This document describes a metal detector with an improved search coil frame. The metal detector comprises a non-conductive housing lined with conductive material like aluminum. A tubular non-conductive frame is secured within the housing and spaced from the housing walls. Balanced transmitting and receiving coils around the frame interior produce an electromagnetic field to detect metal passing through. The improved frame is simpler and cheaper to make than prior frames, and is more durable against stresses from heat.

1. Publication number WO1995011462 A1
Publication type Application
Application number PCT/GB1994/002304
Publication date 27 Apr 1995
Filing date 20 Oct 1994
Priority date 20 Oct 1993
Inventors Richard Grafton
Applicant Richard Grafton, Transfer Technology Ltd
Export Citation BiBTeX, EndNote, RefMan
Patent Citations (3), Referenced by (1), Classifications (3),Legal
Events (7)
External Links: Patentscope, Espacenet
Metal detectors and methods for making the same
WO 1995011462 A1
ABSTRACT
A detector for detecting the presence of metallic objects in, for example, packaged products
comprises a housing (1) of non-conductive material which is lined internally with sheets or foil of a
suitable conductive material such as aluminium. A tubular non-conductive frame assembly (3) is
positioned within the housing and is secured to the front and rear walls of the housing with its sides
spaced from the respective sides of the housing. The frame assembly includes an aperture (2)
through which packaged products to be inspected can pass. Balanced transmitting and receiving coils
are carried about the circumference of the frame assembly and operate to produce within the aperture
an electromagnetic field which is disturbed by a metallic object passing through the aperture in a
packaged product.

2. DESCRIPTION
METAL DETECTORS AND METHODS FOR MAKING THE SAME
This invention relates to metal detectors for detecting the presence of metallic foreign objects in
packaged products such as foodstuffs and pharmaceuticals and is directed more especially to an
improved search coil frame for such a detector and a method for making the same.
Metal detectors are known which comprise a frame assembly carrying transmitting and receiving coils
which are balanced to produce within the detector interior an electromagnetic field which is disturbed
by a metallic body passing through a central aperture of the detector. The presence of metallic foreign
bodies within non-metallic packaging of foodstuffs and pharmaceuticals can thereby be detected and
appropriate action taken to remove detected packages from, for example, a continuous line for more
detailed inspection.
Coil frame assemblies are known which comprise a rigid coil-carrying frame or a plurality of
laminations between which the transmitting and receiving coils are located. Such frame assemblies
are generally complicated and costly to manufacture; additionally and/or alternatively they are
cumbersome and heavy and are often subject to stresses and damage caused by differential
expansion occasioned by high temperatures generated in use. In the case of laminated frames, it has
been found that the individual laminations tend to separate at elevated ambient temperatures.
It is an object of the present invention to provide an improved metal detector and coil frame assembly
which overcomes or at least alleviates the disadvantages referred to above.
According to the present invention in one aspect, there is provided a metal detector comprising a
housing of non-conductive material lined internally with an electrically conductive material, a tubular
non-conductive frame assembly positioned within the housing, means for securing the frame
assembly to the front and rear walls of the housing with the sides of the frame assembly spaced from
the sides of the housing, the frame assembly having a through aperture about the circumference of
which are provided balanced transmitting and receiving coils which operate to produce within the
frame assembly an electro¬ magnetic field which is disturbed by a metallic body passing
therethrough.
The internal lining of the housing may comprise sheets of aluminium or aluminium foil and the frame
may be secured to the housing by mechanical fixings such as aluminium dowels or be bonded with a
structural adhesive.
The frame assembly may be produced from reinforced phenolic sheets. The invention will now be
described by way of example only with reference to the accompanying diagrammatic drawings of
which:-
Figure 1 is a perspective view of a metal detector in accordance with the invention;
Figure 2 is a section taken along II-II of Figure 1;

3. Figure 3 is a detail to an enlarged scale of the detector illustrated in Figures 1 and 2; and
Figure 4 is a side view in section of an alternative metal detector in accordance with the invention.
The metal detector illustrated in Figure 1 of the drawings comprises an outer housing 1 of a non
conductive material, e.g. reinforced phenolic sheet, having end, side, top and bottom walls lined
internally with sheets of aluminium or aluminium foil 5. The aluminium sheets or foil define a screen to
contain electromagnetic fields within the housing interior. The housing 1 is formed with a central
aperture 2 through which packaged products can pass for inspection purposes.
Supported within the interior of the housing 1 is a search coil frame assembly 3 which comprises a
tubular structure whose internal walls define the boundaries of the aperture 2. The frame assembly 3
is constructed from sheets of non conductive material, typically reinforced phenolic sheet, and is
secured to the front and rear walls of the housing 1 by mechanical fixings such as aluminium dowels 4
or bonded with a structural adhesive.
The external surface of the frame assembly is formed with circumferential grooves within which are
wound a transmitting coil 6 connected to a high frequency oscillating circuit and two receiving coils 7
positioned one to each side of the receiving coil in the intended direction of travel of packages through
the aperture 2. The coils 6 and 7 are balanced and are positioned as close to the aperture to minimise
detection sensitivity. Together the coils 6 and 7 produce within the interior of the frame 3
electromagnetic fields which are disturbed by the presence of metal fragments within products
passing through the aperture of the detector.
To achieve accurate and stable location of the coils within the grooves, the grooves are formed with
generally vertical and parallel side walls which are separated by a distance marginally greater than
the diameter of the coil wire. The depth of each groove is selected to achieve the required
characteristics of sensitivity and mechanical strength. Generally a greater groove depth gives
enhanced sensitivity but reduced mechanical strength. After assembly of the coil wire each groove is
filled with an adhesive.
As mentioned above, the frame assembly 3 is secured to the front and rear walls of the housing by
mechanical fixings in the form of aluminium dowels 4 which extend into adjoining surfaces of the front
and rear walls of the housing and the ends of the coil frame assembly 3. The construction may be
retained securely in place by toughened adhesive. The spacing between the external walls of the coil
frame assembly 3 and the internal walls of the housing 2 is selected carefully having regard to the
overall size of metal detector required and the fields to be induced within the detector interior. More
particularly, the spacing is optimised to ensure that the following criteria are satisfied:-
(1) That sufficient magnetic field strength remains in the aperture 2 to enable reliable contamination
detection;
(2) |That any minor deflections in the electro magnetic screen caused by externally induced
mechanical vibrations do not cause the detection system to fail; and

4. (3) That the external dimensions of the system are sufficiently compact to enable installation in space
restricted operating environments.
The optimised dimensions of the detector are calculated from an empirically derived formula as
exemplified below in which "H" and "V" are respectively the height and width of the aperture of the
detector, "R" is the distance between each side of the frame assembly 3 and the respective side of
the outer housing and "X" is the height of the outer housing:- if H <153mm, R = 90mm if H >153mm,
R = 0.1 (H) + 75mm if H <153mm, X = 330mm if H >153mm, X = 1.43 (H) + 112mm if H <153mm, V =
220mm if H >153mm, V = 0.83 (H) + 103mm Advantages of the detector described include an ability
more readily to accommodate mechanical and thermal stresses induced during use, compactness,
simplicity of manufacture, and efficient operation.
In the alternative embodiment illustrated in Figure 4, the dowels 4 of Figures 1 to 3 are replaced by
tongues and grooves 8 to ensure minimum magnetic leakage from the joint faces. The space 9
defined between the frame assembly 10 and the housing 11 is filled with a low density foam to
eliminate bacterial growth during condensation cycles and aluminium rods 12 are provided to reduce
magnetic leakage.
It will be appreciated that the foregoing is merely exemplary of metal detectors in accordance with the
invention and that modifications can readily be made thereto without departing from the true scope of
the invention.
CLAIMS(1)
1. 1. A metal detector comprising a housing of non- conductive material lined
internally with an electrically conductive material, a tubular non- conductive frame
assembly positioned within the housing, means for securing the frame assembly to
the front and rear walls of the housing with the sides of the frame assembly spaced
from the sides of the housing, the frame assembly having a through aperture about
the circumference of which are provided balanced transmitting and receiving coils
which operate to produce within the frame assembly an electro-magnetic field
which is disturbed by a metallic body passing therethrough.
2. A detector as claimed in claim 1 wherein the frame assembly is mounted
generally coaxially within the housing.
3. A detector as claimed in Claim 1 or Claim 2 wherein the internal lining of the
housing comprises sheets of aluminium.
4. A detector as claimed in Claim 1 or Claim 2 wherein the internal lining of the
housing comprises aluminium foil .
5. A detector as claimed in any one of claims 1 to 4 wherein the frame is secured
to the housing by aluminium dowels.

5. 6. A detector as claimed in any one of claims 1 to 4 wherein the frame is secured
to the housing by tongue and groove joints.
7. A detector as claimed in any one of Claims 1 to 6 wherein the frame assembly is
produced from reinforced phenolic sheets.