Patent Publication Number: US-7895999-B2

Title: Cooking appliance

Description:
This application is a continuation application of National Phase patent application filing Ser. No. PCT/NZ2005/000171, having an International filing date of Jul. 13, 2005, which claims priority of NZ534091 having a filing date of Jul. 13, 2004 and U.S. provisional application Ser. No. 60/621,001 having a filing date of Oct. 21, 2004. 
     FIELD OF THE INVENTION 
     The invention relates to improvements to cooking appliances and in particular gas cooktops. 
     BACKGROUND 
     There are a number of methods known in the art of providing heat in a cooktop. One preferred method is to use a gas burner which is able to deliver high levels of heating and which responds fairly quickly to desired changes in level. An example of a typical gas burner is described in WO 01/50065. It is typical that the finest level of control will be desired at the lower levels of output range for various cooking duties. In order to achieve good level control, various fuel gas flow control valves have been proposed such as those described in U.S. Pat. No. 5,009,393 and WO 01/33118. 
     In a typical gas cooktop a trivet is provided to support a cooking vessel above the gas burner and attempts have been made previously to accommodate various shapes of cooking vessels. Examples of trivet arrangements are described in U.S. Pat. No. 6,588,417, WO 02/066899 and U.S. Pat. No. 5,819,719. For gas cooktops, the combustion of fuel gases requires clearance under the cooking vessel to allow flow of the combustion and exhaust gases, which is provided by a trivet to support the cooking vessel the correct distance above the gas flame. A trivet is usually constructed of cast iron or enamel coated steel and comprises a number of narrow prongs to limit interference with the flame and upon which the cooking vessel may rest in a horizontal plane. 
     Trivets and burner components comprise many complex shapes and surfaces which can make cleaning more difficult. These structures are also visually complex. Il order to aid with cleaning, it is known to provide gas heating appliances having removable trivets and removable burner components. However in some cases it may be possible for a user to re-assemble these components incorrectly, which can lead to instability of the cooking vessel and/or incorrect operation of the burner. Incorrect assembly or operation of the gas cooktop components may be hazardous. 
     Further, various constructions of gas burners and burner rings are also disclosed in the prior art. Prior art burners generally have flame outlet openings which are formed as slots, grooves or bore holes which are generally directed outwardly in approximately a radial direction. Fuel gas is supplied through the burner body and exits through the burner ports where it is combusted forming a ring of flames which are used to heat cooking vessels. The efficiency of conventional gas burners is limited by the need to maintain sufficient clearance around the burner head to allow the flame to draw in enough of the surrounding air to achieve complete combustion of the fuel gas. Due to the clearance between the burner head and the cooking vessel, much of the flame has passed the hottest phase of combustion by the time it contacts the surface of the cooking vessel. Much of the flame heat diffuses into the surrounding mass of flowing gases such that the temperature difference between these flowing gases and the surface of the pot is reduced, which in turn reduces the rate of heat transfer to the cooking vessel. 
     The operating range of conventional cooktop gas burners is limited to the performance range of the venturi and the burner ports. The venturi uses the velocity of the fuel gas flowing through a small orifice to draw in an approximately proportional volume of air as required for primary combustion. The fixed geometry of the venturi and fuel gas jet limit the range over which this type of burner will operate. Similarly, the range of operation of burner ports is a function of their cross sectional area and the ability of flame to stay attached to the burner port against the flow of the gases when the burner is at the upper end of its operating range. For these prior art burners, burn back velocity and heat transfer between the flame and the burner head provides the lower limit of the operating range of the port structure by extinguishing the flame. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a cooking appliance or parts therefore which is relatively easy to clean, and/or is visually simple and/or at least provide the public with a useful choice. 
     In a first aspect the invention can broadly be said to consist in a cooking appliance comprising: 
     a substantially horizontal cooktop surface, 
     at least one aperture in said cooktop surface, 
     at least one cooking vessel support, each said support including at least one support location for contacting a cooking vessel, each of said vessel supports passing through a said aperture, 
     said vessel supports moveable between at least a first operating condition and a second condition, wherein 
     in said first operating condition said support locations are located at a first level above said cooktop surface, and wherein, 
     in said second condition said support locations are located at a second level relative to said cooktop surface, said second level being different to said first level, and 
     there are at least three support locations for stably supporting a said cooking vessel above said cooktop surface with said supports in the first operating condition. 
     Preferably when in said second condition said support locations are located at a level substantially flush with said cooktop surface. 
     Preferably said vessel supports are moveable between more than two conditions, each said condition corresponding with a different level of said support locations relative to said cooktop surface. 
     Preferably said supports are moveable to any condition between an upper limit condition where said support locations are located at the highest level above said cooktop surface and a fully retracted condition in which said support locations are at least substantially flush with said cooktop surface. 
     Preferably said gas heating appliance includes a plurality of said apertures in said cooktop surface and a plurality of said cooking vessel supports. 
     Preferably said cooktop surface is substantially planar. 
     Preferably said apertures are arranged regularly on at least one pitch circle. 
     Preferably said gas heating appliance includes a gas burner located substantially concentrically within said circle. 
     Preferably said burner includes at least one aperture in said cooktop surface and a burner head substantially fitting said aperture in said surface and moveable between a position wherein the surface of said burner cap is at least substantially flush with said cooktop surface and a position wherein the surface of said cap is displaced from said cooktop surface. 
     Preferably said cooking vessel supports are constrained such that they all move together between said conditions. 
     Preferably said cooking vessel supports are substantially solid cylindrical in shape. 
     Preferably said support locations are located on a substantially horizontal planar surface, such that said support locations are co-planar with said cooktop surface when in said retracted condition. 
     Preferably said appliance includes an actuator for driving said cooking vessel supports between said conditions. 
     Preferably said appliance includes a controller controlling the supply of power to said actuator, and a user interface, said controller receiving input from said user interface and controlling the supply of power to said actuator as a function of at least one of: 
     inputs from said interface, 
     feedback from said actuator, and 
     signals derived from said vessel supports. 
     Preferably said controller drives the actuator to raise the vessel supports in response to a user operating the user interface to indicate activation of a burner. 
     Preferably said controller causes the actuator to raise the supports on detecting contact of a conductive surface across a plurality of said support locations, and/or lower said supports following removal of such a conducting surface from said support locations. 
     Preferably after a predetermined delay, or after the controller has determined, by sensing or estimation, that said supports have cooled to a touch safe temperature, the controller causes the actuator to lower said supports to said second condition. 
     Preferably said controller operates the actuator between physically fixed upper and lower limits and removes power upon detecting the actuator reaching those limits. 
     Preferably said actuator includes: 
     a first member at least substantially rigidly fixed relative to said cooktop surface, below said cooktop surface, 
     rotating member adjacent said fixed member and rotatable relative to it in a plane substantially parallel with the plane of said cooktop surface, with one or more actuating profiles that act between said rotating member and said supports, by rotation of said member, to move said supports between said conditions, and 
     a rotation actuator to drive rotation of said rotating member relative to said fixed member. 
     Preferably said rotation actuator is configured to drive said rotating member in one direction to raise said supports, and in the reverse direction of rotation to lower said supports. 
     Preferably said actuator includes a structure below said cooktop surface and adjacent said rotating member, said structure rigidly coupling all said supports, said structure being raised and lowered relative to said cooktop surface by rotation of said rotating member. 
     Preferably said structure is constrained to move vertically relative to said fixed member. 
     Preferably said structure is constrained to move only vertically relative to said fixed member. 
     Preferably said rotating member is constrained to only rotate relative to said fixed member. 
     Preferably said support locations are arranged on a circle and said rotating member is mounted to rotate concentric with said circle. 
     Preferably said rotating member includes a plurality of ball slide engagements with said fixed member, said ball slide engagements with said fixed member stably supporting said rotating member relative to said fixed member and within said fixed member so as to allow rotation of said rotating member but no vertical movement. 
     Preferably said structure includes a plurality of ball slide engagements with said fixed member so as to concentrically locate said structure within said fixed member and constrain relative movement of said structure to a vertical movement with no rotation. 
     Preferably there are a plurality of ball slide engagements between said rotating member and said structure, said ball slide engagements each including a ramp section causing relative vertical displacement between said rotating member and said structure by relative rotation therebetween. 
     Preferably said rotation actuator comprises a lead screw and threaded journal, each connected to one or other of said fixed member and said rotating member, and an electric motor connected to drive said lead screw. 
     Preferably said threaded journal is connected with said rotating member and said threaded journal and said lead screw are pivotally connected to their respective components. 
     Preferably said actuator includes a first member at least substantially rigidly fixed relative to said cooktop surface, below said cooktop surface, and a structure below said cooktop surface and adjacent said first member, said structure rigidly coupling all said supports, said structure being raised and lowered relative to said cooktop surface in actuation, 
     said structure being at least substantially annular, and said fixed member accommodating the burner within said substantially annular structure. 
     In a further aspect the invention can broadly be said to consist in a cooking appliance including: 
     gas burner located substantially concentrically within said circle. 
     Preferably said burner comprises: 
     a first gases flow passage including an inlet and an outlet, 
     a second gases flow passage including an inlet and an outlet, at least one fuel gas jet substantially aligned with said inlet of said second passage, 
     a flame locating means within said second passage, 
     a source of oxidising gases at said inlet of said first passage, 
     a source of oxidising gases at said inlet of said second passage, and 
     said outlet of said first passage proximate to said outlet of said second passage. 
     Preferably said inlet of said first gases flow passage and said inlet of said second flow passage are in fluid communication with at least one pressurised gases supply. 
     Preferably the majority of pressurised gases from said pressurised gases supply flows through said first gases flow passage. 
     Preferably said pressurised gases supply is provided by at least one fan. 
     Preferably said burner includes a plenum chamber receiving air from said fan and a burner body having at least an annular end portion projecting into said plenum chamber with an annular air inlet receiving air from said plenum chamber, said burner body being divided into said first gases passage way and said second gases passage way. 
     Preferably said body is divided by a vertically oriented cylindrical tube mounted concentrically within said body, with a tower open end of the tube spaced from the floor of said chamber above said fuel jet. 
     Preferably said burner further comprises a burner head spaced from said outlet of said first flow passage, said burner head extending substantially transversely to said outlet of said first flow passage. 
     Preferably said outlet of said second flow passage is located substantially within said first passage and said outlet of said second passage is in fluid communication with said outlet of said first passage. 
     Preferably said fuel gas jet is spaced from said inlet of said second gases passage, and said jet is in fluid communication with said inlet of said second passage. 
     Preferably said first passage is partially closed, at an inlet end, and said inlet of said first passage comprises of a plurality of apertures toward said inlet end of said first passage. 
     Preferably said apertures are radially spaced slots extending axially with respect to said first passage. 
     Preferably said second flow passage is located concentrically within said first flow passage. 
     Preferably said burner further comprises a movable burner head having a first extended operating condition and a second, retracted non-operating condition, 
     said cap extending substantially transverse to said outlet of said first flow passage. 
     Preferably with said head in said first condition, said head is spaced from said outlet of said first flow passage, and in said second condition said head substantially closes said outlet of said first flow passage. 
     Preferably said burner is located in a substantially horizontal cooktop surface having at least one aperture, and said burner cap substantially fits said aperture in said cooktop surface and is movable between a position wherein the top surface of said cap is at least substantially flush with said cooktop surface and a position wherein said cap is displaced from said cooktop surface to leave an annular opening to said outlet of said first flow passage. 
     Preferably one or more of user controls, burner and cooking vessel supports, rise up from the planar surface of the cooktop for use and retract to provide a substantially planar surface when not in use. 
     In a further aspect the invention can broadly be said to consist in a cooking vessel support assembly comprising: 
     a frame for securing to the underside of a cooktop, 
     at least one cooking vessel support, each said support including at least one support location for contacting a cooking vessel, 
     said vessel supports moveable between at least a first operating condition and a second condition, wherein 
     in said first operating condition said support locations are located at a first level above said frame, and wherein, 
     in said second condition said support locations are located at a second level relative to said frame, said second level being different to said first level, and 
     there are at least three support locations for stably supporting a said cooking vessel above said frame with said supports in the first operating condition. 
     Preferably said vessel supports are moveable between more than two conditions, each said condition corresponding with a different level of said support locations relative to said frame. 
     Preferably said supports are moveable to any condition between an upper limit condition where said support locations are located at the highest level above said cooktop surface and a fully retracted condition in which said support locations are at least substantially flush with said cooktop surface. 
     Preferably said vessel supports are arranged regularly on at least one pitch circle. 
     Preferably said assembly includes a gas burner located substantially concentrically within said circle. 
     Preferably said cooking vessel supports are constrained such that they all move together between said conditions. 
     Preferably said appliance includes an actuator for driving said cooking vessel supports between said conditions. 
     Preferably said actuator includes: 
     a rotating member adjacent said frame and rotatable relative to it in a plane substantially parallel with the plane of said cooktop surface, with one or more actuating profiles that act between said rotating member and said supports, by rotation of said member, to move said supports between said conditions, and 
     a rotation actuator to drive rotation of said rotating member relative to said frame. 
     Preferably said rotation actuator is configured to drive said rotating member in one direction to raise said supports, and in the reverse direction of rotation to lower said supports. 
     Preferably said actuator includes a structure and adjacent said rotating member, said structure rigidly coupling all said supports, said structure being raised and lowered relative to said cooktop surface by rotation of said rotating member. 
     Preferably said structure is constrained to move vertically relative to said frame. Preferably said structure is constrained to move only vertically relative to said frame. 
     Preferably said rotating member is constrained to only rotate relative to said frame. 
     Preferably said support locations are arranged on a circle and said rotating member is mounted to rotate concentric with said circle. 
     Preferably said rotating member includes a plurality of ball slide engagements with said frame, said ball slide engagements with said frame stably supporting said rotating member relative to said frame and within said frame so as to allow rotation of said rotating member but no vertical movement. 
     Preferably said structure includes a plurality of ball slide engagements with said frame so as to concentrically locate said structure within said frame and constrain relative movement of said structure to a vertical movement with no rotation. 
     Preferably there are a plurality of ball slide engagements between said rotating member and said structure, said ball slide engagements each including a ramp section causing relative vertical displacement between said rotating member and said structure by relative rotation therebetween. 
     Preferably said rotation actuator comprises a lead screw and threaded journal, each connected to one or other of said frame and said rotating member, and an electric motor connected to drive said lead screw. 
     Preferably said threaded journal is connected with said rotating member and said threaded journal and said lead screw are pivotally connected to their respective components. 
     Preferably said actuator includes a structure adjacent said frame, said structure rigidly coupling all said supports, said structure being raised and lowered relative to said frame in actuation, 
     said structure being at least substantially annular, and said frame accommodating the burner within said substantially annular structure. 
     In a further aspect the invention can broadly be said to consist in a cooking appliance including a planar cooktop surface wherein one or more of user controls, burner and cooking vessel supports, rise up from the planar surface of the cooktop for use and retract to provide a substantially planar surface when not in use. 
     In a further aspect the invention can broadly be said to consist in a cooking vessel support assembly comprising: 
     a first gases flow passage including an inlet and an outlet, 
     a second gases flow passage substantially concentric with said first flow passage and having an inlet and an outlet, 
     at least one fuel gas supply injecting fuel gas at a controlled rate to flow through said second flow passage, 
     said fuel gas when ignited forming a flame within said second passage, 
     said flame when said burner is in a low power setting extending downstream toward said outlet of said second flow passage and being substantially within said second flow passage, and 
     said flame when said burner is in a high power setting extending downstream through said outlet of said second flow passage and beyond said outlet of said first passage. 
     In a further aspect the invention can broadly be said to consist in a cooking vessel support assembly comprising: 
     an annular exit, 
     a first combustion zone adjacent said annular exit, 
     a second combustion zone separated from said annular exit by said first combustion zone, 
     a flame front locator in said second combustion zone, 
     a gas mixture supply delivering a mixture of combustion fuel gases and air to said second combustion zone, and 
     an air supply delivering a flow of air to said first combustion zone. 
     To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the drawings in which: 
         FIG. 1  is a perspective view of a cooktop of a gas heating appliance according to an embodiment of the present invention showing the cooling vessel supports and burner head extended. 
         FIG. 2  is a perspective view of the cooktop of  FIG. 1  showing the gas burner head and cooking vessel supports retracted. 
         FIG. 3  is a perspective view of the gas heating appliance of  FIGS. 1 and 2 , showing a cooking vessel supported in use on the appliance. 
         FIG. 4  is a perspective view of the underside of a cooktop according to an embodiment of the present invention shown with the cooling vessel supports extended, by a lever lifting mechanism. 
         FIG. 5  is a perspective view of the underside of the cooktop of  FIG. 4  shown with the cooking vessel supports retracted. 
         FIG. 6  is a perspective view of the underside of a ball cam lifting mechanism according to a further lifting embodiment shown with the cooking vessel supports extended. 
         FIG. 7  is a perspective view of the under side of a ball cam lifting mechanism according to a further embodiment, shown with the cooking vessel supports extended. 
         FIG. 8  is a perspective view of the underside of the mechanism of  FIG. 7  showing the cooking vessel supports retracted. 
         FIG. 9  is a perspective view of the stationary support ring of the mechanism shown in  FIGS. 7 and 8 . 
         FIG. 10  is a perspective view of the rotating support ring of the mechanism shown in  FIGS. 7 and 8 . 
         FIG. 11  is a perspective view of the cooking vessel support mounting ring of the mechanism shown in  FIGS. 7 and 8 . 
         FIG. 12  is a perspective view of the top side of the ball cam lifting mechanism of  FIGS. 7 and 8 , showing the cooking vessel supports and a burner head extended. 
         FIG. 13  is a cross sectional view of a burner according to an embodiment of the present invention. 
         FIG. 14  is a partially cut away perspective view of the gas burner and cooktop of  FIG. 13 . 
         FIG. 15  is a schematic view of a gas burner according to an embodiment of the present invention shown with the flame at a high power setting. 
         FIG. 16  is a schematic view of a gas burner according to an embodiment of the present invention shown with the flame at a low power setting. 
         FIG. 17  is a cross sectional view of a burner according to a further preferred embodiment of the present invention. 
         FIG. 18  is a perspective view of a lifting mechanism according to an embodiment of the present invention shown in  FIG. 6 . 
         FIG. 19  is a perspective view of two further alternative preferred embodiments of flame front locator. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout the description reference is made to the accompanying figures which are labelled with numerals in order to more clearly describe the invention. While various different embodiments are described and illustrated representing various combinations of features, where possible like reference numerals have been used across different embodiments to illustrate similar or shared components. In one aspect the present invention provides an easily cleaned gas cooktop surface. The surface is not cluttered by a traditional trivet and can thereby be used for other purposes when not in use for cooking. The cooktop surface is preferably substantially planar but may include raised regions (especially around apertures in the cooktop surface) to contain spillage of food or liquids on the spill plane of the cooktop in order to reduce the potential for spillages to leak into the appliance. It will be readily appreciated that cooktop surfaces usually include multiple burners, which may be of various sizes, types and/or configurations. Such configurations are to be understood as being within the scope of the present invention. The description and Figures following, describe a gas heating appliance having a cooktop surface with a single gas burner, by way of illustration only. 
     With reference to  FIG. 1 , the cooktop consists of a planar cooktop surface  1 , with a plurality of cooking vessel supports  2  and burner head  3  in a first operating condition. The cooling vessel supports are preferably regularly spaced around burner head  3  on one or more pitch circles. In order to provide stable support for the cooking vessel, there are preferably at least three spaced support points for contacting the cooking vessel. In one preferred embodiment, five support locations  4  are provided each by an individual cooking vessel support  2 , extending upwards in a direction substantially normal to planar cooktop surface or spill plane  1 . The extending cooking vessel supports  2  are adapted to support a cooling vessel an appropriate distance above the burner head  3 , and it will be appreciated that the vessel supports  2 , are inherently suitable for providing stable support to a curved or partially curved bottom cooking vessel such as a wok as well as conventional flat bottomed vessels. Burner head  3  may also be operable to provide additional support for the cooking vessel on at least part of its upper surface. 
       FIG. 2  shows the cooktop of the gas heating appliance in a second operating condition, wherein the cooking vessel supports  2 , and burner head  3 , are retracted so that they are substantially flush with the cooktop surface  1 . In this position, when the burner is not being used, the substantially flat planar surface is easy to clean and not visually complex. The retracting burner and cooking vessel supports also allow the cooktop surface to be used as a flat bench top when the burner is not in use. The cooktop surface or burner head may also include a lip around the burner orifice to prevent spills entering and therefore may be substantially flat and not perfectly planar. In the case where an upper support surface of the burner head  3  contacts the vessel  14 , to provide additional support to the vessel, it is important that the vessel is supported the appropriate distance from the flame in order to allow efficient heating. The relative sizes of components and the distances of the flame from the support surfaces will depend on the type of gas burner, and intended use. 
     Preferably the retracting/extending mechanism is automatically driven from below the cooktop surface of the appliance, by a mechanical lifting mechanism including an actuator. Alternatively, the actuator for retracting and/or extending the lifting mechanism of the burner cap and/or cooking vessel supports may be electro-mechanical, hydraulic, pneumatic or operated manually. It is envisaged that both the cooking vessel supports  2  and the burner head  3 , may retract and extend, so that the cooktop surface is completely flat or substantially flat when retracted. Alternatively, only one of the burner head  3  or trivet supports  2  may be actuable to retract and extend. 
     It may be desirable to provide the gas appliance with gas controls located on or in the cooktop surface  1 , which are also actuable to retract and extend relative to the planar cooktop surface  1 . Embodiments wherein all of the moveable components (cooking vessel supports  2 , the burner head  3  or gas controls (not shown)), retract so as to be substantially flush with the planar cooktop surface  1 , result in a cooktop surface which is substantially planar and can be cleaned by wiping down, just as a flat bench top would be. Alternatively, the gas controls may be provided on a surface other than the cooktop surface or may be provided as electronic touch controls which are flush to the cooktop surface. Where the gas controls are moveable, it is envisaged that the lifting mechanism of the burner components may also be used to lift the associated gas controls. Alternatively, the lifting of each burner control may be independent from the other burner controls, and may utilise independent lifting mechanisms from the burner components. 
     In order to aid with cleaning the planar cooktop surface  1 , when the burner head  3  and cooling vessel supports  2  are retracted, the clearance between the retracting/extending elements and the apertures in the cooktop surface which receive them, is preferably as small as practicable. A relatively tight fit between the retracting/extending members and the apertures in the cooktop surface is preferable to minimise the gaps in which food and/or spillages may become trapped and difficult to wipe clean. Bushes may be provided between the cooktop surface and the moveable elements to aid the movement as the components extend and retract. Referring to  FIG. 19 , bushes  32  may also be preferable in order to improve sealing between the vessel supports and cooktop surface. Alternatively a ‘chassis’ in the form of a sheet metal plate may be glued to the underside of the glass cooktop surface  1 . This plate includes threaded inserts to which the lifter mechanism and burners (such as shown in  FIGS. 7-12 ) can be mounted. The bushes in this case may include a flange that is trapped between the glass cooktop surface and the plate to provide a bearing surface extending through the apertures in the cooktop. 
     There are many options suitable for initiating or triggering the extension and/or retraction of the moveable elements of the cooktop of the present invention. Further, it will be appreciated that many of these options are suitable to be employed individually and/or in combination to achieve different desirable effects. For example, the raising of the pot supports and/or gas burners (and/or controls) may be triggered by an electronic touch control or switch, or by the first action of the gas control knob. Where the gas controls are also retractable, electronic touch controls may be used for at least the first stage of operation i.e. raising the gas controls. Further, the retraction of the pot supports  2  (when switched off) may be activated by the last action of the gas control knob or via an electronic touch control or switch. It is envisaged that the gas burners may be fitted with an automatic igniter such as hot surface igniter or spark igniter as is well known in the art. It is envisaged that the automatic start may be configured to ignite the burner automatically once the burner head and pot supports are extended. In order to achieve this, a time delay or a limit switch may be utilised. Alternatively, the igniter may be operated manually as is well known in the art. Further, it is envisaged that a flame detection means may also be incorporated into the cooktop to make sure that unburnt gases do not escape and endanger the user if the flame is extinguished. Flame supervision methods to operate auto reignition and safety shut off functions may be incorporated into the cooktop via flame rectification and/or thermocouples which are well established methods in the art. 
     The retraction and/or extension of the pot supports, burner head(s) and/or the gas control knob(s) may also incorporate a time delay where appropriate, so that the various steps occur in a pre-defined sequence. For example, after the cooktop burner is extinguished, the gas burner head and/or cooking vessel supports and/or gas controls may remain extended for a time period to allow cooling. It is envisaged that the time period may be controlled by temperature sensors or alternatively may be a predetermined or calculated time. The cooktop may also include a sensor in order to determine if a cooking vessel is positioned on the vessel supports over a burner so that retraction and/or extension of the vessel supports  2  may be conditional on the presence or absence of a cooking vessel. The cooking vessel proximity sensors may function in a number of ways, for example, it may sense force or be activated by force applied by the weight of the cooking vessel on the lifting mechanism or alternatively may operate via electrical contact points which utilise the cooking vessel to complete a circuit. Alternatively, induction may be used to sense the presence or absence of a cooking vessel. The cooking vessel proximity sensors may also include an override in order to accommodate unusual cooling situations where this feature may not be desirable for any reason. 
     It is envisaged that gas appliances having multiple burners in the cooktop surface for multiple cooling vessels may be operated together, or separately, or in subgroups. Each of the burners, vessel supports, and/or controls may be extendable/retractable independently or in combination with each of the other burners. 
       FIGS. 4 and 5  show the underside of the gas cooktop where a retracting/extending mechanism can be seen. The cooktop includes five cooling vessel supports  2 , arranged on a single pitch circle. Cooking vessel supports  2  are fixed to a support frame  5  which constrains all of the cooking vessel supports  2  to move together. Movement of these cooking vessel supports  2 , in unison, is desirable so that the cooking vessel cannot be tilted or supported unevenly. The cooling vessel supports have a first extended operating condition and a second retracted operating condition as previously described. Guide frame  6  is provided mounted to the under body of the gas burner housing  7 . The cooking vessel supports  2  are constrained from movement other than along their main axis, by cooktop aperture guides  8  and the guide frame  6 . These guides may include temperature resistant bushes. Movement of the cooking vessel supports  2  in their axial direction is achieved by movement of the support frame  5  relative to the cooktop surface  1 .  FIG. 4  shows support frame  5  in its upper position which corresponds to the cooking vessel supports extended position, substantially as shown in  FIG. 1 .  FIG. 5  shows the support frame  5  in its lower position which corresponds to the retracted position of the cooking vessel supports  2 , substantially as shown in  FIG. 2 . When in their retracted position, the lower ends of cooking vessel supports  2  project downwards from the guide frame  6 . A servo motor  9  is provided with levers  10  rigidly fixed to either side of its shaft for moving the vessel supports  2  between their operating conditions (only one side shown). Actuation of the server motor  9 , rotates the lever  10 , which is coupled to the support frame  5  via a pin  13  engaged within a slot  11  on the lever  10 . Rotation of lever  10 , moves the support frame and its associated vessel supports  2  along their axes between their first and second operating conditions. Pin  13  is further engaged in vertical slot  12  in the gas burner housing  7 . Slot  12  extends parallel to the axis of the cooking vessel supports  2  and constrains the motion of the support frame  5  and vessel supports  2  vertically. This lifting mechanism allows the rotation of the servo motor shaft to retract and extend the cooking vessel supports  2 , via the rotation of the slotted lever  10 . 
     Alternatively, it is envisaged that each of the vessel supports  2  may be actuated independently via a simple linear actuator. Preferably such an actuator would also include a failsafe to prevent collapse and/or tipping of the vessel supports in the event of a fault condition, in one or more vessel supports  2 . 
     A, alternative preferred method of driving the vessel supports and/or burner cap  13  and/or burner controls will now be described with reference to  FIGS. 7-12 . The ball cam lifting mechanism of a preferred embodiment consists of a stationary support ring  19  mounted underneath the cooktop surface of the gas heating appliance. A rotating support ring  20  is mounted inside the stationary support ring  19  and surrounding the cooking vessel support mounting ring  21 . The three rings  19 ,  20  and  21  are interconnected by three sets of three steel ball bearings which run in respective slots between the interconnected rings to constrain their respective relative movements. A lead screw and actuator  22  is connected between stationary support ring  19  and rotating support ring  20  through pin joins  23  and  24 . 
     Movement of the lead screw  25  drives rotation of the rotating support ring  20  with respect to the stationary support ring  19 . This motion is constrained by a ball bearing acting in each of three pairs of cooperating slots  26 ,  29 . In turn, rotation of the rotating support ring  20  results in translation of the cooking vessel support mounting ring  21  along its axis (vertically) via interaction with a ball bearing engaged in each of three respective pairs of angled slots  33 ,  34  in the mounting ring  21  and rotating support ring  20  respectively. Rotation of the rotating support ring  20  via lead screw actuator  22 , enables the cooling vessel support mounting ring  21  which includes a plurality of cooling vessel supports  2  to extend and retract the cooking vessel supports  2  with respect to the cook top surface  1 . 
     With reference to  FIG. 11 , cooking vessel support mounting ring  21  includes five (one for each cooking vessel support  2 ) radially inwardly extending fingers  35 . Each inwardly extending finger  35  is adapted to receive at least one cooling vessel support (not shown in  FIG. 11 ). With reference to  FIG. 7 , it can be seen that each of the extending fingers  35  includes three holes  15  suitable for mounting cooking vessel supports  2  in order to accommodate varying pitch circles which may be desirable for cooking vessels of varying sizes. Vessel supports  2 , are secured in position by a cir-clip  16 . Alternatively, other suitable removable or permanent fastening means can be used such as threading, swaging, welding, press-fitting or clipping. For example, a socket fitting screwed into a thread on the lifting mechanism could be used which receives a ball end on the respective pot support  2 . This releasable ball and socket mechanism allows the coupling to pull apart if the pin  2  becomes stuck. In such a case, the stuck support would remain up (extended) and can then be pulled out from above, cleaned and reinserted into the socket. The socket may also be screwed in and/or out further to allow small adjustments the height of individual pot supports. 
     The inner ends  36  of fingers  35 , extend toward the centre of the mounting ring  21 . The ends  36  are adapted to engage with the burner of the heating appliance such that the burner head  3  may also be extendable and retractable (as previously described) via the ball cam lifting mechanism. The size of the central gap  31  at the ends of the fingers  35 , in the middle of the mounting rings  21 , can be varied according to the size of the burner head utilised. For manufacturing purposes, it may be desirable to manufacture one size mounting ring  21 , and machine out the ends of fingers  36  to accommodate larger burner heads. 
     Mounting ring  21 , also includes three angled slots  33  located at regularly spaced intervals on the outer surface of the mounting ring  21 . Three equally spaced vertical slots  37  are also located in the outer surface of mounting ring  21  between angled slots  33 . With reference to  FIG. 10 , the inner surface of rotating support ring  20  includes three regularly spaced angled slots  34  which correspond to the shape of the three angled slots  33  on mounting ring  21 . Rotating support ring  20 , further includes circumferential slot  26 . With reference to  FIG. 9 , support ring  19  includes regularly spaced downwardly projecting portions  27  which have inwardly facing horizontal slots  29 . Between portions  27 , are equally spaced downwardly projecting portions  28  which include vertical slots  30 . 
     For assembly purposes, at least one of the pairs of co-operating slots in components  19 ,  20 ,  21  which receive a ball bearing, are open ended. In use, a ball bearing is located in each of the three slots  29  on the stationary support ring  19  which engage with slots  26  on the outer surface of rotating support ring  20 . The interaction between the ball bearings and slots  26 ,  29  constrain relative vertical movement allowing the rotating support ring  20  to rotate (coaxially with stationary support ring  19 ) under action of lead screw  25 . In the event of a failure of the lead screw actuator  22 , the mounting ring  21  (and therefore the equivalent vessel supports  2 ) will not collapse or tip the cooling vessel. When in a fully extended position, the flat (horizontal) portions of sloped grooves  33 ,  34  ensure that collapse will not occur even if lead screw  25  failed. Further ball bearings are located in cooperating slots  34  and  33  on the rotating support ring  20  and mounting ring  21  respectively. The cooperating angled slots  33 ,  34  drive mounting ring  21  to translate axially as the rotating supporting  20  is rotated with respect to the mounting ring  21 . The tendency of the mounting ring  21  to rotate about the central axis is prevented by a further steel ball bearing which interlocks into the stationary support ring  19  via each of three pairs of vertical slots  30 ,  37 . 
     The foregoing describes embodiments of lifter mechanisms which can be used to extend or retract burners and/or other moveable components. It will be appreciated that each embodiment is readily capable of use in conjunction with conventional gas burners (as shown in  FIG. 1 ) or with the burner of another aspect of the present invention described later in relation to  FIGS. 7-12  or  13 - 17 .  FIG. 6  also shows a similar lifter mechanism suitable for use with different types of burner. When used in conjunction with typical prior art burners it is envisaged that a length of flexible tubing be used to deliver fuel gas to the burner nozzle which preferable does not move with respect to the burner body as the unit is extended or retracted. 
     It is also envisaged that other support structures may be desirable for supporting cooking vessels above the gas burner. For example, each burner may be fitted with a support ring, either closed or comprising partial annular segments, in place of the rod shaped vessel supports already described. In an extended position (first operating condition), the ring extends up from the horizontal cooktop surface to a preferred distance above the gas burner, substantially as previously described. The ring or partial rings are adapted to contact the surface of the cooking vessel at at least three points to provide a stable support platform. It will be appreciated that upstand rings (either complete or partial) would also be suitable for curved bottom cooling vessels such as woks. It is envisaged in such a case, that the supporting ring may be extendably/retractably supported above the cooktop surface by more or less than three supports extending through apertures in the cooktop surface. In a retracted position, the upstand ring is preferably substantially flush with the cooling surface, as previously described. For this purpose, the support ring may be recessed into the horizontal planar cooktop surface. The ring may be supported by one or more supports which may be substantially the same as cooking vessel supports  2  previously disclosed. Alternatively, a support ring (or segments) may be fitted over vessel supports  2 , if desired, as an accessory. Similarly it will be appreciated that driving (lifting) mechanisms such as those previously disclosed will be inherently suitable for these variations in cooking vessel supports. 
     In a further alternative embodiment, the position of the vessel supports may also be continuously controlled between the first and second operating conditions in order to vary the height above the gas burner head as desired. In a further alternative embodiment the cooling vessel supports  2  may include a third operating condition which is extended further or closer than the first operating condition. The purpose of this third operating condition is to accommodate a curved bottomed cooking vessel such as a wok. The extra or reduced extension above the normal flat bottomed cooling vessel height, allows the curved bottom cooking vessel to extend downwards to a position higher (or lower) than the contact surfaces of the vessel supports above the burner head  3 . This allows the bottom surface of a wok, for example, to be maintained at a proper distance from the burner head. Variation in the height of the support locations may also provide the capability of finer control of the cooling heat, e.g.: below the normal lowest heat setting of the burner, by changing the proximity of the cooking vessel to the burner head. 
     Whether a manual, electromechanical, hydraulic or pneumatic actuating system is used, it is preferabe that a fail safe mechanism is included so that in the event of a failure of the extending/retracting mechanism the cooking vessel is not tilted, which may result in the hazardous spilling of hot material. It will be appreciated by those skilled in the art that the gas heating appliance of the present invention may be constructed from a number of suitable materials. For example, the cooktop surface may be ceramic glass, metal, or stone. Similarly the cooking vessel supports, lifting mechanisms and burner components can be constructed from combinations of ceramics, metal or other appropriate heat resistant materials. 
     With reference to  FIGS. 7 ,  8  and  12 - 18 , in another aspect the present invention provides a gas burner with a high turn-down ratio and/or improved heat output control. The burner of the present invention reduces the clearance needed between the burner head and the cooking vessel by forcing secondary air up from below with a fan and allowing the second phase of combustion to start close to the surface of the cooking vessel and close to the central axis of the burner. This increases the temperature difference between the combustion gases and the cooking vessel at the stage where heat transfer is taking place, which improves heat transfer. Further, the burner of the present invention may include a burner head  3  which is moveable so it can retract into the cooktop surface  1  for easy cleaning as described in relation to other embodiments of the present invention previously. 
     With reference to  FIG. 13  a gas heating appliance consists of an appliance spill plane or planar cooktop surface  1  with a burner head  3  therein. Beneath the cooktop surface  1  is a base pan  39  which substantially surrounds the gas burner housing  40 . The burner housing is supplied with air, via the appliance housing base pan  39 , by one or more fans  41 . The fan  41  may be any suitable type of fan, for example an axial, radial, centrifugal or positive displacement air pump type. Further, the fan(s) may operate at a constant speed or may be operable at variable speeds depending on the burner type and/or burner settings, and/or the configuration of the burners within the cooktop. It is envisaged that a single fan may be utilised to supply multiple burners with air or alternatively each burner, or groups of burners, may have their own fan and respective base pan. It is also envisaged that the fan  41  may pump air into one or more intermediate chambers or plenums connected to each other, or connected to the appliance housing/base pan, or burner body, by restrictive orifices  42 . The restrictive orifices may comprise a single aperture or slot (for example ruining the full length of the baffle  54 ) or a series of apertures. The restrictive orifices  42  connecting the one or more intermediate chambers may also include excess flow valves or surge flaps  18 , or diaphragms or a laminar flow device, in order to prevent the flame from being affected or extinguished by opening or closing cupboards of the kitchen cabinet which the gas appliance may be fitted to. An alternative preferred arrangement is shown in  FIG. 17 . In this embodiment there is a baffle  54  between the fan chamber and the base pan  39 . Air flow from the fan  41  enters the base pan  39  through gap  55  between the top of the baffle and the underside of the cooktop surface  1 . The tortuous path has been found to result in adequately even air flow into the burner body. 
     Air is forced into the base of the burner body housing  40 , optionally through a series of air induction orifices. The air induction orifices may be provided to help the airflow into the base portion of the burner housing  40  to be more evenly distributed. The air induction orifices (if present) are preferably evenly spaced slots  44 , as shown in  FIG. 13 . Alternatively the air induction orifices may be holes or may be in the form of a wire mesh or the like. The slot shape of the air induction slots  44  allow the burner of the present invention to incorporate the extend and retract features described earlier, by allowing the supports to translate through the slots. The embodiment illustrated in  FIGS. 7 and 8  show a gas burner of the present invention without the induction slots as shown in the embodiment of  FIG. 13 . However it can be seen how the mounting ling supports  35  are able to translate vertically between support ribs  45 . Further, it is envisaged that the air flow entering the base of the burner may be modified by deflecting surfaces or vanes or an array of apertures, which may improve the efficiency and/or emissions of the burner, and/or may influence the ratio of fan forced air which flows through the inner and outer passages respectively. 
     At the base of the burner body  40 , is a fuel gas jet  46  which is preferably located on or about, the burner centre line, and directs the jet of fuel gas upwards. Fuel gas is delivered to the fuel gas jet nozzle or injector  46  by fuel gas inlet  47 . The fuel gas flow rate in the fuel gas inlet  47 , is controlled by a control valve (not shown) as is known in the art for varying the output of gas burners. A preferred method of controlling gas flow to each burner in accordance with user settings is with a rotary gas valve mechanically coupled to the rotor shaft of a stepper motor. User adjustments of flame height are received as electronic inputs to a microcontroller. The microcontroller can then control the stepper motor to drive the gas valve to the appropriate angular shaft position to correspond to user-selected flame height level. Software and a user interface display may also be included to aid with user friendliness of the control of the gas burner(s). 
     The fuel gas exits the fuel gas jet  46  (or alternatively, two or more jets), and diverges into a substantially conical shape as it passes through a venturi tube  48 , which is substantially aligned with the fuel gas jet axis (or axes). The venturi tube  48  is open at the top and the bottom, and shelters the diverging cone of gas exiting the fuel gas jet, from some of the fan forced oxidising airflow. The tube  48  divides the burner into two concentric gases passage ways. Through the inner passage way, flows the fuel gas and some entrained and fan forced air which enters the tube  48  through the gap between the burner body  40  and the inlet of tube  48 . In the outer passage flows the majority of the fan forced oxidising air which is separated from the air fuel mixture flowing in the inner passage by the tube walls. 
     It is envisaged that the venturi tube  48  may preferably contain means for locating and/or modifying the flame front. The flame front locators  49  are positioned within the venturi tube  48 , to control the position of the flame and/or reduce the noise in the burner. The flame front locating structure fixes the starting point of the flame which would otherwise move considerably depending on the fuel gas flow rate and burn back velocity. This helps the flame to remain stable and also makes flame detection more reliable. The flame front locators spread the flame front and slow the gases helping mix with air and makes the flame reaction less noisy. It is envisaged that the means for locating the flame may be a variety of structures. For example, a number of elements may be arranged across the venturi opening in a parallel structure or alternatively may be radially oriented like spokes and may also contain apertures in the spoke arrangement. It has also been found that a simple wire mesh works very effectively as the preferred flame locating means. With reference to  FIG. 19   a  &amp;  b , two preferred embodiments of the flame front locators are shown. In  FIG. 19   b , slot  50  is provided to accommodate the tip of a hot surface igniter. The flame front locators shown if  FIG. 19  are formed from a thin flat metal disc, and may be manufactured by any suitable method such as punching, chemically etching, laser cutting or spark erosion. 
     The venturi  48  preferably also contains elements for ignition  51  such as hot surface igniters and/or electrodes for spark ignition and/or flame detection as is well known in the art of gas burners. The venturi tube  48  is designed to provide entrainment of primary air at higher power settings. At lower power settings, primary and secondary air is provided by the small portion of fan forced air that flows through venturi tube  48 . Alternatively, the tube  48  may be a straight walled cylindrical tube which functions primarily to separate the gas flow into two concentric passages and shelter the inner passage from some of the fan forced air. Alternatively, a further smaller venturi tube may be positioned in close proximity to the jet to improve primary air entrainment at lower power settings. 
     In use at high power settings, primary combustion air is drawn up through the venturi  48  predominately by entrainment with the fuel gas flow. The flame front occurs within the venturi tube  48  at a point where the fuel gas cone has spread and mixed with the primary air enough that the mixture is combustible and may be located by flame front locating means  49 , as shown in  FIG. 15 . The primary flame travels up through the burner body  40 , diverging before impinging on the under surface of the burner head  3  (shown approximately as fine array of shading dots). The underside of the burner head  3  may be substantially flat or angled (or curved) such that it directs the primary flame substantially radially outwards, which mixes with the fan forced secondary air flowing upwards in the annular gap  52  between the venturi tube  48  and the burner body  40 . The factors of head diameter (y), distance of head from flame locators (x), and air/fuel gas flow rates, all affect the position at which secondary combustion occurs at higher power. At higher powers, it is preferable for secondary combustion (shown in  FIG. 15  approximately as coarse array of shading dots) to commence on or about the lower circumferential edge  53  of the burner head  3 . The fan forced secondary air flowing through gap  52  eliminates the need for the flame to draw all secondary air from the ambient surroundings in order to achieve substantially complete combustion. Therefore, the burner head  3  can be quite close to, or in contact with, the base of the cooking vessel which allows for improved heat transfer to the cooling vessel due to the proximity of the cooking vessel to the high heat output phase of the flame reaction. 
     It has been found that efficiency is improved by a relatively small diameter of the burner head  3 , as it forces the hot gases to flow radially outwards over an extended distance across the bottom of the cooking vessel. Efficiency is also improved by the relatively high temperature difference between the flowing gases and the surface of the cooking vessel. Further, the secondary combustion which is allowed by the supply of fan forced secondary air, causes the secondary combustion to occur in a concentrated area thereby extending the distance over which the hot gases are in contact with the bottom of the cooking vessel. 
     At lower power settings, the combustion air is predominately provided as forced air from the fan  41  flowing through the venturi. When the burner is turned down to lower levels, the flame recedes diametrically and downwards into the venturi tube  48  where the air flow is sufficient for complete combustion at low power settings (flame shown approximately in  FIG. 16 ). The venturi tube  48  shelters the smaller flame from the ambient air flow and the bulk of the fan forced air flow flowing in gap  52 . This sheltering of the smaller flame is preferable to prevent the smaller flame from being extinguished and may aid the use of a constant speed fan if desired. Alternatively, a variable speed fan (which may be controlled by the power setting) may be utilised and thereby reduce the amount of sheltering of the low power flame necessary. When the burner is turned down to lower settings, the fan forced air flowing through gap  52  mixes with the combustion gases exiting the tube  48 , thereby cooling them and resulting in a lower effective power output (at the burner cap) which contributes to the relatively high turn-down ratio of the gas burner. 
     Due to the relatively high turn down ratios which are achievable by burners of the present invention, it may not be necessary to produce a large number of varying burner sizes in order to achieve desirable maximum and minimum outputs. For example, it may be preferable to produce two burner sizes having respective maximum outputs of approximately 2.5 kilowatts and 6 kilowatts. The high turn down ratio that is achievable with the burner design (approximately 50:1, or better) allows for a great deal of flexibility in output range for burners in a cooktop gas heating appliance. 
     The burner according to the present invention is also suitable for use with conventional type gas cooktops as shown in  FIG. 18  where a conventional trivet is provided and the burner head is not retractable. Alternatively, the burner according to the present invention may be especially suited to easy clean gas cooktops which include retractable burner heads as described earlier. 
     To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as set out in this specification. The disclosures and description herein are purely illustrative and are not intended to be in any sense limiting.