JP7657097B2 - Robot control method and control device - Google Patents

Description

The present invention relates to a method and device for controlling a robot, and in particular to a method and device for controlling the position and speed of a multi-joint robot in which each axis is driven by a motor.

A multi-joint robot (hereinafter simply referred to as a robot), such as a horizontal multi-joint robot in which multiple links or arms are connected via joints, is driven by a motor provided at the joint, and for example, a workpiece is grasped at the tip of the robot, or a workpiece is placed on the tip of the robot. The term "a robot holding a workpiece" is used to refer to a robot grasping a workpiece or having a workpiece placed on the tip of the robot. In a large robot, the moment of inertia of the link around the joint axis changes depending on the mass of the workpiece held at the tip (i.e., the hand), and the characteristics of the robot, which is the object of control, change. If the nominal model for control is fixed regardless of the mass of the workpiece, a model mismatch may occur due to the change in the mass of the workpiece, which may cause the feedback-based control to become unstable or vibration to occur. In particular, in robots, it is common for the motor to be connected to the joint axis via a reducer, and the reducer also has properties as a spring element, so if the control is inappropriate, vibrations are likely to be induced in the robot as it moves due to the spring element including the reducer. There is a need for a control method and control device that can stably control the position and speed of a robot regardless of changes in the mass of the workpiece held by the robot.

Patent Document 1 discloses that a positioning control device used for controlling robots and the like is provided with a classical control means for performing positioning control by P-PI (proportional-proportional integral) control and a modern control means for performing positioning control by state feedback control by a two-inertia system state observer, and that when the moment of inertia around the joint axis is smaller than a threshold value, P-PI control (or PID (proportional integral differential) control) is performed, and when it is equal to or greater than the threshold value, state feedback control is performed, thereby suppressing vibration and performing high-speed and high-precision positioning control. Non-Patent Document 1 discloses that in an R-P-P type robot, PID control is performed under normal conditions, and state feedback control is performed when the arm is extended to suppress vibration at the hand. Patent Document 2 discloses that when controlling a robot by state feedback control using a two-inertia system state observer, the physical model of the state observer, i.e., the control parameters, are switched depending on the mass of the load at the tip of the robot. Patent Document 3 discloses one specific example of state feedback control using a two-inertia system state observer.

Japanese Patent Application Publication No. 7-319546 Patent No. 6097174 International Publication No. 2018/061097

Yoshiaki Ito, "Vibration Control of Large Robots", Toyota Central R&D Labs. R&D Review, March 1995, Vol. 30, No. 1, p. 57

The method described in Patent Document 1 switches between P-PI control and state feedback control depending on the magnitude of the moment of inertia around the joint axis, and when this method is applied to the control of a robot, state feedback control is selected as the control method for the robot when the mass of the workpiece held on the tip side of the joint axis to be controlled is large. According to the inventors' investigations, when the method of Patent Document 1 is applied to a multi-joint robot, particularly a horizontal multi-joint robot, a sufficient vibration damping effect cannot be obtained when the mass of the workpiece held by the robot is large.

The object of the present invention is to provide a control method and control device for controlling an articulated robot that can suppress the occurrence of vibrations and perform stable control regardless of changes in the mass of the workpiece held by the robot.

As a result of the study by the inventors, it was found that in a multi-joint robot, when the mass of the work held by the hand of the robot is large, vibrations generated in the robot can be suppressed more effectively by controlling the motor of the joint axis of the robot using a single-inertia system model than by controlling the motor of the joint axis of the robot using state feedback control, and the robot can be controlled stably. On the other hand, when the mass of the work is small, a greater vibration suppression performance can be obtained by implementing state feedback control. The state feedback control that can be used in the control device of the present invention is, for example, state feedback control using a two-inertia system state observer, and an example of this is the state feedback control described in Patent Document 3. In addition, the control based on the single-inertia system model that can be used in the control method of the present invention is, for example, P-PI control or PID control. The command is, for example, a motor position command.

Therefore, the control method of the present invention performs control by switching according to the mass of at least one motor based on a motor position command, such that a first control is performed when the mass of a workpiece held by the articulated robot is equal to or less than a threshold, and a second control is performed when the mass exceeds the threshold. Here, the first control is state feedback control by a two-inertia system state observer that uses the motor position command and a fed-back motor position, and the second control is P-PI control or PID control that uses the motor position command and a fed-back motor position.

In the control method of the present invention, the operation of the articulated robot is controlled based on an operation program that defines at least a loading operation for holding a workpiece on the articulated robot and an unloading operation for removing the held workpiece from the articulated robot, and the first control and the second control can be switched by the operation program. The operation of a robot used for transporting a workpiece is usually controlled based on an operation program, and such an operation program defines at least a loading operation and an unloading operation. If the operation program defines how to transport the workpiece, the mass of the workpiece can be considered to be known when the operation program is executed, so that the first control and the second control can be switched by the operation program, and the control can be switched with a smaller amount of processing.

In the control method of the present invention, when controlling a robot by an operation program, it is preferable to implement the first control when the articulated robot not holding a workpiece is moved to a workpiece receiving position based on the operation program, and then switch to the second control before the workpiece comes into contact with the articulated robot if the mass exceeds a threshold value. By implementing control in this manner, it is possible to prevent the generation of vibrations caused by holding a heavy workpiece while implementing the first control.

In the control method of the present invention, when the robot is controlled by the operation program, the articulated robot may be moved to a workpiece unloading position while holding a workpiece based on the operation program, and after the unloading operation is performed, the control may be switched to the first control. By controlling in this way, the first control is implemented when the robot is not holding a workpiece, and the robot can be moved quickly and stably.

In the control method of the present invention, the mass of the workpiece can be measured or estimated to switch between the first control and the second control. For example, when a vertical axis motor is provided in an articulated robot, the mass of the workpiece can be determined based on a torque command value for the vertical axis motor. Since the mass of each part of the robot mechanism is known, the mass of the workpiece can be determined by determining the mass corresponding to the torque command value for the vertical axis motor and subtracting the mass of each part on the robot side from the determined mass. Alternatively, when a load sensor that detects whether a workpiece satisfying a predetermined condition is held is provided in the articulated robot, the control can be switched to the second control when the load sensor detects a workpiece satisfying the predetermined condition. The load sensor may be one that detects the mass. In addition, since a workpiece having a large thickness or planar dimension can be estimated to be a workpiece with a large mass, the load sensor may be a sensor that detects the thickness or planar dimension. By measuring or estimating the mass of the workpiece using the torque command value for the vertical axis motor or the load sensor, it is possible to prevent the robot from being controlled by the first control when a heavy workpiece is held, even if there is a program error when switching control by the operation program.

In the control method of the present invention, the articulated robot is, for example, a horizontal articulated robot. Horizontal articulated robots are often used to hold a workpiece in their hands and transport the workpiece. By applying the control method of the present invention to such a horizontal articulated robot, the effect of the present invention, which is to suppress the occurrence of vibrations in the robot and perform stable control, can be more effectively achieved.

The control device of the present invention is a control device for controlling a multi-joint robot whose joint axes are driven by a motor, and includes a first control unit that performs state feedback control by a two-inertia system state observer using a motor position command and a fed-back motor position based on a motor position command , a second control unit that performs P-PI control or PID control using the motor position command and the fed-back motor position based on the motor position command , and a selection unit that selects the first control unit when the mass of a workpiece held by the multi-joint robot is equal to or less than a threshold value, and selects the second control unit when the mass exceeds the threshold value, and controls the motor using the output of the control unit selected by the selection unit depending on the mass.

According to the control device of the present invention, when the mass of the workpiece is small, the robot operates using state feedback control with a two-inertia system state observer , and when the mass of the workpiece is large, the robot operates using P-PI control or PID control, so that the robot can be stably controlled by suppressing the occurrence of vibration regardless of the mass of the workpiece .

A multi-joint robot having a function of holding a workpiece is usually controlled based on an operation program that controls the operation of the multi-joint robot by defining at least a loading operation for holding a workpiece on the multi-joint robot and an unloading operation for removing the held workpiece from the multi-joint robot. Therefore, in the control device of the present invention, it is preferable to provide a calculation unit that executes such an operation program, and to have the selection unit select either the first control unit or the second control unit in response to a switching command from the calculation unit. By configuring in this way, switching between state feedback control and P-PI control or PID control is performed by the operation program, and control switching can be performed with a smaller amount of processing.

In the control device of the present invention, when a calculation unit is provided, it is preferable that the calculation unit selects the first control unit by the selection unit when moving the articulated robot not holding a workpiece to a workpiece receiving position, and then selects the second control unit by the selection unit before the workpiece and the articulated robot come into contact when the mass exceeds a threshold value. By selecting the control unit in this manner, it is possible to prevent the occurrence of vibrations caused by holding a heavy workpiece while state feedback control is being performed.

In the control device of the present invention, when a calculation unit is provided, the calculation unit may move the articulated robot to a workpiece unloading position while holding a workpiece, perform an unloading operation, and then select the first control unit with the selection unit. By selecting the first control unit in this manner, state feedback control is implemented when the robot is not holding a workpiece, allowing the robot to move quickly and stably.

In the control device of the present invention, the mass of the workpiece can be measured or estimated to switch between the first control unit and the second control unit. For example, when the articulated robot is equipped with a vertical axis motor, the calculation unit may obtain the mass of the workpiece based on a torque command value for the vertical axis motor and output a switching command to the selection unit according to the obtained mass . Alternatively, when the articulated robot is equipped with a load sensor that detects whether a workpiece that satisfies a predetermined condition is being held, the second control unit may be selected when the load sensor detects a workpiece that satisfies the predetermined condition. By measuring or estimating the mass of the workpiece using the torque command value of the vertical axis motor or the load sensor, it is possible to prevent the robot from being controlled by state feedback control when holding a heavy workpiece, even if there is a program error when switching control by the operation program.

The articulated robot that is the subject of control in the control device of the present invention is, for example, a horizontal articulated robot. Horizontal articulated robots are often used to hold a workpiece in their hands and transport the workpiece. By using the control device of the present invention to control a horizontal articulated robot, the effect of the present invention of suppressing the generation of vibrations in the robot and performing stable control can be more effectively achieved.

According to the present invention, when controlling an articulated robot, it is possible to suppress the occurrence of vibrations and perform stable control regardless of changes in the mass of the workpiece held by the robot.

1 is a block diagram showing a configuration of a control device according to an embodiment of the present invention; 1A and 1B are diagrams showing an example of the configuration of a horizontal articulated robot, in which FIG. FIG. 11 is a waveform diagram illustrating vibrations of the tip of the robot when state feedback control is performed and when P-PI control is performed. 5A and 5B are pole-zero arrangement diagrams when state feedback control is performed. 13A and 13B are diagrams showing pole-zero arrangements when P-PI control is performed. FIG. 11 is a block diagram showing a configuration of a control device according to another embodiment.

Next, an embodiment of the present invention will be described with reference to the drawings. Figure 1 shows a control device according to an embodiment of the present invention.

The control device 10 shown in FIG. 1 is used for driving control of a multi-joint robot in which a motor 50 is provided for each joint axis to drive that axis. A link 60 is mechanically connected to the motor 50, which is driven by the motor 50 and applies a reaction force or disturbance to the motor 50. The link 60 shown in FIG. 1 does not only show the link or arm directly connected to the motor 50 of interest, but abstractly shows all parts of the robot that have a mechanical effect on the motor 50 of interest. Therefore, the link 60 shown in FIG. 1 includes, in addition to the arm of the robot, in particular, a reducer connected to the motor 50 and a workpiece 70 held by the robot (see FIG. 2). The motor 50 is a motor with an encoder, and the motor position is fed back to the control device 10. In FIG. 1, only the part used for controlling one joint axis in the multi-joint robot is shown, but in reality, a mechanism for controlling the motor 50 is provided for the motor 50 of each joint axis.

The control device 10 controls the position and speed of the motor 50 based on a command, for example, a motor position command. To control the position and speed of the motor 50, the control device 10 includes a state feedback control unit 11 that executes state feedback control of the motor 50 based on the motor position command, a single-inertia system control unit 12 that executes control based on a single-inertia system model of the motor 50 based on the motor position command, and a selection unit 13 that selects either the output of the state feedback control unit 11 or the output of the single-inertia system control unit 12. The state feedback control unit 11 receives feedback of the motor position and executes state feedback control by a two-inertia system state observer, for example, state feedback control described in Patent Document 3. The motor position is also fed back to the single-inertia system control unit 12, and executes, for example, P-PI control or PID control. The selection unit 13 selects the state feedback control unit 11 when the mass of the workpiece 70 held by the articulated robot is equal to or less than a threshold value, and selects the single-inertia system control unit 12 when the mass of the workpiece 70 exceeds the threshold value. The motor 50 is driven by a motor current based on the output selected by the selection unit 13.

Figure 2 shows an example of a multi-joint robot controlled by the control device 10 shown in Figure 1. The multi-joint robot shown here is a horizontal multi-joint robot used to transport a workpiece 70. In Figure 2, (a) is a plan view of the horizontal multi-joint robot, and (b) is a front view of the horizontal multi-joint robot. Figure 2(b) also shows the control device 10 connected to the robot. The illustrated robot includes a first link 60A that is attached at one end to a base 55 and extends horizontally, a second link 60B that is attached at one end to the other end of the first link 60A and extends horizontally, and a hand 61 that is attached at the other end of the second link 60B and serves as an end effector of the robot. The workpiece 70, which is the object to be transported, is placed on the hand 61. The tip of the hand 61 on the opposite side to the connection position with the second link 60B becomes the hand position of the robot.

The connection position between the base 55 and the first link 60A is joint A, and a motor 50A and a reducer 51A are provided in the base 55 to drive the joint A. The first link 60A is driven by the motor 50A via the reducer 51A to rotate in a horizontal plane with the joint A as the center of rotation. Similarly, the connection position between the first link 60A and the second link 60B is joint B, and a motor 50B and a reducer 51B are provided in the first link 60A to drive the joint B. The second link 60B is driven by the motor 50B via the reducer 51B to rotate in a horizontal plane with the joint B as the center of rotation. The connection position between the second link 60B and the hand 61 is joint C, and a motor 50C and a reducer 51C are provided in the second link 60B to drive the joint C. The hand 61 is driven by the motor 50C via the reducer 51C to rotate in a horizontal plane with the joint C as the center of rotation. As a result, in this robot, by driving motors 50A, 50B, and 50C, hand 61 can be moved in a horizontal plane toward or away from base 55 while maintaining the orientation of hand 61, as indicated by the arrow in the travel direction in the figure. The direction in the horizontal plane that is perpendicular to the travel direction is defined as the lateral direction. In the robot, the portion from joint A to hand 61 is a horizontal multi-joint mechanism. Furthermore, the robot is provided with a lifting mechanism 56 inside base 55 to raise and lower hand 61 in the vertical direction. Lifting mechanism 56 includes a ball screw and is driven by vertical-axis motor 50Z via reducer 51Z to raise and lower the horizontal multi-joint mechanism of the robot.

Next, the switching between state feedback control and control by a single-inertia system model in this embodiment will be described with reference to FIG. 3. Consider driving the motors 50A, 50B, and 50C in the robot shown in FIG. 2 to move the hand 61 back and forth along the travel direction shown in FIG. 2 between a state in which the first link 60A and the second link 60B are completely folded and a state in which the first link 60A and the second link 60B are fully extended in a straight line. The waveform shown in FIG. 3 (a) shows the time change in the position command related to the hand end position of the robot. Then, the velocity of vibration in the lateral direction at the hand end position when the hand 61 is moved back and forth in this way was examined. As the workpiece 70 held by the hand 61, one with a mass of 2 kg (corresponding to "workpiece mass: small") and one with a mass of 80 kg (corresponding to "workpiece mass: large") were used. In addition, the velocity of vibration in the lateral direction was examined in both cases where state feedback control and P-PI control are used for controlling the motors 50A, 50B, and 50C based on the position command. The results are shown in Figure 3 (b) to (e).

When state feedback control was used to control motors 50A, 50B, and 50C, a good vibration suppression effect was obtained if the mass of workpiece 70 was small, as shown in Figure 3(b). However, when the mass of workpiece 70 was large, the vibration did not converge, but diverged, as shown in Figure 3(c). In other words, it was found that when the mass of workpiece 70 was large, state feedback control made the control unstable. On the other hand, when P-PI control was used to control motors 50A, 50B, and 50C, a vibration suppression effect was obtained both when the mass of workpiece 70 was small (Figure 3(d)) and when the mass was large (Figure 3(e)). However, when the mass of workpiece 70 was small, state feedback control showed a better vibration suppression effect than P-PI control.

The change in control behavior according to the mass of the workpiece will be explained using Figures 4 and 5. Figures 4 and 5 show the pole-zero point arrangement from the position command of motor 50A of joint A to the angle of joint A in each of the cases shown in Figure 3 (b) to (e), and the pole surrounded by a dashed line rectangle in the figure is the representative pole. Figure 4 shows the case where state feedback control is used for motor control, with (a) being the pole-zero point arrangement diagram when the workpiece mass is small and (b) being the pole-zero point arrangement diagram when the workpiece mass is large. Figure 5 shows the case where P-PI control is used for motor control, with (a) being the pole-zero point arrangement diagram when the workpiece mass is small and (b) being the pole-zero point arrangement diagram when the workpiece mass is large.

In the case of a multi-joint robot, an inter-axis interference force occurs in the torque between the joints. When the mass of the workpiece is large, the inter-axis interference force becomes large, and each axis cannot be approximated by a two-inertia system, so single-inertia system control is more robust than state feedback control. When holding a light workpiece, as can be seen by comparing Figures 4(a) and 5(a), the representative pole from each axis motor position command value to each axis joint angle is farther away from the imaginary axis in the negative direction in state feedback control than in control based on a single-inertia system model (P-PI control). Therefore, state feedback control has higher vibration suppression performance. Conversely, when holding a heavy workpiece, as can be seen by comparing Figures 4(b) and 5(b), the real part of the representative pole is negative in control based on a single-inertia system model, but positive in state feedback control, which is in the so-called unstable region. Therefore, control based on a single-inertia system model is more stable.

In this embodiment, the motor control mode is switched to state feedback control when the mass of the workpiece 70 is small, and to control based on a single-inertia system model when the mass is large, thereby ensuring stability when a heavy workpiece is placed on the workpiece while ensuring vibration control performance when transporting a light workpiece. In state feedback control, control becomes unstable when holding a heavy workpiece, when the spring element component due to a reducer or the like in the system becomes large, or when the distance from the joint of interest to the workpiece becomes long. Therefore, in the robot shown in FIG. 2, the state feedback control according to the mass of the workpiece 70 and the control based on a single-inertia system model may be switched only for the motor far from the holding position of the workpiece 70, for example, only for the motor 50A of joint A, or only for the motors 50A and 50B of joints A and B.

In the embodiment described above, the motor is controlled by switching between state feedback control and control based on a single-inertia system model depending on the mass of the workpiece held by the robot, making it possible to suppress the occurrence of vibrations and perform stable control regardless of changes in the mass of the workpiece held by the robot.

Figure 6 shows a control device 10 according to another embodiment of the present invention. In general, the operation of a multi-joint robot is controlled by executing an operation program. The operation program of a robot that transports or holds a workpiece 70 defines at least a loading operation in which the workpiece 70 is held by, for example, the hand 61 of the multi-joint robot, and an unloading operation in which the held workpiece is removed from the multi-joint robot, and further defines the movement path of the robot. Assuming that the workpiece 70 is transported between user devices, in the loading operation, the robot 10 in a state in which it does not hold the workpiece 70 moves to a receiving position of the workpiece 70 in the user device from which the workpiece 70 is transported. When the workpiece 70 is actually held on the hand 61 of the robot 10 in the loading operation, for example, the workpiece 70 may be placed on the hand 61 by the movement of the robot 10 in the vertical axis direction, or the workpiece 70 may be placed on the hand 61 by the movement of the user device. When a gripping hand is used, the workpiece may be grasped by the hand. Similarly, in the unloading operation, the robot 10 holding the workpiece 70 moves to a loading/unloading position of the workpiece 70 in the user device to which the workpiece 70 is transported. When the workpiece 70 is actually unloaded from the hand 61 of the robot 10 in the unloading operation, the workpiece 70 may be unloaded onto the user device by the movement of the robot 10 in the vertical axis direction, or the workpiece 70 may be unloaded by the movement of the user device. When a gripping hand is used, the hand may release the workpiece that it is holding.

The control device 10 shown in FIG. 6 is similar to the control device 10 shown in FIG. 1, but further includes a calculation unit 14 that executes an operation program to generate a motor position command, and a storage unit 15 that stores the operation program. An operation program is generally created assuming that what processing is to be performed on what type of workpiece, for example, where the workpiece is to be transported from to where, is known, so the mass of the workpiece can be considered known when the operation program is executed. Therefore, if the operation program describes whether the motor is to be controlled by state feedback control or control based on a single-inertia system model, the calculation unit 14 can output a switching command to the selection unit 13 when the operation program is executed, and the selection unit 13 can switch between the output of the state feedback control unit 11 and the output of the single-inertia system control unit 12 based on the switching command.

When the robot is controlled by the operation program executed by the calculation unit 14, the state feedback control unit 11 is selected when the robot is moved to a receiving position for the workpiece 70 while not holding the workpiece 70, and then, when holding a heavy workpiece (a workpiece with a mass exceeding a threshold value), the output of the single inertia system control unit 12 can be selected by the selection unit 13 before the workpiece 70 and the hand 61 come into contact. By controlling in this way, it is possible to prevent the occurrence of vibration caused by holding a heavy workpiece 60 while performing state feedback control. Also, when the robot is controlled by the operation program executed by the calculation unit 14, the robot holding the workpiece 70 is moved to a loading/unloading position for the workpiece 70, and after performing an unloading operation, the output of the state feedback control unit 11 can be selected by the selection unit 13. By controlling in this way, state feedback control is implemented when the workpiece 70 is not held, and the robot can be moved quickly and stably.

When the calculation unit 14 outputs a switching command to the selection unit 13 based on the operation program, there is a risk that a programming error in the operation program will cause state feedback control to be executed when a heavy workpiece 70 is being held. In addition, in manual operation in which the robot is operated without relying on an operation program, it is preferable to be able to automatically switch between state feedback control and control based on a single-inertia system model. Therefore, the control device 10 of this embodiment can also be configured to measure or estimate the mass of the workpiece and switch between state feedback control and control based on a single-inertia system model.

As shown in FIG. 2, the control device 10 for controlling a robot equipped with a vertical axis motor 50Z is provided with a vertical axis control unit 16 for controlling the vertical axis motor 50Z. Even when the robot is not ascending or descending, the vertical axis motor 50Z generates a holding torque for holding the mass of the robot and workpiece 70, and the vertical axis control unit 16 generates a torque command value corresponding to the holding torque and drives the motor 50Z with a motor current corresponding to the torque command value. Since the mass of each part of the robot mechanism is known, the calculation unit 14 can determine the mass corresponding to the torque command value for the vertical axis motor and can determine the mass of the workpiece 70 by subtracting the mass of each part on the robot side from that mass, and can output a switching command according to the mass of the workpiece 70.

The robot may be configured such that a load sensor 62 is provided in the hand 61 to detect whether a workpiece 70 that satisfies a predetermined condition is being held, and the detection result of the load sensor 62 is sent to the calculation unit 14. When such a load sensor 62 is provided, when the load sensor 62 detects a workpiece that satisfies the predetermined condition, the selection unit 13 may select the output of the single-inertia system control unit 12 in response to a switching command. The load sensor may detect mass, but may also detect workpieces 70 whose thickness or planar dimensions are equal to or greater than a predetermined value. Since a workpiece with a large thickness or planar dimensions can be estimated to have a large mass, the robot can be prevented from being controlled by state feedback control when holding a heavy workpiece by switching between state feedback control and control based on a single-inertia system model according to the detection result of the load sensor 62.

By using the control device 10 shown in FIG. 6, the operating program can be used to switch between state feedback control and control based on a single-inertia system model, making it possible to switch between controls with less processing effort.

10...Control device; 11...State feedback control unit; 12...Single inertia system control unit; 13...Selection unit; 14...Calculation unit; 15...Memory unit; 50, 50A, 50B, 50C, 50Z...Motor; 51A, 51B, 51C, 51Z...Reduction gear; 55...Base; 56...Lifting mechanism; 60, 60A, 60B...Link; 61...Hand; 62...Load sensor; 70...Work.

Claims (14)

A control method for controlling an articulated robot in which joint shafts are driven by motors, comprising the steps of:
for at least one of the motors, switching control is performed according to the mass based on a motor position command such that a first control is performed when a mass of a workpiece held by the articulated robot is equal to or smaller than a threshold value, and a second control is performed when the mass exceeds the threshold value;
the first control is a state feedback control by a two-inertia system state observer using the motor position command and a fed-back motor position;
The second control is P-PI control or PID control using the motor position command and the fed back motor position . controlling the operation of the articulated robot based on an operation program that defines at least a loading operation for causing the articulated robot to hold the workpiece and an unloading operation for removing the held workpiece from the articulated robot;
The control method according to claim 1 , wherein the first control and the second control are switched by the operation program. The control method according to claim 2, further comprising the steps of: implementing the first control when the articulated robot not holding the workpiece is moved to a receiving position for the workpiece based on the operating program; and then switching to the second control before the workpiece comes into contact with the articulated robot if the mass exceeds the threshold value. 4. The control method according to claim 2 or 3, further comprising moving the articulated robot to a position for unloading the workpiece while holding the workpiece based on the operation program, executing the unloading operation, and then switching to the first control. A vertical axis motor is provided on the articulated robot,
The control method according to claim 1 , further comprising determining a mass of the workpiece based on a torque command value for the vertical axis motor. The articulated robot is provided with a presence sensor that detects whether the workpiece that satisfies a predetermined condition is being held,
The control method according to claim 1 , further comprising switching to the second control when the workpiece that satisfies the predetermined condition is detected by the load sensor. The control method according to any one of claims 1 to 6, wherein the articulated robot is a horizontal articulated robot. A control device for controlling an articulated robot in which joint shafts are driven by motors, comprising:
a first control unit that executes state feedback control by a two-inertia system state observer using the motor position command and a fed-back motor position based on the motor position command;
a second control unit that executes P-PI control or PID control using the motor position command and the fed-back motor position based on the motor position command;
a selection unit that selects the first control unit when a mass of a workpiece held by the articulated robot is equal to or smaller than a threshold value, and selects the second control unit when the mass exceeds the threshold value;
Equipped with
A control device that controls the motor using an output of a control unit selected by the selection unit in accordance with the mass. a calculation unit that executes an operation program to control the operation of the articulated robot by defining at least a loading operation for causing the articulated robot to hold the workpiece and an unloading operation for removing the held workpiece from the articulated robot;
The control device according to claim 8 , wherein the selection unit selects either the first control unit or the second control unit in response to a switching command from the calculation unit. The control device according to claim 9, wherein the calculation unit selects the first control unit by the selection unit when the articulated robot not holding the workpiece is moved to a position for receiving the workpiece, and then selects the second control unit by the selection unit before the workpiece and the articulated robot come into contact when the mass exceeds the threshold value. The control device according to claim 9 or 10, wherein the calculation unit moves the articulated robot to a position for unloading the work while holding the work, executes the unloading operation, and then selects the first control unit by the selection unit. The articulated robot is equipped with a vertical axis motor,
The control device according to claim 9 , wherein the calculation unit determines the mass of the workpiece based on a torque command value for the vertical axis motor , and outputs the switching command to the selection unit according to the determined mass . The articulated robot is provided with a presence sensor that detects whether the workpiece satisfies a predetermined condition,
The control device according to claim 8 , wherein the second control unit is selected when the workpiece that satisfies the predetermined condition is detected by the load sensor. The control device according to any one of claims 8 to 13, wherein the articulated robot is a horizontal articulated robot.

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