WO2012091670A1

WO2012091670A1 - Force controlled grinding robot system

Info

Publication number: WO2012091670A1
Application number: PCT/SE2011/051603
Authority: WO
Inventors: Anders WANNER
Original assignee: Wanner Anders
Priority date: 2010-12-30
Filing date: 2011-12-29
Publication date: 2012-07-05
Also published as: SE1051392A1

Abstract

The present invention relates to a force controlled grinding robot system for sharpening blades of cutting tools, wherein each blade (101) has a length, a thickness and a height, the force controlled grinding robot system comprising a robot (1) having a memory programmable control, a manipulator (11) moveable in at least four of the six possible translatory and rotational degrees of spatial freedom, a gripper head (12), fixed to the manipulator, having a holding device (13) for holding a cutting tool (10), an edge grinding station (7) having at least one moving grinding surface (71) for receiving and grinding said blade (101). The force controlled grinding robot system is characterised in that a force sensor is adapted to sense the force between the manipulator and an object in at least one translatory degree of freedom and provide an output force signal indicating the magnitude of said force, said memory programmable control being adapted to receive said output force signal and control said force controlled grinding robot system dependent on said force signal. The invention further also relates to a method for sharpening blades and the use of a force controlled grinding robot system.

Description

FORCE CONTROLLED GRINDING ROBOT SYSTEM

Technical field

The present invention relates generally to a device for sharpening blades of cutting tools as e.g. knives and scissors. More particularly, the present invention relates to a force controlled grinding robot system as defined in the introductory parts of claim 1 . The invention further also relates to a method for sharpening blades as defined in claim 8 and the use of a force controlled grinding robot system as defined in claim 14.

Background art

Knives used in industries as e.g. the food industry and especially in the meat industry, tend to get blunt relatively fast. A typical employee using knives in the meat industry will use 3-10 knives per day. When a knife has been used until it's edge is blunt, the user usually have it steeled to remain the sharpness of the edge. After the knife has been steeled for a couple of times it will have to be sharpened by grinding before it can be used again. The sharpening is traditionally made by hand, since prior attempts to grind used knives using a automated process has been unable to reach the quality level that is required. Machine sharpening of the knives using the present state of the art has been found to have a poor result compared to sharpening the blades by hand using normal sharpening tools. For that reason, knives are sharpened by hand, grinding the knife blade against a sharpening stone or belt both in new production of knives and when sharpening used knives.

The patent document EP1091827 shows a sharpening machine for knives having two counter-rotating sharpening stones that are arranged in a self-centering mounting. The shape of the sharpening stones are used to create the wanted edge shape. The force controlled grinding robot system is however intended only for production of new knives, where every subject has the same shape. The machine can be programmed for different shapes depending on the knife model. The machine is, however not suitable to sharpen used knives.

Another machine for sharpening knives in the manufacturing phase is US 6,663,465 by Gross et al. The machine of Gross uses a programmable robot to sharpen knives. The robot has a gripper head to grip the knives in a recess or hole of the fastening part of the knife, the part of the knife where a handle will be mounted later on. The robot will have exactly the same gripping position every time, since it grips the knife substance in the recess or hole of the substance. The robot uses a pneumatic system to regulate the pressure of the force that is applied to the blade, at the same time as a damping effect is achieved. The position of the robot during the grinding procedure is programmed after a certain knife curve or shape and the robot movements are controlled after its position. The grinding is thus controlled after positions defined in a drawing of the knife that is to be grinded. The pneumatic system then partly compensates errors in the applied force in every position due to errors in the knife substance material.

The machine of Gross, however, has a few drawbacks. The machine is designed to grind new knives in the manufacturing process of knives. The pressure regulation using a pneumatic system is relatively slow. The pneumatic compensation system is not designed to handle the large errors in the knife shape that is present by used knives. The machine of Gross requires a blade substance without a mounted knife handle, which makes the machine unfit for grinding used knives.

Summary of the invention

It is an object of the present invention to improve the current state of the art, to solve the above problems, and to provide an improved device for sharpening used cutting tools as e.g. knives and scissors. These and other objects are achieved by a force controlled grinding robot system for sharpening blades, wherein each blade has a length, a thickness and a height, the force controlled grinding robot system comprising a robot having a memory programmable control, a manipulator moveable in at least four of the six possible translatory and rotational degrees of spatial freedom, a gripper head, fixed to the manipulator, having a holding device for holding a cutting tool, a pick-up area where the robot can pick up a cutting tool, an edge grinding station having at least one moving grinding surface for receiving and grinding said blade. The invention is characterised in that a force sensor is adapted to sense the force between the manipulator and an object in at least one translatory degree of freedom and provide an output force signal indicating the magnitude of said force, said memory programmable control being adapted to receive said output force signal and control said force controlled grinding robot system dependent on said force signal.

The sensor is adapted to measure the force between the robot manipulator holding the blade and the grinding surface. The measured force is then used to control the robot manipulator so that the grinding force, naturally equivalent to the grinding pressure, is controlled according to the current programming of the robot. The grinding force preferably changes from the base of the knife to the tip. The grinding force is thus a function of the position of the knife. The force sensor has a fast response time providing a signal of the applied force that is directly coupled to the memory

programmable control of the robot. The robot has a fast response time to the provided signal leading to a quick response time from measured

pressure/force to a possible action of movement of the robot manipulator. Since the system does not have any resilient features, the machine will not suffer from oscillations originating from e.g. a pneumatic compensation system. To handle oscillations in the blade occurring during grinding, the robot uses a control algorithm to control the applied pressure and damp oscillations. The machine will thus be able to keep a correct pressure between the grinding surface and the blade at all times.

The term robot should be construed broadly and may be any

automated machine. The robot could be as simple as an automated collection of linear units, but also an advanced industrial robot. The automated machine or robot has a memory programmable control, a manipulator moveable in at least four of the six possible translatory and rotational degrees of spatial freedom as described above.

The pick-up area is preferably a tray or any other suitable container from which the knives may be easily gripped by the robot gripper head.

Since the grinding procedure is force controlled, using preprogrammed positions only as a starting position, the grinding procedure will handle sharpening of used knives perfectly well. Irregularities in the shape will not affect the grinding procedure since the position of the robot manipulator will be moved to provide the correct force at all times.

The moving grinding surface of the edge grinding station is preferably two counter rotating sharpening stones, but could also be only one

sharpening stone or one or two grinding belts.

The robot with a memory programmable control allows for saving information about a certain knife model for performing the same grinding operation an infinite number of times for the same knife model without adjustments or new measurements of the knife. The first time a knife of a certain model is introduced in the machine, the shape of that knife is measured. The measurement is performed by moving the knife blade, held by the gripper head of the robot manipulator, to a contact point of known position. The knife is placed so that the blade base, the part nearest the handle of the knife, is positioned over the contact point, i.e. the contact point is located under the blade in the direction of its height. The blade is then moved in the direction of its height until it hits the contact point and a predetermined force is measured by the force sensor. The blade position is saved in the memory. Then the blade is moved a short distance in the direction of the knife length and the predetermined force is again applied and the blade position is saved in the memory. The procedure is repeated for the entire knife blade, so that that the memory eventually contain the entire contour of the blade edge. It should, however, be noted that the blade could be measured using other means, e.g. laser, ultrasonic devices etc. within the scope of the present invention.

When the robot memory has the knife shape saved in the memory programmable control, the force controlled grinding robot system can sharpen the edge of the knife in the edge grinding station against the moving grinding surface of the station. Using the known shape, the robot will have the edge grinding station grind the edge at a predefined portion of the knife length, applying a predefined force according to the position of the knife against the moving grinding surface, using a predefined velocity according to the position of the knife of the grinding surface, and for a predetermined time period according to the position of the knife. All the predetermined values of grinding surface velocity, applied force and time duration are thus preferably a function of the position of the blade. The functions of position for velocity, force and time are proportional to the thickness of the blade and the height of the blade at each position. This feature is preferred e.g. due to the differences in thickness and height of the blade at different positions along the length of the blade.

It is further preferred that the force controller is capable of controlling force in two translatory degrees of freedom, preferably three translatory degrees of freedom and more preferably in four translatory degrees of freedom. The three first translatory degrees of freedom are the three spatial dimensions, as described in a normal reference system by x, y, and z. The fourth dimensional degree of freedom is rotation of the gripper head along the longitudinal axis of the manipulator, being one of the three possible rotational degrees of freedom. The grinding operation is not always possible to make by moving the grinding blade in only one direction, since the knife height varies along the knife length and the blade gets thinner near the knife tip. These variations in height and thickness of the blade have to be considered to achieve a result that is equivalent to a grinding operation performed by hand. That is possible if the force of the grinding surface can be measured in more directions than one, preferable in all directions, including rotational force.

Since the manipulator is possible to move in all directions, the grinding angle and force can be controlled in all directions. It should be noted that the force sensor preferably is double acting, i.e. it can sense forces back and forth in all of the mentioned four dimensions.

The force sensor is preferably mounted in the robot manipulator facilitating the integration with the robot memory programmable controller. It should however be noted that the force sensor just as well could be placed on the edge grinding station and not on the robot. The grinding device could e.g. be placed on a scale, measuring the force applied in a direction perpendicular to the scale surface. The computer program executed by the memory programmable naturally has to be adjusted for the hardware used in the force controlled grinding robot system. It is further preferred that the force controlled grinding robot system further comprises a positioning station having a re-gripping holding device for holding said blade during the re-gripping procedure. The manipulator of the robot in the force controlled grinding robot system uses the positioning station to re-grip the knife so that each knife is held in the exact same way. This leads to the benefit that the knife shape does not have to be measured for every knife, when grinding a series of knifes of the same model. The repositioning station includes fastening means that grips around the blade of the knife having an exactly known distance from the clamping device to the grip of the knife. The gripping head of the robot manipulator will then grip the handle of the knife at a predefined position in relation to the fastening means of the gripping device. The result is that the gripping head will grip every knife at the exact same position of the blade for every knife.

It should, however, be noted that if every knife is measured using the knife shape measurement procedure described above, the positioning station is not needed. The positioning station has the purpose to avoid that every knife has to be measured and by that speed up the process of sharpening a large number of knives.

It is further preferred that the force controlled grinding robot system further comprises a hollow grinding station to thin out the blade in the direction of its thickness. The hollow grinding station comprises at least one moving grinding surface. The hollow grinding station is desirable to be able to achieve an acceptable grinding that is equally good for every knife in a series of knifes of the same model. To do that it is important to have a blade that has the right thickness or cross sectional shape before the edge-grinding step is initiated. It will facilitate the edge-grinding if the cross sectional shape of the blade has the same shape every time. The cross sectional shape is often substantially triangular having a very sharp angle. The edge of the triangular shape is set to a certain width that is found optimal for the edge-grinding step for that particular knife model. In the same way as in the edge grinding station the hollow grinding station may grind the blade at a predefined portion of the knife length at a time, applying a predefined force according to the position of the knife against the moving grinding surface, using a predefined velocity according to the position of the knife of the grinding surface, and for a predetermined time period according to the position of the knife. All the predetermined values of grinding surface velocity, applied force and time duration are preferably a function of the position of the blade. The functions of position for velocity, force and time are proportional to the thickness of the blade and the height of the blade at each position. In a preferred embodiment two counter rotating stones produce the correct cross sectional shape of the blade in the grinding procedure of the hollow grinding station and a

predetermined thickness of the edge of that cross section. The stones may be angled to each other, so that their respective axis of rotation are not parallel, leading to a hollow cross section being slightly concave at each side of the blade.

It is further preferred that the force controlled grinding robot system further comprises a polishing station, wherein the polishing station comprising at least one moving polishing surface. The polishing station will remove the burr that may be present on the edge of the blade after the grinding process and provide the last touch of the grinding process to achieve a sharp and even edge as an end result of the grinding process. In the same way as for the edge grinding station and the hollow grinding station the machine may polish the blade at a predefined portion of the knife length at a time, applying a predefined force according to the position of the knife against the moving polishing surface, using a predefined velocity according to the position of the knife of the polishing surface, and for a predetermined time period according to the position of the knife. All the predetermined values of grinding surface velocity, applied force and time duration are preferably a function of the position of the blade. The functions of position for velocity, force and time are proportional to the thickness of the blade and the height of the blade at each position.

It is further preferred that the force controlled grinding robot system comprises a length measurement station adopted to measure the length of the blade. The measurement station comprises a surface having a well defined position that is stored in the memory programmable control. The robot manipulator, holding the knife in the blade using the gripper head, will move the knife towards the surface, in a perpendicular direction to the surface, until the tip or point of the knife touches the surface with a predetermined force. The memory programmable control will receive a signal from the force sensor when the knife touches the surface at a predetermined force and stop the movement to not harm the knife. Since the position of the robot gripper head relative the knife blade is known from the positioning station the length of the blade can be calculated.

The force controlled grinding robot system may further comprise a collection station for rejected cutting tools having a too short blade length. When regrinding a knife a number of times, the blade of the knife will due to the curvature of the knife shape at the tip of the knife become shorter for every time the knife is sharpened. Eventually the blade is to worn and the end of the knife lifetime is thus reached. To avoid that the force controlled grinding robot system grind knifes that are worn out, knifes having a length, as measured by the length measurement station, that is shorter than a

predetermined length for the knife model in question will be rejected by the machine and put in the collection station using the robot manipulator.

It should be noted that the stations described above as separate stations could be combined. E.g. the hollow grinding station and the edge grinding station could use the same grinding stones, the polishing station could be incorporated with either the hollow grinding station or the edge grinding station using the same rotational axis or motor. It should also be noted that in the description above the knife is moved to each and every one of the stations in the machine. It could however be the other way around, that the stations are brought to a stationary knife blade, e.g. positioned in a station like the described positioning station. The different stations could e.g. be realized as tools on a rotary head of the robot manipulator.

The present invention further relates to a method for sharpening blades of cutting tools using a robot having a memory programmable control, a manipulator moveable in at least four of the six possible translatory and rotational degrees of spatial freedom and a gripper head, said gripper head having a holding device, each of said blades has a length, a thickness and a height. The method comprises the steps of: taking a cutting tool from a pick- up area using said holding device to grip said cutting tool, feeding the blade of the cutting tool to an edge grinding station, sensing the force against said blade and providing an output force signal indicating the magnitude of said force, controlling the grinding force during the grinding in the edge grinding station dependent on said force signal.

The signal that the force sensor provide to the memory programmable control is processed according to a computer program, stored in the controller memory. It is understood that a memory programmable control incorporate a memory, a CPU, means to communicate with other electronics as sensors and the robot functionalities. The program that the memory programmable control execute uses input values to perform the correct grinding operation. The values are either measured values, as described above with the force sensor, or stored values as the knife shape. From these values and the computer program stored in the memory, the memory programmable control will be able to control the force controlled grinding robot system so that the grinding operation is performed with the same or equivalent results every time for every used knife that is grinded.

The step of feeding the blade to an edge grinding station is preferably preceded by feeding said blade of the cutting tool to a hollow grinding station, where sides of the blade are grinded to thin out the blade.

The step of feeding the blade to an edge grinding station is preferably followed by a step of feeding the blade to a polishing station, where the edge of the blade is polished.

The step of feeding the blade to an edge grinding station is further preferably preceded by a step of measuring the length of said blade of the cutting tool by moving the blade perpendicular towards a surface until the blade hits the surface, said surface having a well defined position known to the memory programmable control.

The step of taking a cutting tool is preferably preceded by re-gripping the cutting tool in a positioning station, said re-gripping comprising the steps of: placing the blade of the cutting tool in a re-gripping holding device, wherein the blade is positioned in a precise manner with regard to the base of the blade in the longitudinal direction, gripping the cutting using the holding device of the manipulator holding device tool in a part of the cutting tool that is outside the cutting blade in the longitudinal direction.

Blades having a length that is shorter than a pre-determined length are according to the method further preferably rejected and ejected to a collection station for rejected cutting tools.

The present invention further relates to the use of a force controlled grinding robot system for sharpening used blades of cutting tools, the force controlled grinding robot system comprising a robot having a memory programmable control, a manipulator moveable in at least four of the six possible translatory and rotational degrees of spatial freedom, a gripper head, fixed to the manipulator, having a holding device for holding a cutting tool, and a force sensor adapted to control force between the manipulator and an object in at least one dimension and provide an output force signal indicating the magnitude of said force, said memory programmable control being adapted to receive said output force signal and control said force controlled grinding robot system dependent on said force signal.

It should be noted that the inventive method and use of the force controlled grinding robot system may incorporate any of the features described above in association of the inventive device and has the same corresponding advantages.

Brief description of the drawings

The above objects, as well as additional objects, features and advantages of the present invention, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, when taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a topographic view of the force controlled grinding robot system according to the present invention.

Fig. 2 is a perspective view of the force controlled grinding robot system according to the present invention showing the edge grinding station and the robot. Fig. 3 is a perspective view of the force controlled grinding robot system according to the present invention showing the hollow grinding station and the robot.

Fig. 4 is a perspective view of the force controlled grinding robot system according to the present invention showing the positioning station.

Fig. 5 is a perspective view of the force controlled grinding robot system according to the present invention showing the polishing station and the robot. Detailed description of preferred embodiments of the invention

In the various figures of the drawing identical parts are always given the same reference number so that they generally only are described once in the detailed description below.

Fig. 1 shows the setup of the force controlled grinding robot system seen from above. The force controlled grinding robot system contains a robot 1 , a tray or pick-up area 3, a positioning station 4, a length measurement station 5, a hollowing grinding station 6, an edge grinding station 7, a polishing station 8, and a collection station 9.

The robot comprises a manipulator 1 1 , a gripper head 12 having a holding device 13, for gripping the cutting tool that is to be sharpened by the force controlled grinding robot system.

The positioning station 4 comprises a re-gripping holding device 41 holding a knife 10, see Figs. 1 and 4. The knife 10 is received from the robot by the re-gripping holding device 41 so that it grips around the blade 102 of the knife having the handle of the knife 101 abutting the edge 42 of the re- gripping holding device 41 . In that way the re-gripping holding device 41 will have the knife 10 in the exact same position relative the beginning of the blade every time, presenting a defined position for the robot gripper head 12 to pick it up. The positioning device 4 also has a temporary storage holder 43, where the knife 10 can be put e.g. if the knife 10 has to be turned.

The length measurement station 5 comprises a surface that has a well- defined position for the robot to touch with the tip of the knife. By knowing the position that the robot 1 holds the knife 10 and the position of the

measurement station surface, it can calculate the length of the knife 10.

The hollow grinding station 6, as shown in Figs. 1 and 3 comprises two angled counter rotating sharpening stones 61 having a well-defined distance between each other and a well-defined angle between each other.

The edge grinding station 7 also comprises two counter rotating sharpening stones 71 having different predetermined distances to each other to provide optimal grinding of the edge of the knife 10 having a thickness that is set in the hollow grinding station 6. The edge grinding station 7 is shown in Fig. 2.

The polishing station 8 comprises two counter rotating polishing wheels 81 for removing the burr that is created when the edge of the knife 10 is grinded. The polishing station 8 is also shown in Fig. 5.

Referring to all of the figures, the function of the force controlled grinding robot system will now be described. Knives that are blunt and are to be sharpened by the forced controlled grinding robot according to the present invention are inserted into the machine in the tray or pick-up area 3.

Preferably a large number of knives are positioned in the tray 3. The tray 3 itself has holders for the knives to keep them in defined positions that are known to the robot 1 .

The robot 1 then takes the first knife 10 from the tray 3 using the holding device 13 of the gripping head 12. The holding device 13 grips the handle 102 of the knife 10 in the shown embodiment, but could naturally just as well grip it somewhere else instead. The robot 1 then moves the knife 10 to the positioning device 4 and places it in the holding device 41 of the positioning station 4. After placing the knife 10 in the positioning station 4 the holding device 41 of the positioning station 4 grips around the blade 101 . When the positioning station 4 securely grips around the blade 101 , the holding device 13 of the gripper head 12 releases the knife 10 and takes a new grip at a predetermined position in relation to the positioning device, the position being well defined in relation to the base of the blade. When the gripper head 12 has a secure grip around the handle 102, the positioning device 4 releases the knife blade 101 . The robot 1 now holds the knife 10 in its holding device 13 at a well defined distance from the beginning of the knife blade 102.

If the knife 10 was placed upside down in the tray 3, the knife 10 has to be turned so that it can be placed correctly into the holding device 41 of the positioning station 4. In that case the knife is first delivered to the holding device 43 of the positioning station 4 and the robot 1 will re-grip and rotate the knife before it is placed in the holding device 41 as described above.

When the knife 10 is held by the robot 1 in a well-defined position with respect to the starting point of the knife blade 101 , the next step is to measure the length of the knife. This is done by moving the knife 10 to the length measurement station 3 and measure the knife 10 there. The length

measurement station 3 is simply a flat iron or a pole with a well-defined position. The knife 10 is moved towards the surface of the length

measurement device pole in a direction of the normal direction of the surface of the length measurement device pole. When a pre-determined pressure is reached, indicating that the knife blade tip is pressed against the pole, the position of the tip is subsequently known. Since the position of the knife blade tip and the position of the beginning of blade at the handle are known, the length of the knife can be calculated.

If the length, as measured by in the length measurement station 3, is less than a predetermined value, the knife 10 is considered to have been sharpened too many times. The knife is then moved to the collection station 2, where the knife 10 is simply dropped in a container for rejected knifes.

If the knife 10 is longer than the predetermined value, the knife 10 is moved to the hollow grinding station 6. In the hollow grinding station 6, the knife 10 is grinded to have the correct basic shape before the edge is to be sharpened. In the present embodiment according to Fig. 3, the hollow grinding station 6 comprises two counter rotating sharpening stones 61 having a predetermined angle to each other to provide the shape that is desired for the particular knife model that is to be sharpened. The knife 10 is held between and against the two sharpening stones 61 using a

predetermined pressure during a predetermined time at each position of the knife blade 101 . After the hollow grinding procedure is performed, the blade has the predetermined desired shape, suitable as starting point for

sharpening the edge of the knife 10.

When the knife 10 has been thinned out in the hollow grinding step, it is moved to the edge grinding station 7. The knife 10 is held against the two counter rotating sharpening stones 71 . The knife-edge is grinded at each position along the knife length for a predetermined time period at each position, i.e. the knife blade is moved at a predetermined velocity grinding the knife edge along its entire length. The force between the sharpening stones and the knife blade is controlled to predetermined values for each position along the knife length. At the end of the grinding step, if the end of the knife 10 is curved, the robot manipulator 1 1 and gripper head 12 will adjust the angel of the knife 10 so that the angle between the knife edge normal direction and the sharpening stones 71 are the same for every position of the knife grinding.

When the knife-edge has been sharpened in the edge grinding station

7, burr is usually present around the knife-edge. To remove the burr and to make the sharpness of the knife-edge even, the knife 10 is moved to the polishing station 8 where the two polishing rotation stones 81 remove the burr and create an evenly sharp edge of the knife 10.

After the polishing station 8 the sharpening of the knife is complete and the knife is put back into the knife tray 3. The robot gripper head 12 grips the next knife and starts the grinding procedure once again for the new knife.

It is understood that other variations in the present invention are contemplated and in some instances, some features of the invention can be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly in a manner consistent with the scope of the invention.

Claims

1 . Force controlled grinding robot system for sharpening blades (101 ) of cutting and milling tools, wherein each blade (101 ) has a length, a thickness and a height, the force controlled grinding robot system comprising a robot (1 ) having a memory programmable control, a manipulator (1 1 ) moveable in at least four of the six possible translatory and rotational degrees of spatial freedom,

a gripper head (12), fixed to the manipulator (1 1 ), having a holding device (13) for holding a cutting tool (10),

an edge grinding station (7) having at least one moving grinding surface (71 ) for receiving and grinding said blade (101 ),

c h a r a c t e r i z e d in that

a force sensor is adapted to sense the force between the manipulator and an object in at least one translatory degree of freedom and provide an output force signal indicating the magnitude of said force,

said memory programmable control being adapted to receive said output force signal and control said force controlled grinding robot system dependent on said force signal.

2. Force controlled grinding robot system according to claim 1 , said force controller being capable of controlling force in two translatory degrees of freedom, preferably three translatory degrees of freedom and more preferably in four translatory degrees of freedom.

3. Force controlled grinding robot system according to claim 1 or 2, said force controlled grinding robot system further comprising a positioning station (4), said positioning station (4) having a re-gripping holding device (41 ) for holding said blade (101 ) during the positioning procedure.

4. Force controlled grinding robot system according to any one of the preceding claims, said force controlled grinding robot system further comprising a hollow grinding station (6) to thin out the blade (101 ) in the direction of its thickness, said hollow grinding station (6) comprising at least one moving grinding surface (61 ).

5. Force controlled grinding robot system according to any one of the preceding claims, said force controlled grinding robot system further comprising a polishing station (8), said polishing station (8) comprising at least one moving polishing surface (81 ).

6. Force controlled grinding robot system according to any one of the preceding claims, said force controlled grinding robot system further comprising a length measurement station (5) adopted to measure said length of said blade (101 ), said measurement station (5) comprising a surface having a well defined position that is stored in said memory programmable control.

7. Force controlled grinding robot system according to any one of the preceding claims, said force controlled grinding robot system further comprising a collection station (2) for rejected cutting tools having a too short blade length.

8. Method for sharpening blades of cutting tools using a robot having a memory programmable control, a manipulator moveable in at least four of the six possible translatory and rotational degrees of spatial freedom and a gripper head, said gripper head (12) having a holding device (13), each of said blades has a length, a thickness and a height,

wherein said method comprises the steps of:

taking a cutting tool (10) from a pick-up area using said holding device 13 to grip said cutting tool (10),

feeding the blade (101 ) of the cutting tool to an edge grinding station (7),

sensing the force against said blade (101 ) and providing an output force signal indicating the magnitude of said force, controlling the grinding force during the grinding in the edge grinding station (7) dependent on said force signal.

9. Method for sharpening blades of cutting tools according to claim 8, wherein the step of feeding the blade (101 ) to an edge grinding station (7) is preceded by feeding said blade of the cutting tool to a hollow grinding station (6), where sides of the blade are grinded to thin out the blade (101 ).

10. Method for sharpening blades of cutting tools according to any of claims 8-9, wherein the step of feeding the blade (101 ) to an edge grinding station (7) is followed by a step of feeding the blade (101 ) to a polishing station (8), where the edge of the blade (101 ) is polished.

1 1 . Method for sharpening blades of cutting tools according to any of claims 8-10, wherein the step of feeding the blade to an edge grinding station (7) is further preceded by a step of measuring the length of said blade of the cutting (10) tool by moving the blade (10) perpendicular towards a surface until the blade (101 ) is pressed against the surface with a predetermined force, said surface having a well defined position known to the memory programmable control.

12. Method for sharpening blades of cutting tools according to any of claims 8-1 1 , wherein the step of taking a cutting tool (10) is preceded by re- gripping the cutting tool in a positioning station (4), said re-gripping comprising the steps of:

placing the blade (101 ) of the cutting tool (10) in a re-gripping holding device (41 ), wherein the blade is positioned in a precise manner with regard to the base of the blade in the longitudinal direction,

gripping the cutting tool (10) using the holding device (13) of the manipulator (1 1 ), the holding device gripping in a part of the cutting tool (10) that is outside the cutting blade in the longitudinal direction.

13. Method for sharpening blades of cutting tools according to claim 1 1 or 12, wherein blades having a length that is shorter than a pre-determined length are rejected and ejected to a collection station (2) for rejected cutting tools.

14. Use of a force controlled grinding robot system for sharpening used blades of cutting tools, the force controlled grinding robot system comprising a robot (1 ) having a memory programmable control, a manipulator (1 1 ) moveable in at least four of the six possible translatory and rotational degrees of spatial freedom,

a gripper head (12), fixed to the manipulator (1 1 ), having a holding device (13) for holding a cutting tool (10), and

a force sensor adapted to control force between the manipulator and an object in at least one dimension and provide an output pressure signal indicating the magnitude of said pressure,