Robotic hand and method for controlling a robotic hand

Abstract

The present invention relates to a robot hand and a method for controlling a robot hand. A control device includes: a limiting unit configured to execute a limiting process that limits a torque to a torque limit value or less; an estimating unit configured to estimate that a claw has come into contact with a workpiece when a change speed of a rotational position is equal to or less than a threshold value during execution of the limiting process; an increasing unit configured to execute an increasing process that gradually increases the torque to more than the torque limit value after the claw comes into contact with the workpiece; a calculating unit configured to calculate a movement amount from a position at which the claw comes into contact with the workpiece to a current position based on the rotational position during execution of the increasing process; and a maintaining unit configured to execute a maintaining process that maintains the torque equal to or greater than a torque upper limit value that is greater than the torque limit value, or a maintaining process that maintains the torque when the movement amount is equal to or greater than a movement amount upper limit value during execution of the increasing process.

Description

TECHNICAL FIELD

[0001] The present application relates to a robot hand and a method for controlling a robot hand. BACKGROUND

[0002] A robot hand including a gripper for gripping a workpiece is known. Such a conventional robot hand is operated by setting a gripping force so as to grip a workpiece having a predetermined prescribed shape or material. However, when a workpiece of an unspecified kind is gripped by means of the conventional robot hand, if the gripping force of the gripper is too strong with respect to the material of the workpiece, the workpiece can be damaged. As a result, if the gripping force is too weak with respect to the material of the workpiece, the workpiece can fall while being gripped. As described above, in the conventional robot hand, it is difficult to grip a workpiece of an unspecified kind with an appropriate gripping force without changing the gripping force set in accordance with the type of the workpiece. In order to achieve control such that, for a workpiece of an unspecified kind, "a hard workpiece is gripped firmly and a soft workpiece is gripped with a slight force", a pressure sensor for detecting the gripping force can be provided in the robot hand to adjust the gripping force. However, the provision of the pressure sensor increases manufacturing costs. On the other hand, there is a technology in which a deformation ratio of a workpiece in which the displacement amount of the gripper and the gripping force are correlated with each other is obtained in advance, and the gripping force of the gripper is controlled in accordance with the deformation ratio (for example, see Patent Document 1).

[0003] PRIOR ART DOCUMENTS

[0004] Patent Document 1

[0005] Japanese Unexamined Patent Application Publication No. 2018-069381 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] In order to obtain the deformation ratio of the workpiece as described above, it is necessary to prepare in advance.

[0008] Therefore, an object of the present application is to provide a robot hand and a control method for enabling a robot hand to grip a workpiece of an unspecified kind with an appropriate gripping force by a simple method without changing and setting the gripping force for various types of workpieces.

[0009] MEANS FOR SOLVING THE PROBLEM

[0010] The above object is achieved by a robot hand including a motor, a claw configured to grip a workpiece according to rotation of the motor, an encoder configured to detect a rotational position of the motor, and a control device configured to control a torque of the motor such that the claw grips the workpiece according to the rotational position, wherein the control device includes a limiting unit configured to execute a limiting process of limiting the torque to a torque limit value or less, an estimating unit configured to estimate that the claw has come into contact with the workpiece when a speed of change of the rotational position becomes equal to or less than a threshold value during execution of the limiting process, an increasing unit configured to execute an increasing process of gradually increasing the torque to more than the torque limit value after the claw comes into contact with the workpiece, a calculating unit configured to calculate an amount of movement from a position at which the claw comes into contact with the workpiece to a current position based on the rotational position during execution of the increasing process, and a maintaining unit configured to execute a maintaining process of maintaining the torque when the torque becomes equal to or greater than a torque upper limit value that is greater than the torque limit value, or a maintaining process of maintaining the torque when the amount of movement becomes equal to or greater than an amount of movement upper limit value during execution of the increasing process.

[0011] In addition, the above object is achieved by a method for controlling a robot hand, the method including the steps of limiting a torque of a motor that drives a claw to grip a workpiece to a torque limit value or less, estimating that the claw has come into contact with the workpiece when a rate of change of a rotational position of the motor becomes equal to or less than a threshold value during execution of the limiting process, executing an increasing process of gradually increasing the torque to more than the torque limit value after the claw comes into contact with the workpiece, calculating an amount of movement from a position at which the claw comes into contact with the workpiece to a current position based on the rotational position during execution of the increasing process, and executing a maintaining process of maintaining the torque when the torque becomes equal to or greater than a torque upper limit value that is greater than the torque limit value, or a maintaining process of maintaining the torque when the amount of movement becomes equal to or greater than an amount of movement upper limit value during execution of the increasing process.

[0012] Effects of the Invention

[0013] According to the present application, it is possible to provide a robot hand and a control method for gripping a workpiece of an unspecified kind with an appropriate gripping force by a simple method without changing and setting for various types of the gripping force. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1is a schematic configuration diagram of a robot hand;

[0015] Figure 2 is a block diagram illustrating a schematic configuration of a control device for a robot hand;

[0016] Figure 3 is a flowchart illustrating an example of grasp control;

[0017] Figure 4 is a flowchart illustrating an example of a limiting process;

[0018] Figure 5 is a flowchart illustrating an example of an increasing process;

[0019] Figure 6 is a timing chart illustrating a transition between the position of the claw and the torque of the motor when grasping a soft workpiece; and

[0020] Figure 7 is a timing chart illustrating a transition between the position of the claw and the torque of the motor when grasping a hard workpiece. DETAILED DESCRIPTION

[0021] Figure 1 is a schematic configuration diagram of a robot hand 1. The robot hand 1 includes a control device 10, an encoder 20, a motor 30, a drive gear 40, a driven gear 50, and a claw 60. The control device 10 controls the operation of the entire robot hand 1. The motor 30 is a drive source for opening and closing the claw 60, and is, for example, a stepper motor or a brushless DC motor. The encoder 20 is provided at a proximal end of a rotation shaft 32 of the motor 30, and detects the rotational position of the motor 30 (the rotation angle of the rotation shaft 32 of the motor 30). The encoder 20 can be of an optical type or a magnetic type. The drive gear 40 is provided at a distal end of the rotation shaft 32 of the motor 30, and is engaged with the driven gear 50. The rotational force of the motor 30 is transmitted from the drive gear 40 to the driven gear 50 via the rotation shaft 32. The driven gear 50 has a substantially semicircular shape, and is formed with teeth on an arc-shaped outer peripheral surface. The engagement mechanism between the drive gear 40 and the driven gear 50 is, for example, a worm gear, but can also be a helical gear or other gears. A proximal end portion of the claw 60 is fixed to the driven gear 50. Although the claw 60 is fixed to the driven gear 50 in the present embodiment, the claw 60 can be fixed to the drive gear 40. Figure 1 In the present embodiment, only two pairs of the driven gear 50 and the claw 60 are illustrated, but three or more pairs of the driven gear 50 and the claw 60 can be provided.

[0022] When the motor 30 rotates in the forward direction, the driven gear 50 swings in a direction corresponding to engagement with the drive gear 40, and the distal end portions of the claws 60 approach each other. When the motor 30 rotates in the reverse direction, the driven gear 50 swings in the opposite direction to the above-described one, and the tip end portions of the claws 60 separate from each other. When the distal end portions of the claws 60 approach each other, a workpiece serving as a gripping target can be gripped. The workpiece is released by separating the distal end portions of the claws 60 from each other. In this way, the claws 60 are opened and closed by switching the rotation of the motor 30 between the forward direction and the reverse direction.

[0023] Figure 2 is a block diagram illustrating a schematic configuration of the control device 10. The robot hand 1 is used by being fixed to the distal end of a robot arm. Furthermore, the control device 10 controls the drive of the motor 30 in response to a command from a robot controller 100 that controls the entire operation of the robot hand 1 and the robot arm. The control device 10 includes a control unit 11 and a drive circuit 13. The control unit 11 is mainly configured by a microcomputer or the like, and includes a central processing unit, a read only memory, a random access memory, an input / output, a bus connecting these components, and the like, none of which is illustrated. Each process in the control unit 11 can be a software process in which a program that is stored in advance in a tangible memory such as a read only memory (i.e., a readable temporary tangible recording medium) is executed by a central processing unit, or can be a hardware process by a dedicated electronic circuit using a field programmable gate array (FPGA) or the like.

[0024] Based on the detection signal from the encoder 20, the control unit 11 calculates the position and the amount of movement of the pawl 60. As described above, since the opening and closing of the pawl 60 are performed by transmitting the rotational force from the drive gear 40 to the driven gear 50 by means of the rotation of the motor 30, the state of the opening and closing of the pawl 60 is grasped by the rotation angle of the rotational shaft 32 obtained from the detection signal from the encoder 20. When the motor 30 is a stepping motor, the drive circuit 13 includes a switching element that controls the energization of each phase coil, and the drive of the motor 30 is controlled by switching the energization of each phase winding of the motor 30. In the drive circuit 13, in addition to using the function of a general-purpose integrated circuit for controlling the motor 30, a parallel resistor is provided, and the potential difference thereof is analog / digital converted. Thus, the control unit 11 grasps the current of each phase coil of the motor 30. Similarly, in the drive circuit 13, by using the function of a general-purpose integrated circuit for controlling the motor 30, the effective value of the energization of each phase coil of the motor 30 can be set by modulation such as pulse width modulation. Furthermore, based on the current of each phase coil and the detection signal from the encoder 20, the control unit 11 can estimate the torque T of the motor 30. As described above, the control unit 11 can control the current of each phase coil of the motor 30. Thus, the control unit 11 can arbitrarily set the torque T by setting the current of each phase coil of the motor 30 with reference to the detection signal from the encoder 20. In this way, based on the detection signal from the encoder 20, the control unit 11 can control the drive of the motor 30 by issuing a command to the drive circuit 13, and can finally control the opening and closing of the pawl 60.

[0025] Next, gripping control performed by the control unit 11 of the control device 10 will be described. Figure 3 is a flowchart exemplifying an example of the gripping control. The control unit 11 first performs a limiting process for limiting the torque T of the motor 30 (step S10), then performs an increasing process for increasing the torque T of the motor 30 (step S20), and then performs a maintaining process for maintaining the torque T of the motor 30 (step S30). The limiting process will be described below.

[0026] Figure 4is a flowchart illustrating an example of the limiting process. Based on a command from the robot controller 100, the control unit 11 obtains a target position Pt of the gripper 60, a moving speed S of the gripper 60, and a torque limit value Tr of the motor 30 (step Sll). Next, the control unit 11 controls the motor 30 so that the gripper 60 moves from a current position toward the target position Pt at the moving speed S (step S13), while limiting the current applied to the motor 30 by outputting a command to the drive circuit 13 so that the torque T of the motor 30 becomes constant at the torque limit value Tr or less (step S12). As a result, when the torque T of the motor 30 is a relatively weak torque equal to or less than the torque limit value Tr, the gripper 60 moves so as to be closed.

[0027] Next, the control unit 11 determines whether all of the grippers 60 have come into contact with the workpiece in a state in which the torque T of the motor 30 is limited (step S14). Specifically, it is determined whether the rate of change of the rotational position of the motor 30 (an amount of change in the rotational angle of the rotational shaft 32 per unit time) has become equal to or less than a threshold value a. The threshold value a is set to a value smaller than the rate of change of the rotational position of the motor 30 corresponding to the moving speed S described above. That is, it is determined whether the moving speed S has decreased to a moving speed corresponding to the threshold value a. The rate of change of the rotational position is calculated by the encoder 20 based on the amount of rotation of the motor 30 in a predetermined time. When the rate of change of the rotational position of the motor 30 is higher than the threshold value a, it is determined that all of the grippers 60 have not yet come into contact with the workpiece. When the rate of change of the rotational position of the motor 30 decreases to the threshold value a or less, it is determined that all of the grippers 60 have come into contact with the workpiece. In the case where the determination in step S14 is “No”, the process of step S13 is executed again. In the case where the determination in step S14 is “Yes”, the limiting process ends, and then the increasing process described above is executed.

[0028] The increasing process will be described. Figure 5is a flowchart illustrating an example of the increasing process. The control unit 11 obtains the torque upper limit value Tmax and the movement amount upper limit value ΔPmax (step S21). The torque upper limit value Tmax and the movement amount upper limit value ΔPmax can be stored in advance in the above-mentioned read only memory and can be used, or values transmitted from the robot controller 100 can be stored in the above-mentioned random access memory and used. The torque upper limit value Tmax is a value larger than the torque limit value Tr. Next, the control unit 11 temporarily stores in the memory the contact start position Pl at which all the claws 60 determined in the restricting process have come into contact with the work (step S22). Next, the control unit 11 increases the torque T by one step (step S23). Specifically, the control setting is changed so that the torque is increased in accordance with the control characteristics of the motor serving as the motor 30. For example, the absolute value of the current of each phase coil of the motor 30 is increased, or the duty ratio of the pulse width modulation control is increased. Here, "increasing by one step" means increasing the torque T of the motor 30 discretely. "Increasing by one step" means fluctuations before and after the increase in the torque T, and a large amount of energy is transmitted from the motor 30 to the drive gear 40 with respect to the friction loss in the engagement mechanism of the above-mentioned drive gear 40 and driven gear 50. In the engagement mechanism, when the engagement speed is lowered, it is possible that the engagement is stopped due to the friction loss. Specifically, since the friction force acting so far changes from the dynamic friction force to the static friction force, and the static friction force is larger than the dynamic friction force, the engagement mechanism can be caught in a so-called stuck state. In order to make the engagement mechanism start moving again, by increasing the torque T in a discrete value, a torque shock is applied to the engagement mechanism, and thus it is possible to transition from the static friction to the dynamic friction.

[0029] Next, the control unit 11 determines whether the torque T is equal to or larger than the torque upper limit value Tmax (step S24). In the case where "Yes" in step S24, the increasing process ends, and a maintaining process of maintaining the current torque T of the motor 30 at the torque upper limit value Tmax is executed (step S30).

[0030] In the case where "No" in step S24, the control unit 11 temporarily stores the current position P2 of the claws 60 in the memory (step S25). Next, the control unit 11 calculates the movement amount ΔP of the claws 60 (the difference between the contact start position PI and the current position P2) (step S26). Next, the control unit 11 determines whether the calculated movement amount ΔP is equal to or greater than the movement amount upper limit value ΔPmax (step S27). In the case where "No" in step S27, the process of step S23 is executed again. In this case, the torque T is further increased by one step in step S23. As a result, as long as "No" is determined in steps S27 and S24, the torque T is increased at a constant rate of increase. In the case where "Yes" in step S27, the increasing process ends, and a maintaining process of maintaining the current torque T of the motor 30 is executed (step S30).

[0031] Next, the transition between the position P of the claws 60 and the torque T of the motor 30 when the workpiece is gripped will be described. Figure 6 is a time chart illustrating the change in the position P and the torque T of the motor 30 when a soft workpiece is gripped. At time to, the torque T is substantially maintained constant at a torque TO equal to or smaller than the torque limit value Tr, and the position P of the claws 60 is gradually moved from an initial position PO. At time tl, the position P of the claws 60 reaches a contact start position PI at which all the claws 60 come into contact with the workpiece. Since the position P of the claws 60 is not moved, and it is determined that all the claws 60 are in contact with the workpiece at time t2, the torque T is increased, and the position P of the claws 60 starts to move. When the movement amount ΔP (the difference between the current position P2 and the contact start position PI) reaches the movement amount upper limit value ΔPmax at time t3, the torque T is maintained at the torque Tl at that time. In this way, a soft workpiece can be gripped using a weak gripping force that does not damage the workpiece.

[0032] Figure 7 is a time chart illustrating the change in the position P of the claws 60 and the torque T of the motor 30 when a hard workpiece is gripped. Like the case illustrated in Figure 6 After reaching times to, tl, and t2, the torque T is increased, but the current position P2 is not moved from the contact start position PI. Therefore, the torque T is further increased, becomes equal to or greater than the torque upper limit value Tmax at time t3, and the torque T is maintained at the torque upper limit value Tmax. In this way, a hard workpiece can be gripped using a strong enough gripping force to prevent the hard workpiece from falling.

[0033] The above-described limiting process (step S10) is supplemented. The gist of the limiting process is to detect the position of the workpiece. In other words, the limiting process is control of the claw 60 for searching for the presence of the workpiece. In order to detect the position of the workpiece, the torque T is set to a relatively weak value so as not to damage the workpiece. On the other hand, when the torque T is weak (low value), the rotational speed of the motor 30 is slowed down depending on the type or control method of the drive circuit 13 to be used or the type of the motor 30 to be used, and it can take time to detect the position of the workpiece. In order to avoid such inconvenience, it is also possible to select to operate the motor 30 in a high-speed rotation and low-torque control region during the limiting process, and to operate the motor 30 in a low-speed rotation and high-torque control region during the increasing process and the maintaining process. For example, in the case where an inner-rotating brushless DC motor is employed as the motor 30, this is achieved by changing the excitation of the magnetic poles by switching the connection of the stator winding. When the motor 30 is provided with a transmission (not shown) that can change the transmission ratio and the ratio between the rotational speed of the motor 30 and the rotational speed of the drive gear 40, it is also possible to rotate the motor 30 at a low reduction ratio in a high-speed and low-torque state during the limiting process, and to rotate the motor 30 at a high reduction ratio in a low-speed and high-torque state during the increasing process and the maintaining process.

[0034] As described above, it is not necessary to change the setting of the gripping force for each of the unspecified number of workpiece types, to prepare in advance to obtain the deformation ratio of the workpiece, and to provide a pressure sensor for measuring the gripping force based on the rotational position of the motor 30 detected by the encoder 20. Therefore, the workpiece can be gripped with an appropriate gripping force according to the hardness of the workpiece by a simple method.

[0035] In the above-described embodiment, the robot to which the robot hand of the present embodiment is applied has the following advantages. First, one robot hand can dynamically handle workpieces of unspecified kinds. Therefore, when a mass production line is built, the man-hours for teaching and setting changes can be reduced. Further, after the gripping force of each workpiece is determined by the increasing process, the torque T and the movement amount ΔP are obtained. Therefore, for example, in a production line that continuously feeds workpieces of the same type, the robot hand according to the present embodiment can be used as a measuring instrument while gripping the workpieces. As a result, after the gripping force is determined, the torque T and the movement amount ΔP are regarded as physical properties of each workpiece, the quality of the workpieces is statistically determined, and non-standard workpieces are distinguished. As a similar application, in a production line that continuously feeds workpieces of multiple types, it is also possible to classify each workpiece based on the torque T or the movement amount ΔP after the gripping force is determined.

[0036] In addition to the torque T and the movement amount ΔP obtained when the gripping force of each workpiece is determined in the increasing process as described above, the time (t2 to t3) required until the movement amount ΔP becomes equal to or greater than the upper limit value ΔPmax when the positions Pl of all the claws in contact with the workpiece in the restricting process are determined in the restricting process, and the like also indicate the physical characteristics of each workpiece. A plurality of physical quantities representing the physical characteristics of each workpiece are defined as gripping parameters. By using these gripping parameters, the robot hand according to the present embodiment can be widely used. In the case where the same type of workpieces are continuously fed in the production line described above, it is possible to accurately detect defective workpieces by setting a threshold value defining the range of non-defective workpieces so as to determine whether the workpiece is non-defective or defective, and by comparing the gripping parameters obtained by gripping each workpiece with the threshold value. Further, a plurality of non-defective workpieces are prepared in advance as a quality determination reference, and the workpieces of the non-defective group are continuously gripped to obtain the gripping parameters, and a statistical process is performed to generate the threshold value for defining the range of non-defective workpieces. Therefore, it is possible to detect defective products by comparing the gripping parameters of each workpiece with the threshold value at the time of subsequent operation of the production line. Further, since it is not necessary for the production line process designer to handle the gripping parameters as specific numerical values, it is possible to reduce input errors and man-hours, and to save labor.

[0037] The acquisition of the gripping parameters described above, the generation of the threshold value, and the comparison between the gripping parameters and the threshold value can be performed by the control unit 11, or can be co-performed by another robot hand by exchanging necessary information with the external robot controller 100 or the like.

[0038] While the exemplary embodiments of the present application have been described in detail, the present application is not limited to the above-mentioned embodiments, and other embodiments and variations can be made without departing from the scope of the present application.

Claims

1. A robot hand, comprising: a motor; a claw configured to grip a workpiece in accordance with rotation of the motor; an encoder configured to detect a rotational position of the motor; and a control device configured to control a torque of the motor such that the claw grips the workpiece in accordance with the rotational position, wherein the control device includes: a limiting unit configured to execute a limiting process that limits the torque to a torque limit value or less; an estimating unit configured to estimate that the claw has come into contact with the workpiece when a rate of change of the rotational position becomes equal to or less than a threshold value during execution of the limiting process; an increasing unit configured to execute an increasing process that gradually increases the torque to more than the torque limit value after the claw comes into contact with the workpiece; a calculating unit configured to calculate an amount of movement from a position at which the claw comes into contact with the workpiece to a current position based on the rotational position during execution of the increasing process; and a maintaining unit configured to execute a maintaining process that: checks whether the torque becomes equal to or greater than a torque upper limit value that is greater than the torque limit value, and maintains the torque when the torque becomes equal to or greater than the torque upper limit value, and checks whether the amount of movement becomes equal to or greater than an amount of movement upper limit value when the torque is checked and the torque is less than the torque upper limit value, and maintains the torque when the amount of movement becomes equal to or greater than the amount of movement upper limit value, during execution of the increasing process. The limiting unit is configured to maintain the torque at a constant value equal to or less than the torque limit value in the limiting process.

2. The robotic hand of claim 1, wherein, The increasing unit increases the torque at a constant increasing rate.

3. The robotic hand of claim 1 or 2, wherein, 4. A method for controlling a robot hand, the method comprising the steps of: limiting a torque of a motor that drives a claw to grip a workpiece to a torque limit value or less; estimating that the claw has come into contact with the workpiece when a rate of change of a rotational position of the motor becomes equal to or less than a threshold value during execution of a limiting process; executing an increasing process that gradually increases the torque to more than the torque limit value after the claw comes into contact with the workpiece; calculating an amount of movement from a position at which the claw comes into contact with the workpiece to a current position based on the rotational position during execution of the increasing process; and executing a maintaining process that: checks whether the torque becomes equal to or greater than a torque upper limit value that is greater than the torque limit value, and maintains the torque when the torque becomes equal to or greater than the torque upper limit value, and checks whether the amount of movement becomes equal to or greater than an amount of movement upper limit value when the torque is checked and the torque is less than the torque upper limit value, and maintains the torque when the amount of movement becomes equal to or greater than the amount of movement upper limit value, during execution of the increasing process. ​ ​

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