Accelerometers — devices that measure change in velocity — are built into automobiles, airplanes, cell phones, pacemakers, and scores of other products. They warn of potentially destructive vibration in industrial equipment, buildings, and bridges; register seismic shocks; and guide missiles to their targets.
Increasingly, they are miniaturized using microelectromechanical systems (MEMS) technologies with component dimensions on the order of micrometers, and simultaneously register acceleration in all three axes of three-dimensional space. Because errors are additive when calculating velocity from acceleration, even minor errors in output can have very serious consequences.
Yet when three-axis sensitivities and cross-axis sensitivities of a digital three-axis* device are tested at different calibration laboratories, the measurements can vary substantially depending on factors that can be difficult to determine, but often arise from errors with alignment of the test equipment, the internal alignment of the accelerometers in the device, or both.
Now NIST scientists have devised a methodology designed to reduce or eliminate those differences by characterizing intrinsic properties of an accelerometer – those that are unique to it irrespective of the way it is mounted or tested -- thus making possible accurate interlaboratory comparisons.
“Determination of intrinsic properties is part of NIST’s larger effort to help industry develop standard testing protocols for the new MEMS-based device technologies, which do not exist at present,” says Michael Gaitan of NIST’s Physical Measurement Laboratory, which is working in partnership with the MEMS and Sensors Industry Group (MSIG) and the Institute of Electrical and Electronics Engineers. “Testing was reported by MSIG to be as much as half the cost of manufacturing for these sorts of devices. Manufacturers can’t reduce the cost of physical fabrication very much. But they can find savings in the way they package, test, and calibrate the devices.”
When MEMS-based, three-axis accelerometers are tested, they are typically mounted on a gimbal system and rotated about three axes -- x, y, and z -- with measurements taken in different orientations. The measurements are formatted in a three-by-three grid, called a “cross-sensitivity matrix,” used by manufacturers to evaluate device performance. It specifies the relation between the acceleration response along the gimbal axes to the response along the axes of the device under test (DUT).
That process, however, assumes that the DUT’s three axes are perfectly orthogonal – at right angles to each other – and that the device has been mounted in perfect alignment with the gimbal axes, which are themselves perfectly aligned. And in the case of testing accelerometer packages after they have been integrated into products, such as smart phones, it assumes that the package was installed in exact alignment with the axes of the phone case. But none of those conditions is guaranteed, and slight deviations in any of the variables can explain why measurements of the same test unit made at different laboratories produce different values.
“So instead of using the cross-sensitivity matrix alone,” Gaitan says, “we’re defining the device as having intrinsic properties in which the axes of the device are not assumed to be completely orthogonal. There might be some variation in their alignment.”
In NIST’s measurement protocol, the DUT is mounted on the position and rate table which very accurately rotates the device in specific gradations through 360 degrees on each of the gimbal’s three axes while measuring the device response at each interval. The protocol reveals the DUT’s internal axis alignment, the magnitude of response of each axis in different orientations, and its “signal offset” – the constant amount by which measured readings differ from the “true” value.
With that information, a central standards laboratory such as NIST could fully characterize the intrinsic properties of one or more DUTs and distribute the devices to other labs, which would use them to compare results and determine, for example, whether readings were skewed because of instrument-related measurement errors.
Earlier this year, NIST acquired a new position and rate table large enough to permit measurements on entire products that have accelerometers installed. “Our initial gimbal system was a smaller instrument that was useful for making static measurements,” Gaitan says.
“But now we can make dynamic measurements on objects as large as a cell phone. We can set it to steady-state rotation like a record player, and we can accelerate the rotation rate. That will enable us to make measurements above the 1g acceleration of gravity and measure acceleration by rotation.”
* Although it is called a “three-axis accelerometer,” the device in fact contains three separate accelerometers, each of which measures velocity change along one axis. Those signals are merged to register movement in three dimensions.