60 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



(fatigue) amounting to as much as 2 per cent per month has been 

 reported. 



Temperature variation is of twofold influence upon the transducer 

 performance: First, an increase of temperature causes an increase of 

 inductance at zero stress, which is in general 0. 1 per cent/°C; second, 

 an increase of temperature changes the stress sensitivity of the trans- 

 ducer by about 0.2 per cent for a variation of temperature of 1°C. 



Variation of the supply voltage causes relatively small variation 

 in the transducer output, since the inductance variation depends 



(besides the mechanical stress) upon the 

 magnetic field strength in the probe; it first 

 increases with increased field strength and 

 then decreases beyond a certain operating 

 point. There exists, therefore, an operating 

 region where small supply- voltage variations 

 have no influence upon the transducer 

 "* 3 output. 



The material for the core should have 

 Fig. (1-2)42 Synchro- hi h tensile stre ngth, and high electrical 



transducer, schematic dia- ° ° ° 



gram. resistivity to decrease eddy currents; the 



maximum permeability should occur at high 

 values of field strength to provide a high power output from the 

 transducer. 



Magnetoelastic transducers have been used primarily for the direct 

 measurement of large forces, up to several tons, and for fast transient 

 phenomena where frequency responses in the order of several thou- 

 sand cycles per second are required. They can be built as rugged 

 instruments that can withstand accelerations of several thousand 

 g, but their characteristic depends upon temperature, and — like 

 many transducer systems based upon variation of the physical prop- 

 erties of materials — they need individual calibration, i.e., their out- 

 put cannot be computed and their characteristic can be changed, 

 even irreversibly, by environmental influences. 



e. Synchros. Synchros (trade names: Selsyn, Teletorque, Auto- 

 syn, etc.) are mutual-inductance transducers with multiple outputs 

 and are used for rotary displacements. A synchro system with two 

 pairs of output coils is shown schematically in Fig. (1-2)27.^-4, 

 another system with three output coils in Fig. (1-2)42. The synchro 

 contains an armature or rotor with connections made through slip 

 rings, and a stator carrying two or more (usually three) pairs of 

 windings. In the conventional use, an a-c voltage is applied to the 

 rotor causing a current and a magnetic flux in the rotor. The flux 



