TRANSDUCERS 



With suitably shaped vanes in front of plain apertures it is possible to 

 obtain almost any non-linear relationship between displacement and voltage 

 output. 



ELECTROMECHANICAL TRANSDUCERS 



All the dynamic m-e transducers described in an earlier section may be used 

 the other way round, i.e. to provide a mechanical output for an electrical 

 input. Since general descriptions have already been given, the present section 

 will deal only with the characteristics which the devices display when used 

 as e-m transducers. Certain configurations suitable only for e-m use will also 

 be mentioned. 



Piezo-electric transducers 



Although piezo-electric transducers used electromechanically are only 

 capable of minute movements, the large inherent stiffness of the piezo- 

 element results in a very high mechanical output impedance, and the move- 

 ments can be sustained against large opposing forces. 



An interesting use of a piezo-electric transducer is due to Pascoe^, who 

 had experienced difficulty in inserting microelectrodes through cell walls 

 without breaking the tip of the electrode. He then mounted the electrode on 

 a 'bender' crystal and was able to jerk it forwards about 20 jjl by the applica- 

 tion of a d.c. voltage to the crystal. This sudden movement effected the 

 penetration without damage to the electrode. 



Electrodynamic transducers 



Mechanoelectric transducers are usually operated into the input of an 

 amplifier. Thus when an electrodynamic transducer is used mechano- 

 electrically a negligible current is drawn from the transducer. If a current 

 were allowed to flow the transducer would then behave electromechanically 

 and a reaction on the preparation would occur: the mechanical impedance 

 of the transducer would in fact be modified by the electrical impedance into 

 which it worked. This effect does, in fact, occur when an electrodynamic 

 transducer is used electromechanically, as it will be fed from a source of 

 electrical power having a definite impedance. This impedance will modify 

 the mechanical output impedance of the transducer. 



To illustrate this effect, consider an ideal electrodynamic transducer 

 consisting of a length of wire of zero mass and resistance, freely situated in 

 a magnetic field. If a constant current generator is connected to the wire it 

 will experience a constant force regardless of any movement it may suffer. 

 On the other hand, if a constant voltage generator is used the wire will 

 move until it generates a back e.m.f. equal to that of the generator, i.e. it 

 will move with constant velocity regardless of load. Thus, a transducer fed 

 from a high-impedance electrical supply has a low mechanical output 

 impedance; a low-impedance drive to the same transducer gives it a high 

 mechanical output impedance. It can be shown that if the electrical supply 

 has a resistive impedance, as is usual, the transducer will also have a resistive 

 mechanical impedance, and that the mechanical resistance will be inversely 

 proportional to the electrical resistance. 



Actual transducers differ considerably from the idealized one described 



492 



