Process and system for monitoring backlash in a gear mechanism of a joint of an industrial robot

Abstract

Described herein is a process for monitoring backlash in a gear mechanism of a joint (50) of an industrial robot (10), wherein the joint comprises: a first joint body (52) and a second joint body (54), a motor (51) and a motion transmission assembly (53, 55), the first joint body (52) and the second joint body (54) being coupled together with the possibility of movement relative to each other, the motor (51) being provided with an encoder (56), the motion transmission assembly (53, 55) being designed to transmit a torque generated by the motor (51) to the second joint body (54) in order to bring about a movement of the second joint body (54) relative to the first joint body (52), the transmission assembly (53, 55) comprising the gear mechanism. The process is characterized in that the signal of the encoder (56) of the motor (51) for driving the joint is used without providing any additional sensor specifically dedicated to monitoring backlash.

Description

Technical Field

[0001] The present invention relates to a solution for monitoring the backlash of a gear mechanism of a joint of an industrial robot. Background Art

[0002] As is known, backlash is the angular play between the teeth of two intermeshing gears; ie, in other words, it is the distance by which the gap between the teeth of a first gear exceeds the thickness of the teeth of a second gear.

[0003] Generally speaking, a minimum amount of backlash is usually provided in a gear mechanism to ensure smooth movement. However, during use of the gear mechanism, this amount of backlash tends to increase due to wear on the sides of the teeth caused by the repeated impacts and sliding to which they are subjected.

[0004] Above a given limit, backlash may result in degradation of performance and may also cause jams and malfunctions.

[0005] Now with reference to the field of industrial robots, solutions have been proposed for monitoring the backlash in the gear mechanisms of the robot's joints, which envisage mounting sensors on the robot board, said sensors being specifically provided for directly measuring the effects of the backlash during the movements of the joint, and therefore for measuring the backlash degree, for example for measuring the deviation between the actual position of the moving body of the joint and the theoretical position envisaged by the control unit of the robot. Summary of the invention

[0006] In this context, the present invention proposes a solution for monitoring the backlash which presents an improvement over the known solutions referred to above, without envisaging the installation of additional sensors on the robot board.

[0007] Therefore, the solution described herein envisages a simpler arrangement and can also be easily applied on industrial robots that are already installed and operating.

[0008] In particular, the present invention relates to monitoring processes and monitoring systems.

[0009] The appended claims form an integral part of the technical teaching provided herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Further features and advantages of the invention will emerge clearly from the ensuing description, with reference to the accompanying drawings, which are provided purely as non-limiting examples and in which:

[0011] - Figure 1 is a schematic illustration of an example of a system for monitoring backlash according to the solution described herein;

[0012] - Figure 2 An example of a joint of an industrial robot is illustrated;

[0013] - Figure 3 illustrates a flow chart representing an example of a process for monitoring backlash according to the solutions described herein;

[0014] - Figure 4 illustrates a characteristic oscillation curve used by a process for monitoring backlash according to the solution described herein; and

[0015] - Figure 5 A diagram illustrating signal processing performed during monitoring according to the solution described herein is shown. DETAILED DESCRIPTION

[0016] In the following description, various specific details are illustrated in order to provide a deeper understanding of the embodiments. The embodiments may be provided without one or more of the specific details or with other methods, components, materials, etc. In other cases, well-known structures, materials, or operations are not illustrated or described in detail so as not to obscure various aspects of the embodiments.

[0017] The references used herein are provided for convenience only, and therefore do not limit the scope of protection or the scope of the embodiments.

[0018] As an example, Figure 1 An industrial robot consisting of an articulated robot arm 10 having six axes of rotation A1 - A6 is illustrated.

[0019] In particular, the robot arm 10 comprises a series of bodies 1-6 coupled together, wherein each body of the series is rotatably mounted on the other body behind the latter so that it can turn about the aforementioned axis. Each pair of bodies 1-6 constitutes a joint of the arm 10.

[0020] The robot illustrated in the figures has rotational joints, but it should be noted that the solution described herein may also be employed for monitoring translational joints, such as the joints of a Cartesian robot.

[0021] Reference again Figure 1 The control unit 20 is connected to the robot arm 10 for driving corresponding motors, which drive the joints of the robot. Through the coordinated control of various joints, the tool check point (TCP) of the robot located on the last body 6 can be displaced along a predefined path and according to a predefined orientation of each point of the path.

[0022] Again as an example, Figure 2 The structure of the joint of an industrial robot can be used to Figure 1Each of the joints of the articulated arm 10 shown in FIG.

[0023] The illustrated joint, designated as a whole by reference numeral 50 , comprises a first body 52 and a second body 54 , the latter being rotatably mounted on the first body 52 about an axis of rotation Ai.

[0024] In addition, the joint 50 includes a motor 51 and a transmission assembly, wherein the motor 51 is carried by the main body 52, the transmission assembly is designed to transmit the torque generated by the motor 51 to the main body 54, the transmission assembly includes a reducer 53 and a belt transmission assembly 55, the reducer 53 is located in the coupling area of ​​the two main bodies 52 and 54, and the belt transmission assembly 55 connects the output shaft of the motor 51 to the input shaft of the reducer 53.

[0025] Provided on the motor 51 is an encoder 56 for measuring the angular position and rotational speed of the output shaft of the motor itself.

[0026] Figure 1 The control unit 20 illustrated in FIG. 4 uses the signals of the encoder 56 to provide closed-loop control for driving the joint.

[0027] The problems discussed at the beginning with regard to the increase in backlash are obviously related to Figure 2 The gear mechanism of the reducer 53 of the joint.

[0028] The above issues have repercussions on the accuracy and efficiency of joint operation.

[0029] refer to Figure 1 Each of the joints of the articulated arm 10 may suffer negative effects due to an increase in backlash in the gear mechanism of its motion transmission assembly and moreover in a manner substantially independent of the state of the other joints.

[0030] The solution described herein has precisely the purpose of monitoring the increase of backlash in the joints of an industrial robot and is characterized in that it uses the same encoders associated with the motors used to drive the joints, without envisaging any additional sensors specifically dedicated to monitoring backlash.

[0031] refer to Figure 3 According to the solution described herein, a process for monitoring backlash in a gear mechanism of a joint of an industrial robot comprises the following steps:

[0032] - driving (101) the movement of the second joint body relative to the first joint body via the motor of the joint;

[0033] - during said movement, acquiring (102) a signal of an encoder;

[0034] - receiving (103) a characteristic oscillation curve;

[0035] - processing (104) the signal of the encoder to identify an oscillation curve in said signal corresponding to the aforementioned characteristic oscillation curve;

[0036] - determining (105) a parameter of the identified oscillation curve, said parameter being indicative of the amplitude of the oscillation; and

[0037] - Determining (106) a backlash condition based on the determined parameters.

[0038] The solution described herein originates from the observation that the presence of backlash causes mechanical vibrations that propagate along the entire kinematic chain of the joint and are also reflected in the signal generated by the encoder associated with the motor used to drive the joint in the form of oscillations exhibiting characteristic modes.

[0039] The characteristic oscillation mode or curve may be defined via one or more parameters which vary with increasing value of the backlash according to a given dependency.

[0040] Therefore, by monitoring the signal of the encoder, and in particular the evolution of the aforementioned characteristic oscillation curve, it is possible to monitor the state of the backlash of the gear mechanism of the joint.

[0041] The encoder's signal can be proportional to the angular position of the motor's output shaft, or proportional to its rotational speed.

[0042] Preferably, the process described herein uses a signal that is proportional to the rotational speed.

[0043] Therefore, in the case where the signal of the encoder is a signal proportional to the angular position of the motor, the process envisions performing a differential operation of the signal of the encoder to accurately obtain a signal proportional to the rotation speed. In subsequent processing, the signal obtained via the differential operation is also regarded as the signal of the encoder.

[0044] Figure 4 An example of the characteristic oscillation curve discussed above is illustrated. In general, this curve corresponds to a damped oscillation curve, which can be defined by the following formula:

[0045]

[0046] in:

[0047] -d b It is the oscillation caused by the backlash;

[0048] -A is the maximum oscillation amplitude;

[0049] -t1 is the time when oscillation starts;

[0050] -t2 is the moment when the oscillation ends; and

[0051] -T is the oscillation damping factor.

[0052] This can be explained by the fact that the mechanical vibrations discussed above caused by backlash are actually caused by impacts occurring between the teeth of the gears of the gear mechanism.

[0053] The mechanical vibration generated by this impact has a damped vibration mode and is therefore reflected in the signal of the encoder as a damped oscillation.

[0054] In order to identify the characteristic curve in the signal of the encoder, it is possible to adopt the commonly used techniques of multivariate statistical analysis. For example, the applicant has identified the CMA-ES (Covariance Matrix Adaptation - Evolution Strategy) algorithm as a technique suitable for this purpose. In this regard, as an example, Figure 5 The diagram shows Figure 2 Graph of the encoder signals obtained during tests on a joint of the type illustrated, with the characteristic oscillation curves identified using the CMA-ES algorithm superimposed on them.

[0055] With reference to the formula given above, the parameter of the characteristic oscillation curve that can be associated with the backlash is represented by the parameter A, which corresponds to the maximum oscillation amplitude.

[0056] The correlation in question can be obtained experimentally.

[0057] For example, it is possible to set up a test bench which reproduces the structure of the robot joint and subjects the recreated joint to various working cycles, on the one hand, as mentioned above, analyzing the oscillations of the encoder for each working cycle, and on the other hand, directly measuring the backlash on the gear mechanism.

[0058] Based on the determined parameter A and the direct measure of the backlash, it is possible to identify a correlation between the two quantities, for example by regression analysis.

[0059] Purely as an example, refer again to the above reference to Figure 2 In tests carried out on joints of the type illustrated, the applicant has identified the following functions:

[0060] δ(A)=10 -4 (-0.000009A 3 +0.003122A 2 +0.138764A-0.138809)

[0061] Where δ is the value of the backlash, and A is the same parameter as A of the formula given above.

[0062] In view of the foregoing, in order to implement the solution described herein, it is sufficient to provide means for storing and processing the signals of the encoders, and thus no intervention of the robot is necessary.

[0063] Since, among other things, the control unit of the robot already normally uses the signals of the encoders for driving the motors of the joints, it may be sufficient to program the control unit of the robot so that it will store and possibly process the above signals.

[0064] The processing of the signals can in any case also be carried out by a processing unit which is completely independent of the control unit of the robot.

[0065] For example, the above processing unit may belong to a remote processing center.

[0066] Generally speaking, the control unit of the robot may be prearranged for transmitting the stored data to the processing unit via a communication network, such as the Internet.

[0067] According to a preferred embodiment, the solution described herein envisages a test mode for driving the joint during the acquisition of the encoder signals.

[0068] The above mode envisages driving the motor to obtain a rotational movement at a constant speed. Furthermore, according to an even more preferred embodiment, the aforementioned test mode envisages arranging the joint so that the axis of rotation of the gear mechanism to be tested is oriented horizontally. This means that gravity may contribute to inducing impacts between the teeth of the gears of the gear mechanism, thereby generating encoder signals in which a more precise identification of the characteristic oscillation curve is possible.

[0069] According to this preferred embodiment, the control unit of the robot may be configured to drive the motor of the joint to be tested, and possibly also the motors of other joints of the robot, so as to obtain driving of the joints according to the test mode.

[0070] It is clear that, in this case, the acquisition of the encoder's signals is possible only during the actuation of the joint in the above test mode.

[0071] As has been seen above, each joint of the robot may suffer adverse effects due to an increase in backlash in its gear mechanism and therefore each joint of the robot must be tested.

[0072] In a preferred embodiment, the control unit of the robot can be configured for continuously executing a test mode for the various joints of the robot, so that it is possible to envisage a single machine shutdown, during which all joints of the robot are tested.

[0073] On the other hand, the control unit can be configured to perform a test of the joints of the robot in a programmed manner, for example after a given number of working cycles.

[0074] In view of the foregoing, it is clearly evident that the solution described herein can be easily applied to any type of industrial robot, and also to robots already installed and operating, and makes it possible to obtain a monitoring of the tooth gap that is effective and reliable and that can be easily adapted to the different needs of the various environments in which the industrial robot may operate.

[0075] Of course, without prejudice to the principle of the invention, the details of construction and of the embodiments may vary (even significantly) with respect to what has been illustrated herein purely as a non-limiting example, without thereby departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for monitoring backlash in a gear mechanism of a joint (50) of an industrial robot (10), wherein the joint comprises: A first joint body (52) and a second joint body (54), a motor (51) and a motion transmission assembly (53, 55), wherein the first joint body (52) and the second joint body (54) are coupled together with the possibility of movement relative to each other, the motor (51) is provided with an encoder (56), the motion transmission assembly (53, 55) is designed to transmit the torque generated by the motor (51) to the second joint body (54) to bring about the movement of the second joint body (54) relative to the first joint body (52), and the transmission assembly (53, 55) includes the gear mechanism, The method comprises the following steps: - driving (101) the movement of the second joint body (54) relative to the first joint body (52) via the motor (51); wherein driving the movement of the second joint body comprises driving the movement according to a test mode, wherein the speed of the movement is constant, and wherein the gears of the gear mechanism are arranged with their own axes of rotation oriented horizontally so that gravity can contribute to inducing impacts between teeth of the gears of the gear mechanism; - during said movement, acquiring (102) a signal of said encoder; - receiving (103) a characteristic oscillation curve, the characteristic oscillation curve being a damped oscillation curve; - processing (104) the signal for identifying an oscillation curve in the signal corresponding to the characteristic oscillation curve and reflecting mechanical vibrations generated by impacts between teeth of a gear of a gear mechanism; - determining (105) a parameter (A) of the identified oscillation curve, said parameter (A) being indicative of a maximum amplitude of the identified oscillation curve; and - Determining (106) a backlash condition based on the determined parameters. 2 . The method according to claim 1 , for determining the backlash conditions of a plurality of joints of the industrial robot, wherein the driving in the test mode is performed continuously for individual joints of the plurality of joints.

3. A method according to any one of the preceding claims, wherein the signal of the encoder is a signal proportional to the rotational speed of the output shaft of the motor.

4. A method according to any one of claims 1-2, for monitoring the tooth gap in multiple joints of the industrial robot (10), wherein the step of driving the movement includes driving the movement according to a test mode, and wherein the driving in the test mode is continuously performed for individual joints of the multiple joints.

5. A system for monitoring backlash in a gear mechanism of a joint of an industrial robot, comprising: - An industrial robot comprising at least one joint (50), wherein the joint comprises a first joint body (52) and a second joint body (54), a motor (51), and a motion transmission assembly (53, 55), wherein the first joint body (52) and the second joint body (54) are coupled together with the possibility of movement relative to each other, the motor (51) is provided with an encoder (56), the motion transmission assembly (53, 55) is designed to transmit the torque generated by the motor (51) to the second joint body (54) to bring about the movement of the second joint body (54) relative to the first joint body (52), and the transmission assembly (53, 55) comprises the gear mechanism, a control unit (20) for driving the motor (51) in accordance with a test mode in order to monitor the backlash of the gear mechanism, wherein the speed of movement in the test mode is constant and the gears of the gear mechanism are arranged with their own axes of rotation oriented horizontally so that gravity can contribute to inducing impacts between the teeth of the gears of the gear mechanism; as well as - a processing unit configured to: - receiving, directly or through the control unit, a signal from the encoder regarding the step of driving the motor in a test mode; - receiving a characteristic oscillation curve, the characteristic oscillation curve being a damped oscillation curve; - processing the signal for identifying an oscillation curve in the signal corresponding to the characteristic oscillation curve and reflecting mechanical vibrations generated by impacts between teeth of a gear of a gear mechanism; - determining a parameter (A) of the identified oscillation curve, said parameter (A) being indicative of a maximum amplitude of the identified oscillation curve; and - Determining a backlash condition based on said determined parameters.

6. The system according to claim 5, wherein the control unit is configured to store the signal of the encoder (56) regarding the step of driving the motor (51) in a test mode.

7. The system according to claim 5 or claim 6, wherein the control unit (20) is configured for transmitting the stored data of the signal to the processing unit via a communication network.

8. A computer program product loadable into a memory of at least one processor and comprising portions of software code for implementing the steps of the method according to any one of claims 1 to 4.

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