Sec. 1-2] 



.1 / E( HA XI( 'A L IX ITT T HA XS DVC E US 



41 



Carrier 

 Carbon film 



The sensitivity of such gauges, i.e., the fractional resistance change 

 ARjR per unit strain AL/L, varies from about 30 to 150 (highest 

 value, for vapor carbon, 535), compared to a value of the order of 2 

 for wire strain gauges. The transfer function ARj R versus AL/L is, 

 in general, nonlinear. A material which shows the least deviation 

 from linearity consists of 9 parts, by weight, of cellulose acetate to 

 1 part of Dixon Micronized Graphite No. 200-08. All materials 

 exhibit hysteresis and aftereffects, as shown in Fig. (1-2)21. 



A variation of the carbon-composition transducer is shown sche- 

 matically in Fig. (1-2)22. The composition is deposited to the inside 

 or the outside of a ring-shaped 

 elastic carrier. Four electrodes are 

 applied so that the parts a, b, c, 

 and d of the conductive layers 

 between the electrodes form the 

 arms of a Wheatstone bridge. If a 

 force F deforms the ring, it will cause 

 a compression in the arms a and c 

 and an extension in the arms b and 

 d, leading to an unbalance of the 

 bridge and causing an output 

 signal. 



The contact resistance of the 

 carbon disks and the resistivity of 

 the carbon composition vary strong- 

 ly with temperature. The resistance 

 temperature coefficient is negative and of the order of 3 X 10~ 4 to 

 10 _3 /°C. The above-described method of mounting two transducers 

 in adjacent arms of a bridge reduces the temperature influence con- 

 siderably. Differences of the thermal expansion between the mount- 

 ing frame and the transducer also produce changes in output. The 

 carbon-disk transducer is also sensitive to vibration and to humidity 

 and is easily damaged by oil and vapors. Heat caused by the passage 

 of current through the contact causes an erratic behavior. 



The semiconducting transducer is an intriguing device; it has 

 inherently many of the advantages of an ideal transducer, such as 

 high sensitivity, small mass, good dynamic response, low actuating- 

 force requirements, and a wide range for displacement and force. The 

 output impedance can be varied widely to suit the requirements of 

 the user, and the transducer itself and the associated equipment are 

 very simple. But the semiconducting transducer is inherently un- 

 stable and shows aftereffects, hysteresis, and drift. It seems that no 



Fig. (1-2)22. Ring-type semiconduc- 

 tive displacement transducer (from 

 A. Theiss, Arch, techn. Messen, V 

 1121-2 and 3, May and June 1943; by 

 permission). 



