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How to reduce the influence of quadrature error by using industrial grade IMU
While the MEMS Inertial Measurement Unit (IMU) plays a key role in personal vehicle platforms, especially in self-balancing systems, it’s important to recognize that achieving perfect alignment across all sensors is a tall order, even in consumer-grade designs. There's a common misconception that integrating all core sensor components onto a single chip can eliminate all alignment errors, but this isn't typically realistic or expected.
For industrial-grade IMUs, which are designed for robustness and precision, the level of alignment accuracy surpasses that of consumer-grade counterparts. These industrial IMUs, like the ADIS16490, use advanced packaging and optimization techniques alongside factory calibration to ensure superior performance. Consumer and industrial IMUs often describe their alignment characteristics differently. Consumer IMUs tend to group all alignment errors into a single cross-axis sensitivity (CAS) specification, whereas industrial IMUs, such as the ADIS16490, break this down further using two distinct metrics: axis-to-axis alignment error and axis-to-package alignment error.
Axis-to-package alignment error measures how well the axes align with the physical structure of the IMU package itself, while axis-to-axis alignment error assesses how accurately the individual axes of accelerometers and gyroscopes are orthogonal to each other. The latter is often referred to as quadrature error.
Mathematically, these specifications are interconnected:
\[ \text{CAS} = \sin(\text{A2A\_MAE}) \]
\[ \text{A2A\_MAE} = \arcsin(\text{CAS}) \]
Misalignments can occur at various stages, including during sensor fabrication or when the sensor is mounted in its housing. While industrial-grade IMUs are meticulously factory-calibrated and provide detailed specs in their datasheets, consumer-grade devices don’t account for these potential assembly discrepancies as comprehensively.
It's worth noting that even though many inertial sensors in consumer-grade devices might be fabricated on a single chip, this doesn’t guarantee perfect orthogonality. Manufacturing variances can introduce significant quadrature errors, making industrial-grade IMUs far superior in terms of alignment accuracy.
For instance, consumer IMUs often exhibit cross-axis sensitivity in the range of 1% to 5%, translating to an equivalent axis-to-axis alignment error of around 0.57° to 2.87°. Industrial-grade IMUs, however, typically offer far greater precision, with some models boasting alignment errors as low as 0.018°, which equates to a CAS of merely 0.031%.
Take the ADIS16489 as an example. Despite its components not being confined to a single chip, it outperforms the best consumer-grade devices by a factor of about 32. Quadrature errors, while stable, can be mitigated through calibration. However, this requires precise conditions and specialized equipment, something that isn't feasible for every consumer-grade MEMS device once mounted on a PCB.
Industrial-grade IMUs undergo rigorous factory calibration post-mounting, addressing not only the intrinsic errors of the MEMS device but also those caused by assembly deviations. This results in significantly reduced errors related to cross-axis sensitivity, temperature, and assembly. The ADIS16489, for example, boasts a gyroscope axis-to-axis alignment error of ±0.018° and an accelerometer axis-to-axis error of ±0.035°. Additionally, its parylene coating provides protection against environmental factors, enhancing reliability.
In conclusion, while consumer-grade IMUs may seem convenient due to their integration on a single chip, industrial-grade solutions remain indispensable for applications requiring high precision, such as navigation, robotics, and platform stabilization.