Column: Design problem solvers
range, this may not even be possible. From 25°C to 85°C, the tempco offset drift is 1.375° – already exceeding the requirement for less than 1° of tilt accuracy:
Table 2: ADXL355 error source estimates
tions and corresponding tilt errors. When trying to achieve the best possible tilt accuracy, it is imperative to apply some form of temperature stabilisation or compensation. For this example, a constant temperature of 25°C is assumed. The largest error contributors that can’t be fully compensated out are offset over temperature, bias drift and noise. Bandwidth can be lowered to reduce the noise, since inclination applications typically require bandwidths below 1kHz. Table 2 shows the same criteria for the ADXL355.
Short-term bias values were estimated from the root Allan variance plots in the ADXL355 data sheet. At 25°C, the compensated tilt accuracy is 0.1o ADXL345, compared with 0.005o
for the general-purpose for the industrial-grade
ADXL355. Comparing the ADXL345 and ADXL355, it can be seen that larger error contributors like noise have been reduced significantly from 0.05° to 0.0045°, and that bias drift has been reduced from 0.057° to 0.00057°. This shows the massive leap forward in MEMS capacitive accelerometer performance in terms of noise, temperature coefficient, offset and bias drift, enabling much higher levels of inclination accuracy under dynamic conditions. The importance of selecting a higher-grade accelerometer
is crucial in achieving required performance, especially if your application demands < 1° tilt accuracy. Application accuracy can vary depending on application conditions (large temperature fluctuations, vibration) and sensor selection (consumer grade vs. industrial or tactical grade). In this case, the ADXL345 will require extensive compensation and calibration effort to achieve < 1° tilt accuracy, adding to overall system effort and cost. Depending on the magnitude of vibrations in the end environment and the temperature
For the ADXL355 the tempco offset drift from 25°C to 85°C is:
Vibration rectification error (VRE), shown in Table 3, is the offset error introduced when accelerometers are exposed to broadband vibration. When an accelerometer is exposed to vibrations, VRE contributes significant error in tilt measurements when compared to 0g offset over temperature and noise contributions. This is one of the key reasons it is left off data sheets, as it can very easily overshadow other key specifications. In environments with higher magnitude vibrations, it is
essential to have a higher g range accelerometer in order to minimise clipping leading to offsets. Table 4 shows the ADXL35x family of accelerometers and their corresponding g range and bandwidths. Choosing a part from the ADXL35x family for tilt
applications will provide high stability and repeatability, and the family is robust to temperature fluctuation and broadband vibration, as well as requiring less compensation and calibration compared to lower cost accelerometers. The hermetic package helps ensure that the end product conforms to its repeatability and stability specifications long after it leaves the factory. By providing repeatable tilt measurement under all
conditions, Analog Devices’s next-generation accelerometers provide minimal tilt error without extensive calibration in harsh environments, as well as minimising the need for post-deployment calibration.
±2g range in a 1g orientation offset due to 2.5g rms vibration Table 3: Errors shown in degrees of tilt Table 4: ADXL354/355/356/357 measurement ranges
www.electronicsworld.com December/January 2021 17
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