Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

• Accelerometers are used in Air Bag sensors.
• The basic configuration of an accelerometer is shown in the following figure.
• It consist of the 3 components:-
  • Inertial Mass
  • Suspension.
  • Sensing Element.
• The suspension supports the inertial mass and will deflect under the acceleration.
• Sensing elements will transducer the deflection of the suspension to an electric signal.
• The transduction can be implemented using Piezo resistive, Piezo electric or capacitive means.
  • The relative displacement ‘z’ can be represented as z=x-y where x and y represent the inertial mass and the motion of the housing system.

• The Fig. Shows micro mechanical capacitive structure where devices is divided into two parts:-
  • Mechanical Capacitive sensor.
  • Signal Detection Circuit.
• The Mechanical Structure is made up of polysilicon material about 2um thick.
• A central mass plate with finger on both sides is suspended over substrate by 4 thin tethers.
• The mass plate can be displayed by an inertial force in the surface plate and in perpendicular to the tether.
• There are 2 fix strips A & B which are connected to form a stationary electrode.
• The moveable electrode and the stationary electrode constitute two mechanical capacitor C1 & C2.
• Due to acceleration, the plate is displaced and the capacitances C1 and C2 will experience the change.
• The differential capacitance C1-C2 gives the acceleration measurements.

• In the air bag sensor, the operation takes place as explained in the following points:-
  • When car hits something, it start to de accelerate i.e. it loses speed very rapidly.
  • An accelerometer detects the change in speed.
  • If the de acceleration is high enough, then the accelerometers triggers the air bag circuit.
  • The air bag circuits pass an electric current through a heating element.
  • The heating element ignites a chemical explosive.
  • As the explosive burner, it generates a massive amount of harmless gas, typically nitrogen or Argon that floods into a nylon bag passed behind the steering wheel.
  • As the bag expands, it blows the plastic cover of the steering wheel and inflates in front of the driver.
  • The driver moves forward due to the impact and pushes against the bag.
  • This will deflate the bag, the gas will escape through small holes around its edges.
  • When the car stops, the bag may be completely deflated.

Pressure Sensor:

• Micro pressure sensors are used to monitoring and measure gas pressure in environment or engineering system for e.g. pressure in automobile engine.
• Micro Pressure sensor works on the principle of mechanical bending of thin silicon diaphragm by the contact air on the gas pressure.

• The strains associated with the deformation of the diaphragm are measured by tiny piezo resistor which is placed in strategic location on the diaphragm.
• Doped silicon is used for fabricating piezo resistors; they work on same principle like wheat stone bridge for signal transduction.
• They have higher sensitivity and resolution

Wheatstone Bridge for signal transduction:-

$V_0 = vin \bigg(\frac{R_1}{R_1+R_4} - \frac{R_3}{(R_2+R_3} \bigg)$

Where $R_1$& $R_3$ = Resistance induced by longitudinal & transverse stresses.

$R_2$ & $R_4$ = Reference resistor