THE USE OF TRANSDUCERS IN BIOLOGY 



output impedance) ; (b) to carry out integration of a voltage on a long-time 

 scale. Since the angular velocity of the shaft is proportional to the input 

 voltage, its number of revolutions (determined with a mechanical counter) 

 measures the time integral of the voltage. Thus, for example, a linear 

 photocell connected to a velodyne could give a measure of the total light 

 energy falling on the cell over a period of as long as a year — a measurement 

 otherwise very difficult to make. 

 It will be seen from Figure 33.39 that motor and tachometer generator 



MechanicaUy 

 coupled 



Generator 



Motor 



CUW) 



■JJi Outpu 



t shaft 



nmr|_ (I 



Input Subtract -"^Amplifier—' 



voltage 



Figure 33.39 The velodyne — blocI< diagram 



are mechanically coupled, while the difference between the input voltage and 

 the tachometer output is amphfied to drive the motor. Since this difference 

 is therefore held small, the angular velocity of the shaft must be closely 

 proportional to the input voltage. 



THE USE OF TRANSDUCERS IN BIOLOGY 



In this section some examples will be given illustrating the use of trans- 

 ducers in biological research. 



A capacitance transducer manometer 



An application of the capacitance m-e transducer which is notable both 

 for the elegance of its construction and for the completeness of the mathe- 

 matical and experimental performance analysis is due to Hansen^^~^°. 

 His design for a capacitance manometer is shown diagrammatically in 

 Figure 33.40. A change of capacitance of about 0-05 pF is produced by 

 a pressure change of 1 cm of mercury. Using a circuit similar to that 

 described on page 503, this pressure change can be arranged to give an 

 output of 0-5 V. 



The manometer with hypodermic needle can have a resonant frequency 

 as high as 100 c/s, with nearly critical damping. This response is mxore 

 than adequate for delineating accurately the time-course of human blood 

 pressure (the purpose for which the instrument was designed), and is 



505 



