Sec. 1-2] 



MECHANICAL INPUT T HANS I )UCERS 



11 



An electric output can arise either in response to a compression- 

 expansion motion (( 1 -2)59 a and b) or in response to a shearing motion 

 ((l-2)59c and d). Depending upon the material, a compression of the 

 plate (a and b) can cause a diminution of the plate volume, or it 

 can cause an expansion of the plate in other directions (transverse 

 expansion) , so that the plate volume remains constant. Most crystals 

 exhibit a certain transverse ex- 

 pansion and a small net-volume 

 change. Internal mechanical cou- 

 pling within the crystal can cause 

 the simultaneous response to com- 

 pression-expansion and shear 

 action. By the appropriate orien- 

 tation of the cut and application of 



F 



Fig. (1-2)60. Stacked piezoelectric 

 crystals: (a) series connection; (b) and 

 (c) parallel connections. 



id) 



Fig. (1-2)61. Piezoelectric crys- 

 tals combined to form "bimorph 

 benders." 



the electrodes, crystals can also be used for flexing or torsional 

 movements. 



The piezoelectric effect is reversible; the application of an electric 

 field to a crystal causes a mechanical displacement of the crystal. 



Multiple arrangements (stacks) of piezoelectric elements operating 

 in the compression-expansion mode are shown in Fig. (1-2)60. The 

 elements may be connected in series, leading to a higher output 

 voltage for the same force, or in parallel, resulting in lower output 

 impedance than a single element. Crystal stacks can also be arranged 

 so that the response of neighboring crystals to an external vibration 

 or an unwanted force is electrically compensated. 



The single-plate piezoelectric transducer as shown in Fig. (1-2)59 

 or the multiple arrangements in Fig. (1-2)60 are useful for large 

 driving forces but small displacements (order of microinches). If 

 smaller forces or larger displacements are to be converted into 



