In order to build a tilt sensor having a desired sensitivity and measuring range, one should select an appropriate type, orientation, and initial position of an accelerometer. Various cases of tilt measurements are considered: determining exclusively pitch, axial tilt, or both pitch and roll, where Cartesian components of the gravity acceleration are measured by means of low-g uni-, bi-, tri-, or multiaxial micromachined accelerometers. 15 different orientations of such accelerometers are distinguished (each illustrated with respective graphics) and related to the relevant mathematical formulas. Results of the performed experimental study revealed inherent misalignments of the sensitive axes of micromachined accelerometers as large as 1°. Some of the proposed orientations make it possible to avoid a necessity of using the most misaligned pairs of the sensitive axes; some increase the accuracy of tilt measurements by activating all the sensitive axes or reducing the effects of anisotropic properties of micromachined triaxial accelerometers; other orientations make it possible to reduce a necessary number of the sensitive axes at full measurement range. An increase of accuracy while using multiaxial accelerometers is discussed. Practical guidelines for an optimal selection of a particular micromachined accelerometer for a specific case of tilt measurement are provided.
Natural aging of commercial triaxial low-g MEMS accelerometers, manufactured by surface micromachining, was evaluated in terms of changes of their offset voltages and scale factors, assigned to each sensitive axis. Two pieces of two models of triaxial accelerometers (ADXL 330 and ADXL 327 by Analog Devices Inc.) with analog outputs were tested within a period of ca. 4.5 years. Two different computer-controlled test rigs were used for performing relevant experimental studies, employing tilt angles as the reference source. Methodology of determining the proposed indicators of aging was based on cyclic repetition of the calibration procedure for each accelerometer. Changes of the output signals of the tested accelerometers were observed, resulting in respective indication errors of ca. 0.8% or even 2.2% while related to determining tilt. Since the accelerometers were operated under mild conditions while tested, much bigger errors are to be expected in the case of harsh conditions. Both pieces of ADXL 330 accelerometers ceased to operate properly within the testing period, approximately at the same time, for no apparent reason; thus, it is recommended to introduce redundancy in relevant reliable measuring circuits by doubling the number of the applied accelerometers.
The paper addresses the problem of experimental studies of miniature tilt sensors based on low-range accelerometers belonging to Microelectromechanical Systems (MEMS). A custom computer controlled test rig is proposed, whose kinematics allows an arbitrary tilt angle to be applied (i.e. its two components: pitch and roll over the full angular range). The related geometrical relationships are presented along with the respective uncertainties resulting from their application. Metrological features of the test rig are carefully evaluated and briefly discussed. Accuracy of the test rig is expressed in terms of the respective uncertainties, as recommended by ISO; its scope of application as well as the related limitations are indicated. Even though the test rig is mostly composed of standard devices, like rotation stages and incremental angle encoder, its performance can be compared with specialized certified machines that are very expensive. Exemplary results of experimental studies of MEMS accelerometers realized by means of the test rig are presented and briefly discussed. Few ways of improving performance of the test rig are proposed.
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