Have you ever wondered how many individual components are in an iPhone?
The accelerometer and gyroscope can detect the movement of the phone; light sensors, temperature sensors, pressure sensors, and humidity sensors can measure the phone’s environment; there are also sensors for connectivity like RFID, Bluetooth, and WiFi; as well as microphones and speakers for sound.
Most of these components are Micro-Electro-Mechanical Systems (MEMS).MEMS technology drives sensors to become smaller, and as sensors become smaller, they extend the reach of the Internet of Things (IoT) even further.
What is MEMS technology?
Micro-Electro-Mechanical Systems (MEMS) technology uses semiconductor manufacturing processes to produce miniaturized mechanical and electromechanical components ranging in size from less than one micron to several millimeters.MEMS devices can range from relatively simple structures with no moving parts to complex electromechanical systems with multiple moving parts.
MEMS are used not only for sensors but also in many fields. In addition to sensing, they are used in communication modules, actuators, and data processing devices. They are all miniature machines, with component sizes ranging from micrometers (millionths of a meter) to millimeters.
MEMS devices come in a wide range, from simple machines with no moving parts to complex electromechanical systems with multiple moving parts. These systems can be of many different types: magnetic, electrical, thermal, chemical, optical, and mechanical systems.
Manufacturing MEMS devices requires many of the same techniques used to manufacture other semiconductor circuits: oxidation, diffusion, ion implantation, low-pressure chemical vapor deposition (LPCVD), sputtering, etc. Additionally, MEMS use specialized processes such as micromachining.
Compared to earlier methods of achieving the same functions, MEMS devices are smaller, cheaper, and consume less power. They are also very sensitive and highly accurate.MEMS devices also offer excellent repeatability, benefiting from the strict tolerances inherent in semiconductor process technology.
The downside is that while the production cost of the parts is very low, the investment associated with designing, qualifying, and manufacturing MEMS products is substantial. As a result, manufacturers are less likely to develop parts for small-batch applications.
Types of MEMS Devices and MEMS Applications
A typical MEMS sensor employs a mechanical structure that responds to mechanical or electrical stimuli (pressure, motion, acceleration, magnetic fields, etc.) by moving in a controlled manner. A typical technology uses movement to change the distance between the plates of a variable capacitor.
Gyroscopes require multiple MEMS structures to measure angular motion.
Outputs can take various forms: analog voltage; output voltage; standard serial buses such as SPI or I2C; or dedicated protocols popular in automotive airbag applications (like DSI or PSI5); wireless connectivity options include Bluetooth Low Energy (BLE).
MEMS devices can serve as single function sensors.
MEMS Gyroscopes measure angular rotation by utilizing Coriolis acceleration, which generates forces on the MEMS frame as the mass moves toward and away from the center of rotation. Gyroscopes come in single-axis, dual-axis, and tri-axis versions for different applications: for example, dual-axis gyroscopes are used for gaming and optical image stabilization, while tri-axis gyroscopes meet the needs of automotive telematics and navigation.
Accelerometers also use mass within the frame to measure static acceleration (i.e., gravity) and dynamic acceleration (e.g., vibration, motion, tilt, shock, etc.). Devices classified as accelerometers include inclinometers, vibration sensors, concussion sensors, tilt sensors, and motion sensors. Accelerometers also come in different axis combinations: single-axis devices are found in automotive crash sensors, while three-dimensional units appear in robotics, vibration monitoring, and tamper-proof applications.
Pressure Sensors measure pressure through the deflection caused in the MEMS structure. Some versions can measure pressure relative to atmospheric pressure and can also measure absolute pressure relative to a vacuum-sealed chamber. MEMS pressure sensors can also indirectly measure other quantities such as fluid flow, altitude, and water level.
Magnetometers measure mechanical effects caused by magnetic fields using various physical phenomena, such as the Hall effect.
Inertial Measurement Units (IMUs) measure linear and angular acceleration by combining tri-axis accelerometers and gyroscopes into a single unit; IMUs can also include magnetometers and pressure sensors to provide information about the device’s three-dimensional orientation and motion: acceleration on the x, y, and z axes; pitch, roll, yaw, altitude, etc. Applications include unmanned aerial vehicles (UAVs), robotics and factory automation, avionics, smartphones and tablets, virtual reality, and gaming.
MEMS Microphones work by measuring changes in capacitance when sound waves strike a variable capacitor made up of a movable diaphragm and a fixed backplate. They are widely used in space-constrained consumer applications such as smartphones and tablets.
MEMS Biosensors detect measurable movements in the MEMS structure caused by interactions between biomolecules. For example, in tuberculosis (TB) detection, a MEMS cantilever coated with TB antibodies will deflect when an infected blood sample is placed on it.
MEMS Gas Sensors detect the presence of gases by measuring changes in resistance induced on the surface of the coated sensor. These sensors can detect low concentrations of target gases, typically with a response time of less than a second. Humidity sensors are optimized for detecting water vapor.
RF MEMS Switches combine electrostatically driven cantilevers with separate driver ICs to replace unreliable, bulky electromechanical relays in RF switching applications. Various switch configurations can be used: for example, ADI’s ADGM1304 uses an SP4T configuration and can handle signals from DC to 14GHz.
MEMS Optical Actuators, such as Texas Instruments’ Digital Micromirror Device (DMD), use MEMS technology to form a large number of independently controlled mirrors. Each mirror can tilt under electronic control to switch between “on” and “off” states. When enabled, the pixel reflects light from the projector lamp into the lens, making it appear bright. In the off state, the light is directed elsewhere, making the pixel appear dark.
MEMS Oscillators contain a resonator that vibrates under electrostatic excitation from an analog driver chip. MEMS oscillators can generate frequencies from 1Hz to several hundred MHz with excellent stability, low power consumption, and high resistance to electromagnetic interference (EMI).
MEMS in the Internet of Things
The IoT has a huge demand for monitoring production aspects of miniature, low-cost sensors. These sensors must communicate information to other nodes in the factory network and must operate reliably in harsh electrical and mechanical environments. MEMS devices are tailor-made for this purpose: they are compact, robust, and can include additional circuit blocks for wired or wireless connectivity within the same package.
MEMS devices can effectively meet the demands of many IoT applications:
1: Low Power Consumption
IoT sensors and gateways often require wireless and battery-powered operation. Because unit costs are low, replacing the entire unit is often cheaper than reinstalling new batteries. Therefore, any reduction in power consumption extends the life of the device. Some MEMS face the same power requirements as larger MEMS. Others leverage different forces in electromagnetics or fluid dynamics to reduce power consumption without sacrificing functionality (e.g., consider surface tension situations when delivering water through small pipes).
2: Small Size
Users typically want IoT devices to be small and unobtrusive in office and home environments. By definition, MEMS are unobtrusive. However, beyond user demand, in certain IoT applications, there may be a need to add the device to existing machinery (such as automobiles), where hardware space is limited. In other cases, such as wearable devices and biomedical applications, small size is a critical requirement that must be met. Due to their smaller nature, MEMS meet and exceed these requirements.
3: Cost-Effectiveness
When deploying IoT solutions, scale is often a major concern. For example, when placing sensors in farmland to monitor weather and humidity, many devices will be required per acre. Or consider an asset tracking solution where a very large (and variable) number of assets may need to be tracked. In other applications such as transportation, devices may simply be disposable. MEMS are made using a process called photolithography, which makes mass production easy and cost-effective.
As more devices and applications are added to the IoT, MEMS will become a more viable solution.
For example, numerous applications of MEMS sensors in the Industrial IoT:
Industrial robots use MEMS-based 3-D gyroscopes and accelerometers to continuously measure changes in angular velocity and direction, replacing expensive rotary sensors and encoders. They can also detect excessive vibrations in joints and actuators, which may be a sign of premature failure.
MEMS accelerometers can detect harmful vibrations in other industrial machinery or sense harmful shocks; MEMS pressure sensors measure water flow and gas pressure; MEMS gas sensors check for toxic gas emissions; MEMS temperature sensors are a key part of many industrial processes.
In the IoT network infrastructure, MEMS oscillators are popular in programmable logic controllers (PLCs) that supervise the operation of robots and other units. Optical devices are suitable for human-machine interface (HMI) displays.
Factories themselves use MEMS in various ways. Pressure, temperature, and humidity sensors help control HVAC systems. Tamper-proof sensors are installed in smart meters; MEMS vibration sensors can help cut off gas supply in the event of an earthquake.
Calibrated, temperature-compensated MEMS sensors can measure gas pressure in LPG and CNC-driven vehicles that transport products to loading and unloading platforms. Once products leave the factory, asset tracking systems use MEMS to monitor the impact and vibration of goods.
Future Areas of MEMS Sensor Applications Worth Watching
Autonomous vehicles require dozens of sensors to interact with their surroundings. These sensors must also be as small as possible to avoid taking up space needed for other devices (both human and battery). Therefore, the automotive industry is one of the areas where MEMS sensor production and usage are expected to grow the most.
MEMS-based accelerometers offer high sensitivity and have fail-safe performance in extremely critical applications, such as airbag activation and vehicle stability within the end-user’s automobile.
Another area being explored for the use of MEMS technology is microphones. MEMS microphones can, for example, reduce the size of hands-free devices for phones and create more compact and discreet aids for people with hearing impairments.
The most spectacular and interesting use cases may be in biotechnology and medical technology. Procedures such as gastrointestinal examinations can be performed using cameras and tubes, aided by small guiding robots.
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