GB2207270A - Determining the characteristics of conducting objects - Google Patents

Abstract

An apparatus and method for determining the characteristics of conducting objects such as coins, metal foil, machine parts or wire based on the pulsed coil induction principle. Each coil 41, 42 is both pulsed and monitored for return voltage so that magnetic fields add, thus reducing the effect of movement of the object relative to the coils. A transverse coil 43 may be used for obtaining extra information about edge dimensions. The coils are of a rectangular shape and are all similar, thus providing for a uniform magnetic field over a sufficiently large area of an object path. <IMAGE>

Description

IMPF\sUE. TS t 'ralD RELATING TO DETERMINING THE CHARACTERITICS CF CONDUCTING OBJECTS The present invention relates to determining the characteristics of electrically conducting objects.

In this specification, the term "object" is intended to cover any electrically conducting object such as a coin, metal foil or wire. Further, the term "characteristics when used in connection with the term "object" is intended to cover any or all of object dimensions, material type, surface detail or electrical resistance per unit length.

In recent years, a technique has been developed for determining the characteristics of conducting objects, which technique uses an inductive coil which is fed with on/off drive current pulses. This is generally referred to as the pulsed coil induction technique. When the current in the inductive coil suddenly changes, a magnetic field linking the object to the coil also changes abruptly. When this happens, a voltage is created which causes eddy currents to flow in the object to try to maintain the original magnetic field. These eddy currents decay at a rate depending on the conductivity, permeability and dimensions of the object. As they decay, they create a time varying magnetic field linking the inductive coil and this in turn generates a return voltage signal in the coil. It is this return voltage signal which is monitored to obtain information about the object.

Devices using the pulsed coil induction technique for coin discriminations are described in U.K. Patent Specification Nos. 2135095, 2135,492, 2020469, and 2078420. There are, however, some major disadvantages associated with the techniques and devices of these Patent Specifications.

For example, they do not accurately address the problem of transverse movement of an object towards or away from coils while it is being monitored, which movement causes inaccuracies in the return voltage. One example of this is wobble of a coin in a chute. In an effort to overcome this problem, it is known to provide coin chutes having one off-set wall which a coin leans against while passing between coils. This arrangement, however, slows down movement of the coin along the chute and severely restricts the amount of coins which may be checked in a given time.

A further problem with known devices using the pulsed coil induction technique is that the coils used are not of a suitable shape and size to create a magnetic field which is constant over a significant length of an object path.

As return voltage signals must be sampled while there is a constant magnetic field it is accordingly difficult to take enough return voltage samples. A still further problem is that the induced magnetic field provides little information about object thickness as eddy currents tend to flow adjacent the object surfaces next to the coils, for example, the two faces of a coin.

It is thus the object of the.present invention to provide an apparatus and method for determining the characteristics of conducting objects which overcomes these problems.

According to the invention there is provided an apparatus for determining the characteristics of a conducting object comprising: a coil assembly comprising a plurality of inductive coils, each coil having dimensions large enough to form a constant magnetic field over an area of the object path sufficient to encompass the object, there being at least two side coils positioned opposite each other forming an object path therebetween; pulsing means for supplying drive current pulses to each coil of the coil means; monitoring means for monitoring return voltage signals induced in each coil of the coil means; and processing means for determining the characteristics of the object in response to the monitored return voltage signals.

In one preferred embodiment of the invention, all of the coils are substantially similar.

Ideally, the pulsing means comprises means for supplying drive current pulses simultaneously to each side coil.

In a still further embodiment, there is provided a method of determining the characteristics of a conducting object adjacent a plurality of inductive coils including at least two side coils positioned opposite each other forming an object path therebetween, the method comprising the steps of: supplying drive current pulses to each coil; and monitoring the return voltage signals in each coil.

The invention will be more clearly understood from the following description of some preferred embodiments thereof given by'way of example only with reference to the accompanying drawings in which: Fig. 1 is a block diagram of an apparatus according to the invention for determining the characteristics of conducting objects; Fig. 2 is a plan view of a coil assembly of the apparatus of Fig. 1; Fig. 3 is a graph illustrating the magnetic field linking an object and the coil assembly of Fig. 2 as a function of the object position; Figs. 4(a) and (b) are graphs of current and voltage wayforms of the apparatus of Fig. 1; and Fig. 5 is a perspective view from above of an alternative construction of coil assembly according to the invention.

Referring to the drawings, and initially to Figs. 1 to 4 there is illustrated an apparatus for determining the characteristics of conducting objects, indicated generally of the reference numeral 1. The apparatus 1 is of the type for inducing eddy currents in an object and monitoring the corresponding return voltage signals in an inductive coil.

The apparatus 1 comprises a coil assembly indicated generally by the reference numeral 2 and an associated pulsing means, in this case a current source 3. Monitoring means are in this embodiment provided by an amplifier 5 connected at its input to the coil assembly 2 and that its output to an analog to digital circuit 6 having timing circuits 4 which also control the current source 3. The analog to digital converter circuit 6 is connected to processing means comprising storage and processor circuits 7.

Referring specifically to Figs. 2 and 3, the coin assembly 2 comprises a pair of rectangular side coils 11 and 12 positioned opposite each other forming an object path 13 therebetween. In this case, a coin 14 is illustrated in the object path 13, midway between the two side coils.

The two side coils 11 and 12 are connected in series with the current source 3 in this embodiment. The length of each of the side coils 11 and 12 is indicated by the letter "L" and the width is referred to hereinafter by the letter "B". The distance between an object such as the coin 14 and the side coil 11 is referred to by the lower case letter "d". In this embodiment, the side coils have been positioned a distance 0.9B from each other and the length of each coil is double the width. The length L is 6cms and the width B is 3cms and accordingly the coils are sufficiently large to form a constant magnetic field over an area of the magnetic path sufficient to encompass the object.

In use, an object such as the coin 14 moves along the object path 13 in the direction indicated by the arrow and the distance along the path is denoted "x". While this is happening, the current source 3 supplies pulses similar to the pulse illustrated in Fig. 4(a) to both of the side coils 11 and 12. Essentially, the magnetic field linking the coin 14 increases and decreases instantaneously causing eddy currents to flow in the coin. These eddy currents decay at a rate depending on the conductivity, permeability and dimensions of the object to create a time varying magnetic field which in turn generates a return voltage signal in the coils similar to that illustrated in Fig. 4(b).

In this case the return voltage signal is sampled twice, at times tl and t2 and the corresponding voltage values are V1 and V2 respectively.

Fig. 3 illustrates the relationship between the magnetic field linking the coin 14 and the side coils 11 and 12, and the distance x along the object path 13. Because the current source 3 is connected in series to the coils 11 and 12, the magnetic fields add. Accordingly, it will be seen that there is a much smaller variation in magnetic field strength due to variation of the distance d which may, for example, be caused by wobble of the coin in a chute. It will be seen that there is vary little variation in the magnetic field when d varies from 0.3B to O.4SB. For example, we have found that if the width, B, of each coil is 3 cms, a 3 mm variation in d will cause a 2% change in return voltage magnitude. If a single coil acting as a transmitter and receiver were used, a 44% variation in return voltage would be monitored due to this variation in d.This is a particularly important advantage of the invention as it solves the problem of coins wobbling in a chute, or of wire or metal foil moving relative to the coils. This advantage of the invention is achieved because both of the coils are supplied with drive pulses and because the return voltage signals are monitored in each coil and accordingly the total return voltage signal is the sum of that on each coil.

Another advantage of the arrangement illustrated is that because the coils are of rectangular construction and are similar, an even magnetic field strength is achieved over a significant length of the object path. In this embodiment, the magnetic field is even for 0.6L. If circular coils where used, it would be necessary to have coils with a diameter much larger than the length of the rectangular coils. This is a significant advantage where space is at a premium, for a example, in telephone coin recognition devices. We have found that if the coil length L is 6 cms there will generally be a uniform magnetic field for at least 50 ms, allowing 50 return voltage signals to be monitored if the drive pulses are repeated every 1 ms, as the return voltage signals generally decay in 0.5 ms.If it is desired to take slightly fewer measurements it will be possible to turn the coils 11 and 12 through 90 degrees and allow coins drop between them thus greatly increasing the handling capacity of the apparatus according to the invention.

Needless to say, the monitored return voltage signals may, alternatively, be used for comparison with those of a standard object or objects which are in a similar magnetic field. The apparatus of the invention may include audible or visible indicator outputs and further, a keypad may be provided for inputting program instructions.

The apparatus of the invention may be used in various different manners depending on the conducting object being monitored. For example, if it is desired to measure the thickness of a rectangular sheet of aluminum foil of known length and width, the conductivity and permeability of aluminum together with the length and width of the sheet are entered and stored in the storage and processing circuits 7. The return voltage signal from the sheet is then sampled as outlined above. We have found that it is advantageous to subtract the last sample of a single return voltage signal from each of the previous ones for that signal to yield a set of different samples. Accuracy is also improved if each sample is normalised.

Referring now to Fig. 5 there is illustrated an alternative construction of coil assembly indicated generally by the reference numeral 40. The coil assembly 40 comprises a pair of side coils 41 and 42, again connected in series to a current source, not shown. The coil assembly 40 further comprises a transverse coil 43 which is perpendicular to the side coils 41 and 42 and extends between them. The transverse coil 43 is connected to a separate current source, not shown. All three coils of the coil assembly 40 are of similar rectangular construction.

In this embodiment, the side coils 41, 42 and the transverse coil 43 are driven independently and in antiphase to prevent the magnetic fields from interfering.

The return voltage signals in the side coils and in the transverse coil are processed separately. The transverse coil 43 induces a magnetic field perpendicular to edge surfaces of conductive objects in a path, which in this case is above the transverse coil and between the two side coils. A coin 44 is illustrated in the object path and the direction of the magnetic field from the transverse coil 43 is indicated by the arrow A. The eddy currents which are induced in the coin 44 by the magnetic field of the transverse coil 43 have a much smaller area to flow over and accordingly they decay much more rapidly than those induced by the side coils. This return voltage is extremely sensitive to changes in coin thickness, especially for relatively thick coins. In this way, the dimensions of the edge surface may be much more accurately monitored and classified than heretofore.

It is envisaged that the transverse coil may be positioned at a different position along the object path. Indeed, this arrangement overcomes the problem of interfering magnetic fields and the side coils and transverse coils may then be connected in series and driven by the same current source simultaneously. This is a simpler arrangement.

The methods of calculating object characteristics using return voltage samples are programmed into the storage and processing circuits 7 and may be of any suitable type.

The invention is not limited to the embodiments hereinbefore described but may be varied in construction and detail.

Claims (14)

1. An apparatus for determining the characteristics of a conducting object comprising: a coil assembly comprising a plurality of inductive coils, each coil having dimensions large enough to form a constant magnetic field over an area of the object path sufficient to encompass the object, there being at least two side coils positioned opposite each other forming an object path therebetween; pulsing means for supplying drive current pulses to each coil of the coil means; monitoring means for monitoring return voltage signals induced in each coil of the coil means; and processing means for determining characteristics of an object in the object path in response to the monitored return voltage signals.

2. An apparatus as claimed in claim 1 in which all of the coils are substantially similar.

3. An apparatus as claimed in claims 1 or 2, in which the pulsing means comprises means for supplying drive current pulses simultaneously to each side coil.

4. An apparatus as claimed in claim 3, in which each side coil is connected in series to the pulsing means.

5. An apparatus as claimed in any preceding claim, in which the coil assembly comprises at least one pair of side coils and at least one transverse coil positioned to create a magnetic field perpendicular to the magnetic fields of the side coils.

6. An apparatus as claimed in claim 5, in which there is one pair of side coils and one transverse coil extending between and positioned perpendicular to the side coils.

7. An apparatus as claimed in claims 5 or 6, in which the pulsing means comprises separate current sources for the side and transverse coils.

8. An apparatus as claimed in claim 7 in which the separate current sources are arranged to supply drive current pulses in anti-phase to prevent interference of magnetic fields from the side and transverse coils.

9. An apparatus as claimed in any preceding claim, in which each coil of the coil assembly is of rectangular end cross-sectional shape.

10. An apparatus as claimed in claim 9, in which the spacing between the side coils in approximately 90% of the width of the coils.

11. An apparatus as claimed in claims 9 or 10, in which the width to length ratio of each rectangular coil is approximately 0.5.

12. A method of determining the characteristics of a conducting object adjacent a plurality of inductive coils including at least two side coils positioned opposite each other forming an object path therebetween the method comprising the steps of: supplying drive current pulses to each coil; monitoring the return voltage signals in each coil.

13. An apparatus for determining the characteristics of a con ducting object substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 1 and 5 of the accom panying drawings.

14. A method according to claim 12 substantially as hereinbefore described.