Food service system utilizing reflected infrared signals to identify type of dish

Abstract

In a food service system, prepared meals including chilled foods are assembled in individual meal trays (10) and held in the trays for a period prior to regeneration to prepare the food for heating. A control system of a rethermalizing trolley (12), which accommodates a large number of trays, can discern for itself which dishes (30) in the trays need heating and which are to remain cool. Dishes for hot and cold foods have white and black markings respectively, which results in reflected infra-red signals from sensors (42, 44) of the control system to be of discernably different strengths, so identifying the hot food dishes. Dish temperature is continually monitored by the control system by means of thermal contact with thermistors (46), which enables the system to oversee food temperatures both prior to and during regeneration.

Description

cross-section aluminum heating plates 38 which correspond in size and position with the heat transfer plates 28 of two trays 10 installed on the shelf. The heating plates 38 are raised slightly above the upper surface of the shelf in order to locate in the similar recesses in the bottoms of the trays, beneath the transfer plates 28; the parallel heating plates 38 and heat transfer plates 28 are so arranged to abut face-to-face. Electric heating elements 39 (FIGS. 5 and 8) are arranged in a conventional manner for heating the heating plates 38, and so the heat transfer plates 28 and the food dishes 30 standing on them in the trays 10.

Operation of selected heating elements at required times is controlled automatically by an electronic control system of the trolley which includes a microprocessor. In order that the control system can know which heating elements are required to be operated and can continually monitor the temperatures of the food dishes in the trays, the system includes sensor units 40 fitted into the heating plates 38.

As indicated by FIG. 1 (which shows only the heating plates for one shelf, but all the shelves are in fact the same) each heating plate 38 has an opening at its centre into which its associated sensor unit 40 is fitted. A flat top surface of the sensor unit is flush with the upper surface of the heating plate.

Each sensor unit 40 (FIG. 4) comprises an infra-red emitter 42, an infra-red detector 44 and a thermistor 46, the three components being mounted on a printed circuit board 48 and potted together in a clear casting resin. An uppermost edge of the thermistor 46 is coincident with the top surface of the sensor unit so as to engage the undersurface of the associated heat transfer plate 28 of an installed tray; the thermistor is so installed to be electrically responsive to variations in the temperature of the transfer plate 28. The three components 42, 44 and 46 of the unit are electrically interconnected by the printed circuit board 48 and three leads 50, 52 and 54 from the board provide for common ground, input and output connections respectively.

With reference to FIG. 5, the control system comprises in addition to the microprocessor 100 (CPU) and system clock 101, a real-me (R.T.) clock 102 which enables the system to keep track of the time of day. A digital display 103 and keypad 104, communicating with the microprocessor through an interfacing chip 105 forming a peripheral interface adapter (PIA), enable communication between the microprocessor and an operator. Memory capacity is provided through EPROM 106 and RAM 107 facilities. An analog to digital converter 108 (ADC) enables voltage signals from the sensor units 40 to be read by the CPU. A second interfacing chip 109 forming a versatile interface adaptor (VIA) links the CPU to two wiring matrices 56 and 58 for control of the heating elements and the sensor units 40, respectively.

The matrices 56 and 58 enable the ninety-six heating elements and the ninety-six sensor units (there being six of each on each of sixteen shelves) to be controlled individually. Each matrix is an 8×12 arrangement of twenty distinct conductive paths considered in columns and rows, each heating element or sensor unit being connected across a unique combination of column and row paths for individual control.

Considering the sensor units 40 in the matrix 58 first, the twelve row paths lead from a transistor switching device 60 and the eight column paths lead to an analogue switching device 62, both of which devices are under the control of the VIA chip. Output signals from the analog switching device 62 are passed to the ADC for conversion and onward transmission to the CPU.

Considering the heating elements in the matrix 56, the twelve row paths lead from a bank 64 of twelve triacs and the eight column paths lead to a bank 66 of eight triacs, both banks of triacs being under the control of the VIA chip. Mains voltage is applied to energise energize the heating elements, but to prevent mains voltage reaching the CPU, the triacs are not directly connected but are isolated optically from the main circuitry by optotriacs. To increase heating element life and reduce interference, the elements are switched on and off only when the mains voltage cycle is at zero. The total loading of all the heating elements of the trolley would be 288 kw, but by using a pulsed power-sharing technique the trolley is enabled to operate from an ordinary 13 amp 3 kw power supply. The sensor units 40 serve two purposes. As hereinbefore referred to, through use of the thermistors 46 they are adapted to signal variations in temperature of the heat transfer plates 28 and so (indirectly) to monitor food temperature. However, they further have an essential role in identifying which dishes require heating and which do not when the time comes for regeneration. This latter function is effected through use of the infrared emitter 42 and detector 44 of each unit.

Each heat transfer plate 28 has in it a small aperture 68 (FIG. 3) which is so aligned with the sensor unit 40 of the associated heating plate 38, when the tray is installed on a trolley shelf 36, that it forms a window above the infra-red elements 42 and 44 of the unit. The sensor unit 40 can so "view" the bottom of any food dish 30 in the heating compartment 24 of the tray by reflecting an infra-red signal off the dish by means of its infra-red elements 42 and 44. [The thermistor 46 is positioned aside from the window 68, so as still to engage the underside of the heat transfer plate 28.] Each of the food dishes 30 used in the compartments 24 of the tray is identified as one to be heated or not, as the case may be, by means of a white or a black (respectively) identifying patch on its bottom surface opposite the window 68. The reflected signal which the infra-red detector 44 of an activated sensor unit receives from a dish with a white marking is of a markedly different level from that which would be received from a dish with a black marking (or similarly in the absence of a dish from the compartment) and the output signal from the sensor unit 40 as a whole is consequently of a quite different order in the two cases. Accordingly, provided that in preparation of the meal trays dishes with white markings are used to contain those foods which need to be regenerated, and dishes with black markings to contain those foods which do not, the control system is able to determine automatically which heating elements need to be operated by distinguishing between the two levels of signals being produced by the sensor units 40.

Power for the control system comes principally from an internal rechargeable battery, mains power being required only for the heating elements during food regeneration, during which time the battery can also be recharged.

The system is under the control of a program stored in the EPROM, which program can be changed to vary or update the system when required, for example to meet statutory regulations for the safe storage, handling and regeneration of chilled meals. When the system is first powered up, the program sets all the hardware to a predetermined set of conditions. Once this is done the program stays in a loop constantly reading the keypad to check for any input by the operator, and checking whether the time has been reached for regeneration; if an input is detected the program will branch off from the main loop to perform the required task.

Every two minutes the program jumps out of the main loop in order to activate and read the sensors 40 (one by one) and so monitor the arrangement of food dishes 30 and their temperatures. This information is recorded by the CPU in the memory. The CPU is thereafter able to detect any change in the arrangement and temperature of the food dishes by comparing the new information each time with that previously recorded. One minute before it is time to regenerate, the program branches off to make a last check of the food dishes prior to regeneration, and if all is satisfactory the program then branches off into a regeneration routine.

Continual monitoring of the food dish temperatures prior to regeneration can be important, as it may be essential that the temperature of chilled food held in the trays does not rise above, say, 10°C Should the control system detect that such a rise in temperature has occurred it will cause an alarm signal to be given together with an indication to the operator as to which dish is affected.

During the regeneration routine, the system continually activates each of the required heating elements in turn, also continually monitoring the temperatures of the dishes. Towards the end of the routine, the heating elements associated with dishes which have already reached their required temperature are activated by the control system only as much as may be required to maintain that temperature.

This food service system so takes account of varying loads of meal components to enable the system to use a minimum of energy during regeneration. Regardless of the type of food being regenerated the final temperature of all the heated food compartments will be substantially the same.

Instead of individual programming of trolleys by keypads on each, collective programming by an infrared or other transmitter could of course be employed.

Further discussion of the operation of the sensor units now follows. FIG. 6 is a circuit diagram for a single sensor unit, which may be considered as two variable resistors forming a potential divider, the two variable resistors being formed by the infra-red detector 44 and the thermistor 46. As a whole unit, the sensor can be considered as a device that produces an output voltage that is related to its temperature and position relative to a reflective surface, i.e. the bottom surface of the dish 30.

The diode D1 is included to prevent the interaction of other sensors in the matrix, by causing the signals to flow in one direction only.

As hereinbefore described and shown also in FIG. 7, the sensor units 40 are connected so as to form a matrix. The matrix consists of twelve power supply lines (the row paths) each one being controlled by the transistor switching device 60, and eight signal lines (the column paths) connected to the analog switching device 62. The analog switch routes the signal on one of the eight signal lines to the analog to digital converter (ADC). Exactly which signal line is selected depends on a binary number from 0-7 being sent to the analog switch by the CPU.

In order for the value of a particular sensor unit to be read, the CPU must first switch on the transistor that powers the sensor unit in question through the corresponding power supply line, and then send the required binary value to the analog switch to connect the output signal from the sensor unit, on the corresponding signal line, to the analog to digital converter; although there are eleven other sensor outputs connected to this signal line, they are all switched off leaving only the signal from the sensor unit in question to be present. These two operations actually happen simultaneously with the CPU sending a single binary value to the VIA that controls the sensor matrix.

It can be seen from this that only one sensor may be read at a time. To increase the accuracy, each sensor is actually read sixty-four times and an average taken.

The output from each sensor unit 40, once read by the CPU, is a numerical value between 0 and 255. This value is a representation of the type and temperature of the dish 30 with which the sensor is associated.

Initially, before the trolley is loaded, all sensor units will return a low value (e.g. 20) there being slight variations between the ninety-six readings. Any value very different from the rest would indicate that a problem exists, an object obscuring the sensor for example, and a warning buzzer would be sounded to alert the operator.

Once the trolley is loaded and the sensors have been read, all the values should have dropped due to the lower temperature of the dishes, whether requiring heating or not, or remained the same if no plate is present; sensor units associated with dishes that require heating, having white patches viewed by the sensor units, will return a value rather lower than those that are to remain cold (i.e. those having black patches). Any rise in value (i.e. when the trolley is first loaded) would indicate a plate of too high a temperature, the operator being warned should this be the ease.

Subsequent readings, between the times of loading and regeneration, will show a gradual increase in values as the temperature of the food on the dishes rises towards the ambient temperature, this being slowed by the insulated tray. A wildly different value from the rest would point towards a problem with the tray, such as an improperly located lid reducing the amount of insulation and causing the greater increase in temperature.

From the readings before the trolley was loaded the CPU is able to calculate a value that corresponds to the 10°C limit that the food must remain below before regeneration. Each sensor reading is compared with this value and warning given should any exceed it.

During regeneration the readings will rise fairly rapidly with the food temperature. Readings are now compared with another predetermined value that indicates the 70°C that the food must reach to be properly regenerated.

The CPU is able to differentiate between food products of greatly different heat capacities by their temperature readings during regeneration, and regulates the amount of heating given to each dish accordingly in order to complete the regeneration of all the meals at the same time. However, in practice most food products require the same amount of heating and little such intervention by the CPU generally occurs.

A more detailed description now follows, with reference to FIG. 8, of the control matrix 56 for the heating elements 39.

The twelve row paths a to 1 lead to the bank 64 of twelve triacs and the eight column paths A to H lead from the bank 66 of eight triacs. Mains s.c. voltage is applied to the matrix through the triac banks. The particular heating element 39 indicated in the drawing will, for example, be activated only by the matrix combination Ce.

Whilst by means of such a matrix, a primary conductive path through a single element is created by any selected combination of single column and row paths, a plurality of secondary conductive paths are potentially created through combinations of elements in series. This would result in power wastage, and some heating of elements which are intended to remain inactive, unless measures are taken to block all such secondary paths.

By means of a pair of oppositely orientated diodes 70 and 72 at the a.c. input of the triac bank 66 at the head of the column paths A to H, the mains current is split into its negative and positive components. One component supplies the four paths A to D and the other component supplies the paths E to H. Each of the transfer paths, between a row and column, includes a diode 74 which is orientated appropriately to match the polarity of the column path to which it is connected. All twelve of the transfer paths connected to each of the columns A to H are therefore unidirectional in the same sense. Since the creation of a secondary conductive path requires one of two transfer paths connected to the same column path, and one of two transfer paths connected to the same row path, to be a reverse current path, blocking of all such secondary paths is achieved.

Claims

1. For use in a food service system in which prepared meals including chilled foods are assembled in individual meal trays and held in the trays for a period prior to regeneration in the trays to prepare the food for eating, rethermalizing apparatus comprising:

(i) a meal tray forming a plurality of compartments in which removable food dishes can be located;

(ii) dish-heating means comprising a plurality of heating units which for effecting thermalization are positioned beneath food dishes in said compartments in said tray and are independently operable for selective heating or non-heating of said dishes as required;

(iii) food dishes of a first type (hot food dishes) which can be located in said compartments in said tray to hold food which is to be heated;

(iv) food dishes of a second type (cold food dishes) which can be located in said compartments in said tray to hold food which is to remain unheated;

(v) heating control means operative to activate said heating units selectively so as to heat any hot food dish located thereover in said tray but not to heat any cold food dish located thereover;

said control means comprising a plurality of sensor units which are associated one with each of said heating units to reflect radiated sensing signals off dishes in said compartments in said tray and to respond to the reflected signals, said hot food dishes and said cold food dishes being distinctive as to the signals caused to be reflected from them whereby enabling the control means to distinguish between them for operation of those heating units alone positioned beneath hot food dishes.

2. Apparatus according to claim 1 characterized in that the sensing signals are of infra-red radiation.

3. Apparatus according to claim 1 characterized in that it comprises a rethermalizing trolley having a plurality of shelves on which trays can be lodged, each shelf comprising a plurality of heating units which are arranged to be in registry with the dishes in a tray installed on the shelf in order to heat those dishes if required.

4. Apparatus according to claim 1 in which said heating units comprise thermally conductive heating plates and electric heating elements for heating the plates.

5. Apparatus according to claim 4 in which there are openings in said heating plates for said sensor units to reflect sensing signals off dishes thereabove.

6. Apparatus according to claim 5 in which said sensor units are mounted in said openings within said heating plates.

7. Apparatus according to claim 5 in which each sensor unit comprises an infra-red emitter and an infrared detector which are potted together in a casting resin and mounted within said opening.

8. Apparatus according to claim 7 in which each sensor unit comprises also a thermistor which is potted together with said emitter and said detector, an uppermost edge of said thermistor being coincident with a top surface of the unit and said top surface being flush with an upper surface of said heating plate.