Apparatus and method for the automatic removal of grinding discs

A device for automatically removing a grinding wheel from a grinding machine mounted on a manipulator is described. According to one embodiment, the device has the following: a support plate with a surface for depositing a grinding wheel; a movable clamping element that is raised in a first position with respect to the support plate; an actuator that is coupled to the clamping element and is configured to move the clamping element into a second position in which the clamping element is pressed against the support plate such that the grinding wheel is clamped between the support plate and clamping element; and a release element that is arranged in such a way relative to the support plate that the release element is actuated when the grinding wheel is placed on the surface of the support plate and is pressed against the latter. The release element and the actuator are coupled (directly or indirectly, electrically or mechanically) such that, when the release element is actuated, the actuator moves the clamping element from the first position into the second position.

TECHNICAL FIELD

The present disclosure relates to a changing station which enables a robot-supported grinding device to automatically change the grinding elements (i.e. grinding discs).

BACKGROUND

Grinding machines are frequently employed in industry and trade. Eccentric grinders are grinding machines in which a rotating movement around a rotational axis is superimposed on an oscillating movement (vibration). They are often used for the final machining of surfaces when high demands are placed on the surface quality, such as for spot repairs of surface defects on painted surfaces. In order to fulfill these demands, irregularities in the grinding process must be avoided as much as possible. In the field this is generally realized by having skilled professionals carry out these tasks in small batches.

In robot-supported grinding devices a grinding tool (e.g. an orbital grinding machine) is guided by a manipulator, for example, an industrial robot. In the process, the grinding tool may be coupled to the so-called tool center point (TCP) of the manipulator in various ways that enable the manipulator to adjust the tool to virtually any position and orientation. Industrial robots are generally position-controlled, which makes it possible to move the TCP precisely along the desired trajectory. In order to achieve good results from robot-support grinding, in many applications the processing force (grinding force) must be regulated, which is often difficult to realize with sufficient accuracy using conventional industrial robots. The moment of inertia of the large and heavy arm segments of an industrial robot is too large for a closed-loop controller to be able to react quickly to fluctuations in the processing force. To solve this problem, a linear actuator, smaller than the industrial robot, may be disposed between the TCP of the manipulator and the grinding tool in order to couple the TCP of the manipulator to the grinding tool. In this case the linear actuator only controls the processing force (that is, the application force between the tool and the workpiece), while the manipulator moves the grinding tool, together with the linear actuator, along the specifiable trajectory in a position-controlled manner.

Grinding machines such as, e.g. eccentric grinders, operate using thin, flexible and removable grinding discs which are attached to a backing pad. One very commonly used type of backing pads are the so-called daisy discs. A grinding disc is typically made of paper (or of a different fiber composite material) which is coated with abrasive particles and it can be attached to the carrier plate, e.g. by means of an adhesive layer, a hook and loop fastener or a Velcro fastener. Worn grinding discs are frequently changed manually, even in robot-supported grinding devices. Although a few concepts for robot-supported changing stations for changing grinding discs do exist, the known solutions are in general very complex and their realization is time-consuming and expensive.

The inventors identified a need for a changing station which makes it possible for a robot-supported grinding device to automatically change grinding discs in a relatively easy manner.

SUMMARY

An apparatus for automatically removing a grinding disc from a grinding machine mounted on a manipulator is described herein. In accordance with one embodiment, the apparatus comprises the following; a bearing plate with a surface on which a grinding disc can be placed; a moveable clamping element which, in a first position, is elevated above the bearing plate; an actuator which is coupled to the clamping element and which is configured to move the clamping element into a second position in which the clamping element is pressed against the bearing plate such that the grinding disc is clamped in place between the bearing plate and the clamping element; and a release element which is arranged relative to the bearing plate such that the release element is actuated when the grinding disc is placed onto the surface of the bearing plate and is pressed against it. The release element and the actuator are (directly or indirectly, electrically or mechanically) coupled to each other such that, when the release element is actuated, the actuator moves the clamping element from the first position into the second position.

Further, a method for the automatic removal of a grinding disc from a grinding machine mounted on a manipulator is described herein. In accordance with one embodiment, the method comprises the following: Placing a grinding disc that is mounted on a grinding machine onto a bearing plate of a removal device by means of a manipulator, wherein, by placing the grinding disc onto the bearing plate, a release element of the removal apparatus is actuated. The method further comprises clamping the grinding disc in place between the bearing plate and a moveable clamping element which, in reaction to the actuation of the release element, is pushed into the direction of the bearing plate, and raising the grinding machine by means of the manipulator, by means of which the clamped grinding disc is removed from a carrier plate of the grinding disc.

DETAILED DESCRIPTION

Before explaining various embodiments in detail, first a general example of a robot-supported grinding apparatus will be described. The example encompasses a manipulator1, for example an industrial robot, and a grinding machine10with a rotating grinding tool (e.g. an orbital grinding machine), wherein the latter is coupled to the tool center point (TCP) of the manipulator1via a linear actuator20. In the case of an industrial robot possessing six degrees of freedom, the manipulator may be constructed of 4 segments;2a,2b,2cand2d, each of which is connected via joints3a,3band3c(the first segment is usually rigidly attached to a base41, which, however, need not necessarily be the case). Joint3cconnects the segments2cand2d. Joint3cmay be biaxial and allow a rotation of segment2caround a horizontal axis of rotation (elevation angle) and around a vertical axis of rotation (Azimuth angle). Joint3bconnects the segment2band2cand allows a swivel movement of segment2brelative to the position of segment2c. Joint3aconnects the segments2aand2b. Joint3amay be biaxial and may (as in the case of joint3c) allow a swivel movement in two directions. The TCP is at a permanent relative position in respect to segment2a, wherein the latter usually also encompasses a rotational joint (not shown) which allows a rotational movement around a longitudinal axis of segment2a(designated inFIG.1using a dot-dashed line and which corresponds to the axis of rotation of the grinding tool). An actuator is assigned to every axis of a joint in order to effect a rotational movement around the respective joint axis. The actuators in the joints are controlled by a robot controller4in accordance with a robot program.

The manipulator1is usually position-controlled, i.e. the robot controller can determine the pose (position and orientation) of the TCP and can move it along a previously defined trajectory. When the actuator20comes to rest against an end stop, the pose of the TCP also defines the pose of the grinding tool. As previously mentioned, the actuator20serves to adjust the contact force (processing force) between the tool (grinding machine10) and the workpiece40to a desired value during the grinding process. Controlling the force directly using the manipulator1is generally too inaccurate for grinding applications because the large moment of inertia of segments2a-2cof the manipulator1renders it virtually impossible for conventional manipulators to quickly compensate force peaks (e.g. such as occurs when the grinding tool is placed on the workpiece40). For this reason the robot controller is configured to adjust the pose of the TCP of the manipulator, while the force adjustment is carried out exclusively by the actuator20.

As previously mentioned, during the grinding process the contact force FK between the tool (grinding machine10) and the workpiece40can be adjusted with the aid of the (linear) actuator20and a force controller (which, for example, may be implemented in the controller4) such that the contact force between the grinding tool and the workpiece corresponds to a specifiable target value. The contact force is in this case a reaction to the actuator force with which the linear actuator20presses against the surface of the workpiece. If contact between the workpiece40and the tool is absent, the actuator20comes to rest, due to the lack of contact force on the workpiece40, against an end stop. The position control of the manipulator1(which may also be implemented in the controller4) can operate completely independently of the force control of the actuator20. The actuator20is not responsible for positioning the grinding machine10, but instead only for the adjustment and maintenance of the desired contact force during the grinding process and for detecting a contact between the tool and the workpiece. The actuator may be a pneumatic actuator, e.g. a double-acting pneumatic cylinder. Other pneumatic actuators, however, may also be employed such as, e.g. bellows cylinders and air muscles. Direct electric drives (without transmissions) may also be taken into consideration.

If a pneumatic actuator is employed, the force can be adjusted in a conventionally known manner using a control valve, a controller (implemented in controller4) and a compressed air reservoir. The specific implementation, however, is of little relevance for the remaining discussion and will therefore not be described here in further detail. As an alternative to the actuator20and depending on the application, a passive yielding element such as, e.g. a spring, may also be used. The actuator20may also be omitted if the manipulator itself is capable of providing force regulation of a satisfying quality.

The grinding machine10comprises a grinding disc11which is mounted on a backing pad12. The surface of the backing pad12or the back surface of the grinding disc11, or both surfaces, are configured such that the grinding disc11easily adheres to the backing pad12upon contact. For example, a hook and loop fastener or a Velcro fastener may be used to ensure that the grinding disc11remains adhered to the backing pad12. One commonly used alternative to a hook and loop fastener is an adhesive coating on the backside of the grinding disc11which adheres to a corresponding surface of the backing pad12.FIG.2illustrates an example of a grinding apparatus10which can be mounted on a manipulator and in which the grinding machine10is positioned relative to a grinding disc removal device30such that the grinding disc11rests against the surface of a bearing plate35of the grinding disc removal device30and is pressed against this bearing plate35(e.g. with an adjustable force). In the following, one embodiment of the grinding disc removal device30will now be described in detail with reference to theFIGS.3to6.

FIG.3is a perspective view of the grinding disc removal device30fromFIG.2.FIG.4is a corresponding sectional view which shows the components located on the inside of the housing31of the removal device30. It should be understood that the housing31of the removal device30need not necessarily be a closed housing. On the contrary, housing should be understood to designate any mechanical structure on which other components of the removal device30can, directly or indirectly, moveably or immovably, be mounted. The housing may comprise a frame to which (in the case of an at least partially closed housing) one or more covers are attached. In the examples illustrated here, the housing31comprises numerous parts which are connected by means of screws. It is to be understood, however, that other connection techniques such as, e.g. rivets, snap-in connections, etc. may be employed as well. Depending on the application, the shape of the housing may also be designed in a different fashion than the embodiments described here. In the example shown here, the housing comprises a base plate310with holes311. The base plate310(and with it the entire device30) can be mounted onto the floor or another supporting surface by means of screws (not shown) which are inserted through the holes311.

As can be seen inFIG.3, the bearing plate35, against which the robot presses the grinding disc11mounted on the grinding machine10during a removal step, comprises an opening through which the end of a trigger element33has been inserted. The end of the trigger element33protrudes from the opening in the bearing plate35such that the protruding end of the trigger element33is pushed into the opening (seeFIG.4, application force FA) when the grinding disc11is pressed against the bearing plate35and comes to lie flatly on it during a removal process. The opening can also be implemented as a slit. It should be understood that the end of the trigger element33need not necessarily run through an opening in the bearing plate35. Alternatively, the trigger element33may be arranged next to the bearing plate. It is only important that the trigger element33be arranged such that it is actuated when the robot presses the grinding disc11against the surface of the bearing plate35.

The actuation of the trigger element33(when the grinding disc is pressed against the bearing plate35) triggers a mechanism that results in the edge of the grinding disc11being clamped in between the bearing plate35and a clamping plate34. When the robot again moves the grinding machine11away from the removal device30, the grinding disc11is held in place by the clamping plate34, while the backing pad12of the grinding machine10is lifted away from the surface of the bearing plate35. By raising the backing pad12, the (clamped) grinding disc11is removed from the backing pad12. In the following, an example of this described mechanism will be described in detail with reference toFIGS.4and5.

As is illustrated inFIG.4, the clamping plate34(generally referred to here as clamping element) is mounted on a first end of a rocking lever342which is rotatably mounted on a part of the housing31by means of a joint341. This means that the rocking lever342, which may also be called a rocker, can be swiveled around a pivot point (which is defined by the joint341). The clamping plate34(clamping element) may be attached to the rocking lever342, e.g. by means of one or more screws. In other embodiments the clamping plate34and the rocking lever342may be constructed in one piece. In the situation illustrated inFIG.4, the rocking lever342is positioned (first position) such that the clamping plate34is lifted away from the bearing plate35. In the situation illustrated inFIG.5, the rocking lever342is position (second position) such that the clamping plate34is pressed against the surface of the bearing plate35and a grinding disc (provided it has been correctly positioned on the bearing plate35) is clamped in between the clamping plate34and the surface of the bearing plate35.

The movement of the rocking lever342from the first position (clamp released) into the second position (clamp tightened) is triggered by the actuation of the trigger element33. In the example illustrated inFIGS.4and5, the rocking lever342comprises an end stop343which rests against a corresponding bearing surface of the trigger element33. Similar to the rocking lever342, the trigger element33is pivotally mounted on a part of the housing31(rotational joint331) and is pressed, by means of a spring332, into a standard position in which an end of the trigger element33, as illustrated inFIG.4, protrudes beyond the surface of the bearing plate35. In this standard position the trigger element33functions as a pawl which prevents movement of the rocking lever342into the second position (clamp tightened). When the end stop343of the rocking lever342comes to rest against the trigger element33(functioning here as a pawl), the movement of the rocking lever342is blocked. When the protruding trigger element33is pushed, contrary to the spring force of the spring332, towards the surface of the bearing plate35, the trigger element33is swiveled such that the end stop343of the rocking lever342moves away from the trigger element33and a movement of the rocking lever342into the second position is no longer blocked.

In the example illustrated inFIGS.4and5, when the rocking lever341is in the first position (clamp released, the pawl blocks the movement of the rocking lever), a biasing force FBis exerted onto the rocking lever341. When the trigger element33is actuated/moved, the pawl (release element) releases the rocking lever341and the latter abruptly swings, due to the biasing force FB, into the second position in which the grinding disc is clamped in place. This situation is illustrated inFIG.5.

The aforementioned biasing force FBcan be provided by various biasing mechanisms. In the example fromFIGS.4and5this biasing mechanism comprises a pneumatic cylinder37disposed between a second end of the rocking lever341and a part (e.g. mounting bracket311) of the housing31. The cylinder37is connected to the mounting bracket311(which can be regarded as a part of the housing) by means of a joint374and the piston rod372of the piston371disposed in the cylinder is connected to the second end of the rocking lever341by means of a joint373. When the cylinder chamber (designated V1inFIGS.4and5) is filled with compressed air, the pneumatic cylinder37generates (with its respective piston371) the biasing force FBwhich pushes, by actuating the trigger element33, the rocking lever341into the second position and clamps the grinding disc in place.

As previously mentioned, after clamping the grinding disc11, the grinding machine10is again moved away from the removal device30, by means of which the (clamped) grinding disc11is pulled off the backing pad12. After this the rocking lever341(and with it the clamping plate34) have to be moved back from the second position (clamp tightened,FIG.5) into the first position (clamp released,FIG.4). This movement can be performed by various kinds of reset mechanisms. In the example illustrated inFIGS.4and5, the reset mechanism is the pneumatic cylinder37. In this case, the biasing mechanism and the reset mechanism form one unit. The cylinder37may be a double-acting cylinder. This means that, when the cylinder chamber (designated V2inFIGS.4and5) is filled with compressed air, the pneumatic cylinder37generates a reset force FRthat acts in the exact opposite direction of the biasing force FB. The reset force FReffects a swivel movement of the rocking lever341back into the first position, by means of which the clamp on the grinding disc is released. The spring332pushes the trigger element33back into the standard position so that, when the biasing force FBagain takes effect during the next removal process, the movement of the rocking lever341will again be blocked (as illustrated inFIG.4).

Air can be blasted onto the grinding disc11at a high velocity through the compressed air nozzle32, blowing it in the direction of the baffle plate312and onwards, e.g. into a container. The compressed air nozzle32and the baffle plate312are both optional, but in actual practice they can improve the robustness of the device30. The reset mechanism (e.g. switching the compressed air from cylinder chamber V1to cylinder chamber V2), as well as the blasting of compressed air out of nozzle32, can be triggered by the robot controller (seeFIG.1, controller4), as the robot controller “knows” when the grinding machine10has been moved away from the removal device30. As an alternative, the reset mechanism can also be triggered by the trigger element33swiveling back into the standard position. For this purpose, an electric switch could be coupled to the trigger element33and actuating the electric switch can trigger the switching of the compressed air from cylinder chamber V1to cylinder chamber V2, as well as the blasting of compressed air from nozzle32. A corresponding nozzle controller with its respective nozzles is not illustrated in the figures as various possibilities for implementing the nozzle control fall within the scope of the skilled person's professional capabilities.

Some grinding discs stay adhered to the backing pad12(seeFIG.2) by means of an adhesive layer. In such cases it may occur that the grinding disc remains adhered to the clamping plate34. In order to be able to reliably remove the grinding disc from the device30(e.g. by blasting compressed air out of the nozzle32), one or more pins38may be (directly or indirectly) attached to the housing31which are arranged such that a grinding disc11that remains adhered to the clamping plate34is pushed away from the clamping plate34when the latter is moved back into the first position. The pins38are shown inFIGS.4and5. In the overhead view fromFIG.6, which corresponds toFIG.5, one can see that the clamping plate34possesses small recesses38′, through which the pins38are forced when the clamping plate34is moved (away from the bearing plate35) into the first position. If a grinding disc remains adhered to the clamping plate34when this movement is carried out, it will be pushed away by the pins38when the movement is completed and thereby released from the clamping plate34, and the grinding disc is then transported away by the compressed air.

In order to verify whether the grinding disc removed from the grinding machine10has actually been transported away from the removal device30, the removal device30may comprise a sensor36. The sensor36can be seen inFIGS.4-6and may be implemented, for example, as reflex light barrier. InFIG.6the corresponding reflector361of the reflex light barrier36is also shown. The sensor36(e.g. a module with a light emitting diode and a photodiode) and the reflector361are positioned relative to each other such that the light beam emitted from the sensor36is interrupted by a grinding disc. In this way the sensor can detect whether or not the grinding disc has been transported away by the compressed air. If it has not been, one or more additional blasts of compressed air can be emitted from the nozzle32. If the grinding disc then still remains adhered to the removal device, a warning signal, for example, might be triggered. The sensor36need not necessarily be implemented as a light barrier. Since the grinding discs are usually of a certain color, instead an optical color sensor may also be employed to detect the presence of a grinding disc. Alternatively, one or more sensors may be employed to monitor whether the grinding disc falls out of the device30from underneath the baffle plate312.

It should be understood that the function of clamping the grinding disc11between the bearing plate35and the clamping element34(clamping plate) and of the trigger element33releasing the movement of the clamping element can be realized in other ways than as those illustrated in the example fromFIGS.2-6. In the following, a few important aspects of the removal device30will be summarized and further embodiments will be discussed in which certain functions are implemented differently than in the example fromFIGS.2-6.

In general terms, the removal device comprises a bearing plate (see, e.g.FIGS.2and6, bearing plate35) which has a surface on which a grinding disc11can be placed. As shown inFIG.2, the grinding disc11can be placed onto the surface of the bearing plate35with the aid of a robot. As one can see, for example, inFIG.6, the entire grinding disc11need not be placed onto the surface of the bearing plate35. It suffices if a part of the grinding disc rests on the bearing plate35. The removal device further comprises a moveable clamping element (see, e.g.FIGS.4and5, clamping element34) which, in a first position, is raised above the bearing plate. This means that, in the first position, the clamping element does not come into contact with the bearing surface. The removal device further comprises an actuator, which is coupled to the clamping element, and which is configured to move the clamping element into a second position in which the clamping element is pressed against the bearing plate such that the grinding disc is clamped in place in between the bearing plate and the clamping element (seeFIG.5). A trigger element is coupled (directly or indirectly and, depending on the actuator, mechanically or electrically) to the actuator such that, when the trigger element is actuated, the actuator moves the clamping element from the first position into the second position. The trigger element protrudes beyond the surface of the bearing plate such that the trigger element is actuated when the grinding disc (mounted on the grinding machine) is positioned on the surface of the bearing plate and is pressed onto it.

In the simplest of cases, the actuator may be a biased spring. A pneumatic actuator (pneumatic cylinder-piston unit) can also fulfill the same function as a biased spring when it is filled with compressed air. In some embodiments, the clamping element blocks, until it is actuated, the movement of the actuator (seeFIG.4, cylinder preloaded with compressed air), which then abruptly moves the clamping element from the first position (clamp released) into the second position (clamp tightened) when the trigger element is actuated (seeFIG.5). In this case, the trigger element is a purely mechanical machine element that essentially fulfills the function of a pawl. In order to move the clamping element back into the first position, a reset mechanism may be provided. If a double-acting pneumatic cylinder is used as an actuator, it can generate enough reset force to move the clamping element back into the first position. If a single-acting pneumatic cylinder is used, a spring can also generate the needed reset force to move the clamping element back into the first position when the pressure is released from the single-acting pneumatic cylinder. In the aforementioned example, in which the actuator is a simple biased spring, the reset force can be generated, for example, by a solenoid which is capable of re-biasing the spring. Possible is also a unit consisting of two (single-acting) pneumatic cylinders, whereby one cylinder serves as a (biased) actuator and the other is responsible for the resetting movement into the first position.

In other embodiments the actuator does not need to generate a biasing force while the movement of the actuator is mechanically blocked by the trigger element. Instead, the actuator is actively controlled to move the clamping element from the first position into the second position when the trigger element is actuated which, in this case, may also be an electric switch (e.g. a probe), which in turn is positioned such that it protrudes beyond the bearing plate and is thereby “automatically” actuated when the grinding disc mounted on the grinding machine is placed onto the surface of the bearing plate. In this case the actuator may be any given actuator (an electromotor, a linear motor, a pneumatic actuator, a solenoid, etc.) which is configured to move the clamping element from the first position into the second position. Instead of a simple switch like, e.g. a probe, a different sensor element may also be used which is capable of detecting that a grinding disc has been placed on the bearing plate.

In the embodiments described here, the clamping element is mounted on an end of a rocking lever (seeFIG.5). It should be understood, however, that the clamping element and the rocking lever may also be one integrated component. In such a case, the clamping element and the rocking lever are constructed in one piece. The clamping element can be implemented as a small slide, referred to above as clamping plate. However, the clamping element need not necessarily be a slide but instead may be formed, for example, of numerous short pins that protrude from the rocking lever and which can clamp the grinding disc to the bearing plate.

In the following an example of a method for the removal of a grinding disc from a grinding machine mounted on a manipulator will be summarized with reference to the flow chart shown inFIG.7. In accordance with the embodiment ofFIG.7, the method comprises placing a grinding disc mounted on a grinding machine onto a bearing plate of a removal device by means of a manipulator (cf.FIG.7, step S1). This situation is also illustrated inFIG.4. By placing the grinding disc onto the bearing plate, a trigger element of the removal device is also actuated (cf.FIG.4, trigger element implemented as pawl). The method further comprises clamping the grinding disc in between the bearing plate and a moveable clamping element (FIG.7, step S2), which is pushed towards the bearing plate in reaction to the actuation of the trigger element. This situation is also illustrated inFIG.5. Following this, the grinding machine is lifted up with the aid of the manipulator, whereby the clamped grinding disc is pulled off a backing pad of the grinding machine (cf.FIG.7, step S3).

Following this, the clamping element can once again be raised in order to release the clamped grinding disc. In the process it may happen, as described earlier, that the grinding disc remains adhered to the clamping element, which is undesirable, as it hampers the further transport of the grinding disc. In such cases, the grinding disc can be released using one or more pins (seeFIG.5, pins38). The pins38block the movement of the grinding disc adhered to the clamping element when the clamping element is raised, whereby the grinding disc is released from the clamping element. The pin or pins may be mounted in the housing of the removal device such that the pins penetrate one or more recesses (seeFIG.6, recesses38′) on the edge of the clamping element when the clamping element is raised. Various further aspects of the method have already been described above with reference toFIGS.2-6and, in order to avoid repetition, reference is made to the above description.

Although various embodiments have been illustrated and described with respect to one or more specific implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. With particular regard to the various functions performed by the above described components or structures (units, assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond—unless otherwise indicated—to any component or structure that performs the specified function of the described component (e.g., that is functionally equivalent), even if it is not structurally equivalent to the disclosed structure that performs the function in the herein illustrated exemplary implementations of the invention.