HYDROMHANICS
0
AERODYNAMIGS 0 StRUG11JRAI MECHANICS0
APPLIED MATHEMATICSTechnche H
it
DelfLA MODEL TRACKING SYSTEM FOR THE DTMB MANEUVERING BASIN
by
F.E. Friliman
INSTRUMENTATION DIVISION
RESEARCH AND DEVELOPMENT REPORT
A MODEL TRACKING SYSTEM FOR THE DTMB MANEUVERING BASIN
by
F.E. Frilirnan
TABLE OF CONTE PITS
Page
ABSTRACT ...-
1INTRODUCTION 1
BACKGROUND 1
TRACKING SYSTEM REQUIREMENTS 3
ULTRASONIC TRACKING METHODS 3
THE SPHERICAL TRACKING PRINCIPLE 4
THE HYPERBOLIC TRACKING PRINCIPLE 5
GEOMETRY OF HYPERBOLIC SYSTEM 7
DISCUSSION OF ERRORS . 11
INSTRUMENTATION TECHNIQUES 12
ULTRASONIC PULSE CONSIDERATIONS 15
REVERBERATIONS 18 TRANSDUCER CONSIDERATIONS 22 PROTOTYPE iNSTRUMENTATION 24 ThANSDUCERS 24 TRANSMITTER 25 RECEIVERS 30
EVALUATION OF PROTOTYPE INSTRUMENTS' 33
SYSTEM DESIGN CONSIDERATIONS 36
CONCLUSIONS.. 38
ACKNOWLEDGMENTS 38
REFERENCES 38
APPENDIX A - TRANSDUCER CALIBRATION DATA 39
LIST OF FIGURES
Page
Figure 1 - Spherical Tracking Principle .4
Figure 2 - Principle of Hyperbolic Trac1ing System 5
Figure 3 - Geometry of Hyperbolic Tracking System 6
Figure 4 - Three-Dimensional Aspect of Tracking Scheme 9
Figure 5 - Error Data 10
Figure 6 - Simplified Block Diagram of Tracking Instrumentation 12
Figure 7 - Waveform of an Ultrasonic Pulse 16
Figure 8 - Ultrasonic Pulse Detection Error 16
Figure 9 - Reverberation Patterns 19
Figure 10 - Reverberation Interaction 20
Figure 11 - Cylindrical Transducer Configuration 23
Figure 12 - LC-33 Pressure: Transducer, . 24
Figure 13 - Transducer Frequency Response Characteristics ...25
Figure 14 - Block Diagram of Experimental Ultrasonic Pulse Transmitter 26
Figre 15 - Transmitter Oscillator Circuit 26
Figure 16 - Transmitter Pip Generator Circuit 27
Figure 17 - Transmitter Monostable Gate Circuit . 27
Figure 18 - Transmitter Power Amplifier Ciràuit 29
Figure 19 - Block Diagram of Receiver Components 30
Figure 20 - High-Pass Filter
...31
Figure 21 - Detection Level Amplifier 32
Figure 22 - Experimental Tracking Data 34
Figure 23 - Preliminary Design of DTMB Model Tracking System 35
ABSTR ACT
This report explains the fundamentals of a system for tracking both submarine and surface models in the maneuvering basin at the David Taylor Model Basin. The system employs underwater ultrasonic ranging techniques to establish, repetitively, the horizontal rectangular coordinates of a model-borne transmitter. The discussiOn includes mathematical considerations,
instrumentation measurement techniques, a description of experimental
instru-ments, and the results of evaluation experiments.
INTRODUCTION
The acquisition at the David Taylor Model Basin of a maneuvering and seakeeping
facility (MASK) with a usable test area approximately 320 by 200 ft opened the way for a new variety of model maneuvering tests. 'It also required the development of methods for
tracking the path traveled by a model during maneuvers.
This report explains the fundamentals of a hyperbolic tracking scheme which uses underwater ultrasonic ranging techniques. Instrumentation-measurement techniques appli-cable to the design of a tracking system are explained, and the experimental instruments used to verify the feasibility of a suitablö tracking system are described, together with the results of experimental work, Certain significant problems of a physical nattire created by transmitting and receiving ultrasonic pulses in a water mediUm are discussed briefly in the
hopethatother investigations will pursue them more rigorously than was possible within the scope Of this study.
BACKGROUND
Soon after the MASK facility became available, tests were planned to evaluate
the performance of self-propelled, free-running, radio-controlled surface models1 in all types
of surface maneuvers. In 1957, the Hydromechanics Laboratory requested an engineering
study2 to explore Various methods of tracking a surface model todötermine the most practical scheme for the development of an automatic tracking system. The various schemes
consid-ered included, underwater sound-ranging techniques, optical light-sensing,systems, closed
circuit television, radar, photography, and inertial navigatiOn. Of these, the optical light-sensing systems were deemed most practical. Thereupon, experimental development work was initiated on a system employing: shore-based, optical-scanning mechanisms for sensing a light in a model to establish its angular position. The reduction of the angularposition
data would reveal the course traveled by the mOdel. EAperimental instrumentation was developed and. evaluated. The results were favorable although some of the engineering
problems were not completely solved.
owever, in .1958, the Pydromechanics Laboratory launched an ambitious program (DTMB Problem 503-907) for the development of completely free-running, self-propelled
submarine models. The models are to contain the necessary data-measuring and recording
instrumentation, .automati c-programming instrumentation, and a shore-operated underwater
electromagnetic remote-control system. In conjunction with this difficult test program, a means of tracking submarine models was required. The light-sensing system was not
adapt-able for tracking underwater models.
Of all the schemes previously considered, the one employing underwater ultrasonic ranging techniques had the best potential for tracking underwater models. Although many
problems and some doubts were. acknowledged, the results of a preliminary engineering
study indicated that development of a suitable underwater ultrasonic tracking system was feasible. As a result, a recommendation was made to proceed with experimental
develop-ment work which would lead to a better appraisal of many of the problems.
Early in 1959, the Hydromechanics Laboratory authorized further work which began
with the design and development of experimental instruments for transmitting and receiving underwater ultrasonic pulses. The first opportunity to evaluate these instruments in the maneuvering basin came in March of 1960. The tests were not successful. The instruments were not capable of reliable pulse transmission and detection for the maximum required range of about 350 ft although they did work satisfactorily at shorter ranges, i.e., under 100 ft.
Further attempts to satisfactorily improve the first experimental instruments failed, and the project suffered a setback in time while the underwater pulse transmission problem
was reconsidered.
After further study and deliberation, a second set of experimental instruments was evolved for transmitting and receiving underwater pulses. These instruments included newly purchased transducers, redesigned transmitting circuitry, and new -pulse receiving amplifiers. Preliminary tests in March of 1961 proved that they were capable of transmitting, and reliably detecting, ultrasonic pulses for maximum distances in the maneuvering basin.
By May of 1961, further development work resulted in a simplified experimental
track-ing system. The results of maneuvertrack-ing basin experiments showed that this prototype system was capable of locating the position of the underwater transmitter with adequate accuracy. It was concluded that the tracking scheme arid the type of hardware that had been developed and evaluated offered a suitable means of tracking underwater,and surface models. There were, however, certain reservations, pending a thorough evaluation of a completed tracking
system For example, some doubts remained regarding the capability of the system to reliably track urfác models riding on waves.
In June 1961, a decision was made to begin work on the final design and fabrication of a complete automatic underwater ultrasonic tracking system for permanent installation in the maneuvering basin facility.
TRACKING SYSTEM REQIIIR EM ENTS
The primary functiOnal requirements of. the tracking system for the DT'B Maneuvering
Basin facility are: -
-1. The system hal1 resolve the course of submarine or surface models in the horizontal plane only.'
2 The system shall function to establish the rectangular coordinate positlon** ofa point
on the model below the water. The repetition rate for determining the positionshall be three times per sec.
In the final data reduction, each rectangular coordinate of the model position shall be accurate to within 0.25 ft.
The tracking data shall be recorded initially on magnetic tape in digital form accept-able to data-analysis equipment availaccept-able in the Applied Mathematics Laboratory of the Model
Basin.
There are, of course, many requirements of secondary importance which will not be
enumerated herein. Some of these strongly influence the design or even the choice of alter-native tracking methods. For example, Within the fully instrumented models to be tracked, there will be little space available solely for tracking instrumentation. A promising
track-ing scheme which, would require a space of 1 cu ft or more might be rejected in favor of an alternative system requiring considerably less model space.
As a result of work discussed in this report as well as some considerations beyond the scope of this report, it has been concluded that a suitable tracking system to meet all
of the Model Banin requirements can be designed employing the methods disóussed herein.
ULTRASONIC TRACKING METHODS
The model can be geometrically located if distances between it and several fixed points are known; if the model position is determined periodically, it may be tracked. The fact that ultrasonic energy radiates through the water at a constant velocity of approximately
4800 ft/sec provides the basis foran underwater tracking system. The distance d between
'Veijc plane trécking data (depth) can be readily obtained by means of a depth transducer in the iodeL
**Refers to receiver arrays located along two right-angle walls of the basin.
an underwater ultrasonic pulse transmitter and receiver can be determined by the formula:
d=Vt
Eli
where V is the propagation velocity of the ultrasonic pulse
and t is the time required for the pulse to travel from transmitter to receiver.
Under controllable conditions, the pulse propagation velocity V is a constant which can be accurately determined. The pulse travel time t can be measured with precision by appropriate electronic instrumentation.
On this basis, a model may be tracked by means of a model-borne ultrasonic transmit-ter,an appropriate array of ultrasonic receivers, and electronic time-measuring and recording instrumentation. Two different schemes which use this approach are the "spherical" princi-ple and the "hyperbolic" principrinci-ple.
THE SPHERICAL TRACKING PRINCIPLE
The spherical tracking principle is illustrated in Figure 1. Two receivers are situated alOng a line parallel to the X-axis, spaced a known distance (2 times D) apart. For a given transmitter location, the distances d1 and d2 are measured. These are the radii of two inter-secting circles with centers at (-0, k) and (+D,k), respectively. A simultaneous solution of the equations of the two circles yields the equation in Figure 1 which establishes the x-coord-mate of the transmitter. Similarly the y-coordinate of the transmitter location can be resolved by means of two receivers situated along a line parallel to the Y-axis.
In accordance with Equation [1], the distance d1in Figure 1 can be resolved in term of the time required for the pulse to travel the distance from transmitter to Receiver 1. To determine the pulse travel time, a timer is started at.the instant the pulse is transmitted and is-stopped at the instant the pulse is detected at the receiving transducer. Similarly, a sec-ond timer and Receiver2 can be employed to measure the time for the pulse to travel the distance d2. The two pulse travel times can be used in Equation [1 to resolve the two
distances d1 and d2. These distances
are then used in the equation (Figure 1) to compute the z-coordinate of the trans-mitter. For this scheme, one should be
fully aware that it is necessary to estáb-lish, at the shore based instrumentation, the precise time that each pulse is
trans-mitted from the model-borne transmitter.
This consititutes an engineering problem
in the case of. completely free-running models.
Figure 1 - Spherical Tracking Principle ID
TRANSMITTER (x,y)
[V4 - (d1-d2)(d2-d3)] [(d1-.d2)+(2-d3J
-P [(d1
d2) - (d2 - d)] -.-Figure 2 -. !riiiciple of Hyperbolic Tracking System
5
RECEIVER 3
THE HYPERBOLIC TRACKING PRINCIPLE
The hyperbolic principle is discussed in detail later in the report. Figure 2 illustrates the essence of the. scheme and shows the equation used to compute the .z-coordinate. With the hyperbolic scheme, it is not necessary to uniquely determine the distances d1, d2, and
d3 in Order to find the distance differences. For example, the difference d1 - d2 can be
determined as follows:
d1 = V (t - t0)
where V is the pulse propogation velocity, to is the time a pulse is transmitted, and
i is the time the pulse reaches Receiver 1.
Similarly,
d2 = V (t2-t0) [31
where t2 is the. time the same pulse reaches Receiver 2
Subtracting EqUation [31 from Equation [21,
(d1-d2)=V(t1-t2)
[41Notice that the time t0, appearing in Equations [21 and [31 cancels out in Equation [41. Thus it is not necessary to establish when a pulse is transmitted in order to determine the quantity
(d1 - d2). It is only necessary to establish the difference in the two times at Which a trans-mitted pulse arrives at Receiver 2 and Receiver 1, respectively, (t1t2).
A transmitted pulse can be used to start a timer when it arrives at Receiver 2 and to stop the timer when it arrives at Receiver 1.
The indicated time will correspond to the quantity (t1 -t2) in Equation [4] which yields (d1 d2). The distance differences (d2 d3) can be determined in a similar.manner. The two distance differences used in the equation in Figure 2 yield a solution for the coordinate distance z.
The ycoordinate of transmitter can be resolved by means of an array of three. receivers parallel to the Y-axis.
Of the two tracking schemes discussed, the hyperbolic system was chosen for the Model Basin application. A major factor in the selection was that one does not need to determine the time of pulse transmission t0.
GEOMETRY OF HYPERBOLIC SYSTEM
This section discusses the geometry of the hyperbolic tracking system assuming that the difference in distances from an underwater transmitter to a pair of receivers can be determined with suitable instrumentation. Equations for resolving the coordinate position of the transmitter in terms of the distance differences are developed and discussed.
In the. rectangular coordinate diagram shown in Figure 3a, the point T (z, y) represents a movable transmitter. The points R2 and !?3 represent fixed receivers located, respectively, at the center of the coordinate system and a distance D from the Y-axis. The quantity
(d2d3) is the known variable. Considering a given value Of thequantity (d2d3), the locus of all of the possible positions of T (x, y) is defined as a hyperbola illustrated by the dotted curves in Figure 3a. The receiver positions R2 and R3 correspond to the focuses of the
hyperbola.
D
The center ofthe hyperbola lies at the point C, (j. 0). The conjugate axis of the D
hyperbola lies along the line z
The standard equation given in most texts. for a hyperbola having its conjugate axis
parallel to the Y-axis may be recognized as
(y....k)2 1
[5] a2
Corresponding parameters in Figure 3amay beexpressed in the form of Equation [5] as
/
D2
(z
. 2
Figure 3a - Application of Two Receivers [D2(d1_ d2)(d2_d3)] [(d1d2)+(d2_d3))
Ndi d2)(d2d3)]
D -2 V-'
\
Negative \ Curve I'1Z1
Figure 3b - Application of Three Receivers
Figure 3 - Geometry of }TyperbOlic Tracking System
/
Positive/
Curve T (a, y) I 0I x=
12
Note that the sign of the quantity (d2-d.3) can be positive or negative depending upon. which receiver (R2 Or R3) is nearer to the transmitter T. In Figure 3a, if 7' is nearer to (R3) as shown, the sign of the quantity (d2-d3) will be positive and Twill be somewhere along the "positive" hyperbolic curve.. On the other hand, if 7' is nearer to i?2, the quantity (d2-d3) will be negative whereby T will lie on the "negative" cirve in Figure 3a. Thus, time-measur-ing instrumentation must establish the signs as well as the values of the correspondtime-measur-ing dis-tance difference data.
Data obtainable with only two receivers are not sufficient to determine uniquely either of the two rectangular coordinates of the transmitter location. However, one of the coordinates can be resolved with the addition of a third receiver.
Figure 3b, similar to Figure 3a, illustrates the application of three receivers, R1, !?, and R3, which provide a pair of distance differences (d1 -d2) and (d2-d3) for given location of the transmitter 7' (x, y). The locus of (d.2-d3) is a hyperbolic curve which is defined by
Equation [61. Similarly, the locus of (d1-d2) is a second hyperbolic cUrve defined by the
equation I D\2
Lx+I
2\
2/
Y..
=1
[71 d 2 fd1-d\2
D2 (i-2
21
4'2
The intersection of the two hyperbolic curves identifies the coordinate position of T (z, y). Equations [6] and [7] can be solved simultaneously to yield a solution for the -coordinate. Thö resulting equation is
[D2-(d1--d2) (d2-d3)] [(d1-d2) + (d2-d3)1
[8]
2D [(d1-d2)
(d2-d3)1 where the parameters are illustrated in Figure 3bIn accordance with the.above procedure, it is evident that the .ycoordinate of 7' (x, y) in Figure 3b can be determined with an additional array of three receivers (not shown) arranged along a line parallel to the Y-axis. One might observe that an additional array of receivers is not necessary and that.a solution for the y-coordinate as well as the x-coordinate can be ob-tained by using the three receivers, R1, !?2, and I?3 (Figure 3b). This is true if the discussion is limited to a single X-Y plane.. The model tracking problem, however, involves
three-dimen-sional considerations. . .
-The tracking system requirement is to tradc models only in the horizontal plane which is referred to as the X- Y plane. T-Towever, the model is not necessarily restricted to a single
X-Y plane. Model depth, corresponding to a third dimension (z.), is a variable which has to be confronted in some tests This is true in the case of submarine models undergoing vertical
riianeUvers. It is also trUe to a more.limited xtent in the caeofurfaôe model.s riding on
waves In the case of surface model tests in still water, the model is limited to a single X-Y plane where the z parameter is a constant; in this case, however, the receivers, lOcated several feet below the water surface, will be situated in a different X-Y plane from the
trans-mitter.
[D2(d1d2) (d2d3)] [(d1-.d2) + (d2d3)1
2 D [(d1d2) - (d d3)]
Figure 4 - Three-Dimensioiial Aspect of Tracking Scheme
The tracking schethe is illustrated in three dimensions (X, Y, and Z) in-Figure 4. Three receivers, R1, R2,. and R3, are situated along one wall of.a test tank and along a line' paralle1 to the Xaxis. The transmitter is located at a point T (x, y, ) at a distance x from the Y-Z reference plane. 3y three-dimensional definitiOn, the x-coordinate of the transmitter
is the distance between the YZ reference plane and the Y-Z plane in which the 'transmitter is located.
'quation [8] is valid for resolving the i-coordinate. It makes no difference where the transmitter is positioned within the Y-Z, plane for 'quation [81 provides the x-coordinate of
the Y-Z plane in which the transmitter is located.
[81
o--_--t
H"
-J Ko -0D -a 160 80Figure 5 Error Data
For each transmitter location identified by a letter in the diagram of the maneuvering basin, the table shows the error in the x-coordinate as a function of:
A +0.1 ft error added to the absolute value of(d1 -d2)and zero error added to(d2-d3).
Zero error added to (d1-d2) and a +0.1 ft error added to the absolute value of(d2-d3).
The worst combination of ±0.1 ft errors which is +0. 1 ft added to one
variable and -0.1 ft added to the other variable
jo
320 ft
0 80 160
Error in x-Coordinate in Feet
No. of Feet +0.1 Foot
--0 1 Fo t +0.1 Foot Error
Transmitter Transmitter
Error E in (d1-d2)
x-Location Located From j (d1d2)
-.0.1 Foot Error XIS (d2.d3) in(drd3) A 140.000 -0.462 +0.059 -0.521 B 140.000 -0.340 +0.055 -0.395 C 140.000 -0.214 +0.110 -0.324 D 140.000 -0.138 +0.183 -.0.321 E 120.000 -0.165 +0.069 -0.234 F 120.000 .0.144 +0.063 -0.207 G 0.000 +0.075 -0.075 +0.150 H 80.000 0.000 +0.123 -0.123 I 0.000 +0056 -0.056 +0.112 J 0.000 +0.050 -0.050 +0.100 K 80.000 0.000 +0.071 -0.07 1 L 80.000 O.000 + 0.052 -0.052
In terms of only three variables, d1, d2, and d.3 (Figure 4), there is no way of deter-mining where, within the Y-Z plane, the transmitter T (x, y, z) lies. Thus, the required y-coordinate cannot be determined only in terms of these three variables.
An independent measurement of the 2-coordinate of the transmitter could be used to
establish the -coordinate of the transmitter. Although this is a feasible solution, it is not
considered practical in view of the alternative of using additional receivers.
The y-coordinate of the transmitter can be determined by means of a second horizontal array of three receivers situated along the wall in a Y-Z plane
DISCUSSION OF ERRORS.
A hyperbolic tracking instrumentation system will be subject to a variety of different
types of measurement errors. Any one type of error can be equated to a corresponding error
in the distance data variables used to compute the model coordinates. On this basis, this section will discuss errors strictly in terms of distance difference data. Various instrument errors discussed in ensuing sections can be equated to distance errors and referred to this section for appraisal. In this manner, the effects. of any type of error in resolving the coordi-nates of the model-borne transmitter can be evaluated.
The maneUvering basin has a usable horizontal area of about 320 by 200 ft. The
tentative tracking scheme for determining the v-coordinate of the model-borne transmitter
will involve three receivers mounted along a line about 1 ft away from one wall of the basin, 16 ft below the water surface. Two of the receivers will be situated near the corners of the basin with the third receiver located near the center of the 320-ft length. Thus, the distance between each pair of receivers may be fixed at 160 ft, as shown in Figure 5.
It is convenient to assume a small error in the two variables (d1 d2) and (d2d3). Then the error in resolving the i-coordinate as a function of the errors in the variables can be computed in acôordance with Fquation [8]. Such computations are listed in the tabula-tion of Figure 5.
The data in Figure 5 show that the greatest errors result when the transmitter is near a corner receiver. A small error in the variables becomes less significant when the transmit-ter is removed from the corner receivers. It is a favorable as well as a practical assumption that for the vast majority of tests, the models will he confined to an area somewhere within
the middle of the basin, as indicated by the dotted line in Figure 5. The more extreme errors
encountered outside of the assumed test area, particularly near the corners, can be ignored for practical purposes. .
The reader may assume any test area different than that shown in Figure 5 whereby an interpolation of the data will provide an idea of the order of magnitude of worst tracking
errors for assumed errors in the variables (d1d2) and (d2d3). Furthermore, the reader may
assume different errors in the variables than 0.1 ft on the basis that the errors in x are
linearly proportional to the errors in the variables for small magnitudes of errors. For example, a 0.2-ft error instead of a 0.1-ft error in the variables will cause twice the error in z for each case tabulated in Figure 5.
INSTRUMENTATION TECHNIQUES
This section discusses fundamental instrumentation techniques thatcñ be üséd to
determine the distance difference data used to resolve the x'y coordinates of a model-borne
transmitter of underwater ultrasonic pulses.
A block diagram of basic instrumentation is shown in Figure 6. The difference
dis-tances (d1d2) and (d2d3) may be e uated in accOrdance with Equation [4],.i.e.,
d1-d2 = V (t1=t)
where V is the ultrasonic pulse propagation velocity,
t1 is the time at which a pulse arrives at Receiver 1, and
t2 is the time at which a pulse arrives at Receiver 2.
RECEIVER NO. I RECEIVER NO. 2 PREC!S!0N cLocK OSCILLATOR COUNTER COUNTER B A . I
Figure 6 Simplified Block Diagram of Tracking Instrumentation
12
TRANSMITTER
RECEIVER NO. 3
PULSE PULSE PULSE
AMPLIFIER AMPLIFIER AMPLIFIER
NO.1 NO.2 NO. 3
LOGIC LOGIC
The instrumentation (1'iguro 6) provides a means of determining the quantity (t,-t2) in Equation [41. The arrival of a transmitted pulse at either Receiver 1 Or Receiver 2 will cause Logic Gate A to open and allow oscillator pulses to be counted on Counter A. \"hen the ultrasonic pulse reaches the other receiver, it will close the logic gate to stop the count-ing. The total number of cycles registered by the counter corresponds to the difference in
time of arrival (t, -t2) of the Ultrasonic pulse at the two receivers. This time difference can be determined in accordance with the equation
tlt2
-Iwhere n2 is the number of oscillator pulses counted and
f
is the frequency of the oscillator. Substituting this expression for (t, -t2) in Equation [41 givesd1 - V
f [101
The ultrasonic pulse propagation velocity V in Equation [101 can be determined with the aid of the instrumentation by measuring the time required for the pulse to travel a known distance. F'r example, consider positioning the transmitter anywhere between Receiver 2 and Receiver 3 (Figure 6) so that the transmitter is directly in line 'With all three receivers.
In this special case, the distance difference (d,-d2) is equivalent to the known distance D
between Receiver 1 and Receiver 2. In response to a transmitted ultrasonic pulse, CounterY3
will registera count corresponding to. the time required fOr the pulse to travel the known
dis-tance D. The velocity V of the transmitted pulse can be determined from the simpk formula
D
V=.
-t
where D is the known distance between the two receivers and t is the time required for the ultrasonic pulse to travel this known distance. Using the counter reading corresponding to
tin Equation [1111 gives
V=.
DDf
[121d"f Nd
where Nd is the number of clock oscillator cycles counted during the time for the pulse to travel the distance D and fis the frequency of the oscillator. Substitutilg this expression
for V in Equation [10] gives
d1
-where 0 is the fixed distance between receivers, .
Nd is the number, of Counts côresponding to.the distance D,
12 is the number of counts corresponding to the difference in distances (d1 -2)' and
n2,3 is the number of counts corresponding to the difference in distances (d2d3). flnuation 115] does not involve either the oscillator 'frequency or the ultrasonic pulse propa-gation velocity. Both of these vitally important factors are taken into account in the values
of Nd, the counter reading corresponding to the time required for the pulse to travel the k,nown
distance 12.
A practical tracking system might include .a means of automatically repeating a cali-bration procedure for determining value of Nd just prior to recording of tracking data. Thus, any slight change in oscillator frequency over a long time period would automatically be accounted for, by a corresponding change in the value of Na. Similarly, a slight change in propagation velocity of the ultrasonic pulse, possibly due to a change in water temperature, for example, would also be taken into account.
--.., The simple instrumentation illustrated in Figure 6 does not provide the sign of the counter reaTd'ings n12 and n23. Either of these readings can be plus or minus depending upon
the region of the transmitter location. As an example, it is evidentin Figure 6, that' the sign of the counter reading n12 corresponding to the distance difference (d1d2) is positive. In
this case, an ultrasonic pulse arrives at Receiver 2 before arriving at Receiver I) Conversely, when the pulse arrives at Receiver 1 first, the corresponding coUnter reading
illbé ngative.
Therefore, logic circuitry actuated by the received pulses must establish the sign 'of' each
counter reading. ' , '
14
[131
Similarly, the distance difference (d2d3) can be equated as
nD
d d =
1141 aUsing Equation [13] and Equation [141 in substitution for the difference in distances (d1d2) and (d1d3) in Equation [8], (page 9) yields a solution for the i-coordinate of the transmitter location in terms of instrumentation counter readings. That 'is,
(A' n12n23) (n12 + .n3)
x=D
The frequency of the precision clock oscillator in Figure 6 should be chosen to
pro-vide the maximum profitable time-measuring resolution. MOst digital counters have an
ambig-uity of plus or minus one count in the least significant digit which may influence the choice of the counting frequency. Consider as an example, a counting frequency of 100 kc (this is the frequency used in experiments hat will be discussed later), assuming a counter ambiguity
of plus or minuS one count. This plus or minus one count may cause a time measuring dis crepancy of 10 gsec,- i.e., each time difference measurement. corresponding to a distance difference may be in error by as much as 10 sec. Equation [41 may be used to equate a time
measurement error to a èorresponding error in the distance differences. Assuming a pulse pro-pagation velocity of approximately 5000 ft/see, a 10-zsec error in the time measurement Will cause a corresponding error of 0.05 ft in each of the distance differences (d1 . d) and
(d2 - d3). The effect of this 0.05-ft error in resolving the x-coordinate of the model-borne transmitter can be appraised by means of Figure 5. The error due to counter ambiguity could
be decreased by increasing the oscillator frequency, assuming that the counter will still have an ambiguity of only plus or minus one count in the least significant digit.
In order to establish the requirements of the counter, it is necessary to consider the maximum number of digital counts that can occur as a consequence of tracking system para-meters. The maximum possible difference in distances from the transmitter to a pair of receivers is equivalent to the fixed distance between these receivers. Assume a recOiver spacing of 160 ft, a counting frequency of 100 kc, and a pulse propagation velocity of 4800 ft/sec. The corresponding maximum counter reading will be 3300 digital couflts. For binary counting, the counters must have a capacity for 12 binary data bits. If the counting frequency
were increased to 200 ke, the maximum possible count would increase to about 6600 and binary counters would require a 13-bit capacity.
ULTRASONIC PULSE CONSIDERATIONS
The essence of the problem of transmitting and receiving underwater ultrasonic pulses is to determine precisely when a transmitted pulse arrives at the different receiver locations. A measurement of the differences in times at which the transmitted pulse arrives at pairs of receivers makes it possible to. determinéthe coordjnates of the transmitter.
In this application, the ultrasonic pulse consists of seyeral cycles of alternating pres-sure signal transmitted into a water medium; see Figure 7.'It is significant that the pulse requires a few cycles to build up to maximum amplitude. The accuracy of te time difference measurements, which directly affects the accuracy of locating the model-borne transmitter, is mutually dependent upon the frequency of the ultrasonic pulse and the pulse "buildup" time. Consider a transmitter located different distances from a pair of receiving transducers. The nearest receiving transducer will experience a transmitted pulse at a larger amplitude that the further receiver because the amplitude of the transmitted pulse is attenuated as a function of the distance traveled from the transmitter. Since the receivers are equally
Figure 7 Waveform of an Ultrasonic Pulse
The figure' shows the waveform of the 75-kc
volt-age impressed across the terminals of the transmitting
transducer. The transducer imparts -a compressional
pressure signal into the water which fluctuates in a
manner corresponding to the voltage excitation.
.4
Figire 8 - Ultrasonic Pulse Detection Error
An ultrasonic pulse aveform is shown at two differ-- Cntdiffer-- amplitudes as it might appear tO twodiffer-- different.
receivers at -different distances from a transmitter. It
is assumed that each receiver will respond when the received pulse amplitude exceeds the detection level
- indicated-by the dashed -line. The fact that each of'two
receivers respond to a different cycle of the same trans-mitted pulse causes a time measurementerror.
sen'itive in order to detect pulses transmitted from afar, each receiver will respond to a dif ferent portion of the transmitted pulse; this is illustrated in Figure 8. The fact that each of two receivers responds to a different cycle of the same transmitted pulse cauesa time
mea-surement error.
This. error may be expressed as
V
[161 'P
where :\d is the error in the difference of pulse travel times,
is the difference in the number of cycles of the pulse initially detected by a pair of receivers,
V is the pulse propagation velocity, and is the pulse fre4luency.
A typical example of the error encountered can be Computed, assuming practióal values for the dependent parameters suCh as,
= 1 cycle V = 5000 ft/sec
=lOOkc
In accordance with Equation [161, the computed error M is 0.05. ft.
The consequences of a d error can be appraised by means of Figire 5. The 005-ft error in the example above will cause errors about one-half the magnitude of those tabulated in Figure 5. Of course, the amount of error Ad in detecting a transmitted pulse will increase greatly as the transmitter moves closer to one of the corner receivers, since the received signal amplitude at the corner receiver becomes many times greater than the amplitude of the received signal at the further receivçrs.
The error in detecting the arrival of an ultrasonic pulse at different receivers is expect-ed to be the greatest single source of error in tracking system instrumentation.. This error can
be minimized by resorting to a high ultrasonic frequency.
A preliminary appraisal of this error problem led to the conclusion that a pulse frequen
cy well above several hundred kilocycles would safely guarantee Model Basin accuracy require ments.- It becomes increasingly djfflcult to reliably transnit and receive Ultrasonic pu1ses
above the 100.kc frequency range over the distances involved. The eults of the
experi-mental work conducted at the Model Basin indicate that a pulse frequency of 75 kc is ade-quately high considering the pulse buildup time dharacteristic of the experimental equipment.
A higher pulse frequency would be desirable to improve accuracy further.
There is an added advantage of employing a high ultrasonic pulse frequency The higher the pulse frequency, the less the probability of, encountering difficulty with acoustical noises transmitted into the water by thodel machinery or other mechanical sources.
REVERBERATIONS
An omnidirectional underwater ultrasonic pulse transthitted within an enclosed tank such
as the DTMB Maneuvering Basin will readily and repeatedly reverberate from the sides of the
tank, the bottom of the tank, and the sUrface of the water, as well as from submerged objects within the tank; see Figure 9. These reverberations result in a virtually'infinite number of ultrasonic signal paths throughout the tank. As two or more such paths cross, depending up-on their phase relatiup-onship, they can reinforce each other to producea resultant signal larger in amplitude than either of the component signals. Conversely, the crossing signals can can-cel each other. Eventually, all of the reverberations will be attenuated to an insignificant amplitude. Wowever, reverberations are the potential source of problems discussed in this
section.
The function of each receiver is to detect the arrival of the ultrasonic pulses traveling directly from transmitter to receiver. Subsequent to a direct pulse, a multitude of reverbera-tion signals will reach a receiver as illustrated in Figure 9. Fortunately, all reverberareverbera-tions reach a receiver sometime, however small, after the direct: pulse since any reverberation path must be finitely longer than a direct signal path. Assuming that a receiver responds to the direct pulse, then reverberations immediately following are of little concern. However, consider a special case whereby the path of a reverberation signal is only very slightly longer than the path of the pulse traveling directly from the transmitter to the receiver. The rever-beration will intercept the direct pulse at the receiving transducer barely after the direct pulse; see Figure 10.
It is possible that at the instant this signal interception occurs at the receiving trans-ducer, the direct pulse, which is "building up" in amplitude, has not reached a level suffi-cient to be detected by the receiver. Depending upon the precise, phasing of the intercepting signals, the reverberation can cancel the direct sinal, cycle for cycle, as shown in Figure
lOc. In this case, the direct signal may never reach an amplitude sufficient to be detected.
On the other hand, if the phasing of the two signals is favorable, they can add to cause more
certain detection of the resultant signal as shown in Figure lOa. In the case of cancellation of the direct signal, it is inevitable that some later reverberation will ause the receiver to
respond erroneously.
Calculations as well as the experiments conducted at the Model Basin indicate that detrimental signal cancellation can occur under conditions where the transmitting transducer is only a few inches below the water and a maximum distance away from the receiving
trans-ducer. In this case, the reverberation path is barely longer than the direct signal path. Even
in this case, the two signals must be exactly out of phase with each other. The signal phas-ing is dependent upon both the precise spacphas-ing between the transmittphas-ing and receivphas-ing
trans-ducers. and the depth of the transmitting transduc&r. To result in signal cancellation, each of these two dimensions must be critical within a fraction of an inch at the instant a pulse is
I 'iuta.
pI
iiutaui
iiuutiuu
niituuu
FIr1I,i1!I....
- '':;vI
Li1i1IIIUUU
uritiuui
I UTI
p !' !W' i
r
I
A1j
àk ALILi ItiJI I.
ulilti..'.
,r'l tiu
U,
H iti.'..
ULdtIU
Figure 9 - Reverberation Patterns
The oscilograms of signals at the output of an ultrasonic pulse receiver illustrate the nature of multitudinous reveiberations that abound as a consequence of a single ultrasonic pulse transmitted within the DTMB Maneuvering Basin. The transmitting transducer was
fixed at a depth of 10 ft in one corner of the maneuvering basin while the receiving
trans-ducer was fixed 350 ft away in a diagonally opposite corner. The physical conditions are identical for each oscillograrn shown. Only the oscilloscope sweep time was shortened
progressively to define the signal immediately trailing the pulse traveling directly from the transmitting to the receiving transducer. The oscilloscope sweep time, i.e., the total time of observance, is shown alongside of each oscilogram.
19
Figure 9a - 200 msec
Figure 9b - 50 rnsec
Figure 9c - S mac
!I,,1!JJUtI! iI'V.
IIL I1
tiiiui
I20
Figure lOu . Trannitting Transduce! at Depth of 10.5 Feet
Figure lOb - Transmitting Transducer at Depth of 2.8 Feet
Figure lOc -. Transmitting Transducer at Depth of 0.3 Feet
Figure 10 - Reverberation Interaction
The three oscillograms of the signal at the output of a receiver Illustrate reverberations interacting with an ultrasonic pulse traveling directly from transmitter to receiver In each
case, the transmitting transducer was about 350 ft from the receiving transducer fixed several feet below the surface of the water. Only the depth of the transmitting transducer was changed
to produce the different waveforms shown Figure lOu shows the reverberation adding to the direct pulse. Figure lOb shows the reverberation cancelling the direct pulse which has
al-most reached maximum amplitude Figure lOc shows the reverberation arriving immediately
after the direct pulse and cancelling such that the direct pulse does not ge a chance to
reach maximum amplitude.
transmitted. Therefore, even when the transmitting transducer is located only a few inches below the water surface, it is problematical whether detrimental signal cancellatipn will occur. For most model tests, the transmitting transducer will be located deep enough so that cancellation cannot occur before the direct pulse is detected by ill. receivers. The probability of encountering the signal cancellation is increased for the case of tracking surface models riding on large waves.
Those reverberations that linger too long following a transmitted pulse can cause difficulty. A simple receiver cannot discriminate against reverberations prevailing at an amplitude as large or larger than a pulse traveling directly from transmitter to receiver. Therefore, the receiver can respond erroneously to the lingering reverberations unless suffi-cient time is allowed between transmitted pulses for all reverberations to expend to insignifi cant amplitudes. This can restrict the rate at which pulses can be transmitted and reliably detected. . The problem is aggravated by the fact that multiple reverberations can repeatedly reinforce each other and linger with high amplitudes for significantly long periods.
Experiments conducted in the maneuvering basin show that a significant amount of reverberation signal following a 75-kc ultrasonic pulse can persist with high amplitudes for more than 200 msec. As a consequence, the fastest repetition rate at which the pulses can be transmitted and. reliably detected is in the order of three titnes per second This
repeti-tion rate is compatible with Model Basin tracking requirements. It is possible that certain unique locations of the transmitter can result in reinforced reverberation signals that linger up to 300 msec or even longer, causing limited difficulty. Thus far, limited experimental work has not provided a satisfactory appraisal of the extent of the problem. There is, how-ever, some indication that the problem may exist to a very insignificant extent for 75-kc pulses transmitted at the rate of three ner second.
The previous discussion has been concerned with two troublesome aspects of
rever-berations: First, the reverberations immediately following a transmitted pulse may interfere with the detection of the pulse, and secondly, the lingering reverberations are likely to pre-vail too long at large amplitudes. In between these two extremes, discrimination against reverberations can be accomplished as follows: Assume that pulses are transmitted at the
rate of one pulse every 300 msec and that the receiver responds to one of the first couple of cycles of the transmitted pulse. Immediately, the receiver acts to "trigger" time-measuring circuitry and simultaneously closes a gate in the receiver which automatically inhibits
ensu-ing signals, includensu-ing reverberations. After a predetermined time delay, say about 250 rnsec,
the receiver inhibiting gate automatically, opens to allow the receiver to respond to the next transmitted pulse due within the next 50 msec The detection level for each receiver can be adjusted so that the receiver barely responds to tjie smaJest pulse amplitude prevailing when the transmitter is the maximum pOssible distance from the receiver. Thus, reverberation sig-nals prevalent during the uninhibited 50-msec period will be completely ignored as long as they are somewhat smaller in amplitude than the smallest amplitude pulse traveling directly
from transmitter t6 receiver.
TRANSDUCER CONSIDERATIONS
The function of a transmitting transducer is to convert electrical energy into a corres-ponding mechanical vibration capable of imparting ultrasonic pressure variations into water. The function of a receiving transducer is to convert the ultrasonic Pressure variations into corresponding electrical signals. In order to satisfy Model flasin renuirenints, both the trans-mitting and receiving transducers must operate at a frequency in the order of 100 kc or
prefer-ably higher. Furthermore, the transmitting transducer must be essentially omnidirectional, i.e., capable of radiating ultrasonic pulses in all directions. On the other hand, the receiving transducers located close to the walls of the test basin do not require such a broad directivity.
A piezoelectric type of transducer employs a crystal material such as barium titanate which changes its physical dimensins in response to applied electrical energy. Conversely, the material generates electrical energy in response to imposed changes in its physical dimen-sions. Directivity as well as the other requirements of the transmitting and receiving trans-ducers can be satisfied by using piezoelectric material in an appropriate physical
configura-tion.
As a result of empirical research efforts at the '.todel Basin, a hollowed cylindrical piezceIectric type of transducer was found which will satisfy requirements. Therefore, some
general, but important considerations pertinent to the transducer design and application are
discussed.
A cylindrical transducer, as illustrated in Figure 11, has three fundamental modes of
vibration. It can vibrate in the length. dimension 1, in the thickness dimension t, or in the
radial mode where the changing outside diameter dimension is significant. Vibrations inany
of these modes can impart corresponding pressure signals (ultrasonic pulses) into.awater medium. TTowever, a cylindrical transducer designed to vibrate principally in the radial mode offers a means of obtaining nearly omnidirectional characteristics (in the X-Y plane) for
transmitting or receiving underwater Ultrasonic pulses.
In general, the efficiency of a piezoelectric transducer is very poOr. In order to enhance
the efficiency of a cylindrical transducer, it is desirable to design and operate the transducer to resonate in the radial mode at a natural frequency. The radial resonant frequency ofa
cylindrical transducer is a function of the material and the diameter of the cylinder. In
gen-eral, the radial resonant frequency is divorced from the length and thickness of the cylinder. owever, it is possible to have detrimental mechanical interaction3 if the radial resonant frequency is nOt significantly different from the resonant frequencies of the longitudinal and thickness modes. Other aspects of the transducer design may result in dimensions that make
the resonant freqencies sufficintly different. Experience shows that in order to geta radial resonant frequency in the order of 100 kc usng a typca piçge1ectri material such as lead zirconate, the diameter of the Oylinder must .be less than 1/2 in.
In regard to directivity, consider a small cylindrical transducer resonating in the radial mode, in a water medium at an ultrasonic frequency. For practical purposes, it may be
Figure 11 Cylindrical Transducer Configuration
considered a generator of ultrasonic pressure variations. Referring to Figure U, it is logical to expect the pressure variations to radiate out from the transducer center equally in all di-rections in the X-Y plane. Thus, it may be said that the transducer is omnidirectional in the XY plane.
The directivity in the X-Z planes may not necessarily be omnidirectional. i'owever, because of symmetry, the directivity patterns for all of the possible X-Z planes will be identi-cal. Therefore, it is Only necessary to consider the X-Z directivity in terms of a single X-Z
plane. If the transducer is omnidirectional in the X-Z plane as well as the X-Y plane,, the
three-dimensional directivity pattern will be omnidirectional and will meet our requirement.
The X-Z plane directivity of a cylindrieal transducer is a function of the cylinder length and the wavelength of the excitation frequency. It is shown in Reference 4, that the X-Z plaie directivity is very limited where the cylinder length is greater than a wavelength of the excitation frequency. For a given frequency, the X-Z plane directivity becomes less limited as the length of a cylindrical transducer is decreased, and it is possible to obtain essentially Omnidirectional characteristics in the X-Z plane if cylindrical length is onsder-ably legs than a wavelength of the excitation frequency. The prototype transducer described later in this report has a cylindrical diameter of 1/2-in and a length of 1/4-in. The cylinder length is one fifth of the wavelength of the 75kc excitation frequency. Experiments indicate that this transducer is essentially omnidirectional in the X plane when operated at 75 kc.
x
As a general rule, a small piezoelectric traflsducer employed for transmitting ultrasonic pulses has far from ideal electrical impedance characteristics. * "atchins! the impedance
char-actOristics of the transducer to transmitter circuitry consititutes an electronic design problem. The transmitter circuitry may be required to deliver a terminal voltage of several hundred volts rrns and large volt-amnore reactive power in oider to produce an ultrasonic pulse having a power equivalent to a few watts.
It is possible to encounter cavitation around a small transmitting transducer if the transduceris driven too severely in an attempt to generate large amplitude ultrasonic pulses. Although an explanation of cavitational phenomenon** will not be undertaken in this report, it does pose a limitation on the amount of mechanical energy which can be ihiparted into water by a given transducer.
PROTOTYPE INSTRUMENTATION TEANSDUCERs
The prototype transducer, shown in Figure 12, is manufactured by the Atlantic Research
Corporation of Alexandria, Virginia. It is identified as a Model LC-33 Pressure Transducer, and was originally designed to measure air blast pressures. Experiments conducted at the Model 3asin prove that the transducer is suitable for underwater application for transmitting and receiving ultrasonic pulses at pulse frequencies as high as 75 kc.
The active element of the transducer is a lead zirconate crystal in the shape of a
hol-lowed cylindrical ringhaving a diameter of 1/2 in., a length of 1/4 in and a wall thickness of about 1/32 in. The crystal element comes mounted in a 10-in, stainless steel probe. The element is wired and potted and will withstand submersion in water. One end of the probe is machined to accept a coaxial cable connector to facilitate electrical connection to the crystal
element. It is necessary to waterproof the cable connection for underwater application. Factbrycalibration data provided with one of the transducers are shown in Appendix A.
Frequency-response characteristics representative of the crytal element used in the LC-33
-I
0.63" DIA. STANDARD AN 49195
CONNECTOR
RETAINING RING ACOUSTIC CENTER
2.50" ----o 3.50"
Figure 12 LC-33 Pressure Transducer
See Reference 4 for an excellent discussion on transducer impedance characteristics. **See Reference 4 for a discussion of cavitatjonal Phenomenon.
24
0 100 0 a, 110 r0 120 . 0 C .0 0 'I E a, 0 0 4 I., a' .0 a, 0 a, = a, a,
(Response flat athUb to 2 cps.)
10 50
Frequency in kilocycles per SëcDfld
Figure 13a Free field Typical Voltage (Receiving) Response (Open-circuit voltage at end of 25 feet of Coaxial Cable)
100 500
Figure 13b Typical Transmitting Current Response (Pressure atone meter per ampere)
Figure 13 Transducer Frequency Response Characteristics
Transducer are shown in Figure 13 although the actual frequency response of the transducer is not indicated.
On the basis of various experiments conducted with Model LC-33 ttansducer at the. Model 3asin; it is concluded that:
a. The transducer has a radial resonant frequency in the orderof 75 kc.
b Operated at 75 kc lfl water, the transducers are essentially omnldlrectlonal when
employed as transmitters or receivers of ultra.onic pulses.
c. Inwater, at the mechanical resonant frequency, the impedance experienced by transmitting circuitry is equivalent to a O.0027-Ucapacitor in series with an 82-ohm resIstor.
TRANSMITTER
Figure 14 shows a simplified block diagram of the type 0 transmitter used for the
experimental work conducted at the ModelBasin. The oscillator circuit, tuned to the ultrasonic
25
200
0 100 150
OSCILLATOR
75 kc
R12
R13
Figure 14 - Block Diagram of Experimental Ultrasonic Pulse Transmitter
C5 26 Ti POWER AMPLIFIER R16 + 22 R15
Figure 15 - Transmitter Oscillator Circuit
75 kc SINE WAVE OUTPUT TRANSDUCER PIP GENERATOR MO NO ST ABL E GATE
I
SIGNAL
FROM "PIP GENERATOR"
Figure 16a - Circuit
Figure 16 - Transmitter Pip Generator
IO.3
se-fe-O.3 sec
Figure 16b - Waveform at
Figure 16c - Output Waveform at
+22 R OSCILLATOR SIGNAL INPUT C2
---umiuti
-ll1
Figure 17a - Circuit
Jill 111111! Fill
Figure 17b - Gate Signal at © Figure 17c - Output Pulse at
Figure 17 - Transmitter Monostable Gate Circuit
27
OUTPUT
pulse frequency of 75 kc, operates continuously. Upon receiving a sharp pulse from the pip generator, the monostable gate opens to allow several cycles of the oscillator signal to pass through; then after a preset time delay, it closes. The signal that passes through the gate is
amplified by the power amplifier to produce a highvoltage excitation across the. transmitting
-transducer. The transducer responds by imparting an ultrasonic pulse into the water for each occurrence of a pip generator signal. The pip generator frequency is set to about 3 cps, corresponding to the rate at which the ultrasonic pulses are transmitted.
A completely transistorized battery-operated expe ri mental transmitter was developed
and used quite successfully. The circuitry is described in terms of the individual circuits which fulfill the functions of each of the blocks in Figure 14.
The oscillator circuit is shown in Figure 15. The tuning transformer T1 was adapted from a slug-tuned inductor by splitting the winding into two equal parts. The transformer
facilitates direct coupling of the oscillator output into the monostable gate circuit. A few turns of enamel wire were wound on the coil to provide the positive feedback signal into the base of Q5. Adjustient of the tuning slug provides for tuning the oscillator over a limited range. The oscillator output is a continuous, nearly sinusoidal 75-kc signal approximately
20 volts peak-to-peak amplitude.
The pip generator circuit is shown in Figure 16. This circuit is a relaxation oscillator operating at a frequency of about 3 Ops. The unijunction transistor Q1 abruptly discharges capacitor C1 to provide the fast-rising output signal which is required to "trigger" the mono-stable gate circuit.
The monostable gate circuit is shown in Figure 17. The transistors Q2and Q3 are used in a monostable flip-flop circuit. Each time a signal from the pip generator is applied at B, the collectorof Q3 lifts one end of the diode D1 from ground to about + 22 volts; after a time delay determined by B5 and C.3, the collector is automatically returned to ground potential. The diodes D1 and D2 and the resistor R7 constitute an inhibiting "and" gate which holds the base of Q4 at ground level until the circuitry receivesa pulse from the pip generator. For the brief time interval.that diode D1 (at point C) is lifted above ground in response to a pip generator signal5 several cycles of the oscillator signal (applied at point A) are permitted to reach the base of Q4; this results in a 75-kc pulse which appears at the emitter of Q4. The bias network consisting of B8, E, and C4, keeps Q4 completely cut off until the gate is opened. The 75-kc pulses appearing at the output of the monostable gate
circuit are fed to the input of the power amplifier circuit.
The power amplifier circuit is shown in Figure 18. During the absence of a 75-kc input
pulse the base of Q6, held at ground potential, restricts the flow of collector or emitter current
in both transistors Q6 and Q7. As a.75-kc pulse is applied at the circuit input, each cycle of
the pulse rising above ground causes .a small base-to-emitter current to flow through Q6; the corresponding amplified current flowing in the collector of Q6 also flows through the base and
emitter of Q7. This causes a peak current. of 5 amps to flow through the collector of Q7
75-kc
INPUT PULSE
Figure iSa - Circuit.
I!!Il;Ill:lIL!lIl VUU
29 Q7 2N. 1046 +22 TRANSDUCERFigure 18b - Waveform at () Figure 18c Waveform at
7 Volts Peak 500 Volts Peak to Peak
in series with the primary winding of the output transforner T2 A 20-vçlt peak-to-peak signal at the primary of the tuned output transformer is stepped up to 500 volts peak to peak, which
is applied across the transducer. The resulting peak-to-peak current through the tr'ansducer in water is in the order of 0.8 amp.
RECEIVERS
Each ultrasonic pulse' receiver converts an underwater ultrasonic pressure signal into a voltage signal of sufficient amplitude to "trigger" the time measuring circuitry.. Figure 19
shows a block diagram of the receiver elements. The cables are included in the diagram because their electrical characteristics should be considered. In the Model Basin application, each receiving transducer is positioned several feet below the surface of the water. About 50 ft of coaxial cable are required to go from the tansducer, up the wall,. and along the beach toa shore-located preamplifier. The shunt capacity of this 50 ft of coaxial cable is
insignifi-cantly detrimental. It is expedient to preamplify the small transducer voltage signals before transmitting them through several hundred-foot lengths of cable that are required to reach
central Iy located instrumentatiOn.
The voltage signal from the transducer is first preamplified by a Type 140-B Decade Amplifier maiñifactured by W}. Scott, Inc. of Maynard, Massachusetts. This wide-band
amplifier, with its self-contained power supply, has low noise and high input impedance
char-acteristics. The input noise characteristics are improved by usingan external plug-in d-c filament supply purchased from the manUfacturer. Using this filament supply, an amplifier gain setting of X1004 and a 1rnegohrn resistor shunted across the input terminals, the
equi-valent input noise is about 20 v rms. The amplifier output impedance is 30 ohms. The frequency response can be adjusted flat to within ± 1, db between 1 cps and 1 megacycle. The use of negative feedback circuitry assures good gain stability.
50-fl COAX
CABLE
[._i]
TRANSDUCERFigure 19 - i3lock Diagram of Receiver Components
30 500-FT COAX CABLE xi00 PREAMPLIFIER HIGH PASS FILTER X 100 AMPLIFIER DETECTION AMPLIFIER TO TIME MEASURING CIRCUITS
The output of the first preamplifier (Figure 19) is fed through a high-pass filter to reject extraneous signals below the ultrasonic pulse frequency The high-pass filter circuitry, shown in Figure 20, has a band..pass attenuation factor of 2
At the output of the high-pass filter, the transducer signal is further amplified by a second Type 140-B Decade Amplifier identical to the first except that an external d-c
fila-ment supply is not Used.
The second decade amplifier, with an output impedance of 3O'ohrris,'is suitable for driving the amplified sigflal through long cables to the centrally located instrumentation. The longest signal cable is approximately 500 ft. The lumped cable capacity, effectively shuiting the output of the second decade amplifier, is in the order of 0.01 jf. At the 75kc pulse fré-quency, this shunt impedariceis in the order of 200 ohms. The 500-ft cable length is appreci-ably less than a onequarter wave length of the 75-kc signal frequency; therefore, signal reflec-tions within the cable are not anticipated and it is not necessary to terminate the cable in its àharacteristic impedance.
The amplified transducer signal transmitted through the long connecting cables (Figure 19) goes to the 'input of the detection level amplifier circuitry which conditions the receiver
1.2 1.0 w C Lu 06 Lu 0.4 0.2 0. 10 Z) 40 60 80100 200. FREQUENCY INKILOCYCLES
Figure 20a - High-Pass Filter Response
Figure 20b - High-Pass Filter Circuit
Figure 20 High-Pass Filter
31
400 600 800 1000
signals. This circuitry. is adjusted so that a selected minimum amplitude of input signal is
required to produce an output for triggering time measuring circuitry. In effect, the circuit provides discrimination against reverberation or noise signals which are smaller in amplitude than pulses traveling directly from transmitter to receiver.
The detection, level amplifier circuitry shown in Figure 21 functions as follows: The amplified transducer signal applied at the input swings equally above and below the ground potential. Much of the negative half of the signal is shunted to ground by the diode D1. The positive half of the signal is limited by the 4-volt zener diode Z1. The signal appearing at the base of Q1.is a halfwave rectification of the input signal limited to about + 4 volts peak. The transistor Q1 serves as an emitter follower. The resistor network consisting of R4 and the potentiometer 1117 provides a variable bias voltage in the emitter ôircujt of Q1. The base of Q1 mustrise above the bias voltage developed across I? before the transistor will conduct..
In effect5 the signal that reaches the emitter of Q1 has its bottom clipped by an amount de-pendent upon the setting of the variable resistance of This feature provides rejection of signals below a minimum "detection level." The capacitor C5 bypasses the a-c signal around the bias resistor 114. The "clipped" signal is amplified by the transistors Q2 and Q3; the
emitter feedback used in this d-c coupled amplifier provides a somewhat stabilized gain of approximately 50. The transistors Q4 and Q5 are used in a Schmitt trigger-type circuit. A small positive going signal at the base of Q4 "triggers" the circuit and causes a positive pulse in excess of 10 volts at the c011ector of Q5, which serves to actuate time-measuring
circuitry. In summary, a positive going signal, that exceeds a predetermined level at the input of the detection level amplifier will cause a 10-volt pulse at the output of the circuitry.
INPUT R
Wv
ZI QI 10k 1k 32 2.2k L'93 k 2N 502A R9 10kFigure 21 - Detection Level Amplifier
C4
0.01 uf
29 1090
EVALUATION OF PROTOTYPE INSTRUMENTS
Utilizing the experimental instruments that have been described, a simplified version of a traOking system was assembled and evaluated in the maneuvering basin facility. Three receivers were placed 7 ft below the water surface and accurately spaced 140 ft apart along a straight line by suspending them from the bridge located across the center of the basin.
Test conditions permitted "staking" the transmitting transducer at known locations along parallel lines 20 and 100 ft from the receiver array. The ultrasonic pulse frequency was 75 kc. Pulses were transmitted at the rate of three per second.
The time-measuing instrumentation was about as simple as that illustrated in Figure 6, A clock-oscillator frequency of 100 kc was used to obtain a measurement of the difference in the time of arrival of a pulse at each pair of receivers. For. each known location of the
transmitting transducer, the two counter readings corresponding to time difference measure-ments were observed and recorded. The discrepancy due to the plus or minus one count ambiguity inhetent in the counter unit was decreased by averaging the repetitive counter readings that were observed for each transmitter location.
Equation [161 was used to compute the z-coordinate of the transmitter location where
the observed counter readings correspond to n12 and n23 in the equation. The maximum
counter reading corresponding to Nd in 1quation [16] was 2886.
The computed values of the x-coordinate of the transmitter locations are compared with the known values in the tabulation included in Figure 22. The reults in Figure 22 clearly show that as the transmitter moves near an end receiver, the error in determining the z-coordinate of the transmitter locatiOn becomes increasingly large. On the basis of the data
in Table 1, it was concluded that the type of instrumentation and techniques discussed in this report are suitable for satisfying the accuracy requirements for a tracking system for the Model Basin. This conclusion is. conditionally based on the premise that the models to be tracked will be limited to a test area such as the one indicated in Figure 5. Under this con
dition, with the two end receivers positioned in the corner8 of the basin, the errors in deter-mining the two rectangular coordinates of the model-borne transmitter will remain less than
0.25 ft. This complies with Model Basin accuracy requirements.
The evaluation tests did not permit a thorough appraisal of the problem of lingering reverberations. In transmitting the pulses at a repetition rate of three per second, some
difficulty was encountered with lingering reverberations for a couple of the transmitter loca-tions evaluated. The difficulty was overcome by adjustment of the receiver detection level and the length of time that the receivers were inhibited in the period between transmitted pulses. However, the difficulty experienced led to the belief that there may be a few unique transmitter locations within the maneuvering basin in which lingering reverberations could cause an occasional error in locating the model-borne transmitter.
320 FT R1 a a a
0000 00000
AB CD EFG HI
JKLU NO
P000000
0Figure 22 - Experimental Tracking Data
The diagram representing the maneuvering basin shows the fixed
receiver locations (R1, R2, and R3). The lettered points (A, B, C, ... P)
represent known "staked" locations of the transmitter. For each
trans-mitter location, distance difference data were obtained with
Instrumen-tation discussed in the text. These data were used in accordance with
Equation [16] to compute the x-coordinate of the transmitter location.
The computed coordinate distance is compared with the actual coordi nate distance and the discrepancy between the two are shown for each
location of the transmitter.
34 Transmitter Location in Maneuvering Basin Actual Coordinate Distance (x) ft Computed Coordinate Distance ft Discrepancy ft 0 0.074 0.07 B 20 19.929 0.07 C 40 39.971 0.03 D 60 59.976 0.02 E 80 80.03 0.03 F 104 104.14 0.14 G 120 120.28 0.28 H 140 141.05 1.05 156 153.19 3.19 J 0 0.00 0.00 K 20 20.06 0.06 L. 40 40.08 0.08 M 60 60.42? N 80 80.10 0.10 0 100 100.09 0.09 P 140 140.24 0.24
k
I4OFTMANEUVERING BASIN - SHORING RECEIVER TRANSDUCER LOCATIONS
TRANSMITTER (i, y)
0
RECEIVER 5 RECEIVER 4 RECEIVER 3 RECEIVER 2 RECEIVER ICLOCK 400kc
0
GATING: LOG!C CIRCUITS (TIMERESET DIFFERENCE fl I I
-H I ESETL SIGN) RESET I5:BIT BINARY TIME DIFFERENCE REGISTERSfl45H
I RESET -RESETI RESETCOMMAND FORMAT CONTROL READOUT GATES (SERIALIZED DATA) READ AMPLIFIER PARITY GENERATOR
Figure 23 - Preliminary Design of DTMB Model Tracking System
