By: Alex Borochin, FAE, Future Electronics
Accelerometers, gyroscopes and magnetometers implemented as Micro Electro-Mechanical Systems (MEMS) have transformed the operation and the user interface of smartphones and tablets. Because of these nano-scale devices, our everyday interactions with high-performance consumer electronics products feel more natural, convenient and productive.
In fact, MEMS production is now big business: STMicroelectronics, for instance, announced in January 2013 that it had shipped its 3 billionth MEMS device. Paradoxically, however, MEMS technology is found today in a remarkably small number of end-products.
Yet it could be used much more widely. And wider adoption does not need to wait for MEMS technology to become more advanced, or for new MEMS device types to be invented by chip manufacturers, although both will happen in time. In the near future, at least, much of the chip industry’s development effort will be devoted to meeting the requirements of existing users: smartphone and tablet manufacturers.
In fact, however, today’s MEMS technology is already potentially useful to manufacturers of other types of end-products: it simply needs the imagination and creativity of OEM design engineers to uncover its potential for use in devices other than mobile phones and tablets.
To help this effort, this article provides an explanation of the capabilities of the latest MEMS devices. Equipped with an understanding of the functions that MEMS can support, design engineers can then set their imaginations free.
Where MEMS technology wins out
The attraction of MEMS technology is a combination of attributes: the devices are small, robust, and can be manufactured cheaply in high volume for the same reasons that silicon ICs can be manufactured at low cost. Figure 1 shows the internal structure of a MEMS accelerometer at a high magnification. The method of fabricating MEMS sensors also allows for the integration of silicon signal-conditioning and signal processing circuitry in the same device.
The most obvious use for a MEMS sensor is as a replacement for a conventional electro-mechanical sensor. A good example of this is the microphone. Signal-to-Noise Ratio (SNR) is a crucial parameter of a microphone. A conventional Electret Condenser Microphone (ECM) can match the SNR of a MEMS microphone, but only in a package some 30 or more times bigger in volume. Less robust, the ECM is also unable to tolerate the reflow soldering process for board assembly, whereas a MEMS microphone can, thus dramatically reducing production cost. Finally, the fabrication process for making MEMS microphones produces devices with far less variation in frequency response from unit to unit than is found in ECMs.
For any designer using an ECM today, there are obvious reasons to consider replacing it with a MEMS microphone. In fact, the excellent SNR characteristics of MEMS microphones allow them to be used not only in consumer but also industrial applications, where noise suppression is often required and the source of the sound might be some distance from the microphone. Here, digital circuitry providing acoustic-enhancement functions could be integrated into a MEMS device. Examples of applications suitable for such a device include voice and video conferencing equipment and alarm systems.
The other main types of MEMS sensors available today are accelerometers, gyroscopes and magnetometers or digital compasses. MEMS accelerometers are now replacing conventional sensors in the anti-tamper and anti-vandalism protection functions of utility meters and security panels. Mechanical switches are bulky, and inappropriate for today’s miniaturised designs. In addition, when dormant for a long time, mechanical switches are prone to oxidization and corrosion, rendering them inoperable. A MEMS accelerometer is immune to such damage. An accelerometer can measure a sample of each of the three axes and then compare it to internal threshold values. When the threshold is passed, an interrupt can be sent to the host MCU, raising a tamper alert.
In fact, any conventional sensor using piezoelectric, potentiometric, Linear Variable Differential Transformer (LVDT) or variable-reluctance measurement technology could be a candidate for replacement by a smaller, more robust MEMS equivalent.
But the most powerful applications of MEMS technology draw on the more integrated devices recently introduced by leading manufacturers such as STMicroelectronics.
STMicroelectronics is combining different kinds of sensors, such as accelerometers with gyroscopes, together with digital processing circuitry, to provide smart devices which can easily be interfaced to a microcontroller or applications processor. This is accomplished either through integrating functions in a single package, or by combining multiple packages in a module.
Of course, the separate outputs of a discrete accelerometer and gyroscope may be combined by the system developer. This is accomplished far more easily, however, if the two devices are integrated. This is the purpose of the LSM330 from STMicroelectronics, a six Degrees-of-Freedom (DoF) motion-sensor system-in-package. It combines a three-axis accelerometer and a 3D (yaw, pitch and roll) gyroscope.
The device features state machines developed by STMicroelectronics which implement many of the signal-processing functions that an OEM might need. These include:
- Free fall
- 4D/6D orientation
- Pulse counter and step recognition
- Face up/face down
The sensor, in a 3mm x 5.5mm package, provides these measurement outputs via an I2C/SPI port. Here, integration of multiple MEMS sensors with CMOS circuitry and digital IP provides for the quick and easy implementation of advanced motion-sensing functions in a system design.
The iNEMO-M1 module from STMicroelectronics provides a further level of integration, as shown in Figure 2. This combines an L3GD20 threeaxis digital gyroscope, an LSM303DLHC six-axis geomagnetic module (a magnetometer combined with a three-axis accelerometer) and an STM32F103REy ARM® Cortex®-M3 microcontroller providing a digital output via I2C. The result is a 9 DoF inertial system in a 13mm x 13mm module.
Complex end products using these integrated devices will be running an operating system. STMicroelectronics provides native support for Windows 8 through sensor device drivers, a sensor API and Human Interface Device (HID) sensor-class drivers, as well as a Windows 8-based platform. The software supplied by STMicroelectronics supports the HID protocol over I2C or USB.
One of the additional advantages of the integration of MEMS sensors, and other sensors, is reduced power consumption. This is because an internal ADC, serial bus and other resources can be shared, rather than providing them separately for each sensor functional block.
Using the capabilities of integrated MEMS sensors
The development of MEMS sensing devices is very largely a response to demand from smartphone and tablet manufacturers. But examples of new uses of motion sensing and dead-reckoning are starting to emerge, and they show how the designer’s creativity can be unleashed by these new devices.
Indoor navigation is one big opportunity. The Museum of Contemporary Art in Taipei, Taiwan has developed an indoor navigation system to run on mobile devices, for guiding visitors around the building. Dead-reckoning provides accurate co-ordinates of the user’s location, without any requirement for satellite (GPS) location data. The system displays to the visitor relevant information in real time about the artwork in front of them.
In-building pedestrian navigation may be enhanced with the addition of a MEMS pressure sensor, to supplement dead-reckoning. An example of a potential use of this technology is for patient or equipment location in hospitals. An atmospheric pressure sensor can accurately measure altitude, using the calculation:
P = the air pressure in mbars
P0 = the standard atmospheric pressure at sea level (1013.25 mbar)
Altitude = the height above sea level in meters
But every OEM will have its own ideas about how precise location- and altitude-tracking can be used in its own end-products. The remarkable fact about MEMS technology is that integration of sensing elements with digital processing circuitry has enabled a huge extension in functional capabilities combined with easy integration with a host MCU. Design teams can therefore add value and functionality to end products in new, innovative ways with relatively little additional design effort or time, by using one of this new generation of highly integrated systems-in-package or modules from MEMS device manufacturers.