GB2158583A - Metal detecting device - Google Patents

Abstract

There are many applications where it is necessary to sense the presence of metal, and additionally to distinguish between different metals detected, or gauge the distance of the metal detected from the detector to provide size indication. A ferrous or nonferrous metal sensing device that affords such versatility, comprising a sensing head (1) for generating a magnetic field and means for detecting variations in the decay rate of the field so as to give an output signal which is an indication of the presence or absence of metallic objects within the field, is characterised by having means (30) for separating the signal so generated into a plurality of different levels. The separate signals from amplifiers (32), (33), (34) may be supplied to logic S/W unit which is switchable so as to be capable of giving different patterns of parallel output signals (80), (90), (100). <IMAGE>

Description

SPECIFICATION Metal detecting device The present invention concerns devices for detecting metallic objects. The objects may be ferrous or nonferrous.

Such devices are already well known and comprise a sensing head incorporating a magnetic coil. A magnetic field is generated by pulsing a predetermined current through the coil and the rate of decay of the field is measured. The decay rate varies with the presence of metallic objects in the field and measurement of this variation provides the data from which the presence or absence of the objects can be determined.

An object of the invention is to provide a detection device which, whilst utilising the above principles, gives a substantial degree of versatility.

Accordingly from a first aspect of the present invention consists in a metal detection device having a sensing head for generating a magnetic field and means for detecting variations in the decay rate of the field so as to give an output signal which is an indication of the presence or absence of metallic objects within the field, and means for separating the signal so generated into a plurality of different levels.

In accordance with a further feature of the invention, the detection device includes a logic unit to which the separate signals are supplied, the logic unit being switchable so as to be capable of giving different patterns of parallel output signals.

In accordance with a still further feature of the invention a metal detection device is provided in which the output signal or signals is or are opto-isolated from any equipment to which the device may be connected.

In order that the present invention may be more readily understood an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a detection device constructed in accordance with the invention, Figure 2 is a circuit diagram of part of the device of Figure 1, and Figure 3 is a circuit diagram of another part of the device of Figure 1, Figures 2 and 3 being more detailed than Figure 1.

Referring now to Figure 1, this shows a device for detecting metallic objects and which has at least one sensing head 1, including a magnetic coil to which a predetermined pulsating current is applied from an oscillator 2 via a divide-by-100 circuit 3, an inverter 4, a buffer 5 and a pulse amplifier 6. A suitable pulse rate is 100 cycles/sec.

A signal derived from the decaying magnetic field is passed via the pulse amplifier 6 to cascaded signal amplifiers 7 and 8 which together provide a gain of approximately 10,000. The amplified signal is supplied to one input of a switching unit 9 another input of which is connected to the output division circuit 3 via a delay circuit 10. The delay in circuit 10 is set so that the switching unit 9 passes the decay signal from the sensing heads when they are not being energised by the oscillator. The switching circuit 9 also compares the decay signal from the sensing heads with a reference signal generated by a reference supply 11 and supplied to the switching circuit 9 via a buffer 12.The output of switching circuit 9 is taken via another buffer 13 to an integrating circuit 14 the output of which is a signal the magnitude of which is an indication of the presence or absence of a metallic object in the field generated by the sensing heads. The output of the integrating circuit 14 is taken by a switch or link 15 either to an auto tare unit 20 or to a logic unit 30. The auto-tare unit is described and claimed in Application 8407009 of even date herewith, and no claim is made to it herein.

The logic unit 30 is shown in greater detail in Figures 2 and 3 of the drawings. Figure 2 shows that part of the logic unit 30 which separates the output from integrating circuit 14 into three levels.

In Figure 2 the integrated signal is supplied to input terminal 31, and then in parallel to three operational amplifiers 32, 33 and 34. The other inputs of amplifiers 32 - 34 are taken from respective potentiometers 35, 36 and 37 which are in turn connected in parallel between a 24 volt rail 38 and a 0 volt rail 39. The outputs of the amplifiers 32 - 34 are connected via respective diodes 40, 41 and 42 to the bases of transistors 43, 44 and 45 in turn connected to output transistors 46, 47 and 48 so as to give three output signals at terminals 49, 50 and 51. The three parallel signal paths starting at the operational amplifiers 32 - 34 are eseentially identical but by appropriately setting the potentiometers 35 - 37 the signal at terminal 31 can be separated into three different levels.In the embodiment being described the output of amplifier 32, and thus of terminal 49, represents a high level signal, at terminal 50 a medium level, and at terminal 51 a low level signal. The terminals 49 - 50 are the inputs to the second part of logic unit 30, this second part being shown in Figure 3. This second part comprises eleven NAND-gates 60 to 71 interconnected in a network which includes five links or switches A, B, C, D and E. This circuit again gives three outputs and these appear at terminals 72, 73 and 74. In order to isolate the logic unit from any equipment to which the detection apparatus may be connected the terminals 72 - 74 are connected to respective epto-couplers 75, 76 and 77 which in this embodiment are Quad transistors type RIS 307064. The final output signals appear on output terminals 80, 90 and 100.

The logic unit 30 allows the detection device to differentiate betweeen a wide range of objects of different materials, dimensions and ranges. Once the signal levels at the input side of circuit 30 have been set by potentiometers the network of NAND-gates 60 - 71 and of switches A - E enables to following table of options.

Terminal 80 Terminal 90 Terminal 100 SET 1 1 0 0 0 0 0 0 1 0 0 0 1 SET2 0 0 0 1 0 0 0 1 0 0 0 1 SET3 0 0 0 1 0 0 0 1 0 0 1 1 SET4 1 0 0 0 0 0 0 1 0 0 1 1 The various switching patterns so generated can be used to give more precise meanings to the output signals from the sensing head or heads.

Naturally other combinations of output signals are possible. This vesatility has a number of important applications. Thus, there is the ability to sense the presence of different metals. For example in the brewing industry beer is carried in barrels of the same volume but made from different materials, and thus the combined weights of beer and kegs will vary even though the quantity of beer is constant. When check-weighing full barrels of beer it is accordingly important to know the material from which the barrels are made, and which is provided for by the invention.

The sensing head 1 may be used to determine the distance of a known object from the sensing head.

This again is of value in a number of industries where, for example, the diameter of a rod passing the detection unit has to be determined.

In certain industries the detection unit may be used to detect the presence of molten metal. The sensing head may be splashed with the molten metal which on solidifying will give rise to spurious results as the solid metal will remain permanently within the magnetic field of the sensing head. To compensate for this the embodiment being described is provided with the previously mentioned auto tare unit 20.

When this unit is in operation the switch 15 takes the output of integrating circuit 14 via an inverter 21 to an auto zero circuit 22 and then via a summing amplifier 23 back to logic circuit 30. This signal has to be zero when no metal is being detected. To achieve this the auto zero circuit includes a ramp generator driven by an ascillator 24. When a reset button 25 is pressed the ramp generator ramps up until the output of summing amplifier 23 is zero. This condition is detected by a zero crossing detector circuit 26 which shuts off the ramp generator. When the predetermined level has been reached the released signal from logic unit 30 is detected in a negative edge trigger 27, delayed in a delay circuit 28 to allow the system to settle to its predetermined position and recognise thepresence or otherwise of extraneous metal within the field of the sensing head. Should the amount of "tare" metal be so great that the auto tare circuit can no longer cope the unit gives an output at 29 to indicate an "alarm" level. This level is set by selecting the uppermost point to which the ramp generator is to ramp.

Claims (4)

1. A metal detection device comprising a sensing head for generating a magnetic field and means for detecting variations in the decay rate of the field so as to give an output signal which is an indication of the presence or absence of metallic objects within the field, and means for separating the signal so generated into a plurality of different levels.

2. A metal detection device as in Claim 1, wherein the detection device includes a logic unit to which the separate signals are supplied, the logic unit being switchable so as to be capable of giving different patterns of parallel output signals

3. A metal detection device as in Claim 1 or Claim 2, wherein the output signal or signals is or are opto- isolated from any equipment to which the device may be connected.

4. A metal detection device as in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.