Pan Detector For Induction Heating Cooking Unit

Abstract

A pan detection and control arrangement is provided for an induction heating cooking unit by providing a reed switch and a pair of permanent magnets disposed relative to the reed switch to hold the reed switch in a closed position in the absence of a sufficient size cooking vessel in position overlying the work coil, the reed switch operating to an open position to permit energization of the work coil when the ferromagnetic mass provided by the cooking vessel is in place to shunt a sufficient part of the magnetic field through the vessel and away from the reed switch. Opening of the reed switch permits energization of the work coil.

Description

CROSS REFERENCES TO RELATED APPLICATIONS

U.S. patent application Ser. No. 243,715 and the companion U.S. patent applications referred to therein disclose circuit arrangements with which the arrangement according to the present invention may be used.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The invention pertains to the art of pan detection arrangements for use in induction heating cooking apparatus.

2. Description of the Prior Art:

It has been suggested heretofore, as in U.S. Pat. No. 2,497,753, that properly locating a mass to be heated by induction heating apparatus can be used to control a switch controlling the energization of the induction heating apparatus. In the noted patent, the presence of a pressing iron, or a cooking vessel, in a proper location results in the attraction and movement of a permanent magnet toward the mass to thereby move means for closing a switch to permit heating of the iron or vessel. In the arrangement of that patent, the permanent magnet and switch is disclosed as being located to the side of the pressing iron to be heated by the induction heating apparatus. Such an arrangement would be inconsistent with the provision of a smooth top cooking platform such as is currently preferred for induction heating cooking apparatus. The general concept of that arrangement could be employed in connection with current induction heating cooking apparatus by locating a similar arrangement in the core of the work coil of the apparatus. However such an arrangement would have several disadvantages with respect to the arrangement according to the invention. Among these disadvantages are that the magnet in moving would likely make an audible sound at the end of its travel, the magnet so located would be more sensitive to concentrated magnetic masses than to a distributed one, the space provided by the core of the work coil is located in a position making it highly useful for other devices such as a thermal sensor, and the mechanism would be exposed to accidental mishandling by service personnel.

An arrangement according to the present invention is considered to obviate all of these objections.

SUMMARY OF THE INVENTION

In accordance with an arrangement according to the invention, there is provided safety control means responsive to levels of magnetic flux above and below the given range of values for preventing and permitting, respectively, energization of a work coil, permanent magnet means is disposed relative to the safety control means and to the location of the base wall of a cooking vessel properly located over the work coil to direct a magnetic field with a flux level above the given values through the safety control means in the absence of a ferromagnetic mass at least comparable to the smallest cooking vessel to be used on said coil and in the proper location, with the magnetic field being shunted sufficiently in the presence of the mass in the proper location to reduce the flux level of the field through the safety control means to a value below the given values so that unless an adequate load for coupling is present above the coil, the coil is prevented from being energized.

In the currently preferred form of the invention, the safety control means takes the form of a reed switch located generally in a fringe magnetic field between a pair of permanent magnets which have their poles arranged and are spaced apart such that in the absence of a mass corresponding to a properly located cooking vessel the flux level passing through the reed switch causes it to be closed and in the presence of the mass the field is shunted sufficiently that the reed switch opens.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation in the nature of a side elevation showing flux paths in the absence of a magnetic material vessel in position on a work coil;

FIG. 2 is a schematic representation, also in the nature of a side elevation, showing the shunting effect of the magnetic material vessel on the flux paths;

FIG. 3 is a view in the nature of a vertical cross section showing the relationship between the cooking vessel on the cooking surface, the work coil, and the sensing assembly;

FIG. 4 is a plan view of a single cooking unit showing the location of the permanent magnets and the reed switch relative to the cooking location; and

FIG. 5 is a block diagram and partly schematic view of the way in which the arrangement according to the invention may be connected to a circuit for controlling the cooking unit.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the safety control means is shown in its currently preferred form as a sealed, magnetic reed switch 10 having leaf contacts 12 and 14 which extend out through the ends of the glass envelope to provide terminals. The contacts of the reed switch are normally open but are shown as closed in FIG. 1 becuase of the field from the permanent magnets causes sufficient flux to pass through the leaves of the reed switch to force their contact ends to touch each other. The magnetic lines of force, some of which are collectively identified by the numeral 16, are produced by the pair of permanent magnets 18 and 20 which have their poles arranged as shown to provide the magnetic field schematically illustrated. Thus the orientation of the poles of the left magnet 18 is with north at the top and south at the bottom, while the orientation of the right magnet is reversed.

FIG. 2 shows the influence upon the location of the lines of force 16 exerted by placing an extended area mass 22 of magnetic material in proximity to the reed switch 10. As there shown, the extended area mass of magnetice material which corresponds to the bottom wall of a cooking vessel shunts an adequate portion of the field so that the induced magnetism in the leaves 12 and 14 is insufficient to maintain the switch 10 is a closed position.

Incorporation of the arrangement according to the invention in an induction heating cooking unit is best understood in connection with FIGS. 3 and 4. A refractory material such as a glass ceramic sheet 24 provides a cooking surface above the work coil 26. A suitable work coil may comprise litz wire which is wound in a spiral and then molded in a rubber compound to hold the wire in place with the proper spacing between successive convolutions. The work coil also typically includes an underlying layer 28 of the rubber compound. The work coil is supported from below by an underlying Transite sheet or block 30 which supports the work coil. The Transite block 30 is supported by means not shown herein. A slot 32 is grooved out of a generally central part of the block 30 to receive the reed switch 10 which is held in place by potting it in with a rubber compound such as Dow Corning Sylgard 185. The groove is made sufficiently long to accommodate the projecting terminal ends of the leads, which are connected to two lead wires 34 and 36 (Fig. 3).

In the currently preferred form of the invention, the reed switch should be sufficiently long that the requisite sensitivity is provided with respect only to magnetic objects having a sufficiently extended area as to be comparable to the smallest diameter cooking vessel intended for use for the coil. Reed switches which satisfy this requirement are about two to three inches long (including their terminal ends) such as the Hamlin Company DRS-2 and DRT-5. Examples of permanent magnets 18 and 20 which I have found to function satisfactorily are Allegheny General Indox V magnets having top and bottom face areas of about 2 inches by 1 inch, and about 3/8 inch thick. Such magnets are adequate for operating reed switches with closing requirements of 70 ampere-turns and opening values of 50 or more ampere turns and are not heated appreciably by the coil of the range. The spacing between the facing ends of such magnets used with the identified reed switch is about 4 inches. However, since variations may occur between one reed switch and another, and between one pair of magnets and another current practice is to locate the magnets relative to the reed switch by first bringing them together sufficiently close that the contacts close, and then bringing a magnetic mass comparable to a cooking vessel bottom into the proper location, then moving the magnets apart until the contacts open. The magnets are then fixed in place and the operation checked. It is noted in this respect that a differential may typically be found to exist between the levels of magnetic flux effecting closing and opening of the reed switch. Thus it is responsive to levels of magnetic flux above and below a given pair of values, rather than a specific given value.

As may be seen from FIG. 4, it is not necessary that the reed switch and magnet be centered exactly relative to the center of the work coil. It is only necessary that it be responsive to the bottom wall of the cooking vessel which is placed generally coincident with the work coil, the location of which is generally indicated by the locating indicia 38 provided on the top surface of the cooking surface.

The manner in which the arrangement according to the invention is incorporated in general circuit arrangements of the noted patent applications for operating an induction heating cooking unit will be described in connection with FIG. 5. The control circuit portion 40 of the arrangement disclosed includes a timing and firing capacitor 42 for the phase controlled rectifier bridge 44. When the pan detector switch 10 is open due to the presence of a pan bottom 22, the capacitor charges up from various sources as disclosed in the noted applications, and is discharged by a semiconductor switch through a pulse transformer which couples into and turns on the phase controlled rectifier bridge.

When the pan detector switch 10 is closed because of the absence of a pan, the capacitor 42 is shorted out through resistor 46 and accordingly prevented from charging up. Hence the phase controlled rectifier bridge 44 cannot be turned on, so no power can reach the power oscillator 48 and work coil 26.

It will be appreciated that the pan detector switch could be incorporated in the circuit in other ways, such as by controlling a relay which in turn controls power in one part or another of the circuit.

The arrangement according to the invention provides the following safety features. It prevents heating of aluminum foil and possible fire hazards of foil covered packages. It prevents inadvertent heating of metallic cooking utensils such as spoons and forks with consequent burning of the user's hand. It also protects electronic equipment and pacemaker wearers from an energized coil without the shielding provided by a cooking vessel.

It is to be noted that in an arrangement according to the invention the switch is located in what may be considered to be a fringe portion of the magnetic field as distinguished from what could be characterized as a main field if the magnets 18 and 20 were of the type in which the poles were at the ends of the bars, and the north pole of 18 were facing a south pole of 20. By locating the reed switch in the fringe area of the field, placing a relatively extended magnetic mass comprising the bottom wall of the cooking vessel is able to shunt sufficient lines of force thereto to permit the reed switch to be moved to an open position.

While the currently preferred arrangement includes the use of the reed switch, it is to be understood that other devices responsive to a magnetic field shunted into and out of position could be used alternatively, one such example being a Hall generator which could be connected to provide an on-off switching arrangement.

Claims

1. In an induction heating cooking unit of the type including a work coil which is adapted to be coupled electromagnetically to a cooking vessel placed in overlying relation to said coil;

safety control means responsive to levels of magnetic flux above and below a given range of values of said flux for preventing and permitting, respectively, energization of said coil;

permanent magnet means disposed relative to said safety control means, and to the location of the base wall of a cooking vessel when properly located over said coil, to direct a magnetic field with a flux level above said given values through said safety control means in the absence of a ferromagnetic mass at least comparable to the smallest cooking vessel to be used on said coil and in said proper location;

said magnetic field being shunted sufficiently in the presence of said mass in said proper location to reduce the flux level of the field through said safety control means to a value below said given values, so that unless an adequate load for coupling is present above said coil, said coil is

2. In a unit according to claim 1 wherein:

said safety controls means comprises switching means closely adjacent said coil; and

said permanent magnetic means comprise a pair of permanent magnets on opposite sides of said switching means, said magnets being disposed relative to said switching means, and having their poles arranged, and being spaced apart such that in said absence of said mass the flux level passing through said switching means causes said switching means to assume one condition, and in the presence of said mass the field is shunted sufficiently from said switching means that the flux level through said switching means is reduced sufficiently that said switching means assumes

3. In a unit according to claim 1 wherein:

4. In a unit according to claim 2 wherein:

said permanent magnets have their poles oriented generally vertically, with the north pole of one magnet being upwardly facing and the north pole of the other magnet being downwardly facing, and with said magnets being spaced apart sufficiently that said reed switch is located in a fringe

5. In an induction heating cooking unit of the type including a work coil underlying a refractory material sheet forming a cooking surface upon which a cooking vessel is adapted to be supported and electromagnetically coupled to the coil when the coil is energized, an arrangement for controlling the energization of said work coil in accordance with the presence of a ferromagnetic mass at least comparable to the base wall of the smallest cooking vessel to be used with said coil when in overlying relation to said coil comprising:

safety switch means responsive to the presence of levels of magnetic flux above and below a given range of values to assume one and another condition, respectively;

permanent magnet means disposed relative to said safety switch means and to the location of the base wall of the cooking vessel properly located over said coil, to place said safety switch means in a magnetic field providing a level of magnetic flux above said given range of values in the absence of said base wall of said cooking vessel, said permanent magnet means being so disposed that sufficient lines of force are shunted sufficiently in the presence of said base wall of a cooking vessel to reduce the levels of magnetic flux in which said safety switch means is present to a level below said given range of values; and

electrical circuit means for controlling energization of said coil including said safety switch means in said one condition preventing energization of said coil and in said another condition permitting energization of said coil, so that unless an adequate load for coupling is present in the proper location overlying said coil, said coil is prevented from being energized.