DYNAMIC MECHANOELECTRICAL TRANSDUCERS 



change of magnetic field, and therefore with all common configurations to 

 the velocity of the mechanical input. 



If a sinusoidal mechanical input is applied to such a transducer, the result- 

 ing peak velocity will be proportional both to the amplitude of the displace- 

 ment and to its frequency. Thus for a constant amplitude input to an 

 electrodynamic transducer, the voltage output will be proportional to fre- 

 quency; this characteristic is shown in Figure 33.11. Comparison with 



Frequency 



Figure 33.11 Response of electrodynamic transducer 

 for constant input displacement 



Graph 9 shows that there is an essential difference between electrodynamic 

 and piezo-electric transducers, although both give no output at zero 

 frequency. 



Electrodynamic transducers can be constructed with very low mechanical 

 impedance, suitable for use for measuring displacement. However, the 

 rising frequency characteristic implies that the electrical output will not give 

 a true picture of the displacement. Although this frequency characteristic 

 may be linearized by passing the output through an integrating circuit 

 (Chapter 3), it is usually possible to choose a more satisfactory type of 

 transducer for the purpose. However, if information about the velocity of 

 a system is required, the output of an electrodynamic transducer, suitably 

 amplified, can be used to record this quantity directly. This avoids subsequent 

 graphical differentiation of a displacement record and is the most useful 

 application of this type of transducer. 



For qualitative work when, for example, only the frequency of a vibration 

 or the time of occurrence of a transient mechanical disturbance is required, 

 the simplicity and high sensitivity of certain types of electrodynamic trans- 

 ducer are valuable. Thus, if an insect is attached to the armature of a moving- 

 iron transducer (e.g. a magnetic gramophone pick-up), the frequency of the 

 output will correspond to the wing-beat frequency, even though the output 

 waveform gives httle information about the forces to which the insect is 

 subjected. Again, a piece of iron mounted on a rotating device will give 

 several volts output in a nearby coil ; this output may be used, for example, 

 to synchronize a photographic flash with the rotation. 



Moving-iron transducers — In this type of transducer the coil is fixed, and 

 a moving armature varies the reluctance of a magnetic circuit (Chapter 4) 

 passing through it. The m.m.f. is provided by a permanent magnet, which 



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