Hydraulic servo system increases accuracy in applying fatigue loading to a specimen under test. An error sensing electronic control loop, coupled to the hydraulic proportional closed loop cyclic force generator, provides an accurately controlled peak force to the specimen

Dixon, G. V.

Kibler, K. S.

English

NASA Technical Reports Server

December 1967	 Brief 67-10637 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Hydraulic Servo System Increases Accuracy in Fatigue Testing 
Proportional Loop  
Limit
	
Servo Valve	 Comparator S3	
___!IL.si	 Error 
—E Si 
Comm andgnaI
Generator
Servo
Amplifier 
I	 Sensor 
Signal Generator [jti 
For Solid State 
Single-Pole, Double-
Throw Switches
Fatigue Detection 
Specimen	
Force Voltage	 Error 
Track and Store	 /	 Integration 
ens r
rir
S2. 
f+E 
-j 
HIGH FREQUENCY PRECISION CYCLIC LOADING SYSTEM 
The problem: 
To increase accuracy in applying fatigue loading 
to a specimen under test. The conventional hydraulic 
proportional loop loading system has a closed loop 
gain near the ultimate to allow the use of a minimum-
size servo valve for high frequency response. Large 
overshoot and cyclic effects caused by this high gain 
are eliminated by causing the system command to 
change levels whenever the velocity changes sign. 
Manual adjustment of the command level is then re-
quired to achieve the desired peak forces. Such a 
system exhibits relatively large force errors for small 
changes in system gain, component drift, and changes 
in test specimen characteristics.
The solution: 
An error sensing electronic control loop, coupled 
to the hydraulic proportional closed loop cyclic force 
generator, which provides an accurately controlled 
peak force to the specimen. The error sensing loop 
maintains an accuracy of 0.1% at the peak force 
points, thereby eliminating manual adjustment of the 
command levels. 
How it's done: 
The fatigue loading system, shown in the diagram, 
consists of a proportional loop hydraulic system and 
an error sensing electronicl control loop. 
The proportional loop consists of a servo amplifier 
with a limit control, a servo valve which drives a 
(continued overleaf) 
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piston, a velocity sensor, a force sensor, and the 
fatigue specimen. Frequency control is obtained by 
setting the limits on the servo amplifier. This is the 
normal assembly used for fatigue loading of a speci-
men. 
To increase the accuracy of the system, an error-
sensing electronic control loop is added to the pro-
portional loop. This control loop consists of a signal 
generator, a force voltage tracker and storer, an error 
detection comparator, an error integrator, SPDT 
switches, and a command signal generator. 
The electronic error sensor tracks and stores the 
force sensor's output voltage to the point where the 
velocity changes sign, i.e., the point of maximum 
force. The stored output voltages are then compared 
with the desired command reference voltages in the 
error detection comparator. An error signal generated 
for each peak is integrated and added to the appro-
priate command voltage level, and the composite com-
mand signal is fed to the proportional loop hydraulic 
system. 
The velocity sensor of the proportional loop sys-
tem triggers the signal generator which drives the 
SPDT switches. These switches, Si and S2, appro-
priately channel the inputs and outputs of the error
sensing electronic control loop to achieve prosper 
feedback voltages, S3, into the proportional loop. 
The entire loading system is rapidly convergent, and 
continuously strives to drive the system error to, and 
maintain it at zero. 
Notes: 
I. The system was developed for specimen testing re-
quiring dynamic forces of ±5000 pounds at a fre-
quency of 50 Hz. 
2. Inquiries concerning this invention may be di-
rected to:
Technology Utilization Officer 
Langley Research Center 
Langley Station 
Hampton, Virginia 23365 
Reference: B67-10637 
Patent status: 
Inquiries about obtaining rights for the commer-
cial use of this invention may be made to NASA, 
Code GP, Washington, D.C. 20546. 
Source: G. V. Dixon and K. S. Kibler 
(L.AR-2l7) 
Brief 67-10637	 Category 01


Hydraulic servo system increases accuracy in fatigue testing

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