Selection and integration of MEMS-based motion processing devices

Both gyro and accelerometer are needed
To meet the end-user's functional expectations, three axes of rotation and three axes of linear motion are required. There is a common misperception that engineers must select either a gyroscope or an accelerometer to add motion-processing functionality within their handheld systems. Indeed, industry analysts have posed the question, "which motion sensor will win the race?"

The reality is that accurately interpreting both linear and rotational motion requires designers to include both the gyroscope and the accelerometer. Pure gyroscopic solutions can be used for rotation detection with high resolution and quick response. Pure accelerometer-based solutions can be used for applications with fixed reference from gravity as well as linear or tilt movement, constrained to limited rotation. But simultaneous processing by the motion processing solution of linear movement and rotation requires both gyroscopes and accelerometers.

When tracking pitch and roll, accelerometers provide a better measurement when the device is not moving, yet MEMS gyros provide a higher accuracy when the device is in motion. Sensor fusion algorithms are typically used to combine accelerometer and gyroscope data as described in Figure 2. This allows accurate rotational measurements with a wide bandwidth.

The selection of the proper motion processing solution involves careful analysis of many factors, including the device's full scale range, sensitivity, offset performance, noise, cross-axis sensitivity and the effects of temperature, humidity and mechanical g-shock on the product. The next section of this paper focuses on four key factors to consider when making these sensors work together within a CE application.

Motion processing design considerations
Engineers adopting six-axis motion-processing functionality within their CE applications are faced with the choice of either assembling gyroscopes and accelerometers from multiple sources, or selecting a fully integrated solution from a vertically integrated motion-processing supplier. While there are merits and challenges to each approach, the motion processing product selection should consider the following interoperability points:

1. To maximize the value of motion processing functionality, the design may include multiple applications, such as GPS navigation heading assist, mobile gaming and a motion-based user interface. Each of these applications requires different gyroscope data sampling rates and anti-aliasing measures must be employed to ensure motion data accuracy through the use of low pass filtering (LPF) that is specific to the particular application.
2. Accurate timing data is essential to determine the gyroscope's angular data calculations through mathematical integration.
3. Synchronous sampling of the accelerometer and gyroscope data will ensure high quality position coordinate information.
4. The drive, sense and harmonic frequencies of gyroscopes should be designed not to interfere with each other or any other frequencies within the system.

Gyroscopes and accelerometers are typically shown in product selector guides based on their core specifications, yet it is useful to associate sensor specifications with their typical applications. The Table shown below describes applications for analog gyroscopes with their full scale range expressed in degrees-per-second (dps) and the corresponding sensitivity shown in millivolt-per-dps. Digital accelerometers are commonly used in motion processing solutions, describing their full scale range in a g-rating and sensitivity ratings in least-significant-bit (LSB) per g.