MEMS ( Micro-electro-mechanical-systems) is a technology that combines various engineering design and manufacturing techniques covering mechanical, electrical, optical, chemical, fluids, IC fabrication, and packaging technologies. As one of the most promising technologies of the 21st century, it has been widely used to build various kinds of sensors used in control and monitoring systems across a wide range of applications. In this article, we discuss MEMS sensor technology as environmental sensors used in the emerging IoT technology space.
MEMS technology is used to create the electrical and mechanical elements of sensors and actuators in microscopic size. MEMS devices are created using micro-fabrication technology similar to batch processing for integrated circuits. Manufacturers use Computer Aided Design (CAD) tools to design, simulate, and verify the functionality of the MEMS devices.
MEMS devices use various materials for fabrication, including silicon, polymers, and ceramics. The performance of these devices depends on mechanical properties, such as high stiffness, fracture toughness, elasticity, and high fracture strength of the fabrication material. Silicon is used in the majority of MEMS devices as it has excellent mechanical stability, semiconductivity, and availability.
The early MEMS systems used separate chips for sensing and signal conditioning, making the package size large. This approach had several manufacturing disadvantages, such as more assembly steps and multiple packaging technologies needed to build a device for an application. MEMS technology has evolved over time, and an integrated approach to assembling both elements on the same chip provides high performance in very compact packages. They can operate at low voltage and emit less system noise. Sensor fusion technology helps to integrate multiple sensing functions required in a system on a single chip.
MEMS products available today vary in sizes ranging from one micron to several millimeters. These devices are available in different packages, such as ceramic, plastic, and metal.
The MEMS devices are available in two types:
• Simple structure with no moving element: Most of the sensors are simple structure MEMS devices.
• Complex electromechanical systems with multiple moving components: All actuators and some sensors such as accelerometers, micro-relays, and variable capacitors fall under this category.
The sensors and actuators work as transducers that convert one form energy into another. Actuators convert electrical energy into mechanical motion. Sensors measure the physical parameter such as mechanical, thermal, chemical, radiant, magnetic, and electrical from the surrounding environment.
Let’s discuss how a MEMS sensor works by taking the example of an accelerometer. The accelerometer is an electromechanical sensor that measures both static and dynamic accelerations (rate of change of velocity of an object). Static forces include gravity and inclination, whereas dynamic forces include movement and vibrations of the moving element. It measures acceleration in units of G-forces (g) or meters per second squared (m/s2).
MEMS-based accelerometers are compatible with a printed circuit board that can be connected easily with the microcontroller using serial protocols like I2C and SPI.
Figure 1 shows the accelerometer device which contains capacitive plates internally. Some of these plates remain fixed, while other plates are movable and connected to springs. As the acceleration force acts upon the sensor, the movable plates move towards or far from fixed plates. Hence, it changes the capacitance between them. The change in capacitance helps to determine the acceleration.
MEMS Accelerometers are highly sensitive and used in extremely critical applications such as vehicle stability and for air bag activation in automotive systems. They are also used in the industrial, smartphone, aircraft instruments, and tactical guidance systems.
Figure 1: MEMS Accelerometer Sensor
MEMS Sensors Used in a Smart Environmental Monitoring System
A Smart Environmental Monitoring System consists of an Embedded Processing Unit, MEMS Sensors, a Gateway, and a Cloud platform. OMRON offers a MEMS technology based environment sensor, featuring an onboard multifunctioning sensor, a microcontroller unit (MCU), flash memory, and Bluetooth module. Table 1 illustrates the types of MEMS sensors in Omron’s Smart Environmental Monitoring System.
Table 1: Environment Monitoring System
Omron’s environmental sensor offers a temperature, humidity, pressure, sound/noise, and light sensor in a single package. The MCU has a software program used to calculate the alert level for heat stroke and discomfort index by decision making on threshold values using the temperature and humidity sensors. These sensors can generate an emergency alarm signal for earthquake detection using accelerometer data. Inbuilt flash memory helps to store the measured data locally as a data logger to avoid data loss due to poor connectivity. The Bluetooth connectivity allows the sensor to share measured data with a gateway, mobile terminals, and PCs. These devices can communicate with the cloud platform using internet connectivity. The user can access the measured data from the cloud platform using mobile apps. The Omron sensors are available in three packages: Bag, PCB, and USB types.
- Bag and PCB Type: Bag and PCB type devices can measure six environmental parameters, such as temperature, humidity, pressure, light, UV light, and sound noise. These devices can save 26,000 data streams in its flash memory. The bag type device has a 3VDC lithium battery that can provide an uninterrupted power supply for six consecutive months.
- : USB type devices can measure seven environmental parameters, such as temperature, humidity, barometric pressure, light intensity, sound noise, Volatile Organic Compound (VOC), and 3-axis acceleration sensing capabilities. This sensor can store 60,000 data streams in flash memory and supports Bluetooth and USB connectivity, and we can select any one communication mode that depends on the application. It can measure and calculate the seismic information (intensity of ground shaking), vibration count, and spectral intensity (SI) value using accelerometer data. The SI value is equivalent to the magnitude of the breaking energy of earthquake motion against structures and correlates with seismic intensity.
MEMS sensors are widely used in IoT applications such as Smart Homes, Smart Factories and Smart Medical Devices.
- Smart home: In smart home applications, bag and USB type sensors are used to monitor the indoor environment. Environmental sensors consist of temperature, pressure, humidity, and optic and equivalent Total Volatile Organic Compound (eTVOC) sensors. The eTVOC sensors are used to maintain a comfortable environment. The VOCs are chemicals that easily evaporate at room temperature. When VOCs are released into the atmosphere, the gases react with the absorbed oxygen. The MCU generates an alert in case of high VOC concentration.
- Medical: Advanced medical devices use PCB type MEMS sensors for automated drug delivery and health monitoring systems. The optical MEMS sensors are used as artificial retina that is placed inside the eye. It can compensate for the weaknesses and drawbacks of a sense organ. The bio-MEMS are used to identify the chemical contents and genetic diseases in the patient's body. They are also used to monitor the blood cell count, glucose levels, and to perform urinalysis.
- Factory: In factories, PCB and USB type sensors are used to monitor the factory environment. USB type sensors include an accelerometer to detect abnormalities of production equipment by monitoring vibration, orientation, and sound noise. The UV light, pressure, temperature sensors help to monitor the heatstroke level to prevent any hazards to workers.