JPS5836755B2 - How to detect contaminated metals - Google Patents

Description

[Detailed Description of the Invention] Conventionally, the output of a detection head having an oscillating coil and a receiving coil is balanced by a canceling signal from a canceling signal generating circuit, and the balance is determined depending on whether the balance is disrupted when an object to be inspected passes through a magnetic field. 2. Description of the Related Art Metal detectors of a type that detect the presence or absence of mixed metals in an object to be inspected are known.

This type of metal detector is used in a wide range of fields, such as testing for powdered milk, chocolate, tablets, wood, beverages, solid resin raw materials, and other mixed or mixed metals, or checking for possession of a weapon. .

What is important in this metal detector is to accurately balance the induced voltages of both coils making up the receiving coil pair and to increase the receiving sensitivity.

This can be achieved by adjusting the geometry of each coil to equalize the number of linkages.

However, since each receive coil is relatively large and heavy, achieving balance through this geometric arrangement is very difficult and time consuming.

For this reason, it was proposed that the balance be roughly balanced using a geometric arrangement, and that precise balance adjustment be performed externally using an electric circuit (Japanese Utility Model Publication No. 51-136158).
.

As shown in Fig. 1, this is a high-frequency oscillator that excites an oscillating coil L1 by connecting both coils L2 and L3 that make up a receiving coil pair in series with opposite polarities, and connecting the other ends directly to the output terminal. The first variable resistor VRI is connected to the second output terminal.
A series circuit of a second variable resistor VR2 and a phase shift element C is connected to the secondary side of the transformer. The coupling point with the phase element is connected to the output terminal of the receiving coil pair, and the difference signal E1 between the receiving coils L2 and L3 is canceled out by the cancellation signal E2.

However, in this balance adjustment circuit, the phase of the cancellation signal E2 can only be shifted from O to 1800;
Depending on the environmental conditions, especially the environmental temperature, complete equilibration may not be possible.

This is caused by temperature changes that cause the detection head 1 to undergo mechanical changes such as misalignment, expansion and contraction, and stress, as well as changes in electrical characteristics such as coil actance, resistance, and capacitance with the shield surface. It is something to do.

That is, due to such electromechanical changes, the difference signal E
1 may appear as a signal completely 180° out of phase with the previous one, and the cancellation signal E2 can no longer achieve balance.

In response to such demands, the present applicant has already proposed the following balance adjustment circuit (Japanese Patent Application No. 15490/1982).

That is, the cancellation signal generation circuit includes a transformer connected to the oscillation coil, and a real axis signal circuit connected to the secondary side of the transformer and generating signals in the 0° and 18000 phase directions on the vector diagram. , is connected to the secondary side of the transformer and generates signals in the 900 and 27000 phase directions on the vector diagram, and is inserted into both the real axis and imaginary axis signal circuits respectively to generate signals in the input voltage. Depending on the output signal Ex, E of the respective real axis or imaginary axis signal circuit to which it belongs
A controlled element that changes j is provided.

Then, the outputs Ex and Bj of both the real axis and imaginary axis signal circuits are combined to form a cancellation signal.

On the other hand, the output signal from the detection head is passed through a high frequency amplifier,
The obtained head signal So is divided into two signals having a phase difference of 90° from each other.
The respective synchronous detection outputs, that is, both real axis and imaginary axis components Ex and Ej, are input to the gates of the controlled elements of the real axis signal circuit and the controlled elements of the imaginary axis signal circuit, respectively. A delay circuit is provided in the feedback path.

With this configuration, the output of the detection head will be balanced against long-term changes such as changes in environmental temperature due to the function of the delay circuit, while the output of the detection head will be balanced against long-term changes such as changes in environmental temperature. The difference signal can be directly extracted as a detection output.

However, in this force-type balance adjustment circuit, setting the balance adjustment is still troublesome.

This is because, in order to produce a desired cancellation signal in the range of 3600, it is necessary to independently adjust both the outputs Ex and Ej of the real axis and imaginary axis signal circuits to both positive and negative directions on the vector diagram.

Moreover, the difference signal E1 is not always constant, and its amplitude and phase deviate from those at the initial setting due to mechanical changes in the detection head due to the above-mentioned environmental conditions.

In addition, a major problem is that some products that require detection of metal contaminants produce detection signals even when no metal is contaminated.

For example, conductive meat, cheese, etc., and in order to be able to detect metal contamination in these items by distinguishing them from the item itself, it is necessary to remove the influence of the item itself.

Conventionally, as a method for realizing this, a shift circuit is provided on the input side of both of the above-mentioned synchronous detection circuits, and the bias voltage of the drain of the FET in each synchronous detection circuit is changed, thereby controlling the above-mentioned head signal. Only the waveform of the part higher than the bias potential of So is the synchronous detection output E'x
, B'j.

Therefore, in practice, it is also necessary to adjust both of these shift circuits.

An object of the present invention is to provide a detection method based on a new principle that is completely different from conventional methods, and has features such as the above-mentioned balance adjustment setting being extremely easy and the circuit configuration being simple.

The present invention provides a method for detecting objects at each position in the detection head passage when an article is passed through a detection head in which one oscillation coil is disposed on one side of the upper and lower sides of the passage, and two reception coils are arranged side by side on the other side. The difference signal E1 (
It is based on the discovery that when plotted on a pie chart (amplitude and phase), the result is a straight line with a certain slope specific to the item.

In Figure 2, the horizontal axis is the distance l from the entrance of the passage, and the vertical axis is the amplitude and phase difference θ (relative to the transmitting coil side) of the difference signal El (indicated by the voltage ■ after high-frequency amplification). This is a graph when an iron ball with a diameter of 5.5u is sent into the passage every ICrfL.

All solid lines show the amplitude and dotted lines show the phase.Curve A shows the case where no sample is placed in the path of the detection head, B shows the case where ferrite is placed at a certain distance from the entrance of the path, and C shows the case where stainless steel is placed. Indicates the case where .

FIG. 3 schematically shows this as a pie chart in which the amplitude Vo-p is plotted in the radial direction from the center and the phase difference θ is plotted in the circumferential direction.

The horizontal and vertical axes that pass through the center of this pie chart correspond to the (2) will be explained in relation to this rectangular coordinate.

As can be seen from the pie chart in Figure 3, when iron is moved from the entrance to the exit of the detection head, even though the quadrant may change on the xy coordinates, the slope of the locus plotting the difference signal E1 does not change.

In other words, the tip of the voltage vector from the origin simply moves along a straight line with a constant slope, and the slope is approximately 45° for iron.
It is.

Further experiments revealed that this slope differs depending on the type of object being passed through.

Figure 4 shows examples of iron (Fe), stainless steel (SUS),
The inclination angles of cheese CHE and ham HAM are simply shown in complex orthogonal coordinates, and in this way, the inclinations of conductive and magnetic objects differ by about 90 degrees.

The present invention utilizes such properties to detect mixed metals, and will be described below with reference to specific examples.

In FIG. 5, the inclination of the object to be inspected itself is determined in advance by the horizontal axis component Vx and the vertical axis component Vy in FIG. By setting this value, if the value exceeds the range a±b, it is assumed that iron or other metals are mixed into the item.

In FIG. 5, the detection head 10 includes an oscillating part 11 containing an oscillating coil L1 for generating magnetic flux, and two parts disposed in parallel or orthogonally to interlink with the magnetic flux generated by the oscillating coil L1. The receiving section 12 includes two receiving coils L2 and L3.

The object to be inspected moves between the oscillating section 11 and the receiving section 12 with an arrow P.
It is constructed so that it can be passed in both directions.

One end of the oscillation coil L1 is grounded, and the other end is connected to, for example, IOO.
K) Iz is connected to the high frequency oscillator 13.

The receiving coils L2 and L3 are connected in series with opposite polarities, their intermediate connection point is grounded, and their other ends are directly connected to each other.

The difference signal E1 obtained from both receiving coils is sent to the high frequency amplifier 2.
The head signal So obtained from the amplifier is input to the synchronous detection circuits 31 and 32.

The synchronous detection circuits 31 and 32 output the output Eo of the oscillator 13.
Opening/closing is controlled by O0 and gate signals P1 and P2 which are out of phase by 90 degrees with respect to Eo obtained through the phase shift circuit 50.

Therefore, both synchronous detection circuits output detection outputs 81 and 82 having phases different from each other by 90 degrees, as shown in FIG. 6, for example.

70 is a division circuit which outputs the ratio of the magnitudes of the signals input to its input terminals X and y -S 2 /S 1 .

71 is a window comparator, which generates an output when the output -S2/S1 of the dividing circuit 70 exceeds a predetermined value, for example, exceeds the range of a±b mentioned above, and
Activate 2.

According to the detection method shown in FIG. 5, no matter what the equilibrium state is, that is, in which quadrant the difference signal E1 or the head signal So is located on the pie chart, if there is a mixed metal, the head signal Since the tilt angles for SO are different, it can be detected reliably.

Moreover, it has the feature that detection is possible regardless of the magnitude of the amplitude of the head signal So.

In Figure 5, a division circuit is used to detect the variation in slope.
Fluctuations in slope, ie, the presence or absence of mixed metal, can be similarly detected using methods other than division.

FIG. 7 shows a circuit in which a scare root circuit 80 is used instead of the divider circuit 70 to detect mixed metal.

The inherent slope of the item to be inspected is as shown in FIG. 8, and is assumed to be △Vy/△Vx-a.

Try rotating the orthogonal xy coordinates with respect to the inclined straight line G for this item.

Then, in this example, when the straight line G comes to the position shown in the figure in the first quadrant (or third quadrant) of the xy coordinates, the head signal SO
It can be seen that the value of the amplitude ■ is minimum, and maximum at the position shown in the figure in the second quadrant (or fourth quadrant).

When the value of a is 1, that is, when the slope of the straight line G is -45°, the amplitude ■ becomes minimum at a point deviated from the X-axis by about 45°.

In short, the amplitude ■ is a scare root obtained from its X-axis component Vx and y-axis component vy, that is, V=,/'ψkoha'
If the rotational position of the x and y coordinates is determined based on the relationship 〒 and the rotational position of the xy coordinate is set at the point where the amplitude value is the minimum, the detection output of the item itself will be minimized.

This means that if there is any mixed metal in the product, the detection output will suddenly increase at a sensitive equilibrium adjustment point.

The reason why the output increases due to the presence of mixed metals is as follows.

In other words, as shown in Figure 4, the slope of the product itself, for example, HAM, and the slope of the mixed metal, for example, Fe, are different, so when these are combined, a scare root that exceeds the maximum value of the scare root of the product itself will always occur. .

Furthermore, in Fig. 4, the trajectories are drawn centered on one point for convenience in order to show only the differences in the slope of the trajectories due to the differences in the test objects, but in reality, the head signal So for each item is The amplitude differs depending on the article. For example, when comparing the amplitudes of HAM and Fe, the amplitude of Fe is larger than that of HAM, and therefore the scare root of Fe itself is larger than the scare root of HAM itself.

As a result, if Fe is mixed in the HAM, the HA
The scare root indicated by M is larger than the scare root of the HAM itself.

In order to rotate and set the xy coordinates to such a point with the minimum amplitude, specifically, the phases of the gate signals Pi and P2 to the synchronous detection circuits 31 and 32 are set so that they have a phase difference of 90° from each other. All you have to do is change S.

The phase circuit 90 in FIG.
It is configured to output gate signals Pi, P2 with a phase shift.

That is, in the circuit shown in FIG. 7, the slope of the straight line G mentioned above is -45.
This shows the case of detecting whether or not there is a mixed metal in an item.

Without contaminating metal, the output from the scare route circuit 80 is minimal and there is no output from the window comparator 71.

However, if, for example, iron (Fe) is mixed into the product, the slope of the iron will differ from that of the product, for example, ham, as seen in Figure 4.
Since the difference is close to 0°, the output of the scare route circuit 80 increases rapidly.

As a result, the margin range determined by the window comparator 71 is exceeded, and the detection relay 72 is activated.

The feature of metal detection using this scare route method is that, compared to the conventional method of adjusting the Vx and Vy components of the head signal So alone, the variables are narrowed down to one, coordinate rotation, which means that the adjustment is extremely easy. There is something about it.

Another method of detecting slope variations is by subtraction.

That is, it is clear that when one end of the inclined straight line G in FIG. 8 coincides with the coordinate origin, Vy-Vx always becomes a constant value.

Therefore, for example, the subtraction circuit 40 shown in FIG. 9 may be used in place of the division circuit 700 shown in FIG.

The variable resistor 4L42 in the operational amplifiers 43 and 44 provided in front of the differential amplifier 45 adjusts the straight line G to come to the coordinate origin.

Note that it is also possible to set Vy-Vx=0 by using a voltage divider or the like.

Although the above was for the case of non-autobalance, it is clear that all of these can also be applied to the case of autobalance.

FIG. 10 shows an example of the configuration of an autobalance device when a method other than the scare route method is adopted.

When the oscillator coil L1 is excited by the oscillator 13, voltages are induced in the receiving coils L2 and L3 by the lines of magnetic force generated by the coil L1, and the difference signal E1 is
The signal is amplified by the high frequency amplifier 20 and sent to the synchronous detection circuits 31 and 32.

On the other hand, the output EO of the oscillator 13 is input to the primary side of the transformer 510 in the phase shift circuit 50 via the input adjustment circuit 15 consisting of the variable resistor 16.

The transformer 510 secondary winding has its center tap grounded, and the remaining end taps are provided with a real-axis signal circuit 42 for extracting a real-axis signal Ex with respect to the oscillator output EO, and with an imaginary-axis signal circuit 42 with respect to the oscillator output EO. An imaginary axis signal circuit 56 for extracting Ej is provided.

The real axis signal circuit 52 includes a semiconductor element whose resistance value changes depending on the input voltage, such as a FET 53, a variable resistor 54,
It consists of a series circuit with a resistor 55.

The phase circuit 50 is connected between the taps at both ends of the secondary side of the transformer 510.
It has two OCR series circuits connected in antiparallel, and at each connection point of elements C and R, there is an imaginary axis signal circuit 56 configured similarly to the real axis signal circuit 52, that is, a resistor 57 and a variable resistor. A series circuit consisting of a transistor 58 and an FBT 59 is connected.

The real and imaginary signal circuits 52,56 and the mixing means 60 may include adders or other suitable devices.

Signals EX from the real axis and imaginary axis signal circuits 52 and 56
and Ej are synthesized by the variable resistor 60 and sent to the other input terminal of the differential amplifier in the high frequency amplification circuit 20 described above.
It is input as an in-phase cancellation signal E2.

However, E2 does not include a differential amplifier in the circuit 20, and the difference signal E1
When it directly coincides with the signal, the signal is of opposite phase.

Synchronous detection circuits 31 and 32 are connected to the high frequency amplification circuit 20, and their outputs are integrated by integration circuits 41 and 42 and applied to the gates of FETs 53 and 59 of both the real axis and imaginary axis signal circuits. .

The synchronous detection circuits 31 and 32 have the same configuration, and the amplifier 2
It consists of a gate element inserted between the 0 output terminal and the integrating circuit 41 or 420 input terminal.

The voltage of the secondary winding of the converter 510, that is, the gate signal P1 with a phase difference of O0 is applied to the synchronous detection circuit 31, and the synchronous detection circuit 32 is connected to a capacitor C having a phase difference of 90 degrees.
A terminal voltage of , that is, a gate signal P2 is applied.

Therefore, the synchronous detection circuits 31 and 32 are controlled to open and close in synchronization with the secondary output voltage of the transformer 510 and with a phase difference of 90 degrees from each other, as shown in FIG. Output.

The synchronous detection outputs 81 and 82 are respectively output from the integrating circuits 33 . 3
4, and the integrated signals S 1' and S 2' are FE
It is input to the gates of T53 and T59.

When there are correction manual forces S'1 and S'2, FETs 53 and 59 detect the resistance value as output 81 according to the input amount.
．． 82 changes in the direction of decreasing the amplitude.

Therefore, if the variable resistors 54, 58, and 60 are set to appropriate positions in advance, the vector of the cancellation signal E2 will change according to the correction input from the integrating circuits 41 and 42, and eventually the difference signal E1 It settles down to a point where it completely cancels out.

Therefore, no matter how the characteristics of the detection head 10 change due to changes in the environmental temperature and the vector of the difference signal E1 changes, a cancellation signal E2 having the same amplitude and the same phase will be generated. Signal E2 always cancels signal E1.

That is, balance is automatically achieved.

However, this automatic balance function also works when an item is passed through the detection head 10 and a head signal So is generated in the high frequency amplifier 20 due to metal contamination in the item, so it has been inconvenient to detect contaminant metal in X until now. be.

For this reason, a delay circuit consisting of resistors 67, 68 and capacitors 63, 65 is connected to the correction input lines 61, 62 to the gates of both FETs 53, 59, respectively.
It is now inserted by

When the automatic balance changeover switch SW is closed, the head signal S
Even if o occurs, the change in the resistance values of the FETs 53 and 59 does not occur immediately, but occurs after a certain period of time due to the action of the delay circuit, and the head signal So is balanced so as to become zero.

In other words, the head signal So is always balanced against long-term changes such as environmental temperature changes, but when the difference signal E1 appears due to the passage of an article, it remains balanced for a certain period of time. Therefore, detection outputs S1 and S2 are generated.

Therefore, these signals S1 and S2 are passed to the low frequency amplifier 35. 3
6 and inputting it to the aforementioned division circuit 70 or subtraction circuit 80, the desired mixed metal can be detected.

FIG. 11 shows an example of an autobalance device for the scare route method.

The basic configuration is the same as in Fig. 10, but the synchronous detection circuit 3
The difference is that gate signals P1 and P2 of 1.32 mm are generated by a phase shift circuit 90.

The phase shift circuit 90 has a CR series circuit connected to the secondary side of the transformer 920, whereby gate signals P1 and P2 having a phase shift of 900 degrees are taken out.

The primary side of this transformer 92 is connected to the output E of the oscillator 13 via an operational amplifier 91 and a CR circuit including a variable resistor 93.
Connected to O.

By adjusting the variable resistor 93, the transformer 9201
Since the phase of the voltage applied to the next side changes, the gate signal P
1 and P2 change their phases while having a phase difference of 90° from each other.

That is, the xy coordinates will be rotated. Therefore, by adjusting the variable resistor 93, the synchronous detection circuit 31, 32
The output of the scare route circuit 80 connected to can be set to the minimum.

As described above, according to the present invention, contaminant metals can be detected regardless of the length of the amplitude of the difference signal, and regardless of the balance state, that is, in which quadrant of the complex display orthogonal coordinates the difference signal is located. Detection is possible, and balance adjustment is also extremely easy.

[Brief explanation of the drawing]

Fig. 1 is a conventional balance adjustment circuit, Fig. 2 is a diagram showing the relationship between the path position and the difference signal when an object is passed through the detection head, and Fig. 3 is a diagram showing it as a pie chart. Fig. 4 is a diagram showing the slope of the plot trajectory of the difference signal of various objects in complex display coordinates, Fig. 5 is a specific example of the metal detection method of the present invention using division, and Fig. 6 is a diagram of the synchronous detection circuit. A diagram showing an example of operation,
FIG. 7 is a specific example of the detection method of the present invention using a scare root, FIG. 8 is a diagram similar to FIG. 4 for explanation, FIG. 9 is a specific example of the detection method of the present invention using subtraction, and FIG. The figure shows a specific example of an autobalance device using division or subtraction.
FIG. 11 shows a specific example of an autobalance device for a scare route. 10...detection head, 20...high frequency amplifier, 31.32...synchronous detection circuit, 40...
... Subtraction circuit, 50 ... Phase shift circuit, 70.
...Division circuit, 71...Window comparator, 80...Scare route, 90...
...Phase shift circuit.

Claims (1)

[Claims] 1. Contaminated metal detection in which the primary side of the detection head is excited by an oscillator, and the difference signal obtained from the secondary side of the detection head is passed through two synchronous detection circuits having a phase difference of 90° from each other. In the machine, the synchronous detection circuit is connected to the phase of the oscillator and 00
and 90° phase control, dividing the outputs of both synchronous detection circuits, and detecting the presence or absence of mixed metal in the object to be inspected based on whether the obtained result exceeds a predetermined value. 2. In a mixed metal detector in which the primary side of the detection head is excited by an oscillator and the difference signal obtained from the secondary side of the detection head is passed through two synchronous detection circuits with a phase difference of 90 degrees, the above-mentioned two synchronous The output of the detection circuit is taken out through the square root circuit, and the gate signals of both synchronous detection circuits are set at 90° to each other so that the value of the square root is minimized.
A method for detecting the presence or absence of metal contamination in an object to be inspected based on whether the value of the scare root exceeds a predetermined value.