Abstract:
A servomotor control apparatus according to the present invention is a control apparatus for controlling a machine having one or a plurality of movable portions each driven by a servomotor. In the servomotor control apparatus, while an electromagnetic brake is operated, an actual position obtained by a position detector is periodically monitored. The actual position is compared with the actual position at the time of actuating the electromagnetic brake. When the actual positions are different by a predetermined value or more, any of the issue of a motor torque to maintain the position, the actuation of a regenerative brake, or a control means for continuing a control to maintain the position without operating the electromagnetic brake, even if the servomotor stops for a certain period of time during playback operation, is performed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a servomotor control apparatus, and in particular relates to a servomotor control apparatus that controls a machine having one or a plurality of movable portions each driven by a servomotor and that prevents the position of the machine from differing due to a brake malfunction. 
     2. Description of Related Art 
     In servomotor control apparatuses, a method for maintaining the posture of an arm by powering a motor, when an operation command is issued to the motor in a state in which a brake is operated and a reduction in braking performance is detected by a comparison between an operation amount of the motor and a predetermined threshold value, is proposed (for example, Japanese Unexamined Patent Publication (Kokai) No. 2005-254410). 
     However, according to this conventional art, the posture of the arm is maintained by powering the motor, only when the detection of the braking performance is performed and the reduction in the braking performance is detected. Thus, in the event that the brake is fully released due to a sudden brake failure or the like, the conventional art has no means to prevent the position of the machine from differing. 
     SUMMARY OF THE INVENTION 
     The present invention provides a servomotor control apparatus that prevents the position of a machine from differing due to a brake malfunction. 
     A servomotor control apparatus according to an embodiment of the present invention is a servomotor control apparatus that controls a machine having one or more movable portions each driven by a servomotor. In the servomotor control apparatus, in a state in which a servo amplifier is powered off and an electromagnetic brake is operated, an actual position obtained by a position detector is periodically monitored. When the actual position is different from an actual position at the time of actuating the electromagnetic brake by a predetermined value or more, an electromagnetic contactor of a servo feed circuit is closed to supply electric power to the servo amplifier and a motor torque command is issued to maintain the position. 
     A servomotor control apparatus according to another embodiment of the present invention is a servomotor control apparatus that controls a machine having one or more movable portions each driven by a servomotor. In the servomotor control apparatus, in a state in which a servo amplifier is powered on and an electromagnetic brake is operated, an actual position obtained by a position detector is periodically monitored. When the actual position is different from an actual position at the time of actuating the electromagnetic brake by a predetermined value or more, an electromagnetic contactor of a servo feed circuit is opened to power off the servo amplifier and a regenerative brake operation command is issued. 
     A servomotor control apparatus according to yet another embodiment of the present invention is a servomotor control apparatus that controls a machine having one or more movable portions each driven by a servomotor. In the servomotor control apparatus, in a state in which an electromagnetic brake is operated, an actual position obtained by a position detector is periodically monitored. In a case where the actual position is different from an actual position at the time of actuating the electromagnetic brake by a predetermined value or more, a control means that operates the electromagnetic brake and stops an issue of a motor torque is disabled, when the servomotor does not operate for a certain period of time. 
     Furthermore, in the servomotor control apparatus according to the above embodiments, the predetermined value, which is used for determining whether or not a current actual position fed back from the position detector is different from an actual position at the time of actuating the electromagnetic brake, is set lower than a position abnormality determination threshold value for determining a position abnormality between a command position of the servomotor and the actual position fed back from the position detector during normal operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, and advantages of the present invention will be more apparent from the following description of embodiments in conjunction with the attached drawings, wherein: 
         FIG. 1  is a block diagram of essential portions of a servomotor control apparatus and a control system for a servomotor according to a first embodiment of the present invention; 
         FIG. 2  is a flowchart of a malfunction detection in an electromagnetic brake and a subsequent step according to the first embodiment of the present invention; 
         FIG. 3  is a flowchart of a malfunction detection in an electromagnetic brake and a subsequent step according to a second embodiment of the present invention; 
         FIG. 4  is a block diagram of essential portions of a servomotor control apparatus and a control system for a servomotor according to a third embodiment of the present invention; and 
         FIG. 5  is a flowchart of a malfunction detection in an electromagnetic brake and a subsequent step according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A servomotor control apparatus according to the present invention will be described below with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a block diagram of a servomotor control apparatus and a control system for a servomotor according to a first embodiment of the present invention. A servomotor control apparatus  100  according to the first embodiment of the present invention includes one or more servomotors  11  each for driving a movable portion of a machine tool or robot (not-shown), an electromagnetic brake  12  for maintaining the position of the machine, a position detector  13  for feeding back an actual position signal, a servo amplifier  20  for driving the servomotor  11 , a servo feed circuit  30  for powering the servo amplifier  20 , and a central processing unit  40  for commanding and monitoring the servomotor  11 . 
     The servomotor  11  is provided with an encoder as the position detector  13 , which detects an actual position of the servomotor  11  and feeds back the actual position to the central processing unit  40 , and the electromagnetic brake  12  for maintaining a stop position. 
     The servo amplifier  20  includes a main circuit  21  constituted of a inverter circuit (not-shown), a capacitor as a charger and discharger, and the like, and a motor control circuit  22  for controlling the servomotor  11  based on an operation command issued from the central processing unit  40 . 
     The servo amplifier  20  converts a direct current voltage of a power supply  50 , which is supplied through the servo feed circuit  30 , into an alternating current voltage using the inverter circuit (not-shown) contained in the main circuit  21 , and supplies electric power to the servomotor  11 . 
     On the other hand, when operating a regenerative brake, energy is regenerated from the servomotor  11  to the servo amplifier  20 . The energy is charged into the capacitor (not-shown) in the main circuit  21  or consumed as heat by a resistor (not-shown) in the main circuit  21 , thus obtaining a torque to brake the servomotor  11 . 
     The servo feed circuit  30  includes an electromagnetic contactor  31  that switches the supply of electric power to the servo amplifier  20 . The servo feed circuit  30  opens and closes the electromagnetic contactor  31  in response to an open or close command from the central processing unit  40 . 
     The central processing unit  40  includes a motor controller  41 , a brake controller  42 , an actual position monitor  43 , and a feed circuit controller  44 . The motor controller  41  issues a control command for the servomotor  11 . The brake controller  42  issues a brake operation command for the electromagnetic brake  12 . The actual position monitor  43  periodically monitors an actual position based on actual position data fed back from the position detector  13  and determines a difference in position. The feed circuit controller  44  controls the open and close of the electromagnetic contactor  31  of the servo feed circuit  30  in order to control the supply of electric power to the servo amplifier  20 . 
     The control command generated by the motor controller  41  includes at least one of a motor torque command to maintain the position and a regenerative brake operation command. 
     The actual position monitor  43  determines a malfunction in the electromagnetic brake  12  by reference to actual positions of the servomotor  11  based on the brake operation command generated by the brake controller  42  and the actual position signal fed back from the position detector  13 . To be more specific, while the electromagnetic brake  12  is operated, the actual position monitor  43  periodically monitors an actual position and compares the actual position with an actual position at the time of actuating the electromagnetic brake  12 . When a difference equal to or more than a predetermined value is detected by the comparison, a malfunction is determined. 
     Referring to  FIG. 2 , a malfunction detection in the electromagnetic brake and a subsequent step by the servomotor control apparatus according to the first embodiment of the present invention will be described. 
     When the servo amplifier  20  is powered off and the electromagnetic brake  12  is operated to maintain the position, in step S 10 , the actual position monitor  43  periodically monitors an actual position obtained by the position detector  13  and compares the actual position with an actual position that is stored at the time of actuating the electromagnetic brake  12 . 
     When a difference equal to or more than a predetermined value is detected by the comparison, in step S 11 , the feed circuit controller  44  issues a command signal to close a contact of the electromagnetic contactor  31 , so that the servo amplifier  20  is powered on. The motor controller  41  issues a motor torque signal to maintain the position of the servomotor  11 . At this time, a signal indicating a malfunction may be outputted to inform a user of the malfunction. 
     According to the servomotor control apparatus of the first embodiment of the present invention, an actual position obtained by the position detector is periodically monitored. When the actual position is different from an actual position at the time of actuating the electromagnetic brake by a predetermined value or more, electric power is supplied to the servo amplifier and a motor torque is issued to maintain the position. Therefore, it is possible to maintain the position of the machine even if the electromagnetic brake is fully released due to a malfunction in the electromagnetic brake. 
     Second Embodiment 
     A servomotor control apparatus according to a second embodiment of the present invention will be described. The difference between the servomotor control apparatus according to the second embodiment of the present invention and the servomotor control apparatus according to the first embodiment is that in a state in which the servo amplifier  20  is powered on and the electromagnetic brake  12  is operated to maintain a position, when an actual position monitored by the position detector  13  is different from an actual position at the time of actuating the electromagnetic brake  12  by a predetermined value or more, the electromagnetic contactor  31  is opened to power off the servo amplifier  20  and a regenerative brake operation command is issued. The other configurations of the servomotor control apparatus according to the second embodiment of the present invention are the same as those of the servomotor control apparatus shown in  FIG. 1  according to the first embodiment of the present invention, so a detailed description thereof will be omitted. 
     Referring to  FIG. 3 , a malfunction detection in the electromagnetic brake and a subsequent step by the servomotor control apparatus according to the second embodiment of the present invention will be described. 
     In a state in which the servo amplifier  20  is powered on and the electromagnetic brake  12  is operated, in step S 20 , the actual position monitor  43  periodically monitors an actual position obtained by the position detector  13  and compares the actual position with an actual position that is stored at the time of actuating the electromagnetic brake  12 . 
     When a difference equal to or more than a predetermined value is detected by the comparison, in step S 21 , the feed circuit controller  44  issues a command signal to open the contact of the electromagnetic contactor  31 , so that the servo amplifier  20  is powered off. The motor controller  41  issues a regenerative brake operation command. At this time, a signal indicating a malfunction may be outputted to inform a user of the malfunction. 
     According to the servomotor control apparatus of the second embodiment of the present invention, an actual position obtained by the position detector is periodically monitored. When the actual position is different from an actual position at the time of actuating the electromagnetic brake by a predetermined value or more, the servo amplifier is powered off and the regenerative brake is actuated. Therefore, it is possible to maintain the position of the machine even if the electromagnetic brake is fully released due to a malfunction in the electromagnetic brake. 
     Third Embodiment 
       FIG. 4  is a block diagram of a servomotor control apparatus and a control system for a servomotor according to a third embodiment of the present invention. A servomotor control apparatus  101  according to the third embodiment of the present invention includes one or more servomotors  11  each for driving a movable portion of a machine tool or robot (not-shown), an electromagnetic brake  12  for maintaining the position of the machine, a position detector  13  for feeding back an actual position signal, a servo amplifier  20  for driving the servomotor  11 , and a central processing unit  40  for commanding and monitoring the servomotor  11 . 
     The servomotor  11  is provided with an encoder as the position detector  13 , which detects an actual position of the servomotor  11  and feeds back the actual position to the central processing unit  40 , and the electromagnetic brake  12  for maintaining a stop position. 
     The servo amplifier  20  includes a main circuit  21  constituted of a inverter circuit (not-shown), a capacitor as a charger and discharger, and the like, and a motor control circuit  22  for controlling the servomotor  11  based on an operation command from the central processing unit  40 . 
     The servo amplifier  20  converts a direct current voltage applied by a power supply  50  into an alternating current voltage using the inverter circuit (not-shown) in the main circuit  21 , and supplies electric power to the servomotor  11 . 
     On the other hand, when operating a regenerative brake, energy is regenerated from the servomotor  11  to the servo amplifier  20 . The energy is charged into the capacitor (not-shown) in the main circuit  21  or consumed as heat by a resistor (not-shown) in the main circuit  21 , thus obtaining torque to brake the servomotor  11 . 
     The central processing unit  40  includes a motor controller  41 , a brake controller  42 , an actual position monitor  43 , and a motor stop monitor  45 . The motor controller  41  generates a command position of the servomotor  11  and issues a control command. The brake controller  42  issues a brake operation command for the electromagnetic brake  12 . The actual position monitor  43  periodically monitors an actual position based on actual position data fed back from the position detector  13  and determines a difference in position. The motor stop monitor  45  periodically monitors the command position of the servomotor  11  generated by the motor controller  41  and determines a stop of the servomotor  11 . When the servomotor  11  is determined to be stopped, the motor stop monitor  45  commands the brake controller  42  to issue the brake operation command and commands the motor controller  41  to stop issuing a motor torque command for maintaining the position. The control command generated by the motor controller  41  includes at least one of the motor torque command to maintain the position and a regenerative brake operation command. 
     The actual position monitor  43  determines a malfunction in the electromagnetic brake  12  by reference to actual positions of the servomotor  11  based on the brake operation command generated by the brake controller  42  and the actual position signal fed back from the position detector  13 . To be more specific, while the electromagnetic brake  12  is operated, the actual position monitor  43  periodically monitors the actual position and compares the actual position with the actual position at the time of actuating the electromagnetic brake  12 . When a difference equal to or more than a predetermined value is detected by the comparison, a malfunction is determined. 
     The motor stop monitor  45  determines whether or not the servomotor  11  is stopped by reference to the brake operation command generated by the brake controller  42  and the command position of the servomotor  11  generated by the motor controller  41 . To be more specific, the motor stop monitor  45  periodically monitors the command position while the electromagnetic brake  12  is not operated. When the command position does not change for a certain period of time, the servomotor  11  is determined to be stopped. 
     Referring to  FIG. 5 , a malfunction detection in the electromagnetic brake and a subsequent step by the servomotor control apparatus according to the third embodiment of the present invention will be described. 
     In a state in which the electromagnetic brake  12  is operated, in step S 30 , the actual position monitor  43  periodically monitors an actual position obtained by the position detector  13  and compares the actual position with the actual position that is stored at the time of actuating the electromagnetic brake  12 . 
     When a difference equal to or more than a predetermined value is detected by the comparison, in step S 31 , the function of the motor stop monitor  45  is disabled. At this time, a signal indicating a malfunction may be outputted to inform a user of the malfunction. 
     In the embodiments described above, the predetermined value is preferably lower than a position abnormality determination threshold value for determining a position abnormality between the command position of the servomotor  11  and the actual position of the servomotor  11  fed back from the position detector  13  during normal operation. 
     According to the servomotor control apparatus of the third embodiment of the present invention, an actual position obtained by the position detector is periodically monitored. In a case where the actual position is different from the actual position at the time of actuating the electromagnetic brake by a predetermined value or more, a motor torque is continued to be outputted without operating the electromagnetic brake, even if the servomotor does not operate for a certain period of time. Therefore, in the event of detecting a malfunction in the electromagnetic brake even once, it is possible to prevent the electromagnetic brake from being operated unintendedly and the output of a motor torque from being stopped unintendedly, before they happen. 
     Moreover, in the servomotor control apparatuses according to the embodiments of the present invention, the threshold value to determine whether or not the servomotor operates is set lower than the position abnormality determination threshold value used during the normal operation, thus allowing the quick detection of a change in the position of the machine.