This article encompasses type of MEMS and the new advancement. Further the article also takes wearable as case study to explain their architecture and application
By Vishal Goyal
Micro Electro Mechanical System [MEMS] are radically different from other silicon ICs as these IC also exploit the mechanical property of Silicon. They have become an integrated part of our life and widely used in fastest growing electronic market such as Mobile phone, tablet, wearable, IOT, Automotive and healthcare. This article encompasses type of MEMS and the new advancement. Further the article also takes wearable as case study to explain their architecture and application.
MEMS – Type and advancements
Motion MEMS
a) Accelerometer
Also called a g-sensor, accelerometer can be used to detect linear acceleration, vibration, tilt, free fall detection etc. Accelerometer is used to implement features such as horizontal levelling, antitheft, vibration alert, pedometer, remote monitoring of adults and kids etc.
b) Gyroscope
Gyroscope measures angular velocity and used to implement features such as gaming, 3D mouse, athlete training etc.
c) Magnetic Compass
Magnetic Compass can detect earth’s magnetic field and can give direction sense to devices. It is widely used in indoor navigation and map navigation in mobile phone.
Ultra-compact high-performance e-compass : 3D accelerometer and 3D magnetometer module
Motion sensors are also now available in integrated form. ST has introduced a highly integrated MEM IC – LSM9DS0 which combines accelerometer, gyroscope, compass in a single 4X4mm IC.
Enviroment Sensors
d) Pressure Sensor
Pressure sensor can detect earth’s atmospheric pressure. This feature can be used to implement barometer and altimeter. LPS25HB pressure sensor from STMicroelectronics is so accurate that it can even be used to tell the floor of building accurately. This feature can be used to implement indoor navigation, augmented reality and 3D GPS.
e) Humidity Sensor
Humidity sensor can detect humidity parameter which can be used at weather station, condensation level monitoring, air density monitoring and gas sensors measurement correction. It can also be used in HVAC and respiratory equipment.
ST has introduced a humidity sensor which includes a sensing element and an analog front end to provide the measurement information by digital serial interface. The sensing element consists of a polymer dielectric planar capacitor structure able to detect relative humidity variations.
The complete measurement chain consists of a low-noise capacitive amplifier, which converts the capacitive unbalance of the humidity sensor into an analog voltage signal, and an analog-to-digital converter is used to generate digital information. The converter is coupled with a dedicated Hardware (HW) averaging filter to remove the high frequency component and reduce the serial interface traffic.
f) UV Sensor
UV Sensor is used to detection the ultraviolet radiation and targeted to be used in Smartphone, tablet and wearable. It can also be used in Industrial applications such as UV index measurement and weather station equipment.
The UVIS25 is a digital UV sensor to provide an accurate measurement of the Ultraviolet radiation index (UVI) from the sunlight. It includes a sensing element and a mixed signal ASIC to provide the UV Index data by digital serial interfaces.
The sensing element consists of an embedded photodiode. It is extremely stable over time and able to sense the UV-A and UV-B solar radiation erythema weighted. The IC interface is manufactured using a standard CMOS process that allows a high level of integration to design a dedicated circuit that is trimmed to better match the sensing element characteristics
MEMS Microphone
MEMS Microphone is audio sensor that converts sound signals into electrical signals. MEMS Microphones are increasingly preferred over conventional microphones as they offer higher SNR, small form factor, digital interface, better RF immunity and high robustness against vibration.
Microphone makes the interaction easier, faster and smoother. It also enables fashionable and stylish designs by reducing the number of buttons.
Analog MEMS Microphone: Bottom port with high bandwidth, omnidirectional and high SNR
Next microphones trend is to allow dual mode operations
Sniffing/low power mode: the device reduces the power consumption at minimum whilst guaranteeing an adequate set of performances for voice activity detection.
Normal mode: the acoustical parameters are set to the optimal level for voice control applications.
With the help of Switch Microphone can be put is sniff/normal mode by microcontroller
Wearable
Wearable are electronic devices which are small and light weight so can born by the user. They are getting adopted as watches, fashion accessory, fitness tracker, security devices or specialized medical gadgets. A wearable device may be independent in its functioning or it may act as accessory to other device such as tablet or Smartphone. In this case it is mainly used as input or output device with some computing power.
Historically, watches and wearable calculators have been very popular but with the emergence of Bluetooth low energy connectivity, low power technology and sensors wearable are available in various forms such as Smart watches, wrist band, jewelry, arm band, connected clothes etc… This article will elaborate more on wearable architecture and the innovative technologies which are transforming our lives.
Wearable Attributes
As the name suggest wearable are worn by user for an extended period of time and continuously remain on and connected. So a typical wearable device should have following attributes.
a) Always on
Wearables are continuously on the job and monitor or output key data for the user. They may have sleep mode but never OFF.
b) Low Power
As wearables are miniature devices and run on small battery they should consume ultra-low current. A wearable device which requires frequent charging will lose its purpose. The emergence of low power technologies has enabled the revolution in this industry.
c) Compact
As they are worn by user they should be small enough to take various usable and aesthetic forms. A bulky device will be extra burden on user and will lose its acceptance.
d) Environment aware
Environment sensing is emerging as a key driver for wearable acceptance. Today wearable devices can monitor not only user activity information but also temperature, humidity, UV radiation etc.
e) Intelligent
Wearable devices should be intelligent enough to filter out noise from the real data. They should be able to alert user when any important information needs to be shared. For example, a security wrist band should be able to send alert when user has met accident or need SOS help.
f) Connected
Connectivity is the most important factor for emergence and acceptance of wearable. These devices can be connected to smartphones or tablet using technologies such as Bluetooth, wifi etc. Besides, they can also be connected to GPS satellites to monitor user location.
Wearable architecture
A typical wearable device not only contain MEMS devices we discussed but also analog front end – OP-AMPs and switches, power management, battery monitoring as well as wireless connection. We will discuss OP-AMP and BLE in detail as they constitute very important part of an wearable device.
a) Operational Amplifiers
Analog sensors need signal transducers to deliver the information to the MCU. These signal transducers should have following key features
I. Accurate and stable to guarantee the maximum precision of the information
II. Low power to guarantee a longer user experience
III. Small to be integrated in the most stylish and thin designs
A low input offset voltage with zero drift amplifiers is mandatory. Otherwise the electrodes information would be less accurate or lost. Once the signal dynamic has been restored precision and micro power consumption amplifiers are needed before the signal is fed to the MCU
Bluetooth Smart [BLE]
Bluetooth Smart or Bluetooth Low Energy [BLE] is a standard that enables connectivity of low power devices and sensors. It uses short wavelength radio transmission at 2.4GHz band. Using Bluetooth Smart it is possible to enable two way communications between various devices wirelessly with incredible battery life. BLE enabled devices can also work seamlessly with the compatible smartphone and benefit from the rich user interface. Smartphone can also transfer data over the cloud using other connectivity such as GPRS.
Bluetooth low energy technology operates in the same spectrum as Classic Bluetooth technology but it enables ultra-low power connectivity and basic data transfer for applications previously limited by the power consumption
Layered Architecture of BlueNRG – Bluetooth Smart IC from STMicroelectronics
Major Applications of Wearables
a) Fitness and Wellness
Connected Smart watches, activity monitors, pedometers enable users to monitor their fitness activity through their smartphones. Wearable are also available as activity monitors, foot pads, sleep sensors and heart rate monitors. Wearable also allows to optimize sports performance, goal monitoring, weight control, direction data and location sharing.
b) Healthcare
Heart rate monitors, glucose monitors, pulse oximeters can now be connected to Smartphones through BLE. BLE not only allow these gadgets to connect to smartphones wirelessly but also get connected to internet through smartphones. It will enable information to be stored on cloud and share with doctors and family members. BLE has also enabled these gadget to be placed at strategic locations such as shoes, wrist etc. BLE support many healthcare related profiles such as glucose meters, heart rate monitoring, thermometer and blood pressure.
c) Proximity and Anti-theft accessories
A small BLE accessory when tagged to pets and kids can alert the smartphone if they sneak out of reach. BLE enabled car keys can be found easily with the help of connected Smartphone. Wallets and luggage tagged with BLE can alert the Smartphone if some try to steal them.
d) Security and tracking
A wearable watch or tracking devices allows kids location to be constantly monitored by their parents. These devices are also used as SOS device by kids and women when in danger. A connected device will send signal to smartphone which in turn will SMS user location to the concerned authorities and family members.
Conclusion
MEMS is not only integrated part of Smartphone and Tablet but also conquering new territories such as Automotive, healthcare, IOT and wearable. There will be several challenges such as better reliability, filtering noise, ultra-low power and shrink form factor. Presences of wireless technologies are also adding lot of scope of innovation. ST is committed to be at the fore front of developing of MEMS and Sensors and support them with a complete system solution with its offering in Analog and power management.
The author is Senior Manager – Technical Marketing, Analog MEMS and Sensors [AMS], India, Asean and ANZ, STMicroelectronics