The RF output of a voltage controlled oscillator (VCO) is periodically gated to a transducer which produces acoustic waves in a bolt. The reflected acoustic waves are converted to electrical signals by the transducer and gated to a mixer which also receives the output from the VCO and produces an output which is filtered by a low pass filter. The output of filter is a dc signal proportional to the phase difference change from a fixed phase difference between the two input signals to the mixer. This dc signal is sampled at an instant and held by circuit in response to the "P" signal. The output of the circuit is integrated and then applied to the VCO to change the frequency of the VCO such that the phase difference between the two inputs to the mixer remains at the fixed phase difference. The frequency of the VCO is a measure of the change in strain of the bolt

Heyman, J. S.

English

NASA Technical Reports Server

<;: 572 
2 
I ,  
United States Patent [191 [111 4,363,242 
Heyman [45] Dec. 14, 1982 
PULSED PHASE LOCKED LOOP STRAIN 
MONITOR 
Inventor: 
Assignee: 
Joseph S. Heyman, Gloucester, Va. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
Appl. No.: 199,767 
Filed: Oct. 23, 1980 
Int. Cl.3 ........................ F16B 13/02; GOlH 1/00; 
GOlN 29/00 
U.S. c1. ........................................ 73/761; 73/579; 
73/597; 73/629 
Field of Search ..................... 367/125; 343/7 PL; 
73/579,761, 801, 629, 597 
References Cited 
U.S. PATENT DOCUMENTS 
3,759,090 9/1973 McFaul et al. ....................... 73/801 
3,812,709 5/1974 Benson et al. ........................ 73/597 
4,014,208 3/1977 Moore et al. ......................... 73/761 
4,062,227 12/1977 Heyman ................................ 73/801 
4,117,731 10/1978 Heyman ................................ 73/589 
OTHER PUBLICATIONS 
Instrument for Continuous High Resolution Measure- 
ment by Blume in Review of Scientific Instruments, vsl. 
34, No. 12, Dec. 1963. 
Conference: Proceedings of the Conference on Me- 
chanical Properties of Materials at High Rates of Strain, 
by Jumpei Shioiri and Katsuhiko Sato, pp. 154-162, 
Oxford, England, Apr. 1974. 
ISA Transactions, vol. 19, No. 2 in 1980; Measurement 
of Applied and Residual Stresses with Ultrasonic ... by 
B. E. Gordon, Jr. 
Primary Examiner-Edward H. Kazenske 
Assistant Examiner-David V. Carlson 
Attorney, Agent, or Firm-William H. King; John R. 
Manning 
1571 ABSTRACT 
The RF output of a voltage controlled oscillator (VCO) 
11 is periodically gated by gate 15 to a transducer 18 
which produces acoustic waves in bolt 19. The re- 
flected acoustic waves are converted to electrical sig- 
nals by transducer 18 and gated by a gate 20 to mixer 31. 
Mixer 31 also receives the output from VCO 11 and 
produces an output which is filtered by low pass filter 
32. The output of filter 32 is a DC signal proportional to 
the phase difference change from a fixed phase differ- 
ence between the two input signals to mixer 32. This 
DC signal is sampled at an instant and held by circuit 33 
in response to the “P.’ signal. The output of circuit 33 is 
integrated by integrator 34 and then applied to VCO 11 
to change the frequency of VCO 11 such that the phase 
difference between the two inputs to mixer 31 remains 
at said fixed phase difference. The frequency of VCO 11 
is a measure of the change in strain of bolt 19. 
10 Claims, 3 Drawing Figures 
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1 
PULSED PHASE LOCKED LOOP STRAIN 
MONITOR 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 
ployee of the U.S. Government and may be manufac- 
tured and used by or for the Government for govern- 
mental purposes without the payment of any royalties 
thereon or therefor. 
BACKGROUND OF THE INVENTION 
The invention relates generally to a strain measuring 
device and more specifically concerns a device for mea- 
suring changes in strain of any materials (metals, com- 
posites, glass, rock, concrete, etc.) having many geome- 
tries (bolts, rods, plates, blocks, in situ mine shafts). 
The prior art for determining fastener tension using 
torque depends on the repeatability and consistence of 
friction-a fact which is not achievable. Even for criti- 
cal fasteners with extreme process control and care in 
storage and application, torque is capable of at best a 
nominal 15% variation in fastener strain with a given 
torque. In most applications, the errors obtained using 
torque are far greater due to the fact that up to 90% of 
the applied torque overcomes friction-only 10% goes 
into fastener strain. Thus a variation of 10% in friction 
can cause a 100% change in strain. 
Other techniques, which are accurate, measure fas- 
tener elongation or direct strain. The simplist of these 
methods include micrometer measurements which are 
slow, require access to both sides of the fastener or a 
knowledge of the joint compliance and therefore are 
impractical. Electronic strain gaged bolts are costly and 
require highly stable bonds of the strain gage to the 
fastener for accuracy. 
Ultrasonic techniques have been developed for bolt 
tension measurements which use shock excited acoustic 
pulse time of flight measurements or multiple pulse 
overlap/null methods or continuous wave high har- 
monic resonance techniques. 
The shock excited methods are inherently broadband 
and thus launch an acoustic wave containing many 
frequencies. Each frequency propagates with a unique 
diffraction producing many different effective path 
lengths in the group velocity waveform. Also, this tech- 
nique requires high speed electronics to achieve the 
accuracy necessary for bolt tension measurements (typi- 
cally nanosecond resolution for a 10 cm bolt of round 
trip time 35 x 10-6 sec or 3 parts in 104). 
The double pulse overlap method is difficult to auto- 
mate and requires considerable judgment on the part of 
the user. Furthermore, this method is unnecessarily 
complicated and can easily be misread when cycle for 
cycle overlap is shifted by 27~. 
A gated RF tone burst technique has been disclosed 
which requires user judgment to achieve a proper phase 
locking point and an inability to maintain that locking 60 
point under sample changes without user input. In addi- 
tion, this device does not take advantage of instanta- 
neous phase measurement, but rather averages the 
phase signal over several microseconds. This produces 
greater instability for geometrically varying situations 65 
such as occur in a fastener under preloading. Further- 
more, the time delay involved with using the integral of 
the sample during the phase gate does not allow use of 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
2 
layered samples with shifts occuring during the initial 
pulse propagation phase gate window. 
A continuous wave (CW) method which depends on 
resonance techniques requires high Q fastener geometry 
to achieve an acoustic spectra free of structure which 
may cause locking errors. As the bolt geometry be- 
comes nonideal, as may occur in a fastener with a hole, 
markings on its surface or rough nonparallel surfaces, 
the CW approach becomes impractical. 
It is an object of this invention to provide a device for 
measuring changes in strain of any material having 
many different geometries. 
Another object of this invention is to provide a de- 
vice for measuring changes in strain in which no user 
judgment is necessary. 
A further object of this invention is to provide a 
device for measuring strain that takes advantage of 
instantaneous phase measurements. 
Still another object of this invention is to provide a 
device for measuring strain which is not easily misread 
when cycle for cycle overlap is shifted by 27~. 
A still further object of this invention is to provide a 
device for measuring the strain in a sample whose ge- 
ometry is nonideal. 
Other objects and advantages of this invention will 
become apparent hereinafter in the specification and 
drawings. 
SUMMARY OF THE INVENTION 
The invention is an instrument that uses a pulsed 
phase locked loop (P2L2) technique to measure changes 
in strain in a sample. The P2L2 technique propagates a 
gated Radio Frequency (RF) acoustic wave into the 
sample. The acoustic wave propagates through the 
sample, reflecting from an interface and returns to the 
point of origin. The instrument senses the pressure of 
the acoustic signal, gates the electrical signal from the 
sample that is produced by the reflected acoustic wave 
and samples the relative phase of the electrical signal by 
comparing its phase at an instant during each gating 
cycle with that of the continuously running voltage 
controlled oscillator (VCO) from which the initial driv- 
ing signal was gated, A feedback loop is closed thus 
locking the frequency of the VCO to a fixed phase 
relationship with respect to the VCO. When the sample 
is loaded, strain plus sound velocity dependence on 
strain cause an acoustic phase shift producing a fre- I 
quency shift in the VCO. The frequency shift A F  di- 
vided by frequency F, (AF/F) is linearly proportional 
to the applied load (for elastic loading). Thus, the de- 
vice can be used to accurately measure changes in strain 
independent of fastener friction. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a block diagram of the invention; and 
FIGS. 2 and 3 are diagrams of the outputs of some of 
the blocks shown in FIG. 1. 
DETAILED DESCRIPTION OF THE 
INVENTION 
Turning now to the embodiment of the invention 
selected for illustration in the drawings the number 11 
in FIG. 1 designates a voltage controlled oscillator 
(VCO) whose R F  output is split into four isolated sig- 
nals by a splitter 12. The VCO frequency is reduced by 
a count down circuit 13 which outputs a pulse after a 
certain number, N, of VCO cycles have passed with the 
pulse occurring at a fixed phase point of the VCO signal 
4,363,242 
3 
as shown in FIG. 2. The count down pulse starts a logic 
circuit 14 which outputs a sequence of signals that turns 
on and off transmitter gate “T” 15. The output of gate 
15 is power amplified in a buffer 16 to drive a high 
signal level into the impedance of a matching network 
17 which drives a transducer 18 producing an acoustic 
wave propagating in a bolt sample 19. 
The acoustic signal in the bolt consists of several 
cycles of sound which reflect in the bolt (echo) and are 
converted to an electrical signal by transducer 18, eiec- 
trically impedance matched in 17 and fed to a clipped 
gate “R’ 20. The gate 20 prevents the high power 
buffer 16 from damaging an amplifier 21. Gate 20 is set 
to open by logic 14 when the desired echo from the bolt 
occurs. The output of gate 20 is amplified in amplifier 21 
and peak detected by a peak detector 22, the output of 
which enters an enable device 23 which is told to fire 
once by a delay 24 all shown in FIG. 3. The output of 
enable device 23 fans out to four devices: reset enable 25 
4 
The change in frequency AF divided by frequency F 
of VCO 11 is directly related to the fastener change in 
strain: 
5 
where S is the fastener stress, V the velocity of sound, 
lo E the Young’s modulus and dV/dS the change in sound 
velocity with stress. dV/dS has been shown to be equal 
to: 
dV 15 -= dS 
which arms logic 14 to allow it to continue to the next 20 
sequence of pulses; a time delay 26 which closes an 
analog switch 27 in a feedback loop to VCO 11 after an 
integrator 34 receives a signal; “A” sample hold 28 
which measures the amulitude from ueak detector 22 of 
where p is the unstrained material density, h and p are 
the Lam6 constants and 1 and m are the Murnagham 
third order elastic constants. Rewriting equation (1): 
the first echo and displays the amplitide in an amplitude 
read out 29; and “P” sample hold 33. 
The signal to sample hold 33 is level processed in an 
amplitude leveler 30 to assure amplitude insensitivity 
when the signal enters a mixer 31. Mixer 31 which also 
receives an input from VCO 11, provides a sum and 
difference frequency output of the VCO frequency. 
Since the signals into the mixer are both at the VCO 
frequency a DC level and a 2F signal is obtained. The 
2F is filtered in a low pass filter 32 and the remaining 
DC level which contains phase information (phase shift 
between the VCO and the returned echo) is sample held 
in 33 at the instant of the “P” pulse from enable 23. 
The output from sample hold device 33 is integrated 
in integrator 34 and read out by a stress read out device 
35. After pulse “P” from enable 23 has responded, the 
analog switch 27 locks and stays locked until 26 no 
longer continues to receive “P” pulses (several cycles of 
logic from logic 14). Should the locked loop momen- 
tarily lose signal and hop to a different phase stability 
point (a 27r phase shift), a lock-hop detector 36 signals 
faulty operation by sensing a sudden jump in the control 
voltage to VCO 11. 
In automatic operation, if auto enable 23 does not 
receive a signal from peak detector 22 during the delay 
24 window, a “I”’ pulse is not generated. Such a condi- 
tion exists before the transducer is applied to the bolt. 
Reset enable 25, after a long delay (about 0.1 sec) with 
no “P” pulse, allows the cycle to start over with logic 
device 14 initiating a new sequence of signals. If, on the 
other hand, a pulse exists at peak detector 22, auto en- 
able 23 sends out pulse “P” thus properly locking the 
pulsed phase locked loop (P2L2) automatically. Logic 
device 14 awaits the “P” pulse to activate reset enable 
25 before it can start a new sequence on command from 
25 
As=[+] K 
(3) 
where K is the stress acoustic constant and for a simple 
30 fastener under uniform axial strain is: 
(4) 
35 
K is a constant for a given material in its elastic range. 
For the PzL2 technique, the locked loop maintains a 
constant phase condition. The total phase shift at fre- 
quency F1 in a sample of acoustic transit time t, is: 
40 
45 or simply: 
AFl -At1 
Fi tl 
-=- 
50 The mth harmonic resonance is defined as: 
or 
AFm -At1 (8) count down device 13. 60 -=- 
of the integrator 34 adjusts the frequency of the VCO 
11 to a phase condition with respect to the signal which 
holds the integrator voltage constant-nominally zero 
volts out of sample hold 33. Should the phase shift in 65 
bolt 19 change, the frequency of VCO 11 shifts to main- 
tain the locked condition. Thus the change in frequency 
of VCO 11 tracks changes due to fastener strain. 
Under both automatic and manual control, the output Fm f l  
Thus, the P2L2 has a frequency that varies linearly with 
applied stress. 
The functional blocks shown in FIG. 1 are all well 
within the skill of one having ordinary skill in the elec- 
tronic art. Hence the details of these blocks arc not 
disclosed in this specification. 
5 
4,3 63,242 
The advantages of this invention over prior art strain 
measuring devices are: the strain in any material can be 
measured, the shape of the material sample can have 
many different geometries, no user judgment is neces- 
sary, no reading errors are made when cycle for cycle 5 
overlap is shifted by 2 7  and instantaneous phase mea- 
surements or comparisons are made. 
The invention can be used with a two transducer 
system without any loss of generality. In addition, the 
stress read out can incorporate a microprocessor or 10 
other device to calculate actual AF/F values from the 
frequency counter and convert them into stress in MPa 
or PSI notation including proper calibration for K or 
bolt geometry. 
What is claimed is: 15 
1. An instrument for measuring changes in strain of a 
transducer means attached to said sample for convert- 
ing electrical signals into acoustic signals in said 
sample and for converting the acoustic signals re- 20 
flected by said sample into electrical signals; 
sample comprising: 
a continuous wave RF electrical source 
means for periodically gating said RF source to said 
transducer means; 
means for periodically gating away from said trans- 25 
ducer means the signals produced by the acoustic 
signals reflected by said sample such that the times 
that the signals are gated away from said trans- 
ducer means do not overlap the times that the R F  
source is gated to said transducer means; 
means receiving the R F  signals from the R F  source 
and said signals periodically gated away from said 
transducer means for producing a DC electrical 
signal proportional to the change from a fixed dif- 
means receiving said signals periodically gated away 
from said transducer means for producing a pulse 
during each period that signals are gated away 
from said transducer means and during a time that 
a reflected signal is received, the duration of said 40 
produced pulse being substantially less than the 
duration of the gating away time; 
means receiving said DC electrical signal and said 
pulse for sampling said DC electrical signal at the 
time said pulse is received and holding the sampled 45  
signal until the next pulse is received; and 
3 0  
ference in phase of the two received signals; 35 
5 0  
55 
6 
means receiving said sampled and held signal for 
changing the frequency of said RF source to main- 
tain said fixed difference between the phases of said 
RF source to maintain said fixed difference be- 
tween the phases of said RF source and said signals 
periodically gated away from said transducer 
means whereby the changes in frequency of said 
RF source are proportional to the changes in strain 
in said sample. 
2. An instrument according to claim 1 including 
means for measuring the frequency of said RF source to 
provide a measurement of the change in strain of said 
sample. 
3. An instrument according to claim 1 wherein said 
RF electrical source is a voltage controlled oscillator. 
4. An instrument according to claim 1 wherein said 
means for producing a DC electrical signal includes a 
mixer receiving said RF signal and said signals periodi- 
cally gated away from said transducer means and a low 
pass filter connected to the output of said mixer. 
5. An instrument according to claim 1 wherein said 
means for producing a pulse includes a signal peak de- 
tector receiving said signals periodically gated away 
from said transducer means and means for selecting one 
of the peaks produced by said signal peak detector dur- 
ing each of said gating away periods wherein said se- 
lected peak is said pulse. 
6. An instrument according to claim 5 including 
means for measuring said selected peak for providing a 
change in strain of said sample. 
7. An instrument according to claim 5 wherein said 
selected peak is the first peak during each said gating 
away period. 
8. An instrument according to claim 1 wherein said 
means for changing the frequency of said RF source 
includes an integrator at the output of said sampling and 
holding means. 
9. An instrument according to claim 8 including 
means for measuring the output of said integrator to 
provide a stress readout. 
10. An instrument according to claim 8 including a 
lock-hop detector means at the output of said integrator 
for signaling faulty operation by sensng a sudden jump 
in the control voltage to the voltage controlled oscilla- 
tor. * * * * *  
65 


Pulsed phase locked loop strain monitor

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