Magnetoresistor bridge probe for locating the position of a magnetic tube support sheet of a nonmagnetic heat exchange conduit

Apparatus for locating the position of at least one magnetic member relative to a nonmagnetic member includes a probe assembly movably disposed within the nonmagnetic member and including a nonmagnetic head having a pair of spaced permanent magnets mounted thereon. Each magnet has a variable resistor connected thereto, the resistance of each resistor varying directly with the strength of the flux field emanating from each magnet. The resistors are connected to a source of electrical energy for providing an electrical control signal, the magnitude of which varies in response to the movement of the probe assembly including the magnets and resistors relative to the magnetic member.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for locating the position of a magnetic 
member relative to a nonmagnetic member, and in particular to such 
apparatus which is both portable and may be suitably employed in 
relatively confined spaces. 
There are many applications in which it is extremely important to 
accurately fix the position of a magnetic member relative to a nonmagnetic 
member. For example, heat exchange apparatus typically includes 
nonmagnetic heat exchanger tubes extending between and supported by 
magnetic tube support sheets. The tubes are very often mechanically 
expanded into contact with the holes of the support sheet to firmly 
connect the outer surface of the tube to the inner surface of the hole of 
the support sheet. Mechanical expanders are employed to expand the tubes 
in the desired manner. In order to prevent damage to the tubes and to 
insure that the desired mechanical bond between the tubes and the 
surrounding support sheet is properly formed, it is extremely desirable 
that exact location of the magnetic support sheet relative to the 
nonmagnetic tubes is accurately fixed. 
As may be readily recognized, the exact position of the tube support sheets 
relative to the tube is difficult to obtain as the tubes and support sheet 
are located within the housing of the heat exchange apparatus. Thus, some 
suitable means must be employed to accurately locate the position of the 
support sheet relative to the tubes. 
Various devices have heretofore been employed in an attempt to locate an 
otherwise inaccessible or hidden magnetic member relative to a nonmagnetic 
member. Such prior art devices are disclosed in U.S. Pat. Nos. 2,768,684; 
3,017,621; and 3,649,450. While each of these patents disclose various 
means for achieving the desired end, none of the patents utilize the field 
generated by a permanent magnet for locating the position of the magnetic 
member relative to a nonmagnetic member. In addition, each of the prior 
art devices are relatively large making them unsuitable for use in 
relatively small spaces, as for example small diameter tubes. 
SUMMARY OF THE INVENTION 
It is accordingly an object of this invention to provide an apparatus which 
may accurately fix the position of a magnetic member relative to a 
nonmagnetic member. 
It is a further object of this invention to utilize the field generated by 
a permanent magnet to accurately locate the position of a magnetic member 
relative to a nonmagnetic member. 
It is still yet another object of this invention to monitor the field 
generated by a permanent magnet by magneto resistors. 
It is yet another object of this invention to provide a probe assembly for 
locating a magnetic member relative to a nonmagnetic member which is 
compact and thus may be satisfactorily employed in combination with 
relatively small diameter tubes of the heat exchange apparatus. 
These and other objects of the present invention are attained in apparatus 
for locating the position of a magnetic member relative to a nonmagnetic 
member including a probe assembly movably disposed with respect to the 
magnetic member and having a nonmagnetic head having a pair of spaced 
permanent magnets mounted thereon. Each magnet has a variable resistor 
connected thereto, the resistance of each resistor varying directly with 
the strength of the flux field emanating from the magnet. The variable 
resistors are connected to a source of electrical energy for providing an 
electric control signal, the magnitude thereof varying in response to the 
movement of the probe assembly including the magnets and resistors 
relative to the magnetic member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawing, there is disclosed a preferred embodiment of 
the present application as employed to locate a magnetic tube support 
sheet relative to a nonmagnetic heat exchange conduit. However, it should 
be understood that the apparatus of the present invention may be otherwise 
suitably employed in other applications where it is desired to accurately 
fix the position of a magnetic member relative to a nonmagnetic member. 
The apparatus 10 includes a probe assembly 11 having a nonmagnetic head 12. 
The head may be formed from plastic or brass or any other suitable 
nonmagnetic material. When employed to fix the location of a tube support 
sheet 22 relative to a heat exchange conduit 24, the probe assembly 11 is 
movably disposed within the heat exchange conduit. 
The probe assembly 11 further includes a pair of spaced permanent magnets 
14,16 each of which has respectively mounted thereon and fixed thereto a 
variable resistor 18,20. The variable resistors are preferably of the type 
known as magneto resistors which are semiconductors having the 
characteristic that the resistance thereof will increase in the presence 
of a magnetic field. In effect, the stronger the magnetic field, the 
larger the resulting resistance of devices 18, 20. In the preferred 
embodiment, the magneto resistors were obtained from the Siemens 
Corporation and have the Model No. Q65017-L200J. The magneto resistors are 
connected by electrical conduit 26 to an electrical circuit to be more 
fully described hereinafter. When the wiring is completed, the exposed 
sensors 18, 20 and conductor cable 26 are covered with a protective metal 
sheet 25. 
In forming the probe assembly, the two small magnets 14,16 are initially 
demagnetized and pressed into a hole formed in probe head 12. Variable 
resistors 18, 20 are then secured to the magnets 14, 16 by suitable means 
and are connected directly, as by means of solder, to conductor cable 26. 
After sheet 25 is secured to the assembly, all internal voids are filled 
with epoxy or other suitable material. The assembly is thereafter 
subjected to a very strong magnetic field to permanently magnetize magnets 
14 and 16. 
Referring now to FIG. 2, there is disclosed a schematic form of a control 
circuit suitable for use with the probe assembly illustrated in FIG. 1 by 
which the position of a magnetic member relative to a nonmagnetic member 
may be accurately ascertained. 
Magneto resistors 18 and 20 respectively form two legs of a standard bridge 
circuit, with the other legs being formed by fixed resistors 28 and 30. 
The circuit is connected to a suitable source of electrical power 38 which 
may be a nine volt transistor battery. The bridge circuit is connected to 
an amplifier 32 which is connected through suitable electrical conductors 
to a meter 34 having a pointer 36 movably positioned thereon. 
In operation, as the probe head moves toward magnetic support sheet 22, the 
magnetic field of magnet 14 will increase thereby increasing the 
resistance of resistor 18. This will cause an unbalance in the bridge 
circuit causing the pointer 36 to deflect in a first direction relative to 
the midpoint of the meter 34. As the probe assembly continues to move 
relative to the tube support sheet 22, the resistor 18 and magnet 14 will 
move past the support sheet whence resistor 20 and magnet 16 are in closer 
proximity to the support sheet when compared to the first resistor 18 and 
magnet 14. This will cause an unbalance of the bridge circuit in the other 
direction since the resistance of resistor 20 will be greater than the 
resistance of resistor 18. The electrical control signal thus generated as 
a result of the unbalance will cause the pointer 36 of meter 34 to deflect 
in the other direction past the midpoint. The pointer on meter 34 may be 
replaced by lights which will be selectively lit in accordance with the 
position of the probe head relative to the magnetic member. 
An operator can monitor the movement of the pointer in response to movement 
of the probe head and thus move the probe assembly so magnets 14 and 16 
are spaced equidistant relative to the support sheet whereby the 
resistance of resistors 18 and 20 will be of equal value. When this has 
occurred, the pointer 36 of meter 34 will be at the midpoint of the meter 
scale. The operator can thus accurately fix the position of the magnetic 
support sheet relative to the nonmagnetic heat exchange conduit. 
As the probe assembly is relatively compact due to the utilization of 
comparatively small permanent magnets and magneto resistors, the present 
invention may be particularly used in applications having relatively small 
confines, as for example the specific application illustrated wherein 
relatively small diameter conduits are supported by tube support sheets 
within the heat exchange apparatus. The probe assembly can be reduced in 
size to move within the narrow confines of the small diameter tubes. 
The operator, once he has determined the exact location of the tube support 
sheet, can then scribe a mark on the probe head or otherwise identify the 
sheet's location and insert an expander into the tube to expand the tube 
into mechanical engagement with the support sheet. As an alternative, the 
probe assembly may be formed integral with an expander such that the probe 
assembly does not have to be removed once the position of the tube support 
sheet is ascertained, but rather may be moved forward and the expanding 
element of the combination unit be brought into contact with the tube 
support sheet to achieve the desired expansion of the tube for mechanical 
bonding of the tube and sheet. 
While a preferred embodiment of the present invention has been described 
and illustrated, the invention should not be limited thereto, but may be 
otherwise embodied within the scope of the following claims.