Automation, Mechanics, Microelectronics, Robotics, Sensors

What are MEMS?

MEMS are abbreviation of Micro Electro-Mechanical Systems, are miniaturized electromechanical machines. An introduction to MEMS and some of its applications are today’s subject.

Components

A MEMS’s components are: microsensors, microactuators, electronic systems and microstructures. Are made in silicon substrate using manufacturing techniques which will be shown in a future post. Size can vary between 1 and 1000 micrometers (10^{-6}).

Microsensors

Exist a great variety of sensors, are transductors which convert energy from environment in electric signals, these are sent to microelectronic systems. Many of these sensors are encapsulated like integrated circuits.

Here are shown some examples of capacitive, piezoresistive and ressonante types of MEMS sensors. Exist many others types of MEMS sensors, but cannot show all in a single post.

Capacitive sensor

These sensors detect capacitance variation between plates when detect some force. The accelerometer is an example of this type of sensor. When the mobile mass moves with an elastic mechanism, capacitances between walls of mobile mass and static walls change.

An example of accelerometer with variable capacitors. Capacitors are connected to a circuit bridge to measurement.

Piezoresistive sensor

Piezoresistivity is the change of electric resistance when it is submitted to mechanic tension. A pressure sensors for MEMS have piezoresistors which form a Wheatstone bridge and stay above a diaphragm made of silicon.

Resonant sensor

Resonant sensors measure frequency change when receive a stimulus. Have a parte that vibrates, whose sensor’s operation frequency changes when this part moves. This is a resonant gyroscope, disc vibrates in a frequency and spins around the own axis.

Any change of vibration frequency by Coriolis force is detected by capacitance variation between disc and electrodes in the substrate.

Microelectronic systems

Receive electric signals from sensors to processing and decision making. The main component of microelectronic systems is MOSFET transistor. They are field effect transistor, I will write a post explaining the operation of these components. The two existent types of MOSFETs are nMOS and pMOS.

A typical commercial processor has billions of these transistors. MOSFETs are the main construction block to form digital and analog circuits.

Microactuators

Receive commands from microelectronic systems to move mechanical parts or control external components. Can be controlled by magnetic fields, electrostatic forces, thermal expansion and piezoelectric effect.

Some examples of microactuators are:

  • Micromotor controlled by electrostatic forces.

  •  Micromirrors array to direct and modulate light beans.

  • Gear set in micrometric scale.

  • Micropumps for liquid transport.

Microrobots and other applications

MEMS are used in the construction of microrobots with applications in many areas. These are the I-SWARM, microrobots with photovoltaic cells and IR (infrared) communication.

Microrobots will enter in blood stream and apply medicines in the exact spot. This is the ViRob, controlled by an external magnetic field and move inside blood vessels and other human body parts.

These small machines are cheap, have low power and time response. For that, have a big quantity of applications beyond robotics, some examples are:

  • All modern electronic devices have MEMS. Cellphones have accelerometers to know the current position and change screen accordingly.
  • Smartphones and modern sound devices have microfilters to eliminate sound distortion.
  • Cars have many MEMS as: accelerometers to activate airbags, pressure sensors for tires, IR sensors, etc.
  • Accelerometers, gyroscopes and inertial measurement unit (IMU) to guiding missiles and navigation systems to ships, cars, airplanes and drones.
  • Microsensors to disease diagnostic and patient monitoring.

 

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