Column: Design problem solvers


Providing repeatable tilt measurement under all conditions


By Chris Murphy, Applications Engineer, Analog Devices


My consumer-grade accelerometer can


theoretically measure < 1° of tilt. Will this still be possible over temperature changes and in the presence of vibration?


Answer:


Most likely the answer is no. Questions around definitive tilt accuracy values are always difficult to answer, since many environmental factors need to be accounted for when it comes to MEMS sensor performance. Typically, consumer-grade accelerometers struggle to detect less than 1° of tilt in dynamic environments. To show this, a general-purpose consumer-grade accelerometer is compared with a next-generation, low-noise, low-drift and low-power MEMS accelerometer. The comparison looks at a number of error sources present in tilt applications and what errors can be compensated for or removed. Errors such as zero-g bias accuracy, zero-g bias


shift due to soldering, zero-g bias shift due to PCB enclosure alignment, zero-g bias tempco, sensitivity accuracy and tempco, nonlinearity and cross-axis sensitivity are observable and can be reduced by the post-assembly calibration processes. Other error terms such as hysteresis, zero-g bias shift over life, sensitivity shift over life, zero-g shift due to humidity and PCB bend and twist due to temperature variations over time can’t be addressed in calibration, or else they require some level of in-situ servicing to be reduced. For this comparison it is assumed that cross-axis sensitivity, nonlinearity and sensitivity are compensated for, since they require much less effort to minimise compared to tempco offset drift and vibration rectification. Table 1 shows an estimate of the consumer-grade ADXL345 accelerometer’s ideal-performance specifica-


Table 1: ADXL345 error source estimates 16 December/January 2021 www.electronicsworld.com


Question:


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