Design News for Mechanical and Design Engineers

The auto industry is a key example. Several times a day auditors at car plants must check the torque on safety-related fasteners, such as lug nuts, tie rod bolts and seat belt anchors. Their concerns are two-fold: They must ensure that the power tool fastening the joint is behaving correctly and they must guard against shipping hazardous products.

In many cases, technicians still measure residual torque by slowly applying torque to the fastener, releasing the applied torque as soon as the fastener moves and capturing the peak torque during the event. However, this method can produce false high readings if there is excessive overshoot. In other cases, false low readings can occur if the operator mistakes a bump or vibration in the work piece for fastener motion and removes the wrench before fastener motion. These false out-of-spec readings can be expensive, since they trigger unnecessary product quarantines and root-cause investigations.  

A New Standard in Accuracy

To eliminate these problems, Minnesota-based ASI DataMyte has introduced a new line of torque wrenches called LightStar, which features the ADIS16250 iSensor intelligent sensor gyroscope from Analog Devices. The torque sensor, along with a strain-gauge bridge, is mounted directly on the turning axis, providing increased accuracy and making the wrench insensitive to point of load. The wrench also features an LED indicator to notify the operator of good (green) or bad (red) torque readings.

“Since the gyroscope allows the residual torque measurement to be captured based on actual fastener motion, the overshoot problem is eliminated, as are false low readings,” says Frank Skog, product manager for ASI DataMyte. He adds that capturing torque based on fastener motion rather than on human observation also reduces measurement system variability.

One of the challenges designers faced with this new product was the limited room for adding electronics to the torque wrench.  Many of the fasteners that are audited have limited accessibility, so the wrench size must be kept as small as possible. That’s where the high level of integration in the ADIS16250 sensor made a clear difference.

“In one chip, the gyroscope provides angle output, an auxiliary ADC, an on-board reference, and programmable alarm functions,” points out Bob Scannell, iSensors Business Development Manager at ADI.

ASI DataMyte engineers also needed to mechanically integrate the ADIS16250 into the torque wrench assembly so that it could withstand severe handling in industrial environments. “ADI assembled some prototypes for us and performed some additional qualification tests on our end product to help ensure its survivability,” notes Jim Schultz, senior hardware engineer for ASI DataMyte. “This helped in meeting some severe time to market constraints.”

Roots of Invention

The ADIS16250 was specifically designed to make MEMs gyro sensors more accessible and simpler to use for the myriad industrial/instrumentation applications that can benefit from motion sensing. Many MEMs sensors are not factory-calibrated, requiring motion testing at the customer’s site to achieve optimal accuracy. Besides inherent bias or sensitivity variances, such sensors may also be sensitive to voltage, temperature, assembly, and other influences.

“A typical industrial customer, which is perhaps working with MEMs sensors for the first time, wants to avoid high test/calibration set-up costs, as well as the time and risks associated with a long design prototyping,” explains ADI’s Scannell. “Our iSensor intelligent sensor products are designed with this specific target need in mind.”

The ADI family of intelligent sensor gyroscopes provides angular rate and temperature sensing via a simple programmable SPI interface. The core gyro sensor is factory calibrated, delivering an in-run bias stability of 0.016 o/sec. According to ADI, these are the industry’s first devices to provide digital range scaling, with options covering +/-20o/sec to /-320 o/sec, allowing selection of the best device sensitivity for the target application.  In addition, the ADIS16255 sensor includes temperature calibration, improving in-system stability to as tight as 0.005 o/sec/oC. 

Engineers can program all critical parameters over the SPI port, allowing in-system tuning of digital filtering, rate/threshold alarms, sample rate, and power modes. ADI has further reduced design complexity and implementation time with single supply operation, auxiliary I/O ports, and a digital self-test feature. These components are available in 11 mm x 11 mm x 5.5 mm laminate-based land grid array packages.

		The ADIS16250 was specifically designed to make MEMs gyro sensors more accessible and simpler to use for the many industrial/instrumentation applications that can benefit from motion sensing.
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