In situ papermachine web sound velocity test

An on-line instrument in rolling contact with a papermachine web determines the web's strength and elastic modulus properties by intermittently pulsed sonic waves transmitted through the traveling web mass between a full circle, piezoelectric transducer emitter and identical receiving transducers respectively displaced from the emitter along the MD and CD web axes. Transducer signals from the receivers are analyzed by cross-correlation function techniques relative to the original stimulation reference signal to isolate the desired data signal from integral noise.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to nondestructive testing of paper for 
mechanical properties. More particularly, the present invention relates an 
apparatus for continuously testing a paper web ultrasonically while 
traveling within the papermaking machine. 
2. Prior Art 
Paper and paperboard strength properties are important to most converting 
and end-use applications. Mechanical parameters such as ultimate tensile 
strength, burst, and bending stiffness are the strength indicia of 
greatest concern to the papermaker. 
Tests to ascertain these mechanical characteristics of a given paper web 
have, traditionally, been destructive of the test sample or specimen. Loss 
of the sample is of no consequence but the quantity of specialized test 
equipment, skills and time required to perform the full battery of such 
tests is enormous when it is considered that a reliable test average 
requires a large number of test samples. 
Over the past decade, nondestructive ultrasonic methods have been developed 
to measure many of the mechanical properties previously measured by 
destructive tests. By these methods, a vibration induced sonic disturbance 
is transmitted through a sample sheet and the resultant wave velocity is 
measured. Such velocity measurements are taken relative to the test sample 
fiber orientation, both MD (machine direction) and CD (cross-machine 
direction), and used to calculate the sheet in-plane elastic parameters of 
Young's and shear moduli. 
As published by Tappi, Vol. 48, No. 3, March, 1965, the technical paper 
titled "Nondestructive Sonic Measurement of Paper Elasticity" by J. K. 
Craver and D. L. Taylor thoroughly develops the theoretical relationships 
between sonic wave velocity and a paper web modulus of elasticity. From 
the basic theory of Craver and Taylor, a number of authors contributed to 
the evolution of theory and technology embodied by the report of G. A. 
Baum and C. C. Habeger concerning "On-line Measurement of Paper Mechanical 
Properties" appearing in Tappi, Vol. 63, No. 7, July, 1980. U.S. Pat. No. 
4,291,577 issued Sept. 29, 1981 to G. A. Baum and C. C. Habeger, discloses 
the substance of the authors' earlier Tappi report and describes a 
mechanical system for measuring the sonic wave propagation velocity within 
a traveling paper web as found within the papermaking machine. Pursuant to 
the system of Baum and Habeger, three synchronously coordinated idler 
wheels are positioned in friction drive contact with a moving paper web. 
Each of the wheels include a single piezoelectric transducer located on 
the wheel rim for cyclic contact with the traveling web surface. Wheel 
rotation is mechanically timed so that all three transducers are in 
simultaneous contact with the paper web. One of the three transducers 
serves as an ultrasonic signal transmitter whereas the other two are 
receivers. Relative to the transmitter wheel, one receiver is located at a 
known distance along the web in the machine direction and the other at the 
same known distance in the cross-machine direction. When the transducers 
engage the web, an electrical signal to the transmission transducer 
stimulates an ultrasonic mechanical vibration which is transmitted through 
the paper web to the receiver transducers. Responsively, the receivers 
emit electrical signals for receipt by electronic data process equipment 
which compares and determines the time interval between original signal 
emission and signal receipt. This approach to wave velocity measurement 
has been characterized as the "time-of-flight" technique. 
Also applied to paper measurement has been the "wave-phase shift" 
technique. By the wave-phase shift method, the web contacting wheels are 
constructed with four quadrant arc piezoelectric transducers for 
substantially continuous contact with the paper web. A continuously 
emitted 6.75 kH.sub.z sine signal, for example, is electronically 
processed for phase shift determination. From the phase shift, the wave 
velocity is derived. The characterization as a "wave-phase shift" 
measurement technique distinguishes this measurement method from the 
"time-of-flight" technique. 
Both of the prior art wave velocity measurement techniques have respective 
advantages and disadvantages. The time spans between signal stimulation 
and receipt are extremely short: in the microsecond range. It is difficult 
to reliably segregate signal pulses or phase displacements of such short 
duration from extraneous noise always present in a moving paper web. 
Although the "wave-phase shift" signals are continuous and therefore avoid 
the uncertanties of mechanically timed pulsing intervals, other signal 
obfuscation mechanisms are operative. For example, each of the 
piezoelectric transducers are original signal generators of continuous, 
low amplitude signals over a wide frequency spectrum and relatively high 
amplitude signals at a natural frequency determined by the momentary web 
speed driving the transducer wheels. Simultaneously, the transducers 
respond to sympathetic and harmonic vibrations originating from other 
sources around the paper machine but not directly related to the machine 
operation. Even the measurement signal, when received by a reception 
transducer through a carriage conduit other than a direct route through 
the web mass, is a noise source due to a greater or less transmission 
interval depending on the transmission route. 
In the midst of all these spurious signal sources, it is essential to 
instrument reliability that only the original stimulation signal directly 
transmitted through the web mass is the signal processed for sonic 
velocity measurement. It is, therefore, a first object of the present 
invention to teach a process and apparatus for distinguishing these 
desired, web transmitted, acoustic signals from irrelevant noise signals. 
Another object of the present invention is to teach the construction of a 
novel and acoustically quiet transducer assembly for measuring paper web 
strength properties. 
SUMMARY OF THE INVENTION 
These and other objects of the invention are accomplished by a web 
contacting, sonic transducer wheel assembly in which each of the three, 
independently rotating, assembly wheels are fabricated with a full and 
continuous circle, ceramic, piezoelectric annulus that is intimately 
bedded with an aluminum tire. Each wheel is also mounted within an 
acoustic isolation housing. 
Cooperating with the wheel assembly is an electronic equipment combination 
which includes a reference/stimulation signal generator, a programmable 
digitizer and a computational computer. 
One of the transducer wheels is stimulated by the reference signal to emit 
vibratory disturbances through the web mass. The signal characteristics 
are a high frequency pulse generated at a low frequency interval i.e. a 
signal cycle of 50 KH pulse triggered respectively at a 1000 H rate. These 
disturbances stimulate the other two transducers to emit correspondingly 
responsive electrical signals. These responsive signals are filtered, 
integrated, normalized and compared on a time delay sequence for 
probability comparison to the reference signal by cross-correlation 
techniques. The specific time delay that provides the greatest probability 
of synchronization with the reference signal is taken as the "time of 
flight" sonic transit interval through the web mass. 
From such sonic transit interval, the sonic velocity characteristic may be 
calculated and the signal result combined with independently measured 
moisture, basis weight and caliper web characteristic signals to determine 
the strength and elastic modulus characteristics of the web.

PREFERRED EMBODIMENT 
FIG. 1 represents a small portion of dry, papermachine web W in continuous 
transit through the sensory field of the two instruments utilized by the 
present invention. The first instrument is a state of the art 
cross-direction scanner 10 which continuously measures one or more web 
properties such as moisture, basis weight and caliper by means of emission 
and reception field sensors disposed in traverse carriages 11 and 12. 
These carriages reciprocate synchronously across the web width on rigid 
beams 13 and 14. The reception field sensors transmit electrical signals 
proportional to the particular web property measured by a respective 
sensor pair. 
Not specifically shown but as an integral portion of the scanning unit 10 
is a traverse drive mechanism having a cross-direction position 
transmitter represented by element 15 on FIG. 7. It is the drive mechanism 
which moves the traverse carriages 11 and 12 back and forth along the 
beams 13 and 14 without a structural link therebetween to provide clear 
passage space for the web between the carriages. The position signal 
transmitter 15 (FIG. 7) provides the information necessary to correlate a 
particular web property signal to a specific cross-direction location of 
the respective sensor on the web. While there are other web monitoring 
reasons for having a full CD scan of such information, it is only those 
web properties in CD alignment with the modulus measuring unit 20 that 
relate to the invention. Position signal transmitter 15 provides the data 
to make the discrimination. 
Further along the web traveling route are disposed those mechanical, web 
engaging portions of the present modulus measuring instrument 20. This 
mechanical unit 20 will be described in greater detail relative to FIGS. 
2-3, 7 and 8. 
As a discrete unit, these mechanical portions of the invention are attached 
to lower and upper bases 21 and 22, respectively, which are pivotally 
joined by a pin 25. Preferably, the lower base 21 is secured to the 
papermachine frame at the edge plane of the web. The specific mounting 
design is irrelevant except for the characteristics that the entire unit 
20 may be detached and removed from the web proximity when necessary. 
Additionally, the mounting should include a sensitive adjusting mechanism 
for engaging the unit with the web and, perhaps, for adjusting the 
parallelism of the unit relative to the web plane. 
Upper base 22 secures passive idler wheels 23 rotating about spindles at 
the free ends of swing arms 24. These idler wheels are geometrically 
aligned to hold the traveling web in a nip between the idler wheel 
circumference and the circumference of the transducer wheels below. 
It should be noted that the upper base and wheels are pivotally connected 
to the lower base to minimize the possibility of web damage during 
installation and for convenience during web threading. At these times, the 
upper base 22 is rotated 90.degree. away from the web plane. 
To the lower base 21 are attached three active transducer wheels 30, 31 and 
32 and one passive idler wheel 33. None of the lower base wheels are 
rotatively connected: each rotates independently of the others. The idler 
wheel 33 usefulness is limited to that of force balancing: to equalize 
coupling forces in the web plane due to rotational resistance imposed by 
the three active wheels. 
Internal construction of the three transducer wheels, is identical. This 
construction is shown in detail by FIGS. 5 and 6. However, transducer 30 
is electrically connected as an acoustic transmitter whereas transducers 
31 and 32 are acoustic receivers. These relationships will be further 
described relative to the signal schematic of FIG. 7. 
Noting some overall characteristics of the transducer wheels, it will first 
be observed that maximum effort is made to acoustically isolate each 
transducer unit. To this end, vibration dampening material 35 such as soft 
rubber is layered between the lower base plate 21 and the footing surfaces 
of bearing posts 40. Nylon machine screws 41 are used to secure the 
bearing posts 40 to the lower base plate 21. An outer shell 36 of soft 
aluminum is used to enclose each transducer unit. Internally, each 
transducer enclosure is provided with acoustic insulation 42. 
Each transducer wheel assembly is constructed about a flanged axle 44 
having opposed bearing pins 45 and 46. Concentric with the rotational 
axis, a mercury slip-ring 47 is provided on the end of bearing pin 46. A 
bracket appendage 48 from bearing post 40 supports and stabilizes the 
stationary element 49 of the slip ring 47. 
A central bore 51 is provided concentrically through the axle 44 to 
accommodate the rotating signal conductor leads 54 and 55. These signal 
leads are drawn from the axle bore 51 through a radial bore 52 in the 
flange 50. 
A continuous, piezoelectric ceramic ring 60 is the primary active element 
of each transducer wheel. This ring 60 is integrally bedded with a flanged 
aluminum tire 61 by means of an electrically conductive epoxy compound 62. 
In assembly, the ceramic ring edge is electrically insulated from the 
adjacent radial flange of tire 61 by a layer of electrically 
non-conductive epoxy compound 63. 
Independent radial structural links positionally secure the active tire and 
ring elements concentrically to the axle structure. Such links comprise a 
disc-shaped aluminum rim 64 secured to the axle flange 50 and the tire 
flange by nylon machine screws 65 and 66, respectively. However, a sheet 
69 of electrical and acoustic insulating material is clamped between the 
rim 64, the flange 50 and the tire 61 to sonically isolate the tire from 
the rim. 
A similar rim 67, secured to the opposite face of axle flange 50 by nylon 
machine screws 68, completes the wheel enclosure. As on the first wheel 
side, a sheet of electrical and acoustic insulating material 69 separates 
and therefore sonically isolates the rim 67 from the flange and tire 
assembly. 
Signal conductor leads 54 and 55 are electrically connected to the tire 
flange and ceramic ring, respectively. Signals carried thereby are passed 
through the mercury slipring 47 to the static conductor leads 56 and 57. 
The aforedescribed mechanical elements of the invention are electrically 
interconnected in the manner represented by the signal schematic of FIG. 
7. Along the route of web W, the traverse carriages 11 and 12 are shown to 
include sensors for web moisture content, M; web basis weight, B and web 
caliper C. Also shown in connection with the carriage 12, is a 
cross-direction position transducer 15. Each of these signal generators 
transmit the variable quality analog signals proportional to the 
respective conditions to an analog/digital converter 100 which produces a 
corresponding succession of digital values. In signal form, these 
corresponding digital values are transmitted from the converter to a data 
base computer 110 and stored with corresponding elapsed time values. 
Further along the route of web W are the transducer wheels 30, 31 and 32. 
Wheel 30 is connected as the acoustic signal generator whereas wheels 31 
and 32 are acoustic receivers. Wheel 31 is displaced a known distance 
along the web traveling direction (MD) from the wheel 30. Wheel 32 is 
displaced a known distance transversely of the web traveling direction 
(CD) from the wheel 30. These known distances are included with the 
computer 110 data base. 
The electronic components of the invention include a pulse generator 120 
which releases a low frequency trigger pulse signal to a high frequency 
sine function generator 130. For example, pulse generator 120 discharges a 
trigger pulse at the relatively slow rate of 1000 H. This 1000 H trigger 
pulse starts the 50 KH function generation which stops after the 
transmission of one 50 KH cycle. This intermittently emitted 50 KH signal 
is power amplified by element 140 to provide a 300 V, for example, peak 
pulse voltage at the amplifier output terminal 142. From amplifier output 
terminal 142, the signal is divided with a first component directed to the 
transducer wheel 30 and a second component directed to a programmable 
digitizer 160. 
Transducer signals respective to the sonic receiver wheels 31 and 32 are 
conducted to a differential amplifier/filter 150. The filter function of 
the amplifier 150 limits the signals to be amplified to a narrow frequency 
bandwidth centered at 50 KH, for example. The filtered and amplified 
signal product is conducted to an appropriate signal channel of 
programmable digitizer 160. 
The 50 KH reference frequency is the selected product of a particular 
embodiment of the invention and is not a limiting characteristic. In this 
example, the frequency was selected as follows. Relative to the graph of 
FIG. 8, what is seen is a passive power/frequency distribution spectrum 
for a particular sonic receiving transducer 31 or 32 when driven by a 
papermachine web at a known surface velocity. In other words, the FIG. 8 
graph represents the background signal noise coming from transducers 31 or 
32 without sonic stimulation from the sonic transmitter 30. Between 10 and 
15 KH, the passive noise emissions are synergistic. This is the natural 
frequency of the system. It will also be noted that the signal amplitude 
begins to grow at a substantially linear rate as frequency increases 
beyond 50 KH. Obviously, therefore, for this particular profile, 50 KH is 
the frequency most distant from the 10-20 KH natural frequency band and 
simultaneously is accompanied by the lowest voltage amplitude. 
The desired data signals transmitted from the sonic receiving transducers 
31 or 32 will be the respective transducer piezoelectric response to a 
sonic stimulation from transducer 30 at the reference frequency. In a 
noiseless system, the two signals would be identical except for a 
"time-of-flight" delay due to transmission velocity differences through 
the web. The instrument objective is to precisely quantify the magnitude 
of this "time-of-flight." 
In a real system, however, the desired data signals are immersed in a flood 
of noise signals thereby obscuring the exact signal pulse which signifies 
the time-of-flight interval. Nevertheless, the desired data signals 
influence the total signal, desired data plus noise. Consequently, the 
total signal is cross-correlated with the reference signal by the function 
##EQU1## 
where: x(t) is the reference signal at the reference frequency 
y(t+.tau.) is the total signal 
Application of this cross-correlation function is a special case of 
probability where the total amplified signal y(t+.tau.) received by the 
programmable digitizer 160 from the respective transducers 31 and 32 is 
compared by a "fast Fourier transform" analysis to the reference signal 
x(t) received from the function generator 130. 
The mechanics of such a comparison follows the format of a "fast Fourier 
transform" analysis. This comparison is performed by the data processing 
and computational units of the invention represented by FIG. 7. The 
process mechanics of the comparison is explained as follows with reference 
to FIGS. 9 through 15. 
Transducer wheel 30 of FIG. 9 is represented as receiving a distinctive 
electrical stimulation signal s. This signal s is the same as that emitted 
at the amplifier output terminal 142 and is also the x(t) parameter of the 
cross-correlation function. Responsive to the stimulation of electrical 
signal s, transducer 30 emits sonic signals d at the same frequency as s: 
predominately through the web mass but also through the frame structure 
and atmosphere. These sonic signals d are received as stimuli by the 
transducer wheels 31 and 32 which are also being stimulated by other sonic 
sources represented collectively in FIG. 9 as noise. 
Responsive to the sonic signals, d plus noise, transducers 31 and 32 emit 
total electrical signals c and m, respectively. These are the signals 
represented by FIG. 7 as received by amplifier/filter 150. Signals c and m 
are also the parameter y(t+.tau.) of the cross-correlation function. 
Further explanation will delete references to signal c since both signals, 
c and m, are processed identically relative to stimulation signal s. 
Advancing the present explanation to the composite graph of FIG. 10, 
stimulation signal s is represented as an intermittent pulse: a single 50 
KH cycle occurring at 1000 H with a peak voltage value of a.sup.s 
occurring at time t.sub.o. Superimposed on the same time abscissa is the 
total response signal m and the electrical correspondent of sonic signal d 
as a constituent portion of total signal m. Note is given to the fact that 
sonic signal d has the same frequency as stimulation signal s but with a 
pulse apex at voltage value a.sup.m. The absolute value of voltage a.sup.m 
is of no consequence but the time of occurrence relative to the occurrence 
moment of a.sup.s, i.e. .DELTA.t, the process objective 
From superficial observation of FIG. 10, it is seen that although signal 
wave m is far more complex than wave d, there is an average frequency 
correlation between the two. To find the most probable time delay .DELTA.t 
from coincidence between signals s and m, voltage data is taken from both 
signals over a predetermined time span T: arbitrarily selected for this 
example as being the same as the stimulation pulse frequency. This time 
span T is then divided into a number of subdivision increments t.sub.o, 
t.sub.1, t.sub.2, t.sub.3 . . . t.sub.x. Normally, the number of such time 
increments will be an exponential value of 2 i.e. 2.sup.10 =1024 
increments. 
For each of these time increments t.sub.o, t.sub.1 etc., the momentary 
voltage value of each signal s and m is measured, digitialized by the 
converter 100. The digital values are memory stored with the computer 110 
data base in series sequence corresponding to the respective time 
increment. Relative to the graph of FIG. 10 and the table of FIG. 11, it 
is seen that the initial voltage value of signal s is 
##EQU2## 
This value is memory stored in the columnar order of FIG. 11 where where 
the voltage value 
##EQU3## 
is shown under the s column and t.sub.o row. Simultaneously, the initial 
value of signal m, 
##EQU4## 
is measured, digitized and stored under column m, row t.sub.o. 
Sequentially, signal voltage values 
##EQU5## 
are measured, converted and stored as corresponding to signals s and m, 
respectively, and to time increment t.sub.1. 
This process is repeated through time increment t.sub.x or until the end of 
the predetermined time span T. 
These memory stored voltage values are next processed by obtaining the 
product of the s and m signal values respective to each time increment. In 
other words, for the moment t.sub.o, the values 
##EQU6## 
are multiplied to find the product 
##EQU7## 
Similarly, the values 
##EQU8## 
are multiplied to find the product 
##EQU9## 
When all of the voltage values of corresponding time moments are expanded 
to respective products, these products are added to find the total product 
##EQU10## 
This total product 
##EQU11## 
becomes a single data point, .SIGMA.P.sub.t.sbsb.o, on the 
cross-correlation curve of FIG. 14 at the originating time abscissa 
t.sub.o. 
To obtain the second point on the FIG. 14 cross-correlation curve, 
.SIGMA.P.sub.t.sbsb.1, the m signal column data is indexed by one time 
increment relative to the s signal data. This concept is illustrated by 
FIG. 12 which shows the 
##EQU12## 
value combined with the 
##EQU13## 
value for the product 
##EQU14## 
Correspondingly, the 
##EQU15## 
value respective to the s signal at time t.sub.1 is multiplied by the 
##EQU16## 
value respective to the m signal at time t.sub.2. The sum of all such 
indexed data, 
##EQU17## 
becomes the second cross-correlation point .SIGMA.P.sub.t.sbsb.1. 
FIG. 13 illustrates a third example of the foregoing indexing process 
whereby 
##EQU18## 
is multiplied by the m signal value of 
##EQU19## 
from time increment t.sub.2 for the product 
##EQU20## 
The sum of the FIG. 10 indexed products 
##EQU21## 
yields the third point .SIGMA.Pt.sub.2 on the cross-correlation curve of 
FIG. 14. 
Described thus far is the computerized process for cross-correlating the 
voltage values characteristic of signal m to the values of signal s. FIG. 
14 graphically represents a simplified iteration of this process. As 
actually applied, reference is made to FIG. 15 where the process produces 
a series of peaks along the declining slope of reference line R. The first 
departure from this pattern is the peak labeled a.sup.m which falls within 
the window interval between time moments w.sub.1 and w.sub.2 . These time 
window limits are predetermined by the rational extremes of a valid time 
delay for the web under scrutiny considering the physical separation of 
the transducer wheels 30, 31 and 32 and anticipated variations in the web 
characteristics having a known influence over the sonic propagation 
velocity in paper. 
In the specific example of FIG. 15, cross-correlation peak a.sup.m occurs 
at 0.0002 seconds after the reference moment of t.sub.o. This value of 
0.0002 seconds is therefore the assigned value for .DELTA.t in determining 
the sonic velocity of the web in the machine direction according to the 
relationship v=D/.DELTA.t. 
The sonic velocity of the web is combined with the moisture, basis weight 
and caliper characteristics to determine the strength and modulus 
properties of the web. 
Although there are numerous other and stronger cross-correlation peaks to 
follow that of a.sup.m, these peaks are rationally excluded from 
consideration as outside the predetermined time window. Such irrational 
peaks are the products of echoes, harmonics and other transmission media. 
Having fully described our invention,