Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE :CNVENTION
This invention pertains to the art of utensil
detection arrangements for use in an induction heating
apparatus.
BACKGROUND OF TME INVENTION
The art of inductively coupling a work coil
with a ferrous utensil thereby electromagnetically hea-ting
the contents of the utensil has been widely known for
many years. The work coil is located below the
cooking surface and an alternating current through
the coil causes a magnetic field to be generated. The
magnetic flux lines of the field extend through the
cooking surface and cause eddy currents in the utensil
and allow it to heat up.
Many prior art devices have included a sensing
means for determining whether a utensil has been placed
on the work surface above the work coil before the coil
is powered. These sensors prevent high strength electro-
~magnetic fields ~rom emanating from~the heatlng coll,
which fields may interfere with television and radio
reception. ;
Var~ious sensors have been used for this purpose. For
example, U.S. Patent 3,796,850 to Moreland II et al dated
March 12, 1974 utilizes a reed switch coupled to two
magnets. If no utensil is placed over the induction heating
unit, the contacts of the reed switch are ~orced to close
due to the magnetic fluid lines produced by the magnets.
However, if a~utensil is placed over the induction heating
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unit, the magnetic flux lines are not sufficiently skrong
to close the lead contacts of the reed switch ana the
induction unit is allowed to be powered.
Similarly, U.S. Patent No. 3,~93,885 to Kominami et al
dated November 23, 1976 includes a movable magnet, a fixed
magnet and a reed switch situated between the two magnets.
If a ferrous pan is placed upon the induction heating unit,
the movable magnet is attracted towards the pan and the
flux lines near the reed switch are changed allowing power
to be supplied to the heating coil.
U.S. Patent No. 4,013,859 to Peters, Jr dated March
22, 1977, utilizes a very low power oscillator coupled to
a load sensing coil to indicate the presence of a pan over
the heating coil. Furthermore, U.S. Patents No. 3,823,297
to Cunningham dated July g, 1974; 4,016,392 to Kobayashi
et al dated April 5, 1977, and 4,010,342 to Austin dated
March 1, 1977 include current or voltage detectors which
also indicate the presence of a pan above the induction
heating coil.
However, while the above noted U.S. patents include
detectors or sensors which would disable thé inverter
circuit of the induction heating coil if no utensil was
placed upon the cooking surface, none of these patents
is directed to the problem of disabling the inverter
circuit if a utensil is placed off-center of the work
surface with respect to the induction heating coil or if
an improperly sized pan were placed properly above the
heating coil.
U.S. Patent No. 3,843,857 to Cunningham dated October
22, 1974 discloses an induction heating system which
includes an unsymmetrical work coil which varies the
amount of heat conductor to a utensil depending upon the
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location of the utensil relative to the coil. However,
since the user will consciously place a utensil of~-center
with respect to the heating coil to vary the amount of
heat conducted to the utensil, a utensil sensor indicating
that the utensil is off-center and thereby disabling the
inverter circuit is not necessary.
Therefore, none of these references is directed to the
problem of an improperly placed or incorrectly sized pan
above the heating coil. In an induction surface unit, if
the pan is not of the proper type or if the proper type
pan is positioned off-center, the Q of the inverter circuit
might still allow the inverter to operate; however, the
radiated magnetic field is much higher than when the pan
is centered or the pan is matched to the surface unit and
this ~high field can cause radio and television interference.
SUMM~RY OF THE INVENTION
The main objective of the present invention is to
insure that a cooking utensil is properly placed upon an
induction surface unit. To effectuate this end, a sensor
assembly is placed below the cooking or work surface.
The sensor assembly consists of three current sensors
spaced 120 apart and situated beyond the periphery of the
coil. The sensors detect the magnetic flux lines that are
directly affected by the position or size of the pan. If
the output of the detectors does not meet a predetermined
criteria, the inverter is disabled and the user is
advised of the problem.
BRIEF DESCRIPTION OF TE~E DR~WINGS
Further details of the present invention and many
additional advantages of this invention will be apparent
from a detailed consideration of the remainder of the
specification and the accompanying drawings in which:
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FIG. 1 is a vertical cross-sec-tion showing the relationship between
the cooking utensil on the cooking surface, the induction coil and the
sensing assembly;
FIG. 2 is a top plan view of the cooking surface through lines 2-2
showing the position of the sensors below -the cooking surface;
FIG. 3 is a perspective view of a sensing elementi
FIGS. 4, 5 and 6 show the current output from one of the sensors
during different positionings of a utensil;
FIG. 7 shows a t~-pical logic circuit used in conjunction with the
sensing assemblyt and
E'IG. 8 is the truth table used in conjunction with the circuit
shown in FIG. 7.
ETAILED DESCRIPTION OF THE DR~WINGS
Referring to FIG. 1~ a utensil 10 containing a ferrous surface 11 is
shown properly centered upon a cooking surface 13. m is surface is preferably
constructed of a ceramic material such as glass which is waterproof and
preferably electrically non-conductive and non-ferromagnetic in character.
An induction work coil 12 is provided beneath the cooking surface 13, the
coil being mounted such that its imaginary central axis 19, if extended
upwardly through the cooking surface 13, passes through the approximate center
of the cooking area on which utensil 10 is adapted to be located, the relative
locations of the utensll 10 and the coil 16 being shown by dotted lines in
FIG. 12. Three current sensing elements 14, 16, and 18, spaced 120 apart
and situated beyond the periphery of the coil and approximately the same
distance from the center axis of the work coil, as shown in phantom in FIG.
2, detect the magnetic flux lines that are directly affected by the position
or size of the utensil.. The coi.l 12 may include a litz wire which is wound
in a spiral and then molded in a rubber compound to hold the wire into place
with the proper spacing between successive convolutions.
A typical current sensor is shown in FIG. 3 and consists
of a current transformer 20 with only five turns of a wire
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22 placed upon a small torroidal core 24. Since the three
transformers are situated beyond the periphery of the work
coil 12, they do not interfere with the spacing between
the work coil l~ and the surface 13.
FIG. 4 shows the current through one of these cu~rent
transformer sensin~ elements when the work coil is activated
and a utensil is properly centered. When the utensil is
moved one inch closer to the sensing element, the current
decreases as shown in FIG. 5. If the utensil is moved
from the center as shown in FIG. 4 to one inch further
from a sensing element, the current increases as is shown
in FIG. 6. By using three current sensing elements, any
movement of the pan off-center past a predetermined value
will easily be detected. Similarly, when a pan which is
too small to be used on a particular cooking surface is
properly centered upon that surface, the output of the
sensors would be similar to that shown in FIG. 6.
FIG. 7 shows a logic circuit for determining whether
a proper utensil is properly centered above the work coil
12. One slde of each of the sensing elements 14, 16 and
18 (denoted by A, B and C respectively) is conducted to a
common ground and the signal produced by each of the
sensors is transformed by a signal conditioner circuit
26 allowing a suitable signal to be produced as inputs
for the logic circuit. Essentially, the signal conditioner
circuit 26 transforms the varying inputs from the sensors
to a DC voltage by well known reactifying filtering
techniques. The logic circuit shown in FIG. 7 includes
three comparators or operational amplifiers 28, 30 and 32
and an OR gate 34. The conditioned output of sensing
elements A, B and C serve as the positive input to the
comparators. The reference voltage is chosen so that it is
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greater than the signals produced by the sensiny elements
14, 16 and 18 when a properly sized utensil is cen-tered
upon the cooking surface 13 with respect to the work coil
12 but is less than the voltage produced by a sensing
element when the utensil is improperly positioned with
respect to the sensing element. The exact voltage levels
are dependent upon the size oE the sensing elements
employed and the particular work coil utilized.
The reference voltage is compared to each of the
voltages produced by the sensing elements. The output
of each of the comparators 28, 30 and 32 would be lower
of the two inputted voltages. These outputs are conducted
to OR gate 34 which inhibits ~he inverter of the work coil
12 if a properly sized utensil is not properly centered
above the work coil 12. As shown in the truth table of
FIG. 8, the work coil 12,would be disabled in every
instance except when the output of each of the comparators
is the signal produced by their respective sensing
elements. If the inverter is disabled, a visual or audio
alarm 36 is enabled informing the user that either the
utensil is improperly centered or is incorrectly sized.
The logic circuitry shown in FIG. 7 not only
determines whether the utensil is properly centered but
also if a correctly sized utensil is utilized. This is
accomplished since each individual sensing element is
independent of the other sensing elements. If a small
utensil is properly centered upon the cooking surface, all
of the outputs of the sensing elements would be greater
than the reference voltage and the inverter would be
3Q disabled. If a properly sized utensil is improperly
centered, the outputs of one or more of the sensing
elements would be greater than the reference voltage and
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the inverter would also be disabled. Therefore, the
inverter would be disabled in every situation except when
the output of each of the sensing elements is lower than
that of the reference voltage. In this case, the utensil
is properly sized and properly centered.
For example, if a work coil having a radius of 4.15
inches is utilized, it has been noted that a sensing
elements of one inch in radius could be placed 5.65 inches
from the center of the work coil. In this situation,
1~ there is little interaction between the sensing elements
and the work coil. Furthermore, the optimum utensil would
be a pan having a five inch radius and that if utensils
having a radius of less than four inches were utilized,
the inverter would be disabled. ~dditionally, although
three sensing elements are shown and described, it is
important to note that the number of sensing elements
placed beyond the periphery of the work coil could be
increased.
The foregoing description shows only the preferred
embodiments of the present invention. Various modifications
are apparent to those skil]ed in the art without
departing from the scope of the invention. Therefore, the
embodiments shown and described are only illustrative and
not restrictive.
