Method for calibrating a torque sensor in a robot joint

Abstract

The invention relates to a method for calibrating a torque sensor in a robot joint of a robot having one or more robot joints, which each comprise a torque sensor and a rotational position sensor, having the following method steps: A) an end effector of the robot is loaded with a weight of a mass; B) the weight of the mass on the end effector (5) is determined by means of a force sensor; C) a torque measurement value is measured by means of the torque sensor that is to be calibrated; D) the respective rotational position of the robot joints is determined by means of the one or more rotational position sensors; E) based on the determined weight and the determined rotational position of the robot joints, a torque is calculated as a torque reference value, which acts in the robot joint whose torque sensor is to be calibrated; wherein the method steps A, B, C, D and E are optionally repeated once or several times when the position of the robot changes, wherein a correction function for correcting the torque measured by the torque sensor to be calibrated is determined by means of the one torque measurement value or the plurality of torque measurement values and the one torque reference value or the plurality of torque reference values.

Description

[0001] Method for calibrating a torque sensor in a robot joint

[0002] The invention relates to a method for calibrating a torque sensor in a robot joint of a robot having one or more robot joints, each comprising a torque sensor and a rotational position sensor.

[0003] With the advancement of industrial automation, robots are increasingly being used in production facilities. Many such robots feature rotatable robot joints, for example, to enable movement around multiple spatial axes. In addition to drive motors and gear stages, the robot joints also incorporate sensor technologies, such as torque sensors. The torque sensors are typically calibrated during production.

[0004] The functionality, especially the accuracy, of such torque sensors can deteriorate over time, for example, due to wear and tear. As sensor accuracy decreases, the robot's performance can deteriorate and operational reliability can be compromised. Therefore, it is common practice in the state of the art to disassemble the robot during maintenance or when a problem with the sensor accuracy is discovered. The torque sensor can be removed and recalibrated outside the robot. Therefore, recalibrating torque sensors is a significant effort.

[0005] Against this background, the task arises of reducing the effort required to calibrate torque sensors arranged in robot joints.

[0006] This object is achieved by a method for calibrating a torque sensor in a robot joint of a robot with one or more robot joints, each comprising a torque sensor and a rotational position sensor, comprising the following method steps:

[0007] A. an end effector of the robot is loaded with a weight force of a mass,

[0008] B. using a force sensor, the weight force of the mass at the end effector is determined,

[0009] C. using the torque sensor to be calibrated, a

[0010] Torque measurement value measured,

[0011] D. the respective rotational position of the robot joints is determined by means of the rotational position sensor(s), E. based on the determined weight force and the determined rotational position of the robot joints, a torque is calculated as a torque reference value, which torque acts in the robot joint whose torque sensor is to be calibrated, wherein the method steps A, B, C, D and E are optionally repeated once or several times with a changed position of the robot, wherein a correction function for correcting the torque measured by the torque sensor to be calibrated is determined by means of the one torque measured value or the several torque measured values ​​and the one torque reference value or the several torque reference values.

[0012] The robots calibrated using the method according to the invention have at least one torque sensor and at least one rotational position sensor in at least one robot joint. Furthermore, such robots have an end effector, i.e. an effector arranged distally on a robot arm. According to the invention, the end effector of the robot is loaded with a weight force. The weight force is determined using a force sensor. The weight force acting on the robot tends to set the robot arm(s) in motion. To maintain a static equilibrium of the robot, a torque must be provided in each of the robot joints. These torques acting in the robot joints are measured as torque values ​​using one or more torque sensors to be calibrated for each robot joint.The torque measurements can be temporarily stored in a memory unit of the robot for possible future operations. In addition, according to the invention, the current rotational position of the robot joints is detected by the one or more rotational position sensors. Using the rotational position of the robot joints - in combination with geometric information, for example the length of the individual robot arm segments - the position coordinates of the end effector can also be calculated. Furthermore, according to the invention, a torque reference value is calculated for each robot joint or torque sensor based on the determined weight force and the determined rotational position of the robot joints. For optimized calibration of the torque sensors, the aforementioned method steps can optionally be repeated each time the position of the robot or the robot arm segments changes. Using the one torque measurement value orA correction function is finally determined from the multiple torque measured values ​​and the one or more torque reference values. Using the correction function, the

[0013] Torque sensors are calibrated. The goal of determining the correction function is to minimize any discrepancy, deviation, or error between the measured torque value and the calculated torque reference value, for example, using linear regression.

[0014] The method according to the invention offers the advantage that calibrating a torque sensor does not require disassembly and, consequently, a temporary shutdown of the robot. Calibration can be performed during a manufacturing process or robot operation. Furthermore, calibration can be performed, for example, at regular intervals during robot operation, and in particular after a specific operating period at which wear is expected. Overall, resources such as time and labor, and thus costs, can be saved.

[0015] To increase safety, a redundant sensor arrangement can optionally be provided; in particular, several torque sensors and several rotational position sensors can be provided for each robot joint. The robot joints can, in particular, have one degree of freedom of rotation, i.e., they can be designed to rotate about exactly one axis.

[0016] The correction function can be linear or nonlinear. The correction function can include multiple free parameters, whereby the number of repetitions of process steps A to E must be greater than the number of free parameters of the correction function. Furthermore, the correction function can be determined by adjusting the parameters from an existing correction function.

[0017] In a preferred embodiment of the invention, the torque sensors of several, preferably all, robot joints of the robot are calibrated simultaneously. Simultaneous calibration of several torque sensors is advantageous because the total number of robot positions can be reduced during the implementation of the calibration method according to the invention.

[0018] According to an advantageous embodiment of the invention, a Jacobian matrix is ​​determined based on the rotational position of the robot joints, which indicates a correlation between the torques in the robot joints and the weight of the mass on the end effector. The Jacobian matrix is ​​robot- and situation-specific. It depends on the rotational position of the robot joints. It can also include force components, in particular the robot's own weight and thus also the mass moments of inertia of the robot or of the individual robot components, such as robot arm segments. Alternatively, the robot's weight can be considered as an additional mathematical expression when calculating the torque.

[0019] In a preferred embodiment of the invention, the end effector includes the force sensor. The force sensor is preferably designed as a three-axis sensor, allowing forces to be measured in all three spatial directions. By incorporating the force sensor into the end effector, a space-saving and thus compact design can be realized. The end effector can include multiple force sensors.

[0020] In addition, one or more further force sensors - in particular for detecting inertial forces emanating from the robot's own weight, its robot arm segments or robot joints - can be arranged on the robot, in particular in a robot joint or on a robot arm segment.

[0021] According to a preferred embodiment of the invention, the force sensor is connected to the mass. The force sensor or one of the force sensors can preferably comprise an acceleration sensor or be designed as an acceleration sensor and thus detect centrifugal forces, in particular centrifugal forces acting on the mass. This results in the advantage that the forces actually acting on the mass—and thus also on the robot—can be determined more precisely.

[0022] According to an advantageous embodiment of the invention, the end effector is designed as a gripper. The end effector or gripper can have several movable gripping fingers for easy and secure gripping of masses of different sizes.

[0023] A preferred embodiment of the invention provides that the torque reference values ​​calculated during the repetitions of method steps A, B, C, D, and E are weighted differently. Since robots can perform routine movements, certain rotational positions of the robot joints and positions of the robot may be more important during operation than others. Repetitions of method steps A, B, C, D, and E in the more significant rotational positions and positions can be given greater weight when calculating the torque reference values.

[0024] A further subject of the invention is the use of a method according to one of the preceding embodiments for calibrating a torque sensor in a robot joint of a robot in an assembled and functional state of the robot, preferably during operation of the robot. The same advantages and technical effects that were already explained in the context of the description of the invention and its embodiments apply to the use of the method according to the invention.

[0025] Further details and advantages of the invention will be explained below with reference to the exemplary embodiments illustrated in the drawings. Herein:

[0026] Fig. 1 schematically shows a first embodiment of a robot in a side view to illustrate an embodiment of the method according to the invention;

[0027] Fig. 2a schematically shows a second embodiment of a robot in a side view to illustrate an embodiment of the method according to the invention;

[0028] Fig. 2b schematically shows the second embodiment from Fig. 2a in a changed position of the robot.

[0029] Fig. 1 schematically shows a robot 1 to which the method according to the invention can be applied. The robot 1 has several robot joints 3. The robot joints 3 each comprise a torque sensor 3.1 and a rotational position sensor 3.2. By means of the robot joints 3, the robot 1 can be moved along and about all three spatial axes, in particular also rotated about a vertical axis V. In the first embodiment shown in Fig. 1, the robot 1 also has a seventh axis L, along which the robot 1 is designed to be translationally movable.

[0030] The robot 1 has an end effector 5, which is schematically designed as a gripper or pincer-shaped. In a first method step, the end effector 5 is loaded with a weight force G of a mass m (not shown in Fig. 1). The weight force is determined by means of a force sensor 7 integrated into the end effector 5. A torque measurement value is then measured by each of the torque sensors 3.1 arranged in all robot joints 3 that are to be calibrated. Simultaneously or subsequently, the rotational position of the robot joints 3 is determined by the rotational position sensors 3.2. Then, based on the weight force G determined by the force sensor 7 and the respective rotational position of the robot joints 3 determined, a torque reference value is calculated for each robot joint 3, which acts in the respective robot joint 3.The set of previously performed process steps is repeated several times, with the position of the robot 1, i.e., the rotational positions of the robot joints 3, being changed after each set. Since different robot joints 3 may be worn to varying degrees or, for other reasons, the torque sensors 3.1 may each have varying degrees of accuracy, a correction function is determined for each robot joint 3 using the multiple torque measurement values ​​and the multiple torque reference values ​​to correct the torques measured by the torque sensors 3.1 to be calibrated.

[0031] In Fig. 2a, a second embodiment of a robot 1 is shown schematically in a side view to illustrate an embodiment of the method according to the invention. In Fig. 2a, the mass m held by the end effector 5 is visible. Furthermore, the robot 1 is shown in Fig. 2a in a first kinematic state, i.e. in particular that the robot joints 3 each have a specific rotational position, for example α and β. The first kinematic state in conjunction with the weight force G of the mass m results in certain forces, for example F 1 , F 2 and F 3 , which act on the robot 1. From the totality of the forces acting on the robot 1 in combination with the first kinematic state - in particular the rotational positions of the robot joints, but also the length of robot arm segments 1.1 - a torque reference value for each robot joint 3 can be calculated.Using the multiple calculated torque reference values ​​per robot joint 3 and the measured torque values ​​per robot joint 3, a correction function can be determined to correct the torques measured by the torque sensors 3.1 to be calibrated.

[0032] Fig. 2b schematically shows the second embodiment from Fig. 2a in a modified position of the robot 1. In other words, the robot 1 is shown in a second kinematic state. In particular, the robot joints 3 each have a different rotational position. Furthermore, the robot 1 can be loaded by a different mass with a different weight force than by the mass m with the weight force G from Fig. 2a. This results in different forces Fi acting on the robot 1, in particular the forces F4, F5 and Fe in Fig. 2b. As a result of the changed forces F4, F5 and Fe, changed torque reference values ​​result for each robot joint 3. According to the invention, the kinematic state of the robot 1 can be changed multiple times, in particular as often as desired, in order to determine an improved correction function. List of Reference Symbols

[0033] 1 robot

[0034] 3 Robot joint

[0035] 3.1 Torque sensor

[0036] 3.2 Rotary position sensor

[0037] 5 End effector

[0038] 7 Force sensor a, ß Rotational positions per robot joint m mass

[0039] G Weight force of the mass m

[0040] Fi forces on the robot

[0041] V vertical axis

[0042] L Linear axis

Claims

Patent claims 1. A method for calibrating a torque sensor in a robot joint (3) of a robot (1) having one or more robot joints (3), each comprising a torque sensor (3.1) and a rotational position sensor (3.2), comprising the following method steps: A. an end effector (5) of the robot (1) is loaded with a weight force of a mass, B. by means of a force sensor (7) the weight force of the mass on the end effector (5) is determined, C. Using the torque sensor (3.1) to be calibrated, a torque measurement value (M m ) measured, D. the respective rotational position of the robot joints (3) is determined by means of the rotational position sensor(s) (3.2), E. Based on the determined weight force and the determined rotational position of the robot joints (3), the torque reference value (M ref) calculates a torque acting in the robot joint (3) whose torque sensor (3.1) is to be calibrated, wherein the method steps A, B, C, D and E are optionally repeated once or several times with a changed position of the robot (1), wherein by means of the one torque measurement value (M m ) or the multiple torque measured values ​​(M m ) and one torque reference value (M re f) or the multiple torque reference values ​​(M re f) a correction function is determined to correct the torque measured by the torque sensor (3.1) to be calibrated.

2. Method according to claim 1, characterized in that the torque sensors (3.1) of several, preferably all, robot joints (3) of the robot (1) are calibrated simultaneously.

3. Method according to one of the preceding claims, characterized in that a Jacobian matrix is ​​determined on the basis of the rotational position of the robot joints (3), which indicates an assignment of the torques in the robot joints (3) to the weight of the mass on the end effector (5).

4. Method according to one of the preceding claims, characterized in that the end effector (5) comprises the force sensor (7).

5. Method according to one of the preceding claims, characterized in that the force sensor (7) is connected to the mass.

6. Method according to one of the preceding claims, characterized in that the end effector is designed as a gripper.

7. Method according to one of the preceding claims, characterized in that the torque reference values ​​calculated during the repetition of method steps A, B, C, D and E are weighted differently.

8. Use of a method according to one of the preceding claims for calibrating a torque sensor (3.1) in a robot joint (3) of a robot (1) in an assembled and functional state of the robot (1), preferably during operation of the robot (1).

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