Multi-axis sensors are in high demand, with MEMS accelerometers making inroads into medical design.

2020-11-06

Multi-axis inertial sensors have become increasingly popular in the medical applications market. Inertial MEMS components integrating multi-axis sensing capabilities demonstrate excellent performance in size, power consumption, accuracy, and reliability, meeting the stringent requirements of medical applications. They are now being widely adopted in precision medical instruments such as surgical navigation tools.

　　Navigation is relevant to cars, trucks, airplanes, ships, and people. However, it is also beginning to play a significant role in medical technology, with precision surgical instruments and robots requiring navigation. The design requirements of surgical navigation tools share many commonalities with traditional vehicle navigation, but the former also presents unique challenges, such as indoor use, where Global Positioning System (GPS) support is unavailable, thus requiring higher performance.

　Multi-axis MEMS sensors transform medical information

Microelectromechanical systems (MEMS) have become a mature technology encountered by most people daily. They make cars safer, enhance mobile phone usability, and optimize the performance of tools and athletic equipment, thereby improving the level of medical care for patients.

MEMS components used for linear motion detection are typically based on a micromachined polysilicon surface structure formed on a silicon wafer. This structure is suspended above the wafer surface by polysilicon springs, providing resistance to acceleration forces. Under acceleration, the deflection of the MEMS axis is measured by a differential capacitor consisting of a stationary fixed plate and a movable quality-connected plate. Movement thus unbalances the differential capacitor, resulting in a sensor output amplitude proportional to acceleration.

For example, when a car suddenly decelerates sharply due to a collision, the MEMS axis in the airbag sensor undergoes the same motion, causing capacitor imbalance and ultimately generating a signal to trigger the airbag deployment. This basic accelerometer structure, adjusted for different application performance parameters and with added data processing capabilities, can indicate tilt, velocity, or even position. A technologically related structure is the gyroscope, which detects rotational rate, outputting in degrees/second.

The ability to detect and measure motion through an extremely low-power micro-component is valuable for almost any application involving movement. Table 1 lists basic medical applications by motion type.

　While simple motion detection, such as linear motion on a single axis, is valuable, most applications involve multiple types of motion on multiple axes. Capturing this multi-dimensional motion state not only brings new benefits but also maintains accuracy in situations where off-axis disturbances might affect single-axis motion measurements.

　　To measure the motion experienced by an object, various types of sensors (such as linear and rotational) must be combined. For example, accelerometers are sensitive to the Earth's gravity and can be used to determine tilt angle. In other words, when a MEMS accelerometer is rotated in a ±1g gravitational field (±90 degrees), it can convert that motion into an angular representation.

　　However, accelerometers cannot distinguish between static acceleration (gravity) and dynamic acceleration. Therefore, accelerometers can be combined with gyroscopes. Using the additional data processing capabilities of the combined components, linear acceleration and tilt can be distinguished (i.e., when the gyroscope's output shows that rotation coincides with the significant tilt recorded by the accelerometer). As the dynamic nature of the system (number of axes of motion and degrees of freedom) increases, the sensor fusion process becomes more complex.

　　Understanding the environmental impact on sensor accuracy is also important. One obvious factor is temperature, which can be calibrated. In fact, high-precision sensors can be recalibrated and perform dynamic compensation themselves. Another less obvious consideration is potential vibration; even slight vibrations can offset the accuracy of rotational rate sensors. This effect is called linear acceleration effect and vibration correction, and its impact can be significant depending on the quality of the gyroscope. In this case, sensor fusion can also improve performance, using the accelerometer to detect linear acceleration and then using this information and the calibration information of the gyroscope's linear acceleration sensitivity for correction.

CCID Consulting | 2019 Sensor Market Data (Forward)

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