1488 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



winding on the rotor and the secondary windings on the stator or vice 

 versa. The primary winding is excited from an alternating voltage 

 source of, say, 400 cycles per second. The amplitudes of the induced 

 secondary voltages of the synchro resolver are ideally proportional to 

 the sine and cosine of the rotor orientation. These two induced, ampli- 

 tude modulated voltages are the resolver output. 



The accuracy of commercially available synchros is, at best, three 

 minutes of arc. Certainly, this accuracy is sufficient for many applica- 

 tions. In the machining of precision parts and in field applications in- 

 volving the measurement of elevation and azimuth of distant targets, 

 however, accuracies down to 10 seconds of arc are required. 



One might be tempted to try to meet this requirement by merely re- 

 fining the present standard synchro. However, even if this refinement 

 were possible, it still would be a difficult task to transmit this near-per- 

 fect synchro output and also to convert it into other analog forms with- 

 out losing most of the added accuracy because of noise in the system. 

 The transmission and conversion problem can be side-stepped by going 

 to a so-called "two speed" or "vernier" representation of the angle. This 

 representation is obtained by using two synchros; one, the low speed 

 synchro, is positioned directly to the particular angle and the other, the 

 high-speed synchro, is geared up with respect to the former. The angle 

 is now represented by two synchro outputs. Assuming perfect gears the 

 accuracy of this system is improved by the step-up ratio in the gearing. 



This approach has been adopted in the past, but unfortunatel}^, it has 

 major disadvantages to it. First, precision gears of better than one min- 

 ute of arc are expensive, relatively large and of limited life due to wear. 

 Second, considerable torque is required to overcome the gear friction and 

 the inertia effect of the high speed synchro. For these reasons, it is de- 

 sirable to replace the geared-up synchro by a transducer which performs 

 the step-up between input and output electrically. The vernier resolver 

 is such an angle transducer. 



The unit is a reluctance type, variable coupling transformer. By 

 placing all windings on the stator, sliding contacts are eliminated. Both 

 the stator and the rotor are built up of laminations. The step-up ratio 

 is equal to the number of teeth on the rotor lamination. Prototype units 

 have been built with step-up ratios of 26, 27, 32 and 33. The accuracy 

 of these units is characterized by a standard deviation error of less than 

 10 seconds of arc. This high degree of accuracy is due largely to the aver- 

 aging effect inherent in a laminated construction. The unit may be re- 

 garded, simply, as a device which senses the average orientation of 

 all rotor laminations with respect to the stator. Because of the great 



