Accelerometer with FM output signals indicative of mechanical strain on i

Hartenstein, R. G.

Kirchman, E. J.

English

NASA Technical Reports Server

Aug. 10, 1965 R. G. HARTENSTEIN ETAL 
ACCELEROMETER WITH FM OUTPUT 
Filed May 29, 1963 
INVENTORS 
EDWARD J. KIRCHMAN 
RAYMOND G. HARTENSTEIN 
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3 19,9,340 ' 3 United States Patent Ofice , , ,, ,,,, 
3,199,340 
ACCELEROMETER WITH F M  OUTPUT 
Raymond G. Hartenstein, Seabrook, and Edward J. Kirch- 
man, Baltimore, Md., assignors to the United States of 
America as represented by the Administrator of the 
National Aeronautics and Space Administration 
Filed May 29,1963, Ser. No. 284,265 
5 Claims. (Cl. 73-48.5) 
(Granted under Title 35, U.S. Code (1952), sec. 266) 
The invention described herein may be manufactured 
and used by or for the Government of the United States 
of America for governmental purposes without the pay- 
ment of any royalties thereon or therefor. 
The invention relates to an improved transducer and 
in particular to an accelerometer with a frequency modu- 
lated output. 
Many accelerometers can be found in the prior art. 
Most of these prior art accelerometers produce voltages 
whose amplitudes are proportional to the accelerations 
to which they are subjected. If these produced voltages 
are to be transmitted any appreciable distance they must 
be used to modulate a carrier frequency and then trans- 
mitted. In accordance with the present invention there 
is provided an accelerometer which directly produces a 
frequency modulated output which can be transmitted. 
The device is simple, rugged, small in size and has neg- 
ligible weight making it ideal for use on space vehicles. 
It could be attached to a space vehicle at a remote loca- 
tion and transmit its frequency modulated output to a 
more suitable location on the vehicle for recording or re- 
transmitting to ground stations. The signal produced by 
the device is of sufficient amplitude to drive the FM 
modulator of a standard transmitter as now used on flight 
programs. 
It is therefore an object of the invention to provide an 
accelerometer with a frequency modulated output. 
Another object of the invention is to provide an ac- 
celerometer which is simple, small in physical size, rugged 
and has negligible weight making it ideal for use on space 
vehicles. 
A further object of the invention is to provide a trans- 
ducer that will produce a frequency modulated electrical 
signal which is indicative of the mechanical strain to 
which the transducer is subjected. 
Other objects and a fuller understanding of the inven- 
tion may be had by referring to the following specification 
and the accompanying drawings in which: 
FIG. 1 shows a strain sensing device which is used in 
the preferred embodiment of the invention; and 
FIG. 2 shows a circuit diagram of the preferred em- 
bodiment of the invention. 
In describing the preferred embodiment of the inven- 
tion illustrated in the drawings, specific terminology will 
be resorted to for sake of clarity. However, it is not in- 
tended to be limited to the specific terms so selected, and 
it is to be understood that each specific term includes all 
technical equivalents which operate in a similar manner 
to accomplish a similar purpose. 
In the preferred embodiment of the invention a resistor, 
a variable voltage source, a PNPN transistor having a 
negative resistance characteristic and two strain sensitive 
devices are connected in a series circuit. The transistor 
can be the PNPN hook collector type designed to have 
or>l as described in "PNPN Transistor Switches" by 
J. L. Moll et a]., Proceedings of the IRE, vol. 44, Septem- 
ber 1956, or it can be any PNPN transistor having a nega- 
tive resistance characteristic. The strain sensitive devices 
are any devices which will produce changes in resistance 
when they are subjected to a strain. In the preferred 
embodiment two semiconductor strain sensitive devices 
are used, however, one or more of any type strain sensitive 
device could be used. The voltage source is connected 
in the series circuit so as to back bias the intermost NP 
junction of the transistor. The voltage source is varied 
until oscillations are caused in the series circuit because 
5 of the negative resistance characteristic of the transducer. 
These oscillations produce a constant frequency signal 
across the resistor. Whenever the strain sensitive devices 
are subjected to a strain, the resistance connected in series 
with the transistor changes which changes the frequency 
10 of the signal produced across the resistor. This frequency 
change is related to  the strain to which the strain sensitive 
devices are subjected and is the output of the preferred 
embodiment of the invention. 
Turning now to the specific embodiment of the inven- 
15 tion selected for illustration in the drawings, the number 
11 in FIG. 1 designates a beam which is attached to a 
surface 12 to form a cantilever beam. Surface 12 can be 
any surface where a mechanical strain such as a strain 
due to acceleration, is to be measured. Beam 11 is 
20 made from strain sensitive semiconductor material such 
as N-type germanium which undergoes changes in re- 
sistance when it is subjected to a strain. When beam 11 
is under tension it decreases in resistance and when it 
is under compression it increases in resistance. A termi- 
25 nal 13 is connected to one end of beam 11 and a terminal 
14 is connected to the other end of beam 11. A beam 
15 is also attached to surface 12 to form a cantilever 
beam. Beam 15 is made from strain sensitive semi- 
conductor material such as Ptype germanium which 
30 undergoes changes in resistance when it is subjected to 
a strain. When beam 15 is under tension it increases in 
resistance and when it is under compression it decreases 
in resistance. A terminal 16 is connected to one end of 
beam 15 and a terminal 17 is connected to the other end 
35 of beam 15. A spacer 18 is placed between beams 11 and 
15 to transmit any strain on one of the beams to the other 
Spacer 18 can be made from any suitable insulating ma. 
terial which will electrically insulate beams 11 and 15 
from each other. When beam 11 and 15 are subjected 
40 to a strain indicated by arrow 19 the resistances of each 
of the beams will change. The magnitude of these 
changes is indicative of the magnitude of the strain to  
which these two beams are subjected. 
Referring to FIG. 2 there is shown the strain sensitive 
devices disclosed in FIG. 1 connected in an electrical cir- 
4.3 cuit. The electrical circuitry enclosed by the dotted box 
20 in FIG. 2 is the strain sensitive beams 11 and 15 dis- 
closed in FIG. 1. In FIG. 2 beam 11 is represented as 
a variable resistor 11 and beam 15 is represented as a 
variable resistor 15. One end of variable resistor 11 is 
50 connected to a first terminal of a PNPN transistor 21 and 
the other end of variable resistor 11 is connected to a first 
end of a resistor 22. One end of variable resistor 15 is 
connected to a second terminal of PNPN transistor 21 and 
the other end of variable resistor 15 is connected to the 
55 positive terminal of a variable voltage source 23. The 
negative terminal of voltage source 23 is connected to the 
other end of resistor 22. Terminals 24 and 25 are con- 
nected across resistor 22 to provide an output. Variable 
voltage source 23 is connected so as to back bias the inter- 
60 most PN junction of transistor 21. Transistor 21 is de- 
signed to have a negative resistance characteristic such as 
the PNPN hook collector type transistor as described in 
the J. L. Moll et al. reference referred to above. 
In the operation of this invention, variable voltage 
65 source 23 is varied, without any strain on beams 11 and 15, 
until oscillations are setup in the circuit of FIG. 2. These 
oscillations will produce a signal at some frequency across 
output terminals 24 and 25. Whenever beams 11 and 15 
are subjected to a strain the resistances of these beams will 
70 vary which will change the frequency of the signal pro- 
duced across terminals 24 and 25. This change in fre- 
3,109,340 
3 4 
qucncy across output terminals 24 and 25 is indicative of 3. A transducer as claimed in claim 1 wherein said 
thc strain to which beams 11 and 15 are subjected. There- transducer is an accelerometer. 
forc the signal across terminals 24 and 25 is frequency 4. A transducer for producing an electrical output 
modulated in accordance with the strain to which beams which is frequency mo&~lated in accordance with the mag- 
11 and 15 are subjected. 5 nitude of a strain comprising: a resistor, a voltage source, 
It will be apparent to those skilled in the art that the and a PNPN transistor with a negative resistance charac- 
novel principles of the invention disclosed herein with the teristic connected in series in such a manner that the inner- 
specific exemplification thereof will suggest various modi- most PN junction of said transistor is back biased whereby 
fications and applications of the same. For example, the constant frequency oscillaions are produced acrass said 
invention is disclosed as an accelerometer, however, it 10 resistor; and strain ~ensitive means which produces a re- 
could be used to measure any mechanical vibration or sistance change re la td  t~ the magnitude of the strain to  
mechanical strain. With minor modifications the inven- which it is subjected connected in series with said resistor, 
tion c o ~ ~ l d  be used to measure any phenomena which can voltage source and transistor whereby when said strain 
bc translated into mechanical displacement such as pres- sensitive means is subjected to said strain the frequency of 
sure or temperature. The prcferred embodiment of the 15 said oscillations produced across said resistor will change. 
invention is disclosed as having two strain sensing means. 5. A transducer for producing an electrical output 
However, any number could be used. Also, these strain which is frequency modulated in accordance with the mag- 
sensing means could be any type strain gage which will nitude of a strain comprising: an impedance, a voltage 
change in resistance when strain is applied to it. It is also source and a transistor with a negative impedance char- 
possible to make the outermost N and P layers of transis- 20 acteristic connected in series whereby constant frequency 
tor 21 in the form of beams and let them function as the oscillations are produced across said impedance; and strain 
strain sensing means. It is accordingly desired that in con- sensitive means which produces an impedance change re- 
struing the breadth of the appended claims they shall not lated to the magnitude of the strain to which it is subjected 
be limited to the specific exemplification of the invention connected in series with said impedance, voltage source 
described herein. 26 and transistor whereby when said strain sensitive means is 
What is claimed as new and desired to be secured by subjected to said strain the frequency of said oscillations 
Letters Patent of the United States is: produced across said impedarice will change. 
1. A transducer for producing an electrical output 
which is frequency modulated in accordance with the mag- References Cited by the Examiner 
nitude of the strain to which the transducer is subjected 
comprising: a PNPN transistor having a negative resist- 
ance characteristic; strain sensing means for producing a 
resistance change related to the magnitude of said strain to 
which the transducer is subjected; an impedance; and a 
voltage source connected in series with said transistor, said 
strain sensing means, and said impedance in such a way 
as to back bias the innermost PN iunction of said transis- 
UNITED STATES PATENTS 
12/54 Stanton ---------------- 73-517 
6/59 Leblond -------------- 33 1-132 
1/62 Circone ---,---------- 33 1-107 
1/62 Pratt -----,,---------- 73-88.5 
9/62 Rut2 -------,--------- 331-107 
1/63 Li ------------------- 331-107 
tor whereby oscillations having a frequency related to the 
magnitude of said strain to which the transducer is sub- RICHARD C. QUEISSER, Primary Examiner. 
jectcd are produced across said impedance. 40 JAMES I. GILL, Examiner. 
2. A transducer as claimed in claim 1 wherein said 
impedance is a resistor. 


Accelerometer with FM output Patent

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