ELECTROMECHANICAL TRANSDUCERS 



sometimes provided while other designs employ the damping effects of 

 eddy currents induced in a metallic coil former. In all cases some additional 

 mechanical resistance is introduced since the driving amplifier has a resistive 

 output impedance. 



Figure 33.27 Equivalent circuit of pen recorder 



The equivalent circuit of the mechanical parts of a pen recorder is shown 

 in Figure 33.27. Here R^ is the mechanical resistance induced by the output 

 resistance of the driving amplifier, R^ and R^ are the resistances (if any) due 

 to oil and eddy current damping, m^ and m^ are the effective masses of the 

 coil and pen, and Q and Cg are the compliances of the coil control spring 

 and the pen coupling spring. The displacement of the pen is represented 

 by the charge flowing through m^. By suitable choice of the components 

 it can be arranged that the sensitivity of the recorder is constant from zero 

 frequency to, in a typical case, 100 c/s. 



This adjustment depends, of course, on R^ having the correct value. 

 It is usual in the design of driving amplifiers for pen recorders to provide 

 a variable degree of negative feedback. The output impedance of the 

 amplifier may thereby be varied {cf. Chapter 11), and hence the value of 

 R^ may be controlled. If Ry is too small there will be a resonant peak in 

 the response of the recorder and the response to a transient will be oscillatory. 



(a) 



> 



> 



> 



> 



> 



(b) 



(0 



20 



^ 



60 



I 



100 150 c/s 



Figure 33.28 Frequency response and transient response of pen recorder: 

 (a) under-damped; (b) correct damping; (c) slightly over-damped 



If R^ is too high the transient response will be over-damped. A technique 

 for adjusting the damping is thus available; a low-frequency square wave 

 is applied to the recorder, and the feedback control adjusted until it is 

 reproduced most faithfully. In Figure 33.28 the square wave response and 

 frequency response of a pen recorder are shown for three different degrees 

 of damping. 



495 



