WO1999040396A1

WO1999040396A1 - Level sensor for cooking apparatus

Info

Publication number: WO1999040396A1
Application number: PCT/AU1999/000073
Authority: WO
Inventors: Roger Kent
Original assignee: Vos Industries Limited
Priority date: 1998-02-05
Filing date: 1999-02-04
Publication date: 1999-08-12
Also published as: AUPP167998A0

Abstract

Disclosed is a level sensor (161) for sensing level of cooking medium in cooking medium storage means (151) of a cooking apparatus (110). The level sensor (161) includes a tubular housing into which cooking medium may flow from a storage means (151) in which liquid level state is to be sensed; a conductive coil (24) mounted relative to the tubular housing (20); a ferromagnetic float (22) which, when moved into proximity with the coil (24), induces a current to flow therein. The level sensor (161) also includes a resonant detection circuit (24, 26, 28) coupled with the coil (24) such that when current is induced to flow in the coil (24) by movement of the float (22), resonant frequency of the detecting circuit (24, 26, 28) alters. Any frequency alteration outside permissible bounds is detected and provided as a control signal to the cooking apparatus control unit (100). Cooking apparatus (110) including such level sensors and methods of determining level using such level sensors (161) form further aspects of the disclosure.

Description

LEVEL SENSOR FOR COOKING APPARATUS Field of the Invention

This invention relates to electromagnetic level sensors and, in particular, to level sensors suitable for use in cooking apparatus, most preferably of the kind operating on the spray frying principle. Background of the Invention

In the Applicant's co-pending Australian Provisional Patent Application No. PP 5622, filed on 1 st September, 1998, entitled "CONVEYOR FOR COOKING APPARATUS" there is described a particular type of spray frying apparatus in which a heated cooking medium, at temperatures typically above 150°C, is sprayed onto food items located within a cooking chamber of the spray frying apparatus. A preferred cooking medium is oil, for example vegetable oil or melted shortening.

In such cooking apparatus are included a number of oil storage means such as a feeder tank and a buffer tank which serve the following functions. The buffer tank is an intermediate reservoir of oil serving to collect excess cooking medium from the cooking chamber while allowing the required spraying rate by operation of the cooking medium pump. When the level of oil in the buffer tank falls below a certain value, it may be desirable to cause a flow of oil from the feeder tank such that oil level in the buffer tank is sufficient to allow safe and effective operation of the cooking apparatus, in particular, it is essential to avoid a situation in which oil flow fails in the pump and sprayer lines.

Typically, it is also necessary to provide for the situation in which the oil level in the buffer or feeder tank becomes excessive. In this case, the control unit of the cooking apparatus is to be configured to advise the operator of the cooking apparatus such that corrective action may be taken and the cooking operation properly controlled.

To date, the level sensor of reference has been of magnetic type comprising a guide tube for a float. The guide tube has hermetically sealed therein a reed switch and the float incorporates a magnetic material such that, as the float moves into proximity with the reed contact, the switch is activated in accordance with a high or low level condition. Manufacturers typically specify temperature ranges of operation suitable for monitoring of level with such level sensors.

In the case of measurement of buffer tank oil level, two such magnetic sensors may be provided, one being employed to sense a low level condition and the other to sense a high level condition.

The clearance between the guide tube and the float is typically small, this being necessary to enable effective operation of the magnetic sensor, and, accordingly, a problem arises in the case of cooking apparatus operating at high temperature. The oil tends to form gummy deposits in the space between the float and the guide tube which affects the proper operation of the level sensor. The level sensor may thus be said to be poorly compensated for temperature in the cooking application as the manufacturer of a level sensor will dictate a temperature range of operation not encompassing those typically encountered in cooking apparatus. In this respect, the control system of the cooking apparatus is compromised by the inappropriate temperature sensitivity of the level sensors. Summary of the Invention

It is the object of the present invention to provide a level sensor for monitoring of level of cooking medium storage means within a cooking apparatus which provides accurate data for use in control of the cooking apparatus at temperatures typically encountered in such apparatus.

With this object in view, the present invention provides, in a first aspect, a level sensor for sensing level of cooking medium in cooking medium storage means of a cooking apparatus including: • a housing connected to cooking medium storage means in which liquid level state is to be sensed;

• a conductive coil mounted relative to the housing;

• a ferro magnetic float which, when moved into proximity with the coil, induces a current to flow therein; and • a resonant detecting circuit coupled with the coil such that when current is induced to flow in the coil movement of the float, resonant frequency of said detecting circuit alters, frequency alteration being detected and provided as a control signal to the cooking apparatus control unit.

The sensor may be configured so that only frequency alteration outside permissible bands is detected and provided as a control signal. A derivative of frequency alteration may be used as the control signal. The level sensor may function as a switch suitable for detecting a level state such as a level HIGH and/or level LOW condition. In response to such detection, the control unit of a cooking apparatus may take corrective action to overcome the level HIGH or level LOW condition. For example, where a level LOW condition is detected, more oil may be supplied to the oil storage means to ensure that oil level in that means returns to permissible bounds. Analog sensing may also be achieved with sensed data being logged, if desired, by the cooking apparatus control unit.

The float may be made magnetic in any convenient manner, by manufacturing it of a ferromagnetic material or incorporating ferromagnetic material within a substrate material from which the float is fabricated.

The resonant detection circuit may incorporate calibration means such that the sensor may be tuned to provided accurate reading of level in the oil storage means. A convenient form of calibration means includes an oscillator, the frequency of which may be adjusted to achieve the desired reference resonant frequency of the detection circuit, both under conditions where the level is within permissible bounds and where the level is outside the permissible bounds.

With the compensation achieved by use of a resonant detection circuit, sensitivity of the level sensor to high temperature conditions may be reduced to levels allowing valid level data to be obtained in the temperature range of the cooking apparatus, typically in the range 140°C to 200°C, more preferably 150 to 180°C. This enables the level sensor to be manufactured with higher tolerances between the magnetic float and the tubular housing making gumming and inaccurate level readings less likely to occur even at high temperature. These advantages are promoted by omission of a float guide and attainment of acceptable sensitivity.

Analog and digital operation of the level sensor is possible as desired by the manufacturer.

The sensor may be utilised to advantage in any cooking apparatus, especially those operating on the spray frying principle. Such cooking apparatus may conveniently be fitted with such sensor(s) in buffer and/or feed tanks, ln this respect, the storage means encompasses buffer and/or feed tanks. However, it is not intended to limit the nature of vessels or means in which level state is to be sensed. Any storage or holding means which requires level sensing of cooking media is encompassed by this invention. Further, cooking media, in the context of this specification, may include any cooking apparatus fluid, that is any fluid used in the cooking apparatus.

In a further aspect of the invention there is provided a cooking apparatus including cooking medium storage means; and means for determining level of cooking medium in the storage means which further includes at least one level sensor of the first aspect of the invention. Multiple level sensors may be included in any cooking medium storage means of the cooking apparatus. Where multiple level sensors are employed, for example, to sense level HIGH and level LOW in a cooking medium storage means, these sensors may be coupled in differential mode. Differential mode operation may be used to cross-check sensor validity and advise of any necessary corrective action. ln a still further aspect of the invention, there is disclosed a method for sensing level of cooking medium in a cooking medium storage means of a cooking apparatus including a level sensor, of the first aspect of the invention, including flowing cooking medium from the cooking medium storage means into a housing of the level sensor such that a float located within the housing moves in response to changes in level causing current to be induced to flow in a conductive coil arranged about the housing to a resonant detection circuit, the resonant frequency of which alters due to such induction of current, the frequency alteration being detected and provided as a control signal to the cooking apparatus control unit.

Brief Description of the Drawings

The invention will be more fully understood from the following description of a preferred embodiment made with reference to the accompanying drawings in which:

Figure 1 is a process flow diagram of a cooking apparatus having a tank incorporating a level sensor in accordance with an embodiment of the present invention;

Figure 2 is a block diagram of a level sensor in accordance with an embodiment of the present invention; and

Figure 3 is a block diagram of multiple level sensors coupled and operated in differential mode. Detailed Description of Preferred Embodiments of the Invention

Referring first to Figure 1 , there is shown, for the purposes of illustration of a potential use for the level sensor, a cooking apparatus process flow diagram of a cooking apparatus operating on the spray frying principle showing that, during operation of the cooking apparatus (which is shown schematically as a box 110 in dashed outline), oil is delivered from a feeder tank 160 to receiving portion 152 of a buffer tank 151 for intermediate storage of oil recovered from sump 70 which collects excess cooking medium, sprayed by sprayers 30, from the cooking chamber 14 of a spray cooking apparatus. The cooking medium is then filtered, heated and pumped to the sprayers 30. Operation of the cooking apparatus is under the control of electronic control unit (ECU) 100.

The actual design of buffer tank 151 may be varied though the form described in the Applicant's co-pending Australian Provisional Patent Application No. PP 5717 entitled "IMPROVEMENTS TO COOKING APPARATUS", filed 4th September, 1998, the contents of which are hereby incorporated herein by reference, is a most advantageous design. Other aspects of the cooking apparatus therein described are also advantageous and further reference is made to the application for full description thereof.

Most important for present purposes is that feeder tank 160 incorporates a level sensor 161 for monitoring of oil level in that tank. Buffer tank 151 also incorporates such a level sensor 153.

These level sensors may be constructed in accordance with the present invention in accordance with the following description made with reference to Figure 2 which is directed to the level sensor 161.

The level sensor 161 comprises a non-magnetic tubular housing 20 connected by a narrow diameter tube with buffer tank 151 or feeder tank 160. Direct or indirect connection is possible. The tubular housing 20, which may be of stainless steel or other suitable durable, inert material, carries the fluid of which level is to be monitored. The fluid in this case is a heated oil or cooking medium adapted to spray frying, typically at temperature 140°C to 150°C. Temperature of cooking medium delivered from the sprayers is typically in the range 160°C to 170°C. Higher temperatures are possible in tubular housing 20. Housing 20 should accommodate oil or cooking media at pressures likely to be encountered in cooking apparatus operation. The housing 20 should also be air-tight to prevent oxidation of oxidisable cooking media by ambient air. Housing 20 may also be provided with means at its top or bottom to prevent sticking of the float. Pins 20a may be provided for this purpose. The float 22 must be magnetic in character, thus the float 22 may be fabricated wholly from a ferromagnetic material such as soft iron or may be of other material which has incorporated within it, in any conventional manner, a ferromagnetic material. The float 22 will be sufficiently buoyant to move freely in accordance with fluctuations in liquid level in the tubular housing 20 consequential on level changes in feeder tank 160 and may be hollow. It is not intended, in this most preferred embodiment, that the float 22 be guided in any way though it could be mounted relative to such a guide. This avoids the gumming problem mentioned previously. In fact, the tolerances are such as to reduce this problem to a minimum. The direction of travel of float 22 may be along the axis of tubular housing 20 though this is not essential. Housings of other geometry than tubular may be used subject to accuracy.

The float 22 should not be made of a material which includes materials absorbent of oil or other cooking media. This may cause erroneous level readings. Wound around the external circumference of an upper portion 23 of the tubular housing 20 is a coil 24 of conductive material, typically of metallic conductive material of such nature that when float 22 comes within predetermined proximity of the coil 24, a current will be induced to flow within the coil 24 which is connected to a resonant detection circuit of the RLC type having coil 24, connected in parallel with capacitor 26 and in series with resistor 28. Other resonant circuits may be employed. The coil 24 should be arranged such that sufficient sensitivity is achieved in level detection bearing in mind the dielectric constant of the cooking medium for which level is being sensed. The control unit 100 may be programmed to allow for cooking media of different or variable dielectric constant.

The coil 24, capacitor 26 and resistor 28 are connected to a calibration means which enables the resonant frequency of the circuit to be varied to a desired "reference" frequency.

In a particularly preferred embodiment, the calibration means takes the form of an oscillator 30, a device which produces a time variant voltage waveform, particularly a square wave/sinusoidal oscillator, having - in the block diagram - an output impedance of 330 ohms.

It will be understood that the current induced in coil 24 varies in frequency, it is an alternating current, which is rectified by full wave rectifier 60 to provide a first dc voltage input to comparator 40. The comparator second voltage input 41 is connected to a variable voltage reference 70 which serves as a sensitivity control varying the reference voltage as desired by the manufacturer. If the input voltage exceeds the reference voltage of the comparator 40, as will occur when the float comes into proximity with the magnetic coil, a level HIGH condition is flagged by the control unit and any control action contingent on the level HIGH condition such as an alarm signal, pump operation signal or otherwise may be implemented by the cooker apparatus ECU 100 of kind currently employed in cooking apparatus supplied by the Applicant. The comparator 40 may be compensated for noise in the signal by means known in the electronic arts.

The level sensor is calibrated in the following manner. With no float 22 in the system, the frequency of the square wave oscillator 30 is adjusted until a sinusoidal waveform reflecting the resonant frequency of the LC combination is found. At this frequency, the square wave is effectively converted into a sine wave.

When the float 22 comes into proximity with the coil 24, the inductance of coil 24 changes, which causes a corresponding change in the resonant frequency. As a consequence, the amplitude of the sine wave decreases and the comparator 40 output 42 flag on being served causes ECU 100 to a level HIGH or level LOW condition. It is the detection of this amplitude change signal that provides the level state signal.

Thus the level sensor 161 produces a digital output 44 though it is comprehended that such a level sensor could produce an analog signal if desired.

Where an analog signal output is desired, this may be connected before local comparator 40 (as shown by dashed line 43) which could, in such case, be omitted, sensor 161 being operated as a switch or multiple points switch using appropriate comparator software in the ECU 100 rather than local comparator 40.

The coil 24 or level sensor 161 could also be arranged such that a level LOW condition is flagged. Multiple coils could be arranged around the tubular housing for this purpose. More typically, separate level sensors would be provided for the buffer tank with tubes being connected to the sensor at an upper level; and a lower level of the tank. Such multiple coils may be operated in differential mode such that when signal increases on one coil, it decreases on the other in a manner appropriate for linear displacement sensors. Figure 3 shows two such sensors, upper sensor 161 and a lower tank level sensor 163 operated in differential analog mode. Digital operation is also possible. Sensor 163 may, other than being arranged relative to sensor 161 , be arranged with the same componentry though that is not essential. Control strategy may be implemented in accordance with sensed difference. Sensed difference may also be used to check sensor signal validity and allow appropriate ECU 100 response where invalidity is detected. The analog mode is conveniently shown by the block diagram. A rectified signal from each of sensors 161 and 163 may be an input to differential amplifier 200 and the difference signal (if any) used to command appropriate control strategy by ECU 100. A preferred control strategy for cooking apparatus of various types is disclosed in the Applicant's co-pending Australian Provisional Patent Application No. PP2568, filed 24th March, 1998, the contents of which are hereby incorporated herein by reference. In any case, the components of sensor 161 are those as shown in dashed box 161 d. Sensor 163 is likewise arranged.

For example, if level state information is in conflict this may indicate sensor failure. ECU 100 may shut down cooker operation and/or flag the need for system inspection. A safe mode of operation may be ensured.

Further modifications and variations may be made to the level sensor disclosed herein as will be appreciated by those skilled in the art on reading the disclosure. Such modifications and variations are within the scope of the present invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A level sensor for sensing level of cooking medium in cooking medium storage means of a cooking apparatus including a housing connected to cooking medium storage means of which liquid level state is to be sensed, a conductive coil mounted relative to the housing, a ferromagnetic float which, when moved into proximity with the coil, induces a current to flow therein; and a resonant detecting circuit coupled with the coil such that when current is induced to flow in the coil by movement of the float, resonant frequency of the detecting circuit alters, frequency alteration being detected and provided as a control signal to the cooking apparatus control unit.

2. The level sensor of claims 1 wherein the resonant detecting circuit includes a calibration means, preferably an oscillator.

3. The level sensor of claim 1 or 2 having temperature operating range 140┬░C to 200┬░C.

4. A cooking apparatus including cooking medium storage means and means for determining level of cooking medium in the storage means including a level sensor as claimed in any one of claims 1 to 3.

5. The cooking apparatus of claim 4 including a plurality of level sensors as claimed in any one of claims 1 to 3.

6. The cooking apparatus of claim 5 wherein said level sensors are coupled in differential mode.

7. The cooking apparatus of any one of claims 4 to 6 being a spray fryer.

8. A method for sensing level of cooking medium in a cooking medium storage means of a cooking apparatus including a level sensor including flowing cooking medium from the cooking medium storage means into a housing of the level sensor such that a float located within the housing moves in response to changes in level causing current to be induced to flow in a conductive coil arranged about the housing to a resonant detecting circuit, the resonant frequency of which alters due to such induction of current, the frequency alteration being detected and provided as a control signal to the cooking apparatus control unit.