CA1137592A - Food serving system - Google Patents

Abstract

Abstract of the Disclosure A food serving system for rethermalization of the food and for delivering prepared meals to locations remote from the place of preparation, such as to patients in hospitals and in nursing homes. The system includes a mobile cart having its own rechargeable power pack that may be utilized to propel the cart. The cart has separate bays each having a tier or racks for carrying individual food serving trays. Each tray has one or more thermally isolated heat transfer devices which are selectably energized when the tray is in place on a rack in the cart. The trays are constructed to carry conventionally styled, removable dishes, bowls, cups, etc., that may be either disposable or reuseable. Preferably, each tray has a pair of heaters referred to herein as plate and bowl heaters and adapted to be operated by control means in either a continuous mode of operation or a rethermaliza-tion mode of operation. In the rethermalization mode of operation the manually selected heaters associated with trays in all bays are operated for a predetermined period of time of, for example, 25 minutes, to rethermalize the food from a frozen or at least cold state to a temperature on the order of 170°P.-180°P. Thereafter, the heaters in predetermined bays are operated cyclically to hold the food at the desired temperature range.

Description

Back~round o-f the Invention .. ..
The present ;nvention relates in general to a mobile food serving system and pertains, more particularly, to a system for rethermalizing the food, maintaining the food in a heated edible condition, and delivering the prepared meals to locations remote from the place of preparation. The system of this invention may have use in the feeding of patients in hospitals, nursing homes, lairlines and other institutions.
It may also be used in the feeding of hotel guests. Also, although the principles of this invention are preferably applied to a mobile system, they may also be applied to a stationary food serving system.
U.S. Patent 3,924,100 discloses a mobile food serving system that is adapted for the delivery of prepared meals from a central location and with the maintenance of the proper temperature of these meals. This patent discusses in some detail the disadvantages associated with both a centralized system and a decentralized system. It appears that the cen-tralized system may be more desirable from a standpoint of cost and efficiency. Even so, with the present centralized systems, there is still a requirement for a large heating apparatus such as steam pans or convection ovens in the central area. With these systems the food is, after rethermalization, placed on trays and delivered to the patients on different floors.
Many of the other systems now used especially in hospitals employ the centralized control with the use of microwave ovens for heating, preferably at each floor where food is to be served. At first appearance it would seem that the rapid heating available with the microwave system would provide some time and cost saving. However, decentralized microwave system is extremely cost saving. However, decentralized microwave system is extremely labor intensive requiring an excessive number of steps of handling the food. With such a system, once the food has been transported to the floor, ~.

each dinner must be removed and individually heated and then groups o-f dinners are served, usually in relatively small groups in order to maintain the food suitably heated. There is therefore a requirement for substantial help in the hos-pital for handling these steps that totally negates anyadvantage to the use of microwave energy.
It is an object of the present invention to ta~e more full advantage of the centralized system by providing a food serving system that itself provides rethermalization and delivery of the meals. The rethermalization and the delivery system is preferably embodied in a single unit in the form of a self-powered cart.
Another object of the present invention is to provide an improved mobile food serving system that will enable simplification of the main preparation area by substantially reducing the need for large heating apparatus such as steam pans or convection ovens located in the main preparation area. Further, because of the use of this centralized delivery system there will be a significant cost saving in that substantially no extensive support equipment will be required on each floor.

Summary of the Invention To accomplish these and other objects, this invention includes among its features, a self-contained mobile cart for carrying and rethermalizing the food and thereafter delivering the meals. Separate removable food serving trays are used for each patient along with individual dishes, plates, bowls, etc. to be carried on the trays and which may be like those used in the home. The cart carries its own rechargeable low voltage battery pack, which selectively supplies the necessary energy to each tray to maintain selected portions of the tray at the desired temperature. The trays each include preferably two isolated heaters that may be positive temperature coeffi-cient type heaters. In the disclosed embodiment these heatersare referred to as plate and bowl heaters for each tray.

37S~Z

With the system of this invention, the unheated, possibly even frozen, meal is ~laced on a tray hours or possibly even a ~ay or mo~e before the meal is to be served.
The entire cart may be stored in a central holding refrigerator. This arrangement eliminates the extra labor that is necessary to assemble the meal at the time it is to be served. Approximately one half to one hour before meal time the fully assembled cold trays are heated by activating the plate and bowl heaters as required. ~hen the cart is fully loaded and programmed, it is deli~ered to the desired floor. While this deli~ery is taking place, the food is being rethermalized autQmatically, with an indicator -for indicating when the meal is ready to be served. Should the meal not be removed at the time that it is ready5 in accordance with the control of this invention a holding cycle is used so as to provide sufficient heat to hold the meal in a temperature range of about 140F.-160F. In one embodiment of the invention the rethermalization occurs over a period of 25 minutes and for an additional period of say 5 minutes the heaters are operated on an on-off cycle which may be at a 50%
duty cycle. After this 5 minute cycle the food is ready to be served. However, if it is not then served, the on-off cycle of the heaters continues to maintain the food at the proper temperature.
In one embodiment cycling operation associated with each tray is terminated preferably a predetermined period of time after removal of that tray. In an alternate arrangement of the invention all cycling operation is terminated auto-matically upon removal of the last tray in the cart.
In one embodiment of the present invention the ready condition is registered for essentially all tray locations.
However, it may be desirable in some other cases, wherein trays may be loaded at different times, to have a ready condi-tion associated with each tray. Thus, in an alternate embodiment a ready light is associated with each heater or 3~5 ~ ~

heaters corresponding to a tray. In such an embodiment separate time delays are also used in conjunction with each tray heater or heater array.

Brief Figure Description Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a front fragmentary view of a cart constructed in accordance with this invention and showing in particular the tray control panel;
FIGS. 2A-2B show a total electronic system in accordance with the in~ention and employing the cycle control for rethermalization;
FIG. 3 is a circuit diagram o~ an embodiment of the heater control buttons shown in FIG. 2;
FIG. 4 is a circuit diagram of an alternate embodiment of the invention including cycle control; and FIG. 5 shows a further feature of the present invention in an electrical diagram showing means for sensing complete tray removal.

Detailed Description U.S. Patent 3,924,100 shows a typical cart construction, the entire construction of which is not discussed in detail herein. The preferred cart construction of this invention has three bays identified hereinafter as bays #1, #2, and #3, each having racks for supporting a plurality of trays in each bay. FIG. 1 herein is a fragmentary view of a portion of the cart showing two of the bays and, disposed therebetween, a control panel 10. In addition, there is another control panel not specifically shown in the drawings but similar to the control panels shown in FIG. 2 of U.S. Patent 3,924,100 and including, for example, an on-off switch, an operate/load S~

switch, a mode switch and associated indicators all dis-cussed hereinafter with reference to FIG. 2.
The control panel 10 supports three columns of push-to-light switches. Each hori70ntal row which comprises the three switches represent the controls for a single tray associated therewith. The first vertical column of switches 12 are for indicating that the corresponding tray is in place on the rack associated with that position and also serves as an "off" or "program clear" for that station. When the tray is in place, the indicator 12 is illuminated and when the tray is not completely in place, the indicator is not illuminated. The adjacent two vertical columns comprise, respectively, bowl switches 14, and place switches 16. These switches or buttons 14 and 16 are for selectively operating the heat transfer devices or heaters which are independently operated and which are provided in each tray. In an alternate embodiment a single heater may be used in which case only one of the switches 14 and 16 may be necessary.
The push-to-light switches 14 and 16 may be of conventional design. FIG. 3 shows one typical embodiment including a relay 14A in series with the switch 14. A con-tact 14B of the relay is coupled across the switch 14 so that upon momentary operation of the switch 14 the relay 14A
energizes and is latched in that position by way of the con-tact 14B. The light portion of the switch 14 is depicted in FIG. 3C by the indicator 14C.
FIG. 3 also shows a second contact 14D of the relay 14A. This contact 14D is coupled in series with the tray heater 15. FIG. 2 also shows schematically the contact 14D
and associated heater 15. The contact 14D and heater 15 are coupled across the voltage source so long as the particular bay is selected. Ilowever the positive voltage input to terminal 17 of FIG. 3 is coupled from line 19 of FIG. 2 so that the voltage available at the bowl and plate switches or buttons is a voltage that is only present after momentary ~1 3~ Z

movement to the "tray" position and is maintained in all positions of switch 24.
FIG. 2 is an electronic schematic diagram associated with the system of this invention and including the control for the rethermalization or reconstitution mode of operation.
In FIG. 2 an external power supply is coupled between terminals 20 and 21. Ihis power may be provided from a con-ventional AC source coupled by way of a converter to provide a positive voltage at termina~l 20 and a negative or ground voltage at terminal 21. These voltages at terminals 20 and 21 are coupled respectively to lines 20A and 21A for pro-viding power to the three bays disclosed in FIG. 2. The bay #1 is shown in detail while the bays #2 and #3 are shown only in block form as ~hey are substantially of the same con-struction as the first bay.
When the external power is applied, the rela~ Ri islatched. When this occurs its associated contact RlA closes thereby in turn latching the relays C4, C5 and C6. Each of the relays C4, C5 and C6 has associated therewith a coil suppressor circuit comprising a diode and resistor in series coupled across each of these relays as depicted in FIG. 2.
Upon energization o~ these relays C4-C6 the associated normally open contact closes and power is provided from terminal 21 to one side of a further contact ClA, for example. In FIG. 2, the contacts that control power to each of the bays are contact ClA for bay #1, contact C3A for bay #2, and contact C2A for bay #3. When external power is being used, the power i.s coupled to bay #l by way of the contact ClA in series with the contact C4A.
When external power is being used, the connection 22 at terminal 20 also causes the relay R2 to be operated. This relay has associated therewith a contact R2A which is normally closed except when the relay R2 is energized. Thus, when external power is used, the contact R2A is open and all power to the cart drive motor is interrupted even if the S~

switch 24 is operated to its "drive" position. The switch 24 has a first position 24A for controlling the "tray"
position of the switch and a second position 24B for con-trolling the "drive" position of the switch. Thus, if the cart is put into the "drive" position by switch 24 with external power applied, this power will not be coupled to the drive motor because of the interrupting contact R2A.
Once the external connector is removed, the relay R2 de-energizes and then power to the drive motor may occur through the operation of the switch 24 to its "run" position.
In FIG. 3 there is considered an embodiment for one of the many push button switches that are used for selecting the heaters which in the disclosed embodiment comprise a bowl and plate heater associated with each tray. In FIG. 2, for example, there is shown tray #1 in bay #1 havi~g associated therewith two heaters 15 which may be identified as a bowl heater 15B and a plate heater 15P. Each tray also has a shorting conductor wire 25 so that when the tray is plugged in a voltage signal from line 20 by way of a circuit breaker is coupled by way of the conductor wire 25 through the indicator 12 for indicating that the associated tray is in place. Once the tray is in place, then one or the other or both heaters may be selectively operated through their associated push buttons such as the button 14 as shown in FIG. 3. For example~ in bay #1, tray #1, if both of the bowl and plate switches are operated~ then the contacts PRl and BRl are both closed coupling power from the contact ClA
to both of the heaters lSB and 15P.
The heater elements 15B and 15P are schematically shown herein. Ilowever, it is understood that each of these heaters may actually comprise one or more heater elements.
In the preferred embodiment of the invention a plurality of positive temperature coefficient (PTC) pellets are used.
These pellets may be of the type manufactured by Texas Instrument Company. See U.S. Patent 3,924,100 for one detail of an arrangement of these heaters.

s~z - g -The PTC pellets are basically multi-modal resistance heaters. Below their critical temperature which may range from 120. to 400F. depending upon composition, they have a low electrical resistance, while above the range the resistance is very high. These heaters are designed to operate at a relatively narrow range of temperatures depend-ing upon their composition. Thus~ if the plate or bowl is colder than intended, the heater operates at a higher wattage output to quickly heat the bawl or plate. Once the proper temperature has been attained, then the resistance increases and the heating reduces substantially.
In the construction shown in U.S. Patent 3,924,100 the heaters were primarily used for maintaining the temperature of food. However, in accordance with one mode of operation lS of the present invention it is intended that the heaters be used for rethermalization or reconstitution of the food.
Accordingly, the designs of the specific PTC elements are different. For the bowl heater the PTC element is selected so that the surface of the heater is in the temperature range of 270F.-280F. For the plate heater the surface temperature of the tray is on the order of 240F.-250F.
The bowl heater is designed to provide a power output in the range of 35-40 watts while the plate heater is in the range of 60-80 watts primarily because of the larger surface of heating that is necessary with the plate in comparison to the bowl.
In accordance with the invention there is initially a continuous mode of operation for reconstitution followed by a maintaining mode of operation which may ~e accomplished in the preferred embodiment by a cycling of the heaters. In an alternate arrangement the maintenance mode may also be accomplished by a drop in the heater voltage or by providing additional heaters and sequencing between groups of heaters.
The end result is to provide a continuous cycle so that the food can be raised from a temperature of say 40F. to a ~ ~3~5~Z

temperature range on the order of 160F.-180F. Thereafter, the PTC heaters are operated in one embodiment in a cyclic manner so as to maintain the temperature of the ood in the range on the order of 140F.-160F.
It is preferred that the internal battery 28 be used primarily for propelling the cart although this battery is also used for heating, by way of lines 30 and 31 which couple by way of, for exmple, contacts C4B and ClA to each bay to provide power thereto- For bay ~2 this power is coupled by way of contacts C6B and C3A and for bay #3 the power is coupled by way of contacts C5B and C2A. When the external power is removed, the relay Rl becomes de-energized as do the relays C4-C6. Under that condition the contacts C4B, C5B and C6B close and thus all power is coupled from lS the battery 28 and not from the remote terminals 20 and 21.
As previously indicated, the power to the bays is controlled by the relays Cl-C3. An indication of each bay being powered is by means of the indicators Ll-L3. In accordance with one mode of operation wherein the power to the bays is cycled, the indicators Ll and L3 may be illuminated at one time and only the indicator L2 at another time.
The control circuitry o~ FIG. 2 comprises, in addition to the switch 24, a load/operate switch 34, a normal/re-thermalization switch 36, a door interlock switch 38, an of~-on switch 40, and repeat cycle timer switch 42. Also shown in FIG. 2 are time delays TDl and TD2 and the repeat cycle timer RCT.
In the "drive" position o~ the swi~ch 24 when the contact 24B is closed and assuming that external power is not being used, the contact R2A is also closed and power is coupled to the cart drive motor under some type o manual control not disclosed in detail herein. In this mode of operation the contact 24A is open but the relay Kl is in its latched state because there was a previous momentary operation ~..3~5~.~2 of switch 24 to the "tray" position which energized relay Kl.
The relay Kl was maintained in this lathed state by way of its latching contact KlA. The drive by way of contact 24B
can only occur when there is an initial operation of the con-tact 24A when the switch 24 is put in the "tray" position.
When, for example, trays are to be loaded into the cart then the switch 24 is moved to its "tray" position. The con-tact 24B is open and the contact 24A is momentarily closed.
The relay Kl is latched assu~ing that the door interlock switch 38 is closed and that the switch 40 is in its "on"
position. The door interlock switch 38 will be open if the rear door of the cart is opened for the purpose of charging the battery 28. Thus, under the condition of recharging of the battery, the relay Kl is not latched and no type of con-trol is possible. Assuming that the door is closed becausethe battery is not being reharged, then the relay Kl may latch when the switch 40 is closed. When this occurs its associated contact KlA closes providing power to line 19 and also by way of diode 35 to the switches 34 and 36. The power on line 19 couples to the button circuitry in each of the bays as exemplified by the terminal 17 in FIG. 3. Thus, the power to the but~ons such as button 14 in FIG. 3 is selected based upon the position of the switch 24.
When the switch 36 is in its "normal" mode the switch 34 is essentially by-passed and power is coupled to line 40 by way of diode 41 for the purpose of illumination of the indicators Ll-L3, and is also coupled by way o~ the contacts 42A and 42B o switch 42 to the relays or contactors Cl, C2 and C3. Thus, in this mode of operation, all three of the bays are enabled through the contactors Cl, C2 and C3 and any of the selected trays have their heater or heaters operating depending upon the selection that has been made. In the "normal" position of the switch 36 the time delay TDl is not operated and thus its contacts TDlA and TDlC are closed, as shown, and all of the contactors are operated. Accordingly, 1~37S~Z

the associated contacts ClA, C2A and C3A are closed and power is coupled to each bay.
When the mode switch 36 is moved to its "rethermaliza-tion" position, then the power is coupled to the time delay 5 TDl which may be a 25 minute time delay. If it is assumed that the switch 34 is in its "operate" position, then the power is coupled by way of switch 34 and switch 36 to the time delay TDl. On the other hand, if the switch 34 is in its "load" position, then power is interrupted to the switch 36 while the trays are being loaded into the cart. There-after,the switch 34 is moved to its "operate" position for providing the programming in accordance with this invention.
During the initial time delay interval defined by delay TDl which is part of the rethermalization or reconsti-tution interval, the contacts TDlA and TDlC are still closedand thus all of the bays are enabled with any tray capable of receiving heating power as a function of the operation of the switches associated with each tray to select either or both of the heaters on a tray. During the rethermalization inter-val, the contact TDlB is open, as shown, and thus there is nopower provided to the second time delay TD2. Also, because of this open contact, there is no power provided to the ready light 44 which coupled in series with the contacts TDlB
and TD2C. Also, the contact TDlD is still in its closed position, as shown, and thus the repeat cycle timer RCT is not yet operated.
After the termination of the rethermalization interval (25 minute interval), the time delay TDl is operated so as to change the state of the contacts as depicted in FIG. 2. Thus, the contact TDlA and the contact TDlC open interrupting the direct power to the contactors Cl, C2 and C3. However, the contact TDlB is closed providing power to the second time delay TD2. Its contacts TD2A and TD2B are maintained in the closed position until the second time delay has timed out.
The second time delay typically has an interval of 5 minutes.

.

S~2 During the interval of delay TD2, the contact TDlD of the first time delay is closed and thus the repeat cycle timer initiates its operation. This timer may be a conventional timer having a repeating cycle. Associated with this timer are contacts RCTA, RCT~, and RCTC. At the initiation of this timer, the contact RTCB is closed and thus power is pro-vided to the contactors Cl and C2. At the same time the con-tact RCTA is opened and power is interrupted to the contactor C3. After an interval of operation of approximately 1 minute, the timer has its contacts reversed and thus power is then provided to the contactor C3 with power being interrupted to the contactor Cl and C2 through the contact RCTB. Thus, after the termination of the time delay period of TDl, the repeat cycle timer continues to operate on this one minute cycling basis. However, at the end of the 5 minute interval determined by the delay TD2 the ready light 44 is operated through the contacts TDlB and TD2C. Thus, at the end of a total of 30 minutes, the food is ready for serv-ing. However, the cycling operation continues after that period for any trays that are still ~eing maintained heated.
At the end of delay TD2 its contacts TD2B opens extinguish-ing the indicator 47. Furthermore, at the same time the contacts TD2A opens to extinguish the indicator 46 indicating an end to the rethermalization interval which comprised the 25 minute interval plus the 5 minute interval.
After all of the trays have been used, they may be returned to the cart but would not be completely plugged in as it would not be desired to heat any of the empty trays.
At that time the switch 34 may be changed back to its "load"
position so as to reset the control means including the time delays TDl and TD2.
FIG. 4 shows an alternate embodiment of the present invention which provides a "ready" indication associated with each tray position rather than an overall "ready" condi-tion as with the first embodiment in FIG. 2. In the ~3'~S~:

embodiment of FIG. 4 for the sake of simplicity only twotray positions have been shown, it being understood that all other tray positions are of substantially the same con-struction. Also, in FIG. 4 like reference characters have been used to designate like components also found in FIG. 2.
In the embodiment of FIG. 4 only the self-contained battery source 28 is shown, it being understood, however, that external power may also be used with this arrangement and connected in a manner similar to that discussed previously with reference to FIG. 2. The output from the battery 28 couples to the power lines 30 and 31 for providing power to each of the individual trays and also to the repeat cycle timer RCT. Thus, in the embodiment of FIG. 4 the repeat cycle timer is operated all the time. The control for the embodiment of ~IG. 4 is simpler than the control for the embodiment of FIG. 2. However, the individual logic circuitry associated with each tray ls somewhat more complex.
In FIG. 4 each relay board 50 may contain circuitry similar to that shown and discussed previously in FIG. 3.
Thus, with regard to tray #1 the relay board 50 may comprise a bowl button and a plate button each represented by a con-tact 14D (See FIG. 3) and each for coupling power from line 31 to the bowl heater 15B and the plate heater 15P. In this embodiment the bowl heaters 15B are operated continuously as long as the bowl heater is selected by its associated switch.
The plate heater 15P can be operated by way of two different paths including a path by way of contact TRlA directly from the board 50, or a second path by way of contact TRlB and the contactor contacts CRlA, CRlB, CR2A or CR2B. In FIG. 4 each of these contacts are shown as coupling to diferent trays for providing operation thereof.
When a tray is initially engaged in place on its rack the time delay relay lRl is operated and during the time delay interval its contacts are maintained in the position shown in FIG. 4. Thus, initially during this continuous ~37S~Z

operation interval, the contact TRlA is closed while the contacts TRlB and TRlC are open thereby enabling continuous operation of the heater 15P. After the end of this con-tinuous interval which may be 25 minutes, th0 timer relay TRl terminates its operation and its contacts change position. The contact TRlC causes illumination of the ready light RLl. The contacts TRlA interrupts power from the board 50 and the contact TRlB now couples power to the plate heater 15P by way of line 52 from contact CRlB operated from the contactor CRl.
The repeat cycle timer may be a timer having equal 1 minute intervals as in the embodiment of FIG. 2 or may be adjusted to different types of intervals preferably operated at a 50% duty cycle to provide equal overall heat-ing. During one portion of the cycle the contact RCT isopen and during another portion the contact RCT is closed.
When the contactors CRl and CR2 are operated their associated contacts CRlA and CR2A are closed thus coupling power to the appropriate trays such as tray #4. Thereafter, when the contactors CRl and CR2 fall out then the opposite contacts CRlB and CR2B are closed to provide power to the - alternate trays. Thus, if all trays are in position, then essentially half of the trays have heating power applied thereto during one portion of the period and then the other half have power applied thereto during a second portion of the period of the repeat cycle timer. This flip-flop type action continues and the ready lights remain illuminated.
However, it is understood that this repeat cycle timer operation only commences after the continuous portion of the cycle as defined by each of the individual time delay relays associated with each tray. In FIG. 4 only two tray positions are shown and these relays are identified as time delay relays TRl and TR4.
If, for example, the tray #l position is operated and thereafter the tray #4 position is operated, the 25 minute ~3~55~2 intervals will be the same hut will have commenced at different periods of time. At the end of each interval the cycling operation continues so that at the end of the con-tinuous (25 minute) interval the heating will be either interrupted or maintained for a short period of time depend-ing upon the particular state of the repeat cycle timer at that time. However, once the continuous mode is terminated, sometime soon thereafter the control in accordance with the repeat cycle timer will take over.
In the embodiment of FIG. 2 after all of the meals have been served, the load/operate switch 34 may then be changed to its "load" position in order to reset ~he control system including the time delays and the repeat cycle timer. When switch 34 is moved to this position, the power is interrupted to the timer RCT. FIG. 4 shows an alternate form of opera-tion and control which provides an essentially automatic resetting conditioned upon tray removal. In other words, the operation taught in FIG. 5 provides for a termination of the "operate" condition when all of the trays have been served.
In FIG. 5 similar reference characters are used to indentify similar components with regard to the circuitry shown in FIG. 2. The circuit arrangement of FIG. 5 is somewhat different than parts of the circuit arrangement of FIG. 2 but the principle of operation is basically the same.
In FIG. 2 the switch 34 coupled to the opposite polarity side of the source as the on/off switch 40. However, in FIG. 5 it is noted that both the switch 38A and the switch 40A couple directly in series form, in this case, the positive terminal of the battery. The switch 40A is a door interlock switch and the switch 38~ is the on-off switch.
These switches couple to one of the fixed terminals of momentary switch 34A. The other side of switch 34A couples to relay Pl. A contact FlC of relay Fl couples across the momentary switch 34A and functions as a holding contact so ~3~5~Z

that even when the momentary switch is released, a holding path exists through this contact for the relay Fl. One side of the switch 34A also couples to the RCT timer as in an embodiment of FIG. 2.
FIG. 5 also depicts one of the tray positions including the heaters l5B and 15P. When ~he tray is in place, the conductive short 25A provides a path from, in this case, the negative terminal OI the battery to the cathode of diode DN.
Thus, if any one of the trays is in place~ its associated conductive short 25A provides a path by way of its associated diode for the relay Fl. This condition will be maintained as long as there is at least one tray that has not yet been taken from the cart and served. Once the last tray is served then all of the trays are open-circuited and the relay Fl falls out releasing itsassociated contact FlC thus interrupting power to the RCT
timer and the time delays such as the time delays TDl and TD2 shown in FIG. 2.
Having described a limited number of embodiments of this invention, it should now be apparent to those skilled in the art that numerous modifications can be made in these embodiments without departing from the sCQpe of this invention. For example, in place of some of the electro-mechanical means shown herein, electronic circuit means can be used. For example, SCR circuits may be used in place of certain of the relays shown herein. Also, many different battery arrangements may be used and the polarities thereof may even be reversed with regard to the embodiment of PIG. 2 while of course, still maintaining the same potential dif~erence across the heaters such as heaters 15B and 15P o~ FIG. 2.
What is claimed is:

Claims (16)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:-

1. A mobile food serving system for rethermalizing food and maintaining food temperature after rethermalization, said system comprising:
mobile housing means having at least one tray supporting rack, an electrical energy source carried by and movable with the housing means, a tray removably supported on the rack and having a supporting surface for food containers, at least one food heater associated with the supporting surface and in thermal contact with a surface of the tray for good heat transfer thereto, selectable means in series with said heater for enabling coupling of energy source power to the heater, and control means coupled from the energy source for controlling food heating and temperature, said control means including manual mode control switch means having opposite mutually-exclusive single positions including a normal position associated with a normal heating period of the food, and a rethermalization position associated with a rethermalization period, means responsive to the normal operation position for coupling operating power to the food heater at a continuous rate with the switch means set at and maintained at said single normal position throughout said normal operation position, and first means responsive to the rethermalization position and includ-ing first timing means for coupling operating power to the food at a continuous rate for a predetermined interval of sufficient length to rethermalize the food at a first tempera-ture range and second means responsive to termination of the predetermined interval for thereafter operating the heater at a lower power level to maintain the food heated to a second temperature range lower than the first temperature range, said switch means being set at and maintained at said single re-thermalization position throughout both portions of said rethermalization period.

2. A system as set forth in claim 1 wherein said second means includes cyclic operating means including a repeat timer having an "on" period and an "off" period.

3. A system as set forth in claim 2 including a plurality of heaters each associated with a tray upon which the meal is disposed and separate selectable means for each heater wherein said repeat timer operates some heaters "on"
while other heaters are "off".

4. A system as set forth in claim 1 wherein said source includes a rechargeable battery and further including external power receive means and means responsive to said external power for coupling said external power to said heater.

5. A system as set forth in claim 1 including a plurality of heaters each having a separate control line and separate selectable means including a switch means for selecting each heater for operation.

6. A system as set forth in claim 5 including a group of at least two heaters associated with a tray upon which the meal is disposed, each heater of the group having a switch means for selection thereof.

7. A system as set forth in claim 5 wherein the heaters are operated after the predetermined time interval at a fifty percent duty cycle.

8. A system as set forth in claim 5 wherein said control means includes means defining a first path to a heater for continuous operation thereof and means defining a second path to the heater for cycling operation thereof.

9. A system as set forth in claim 8 including a timer means associated with each heater and for controlling use of one of said paths.

10. A system as set forth in claim 9 including ready light means associated with each heater.

11. A system as set forth in claim 6 including means for operating one of the heaters of a group continuously when selected and for operating the other heater of the group first continuously and thereafter cyclically.

12. A system as set forth in claim 5 including means for sensing a tray in position to receive power when so selected and means responsive to an absence of any tray being in power receiving position for terminating the cyclic opera-tion.

13. A system as set forth in claim 1 wherein said heater is a PTC heater, said first temperature range is on the order of from 160°F. to 180°F., and said second temperature range is on the order of from 140°F. to 160°F.

14. A system as set forth in claim 1 wherein the top of the second range coincides with the bottom of the first range.

15. A system as set forth in claim 1 wherein said housing means is in the form of a motor powered cart.

16. A system as set forth in claim 1 including a second switch substantially in parallel with the first switch and having a first power interrupting position for the purpose of loading trays and a second operate position for coupling energy to the one switch.