Method and apparatus for detecting runaway operation of a servo motor

Abstract

This system quickly detects runaway motors controlled by servos while suppressing the occurrence of false detections. [Solution] A combination determination logic is provided that determines whether or not the motor is running out of control based on the combination of indicators obtained by determining a binary or ternary index for each of a plurality of items selected from the command value and the status value obtained for the motor. The combination determination logic includes, for example, sign mismatch detectors 41-44, AND circuits 45-47, OR circuit 48, and selectors 51, 52.

Description

Technical Field

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

Background Art

[0002] A servo motor is controlled and driven based on a position command or a speed command input to a motor control device. For the control of a servo motor, information regarding the exact position (rotation angle) of the motor is required. Therefore, an encoder for detecting the position of the motor is attached to the rotation shaft of the motor. As the encoder, one having a nonvolatile memory for storing the rotation angle of the motor even when the power is turned off is used. However, due to some reason, the data in the nonvolatile memory is damaged, whereby the rotation angle data of the motor stored in the encoder may be lost, or the rotation angle indicated by the data in the encoder may deviate from the actual rotation angle of the motor. When the servo control of the motor is started in such a case, the motor may operate contrary to the command input to the motor control device and run away. Such a runaway is a runaway based on the inconsistency between the motor and the encoder. Even for reasons other than the damage of the rotation angle data, a runaway of the servo motor may occur due to a defect of the encoder or the like. In a runaway of the motor, the motor continuously outputs its maximum torque or starts rotating with a sudden acceleration. Such a runaway is very dangerous, and it is necessary to quickly detect the occurrence of an abnormality and stop the motor. Conventionally, however, it has taken time from the start of the runaway to the detection, and the braking distance until the motor stops has been extended, which may cause a material damage accident or the like.

[0003] Patent Document 1 discloses that a runaway operation is determined when the speed deviation is greater than or equal to a threshold, 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 integrated value of the speed deviation is greater than or equal to a predetermined value. Patent Document 2 discloses that a runaway operation is detected when the speed deviation increases continuously. Patent Document 3 discloses that the motor operation is determined to be normal by inputting the motor speed and comparing the motor speed or a value created based on the motor speed with a preset threshold. Patent Document 4 relates to a technology for detecting reverse runaway in a door driven by a linear motor, and discloses that the speed of the door after a predetermined time is predicted from the acceleration detection value and the speed detection value, and an abnormality is determined when the predicted speed exceeds a positive or negative set speed. Similarly, Patent Document 5 relates to a technology for detecting reverse runaway in a door driven by a linear motor, and discloses that an abnormality is determined when the speed command value exceeds a first set speed, and the speed detection value exceeds a second set speed with the opposite polarity to the speed command value. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2007-209080 [Patent Document 2] Japanese Patent Application Publication No. 10-290521 [Patent Document 3] Japanese Patent Publication No. 2004-318509 [Patent Document 4] Japanese Patent Publication No. 2012-135212 [Patent Document 5] Japanese Patent Publication No. 2008-271705 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The technologies described in Patent Documents 1 and 2 detect motor runaway by calculating the cumulative value of the speed deviation or by detecting a continuous increase in the speed deviation, and in principle, these technologies require a certain amount of time to detect abnormalities. Furthermore, depending on the form of motor runaway, the speed deviation may not increase rapidly, so it is difficult to quickly detect motor runaway using the technologies described in Patent Documents 1 and 2. The technology described in Patent Document 3 depends on the comparison of motor speed and a threshold, so it cannot necessarily be said that it can quickly detect runaway depending on the form of runaway. The technologies described in Patent Documents 4 and 5 relate to the detection of reverse runaway in linear motors, so it is difficult to directly apply them to the detection of runaway in ordinary rotary motors. Furthermore, all of the technologies described in Patent Documents 1 to 5 detect runaway by comparing with a threshold, and depending on the threshold setting, they may not be able to properly detect runaway or false detections may increase. Moreover, adjusting the threshold takes a lot of time, and if the number of thresholds to be set increases, the amount of work required for tuning also increases.

[0006] The object of the present invention is to provide a method and apparatus that can quickly detect runaway behavior in a servo motor while suppressing the occurrence of false detections. [Means for solving the problem]

[0007] According to one aspect of the present invention, a method for detecting a motor runaway in a system that servo-controls a motor based on a command value involves determining a binary or ternary index for each of a plurality of items selected from the command value and the status value obtained for the motor, and detecting the runaway based on a combination of these indexes.

[0008] According to another aspect of the present invention, a detection device for detecting a motor runaway in a system that servo-controls a motor based on a command value includes a combination determination logic that determines whether or not a runaway has occurred based on a combination of indicators, for each of a plurality of items selected from the command value and the status value obtained with respect to the motor, and determines whether or not a runaway has occurred. [Effects of the Invention]

[0009] According to the present invention, it becomes possible to quickly detect runaway behavior in a servo motor while suppressing the occurrence of false detections. [Brief explanation of the drawing]

[0010] [Figure 1] This is a block diagram showing the configuration of a motor system to which one form of runaway detection method is applied. [Figure 2] This is a block diagram illustrating the logical configuration of the runaway detection unit. [Modes for carrying out the invention]

[0011] Next, embodiments for carrying out the present invention will be described with reference to the drawings. Figure 1 is a block diagram showing the configuration of a motor system to which a runaway detection method according to one embodiment of the present invention is applied. The illustrated motor system receives AC power from an AC power source 10 and drives the motor 30 based on commands input from an external source (at least one of a position command and a speed command). A motor control device 20 is provided to control the motor 30 based on the commands. An encoder 31 for detecting the position (i.e., rotation angle) of the motor 30 is attached to the output shaft of the motor 30, and the data indicating the position detected by the encoder 31 is fed back to the motor control device 20. If the motor 30 is a motor that drives each axis of a robot, then the motor system described here will be a robot system, and the motor control device 20 will be a robot control device or robot controller that controls the robot.

[0012] There are various ways in which the motor control device 20 controls the motor 30, and the runaway detection method according to the present invention can detect runaway operation of the motor 30 regardless of which control mode is employed. The motor 30 may be an AC motor or a DC motor. In the following, assuming that each motor 30 is a three-phase synchronous motor (or a three-phase induction motor), runaway detection will be described when the motor 30 is controlled by vector control or the like.

[0013] The motor control device 20 includes a full-wave rectifier circuit 21 that rectifies AC power received from an AC power source, and an inverter circuit 22 that receives DC power from the full-wave rectifier circuit 21 and generates three-phase AC power to drive the motor 30. The voltage of the DC power output by the full-wave rectifier circuit 21 is called the 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 power lines 32 for each phase. Current sensors 23 and 24 are provided on the u-phase and v-phase power lines 32, respectively. Furthermore, the motor control device 20 includes a control calculation unit 26, a current control unit 27 provided for each motor 30 that drives the switching elements in the inverter circuit 22, and a runaway detection unit 28. External position commands and speed commands from the motor control device 20 are input to the control calculation unit 26. Based on the input command values ​​such as position commands and speed commands, and position data input from the encoder 31 attached to the motor 30, the control calculation unit 26 outputs current commands (e.g., d-axis current commands and q-axis current commands) to drive the motor 30. The control calculation unit 26 calculates command values ​​such as voltage commands and torque commands, also called internal command values, for the control of the motor 30. Current commands are also classified as internal command values. Current commands are input to the current control unit 27. The current control unit 27 receives the detected position value of the motor 30 from the encoder 31, and the detected current values ​​of each phase of the motor 30 from the current sensors 23 and 24. The current control unit 27 calculates the electrical angle of the motor 30 from the position of the motor 30, and based on the current values ​​of each phase and the input current commands, drives the switching elements in the inverter circuit 22 to control the three-phase output from the inverter circuit 22.

[0014] Incidentally, the inverter circuit 22 and the motor 30 have rated values ​​such as rated power, and regardless of what the voltage command or current command calculated by the control calculation unit 26 is, the motor 30 cannot be driven beyond these rated values. When the motor 30 rotates, a back electromotive force is generated in the motor 30, and if this exceeds the main circuit power supply voltage, current cannot flow from the inverter circuit 22 to the motor 30, regardless of the voltage command or current command for the motor 30. These conditions can be described as the output of the motor control device 20 being saturated with respect to the motor 30. Such saturation is detected by the control calculation unit 26. Since the current command for the motor 30 is generated based on the torque command, if saturation occurs with respect to the current command, it can be considered that the motor 30 is not generating torque according to the torque command value, and the torque is being limited.

[0015] The runaway detection unit 28 detects the occurrence of runaway in the motor 30 connected to the motor control device 20. As described above, it is difficult to quickly detect runaways while reducing false detections if one attempts to detect runaways based solely on speed deviation or motor rotation speed. Therefore, in this embodiment, when servo-controlling the motor 30 based on command values, binary or ternary indicators are acquired for each of several items relating to the command value and the state value obtained for the motor 30, and a determination is made as to whether or not the motor 30 is runaway based on the combination of acquired indicators. Command values ​​include both command values ​​given to the motor control device 20 from the outside and command values ​​generated internally by the motor control device 20. Examples of command values ​​that can be used in this embodiment include position command values, speed command values, torque command values, current command values, and voltage command values. Examples of status values ​​obtained for the motor include the position measured by the encoder 31 (position feedback value) and the speed calculated from it (speed feedback value), the duty cycle of the PWM (pulse width modulation) in the inverter circuit 22, and whether or not saturation or limitations on torque commands occur in the motor 30.

[0016] If the motor 30 is operating normally without runaway, the command values ​​and state values ​​obtained for the motor 30 will generally exhibit a fixed behavior depending on the control mode of the motor 30. Therefore, by treating each of the multiple command values ​​and state values ​​as a binary index or a ternary index including 0, and then checking them using a combination condition with an AND (logical conjunction) condition, it is possible to determine whether those command values ​​and state values ​​are behaving as expected, and the presence or absence of abnormalities, specifically the presence or absence of runaway behavior in the motor 30, can be quickly detected. It is preferable to define the index for each item in a way that eliminates arbitrariness in threshold setting as much as possible. Therefore, as indexes, binary indexes indicating whether a value exists or not, binary indexes indicating whether a value is 0 or not, binary indexes indicating whether a value is positive or negative, ternary indexes indicating whether a value is positive, 0 or negative, and indexes indicating whether or not saturation or limitation has occurred with respect to the command value can be used. When using a ternary index, being non-zero is equivalent to being positive or negative. In this embodiment, it is preferable to use three or more binary or ternary indexes to detect whether or not the motor 30 is runaway.

[0017] Considering the effects of noise and other factors, especially with values ​​based on actual measurements, even if the value is not exactly zero, if it falls within a certain range including zero, it should be considered as zero. Therefore, strictly speaking, a threshold is required to determine whether a value is zero or not, but in this embodiment, an arbitrarily set threshold is not used to determine binary or ternary indicators. Instead, in this embodiment, a threshold may be set for the duration that the combination of conditions for determining runaway operation is met, and if the duration of this combination of conditions exceeds the threshold, it may be determined that runaway operation has occurred. In cases where the combination of conditions is met momentarily even if the motor 30 is not actually running out of control due to noise or overshoot in servo control, it is preferable to set a threshold for the duration in such cases.

[0018] Hereinafter, specific examples will be described. As a plurality of indicators, for example, a three-value indicator indicating whether the speed command value is positive, zero, or negative, a two-value indicator indicating whether the speed feedback value is positive or negative, a two-value indicator indicating whether the differential value of the speed feedback value, that is, the acceleration, is positive or negative, and a two-value indicator indicating whether the torque command value is positive or negative are used. Whether the differential value of the speed feedback value is positive or negative is actually determined by whether the difference value between the current speed feedback value and the previous speed feedback value is positive or negative. These command values and state values are input from the control calculation unit 26 to the runaway detection unit 28. When the combinations shown in Table 1 are established for these indicators, the runaway detection unit 28 determines that the motor 30 is running away.

[0019]

Table 1

[0020] Abnormalities 1 and 2 are classified according to the positive or negative of the speed command value, that is, whether the rotation direction in which the motor 30 is to be rotated is the positive direction or the reverse direction, and substantially indicate the same state. In this state, since the speed command value and the speed feedback value have different signs, the motor 30 rotates in the opposite direction to the intended rotation direction, and since the speed feedback value and its differential value have the same sign, the rotation of the motor 30 is accelerating. And the torque command value is intended to rotate the motor in the intended rotation direction, and in Abnormalities 1 and 2, it can be evaluated that the motor 30 is running away.

[0021] Abnormalities 3 and 4 are classified according to whether the actual rotation direction of the motor 30 represented by the positive or negative of the speed feedback value is the positive direction or the reverse direction, and substantially indicate the same state. In this state, although the speed command value is 0 and it is intended to keep the motor 30 stopped, the motor 30 is rotating and the rotation is accelerating. And the torque command value has a different sign from the speed feedback value and is in the direction of preventing the current rotation of the motor 30, and in Abnormalities 3 and 4, it can be evaluated that the motor 30 is running away.

[0022] Figure 2 illustrates the configuration for realizing a runaway detection unit 38 that detects runaway operation of the motor 30 based on the combination of indicators shown in Table 1, and shows the logical configuration of the runaway detection unit 28. The runaway detection unit 28 is composed of a combination determination logic that determines whether or not the combination conditions shown in Table 1 are met. It can be configured as hardware by combining discrete components based on the logical configuration shown in Figure 2, but it can also be realized by having a microprocessor or microcomputer execute software that achieves operation equivalent to the logical configuration shown in Figure 2. In the motor control device 20, the control calculation unit 26 and the current control unit 27 are generally also configured by software, so the software that realizes the runaway detection unit 28 may be incorporated into the software that realizes the control calculation unit 26 and the current control unit 27.

[0023] Assuming that a position command value is input to the control calculation unit 26 and the position of the motor 30 is fed back from the encoder 31, the speed command value is the difference between the previously input position command value and the currently input position command value, and the speed feedback value is the difference between the previously input position feedback value and the currently input position feedback value. The runaway detection unit 28 includes sign mismatch detectors 41 to 44, AND (logical conjunction) circuits 45 to 47, OR (logical disjunction) circuit 48, ON delay circuits 49 and 50, and selectors 51 and 52.

[0024] Each of the sign mismatch detectors 41-44 has two inputs and outputs true if the signs of the two input values ​​are different, and false if they are the same. It outputs false if either or both inputs are 0. The sign mismatch detector 41 is input with the speed command value and the speed feedback value, and the sign mismatch detector 42 is input with the speed feedback derivative value and the torque command value. The AND circuit 45 calculates the logical AND of the output of the sign mismatch detector 41 and the output of the sign mismatch detector 42 and outputs the result. The output of the AND circuit 45 is true when either condition 1 or condition 2 above is true, and false otherwise. That is, the output of the AND circuit 45 is true when runaway is occurring while the speed command value is not 0. The output of this 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 a preset on-delay time, but is reset if the output of the AND circuit 45 becomes false during that on-delay time. As a result, the output of the on-delay circuit 49 is true only when the output of the AND circuit 45 remains true for a period longer than the length of the on-delay time.

[0025] Incidentally, the output of the AND circuit 45 is true when the speed command value is positive, the speed feedback value is negative, the speed feedback derivative value is positive, and the torque command value is negative. This state does not correspond to either abnormality 1 or abnormality 2, nor is it a state in which the motor 30 is running out of control. However, this state can occur temporarily due to control system overshoot, which may lead to false detection. Therefore, a selector 51 is provided that outputs true when the torque command is limited and false otherwise, so that a runaway condition is determined only when the torque command is limited. The AND circuit 46 calculates the logical OR of the output of the on-delay circuit 49 and the output of the selector 51 and outputs the result. The output of the AND circuit 46 is true when the conditions for abnormality 1 or abnormality 2 are met and the motor 30 is running out of control, and false otherwise.

[0026] The sign mismatch detector 43 receives the speed feedback value and the torque command value as inputs, and the sign mismatch detector 44 receives the torque command value and the speed feedback derivative value as inputs. The AND circuit 47 calculates the logical AND of the output of the sign mismatch detector 43 and the output of the sign mismatch detector 44 and outputs the result. The sign mismatch detector 44 may be omitted, and instead, the output of the sign mismatch detector 42 may be input 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 condition 3 or condition 4 above is met, and false otherwise. That is, the output of the selector 52 is true when runaway is occurring with a speed command value of 0. The output of the selector 52 is input 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 is true only when the output of the selector 52 remains true for a period of time equal to or longer than the on-delay time. In the two on-delay circuits 49 and 50, their on-delay times are set separately. For example, the on-delay time in on-delay circuit 50 is set to be longer than the on-delay time in on-delay circuit 49.

[0027] The OR circuit 48 calculates the logical OR of the output of the AND circuit 46 and the output of the ON delay circuit 50 and outputs the result. The output of the OR circuit 48 is true when any of abnormalities 1 to 4 are true, and false otherwise. If the output of the OR circuit 48 is true, it indicates that the motor 30 is running out of control, and this detection result is output externally from the runaway detection unit 28.

[0028] The above describes an example of a configuration for carrying out the present invention, but the above technology can take the following configuration.

[0029] (1) A method for detecting a runaway motor in a system that servo-controls a motor based on a command value, A binary or ternary index is determined for each of the multiple items selected from the command value and the state value obtained for the motor. A detection method for detecting the runaway behavior based on a combination of the aforementioned indicators.

[0030] (2) The binary index includes at least one of the following: an index indicating whether a value exists or does not exist, an index indicating whether a value is zero or non-zero, an index indicating whether a value is positive or negative, and an index indicating whether saturation or limitation has occurred. The detection method described in (1), wherein the three-value index is an index that indicates whether the value is positive, zero, or negative.

[0031] (3) The detection method according to (1) or (2), which determines that runaway operation has occurred when the combination of indicators continues to satisfy predetermined conditions for a first period of time.

[0032] (4) The detection method according to (3), wherein the detection sensitivity is adjusted by adjusting the length of the first time.

[0033] (5) The detection method according to any one of (1)-(4), wherein the runaway is caused by a mismatch between the motor and the encoder connected to the motor or by a malfunction of the encoder.

[0034] (6) The detection method according to any one of (1)-(5), wherein with respect to the motor, it is determined that runaway is occurring when the speed command value is zero and the speed feedback value and the speed feedback derivative value have the same sign and the speed feedback value and the torque command value have opposite signs, and when the speed command value and the torque command value have the same sign and the speed feedback value and the speed feedback derivative value have opposite signs with respect to the speed command value.

[0035] (7) A detection device for detecting a runaway motor in a system that servo-controls a motor based on a command value, A detection device comprising a combination determination logic that determines whether or not runaway occurs based on a combination of the command value and a state value obtained for the motor, for each of a plurality of items selected from the command value and the state value obtained for the motor, and determines whether or not runaway occurs based on the combination of the indicators.

[0036] (8) The binary index includes at least one of the following: an index indicating whether a value exists or does not exist, an index indicating whether a value is zero or non-zero, an index indicating whether a value is positive or negative, and an index indicating whether saturation or limitation has occurred. The detection device described in (7), wherein the three-value index is an index that indicates whether the value is positive, zero, or negative.

[0037] (9) The detection device according to (7) or (8), which includes an on-delay circuit at the output of the combination determination logic, and notifies the outside that a runaway has occurred when the state in which the combination determination logic has determined to be a runaway continues for a first time period.

[0038] (10) The detection device according to (9), wherein the first time is adjustable for adjusting the sensitivity of detection.

[0039] (11) The detection device according to any one of (7)-(10), wherein the runaway is caused by a mismatch between the motor and the encoder connected to the motor or by a malfunction of the encoder.

[0040] (12) The detection device according to any one of (7)-(11), wherein the combination determination logic is configured to detect, with respect to the motor, at least one of the following: the speed command value is zero and the speed feedback value and the speed feedback derivative value have the same sign and the speed feedback value and the torque command value have opposite signs; and the speed command value and the torque command value have the same sign and the speed feedback value and the speed feedback derivative value have opposite signs with respect to the speed command value.

[0041] According to the configurations of (1) and (7), by using multiple binary or ternary indicators, it is possible to quickly determine whether the system is behaving as expected, thereby enabling rapid detection of motor runaway while suppressing false positives.

[0042] According to the configuration of (2) and (8), the arbitrariness in threshold setting is eliminated as much as possible when determining the indicator, so that false positives are suppressed and runaway behavior is detected quickly, while preventing a decrease in work efficiency that is associated with tuning multiple thresholds.

[0043] The configurations in (3) and (9) eliminate the possibility of false detections caused by noise, overshoot, etc., and in particular, the configurations in (4) and (10) allow for adjustment of detection sensitivity by adjusting only a single parameter, the first time.

[0044] As described in (5) and (11), runaway operation can be detected particularly effectively when it is caused by a mismatch between the motor and the encoder connected to the motor, or by a malfunction of the encoder. The configurations in (6) and (12) provide a suitable combination of indicators for detecting motor runaway operation, enabling faster detection of runaway operation while further suppressing false detections. [Explanation of Symbols]

[0045] 10...AC power supply, 20...Motor control device, 21...Full-wave rectifier circuit, 22...Inverter circuit (INV), 23,24...Current sensor, 26...Control calculation unit, 27...Current control unit, 28...Runaway detection unit, 30...Motor, 31...Encoder, 32...Power line, 41~44...Sign mismatch detector, 45~47...AND circuits 45~47, 48...OR circuits 48, 49,50...ON delay circuits, 51,52...Selectors 51,52.

Claims

1. A method for detecting a runaway motor in a system that servo-controls a motor based on a command value, A binary or ternary index is determined for each of the multiple items selected from the command value and the state value obtained for the motor. A detection method for detecting the runaway behavior based on a combination of the aforementioned indicators.

2. The aforementioned binary indicator includes at least one of the following: an indicator indicating whether a value exists or does not exist; an indicator indicating whether a value is zero or non-zero; an indicator indicating whether a value is positive or negative; and an indicator indicating whether saturation or limitation has occurred. The detection method according to claim 1, wherein the three-value index is an index that indicates whether the value is positive, zero, or negative.

3. The detection method according to claim 1 or 2, wherein it is determined that the runaway operation has occurred when the combination of the indicators continues to satisfy predetermined conditions for a first period of time.

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

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

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

7. A detection device for detecting runaway motor in a system that servo-controls a motor based on a command value, A detection device comprising a combination determination logic that determines whether or not runaway occurs based on a combination of the command value and a state value obtained for the motor, for each of a plurality of items selected from the command value and the state value obtained for the motor, and determines whether or not runaway occurs based on the combination of the indicators.

8. The aforementioned binary indicator includes at least one of the following: an indicator indicating whether a value exists or does not exist; an indicator indicating whether a value is zero or non-zero; an indicator indicating whether a value is positive or negative; and an indicator indicating whether saturation or limitation has occurred. The detection device according to claim 7, wherein the three-value index is an index that indicates whether the value is positive, zero, or negative.

9. The detection device according to claim 7 or 8, further comprising an on-delay circuit at the output of the combination determination logic, and notifying an external party that a runaway condition has occurred when the state in which the combination determination logic has determined a runaway condition continues for a first period of time.

10. The detection device according to claim 9, wherein the first time is adjustable for adjusting the sensitivity of the detection.

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

12. The detection device according to claim 7 or 8, wherein the combination determination logic is configured to detect, with respect to the motor, at least one of the following: the speed command value is zero and the speed feedback value and the speed feedback derivative value have the same sign and the speed feedback value and the torque command value have opposite signs; and the speed command value and the torque command value have the same sign and the speed feedback value and the speed feedback derivative value have opposite signs with respect to the speed command value.

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