In an induction balance transmit/receive type
Contents
1. 4 evt 5 Sheet 7 of 15 Nov 8 1988 US Patent 4 783 630 Q 2 te S mig tn OL nsz AS Z 2 d 6 LLI 14 5 H3MOd wor HY ININ 8 5 3ov143N 9 5 238 ia 4 783 630 Sheet 8 of 15 Nov 8 1988 US Patent me e e o m 8 6 lt lt 6 6 93959151 7109 AIMS 375501 NO 440 NO 60tQo 5 NI 6 5 V 8 6 c ASt er YOLWNIWIMDSIC 14 US Patent 8 1988 Sheet 90f15 4 783 630 FIG 3A5 75y OVERLOAD DETECTOR 8 5 EY R73 390 gt 20 1 0 12 ind DEPTH gt PX ACTIVE LOW 878 75 RIOS DMNLM 339 870 gt SYNC DRIVE C38 1 h 3 223088 Lepaoss 25 2 3 R 8 2 02 is METER P359 UPDATE 2 7 5 2 2 R82 5 4 2 0 4093 IK G2 15V DISC GF 1 5 IED K GATE R83 4 CER 5 7 NPO 1 4 93 145094 TN 75v Q2 f 5 retest R79 2 2 7 TO TARGET INPUT 4 P 3 41 A 3 625 _ 17 54 7 5 RIO62. 40 44 SPEAKER 4 783 630 Sheet 1 of 15 Nov 8 1988 US Patent u3Xv3ds gt 304 9 ONINNL TOULNOD 710414405 5 7108405 ALIALLISN3S 14 5 5 SLINDYID 34V NIWISOSIG QNV 301531340 10 53 1 06 TOYLNOQ 9NIANWRS 9 539415 9 49415 NIV9 3NIS 7 2689 4001 950 LINX 4001 5 4019313 3SVHd 55 4 os nana 4001 4 783 630 Sheet 2 of 15 Nov 8 1988 US Patent 3AVM 5 72609 7 2669 H31H3ANOO 4001 JOVIIOA NV 319A2 A1nQ 1051000 9d OL JIVA 2 2699 3uvnos 23 312349 318VIMVA P5 2041409 2041409 443440 39uvHOSId 31dWvS 1 x 135330 X 590103130 3SvHd 1 915 04 4001 913 4 783 630 Sheet 3 of 15 8 1988 US Patent 8 913 9195 1 915 1399 91907 10U LNOO aig Masn oll 291 1081NO9 201 5 141 isnray 1574 OL 333345
3. 1 554 s SENS n 840 7 5v RIO 500 4 783 630 93 5399151 o Oianv oum 4 9 oo gt 5 E 9 US Patent 4 783 630 Sheet 11 of 15 Nov 8 1988 US Patent 98 4 225 OL 55 gt 9 US Patent Nov 8 1988 Sheet 120415 4 783 630 FIG 3BO 4 783 630 Sheet 13 of 15 Nov 8 1988 US Patent 9 SWHO 801 p i 5 P Lae 4 783 630 Sheet 14 of 15 8 1988 US Patent 1594 415440 119702 y GNnoH9 268702 1 gt 0 9 et 5 4 6 p ECY 13988 ONNOYIS 1 XV 108903 9 1 839 839 108903 1 Et HILIAS 399982510 08903 971 6 H211AS 2510 668903 9 1 Po 148903 971 4 Lj 1539 S800 m HILIAS 3ouviDsia 135430 8 X isnrav isnrqv 231 US Patent 8 1988 Shetisotis 4 783 630 FIG 3B3 1 3 604853 4 783 630 1 METAL DETECTOR WITH CIRCUITS FOR AUTOMATICALLY SCREENING OUT THE EFFECTS OF OFFSET AND MINERALIZED GROUND BACKGROUND OF THE INVENTION The
4. 172 from the Control Logic mod ule forces the output of the GEB Sample Hold to match the G signal supplied from the Summing Ampli fier 60 to the output 164 of the Air Sample Hold With this matching accomplished the G signal in ground balance mode has been automatically modified through adjustment of the X component signal to equal the G signal level in air balance mode of the detector system This corresponds to the static balance operation de scribed earlier When this step is completed the GEB Sample Hold 170 is caused to hold its output level 131 by the control signals 171 172 at the same time the X 20 25 Offset Cancel 100 is disabled by the control signal 104 An audio beep indication signaling that this has oc curred is provided by the Audio Drive 30 in response to a pulse on the reset line 35 provided by the Control Logic module If it is desired that the system perform a dynamic ground exclusion balance as earlier described then the user selects the Gnd position at 115 otherwise this feature is disabled In the dynamic GEB setting a Slope Detector 180 responsive to the derivative of the X component signal provides a Ground Signal 182 whose polarity is indicative of the direction of the X compo nent s slope For example a first polarity signifies the Search Loop 20 approaching the ground and the other polarity moving away or no movement relative to ground A change in that signal from the first to the
5. 5 2 839 5 a Hlvd 80333 39v 2051409 1 ue 95 31vNIWIHOSIO Seve Nive 6 6 77 31VNINRIOSIQ EXE WE 5 9 3IVNINIHOSIQ 1 96 a GIOHS3UHL 59311154 ALIALLISN3S US Patent 8 1988 Sheet 4 15 4 783 630 FIG 0 4 783 630 Sheet 5 of 15 8 1988 US Patent 43 O4 9 b e si se pem Ol 1 27 9 Wee 82H 028 8 72 8 2 o dd OOS ege Nene TE WI 4 4 9 S Pu p lt xe L 33 14 m M 77 CM 9001 74 4001 IJ b lt H 2 09 aasoau __5 4 783 630 Sheet 6 of 15 Nov 8 1988 US Patent ANALNO 439 2 7 2 AC T 1 95 1 6 0 5581 0 NIHLIM Q3HOLVW 5 LOCO NIHLIM Q3HOLVW PICO 8055 NIHLIM Qd3HOLVW 5120 9 6020 NIHLIM 9120
6. automating this GEB operation both staticly and dynamically the latter referring to the GEB func tion being performed by the instrument on a continual automatic basis as the detector is used to search over an area As a consequence the operator using a metal detector with this feature can quickly set the instru 10 15 20 25 30 40 45 55 60 65 2 ment for search and find activity with the offset adjustment in the loop circuit required to screen out the ground mineralization effects provided automatically and then have this screening in effect without further ado as he she proceeds over an area even though the ground s mineralization content might change dramati cally In an exemplary embodiment of the present invention an air balancing operation is performed by the instru ment s circuit in response to operator switch actuation while momentarily holding the unit with its search head up in the air and away from any metal Objects audi beep tone signals the completion of this air bal ance cycle The operator then places the loop on or very near the ground and switches the instrument to the ground balance cycle The instrument s control logic circuit then proceeds to automatically make the neces sary changes in the loop circuit parameters to accom plish ground effect elimination and when this step is completed a second audible beep occurs thus signaling that the instrument
7. is ready for use The user can then if desired empirically confirm that the instrument is indeed ground balanced by listening for any change in either direction in the threshhold tone as the loop is moved over the ground in an area free of targets After this static GEB has been accomplished the operator can if desired maintain the detector in ground balance automatically as he she uses it The instrument is pro vided with a self adjusting or dynamic ground balanc ing feature which as the loop is swept over the ground a tracking circuit operates under operator switch selec tion to automatically incrementally adjust the loop s circuit parameters as needed to maintain the unit in ground balance even as the mineralization content of the soil chariges The instrument in the illustrative em bodiment herein described is also provided with a sen Sitivity adjustable knob setting to enable the user to regulate the operation of the tracking circuit so that target objects within the instrument s range will not inadvertently be tuned out In a preferred form of the invention the detector is additionally provided with a user selectable preset or factory fixed setting on its sensitivity control which enables a novice operator to obtain satisfactory performance with the unit with a minimum of fuss however an experienced user will invariably desire to set his her own level of sensitivity in the instrument particularly when searching un
8. present invention relates to an apparatus and method for providing automated ground exclusion bal ance in portable hand held metal detectors of the induc tion balance transmit receive type in popular use today for searching and finding metal objects lying on or buried just beneath the surface of the ground Such detectors are held by the operator and moved or swept over the surface of the ground and when the detector passes over a metal object the disturbance of a mag netic field created in the area of the detector s search loop as generated by a transmit coil signal and moni tored by a receive coil unbalances the coil loop circuit and causes the detector to produce a visual or audible indication of the presence of the metal object Such instruments have in recent years become quite sophisti cated in operation sufficiently so that by responding not only to the amplitude of the magnetic field distur bance but also to the shift in the phase angle of the received or target signal relative to the transmitted signal an approximate indication can be provided by the detector as to the type of metal object located e g coins as opposed to aluminum pull tabs and even as to the type of metal coin involved penny nickel dime quarter etc However before utilizing these detectors to search for and locate metal objects it is first necessary to adjust the loop circuit to screen out the effects of the mineralized ground in the area The compe
9. the amplitude of said phase detec tor signal the improvement comprising a air balance circuit means responsive to said phase detector signal when said search head is held away and remote from the ground for automatically adjusting said phase detection signal level so as to cancel out offsets occurring in said loop return signal b static ground balance circuit means for screening out on a one time user actuatable basis ground effects said ground effects being the effects on said loop return signal of the mineralization content of the ground in the vicinity of said metal detector c dynamic ground balance circuit means for screen ing out on a continuing repetitive basis said ground effects and d user selectable switch means for transferring said metal detector being two modes of operation the first mode utilizing said air balance circuit means to adjust said phase detection signal to an air balance level of said detector and the second mode utiliz ing said static and said dynamic ground balance circuit means to provide a screening out from said phase detection signal of said ground effects 2 The metal detector of claim 1 including control means for varying the sensitivity of said detector in responding to the mineralization content of the ground in the vicinity of said detector 3 The metal detector of claim 1 including means for providing an indication when said air balance circuit means has completed said adjustmen
10. 30 6 exemplary electrical schematic of the circuitry of the system depicted in block diagram form in FIGS 2A and 2B is shown in FIGS 3A1 3A7 and 3B1 3B3 Sections of the circuitry have been partitioned off by dashed lines and labeled with reference numerals corre sponding to those of the corresponding elements in the block diagram of FIG 2 Component elements in the Schematic and their respective values are shown using standard industry nomenclature The terms and expressions which have been em ployed in the foregoing specification are used therein as terms of description and not of limitation and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and de scribed or portions thereof it being recognized that the Scope of the invention is defined and limited only by the claims which follow What is claimed is 1 In an induction balance transmit receive type of portable metal detector having an oscillator a search head with a transmit coil driven by said oscillator to produce a magnetic field and a receive coil producing as an output a loop return signal responsive to changes in said magnetic field caused by the nearby environment a pair of phase detectors receiving said loop return signal and producing a pair of component signals in quadra ture means for adding said component signals together to produce a phase detection signal and indicator means responsive to
11. United States Patent Shoemaker 54 METAL DETECTOR WITH CIRCUITS FOR AUTOMATICALLY SCREENING OUT THE EFFECTS OF OFFSET AND MINERALIZED GROUND 75 Inventor Donald K Shoemaker Sweet Home Oreg 73 Assignee White s Electronics Inc Sweet Home Oreg 21 Appl 844 797 22 Filed Mar 27 1986 51 Int 12 01 3 11 GOIR 33 00 52 US Cb 324 329 324 233 58 Field of Search 324 326 329 324 233 56 References Cited U S PATENT DOCUMENTS 4 303 897 12 1981 Podhrasky 324 324 4 387 338 6 1983 Hecht et al 324 236 4 470 015 9 1984 Hirschi et 324 233 Primary Examiner Reinhard J Eisenzopf Assistant Examiner Walter E Snow Attorney Agent or Firm Chernoff Vilhauer McClung amp Stenzel LOOP RE TURN 50 SIGNAL Y BANDPASS AND PHASE ___ SEARCH LOOP 20 10 XMIT OSC AND 65922 LOOP DRIVE SINE WAVE AUDIO 6592HZ DRIVE SQUARE WAVE x 6 SUMMING RETI 60 CHOPPER 120 oer FEEDBACK PATH 170 FIER STAGE 150 11 Patent Number 45 Date of Patent 4 783 630 8 1988 57 ABSTRACT A portable transmit receive induction balance type metal detector having an automatic ground exclusion balance GEB feature to facilitate the screening ou
12. e field of the search loop Knob switch set ting controls 90 and 91 are provided to enable the oper ator to vary respectively the degree of sensitivity of the instrument and the degree of discrimination as be tween target objects of different response characteris tics The foregoing description is of a conventional metal detector system circuitry in which GEB is effected by the user s manual twiddling and adjustment of the GEB Control setting 65 in the manner described earlier FIGS 2A and 2B depict the same simplified metal detector system with the addition of the AGEB feature In this diagram elements labeled with the same refer ence numerals are substantially the same as the similar ly labeled elements in FIG 1 In providing AGEB offsets in the loop circuit present in the X or ground component of the loop return signal as derived by the Quadrature Phase Detectors 50 are cancelled out by the X Offset Cancel 100 in response to an appropriate actuating signal 102 from the Control Logic Module 110 when the user selects the Air position at 115 Offsets refer to the residual DC levels in the output of the Phase Detectors The offset to the X component signal is maintained by the Sample Hold portion of this element 100 until a subsequent time to be described later when a disable signal is supplied thereto from the Control Logic module via Discharge Control line 104 The Summing Amplifier 60 determines the phase G of the loo
13. es in that magnetic field caused by 10 15 20 25 30 35 40 45 50 60 65 4 the nearby environment Targets coming within the area of the transmit coil s field change the loop return signal produced in the receive coil s windings which are connected to a Band Pass Preamplifier and Phase Quad rature Detectors 50 The Band Pass Preamplifier por tion boosts and buffers the loop return signal and re duces the gain for frequencies far from the Transmit Oscillator s frequency The Phase Detectors are sub stantially identical and serve as a balanced demodulator X and Y component quadrature phase detector system The two respective outputs X and Y of the Phase De tectors are summed in GEB Summing Amplifier 60 and output as a phase shift signal G which is a function of the amplitude of the loop return signal as well as of the difference in phase between the respective transmit and loop return signals A GEB Control 65 enables the user prior to the summing to vary the gain of one of the quadrature signal components exemplarily X corre sponding to the ground component while maintaining the gain of the other component at unity These three phase signals X Y and G are then passed through Filters and Gain Stages 70 and supplied to a Threshold Detector 80 which also includes Discriminater Circuits for responding to and generating as an output to the Speaker 40 an audio indication of the presence of target objects in th
14. nsation or adjustments to the detector s search head loop circuit for such earth effects referred to herein as ground exclusion balance or more simply GEB was com monly carried out prior to the present invention by cut and try manual techniques in which the operator would first reset the detector instrument in air to cancel all offsets and to set a low but discernible audio tone level in the detector and then hold it on or just above a patch of ground known or believed to be free of metal objects The change from air to ground typically would cause either a marked decrease or increase in the audio tone and the operator would next respond by changing through manipulation of a knob setting on the instru ment the parameters of its detector circuitry to change the tone level in the opposite direction The operator would then repeat the reset step again change the ground balance setting and continue this sequence as many times as necessary until the change in tone level in going from air to ground or back was either eliminated or reduced to an acceptable minimum level Such man ual GEB adjustment as can readily be appreciated from the foregoing description was tedious and time con suming and the procedure had to be repeated whenever the mineralization content of the ground changed ap preciably as the operator worked over an area SUMMARY OF THE INVENTION The present invention provides a control logic circuit means for
15. onsideration of the fol lowing detailed description of the invention taken in conjunction with the accompanying drawings BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a block diagram of an illustrative example of a metal detector of the induction balance transmit receive balance type with no provision for an auto matic ground exclusion balance feature FIGS 2A and 2B block diagram of the metal detec tor embodiment of FIG 1 modified to include an auto matic ground exclusion balance feature in accordance with the present invention FIGS 3A0 3A7 and 3B0 3B3 are electrical sche matic diagrams of an exemplary circuit embodiment of the metal detector of FIG 2 having the automatic ground exclusion balance feature of the present inven tion FIGS 3A0 and 3 0 are diagrams illustrating the layout arrangement of the electrical schematic diagrams of FIGS 3A1 to and 3 1 to 3B3 respectively DETAILED DESCRIPTION OF THE INVENTION An illustrative embodiment of a metal detector of the described type having the automatic ground exclusion balance AGEB feature of the present invention is currently manufactured and marketed by applicant s assignee White s Electronics Inc of Sweet Home Oreg under the model name and trademark COIN MASTER 6000 Di Professional hereinafter referred to as the 6000 Di Pro The operator s manual for the 6000 DiPro instru ment published by White s Electronics Inc describes the manne
16. p return signal relative to the transmit signal by inversion summing of a fixed multiple of the Y compo nent with a variable multiple of the X component The adjustment a applied to the X component at the Sum ming Amplifier is accomplished through the use of a Chopper 120 whose duty cycle between zero and 100 is determined by the DC Control Voltage level 131 applied to Converter 130 The square wave signal 132 determines the frequency of the Converter s output In other words the duration of the duty cycle corre sponding to the adjustment a is responsive to the out put 131 of the GEB Sample Hold 170 In the air balance mode of operation the Error Am plifier 150 through a feedback loop which is enabled by a control pulse 162 from the Control Logic module 5 48 forces the output 164 of the Air Sample Hold 160 to match the level of the G signal supplied from the Sum ming Amplifier 60 A beep indication signifying com pletion of the air balance step is provided by the Audio Drive 30 upon the enabling of the feedback loop in response to a pulse on the reset line 35 provided by the Control Logic module Output 164 is held until the ground balance step described later is completed The Error Signal output 152 is provided as an input to GEB Sample Hold 170 whose function is to provide the Control Voltage level 131 In the ground balance mode the Error Amplifier through an AGEB Feedback Path which is enabled by control signals 171
17. r in which the feature is utilized by the operator through panel switch settings and manipu lations of the instrument s search head While such description has been summarized earlier herein and is non essential to an understanding of the nature and manner of operation of the present invention it is useful in understanding its background and operational envi ronment Accordingly that publication to the extent pertinent hereto in respect to the feature is herein incorporated by this reference Referring first to FIG 1 there is shown a simplified block diagram of a transmit receive type induction balance metal detector corresponding to the Model 6000 Di Pro but without the feature included The instrument comprises a Transmit Oscillator and Loop Drive 10 which causes a sinusoidal waveform to be produced in to the Search Loop 20 as well as provid ing an audio tone signal supplied via Audio Output 30 to the Speaker 40 which provides an audible indication to the operator of the status of the loop s balance circuit The Audio Output 30 is preferably provided with a user actuatable Sample Hold circuit a conventional feature to allow offsets in the input to the audio stage to be cancelled The Search Loop 20 as is conventional comprises a pair of coils a transmit coil driven by the Transmit Oscillator which generates a magnetic field in the area of the search loop and a receive coil which is responsive to chang
18. second polarity signifies the point at which the search loop is closest to the ground As the Search Loop moves away from ground or is static as monitored by the Slope Detector the Error Amplifier 150 functions as when in the air balance mode to force the Air Sam ple Hold output 164 to match the Summing Amplifier output G When the Search Loop approaches the ground the output 164 is held as before and in re sponse to a control signal from the Control Logic on the Slow Adjust line 172 the AGEB Feedback Path is closed and the DC Control Voltage 131 is adjusted as described before in response to the Error Signal 152 produced upon the matching of Summing Amplifier output G to the Air Sample Hold output 164 In this dynamic GEB mode of operation a slow adjustment rate exemplarily about ten times the time constant of the fast adjustment rate used in the static balance mode is utilized to limit the speed of the correction provided by the AGEB Feedback Path This is preferably done while in this mode so as to minimize the error which would otherwise occur due to the presence of offsets in the X component signal During dynamic balance it will be recalled the X Offset Cancel 100 is disabled If the unit were left at a fast adjustment rate for example a circuit time constant of one second as opposed to ten seconds the circuit would hunt back and forth around the correct GEB setting 30 35 40 45 50 55 60 65 4 783 6
19. t 4 The metal detector of claim 1 wherein said dy namic ground balance circuit means has a circuit time constant which is at least an order of magnitude larger than the circuit time constant of said static ground bal ance circuit means so as to slow the response and thereby avoid hunting in the operation of said dynamic ground balance circuit means 5 The metal detector of claim 1 wherein a first indi cation is given when said metal detector has completed 4 783 630 T an air balance operation and a second indication is given when said metal detector has completed a static ground balance operation 6 The metal detector of claim 1 characterized in that said dynamic ground balance circuit means includes slope detector means for producing an output signal 10 15 20 25 30 35 45 50 55 60 8 responsive both a change the distance of said search head from the ground and the direction of said change said output signal being utilized in said dynamic ground balance circuit means for screening out said ground effects 65
20. t of the effects of mineralized ground The circuitry pro vides a static GEB in a single try and then if selected by the user continual automatic GEB on a dynamic basis as the system is used The circuit includes a pair of quadrature phase detectors one of whose outputs is approximately in phase with the ground component of the loop return signal That output is modified during an air balance of the detector to remove the residual offsets in the circuit This modified output in turn is summed with the other phase detector output to pro duce a phase detection signal that can be adjusted by automatic means to be precisely in quadrature with the ground component of the loop return signal A control ler is provided with selectable user inputs to initiate the static and dynamic GEB operations An error am plifier in conjunction with two sample hold elements and a variable duty cycle chopper operate in a feedback loop to appropriately adjust the phase detection signal Audible beep indications are generated to signal the user when the steps in the automatic GEB are com pleted 6 Claims 15 Drawing Sheets 100 lt CANCEL SAMPLE HOLD DISCHARGE CONTROL 104 VARIABLE DUTY CYCLE SQUARE WAVE 6592HZ VOLTAGE TO DUTY CYCLE DC CONTROL CONVERTER VOLTAGE 130 SENSITIVITY 907 CONTROL DISCRIMINATE CONTROL DETECTOR GAIN sms SI MINATE VOLUME CONTROL TUNING CONTROL
21. usual terrain Selection of the preset setting on sensitivity locks out the sensitivity knob control and prevents the user from turning the sensitivity control up so much that the ground balance tracking circuit does not work properly Use of the preset setting enables the instru ment to be used effectively after a short while even without performing the GEB steps referred to previ ously as sweeping of the detector back and forth over the ground in searching will cause the tracking circuit to eventually in about 30 to 50 sweeps the number required being determined by the amount the instru ment is initially off range from the GEB condition bring the loop circuit into ground balance In the track ing circuit provided in the automated GEB system of the present invention the adjustment in the detector circuit s parameters is made on each sweep of the search head as it approaches the low point nearest to the ground Thus for best results the operator should im plement the initial or static GEB with the loop resting On or very near the ground and then search by Sweep 4 783 630 3 ing the loop as close to the ground as possible If condi tion of the ground changes as the operator moves about the area the tracking circuit will adjust the loop circuit to keep the instrument in ground balance at all times without need for operator intervention Other objectives features and advantages of the in vention will be apparent upon c
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