Apparatus for positioning receptacles of ferromagnetic material and having a welding seam

When receptacles (1) having a substantially cylindrical side wall and a welding seam extending in the direction of the axis are to be angulary positioned it is known to rotate the receptacle about the axis and detect the passage of the welding seam past a probe and thereafter stop the receptacle a predetermined number of degrees after the detection. Provided that the receptacle wall is of a ferromagnetic material and that there is a substantial material overlapping at the welding seam the detection can take place by means of a magnetic probe for measuring the material thickness. However this method can not be used if there is only a slide material overlapping at the welding seam. In this case the use of a magnetic probe (11) urged against the receptacle wall (1) by spring means is suggested. The effect is based on the fact that the tip (17) of the probe on passage of the welding seam loses contact with the receptacle wall whereby a comparatively great pulse is produced in the output signal of the probe, said pulse being used as an energizising signal for stopping means (6) for the receptacle (1).

The invention relates to an apparatus for positioning receptacles having a 
substantially cylindrical side wall of ferromagnetic material and a 
longitudinal welding seam, comprising means for rotating the receptacle 
about the cylinder axis, detecting means for detecting the welding seam 
and including a magnetic probe having a tip of ferromagnetic material 
projecting from a housing and being adapted to produce a magnetic field 
through the receptacle wall and having means for determining the strength 
of the field, and means for stopping the rotational movement a 
predetermined number of degrees, including 0 degrees, after the detection. 
It is known to detect the thickness of ferromagnetic material by means of a 
magnetic field which is sent through the material from a probe. The probe 
comprises an electric winding inserted in an electric circuit arrangement 
in which the winding may for instance constitute a frequency-determining 
part of an oscillatory circuit. The circuit arrangement may for instance 
be adapted in such a way that the oscillatory circuit is at resonance when 
in the magnetic field of the probe a single layer of ferromagnetic 
material of a predescribed thickness is present. If the material is 
removed from the magnetic circuit of the probe, or if two or more material 
layers are inserted, the oscillation amplitude will be reduced, or the 
oscillations will cease completely. 
For positioning of welded steel tubes it is furthermore known, as shown in 
U.S. Pat. No. 3,972,432, to use a probe having two adjacent windings, each 
one provided with a ferromagnetic core. The probe is maintained at a 
certain distance from the tube wall. The windings are included in an 
alternating current bridge, the equilibrium of which is disturbed when the 
probe passes the welding seam because of the variation of the material 
thickness. 
Such known devices can normally also be used to detect the welding seam in 
the above-mentioned receptacles, because at the place of the seam there is 
considerable overlapping of the welded edges. 
However, during recent years a new welding method, the so called 
"Opprecht-method", disclosed in U.S. Pat. No. 4,160,892, has appeared by 
which a welding seam is produced having a comparatively slight material 
overlapping and a thickness which is considerably smaller than twice the 
material thickness of the side wall. Such a welding seam cannot be 
detected with certainty by means of the devices described above which 
require a substantial overlapping and a considerable variation of the 
material thickness, for instance to twice the value. 
It is the object of the present invention to provide an apparatus which can 
detect with certainty welding seams produced by the "Opprecht-method". 
According to the invention this is obtained by means of spring members for 
pressing the tip of the probe elastically against the cylindrical 
receptacle wall and by an adaption of the detecting means in such a way 
that by a sudden variation of the magnetic field through the receptacle 
wall an activating signal for the stopping means is produced. The 
apparatus according to the invention is based on another principle than 
said known devices. The invention is based on the recognition that when 
the tip of the probe, which is urged against the receptacle wall by spring 
power, passes the welding seam an air gap will for a short moment be 
produced between the tip and the receptacle wall, since the tip cannot, 
because of the inertia of the probe, follow the wall when the probe has 
been pushed back by the rise formed by the welding seam. Therefore, a 
great electrical pulse will be produced in the winding of the probe. In 
this case it is not the material thickness which is measured. 
In addition to the advantage indicated above, namely the reliable detecting 
of the welding seam, the further advantage is obtained by the invention 
that the electrical circuit arrangement can be extremely simple, because 
it only has to give the pulse produced in the winding of the probe such a 
magnitude and shape that it can be used as an energizing signal for the 
stopping means. 
The detecting means can be especially simple if the probe contains a 
permanent magnet, since according to the invention in that case the 
detecting means need only contain the electrical amplifier and an 
amplitude-filter. There is no need for a current supply to the probe. The 
permanent magnet produces a permanent magnetic field through part of the 
receptacle wall and by any variation in this magnetic field an electrical 
output signal will come from the probe. During rotation of the receptacle 
a variation of the magnetic transitional resistance between the tip of the 
probe and the receptacle wall will constantly occur and a certain output 
signal from the probe will therefore always be present. In the moment when 
the tip of the probe has just passed the welding seam, and therefore 
temporarily out of contact with the receptacle wall, a significantly 
greater variation of the magnetic resistance occurs and this manifests 
itself in a probe output signal which has a significantly greater 
amplitude than the other output signals. By means of a simple amplitude 
filter the high signal occuring on passage of the welding seam can be 
separated from the remaining signals and after amplification it can be 
used as a control signal for the stopping means.

The apparatus shown in FIG. 1 for positioning receptacles 1, such as 
tinboxes and pails, of ferromagnetic material comprises an apparatus base 
2 in which a turntable 3 is journaled for free rotation about a vertical 
axis. 
Above the turntable and with the same axis of rotation as turntable 3 is 
mounted a driving disc 4 arranged on a shaft 5 which, via a gear and brake 
mechanism 6, can be driven from an electric motor 7 connected to the gear 
and brake mechanism via a shaft 8. The shaft 5 and the driving disc 4 can 
furthermore be raised and lowered. 
The driving disc 4 has an edge 9 for gripping the bottom of the receptacle 
and thereby rotating same. 
In a part 10 of the apparatus frame which is in firm connection with the 
basis 2 a magnetic probe 11 is journaled. This probe is firmly arranged in 
a carrier 12 which is rigidly connected to a member 13, as shown in FIG. 
2, which can rotate on a shaft 14 and which is biased by a pressure spring 
15. As a result of this construction the tip 17 of the probe is constantly 
urged against the substantially cylindrical sidewall of the receptacle 1. 
The probe contains a permanent magnet producing a magnetic field through 
the ferromagnetic tip 17 and part of the receptacle wall, cf. FIG. 2. 
When the apparatus is in operation receptacles 1 are supplied to the 
turntable 3 by means of some sort of conveyor mechanism, e.g. conveyer 
band. When one of the receptacles is placed on the turntable the shaft 5 
and the driving disc 4 are lowered towards the receptacle whereby the 
driving edge 9 engages the up-turned bottom edge of the receptacle. 
Thereafter the shaft 5 is coupled to the driving motor 7 via the gear and 
brake mechanism 6 so that the driving disc 4 rotates the receptacle 1 and 
the turntable 3. During this rotation a magnetic field is constantly 
propagating from the tip 17 of the probe through the receptacle wall 1 and 
back to the housing of the probe, as shown in FIG. 2. The probe comprises 
a permanent magnet for producing the said magnetic field together with a 
coil encircling the magnetic flux whereby an electric voltage is induced 
in the coil by each variation of the magnetic field. The magnetic field 
varies when the magnetic resistance between the tip of the probe and the 
wall of the receptacle varies which will constantly take place during the 
rotation of receptacle, i.a. because of irregularities in the surface of 
the receptacle. When the tip 17 of the probe passes the welding seam 18, 
representing a certain rise, the whole probe 11 will be urged back against 
the influence of the spring 15. 
If the rotation of the receptacle takes place at a certain speed the tip 17 
of the probe cannot, because of the inertia of the probe, exactly follow 
the surface of the receptacle immediately after the welding seam and an 
air gap between the tip 17 of the probe and the wall of the receptacle 
will therefore be formed for a short time. This causes a variation of the 
magnetic field which is substantially greater than the variations produced 
because of random irregularities in the wall of the receptacle and an 
electric pulse is therefore produced which is substantially greater than 
the remaining electric signals from the probe. 
This is illustrated in FIG. 3a where the curve 19 shows the amplitude A of 
the electrical signal as function of time t. The electrical signal is 
supplied from the probe 11 via a conductor 20 to an electric circuit 
arrangement 21 comprising an amplitude filter and an amplifier. If the 
amplitude filter for example has a threshold value represented by the 
dotted line 22 in FIG. 3a only the pulse 23 produced on passage of the 
welding seam will pass the amplitude filter. After passage of the 
amplitude filter the signal will therefore contain only a pulse 23', as 
shown in FIG. 3b, representing the passage of the welding seam. The 
amplification of the signal can take place either before or after the 
amplitude filter or both before and after. 
By means of the pulse 23' or and electrical signal derived therefrom a 
decoupling of the shaft 5 from the driving motor 7 and a braking of the 
shaft 5 and thereby the driving disc 4 and the receptacle 1 is produced in 
the gear and brake mechanism 6. These operations can be initiated at the 
moment when the pulse 23' occurs. However, a certain time delay may be 
introduced so that the stopping does not take place until after a 
predetermined, possibly adjustable, time after detection. Under the 
assumption that the receptacle rotates at a constant speed, it is thereby 
possible to cause the receptacle to stop upon having rotated a 
predetermined number of degrees after the probe has past the welding seam. 
The invention is not limited to the embodiment shown on the drawing and 
described above but can be modified in different ways within the scope of 
the following claims.