Metal detector for detecting product impurities

In a metal detector, such as may be used in a bakery, an adjustable phase shifter is provided to adjust the detectors for different products. The adjustable phase shifter is controlled electronically from the output signal so that the arrangement is self-adjusting.

This invention relates to a detector, such as a detector adapted to detect 
metal or other impurities in products. 
In places such as bakeries it is necessary to be able to detect impurities, 
such as metal impurities, in the products, which may for example be bread 
or cakes. 
It has been proposed to provide a detector system in which a first 
rectangular coil 1 is provided, as shown in FIG. 1 of the accompanying 
drawings, which is a diagrammatic view of part of the prior proposed 
device. Two similarly dimensioned coils 2, 3 are located on opposite sides 
of the first coil 1. A conveyor belt schematically indicated by the arrow 
4, or some other device, is provided to convey items to be inspected 
through the three coils sequentially. The central coil 1 has terminals 5, 
and the outer coils are effectively connected in series opposition and 
have terminals 6. 
As shown in FIG. 2, which is a block circuit diagram, an oscillator 5A is 
connected to the terminals 5 and thus energises the first coil. A field is 
established which energises the two other coils,and thus a signal is thus 
provided at the terminals 6 which is amplified by the amplifier 6A. An 
output terminal of the oscillator is connected to an input of a first 
phase detector or phase comparator 7, and also to a first input of a 
second phase detector or comparator 8. The output of the amplifier 6A is 
connected directly to a second input of the first phase detector 7, and 
through a fixed phase shifter 9 which imparts a 90.degree. phase shift to 
the signal, and an adjustable phase shifter 10 which imparts a selectable 
phase shift to the signal, to the other input of the phase detector 8. 
Each phase detector 8 provides a null output when the signals fed to it 
have a 90.degree. relative phase shift. 
When a typical product, such as a loaf of bread is passed through the 
coils, it is found that since the loaf is slightly conductive a product 
signal 11 is produced, as shown on FIG. 3, which is a diagrammatic figure 
given for purposes of explanation. This signal largely corresponds with 
the resistive loss vector, but will have an equal and opposite component 
11' depending upon whether the product is between coils 3 and 1 or between 
coils 2 and 1. When the phase adjuster 10 does not provide any phase shift 
a signal 8' is provided at 90.degree. to the product signal. By adjusting 
the phase shifter 10 to provide an angular shift of .phi. the signal 8' 
can be made to be purely inductive thus having no resistive component. 
Thus the phase detector 8 has a constant output. 
Initially the described apparatus is set up so that as a proper or 
uncontaminated product passes through the coils there is no fluctuation on 
the output of the phase detector 8. However, if there is a metallic or 
other similar impurity in one product, the product signal 11 will have an 
impurity signal 15 added to it, leading to a new composite signal 16. This 
gives rise to a new inductive signal 16' which is angularly displaced from 
the inductive axis, and which thus has a resistive component. Consequently 
there is a signal fluctuation present on the output of the phase detector 
8, and such a signal can be used to identify a faulty product. 
Where various different products, such as different types of loaf, are 
inspected on one apparatus it is desirable to reset the phase shifter 10 
when each different product is inspected. This is time consuming. It is 
possible to provide the phase shifter 10 with a number of selected pre-set 
phase shifts, but often operators do not use this facility, choosing 
merely to adjust the sensitivity of the device, so that all the different 
products are accepted on one sensitivity setting. This has the 
disadvantage that certain products containing impurities will not be 
identified as containing impurities. 
As can be seen in FIG. 4 the output, in volts, of the phase detector 8 as a 
product passes through the coils before the phase shifter 10 is adjusted 
is effectively a wave, with time. The first peak appears as the product 
passes between coils 3 and 1 with the null 13 appearing as the product 
passes through coil 1. The reverse peak 14 appears as the product passes 
between coils 1 and 2. It is desired that this wave should be as close to 
possible as a straight line as an uncontaminated product passes thought 
the described device, and that when the wave differs from a straight line 
configuration by a predetermined amount the product is identified in some 
way as being contaminated. 
According to this invention there is provided a detector comprising a first 
energised coil through the field of which an item to be inspected is 
passed, a second coil arrangement within the field of the first coil which 
produces an output signal, part of the signal energising the first coil 
and the output signal of the second coil being fed to the inputs of two 
phase detectors, oneinput of a phase detector being associated with a 
90.degree. phase shifter and one input of a phase detector being 
associated with an adjustable phase shifter, the output of at least one 
phase detector being monitored to produce a control signal, the control 
signal being used to adjust said adjustable phase shifter. 
Preferably the 90.degree. phase shifter and the adjustable phase shifter 
are associated with one input of one phase detector. 
Conveniently the voltage of the output of one phase detector is monitored 
by a voltage monitor to produce said control signal in response to 
variations in said voltage. 
Preferably the output of the other phase detector is fed to a polarity 
detector which provides signals to the voltage monitor indicative of the 
polarity of the input to the said other polarity detector. 
Conveniently the voltage monitor includes signal storing means, and 
switches controlled by the polarity sensor so that when one polarity is 
sensed the signal storing means are supplied with said voltage in one way, 
and when another polarity is sensed the signal storing means are supplied 
with said voltage in another way. 
Advantageously the signal storing means comprise a first capacitor which is 
charged with the output of said phase detector when one polarity is 
sensed, and which is charged with the inverted output of that phase 
detector when the other polarity is sensed, there being a second capacitor 
which is charged with the output of said phase detector when no polarity 
is sensed. 
In an alternative embodiment the signal storing means comprise two 
capacitors, one of which is charged with the outpt of said phase detector 
when one polarity is sensed, the other being charged with the output of 
said phase detector when the other polarity is sensed. 
Preferably the capacitors are connected to a differential amplifier, the 
output of which provides the control signal for the adjustable phase 
shifter. 
Preferably a delay is present between the voltage monitor and the 
adjustable phase shifter. 
In an alternative embodiment of the invention the outputs of the first 
phase detector and the second phase detector are fed to the inputs of a 
third phase detector, the output of the third phase detector being 
connected by means of an RC delay network to the control terminal of said 
adjustable phase shifter. 
Preferably means are provided to reduce the time constant of the RC network 
to enable a rapid setting of the detector to be obtained. 
Advantageously said means to shorten the RC time constant comprise 
switchable means.

Referring to the drawings the present invention is a modification or 
addition to the arrangement described above with reference to FIGS. 1 to 
4. Thus the apparatus of FIGS. 1 and 2 is present, but will not be 
re-described. 
In this invention the output of the phase detector 8 is monitored, and if 
the waveform departs from the linear by more than a predetermined amount, 
the product is rejected. However, if the variation from the linear occurs 
more than a predetermined number of times when a number of products to be 
examined pass through the coils, the voltage monitor produces a control 
signal which is fed to the adjustable phase shifter 10, which in this 
invention is an electronically controllable phase shifter. 
As can be seen in FIG. 5 the output of the phase detector 8 is fed to a 
voltage monitor 18. In the described embodiment the voltage monitor 18 
also receives signals from a polarity detector 15 which is connected to 
the output of the phase detector 7. The polarity detector has a first 
output 16 which carries a signal whenever the input is positive, and a 
second output 17 which carries a signal whenever the input is negative. 
The output of the voltage monitor is fed through a delay line 19, to 
prevent undesired `hunting`, and through a lead 21 to the electronically 
controllable adjustable phase shifter 10. 
FIG. 6 shows one type of voltage monitor. The output of phase detector 8 is 
fed through input 22 to an amplifier 23. The output of the amplifier is 
connected through an inverter 24 and a resistor 25 and a switch 26 which 
is only closed when a signal is present on the line 17 to a device adapted 
to store the signal, in this embodiment a capacitor 28, the other terminal 
of which is earthed. The output of the amplifier 23 is also connected 
through a resistor 29 and a switch 30 that is closed when a signal is 
present on the line 16 to the input terminal of the capacitor 28. The 
output of the amplifier 23 is also connected through a switch 31, which is 
arranged to be closed whenever both the switches 26 and 30 are open, to a 
second device adapted to store the signal, in the form of a second 
capacitor 32, the other terminal of which is earthed. The input terminals 
of the capacitors 28 and 32 are connected to a differential amplifier 33, 
the output of which is fed to the line 19. 
In the voltage monitor a first signal representative of the average phase 
shift measured by the phase detector 8 whilst an item passes through the 
detector coils is stored, and a second signal representative of the 
average phase shift measured by the phase detector 8 is stored, and the 
stored signals are compared to produce a control signal for the adjustable 
phase shifter 10. 
As can be seen from FIG. 3 the resistive component of the signals from the 
two phase detectors are always in phase or 180.degree. out of phase. Thus 
as the signal from phase detector 7 goes positive the switch 30 is closed 
and the capacitor 28 charges up with the signal of the polarity (the first 
polarity) then provided by the phase detector 8. In the example of FIG. 4 
this will be X volts plus the amplitude of the peak 12. When the output of 
the phase detector 7 goes negative the switch 30 is opened and the switch 
26 is closed. The peak 14 is then inverted relative to the X volt line and 
the resultant is fed to the capacitor 28. During the period between 
products passing between the coils the switch 31 is closed and capacitor 
32 is charged up to X volts. It will be seen that if the output of phase 
detector 8 is linear both capacitors 28 and 32 will carry the same voltage 
and there will be no output from the differential amplifier. Equally it 
will be understood that the magnitude of any non linearity (which is 
dependant on the phase shift) is reflected in the magnitude and polarity 
of the output of the differential amplifier and thus this signal can 
sensibly be used to control the adjustable phase shifter 10 to restore the 
desired operating condition. 
In this arrangement the value of the signal from the phase detector 8 when 
the phase detector 7 has one polarity is stored and is compared with the 
value of the signal from the phase detector 8 when the phase detector 7 
has the opposite polarity. 
FIG. 7 illustrates another form of voltage monitor 18. In this case the 
input 22 is connected through a resistor 34 to a first switch 35, which is 
closed when a signal is present on line 16, and thus to a first capacitor 
36, and through a second switch 37 which is closed when there is a signal 
present on the line 17 to a second capacitor 38. The capacitors are 
connected to the inputs of a differential amplifier 39, the output of 
which is fed to the line 19. It will be appreciated that the capacitor 36 
will effectively store the value of the peak 12 of FIG. 4, which is X 
volts plus the size of the peak, and the capacitor 38 will store the value 
of the peak 14, that is X volts minus the value of the peak. It is only 
when the peaks are of insignificant size, i.e. when the output of the 
phase detector 8 is substantially linear, that no signal is generated at 
the output of the differential amplifier. 
FIG. 8 illustrates a modified embodiment of the invention. It is to be 
understood that a problem may arise when the product being monitored by 
the described apparatus is electrically conductive due to a high moisture 
content combined with salt. This is a common situation with many food 
products. In such a case a very large resistive signal may be generated, 
and it is then critical that the adjustment of the variable phase shifter 
10 is effected rapidly and accurately since short term errors may occur 
during warm up of the equipment. Of course, the setting of the adjustable 
phase adjuster varies with different product types, due to different 
moisture or salt content. 
As in the previous embodiments the arrangement shown in FIG. 8 incorporates 
the phase detectors 7 and 8 as present in FIG. 1. The output of the two 
phase detectors 7 and 8 may be fed, optionally through amplifiers such as 
the amplifier 39, to a further phase detector 40. The output of the phase 
detector 40 is fed by means of an RC network 41, 42 and a further 
amplifier 43 to a lead 44 which is connected to the control input of the 
adjustable phase shifter 10. When an item such as a piece of metal passes 
through the coils the phase change that occurs is not immediately passed 
through the RC network, and thus the RC network operates as a combined 
smoothing and delay circuit. Thus there is no significant change in the 
output of the RC circuit when a single impurity is detected, but if the 
nature of the product being monitored changes, then the output of the RC 
network changes in response to the change in the product. The RC network 
may be by-passed by means of a switch 45 to shorten the RC time constant 
during an initial warm up period or when the product is suddenly changed. 
An output 46 is taken directly from the phase detector 8 to activate means 
responsive to a detected impurity. 
FIG. 9 illustrates an embodiment which is a modification of the embodiment 
of FIG. 6. Instead of signals being stored on capacitors the switches 26 
and 30 are connected to an analog to digital converter 45 which produces a 
digital signal representative of the instantaneous amplitude of the 
signal. The digital signal is fed to a sampling adding counter 46 which 
obtains a count equivalent to the integral of the signal. The switch 31 is 
connected to a corresponding analog to digital converter 47 and counter 
48. The outputs of the counters 46 and 48 are compared by a comparator 49 
to produce a control signal. 
FIG. 10 illustrates an embodiment which is a modification of the embodiment 
of FIG. 6. The switches 35, 37 are each connected, via respective analog 
to digital converters 49, 50 to respective counters 51, 52 (corresponding 
to the counters 48, 46). The outputs of the counters are passed to a 
comparator 53 to generate the control signal.