DYNAMIC MEASUREMENTS ON ELECTROMAGNETIC DEVrCES 1453 



360 



320 



280 



240 



200 



160 



120 



80 



40 







-40 



0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.05 0.10 0.15 0.20 0.25 0.30 0.35 

 TIME IN MILLISECONDS 



Fig. 20 — Output amplifier square wave response. 



followed by (2) an oscillation caused by the frequency response of the 

 amplifier deviating from the ideal for frequencies above 10 kc. The for- 

 mer effect is caused by the finite power supply voltages available in the 

 final stage of the output amplifier. 



The current in the primary of the output transformer represents the 

 function being measured. If a step discontinuity occurs, as during ve- 

 locity measurements, then the current suddenly has to be changed to a 

 different value. The rate of change of an increase in current which can 

 be developed is proportional to the power supply voltage, and hence is 

 finite. For a decrease, the distributed capacitance of the winding and 

 the shunting resistors delay the current decay. These result in a delayed 

 transition from one condition to the other, occuring, from Fig. 20, in 

 about 0.1 millisecond. 



A discussion of the operating conditions of the system leads to a method 

 of evaluating these forms of error and determining what limitations are 

 imposed upon the use of the measured data. It will be shown that only 

 immediately following velocity discontinuities are the data not usable. 

 Displacement data are never in difficulty from an overload standpoint 

 and fortunately we ordinarily are not too interested in velocities after 

 impacts. 



Discontinuity Errors Dm to Overloading 



The arbitrary gain setting for the output amplifier is chosen to provide 

 about the full scale of 200 microamperes to represent the full displace- 



