Automatic self-cleaning ferromagnetic metal detector

A ferrous metal detecting apparatus is disclosed for detecting metal particles in a flowing fluid, such as engine oil. The detecting apparatus has soft-iron pole pieces with a bridging gap therebetween. The soft-iron pole pieces are electrically insulated from each other and magnetized by direct current through an electromagnetic coil. After a sufficient amount of ferrous metal has accumulated on the bridging gap, a level detector will give a warning signal and cause the electromagnetic coil to degauss the soft-iron pole pieces by applying an alternating current to the electromagnetic coil. After the flowing fluid has washed away the ferrous metal, the direct current will remagnetize the soft-iron pole pieces. If repeated warning signals are within predetermined time intervals, an indication of the amount of ferrous metal contained in the fluid can be interpolated. The recycling for repeated warning signals may be automatically provided through a timing circuit and control logic.

BACKGROUND OF THE INVENTION 
This invention relates to a ferrous metal detecting apparatus, commonly 
called a "chip detector," for detecting ferrous metal particles in a 
flowing fluid and, more particularly, to an automatic self-cleaning 
integrating ferromagnetic metal detector that will provide a series of 
step warnings to indicate the quantity of metal particles in the fluid. 
The detecting apparatus includes soft-iron pole pieces that can be 
magnetized and demagnetized. Upon collection of a sufficient quantity of 
metal particles to bridge a gap between the soft-iron pole pieces, a level 
detector will activate a logic circuit to demagnetize (degauss) the soft 
pole pieces thereby allowing the collected metal particles to be washed 
away by the fluid. The time required for repeated triggering of the level 
detector after remagnetization of the soft pole pieces is monitored by a 
timing circuit for an appropriate warning if the repeated metal particle 
collection to bridge the gap is within a predetermined time interval. 
BRIEF DESCRIPTION OF THE PRIOR ART 
Prior to the present invention, many different types of chip detectors 
embodying numerous principles have been used to determine the amount of 
metal particles (chips) in nonconductive fluids, particularly engine oil. 
Many of the prior chip detectors used some type of insulated permanent 
magnet with a portion being exposed to fluid being monitored. If a 
sufficient amount of conductive ferromagnetic chips is collected on the 
tip of a permanent magnet to bridge insulation to a metal housing, current 
would begin to flow thereby giving an appropriate warning signal. A 
typical magnetic chip detector is shown in Pool, et al. (U.S. Pat. No. 
3,404,337) wherein the permanent magnet is located in the insulated center 
portion of a plug to be screwed into the housing. The plug housing is 
grounded, and, if a current begins to flow between the permanent magnet 
and the housing, it will be detected by an appropriate metering system. 
Another similar chip detector is shown in Prestel (U.S. Pat. No. 
3,193,815). Normally the prior art shows some type of DC voltage being 
applied to the permanent magnet via an appropriate warning indicator so 
that upon current flow through the permanent magnet to ground, the warning 
device would be activated. 
In Huigens (U.S. Pat. No. 3,373,352), a coil is inserted into the fluid 
flow. Upon collection of metal particles on the coils, various turns of 
the coils would be shorted out thereby reducing the impedance of the coil. 
By measuring the impedance or voltage drop of the coil, the amount of 
particles collected on the coil could be interpolated. By stopping any 
current flow through the coil, some of the metal particles would be 
removed from the coil by continued fluid flow. 
None of the prior art known to applicant discloses semipermanent magnets 
used in a chip detection device wherein the semipermanent magnets are 
magnetized and demagnetized upon collection of sufficient ferromagnetic 
particles to bridge a gap therebetween. The demagnetization of the 
semipermanent magnets (soft-iron pole pieces) allows fluid flowing over 
the tips of the semipermanent magnets to remove any metal particles 
collected thereon. 
All of the prior devices for chip detection simply give a single warning, 
which may or may not be true, to indicate the quantity of metal particles 
contained in the fluid. In many situations, it is very critical to know 
with some degree of certainty the quantity of metal particles contained in 
the fluid. For example, it is essential to know the amount of metal 
particles contained in the oil of a helicopter engine. If a helicopter is 
in use in a combat situation, a false warning of metal particles in the 
engine oil could result in a premature termination of the mission. 
However, if a critical level of metal particles in the oil has been 
reached, it is essential to terminate the mission to avoid engine failure 
which could result in the loss of numerous lives. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide an automatic 
self-cleaning ferromagnetic metal detector. 
It is another object of the present invention to provide a ferromagnetic 
detector, the cycle of which may be repeated for subsequent indications to 
verify the accuracy of a prior warning. 
It is yet another object of the present invention to provide an automatic 
ferromagnetic detector wherein soft-iron pole pieces are magnetized 
thereby causing metal particles to collect on the tips thereof. After a 
sufficient amount of metal particles have collected on the tips of the 
soft-iron pole pieces to bridge a gap therebetween, a level detector will 
trigger appropriate control logic to give a warning indication. The 
control logic also degausses the soft-iron pole pieces via an 
electromagnetic coil. Fluid flow over the tips of the soft-iron pole 
pieces washes away the metal particles collected thereon. Thereafter, upon 
remagnetizing the soft-iron pole pieces by the control logic, if 
sufficient metal particles to trigger the level detector are again 
accumulated to bridge the gap within a predetermined time period, the 
control logic will give a second-stage warning. The cycle may again be 
repeated for as many stages or warning levels as would be desirable for 
the particular operating condition. 
The control logic first applies a direct current to the electromagnetic 
coil surrounding the soft-iron pole pieces thereby creating a 
semipermanent magnet. After the level detector has been triggered by a 
sufficient amount of metal particles on the tips of the soft-iron pole 
pieces, DC voltage being applied to the soft-iron pole pieces is removed, 
and an alternating current is applied to the electromagnetic coils to 
demagnetize or degauss the soft-iron pole pieces. After a sufficient 
period of time has passed for the washing of metal particles from the tips 
of the soft-iron pole pieces, the direct current is again applied to the 
electromagnetic coils to remagnetize the soft-iron pole pieces for 
repeated cycles.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2 in combination, a ferromagnetic metal detector, 
represented generally by reference numeral 10, is shown. The metal 
detector 10 is designed for use in the flow line of a fluid having low 
electrical conductivity, such as an oil line for oil flowing through an 
engine. While many other uses may be made of the ferromagnetic metal 
detector 10, one of the primary design considerations is for use in the 
flow lines of aircraft engines, particularly helicopters. 
The ferromagnetic metal detector 10 includes a probe portion 12, mounting 
portion 14 and a probe retaining portion 16. The metal detector 10 may be 
very easily mounted into a flow line (not shown) by means of threads 18 
and nut 20 of mounting portion 14. The probe portion 12 has a pair of 
probe tips 22 extending from the end 21 thereof. The probe portion 12 has 
a cylindrical extension 24 formed integral with a flange 26 for mounting 
purposes. The cylindrical extension 24 has a beveled surface 28 to allow 
for ease of insertion into a flow line, and to decrease resistance to 
fluid flow. An electrical cable 30 connects to the probe tips 22 as will 
be subsequently explained in more detail. The electrical cable 30 extends 
through the flanged end of the probe portion 12 and through probe 
retaining portion 16. The probe retaining portion 16 has a mounting nut 32 
with a strain relief 36 to hold the electrical cable 30 securely in 
position. The mounting nut 32 is threadably connected to mounting portion 
14. 
Referring now to the probe portion 12, the probe tips 22 form a portion of 
a pair of soft-iron pole pieces 44 and 46 arranged as shown in FIG. 2. 
Surrounding the soft-iron pole pieces 44 and 46 are coils 48 and 50, 
respectively. One side of coil 48 is connected to lead 52, and the other 
side of coil 48 connected to coil 50. The opposite side of coil 50, after 
series connection with coil 48, is connected to lead 54. Soft-iron pole 
pieces 44 and 46 are connected to leads 56 and 58, respectively. All of 
the leads 52, 54, 56 and 58 form a part of cable 30 described hereinabove. 
While the particular construction of the probe portion 12 may vary without 
varying from the principles of this invention, applicant mounts the 
soft-iron pole pieces 44 and 46 with their respective coils 48 and 50 by 
means of a nonconducting epoxy resin which will provide the insulation and 
strength desired. By use of a nonconducting epoxy resin upon forming of 
the cylindrical extension 24, all of the components forming a part of the 
probe portion 12 are securely located in a permanent position. 
The space (hereinafter called sending gap 118) between probe tips 22 may 
also be filled with the nonconducting epoxy resin and made flush 
therewith. The filling of the sensing gap 118 would prevent the sticking 
of particles during a demagnetization cycle which will be explained 
hereinbelow. Only the very ends of probe tips 22 need to be exposed to the 
flowing fluid. 
Referring now to FIG. 3 of the drawings, there is shown a partial sectional 
view of the cylindrical extension 24 of the probe portion 12. Control 
logic 60 is connected to leads 52, 54, 56 and 58, which control logic 
cycles the ferromagnetic metal detector 10 in a manner as will be 
subsequently described. The control logic 60 is divided into general block 
diagram subsections. Current flows through coils 48 and 50 via leads 52 
and 54 from coil power switch 62. The coil power switch 62 receives its 
voltage from an alternating current (AC) power supply 64 and a direct 
current (DC) power supply 66. The soft-iron pole pieces 44 and 46 connect 
via leads 56 and 58 to conductivity sensor circuit 68, which also receives 
power from DC power supply 66. The output of conductivity sensor circuit 
68 operates logic circuit 70 via coil power switch 62 in conjunction with 
timer 72. The logic circuit 70 controls the operation of a series of 
warning lights on warning light panel 74. 
Referring now to FIG. 4 of the drawings, a more detailed functional block 
diagram for the control logic 60 is shown. It should be understood that 
other types of control logic circuits may be utilized; however, the 
control logic as shown in FIG. 4 provides an automatic, self-cleaning 
ferromagnetic metal detector. The control circuit as shown in FIG. 4 has 
six operating conditions, which are as follows: 
1. Power-on reset. 
2. Initial metal detection event. 
3. Second metal detection event occuring within a preset time delay. 
4. Third metal detection event occuring within a preset time delay. 
5. Preset time delay expires before next metal detection event occurs 
(applies only after first and second metal detection events). 
6. Authorized reset following Condition 4 above. 
Functions of block diagram shown in FIG. 4 are described hereinbelow. 
When the DC power supply 66 is turned ON, it will be sensed by power-on 
sensor 76. The power-on sensor 76, which may be a standard component item, 
provides an output signal to reset the control logic 60 each time DC power 
supply 66 is turned ON. This is the first of the previously listed 
operating conditions. 
An astable multivibrator 78, which may be temperature compensated to 
provide a stable time based generator, is the heart of the timer 72. While 
any time based astable multivibrator may be used, the astable 
multivibrator 78 delivers an output pulse every minute to a preset counter 
80. Prior to receiving the first metal detection event, the preset counter 
80 is maintained in its preset condition by an output from set-reset 
flip-flop 82. Set-reset flip-flop 82 is reset by the initial signal 
received from power-on sensor 76 via OR gate 84. The output from set-reset 
flip-flop 82 connects to the preset counter 80 via OR gate 86. While the 
preset counter 80 is in the preset condition as controlled by set-reset 
flip-flop 82, the preset counter 80 will ignore pulses received from the 
astable multivibrator 78. 
A two-stage binary counter 88 is also reset by set-reset flip-flop 82. Upon 
occurrence of a first metal detection event (as will be subsequently 
described in more detail), the two-stage binary counter 88 will count the 
event on the terminating transient of the signal voltage. The output of 
the two-stage binary counter 88 controls a standard binary to one-of-four 
decoder 90 via OR gates 92 and 94. Outputs from the binary to one-of-four 
decoder 90 control lamp drivers 96, 98, 100 and 102 by each successive 
output, either directly or through intervening logic. Lamp drivers 96, 98, 
100 and 102 control warning lights 104, 106, 108 and 110, respectively. 
Warning light 106 is maintained in the ON condition even when the third 
output pulse from binary to one-of-four decoder 90 has activated warning 
light 108 by means of OR gate 112. To correspond with normal warning 
signals, warning light 104 is green indicating the engine is being 
monitored for detection of metal particles. Upon receiving the first and 
second metal detection events, the warning lights 106 and 108 which are 
yellow will be illuminated to indicate a further cautionary state in 
operation of the engine. Upon receiving the third metal detection event, 
warning light 110 is red to indicate ceasing of engine operation 
immediately. 
A non-volatile memory 114 receives the output from binary to one-of-four 
decoder 90 intended for lamp driver 102. The non-volatile memory 114 will 
give an output for lamp driver 102 to maintain the red warning light 110 
in the ON condition. The non-volatile memory 114 may be (1) a mechanical 
or magnetic latching relay, (2) an erasable programmable read only memory, 
or (3) a complementary metal-oxide-silicon set-reset flip-flop operating 
from a rechargable battery power supply. The non-volatile memory 114 
prevents the unauthorized resetting of the conrol logic 60 thereby 
eliminating the red warning signal 110. The most critical alarm condition 
as indicated by the red warning light 110 cannot be overridden by simply 
turning power to the circuit OFF and then ON again. The non-volatile 
memory 114 can only be reset by the authorized manual reset 116, which is 
not accessible to the operator of the vehicle, but is only accessible to 
maintenance personnel. The authorized manual reset 116 also resets 
set-reset flip-flop 82, the two-stage binary counter 88 and the preset 
counter 80. 
The output of the DC power supply 66 (connection indicated by the letter 
"e" in a circle) applies a DC voltage through lead 56 to the sensing gap 
118 between the probe tips 22 of the soft-iron pole pieces 44. When 
sufficient ferromagnetic metal particles have collected on sensing gap 118 
to bridge the probe tips 22, current will flow through lead 56, through 
the ferromagnetic metal particles collected between probe tips 22 forming 
sensing gap 118, through lead 58 and through resistor 120 to ground. Once 
the current flow through resistor 120 reaches a predetermined value, level 
detector 124 will be activated by the voltage developed there-across. 
The coil power switch 62 includes a monostable multivibrator 126 and a 
triggered ramp generator 128, both of which receive the output from the 
level detector 124. An analog multiplier 130 receives an alternating 
voltage from AC source 64, as well as the output from triggered ramp 
generator 128. An output from the analog multiplier 130 connects to a 
single-pole double-throw analog switch 132 which receives its control 
signal from monostable multivibrator 126. By the output received from 
monostable multivibrator 126, set-reset flip-flop 82 is set and two-stage 
binary counter 88 counts its first input signal indicating a collection of 
ferromagnetic particles on the probe tips 22 of the metal detector 10. 
Also, the output of the monostable multivibrator 126 triggers the analog 
switch 132 so that an output from the analog multiplier 130, which 
consists of an alternating voltage received from AC source 64, will be fed 
through DC-coupled power amplifier 134 to coils 48 and 50 via leads 52 and 
54. The analog switch 132 receives a DC bias from DC power supply 66. The 
DC bias level may be set by a suitable DC bias adjust 133. 
During normal operation, the coil power switch 62 passes direct current 
through the coils 48 and 50 of the metal detector 10. The direct current 
will magnetize the soft-iron pole pieces 44 and 46 to cause ferromagnetic 
particles from a flowing fluid (such as lubricating oil) to collect on the 
probe tips 22 thereby bridging sensing gap 118. On command from the level 
detector 124 via monostable multivibrator 126, a gradually decreasing 
alternating current flows through coils 48 and 50 to demagnetize (degauss) 
the soft-iron pole pieces 44 and 46. The flowing fluid will wash away the 
ferromagnetic particles collected on the probe tips 22 thereby providing 
the self-cleaning operation. 
METHOD OF OPERATION 
In the normal mode of operation, the analog switch 132 applies a DC bias to 
the DC-coupled power amplifier 134 which, in turn, causes direct current 
to flow through the coils 48 and 50, thus providing a constant, 
unidirectional magnetic field in the soft-iron pole pieces 44 and 46, and 
across the sensing gap 118 between probe tips 22. Whenever a sufficient 
number of ferromagnetic particles have collected between the probe tips 22 
so that a predetermined current will flow through resistor 120, the level 
detector 124 will give an output signal at point A of FIG. 4 that 
corresponds to waveform A in FIG. 5. Waveform A has a fast transition time 
as can be seen in FIG. 5. The leading edge of waveform A from level 
detector 124 triggers both the monostable multivibrator 126 and the 
triggered ramp generator 128. The output of the monostable multivibrator 
126 at point B of FIG. 4 is the same as waveform B of FIG. 5. The output 
from the monostable multivibrator 126 triggers the analog switch 132 so 
that the output of the analog multiplier 130 is fed to the DC-coupled 
power amplifier 134. 
The triggered ramp generator 128 produces a special waveform at point C of 
FIG. 4 as illustrated by waveform C of FIG. 5. Waveform C, when multiplied 
by a sinusoid in the analog multiplier 130, produces a modulated sine wave 
voltage at point D of FIG. 4 as illustrated in waveform D in FIG. 5. The 
voltage waveform at point D and the current waveform through the coils 48 
and 50 are identical. Waveform D is amplified prior to being received by 
the coils 48 and 50. The modulated sinusoid current through the coils 48 
and 50 demagnetizes the core material, namely soft-iron pole pieces 44 and 
46. Following demagnetization, sufficient time is allowed for the flowing 
fluid to wash away any ferromagnetic particles collected at the probe tips 
22 in sensing gap 118. At the conclusion of the washing period, monostable 
multivibrator 126 returns to its zero state thereby allowing the analog 
switch 132 to return to its original state wherein a DC voltage is applied 
to coils 48 and 50. The DC voltage applied to coils 48 and 50 will again 
magnetize the soft-iron pole pieces 44 and 46 to repeat the collection of 
ferromagnetic particles. At some point between the time the alternating 
current demagnetization waveform D is reduced to zero, and termination of 
the output (waveform B) of the monostable multivibrator 126, bridging of 
the sensing gap 118 will cleared by washing action of the fluid. This 
permits the level detector output (waveform A) to return to its normal 
zero state. The demagnetization cycle is terminated when the output of the 
monostable multivibrator 126 returns to its zero state. 
After turning the power ON and the occurrence of the poweron reset by the 
power-on sensor 76, the initial operating condition has occurred. 
Thereafter, when sufficient ferromagnetic material has collected to bridge 
the sensing gap 118, the second operation condition (initial metal 
detection event) has been met. A demagnetization cycle is initiated by the 
level detector 124 and the monostable multivibrator 126 sets the set-reset 
flip-flop 82 to remove the signal from OR gate 86. The output of the 
monostable multivibrator 126 maintains the preset condition in preset 
counter 80 for the duration of the output pulse of the monostable 
multivibrator 126. The trailing edge of the monostable multivibrator 126 
causes the two-stage binary counter 88 to advance one count, and removes 
the preset signal from the preset counter 80 thereby permitting the preset 
counter 80 to count pulses received from the astable multivibrator 78 at 
the rate of 1 pulse per minute. 
Prior to the occurrence of the initial metal detection event, the output of 
the binary counter 88 (which is initially set at zero) will maintain the 
green warning light 104 in its illuminated condition through binary to 
one-of-four decoder 90. When the two-stage binary counter advances to its 
first count position, the green warning light 104 is extinguished and the 
yellow warning light 106 is illuminated. 
If another metal detection event occurs before the preset counter 80 has 
reached the end of its count, then another demagnetization cycle by the 
coil power switch 62 is initiated. This time, however, the set-reset 
flip-flop 82 is already set and the leading edge of waveform B from the 
monostable multivibrator 126 only restores the preset condition in preset 
counter 80. The trailing edge of the output of the monostable 
multivibrator 126 removes the present signal from preset counter 80 
thereby allowing it to again begin counting pulses received from the 
astable multivibrator 78. Also the trailing edge of waveform B from the 
monostable multivibrator 126 causes the two-stage binary counter to 
advance one count. The advancing of the binary counter 88 one additional 
count will cause the binary to one-of-four decoder 90 to give its second 
output to illuminate yellow warning light 108. OR gate 112 will also 
maintain yellow warning light 106 in the ON condition. 
If a third metal detection event occurs before the preset counter has 
reached the end of its second count, another demagnetization cycle as 
previously described is again initiated. Again, the leading edge of the 
output (waveform B) of the monostable multivibrator 126 presets the preset 
counter 80, and the trailing edge of waveform B of the monostable 
multivibrator causes the two-stage binary counter 88 to advance one count. 
Upon advancing of the two-stage binary counter 88 by one additional count, 
an output of the binary to one-of-four decoder 90 is energized thereby 
activating non-volatile memory 114. At the same time, yellow warning 
lights 106 and 108 are extinguished and red warning light 110 s 
illuminated. Also by means of a feedback from the output of non-volatile 
memory 114 through OR gates 92 and 94, the binary to one-of-four decoder 
90 is locked to its current state independent of changes which may occur 
elsewhere in the control logic 60. Even though additional metal detection 
events may occur, the red warning light 110 remains latched to the ON 
condition. 
Once the red warning light 110 has been illuminated by the third successive 
metal detection event within preset time limits, the automatic 
ferromagnetic metal detector 10 and associated control logic 60 are 
latched into a permanent alarm condition by the non-volatile memory 114. 
Even though power may be turned OFF and then back ON to the control logic 
60, the warning light 110 will remain illuminated. Only authorized manual 
reset 116, which is inaccessible to crew members that may be operating the 
vehicle, can be used to erase the non-volatile memory 114 during 
maintenance operations. For example, if the non-volatile memory 114 is an 
erasable, programmable read-only memory, an authorized reset of the 
non-volatile memory 114 could be accomplished by ultraviolet radiation 
through a small window in the non-volatile memory 114. Regardless of the 
non-volatile memory 114 that is being utilized, authorized maintenance or 
supervisory personnel should be the only individuals that could restore 
ferromagnetic metal detector 10 and its control logic 60 to normal 
operation. This insures the reporting of critical ferromagnetic metal 
detection events. 
ALTERNATIVE EMBODIMENT 
Referring to FIG. 6 of the drawings, there is shown an alternative control 
logic 60 that may be used with the ferromagnetic metal detector 10. Like 
components as shown in FIG. 4 of the preferred embodiment will be given 
the same reference numerals. The alternative control logic 60 as shown in 
FIG. 6 has four fundamental conditions which are as follows: 
1. Initial metal detection event. 
2. Second metal detection event occuring within a preset time interval. 
3. Preset time delay expires before second metal detection event occurs. 
4. Authorized reset following Condition 2 hereinabove. The functions of the 
block diagram shown in FIG. 6 that are different from the block diagram 
shown in FIG. 4 are described hereinbelow. 
The astable multivibrator 78 may be of the same type previously used to 
deliver one pulse per minute to a 3-decade BCD counter 137 via AND gate 
136. The 12 output lines of BCD counter 137 drive the "x" inputs of a 
12-bit magnitude comparator 138. The "y" inputs of the magnitude 
comparator 138 are controlled by a preset count limiter 140, which may be 
set by (1) adjustable controls means, such as a selector switch, or (2) 
hard-wired connections. When the count stored in the BCD counter 137 
equals the count of the present count limiter 140, an output is given by 
the magnitude comparator 138 indicating that the "x" input equals the "y" 
input. 
Control of the BCD counter 137 is accomplished by the previously described 
set-reset flip-flop 82, monostable multivibrators 142, 144 and 146, and 
associated logic. The AND gate 136 between the astable multivibrator 78 
and the BCD counter 137 is controlled by the output of set-reset flip-flop 
82. Under normal operating conditions, no output is received from the 
set-reset flip-flop 82, therefore no clock pulses from astable 
multivibrator 78 feed through AND gate 136 to the BCD counter 137. 
Monostable multivibrator 144, which is triggered by the fall of the output 
of monostable multivibrator 142, resets the BCD counter 137 to zero at 
appropriate times as described hereinbelow. Monostable multivibrator 142 
is triggered by the fall of the output of the set-reset flip-flop 82, and 
delays reset of the BCD counter 137 until after the count contained in the 
BCD counter 137 has been fed to 12-bit latch 148 by means of a strobe 
output from set-reset flip-flop 82. Monostable multivibrator 146 resets 
the set-reset flip-flop 82 via OR gate 150. The set-reset flip-flop 82 can 
also be reset by the output from magnitude comparator 138 indicating that 
the count output from BCD counter 137 is equal to the count set into 
preset count limiter 140 via OR gate 150. 
The 12-bit latch 148 is used to capture and store the count received from 
BCD counter 137 should a second metal detection event occur before the 
preset count of the preset count limiter 140 is reached. While counting 
pulses from the astable multivibrator 78 is taking place, the strobe input 
of the 12-bit latch 148 is held in the logic state which permits the 
output of the latch 148 to follow the input directly. When the strobe 
input of the 12-bit latch 148 falls, the count contained in the 12-bit 
latch 148 is captured and held. 
Outputs of the 12-bit latch 148 drive the input of a 3-digit 
BCD-to-seven-segment decoder 152 which, in turn, controls a 3-digit 
numeric display 154. Any type of readout in the 3-digit numeric display 
154 could be utilized including, but not limited to, light emitting 
diodes, gas discharge, fluorescent, liquid-crystal or incandescent lights. 
A blanking input to the BCD-to-seven-segment decorder 152 received via OR 
gate 156 provides for (1) maintaining the display in a normally OFF 
condition, and (2) turning the display ON whenever counting is in process 
or a second metal detection event occurs within the preset time period 
controlled by preset count limiter 140. 
The event counter function of the control logic 60 comprises two 
edge-triggered flip-flops 158 and 160, and associated logic. When the 
output from edge-triggered flip-flop 160 is zero, inverter 164 will 
maintain AND gate 162 in the open condition, thereby permitting output 
signals received from level detector 124 via monostable multivibrator 126 
to trigger edge-triggered flip-flop 158, and to set set-reset flip-flop 
82. Edge-triggered flip-flop 158 responds to the leading edge of the 
output of the level detector 124 and monostable multivibrator 126. 
Edge-triggered flip-flop 160, on the other hand, responds to the trailing 
edge of the output of edge-triggered flip-flop 158. When the output of 
edge-triggered flip-flop 160 changes state, AND gate 162 is closed via 
inverter 164. Therefore, additional output pulses received from level 
detector 124 via monostable multivibrator 126 will not pass through AND 
gate 162. If the output of both edge-triggered flip-flops 158 and 160 are 
OFF, then OR gate 156 will have an output that is received in 
BCD-to-seven-segment decoder 152 as a blanking signal. The blanking signal 
received by BCD-to-seven-segment decoder 152 from OR gate 156 extinguishes 
the 3-digit numeric display 154. An output from the 12-bit magnitude 
comparator 138, or an authorized reset 116 via OR gate 166, resets both 
edge-triggered flip-flops 158 and 160 to their zero state. 
A special power supply 168 is necessary to provide an accurate logging of 
metal detection events, especially in cases of loss of power when the 
equipment being monitored is not in use. The special power supply 168 
would comprise secondary cells, such as rechargable batteries, that may be 
recharged when power to the control logic 60 is ON. The function elements 
which require continuous power include (1) the set-reset flip-flop 82, (2) 
both edge-triggered flip-flops 158 and 160, (3) the 3-decade BCD counter 
137, and (4) and 12-bit latch 148. The connection from the special power 
supply 168 to these subcomponents prevents their being reset except by an 
authorized manual reset 116. 
METHOD OF OPERATION OF ALTERNATIVE EMBODIMENT 
The detection of ferromagnetic particles at the sensing gap 118 by the 
level detector 124, and the magnetization and demagnetization of coils 48 
and 50 is the same as described in the preferred embodiment. The leading 
edge of the output of monostable multivibrator 126 passes through AND gate 
162 with the leading edge triggering edge-triggered flip-flop 158 and 
setting set-reset flip-flop 82. The output of edge-triggered flip-flop 158 
through OR gate 156 removes the blanking from the BCD-to-seven-segment 
decoder 152 and 3-digit numeric display 154. The output of set-reset 
flip-flop 82 enables AND gate 136 thus permitting clock pulses from 
astable multivibrator 78 to be received into the 3-decade BCD counter 137. 
In addition, the output of the set-reset flip-flop 82 gives a strobe input 
to 12-bit latch 148 so that the output of the BCD counter 137 is 
transferred directly into the BCD-to-seven-segment decoder 152. 
If the second metal detection event occurs before the count in the BCD 
counter 137 reaches the count preset into preset count limiter 140, a 
demagnetization cycle is automatically initiated and the output of the 
monostable multivibrator 126 passes through AND gate 162 to trigger 
edge-triggered flip-flop 158. Since edge-triggered flip-flop 158 is 
already in the set condition, the output will fall thereby triggering 
edge-triggered flip-flop 160 which triggers on the trailing edge. The 
output of edge-triggered flip-flop 160 by way of inverter 164 disenables 
AND gate 162 thereby terminating its output and preventing any further 
metal detection pulses from passing therethrough. In addition, the output 
of the edge-triggered flip-flop 160 via OR gate 156 maintains the 3-digit 
numeric display 154 in the unblanked condition. The falling transition of 
the output of edge-triggered flip-flop 158 also triggers monostable 
multivibrator 146 which resets set-reset flip-flop 82 via OR gate 150. As 
the output of set-reset flip-flop 82 falls, AND gate 136 is closed thereby 
terminating the count from astable multivibrator 78, and the strobe input 
for 12-bit latch 148 is disabled thereby capturing the count existing in 
the BCD counter 137. The fall of the output of set-reset flip-flop 82 also 
triggers monostable multivibrator 142 which, at the termination of its 
output pulse, triggers monostable multivibrator 144. The output of 
monostable multivibrator 144 resets the BCD counter 137 to zero. 
Thereafter, the control logic 60 remains in this condition until receiving 
an authorized manual reset 116. 
If the count of the 3-decade BCD counter 137 reaches the preset count set 
into preset count limiter 140 before a second metal detection event 
occurs, then a pulse appears at the output of the 12-bit magnitude 
comparator 138. This output resets the set-reset flip-flop 82 via OR gate 
150, and resets both edge-triggered flip-flops 158 and 160 thereby causing 
OR gate 156 to blank the output of BCD-to-seven-segment decoder 152. The 
resetting of set-reset flip-flop 82 closes AND gate 136 thereby 
terminating the count from astable multivibrator 78 and triggering 
monostable multivibrator 142. After a suitable delay, monostable 
multivibrator 142 triggers monostable multivibrator 144. The output of 
monostable multivibrator 144 resets the BCD counter 137 to zero thereby 
terminating the cycle. If the cycle is completed in this fashion, the 
control logic 60 remains ready to receive a new initial metal detection 
event. 
If a second metal detection event occurs within the preset time interval 
controlled by preset count limiter 140, the control logic 60 latches in an 
alarm state, and it cannot be reset by turning power OFF and then ON 
again. Thus, authorized maintenance or supervisory personnel are required 
to restore the control logic 60 to its normal operation; therefore, a 
critical metal detection event cannot pass unnoticed. 
The authorized manual reset 116 resets both edge-triggered flip-flops 158 
and 160 via OR gate 166 thereby enabling AND gate 162 so that the control 
logic 60 may again receive metal detection pulses. Also, the authorized 
manual reset 116 resets edge-triggered flip-flops 158 and 160 which, via 
OR gate 156, blank the numeric display 154.