Abstract:
A rotatable gripper grips and turns a container cap. When locked, a collet clamp engages the cap, and when unlocked, the clamp releases the cap. The collet clamp is elongate with a first and second ends. The first end has a substantially rigid section. The second end has a gripper section with a substantially cylindrical recess that expands in width to hold the cap when the clamp is moved from locked to unlocked. Between the rigid and gripper sections is a clamping section. An outside surface of the clamping section is conical with a width increasing towards the gripper section. Sliding a ring on the conical clamping section deforms it into the locked position, constricting the recess tightly onto a container cap. When unlocked, the recess widens, permitting the cap to be removed from the recess.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to benefit of a right of priority under 35 USC §119 from European patent application 13164482.5, filed on 19 Apr. 2013, the content of which is incorporated by reference as if fully recited herein. 
     TECHNICAL FIELD 
     The present invention concerns a rotatable gripper which serves to open and close rotatable container caps. The rotatable gripper according to the invention is intended primarily for use in a sample preparation system. 
     BACKGROUND 
     The sample preparation system serves for the preparation of a large number of samples that are filled into containers. The empty containers are arranged in a sample rack. To perform the fill operation, an empty container which is closed with a screw cap is picked up with a tool and placed on a load receiver of a balance. Next, the cap is removed and the desired amount of the required substance is filled into the container, with the mass of the substance being measured by the balance. Then, the container is closed with the cap and returned to the sample rack. 
     This describes the operation in which a rotatable gripper is used. The rotatable gripper is parked in a base station. From there, the rotatable gripper is picked up by the robot arm and moved to the container that is to be fetched. The rotatable gripper takes hold of the container, and the robot arm moves the rotatable gripper with the container to the load receiver, where the container is set down. Next, the rotatable gripper opens the screw cap through a rotating action. Then, the rotating gripper with the cap is temporarily parked at the tool base station and the open container is filled with the desired substances. The filling is performed with a dosage-dispensing device, for example a dispensing head which contains or is connected to a supply of the substance. If the container is to be filled with a plurality of different substances, different dispensing devices will be used which are transported by the robot arm between the container and a dispensing device base station. After the filling process has been concluded, the rotating gripper is picked up again by the robot arm and moved to the container. The cap, which is still held by the rotating gripper, is screwed back onto the container, and the filled container is returned to the sample rack. 
     Typically, the sample amount dispensed into the container is in the range between 0.5 mg and 5000 mg. The weighing of such a small amount of mass requires a balance of high accuracy. 
     The sample preparation system is used in a laboratory environment. In the preparation of samples, critical substances are dispensed into a container. These substances must not be contaminated with lubricants. 
     The existing state of the art offers a variety of rotatable grippers. 
     Rotatable grippers that hold the cap by means of swivel jaws are disclosed in U.S. Pat. No. 7,836,664 B2, EP 2179758 B1, WO 2008043498 A1 and US 2007/0068117 A1. These rotatable grippers have the disadvantage that the jaws have to be greased with a lubricant, which can lead to contamination of the substances being dispensed. In addition, the jaws are pushed down on the cap by a spring force. Thus, the vertical force of the jaws bears against the cap and thus pushes down on the balance. As the balance is highly sensitive, it is therefore possible that the overload protection device of the balance is triggered, which can lead to a shift of the zero point of the balance. Subsequent measurements could thus be inaccurate. 
     Other rotatable grippers, which are disclosed in U.S. Pat. No. 7,765,772 B2 and U.S. Pat. No. 5,417,031 A, push down on the screw cap during the closing operation through the elastic force of a spring. This spring force would also act on the load receiver and trigger the overload safety device of the balance. 
     It is therefore the object of the present invention to provide a rotatable gripper which does not cause contamination and does not activate the overload safety device of the balance. 
     SUMMARY 
     This task is solved by the rotatable gripper according to the independent claim and the subordinate claims depending on it. 
     The rotatable gripper according to the invention is capable of gripping and turning a container cap. It is equipped with a rotatably supported collet clamp which has a locked operating position where the cap can be clamped in the collet and turned by the latter, and an unlocked operating position where the cap can be released from the collet. The collet clamp is of an elongate configuration, with a first and a second end. The first end comprises an essentially rigid section, while the second end comprises a gripper section with an essentially cylindrical recess that serves to hold the cap and can be expanded in width in the change from the locked to the unlocked operating position. In a preferred embodiment, the basic configuration of the cylindrical recess is circular. However, other configurations are also possible, such as for example triangular, quadrangular, or polygonal. According to the invention, a clamping section is arranged between the gripper section and the rigid section of the collet. The gripper section and the clamping section have an inside surface and an outside surface, wherein the outside surface of the clamping section has the shape of a cone whose width increases towards the gripping section and which is designed to be elastically deformable. The inside surface of the clamping section and the inside surface of the gripping section adjoin each other directly. The same applies to the outside surface of the clamping section and the outside surface of the gripping section. In the locked operating position, the deformable cone can be compressed by means of a ring that is slidably seated on the cone, whereby the recess can be compressed in such a way that a cap that is located in the recess can be clamped tightly. By loosening the ring, the recess can be made wider, so that the cap can be removed again from the recess. The ring is made of an essentially rigid material. The operating position of the collet clamp is essentially vertical. Accordingly, in the operating position the first end with the rigid section is located above the second end with the gripper section. The ring and cone are coaxial with the axis of the collet clamp. As the cone has a smoothly narrowing taper towards the clamping section and as the collet is oriented vertically, the ring is pushed by gravity onto the cone and thereby constricts the recess in the gripper section. Thus, the container cap is locked into the gripper section of the collet clamp. When the ring is moved upwards, the pressure on the cone is released and the recess in the gripper section expands, so that the cap can be removed from the gripper section. 
     The rotating gripper according to the invention has the advantage that no lubricant needs to be used between the ring and the cone. Thus, the sample substance cannot be contaminated by lubricant. 
     As a further advantage of the invention, the cap is locked into the recess by the ring sitting on the cone. No additional force is acting on the load receiver, and the overload safety device of the balance is not being triggered. 
     The ring is preferably held on the cone in the locked operating position by means of a spring that coaxially surrounds the collet clamp. As a result of this arrangement, the spring force is acting likewise in the vertical direction, and the spring occupies for the most part the space around the clamping section of the collet. The spring force is designed to be sufficiently large to hold the ring in place in the locked operating position. The locked operating position is maintained only by the pre-tensioning force of the spring. To change to the unlocked operating position, the spring needs to be compressed further. Thus, the locked state of the ring is a passively occurring condition. To be released, the ring has to be pushed up by an active force against the opposition of the spring force. The ring can be retracted from the cone by means of a retractor rod against the force of the pre-tensioned spring in order to attain the unlocked operating position. Alternative embodiments can have a hydraulically or pneumatically operated mechanism acting in conjunction with a rotary clutch. 
     The retractor rod is essentially of an elongate shape and is arranged in a hollow space inside the collet clamp. The retractor rod has a first and a second end. The first end is arranged essentially at the height of the ring. The first end has fingers which can reach through cutouts of the gripper section and are configured so that the ring is raised up when the retractor rod is moved upwards and the collet clamp can take its unlocked position. In the locked position, neither the retractor rod nor the fingers are touching the ring and the collet clamp. The spring force does not act on the load receiver in either the locked or the unlocked operating position, which assures that the overload safety device of the balance is not being triggered. 
     The second end of the retractor rod is arranged inside the collet clamp. A driving mechanism engaging the second end serves to raise and lower the retractor rod and thus serves to control the switching between the unlocked and the locked operating position. 
     The move to the unlocked operating position is facilitated by the fact that the inside surface of the collet clamp has an upwardly narrowing conical taper and the retractor rod has a conical, upwardly narrowing tapered section which cooperates with the conical inside surface of the collet clamp in such a way that in the change from the locked to the unlocked operating position the width of the recess is expanded by the spreading-apart of the gripper section of the collet clamp. The collet clamp is constructed advantageously of a material that springs back easily after deformation. It is important for the collet clamp to have springlike resilience in the radial direction, which can be significantly enhanced by lengthwise directed kerf slits which are arranged in the clamping section and in the gripper section. Ideally, the slits run out open-ended in the axial direction at the end of the collet clamp in the gripper section. This makes it easier to compress the gripper section. In the unlocked operating position, the collet clamp is free of elastic tension. In the locked operating position, the collet clamp is in a state of spring tension. Due to material fatigue it is possible that the elastic resilience may decrease over the life of the collet clamp. To ensure that the collet clamp can still be released, the conical or upwardly narrowing section of the retractor rod and the conical inside surface of the gripper section cooperate in the manner described above. This action can also be described as a forced opening of the collet clamp. This feature is most effective if the two conical surfaces run essentially parallel to each other. 
     In an alternative embodiment, the collet clamp is made of a material without elastic spring properties. In such a design, the collet clamp is always spread apart by the cone of the retractor rod in order to attain the unlocked operating position. 
     With preference the collet clamp and the ring are arranged in a sleeve, wherein the collet clamp is solidly connected to the sleeve. The connection between the sleeve and the collet clamp can be achieved with a form-locking or force-based engagement of the two parts. Advantageously, the collet clamp, the ring and the sleeve are arranged coaxially. The sleeve is preferably of a concentric design and rotatably supported in a housing. Due to the concentric configuration, the circular ring can be enclosed by the sleeve with a precise fit. 
     When the cap is opened or closed, the screw thread of the cap will cause the latter to move, respectively, up or down over a height difference of typically about 8 millimeters. This vertical movement of the cap and of the collet clamp holding the cap can cause the collet to become misaligned in the housing. This tendency can be counteracted with a design where the sleeve in its operating position is supported in the housing with vertical mobility. This arrangement offers the advantage that the sleeve with all of the components arranged inside it is moved up and down together with the cap. The problem of the collet clamp getting out of alignment in the housing is thereby avoided. 
     The rotatable gripper according to the invention includes a first drive mechanism to raise the ring and a second drive mechanism to rotate the collet clamp. Advantageously, the first as well as the second drive mechanism are arranged laterally of the collet clamp. This arrangement offers the advantage that the rotatable gripper has a compact design. The compact configuration is especially advantageous if the rotatable gripper is used with a robot arm. In rotatable grippers of the prior art, for example EP 0876991 B1, the drive mechanism is arranged above the collet clamp. These known rotatable grippers do not need to be moved as they are firmly installed. However, these embodiments of the state of the art have the disadvantage that the rotatable gripper is too tall. Consequently, the robot arm would require more room, the transfer distances would be longer, and a larger space would be needed to store the rotatable gripper. As a consequence of their design configuration, rotatable grippers of the existing state of the art have the further disadvantage that the force transmission levers would be larger. A larger lever ratio, in turn, could cause inaccuracies, which can have a big influence especially in critical laboratory applications. 
     Advantageously, in the operating state of the rotatable gripper, the first drive mechanism generates a movement in the vertical direction. This vertical movement raises the ring by way of a lever and the retractor rod. The lever has a first end and a second end. The first end of the lever is connected to the first drive mechanism, and the second end of the lever is connected to the retractor rod by way of a guiding system. The guiding system is configured in such a way that the rotary movement of the lever is converted by two movably supported guide pins into a purely vertical movement of the retractor rod. Advantageously, a ball joint is arranged between the lever and the first drive mechanism, wherein the ball joint transmits a rotary movement to the retractor rod. Due to this arrangement, the first drive mechanism can remain in the same position; only the lever is tilted, and the ball joint prevents jamming of the sleeve during the vertical movement. 
     The second drive mechanism preferably moves a drive gear which, in the operating state, is oriented horizontally and is arranged to cooperate with a collet gear that is connected to the sleeve. The drive gear is designed thicker than the collet gear, so that the latter will remain in mesh with the drive gear also when the collet moves up or down. The vertical height of the drive gear is selected so that the collet gear can mesh with the drive gear when the cap is completely screwed on as well as when it is completely screwed off. The drive mechanism together with the rotatable gripper is mounted so that it stays in a fixed position during the opening and closing action. The collet is supported with vertical mobility in the rotatable gripper. Consequently, the drive mechanism is able to move vertically in relation to the collet. 
     It is advantageous if the collet clamp is supported in a floating condition by means of a vertically oriented housing spring which is arranged between the housing and the collet clamp, wherein in the operating state the collet clamp is pushed downward against the spring force of the housing spring. When the collet clamp is used on a balance of high precision it is important that the balance is never exposed to the entire weight of the rotating gripper whereby the overload safety device could be triggered. When the collet clamp is moved downward against the spring force of the housing spring as a result of the rotation of the screw cap, the collet clamp springs back up after the clamping section is released. This ensures that the weight of the collet clamp never rests on the balance. During this operation, the housing spring is typically compressed by about 2 millimeters. 
     The retractor rod advantageously has a central bore hole for a sensor pin which is seated in such a way in the passage that the sensor pin is pushed upwards along the axis of the central bore hole when a cap is present in the recess. This displacement can be detected by means of a suitable sensor, and a corresponding signal can be transmitted to the user or to a computer system. Typically, an optical sensor with a light gate is used for this purpose, but other sensors could also be considered. 
     Ideally, the collet clamp and the ring are made of anodized and/or PTFE-coated aluminum (PTFE=polytetrafluoroethylene, commonly known as Teflon™). Other parts such as for example the sleeve can also be made of the same materials, which offer the advantage of having a smooth surface and thus a small coefficient of friction. Due to the low friction between the components it is unnecessary to add lubricants which could lead to contamination of the substances being filled into the containers. As a further advantage, there is less friction between the components of the rotatable gripper according to the invention. Consequently, the rotatable gripper is less susceptible to wear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Details of the rotatable gripper are presented in the description of an embodiment which is illustrated in the drawings, wherein identical parts are identified with identical part numbers and wherein: 
         FIG. 1  shows a sample preparation system in which an embodiment of the rotatable gripper is employed; 
         FIG. 2  shows a sectional view of embodiment of the rotatable gripper; 
         FIG. 3  shows an exploded view of the embodiment of the rotatable gripper; and 
         FIG. 4  shows an exploded view of the collet clamp. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a sample preparation system  10  in which the rotatable gripper  1  according to the invention is employed for the preparation of samples. The sample preparation system  10  includes a tool base station  2  in which a variety of tools, such as for example the rotatable gripper  1  or a gripper for a powder-dispensing head, are parked temporarily. A sample rack  5  holds a plurality of containers  81 . In the storage rack  7  for dosage-dispensing heads, different materials contained in individual dispensing heads can be kept ready for use. When samples are to be prepared, containers  81  can be placed on the load receiver  6 . The motion module  4  allows the tool holder  3  to be moved in the x-, y- and z-directions. 
     To begin the preparation of a sample, the tool holder  3  is moved by means of the motion module  4  to the tool base station  2 , where the tool holder  3  receives the rotatable gripper  1 . A closed empty container  81  is taken out of the sample rack  5  and placed on the load receiver  6  by means of the rotatable gripper  1 , where the cap  80  is removed from the container  81  by means of the rotatable gripper  1 . After the cap  80  has been removed, the rotatable gripper  1  together with the cap  80  is parked in the tool base station  2 . Next, the prescribed substances are dispensed into the open container  81 , as the masses of the dispensed substances are monitored and the dispensing operation is controlled by means of a balance which is connected to the load receiver  6 . After the desired substances have been filled into the container  81 , the tool holder  3  picks up the rotatable gripper  1  containing the cap  80 , closes the container  81  on the load receiver  6  with the cap  80  held in the rotatable gripper  1 , and then moves the closed container  81  back to the sample rack  5 . Subsequently, further containers  81  are filled with the prescribed substances, if desired. 
       FIG. 2  shows a sectional view of the rotatable gripper  1  according to the invention with a closed container  81  held by the collet clamp  20 . The same components are also illustrated in  FIG. 3  in an exploded view. 
     The rotatable gripper  1  includes a housing  90  containing the collet clamp  20  and the drive mechanisms  100 ,  110  through which the collet clamp  20  is actuated. The housing  90  has side walls and a cover plate. In the operating state, the cover plate is essentially horizontal and located above the collet clamp  20 . Arranged on the cover plate are elements for the attachment of the housing  90  to the tool holder  3 . These elements can be configured as pins  93 , but other configurations are also conceivable. Also arranged on the housing  90  is an electrical connection  92  through which the rotatable gripper  1  is supplied with electrical power and/or data signals from the tool holder  3  to power and control the drive mechanisms  100 ,  110 . The collet clamp  20  which is inside the housing  90  has a first and a second end and is of an elongate configuration. In the operating state the longitudinal axis of the collet clamp  20  is essentially vertical. The first end of the collet clamp  20  comprises an essentially rigid section  21 . The second end comprises the gripper section  22  which has an essentially cylindrical recess  25  that serves to hold the cap  80 . The gripper section  22  can be spread open or tightened, whereby the recess  25  is expanded or constricted. The constricted condition of the recess  25  corresponds to the locked operating position where a cap  80  can be held tight in the gripper section  22  and turned by it. The spread-open condition of the recess  25  corresponds to the unlocked operating position where the cap  80  can be released from the gripper section  22 . A clamping section  23  is arranged between the rigid section  21  and the gripper section  22 . By way of the clamping section  23  the gripper section  22  can be tightened and loosened. The tightening and loosening is achieved through an outside surface  27  of the clamping section  23  which at least in part has the shape of a cone  24  whose width increases towards the gripping section  22 . The cone  24  is designed to be elastically deformable in the radial direction, i.e. to have radial flexibility. The gripper section  22  and the clamping section  23  are designed so that compressing the cone  24  causes a constriction of the recess  25 , putting the rotatable gripper into the locked operating position. Expanding the cone causes a widening of the recess  25 , putting the rotatable gripper into the unlocked operating position. An essentially rigid ring  50  which can slide up and down is resting on the flexible cone  24 . Pushing the ring  50  downward in the operating position causes the cone  24 , and consequently the recess  25 , to be constricted, putting the rotatable gripper into the locked operating position. On the other hand, moving the ring  50  upward causes the cone  24 , and consequently the recess  25 , to expand, putting the rotatable gripper into the unlocked operating position. The geometry and the material for the collet clamp  20  are selected so that the latter is clamped tightly in the locked operating position and released in the unlocked operating position. Thus, pushing the ring  50  onto the cone  24  has the effect of compressing the cone  24 . Retracting the ring  50  allows the tension in the material of the collet clamp  20  to relax so that the recess opens up. 
     The ring  50  is pushed onto the cone  24  by a pre-tensioned spring  30 . The spring  30  is arranged coaxially around the collet clamp  20 . To allow the rotatable gripper to take on the unlocked operating position, the ring  50  has to be pushed upwards against the spring force of the spring  30 . The spring force is stronger in the released position than in the locked position. To put the gripper into the released position, the ring  50  has to be actively pushed upward. This action occurs by way of a retractor rod  60 . The retractor rod  60  has a first end  62  and a second end  63 . The retractor rod  60  is arranged in a hollow space inside the collet clamp  20 . The retractor rod  60  has a first end  62  and a second end  63 . The first end  62  has fingers  64  which reach through slits  28  that are arranged in the gripper section  22  and clamping section  23 . When the retractor rod  60  is moved upwards the ring  50  is pushed up against the pre-tensioning force of the spring  30 . To ensure that the recess  25  will open up even as the material of the collet clamp  20  loses some of its elasticity over time, the inside surface  26  of the collet clamp  20  has an upwardly narrowing conical section  24  which cooperates with the upwardly narrowing conical surface  65  of the retractor rod  60  in such a way that when the retractor rod is moved upward the gripper section  22  is spread apart. 
     The second end  63  of the retractor rod  60  is arranged inside the rotatable gripper  1  and connected to a first drive mechanism  100 . The first drive mechanism  100  moves the retractor rod  60  up and down in a controlled manner. The first drive mechanism  100  is arranged to the side of the retractor rod  60  and the collet clamp  20 . The first drive mechanism  100  generates a rotary movement in a horizontal plane relative to the operating state of the rotatable gripper. This rotary movement is converted into a vertical movement by means of a ball joint  102  and a lever  101  which moves the retractor rod  60 , and consequently also the ring  50 , up and down relative to the normal operating position. 
     The collet clamp  20 , the retractor rod  60  and the ring  50  are arranged inside a sleeve  70 . The rigid section  21  of the collet clamp  20  is solidly connected to the sleeve  70 . Ideally, the collet clamp  20  as well as the sleeve  70  are for the most part of rotationally symmetric configuration. The collet clamp  20 , the ring  50  and the sleeve  70  are arranged coaxially with one another. The sleeve  70  is rotationally symmetric and rotatably supported in the housing  90 . Due to the solid connection between the collet clamp  20  and the sleeve  70 , the collet clamp  20  and the sleeve turn together. The container cap  80  is opened by rotating the sleeve  70  with the collet clamp  20  holding the cap  80 . 
     Screwing the cap  80  onto or off the container causes the collet clamp  20 , and consequently also the sleeve  70  with the components contained in it, to move, respectively, downwards or upwards. To take up this vertical displacement, the sleeve  70  is held in the housing  90  through a floating kind of suspension. The first drive mechanism  100  is supported in the housing  90  in a floating arrangement together with the collet clamp  20 . To prevent the collet clamp  20  or the drive mechanism from becoming misaligned, a lever  101  which is rotatable about a horizontal fulcrum axis is arranged between the first drive mechanism  100  and the collet clamp  20 . As an alternative, one could also use a rigid linear guiding constraint instead of the rotatable lever. 
     The rotary movement that is needed to open and close the cap  80  is generated by means of a second drive mechanism  110 . The second drive mechanism  110  is likewise arranged to the side of the collet clamp  20 . Due to this lateral arrangement, a compact design of the rotatable gripper  1  is achieved. The second drive mechanism  110  acts through a drive gear  111  extending horizontally in the operating state. This drive gear  111  meshes with a collet gear  112  that is connected to the sleeve  70 . Rotation of the drive gear  111  turns the collet gear  112 , whereby a cap  80  clamped in the collet  20  is screwed onto, or unscrewed from, the container  80 . The second drive mechanism  110  is solidly connected to the housing, which means that the second drive mechanism  110  does not move up or down together with the cap  80 . To ensure that the two gears  111 ,  112  mesh with each other in any position, the drive gear  111  is designed thicker than the collet gear  112 . It is also possible that the collet gear is designed thicker than the drive gear  111 . 
     Between the collet clamp  20  and the housing  90 , a vertically oriented housing spring  91  is arranged in such a way that in the operating state the collet clamp  20  has to be pressed downward against the spring force of the housing spring  91 . 
     The retractor rod  60  has a central bore hole  61  for a sensor pin  40  arranged in such a way that the sensor pin  40  is pushed upwards along the axis of the central bore hole when a cap  80  is present in the recess  25 . This displacement can be detected by means of a suitable sensor, and a corresponding signal can be transmitted to the user or to a computer system. An optical sensor  41  with a light gate is used for this purpose, but other sensors could also be considered. 
       FIG. 4  shows an exploded view of the collet clamp  20  of the rotatable gripper  1  according to the invention. The exploded view serves to more clearly illustrate the function of the collet clamp shown in  FIGS. 2 and 3 . 
     Although the invention has been described through the presentation of a specific embodiment, it is evident that numerous further variant versions could be created based on the teachings of the present invention.