DE102019107967B3 - Learning a holding force for an object in a robotic gripper - Google Patents

Abstract

The invention relates to a method for teaching in a holding force for holding an object (9) by a gripper of a robot manipulator (3), comprising the steps: a) closing gripper jaws (7) of the gripper (5) until the gripper jaws (7) reach the object (9) contact with a predetermined closing force, b) check whether the object (9) remains between the gripper jaws (7) due to the predetermined closing force, and- repeating (S1) steps a) and b) with one in the repeated step a ) compared to the previous step a) increased predetermined closing force, as long as step b) shows that the object (9) does not remain in the gripper jaws (7), and - storage (S2) of the current predetermined closing force, if step b) has shown, that the object (9) remains in the gripper jaws (7).

Description

The invention relates to a method for teaching in a holding force for holding an object by a gripper of a robot manipulator and a robot system with a robot manipulator and with a computing unit for teaching in a holding force for holding an object by a gripper of the robot manipulator.

A large number of gripper devices are known from the prior art.

So that affects DE 10 2017 213 824 A1 a gripper device for handling a workpiece, in particular for a robot, with a gripper, the gripper having at least a first gripper jaw and a second gripper jaw, the first gripper jaw and the gripper for holding, and / or for depositing the workpiece second gripper jaw can be displaced relative to one another in a gripping direction, characterized in that the first gripper jaw and / or the second gripper jaw has an elastomer section for elastic gripping and / or holding the workpiece.

The DE 10 2017 201 057 A1 relates to a gripping device, with three gripping fingers arranged on a base structure and jointly delimiting a gripping space, which are movable relative to one another for holding and releasing an object which is at least partially arranged in the gripping space, a first of the gripping fingers executing a working movement in a through a vertical axis and a movement plane spanned at right angles to the vertical axis is movable and a second and a third of the gripping fingers are arranged at a distance from one another on opposite sides of the movement plane.

The DE 10 2016 208 362 A1 also relates to a method for automatic control of an industrial robot by means of a robot control, the industrial robot comprising an end effector which is designed as a clamping gripper for gripping objects, and which can be opened and closed automatically by means of a motor controlled by the robot control.

The DE 10 2016 111 173 A1 further relates to a method for operating a gripping or clamping device with at least one jaw movable in one direction of movement, and a gripping or clamping finger arranged on the jaw.

And the DE 10 2015 008 577 A1 relates to a robot control device configured to detect a state transition between a state where a load of a workpiece is not transferred to a robot and a state where the entire load of the workpiece is transferred to the robot by a hand, and it defines an area that includes the robot and hand at the time the state transition begins, within a state space that expresses the state of the robot and hand. The collision judgment part of the robot control device compares an estimated disturbance value and a first threshold value when the robot and the hand are within the defined range, and compare the disturbance value and a second threshold value that is different from the first threshold value when the robot and the hand is outside of the defined area.

The DE 10 2012 111 701 A1 relates to a handling device which is designed for gripping a workpiece and a gripping device with at least two gripping arms which, under the action of a closing force with gripping surfaces, can be moved relative to a workpiece to be arranged between the gripping arms and with a pressure force sensor device by means of which one of the gripping surface at least one gripping arm on the workpiece can be determined, the pressure force sensor device has at least one capacitive sensor on the gripping surface of the sensor arm and at least one evaluation device evaluating the capacitance value of the sensor.

The object of the invention is to determine a holding force for holding an object by a gripper of a robot manipulator in an automatic process.

The invention results from the features of the independent claims. Advantageous further developments and refinements are the subject of the dependent claims.

1. a) Schließen von Greiferbacken des Greifers bis die Greiferbacken den Gegenstand mit einer vorgegebenen Schließkraft kontaktieren,
2. b) Prüfen, ob der Gegenstand aufgrund der vorgegebenen Schließkraft zwischen den Greiferbacken verbleibt, und
   • - Wiederholen der Schritte a) und b) mit einer im wiederholten Schritt a) gegenüber dem vorangegangenen Schritt a) erhöhten vorgegebenen Schließkraft, solange Schritt b) ergibt, dass der Gegenstand nicht in den Greiferbacken verbleibt, und
   • - Abspeichern der aktuellen vorgegebenen Schließkraft, wenn Schritt b) ergeben hat, dass der Gegenstand in den Greiferbacken verbleibt.

A first aspect of the invention relates to a method for teaching in a holding force for holding an object by a gripper of a robot manipulator, comprising the steps:

1. a) closing the gripper jaws of the gripper until the gripper jaws contact the object with a predetermined closing force,
2. b) Check whether the object remains between the gripper jaws due to the predetermined closing force, and
   • - Repeating steps a) and b) with a predetermined closing force increased in repeated step a) compared to the previous step a), as long as step b) shows that the object does not remain in the gripper jaws, and
   • - Saving the current predetermined closing force when step b) has shown that the object remains in the gripper jaws.

The closing force required to hold the object is defined in particular by the static friction force between the gripper jaws and the object. The static friction force is approximately linearly dependent on the closing force which the gripper jaws exert on the object in the direction of the direction of travel of the gripper jaws. In particular, a linear relationship between the closing force and the static friction force can be expressed by a constant static friction factor. If this friction factor is equal to one, for example, the static friction force always corresponds exactly to the closing force with which the gripper jaws press against the object. The static friction force, in particular, must fully compensate for gravity in order to prevent the object from slipping out of the gripper jaws. During the execution of step b), an additional force, which is directed parallel to the force of gravity, is advantageously applied to the object in order to determine a safety distance defined by the user from the closing force of the gripper on the object, which is only used to compensate for the gravity of the Object is necessary. This method advantageously determines a safety margin for the user in an intuitive way in order to hold the object securely in the gripper, as the user deems it to be.

The gripper preferably has exactly two gripper jaws which are designed to move towards one another in order to close the gripper and, analogously, to move away from one another in order to open the gripper. Furthermore, the gripper jaws of the gripper are preferably closed until the gripper jaws contact the object by controlling a gripper drive. The gripper drive is designed, in particular, to move the gripper jaws in response to a control signal and to serve as a source of a moment or a force which can be transmitted as a closing force to the object via the gripper jaws.

Steps a) and b) are therefore repeated as often with increasing predetermined closing force until the closing force generates a sufficient static friction force between the gripper jaws and the object so that the object no longer falls out of the gripper jaws. There is a cycle of closing the gripper with increased closing force in each closing cycle, checking whether the object remains between the gripper jaws due to the applied and maintained closing force, and opening the gripper until the closing force is sufficiently high that the The object remains between the gripper jaws. In the first embodiment of this cycle, only a minimally higher closing force above zero is preferably specified in order to obtain the smallest necessary closing force without starting with an unnecessarily high closing force.

It is an advantageous effect of the invention that the closing force which is necessary for holding the object takes place empirically and nevertheless automatically. This advantageously determines a closing force that is only as high or only slightly higher than the minimally required closing force to hold the object between the gripper jaws. Furthermore, if the contact plane between the gripper jaws and the object is not perpendicular but at an angle to the force of gravity, the necessary static friction force between the gripper jaws and the object is smaller than if this plane were to lie exactly along the gravity vector. If the gripper jaws and the object contact each other in a plane that does not run along the gravity vector, but has an angle with respect to the gravity vector, then the necessary static friction force is replaced by a normal force component between the object and at least one of the two gripper jaws. The decrease in the necessary closing force advantageously does not have to be calculated on the basis of this angle, but is automatically determined by the method according to the invention in an empirical procedure. Furthermore, uncertainties in the coefficient of friction and generally in the function between the force of the gripper jaws on the object to the static friction force do not need to be modeled or estimated, since regardless of the boundary conditions and regardless of the surface of the object and other uncertainties due to the empirical nature of the method according to the invention a closing force for holding the object is found as long as it lies within the saturation limits of the actuators for the gripper jaws in order to hold the object between the gripper jaws against the force of gravity.

According to an advantageous embodiment, the object lies on a surface during step a) and step b) is carried out by lifting the gripper from the surface against the direction of gravity. The gripper is lifted away from the surface in particular by appropriately controlling the robot manipulator. The lifting is advantageously carried out with only very small accelerations, in order not to determine a closing force which is the necessary closing force Compensation of the object's gravity exceeds because the determined closing force also had to compensate for the inertia of the object. Alternatively, the object is preferably lifted from the surface at a certain predetermined acceleration in order to determine a closing force which generates such a high static friction force between the gripper jaws and the object that the object is still held by the gripper jaws even when the The gripper jaws accelerate the object in the room by activating the robot manipulator.

According to a further advantageous embodiment, the object is held between the gripper jaws during step a) and released in step b). In particular, the object is held between the gripper jaws by manual holding; alternatively, the object is preferably held between the gripper jaws by a second robot manipulator, which advantageously allows the entire method to be carried out automatically. The latter alternative in particular advantageously enables the automatic determination of a necessary holding force for a large number of objects.

According to a further advantageous embodiment, the closing force specified in the repeated step a) is increased in each case by a constant predetermined amount compared to the closing force specified in the previous step a).

According to a further advantageous embodiment, the closing force specified in repeated step a) is increased in each case by a predetermined amount that becomes smaller with each repetition compared to the closing force specified in previous step a).

According to a further advantageous embodiment, the closing force specified in the repeated step a) is increased in each case by a predetermined amount that increases with each repetition compared to the closing force specified in the previous step a).

The last three embodiments mentioned above represent possibilities of how the closing force is increased with each repetition of step a). The closing force is preferably increased at constant intervals, decreasing intervals, or at increasing intervals. A corresponding function is preferably stored in a memory unit of the robot manipulator, so that this function can be used repeatedly. Each form of increasing the closing force offers its own advantages - the progressive increase in the closing force in a very short time can lead to a determined necessary closing force for holding the object between the gripper jaws, while a smaller increase in the closing force more easily leads to a determined closing force Holding the object between the gripper jaws leads, which is only minimally above the smallest necessary closing force to hold the object. If the closing force is increased in constant steps, a compromise is advantageously made and the two advantages mentioned above are combined in a somewhat milder form.

According to a further advantageous embodiment, step b) is carried out by comparing the current closing force with the predetermined closing force. The gripper jaws are advantageously controlled in a force-controlled manner, whereby there is always feedback of a measured gripping force currently present on the gripper jaws. When the object falls out of the gripper jaws, this force on the gripper jaws decreases in a short time, and the gripper jaws would move towards one another with high acceleration. This movement is advantageously stopped. Furthermore, the detection of the sudden drop in this force acting on the gripper jaws is a reliable indication that the object has fallen out of the gripper jaws.

According to a further advantageous embodiment, step b) is carried out by comparing a position of the gripper jaws at the end of step a) with a current position of the gripper jaws. A simple position measurement of the gripper jaws advantageously serves to determine whether the object has fallen out between the gripper jaws, in particular when the gripper jaws move towards one another by force regulation in order to apply the predetermined closing force. A position sensor is very inexpensive and technically simple to arrange on a gripper.

According to a further advantageous embodiment, step b) takes place by means of a sensor, the sensor detecting whether the object is located between the gripper jaws. The sensor can advantageously be an optical sensor, an ultrasonic sensor, a distance sensor based on cameras, on stereo cameras, or a laser distance sensor. The sensor advantageously provides a very reliable detection of whether the object is still between the gripper jaws.

1. a) die Greiferbacken zum Schließen des Greifers anzusteuern bis die Greiferbacken den Gegenstand mit einer vorgegebenen Schließkraft kontaktieren,
2. b) zu prüfen, ob der Gegenstand aufgrund der vorgegebenen Schließkraft zwischen den Greiferbacken verbleibt, und
   • - die Schritte a) und b) mit einer im wiederholten Schritt a) gegenüber dem vorangegangenen Schritt a) erhöhten vorgegebenen Schließkraft zu wiederholen, solange Schritt b) ergibt, dass der Gegenstand nicht in den Greiferbacken verbleibt, und
   • - die aktuelle vorgegebene Schließkraft abzuspeichern, wenn Schritt b) ergeben hat, dass der Gegenstand in den Greiferbacken verbleibt.

A further aspect of the invention relates to a robot system with a robot manipulator and with a computing unit, a gripper being arranged on the robot manipulator, the robot system being used for teaching in a holding force for holding an object by the gripper,
the computing unit is designed for this:

1. a) actuate the gripper jaws to close the gripper until the gripper jaws contact the object with a predetermined closing force,
2. b) to check whether the object remains between the gripper jaws due to the predetermined closing force, and
   • - Repeat steps a) and b) with a predetermined closing force increased in the repeated step a) compared to the previous step a), as long as step b) shows that the object does not remain in the gripper jaws, and
   • - to save the current predetermined closing force when step b) has shown that the object remains in the gripper jaws.

Advantages and preferred developments of the proposed robot system result from an analog and analogous transmission of the statements made above in connection with the proposed method.

Further advantages, features and details emerge from the following description, in which - if necessary with reference to the drawing - at least one exemplary embodiment is described in detail. Identical, similar and / or functionally identical parts are provided with the same reference symbols.

• 1 ein Verfahren zum Einlernen einer Haltekraft zum Halten eines Gegenstands durch einen Greifer eines Robotermanipulators gemäß einem Ausführungsbeispiel der Erfindung,
• 2 einen Greifer eines Robotermanipulators gemäß einem weiteren Ausführungsbeispiel der Erfindung, und
• 3 ein Robotersystem mit einem Robotermanipulator und mit einer Recheneinheit gemäß einem weiteren Ausführungsbeispiel der Erfindung.

Show it:

• 1 a method for teaching a holding force for holding an object by a gripper of a robot manipulator according to an embodiment of the invention,
• 2nd a gripper of a robot manipulator according to a further embodiment of the invention, and
• 3rd a robot system with a robot manipulator and with a computing unit according to a further embodiment of the invention.

The representations in the figures are schematic and not to scale.

1. a) Schließen von Greiferbacken 7 des Greifers 5 bis die Greiferbacken 7 den Gegenstand 9 mit einer vorgegebenen Schließkraft kontaktieren, wobei der Gegenstand 9 während Schritt a) auf einer Oberfläche liegt und Schritt b) durch Heben des Greifers 5 von der Oberfläche entgegen der Richtung der Schwerkraft erfolgt,
2. b) Prüfen, ob der Gegenstand 9 aufgrund der vorgegebenen Schließkraft zwischen den Greiferbacken 7 verbleibt, wobei Schritt b) durch Vergleich der aktuellen Schließkraft mit der vorgegebenen Schließkraft erfolgt, und
   • - Wiederholen S1 der Schritte a) und b) mit einer im wiederholten Schritt a) gegenüber dem vorangegangenen Schritt a) erhöhten vorgegebenen Schließkraft, solange Schritt b) ergibt, dass der Gegenstand 9 nicht in den Greiferbacken 7 verbleibt, wobei die im wiederholten Schritt a) vorgegebene Schließkraft gegenüber der im vorangegangenen Schritt a) vorgegebenen Schließkraft jeweils um einen konstanten vorgegebenen Betrag erhöht ist, und
   • - Abspeichern S2 der aktuellen vorgegebenen Schließkraft, wenn Schritt b) ergeben hat, dass der Gegenstand 9 in den Greiferbacken 7 verbleibt.

1 shows a method for teaching a holding force for holding an object 9 by a gripper 5 a robot manipulator 3rd , comprising the steps:

1. a) Closing jaws 7 of the gripper 5 until the gripper jaws 7 the object 9 contact with a predetermined closing force, the object 9 during step a) lies on a surface and step b) by lifting the gripper 5 from the surface against the direction of gravity,
2. b) Check that the item 9 due to the specified closing force between the gripper jaws 7 remains, step b) being carried out by comparing the current closing force with the predetermined closing force, and
   • - To repeat S1 of steps a) and b) with a predetermined closing force increased in the repeated step a) compared to the previous step a), as long as step b) shows that the object 9 not in the gripper jaws 7 remains, the closing force specified in the repeated step a) being increased by a constant predetermined amount in each case compared to the closing force specified in the previous step a), and
   • - Save S2 the current predetermined closing force if step b) has shown that the object 9 in the gripper jaws 7 remains.

2nd shows a gripper 5 with two gripper jaws 7 , and one between the gripper jaws 7 located object 9 , as in step a) or after storing the currently specified closing force, if step b) has shown that the object 9 in the gripper jaws 7 remains. The gripper 5 has two gripper jaws 7 on, which are made of a resilient elastic material.

1. a) die Greiferbacken 7 zum Schließen des Greifers 5 anzusteuern bis die Greiferbacken 7 den Gegenstand 9 mit einer vorgegebenen Schließkraft kontaktieren,
2. b) zu prüfen, ob der Gegenstand 9 aufgrund der vorgegebenen Schließkraft zwischen den Greiferbacken 7 verbleibt, und die Schritte a) und b) mit einer im wiederholten Schritt a) gegenüber dem vorangegangenen Schritt a) erhöhten vorgegebenen Schließkraft zu wiederholen, solange Schritt b) ergibt, dass der Gegenstand 9 nicht in den Greiferbacken 7 verbleibt, und die aktuelle vorgegebene Schließkraft abzuspeichern, wenn Schritt b) ergeben hat, dass der Gegenstand 9 in den Greiferbacken 7 verbleibt.

3rd shows a robot system 1 with a robot manipulator 3rd and with an arithmetic unit 11 , on the robot manipulator 3rd a gripper 5 is arranged. Such a gripper 5 is in 2nd shown. The robot system is used to automatically teach in a holding force for holding an object 9 by the gripper 5 , the arithmetic unit 11 is carried out:

1. a) the gripper jaws 7 to close the gripper 5 to be actuated until the gripper jaws 7 the object 9 contact with a specified closing force,
2. b) to check whether the object 9 due to the specified closing force between the gripper jaws 7 remains, and repeating steps a) and b) with a predetermined closing force increased in the repeated step a) compared to the previous step a), as long as step b) reveals that the object 9 not in the gripper jaws 7 remains, and save the current predetermined closing force when step b) has shown that the object 9 in the gripper jaws 7 remains.

Reference list

1

Robot system

3rd

Robot manipulator

5

Gripper

7

Gripper jaws

9

object

11

Arithmetic unit

a)

Conclude

b)

Check

S1

To repeat

S2

Save

Claims (10)

Method for teaching a holding force for holding an object (9) by a gripper of a robot manipulator (3), comprising the steps: a) closing the gripper jaws (7) of the gripper (5) until the gripper jaws (7) contact the object (9) with a predetermined closing force, b) Check whether the object (9) remains between the gripper jaws (7) due to the predetermined closing force, and - repeating (S1) steps a) and b) with a predetermined closing force increased in repeated step a) compared to the previous step a), as long as step b) shows that the object (9) does not remain in the gripper jaws (7), and - Saving (S2) the current predetermined closing force when step b) has shown that the object (9) remains in the gripper jaws (7). Procedure according to Claim 1 wherein the object (9) lies on a surface during step a) and step b) is carried out by lifting the gripper (5) from the surface against the direction of gravity. Procedure according to Claim 1 , the object (9) being held between the gripper jaws (7) during step a) and being released in step b). Procedure according to one of the Claims 1 to 3rd , wherein the closing force specified in the repeated step a) is increased in each case by a constant predetermined amount compared to the closing force specified in the previous step a). Procedure according to one of the Claims 1 to 3rd , wherein the closing force specified in the repeated step a) is increased in each case by a predetermined amount that becomes smaller with each repetition compared to the closing force specified in the previous step a). Procedure according to one of the Claims 1 to 3rd , wherein the closing force specified in the repeated step a) is increased in each case by a predetermined amount that increases with each repetition compared to the closing force specified in the previous step a). Procedure according to one of the Claims 1 to 6 , step b) being carried out by comparing the current closing force with the predetermined closing force. Procedure according to one of the Claims 1 to 6 Step b) is carried out by comparing a position of the gripper jaws (7) at the end of step a) with a current position of the gripper jaws (7). Procedure according to one of the Claims 1 to 6 , wherein step b) takes place by means of a sensor, the sensor detecting whether the object (9) is located between the gripper jaws (7). Robot system (1) with a robot manipulator (3) and with a computing unit (11), a gripper (5) being arranged on the robot manipulator (3), the robot system for teaching in a holding force for holding an object (9) by the gripper ( 5), the computing unit (11) being designed for this: a) trigger the gripper jaws (7) to close the gripper (5) until the gripper jaws (7) contact the object (9) with a predetermined closing force, b) to check whether the object (9) remains between the gripper jaws (7) due to the predetermined closing force, and - Repeat steps a) and b) with a predetermined closing force which is increased in each case in repeated step a) compared to the previous step a), as long as step b) shows that the object (9) does not remain in the gripper jaws (7), and - to save the current predetermined closing force when step b) has shown that the object (9) remains in the gripper jaws (7).

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