Capacitive strain gage and signal conditioning system measures stress-induced strain and cancels thermal expansion strain at temperatures to 1,500 F (815 C). Gage does not significantly restrain or reinforce specimen

Egger, R. L.

Wilson, E. J.

English

NASA Technical Reports Server

My 1975	 B75-10069 
•	 NASA TECH BRIEF	 al. 
Flight Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
High -Temperature Capacitive Strain Measurement System 
.
The problem: 
Bonded-resistance strain gages provide very effi-
cient means of measuring stress-induced strain and 
canceling thermal expansion strain at temperatures 
up to about 6500 F (343° Q. The means for measuring 
stress-induced strain at temperatures above 650° F are 
very limited. 
The solution: 
A capacitive strain gage and signal conditioning 
system that measures stress-induced strain and 
cancels thermal expansion strain at temperatures to 
1,500° F (815° C) has been designed. 
How it's done: 
The system consists of a capacitive strain gage 
(Figure 1) that mounts directly on the specimen 
surface and converts the stress-induced dimension 
changes into: (a) changes in capacitance; (b) 
conditioning electronics (Figure 2) which provide a 
carrier voltage to the capacitive gage and demodulate 
the signal from the gage, producing an analog voltage 
proportional to strain; and (c) a three-conductor lead 
wire assembly connecting the gage and the condition-
ing electronics. The gage consists of a link about 1 in. 
(2.54 cm) long, which spans that portion of the 
specimen over which stress-induced strain is to be 
measured, and a capacitance-based linear differential
displacement sensor. The link is rigidly attached to 
the specimen at one end and is left relatively free (in 
the strain-sensing direction) at its other end. This link 
serves to sum the accumulated specimen strain 
beneath it and to transfer the relative motion to the 
displacement sensor located at its free end. 
When the link is fabricated from the same material 
as the test structure, the thermal expansion strain in 
the structure is canceled by the thermal expansion 
strain in the link, so that only stress-induced strain is 
transferred to the differential capacitive displacement 
sensor. Errors resulting from specimen bending are 
nullified by flexures that cause the gage to measure 
the chord strain between its points of attachment to 
the test specimen, even though the link and the 
displacement sensor are located above the specimen 
surface. 
Sensor Flexure
I- '\\ J	 Capacitive Sensor 
Attachment 
Ribbon 
Link	 NSensor Lead Wire 
Terminal 
Excitation Lead 
Wire Terminals 
Attachment 
Ribbon 
.
	
Figure 1. Capacitive Strain Gage Assembly
(continued overleaf) 
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. 
Figure 2. Signal Conditioning System 
The gage operates at temperatures to 1,500° F and 
does not significantly restrain or reinforce the speci-
men. The system has a resolution of better than 1 r, a 
range of over ±20,000 jAF, and excellent linearity and 
temperature stability. It measures static as well as 
dynamic strain and changes sensitivity less than 5 
percent between 70° and 1,500° F (21° and 815° Q. 
System performance is not critically dependent on the 
length of the cable between the gage and the 
conditioning electronics. 
Notes: 
1. The high-temperature capacitive strain gage was 
originally conceived and developed by The Boeing 
Aerospace Company under NASA Flight Research 
Center contract NAS4-1403. Further evaluation 
and improvement of the system were accomplished 
by Boeing under NASA contracts NAS4-1684 and 
NAS4-1812. Still further improvements have been 
and are being made at Boeing. 
2. Boeing has licensed its rights to the capacitive 
gage system to HITEC Corporation which is now 
B75-10069
manufacturing and commercially marketing the 
system. Information about the system can be 
obtained from: 
HITEC Corp. 
Snake Road 
Westford, Massachusetts 01886 
Phone: (617) 692-4793 
Patent status: 
Title to the inventions of the portions of the system 
developed under NASA contracts has been waived 
under the provisions of the National Aeronautics 
and Space Act [42 U.S.C. 2457W], to The Boeing 
Company, Seattle, Washington 98124. 
Source: Earl J. Wilson
Flight Research Center and
Richard L. Egger of
The Boeing Aerospace Co. 
under contract to
Flight Research Center 
(FRC- 10053) 
Categories: 02 (Electronics Systems) 
01 (Electronics - Components 
and Circuitry) 
03 (Physical Sciences) 
06 (Mechanics)
.


High-temperature capacitive strain measurement system

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