Accelerometers are a key technology in wearable design. Almost any device can benefit from knowing its orientation, and adding an accelerometer provides a new way to implement complex user interfaces. If the system knows how it is moving, it can respond in different ways, from changing the screen to reflect position or orientation to supporting gesture control. The processing power that has been packaged in accelerometers to provide positional data is also used to provide sensor fusion hubs of such designs. This can save space, weight and power by allowing other sensors to be connected to the accelerometer without having to use processing cycles in the central controller or application processor.
All of these were pioneers in the mobile phone industry, but accelerometers are making their way into more designs from fitness systems to smartwatches. Interfaces are also changing, moving from analog interfaces to simple digital links to controllers that contain data from other sensors. Wearable accelerometers range from 2-axis and 3-axis devices to 6-axis, integrating temperature and magnetic sensors to provide more functionality in the design.
The MMA32 series of dual-axis (X and Y) silicon capacitive, micromachined accelerometers from Freescale Semiconductor feature signal conditioning, 4-pole low-pass filter and temperature compensation, and independent outputs for both axes. The zero-G offset full-scale span and filter cutoff are factory set and require no external equipment, making it easier to include in designs as no adjustments are required on the production line. Similarly, a complete system self-test capability verifies system functionality.
Freescale MMA32 dual-axis accelerometer schematic
Figure 1: The MMA32 dual-axis accelerometer has different sensitivities in the X and Y axes.
The accelerometer is a surface micromachined capacitive sensing cell (G-cell) and a CMOS signal conditioning ASIC contained in a single package that is sealed at the wafer level using a bulk micromachined "cap" wafer.
A G cell is a mechanical polysilicon structure that can be thought of as two fixed plates with a movable plate in between. The center plate can be deflected from its resting position by accelerating the system, and this change is captured as a change in the inter-plate capacitance. The CMOS ASIC uses a switched capacitor technique to measure the G-battery capacitor and extract acceleration data from the difference between the two capacitors. The ASIC also provides the signal and filter (switched capacitor) signals, providing a high level output voltage proportional to acceleration of different sensitivities of 100 grams on the X-axis and 50 grams on the Y-axis.
Freescale MMA32 dual-axis accelerometer image
Figure 2: Integrating the MMA32 dual-axis accelerometer into one design.
The 2-axis device can easily be connected as part of a wearable design to a 0.1 μF capacitor on the power line as close as possible to the central microcontroller to decouple the power supply. Ensuring that there is a ground plane under the accelerometer will help reduce noise and that this ground plane should be connected to all open terminals in the interface.
The 1 KΩ and 0.01 μF RC filter of the accelerometer output will help minimize the clock noise of the switched capacitor filter circuit. Also, it is important to ensure that the PCB layout for power and ground does not couple power supply noise, and that the accelerometer and microcontroller are not high current paths. Choosing the A/D sampling rate and the switching frequency of any external power supplies so that they do not interfere with the internal accelerometer sampling frequency will prevent aliasing errors that can give incorrect results from the sensor and give spurious responses from the system.
Accelerometers can also be used for navigation in advanced wearable designs, and the ADS16305 ISISOR, now from Invensys, offers a complete inertial system, including gyroscopes and three-axis accelerometers. Each sensor combines IMEMS technology with signal conditioning to optimize dynamic performance, and factory calibration characterizes each sensor's sensitivity, bias, alignment, and linear acceleration. As a result, each sensor has its own dynamic compensation formula that provides accurate sensor measurements under a variety of conditions.
The ADIS16305 provides a simple, cost-effective way to integrate accurate, multi-axis inertial sensing, greatly reducing system integration time because all necessary motion testing and calibration are part of the factory production process.
An improved SPI interface and register structure provide faster data collection and configuration control, and the ADIS16305 uses pins compatible with the ADIS1635X, ADIS1636X, and ADIS1640X families when used with the Interface Flex connector.
用户寄存器为SPI接口上的所有输入/输出操作提供寻址。每个16位寄存器都有两个7位地址：一个用于高位字节，一个用于下字节。虽然ADIS16305独立地产生数据，但它作为SPI从设备工作，它使用16位段作为主处理器与系统处理器通信。单个寄存器读取需要这16个位序列中的两个，其中第一个提供读命令位(R/W=0)和目标寄存器地址(A6到A0)。第二序列在数据输出(DUT)线上发送寄存器内容(D15到D0)。SPI在全双工模式下工作，这意味着主处理器可以使用DoT读取输出数据，同时使用相同的系统时钟脉冲来传输DIN上的下一个目标地址。
同时，来自意法半导体的A3G4250D是一种低功率的三轴角速率传感器，具有零速率级的高稳定性和对温度和时间的敏感度。传感元件与接口芯片相结合，该接口芯片通过标准SPI数字接口向外部世界提供测量的角速率，以简化与控制器的集成。一个I/C兼容接口也是可用的。该传感元件是利用意法半导体开发的专用微加工工艺制造硅晶片上的惯性传感器和致动器。A3G4250D的满刻度为±245 dPS，并且可以用用户可选带宽测量速率，以便仅使用应用所需的功率。
从意法电子学看A3G4250D三轴加速度计
图3：来自意法半导体的A3G4250D三轴加速度计的结构。
来自飞思卡尔的XTrFISH FXOS9000CQ是一个具有集成线性加速度计和磁强计的6轴传感器，可用于便携式导航设备到医疗监控设备的可穿戴设计。虽然该规范称为6轴，塑料封装将一个3轴直线加速度计和3轴磁力计与一个可选择的Ii C或点对点SPI串行接口相结合，具有14位加速度计和16位磁力仪ADC分辨率，以及其他来自数字的嵌入式函数。铝信号处理器
FXOS9000CQ具有动态可选择的加速度范围为±2克/±4克/±8克，固定磁测量范围为±1200μt。输出数据速率(ODR)范围为1.563 Hz至800 Hz，可由用户对每个传感器进行选择。交错的磁和加速度数据可在ODR率高达400赫兹。可编程的自动ODR改变还使用自动唤醒和返回休眠功能来节省功率，这既适用于磁性事件，也适用于加速事件中断源。
XXF型FXO7900CQ六轴加速度计简图
图4：XFrutoFXO7900CQ六轴加速度计和磁传感器可用于构建可穿戴系统的电子罗盘。
结论
当今许多加速度计提供了从加速度计到中央处理器的简单SPI或I C接口，使得对于可穿戴系统的设计者来说，集成是更简单的，无论使用2、3或6轴传感器。然而，需要注意的是传感器的放置。避免高电流路径，并确保采样率被选择以避免切换模式电源的抗混叠，并确保数据尽可能精确。这允许设计者为最新的可穿戴设备添加广泛的新的用户界面技术和定位功能。