Abstract:
In cutting devices ( 10 ), it is necessary to preset the specimen ( 14 ) onto the knife ( 16 ). The use of a light barrier ( 30 ) is proposed in order to facilitate and automate this presetting operation, which is intended to ensure maximally rapid and accurate positioning of the knife ( 16 ) relative to the specimen ( 14 ). The light barrier ( 30 ) is positioned between the knife ( 16 ) and the specimen ( 14 ), and is preferably arranged parallel to the knife edge ( 15 ). As soon as the light barrier ( 30 ) is interrupted, a defined spacing ( 25 ) between knife ( 16 ) and specimen ( 14 ) is ascertained, and is used to shut off the automatic feed or to switch over to a different drive system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority of the German patent application 102 58 553.9-52 which is incorporated by reference herein.  
         FIELD OF THE INVENTION  
         [0002]    The invention concerns an apparatus for cutting into specimens, in particular a microtome or ultramicrotome, as well as a method for bringing a specimen close to a knife of a microtome or ultramicrotome.  
         BACKGROUND OF THE INVENTION  
         [0003]    In the operation of cutting apparatuses, in particular a microtome or ultramicrotome, it is routinely necessary to position the specimen exactly with respect to the knife in accurately positioned and rapid fashion. Care must be taken that neither the knife nor the specimen is damaged in this so-called “presetting” operation. It is correspondingly necessary to prevent inadvertent contact from occurring between knife and specimen. It has therefore been common for some time, in the context of the presetting operation between knife and specimen, to observe through a stereomicroscope as the knife and specimen approach one another. This observation does not always, however, result in a reliable estimate of the spacing between the specimen and the knife edge. Technical presetting aids have therefore also been used for some time, for example a base-mounted illumination system, with which it is possible to illuminate a gap between the knife and specimen, and on the basis of the illuminated gap to allow a better evaluation of the spacing between specimen and knife. The surface of the specimen to be cut into is, however, usually irregularly shaped before the first cut, so that this action as well only partially achieves the goal.  
           [0004]    To solve this problem, it is already known from DE 41 11 689 to provide a force sensor that is mounted on the specimen or knife. The cutting force is sensed with the aid of this force sensor, so that it is possible to ascertain when the first cut occurs. In order to carry out the method, once an initial coarse positioning of the knife with respect to the specimen has been performed, the spacing between specimen and knife is decreased at a high feed and cutting speed. As soon as the specimen touches the knife for the first time, the force sensor responds. From that time on, operation switches to a selectable (usually slower) cutting speed, and a specific desired cut thickness is maintained. In order to minimize stress on the specimen and knife, however, the first cuts after presetting must also not be too thick. For many diamond knives, a cut thickness of 0.3 μm is considered the upper limit. This means that before cutting operation begins, alignment of the knife and specimen must be accomplished to an accuracy of a few micrometers without contact between the knife and specimen. The method proposed in DE 41 11 689 cannot guarantee this, however. The first contact between knife and specimen takes place at high speed, so that damage to the knife and specimen can thereby occur.  
           [0005]    In order to improve the presetting operation and to automate presetting, with the intent of simultaneously avoiding uncontrolled contact between the knife and specimen, EP 544 181 has proposed a method and an apparatus for automatic presetting. For this, there is mounted on the knife holder a so-called limiting device which is provided in order automatically to ascertain the proximity of the specimen holder along with the specimen secured therein. At the same time, the drive device is intended to switch off in timely fashion so that damage to the knife blade and to the specimen is reliably prevented. To ensure this, a movable plate having a microswitch located behind it is provided on the knife holder. In the presetting operation, the specimen is moved toward this plate until the switching point of the microswitch is reached. A disadvantage of this method, however, is the fact that contact between the specimen and the plate is necessary. In addition, the microswitch does not have the requisite repeatability in the micrometer range.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore the object of the present invention to propose a reliable measurement system that permits non-contact and accurate measurement of the spacing between knife and specimen.  
           [0007]    According to the present invention, this object is achieved by a cutting apparatus for cutting a specimen comprising: a knife, defining a knife edge, a knife holder for clamping the knife, a specimen holder for holding the specimen, a feed device for generating a relative motion between the knife and the specimen, a light barrier being arranged parallel to the knife edge and located between the knife and the specimen, the arrangement of the light barrier is such that the relative motion between the knife and the specimen penetrates the light barrier and thereby ascertains a spacing between the knife and the specimen.  
           [0008]    It is a further object of the present invention to propose a microtome or an ultramicrotome that permits non-contact and accurate measurement of the spacing between the knife and the specimen.  
           [0009]    The above object is achieved by a microtome or ultramicrotome comprising: a knife, defining a knife edge, a knife holder for clamping the knife, a specimen holder for holding a specimen, a feed device for generating a relative motion between the knife and the specimen, a light barrier being arranged parallel to the knife edge and located between the knife and the specimen, the arrangement of the light barrier is such that the relative motion between the knife and the specimen penetrates the light barrier and thereby ascertains a spacing between the knife and the specimen.  
           [0010]    It is as well an object of the invention to provide a method which allows a defined spacing between the specimen to be cut and the specimen holder.  
           [0011]    The object is achieved by a method for bringing a specimen close to a knife of a microtome or ultramicrotome, comprising the steps of:  
           [0012]    securing the specimen in a specimen holder and the knife in a knife holder ( 24 );  
           [0013]    moving the specimen and the knife toward one another with the aid of a feed device;  
           [0014]    providing a light barrier between the knife and the specimen, wherein the light barrier being arranged parallel to a knife edge of the knife;  
           [0015]    interrupting the motion of the specimen and the knife toward one another at a defined; and  
           [0016]    ascertaining a defined spacing between the knife and the specimen upon the interruption of the light barrier.  
           [0017]    According to the present invention, therefore, there is provided between the knife and the specimen a light barrier which is penetrated as the knife approaches the specimen, i.e. as the spacing between knife and specimen becomes shorter. The light barrier is arranged between the knife and the specimen in such a way that upon penetration of the light barrier, a defined spacing between the knife and the specimen can be deduced. Although the light beam of the light barrier can in principle be arranged in any desired direction in a plane that lies perpendicular to the displacement direction of the feed device, it is particularly advantageous if the light beam extends parallel to the blade of the knife. At the same time, the light barrier is arranged in such a way that upon interruption of the light barrier, a predetermined spacing still exists between the knife and the specimen. This spacing is preferably selected so that upon penetration of the light barrier there is no contact between the specimen and the knife, but instead the spacing (preferably a few micrometers) between specimen and knife is maintained. For that purpose, the light barrier can be e.g. mechanically coupled to the knife and oriented at a defined spacing and in a defined direction with respect to the knife edge. It is thereby possible either to move the knife toward the specimen or the specimen toward the knife, or to move the specimen and knife toward one another. The light barrier can also be mounted on the specimen in such a way that it is at a defined spacing from the specimen. With this solution as well, it is possible to move the knife toward the specimen, the specimen toward the knife, or the specimen and knife toward one another. Lastly, it is also possible to install the light barrier adjustably in a side wall of the cutting apparatus, although it is then favorable to move either the specimen toward the knife or the knife toward the specimen.  
           [0018]    In all the configurations mentioned above, the light barrier is penetrated by the knife or by the specimen. A knowledge of the exact distance between the knife blade and the light beam then also yields the spacing between the knife and the specimen, which in this case is known and defined. The presetting operation is complete, and the cutting operation can be initiated.  
           [0019]    The use, according to the present invention, of the light barrier also makes possible a completely automated presetting operation. For this purpose, after a coarse positioning between knife and specimen at a spacing of a few millimeters, the motorized cutting motion is activated. The spacing between specimen and knife is thereby decreased. Feed can be performed on both the specimen side and the knife side. The feed operation remains active until an interruption of the previously positioned light barrier ( 30 ) occurs for the first time. Once the light barrier has been penetrated, a defined spacing between knife and specimen is detected, so that the first approach operation is complete. The feed can now be switched off. Alternatively, it is also possible to switch automatically to a desired cut thickness and/or speed. This means that after a few cycles without cutting, the specimen is cut into at the desired cut thickness.  
           [0020]    The light barrier can be implemented using any desired light transmitter and a receiver tuned thereto. It is inherently possible to use any desired and suitable electromagnetic radiation. Preferably, however, a laser beam is used in order to achieve the smallest possible beam cross section for the sake of the requisite accuracy. A collimated LED beam can also be utilized.  
           [0021]    The entire presetting operation can be automated using the spacing measurement system proposed according to the present invention. This also reliably prevents the specimen and knife from accidentally or undesirably coming into contact, thereby ruling out any damage to the knife and specimen. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    Further advantages and advantageous embodiments of the invention are the subject matter of the Figures below and their descriptions.  
         [0023]    In the individual Figures:  
         [0024]    [0024]FIG. 1 shows an apparatus according to the existing art for cutting specimens;  
         [0025]    [0025]FIG. 2 shows an apparatus according to the present invention for cutting specimens, having a light barrier;  
         [0026]    [0026]FIG. 3 shows an enlarged portion of an apparatus according to the present invention for cutting specimens, having a light barrier. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    [0027]FIG. 1 shows a cutting apparatus  10  with which a specimen  14  is to be cut using a knife  16 . Specimen  14  is clamped in a specimen holder  22 , while knife  16  is secured in a knife holder  24 . For cutting, specimen  14  is guided past knife  16 , knife holder  24  being moved by means of carriage  26  on a carriage plane  28 . A relative motion is thus generated between knife  16  and specimen  14 . Specimen  14  can thereby be preset onto knife  16 . In order to minimize stress on specimen  14  and knife  16 , the first cuts after the presetting operation should also not be too thick. For many diamond knives, the upper limit of cut thickness can be assumed to be 0.3 μm. This means that before cutting begins, knife  16  must be aligned with specimen  14  to within a few micrometers, and knife  16  and specimen  14  must not touch. This alignment operation is performed under observation through stereomicroscope  12 . The spacing between specimen  14  and knife  16  appears in stereomicroscope  12 , because of the illumination from below, as a bright light gap. The illumination system is implemented, for example, as a lamp  20  whose light is guided to the desired point using light guide  18 .  
         [0028]    In an ultramicrotome of this kind it is often impossible, despite lamp  20  and light guide  18  as aids, to estimate exactly the gap between specimen  14  and knife  16  using stereomicroscope  12 . Automation of the presetting operation is moreover extremely difficult.  
         [0029]    [0029]FIG. 2 shows an apparatus according to the present invention for cutting specimens, such as can be implemented e.g. on a microtome or ultramicrotome. With the apparatus according to the present invention the purpose is once again to bring specimen  14  close to knife  16 , the spacing between specimen  14  and knife  16  being decreased by a relative motion between specimen  14  and knife  16 . Arranged between specimen  14  and knife  16  is a light barrier  30  that comprises a light transmitter  29  and a light receiver  30 . Transmitter  29  and receiver  30  are aligned with one another in such a way that the light beam proceeding from transmitter  29  is incident on receiver  30 . The light barrier can be mechanically coupled to an external housing (not shown) or to the specimen holder  22 , but preferably to knife holder  24  or carriage  28 , i.e. mounted thereon. As specimen  14  approaches knife  16 , light barrier  30  is penetrated at a defined position. The interruption can be accomplished either by knife  16  or by specimen  14 . In the example shown in FIG. 2, the light barrier is mechanically coupled to knife holder  24 . A defined, fixed spacing thus exists between light barrier  30  and knife  16 , and is not changed even by a translating motion of knife  16 . Correspondingly, in the event of a change in the spacing between specimen  14  and knife  16 , light barrier  30  is interrupted by specimen  14 .  
         [0030]    As depicted in FIG. 3 in an enlargement of a portion of apparatus  10  according to the present invention for cutting specimens  14 , light barrier  30  is preferably arranged between knife  16  and specimen  14  in such a way that light beam  32  extends parallel to knife edge  15  of knife  16 . Based on an accurate knowledge of spacing  25  between knife edge  15  and light beam  32 , upon penetration of the light barrier by specimen  14  it is possible to ascertain when specimen  14  is at exactly that spacing  25  from knife edge  15 . It is correspondingly important to set spacing  25  as accurately as possible. This can be achieved on the one hand by exact mechanical immobilization of light barrier  30  with respect to knife edge  15 . On the other hand, the cross section of the light beam should be kept as small as possible, for which purpose a collimated LED light beam or the light of a laser is a good choice. There are fundamentally no limitations of any kind in terms of the light that is used. The transmitter and receiver must simply be coordinated with one another so that the penetration of light barrier  30  can be detected.  
         [0031]    The presetting operation can be automated using the device according to the present invention. For this, firstly knife  16  and specimen  14  are roughly positioned relative to one another; this is preferably done at a spacing of a few millimeters. A motorized cutting motion  33 , comprising substantially an up-and-down motion of specimen holder  22 , is then activated. At the same time, the spacing between specimen  14  and knife  16  is decreased by way of a feed unit which generates a relative motion between specimen  14  and knife  16 . A stepping motor and a spindle, which move knife  16  together with knife holder  24  on a carriage  28  in the direction of specimen  14 , can be used, for example, for this purpose. The feed operation remains activated until an interruption of light barrier  30  occurs for the first time.  
         [0032]    Accurate detection of the interruption point of light barrier  30  can be further improved by the fact that after light barrier  30  is penetrated for the first time, the feed unit is moved back in the opposite direction in small steps in order to accurately ascertain the interruption point of light barrier  30  once again. The forward or reverse feed speed can also be modified, in particular lowered, for this purpose. After definite detection of the light barrier, a feed action then occurs over the entire spacing  25  minus a few micrometers. This is possible because the exact spacing  25  is known. Once a feed action has occurred over spacing  25  minus a few micrometers, it is possible to switch over automatically to the desired cut thickness and the corresponding feed speed. At the same time, motion  33  of specimen can also be set to the desired cut thickness. After a few cycles without cutting, the first cuts then occur at the desired cut thickness.  
         [0033]    Microtomes and ultramicrotomes are usually equipped with an alternating drive system with which the sample is moved slowly at the adjustable cutting speed when within the so-called cutting window, and at a higher return speed when outside the cutting window. The cutting window must usually be adapted to the specimen position and specimen size. A coding device on the alternating drive system permits adjustment of the cutting window.  
         [0034]    In a further embodiment of the invention, this cutting window can likewise be set automatically. This is done by arranging light barrier  30  between specimen  14  and knife  16  in such a way that in addition to the detection of spacing  25 , it is also possible to detect the position of specimen  14  during its up-and-down motion  33 . The interruption of light barrier  30  during up-and-down motion  33  of specimen  14  can be used, by way of the coding device of the alternating drive system, to determine the specimen size and to set the cutting window. In this context, for example, the point in time and duration of the interruption of light barrier  30  by specimen  14  is sensed. Those values can then be conveyed to the coding device of the alternating drive system in order to set the cutting window. The interruption of light barrier  30  can thus be employed to sense the specimen size and position so that a switchover between cutting speed and return speed, and thus the setting of the cutting window, can be accomplished automatically.