Servo motor monitoring unit

Abstract: The invention relates to a monitor system for a servo controller used in connection with servo motors utilized in multivarious machines, such as machine tools. In addition to detecting a servo system fault when the difference between the commanded position and the actual position exceeds a predetermined value, the system monitors several servo system parameters to distinguish between different causes of servo faults. Identifying the source of the fault leads to improved maintenance and decreased system down time when a fault occurs.

Description: This invention relates to a servo motor monitoring unit for monitoring a servo motor which drives a load, such as a machine tool, and more particularly to a servo motor monitoring unit with a fault detection and cause determination function. The PCT publication WO 88/05570 discloses a process and device for monitoring control elements such as safety devices in an automobile. Position command signals from a microprocessor as well as feedback position signals from the control element are fed to a common discrimination circuit. The received signals are continuously compared with set signals. When deviations are found, switching signals are generated and supplied to the microprocessor. The UK patent application GB-A-2 006 990 discloses a method and device for monitoring a controlled variable. The monitoring device comprises a logic unit which compares command signals with detected position and rotary speed signals of a motor. The device further comprises means for forming the first differential of the controlled variable, for example position or speed, with respect to time. Comparison means compare this differential value with predetermined command parameters and indicate a fault when a predetermined deviation is present. In the prior art, a troubleshooting unit is also known for use with a motor controller (see Japanese Patent Disclosure Publication No. 291682 of 1989) which comprises a plurality of status observers for selectively monitoring control signals, e.g. voltage, current, speed and other signals, and estimating the disturbance torque of a motor in different modes. A fault location is guesstimated from the estimated values. Since such unit employs a plurality of status observers, the constants of the motor must be exactly known. In general, however, the motor constants are easily affected by individual differences and temperature, leading to errors. In addition, what is essential in controlling a servo motor is whether the actual position is tracking the position commands. For this purpose, it is necessary to continuously compare the position command and a position detection feedback signal incoming from a position detector, to provide an alarm such as "excessive error", "excessive deviation," or the like if the difference therebetween is larger than a predetermined threshold value, and to alert the operator to any fault. The above unit, however, does not monitor the position itself, which is an essential factor in monitoring a servo motor. On the other hand, prior art is also known for monitoring position. As shown in Fig. 6, a servo motor monitoring unit 601 comprises a counting section 601a for receiving a position command signal P R and a position detection feedback signal P F and operating on a difference therebetween, and a range determining section 601b for determining fault if the difference obtained by the counting section 601a is greater than a predetermined threshold value, and outputting a fault alarm such as "excessive error." Referring to Fig. 6, numeral 602 indicates a servo motor, 603 a position detector for detecting the position of the servo motor 602, 604 a position command generator for outputting the position command signal, and 605 a servo controller for controlling the driving of the servo motor 602 in accordance with the position command signal P R and the position detection feedback signal P F . The operation of the unit configured as described above will now be described. The servo controller 605 compares the position command signal P R output by the position command generator 604 and the position detector 603 and controls the drive current of the servo motor 602. The position detector 603 outputs the position detection feedback signal P F in accordance with the operation of the servo motor 602. In the servo motor control system as described above, the position detection feedback signal P F cannot track the position command signal P R when: (1) the load is too heavy to generate acceleration; (2) the polarity of the position detection feedback signal P F from the position detector 603 is reversed; and (3) electrical connections to the servo motor 602 are improper. In any of such cases 1 to 3, the servo motor monitoring unit 601 causes the counting section 601a to operate on the difference between the position command signal P R and the position detection feedback signal P F , and causes the range determining section 601B to compare that difference with a predetermined threshold value, determine that a fault has occurred if the difference is larger than the threshold value, and output a fault alarm. Fig. 7 is a flowchart illustrating the sequence of said operation. First, the difference D between the position command signal P R and the position detection feedback signal P F is found (step 701). Then, whether the difference D is within the range of the predetermined threshold value is determined (step 702). The fault alarm "excessive error" is output if the difference D is outside the threshold value range (step 703). On the other hand, if the difference D is within that range, the operation returns to step 701 and repeats processing. The known servo motor monitoring unit may be able to determine the occurrence of a fault in accordance with the difference D between the position command signal P R and the position detection feedback signal P F , but cannot determine the cause thereof. It cannot determine whether the difference D has increased due to insufficient torque because the machine (load) is too heavy or has collided with an obstacle, or due to opposite servo because of incorrect connection to the servo motor, or because the feedback of the equipment has been connected reversely. Hence, when the fault alarm "excessive error" is output, the cause of the fault must be investigated, taking much time. It is, accordingly, an object of the present invention to overcome the disadvantages in the prior art by providing a servo motor monitoring unit which allows the servo motor controller to be easily restored in a short time when the difference D between the position command signal P R and the position detection feedback signal P F is determined to be excessive. According to the present invention, a servo motor monitoring device is provided as defined in claim 1. Fig. 1 illustrates a servo motor monitoring unit according to one embodiment of the present invention; Fig. 2 is a flowchart of the operation for the servo motor monitoring unit of one embodiment of the present invention, and Fig. 2(a) is a flowchart illustrating operations in an alternative embodiment; Fig. 3 is a timing chart of a servo control system during normal operation; Fig. 4 is a timing chart of a servo control system when the torque is insufficient; Fig. 5 is a timing chart of the servo control system when servo is opposite; Fig. 6 illustrates a servo motor monitoring unit of the prior art. Fig. 7 is a flowchart of the operation of the servo motor monitoring unit of the prior art; and Fig. 8 is a timing chart illustrating the servo control system when the controlled machine has collided with an object. An embodiment of a servo motor monitoring unit according to the present invention will now be described in detail with reference to the drawings. Fig. 1 illustrates the configuration of a servo system to which the servo motor monitoring unit 101 has been applied. The servo motor monitoring unit 101 comprises a counting section 101a for receiving a position command signal P R and a position detection feedback signal P F and determining the difference D therebetween, a range determining section 101b for determining fault if the difference D obtained by the counting section 101a is greater than a predetermined threshold value, and a determining section 101c for determining the cause of fault occurrence in accordance with a sign of the position detection feedback signal P F (i.e., the sign of the acceleration of this signal) and that of a current feedback value P 0 . In Fig. 1, numeral 102 indicates a servo motor; 103, a position detector for detecting the position of the servo motor 102; 104, a position command generator for outputting position command signals, and 105, a servo controller for controlling the power delivered to the servo motor 102 in accordance with the position command signal P R and the position detection feedback signal P F . The operation of the servo motor monitoring unit according to the present embodiment configured as described above will now be described in greater detail. Referring to Fig. 1, the servo controller 105 compares the position command signal P R output by the position command generator 104 and the position detection feedback signal P F output by the position detector 103 to control the current used in driving the servo motor 102. As the servo motor 102 runs, the position detector 103 outputs the position detection feedback signal P F accordingly. In the meantime, the servo motor monitoring unit 101 causes the counting section 101a to determine the difference between the position command signal P R and the position detection feedback signal P F , cause the range determining section 101B to compare that difference with a predetermined threshold value and determine the occurrence of fault if the difference is larger than the threshold value. Further, the determining section 101c compares the sign b of the acceleration of the position detection feedback signal P F and that of the current feedback value P 0 , and determines the cause of the excessive position error as "insufficient torque" if the signs match, or "opposite servo" if the signs do not match. Fig. 2 is a flowchart illustrating the sequence of the above operation. First, the difference D between the position command signal P R and the position detection feedback signal P F is found (step 201). Then, whether the difference D is within a given range of a predetermined threshold value (determination value) or not is determined (step 202), and the sign b of the acceleration of the position detection feedback signal P F is compared with the current feedback sign (sign of the current feedback value P 0 ) if the difference D is outside the threshold value range (step 203). If the above signs do not match, an "opposite servo" fault alarm is output (step 204), or if they match, an "insufficient torque" fault alarm is output (step 205). On the other hand, if the difference D is within the threshold value range, the operation returns to step 201 and repeats processing. The basis for determining "insufficient torque" and "opposite servo" in the sign determining section 101C will now be described with reference to graphs shown in Figs. 3, 4 and 5. Fig. 3 illustrates the waveforms of the output signals (position command signal P R , position detection feedback signal P F and current feedback value P 0 ) provided by the corresponding portions of the servo control system when operating without fault, and also shows the acceleration of the position feedback signal. Note that the above embodiment assumes that a positive current flows when the servo motor accelerates in the forward direction. The top graph in Fig. 3 gives the relationship between the position command signal P R and position detection feedback signal P F , the next graph indicates the difference therebetween, and the bottom graph indicates the change in the current feedback value P 0 . Fig. 4 provides an example of "insufficient torque," wherein acceleration is initiated at t 0 , but due to a current limitation at t 1 , the position detection feedback signal P F cannot track the position command signal P R normally, and the difference D therebetween exceeds the threshold value at t A , resulting in a fault alarm. Since the sign of the position detection feedback signal (i.e., the sign of the acceleration of this signal) matches that of the current feedback value P 0 in this case, the cause of the fault can be determined as "insufficient torque". Fig. 5 indicates an example of "opposite servo," wherein the position command signal P R has been output at t 0 , but the motor runs abnormally in a direction opposite to the command of the position command signal P R due to positive feedback caused by opposite servo, and the difference D exceeds the threshold value, resulting in a fault alarm. Since the sign of the position detection feedback signal (i.e., the sign of the acceleration of this signal) does not match that of the current feedback value P 0 in this case, the cause of the fault can be determined as "opposite servo". In the above embodiment, it has been assumed that positive current feedback (i.e. the current feedback value P 0 is positive) flows when the motor is accelerated in the forward direction. When the opposite assumption is made, it will be appreciated that the cause of the fault will be determined as "opposite servo" if the position detection feedback signal sign a matches the current feedback value P 0 sign, and as "insufficient torque" if they do not match. In the present embodiment, inability to accelerate the servo motor due to a heavy machine load is not differentiated from collision of the machine with an obstacle and both are determined as "insufficient torque." Since the acceleration of the position detection feedback signal suddenly drops toward zero at the time of collision, it is also possible to distinguish "machine collision" from other "insufficient torque" situations if the acceleration value falls below a certain threshold value. A flowchart showing the operation of this alternative is shown in Fig. 2(a), and a timing chart is depicted in Fig. 8. In this example, the possibility of collision is checked by calculating the acceleration value within the difference detection routine (see step 202(a)), and branching at step 202(b) if the acceleration falls below a given threshold. In this instance, machine collision is discriminated and a "machine collision" fault alarm is raised. The flowchart of Fig. 2(a) is otherwise the same as that of Fig. 2. That, is, if the acceleration has not dropped, comparison of the acceleration sign with that of the current feedback is carried out at step 203. In this alternative embodiment, determining section 101c performs the additional function of acceleration value threshold comparison. As described above, a servo motor monitoring unit includes fault determining means for determining a fault in servo motor operating status and the cause of the fault in accordance with a position command signal, a position detection feedback signal and the feedback value of motor current supplied to said servo motor. The monitoring unit therefore allows the servo motor controller to be easily restored within a short period when the difference D between the position command signal P R and position detection feedback signal P F becomes excessive. In other words, the servo motor monitoring unit provides quick troubleshooting at occurrence of any fault, ensuring improved maintenance performance.

Claims: A servo motor monitoring device comprising; a position detector (103) for monitoring a position of the servo motor (102) and for generating a position detection feedback signal (P f ) ; detecting means (101a) for detecting a difference between a position command signal (P r ) issued from a position command generator (104) to a servo controller (105) for said servo motor (102) and said position detection feedback signal (P F ) ; and range determining means (101b) for comparing said difference with a predetermined range of acceptable difference values (D) and, when said difference is outside of said range, generating a fault signal indicative of a fault; and characterized by sign determining means (101c) responsive to said fault signal for comparing the sign (b) of the second derivative (acceleration) of the position detection feedback signal (P f ) with the sign of the current (Po) supplied to the servo motor (102), said current (Po) being fed to the sign determining means (101c) as a current reference value from the servo controller (105), wherein said sign determining means (101c) is adapted to discriminate the fault based on the comparison of signs by outputting a first fault alarm indicating a heavy load condition ("insufficient torque") if the signs match and by outputting a second fault alarm indicating a reverse servo condition ("opposite servo") if the signs do not match. The device of claim 1, wherein said servo motor (102) is used to control the position of at least one moveable element of a machine and wherein said determining means (101c) comprises means for discriminating between a heavy load condition ("insufficient torque") and a machine collision condition by detecting a sudden drop in the second derivative (acceleration) of said position detection feedback signal (P f ).

Inventors: KAZATO HIROHIKO, KAZATO, HIROHIKO, Kazato, Hirohiko, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: G05B19
IPC codes: G05B19, H02P29