Accelerometers, Piezoelectric material , Hall effect , Encoders, Mechatronics , UPSC ESE

 Piezoelectric accelerometer 

• It is a sensor which is used to measure amplitude of acceleration as well as amplitude of vibration.
• The output voltage of the piezoelectric accelerometer is directly proportional to the amplitude of acceleration.
• It consists of piezoelectric material which works on piezoelectric property .

Piezoelectric property

• If the piezoelectric material is subjected to force or pressure then the material will be deformed ,if the piezoelectric material deforms then electrical charge will be developed across the material which leads to voltage generation.
• It generates sinusoidal voltage when it is subjected to  dynamic load .
• When static force ,voltage cannot be hold for a long time .
• Consider the 3d piezoelectric material shown below 
  
  
  
  
  
  
  

Seismic accelerometer 

• Generally accelerometer consist of mass damper spring system .
• If the accelerometer is mounted on vibrating structure than the mass present in the mass damper spring system displaces from its mean position .
• The amplitude of relative displacement of the mass is directly proportional to the amplitude of input acceleration 
• A piezoelectric accelerometer, the piezoelectric material converts the relative displacement of the mass into voltage .
• The amplitude of the output voltage of the piezoelectric material is directly proportional to the amplitude of relative displacement of the mass thereby it is also directly proportional to amplitude of input acceleration .
• Piezoelectric effect is reversible, means mechanical to electrical or vice versa , also called as inverse sensor .
• Piezoelectric material can act like sensor as well as actuator .
  
  
  
  
  
  

From piezoelectric material 

For accurate acceleration measurement 

For accurate vibration measurement

 

Piezoelectric accelerometer 

Advantage 

• Instrument is quite small in size 
• Wn is very high 
• Useful for high input frequency and their response is poor at low frequencies .

Application 

• Used to measure vibration of aeroplanes and rockets 
• Detect vibrations of engine in cars 
• Use to study high speed waves

Hall sensor 

• It is a proximity sensor which is used to detect the presence of magnetic field .
• Hall sensor is made up of conductor as well as semiconductor 
• Hall sensor is also called as position sensor and it works on hole effect.

Hall effect

•  If the current carrying conductor placed in perpendicular magnetic field then voltage will be generated across the material, that developed voltage is called as hall voltage  and the hall voltage will be perpendicular to the  applied current as well as applied magnetic field .

Electric field 

• Voltage gradient available across the metal plates of the capacitor is called as electric field in the capacitor .
• Force acting on Lorentz charge particle because of magnetic field (B)
  
  
  
• Force acting on charged particle because of electric field (E)
  
  
  
• Consider 3 d conductors shown below 
  

Consider the  material 

• Hall sensor is a position sensor and it is capable to measure linear displacement ,angular displacement , shaft speed and linear speed ,water level in the tank .

Optical encoder

• It is a digital transducer which is used to measure angular displacement and  linear displacement.
• Optical encoder is also called as position sensor ,it converts the input position information into electrical pulses .
• Optical encoder consists of light emitting diode ,light detector, rotating disc to which shaft is attached as shown in the figure 
• Optical encoders are classified into two types :incremental type and  absolute type 
• It convert input position information into digital output .

Incremental type encoder

• This encoder consist of rotating disc which has a rectangular holes of equal length which are separated at equal distances as shown below 
• Incremental encoder is used to measure the angular displacement of the stepper motor provided that step angle should be equal to the resolution of the incremental encoder .
  
  
  
• If the hole comes in the path of light then light passes through the whole and reaches to the detector .

Disadvantage of incremental and encoder 

• To improve the resolution number of holes on the disk should be increased but it is not suggestible to increase the number of holes on the disk beyond some limit.
• Incremental encoder cannot find direction of rotation ,to overcome these disadvantages we generally prefer absolute encoder.

 Absolute encodrer

• It provides accurate information of direction of rotation as well as position of the shaft.
• Absolute encoders generates binary code corresponding to the position of shaft.
• As the output of absolute encoder is binary form ,it is possible to interface output of the encoder to microprocessor without the help of analog to digital converter .

Single track encoder

Two track encoder

 Note:

• If  absolute encoder consist of N tracks on the disk then it is capable  to provide N bit binary code for each and every position of the shaft, then resolution equals to  360 degree/2^n .

8 track encoder 

• Each position of shaft is converted into 8 bit digital code.

 Question :a shaft encoder which is attached to a wheel has a sensitivity of 500 pulses per Revolution a digital pulse counter connected to the encoder indicates 5500 pulses in one second then the shaft speed in RPM is -------and the resolution of the encoder is ------------

Question: a stepper motor which is rotating at 200 steps per revolution was subjected with input pulse rate of thousand pulses per second if a to track absolute optical encoder is used then the binary display indicated just after 1.25 seconds  is ---------

Measurement of speed of a vehicle