TRANSDUCERS 



inherent in the preparation, Cg is the very low compHance of the added spring, 

 and C3 is the very high comphance of the transducer itself. 



This system will display the resonant behaviour described on page 476; 

 the resonant frequency will be governed by the mass of the preparation and 

 by the compliances of the preparation and added spring. 



Similar results are obtained when springs are used to modify the impedance 

 of e-m transducers. Thus the output of a high-impedance transducer may be 

 applied through a weak spring; the propagation velocity along the spring 

 must be taken into account. Alternatively, a low-impedance transducer 

 may have a stiff spring across it, giving rise to a resonance controlled by the 

 compliance of the spring and the mass of the transducer. 



All practical transducers behave in these ways to a certain extent. For 

 example, a low-impedance transducer may well have an operating arm 

 between the preparation and the point at which the mechanoelectrical 

 conversion is carried out ; the upper frequency limit will be governed by the 

 velocity of propagation along this arm. Similarly, the control spring of a 

 high-impedance transducer will not have zero compliance, and resonance 

 effects must be expected. In general, a high value of upper frequency limit 

 involves low sensitivity, and vice versa. Much of the technique of the use of 

 transducers is concerned with attaining maximum sensitivity by setting the 

 upper frequency limit only as high as is absolutely essential. 



CLASSIFICATION OF TRANSDUCERS 



Transducers have been classified in this chapter according to the scheme 

 shown in Table 2. 



TABLE 2 



Transducers 



limited movement continuous rotation 



mechanoelectrical electromechanical mechanoelectrical electromechanical 



(m-e) (e-m) (m-e) (e-m) 



dynamic static piezo-electric electrodynamic 

 , I \ _J 



piezo- electro- variable variable variable niiscel- moving-iron moving-coil 

 electric dynamic resistance inductance capacitance laneous 



I 1 



moving-iron moving-coil 



Mechanoelectrical transducers are divided into dynamic and static types; 

 the former gives an output only while moving, the latter's output is propor- 

 tional to displacement even when stationary. Dynamic transducers are self- 

 generating, i.e. no external supply is needed. Static transducers, on the other 

 hand, consist of an electrical component (resistance, inductance or capaci- 

 tance) which can be varied mechanically; suitable active circuits are needed 

 to convert this variation into a voltage or current. 



No distinction will be made between transducers for linear motion and 

 those for limited rotary motion, but examples of each will be given later. 

 The choice of linear or rotary input is usually a matter of convenience, and 



480 



