WO2016145472A1

WO2016145472A1 - Method and device for robot-supported surface processing

Info

Publication number: WO2016145472A1
Application number: PCT/AT2016/050068
Authority: WO
Inventors: Ronald Naderer
Original assignee: Ferrobotics Compliant Robot Technology Gmbh
Priority date: 2015-03-19
Filing date: 2016-03-18
Publication date: 2016-09-22
Also published as: DE102015104164A1; DE102015104164B4

Abstract

Disclosed is a method for the automated surface processing of a workpiece (301). According to an embodiment of the invention, the method comprises the generation of a contact force (FK) between a rotating tool (202) and a workpiece (301) by means of a linear actuator (401) and the measurement of the contact force (FK) between the rotating tool (202) and the workpiece (301). A motor (204) that drives the tool (202) is controlled according to a measurement value (FA, p) which represents the contact force (FK), in order to adjust a rotational speed of the tool (202) according to the contact force (FK).

Description

METHOD AND DEVICE FOR

ROBOT-BASED SURFACE MACHINING

TECHNICAL FIELD [0001] The invention relates to a device for robot-assisted surface processing, in particular to a grinding device guided by means of a manipulator, for example an orbital grinding machine.

BACKGROUND

Grinders and especially Orbitalschleifmaschinen are widely used in industry and craft. Orbital grinding machines are grinding machines in which an oscillation movement (vibration) is superimposed on a rotational movement about an axis of rotation. They are often used to finish metallic surfaces with high surface quality requirements. In order for these requirements to be realized, irregularities during the grinding process should be avoided as far as possible. This is done in practice mostly by the fact that this

Tasks especially in the production of small quantities by experienced skilled workers to be executed. These detect irregularities that arise due to fluctuations in the speed of the machine and can affect them by continuously changing the contact pressure. This requires a certain amount of experience and practice. The speed (of the rotary motion) in orbital grinding machines usually depends on a contact force between the grinding machine and the workpiece (grinding force).

Such grinding machines are usually not speed-controlled, and consequently causes a higher contact force a corresponding reduction of the speed and vice versa (according to the motor characteristic). This dependence of the speed of the contact force can be a particular difficulty in the automation of such a process. Industrial robots (at least in the lower price segment) are usually position-controlled and thus for grinding tasks without further measures not suitable. Even if a force control is provided for controlling the contact force, speed fluctuations due to variations in the contact force can not be excluded, especially when contacting the workpiece or when releasing the contact between the grinding machine and the workpiece. The speed variations have a negative effect on the resulting surface quality after machining. A rapid increase in speed can scratch the surface.

The object underlying the invention is to provide an apparatus for machining surfaces that allows machining of workpieces with improved accuracy. This object is achieved by a device according to claim 1 and a method according to claim 11. Various embodiments and further developments of the invention are the subject of the dependent claims.

SUMMARY

A method for automated surface processing of a workpiece is described. According to one example of the invention, the method comprises generating a contact force between a rotating tool and a workpiece by means of a linear actuator and measuring the contact force between the rotating tool and the workpiece. A motor driving the tool is driven depending on a measured value representing the contact force to set a rotational speed of the tool depending on the contact force.

Furthermore, a device for automated surface processing of a workpiece is described. According to one example of the invention, the device has a processing machine with a rotating tool, which is driven by a motor, and a linear actuator for generating a contact force between the tool and the workpiece. There is provided a device for measuring the contact force and a control unit, which is designed to control the motor depending on a measured value representing the contact force. According to one embodiment, the device comprises a manipulator for positioning the processing machine or the workpiece. The manipulator can thereby position the processing machine (relative to the workpiece) or workpiece (relative to the processing machine) on a predeterminable trajectory. In the first case, the processing machine is mechanically connected to the manipulator via the linear actuator and the workpiece is stationary. In the second case, the processing machine is mounted on a stationary frame via the linear actuator and the workpiece is moved by the manipulator.

In another exemplary embodiment, the device has a force control unit which is designed to regulate the actuator force caused by the linear actuator in such a way that it corresponds to a predefinable desired value. In this case, the force control unit may be designed to detect whether there is contact between the workpiece and the tool, wherein the desired value is increased from a minimum value to a nominal value when a contact between the workpiece and the tool is detected. The engine has a speed / torque characteristic. The control unit is designed to change the speed / torque characteristic curve of the motor depending on the measured value representing the contact force. For example, the speed / torque characteristic curve of the motor can be adapted such that a speed change of the motor caused by a change in the contact force is at least partially compensated. The control unit does not need to measure the speed of the motor.

The linear actuator may be a pneumatic actuator, and the device for measuring the contact force may comprise a pressure sensor for measuring the pressure in the pneumatic actuator, wherein the measured value representing the contact force depends on the pressure in the pneumatic actuator. The device for measuring the contact force may be supplied with a signal which indicates whether there is contact between the tool and the workpiece, wherein the measured value representing the contact force is zero when there is no contact between the tool and the workpiece. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the examples shown in the figures. The illustrations are not necessarily to scale and the invention is not limited to the aspects presented. Rather, emphasis is placed on presenting the principles underlying the invention. To the pictures::

Figure 1 shows an embodiment according to the invention, in which an orbital grinding machine is guided via an actuator and the rotational speed is controlled in dependence on the pressing force. Figure 2 shows an exemplary speed / torque characteristic of the drive of the orbital grinding machine.

Figure 3 shows the change in the speed / torque characteristic of Fig. 2 by a suitable control of the drive.

In the figures, like reference numerals designate the same or similar components, each having the same or similar meaning.

DETAILED DESCRIPTION

The examples of the invention described here relate to a device for the automated processing of surfaces. By way of example, the invention will be explained with reference to an orbital grinding machine guided by a manipulator. However, the principles underlying the invention can be easily transferred to other machines with a rotating tool for machining surfaces (eg polishing machines, belt grinders, etc.), in particular machines which have a drive with slip or whose drive has an engine characteristic according to the (rotational) speed of the tool depends on the load of the motor. A rotating tool can also be understood to mean a circulating belt, for example a grinding belt of a grinding machine. The invention is thus not limited to applications with orbital grinding machines. The quality of the surface of the machined workpiece depends on different parameters of the machining process. One of these process parameters is, for example, the rotational speed of the tool (eg a sanding pad equipped with abrasive paper or a grinding wheel) and the contact force F _K between see tool and workpiece, which corresponds to an actuator force FA, with which the tool is pressed onto the surface of the workpiece ( FA + F _K = 0, see Fig. 1). The speed of removal of material removal can also be a relevant process parameter.

Figure 1 shows an embodiment of an automated using a manipulator 150 device 100 for fully or partially automatic machining of workpieces. A grinding machine 201 - in the present example an orbital grinding machine - is mechanically coupled to the manipulator 150 via a linear actuator 401. The manipulator may be, for example, a standard industrial robot. Such industrial robots usually have 6 degrees of freedom and are able to set the position and orientation of a so-called tool center point (TCP) at the end of the robot arm and to move along a predetermined trajectory at a predeterminable speed. Depending on the application, the manipulator 150 can also be made simpler and, for example, have less than six degrees of freedom. In simple applications, even one degree of freedom can suffice. Alternatively, an arrangement is also possible in which the grinding machine 201 is mounted via a linear actuator 401 on a stationary (fixed) frame and the manipulator 150 moves the workpiece 301 relative to the grinding machine 201. The workpiece is located on a support 101, which is either stationary, or (eg in the case of a conveyor belt) is itself movable. However, a simple position control of the grinding machine 201 is generally unsuitable for a grinding process, since with a position control, the contact force F _K between the grinding tool 202 and workpiece 301 is not controllable. The regulation of the contact force F _K between the grinding tool 202 and the workpiece 301 is accomplished by means of the linear actuator 401 and a force regulator 410, whose operation will be discussed in more detail later. A motor 204 (eg, an electric motor) of the grinding machine 201 causes a rotation of the grinding tool 202 about an axis of rotation 203, the rotational speed generally of the load (load torque) of the drive and thus the contact force F _K between the grinding tool 202 and workpiece top - Area 301a depends. The relationship between the load torque M _{L of} the motor 204 and the rotational speed (rotational speed n) of the grinding tool 202 usually depends on the engine characteristic of the drive, with a proportionality between the contact force F _K and load torque M _{L of} the motor 204 being assumed for the further discussion. In the case of an orbital grinding machine, the grinding tool 202 further performs an oscillatory motion about the rotation axis 203 (or another axis of rotation parallel to the axis 203), the oscillation motion being superimposed on the rotation motion. For the grinding process, in particular, the oscillation movement is responsible, whereas the rotational movement causes a uniform wear of the grinding tool and the removal of the material removal. As already mentioned, during the grinding process, the contact force F _K between the tool 202 and the workpiece 301 can be adjusted by means of the linear actuator 401 and a force control such that the contact force F _K corresponds to a predefinable desired value. The contact force F _K is a reaction to the actuator force FA (- F _K = FA in quasi-stationary state), with which the linear actuator 401 presses on the workpiece surface. In the absence of contact between workpiece 301 and tool 202, the actuator moves due to the lack of contact force F _K against an end stop and presses against this with the corresponding set target force. The position control of the manipulator 150 can operate completely independently of the force control of the linear actuator 401. The linear actuator 401 is not responsible for the positioning of the grinding machine 201, but only for setting and maintaining the desired contact force F _K. The linear actuator can be a pneumatic actuator, eg a double-acting pneumatic cylinder. However, other pneumatic actuators are applicable such as bellows cylinder and air muscle. As an alternative, electrical direct drives (gearless) come into consideration. In the present example, the force control is realized in a conventional manner with the aid of a control valve 406, a regulator 410 and a compressed air reservoir 405. The compressed air reservoir 405 can also be replaced by a compressor. The control valve 406 is configured to control the air supply to the actuator 401. The controller 410 controls the valve 406 depending on a measurement value representing the contact force FK such that the actuator force FA corresponds to a desired value. The measured value representing the contact force FK (and thus the actuator force FA) can be determined, for example, via a force sensor (eg a load cell, load cell), in which case a pressure sensor connected to the actuator can serve as a force sensor in the case of a pneumatic actuator; the pressure in the actuator 401 is also a measured variable for the actuator force FA and thus for the contact force FK. The controller 410 may also be configured to detect a contact between the workpiece 301 and the tool 202 and, depending on whether there is contact, to adjust the setpoint for the force control. For example, the set point may be set to very small force values (approximately zero Newton) when not in contact, and slowly increased to a nominal value after contact detection. However, the force control is known as such and will therefore not be explained further. For contact detection, for example, a displacement sensor (disposition sensor) can be used, with the aid of which it can be detected whether the linear actuator 401 is located at the end stop. Despite the aforementioned regulation of the actuator force FA (and thus the contact force FK), the contact force can fluctuate. Such fluctuations can be caused on the one hand by external disturbance forces, and on the other hand, the contact force FK changes from the time of contact between the tool 202 and workpiece 301 until reaching a desired contact force. When releasing the contact between the tool 202 and workpiece 301, the contact force drops to zero. The force regulator 410 may also be designed to detect a contact between the tool 202 and the workpiece 301 and then slowly increase the setpoint for the force control from a predeterminable minimum contact force (eg zero) to the mentioned desired contact force. With each change in the contact force FK is a change in the load torque of the drive the grinding machine 201 and consequently also a change in the rotational speed of the tool 202 connected (corresponding to the motor characteristic). As already mentioned, fluctuations in the rotational speed can have a negative effect on the processing result. One way to prevent such fluctuations is the control of the rotational speed of the tool 202 to a constant value by means of a control loop. In this case, however, the measurement of the rotation speed is necessary, which causes additional expense.

In the following it is described how with a simple control (without measurement and feedback of the rotational speed), the rotational speed of the grinding tool 202 can be kept approximately constant despite fluctuations in the contact force F _K. The linear actuator 401 comprises a device for measuring the actuator force FA, which is needed anyway for the force control. In the case of a pneumatic actuator, the device for measuring the actuator force FA may be a simple pressure sensor which measures the pressure p in the pneumatic actuator (eg the pressure in the pneumatic cylinder). This pressure can be converted into the force FA in a manner (in the case of a pneumatic cylinder: FA = P-A, where A is the effective cross-sectional area of the cylinder). Depending on the measurement force representing the actuator force FA (eg pressure p), the drive of the motor can now be changed taking into account the motor characteristic of the motor of the grinding machine 201 such that a change of the rotational speed of the drive due to a change of the actuator force FA by a corresponding change of the Control of the motor (eg change in the armature voltage UA, change of the exciter field) is at least partially compensated. This is a control of the engine speed without feedback of the controlled variable (speed), since the actuator force FA or the pressure p in the actuator 401 are not measured values representing the rotational speed. The actual speed of the motor of the grinding machine depends on other parameters such as the friction between the grinding tool and the workpiece surface and thus also on the nature of the workpiece and the grinding tool. The control of the rotational speed of the motor of the grinding machine 201 is explained in more detail with reference to the characteristic curve shown in Fig. 2. The characteristic curve is only to be understood as an example and may also look different depending on the type of motor used. The load torque M _L is assumed to be proportional to the actuator force FA (with a not exactly known proportionality factor), which - in the case of a pneumatic actuator - depends directly on the pressure p. In the present example, a shunt machine is assumed in which the rotational speed n decreases approximately linearly, depending on the load torque M _L (and thus also on the force FA). The characteristic curve shown in FIG. 2 applies to an armature voltage UA = UAO. In the case of a (nominal) actuator force FA = FO, the armature of the motor and thus the grinding tool index 202 rotate with a (nominal)

Speed n = no (operating point Po). If the actuator force FA increases to a value Fi (Fi> Fo), the speed n would decrease (without control) to a value m (m <no) (operating point Pi). To compensate for this effect, the control unit 402 may, for example, increase the armature voltage UA at the motor from the original value UAO to the value UAI, whereby the motor characteristic is shifted parallel to higher rotational speeds (see FIG. 3). The armature voltage UAI (UAI> UAO) is just high enough that the motor speed at an actuator force FA = FI again the original target speed no corresponds (operating point P ₂ ). This situation is shown in FIG.

The mentioned characteristic shift can also be achieved differently depending on the type of motor, for example by a weakening of the excitation field, by changing the armature resistance, etc. In general, the speed / torque characteristic is dependent on a contact force F _K representing the measured value ( For example, actuator force FA or pressure p) influenced such that a change in the rotational speed n due to a change in the contact force F _{K is} at least partially compensated. In the absence of contact between the grinding tool 202 and the workpiece 301, the measured value representing the contact force F _{K is} zero, and the actuator 402 presses against an end stop. As soon as a contact between the grinding tool 202 and the workpiece 301 is detected (eg due to the change in the deflection of the actuator), the actuator force FA (ZB measured via the pressure p) is a measured value for the contact force F _K (-f _K = FA). Because no speed control (with feedback of a speed measurement) takes place, fluctuations in the speed can not be compensated, due to the force-dependent actuations larger speed fluctuations due to (intentional and caused by the force controller) changes in the actuator force FA can be easily reduced.

Claims

A device (100) for automated surface machining of a workpiece (301), comprising:

a processing machine (201) having a rotary tool (202) driven by a motor (204);

a linear actuator (401) for generating a contact force (F _K ) between the tool (202) and the workpiece (301);

a device for directly or indirectly measuring the contact force (F _K ); and a control unit (402) which is designed to control the motor (204) as a function of a measured value (FA, p) representing the contact force (FK).

2. Apparatus according to claim 1, further comprising:

a manipulator (150) for positioning the processing machine (201) or the workpiece (301) on a predeterminable trajectory.

3. A device according to claim 2, wherein

the manipulator (150) is mechanically connected to the processing machine (201) via the linear actuator (401) and

the manipulator (150) is adapted to the processing machine (201) together with

Linear actuator (401) to position on a predetermined trajectory.

4. Apparatus according to claim 2, wherein

a stationary frame, on which the linear actuator (401) is mechanically connected to the processing machine (201), and in which

the manipulator (150) is designed to position the workpiece on a predeterminable trajectory.

5. Apparatus according to any one of claims 1 to 4, further comprising:

a force control unit (410) which is designed to regulate the actuator force (FA) SO such that it corresponds to a predefinable desired value.

6. Apparatus according to claim 5, wherein the force control unit (410) is adapted to detect whether there is a contact between the workpiece (301) and tool (201), and wherein the setpoint is increased from a minimum value to a nominal value, if a Contact between workpiece (301) and tool (201) is detected.

7. Device according to one of claims 1 to 6, wherein

the motor (204) has a speed / torque characteristic, and the control unit (402) is adapted to, depending on the contact force

(FK) representing measured value (FA, p) the speed / torque characteristic of the

Motors (204) to change.

8. Apparatus according to claim 7,

wherein the speed / torque characteristic of the motor (204) is adjusted so that a caused by a change in the contact force (F _K ) speed change of the motor is at least partially compensated.

9. Device according to one of claims 1 to 8, wherein

the linear actuator (401) is a pneumatic actuator and

the device for measuring the contact force (F _K ) has a pressure sensor for measuring the pressure (p) in the pneumatic actuator,

wherein the measured value representing the contact force (F _K ) depends on the pressure (p) in the pneumatic actuator.

10. The device according to claim 9, wherein the device for measuring the contact force (F _K ) is supplied with a signal indicating whether contact between the tool (202) and the workpiece (301) consists, and wherein the contact force (F _K ) representing measured value Zero is when there is no contact between tool (202) and workpiece (301).

11. A method of automated surface machining of a workpiece (301), the method comprising:

Generating a contact force (F _K ) between a rotating tool (202) and a workpiece (301) by means of a linear actuator (401);

Measuring the contact force (F _K ) between the rotary tool (202) and the workpiece (301);

Driving a motor (204) driving the tool (202) in dependence on a measured value (FA, p) representing the contact force (FK) in order to set a rotational speed of the tool (202) in dependence on the contact force (F _K ).

12. The method of claim 11, further comprising:

Positioning of the processing machine (201) together with linear actuator (401) or the workpiece (301) on a predetermined trajectory by means of a manipulator.

13. The method according to claim 11 or 12,

wherein the linear actuator (401) is a pneumatic actuator and

wherein measuring the contact force (F _K ) comprises measuring the pressure (p) in the pneumatic actuator.

14. The method according to any one of claims 11 to 13,

wherein the linear actuator (401) acts directly on the rotating tool (202).

15. The method according to any one of claims 11 to 14, further comprising:

Controlling an actuator force (FA) effected by the linear actuator (401) such that the contact force (F _K ) corresponds to a predefinable setpoint value.