January 4, 2012 — Recent advances in micro electro mechanical system (MEMS) sensor technology and manufacturing have enabled high-performance, small, low-cost sensors. These attributes encourage integration into handheld devices, including smart phones and tablets. Features such as interrupts and first-in/first-out (FIFO) functions have been integrated into MEMS sensors. The new trend is to integrate multiple sensors and a microcontroller in a tiny single package with embedded algorithms. Some of the smart features built into digital MEMS inertial sensors available on the market today are explained here, with an overview of future trends of sensor integration.
Each MEMS sensor comprises the MEMS sensing structure, an application-specific integrated circuit (ASIC), and device package. The sensing structure is responsible for detecting capacitance or resistance change when the proof mass moves from the center position due to external motion or applied force [1]. The ASIC consists of a charge amplifier to convert the output of the mechanical sensing part into an analog output voltage that can be digitized through an A/D convertor and presented in a digital format. The package, in addition to housing the sensing and processing die, influences device performance, defining stability over temperature and time.
Figure 1 shows the typical internal structure of a MEMS accelerometer and a gyroscope as an example based on the capacitive principle technology.
 |
| Figure 1. Structure inside a MEMS accelerometer and gyroscope. |
The host processors in smart systems, e.g., smart phones and tablets, have limited resources for sensor data acquisition and processing. Therefore, MEMS sensors need to include more computing power and embedded features to reduce the load of the host processors.
Self-test. Most MEMS sensors have built-in self-test (BIST). The self-test can be used to verify if the sensor is functioning or not after PCB assembly. This functional test (FT) doesn’t require physically tilting or rotating the PCB for inertial sensors.
Figure 2 shows an example self-test procedure for accelerometers and gyroscopes. The sensor data acquisition when self-test is enabled and disabled should be performed at the same arbitrary and stationary position.
 |
| Figure 2. Self test procedure for digital accelerometer and gyroscope. |
Interrupt feature. Most MEMS sensors have one or two interrupt output pins available for connecting to the GPIO ports of the host processor. The host processor is not required to keep acquiring sensor data to determine the device’s current status; the sensor is running in the background. When the predefined criteria are met, the sensor will generate an interrupt signal on its output pin to notify the host processor. The host processor can then decide if this interrupt needs to be serviced or not.
FIFO feature. FIFO is another power-saving feature that can be implemented in ASICs. The host processor doesn’t need to acquire sensor data all the time. Instead, the sensor can collect data and store it into the FIFO in the background.
When the FIFO interrupts are generated, the host processor can wake up and read all FIFO data samples at once. Then the host processor can process the sensor data to see if further action needs to be taken.
Sensor integration trends
As some interrupt features embedded in an accelerometer cannot distinguish fake motion from the real one, the processor needs to acquire sensor data to determine the nature of the motion. Future smart sensors will have more advanced computing power such as finite state machine (FSM) for reliable interrupt generation.
A low-power microcontroller can be integrated into an inertial module unit to run the sensor fusion algorithms so that the final dynamic accurate pitch/roll/yaw angles can be available to the host processor directly [2].
With respect to the applications such as 3D gaming, indoor pedestrian dead reckoning, etc., 9- or 10-axis sensors are required. In the future, such MEMS sensors and the programmable microcontroller will be combined into a single package as shown in Fig. 3. The wireless link and some other sensors may be integrated in the same package too.
 |
| Figure 3. Multiple sensors integrated in one package. |
Conclusion
Embedded features and computing power are required for future sensors and embedded features and sensor integration will determine the future applications of MEMS sensors. A dedicated microcontroller is needed to handle the complex algorithms of sensor fusion.
Driven by MEMS technology and market needs, the multiple sensors with lower power consumption and low-cost microcontroller in one package will appear soon.
References:
1. J. Esfandyari et al., Introduction to MEMS gyroscopes, November 2010, http://www.electroiq.com/articles/stm/2010/11/introduction-to-mems-gyroscopes.html.
2. J. Esfandyari et al., “Solutions for MEMS sensor fusion,” Solid State Technology, Volume 54, Issue 7, July 2011, http://www.electroiq.com/articles/sst/print/volume-54/issue-7/features/cover-article/solutions-for-mems-sensor-fusion.html.
Jay Esfandyari received his Master’s Degree and Ph.D. in EE from the University of Technology in Vienna and is MEMS Product Marketing Manager at STMicroelectronics, 750 Canyon Dr., Coppell, TX, 75019 USA; ph.: 972-971-4969; [email protected].
Fabio Pasolini received his Engineering Degree at the University of Pavia, Italy, in 1994 and is the General Manager of the Motion MEMS at STMicroelectronics.
Gang Xu received his Ph. D from Shanghai Jiao Tong University and is Senior Application Engineer at STMicroelectronics.