer, FAST Technology (Livonia, Michigan), offers standard torque sensors in two versions: the TM-HR-Sq series with square drive shafts (male/female) for testing, calibration and verification applications; and the TM-HR-Rd series with round shafts (male/male) equipped with keyways for more rigorous applications, such as integration into machine tools. Both versions are available in ratings up to 110.6 foot-pounds, with 221, 369 and 737 foot-pound versions in the works. The firm is also willing to develop custom sensors for OEM applications.

According to the developer, the EMD sensors are smaller, lighter and less costly than competing products. Because there is no contact with the rotating shaft, integration of the sensor into existing designs is simplified. The sensors provide torque measurements at a high sampling rate with both high resolution and accuracy. They are also robust enough to withstand hostile environments that frequently discourage the use of such instruments. The EMD sensors are unaffected by dirt, fluids and similar materials found in a manufacturing environment, and they can withstand working temperatures from -40ºF to 132ºF.

FAST Technology considers machine tools a major potential market for its sensors. “Hundreds of thousands of multiple-spindle machines in the field can be significantly improved with the addition of our sensors,” explains Rhex Edwards, marketing manager for the firm. “We can insert our torque sensor between the spindle head and toolholder of a machine tool and monitor the torque resulting from, say, a drilling or tapping operation. We can then compare the measurement to an ideal profile to determine when it is time to index the insert or change the tool.

“Our round-shaft sensors can be easily integrated into existing machines so that customers can upgrade their machines quickly and inexpensively,” Mr. Edwards continues. “The accuracy with which they can measure torque is significantly improved over what they previously experienced. Therefore they can respond faster to changes in the process that affect part quality.”

While EMD sensors are relatively new, firms interested in adopting the technology need not feel like pioneers. Mr. Edwards notes that his firm has been working closely with a major metalworking customer, making improvements to its sensors suggested by actual production machining situations.