GRINDING MACHINE FOR ROBOT-ASSISTED GRINDING

Description

TECHNICAL FIELD

[0001] The present invention relates to a grinding machine for robot-assisted grinding, in particular to a compact and lightweight grinding machine for mounting on a manipulator. BACKGROUND

[0002] In robot-assisted grinding systems, a grinding tool (e.g., an electrically operated grinding machine with a rotating grinding wheel) is guided by a manipulator, such as an industrial robot. The grinding tool can be controlled in various ways using the so-called TCP (Tool Center Point)of the manipulator, so that the manipulator can adjust the position and orientation of the tool practically as desired. Industrial robots are usually position-controlled, which enables precise movement of the TCP along a desired trajectory. To achieve good results in robot-assisted grinding, in many applications, control of the process force (grinding force) is necessary, which is often difficult to achieve with sufficient accuracy using conventional industrial robots. The large and heavy arm segments of an industrial robot have too great a mass inertia for a controller to (closed-loop controller)to react quickly enough to fluctuations in the process force. To solve this problem, a linear actuator, smaller than that of an industrial robot, can be arranged between the manipulator's TCP and the grinding tool, coupling the manipulator's TCP with the grinding tool. During grinding, the linear actuator only controls the process force (i.e., the contact force between the tool and the workpiece), while the manipulator moves the grinding tool and linear actuator along a predefined trajectory with position control.

[0003] Publication DE 10 2005 036 195 A1 shows a hand-held sander with a pneumatic drive, in which the suction provided by a suction device drives a turbine wheel, which in turn drives a sanding disc. Publication US 3,824,745 A also shows a hand-held sander with a suction device. Further reference is made to publications WO 2016 / 145472 A1 and US 6,969,310 B1.

[0004] Conventional orbital or random orbital sanders are comparatively less powerful because they are designed for manual work. The robot can work faster and therefore with greater force, requiring more sanding power. However, a small and compact design can cause thermal problems with orbital or random orbital sanders. Furthermore, the bending forces of hoses and cables can cause disruptive forces that change the process force during sanding but cannot be eliminated by the controller.

[0005] The inventors have set themselves the task of developing a compact grinding machine that is suitable for robot-assisted grinding and enables comparatively precise control of the process force during grinding. SUMMARY

[0006] The above-mentioned object is achieved by the grinding machine according to claim 1, the method according to claim 7, and the device according to claim 10. Different embodiments and further developments are the subject of the dependent claims.

[0007] A grinding machine is described which is suitable for a robot-assisted grinding process. According to one exemplary embodiment, the grinding machine has a housing, a motor arranged inside the housing, a fan wheel arranged on a motor shaft of the motor inside the housing, and a support plate coupled to the motor shaft for receiving a grinding wheel. The support plate has openings for sucking grinding dust into the interior of the housing. The grinding machine further has an outlet arranged in a wall of the housing for sucking the grinding dust out of the interior of the housing and a check valve arranged in the wall of the housing. The check valve allows air to escape from the interior of the housing but prevents air from being sucked into the interior of the housing.

[0008] Furthermore, a device for robot-assisted grinding is described. According to one embodiment, the device comprises a manipulator, a grinding machine, a linear actuator that couples the grinding machine to a TCP of the manipulator, and an extraction system connected to an outlet in the housing of the grinding machine.

[0009] Finally, a method for cooling a grinding machine with a rotatable backing plate for holding a grinding wheel is described. According to one embodiment, this comprises generating a negative pressure inside the housing of the grinding machine by means of an extraction system that is connected to the interior of the housing via an air outlet in a housing wall. The negative pressure generates an air flow through openings in the backing plate, which transports grinding dust into the interior of the housing. This dust is then extracted via the outlet in the housing wall. At the same time, the air flow also cools a motor located inside the housing.In the case of an inactive extraction system, the method comprises generating an additional air flow for cooling the motor through the openings in the support plate by means of a fan wheel, whereby a back pressure is generated inside the housing so that a check valve arranged in the housing wall opens and the additional air flow can escape. SHORT DESCRIPTION OF THE DRAWINGS

[0010] The invention is explained in more detail below using the examples shown in the figures. The illustrations are not necessarily to scale, and the invention is not limited to the aspects shown. Rather, emphasis is placed on illustrating the principles underlying the invention. The figures show: Figure 1 shows a schematic example of a robot-assisted grinding device. Figure 2 shows a schematic example of an orbital grinding machine with combined air cooling and extraction. Figure 3 shows the example from Fig. 2 in a situation with an inactive extraction system. Figure 4 shows the rotating support (support plate) with openings made of Fig. 2 in a view from below. Figures 5A and 5B , together Fig. 5 , show an example of a cable guide on a robot-assisted grinding device. DETAILED DESCRIPTION

[0011] Before various embodiments of the present invention are explained in detail, an example of a robot-assisted grinding device will first be described. This comprises a manipulator 1, for example an industrial robot, and a grinding machine 10 with a rotating grinding tool (e.g. an orbital grinding machine), which is connected to the so-called Tool Center Point(TCP) of the manipulator 1 via a linear actuator 20. In the case of an industrial robot with six degrees of freedom, the manipulator can be constructed from four segments 2a, 2b, 2c and 2d, which are each connected via joints 3a, 3b and 3c. The first segment is usually rigidly connected to a foundation 41 (although this does not necessarily have to be the case). The joint 3c connects the segments 2c and 2d. The joint 3c can be 2-axis and enable rotation of the segment 2c about a horizontal axis of rotation (elevation angle) and a vertical axis of rotation (azimuth angle). The joint 3b connects the segments 2b and 2c and enables a pivoting movement of the segment 2b relative to the position of the segment 2c. The joint 3a connects the segments 2a and 2b. The joint 3a can be 2-axis and therefore (similar to the joint 3c) allow a pivoting movement in two directions.The TCP has a fixed relative position to the segment 2a, which usually also comprises a rotary joint (not shown) that enables a rotary movement about a longitudinal axis A of the segment 2a (in . Fig. 1 (shown as a dot-dash line, corresponds to the rotational axis of the grinding tool). Each axis of a joint is assigned an actuator that can cause a rotational movement around the respective joint axis. The actuators in the joints are controlled by a robot controller 4 according to a robot program.

[0012] Manipulator 1 is usually position-controlled, meaning that the robot controller can determine the pose (location and orientation) of the TCP and move it along a predefined trajectory. Fig. 1is the longitudinal axis of segment 2a, on which the TCP lies, designated A. When actuator 20 rests against an end stop, the pose of the TCP also defines the pose of the grinding tool. As already mentioned at the beginning, actuator 20 is used to set the contact force (process force) between the tool (grinding machine 10) and workpiece 40 to a desired value during the grinding process. Direct force control by manipulator 1 is generally too imprecise for grinding applications, since the high mass inertia of segments 2a-c of manipulator 1 makes rapid compensation of force peaks (e.g. when placing the grinding tool on workpiece 40) practically impossible with conventional manipulators. For this reason, the robot controller is designed to regulate the pose of the manipulator's TCP, while the force control is carried out exclusively by actuator 20.

[0013] As already mentioned, during the grinding process, the contact force FK (also referred to as process force) between the tool (grinding machine 10) and the workpiece 40 can be adjusted with the aid of the (linear) actuator 20 and a force control (which can be implemented, for example, in the controller 4) so ​​that the contact force (in the direction of the longitudinal axis A) between the grinding tool and the workpiece 40 corresponds to a predeterminable target value. The contact force is a reaction to the actuator force with which the linear actuator 20 presses onto the workpiece surface. If there is no contact between the workpiece 40 and the tool, the actuator 20 moves against an end stop (not shown since it is integrated in the actuator 20) due to the lack of contact force on the workpiece 40. The position control of the manipulator 1 (which can also be implemented in the controller 4) can operate completely independently of the force control of the actuator 20.The actuator 20 is not responsible for the positioning of the grinding machine 10, but only for setting and maintaining the desired contact force during the grinding process and for detecting contact between the tool and the workpiece.

[0014] The actuator can be a pneumatic actuator, e.g., a double-acting pneumatic cylinder. However, other pneumatic actuators are also applicable, such as bellows cylinders and air muscles. Alternatively, electric direct drives (gearless) can also be considered. It should be noted that the effective direction of the actuator 20 does not necessarily have to coincide with the longitudinal axis A of the segment 2a of the manipulator. In the case of a pneumatic actuator, the force control can be realized in a conventional manner using a control valve, a regulator (implemented in the controller 4), and a compressed air reservoir. However, the specific implementation is not important for the further explanation and will therefore not be described in detail. Grinding machines typically have an extraction system to extract grinding dust. Fig. 1A connection 15 for a hose of a suction device is shown. This suction device will be discussed in more detail later.

[0015] As mentioned at the beginning, the inertia of the grinding machine can play a role in precise control of the contact force (process force). However, a small and compact design of the grinding machine results in higher power densities, which in turn causes high heat dissipation (and correspondingly high temperatures) in a comparatively small space. In the case of an orbital grinding machine, the heat loss arises partly in the electric motor of the grinding machine (e.g., ohmic losses, iron losses, friction losses in the bearings) and partly in the eccentric bearing that enables the orbital movement. In the examples presented here, compactness is achieved, among other things, by combining cooling and extraction. This means that during "normal" operation, the airflow generated by the grinding dust extraction system is also used for cooling.

[0016] Figure 2shows a schematic example of a grinding machine 10 with combined cooling and extraction. The grinding machine 10 can be Fig. 1 shown mounted on a manipulator 1 to enable a robot-assisted grinding process. Fig. 2 is a schematic longitudinal section along a longitudinal axis A of the grinding machine 10 (may coincide with the axis of the segment 2a, cf. Fig. 1 ). The grinding machine 10 comprises a housing 11, which may (but need not) have a substantially cylindrical basic shape. An electric motor 12 is arranged in the housing 11. The axis of rotation of the motor shaft of the motor 12 also corresponds to the longitudinal axis A of the grinding machine 10. In the present example, the motor shaft is connected via an eccentric bearing 16 to a rotating support 19 (support plate, backing pad) to which a grinding wheel is attached during operation. For example, the grinding wheel can be attached using a Velcro fastener (Velcro, hook-and-loop fastener)be attached to the rotating support. The eccentric bearing 16 enables rotation of the rotating support around an eccentric axis of rotation A', which rotates around the longitudinal axis A. The axial offset between the motor shaft (axis of rotation A) and the eccentric shaft (axis of rotation A') is in Fig. 2 denoted by e. The basic structure of the drive of the rotating support 19 is known per se and will therefore not be explained in detail.

[0017] As mentioned, grinding machines, in particular orbital grinding machines, can be coupled with an extraction system for extracting grinding dust. The extraction system generates a negative pressure, similar to a vacuum cleaner, and is connected to the interior of the housing 11 by means of a hose. In the present example, the hose of the extraction system can be connected to the air outlet 15. During grinding operation, air is sucked in through openings 17 in the rotating support 19, with dust particles being transported by the air flow through the openings 17 into the interior of the housing 11 and finally extracted through the air outlet 15. The air flow through the housing 11 is in Fig. 2 represented by the dashed arrows. Due to the negative pressure generated by the extraction system inside the housing (pressure pi inside is lower than the ambient pressure pa ), the Fig. 2The check valve 14 shown, which connects the interior of the housing 11 with the environment, is closed. In the illustrated situation with activated extraction and closed check valve 14, the axial fan 13 attached to the motor shaft is actually redundant and amplifies the air flow generated by the extraction system.

[0018] The airflow created by the extraction system simultaneously cools the interior of the housing and transports heat away from the motor 12 and the eccentric bearing 16. Due to the compact design, this cooling is necessary to prevent the grinder from overheating. Without cooling, temperatures exceeding 150°C could cause damage to the motor or mechanical components. During operation, there is a risk that the extraction system may not function properly for some reason, for example, because a worker forgets to turn it on, or because an air hose has come loose, etc. This is generally not a problem with conventional grinders. This is due to the less compact design, which means less waste heat, and extraction and cooling are two independent subsystems.However, the compact design of the grinding machine described here with combined cooling and extraction depends on a functioning extraction system unless further measures are taken to prevent the grinding machine from overheating when the extraction system is not functioning. If the extraction system fails, the axial fan 13 is not always able to generate sufficient convection, especially if the flow resistance through the air outlet 15 is too high. This can be the case, for example, in situations where the extraction system is connected to the air outlet 15 via a hose, but the extraction system is switched off or has failed.

[0019] Figure 3 shows the example from Fig. 2in a situation with an inactive extraction system. To prevent overheating of the grinding machine 10, an axial fan (fan wheel, propeller) is mounted on the motor shaft of the motor 12 (actually redundant for normal operation with extraction), which can generate an air flow through the interior of the housing for cooling purposes. As shown in Fig. 3 As shown, the axial fan also draws in air through the openings 17 and generates an airflow to cool the interior of the housing 11. As mentioned, situations may arise in which it is not guaranteed that the airflow generated by the axial fan 12 can be blown out via the outlet 15. If, for example, a hose is attached to the outlet 15, the air resistance of the hose may be so great that the axial fan 12 cannot generate enough volume flow to adequately cool the interior of the housing. For this reason, the Fig. 2 and 3The grinding machine shown has a check valve 14 which allows air to flow out of the interior of the housing 11 into the environment. When the extraction system is inactive, the axial fan generates an internal pressure pi (dynamic pressure) in the housing which is greater than the ambient pressure pa. Consequently, the check valve opens and air can escape from the housing via the check valve 14 with comparatively low air resistance, thereby dissipating heat. In machines without separate cooling (independent of the extraction system), the axial fan 13 - in combination with the check valve 14 - is a safety feature which prevents the motor from overheating if the extraction system does not generate a sufficient air flow to cool the motor. The axial fan can (but does not have to) be the only fan in the housing 11.At this point it should be emphasized again that in "normal" operation with active extraction the air flow cooling the motor is generated by the extraction.

[0020] Figure 4 shows the rotating support 19 (backing plate) with openings 17 in a view from below. As mentioned, the surface of the support 19 can be adhesive (e.g., by means of a Velcro fastener) to attach a grinding wheel to it. Fig. 4 The rotational axis A of the motor shaft and the eccentric rotational axis A' of the support 19 are also shown. It should be noted that the embodiments described here are not limited to orbital grinding machines. The Fig. 2 and 3 The combined cooling and extraction described is also applicable to other grinding machines with rotating grinding tools (grinding wheels), even if the grinding tool performs a purely rotary movement instead of an orbital movement.

[0021] As mentioned at the beginning, a compact and lightweight design of the grinding machine can help reduce inertia forces and improve contact force control. Another aspect that can play a role in contact force control is interference forces caused by bending of cables and hoses. These interference forces act parallel to actuator 20 (between grinding machine 10 and the TCP of manipulator 1) and therefore cannot be easily compensated by the actuator. Figures 5A and 5B show an example of a cable routing on a robot-assisted grinding device with grinding machine 10 and actuator 20, wherein the cables (electrical lines) required for the operation of the grinding machine are guided approximately along a spiral curve around the longitudinal axis A. Fig. 5A is a schematic view from the front and Fig. 5B a schematic view from below.

[0022] The routing of the cable(s) 18 along an approximately spiral curve (at least partially) around the grinding machine 10 and the actuator 20 causes that when the deflection changes a of the actuator 20, the deflection of the cable 18 should change as little as possible. Furthermore, the spiral cable guide generally enables very low disturbance forces along the longitudinal axis A (i.e., along the effective direction of the actuator) due to the bending of the cables 18. In comparison, with a conventional cable guide, in which the cable 18 (or cables) is (are) bent into a loop on one side of the actuator 20, the disturbance forces are considerably greater and more variable.

[0023] The one at outlet 15 (see Fig. 3) connected hose (not shown), through which the grinding dust is extracted, can be guided along a spiral curve in a similar manner as the cable(s) 18. Several cables 18 can be guided together in a cable conduit. As in Fig. 5 As shown, one end of the cable 18 is connected to the grinding machine 10 (and passed through a wall of the housing 11), whereas the other end of the cable 18 is mechanically connected to the outermost segment 2a of the manipulator 1.

[0024] Below, some important aspects of the exemplary embodiments described here are summarized. However, the following is not to be understood as a complete list, but merely as an example. One exemplary embodiment concerns a grinding machine 10 that is suitable for a robot-assisted grinding process (see FIG. Fig. 2). The grinding machine has a housing 11, a motor 12 arranged inside the housing, a fan wheel 13 arranged on a motor shaft of the motor 12 inside the housing and a rotatable holder 19 (backing plate) coupled to the motor shaft 12 for holding a grinding wheel. The backing plate has openings 17 for sucking grinding dust into the interior of the housing 11. The grinding machine 10 further has an outlet 15 arranged in a wall of the housing for sucking the grinding dust out of the interior of the housing 11 and a check valve 14 arranged in the wall of the housing 11. The check valve 14 allows air to escape from the interior of the housing, but prevents air from being sucked into the interior of the housing (cf. Fig. 2 and 3 ).

[0025] In one embodiment, the motor 12 is arranged in the interior of the housing 11 in such a way that the air flow from the openings 17 in the support plate 19 to the outlet 15, which removes the sanding dust, also cools the motor 12. The air flow generated for the extraction is thus also used to cool the motor. In the case of an orbital sander, the sander has an eccentric bearing 16 that connects the motor shaft to the support plate 19, allowing the support plate 19 to perform an orbital movement. The aforementioned air flow from the openings 17 in the support plate 19 to the outlet 15, which removes the sanding dust, also cools the eccentric bearing 16.

[0026] In case of lack of suction through the outlet 15, air which is sucked in by means of the fan wheel 13 through the openings 17 in the support plate 19 exits through the check valve 14 (see Fig. 3). In other words, the fan impeller 13 creates a back pressure inside the housing 11, so that air from the inside of the housing 11 can escape into the environment through the check valve 14.

[0027] In order to reduce the disturbing forces due to cables and hoses connected to the grinding machine, a cable 18 for supplying power to the motor 12 can be routed approximately spirally around the housing 11.

[0028] A further aspect relates to a device for robot-assisted grinding with a manipulator 1 (e.g. an industrial robot), a grinding machine 10 according to the examples described here, a linear actuator 20 which couples the grinding machine 10 to a TCP of the manipulator 1, and an extraction system which is connected to an outlet in the housing of the grinding machine (cf. Fig. 1 and 2). A further aspect relates to a method for cooling a grinding machine 10 with a rotatable support plate 19 for holding a grinding wheel. The method comprises generating a negative pressure inside the housing 11 of the grinding machine 10 by means of an extraction system which is connected to the interior of the housing via an air outlet 15 in a housing wall (see Fig. 2 and 3). The negative pressure creates an air flow through openings 17 in the support plate 19, which transports grinding dust into the interior of the housing 11. This dust is then extracted via the outlet 15 in the housing wall. At the same time, the air flow also cools a motor arranged inside the housing. In the case of an inactive extraction system, the method comprises generating an additional air flow for cooling the motor 12 through the openings 17 in the support plate 19 by means of a fan wheel 13, whereby a back pressure is generated inside the housing 11, so that a check valve 14 arranged in the housing wall opens and the additional air flow can escape.

Claims

1. A grinding machine having the following: a housing (11); a motor (12) arranged inside the housing (11); a fan wheel (13) arranged on a motor shaft of the motor (12) inside the housing; a support plate (19) coupled to the motor shaft for receiving a grinding wheel, wherein the support plate (19) has openings (17) for sucking grinding dust into the interior of the housing (11); an outlet (15) arranged in a wall of the housing (11) for extracting the grinding dust from the interior of the housing, wherein the outlet (15) is connectable to an extraction system that can generate a negative pressure inside the housing (11) of the grinding machine; and a check valve (14) arranged in the wall of the housing (11), which allows air to escape from the interior of the housing (11) but prevents air from being sucked into the interior of the housing (11), wherein the motor (12) is arranged in the interior of the housing (11) in such a way that the air flow from the openings (17) in the support plate (19) to the outlet (15), with which the grinding dust is removed, also cools the motor (12).

2. The grinding machine according to claim 1, wherein the motor (12) is an electric motor.

3. The grinding machine according to claim 1, further having: an eccentric bearing (16) which connects the motor shaft to the support plate (19) so that the support plate can perform an orbital movement, wherein the air flow from the openings (17) in the support plate (19) to the outlet (15), with which the grinding dust is removed, cools the eccentric bearing (16).

4. The grinding machine according to any one of claims 1 to 3, in which the fan wheel (13) and the check valve (14) are arranged in such a way that - in the event of no extraction through the outlet (15) - air sucked in by means of the fan wheel (13) through the openings (17) in the support plate (19) exits through the check valve (14).

5. The grinding machine according to any one of claims 1 to 4, in the event of a back pressure inside the housing (11) caused by the fan wheel (13), air can escape from the inside of the housing (11) through the check valve (14) into the environment.

6. The grinding machine according to any one of claims 1 to 5, further having: a cable for supplying power to the motor (12), wherein the cable is guided approximately spirally around the housing (11).

7. A method for cooling a grinding machine with a rotatable support plate (19) for receiving a grinding wheel; the method comprises: generating a negative pressure inside a housing (11) of the grinding machine by means of an extraction system which is connected to the interior of the housing (11) via an air outlet (15) in a housing wall, wherein the negative pressure generates an air flow through openings (17) in the support plate (19), which transports grinding dust into the interior of the housing (11), which is sucked out via the outlet (15) in the housing wall, and wherein the air flow also cools a motor (12) arranged inside the housing (11); and wherein the method comprises, in the case of an inactive extraction system: generating a further air flow for cooling the motor through the openings (17) in the support plate (19) by means of a fan wheel (13), whereby a back pressure is generated inside the housing (11) so that a check valve (14) arranged in the housing wall opens and the further air flow can escape.

8. The method according to claim 7, wherein the fan wheel (13) is attached to the motor shaft of the motor (12) and forms an axial fan.

9. The method according to claim 7 or 8, wherein the check valve (14) is closed when the extraction system is active.

10. A device for robot-supported grinding, having; a manipulator; a grinding machine according to any one of claims 1 to 6; a linear actuator (20) coupling the grinding machine to a TCP of the manipulator; an extraction system which is connected to the outlet (15) in the housing (11) of the grinding machine.

11. The device according to claim 10, wherein the direction of action of the linear actuator (20) runs substantially parallel to the axis of rotation of the motor (12).

12. The device according to claim 10 or 11, wherein the extraction system is connected to the outlet (15) via a hose which is arranged approximately spirally around the housing (11) of the grinding machine and the linear actuator.

13. The device according to any one of claims 10 to 12, wherein a cable required for the operation of the grinding machine is arranged approximately along a spiral curve around the longitudinal axis of the housing (11) of the grinding machine and of the linear actuator (20), wherein one end of the cable is mechanically coupled to the manipulator.

14. The device according to claim 13, wherein the cable is arranged together with other cables in a cable hose (18).

15. The device according to any one of claims 10 to 14, wherein the extraction system is connected to an outlet (15) on the grinding machine via a hose (18) which is arranged approximately spirally around the housing (11) of the grinding machine and the linear actuator (20) and is fastened at one end to the manipulator.