TRANSDUCERS 



added together. This sum is converted into a current in the moving coil, 

 and hence to the force F on the output member. 

 We therefore have that 



F az ax + fix + yx 



By comparison with the equation 



1 



F = mx -\- Rx -\- —X 



it follows that the parameters a, [i, y, controlled by the potentiometers, 

 set the values of the apparent compliance, viscous resistance and mass 



Muscle 

 attached here 



' Photoelectric 

 transducer 



Moving coil 



e-m 

 transducer 



Current 

 C3C orx+zyx+zV 



Figure 33.41 A variable mechanical load — block diagram 



exhibited by the output member. It should be noted that the mass thus 

 generated has the usual inertial properties, but is not subject to gravitational 

 forces. Any static force required can be fed into the adding circuit in the 

 form of a constant voltage. 



This circuit shows once more how the output impedance of a transducer 

 is modified by feedback (in this case either positive or negative). This 

 application of transducers is unusual for the following reason. The complex 

 arrangement of two transducers and associated electronic circuits is not 

 employed to 'convert quantities of one type into equivalent quantities of 

 another type' (page 471); it merely serves to modify the mechanical para- 

 meters of the input shaft. 



CONCLUSION 



The biologist who decides to use a transducer should first ask himself 

 whether the experiment could not be carried out better with a lever and a 

 piece of string. Only when he is satisfied that a transducer is necessary, 

 either for reasons of sensitivity or of convenience, should he proceed to 

 choose a suitable design. The impedance of the transducer must be con- 

 sidered at this stage. If displacement is to be measured, how much loading 

 will the preparation stand? If an electromechanical transducer is to be 



508 



