CA1084634A - Method of and apparatus for current-tracing the location of faults on printed circuit boards and similar systems - Google Patents
Method of and apparatus for current-tracing the location of faults on printed circuit boards and similar systemsInfo
- Publication number
- CA1084634A CA1084634A CA286,335A CA286335A CA1084634A CA 1084634 A CA1084634 A CA 1084634A CA 286335 A CA286335 A CA 286335A CA 1084634 A CA1084634 A CA 1084634A
- Authority
- CA
- Canada
- Prior art keywords
- track
- probe
- phase
- current
- signal
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
- G01R19/145—Indicating the presence of current or voltage
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/28—Testing of electronic circuits, e.g. by signal tracer
- G01R31/2801—Testing of printed circuits, backplanes, motherboards, hybrid circuits or carriers for multichip packages [MCP]
- G01R31/2805—Bare printed circuit boards
Abstract
ABSTRACT OF THE DISCLOSURE
This disclosure is concerned with novel current-tracing of short circuits in printed circuit boards and similar systems by novel test excitation of the conductors with tracing of the phase polarities of the fields generated therein.
This disclosure is concerned with novel current-tracing of short circuits in printed circuit boards and similar systems by novel test excitation of the conductors with tracing of the phase polarities of the fields generated therein.
Description
`` lOb~t~34 , The present invention relates to mç~hDds o~
and apparatus for current-tracing the location o~ faults in conductor tracks on printed circuit boards and si~i-lar systems, being more particularly concerned with de-tecting the location of short circuits and the like there-n .
There have been previous systems evolved for ena-bling the finding of short circuits and the like in print-ed circuit board and similar systems, where conductors are provided on the board at very close sep~rations and where it is often difficult to locate the point of thé
fault. Included in such prior proposals are current tracer signal-amplitude measuring probes as described, for example, in the Hewlett-Packard bulletin entitled "Current Tracer, Model 547A" November 1975; the Testline of Titusville, Florida, "Shortstop" probe apparatus; and related systems as disclosed in the followi~ng U.S. Letters Patent:
Patent No. Issue Date Inventor(s) 1,158,086 Oct, 26, 1915 James ~. Vahey
and apparatus for current-tracing the location o~ faults in conductor tracks on printed circuit boards and si~i-lar systems, being more particularly concerned with de-tecting the location of short circuits and the like there-n .
There have been previous systems evolved for ena-bling the finding of short circuits and the like in print-ed circuit board and similar systems, where conductors are provided on the board at very close sep~rations and where it is often difficult to locate the point of thé
fault. Included in such prior proposals are current tracer signal-amplitude measuring probes as described, for example, in the Hewlett-Packard bulletin entitled "Current Tracer, Model 547A" November 1975; the Testline of Titusville, Florida, "Shortstop" probe apparatus; and related systems as disclosed in the followi~ng U.S. Letters Patent:
Patent No. Issue Date Inventor(s) 1,158,086 Oct, 26, 1915 James ~. Vahey
2,226,021 Dec. 24, 1940 Edmund O. Schweitzer,Jr.
2,249,166 July 15, 1941 George B. Parker;Lloyd L. Parker;John P.Meehan 2~586,781 Feb. 26~ 1952 Cecil L. Brownlow 2,698,921 Jan. 4, 1955 Donald A. Wharton 2,769,868 Nov. 6, 1956 Cecil L. Brownlow 2,974,278 March 7, 1961 ~ohn B. Kennon
2,249,166 July 15, 1941 George B. Parker;Lloyd L. Parker;John P.Meehan 2~586,781 Feb. 26~ 1952 Cecil L. Brownlow 2,698,921 Jan. 4, 1955 Donald A. Wharton 2,769,868 Nov. 6, 1956 Cecil L. Brownlow 2,974,278 March 7, 1961 ~ohn B. Kennon
3,753,089 ~ug. 1~, 1973 ~ohn B. Gunn;
John L. Staples 3,831,086 ~ug. 20, 1974 William Steve Pesto 3,882,287 ~ay 6, 1975 ~ames F. Simmonds Devices such as the first-named current tracers rely on a movable coil in the form of a probe, picking up the voltage in-~, "
pg/~, ;'f , ,, ,, ,,. -, ,,, ,., ~ , ' , . ' ,. ', : ' , , ~ " ' ,' . ' ' ,'"', ' '''~ ' ' ', ' ' ,', . '' "' ' ~ ,' ' , ' " ' ', ' " :
, , , ," , ..
,. , '", ,;'" ,, '' , ' ' ',' duced in such a coil by current flowing in a particular track on the device being examined. The intent in the use of such a device is to position the probe coil so that the greatest possible signal amplitude is induced in -that coil. In such case, the coil is in closest possible proximity to the track of interest, i.e. the track carry-ing the current. ~y observing where the maximum signal is picked up as the probe is moved, one can, in the ideal, -trace the path of the current.
Diff~culties with such amplitude-measuring tech-niques and the limitations thereof are as follows. One is looking with such a probe for a peak, a maximum in the induced signal that will indicate that the probe coil is closest to the track under test, and such peak is not a particularly sharp one. In fact, within close proximity to the track carrying the current, the signal amplitude falls off relatively slowly as the probe is moved away.
., .
The distance between the probe coil and the track under te~t in any direction, moreo~er, is of prime importance;
and if the track passes under a component or through some other constricted area where the probe cannot direc-tly reach, such constraints impose even further amplitude reduction. It can thus be less than straightforward for the operator of such an apparatus to discriminate, when an amplitude drop~, whether this is because one is getting away physically from the direction the track is taking, or whether one has moved the probe, for example, further above the board to get over some obstruction, requiring a fair degree of interpretation in the results from such a device.
','' ',.',' ',', ,'. ' ,'"', , ',, , . , : ..
.:; ;,' ' ' . ' ., ' ' ' ,'.' , , . ' . ,- . ,: , ' , . . . ~,. .. . . . . .. . ..
, ,. ,, . . , , , , '. ; . ' , . ' ' ' ' ,.. . . . . . .
, ~ : , . , . , , :
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3~
In addition, such devices generally operate in connection with the fields produced by the actual opera-ting currents and voltages in the circuit board. ~mong the limitations of such a concept are that it is hard to ascertain that the particular track of interest is the only one in the immediate area that is carrying current in such an operating board. The chances are that there is more than one signal occuring at the same time, and, in simply looking for a signal in a track, the operator can lose the path of the track o~ interest. This can be overcome at least in part by supplyïng a particular signal to a ~oard that is otherwise inactive, and only to the track of interest. The current-tracing probe may also be sued with a separate pulse source probe such that current pulses inserted by the current-pulse probe may be followed with the current tracer.
Underlying the present invention, however, is a novel philosophy of operation in connection with phase polarities of fields generated with a rather critically designed excitation concept for placing a desired test current in the portions of the board conductors to be examined. This is not to say that probes have not here-tofore been otherwise used with phase detectors as, for example, in U.S. Letters Patent No. 3,860,866 (Dornber~er, January 14, 1975); or that directional pick-up coils have not been otherwise used as, for example, in U.S. Letters Patent No. 3,889,179 ~Culter, June 10, 1975). But these do not in-volve the t~pè of phase polarity measurement or particular Pg/
-- , _ ~ _ ,, ". - . . " ..... ' ,. . , " , :, .
.: , "
, . . . . ..
" . , , , ~ , .
. . .. . . . ..
excitation current concepts that have been found to produce the highly improved and novel results of the invention. The use of a pick-up coil broadly responsive to phase, of course, is decades old; a more recent application ~eing as the stylus of a graphic-input tablet using phase detection as part of a sche~e by which the X and Y coordinates of - -the pen of the stylus are determined and presented as digital information, as in U.S. ~etters Patent No. 3,705,956 l~ertouzos, Decemker 12, 1972). ~ -But this is unrelated to the particular usages and connections in the different proble~ underlying the present invention.
lG The magnetic field around a conductor track carrying current changes its perceived direction as one passes over the conductor. This assumes a conductor substantially in a plane, so that a point a fairly small distance to one side of a current-carrying conductor will perceive a field opposite in direction to that perceived by a p~int a small dis-tan oe on the other side of the conductor. By measuring the direction -~
of the field and locating the position at which the measured field appears to change direction, a very sharp and precise indication as to the location of the conductor is attainable. In the event that the signal used to excite the conductor, i.e. the signal flowing in the con-ductor track, is an alternating current signal, then the direction of the measured field is e~uivalent to the phase picked up b~ the measuring or prcbe coil relative to the phase of the current flcwlng in that track;
and b~ driving the track with a signal of known phase and oomparing the signal measured by the probe coil to that signal of kncwn phase, a very precise indication of the physical position of the current-carrying conduc-pg/l,,/:,) _ ~_ b ; ,. . . . . . . . . . . . ..
., " , ... ..
. , .. ~ . ..
. . .
~ .
tor may be obtalned as the physical polnt at which the perceived phase changes is noted. The fault may then be traced along the conductor.
Added advantages of using phase as the parameter of interest are increased discrimination of locatlon, and relative insensitivity, within broader ranges, to the posi-tion of the coil above the circuit board. While the prlor art amplitude-sensing current tracers the distance between the probe and the curren~-carrying conductor directly affects the amplitude Or the measured signal, the measurement of the phase of the signal in accordance with the invention remains unaffected by sheer distance between the conductor and the probe. This is not to say that the probe can be arbitrarily far away, because as the distance increases, the probe becomes more sensitive to signals generated by other conductors carrying current lncluding the leads bringing the excitation signal to the track under test.
But within a reasonable range o~ dlstances, comparable ~to, say, several times the size of the probe coil, the measured phase, and hence the validity of the information as to where the current-carrying conductor is, remalns un-changed wlth variatlons in distance between probe and con-ductor; i.e., the distance o. the probe from the conductor.
An obJect of te invention, aocordlngly, is to provlde a new and lmproved method of and apparatus for cur-rent tracing circults, and more particularly locating short ¢lrcuit~ or the llke in prlnted clrauit boards and slmllar apparatus, that obvlate the above-descrlbed dis-, .
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.. . . . . . .
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~, .
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advantages and limitations of prLor art ampllt~ld~-meastlring and other approaches.
A further object is to provide a novel current-tracing apparatus operating upon phase measurement and of more general ~ -applicability, as Mell. --Other and further objects will be explained hereinafter and are more fully delineated in the appended claims.
In summary, from one of its view points, the invention embraces a method of current-tracing the location of faults such as short circuits on printed circuit boards and the like, that comprises-, applying an a.c. test excitation signal along a pre-determined conductive track among other closely disposed tracks ~-on a printed circuit board to generate a magnetic field about the predetermined track; probing back and forth on either side of the predetermined track in the proximity of and along the same to pick up said field and produce voltages induced therefrom; indicat-ing the point of phase change of said voltages as the probing crossea the track to pin point the track; and tracing the same to find the fault location.
Furthermore, from another of its view points, the present invention provides apparatus for current-tracing the location of faults auch as short circuits on printed circult boards and the like, having, in combination, means for applying an a.c. test excitation signal along a predetermined conductive track among other closely disposed tracks on a prlnted circuit board to generate a magnetic field about the predetermined track; magne~ic field probe means adapted to be moved along and back and forth on either side o the proximity of the predetermined conductor to induce voltages therein ~rom the field; means for detecting the phase of dap/~ 6 - `~
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the incluced voltagc ancl tlle point o~ reversal cllange thereof as the probe means crossed the predeterm-ined track; and means for indicating such point of change.
The invention will now be described with reference to the accompanying drawings, Fig. 1 of which is a combined schematic and block circuit diagram illustrating a preferred embodiment of the invention operating in accordance with the method underlying the same;
Fig. 2 is a similar view illustrating the application of the system of Fig. 1 to an actual short-circuit-~ 6a -, ~ ~i dap/f~
, ,~ ' ' ' ' ' ' ' ' ''' "
, 6;~
detection usage with a printed circuit board; and Figs. 3A, 3B, and 3C are waveform diagrams illus-trating, respectively, the operation of the prior-art am-plitude-detecting technique (Fig. 3A), the polarity opera-tion of the phase system of the invention (Fig. 3B), and the relative insensitivity of the same to ~he position above the board (Fig. 3C).
Re~erring to Fig. 1, there is shown a function-al block diagram of a system 10 embodying the approach underlying the invention. A cloc~ 1 generates the basic electrical oscillation excitation signal that is applied as a current through a driver 2 via path 11 to the traaks ~ -3 under test, and also the reference signal for determining the phase of the signal from the probe coil 4. The pickup or probe coil 4 is positioned in the vicinity of the tracks 3 under examination and receives an a.c.
voltage induced in it by the magnetic field of the tracks. This voltage is applied to an amplifier 5, pre-ferably an amplifier tuned to said oscillations to improve ;
sensitivity without picking up extraneous signals. From the ampli~ier 5, it is applied to a comparator 6 which effectively changes the amplified a.c. signal from the probe to a logic signal. The comparator output is applied to a threshold detector 7 the purpose of which is to determine that the probe 4 is actually in proximity to the tracks 3 under examination and to suppress extraneous indications to the operator if insufficient signal is being received.
The comparator output is also applied to a phase detector 8 which receives as its other input, the original clock sig-,, jrc: ~
'~', , '' ,' . . . ~' ;,:'''''~ '' ': ' .
~,.. .. . ..
G3~
nal from 1. The phase detector 8 produces outputs that drive one of two indicators 9, such as vlsual lndicators;
one belng energized ir the measured signal is in phase with the clock signal, and the other, i~ the measured slgnal is out of phase.
The probe coil 4 is wound in such a manner that its axis is substantially perpendicular to the plane ' containing the track conductors 3. The line derined by this axis., when extended down to the surrace containing the conductor, is the line that efrectively denotes the position of that conductor. ~-The comparator 6 is ad~usted to compare the instant- ' aneous value Or the a.c. signal output of the ampli~ier 5 against a voltage approximately at its midpoint, such that when the instantaneous value o~ the output from the .
amplirier is greater than this threshold, a digltal lo-~gic "one" is produced or converted by the comparator.
When the amplifier output is less, a logic "zero" is produced. The result is that-the comparator 6 produces a square wave whose transition polnts correspond to the zero crossings Or the amplifier output about lts steady-state level. The comparator 6, for example, may be of the integrated circuit zero-crossing detector type as des-'cribed ln Signetics Corp. Bulletln "Linear Integrated Cir-cuit LM 311"; the threshold detector 'I o~ slmilar com-parator chip type and an R-C low pass filter; and the phase detector 8, Or the sampllng type Or the assignee's ' ''' GR~1683 brldge and GR 1161 rrequency syntheSiZer implemented with a dlgltal rllp-rlop. Other well known components may ~: --8~
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also clearly be used.
The importance of magnitude of the excitation signal resides ln the fact that the greater the excitation signal, the larger the magnetlc field around the track conductor of interest, and hence the greater the voltage induced in the pickup coil 4. The actual value chosen de-pends on several details of implementation, principally the signal required from the pickup coil 4 to be readily discernible in the presence o~ noise or uncertainties that might arise in the amplifier circuitry. It has been ~ound that ana~c. signal level of the order of lO milliamperes is sufficiently large to give good signal pickup wlth adequate noise margin, yet w~h~u~ producing current levels that can either damage a device under test or perhaps give false indications by providing su~iciently high levels to enable the development of erroneolls paths. A clock oscilla-tlon frequency in the neighborhood of 600 kHz has proven satisfactory. This is determined primarily by the tuned components in the received signal chain. For the purposes of signal detection and noise exclusion, the tuned ~ -ampllfier 5 may runction with the coil in the probe 4 made resonant, as with suitable capacitance, in the neigh-borhood of, ror exarnple~ sald 600 kHz.
Turning, now~ to Fig. 2, the system of Flg. l involvlng the probe houslng with lts plckup coll 4 and the a~soclated ¢lrcult system lO, are lllustrated as applled to a typical use in ~earch ~or short clrcults ln a prlnted clrault board. The hoUslng or box contalning the electro-nlc clrcultry 10 drives the tracks 3 under test through two slenal leads ll whlch ¢arry the excltatlon slgnal;
_9_ .. . . . . . . ..
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one lead belng attached to one conductor, and the other lead to the other conductor, where the two conductors are the tracks 3 between which there is an unwanted short circuit such that they e~ectively appear to be one conductor. The object, of course, ls to find the location o~ the short so that the unwanted part o~ the path can be located and removed.
The housing of the probe 4 is illustrated ln Fig. 2 as also containing the indicator lamps 9, and is shown located above a segment of the track of interest.
One of the two indicators 9 is schematically illustrated as illuminated. In operation, the cperator would hold the probe and move it in the proximity of the track until one indicator went out and the other one came on. At that point, the prcbe would be located directly over the track - carrying the current. Re~erring to Fig. 3B, plotting the magnitude of the received or induced signal in the probe coil 4 as a ~unction o~ the probe position relative to the track 3 carrying the current, it can be seen that as the probe approaches the track ~rom a ~alr distance away, the magnltude of the signal increases in one direc-tion tsay positlve or ~-) until the probe is extrernely close to the track; that is, extremely close ln terms o~
within a fractlon of the diameter o~ the probe coil 4. At that point, the amplitude o~ the received signal ~alls Or~ very sharply, and ln a very short lncremental distance, goes rrom a peak o~ posltive phase, through zero to a peak o~ the negatlve ~-) phase. ~he sensltlvlty o~ the slgnal-dete¢t clrcuitry is such that one indicator 9 wlll remaln llt until the positive recelved slgnal i8 almost . ~:
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' : ' . ' , - ,' : ' 3~
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zero; and the other indica~or 9 will illuminate when the negative signal reaches above a certain, very small thresh- .
old. As a result, there is an extremely small range of un-certalnty about the point directly above the track in which neither indicator is llt, and outside of this posi-tion Or uncertainty there is a very clear indication-one indicator or the other being illuminated-that the probe is to one side or the other of the current-carrying conductor.
This is to be contrasted with the previously mentioned amplitude-sensing,the operation of which is illus-trated in Fig. 3A in which the signal strength increases much as it does in the phase scheme, when the probe approa-ches from a great distance; but, as the probe reaches the near vicinity of the track of interest, the signal continues to increase to a peak P directly above the track. Because, again, the distances are comparable to the dimensions of the probe coil, this peak tends not to be particularly sharp; it requires fair discrimination to determine it directly; and, as mentioned, is a function of the distance between the probe coil and the track. There is potential for error here as a change in this distance may readily be caused by intentional motlon Or the probe, operator uncer-tainty, or the need to avoid some component of the board.
This can be contrasted, as in Flg. 3C, wlth the operatlon Or the phase soheme Or Flg. 3B. The two tra¢es ln Flg. 3C
show the ~ignal lnduced in the probe coil when the probe coll is dlrectly above the plane carrylng the conductor (solid line) or 18 some dlstance away (dotted llne). The recelved slgnal where the coll ls some dl~tance away ls ~ust smaller. ~he behavlor ln terms Or changlng slgn as --11-- , ,''''~ ' , ' .
- , . ..
.
, -
John L. Staples 3,831,086 ~ug. 20, 1974 William Steve Pesto 3,882,287 ~ay 6, 1975 ~ames F. Simmonds Devices such as the first-named current tracers rely on a movable coil in the form of a probe, picking up the voltage in-~, "
pg/~, ;'f , ,, ,, ,,. -, ,,, ,., ~ , ' , . ' ,. ', : ' , , ~ " ' ,' . ' ' ,'"', ' '''~ ' ' ', ' ' ,', . '' "' ' ~ ,' ' , ' " ' ', ' " :
, , , ," , ..
,. , '", ,;'" ,, '' , ' ' ',' duced in such a coil by current flowing in a particular track on the device being examined. The intent in the use of such a device is to position the probe coil so that the greatest possible signal amplitude is induced in -that coil. In such case, the coil is in closest possible proximity to the track of interest, i.e. the track carry-ing the current. ~y observing where the maximum signal is picked up as the probe is moved, one can, in the ideal, -trace the path of the current.
Diff~culties with such amplitude-measuring tech-niques and the limitations thereof are as follows. One is looking with such a probe for a peak, a maximum in the induced signal that will indicate that the probe coil is closest to the track under test, and such peak is not a particularly sharp one. In fact, within close proximity to the track carrying the current, the signal amplitude falls off relatively slowly as the probe is moved away.
., .
The distance between the probe coil and the track under te~t in any direction, moreo~er, is of prime importance;
and if the track passes under a component or through some other constricted area where the probe cannot direc-tly reach, such constraints impose even further amplitude reduction. It can thus be less than straightforward for the operator of such an apparatus to discriminate, when an amplitude drop~, whether this is because one is getting away physically from the direction the track is taking, or whether one has moved the probe, for example, further above the board to get over some obstruction, requiring a fair degree of interpretation in the results from such a device.
','' ',.',' ',', ,'. ' ,'"', , ',, , . , : ..
.:; ;,' ' ' . ' ., ' ' ' ,'.' , , . ' . ,- . ,: , ' , . . . ~,. .. . . . . .. . ..
, ,. ,, . . , , , , '. ; . ' , . ' ' ' ' ,.. . . . . . .
, ~ : , . , . , , :
.,';',',' . , , ~ ,' , ,. . . . . . . . .
3~
In addition, such devices generally operate in connection with the fields produced by the actual opera-ting currents and voltages in the circuit board. ~mong the limitations of such a concept are that it is hard to ascertain that the particular track of interest is the only one in the immediate area that is carrying current in such an operating board. The chances are that there is more than one signal occuring at the same time, and, in simply looking for a signal in a track, the operator can lose the path of the track o~ interest. This can be overcome at least in part by supplyïng a particular signal to a ~oard that is otherwise inactive, and only to the track of interest. The current-tracing probe may also be sued with a separate pulse source probe such that current pulses inserted by the current-pulse probe may be followed with the current tracer.
Underlying the present invention, however, is a novel philosophy of operation in connection with phase polarities of fields generated with a rather critically designed excitation concept for placing a desired test current in the portions of the board conductors to be examined. This is not to say that probes have not here-tofore been otherwise used with phase detectors as, for example, in U.S. Letters Patent No. 3,860,866 (Dornber~er, January 14, 1975); or that directional pick-up coils have not been otherwise used as, for example, in U.S. Letters Patent No. 3,889,179 ~Culter, June 10, 1975). But these do not in-volve the t~pè of phase polarity measurement or particular Pg/
-- , _ ~ _ ,, ". - . . " ..... ' ,. . , " , :, .
.: , "
, . . . . ..
" . , , , ~ , .
. . .. . . . ..
excitation current concepts that have been found to produce the highly improved and novel results of the invention. The use of a pick-up coil broadly responsive to phase, of course, is decades old; a more recent application ~eing as the stylus of a graphic-input tablet using phase detection as part of a sche~e by which the X and Y coordinates of - -the pen of the stylus are determined and presented as digital information, as in U.S. ~etters Patent No. 3,705,956 l~ertouzos, Decemker 12, 1972). ~ -But this is unrelated to the particular usages and connections in the different proble~ underlying the present invention.
lG The magnetic field around a conductor track carrying current changes its perceived direction as one passes over the conductor. This assumes a conductor substantially in a plane, so that a point a fairly small distance to one side of a current-carrying conductor will perceive a field opposite in direction to that perceived by a p~int a small dis-tan oe on the other side of the conductor. By measuring the direction -~
of the field and locating the position at which the measured field appears to change direction, a very sharp and precise indication as to the location of the conductor is attainable. In the event that the signal used to excite the conductor, i.e. the signal flowing in the con-ductor track, is an alternating current signal, then the direction of the measured field is e~uivalent to the phase picked up b~ the measuring or prcbe coil relative to the phase of the current flcwlng in that track;
and b~ driving the track with a signal of known phase and oomparing the signal measured by the probe coil to that signal of kncwn phase, a very precise indication of the physical position of the current-carrying conduc-pg/l,,/:,) _ ~_ b ; ,. . . . . . . . . . . . ..
., " , ... ..
. , .. ~ . ..
. . .
~ .
tor may be obtalned as the physical polnt at which the perceived phase changes is noted. The fault may then be traced along the conductor.
Added advantages of using phase as the parameter of interest are increased discrimination of locatlon, and relative insensitivity, within broader ranges, to the posi-tion of the coil above the circuit board. While the prlor art amplitude-sensing current tracers the distance between the probe and the curren~-carrying conductor directly affects the amplitude Or the measured signal, the measurement of the phase of the signal in accordance with the invention remains unaffected by sheer distance between the conductor and the probe. This is not to say that the probe can be arbitrarily far away, because as the distance increases, the probe becomes more sensitive to signals generated by other conductors carrying current lncluding the leads bringing the excitation signal to the track under test.
But within a reasonable range o~ dlstances, comparable ~to, say, several times the size of the probe coil, the measured phase, and hence the validity of the information as to where the current-carrying conductor is, remalns un-changed wlth variatlons in distance between probe and con-ductor; i.e., the distance o. the probe from the conductor.
An obJect of te invention, aocordlngly, is to provlde a new and lmproved method of and apparatus for cur-rent tracing circults, and more particularly locating short ¢lrcuit~ or the llke in prlnted clrauit boards and slmllar apparatus, that obvlate the above-descrlbed dis-, .
, ,: . ' ' ', ' ' , ' ' .. ' . ~ ' . ,' " ~ ' ' .. .. . . .. . . . . .
.. . . . . . .
''' ' ' ' ,, ',', , ' ' '. ' , " ' ' , ' . , ' '' .
~, .
1t)~4~;3~
advantages and limitations of prLor art ampllt~ld~-meastlring and other approaches.
A further object is to provide a novel current-tracing apparatus operating upon phase measurement and of more general ~ -applicability, as Mell. --Other and further objects will be explained hereinafter and are more fully delineated in the appended claims.
In summary, from one of its view points, the invention embraces a method of current-tracing the location of faults such as short circuits on printed circuit boards and the like, that comprises-, applying an a.c. test excitation signal along a pre-determined conductive track among other closely disposed tracks ~-on a printed circuit board to generate a magnetic field about the predetermined track; probing back and forth on either side of the predetermined track in the proximity of and along the same to pick up said field and produce voltages induced therefrom; indicat-ing the point of phase change of said voltages as the probing crossea the track to pin point the track; and tracing the same to find the fault location.
Furthermore, from another of its view points, the present invention provides apparatus for current-tracing the location of faults auch as short circuits on printed circult boards and the like, having, in combination, means for applying an a.c. test excitation signal along a predetermined conductive track among other closely disposed tracks on a prlnted circuit board to generate a magnetic field about the predetermined track; magne~ic field probe means adapted to be moved along and back and forth on either side o the proximity of the predetermined conductor to induce voltages therein ~rom the field; means for detecting the phase of dap/~ 6 - `~
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the incluced voltagc ancl tlle point o~ reversal cllange thereof as the probe means crossed the predeterm-ined track; and means for indicating such point of change.
The invention will now be described with reference to the accompanying drawings, Fig. 1 of which is a combined schematic and block circuit diagram illustrating a preferred embodiment of the invention operating in accordance with the method underlying the same;
Fig. 2 is a similar view illustrating the application of the system of Fig. 1 to an actual short-circuit-~ 6a -, ~ ~i dap/f~
, ,~ ' ' ' ' ' ' ' ' ''' "
, 6;~
detection usage with a printed circuit board; and Figs. 3A, 3B, and 3C are waveform diagrams illus-trating, respectively, the operation of the prior-art am-plitude-detecting technique (Fig. 3A), the polarity opera-tion of the phase system of the invention (Fig. 3B), and the relative insensitivity of the same to ~he position above the board (Fig. 3C).
Re~erring to Fig. 1, there is shown a function-al block diagram of a system 10 embodying the approach underlying the invention. A cloc~ 1 generates the basic electrical oscillation excitation signal that is applied as a current through a driver 2 via path 11 to the traaks ~ -3 under test, and also the reference signal for determining the phase of the signal from the probe coil 4. The pickup or probe coil 4 is positioned in the vicinity of the tracks 3 under examination and receives an a.c.
voltage induced in it by the magnetic field of the tracks. This voltage is applied to an amplifier 5, pre-ferably an amplifier tuned to said oscillations to improve ;
sensitivity without picking up extraneous signals. From the ampli~ier 5, it is applied to a comparator 6 which effectively changes the amplified a.c. signal from the probe to a logic signal. The comparator output is applied to a threshold detector 7 the purpose of which is to determine that the probe 4 is actually in proximity to the tracks 3 under examination and to suppress extraneous indications to the operator if insufficient signal is being received.
The comparator output is also applied to a phase detector 8 which receives as its other input, the original clock sig-,, jrc: ~
'~', , '' ,' . . . ~' ;,:'''''~ '' ': ' .
~,.. .. . ..
G3~
nal from 1. The phase detector 8 produces outputs that drive one of two indicators 9, such as vlsual lndicators;
one belng energized ir the measured signal is in phase with the clock signal, and the other, i~ the measured slgnal is out of phase.
The probe coil 4 is wound in such a manner that its axis is substantially perpendicular to the plane ' containing the track conductors 3. The line derined by this axis., when extended down to the surrace containing the conductor, is the line that efrectively denotes the position of that conductor. ~-The comparator 6 is ad~usted to compare the instant- ' aneous value Or the a.c. signal output of the ampli~ier 5 against a voltage approximately at its midpoint, such that when the instantaneous value o~ the output from the .
amplirier is greater than this threshold, a digltal lo-~gic "one" is produced or converted by the comparator.
When the amplifier output is less, a logic "zero" is produced. The result is that-the comparator 6 produces a square wave whose transition polnts correspond to the zero crossings Or the amplifier output about lts steady-state level. The comparator 6, for example, may be of the integrated circuit zero-crossing detector type as des-'cribed ln Signetics Corp. Bulletln "Linear Integrated Cir-cuit LM 311"; the threshold detector 'I o~ slmilar com-parator chip type and an R-C low pass filter; and the phase detector 8, Or the sampllng type Or the assignee's ' ''' GR~1683 brldge and GR 1161 rrequency syntheSiZer implemented with a dlgltal rllp-rlop. Other well known components may ~: --8~
, " ,- - . , .
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, :,, :,, ,, , , :
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.. . . .
also clearly be used.
The importance of magnitude of the excitation signal resides ln the fact that the greater the excitation signal, the larger the magnetlc field around the track conductor of interest, and hence the greater the voltage induced in the pickup coil 4. The actual value chosen de-pends on several details of implementation, principally the signal required from the pickup coil 4 to be readily discernible in the presence o~ noise or uncertainties that might arise in the amplifier circuitry. It has been ~ound that ana~c. signal level of the order of lO milliamperes is sufficiently large to give good signal pickup wlth adequate noise margin, yet w~h~u~ producing current levels that can either damage a device under test or perhaps give false indications by providing su~iciently high levels to enable the development of erroneolls paths. A clock oscilla-tlon frequency in the neighborhood of 600 kHz has proven satisfactory. This is determined primarily by the tuned components in the received signal chain. For the purposes of signal detection and noise exclusion, the tuned ~ -ampllfier 5 may runction with the coil in the probe 4 made resonant, as with suitable capacitance, in the neigh-borhood of, ror exarnple~ sald 600 kHz.
Turning, now~ to Fig. 2, the system of Flg. l involvlng the probe houslng with lts plckup coll 4 and the a~soclated ¢lrcult system lO, are lllustrated as applled to a typical use in ~earch ~or short clrcults ln a prlnted clrault board. The hoUslng or box contalning the electro-nlc clrcultry 10 drives the tracks 3 under test through two slenal leads ll whlch ¢arry the excltatlon slgnal;
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one lead belng attached to one conductor, and the other lead to the other conductor, where the two conductors are the tracks 3 between which there is an unwanted short circuit such that they e~ectively appear to be one conductor. The object, of course, ls to find the location o~ the short so that the unwanted part o~ the path can be located and removed.
The housing of the probe 4 is illustrated ln Fig. 2 as also containing the indicator lamps 9, and is shown located above a segment of the track of interest.
One of the two indicators 9 is schematically illustrated as illuminated. In operation, the cperator would hold the probe and move it in the proximity of the track until one indicator went out and the other one came on. At that point, the prcbe would be located directly over the track - carrying the current. Re~erring to Fig. 3B, plotting the magnitude of the received or induced signal in the probe coil 4 as a ~unction o~ the probe position relative to the track 3 carrying the current, it can be seen that as the probe approaches the track ~rom a ~alr distance away, the magnltude of the signal increases in one direc-tion tsay positlve or ~-) until the probe is extrernely close to the track; that is, extremely close ln terms o~
within a fractlon of the diameter o~ the probe coil 4. At that point, the amplitude o~ the received signal ~alls Or~ very sharply, and ln a very short lncremental distance, goes rrom a peak o~ posltive phase, through zero to a peak o~ the negatlve ~-) phase. ~he sensltlvlty o~ the slgnal-dete¢t clrcuitry is such that one indicator 9 wlll remaln llt until the positive recelved slgnal i8 almost . ~:
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.
zero; and the other indica~or 9 will illuminate when the negative signal reaches above a certain, very small thresh- .
old. As a result, there is an extremely small range of un-certalnty about the point directly above the track in which neither indicator is llt, and outside of this posi-tion Or uncertainty there is a very clear indication-one indicator or the other being illuminated-that the probe is to one side or the other of the current-carrying conductor.
This is to be contrasted with the previously mentioned amplitude-sensing,the operation of which is illus-trated in Fig. 3A in which the signal strength increases much as it does in the phase scheme, when the probe approa-ches from a great distance; but, as the probe reaches the near vicinity of the track of interest, the signal continues to increase to a peak P directly above the track. Because, again, the distances are comparable to the dimensions of the probe coil, this peak tends not to be particularly sharp; it requires fair discrimination to determine it directly; and, as mentioned, is a function of the distance between the probe coil and the track. There is potential for error here as a change in this distance may readily be caused by intentional motlon Or the probe, operator uncer-tainty, or the need to avoid some component of the board.
This can be contrasted, as in Flg. 3C, wlth the operatlon Or the phase soheme Or Flg. 3B. The two tra¢es ln Flg. 3C
show the ~ignal lnduced in the probe coil when the probe coll is dlrectly above the plane carrylng the conductor (solid line) or 18 some dlstance away (dotted llne). The recelved slgnal where the coll ls some dl~tance away ls ~ust smaller. ~he behavlor ln terms Or changlng slgn as --11-- , ,''''~ ' , ' .
- , . ..
.
, -
4~i34 the probe crosses the conductor remains unchanged such that with certain minimum sensitivity in the signal pro-cessing path, the circuit can function properly even - ~ -with a reduced signal.
Typlcal track spacing on printed circuit boards in use is on the order of 20 to 40 thousandths of an inch.
The spacing resolvable by the short-detecting probe de-pends, of course, on the dimensions o~ the probe coll;
but with the probe coil 4 of the present invention, which may be approximately one tenth of an inch in dlameter (wlth a sensltlve region o~ this size), there is no di~ficulty in resolving which one Or several conductors spaced 20 to 30 thousandths apart is carrying the current. This may be improved further by making the coil smaller in diameter, but at the cost o~ reducing the signal induced in it.
The short-detecting probe using phase in accord-ance with the invention thus of~ers primarily two advanta- -ges. There is a very clear indication when the probe crosses the track of interest, and there is an indication as to which slde the probe is on, maklng lt substantlally lmposslble that a particular ¢rosslng can go unnotlced.
It is thus a very clear, dlrect operatlon requlrlng little or no lnterpretatlon, to move the probe essentially ln a zlg~ag path, back and ~orth~ along a path Just to one slde and the other o~ the conductor o~ lnterest, so as to veri~y at any polnt the particular conductor under ob-servation by thé phase changes and accordlngly to trace out the path of the current until the unwanted sectlon o~ path i8 found. In techniques that use amplltude, on the other hand, the locatlon Or the peak is subJect to lnterpretatlon because ls 1~ not a sharp peak and because .. . .
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.
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., , ,, '' ' . ;.
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~0~ 4 it depends on factors such as height of the probe coil above the board. The operator must be care- -ful to ensure that a peak does not go unnoticed such that the probe is actually on the wrong side of the current-carrying conductor from the side that operator has in mind. Because the indications of the short-detecting probe of the invention are relatively simple, moreover, the two lights are easily placed directly in the probe, as indicated in the implementation of Fig. 2, resulting -in the operator not having to lift his eyes from the direct probe position during measurement. This is as contrasted with prior art amplitude signal indication by a meter or the like wherein the operator needs to look at things in two places; or in connection with techni-ques as used by Hewlett Packard, supra, the operator must effectively use a threshold detection adjustment con-tinually subjectively to determine what the measured field strength is.
While the invention has been described in con-~ . .
nection with its important application to the detection of short circu~ts on printed circuit boards and the like, it appears that the method underlying the same and this type of apparatus may also haYe okher applications such as, or example, to any situation which requires that the path of a particular conductor be determined. The techni-que is applicable not only to paths on a printed circuit board but to conductors on a cable or in a bus system or other aggregation of electrical conductors. It ha~ al90 been successfully used to find the location of short cir-cuits within a cable, and can be extended to the location . : . .
.:
, ' , . .
. . ~ . . .
, 1~ 63~
~, o~ other conducting objects which are not prlmarily inten-ded as electrical conductors, such as determina~ion of the presence of a metal structural member or a water pipe that is buried so as not to be visible. Further modifica-tions will also suggest themselves to those skilled in this art, including other indicating means or automatic mechanical positioning under computer control; such being considered to ~all within the spirit and scope on the in-vention as defined in the appended clalms.
What is claimed is:
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Typlcal track spacing on printed circuit boards in use is on the order of 20 to 40 thousandths of an inch.
The spacing resolvable by the short-detecting probe de-pends, of course, on the dimensions o~ the probe coll;
but with the probe coil 4 of the present invention, which may be approximately one tenth of an inch in dlameter (wlth a sensltlve region o~ this size), there is no di~ficulty in resolving which one Or several conductors spaced 20 to 30 thousandths apart is carrying the current. This may be improved further by making the coil smaller in diameter, but at the cost o~ reducing the signal induced in it.
The short-detecting probe using phase in accord-ance with the invention thus of~ers primarily two advanta- -ges. There is a very clear indication when the probe crosses the track of interest, and there is an indication as to which slde the probe is on, maklng lt substantlally lmposslble that a particular ¢rosslng can go unnotlced.
It is thus a very clear, dlrect operatlon requlrlng little or no lnterpretatlon, to move the probe essentially ln a zlg~ag path, back and ~orth~ along a path Just to one slde and the other o~ the conductor o~ lnterest, so as to veri~y at any polnt the particular conductor under ob-servation by thé phase changes and accordlngly to trace out the path of the current until the unwanted sectlon o~ path i8 found. In techniques that use amplltude, on the other hand, the locatlon Or the peak is subJect to lnterpretatlon because ls 1~ not a sharp peak and because .. . .
,,,, - . ~ :
~, .. , . -, ,, .. . . , , ' ' . ' 1 '. ' . , ,, ' ' ' . , " , ,,,, , : , ,, ,:: :
.... . . .
.
: .. . ,, - , . ' :
, ( ,., , . , , , ' ': '., . ' ' ' ' ' . .
., , ,, '' ' . ;.
',, ,' ,':' " ','" ' ', , , "
~0~ 4 it depends on factors such as height of the probe coil above the board. The operator must be care- -ful to ensure that a peak does not go unnoticed such that the probe is actually on the wrong side of the current-carrying conductor from the side that operator has in mind. Because the indications of the short-detecting probe of the invention are relatively simple, moreover, the two lights are easily placed directly in the probe, as indicated in the implementation of Fig. 2, resulting -in the operator not having to lift his eyes from the direct probe position during measurement. This is as contrasted with prior art amplitude signal indication by a meter or the like wherein the operator needs to look at things in two places; or in connection with techni-ques as used by Hewlett Packard, supra, the operator must effectively use a threshold detection adjustment con-tinually subjectively to determine what the measured field strength is.
While the invention has been described in con-~ . .
nection with its important application to the detection of short circu~ts on printed circuit boards and the like, it appears that the method underlying the same and this type of apparatus may also haYe okher applications such as, or example, to any situation which requires that the path of a particular conductor be determined. The techni-que is applicable not only to paths on a printed circuit board but to conductors on a cable or in a bus system or other aggregation of electrical conductors. It ha~ al90 been successfully used to find the location of short cir-cuits within a cable, and can be extended to the location . : . .
.:
, ' , . .
. . ~ . . .
, 1~ 63~
~, o~ other conducting objects which are not prlmarily inten-ded as electrical conductors, such as determina~ion of the presence of a metal structural member or a water pipe that is buried so as not to be visible. Further modifica-tions will also suggest themselves to those skilled in this art, including other indicating means or automatic mechanical positioning under computer control; such being considered to ~all within the spirit and scope on the in-vention as defined in the appended clalms.
What is claimed is:
~ ' .
, .
' ; ;., .
:
~: -14- . ' ' , ' ' ~ ' ' '" ' , . . ~ , .
',' '~,' ':' "' .,'. ' ' ' '. ', '.,. '' . ~ .,' ''. ' - , . . .
. .
. .. . . . ... . . . . . .. . . . . .
~ , ~ - . . , ' ' ' ' , ':
,. . .
Claims (7)
1. A method or current-tracing the location of faults such as short circuits on printed circuit boards and the like, that comprises, applying an a.c. test excitation signal along a predetermined conductive track among other closely disposed tracks on a printed circuit board to generate a magnetic field about the predetermined track;
probing back and forth on either side of the predetermined track in the proximity of and along the same to pick up said field and produce voltages induced therefrom; indica-ting the point or phase change or said voltages as the probing crosses the track to pin point the track; and tracing the same to find the fault location.
probing back and forth on either side of the predetermined track in the proximity of and along the same to pick up said field and produce voltages induced therefrom; indica-ting the point or phase change or said voltages as the probing crosses the track to pin point the track; and tracing the same to find the fault location.
2. A method as claimed in claim l and in which the fur-tour step is performed of discriminatingly indicating induced voltages of opposite phase on either side or said track.
3. A method as claimed in claim 2 and in which the sen-sitive pick-up region is adjusted to be of the order of approximately one tenth inch and the signal strength or said excitation signal is adjusted to a level of the order or approximately 10 milliamperes.
4. Apparatus for current-tracing the location of faults such as short circuits on printed circuit boards and the like, having, in combination, means for applying an a.c.
test excitation signal along a predetermined conductive track among other closely disposed tracks on a printed circuit board to generate a magnetic field about the pre-determined track; magetic field probe means adapted to be moved along and back and forth on either side or the proximity or said predetermined conductor to induce voltages therein from said field; means for detecting the phase of the induced voltage and the point of reversal change there-of as the probe means crossed the predetermined track; and means for indicating such point of change.
test excitation signal along a predetermined conductive track among other closely disposed tracks on a printed circuit board to generate a magnetic field about the pre-determined track; magetic field probe means adapted to be moved along and back and forth on either side or the proximity or said predetermined conductor to induce voltages therein from said field; means for detecting the phase of the induced voltage and the point of reversal change there-of as the probe means crossed the predetermined track; and means for indicating such point of change.
5. Apparatus as claimed in claim 4 and in which the in-dicating means includes means for discriminatively indica-ting the opposite phases of the voltages induced on oppo-site sides of said predetermined track.
6. Apparatus as claimed in claim 5 and in which said probe means and indicating means are carried by a common movable probe housing.
7. Apparatus as claimed in claim 4 and in which said phase detecting means comprises comparator means connected to receive said voltages induced in the magnetic field probe means; threshold detector means cooperative with said comparator means for generating therefrom a square wave the transition points of which correspond to zero crossings of the induced voltage about a steady-state level; and a phase detector connected to receive the gener-ated comparator means square wave and reference phase in-formation from the said a.c. test excitation signal and correspondingly to signal the indicating means.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/751,103 US4186338A (en) | 1976-12-16 | 1976-12-16 | Phase change detection method of and apparatus for current-tracing the location of faults on printed circuit boards and similar systems |
| US751,103 | 1976-12-16 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1084634A true CA1084634A (en) | 1980-08-26 |
Family
ID=25020496
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA286,335A Expired CA1084634A (en) | 1976-12-16 | 1977-09-08 | Method of and apparatus for current-tracing the location of faults on printed circuit boards and similar systems |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4186338A (en) |
| JP (1) | JPS6037900B2 (en) |
| CA (1) | CA1084634A (en) |
| DE (1) | DE2749529A1 (en) |
| FR (1) | FR2374650A1 (en) |
| GB (1) | GB1578798A (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2744232A (en) * | 1952-03-06 | 1956-05-01 | Elbert N Shawhan | Magnetic and conducting materials detector |
| US2946939A (en) * | 1958-06-02 | 1960-07-26 | Arden L Lind | Curve follower |
| US2921179A (en) * | 1958-11-20 | 1960-01-12 | Air Reduction | Electromagnetically locating and following workpart configurations |
| US3076931A (en) * | 1959-12-23 | 1963-02-05 | Jasper Cronje | Apparatus for identifying and phasing electrical conductors |
| US3303400A (en) * | 1961-07-25 | 1967-02-07 | Fairchild Camera Instr Co | Semiconductor device complex |
| US3201562A (en) * | 1963-02-28 | 1965-08-17 | Air Reduction | Stabilized eddy current seam follower |
| US3344347A (en) * | 1963-08-23 | 1967-09-26 | Bell Inc F W | Method and apparatus for determining displacement utilizing a hall plate positioned tangential to an arcuate magnetic field |
| DE2050705A1 (en) * | 1969-10-15 | 1971-04-22 | Olivetti & Co Spa | Method and arrangement for locating short circuits |
| DE1960103A1 (en) * | 1969-11-29 | 1971-06-03 | Volkswagenwerk Ag | Method for testing the electrical continuity of parallel-connected heating filaments embedded in a pane |
| US3705956A (en) * | 1971-01-25 | 1972-12-12 | Computek Inc | Graphic data tablet |
| DE2311903A1 (en) * | 1973-03-09 | 1974-09-12 | Siemens Ag | PROCEDURE AND EQUIPMENT FOR LOCATING SHORT CIRCUITS IN MULTI-LAYER WIRING |
| US3860866A (en) * | 1973-05-04 | 1975-01-14 | Western Electric Co | Methods and apparatus for locating an open section in a conductor |
| US3889179A (en) * | 1974-01-21 | 1975-06-10 | Cranleigh Electro Thermal Inc | Directional pickup coil and oscillator apparatus for the location of buried electrically conducting elements |
| US3924179A (en) * | 1974-04-24 | 1975-12-02 | William A Dozier | Method for certifying dead cables or conductors by determining current pulse polarity |
| US3988663A (en) * | 1975-03-19 | 1976-10-26 | Texaco Inc. | Locator for metallic pipe with AC signals impressed |
| US4074188A (en) * | 1975-08-01 | 1978-02-14 | Testline Instruments, Inc. | Low impedance fault detection system and method |
| GB1537870A (en) * | 1975-09-05 | 1979-01-04 | Ericsson L M Pty Ltd | Printed circuit board testing method and apparatus |
-
1976
- 1976-12-16 US US05/751,103 patent/US4186338A/en not_active Expired - Lifetime
-
1977
- 1977-07-01 GB GB27656/77A patent/GB1578798A/en not_active Expired
- 1977-08-03 JP JP52093327A patent/JPS6037900B2/en not_active Expired
- 1977-08-31 FR FR7726487A patent/FR2374650A1/en active Granted
- 1977-09-08 CA CA286,335A patent/CA1084634A/en not_active Expired
- 1977-11-02 DE DE19772749529 patent/DE2749529A1/en not_active Ceased
Also Published As
| Publication number | Publication date |
|---|---|
| FR2374650B1 (en) | 1983-01-07 |
| JPS5375473A (en) | 1978-07-04 |
| US4186338A (en) | 1980-01-29 |
| FR2374650A1 (en) | 1978-07-13 |
| JPS6037900B2 (en) | 1985-08-29 |
| GB1578798A (en) | 1980-11-12 |
| DE2749529A1 (en) | 1978-06-22 |
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