Sec. 1-2] 



MECHANICAL INPUT TRANSDUCERS 



89 



electrodes A and B; the other side of the source S is connected to a 

 ring electrode R which externally surrounds the tube. A discharge 

 takes place between A and R as well as B and R. Any asymmetry 

 of this arrangement brought about by longitudinal shifting of the 

 ring electrode causes a d-c voltage to arise between A and B. The 

 purpose of the capacitors C 1 and C 2 is to prevent shorting of this d-c 

 voltage. The output voltage E arising between the internal elec- 

 trodes A and B varies linearly with the displacement of the external 

 ring electrode over a wide range. The sensitivity is in general between 

 1 and 10 volts/mm. which is con- 

 siderably lower than in the trans- 

 ducer system of Fig (1-2)68. Also 

 this system is relatively insensitive 

 to variation of the supply source; 

 a change of 10 per cent of the fre- 

 quency and also a variation of 1 

 per cent of the supply voltage are 

 barely noticeable in the output. 



Variation of the symmetry can 

 also be obtained by capacitance- 

 variation circuits. The output 

 impedance of the device is of the 

 order of 1 MQ; therefore, the 

 succeeding stage must have a high 



input impedance. The system is applicable for mechanical displace- 

 ments of a steady-state character and up to mechanical frequencies 

 of several thousand cycles per second. 1 



d. Mechanic-Optical Transducers. Mechanical displacements can 

 be converted into electric signals by optic-electrical means, for 

 instance, by an arrangement consisting of an illuminated slit and a 

 photoelectrical transducer; the moving object obscures a part of the 

 slit and causes a variation of the light intensity reaching the 

 photoelectrical transducer and hence a variation of the transducer 

 output. 



An arrangement of this type used primarily for the conversion of 

 rotary displacement into a digital output ("shaft-position encoder") 

 is shown schematically in Fig. (1-2)71. An optical system L produces 

 a number of parallel light beams which are directed upon corre- 

 sponding photo transducers P. The light beams traverse an encoder 



1 K. S. Lion, Rev. Sci. Instr., 27, 222 (1956). A commercial ionization trans- 

 ducer tube (T-42, The Decker Corporation, Philadelphia) is shown in Fig. 

 (1-2)70. 



Fig. (1-2)71. Mechanic-optical trans- 

 ducer, shaft-position encoder with digi- 

 tal read out; L, light source; D, encoder 

 disk; P. phototransducers. 



