Abstract:
The invention relates to a device that comprises a tool holder that can be adjusted in an x-axis, a y-axis which is perpendicular thereto, and a z-axis that is perpendicular both to the x-axis and the y-axis and that can be pivoted about the z-axis. A dispense head for solid material is mounted on the tool holder as the tool. Two scales are disposed on the dispense head for solid material, said scales weighing the material which is or is to be delivered by the dispense head for solid material. The inventive design with two scales directly mounted on the dispense head for solid material allows for weighing of the material without the dispense head for solid material or the material having to be placed on separate scales.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a device having a tool holder, which can be displaced in an x direction and a z direction which is perpendicular to the x direction, and a tool in the form of a metering head, which is secured to the tool holder. A further aspect of the invention relates to weighing out a desired quantity of substance using a device of this type. 
     Devices of this type are used, inter alia, for automatically metering substances into a plurality of reaction vessels or test tubes which are arranged, for example, next to one another. 
     In a device which is known as Caco-2 Assay produced by Mettler Toledo Bohdan, Greifensee, Switzerland, there are two tool holders with different tools. The tool holders can be displaced in a horizontal x direction, a horizontal y direction which is perpendicular to the x direction, and a vertical z direction which is perpendicular to the x and y directions, and in this way can serve reaction vessels arranged next to one another under the control of software. One of the tools is designed for metering liquid as a metering head in the form of a four-needle head with four parallel hollow needles which can be spread apart. The other tool is a gripper for handling substance plates which have a multiplicity of recesses for holding substance. To weigh matter which can be handled by the device, there is a balance, on which, by way of example, a corresponding substance plate or a test tube is placed. 
     Although the two fixedly installed tools do make it possible to handle liquids and solids, they do not, for example, allow a solid to be metered directly into a reaction vessel. Moreover, there are two tool holders which have to be able to move independently of one another, in which context it must be ensured that they do not collide with one another. Finally, accurate weighing out of a defined quantity of substance is relatively complex. 
     DE 40 02 255 A1 has disclosed a fixedly mounted device for metering liquids by dispensing them from at least one metering valve connected to a liquid reservoir, which device has a main balance on which a vessel for holding liquid can be positioned. This main balance has a wide weighing range of, for example, several tons and therefore a relatively low accuracy of, for example, ±100 g. Between the metering valve and the liquid reservoir there is a buffer vessel, the weight of which can be determined by means of a precision balance and which can be sealed off with respect to the liquid reservoir in order to dispense small quantities of liquid from the metering valve. The precision balance can be used to determine the weight of the buffer vessel and the liquid which is present therein, according to the disclosure with an accuracy of, for example, ±0.1 g, and from this determination to determine the quantity of liquid which has been dispensed. The accuracy of the weight of the quantity of liquid dispensed is limited firstly by the fact that the buffer vessel is connected to the storage reservoir and the metering valve via flexible lines, which has an adverse effect on the measurement, and secondly by the fact that the liquid is not dispensed directly from the buffer vessel, but rather firstly passes via a line to the metering valve and is only dispensed by the latter. Moreover, the complex structure with storage vessel, buffer vessel and metering valve, which are connected via lines, in practice prevents the metering device from being of mobile design or being fitted to a robot arm or a linear axis system. 
     In view of the drawbacks of the devices of the prior art which have been described above, the invention is based on the object of providing a device which is intended to allow simplified weighing out of a desired quantity of substance. 
     SUMMARY OF THE INVENTION 
     The essence of the invention consists in the fact that, in a device having a tool holder, which can be displaced in an x direction and a z direction which is perpendicular to the x direction, and a tool in the form of a metering head, which is secured to the tool holder, a balance, by means of which substance or capsules which has/have been taken up or dispensed or is/are to be dispensed by the tool can be weighed, is arranged on the tool or on the tool holder. 
     The fact that a balance is arranged directly on the tool or on the tool holder allows a substance which has been taken up or dispensed or is to be dispensed, a substance capsule or another object to be weighed without the substance, the substance capsule or the other object or the tool for this purpose having to be placed onto a separate balance. This significantly simplifies the weighing operation and also means that the weighing is virtually location-independent within the range of action of the device and can even take place where, for technical reasons, it is difficult or impossible to position a balance, for example beneath a shaken reaction vessel. 
     The balance used may, for example, be a balance having at least a weighing range from 0 to 2 kg and an accuracy of 0.1 g. Balances of this type are available, for example, from Sartorius AG, 37070 Göttingen, Germany. However, it is preferable to use a more accurate balance with an accuracy of 0.1 mg. 
     Preferably, the substance or capsule(s) can be dispensed or taken up by a metering means which is also weighed by the balance. As a result, any substance which has remained attached to the metering means is always weighed as well and is not recorded as already having been metered in. 
     Advantageously, the metering head carries all the substance which is to be dispensed with it. Consequently, it does not have to be supplied, for example via flexible hoses, which would have an adverse effect on the weighing accuracy. 
     In an advantageous exemplary embodiment, the balance is arranged on the tool, and the tool can be detached from and refitted to the tool holder without screws having to be undone. 
     Preferably, the metering means is arranged on the balance in such a way that the metering means can be detached from and refitted to the balance without screws having to be undone, in particular by being lifted off and put back on. As a result, it is easy to use different types of metering means in order, for example, to meter liquids or solid substances in succession. The handling of the metering means may take place manually or automatically. 
     Advantageously, the metering means has a metering unit, which comprises a storage vessel, and a drive unit, it being possible for the metering unit to be removed from and refitted to the drive unit without screws having to be undone, in particular by being lifted off and put back on. As a result, it is possible to prepare different substances in a plurality of metering units and then to meter them successively using the same drive unit. The metering units can be handled manually or automatically. 
     In a variant embodiment which is advantageous for certain tools, the balance bears a vessel for temporarily holding substance which is to be dispensed, which vessel can be completely emptied, the vessel preferably being the concave part of a spoon which can be tilted in order to be completely emptied. This allows substance which is to be dispensed to be weighed accurately in a vessel, which then, depending on the results, is either emptied completely at the metering location, for example into a reaction vessel, i.e. the substance is definitively discharged, or is filled further or, in particular if an excessive quantity of substance has been measured, is emptied at a location other than the metering location and is then refilled. 
     Advantageously, in addition to the first balance arranged on the tool or on the tool holder, the device according to the invention also has a second balance, the second balance preferably bearing the vessel for temporarily holding substance which is to be dispensed and being used to measure the weight of substance to be dispensed which is being temporarily held, while the first balance can be used also to measure the weight of substance which has not yet been dispensed to the vessel for temporarily holding substance which is to be dispensed. This allows more accurate weight measurement, in particular of substance which is to be dispensed, with the aid of checking measurements carried out by the second balance. 
     In a preferred exemplary embodiment, the tool holder can rotate about the z direction. This in particular allows the tool to rotate through, for example, 90°, i.e. allows, by way of example, a multi-needle head having a plurality of hollow needles arranged next to one another to be used to meter substances, which may differ according to the hollow needle used, to vessels belonging to a matrix in rows, then allows the multi-needle head to be rotated through 90° and substances, which once again may differ according to the hollow needle used, to be metered to the vessels of the matrix in columns. It is thus possible for a different combination of substances to be metered to each vessel of the matrix in a simple way. Moreover, the rotation allows reaction vessels, starting-material bottles, etc. to be arranged over an area and not just on a straight line. 
     Preferably, the tool holder can additionally be displaced in a y direction, which is perpendicular to the x direction and the z direction. This enables reaction vessels, starting-material bottles, etc. to be arranged over a larger area. 
     In an advantageous variant embodiment, the tool is secured to the tool holder by means of magnets, in which case it is preferable, where there are two permanent magnets which attract one another, for one of the two permanent magnets to be arranged on the tool holder and the other of the two permanent magnets to be arranged on the tool, and for it to be possible for the action of the attraction between the two permanent magnets to be cancelled out by means of at least one electromagnet. Connecting tool and tool holder by means of magnets allows automatic securing of the tool to the tool holder, for example by the tool holder being guided over the tool and then lowered onto it or the tool holder being moved laterally onto the tool. Detaching the tool from the tool holder by activating the at least one electromagnet by means of current pulses also contributes to enabling the tool change to take place automatically. 
     In alternative advantageous variant embodiments, the tool is secured to the tool holder by screw connection, by means of a bayonet connection or by means of a clamping connection, etc. Although these methods of securing are normally more complex to implement, they are relatively simple to automate, in particular if the tool holder can be rotated about the z direction. 
     Preferably, the tool is a screw metering head, which comprises a screw which can rotate forward and backward about the z direction in a tube which is at least partially open at its lower end and which can be used to take up and dispense substance. A screw metering head of this type can be used for targeted removal of pulverulent or liquid substance from a storage vessel and also for targeted dispensing of this substance. 
     Advantageously, the lower open end of the tube can be closed off by a diaphragm provided with holes, and there is preferably a ram, which runs on the screw and presses substance through the diaphragm as the screw rotates when substance is being dispensed, arranged in the tube. The use of a diaphragm leads to more uniform dispensing of substance, since the substance is forced uniformly through the holes in the diaphragm. This in turn has the advantage that metering can be carried out more accurately. 
     Preferably, at the diaphragm there is a stripper which periodically strips off any substance adhering to the diaphragm. This allows more accurate metering. 
     Advantageously, the tool is a capsule-transporting head, by means of which a capsule can be picked up and released, preferably by suction. A tool of this type makes it possible to transport substances in capsules or similar containers. 
     Preferably, the tool is a matrix-capsule-transporting head, by means of which capsules which are arranged in the manner of a matrix can be picked up, preferably by suction, and the capsules can be released individually, together or in groups. The matrix-capsule-transporting head also makes it possible to transport substances in capsules, it being possible for a large number of capsules which are arranged in matrix form to be handled at the same time. 
     Advantageously, the tool is a capsule-handling head, by means of which at least one capsule can be picked up, which capsule can be opened in the tool, preferably by means of a hollow needle, and in which tool the contents of the capsule can preferably be mixed with another substance, in particular a solvent. The mixing can be effected, for example, by adding solvent to the capsule, sucking up substance and solvent from the capsule and returning the material which has been sucked up into the capsule. Alternatively, the hollow needle can also be used to suck substance out of the capsule and dispense it again at another location. The capsule-handling head according to the invention makes it possible to prepare even more successfully for chemical reactions outside a reaction vessel. 
     In a preferred variant embodiment, the tool is a matrix-capsule-handling head, by means of which a plurality of capsules which are arranged in the form of a matrix can be picked up, which capsules can be opened in the tool, preferably using hollow needles, and in which tool the contents of one capsule can preferably in each case be mixed with another substance, in particular a solvent. The mixing can be effected, for example, by adding solvent to the capsule, sucking up substance and solvent from the capsule and returning the material which has been sucked up into the capsule. Alternatively, the hollow needle can also be used to suck substance out of the capsule and dispense it again at another location. The matrix-capsule-handling head also makes it possible to handle substances in capsules and to prepare for chemical reactions, it being possible for a multiplicity of capsules which are arranged in the form of a matrix to be picked up and processed simultaneously. 
     In another preferred variant embodiment, the tool is a capsule-dispensing head, in which a multiplicity of capsules are stored and can be dispensed individually, together or in groups, it preferably being possible for the capsules to be opened in the capsule-dispensing head, and it even more preferably being possible for the contents of the capsules to be mixed with another substance, in particular a solvent, in the capsule-dispensing head. The capsule-dispensing head according to the invention makes it possible to prepare for chemical reactions largely outside a reaction vessel and means that the appropriate capsules or the contents thereof simply have to be added to the reaction vessel in order to carry out these chemical reactions. 
     Advantageously, the tool is a needle head with a hollow needle, a multi-needle head with a plurality of hollow needles, which can preferably be displaced individually in the z direction and/or the distance between which can preferably be adjusted, or a solids-metering head. 
     Advantageously, the tool is a combination head having at least two identical or different tool parts, one of the tool parts preferably being a needle head, multi-needle head, capsule-transporting head, matrix-capsule-transporting head, capsule-handling head, matrix-capsule-handling head, capsule-dispensing head, screw metering head or solids-metering head. This allows a plurality of method steps to be carried out in succession or simultaneously using a single tool. 
     Advantageously, the device according to the invention has a camera, which is preferably arranged on the tool holder and which can be used to film an area below the tool holder, as well as a control computer having an image-processing unit, which evaluates images which have been filmed by the camera, it being possible for the displacement of the tool holder and, if necessary, a change of tool to be controlled preferably on the basis of the evaluation result. 
     In an advantageous variant embodiment, the device according to the invention has an infrared analysis unit, which is preferably arranged on the tool holder and has an infrared transmitter, by means of which infrared waves can be radiated into an area below the tool holder, and an infrared sensor, which can be used to measure reflected infrared waves, as well as a control computer having a measured-value-processing unit, which evaluates the reflected infrared waves measured by the infrared sensor, it preferably being possible for the displacement of the tool holder and, if necessary, a change of tool and/or the quantity of substance to be metered to be controlled on the basis of the evaluation result. The precise way in which an infrared analysis unit of this type functions is described, for example, in U.S. Pat. No. 6,031,233, which is hereby specifically incorporated by reference in the present description. 
     The camera or the infrared analysis unit, together with the control computer, allows the device to operate completely automatically without an operator having to evaluate the substance or capsule to be handled and then actively control the displacement of the tool holder and/or any change of tool which may be required. 
     In an advantageous variant embodiment, the device according to the invention comprises a further tool holder for attachment of a further tool which can be displaced in an x direction and in a z direction which is perpendicular to the x direction, it preferably additionally being able to rotate about the z direction and/or to be displaced in a y direction which is perpendicular to the x direction and to the z direction. The second tool holder may be designed and controlled in the same way as the first. With two or even more tool holders with tools attached to them, it is possible to multiply the speed of the device; at the control, it must be ensured that the various tool holders and tools do not impede one another. 
     A method according to the invention for weighing out a desired quantity of substance using a device having a tool holder, which can be displaced in an x direction and a z direction which is perpendicular to the x direction, and a tool in the form of a metering head, which is secured to the tool holder, and a balance arranged on the tool or on the tool holder, by means of which substance which has been taken up by the tool can be weighed, is characterized by the steps that
     a) substance is taken up by the tool;   b) the substance is weighed;   c) the difference between the weighed value obtained and the desired set value is calculated; and   d) if the difference lies outside the range of a desired level of accuracy, the tool is used to discharge substance or take up additional substance depending on this difference;
 
steps b) to d) being repeated until the difference is equal to zero within the range of a desired level of accuracy.
   

     A similar method according to the invention for dispensing a desired quantity of substance using a device having a tool holder, which can be displaced in an x direction and a z direction which is perpendicular to the x direction, and a tool in the form of a metering head, which is secured to the tool holder, and a balance which is arranged on the tool or on the tool holder and can be used to weigh substance which is to be dispensed from the tool, the balance bearing a vessel for temporarily holding substance which is to be dispensed, which can be completely emptied, is characterized by the steps that
     a) a quantity of substance is placed into the vessel for temporarily holding substance which is to be dispensed;   b) the substance in the vessel is weighed;   c) the difference between the weighed value obtained and the desired set value is calculated; and   d) if the difference lies outside the range of a desired level of accuracy, additional substance is added to the vessel or the vessel is at least partially emptied at a location other than an intended metering location and then substance is added to it again, depending on this difference;
 
steps b) to d) being repeated until the difference is equal to zero within the range of a desired level of accuracy, after which the substance which is present in the vessel is dispensed by the vessel being completely emptied.
   

     A further similar method according to the invention for selecting a capsule with a desired quantity of substance using a device having a tool holder, which can be displaced in an x direction and a z direction which is perpendicular to the x direction, and a tool in the form of a metering head, which is secured to the tool holder, and a balance which is arranged on the tool or on the tool holder and can be used to weigh capsules which have been picked up by the tool, is characterized by the steps that
     a) the tool is used to pick up a capsule containing substance;   b) the capsule with substance is weighed;   c) the difference between the weighed value obtained and the desired set value is calculated; and   d) if the difference lies outside the range of a desired level of accuracy, the capsule is released again from the tool and a new capsule containing substance is picked up;
 
steps b) to d) being repeated until the difference is equal to zero within the range of a desired level of accuracy.
   

     These three weighing methods which operate in accordance with the test principle make it easy to weigh out a desired quantity of substance or a desired object with the desired level of accuracy at any desired location within the area of action of the device. Moreover, for example when substance is being dispensed into, for example, a reaction vessel, a test tube, a substance plate, etc., it is possible for the weight of the quantity of substance which has effectively been dispensed to be measured again. This has two important advantages: 1) Monitoring and more accurate determination of the effective value. 2) If, for example, small quantities of substance remain attached to the tool, this is determined and can be corrected, for example by vibration or topping up the metering. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The devices according to the invention are described in more detail below with reference to the appended drawings and on the basis of exemplary embodiments. In the drawings: 
         FIG. 1  shows a tool holder which can be displaced in all three spatial directions x, y and z on a linear axis system and can rotate about the z direction; 
         FIG. 2  shows the tool holder from  FIG. 1 , but additionally with a balance arranged thereon, having a needle head with a hollow needle as tool; 
         FIG. 3  shows the tool holder from  FIG. 1 , but additionally with a balance arranged thereon, having a needle head with four hollow needles which can be displaced with respect to one another as tool, the four hollow needles being at a minimum distance from one another; 
         FIG. 4  shows the tool holder with needle head from  FIG. 3 , with the four hollow needles at a maximum distance from one another; 
         FIG. 5  shows the tool holder from  FIG. 1  with a capsule-transporting head as tool; 
         FIG. 6  shows the capsule-transporting head from  FIG. 5  when it is holding a capsule; 
         FIG. 7  shows the capsule-transporting head from  FIG. 5  when a capsule is being placed in a reaction vessel arranged in a matrix; 
         FIG. 8  shows the tool holder from  FIG. 1  with a matrix-capsule-transporting head as tool; 
         FIG. 9  shows a sectional view of a tool in the form of a capsule-handling head with hollow needle; 
         FIG. 10  shows the capsule-handling head from  FIG. 9  on the tool holder from  FIG. 1  with a closed capsule which has been picked up; 
         FIG. 11  shows the capsule-handling head with a capsule which has been picked up as shown in  FIG. 10  during the addition of solvent after the capsule has been punctured by the hollow needle; 
         FIG. 12  shows the capsule-handling head with punctured capsule as shown in  FIG. 11  when the capsule, which now contains dissolved substance, is being dispensed; 
         FIG. 13  shows the tool holder from  FIG. 1 , but additionally with a balance arranged thereon, with a diagrammatically depicted matrix-capsule-handling head as tool and capsules arranged in a matrix; 
         FIG. 14  shows a sectional view of a tool in the form of a first exemplary embodiment of a capsule-dispensing head having a multiplicity of stored capsules at the tool holder shown in  FIG. 1 ; 
         FIG. 15  shows a sectional view of a tool in the form of a second exemplary embodiment of a capsule-dispensing head having a multiplicity of stored capsules which can be opened in the capsule-dispensing head, at the tool holder shown in  FIG. 1 ; 
         FIG. 16  shows the tool holder shown in  FIG. 1  with a screw metering head as tool, with a diaphragm which has been pivoted away, in a partially sectional illustration; 
         FIG. 17  shows the tool holder with screw metering head from  FIG. 16  with a diaphragm which has been pivoted under the screw, in a partially sectional view; 
         FIG. 18  shows the tool holder from  FIG. 1  with a solids-metering head as tool; 
         FIG. 19  shows a tool holder with an alternative screw metering head with weighing unit, metering unit and drive unit as tool; 
         FIG. 20  shows the weighing unit of the screw metering head shown in  FIG. 19 ; 
         FIG. 21  shows a perspective view of the metering unit of the screw metering head shown in  FIG. 19 ; 
         FIG. 22  shows the metering unit of the screw metering head shown in  FIG. 19  in an exploded view; and 
         FIG. 23  shows the drive unit of the screw metering head from  FIG. 19 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     
       FIG. 1 
     
     A linear axis system for holding and displacing a tool holder  1  comprises two guide rails  6 ,  61 , which run parallel to one another in the y direction and are anchored in a fixed position in a manner which is not illustrated. The first ends of the two guide rails  6 ,  61  are connected by a rotary rod  7 , which can be rotated by means of a stepper motor  71 . An upper running rail  5  is secured to the two guide rails  6 ,  61  in such a manner that it can be displaced in the y direction. The upper running rail  5  is fixedly connected to a lower running rail  51  by means of two end plates  52 ,  53 . As a result of the rotary rod  7  being rotated by means of the stepper motor  71 , in each case one toothed belt in the interior of the guide rails  6 ,  61  is driven, causing the running rails  5 ,  51  to be displaced in the y direction. In the present context, the term displacement in the y direction is to be understood as meaning both a displacement in the +y direction and in the −y direction (the opposite direction). 
     A carriage  4  is secured to the two running rails  5 ,  51  in such a manner that it can be moved in the x direction. In the present context, the term movement in the x direction is once again to be understood as meaning both a movement in the +x direction and in the −x direction (the opposite direction). The carriage  4  is driven by a stepper motor  54  via a toothed belt arranged in the hollow upper guide rail  5 . 
     A tool rod  3  is secured to the carriage  4  in such a manner that it can move in the z direction. In the present context, the term movement in the z direction is once again to be understood as meaning both a movement in the +z direction and in the −z direction (the opposite direction). In order for the tool rod  3  to be displaced, a stepper motor  31  is attached to it via a hollow plate  32 , and a toothed belt is arranged in the hollow plate  32  and the tool rod  3 . 
     At the lower end of the tool rod  3  there is a rotary drive  2 , to which the tool holder  1  is secured. The tool holder  1  can be rotated both ways about the z direction, as indicated by the arrow c, with the aid of a rotary motor  21 . In order to secure a tool, the tool holder  1  substantially consists of a permanent magnet, in which an electromagnet is arranged. 
     A camera  10 , which is directed downward in the z direction and can be used to film an area below the tool holder  1 , is attached to the tool holder  1 . The images which are filmed by the camera  10  are transmitted via a data line to an image-processing unit of a control computer  11 , which evaluates these images. The control computer  11  can then control the displacement of the tool holder  1  in the x, y, z and c directions by means of the motors  54 ,  71 ,  31  and  21  and the selection, securing or release of a tool on the basis of the evaluation results. 
     The following consideration applies to the whole of the remainder of the description. If a figure includes reference symbols which are provided for the purpose of clarity of the drawing but these reference symbols are not mentioned in the immediately associated text of the description, or vice versa, reference is made to the corresponding explanations given in preceding descriptions of figures. 
     
       FIG. 2 
     
     In this case, a needle head  100  is removably secured as the tool to the holder  1  by means of a permanent magnet  101 . The permanent magnet  101  of the needle head  100  and the permanent magnet of the tool holder  1  attract one another, so that when the needle head  100  is removed it can be secured to the tool holder  1  by being placed onto the latter, an operation which can be performed automatically, i.e. the needle head  100  does not have to be attached to the tool holder  1  manually. The needle head  100  is detached from the tool holder  1  by means of the electromagnet which is arranged in the tool holder  1 , cannot be seen and, when it receives a current pulse, cancels out the action of the attraction between the permanent magnet  101  of the needle head  100  and the permanent magnet of the tool holder  1 . 
     A linear drive  103  is attached to the permanent magnet  101  via a plate  102 . A hollow needle  105  is secured to the outer cylinder of the linear drive  103  by means of two holding parts  104 , which are provided with continuous receiving holes for the hollow needle  105 . With the aid of the linear drive  103 , the hollow needle  105  can be displaced in the z direction. 
     A hollow needle  105  of this type can be used, for example, to meter or remove liquid substances into or from reaction vessels. In particular, for this purpose a suction and/or blowing means can be connected to the top end of the hollow needle  105 . 
     Unlike in  FIG. 1 , a balance  9 , which can be used to measure the total weight of the tool holder  1 , the needle head  100  and the substance which is present in the hollow needle  105 , is additionally arranged on the tool holder  1  below the rotary drive  2 . If the weight of the tool holder  1  and the needle head  100  is subtracted from this total weight, the result is the weight of the substance which is present in the hollow needle  105 . The weight of substance which has been taken up or dispensed can be determined by differential measurements. 
     
       FIGS. 3 and 4 
     
     The tool is in this case formed by a needle head  120  with four hollow needles  125 , which can be individually displaced in the z direction and the distance between which can be adjusted from a minimum distance a min  to a maximum distance a max , the distance between each pair of adjacent hollow needles  125  always being identical. To this end, the hollow needles  125  are each secured to the outer cylinder of a linear drive  123  by means of two holding parts  124  which are provided with continuous hollow-needle receiving holes. The linear drives  123  which can be used to displace the hollow needles  125  individually in the z direction are for their part in each case attached to an associated plate  122 . The four plates  122  are arranged movably in two grooves in a permanent magnet  121 , the drive for this purpose being effected by means of two spindles which are driven by a motor and are located inside the permanent magnet  121 . The needle head  120 , as described in connection with  FIG. 2 , is connected to the tool holder  1  via the permanent magnet  121 . Once again, the needle head  120  is detached from the tool holder  1  by means of the electromagnet (not visible) arranged in the tool holder  1 . 
     A needle head  120  of this type can be used, for example, to meter liquid to or remove liquid from a plurality of reaction vessels simultaneously. In particular, suction and/or blowing devices can be connected to the top end of the hollow needles  125  for this purpose. 
     Unlike in  FIG. 1 , a balance  9 , which can be used to measure the total weight of the tool holder  1 , the needle head  120  and the substance which is present in the hollow needles  125 , is additionally arranged on the tool holder  1  below the rotary drive  2 . If the weight of the tool holder  1  and of the needle head  120  is subtracted from this total weight, the result is the weight of the substances which are present in the hollow needles  125 . The weight of substances which have been taken up or dispensed can be determined by means of differential measurements. 
     
       FIGS. 5 to 7 
     
     The tool is in this case formed by a capsule-transporting head  140 , by means of which a tightly closed capsule  150 , which is in the form of a small tube and contains a pulverulent substance  151 , can be picked up by suction. The capsule-transporting head  140  comprises a permanent magnet  141 , by means of which, as described in a corresponding way in connection with  FIG. 2 , it is connected to the tool holder  1 . It can be released by means of the electromagnet arranged in the tool holder  1 . A suction tube  143  having a capsule-holding end piece  144  is attached to the permanent magnet  141  via a balance  145  and an intermediate part  142 . A reduced pressure can be generated in the suction tube  143  by means of a conventional suction means (not shown). 
     To pick up a capsule  150 , the capsule-transporting head  140  is moved such that the capsule-holding end piece  144  is above the top end of the capsule  150 , and then the capsule  150  is picked up as a result of a reduced pressure being generated in the suction tube  143 , as illustrated in  FIG. 6 . Then, the capsule  150  is transported by the linear axis system to the intended location, in  FIG. 7  a reaction vessel  171  arranged in a matrix  170 , where it is released into the reaction vessel  171  as a result of the reduced pressure in the suction tube  143  being eliminated. 
     The balance  145  can be used to measure the total weight of the intermediate part  142 , the suction tube  143  with the capsule-holding end piece  144  and the capsule  150  filled with substance  151  which it has picked up. If the weight of the intermediate part  142  and the suction tube  143  with the capsule-holding end piece  144  are subtracted from this total weight, the result is the weight of the capsule  150  filled with substance  151 . The weight of the substance  151  in the capsule  150  can be determined by differential measurements using an empty capsule  150 . 
     
       FIG. 8 
     
     The tool is in this case formed by a matrix-capsule-transporting head  160  which comprises a permanent magnet  161 , by means of which, as has been described in a corresponding way in connection with  FIG. 2 , it is connected to the tool holder  1 . It is released by means of the electromagnet arranged in the tool holder  1 . Sixteen suction tubes  163 , which are arranged in the form of a matrix and each have a capsule-holding end piece  164 , are attached to the permanent magnet  161  via a balance  165  and a suction-tube plate  162 . A reduced pressure can be generated in the suction tubes  163  via the suction-tube plate  162  by means of a conventional suction means (not shown). 
     To pick up capsules  150 , the matrix-capsule-transporting head  160  is moved such that the capsule-holding end pieces  164  are above the top ends of the capsules  150 , and then the capsules  150  are picked up as a result of a reduced pressure being generated in the suction tubes  163 . Then, the capsules  150  are transported by the linear axis system to the intended location, in this case reaction vessels  171  arranged in a matrix  170 , where the capsules  150  are dispensed into the reaction vessels  171  as a result of the reduced pressure in the suction tubes  163  being eliminated. 
     The balance  165  can be used to measure the total weight of the suction tube plate  162 , the suction tubes  163  with the capsule-holding end piece  164  and the capsules  150  filled with substances which they have picked up. If the weight of the suction tube plate  162  and the suction tubes  163  with the capsule-holding end pieces  164  is subtracted from this total weight, the result is the weight of the capsules  150  filled with substances. The weight of the substances in the capsules  150  can be determined by differential measurements using empty capsules  150 . 
     
       FIGS. 9 to 12 
     
     In this case, the tool is formed by a capsule-handling head  220 , which comprises a cylindrical housing  221  which is divided into two compartments  223  and  224  by a partition  222  and is closed off at the top by an end wall  227 . At the open end of the bottom compartment  223 , in the cylindrical housing  221 , there is an air-filled sleeve  225 , for example made from rubber, which in the unladen state as shown in  FIG. 9  has an internal diameter d min . In the upper compartment  224  there is a plunger  226 , to which a plunger rod  228 , which projects out through the end wall  227  and is provided at its top end with an outer push-button  229 , is attached. Between the plunger  226  and the cylindrical housing  221  and between the plunger rod  228  and the end wall  227  there is in each case an annular seal  230 ,  231 . Between the plunger  226  and the partition  222  there is a coil spring  232 , which in the unladen state holds the plunger  226  in the position shown in  FIG. 9 . Between the plunger  226  and the end wall  227  there is an air-filled space  233 , which is in communication with the interior of the sleeve  225  via an air line  234 . 
     In addition, the capsule-handling head  220  comprises a hollow needle  235 , to which an inner push-button  236  is attached. The inner push-button  236  is mounted movably in a recess  237  in the outer push-button  229 , a coil spring  238  being arranged in the recess  237  below the inner push-button  236 , which coil spring  238 , in the unladen state, holds the inner push-button  236  and the hollow needle  235  in the position shown in  FIG. 9 . The hollow needle  235  passes through the plunger rod  228 , the plunger  226  and the partition  222 . It is in communication with the internally hollow inner push-button  236 , which can be fed, for example, with a solvent or another liquid via a feed line  239 . 
       FIG. 10  shows the capsule-handling head  220  after it has picked up a capsule  150 , an operation which can be effected by placing the capsule-handling head  220  onto the capsule  150 . The capsule  150  is held by the sleeve  225 , which now has an internal diameter d which corresponds to the external diameter of the capsule  150  and is greater than the internal diameter d min  in the stress-free state. 
       FIG. 10  also illustrates that the capsule-handling head  220  comprises a balance  241  and a permanent magnet  240 , via which, as described in a corresponding way in connection with  FIG. 2 , it is connected to the tool holder  1 . The capsule-handling head  220  is detached from the tool holder  1  by means of the electromagnet arranged in the tool holder  1 . Moreover, the figure diagrammatically indicates that the inner push-button  236  can be actuated by a rotary lever  242  and the outer push-button  229  can be actuated by a rotary lever  244 , the two rotary levers  242 ,  244  being articulatedly mounted on a rod  243 , which is secured to the balance  241  by means of a bearing part  245 , in such a manner that they can rotate in the direction indicated by the arrows. The drives for the two rotary levers  242 ,  244 , which are controlled by the control computer, are not shown.  FIGS. 9 ,  11  and  12  do not show the permanent magnet  240 , the balance  241 , the two rotary levers  242 ,  244 , the rod  243 , the bearing part  245  and the tool holder  1 , for reasons of clarity. 
     The balance  241  can be used to measure the total weight of the capsule  150  which has been picked up by the capsule-handling head  220  and is filled with substance and of the capsule-handling head  220  with the exception of the permanent magnet  240  and the balance  241  itself. If the weight of the capsule-handling head  220  with the exception of the permanent magnet  240  and the balance  241  is subtracted from this total weight, the result is the weight of the capsule  150  filled with substance. The weight of the substance in the capsule  150  can be determined by differential measurements using an empty capsule  150 . 
     The coil spring  238  is compressed as a result of the inner push-button  236  being pushed downward, and as a result the hollow needle  235  is forced into the capsule  150 , as illustrated in  FIG. 11 . As a result, the capsule  150  is opened and it can be supplied, via the hollow needle  235 , with a substance from the inner push-button  236 , which is fed via the feed line  239 . Alternatively, the feed line  239  could also be connected directly to the hollow needle  235 . The substance supplied, in particular a solvent, can be mixed with the substance which is already present in the capsule  150 , for example by the capsule-handling head  220  being shaken. If a sufficiently long hollow needle is used, the mixing could also be effected by the substances which are present in the capsule  150  being sucked up and discharged again a number of times. 
     If pressure is no longer being exerted on the inner push-button  236 , the coil spring  238  forces it back upward into the starting position. 
     In order for the capsule  150  to be released, the outer push-button  229  is pressed downward, as illustrated in  FIG. 12 . In the process, the plunger rod  228  and the plunger  226  are moved downward so as to compress the coil spring  232 , with the result that the size of the space  233  between the plunger  226  and the end wall  227  is increased greatly and a reduced pressure is generated therein. This reduced pressure causes air to be extracted from the interior of the sleeve  225  via the air line  234 , with the result that the internal diameter of the sleeve  225  is increased to a maximum value d max , which is greater than the external diameter of the capsule  150 , so that the capsule  150  is no longer held by the sleeve  225  and drops downward under the force of gravity. 
     If pressure is no longer being exerted on the outer push-button  239 , the coil spring  232  forces it back upward into the starting position shown in  FIG. 9 . 
     
       FIG. 13 
     
     The tool is in this case formed by a matrix-capsule-handling head  250 , which comprises a holding plate  255  which is removably connected to the tool holder  1  by means of a permanent magnet, in a manner which is not illustrated. The matrix-capsule-handling head  250  is detached from the tool holder  1  by means of the electromagnet which is arranged in the tool holder  1  and the power supply line  8  of which can be seen. Two rods  252 ,  253 , which are fixedly connected to the holding plate  255 , extend upward in the z direction, i.e. vertically, from two diagonally opposite corner regions of the holding plate  255 . A release plate  254 , which can be displaced in the z direction and is guided by the rods  252 ,  253  in two diagonally opposite corner regions, is arranged above the holding plate  255 . A trigger plate  251  located above the release plate  254  can likewise be displaced in the z direction and is guided by the two rods  252 ,  253 . The vertical displacement of the release plate  254  and of the trigger plate  251  is effected by two motors (not shown), although in principle it could also be brought about manually. 
     Sixteen capsule-handling elements  256  are secured in the holding plate  255 . The capsule-handling elements  256 , which are only diagrammatically depicted in this figure, apart from the connecting part  241  and the permanent magnet  240 , are constructed in substantially the same way as the capsule-handling heads  220  shown in  FIGS. 9 to 12  and each comprise, in addition to a cylindrical housing  221 , an outer push-button  229  and an inner push-button  236 . The inner push-buttons  236  with the hollow needles attached to them can be actuated jointly as a result of the trigger plate  251  being lowered. The joint actuation of, the outer push-buttons  229  is effected as a result of the release plate  254  being lowered. The matrix-capsule-handling head  250  can be used to take hold of sixteen capsules  150  arranged in a matrix  149  together, to open each of them by means of a hollow needle  235  and if appropriate to mix the substances contained therein with other substances and release them again. 
     Unlike in  FIG. 1 , a balance  9 , which can be used to measure the total weight of the tool holder  1 , the matrix-capsule-handling head  250  and the capsules  150 , which have been picked up by it and are filled with substances, is additionally arranged on the tool holder  1  beneath the rotary drive  2 . If the weight of the tool holder  1  and of the matrix-capsule-handling head  250  is subtracted from this total weight, the result is the weight of the capsules  150  filled with substances. The weight of the substances in the capsules  150  can be determined by means of differential measurements using empty capsules. 
     
       FIG. 14 
     
     The tool is in this case a first exemplary embodiment of a capsule-dispensing head  280 , which comprises a balance  296  and a permanent magnet  295 , by means of which, as has been described in a corresponding way in connection with  FIG. 2 , it is connected to the tool holder  1 . The removal of the capsule-dispensing head  280  from the tool holder  1  is effected by means of the electromagnet arranged in the tool holder  1 . 
     The capsule-dispensing head  280  comprises a substantially cylindrical housing  281 , the lower part of which narrows to form a neck  282  and in which a large number of capsules  150 , which each contain a substance  151 , are stored. One of the capsules  150  is held by an air-filled sleeve  283 , which is arranged in the neck  282  and is made, for example, from rubber. In a separate cylinder  284  there is a plunger  285 , to which a plunger rod  286 , which projects out through an end wall  287  of the cylinder  284  and is provided at its top end with a push-button  288 , is attached. Between the plunger  285  and the cylinder  284  and between the plunger rod  286  and the end wall  287  there is in each case an annular seal  289 ,  290 . Between the plunger  285  and the base  291  of the cylinder  284  there is a coil spring  292 , which in the stress-free state holds the plunger  285  in the position illustrated. Between the plunger  285  and the end wall  287  there is an air-filled space  293 , which is in communication with the interior of the sleeve  283  via an air line  294 . 
     In order for the capsule  150  which is being held by the sleeve  283  to be released, the push-button  288  is pressed downward. In the process, the plunger rod  286  and the plunger  285  are moved downward so as to compress the coil spring  292 , with the result that the size of the space  293  between the plunger  285  and the end wall  287  is increased greatly and a reduced pressure is generated therein. This reduced pressure causes air to be extracted from the interior of the sleeve  283  via the air line  294 , with the result that the internal diameter of the sleeve  283  is increased to a value which is greater than the external diameter of the capsule  150 , so that the capsule  150  is no longer held by the sleeve  283  and drops downward under the force of gravity. At the same time, a second capsule  150  moves up to take the place of the first capsule  150 , it being important for the pressure on the push-button  288  to be released again sufficiently quickly, so that the coil spring  292  moves the plunger  285  back upward into the starting position, the size of the space  293  is reduced again and air is fed back to the sleeve  283  via the air line  294  sufficiently quickly for the capsule  150  to be gripped by the sleeve  283 . 
     Moreover, the figure diagrammatically indicates that the push-button  288  can be actuated by a rotary lever  297 , the rotary lever  297  being articulatedly mounted on a rod  298  in such a manner that it can rotate in the direction of the arrow, this rod in turn being secured to the balance  296  by means of a bearing part  299 . The drive of the rotary lever  297 , which is controlled by the control computer, is not illustrated. 
     The balance  296  can be used to measure the total weight of the capsules  150  which are present in the capsule-dispensing head  280  and are filled with substances and of the capsule-dispensing head  280 , with the exception of the permanent magnet  295  and the balance  296  itself. The weight of a capsule  150  filled with substance can be measured by measuring the weight difference before and after a capsule  150  has been dispensed. The weight of the substance in the capsule  150  can be determined by differential measurements using an empty capsule  150 . 
     
       FIG. 15 
     
     The tool is in this case a second exemplary embodiment of a capsule-dispensing head  300 , which comprises a balance  318  and a permanent magnet  317 , by means of which, as has been described in a corresponding way in connection with  FIG. 2 , it is connected to the tool holder  1 . The removal of the capsule-dispensing head  300  from the tool holder  1  is effected by means of the electromagnet arranged in the tool holder  1 . 
     The capsule-dispensing head  300  comprises a substantially cylindrical housing  301 , which in its lower part narrows to form a neck  302  and in which a multiplicity of capsules  150 , which each contain a substance  151 , are stored. One of the capsules  150  is held by an air-filled sleeve  303 , which is arranged in the neck  302  and is made, for example, from rubber, while the other capsules  150  are arranged in the cylindrical housing  301  in a chamber part  315  which can rotate in the manner of a revolver as indicated by arrow E. In a separate cylinder  304  there is a plunger  305 , to which a plunger rod  306 , which projects out through an end wall  307  of the cylinder  304  and is provided at its top end with a push-button  308 , is attached. Between the plunger  305  and the cylinder  304  and between the plunger rod  306  and the end wall  307  there is in each case an annular seal  309 ,  310 . Between the plunger  305  and the base  311  of the cylinder  304  there is a coil spring  312 , which in the stress-free state holds the plunger  305  in the position illustrated. Between the plunger  305  and the end wall  307  there is an air-filled space  313 , which is in communication with the interior of the sleeve  303  via an air line  314 . 
     In addition, the capsule-dispensing head  300  comprises a hollow needle  316 , which passes through the push-button  308 , the plunger rod  306 , the plunger  305  and the base  311 . As a result of the hollow needle  316  being forced downward, the capsule  150  which is located above the capsule which is held by the sleeve  303  can be punctured. If necessary, another substance, in particular a solvent, can be fed to the open capsule  150  via the hollow needle  316 . 
     In order for the capsule  150  which is being held by the sleeve  303  to be released, the push-button  308  is pushed downward. In the process, the plunger rod  306  and the plunger  305  are moved downward so as to compress the coil spring  312 , with the result that the size of the space  313  between the plunger  305  and the end wall  307  is increased greatly and a reduced pressure is generated therein. This reduced pressure causes air to be extracted from the interior of the sleeve  303  via the air line  314 , with the result that the internal diameter of the sleeve  303  is increased to a value which is greater than the external diameter of the capsule  150 , so that the capsule  150  is no longer held by the sleeve  303  and drops downward under the force of gravity. At the same time, the capsule located above this capsule  150  drops into the position which was occupied by the capsule  150  which has been released, it being important for the pressure on the push-button  308  to be released again sufficiently quickly, so that the coil spring  312  moves the plunger  305  back upward into the starting position, the size of the space  313  is reduced again and air is fed back to the sleeve  303  via the air line  314  sufficiently quickly for the next capsule  150  to be gripped by the sleeve  303 . Then, the chamber part  315  is rotated one step onward, so that a new capsule  150  moves into the position directly above the neck  302 . The rotation of the chamber part  315  may be effected externally, for example by hand, or may be triggered by the actuation of the push-button  308 . For this purpose, if necessary the cylindrical housing  301  has access openings. 
     Moreover, the figure diagrammatically indicates that the hollow needle  316  can be actuated by a rotary lever  319  and the push-button  308  can be actuated by a rotary lever  322 , the two rotary levers  319 ,  322  being articulatedly mounted on a rod  321 , which is secured to the balance  318  by means of a bearing part  323 , in such a manner that they can rotate in the direction indicated by the arrows. The drives of the two rotary levers  319 ,  322 , which are controlled by the control computer, are not shown. 
     A cuboidal housing, in which the capsules  150  are arranged in a plate which can be moved in the x direction and in the y direction, may also be provided instead of the cylindrical housing  301  and the chamber part  315  which can rotate in the manner of a revolver. 
     The balance  318  can be used to measure the total weight of the capsules  150  which are filled with substance and are present in the capsule-dispensing head  300  and of the capsule-dispensing head  300  with the exception of the permanent magnet  317  and the balance  318  itself. The weight of a capsule  150  filled with substance can be measured by measuring the weight difference before and after a capsule  150  has been dispensed. The weight of the substance in the capsule  150  can be determined by differential measurements using an empty capsule  150 . 
     
       FIGS. 16 and 17 
     
     The tool is in this case formed by a screw metering head  320 , which comprises a permanent magnet  321 , by means of which, as has been described in a corresponding way in connection with  FIG. 2 , it is connected to the tool holder  1 . The removal of the screw metering head  320  from the tool holder  1  is effected by means of the electromagnet arranged in the tool holder  1 . 
     A motor part  326  is attached to the permanent magnet  321  by means of a balance  333  and a connecting part  322 , and an open tube  323 , in which a screw  324 , which can rotate forward and backward about the z direction as indicated by arrow F, with screw shaft  325  is mounted at its bottom end. The screw  324  is anchored by means of the screw shaft  325  in such a manner that it can be rotated by a motor arranged in the motor part  326  and is stable in the z direction. Rotation of the screw  324  results in a ram  327  which runs on the screw moving up or down. The lower, open end of the tube  323  can be closed off by means of a diaphragm  328  which is provided with holes  329  and is secured to two pivot arms  330 ,  331  which are mounted pivotably in a suspension  332  on the motor part  326 . In  FIG. 16 , the diaphragm  328  has been removed from the open end of the tube  323  and can be moved into the closed position illustrated in  FIG. 17  by being pivoted in the direction indicated by arrow I. 
     To take up substance, the open end of the tube  323  is moved onto the substance with the diaphragm  328  in its pivoted-away position. Rotation of the screw  324  in the direction which moves the ram  327  upward causes substance to be carried upward directly by the screw  324 . 
     To dispense substance, the diaphragm  328  is pivoted under the screw  324  to cover the open end of the tube  323 . Then, the screw  324  is rotated in the direction which moves the ram  327  downward, with the result that substance is forced out downward through the holes  329  in the diaphragm  328  on the one hand directly by the screw  324  and on the other hand by means of the ram  327 . A stripper  334 , in the shape of a U-shaped wire, part of which bears against the underside of the diaphragm  328 , is, like the two pivot arms  330 ,  331 , mounted pivotably on the suspension  332 . Pivoting the stripper  334  in the direction indicated by the arrow K ensures that any substance which has remained attached to the bottom of the diaphragm  328  is periodically stripped off, allowing more accurate metering. 
     The diaphragm  328  is responsible for continuous delivery of substance, but in principle metering is also possible without a diaphragm  328 . 
     The balance  333  can be used to measure the total weight of the substance which has been taken up by the screw  324  and of the screw metering head  320  with the exception of the permanent magnet  321  and the balance  333  itself. If the weight of the screw metering head  320  with the exception of the permanent magnet  321  and the balance  333  itself is subtracted from this total weight, the result is the weight of the substance which has been taken up. The weight of substance which has been additionally taken up or dispensed can be determined by differential measurements. 
     
       FIG. 18 
     
     The tool is in this case formed by a solids-metering head  350 , which comprises a permanent magnet  351 , by means of which, as has been described correspondingly in connection with  FIG. 2 , it is connected to the tool holder  1 . The removal of the solids-metering head  350  from the tool holder  1  is effected by means of the electromagnet arranged in the tool holder  1 . 
     On the permanent magnet  351  there is a bearing part  352 , on which a carriage  353  is mounted in such a manner that it can move in the z direction. A holding plate  354  has been pushed laterally into the carriage  353  and has attached to it a metering housing  355 , the internal diameter of which decreases in steps toward the bottom and which has an intermediate base  371  with a conical metering opening which tapers upward. The holding plate  354  with the metering housing  355  can be detached from the carriage  353  by means of a horizontal movement involving little force. 
     A rotating metering shaft  357 , which drives a stripper  356  and can be displaced in the z direction, runs in the z direction centrally through the metering housing  355  and the conical metering opening in the intermediate base  371 . At the lower end of the metering shaft  357  there is a closure cone  372  which tapers upward and partially or completely closes off the conical metering opening in the intermediate base  371  depending on the z position, substance which flows downward when the metering opening is partially open being fed to the stripper  356 . 
     The rotating metering shaft  357  is fixedly connected to a co-rotating bearing part  368 , projects from below into a shaft  359  driven by a motor  360  and is rotated with the shaft  359 . A rotating stripper  358  which is arranged in the upper part of the metering housing  355  runs through the bearing part  368  and likewise projects into the shaft  359  from below. The stripper  358  can move in the z direction in the bearing part  368  and is driven, together with the metering shaft  357 , by the shaft  359 . 
     The displacement of the metering shaft  357  in the z direction is brought about by two electromagnets  362  and  363 , which are mounted on the holding plate  354  and bear a cover plate  366  via two support parts  364 ,  365 . The cover plate  366  is connected to the bearing part  368  fixedly in the z direction, a ball bearing  361  enabling the bearing part  368  to rotate on the rotationally fixed cover plate  366 . On activation, the electromagnets  362 ,  363  generate a force in the z direction and raise or lower the cover plate  366  and as a result the bearing part  368  and the metering shaft  357 . 
     The motor  360  and the electromagnets  362 ,  363  are controlled by a control part  367 , which is arranged laterally on the bearing part  352  and to which the motor  360  is secured. 
     Moreover, a balance  369  with a minimum weighing range from 0 to 2 kg and an accuracy of 0.1 g, which is in contact with the carriage  353  via a pin  370 , is attached to the bearing part  352 . Balances of this type are commercially available, for example from Sartorius AG, 37070 Göttingen, Germany. However, it is preferable to use a more accurate balance with an accuracy of 0.1 mg. 
     If substance which is stored in the metering housing  355  is dispensed via the conical metering opening in the intermediate base  371 , the weight load applied to the carriage  353  is reduced and the carriage  353  is pulled downward less strongly, a fact which is measured by the balance  369  via the pin  370 . 
     A second balance  374  is secured to the control part  367  by means of a connecting part  373 . The balance  374  bears, via a rotary axle  376  extending in the z direction, a tiltable spoon  375 , the concave part of which is located vertically below the metering housing  355 . 
     Substance which has been dispensed by the metering housing  355  firstly drops into the concave part of the spoon  375 , so that its weight can be measured there by means of the balance  374 . If the measured weight corresponds to a quantity of substance which, by way of example, is to be metered to a reaction vessel, the substance is added to the reaction vessel as a result of the spoon  375  being tilted through 180° as indicated by arrow G. If the weight corresponds to a quantity of substance which is smaller than the quantity desired, either first of all the quantity of substance which is present is added to the reaction vessel as a result of the spoon  375  being tilted, and then the spoon  375  is rotated back into the receiving position and the differential quantity which is still missing is weighed out in a second step, and finally this quantity is added to the reaction vessel, once again as a result of the spoon  375  being tilted, or, as an alternative, more substance is fed direct to the concave part of the spoon  375  until the desired quantity is reached. On the other hand, if the measured weight corresponds to a quantity of substance which is greater than the desired quantity, either the concave part is, as a result of rotation of the rotary axle  376  and therefore of the spoon  375  attached to it in the direction of arrow H, rotated away, emptied, rotated back under the metering housing  355  and refilled with substance, or, as an alternative, the entire solids-metering head  350  is displaced over the tool holder  1 , the concave part is emptied, is guided back under the metering housing  355  as a result of displacement of the solids-metering head  350  and is refilled with substance. 
     The balances  369  and  374  can in each case either be used on their own or together in order to check one another, the balance  374  having the advantage of measuring a smaller total weight. In principle, however, it would also be possible for the rotary axle  376  to be mounted directly on the connecting part  373  and for it, together with the spoon  375 , to be controlled purely on the basis of the measurement results from the balance  369 . 
     As an alternative to the spoon  375 , by way of example a vessel, e.g. a funnel, which has a closable opening at the bottom, is also conceivable. 
     A solids-metering head of this type, but without magnet coupling to the tool holder  1 , without spoon  375  and without balances  369  and  374  arranged directly on the solids-metering head, is marketed by Auto Dose SA, CH-1228 Plan-les-Ouates. 
     
       FIGS. 19 to 23 
     
     In this exemplary embodiment, the tool is formed by a screw metering head  420 , which can be connected to a tool holder  401 , which is secured to the rotary drive  2 , by means of a bayonet connection. The bayonet connection comprises, on the tool holder side, an annular connecting part  411  with a connecting bolt  412  and, on the tool side, an annular connecting part  421  with a recess  422  for receiving the connecting bolt  412 . Moreover, on the tool side there is a mandrel  424  which is intended to engage in the annular connecting part  411  and stabilizes the bayonet connection. 
     Via eight contact locations  413 , which are distributed over the outer circumference, on the annular connecting part  411  on the tool holder side and eight contact locations  423 , which are distributed over the inner circumference, on the annular connecting part  421  on the tool side, the screw metering head  420  can be supplied with power via the annular connecting part  411  and data communication can take place. For its part, the annular connecting part  411  is connected via a cable  414  to the fixed part of the device. 
     The screw metering head  420  comprises a weighing unit with a housing  425 , in which the control electronics  426  and a balance  427  are arranged. It is preferable to use a balance with an accuracy of 0.1 mg. As can be seen from  FIG. 20 , a bearing part  428  of the balance  427  projects out of the housing  425 . A metering unit  430  rests on the bearing part  428  via a drive unit  440  and in this way is weighed, together with the drive unit  440 , by the balance  427 . 
     To increase the weighing accuracy, a second balance, which measures the influence of any vibrations, which is then subtracted from the measurement result of the balance  427 , can be used in addition to the balance  427 . 
     A filling connection piece  450  is held removably beneath the metering unit  430  by a holder  451  which is fixedly connected to the housing  425 . The filling connection piece  450  does not touch the metering unit  430  and therefore does not have any adverse effect on the weighing operation. The fact that it is separate from the metering unit  430  means that the balance  427  is subjected to load from a lower weight, with the result that the weighing accuracy is increased. Moreover, the metering unit  430  and the filling connection piece  450  can be removed and stored separately from the drive unit  440  and the holder  451 , respectively. 
     Alternatively, it would also be possible to use a filling connection piece which is connected to the metering unit  430 , which would have the advantage that any residual substance which has remained in the filling connection piece would also be weighed. 
     The structure of the metering unit  430  can be seen from  FIGS. 21 and 22 . The metering unit  430  comprises a storage vessel  431 , an extruder  432  having a screw part  4322  and a web part  4321 , a metering funnel  433  and a cover  434  which is provided with toothing. The screw part  4322  tapers from the top downward, i.e. away from the web part  4321 , with the result that when pulverulent substance is being metered, this substance does not clump together as it passes through the metering funnel  433 . The toothed cover  434  has an internal screw thread and is screwed onto a screw thread  4311  of the storage vessel  431 , the extruder  432  being clamped between cover  434  and storage vessel  431 . The clamping is effected by means of the web part  4321 , from which, moreover, strippers, which are not shown in  FIG. 2 , preferably extend toward the screw part  4322 . The metering funnel  433  is held rotatably between cover  434  and extruder  432  and has lugs  4331  which, when the metering unit  430  is inserted in the drive unit  440 , engage in recesses  4411  of a metering-unit receiving part  441  of the drive unit  440 . 
     The drive unit  440  also comprises a motor  442  which is secured to a printed-circuit board  443  provided with control electronics and actuates a transmission gearwheel  444 . The transmission gearwheel  444  engages through a gap in the metering-unit receiving part  441  in the toothed cover  434  of the metering unit  430  and rotates the toothed cover  434  together with the storage vessel  431  and the extruder  432 , while the metering funnel  433  is held in a fixed position by the lugs  4331  engaging in the recesses  4411 . The resultant relative movement between metering funnel  433  and extruder  432  causes substance to be conveyed out of the storage vessel  431  through the metering funnel  433  into the filling connection piece  450 . 
     The motor  442  is fed by two storage batteries  445  and  446 , which, by way of example, can be recharged by the charging means  429  which is shown in  FIG. 19  and is attached to the housing  425 . The charging device  429  is designed as a switch and is only in contact with the storage batteries  445 ,  446  while they are being charged. During the weighing operation, the charging means  429  does not touch the storage batteries  445 ,  446 , so that the weighing operation is not adversely affected. 
     Alternatively, the charging of the storage batteries  445 ,  446  could also take place in a separate charging station which is separate from the screw metering head  420 , in which case the drive unit  440 , to this end, would simply have to be lifted off the bearing part  428  of the balance  427  and transported to the charging station. 
     The motor  442  is controlled by means of the printed-circuit board  443 , which for its part receives control signals from the control electronics  426  arranged in the weighing unit. The transmission of signals from the weighing unit to the printed-circuit board  443  is effected by means of light through an opening  4251 , which can be seen in  FIG. 20 , in the housing  425 , so that mechanical contact between weighing unit and drive unit  440  is avoided and the weighing operation is not adversely affected. 
     The screw metering head  420  can be modified in various ways. In particular, by way of example, the storage vessel  431  can be fixed in such a way that it does not also rotate during the metering operation. In this case, it is also preferable for a driver to extend into the storage vessel  431  from the rotating extruder  432 . 
     The metering may generally take place continuously, but periodic additions of substance and a weighing operation between the individual addition operations are also possible. Moreover, it is conceivable for the storage vessel  431  to be shaken during the metering operation, so that the pulverulent substance contained therein is loosened. 
     It is possible to execute further design variations on the devices according to the invention which have been described above. Express mention should also be made of the following at this point:
         In all the exemplary embodiments described, the balance or balances may be provided either on the tool or on the tool holder  1 . Arranging the balance on the tool holder  1  has the advantage that, in the event of a tool change, there is no need for each tool to have a balance. However, this solution means that the weight of the entire tool is always measured as well. By contrast, arranging the balance on the tool has the advantage that in each case a lower overall weight is measured. This tends to make the measurements more accurate.   The connection between tool holder  1  and tool may also be formed in a different way than with magnets or bayonet connections. By way of example, screw connections or clamping connections are conceivable. However, it should be possible for the connection to be produced and released again automatically, i.e. not by hand.   In addition to the tools described, it is also possible to use further tools which are equipped with a connection point to the tool holder and possibly a balance.