Abstract:
A dental furnace ( 10 ), with a furnace head (including firing space  16 ) and a firing space bottom ( 12 ) is provided which is suited to accommodate a dental restoration part ( 40 ). At least one optical sensor ( 22 ) is positioned at the dental furnace ( 10 ) or in its vicinity, which sensor comprises an output port ( 26 ) which is connected to an evaluation device for evaluating the dental restoration part ( 40 ) and/or a muffle ( 14 ) and/or a press plunger ( 52 ) and/or a firing-charge carrier as far as its dimensions and/or its shape and/or its position are concerned.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of European Patent Application No. 11175231.7 filed Jul. 25, 2011, which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates generally to a dental furnace and a process for operating a dental furnace. 
       BACKGROUND 
       [0003]    Dental furnaces for firing dental restoration parts have been known for a long time. In dental furnaces, a dental restoration part or a number of dental restoration parts are subjected to a firing process or sintering process, in accordance with a precisely prescribed temperature profile. 
         [0004]    The quality of the dental restoration parts produced depends essentially on the intended parameters which are optimally adapted for the dental restoration material and are exactly adhered to during the firing process. This comprises not only the temperature profile to be adhered to, not inside the furnace but at the dental restoration part itself, but also the existing vacuum conditions during the firing cycle. 
         [0005]    Such dental restoration parts comprise dental restoration parts to be manufactured of plastics, of metal, of composites, and, in particular, also of ceramics. 
         [0006]    Ceramic dental restoration parts are often produced in what is referred to as muffles, in which hollow molds correspond to the precise shape of the future dental restoration part as negative impressions. Such patterns are made of wax or similar materials as positive patterns, are mounted on a common basis via what is referred to as waxing sprues, and are then cast out with a material which cures true to shape and which in most cases contains plaster. 
         [0007]    After the wax has been melted out, hollow spaces are available, with the waxing sprues being connected with one another via a common press channel which in most cases is of cylindrical shape, and ending at the hollow spaces in which the dental restoration parts are to be produced. 
         [0008]    When a dental restoration part is to be pressed, a green body or press blank is introduced into the press channel and is subjected to pressure with the help of a molding plug. During the process of heating to reach the sintering temperature of the ceramic material of which the press blank consists, the press blank begins to take a plastic state and is introduced into the hollow shaping spaces for the production of dental restoration parts by means of pressure exerted on the molding plug. 
         [0009]    This process has been known for a long time, but lately attempts have been made to improve the throughput of dental press furnaces, for instance by pressing more or larger dental restoration parts together in one muffle. 
         [0010]    Larger hollow shaping spaces or a larger number of shaping spaces, however, lead to a certain weakening of the muffle. For this reason, the furnace parameters for preventing the formation of cracks in the muffle or other damage of the dental restoration parts must be adhered to even more precisely. 
         [0011]    It has already been suggested as well, as can be seen from DE 10 2006 050 830 A1 and corresponding US Published Application No. 2008099939, which is hereby incorporated by reference, to monitor whether the muffle which is subject to pressure exhibits a crack during the pressing process. This evaluation, however, can basically be done after the fact, and it would be desirable to safely prevent cracks in the muffle already in advance. 
         [0012]    In order to guarantee this with a certain safety margin, the muffles produced are subjected to a prescribed press force to be set by the user, which force is clearly lower than the press force at which cracks might appear. 
         [0013]    A reduced press force, on the other hand, offers the risk of the dental restoration parts having to remain in the firing furnace for too long, so that the hollow shaping spaces are completely filled, and also the risk of the user by accident interrupting the pressing process because the user assumes that the pressing process has been completed, for instance when the molding plug does not move further downwards. This can happen, in particular, when there is comparably high friction inside the shank. A dental furnace operated at reduced press force is more vulnerable for such friction faults in comparison to a dental press furnace operated at high press force. 
         [0014]    Another quality problem is that parameters are by accident set wrongly for the firing process and/or the pressing process to be carried out. This can result in the press force being set too high or too low, or in the firing temperature being set too high or too low. The dental restoration part produced will then be of minor quality, which in many cases will go unnoticed because it will not recognizable in the finished dental restoration part. Damage will in this case occur only after the part has been used inside the mouth of the patient, in different cases often for years, for which then the dentist or the producer of the dental restoration material will be blamed, even if it was indeed the dental technician operating the press furnace who made an error. 
         [0015]    In large dental laboratories, the work process is often split, such that certain dental technicians do only certain jobs with given parameters, which to put into practice will quite possibly also be feasible at the same firing furnaces which in most cases cover a rather large range in accordance with the client&#39;s wishes. In order to limit the range of parameters for the necessary dental technical work, it has already been suggested to draw up what is referred to as user profiles which make it necessary for the respective user to register with the dental furnace before firing curves and press curves available for selection within his range are presented to him. For a correct assignment, however, it is vital that the user also logs off when he has finished his job at the firing furnace concerned, which is thought to be awkward, such that it is to be feared that another user will accidentally carry out some wrong operation, assuming that the dental furnace concerned is presenting him the “correct” firing curves and press curves for selection. 
       SUMMARY 
       [0016]    The invention has the task of providing a dental furnace with the help of which work in a dental laboratory is improved even with complex applications and high expectations of the users as to quality, without making the operation of the dental furnaces any more complicated than is relevant in practice. 
         [0017]    This task is fulfilled in accordance with the invention by the attached claims, which are herein incorporated by reference. 
         [0018]    In accordance with an embodiment of the present invention, it is particularly favorable to incorporate an optical sensor in order to detect certain conditions relevant for the operation of the dental furnace and control the dental furnace accordingly based on the detection result. Here, the expression “optical sensor” shall be understood to be an alignment of identical or essentially identical elements sensitive to radiation. 
         [0019]    In case of a one-dimension alignment, such an optical sensor can also be composed of a number of individual sensor elements which can for example also include infrared sensors, in case of an alignment of the individual sensor elements in two dimensions, in contrast, picture recording and imaging in these two dimensions is possible as well, as can for instance be put into practice with the help of a CCD unit of a digital camera. If realized as a CCD unit, it is also possible in a favorable embodiment to realize what is referred to as a thermal imaging camera which has its maximum sensitivity in the infrared range, or as a digital camera for the range of visible light, i.e. between 300 nm and 750 nm. 
         [0020]    In accordance with an embodiment of the present invention, it is particularly favorable that non-contact measurement is possible thanks to the provision of the dental furnace, in accordance with an embodiment of the present invention, with an optical sensor. For instance, the size of a dental restoration part, a press plunger, a muffle or a firing charge carrier can be measured, also their shapes, respectively, and their positions, respectively, as well. Here too the optical sensor, which can very well reach into the range of infrared in its measuring range, makes possible a measurement of the temperature, and it is also possible to transmit the measuring result over larger distances. 
         [0021]    Mechanical deformations or other deviations do not have any influence on the measurement. 
         [0022]    With the help of the monitoring provided in an advantageous embodiment, control of the dental furnace is possible by the user via a display device or in any other suitable form. When the optical sensor is directed towards the dental restoration part or the muffle, the control device also recognizes without any problem when the muffle or the dental restoration part is introduced and removed. 
         [0023]    This is also true for firing charge carriers, and it is possible, for instance, to detect, in accordance with color, whether the correct firing charge carrier has been introduced. 
         [0024]    Monitoring is not restricted to the muffle. It is also possible, for example, to monitor the position of an aluminum-oxide plunger or a molding plug inside the muffle. As is known, the latter must be positioned deeper inside the muffle after the pressing process, which can be detected via position detection 
         [0025]    In a first embodiment of the solution in accordance with an embodiment of the present invention, it is intended to drive a control device for the dental furnace with the help of the optical sensor. The control device then triggers the desired function depending on the image recorded or other signal output as is transmitted by the optical sensor. 
         [0026]    Thus it is possible, with a correspondingly high-resolution realization of the CCD unit, to recognize the object to be treated. This can for example be a dental restoration part to be fired, and the control device then triggers the related program. It is also possible to realize the corresponding recognition in a supported fashion, i.e., for instance, with the help of bar codes on the packing of the product. However, it is preferred for the optical sensor to recognize the object directly and trigger the desired program function. 
         [0027]    For example, it is possible to recognize the size of a dental restoration part to be fired by means of image recognition and to determine the corresponding parameters. 
         [0028]    Another possibility is the automatic identification of muffles. Their sizes, lot numbers or the like can be recognized, and furnace control can be adapted in a suitable fashion. 
         [0029]    The optical sensor in accordance with an embodiment of the present invention can either be firmly connected with the housing or the lower portion of the dental furnace, or it can be connected with the control device of the dental furnace via a suitable connection, such as, for example, a wireless connection, an electric cable or the like. 
         [0030]    In an advantageous embodiment of the invention, the optical sensor is provided as a multiple arrangement of individual light-sensitive elements in a row and specially adapted to recognizing the diameter of a muffle. For this purpose, the arrangement with light-sensitive elements of the optical sensor preferably extends horizontally, corresponding to the vertical positioning direction of a muffle in a dental furnace. It is also possible to introduce a basically known temperature measuring camera and merely evaluate one line of the matrix provided there. Each detection element will then correspond to one pixel. The number of pixels which are hit by optical radiation or thermal radiation from the muffle will then allow conclusions to the diameter of the muffle. The desired precision is already sufficient if the width of a pixel corresponds to a difference in the diameter of, for example, 4 mm. 
         [0031]    Based on the measuring result present, the control device is provided with the corresponding information and will drive the furnace in a suitable fashion, for instance by selecting a longer firing cycle for muffles with larger diameters. 
         [0032]    In an advantageous embodiment, it is intended to evaluate not only the diameter of the muffle, but also the height of the muffle. For this purpose, a two-dimensional detection of the muffle outlines via the optical sensor is provided. In an alternative embodiment, it is intended to evaluate a pixel line of the CCD unit, as described above, as well as a pixel column of the unit. From the evaluation of the pixel column, the height of the muffle will result here, and from the pixel width the diameter will result. 
         [0033]    It is to be understood that the optical sensor takes a given position relative to the muffle. For example, the muffle can be positioned at a given recess in the firing chamber bottom, and the optical sensor can be firmly mounted to the lower portion of the furnace in such a fashion that it is directed towards the muffle. 
         [0034]    In an advantageous embodiment, it is intended that the format selection of the optical sensor can be adapted to the requirements. If, for example, in the vertical direction pixel resolution is higher than in the horizontal direction, the picture evaluation of an image standing in an upright position is favorable. If, vice versa, the number of pixels in the horizontal direction is larger than in the vertical direction, a higher resolution is available for an evaluation in the horizontal direction, such that even objects positioned next to each other, such as, for instance, multi-member crowns lying flatly can well be evaluated. 
         [0035]    It is possible to enlarge or reduce the size of the image area with the help of zoom optics. 
         [0036]    In an advantageous embodiment it is intended that the control device has a comparison device which compares images recorded with images detected by the optical sensor. This also includes, for instance, certain muffle sizes, and also certain sizes of dental restoration parts. 
         [0037]    In a preferable embodiment, the evaluation device is connected to a display device with the help of which information about the dimensions and/or the shape and/or the position can be displayed, and/or to a control device for controlling the dental furnace. 
         [0038]    In a preferable embodiment, a monitoring device of the dental furnace is connected to the sensor, with the help of which device a temporal change in the dimensions and/or shape and/or position can be detected. 
         [0039]    In a preferable embodiment, the sensor comprises a detection area which extends, at least partially, inside the firing space above the firing space bottom of the dental furnace and through which, in particular, the central longitudinal axis of the firing space extends as well. 
         [0040]    In a preferable embodiment, the furnace head is removable from the firing space bottom, and can be lifted off the latter, such that the sensor is directed towards a detection area which extends between the firing space bottom and the furnace head in the open state of the furnace. 
         [0041]    In a preferable embodiment, the sensor is mounted in such a fashion that it can be swiveled, and is, in particular, attached to the dental furnace in such a fashion that it can be swiveled. 
         [0042]    In a preferable embodiment, the sensor is formed to be a picture recording device with the help of which an image of the dental restorations and/or the muffle and/or the press plunger and/or the firing charge carrier can be recorded. 
         [0043]    In a preferable embodiment, the sensor is formed to be a picture recording device which evaluates the recorded image line-wise and/or column-wise and determines its dimensions based on the evaluation result in a width extending in a parallel fashion to the firing space bottom or in a height extending in a vertical fashion to the firing space bottom of the dental restoration part and/or the muffle and/or the press plunger and/or the firing charge carrier, in particular, by determining the number of pixels which correspond to the width or height, respectively, of the object recorded, with the number of pixels being realized by a comparison of the pixels corresponding to the object as opposed to the pixels corresponding to the background, based on a pre-set criterion. 
         [0044]    In a preferable embodiment, the criterion mentioned is the temperature and/or the brightness and/or the color of the object. 
         [0045]    In a preferable embodiment, the sensor detects electromagnetic waves within a range of wavelengths of 380 nm and 18 μm and, in particular, comprises a multitude of sensor elements. 
         [0046]    In a preferable embodiment, the optical sensor comprises at least two sensor elements which are positioned at an angle to one another and, in particular, have detection areas which at least partly overlap each other. 
         [0047]    In a preferable embodiment, the optical sensor forms part of a light barrier which, in particular, comprises at least two sensor elements as well as at least one transmitter of electromagnetic rays, the presence of which can be detected by the sensor elements. 
         [0048]    In a preferable embodiment, the sensor is connected to a control device via which a dental restoration part or a muffle can be detected in view of its size, and wherein the control device determines furnace parameters depending on the size of the dental restoration part, in particular, depending on the width of the muffle thus detected. 
         [0049]    In a preferable embodiment of the process In accordance with an embodiment of the present invention, the control comprises a pre-selection of processing programs of the dental furnace after the control device has recognized the object recorded, in particular, the size of a muffle positioned upon the firing space bottom. 
         [0050]    In a preferable embodiment, the temperature of the dental restoration part and/or the muffle and/or the press plunger and/or the firing charge carrier can be detected by the sensor, and wherein the control device controls the dental furnace depending on the temperature detected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    Embodiments of the present invention will be more fully understood and appreciated by the following Detailed Description in conjunction with the accompanying drawings, in which: 
           [0052]      FIG. 1  is a schematic view of an embodiment of a detail of a dental furnace in accordance with the present invention; 
           [0053]      FIG. 2  is a schematic view of a detail of the optical sensor belonging to  FIG. 1 ; 
           [0054]      FIG. 3  is a schematic view of another embodiment of a portion of a dental furnace in accordance with the present invention; 
           [0055]      FIG. 4   a  to  FIG. 4   f  are schematic views of muffles which are destined for an embodiment of the dental furnace in accordance with the present invention; 
           [0056]      FIG. 5  is another schematically depicted embodiment of a dental furnace in accordance with the present invention; 
           [0057]      FIG. 6  is another schematically depicted embodiment of a dental furnace in accordance with the present invention, with the furnace hood opened in the depiction; 
           [0058]      FIG. 7  is another schematically depicted embodiment of a dental furnace in accordance with the present invention; 
           [0059]      FIG. 8  is a schematic depiction of an optical sensor in accordance with the present invention during operation; 
           [0060]      FIG. 9  is a schematic depiction of an optical sensor in accordance with the present invention, however in a modified embodiment; 
           [0061]      FIG. 10  is a schematic top view on a partially depicted further embodiment of a dental furnace in accordance with the present invention; and 
           [0062]      FIG. 11  is a schematic perspective view on a partially depicted further embodiment of a dental furnace in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0063]    A dental furnace  10  in accordance with an embodiment of the present invention has a firing space bottom  12  which is intended to take up a muffle  14 . For the firing process, the muffle  14  is taken up in a firing space  16  which is depicted schematically and by broken lines in  FIG. 1 , and which is formed in a furnace hood which is not depicted. 
         [0064]    In the condition depicted in  FIG. 1 , the furnace hood (including the firing space  16 ), however, is lifted such that the muffle  14  is visible. 
         [0065]    The muffle  14  is destined to form dental restoration parts. For this purpose, it is provided with a press channel  20  which is intended for taking up a press plunger. Inside the muffle  14 , hollow shaping spaces are formed in an also basically known fashion, which hollow spaces determine the dental restoration parts in their shape. 
         [0066]    In accordance with an embodiment of the present invention, an optical sensor  22  is positioned laterally with respect to the firing space, however a little above the latter. The optical sensor  22  is directed towards the muffle  14  in the exemplary embodiment depicted, and its detection area  24  covers the maximum possible size of a muffle in the firing space  16 . The detection area  24  of the optical sensor  22  is basically cone-shaped for this purpose. The optical sensor  22  thus detects the width of the muffle  14 , and in a preferable embodiment also the height of the muffle  14 . 
         [0067]    The optical sensor is provided with an output port  26  which is connected with a control device  28  for the dental furnace via any connection suitable for this purpose. The control device  28  drives the dental furnace in such a fashion as is optimal for the muffle  14  detected. If, for instance, a larger muffle  14  is detected, the firing cycle will automatically extend, in order to account for the increased heat capacity of the muffle and in order to achieve a constant dental restoration result. 
         [0068]    , an image recognition means is provided for this purpose inside the control device, which evaluates the image recognized and recorded by the optical sensor and makes the size of the muffle detected the basic evaluation scale. 
         [0069]    Even if a muffle size recognition means is provided inside the dental furnace or, to be more precise, above the lower portion of the dental furnace, it is to be understood that in an equivalent fashion, a muffle size recognition means can also be provided outside the lower portion of the dental furnace. In a preferable embodiment of the invention, it is also possible to realize the evaluation both in the position in accordance with  FIG. 1  and with respect to other positions not shown. For this purpose, the optical sensor  22  can either be firmly mounted or can be supported separately in front of the dental furnace  10 . How this can be realized can be taken schematically from  FIG. 3 . 
         [0070]    From  FIG. 2  the schematic alignment of individual detection elements  30  of the light-sensitive unit of the optical sensor  22  can be taken. As can be seen, a multitude of individual detection elements  30  is aligned one after another, with the minimum necessary resolution—corresponding to the number of individual detection elements, being adaptable to a great extent to the requirements. The light-sensitive unit  32  in accordance with  FIG. 2  can also form part of a CCD unit which is provided in the form of a matrix of the optical sensor  22 . In this case, merely one line of the CCD matrix is evaluated. 
         [0071]    The individual detection elements  30  in accordance with  FIG. 2  recognize through an optics element  34  of the optical sensor the width or the diameter of the muffle  14 . For this purpose, the exposure of the individual detection elements  30  activated by the muffle  14  differs from that of those not activated. The number of detection elements  30  activated corresponds to the diameter of the muffle and is for which a signal is sent to the control device  28 . 
         [0072]    From  FIG. 3  it can be shown in which fashion a muffle can be detected by the optical sensor  22 . The optical sensor  22  is provided with a detection area  24  which essentially extends in a conical shaped manner from the optical sensor  22  towards the front, i.e., in the direction towards the muffle  14 . As shown in  FIG. 3 , a hollow space for a dental restoration part  40  is provided inside the muffle  14 , with several hollow spaces being realized in the exemplary embodiments depicted. 
         [0073]    A press plunger  42  is also positioned within the detection area  24  and can also be detected by the optical sensor  22 . Thus, it is also possible to detect, with the help of the optical sensor  22 , to what extent the press plunger  22  has already been pushed inside the muffle  14 . 
         [0074]    The muffle  14  is adapted to a firing space bottom  12  or  46  in accordance with  FIG. 3 . The firing space bottom is provided with a recess, for supporting or holding the muffle  14 . For supporting or holding muffles of different diameters, a number of recesses is provided in a stepped or graded fashion, which recesses are each adapted to the possible sizes of muffles and with the help of which it is made sure that the muffle is placed in a centered position. 
         [0075]    The detection area  24  of the optical sensor  22  extends not only in a vertical direction, but, in particular, also in a horizontal direction. Thus it is also possible to detect the position and also the width of the muffle  14 . Thus, if for instance the muffle is positioned too far sideward, i.e., not in a centered position, inside the recess, which could result in faulty pressing, this can also be detected with the help of the optical sensor  22 . 
         [0076]    As can be taken from  FIGS. 4   a  to  4   f , differently shaped muffles, respectively, can be classified and evaluated in order to select the suitable program for the dental furnace  10 .  FIG. 4   a  differs from  FIG. 4   b  in that the muffle in accordance with  FIG. 4   a  has a distinctly smaller diameter, but has the same diameter of the press channel. The thermal capacity of the muffle  14  in accordance with  FIG. 4   b  typically is considerably larger compared to that in accordance with  FIG. 4   a , and fast heating with a temperature-balancing phase is recommended, during which phase the temperature in the muffle  14  can be balanced. 
         [0077]    In accordance with  FIG. 4   c , a muffle is provided which bears a marking ring  46 . The marking ring  46  permits, for example, indications as to what dental material is used, such as, for instance, lithium disilicate or zirconium dioxide, and thus significantly influences the press curve. 
         [0078]    In accordance with  FIG. 4   d , a muffle with a beveling  48  at its upper end is provided. Such a muffle permits a lifting/pivoting movement even with a firing space whose diameter is only scarcely larger than the outside diameter of the muffle. 
         [0079]    A marking  50  of a different type is provided as a flattened portion which differentiates the muffle  14  depicted there from other muffles. 
         [0080]    Also, a press plunger  52  which can be introduced into the press channel  20  can be provided with a marking  54  in any suitable fashion, which marking can be detected by the optical sensor  22 . 
         [0081]    The dental furnace  10  depicted in  FIG. 5  is provided with an optical sensor  22  which is directed towards the dental restoration part  40  which is put next to a furnace hood  54  on a furnace bottom part  56 . As can be taken from  FIG. 5  schematically, the optical sensor  22  can be swiveled around a vertical axis not depicted, such that it can also be directed towards the firing space when the furnace hood is open and can recognize a dental restoration part  40  or a muffle placed there. 
         [0082]    The dental furnace  10  in accordance with  FIG. 5  is provided with a display device  58  as well as operation keys  60 , and the furnace hood  54  is mounted in such a fashion that it can be swiveled around a horizontal axis or can be lifted. 
         [0083]    The embodiment in accordance with  FIG. 6  also shows an embodiment of a dental furnace  10  with a furnace hood  54 . The furnace hood  54  is formed in such a fashion that it is suitable for the maximum possible size of the muffle  14 . The optical sensor  22  is attached laterally to the lower portion  56  of the furnace and is directed towards the muffle  14 . The detection area of the optical sensor makes it possible to notice that the muffle  14  does not exceed the admissible size, and also that it is not placed in a position which would collide with the inner side of the furnace hood  54  when the latter is closed. 
         [0084]    A modified version of a dental furnace  10  with a vertically lowerable lower portion  56  of the furnace can be taken from  FIG. 7 . In this embodiment, two optical sensors  22  and  23  are provided which are both directed towards the firing space bottom and a little above the firing space bottom, respectively, in order to be able to detect dental restoration parts  40  positioned there. Here, the optical sensor  23  can be moved in a vertical direction, and the optical sensor  22  can be swiveled, each in order to be able to detect the desired information about the dental restoration parts and their arrangement on the firing space bottom  46  in the best possible fashion. 
         [0085]    In  FIG. 8  it is depicted in which way the detection area  24  of the optical sensor can extend beyond a muffle  14 . The optical sensor  22  detects the width of the muffle  14  in a horizontal direction. For this purpose, a multitude of pixels  66  is evaluated, and in the exemplary embodiment depicted this is done at an upper vertical position  68  and at a lower vertical position  70 . By comparing the pixel values determined, it is also possible to roughly determine the shape of the muffle  14 . 
         [0086]    It is to be understood that the optical sensor can be adapted to compare the reflection properties of the muffle  14  with the reflection properties of the background and determine differences. Typically, for instance, a muffle is white, while the background is typically of a different color than white. Thus a differentiation is also possible, even if only the range of visible light is evaluated by the optical sensor  22 . 
         [0087]    In a modified embodiment, however, infrared radiation is evaluated instead or in addition. This makes it possible to detect a hot muffle, which comes, for example, from a preheating furnace and is positioned on the firing space bottom  46 . 
         [0088]    Compared with the muffle which has a temperature of, for instance, 600° C., the surrounding environment is distinctly colder, such that a particularly good differentiation is possible. 
         [0089]    The embodiment in accordance with  FIG. 9  corresponds to the embodiment of  FIG. 8 , with, however, a horizontal detection position  72  being realized instead of the second vertical position. It is possible by detecting the corresponding pixels  66  to determine also the height of the muffle  14 . 
         [0090]    Also non-circular muffles can be detected, such as in the embodiment of the muffle  14  in accordance with  FIG. 10 . Also in this embodiment, two optical sensors  22  and  23  are employed, with one of the optical sensors being mounted in such a fashion that it can be swiveled around a vertical axis  80 . 
         [0091]    A detection of a size of the muffle via a light barrier  82  is provided in the embodiment in accordance with  FIG. 11 . The light barrier is provided with a transmitter  84  and the optical sensor  22  as a receiver. The light barrier  82  can, for example, remain stationary and then detect whether a muffle is introduced or not. Alternatively, it can also be moved in a horizontal direction, and with the help of the traveling way which is necessary in order for radiation emitted by the transmitter  84  to reach the optical sensor  22  again after coverage by the muffle  14 , it is determined how large the diameter of the muffle is. 
         [0092]    Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.