Detection method and detection device

Abstract

The present invention addresses the problem of providing a detection method and a detection device that suppress the occurrence of erroneous detection and quickly detect out-of-control of a servo-controlled motor. In the present invention, a combination determination logic is provided that obtains a binary index or a ternary index for each of a plurality of items selected from a command value and a state value obtained with respect to a motor, and determines the presence or absence of runaway of the motor on the basis of a combination of the indexes. The combination determination logic includes, for example, symbol inconsistency determiners (41-44), AND circuits (45-47), an OR circuit (48), and selectors (51, 52).

Description

TECHNICAL FIELD

[0001] The present application relates to a method and apparatus for detecting runaway in a servo motor. BACKGROUND

[0002] A servo motor is controlled and driven based on a position command or a speed command input to a motor control device. The control of the servo motor requires information on the accurate position (rotation angle) of the motor, and therefore, an encoder that detects the position of the motor is installed on the rotation shaft of the motor. As the encoder, an encoder including a nonvolatile memory that stores the rotation angle of the motor even when the power is turned off is used, but for some reason, the data in the nonvolatile memory is broken, and thus, there are cases where the rotation angle data of the motor stored in the encoder disappears or the rotation angle indicated by the data in the encoder deviates from the actual rotation angle of the motor. If the servo control of the motor is started in such a case, there is a case where the motor runs against the command input to the motor control device, that is, the motor runs out of control. Such a runaway is a runaway based on the mismatch between the motor and the encoder. Even if it is not the breakage of the rotation angle data, there are cases where the failure of the encoder and the like cause the servo motor to run out of control. In the runaway of the motor, the motor continues to output the maximum torque thereof or starts to rotate with a steep acceleration. Such a runaway is very dangerous, and it is necessary to quickly detect the occurrence of the abnormality and stop the motor, but in the past, it takes time from the start of the runaway to the detection, and the braking distance before the motor is stopped is extended, and there is a risk of an accident of damage to an article or the like.

[0003] Patent Literature 1 discloses that in a case where the speed deviation is equal to or greater than a threshold value, the sign of the product of the torque command value obtained from the integral value of the speed deviation and the acceleration is negative, and the cumulative value of the speed deviation is equal to or greater than a prescribed value, it is determined that the degree of runaway is equal to or greater than a prescribed degree. Patent Literature 2 discloses that the runaway is detected when the speed deviation continuously increases. Patent Literature 3 discloses that the motor speed is input, and the motor speed or a value made based on the motor speed is compared with a threshold value set in advance, and thereby it is determined whether the operation of the motor is in a normal state. Patent Literature 4 relates to a technology for detecting the reverse runaway in a door driven by a linear motor, and discloses that the speed of the door after a prescribed time is predicted from the acceleration detection value and the speed detection value, and it is determined that the abnormality occurs when the predicted speed exceeds a set speed that is positive or negative. Similarly, Patent Literature 5 relates to a technology for detecting the reverse runaway in a door driven by a linear motor, and discloses that it is determined that the abnormality occurs when the speed command value exceeds a first set speed and the speed detection value exceeds a second set speed whose polarity is opposite to that of the speed command value.

[0004] [Related Art Documents]

[0005] [Patent Literature]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open (JP A) No. 2007-209080

[0007] [Patent Document 2] Japanese Patent Application Laid-Open (JP A) No. H10-290521

[0008] [Patent Document 3] Japanese Patent Application Laid-Open (JP A) No. 2004-318509

[0009] [Patent Document 4] Japanese Patent Application Laid-Open (JP A) No. 2012-135212

[0010] [Patent Document 5] Japanese Patent Application Laid-Open (JP A) No. 2008-271705 SUMMARY

[0011] [PROBLEMS TO BE SOLVED BY THE INVENTION]

[0012] The technologies described in Patent Documents 1 to 2 are technologies that calculate a cumulative value of a speed deviation, or detect a runaway of a motor by detecting a continuous increase in a speed deviation value. In principle, a certain time is required until an abnormality is detected. Furthermore, depending on the form of the runaway of the motor, there are cases in which the speed deviation does not increase drastically, and thus it is difficult to quickly detect a runaway of the motor by the technologies described in Patent Documents 1 to 2. The technology described in Patent Document 3 relies on a comparison of the motor speed with a threshold value, and thus depending on the form of the runaway, it is not necessarily possible to quickly detect a runaway. The technologies described in Patent Documents 4 to 5 are technologies related to detection of a reverse-rotation runaway of a linear motor, and thus it is difficult to directly apply the technologies to detection of a runaway in a general rotary motor. Furthermore, the technologies described in Patent Documents 1 to 5 all detect a runaway by comparison with a threshold value, and depending on the setting of the threshold value, there are cases in which a runaway is not appropriately detected, or false detection increases. Furthermore, adjustment of the threshold value takes a large amount of time, and if the number of threshold values to be set increases, the amount of work for adjustment also increases accordingly.

[0013] An object of the present application is to provide a method and a device that can suppress occurrence of false detection and quickly detect a runaway of a servo motor.

[0014] [TECHNICAL MEANS FOR SOLVING THE PROBLEMS]

[0015] According to an embodiment of the present application, a method of detecting a runaway of a motor in a system that performs servo control of the motor based on a command value is a method of calculating a binary index or a ternary index for each of a plurality of items selected from the command value and a state value obtained with respect to the motor, and detecting the runaway based on a combination of the indexes.

[0016] According to another embodiment of the present application, a detection device that detects runaway of a motor in a system that servo-controls the motor based on an instruction value includes combination determination logic that respectively calculates a binary index or a ternary index for a plurality of items selected from the instruction value and a state value obtained with respect to the motor, and determines the presence or absence of runaway based on a combination of the indexes.

[0017] [Effects of the Invention]

[0018] According to the present application, it is possible to suppress occurrence of false detection, and to rapidly detect runaway in a servo motor. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a block diagram showing the structure of a motor system that is a form of application of the runaway detection method.

[0020] Figure 2 is a block diagram that explains the logic structure of the runaway detection section.

[0021] BRIEF DESCRIPTION OF DRAWINGS

[0022] 10: AC power supply

[0023] 20: motor control device

[0024] 21: full-wave rectifier circuit

[0025] 22: inverter circuit (INV)

[0026] 23, 24: current sensor

[0027] 26: control operation section

[0028] 27: current control section

[0029] 28: runaway detection section

[0030] 30: motor

[0031] 31: encoder

[0032] 32: power line

[0033] 41-44: sign inconsistency determiner

[0034] 45-47: AND circuit

[0035] 48: OR circuit

[0036] 49, 50: on-delay circuit

[0037] 51, 52: selector DETAILED DESCRIPTION

[0038] Next, a mode for carrying out the present application will be described with reference to the drawings. Figure 1 is a block diagram showing the configuration of a motor system that is a mode of carrying out the present application. The illustrated motor system receives a supply of alternating current from an alternating current power supply 10 and drives a motor 30 based on a command (at least one of a position command and a speed command) input from the outside. In order to control the motor 30 based on the command, a motor control device 20 is provided. An encoder 31 that detects the position (i.e., the rotational angle) of the motor 30 is attached to the output shaft of the motor 30, and data indicating the position detected by the encoder 31 is fed back to the motor control device 20. In the case where the motor 30 is a motor that drives each axis of a robot, the motor system described here is called a robot system, and the motor control device 20 is called a robot control device or a robot controller that controls the robot.

[0039] There are various modes of the motor control device 20 that controls the motor 30, and the runaway detection method based on the present application is able to detect the runaway of the motor 30 in any of these control modes. The motor 30 can be an alternating current motor or a direct current motor. Hereinafter, each motor 30 will be assumed to be a three-phase synchronous motor (or a three-phase induction motor), and the runaway detection in the case where the motor 30 is controlled by vector control or the like will be described.

[0040] The motor control device 20 includes a full-wave rectification circuit 21 that rectifies alternating current received from an alternating current power supply, and an inverter circuit 22 that is supplied with direct current from the full-wave rectification circuit 21 and generates three-phase alternating current that drives the motor 30. The voltage of the direct current output by the full-wave rectification circuit 21 is referred to as a main circuit power supply voltage. The outputs of the u-phase, v-phase, and w-phase of the inverter circuit 22 are connected to the motor 30 via the power lines 32 of each phase. Current sensors 23, 24 are provided in the power lines 32 of the u-phase and v-phase, respectively. Further, the motor control device 20 includes a control calculation section 26, a current control section 27 provided in each motor 30 to drive the switching elements in the inverter circuit 22, and a runaway detection section 28. A position command or a speed command from the outside of the motor control device 20 is input to the control calculation section 26, and the control calculation section 26 outputs a current command (for example, a d-axis current command and a q-axis current command) for driving the motor 30 based on a command value such as the input position command or speed command, and position data input from an encoder 31 attached to the motor 30. In the control calculation section 26, a command value such as a voltage command, a torque command, or the like, also referred to as an internal command value, is calculated in order to perform control of the motor 30. The current command is also classified as an internal command value. The current command is input to the current control section 27. To the current control section 27, a detected value of the position of the motor 30 is input from the encoder 31, and detected values of the currents of each phase of the motor 30 are input from the current sensors 23, 24. The current control section 27 calculates the electrical angle of the motor 30 from the position of the motor 30, and controls the switching elements in the inverter circuit 22 based on the current values of each phase and the input current command, to control the three-phase output from the inverter circuit 22.

[0041] In addition, there is a rated value such as a rated power of the inverter circuit 22 or the motor 30, and regardless of the voltage command or current command calculated in the control calculation section 26, the motor 30 cannot be driven if the rated value is exceeded. If the motor 30 rotates, the motor 30 generates a counter electromotive force, and if the counter electromotive force exceeds the main circuit power supply voltage, current does not flow from the inverter circuit 22 to the motor 30 regardless of the voltage command or current command related to the motor 30. These states can be considered to be states in which the output of the motor control device 20 is saturated for the motor 30. Saturation occurs in the control calculation section 26. The current command for the motor 30 is generated based on the torque command, and therefore if saturation occurs with respect to the current command, it can be considered that torque is not generated in the motor 30 in accordance with the torque command value, and that torque is limited.

[0042] The runaway detection unit 28 detects the occurrence of a runaway in the motor 30 connected to the motor control device 20. As described above, even if the detection of a runaway is intended to be performed only on the basis of a speed deviation or a rotational speed of the motor, it is difficult to reduce false detection and promptly perform the detection of a runaway. Therefore, in the present embodiment, in a case where the motor 30 is servo-controlled on the basis of a command value, a binary or ternary index is acquired for each of a plurality of items related to the command value and a state value obtained with respect to the motor 30, and on the basis of a combination of the acquired indices, it is determined whether or not the motor 30 has a runaway. The command value is present in two kinds of a command value externally given to the motor control device 20 and a command value internally generated by the motor control device 20. As the command value that can be used in the present embodiment, for example, there are a position command value, a speed command value, a torque command value, a current command value, a voltage command value, and the like. As the state value obtained with respect to the motor, for example, there are a position (position feedback value) measured by the encoder 31 or a speed (speed feedback value) calculated therefrom, a duty ratio of pulse width modulation (PWM) in the inverter circuit 22, whether or not saturation occurs in the motor 30 or a limit with respect to a torque command, and the like.

[0043] If the motor 30 is not in a runaway and is normally operated, depending on a control mode of the motor 30, the command value or the state value obtained with respect to the motor 30 indicates a substantially determined behavior. Therefore, after a plurality of command values or state values are respectively set to binary indices or ternary indices including 0, it is confirmed under a combination condition obtained by using AND (AND gate) conditions, whereby it is possible to determine whether or not a behavior expected from the command values or the state values is performed, and it is possible to promptly detect the presence or absence of an abnormality, specifically, the presence or absence of a runaway in the motor 30. The index of each item is preferably determined in a manner that can eliminate arbitrariness in threshold setting as much as possible. Therefore, as the index, there are a binary index indicating the presence or absence of a value, a binary index indicating whether or not a value is 0, a binary index indicating whether or not a value is positive or negative, a ternary index indicating whether or not a value is positive, 0, or negative, an index indicating whether or not a command value has saturation or a limit, and the like. When a ternary index is used, not zero is equivalent to positive or negative. In the present embodiment, it is preferable to detect the presence or absence of a runaway in the motor 30 using three or more binary or ternary indices.

[0044] If the influence of noise and the like is taken into consideration, even if the value is not strictly 0, it is necessary to consider the value as 0 as long as it is within a certain range including 0, particularly in a value based on a measured value. Therefore, in order to determine whether the value is 0, a threshold value is necessary in a strict sense, but basically, in the present embodiment, in order to obtain a two- or three-valued index, a threshold value that can be arbitrarily set is not used. Instead, in the present embodiment, a threshold value can be set with respect to the time for which the state in which the combined condition for determining the occurrence of runaway is established is continued, and it is determined that runaway has occurred in the case where the duration of the combined condition exceeds the threshold value. Depending on noise or overshoot in servo control and the like, there can be a case where the combined condition is momentarily established even if the motor 30 is not actually in a runaway state, and therefore, in such a case, it is preferable to set a threshold value with respect to the duration.

[0045] Next, a specific example will be described. As the plurality of indexes, for example, a three-valued index indicating whether the speed command value is positive, 0, or negative, a two-valued index indicating whether the speed feedback value is positive or negative, a two-valued index indicating whether the differential value of the speed feedback value, that is, the acceleration, is positive or negative, and a two-valued index indicating whether the torque command value is positive or negative can be used. Whether the differential value of the speed feedback value is positive or negative can actually be determined by whether the difference between the present speed feedback value and the previous speed feedback value is positive or negative. These command values or state values are input from the control operation portion 26 to the runaway detection portion 28. The runaway detection portion 28 determines that the motor 30 is in a runaway state when the combination shown in Table 1 is established among the indexes.

[0046] [Table 1]

[0047] Abnormality 1 Abnormality 2 Abnormality 3 Abnormality 4 Speed command value Positive Negative 0 0 Speed feedback value Negative Positive Negative Positive Differential value of speed feedback value Negative Positive Negative Positive Torque command value Positive Negative Positive Negative

[0048] Abnormity 1 and Abnormity 2 are obtained depending on whether the rotation direction in which the motor 30 is intended to move is the forward direction or the reverse direction, that is, the sign of the speed command value, and substantially indicate the same state. In the state, the sign of the speed feedback value is different from that of the speed command value, and therefore, the motor 30 rotates in the direction opposite to the intended rotation direction, and further, the sign of the speed feedback value is the same as that of the differential value of the speed feedback value, and therefore, the rotation of the motor 30 accelerates. Also, the torque command value is a value intended to rotate the motor in the intended rotation direction, and it can be evaluated that the motor 30 is in a runaway state in Abnormity 1 and Abnormity 2.

[0049] Abnormity 3 and Abnormity 4 are obtained depending on whether the actual rotation direction of the motor 30 indicated by the sign of the speed feedback value is the forward direction or the reverse direction, and substantially indicate the same state. In the state, although the speed command value is 0 and the motor 30 is intended to be kept in a stopped state, the motor 30 rotates and the rotation accelerates. Also, the sign of the torque command value is different from that of the speed feedback value, and becomes a direction that hinders the current rotation of the motor 30, and it can be evaluated that the motor 30 is in a runaway state in Abnormity 3 and Abnormity 4.

[0050] Figure 2 The structure of the runaway detection section 38 for detecting the runaway of the motor 30 based on the combination of the indexes shown in Table 1 will be described, and the logic structure of the runaway detection section 28 will be shown. The runaway detection section 28 includes a combination determination logic for determining whether the combination conditions shown in Table 1 are satisfied, and can be implemented as hardware in which discrete parts are combined, or as software in which a microprocessor or microcomputer is used to execute operations equivalent to the logic structure shown in Table 1. Figure 2 Figure 2 The structure of the runaway detection section 38 for detecting the runaway of the motor 30 based on the combination of the indexes shown in Table 1 will be described, and the logic structure of the runaway detection section 28 will be shown. The runaway detection section 28 includes a combination determination logic for determining whether the combination conditions shown in Table 1 are satisfied, and can be implemented as hardware in which discrete parts are combined, or as software in which a microprocessor or microcomputer is used to execute operations equivalent to the logic structure shown in Table 1.

[0051] The position command value is input to the control operation section 26, and if the position of the motor 30 is fed back from the encoder 31, the speed command value is the difference between the last input position command value and the present input position command value, and the speed feedback value is the difference between the last input position feedback value and the present input position feedback value. The runaway detection section 28 includes sign disagreement determiners 41 to 44, AND circuits 45 to 47, an OR circuit 48, an on-delay circuit 49, an on-delay circuit 50, and selectors 51 and 52.

[0052] Each of the sign disagreement determiners 41 to 44 has two inputs, and outputs true if the signs of the two input values are different, and outputs false if they are the same. False is output when either of the inputs, or both of the inputs, is zero. The sign disagreement determiner 41 is input with the speed command value and the speed feedback value, and the sign disagreement determiner 42 is input with the speed feedback differential value and the torque command value. The AND circuit 45 outputs the logical product of the outputs of the sign disagreement determiners 41 and 42. The output of the AND circuit 45 is true when either of the conditions 1 or 2 is satisfied, and is false otherwise. That is, the output of the AND circuit 45 is true when the runaway occurs in a state where the speed command value is not zero. The output of the AND circuit 45 is input to the on-delay circuit 49. The on-delay circuit 49 outputs the output of the AND circuit 45 after delaying a predetermined on-delay time, but is reset if the output of the AND circuit 45 becomes false during the on-delay time. As a result, the output of the on-delay circuit 49 also becomes true only when the output of the AND circuit 45 is true for a length of the on-delay time or more.

[0053] ​Further, the output of the AND circuit 45 becomes true when the speed command value is positive, the speed feedback value is negative, the speed feedback differential value is positive, and the torque command value is negative. The state does not conform to the abnormality 1 or the abnormality 2, and also, it is not a state where the motor 30 is out of control, but there is a possibility that it becomes such a state temporarily due to an overshoot of the control system or the like. Therefore, the selector 51 that outputs true when the torque command is limited and outputs false otherwise is provided, and it is determined that the motor 30 is out of control only when the torque command is limited. The AND circuit 46 outputs the logical sum of the output of the on-delay circuit 49 and the output of the selector 51. The output of the AND circuit 46 is true when the conditions of the abnormality 1 or the abnormality 2 are satisfied and the motor 30 is out of control, and false otherwise.

[0054] The sign disagreement determiner 43 is inputted with the speed feedback value and the torque command value, and the sign disagreement determiner 44 is inputted with the torque command value and the speed feedback differential value. The AND circuit 47 outputs the logical product of the output of the sign disagreement determiner 43 and the output of the sign disagreement determiner 44. Instead of the sign disagreement determiner 44, the output of the sign disagreement determiner 42 can be inputted to the AND circuit 47. The selector 52 outputs the output of the AND circuit 47 when the speed command value is 0, and outputs false otherwise. The output of the selector 52 is true when either of the condition 3 or the condition 4 is satisfied, and false otherwise. That is, the output of the selector 52 is true when the motor 30 is out of control in a state where the speed command value is 0. The output of the selector 52 is inputted to the on-delay circuit 50. The on-delay circuit 50 has the same function as the on-delay circuit 49, and the output of the on-delay circuit 50 becomes true only when the output of the selector 52 is true for the length of the on-delay time. The on-delay times of the two on-delay circuits 49 and 50 are set respectively. As an example, the on-delay time in the on-delay circuit 50 is set to be longer than the on-delay time in the on-delay circuit 49.

[0055] The OR circuit 48 outputs the logical sum of the output of the AND circuit 46 and the output of the on-delay circuit 50. The output of the OR circuit 48 is true when either of the abnormality 1 to the abnormality 4 is satisfied, and false otherwise. When the output of the OR circuit 48 is true, it indicates a state where the motor 30 is out of control, and is outputted to the outside as a detection result from the out-of-control detection section 28.

[0056] The above has described an example of the structure for implementing the present application, but the technology can employ a structure as follows.

[0057] (1) A detection method for detecting an out-of-control of a motor in a system that performs servo control of the motor based on a command value, in the detection method,

[0058] for a plurality of items selected from the command value and a state value obtained with respect to the motor, a binary index or a ternary index is respectively calculated,

[0059] and the runaway is detected based on a combination of the indexes.

[0060] (2) The detection method according to (1), wherein the binary index includes at least one of an index indicating presence or absence of a value, an index indicating whether a value is zero or not, an index indicating whether a value is positive or negative, and an index indicating whether saturation or limitation has occurred,

[0061] the ternary index is an index indicating whether a value is positive, zero, or negative.

[0062] (3) The detection method according to (1) or (2), wherein when the combination of the indexes satisfies a prescribed condition for a first time, it is determined that the runaway has occurred.

[0063] (4) The detection method according to (3), wherein the sensitivity of detection is adjusted by adjusting the length of the first time.

[0064] (5) The detection method according to any one of (1) to (4), wherein the runaway is caused by a mismatch between the motor and an encoder connected to the motor or by a failure of the encoder.

[0065] (6) The detection method according to any one of (1) to (5), wherein with respect to the motor, it is determined that the runaway has occurred when a speed command value is zero, a speed feedback value and a speed feedback differential value have the same sign, and the speed feedback value and a torque command value have different signs, and when a speed command value and a torque command value have the same sign, the speed feedback value and the speed feedback differential value have different signs with respect to the speed command value.

[0066] (7) A detection device that detects a runaway of a motor in a system in which the motor is servo-controlled based on a command value,

[0067] the detection device includes combination determination logic that, for a plurality of items selected from the command value and a state value obtained with respect to the motor, respectively calculates a binary index or a ternary index, and determines the presence or absence of the runaway based on a combination of the indexes.

[0068] (8) The detection device according to (7), wherein the binary index includes at least one of an index indicating presence or absence of a value, an index indicating whether a value is zero or not, an index indicating whether a value is positive or negative, and an index indicating whether saturation or limitation has occurred,

[0069] The three-value index is an index indicating whether the value is positive, 0, or negative.

[0070] (9) The detection device according to (7) or (8), wherein the output of the combination determination logic includes an on-delay circuit, and when the combination determination logic determines that the runaway state has continued for a first time, informs an outside that the runaway has occurred.

[0071] (10) The detection device according to (9), wherein the first time is adjustable in order to adjust the sensitivity of the detection.

[0072] (11) The detection device according to any one of (7) to (10), wherein the runaway is caused by a mismatch between the motor and an encoder connected to the motor or by a failure of the encoder.

[0073] (12) The detection device according to any one of (7) to (11), wherein the combination determination logic is configured in such a manner that at least one of the following can be detected with respect to the motor: a speed command value is zero, a speed feedback value and a speed feedback differential value have the same sign, and the speed feedback value and a torque command value have different signs; and the speed command value and the torque command value have the same sign, and the speed feedback value and the speed feedback differential value have different signs with respect to the speed command value.

[0074] According to the structures of (1), (7), by using a plurality of binary or three-value indexes, it is possible to quickly determine whether the system is performing the expected action, thereby suppressing false detection and quickly detecting a runaway of the motor.

[0075] According to the structures of (2), (8), the indexes are obtained while trying to eliminate arbitrariness in threshold setting, so it is possible to balance suppression of false detection and quick detection of a runaway, and prevent a decrease in workability that accompanies adjustment work of a plurality of thresholds.

[0076] According to the structures of (3), (9), it is possible to remove the possibility of false detection caused by noise or overshoot, and according to the structures of (4), (10), it is possible to achieve adjustment of the detection sensitivity by adjustment of only the first time, which is a single parameter.

[0077] When the runaway is caused by a mismatch between the motor and an encoder connected to the motor or by a failure of the encoder, as described in (5), (11), it is possible to effectively perform detection of the runaway. According to the structures of (6), (12), a combination of indexes suitable for detection of a runaway of the motor is obtained, it is possible to further suppress false detection, and more quickly detect a runaway.

Claims

1. A detection method of detecting runaway of a motor in a system in which the motor is servo-controlled based on command values, in the detection method, for a plurality of items selected from the command values and state values obtained with respect to the motor, a binary index or a ternary index is respectively calculated, and the runaway is detected based on a combination of the indexes.

2. The detection method according to claim 1, wherein the binary index includes at least one of an index indicating presence or absence of a value, an index indicating whether a value is zero or not, an index indicating whether a value is positive or negative, and an index indicating whether saturation or limitation has occurred, the ternary index is an index indicating whether a value is positive, zero, or negative.

3. The detection method according to claim 1 or 2, wherein when the combination of the indexes satisfies a prescribed condition for a first time, it is determined that the runaway has occurred.

4. The detection method according to claim 3, wherein the sensitivity of detection is adjusted by adjusting the length of the first time.

5. The detection method according to claim 1 or 2, wherein the runaway is caused by a mismatch between the motor and an encoder connected to the motor or by a failure of the encoder.

6. The detection method according to claim 1 or 2, wherein with respect to the motor, it is determined that the runaway has occurred when a speed command value is zero, a speed feedback value and a speed feedback differential value have the same sign, and the speed feedback value and a torque command value have different signs, and the speed command value and the torque command value have the same sign, the speed feedback value and the speed feedback differential value have different signs with respect to the speed command value.

7. A detection device of detecting runaway of a motor in a system in which the motor is servo-controlled based on command values, the detection device includes combination determination logic that, for a plurality of items selected from the command values and state values obtained with respect to the motor, calculates a binary index or a ternary index, and determines the presence or absence of the runaway based on a combination of the indexes.

8. The detection device according to claim 7, wherein the binary index includes at least one of an index indicating presence or absence of a value, an index indicating whether a value is zero or not, an index indicating whether a value is positive or negative, and an index indicating whether saturation or limitation has occurred, the ternary index is an index indicating whether a value is positive, zero, or negative.

9. The detection device according to claim 7 or 8, wherein an output of the combination determination logic includes an on-delay circuit that, when the combination determination logic determines that the state of the runaway has continued for a first time, notifies an outside that the runaway has occurred.

10. The detection device according to claim 9, wherein the first time is adjustable in order to adjust the sensitivity of detection.

11. The detection device according to claim 7 or 8, wherein the runaway is caused by a mismatch between the motor and an encoder connected to the motor or by a failure of the encoder.

12. The detection device according to claim 7 or 8, wherein the combination decision logic is constituted in a manner that at least one of the following situations with respect to the motor can be detected: the speed command value is zero, the speed feedback value and the speed feedback differential value have the same sign, and the speed feedback value and the torque command value have different signs; and the speed command value and the torque command value have the same sign, the speed feedback value and the speed feedback differential value have different signs with respect to the speed command value.

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