JP2014065086A - Robot system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a robot system capable of restraining an element of an inverter from being damaged, even if an excessive load-applied state to a driven part of a robot continues.SOLUTION: The robot system 10 comprises an inverter circuit 30 having a robot 11 having a motor 52 and an arm 50 and a bridge circuit for connecting a set of two switching elements in parallel, connecting the switching elements SW1-SW3 to the high potential side of a power source 14, connecting the switching elements SW4-SW6 to the low potential side of the power source 14 and respectively connecting the mutual two switching elements in respective series connection bodies to a U phase, a V phase and a W phase of the motor 52 and a control part 60 for switching the switching elements put in a closed state in order while maintaining a position of the arm 50 on condition that an electric current of flowing to one phase among the U phase, the V phase and the W phase is 0 and an equal state of the electric current flowing to the residual two phases continues by exceeding a predetermined period.

Description

  The present invention relates to a robot system that applies a voltage to a motor of a robot by a three-phase inverter.

  Conventionally, when a collision between an arm (driven part) of a robot and an object is detected, the current to the motor is stopped and the motor is stopped by a brake (for example, see Patent Document 1).

JP2012-110971

  By the way, in the thing of patent document 1, when a collision is not detected even if the arm of a robot collides with an object, an electric current continues flowing into a motor in the state where the operation of the arm was forcibly stopped. It becomes. In general, a motor for driving a robot arm is supplied with electric power from a three-phase inverter in which six switching elements are connected in a three-phase bridge and a full-wheel diode is connected in parallel with each switching element. In the above case, the three-phase inverter may be balanced in a state in which current flows only by a specific phase. As a result, an excessive current continues to flow through some of the elements that control the applied voltage in the inverter, which may damage the elements.

  Also, even when the arm is stationary while holding a workpiece that is heavier than the predetermined weight, it is balanced in a state where current flows only by a specific phase of the inverter, and an excessive current is applied to some elements of the inverter. May continue to flow.

  The present invention has been made in view of such circumstances, and its main purpose is to suppress damage to the elements of the inverter even if an excessive load continues on the driven part of the robot. It is to provide a robot system capable of performing the above.

  The present invention employs the following means in order to solve the above problems.

  The first means is a robot system, a robot having a motor and a driven part driven by the motor, a series connection body of a first switching element and a fourth switching element, a second switching element, and a second switching element. 5. A bridge circuit in which a serial connection body with 5 switching elements and a serial connection body with a third switching element and a sixth switching element are connected in parallel, and the first, second, and third switching elements are power sources. The fourth, fifth, and sixth switching elements are connected to the low potential side of the power source, and the U-phase, V of the motor is between two switching elements in each series connection body. By controlling the open / closed state of the switching element and the three-phase inverter connected to the phase and the W phase respectively, the voltage applied to each phase of the motor is controlled. And the current flowing in one of the U phase, V phase, and W phase is 0, and the current flowing in the remaining two phases is equal for more than a predetermined period. And the control unit that sequentially switches the switching elements to be closed while maintaining the position of the driven unit.

  As described above, when the driven part of the robot collides with an obstacle and is forcibly stopped, or when the driven part holds a workpiece that is heavier than a predetermined weight and stops, only a specific phase in the inverter May cause equilibrium in the state of current flow.

  In this regard, according to the above configuration, the current flowing in one of the U phase, V phase, and W phase of the motor is 0, and the current flowing in the remaining two phases is equal to or longer than a predetermined period. When it continues, the switching element to be closed is sequentially switched while the position of the driven part is maintained. Therefore, while maintaining the position of the driven part that is stopped, it is possible to sequentially switch the elements that generate heat by energization and to distribute the thermal load to each element. As a result, even if the excessive load is applied to the driven part, it is possible to prevent the inverter element from being damaged.

  In the second means, the control unit maintains the position of the driven unit by sequentially switching the switching elements to be closed at a speed at which the operation of the driven unit cannot follow.

  According to the above configuration, the position of the driven unit is maintained by sequentially switching the switching elements to be closed at a speed at which the operation of the driven unit cannot follow, that is, at a high speed faster than a predetermined speed. Generally, in a robot arm (driven portion), the rotation of a motor is decelerated by a speed reducer. For example, when the motor rotates three times (1080 ° rotation), the arm rotates only 1 °. For this reason, even if the motor is rotated forward and reverse alternately within 3 rotations, the arm hardly moves in appearance. Therefore, by switching the switching element at a high speed, the operation of the driven part can be prevented from following the switching of the switching element, and the position of the driven part can be maintained. As a result, the time during which the temperature of the energized element continuously rises can be shortened, the maximum temperature of each element can be suppressed, and the thermal load on each element can be reduced.

  In the third means, the control unit switches the switching unit to a closed state while maintaining the position of the driven unit, on the condition that a current flowing through at least one of the switching elements is larger than a rated current. Switch elements in order.

  According to the above configuration, in performing the control to sequentially switch the switching elements to be closed while maintaining the position of the driven part, it is further necessary that the current flowing in at least one of the switching elements is larger than the rated current. Is done. For this reason, when the electric current which flows into a switching element is below a rated electric energy, it can suppress performing the said control unnecessarily.

  In a fourth means, the control unit closes the first, third, and fifth switching elements as an aspect of sequentially switching the switching elements to be closed while maintaining the position of the driven part. And a state in which the second, third, and fourth switching elements are closed, and a state in which the first, second, and sixth switching elements are closed.

  According to the above configuration, since the first to sixth switching elements are switched to the open state after being switched to the closed state twice at the latest, it is possible to suppress the heat load from being concentrated on a specific element. Furthermore, since the fifth, fourth, and sixth switching elements through which currents from two phases flow are switched to the closed state with two open states after the closed state, the temperature of the element having a large heat load. The rise can be suppressed.

  Specifically, as in the fifth means, in the control unit, the current flowing in one phase of the U phase, the V phase, and the W phase is 0, and the currents flowing in the remaining two phases are equal. When the state continues for longer than a predetermined period, it is possible to adopt a configuration in which it is determined that the driven unit has collided with an obstacle and the operation is forcibly stopped.

  Further, specifically, as in the sixth means, the control unit is configured such that the current flowing in one phase of the U phase, the V phase, and the W phase is 0 and the current flowing in the remaining two phases. A configuration may be adopted in which it is determined that the driven portion is in a stationary state while holding a work heavier than a predetermined weight when the state where the two are equal is longer than a predetermined period.

The block diagram which shows a robot system. The time chart which shows an electric current lock state. The graph which shows the temperature of the switching element in an electric current lock state. The flowchart which shows the process sequence of heat dispersion | distribution control. The circuit diagram which shows the aspect which switches the switching element of a closed state in order.

  Hereinafter, an embodiment will be described with reference to the drawings. This embodiment is embodied as a robot system that controls the operation of an articulated robot.

  FIG. 1 is a block diagram showing a robot system 10. The robot system 10 includes a robot 11, a power supply unit 20, an inverter circuit 30, a control unit 60, and the like. The robot 11 includes an arm 50 having a plurality of joints. The power supply unit 20, the inverter circuit 30, and the control unit 60 constitute a robot controller.

  The power supply unit 20 (voltage adjusting unit) is connected to a three-phase 200V commercial AC power supply 14 (power supply). The power supply unit 20 includes a rectifier circuit, a coil, a capacitor 21, switching elements 22, 23, and the like. The capacitor 21 is connected in parallel to the power supply lines 15a and 15b. The switching elements 22 and 23 are formed of a field effect transistor (FET) or the like. The switching element 22 is connected in series to the power supply lines 15a and 15b, and the switching element 23 is connected in parallel to the power supply lines 15a and 15b. Then, the power supply unit 20 performs the PWM control (duty control) of the switching element 23 with the switching element 22 turned on, thereby increasing the height of the power supply voltage Vb supplied from the power supply 14 to the power supply lines 15a and 15b. Adjust.

  A series connection body in which a resistor 25 (regenerative resistor) and a switching element 26 (switch unit) are connected in series is connected in parallel to the power supply lines 15a and 15b. The switching element 26 is formed of a field effect transistor or the like. A voltage sensor 27 is connected in parallel to the power supply lines 15a and 15b. The voltage sensor 27 (voltage detection unit) detects the power supply voltage Vb between the power supply lines 15a and 15b.

  A plurality of inverter circuits 30 (only one is shown) are connected in parallel to the power supply lines 15a and 15b. A plurality of motors 52 that respectively drive a plurality of rotating portions 51 in the arm 50 (driven portion) are connected to the plurality of inverter circuits 30 (three-phase inverter). Each motor 52 is provided with a speed reducer that decelerates the rotation of the motor 52. For example, when the motor 52 rotates three times (1080 ° rotation), the rotating unit 51 rotates 1 °. Each motor 52 is provided with an encoder 53 (position detection unit) that detects a rotational position.

  Each inverter circuit 30 is a known circuit in which six switching elements SW1 to SW6 are connected in a three-phase bridge, and a flywheel diode 32 is connected in parallel with each switching element. Switching elements SW1 to SW6 are formed of bipolar power transistors or the like. Each inverter circuit 30 includes a current sensor 35 that detects a current supplied to each motor 52.

  Specifically, the inverter circuit 30 is a bridge circuit in which a serial connection body of switching elements SW1 and SW4, a serial connection body of switching elements SW2 and SW5, and a serial connection body of switching elements SW3 and SW6 are connected in parallel. The switching elements SW1 to SW3 are connected to the positive side (high potential side) of the power source 14, and the switching elements SW4 to SW6 are connected to the negative side (low potential side) of the power source 14 and GND. Further, the switching element SW <b> 1 and the switching element SW <b> 2 in the series connection body are connected to the U phase of the motor 52. Similarly, the switching element SW2 and the switching element SW5 in the series connection body are connected to the V phase of the motor 52, and the switching element SW3 and the switching element SW6 in the series connection body are connected to the W phase of the motor 52. It is connected.

  In each inverter circuit 30, the switching elements SW <b> 1 to SW <b> 6 are PWM-controlled, whereby the voltage supplied from each inverter circuit 30 to each motor 52 is controlled. A current sensor 35 detects a current supplied to each motor 52. In addition, when two switching elements of one serial connection body, for example, switching elements SW1 and SW4 are simultaneously closed, a short-circuit state is established. For this reason, when one switching element of one series connection body is made into a closed state, the other switching element is made into an open state.

  The control unit 60 includes a power supply control unit 61, a trajectory generation unit 62, a position / speed control unit 63, a current control unit 64, a PWM circuit 65, a drive circuit 66, and the like.

  The output of the encoder 53 provided in each motor 52 is input to the trajectory generation unit 62, the position / speed control unit 63, and the current control unit 64, respectively. The trajectory generating unit 62 is based on the operation performed by the arm 50 and the rotational position of each motor 52 detected by each encoder 53 (the rotational position of each rotating unit 51). A target rotation position of the motor 52 (target rotation position of each rotating unit 51) is calculated. The position / speed control unit 63 determines the target rotation speed of each motor 52 (each rotation unit 51 based on the target rotation position of each motor 52 calculated by the trajectory generation unit 62 and the detected rotation position of each motor 52. Target rotation speed) and target torque of each motor 52 are calculated. The current control unit 64 is based on the target torque of each motor 52 calculated by the position / speed control unit 63, the current detected by each current sensor 35, and the detected rotational position of each motor 52. To calculate the target current to be supplied to the motor 52. The PWM circuit 65 outputs a PWM signal for controlling the switching elements SW <b> 1 to SW <b> 6 of the inverter circuits 30 based on the target current calculated by the current control unit 64. The drive circuit 66 opens and closes each switching element of each inverter circuit 30 based on the PWM signal output from the PWM circuit. Thereby, the drive of each motor 52 is controlled so that each motor 52 (arm 50) is operated to the target rotation position (target position).

  Here, when the arm 50 of the robot 11 collides with an obstacle, the arm 50 cannot move any further and may be forced to stop. In that case, since the rotational position of each motor 52 does not change, the state of opening and closing each switching element of each inverter circuit 30 does not change. For this reason, in the inverter circuit 30, it balances in the state which sends an electric current only by a specific phase, and it will be in the electric current lock state to which the phase which sends an electric current was fixed. Further, since the deviation between the target rotational position of each motor 52 and the detected rotational position of each motor 52 increases, the position / speed control unit 63 determines the target rotational speed of each motor 52 and the target torque of each motor 52. Increase. For this reason, the target current supplied from the inverter circuit 30 to the motor 52 is increased by the current control unit 64.

  FIG. 2 is a time chart showing a current lock state. For example, when the motor 52 is balanced in a state where current flows from the U phase to the W phase, the switching elements SW1 and SW6 are closed, and the switching elements SW2 to SW5 are opened. As a result, as shown in the figure, the voltage applied to the switching elements SW1 and SW6 becomes a high voltage (for example, 280V), and the current flowing through the switching elements SW1 and SW6 becomes a large current (for example, 10A). On the other hand, the voltage applied to the switching elements SW2 to SW5 becomes a low voltage (for example, 0 V), and the current flowing through the switching elements SW2 to SW5 becomes a small current (for example, 0 A). Therefore, the temperature of the switching elements SW1 and SW6 rises excessively, while the temperature of the switching elements SW2 to SW5 falls. In normal control of the inverter circuit 30, the switching elements SW1 to SW6 are driven to open and close so that the sum of the currents flowing in the U phase, the V phase, and the W phase becomes zero.

  FIG. 3 is a graph showing the temperatures of the switching elements SW1 and SW6 in the current lock state of FIG. As indicated by a broken line in the figure, when the current lock state is established in the conventional control, the temperatures of the switching elements SW1 and SW6 in the closed state may rise with time and exceed the upper limit temperature Th. . Therefore, in the present embodiment, when the current lock state is established, heat dispersion control is performed in which the switching elements to be opened are sequentially switched at high speed while maintaining the position of the arm 50 of the robot 11. As a result, heat is generated by being dispersed in the switching elements SW1 to SW6, and the temperature of the switching elements SW1 to SW6 can be made lower than the upper limit temperature Th as shown by a solid line in FIG. In addition, it can prevent that the arm 50 falls by gravity by generating the torque which maintains the position of the arm 50. FIG.

  FIG. 4 is a flowchart showing a processing procedure of heat dispersion control. This series of processing is repeatedly executed by the control unit 60 at a predetermined cycle.

  First, currents flowing through the U phase, the V phase, and the W phase are detected (S10). Specifically, currents flowing in the U phase, the V phase, and the W phase are detected based on the detection value of the current sensor 35.

  Subsequently, it is determined whether or not the magnitude of the current flowing in the U phase is equal to the magnitude of the current flowing in the W phase (S11). In this determination, when it is determined that the current flowing in the U phase is equal to the current flowing in the W phase (S11: YES), it is determined whether or not the current flowing in the V phase is 0 (S12). ). In this determination, if it is determined that the current flowing in the V phase is 0 (S12: YES), whether or not the magnitude of the current flowing in the U phase (= W phase) is greater than the rated current (predetermined current). (S13). In this determination, when it is determined that the magnitude of the current flowing in the U phase is larger than the rated current, the state in which S11 to S13 are all affirmed (S11 to S13: YES) continues beyond a predetermined period. It is determined whether or not (S14). This predetermined period is set to a period during which it can be determined that there is a possibility that the switching elements SW1 to SW6 may be damaged by continuing a state where a current exceeding the rated current flows through the switching elements SW1 to SW6. ing.

  If S11 to S14 are all positive (S11 to S14: YES), it is determined that the arm 50 of the robot 11 is colliding with an obstacle (S15). Then, the process which switches the switching element made into a closed state in order at high speed is performed (S16).

  Specifically, as shown in FIG. 5, the states (a) to (c) are sequentially switched at a speed at which the operation of the arm 50 cannot follow. (A) is a state where the switching elements SW2, SW4, SW6 do not generate heat, that is, a state where the switching elements SW1, SW3, SW5 are closed. In this state, current flows from the U phase to the V phase as indicated by the broken line, and current flows from the W phase to the V phase as indicated by the alternate long and short dash line. (B) is a state in which the switching SW1, SW5, SW6 does not generate heat, that is, a state in which the switching elements SW2, SW3, SW4 are closed. In this state, current flows from the V phase to the U phase as indicated by the broken line, and current flows from the W phase to the U phase as indicated by the alternate long and short dash line. (C) is a state where the switching elements SW3, SW4, SW5 do not generate heat, that is, a state where the switching elements SW1, SW2, SW6 are closed. In this state, current flows from the V phase to the W phase as indicated by the broken line, and current flows from the U phase to the W phase as indicated by the alternate long and short dash line. And after switching to (a), (b), (c) in order, it switches to (a) again.

  Returning to FIG. 4, if negative in any of S11 to S14 (NO in any of S11 to S14), the process according to S11 to S14 is performed for different combinations of U phase, V phase, and W phase. Execute (S21 to S24). That is, there is a state in which the current magnitudes of the two phases of the U phase, the V phase, and the W phase are equal to each other, the current of the remaining one phase is 0, and the current magnitude exceeds the rated current. Then, it is determined whether or not it has continued beyond a predetermined period. If S21 to S24 are all affirmed (S21 to S24: YES), it is determined that the arm 50 of the robot 11 is colliding with an obstacle (S15). Then, the process which switches the switching element made into an open state in order at high speed is performed (S16).

  If the result is negative in any of S21 to S24 (NO in any of S21 to S24), the process according to S11 to S14 is executed for further different combinations of the U phase, the V phase, and the W phase. (S31-S34). If S31 to S34 are all affirmed (S31 to S34): YES, it is determined that the arm 50 of the robot 11 is colliding with an obstacle (S15). Then, the process which switches the switching element made into an open state in order at high speed is performed (S16).

  And while it determines with the arm 50 of the robot 11 colliding with the obstruction in S15, the process of S16 is continued and performed. If the result is negative in any of S31 to S34 (NO in any of S31 to S34), the process of S16 is stopped, and this series of processes is temporarily ended (END).

  The embodiment described in detail above has the following advantages.

  When the current flowing in one of the U-phase, V-phase, and W-phase of the motor 52 is 0 and the current flowing in the remaining two phases is the same for more than a predetermined period, the robot 11 The switching elements to be closed are sequentially switched while the position of the arm 50 is maintained. Therefore, while maintaining the position of the arm 50 that has been stopped, the elements that generate heat when energized can be sequentially switched to distribute the thermal load to each switching element. As a result, even if an excessive load is applied to the arm 50, the switching elements SW1 to SW6 of the inverter circuit 30 can be prevented from being damaged.

  The position of the arm 50 is maintained by sequentially switching the switching elements to be closed at a speed at which the operation of the arm 50 cannot follow, that is, at a speed higher than a predetermined speed. In the arm 50, the rotation of the motor 52 is decelerated by the speed reducer. When the motor 52 rotates three times (1080 ° rotation), the rotating portion 51 of the arm 50 rotates 1 °. For this reason, even if the motor 52 is rotated forward and reverse alternately within three rotations, the rotating portion 51 hardly moves in appearance. Therefore, by switching the switching element at high speed, the operation of the rotating unit 51 can be prevented from following the switching of the switching element, and the position of the rotating unit 51 can be maintained. As a result, the time during which the temperature of the energized switching element continuously rises can be shortened, the maximum temperature of each switching element can be suppressed, and the thermal load on each switching element can be reduced.

  Further, when the switching elements SW1 to SW6 are switched, lost motion or friction is generated in the gears of the reduction gear or the like even if the rotating unit 51 of the arm 50 tries to start the operation from the stationary state. For this reason, the time constant (reaction time) of the electrical part and the time constant (reaction time) of the mechanical part differ by about 1000 times in time order. For example, the time constant of the mechanical part is on the order of several ms compared to the time constant of the electrical part that reacts on the order of several μs to ns. Also from this point, the switching element to be closed can be easily switched at a speed that the operation of the arm 50 cannot follow.

  In the control for sequentially switching the switching elements to be closed while maintaining the position of the arm 50, it is further required that the current flowing in at least one of the switching elements SW1 to SW6 is larger than the rated current. For this reason, when the electric current which flows into switching element SW1-SW6 is below a rated electric energy, it can suppress performing heat distribution control unnecessarily.

  -As shown to Fig.5 (a)-(c), since switching element SW1-SW6 is switched to an open state after switching to a closed state twice at the latest, a thermal load concentrates on a specific switching element. This can be suppressed. Furthermore, since the switching elements SW5, SW4, SW6 through which currents from the two phases flow are switched to the closed state with two open states after being closed, the switching elements SW5, SW4, SW4, SW4, SW4, SW4, SW4, SW4, SW4, SW4 The temperature rise of SW6 can be suppressed.

  The arm 50 becomes an obstacle when the current flowing in one of the U phase, V phase, and W phase is 0 and the current flowing in the remaining two phases is longer than the predetermined period. It can be determined that the operation is forcibly stopped due to a collision.

  Note that the above embodiment can be modified as follows.

  When the current flowing in one phase of the U phase, the V phase, and the W phase is 0 and the current flowing in the remaining two phases is equal to or longer than a predetermined period, the control unit 60 It can also be determined that 50 is in a stationary state while holding a workpiece heavier than a predetermined weight. Even in such a case, it is preferable to execute heat dispersion control because the current may be locked.

  In the flowchart of FIG. 4, the processes of S13, S23, and S33 may be omitted, and the predetermined periods of S14, S24, and S34 may be variable according to the magnitudes of the U-phase, U-phase, and V-phase currents, respectively. Specifically, it is preferable to shorten the predetermined period as the current of each phase is larger. In this case, it can be appropriately determined that each switching element may be damaged depending on the magnitude of the current flowing in each phase.

  In the above embodiment, as a mode for maintaining the position of the arm 50, the states of FIGS. 5A to 5C are sequentially switched at a speed at which the operation of the arm 50 cannot follow. You may switch at the speed of

  In the above embodiment, the states of FIGS. 5A to 5C are sequentially switched, but the switching elements SW1 and SW6 are closed, the state of FIG. 5A, and the state of FIG. 5B. And may be switched in order. In this case, the switching elements SW1 and SW6 are switched from the closed state to the opened state of the switching element SW6 in FIG. 5A, and then the switching element SW1 in FIG. 5B is switched to the opened state. . Therefore, when the switching elements SW1 and SW6 are closed and the current lock state is established, the switching elements SW1 and SW6 can be sequentially opened. Moreover, switching element SW1-SW6 can also be sequentially switched to a closed state (open state) with another combination.

  DESCRIPTION OF SYMBOLS 10 ... Robot system, 11 ... Robot, 14 ... Power supply, 30 ... Inverter circuit (three-phase inverter), 50 ... Arm (driven part), 52 ... Motor, 60 ... Control part, SW1 ... Switching element (1st switching element) ), SW2 ... switching element (second switching element), SW3 ... switching element (third switching element), SW4 ... switching element (fourth switching element), SW5 ... switching element (fifth switching element), SW6 ... switching element (Sixth switching element).

Claims (6)

A robot having a motor and a driven part driven by the motor;
A series connection body of the first switching element and the fourth switching element, a series connection body of the second switching element and the fifth switching element, and a series connection body of the third switching element and the sixth switching element are in parallel. Having a connected bridge circuit, wherein the first, second and third switching elements are connected to the high potential side of the power supply, and the fourth, fifth and sixth switching elements are connected to the low potential side of the power supply; A three-phase inverter in which the two switching elements in each series connection body are respectively connected to the U phase, V phase, and W phase of the motor;
A control unit that controls a voltage applied to each phase of the motor by switching an open / close state of the switching element, and a current flowing in one of the U phase, the V phase, and the W phase is zero. And the control unit that sequentially switches the switching elements to be closed while maintaining the position of the driven unit, on condition that the state where the currents flowing in the remaining two phases are equal to each other continues for a predetermined period of time. When,
A robot system comprising:   The robot system according to claim 1, wherein the control unit maintains the position of the driven unit by sequentially switching the switching elements to be closed at a speed at which the operation of the driven unit cannot follow.   The control unit sequentially switches the switching elements to be closed while maintaining the position of the driven part, on the condition that a current flowing in at least one of the switching elements is larger than a rated current. The robot system according to 1 or 2.   As a mode in which the control unit sequentially switches the switching elements to be closed while maintaining the position of the driven part, the control unit closes the first, third, and fifth switching elements, and The robot system according to claim 1 or 2, wherein a state in which the third and fourth switching elements are closed and a state in which the first, second, and sixth switching elements are closed are sequentially switched.   When the current flowing in one phase of the U phase, the V phase, and the W phase is 0 and the current flowing in the remaining two phases is equal to or longer than a predetermined period, the control unit The robot system according to any one of claims 1 to 4, wherein the driven unit collides with an obstacle and determines that the operation is forcibly stopped.   When the current flowing in one phase of the U phase, the V phase, and the W phase is 0 and the current flowing in the remaining two phases is equal to or longer than a predetermined period, the control unit The robot system according to claim 1, wherein the driven unit is determined to be in a stationary state while holding a workpiece heavier than a predetermined weight.

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