Air Bag Sensor (Accelerometer)

Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10

1 Answer
  • 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 ā€˜zā€™ can be represented as z=x-y where x and y represent the inertial mass and the motion of the housing system.

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  • 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.

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  • 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.

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  • 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

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Wheatstone Bridge for signal transduction:-

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

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

$R_2$ & $R_4$ = Reference resistor

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