Abstract:
A drive device for generating oscillations, especially for the blade of a microtome, with a power generator and a power transmission element, which with the power generator is movable in a predetermined oscillation direction (S), whereby the power generator includes a combination of a permanent magnet part and a coil part, whereof one part is fixed stationary on a holder and the other part on the power transmission element, respectively, and the power transmission element is formed by a vibrational arm, which is fixed at one end to the holder and which is elastically bendable in the oscillation direction and still in all other directions.

Description:
FIELD OF THE INVENTION 
   The invention relates to a drive device for generating vibrations, in particular for precision tools and manipulators, such as for example a drive for the cutting tool of a microtome, and a microtome that is equipped with such a drive device. 
   BACKGROUND 
   Conventional microtomes for producing thin tissue sections are as a rule equipped with an eccentric drive, which sets a blade in a horizontal oscillation. By means of a feed device, the oscillating blade is passed through an object to be processed, fresh tissue for example, so that the desired section of the object is severed. The use of eccentric drives is bound up with a number of drawbacks. The oscillation frequency of the blade is limited, because with the eccentric drive the masses of a knife holder and guides also have to be accelerated. Furthermore, the oscillation amplitude of the blade is also restricted by the eccentric itself to around 1 mm as a rule. An essential drawback in the production of precision sections of biological materials is caused by the formation of vertical movements (vertical beats), which diverge from the horizontal oscillation direction of the blade. The eccentric drive generates a vertical beat at its reversal points. Furthermore, the tendency towards vertical oscillation increases at higher oscillation frequencies, at which the effective forces increase. As a result of the vertical oscillations, the tissue to be processed is separated not only in the feed direction. Cells in the tissue surface are also damaged. This makes the subsequent examination of the tissue section more difficult. 
   In a modified design of a microtome, such as is commercially available for example under the name “Vibracut 3”, manufacturer: FTB Feinwerktechnik, the knife holder is caused to oscillate with a moving lift magnet. This form of drive is disadvantageous, since the vibration frequency of the blade is tied to the resonance frequency of the knife holder. Furthermore, it is in turn not possible to rule out vertical oscillations to the extent required for precision applications, especially in microbiology and neurology. 
   The stated problems with the vibrational drive of a microtome blade also arise with other precision tools and manipulators with which a linear oscillatory motion in a predetermined oscillation direction is desired, whilst in all other directions no deflection movements take place. This concerns, for example, tools for the processing of micro-systems or micro-surgical instruments. 
   It would accordingly be advantageous to provide an improved drive device for generating vibrations, with which the drawbacks of the conventional drives for precision tools or manipulators are overcome and which in particular enables a generation of vibrations in a predetermined oscillation direction with an adjustable oscillation frequency and oscillation amplitude and without lateral deflections. It would also be advantageous to provide an improved microtome, with which the thickness of tissue sections can be reduced and the damage to tissue parts outside in the sectioning direction can be reduced. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, a drive device (precision vibrational drive) in particular is created, which contains as a power generator a combination of a magnet with a permanent magnetic field and a magnet with a periodically variable magnetic field, with which a power transmission element can be set into an oscillatory motion relative to a stationary holder according to a predetermined oscillation direction. The power transmission element is formed in particular by a vibrational arm fitted on the holder, which vibrational arm comprises at least one plane-shaped strip of elastic material which is flexible in the oscillation direction and inflexible or stiffened in all other directions. The vibrational arm is preferably formed as a U-section. The open end of the U-section is fixed to the holder and the closed end of the U-section is elastically bendable in the section plane corresponding to the oscillation direction. According to a preferred embodiment of the invention, the magnet with the permanent magnetic field is a bell-shaped magnet and the magnet with the variable magnetic field is a plunger coil which is arranged in the bell-shaped magnet. The plunger coil is fixed at or near to the closed free end of the U-section of the power transmission element and is connected electrically to an oscillation generator. 
   According to a further aspect of the invention, the precision vibrational drive is driven in such a way that a forced oscillation is imparted to the power transmission element. The frequency and amplitude of the forced oscillation can be adjusted according to the desired motion characteristics of the tool or manipulator to be operated. According to a preferred embodiment, a control circuit is implemented for the frequency and amplitude adjustment using a bending sensor on the power transmission element. 
   According to a still further aspect of the invention, a microtome is described which is equipped with the aforementioned drive device. The power transmission element serves as a support of a microtome blade. The stationary holder of the power generator or the power transmission element is fitted on a precision feed in the form of a linear table. 
   The invention has the following advantages. The drive device according to the invention delivers an oscillatory motion in a fixed oscillation direction without lateral deflections, whereby the power transmission element is tied to the oscillation direction without mechanical guides solely by means of its shape, in particular its sectional shape. The oscillation frequency and amplitude are variable and adjustable independently of one another. The microtome according to the invention is distinguished by a greatly reduced vertical stroke. It is thus possible to produce tissue sections with minimum damage to the surface. The drive device according to the invention has an extended area of application also outside the microtome application. 

   
     BRIEF SUMMARY OF THE DRAWINGS 
     Further advantages and details of the invention will become apparent from the description of the appended drawings. 
     They show: 
     FIG.  1 : a sectional view of an embodiment of the drive device according to the invention, 
       FIGS. 2   a  &amp;  b:  details of the power generator of the drive device according to  FIG. 1 , 
     FIG.  3 : an outline diagram of a microtome according to the invention, and 
       FIGS. 4   a  &amp;  b:  graphical representations to characterize the oscillatory motion of a blade of a microtome according to the invention. 
   

   DETAILED DESCRIPTION 
   The invention is described in the following by reference to a drive device for a microtome with given oscillation parameters of the blade. The invention, however, is restricted neither to the microtome application nor to the oscillation parameters given by way of example. 
   Drive device  100  according to the invention comprises, according to the diagrammatic sectional view in  FIG. 1 , a power generator  10  and a power transmission element  20 , which is movable in a predetermined oscillation direction (arrow S). The power generator comprises a permanent magnet part and a coil part. The permanent magnet part is formed by the bell-shaped magnet fitted stationary on a holder  30 , the details of which bell-shaped magnet are explained by reference to  FIGS. 2   a  &amp;  b.  The coil part is a plunger coil  12 , which projects into bell-shaped magnet  11 . Plunger coil  12  is fixed to power transmission element  20 . 
   Power transmission element  20  is formed by a vibrational arm  21 , which is fixed at one end to holder  30  and is freely mobile at opposite end  22 . Vibrational arm  21  is a U-profiled strip of a flat elastic material. This adjusts the mobility of free end  22  to bending in oscillation direction S. In all other directions diverging from oscillation direction S (in the drawing plane), vibrational arm  21  is a rigid structure immobile relative to holder  30 . At free end  22  or close to the latter, plunger coil  12  is fixed with a fixing flange  23  to vibrational arm  21 . Bell-shaped magnet  11  in the U-section and plunger coil  12  are position such that they are movable relative to one another in oscillation direction S. Generally, power transmission element  20  is designed as a support for a tool to be moved periodically or a manipulator or a tool holder. For this purpose, it has holding elements (not shown) on free end  22 , depending on the application. 
   For use in a microtome, the drive device according to the invention essentially has dimensions shown in  FIG. 1  (scale 1:1). The vibrational arm is made for example of aluminum sheet with a width of around 40 mm and a thickness of around 1 to 2 mm. The length of the vibrational arm is selected according to the application. With a length of around 10 cm, the natural frequency of represented vibrational arm  21  with a U-section amounts to around 80 Hz. If, depending on the application, larger or smaller frequency ranges (e.g. at around 10 Hz) are required, the vibrational arm should be designed accordingly with a smaller or greater length. 
   The parts of the drive device can be modified according to different embodiments. In the case of the power generator, it is possible to fix the coil part stationary on the holder and the permanent magnet part on the power transmission element. Furthermore, it is possible to use, as a permanent magnet part, a differently formed permanent magnet or also a coil which is driven with a constant excitation current. Finally, a piezo drive could also be used alternatively as a power generator. The power transmission element may also be formed by a single strip-shaped sheet (see  FIG. 2   b,  which is flexible only at right angles to the sheet plane and otherwise rigid, or a complex profiled structure with only one flexural degree of freedom. 
   Reference numeral  24  relates to a piezo bending element, which if need be is fitted as a sensor on vibrational arm  21 . The current oscillation amplitude of vibrational arm  21  can be detected with piezo bending element  24 . 
     FIG. 2  shows details of power generator  10  in the disassembled ( FIG. 2   a ) and assembled ( FIG. 2   b ) state. Bell-shaped magnet  11  is made of highly permanent material (e.g. of neodymium, B=1.17 T). The structure of the bell-shaped magnet preferably corresponds to the structure of a bell-shaped magnet known per se, such as is used in loud speakers. Plunger coil  12  comprises a coil holder  13 , coil winding  14  and a coil flange  15 . Fixing flane  23 , to which plunger coil  12  is fixed (e.g. glued) with coil flange  15 , is fitted to vibrational arm  21  of the power transmission element. 
   Plunger coil  12  is connected electrically to a supply device. This contains an adjustable oscillation generator and a power amplifier (power e.g. 25 W). The oscillation generator is designed to generate electrical oscillations in a frequency range from 30 to 130 Hz. Through the power of the power amplifier, the oscillation amplitude of vibrational arm  21  can be adjusted continuously in the range from 0 to 1.5 mm. A particular advantage of the invention consists in the adjustment of forced oscillations of the power transmission element. In the presence of excitation at a frequency, selected according to application, which is different from the natural frequency of the power transmission element (with the respective tool or manipulator), its oscillation frequency and amplitude can be freely adjusted. 
   According to a preferred embodiment of the invention, a control circuit is arranged for the adjustment of the oscillation amplitude and/or frequency. The piezo bending element  24  on vibrational arm  21  is used as a sensor. Piezo bending element  24  is connected electrically to the supply device. Depending on the current oscillation amplitude of vibrational arm  21 , the parameters of the supply device are changed for the adjustment of a certain target value. 
     FIG. 3  illustrates in a diagrammatic side view a microtome  200  according to the invention, which is equipped with drive device  100  described above. Drive device  100  is constructed in accordance with the embodiment illustrated in  FIGS. 1 and 2 . In particular, vibrational arm  21  with fixing flange  23 , plunger coil  12 , permanent magnet  11  (dashed) and holder  30  are illustrated in  FIG. 3. A  tool  40  is fixed to free end  22  of vibrational arm  21 . Tool  40  comprises a vibrating fork  41 , which forms an extension arm for the blade. Knife holder  42  with blade  43  is fitted at the end of the extension arm. Knife holder  42  is fitted in a swivelling manner relative to vibrating fork  41 . With drive device  100 , horizontally aligned blade  43  is mobile in an oscillating manner in the oscillation direction at right angles to the drawing plane. 
   Furthermore, microtome  200  comprises a feed device  50  and a dissecting area  60 , which are accommodated on a lifting table  70 . Feed device  50  includes a linear table  51  known per se, which is driven by a spindle, gearing  52  and a d.c. motor  53 . The d.c. motor is equipped with a d.c. tacho unit  54 . A speed control is not required due to the provision of a large gear reduction of around 200:1. The d.c. tacho unit generates a direct voltage which is proportional to the rate of feed and is displayed for example by a digital voltameter. 
   Dissection area  60  comprises a dissection tank  61 , in which the object to be processed, a biological tissue for example, is accommodated. In place of tank  61 , any other form of specimen holder may also be provided (e.g. a platform or a clamping support). With an adjustment device  71  on lifting table  70 , the height of blade  43  is adjusted relative to dissection tank  61 . For a rough adjustment, dissection area  60  may also be displaceable relative to lifting table  70 . Adjustment device  71  is, for example, a micrometer screw. A graduation mark corresponds to 1 μm in the example shown. 
   To use microtome  200 , a specimen is positioned in dissection tank  61 . Blade  43  is returned to a starting point by feed device  50 . The desired height of the blade is then adjusted with adjustment device  71 . Drive device  100  is actuated, so that blade  43  performs horizontal oscillations. Simultaneously, oscillating blade  43  is driven by feed device  50  through the specimen, so that a section of the specimen is lifted off and lies on blade  43 . 
   Any suitable cutting tool, for example a razor blade, can be used as blade  43 . To make optimum use of the oscillation capacity of the drive device according to the invention, which is essentially free from vertical strokes, a blade made of an inflexible material, for example a ceramic knife, is however preferred. 
   An important advantage of the invention, i.e. the considerable reduction in the vertical beat of the blade, is illustrated in  FIGS. 4   a  &amp;  b.  With a contactless inductive path sensor (e.g. sensor system IS 115, firm Micro-Epsilon), the vertical vibrations of the blade of a conventional microtome (with eccentric drive,  FIG. 4   a ) and a microtome according to the invention ( FIG. 4   b ) are recorded as a function of time. The graphical representations show a vertical beat of around 24 μm for the conventional microtome. The vertical beat of the blade with the microtome according to the invention is 5 to 2 μm in the frequency range from 30 to 130 Hz with amplitude of 1 mm. 
   In alternative applications, the drive device according to the invention is used for the actuation of tools or manipulators for microsystem technology or of microsurgical tools or instruments. Several drive devices can also be combined with one another, in that the holder of a first drive device is itself caused to oscillate with another drive device in order to generate variable oscillation directions or periodic movements of the tool or manipulator along predetermined curved paths.