DETECTION DEVICE AND DETECTION METHOD

Abstract

A sensor device (detection device) (100) includes an acceleration sensor (sensor unit) (1) that detects an X-axis acceleration and a Y-axis acceleration. The sensor device (100) further includes a signal processor (state detection unit) (2) that detects a tightening state of each nut (240) based on a comparison result between a rotation angle of the sensor device (100), which is calculated based on the X-axis acceleration and the Y-axis acceleration, and an initial rotation angle of the sensor device (100), which is calculated based on an initial value (reference acceleration) of each of the X-axis acceleration and the Y-axis acceleration.After each of the X-axis acceleration and the Y-axis acceleration becomes equal to a value that can be regarded as a zero value, the signal processor (2) defines an acceleration detected by the acceleration sensor (1) that is larger than the zero value as an initial acceleration.

Description

TECHNICAL FIELDThe present invention relates to a detection device and a detection method.BACKGROUND OF THE INVENTIONJP 2005-329907 (PTL 1) discloses a detection device that detects a mounted state of a tire (a nut for fastening a wheel rim) based on a detection value of a detector (G sensor) mounted on the tire or the wheel rim.PATENT LITERATUREPTL 1: JP 2005-329907SUMMARY OF THE INVENTIONTECHNICAL PROBLEMIn the detection device described in PTL 1, the loosening of a nut fixed to the wheel rim is detected based on a detection value of the G sensor. However, the vibration of the wheel rim generated when the nut is loosened may vary depending on the type of vehicle or tire. Therefore, in order to detect the loosening of a nut (fastener) regardless of the type of the vehicle or tire (rotating body), the loosening of the nut is detected based on an amount of change in the value based on the rotation angle of the nut. In this case, in order to detect the loosening of the nut, it is necessary to set a reference value (initial value) for the value based on the rotation angle of the nut. Therefore, it is desired to provide a detection device and a detection method with which a reference value for the value based on the rotation angle of the nut (the fastening member) can be easily set.The present invention has been made to solve the above problem, and an object of the present invention is to provide a detection device and a detection method that can easily set a reference value for a value based on a rotation angle of a fastener.SOLUTION OF PROBLEMA detection device according to a first aspect of the present invention is a detection device that detects a fixing state of a fixing member that fixes a predetermined member to a rotating body having a rotation axis intersecting a gravity direction, the detection device including: a sensor unit that detects acceleration in at least one axis along a plane intersecting the rotation axis of the rotating body when the predetermined member is fixed to the rotating body by the fixing member; and a state detection unit that detects the fastening state of the fastener based on a comparison result between a value based on the acceleration detected by the sensor unit and a value based on a reference acceleration defined as a reference value of the acceleration, wherein after an absolute value of the acceleration in the at least one axis becomes equal to a value that can be regarded as a zero value, the state detection unit defines a value based on the absolute value of the acceleration detected by the sensor unit and greater than the zero value as the reference acceleration.As described above, in the detection device according to the first aspect of the present invention, after the absolute value of the acceleration in at least one axis becomes equal to a value that can be regarded as a zero value, the acceleration detected by the sensor unit and larger than the zero value is defined as a reference acceleration. In the present invention, the fixing member for fixing the fixing member is first removed from the rotating body and then placed horizontally on the ground, whereby the acceleration of the sensor unit in at least one axis becomes zero. After the fastening operation of the fastening member is completed, the rotation of the rotating body is started with the fastening member being subjected to centrifugal acceleration of a predetermined magnitude or greater. Therefore, after completion of the operation of fixing the fixing member and starting the rotation of the rotating body, the acceleration of the fixing member is automatically set as the reference acceleration. Accordingly, it is possible to reduce a user's time and effort compared to a case where the user sets the reference acceleration of the fastener by performing a predetermined operation, for example. Thereby, it is possible to easily set the reference acceleration for the acceleration (the value based on the rotation angle) of the fastener.A detection method according to a second aspect of the present invention is a detection method for detecting a state of attachment of a fixing member that fixes a predetermined member to a rotating body having a rotation axis intersecting a gravity direction, the detection method including: detecting, by a sensor unit that detects the acceleration in the at least one axis along a plane intersecting the rotation axis of the rotating body when the predetermined member is fixed to the rotating body by the fixing member; after an absolute value of the acceleration in the at least one axis becomes equal to a value that can be regarded as a zero value, defining, as a reference acceleration, a value based on the absolute value of the acceleration detected by the sensor unit and greater than the zero value; detecting the fastening state of the fastening element based on a comparison result between a value based on the acceleration detected by the sensor unit in the at least one axis and a value based on the reference acceleration.As described above, in the detection method according to the second aspect of the present invention, after the absolute value of the acceleration in at least one axis becomes equal to a value that can be regarded as a zero value, the acceleration detected by the sensor unit and larger than the zero value is defined as a reference acceleration. Therefore, it is possible to provide a detection method with which the reference acceleration for the acceleration (the value based on the rotation angle) of the fastener can be easily set.ADVANTAGEOUS EFFECTS OF THE INVENTIONAccording to the present invention, it is possible to easily set the reference acceleration for the value based on the rotation angle of the fastener.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a diagram illustrating a vehicle provided with a sensor device according to an embodiment. FIG. 2 is a cross-sectional view of a nut according to the embodiment. FIG. 3 is a diagram illustrating a configuration of a sensor device according to the embodiment. FIG. 4 is a functional block diagram of a signal processor according to the first embodiment. FIG. 5 is a front view illustrating a configuration of a tire of a vehicle (in an initial state) according to the embodiment. FIG. 6 is a diagram illustrating a relationship between acceleration and rotation angle of a wheel rim when the centrifugal force is 0. FIG. 7 is a diagram illustrating a relationship between acceleration and rotation angle of the wheel rim when the centrifugal force is 6 G. FIG. 8 is a front view illustrating a configuration of a tire of the vehicle (in a state where a nut is loosened) according to the embodiment. FIG. 9 is a diagram illustrating a relationship between acceleration and rotation angle of the wheel rim when the centrifugal force is 6 G and a nut is loosened. FIG. 10A is a diagram illustrating a relationship between an average acceleration and a sensor angle when the centrifugal force is 6 G. FIG. 10B is a diagram illustrating a relationship between an average acceleration and a sensor angle when the centrifugal force is 10 G. FIG. 11 is a flowchart illustrating a method of detecting a fastening state of a nut using a sensor device according to the embodiment. FIG. 12 is a view illustrating a state in which the sensor device according to the embodiment is placed horizontally on the floor. FIG. 13 is a cross-sectional view of a nut according to a first modification of the embodiment. FIG. 14 is a cross-sectional view of a nut according to a second modification of the embodiment.DETAILED DESCRIPTION OF THE EMBODIMENTSHereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and the description of these parts is not repeated.FIG. 1 is a diagram illustrating a vehicle 200 on which a sensor device 100 (see FIG. 2 ) according to an exemplary embodiment of the present invention is mounted. The vehicle 200 includes a plurality of wheels 210. The vehicle 200 further includes a communication terminal 201 capable of communicating with a communication unit 3 (described below) and including a display unit (not illustrated). The sensor device 100 is an example of a "detection device" in the present invention.The wheel 210 includes a wheel rim 220 and a tire 230 mounted on the wheel rim 220. The wheel rim 220 is fixed to a wheel hub 250 a(see FIG. 2 ) by a plurality of (five in FIG. 1 ) nuts 240. The number of the nuts 240 is not limited to the above-mentioned number. The wheel hub 250 ais an example of a "predetermined member" and a "vehicle body" in the present invention. The wheel rim 220 is an example of a "rotating body" in the present invention, and each nut 240 is an example of a "fastener" in the present invention.As shown in FIG. 2, each nut 240 secures a bolt 250 to the wheel rim 220. Specifically, the wheel rim 220 is provided with a plurality of (five) wheel holes 221, and the bolt 250 is inserted into (penetrates through) each wheel hole. Each nut 240 secures the bolt 250 (see FIG. 2 ) inserted into each wheel hole 221 to the wheel rim 220. The bolt 250 is fixed to the wheel hub 250 a.FIG. 2 illustrates a dual tire as an example, and the wheel rim 220 is composed of an inner wheel rim 222 and an outer wheel rim 223.The nut 240 is open on one side. A nut cap 241 is attached to the nut 240. The sensor device 100 may be attached to the nut cap 241, for example, and is thereby indirectly provided in the nut 240. The nut 240 is an example of a "fastener" of the present invention.Specifically, the mother cap 241 includes an upper portion 241 aand a side portion 241 b. The side portion 241 bis provided so as to circumferentially surround a portion of the bolt 250 passing through the wheel hole 221. The upper portion 241 ais provided to face a tip end 251 of the screw 250 (in the insertion direction of the screw 250). The upper portion 241 ais continuous with the side portion 241 b. A washer 243 may be disposed between the nut 240 and the wheel rim 220.The sensor device 100 is attached (bonded) to an inner surface 241 cof the upper portion 241 aof the mother cap 241. Therefore, the sensor device 100 is disposed in a space S of the nut cap 241 in which the bolt 250 is accommodated.The sensor device 100 is provided in some of the plurality of nuts 240 provided in each wheel 210. Note that the sensor device 100 may be provided in each of the plurality of nuts 240 provided in each wheel 210.As illustrated in FIG. 3, the sensor device 100 includes an acceleration sensor 1, a signal processor 2, a communication unit 3, and a power supply unit 4. the acceleration sensor 1 is an example of a "sensor unit" of the present invention, and the signal processor 2 is an example of a "state detection unit".As illustrated in FIG. 5, the acceleration sensor 1 detects acceleration of both an X axis and a Y axis that are orthogonal to each other in a plane orthogonal to a rotation axis (not shown) of the wheel rim 220 extending in a direction perpendicular to the paper surface of FIG. 5. The acceleration detected by the acceleration sensor 1 has a positive or negative magnitude (direction). An X-axis arrow and a Y-axis arrow shown in FIG. 5 indicate an X-axis positive direction and a Y-axis positive direction, respectively. When viewed from the paper surface of FIG. 5, a direction of the Y axis when rotated by 90 degrees counterclockwise with respect to the X axis is referred to as a positive direction.The Z direction illustrated in FIG. 5 indicates the vertical direction (up-down direction). When the wheel 210 rotates, a centrifugal acceleration is applied to the nut 240A in response to the rotational speed of the wheel 210. In the present embodiment, for the purpose of convenience of description, it is assumed that the centrifugal acceleration is sufficiently larger than the gravitational acceleration (in other words, it is assumed that the gravitational acceleration can be ignored). In FIG. 5, one nut 240 of the five nuts 240 disposed at the farthest position in the Z 1 direction is referred to as nut 240A. In the following description, an angle (rotation angle) of the sensor device 100 is 0 degrees when the sensor device 100 is oriented as illustrated in FIG. 5.The signal processor 2 detects a state (attachment state) of the nut 240 based on a detection signal of the acceleration sensor 1. as illustrated in FIG. 4, the signal processor 2 includes a root sum square calculation unit 2 a, a normalization unit 2 b, a rotation angle calculation unit 2 c, an attachment state detection unit 2 d, an initial value setting unit 2 e, and a detection period setting unit 2 f. Note that each of the root sum square calculation unit 2 a, normalization unit 2 b, rotation angle calculation unit 2 c, attachment state detection unit 2 d, initial value setting unit 2 e, and detection period setting unit 2 fillustrated in FIG. 4 is software in which the functional characteristics of the signal processor 2 are divided into blocks. The details of each function will be described below.The communication unit 3 transmits a processing result of the signal processor 2 or information based on the processing result to the communication terminal 201 (see FIG. 1 ) of the vehicle 200 by wireless communication.The power supply unit 4 supplies power to each of the acceleration sensor 1, the signal processor 2, and the communication unit 3.The acceleration sensor 1 detects an X-axis acceleration (Xg) which is an acceleration (vector) in the X-axis and a Y-axis acceleration (Yg) which is an acceleration (vector) in the Y-axis after the wheel hub 250 ais fastened to the wheel rim 220 with the nuts 240. The X-axis acceleration and the Y-axis acceleration are each represented by a G value (e.g., the gravitational acceleration is denoted by 1 G).FIG. 6 is a graph illustrating a relationship between the rotation angle about the rotation axis of the tire 230 (the wheel rim 220) and the X-axis acceleration and the Y-axis acceleration when the vehicle speed of the vehicle 200 is zero (i.e., the centrifugal force applied to the nut 240). In this case, each of the X-axis acceleration and the Y-axis acceleration sinusoidally fluctuates in a range of ±1 G. This is because each of the X-axis and the Y-axis includes only one acceleration component based on the gravitational acceleration in the Z2 direction. FIG. 6 illustrates a result of the sensor device 100 provided in the nut 240A illustrated in FIG. 5.FIG. 7 is a diagram illustrating a relationship between a rotation angle of the tire 230 (the wheel rim 220) and each of the X-axis acceleration and the Y-axis acceleration when the vehicle travels at a predetermined speed, thereby applying a centrifugal force having a centrifugal acceleration of 6 G to the nut 240. In the present invention, the magnitude of the centrifugal force may be indicated by the G value. When the Y axis is oriented as in FIG. 5, the force component of the centrifugal force is not applied to the Y axis, and thereby the Y axis acceleration is the same as illustrated in FIG. 6. Since the component force of the centrifugal force acts on the X-axis, the X-axis acceleration is equal to a value obtained by adding 3G to the X-axis acceleration shown in FIG. 6. In this case, the waveform of the root sum square of the X-axis acceleration and the Y-axis acceleration is the same as the waveform of the X-axis acceleration. In Fig. 7, the waveform of the X-axis acceleration and the waveform of the root sum square are slightly shifted from each other for better understanding. FIG. 7 also illustrates a result of the sensor device 100 provided in the nut 240A illustrated in FIG. 5.FIG. 8 is a view illustrating a state in which the nut 240 is rotated by 135 degrees in the clockwise direction (rotated by 225 degrees in the counterclockwise direction and in the loosening direction), from the state illustrated in FIG. 5. FIG. 9 is a graph illustrating a relationship between an angle of the tire 230 (the wheel rim 220) and each of the X-axis acceleration and the Y-axis acceleration when a centrifugal force of 6 G is applied to the nut 240 in the state of FIG. 8. In this case, the amplitude of the waveform of the X-axis acceleration and the Y-axis acceleration is the same as illustrated in FIG. 7, but each of the average X-axis acceleration and the average Y-axis acceleration is different from that illustrated in FIG. 7. The X-axis average acceleration and the Y-axis average acceleration each reflect the rotation angle of the nut 240 (the sensor device 100). On the other hand, the waveform of the root sum square of the X-axis acceleration and the Y-axis acceleration is the same as illustrated in FIG. 7, and does not change in response to the rotation angle of the nut 240 (the sensor device 100). FIG. 9 illustrates a result of the sensor device 100 provided in the nut 240A illustrated in FIG. 8.FIG. 10A is a diagram illustrating an average acceleration with respect to an angle (rotation angle) of the sensor device 100 when the centrifugal force is 6 G. FIG. 10B is a diagram illustrating an average acceleration with respect to an angle (rotation angle) of the sensor device 100 when the centrifugal force is 10 G. As illustrated in FIGS. 10A and 10B, the waveform of the X-axis average acceleration and the Y-axis average acceleration each has an amplitude corresponding to the centrifugal force (the scales of the vertical axes are different from each other), but has the same shape. In each of FIGS. 10A and 10B, the root sum square of the X-axis average acceleration and the Y-axis average acceleration is a constant value corresponding to the centrifugal force. Therefore, a value obtained by dividing the average X-axis acceleration by the root sum square and a value obtained by dividing the average Y-axis acceleration by the root sum square become equal regardless of the magnitude of the centrifugal force.In the present embodiment, the signal processor 2 (the root sum square calculation unit 2 a) calculates the root sum square of the X-axis acceleration (Xg) and the Y-axis acceleration (Yg). The signal processor 2 (the normalization unit 2 b) calculates a normalized X-axis value by dividing the X-axis acceleration by the root sum square. The signal processor 2 (the normalization unit 2 b) calculates a normalized Y-axis value by dividing the Y-axis acceleration by the root sum square.Then, the signal processor 2 (the rotation angle calculation unit 2 c) calculates a rotation angle of the nut 240 (the sensor device 100) based on both the X-axis normalized value and the Y-axis normalized value. As described above with reference to FIGS. 10A and 10B, when the vehicle speed is equal to or greater than a predetermined value, the X-axis normalized value and the Y-axis normalized value depend on the sensor angle regardless of the centrifugal force (the vehicle speed). Therefore, by using the X-axis normalized value and the Y-axis normalized value, it is possible to determine the rotation angle of the nut 240 regardless of the vehicle speed. The signal processor 2 (the rotation angle calculation unit 2 c) acquires information on the X-axis acceleration and the Y-axis acceleration from the acceleration sensor 1, and calculates the rotation angle of the nut 240 every predetermined period (e.g., every 20 seconds to 120 seconds). The rotation angle of the nut 240 is an example of a "value based on the acceleration" of the present invention.The signal processor 2 (the fastening state detection unit 2 d) detects the fastening state of the nut 240 based on a difference between a rotation angle of the nut 240 calculated at the current time and a previous rotation angle of the nut 240. When the difference is outside a predetermined allowable range, the signal processor 2 (the fastened state detection unit 2 d) determines that the nut 240 is loosened (not fastened). In this case, the signal processor 2 notifies the communication terminal 201 (see FIG. 1 ) via the communication unit 3 (see FIG. 3 ) that the nut 240 is loosened. This may result in the communication terminal 201 displaying a warning on a display unit (not shown) or the communication terminal 201 outputting a warning sound. On the other hand, when the difference is within the predetermined allowable range, the signal processor 2 (the fastening state detection unit 2 d) determines that the nut 240 is fastened. In this case, the signal processor 2 does not notify the communication terminal 201. The previous rotation angle may be a rotation angle of a previous time point or may be an average value of rotation angles for a plurality of previous time points including the previous time point.The signal processor 2 (the mounting state detection unit 2 d) determines that the rotation of the wheel rim 220 (the tire 230) is stopped when both the current detection value and the previous detection value having a larger absolute value of the X-axis acceleration and the Y-axis acceleration detected by the acceleration sensor 1 are within a range of ±1 G. The previous detection value may be a detection value of a previous time point or may be an average value of detection values for a plurality of previous time points including the previous time point. It is possible to determine that the rotation of the wheel rim 220 (the tire 230) is stopped based on the X-axis acceleration or the Y-axis acceleration. It is possible to determine that the rotation of the wheel rim 220 (the tire 230) is stopped when the current detection value of each of the X-axis acceleration and the Y-axis acceleration and the previous detection value of each of the X-axis acceleration and the Y-axis acceleration are both within the range of ±1 G.When it is determined that the rotation of the wheel rim 220 (the tire 230) is stopped, the signal processor 2 increases a detection period (the predetermined period) of the sensor device 100 (e.g., increases the detection period to 30 minutes).The signal processor 2 acquires information on each of the X-axis acceleration and the Y-axis acceleration detected by the acceleration sensor 1. After each of the X-axis acceleration and the Y-axis acceleration becomes equal to a value that can be regarded as a zero value, the signal processor 2 (the initial value setting unit 2 e) defines each of the X-axis acceleration and the Y-axis acceleration as an initial value when at least a larger absolute value of the X-axis acceleration and the Y-axis acceleration becomes equal to or larger than a predetermined value (for example, 2 G) that is larger than the zero value. The initial value is an example of "reference acceleration" of the present invention. The value that can be considered a zero value refers to such a value centered around the zero value within a predetermined range (for example, 0±0.1 G).For example, if a torque wrench is used to tighten the nut 240, the nut 240 is first removed from the wheel rim 220 and then placed horizontally on the ground, causing both the X-axis acceleration and the Y-axis acceleration to be zero. After the nut 240 is fastened to the wheel rim 220, the vehicle 200 travels at a predetermined speed or more, whereby a centrifugal acceleration of 3 G or more acts on the nut 240. At this time, both the X-axis acceleration and the Y-axis acceleration are set as initial values.Specifically, after the state in which each of the X-axis acceleration and the Y-axis acceleration can be regarded as a zero value continues for a predetermined period of time or longer (for example, 30 minutes or longer), the signal processor 2 (the initial value setting unit 2 e) sets each of the X-axis acceleration and the Y-axis acceleration as an initial value when at least a larger absolute value of the X-axis acceleration and the Y-axis acceleration becomes equal to or larger than the predetermined value. Thus, it is possible to prevent the initial value from being set when both the X-axis acceleration and the Y-axis acceleration become zero (immediately) due to wrong detection of the acceleration sensor 1.Specifically, in a plurality of detections, after each of the X-axis acceleration and the Y-axis acceleration becomes zero, when at least a larger absolute value of the X-axis acceleration and the Y-axis acceleration becomes equal to or larger than the predetermined value, each average value of the X-axis accelerations obtained in the plurality of detections and an average value of the Y-axis accelerations obtained in the plurality of detections are set as an initial value. The plurality of detections may be a plurality of continuous detections.The signal processor 2 (the detection period setting unit 2 f) increases the detection period after both the X-axis acceleration and the Y-axis acceleration become zero. For example, the signal processor 2 changes the detection period from 20 to 120 seconds to 30 minutes (constant value).The signal processor 2 (the fastening state detection unit 2 d) detects the fastening state of the nut 240 based on a difference between a current rotation angle of the nut 240 (the sensor device 100) and a rotation angle of the nut 240 (the sensor device 100) calculated based on the initial value. Specifically, the signal processor 2 (the fastening state detection unit 2 d) determines that the nut 240 is loosened (not fastened) when the difference is outside the predetermined allowable range. In this case, the signal processor 2 notifies the communication terminal 201 (see FIG. 1 ) that the nut 240 is loosened via the communication unit 3 (see FIG. 3 ). This may cause the communication terminal 201 to display a warning on a display unit (not shown) or the communication terminal 201 to output a warning sound.(Method for Detecting Fastening State of Nut)Next, a method for detecting the fastening state of a nut 240 will be described with reference to the flowchart of FIG. 11.First, in step S 1, a nut 240 (a nut cap 241) is placed on a horizontal surface 900 perpendicular to the vertical direction (see FIG. 12 ). Thus, each of the X-axis acceleration and the Y-axis acceleration detected by the acceleration sensor 1 becomes zero. Next, at step S 2, the signal processor 2 acquires information indicating that each of the X-axis acceleration and the Y-axis acceleration from the acceleration sensor 1 is zero (or may be regarded as a zero value). Next, at step S 3, the signal processor 2 (the detection period setting unit 2 f) changes the detection period from 20 to 120 seconds to, for example, 30 minutes.Next, at step S 4, the signal processor 2 determines that the state in which each of the X-axis acceleration and the Y-axis acceleration is zero continues for a predetermined period of time (for example, 30 minutes) or longer, based on the information from the acceleration sensor 1.Thereafter, at step S 5, after the wheel hub 250 ais fastened to the wheel rim 220 with the nuts 240, at least one of the X-axis acceleration and the Y-axis acceleration is set to the predetermined value (for example, 2 G) or more. Specifically, after the wheel hub 250 ais fastened to the wheel rim 220 by the nuts 240, the vehicle 200 travels at a predetermined speed or more, and thereby a centrifugal acceleration of a predetermined magnitude or greater is applied to the nut 240.Next, at step S 6, the signal processor 2 acquires information indicating that at least one of the X-axis acceleration and the Y-axis acceleration is equal to or greater than the predetermined value.Next, at step S 7, the signal processor 2 (initial value setting unit 2 e) sets each of the X-axis acceleration and the Y-axis acceleration as an initial value when it is determined that at least one of the X-axis acceleration and the Y-axis acceleration is equal to or greater than the predetermined value (when the information is acquired at step S 6). Specifically, the signal processor 2 (the initial value setting unit 2 e) sets, as the initial value, an average value of the X-axis accelerations or the Y-axis accelerations acquired a plurality of times (for example, three times) after the information is acquired at step S 6.At step S 8, the signal processor 2 (the fastening state detection unit 2 d) detects the fastening state of the nut 240 based on the initial value of both the X-axis acceleration and the Y-axis acceleration set at step S 7. Specifically, the signal processor 2 (the fastening state detection unit 2 d) detects the looseness of the nut 240 based on a difference between a rotation angle of the nut 240 calculated based on the current X-axis acceleration and the current Y-axis acceleration and a rotation angle of the nut 240 calculated based on the initial value.Steps S 1 and S 5 are performed by a user, and the other steps are performed by the signal processor 2.As described above, in the present embodiment, after each of the X-axis acceleration and the Y-axis acceleration becomes equal to a value that can be regarded as a zero value, each of the X-axis acceleration and the Y-axis acceleration is set as an initial value when at least a larger absolute value of the X-axis acceleration and the Y-axis acceleration becomes equal to or larger than a predetermined value that is larger than the zero value. Since the initial value is automatically set for the X-axis acceleration and the Y-axis acceleration, the time and labor for the user can be reduced. Thereby, it is possible to easily set the initial value for the X-axis acceleration and the Y-axis acceleration.Since a mechanical switch or the like for registering the initial value does not need to be provided, it is possible to reduce the number of components in the vehicle 200, whereby the configuration of the vehicle 200 can be simplified. Moreover, it is also acceptable to provide a mechanical switch as described above or a switch that is turned on and off by magnetic force in the vehicle to register the initial value.In the present embodiment, it is described that the signal processor 2 sets the detection period to 30 minutes (constant value) after both the X-axis acceleration and the Y-axis acceleration become equal to a value that can be regarded as a zero value, but the present invention is not limited thereto. The signal processor 2 may gradually increase the detection period after both the X-axis acceleration and the Y-axis acceleration become equal to a value that can be regarded as a zero value. For example, the signal processor 2 may gradually increase the detection period to 1 minute, 5 minutes, 30 minutes, 2 hours, or 6 hours (hereinafter, every 6 hours). In the present embodiment, it is described that the initial value is set after the state in which each of the X-axis acceleration and the Y-axis acceleration can be regarded as a zero value continues for 30 minutes or longer, for example, but the present invention is not limited thereto, and the initial value may be set after the state continues for 5 minutes or longer.In the present embodiment, it is described that the acceleration sensor 1 detects the X-axis acceleration and the Y-axis acceleration, but the present invention is not limited thereto. For example, the acceleration sensor may detect accelerations in three or more axes intersecting each other in a plane orthogonal to the rotation axis of the wheel rim 220. The acceleration sensor may detect only one of the X-axis acceleration and the Y-axis acceleration.In the present embodiment, it is described that the average value of the X-axis accelerations or the Y-axis accelerations at a plurality of detections is set as the initial value after both the X-axis acceleration and the Y-axis acceleration become zero, but the present invention is not limited thereto. The X-axis acceleration or the Y-axis acceleration calculated in detection may be set as the initial value after both the X-axis acceleration and the Y-axis acceleration become zero.In the present embodiment, it is described that the fastening state of the nut 240 provided on the wheel rim 220 of the vehicle 200 is detected, but the present invention is not limited thereto. For example, the fastening state of a fastening member such as a nut fastened to a pulley of an elevator, a pulley of a conveyor belt, a coffee cup, and a carousel provided in an amusement park or a rotating toy provided in a park may be detected. In the above examples, in the case of a rotating body rotating along a plane perpendicular to the gravity direction, since the centrifugal force is not influenced by the gravitational force, the attachment state of the attachment member can be easily detected even at low centrifugal acceleration. In a case where a rotating body rotates along a plane perpendicular to the gravity direction, the X-axis acceleration (Y-axis acceleration) becomes zero when the rotation of the rotating body is stopped. In this case, the reset condition (the condition corresponding to step S 2) is always satisfied when the rotation of the rotating body is stopped. Therefore, in this modification, it is preferable to swap the reset condition and the initial value setting condition (the condition corresponding to step S 6).In the present embodiment, it is described that the nut cap 241 is attached to the nut 240, but the present invention is not limited thereto. As illustrated in FIG. 13, the sensor device 100 may be attached to a nut 340, which is a cap nut. The nut 340 is an example of a "fastener" of the present invention.In a second modification illustrated in FIG. 14, a nut 440 is open on one side and does not include a nut cap. In the second modification, the sensor device 100 may be provided on a side surface 441 of the nut 440 (a surface orthogonal to the wheel rim 220). The nut 440 is an example of a "fastener" of the present invention.In the present embodiment, it is described that the sensor device 100 is provided in the nut 240, but the present invention is not limited thereto. The sensor device 100 may be provided in a bolt (a bolt separated from the wheel hub). In this case, the screw is an example of a "fastener" of the present invention.In the present embodiment, it is described that the loosening of the nut 240 is detected by the signal processor 2 provided in the sensor device 100, but the present invention is not limited thereto. For example, the detection value of the acceleration sensor 1 may be transmitted to an electronic control unit (ECU) provided in the vehicle 200 via the communication unit 3, and the ECU may detect the loosening of the nut 240 based on the detection value.In the present embodiment, it is described that the fastening state (the looseness) of the nut 240 is detected based on a change in the rotation angle of the nut 240, but the present invention is not limited thereto. The fastening state (the looseness) of the nut 240 can be detected by comparing an amount of change of at least one of the X-axis normalized value (X-axis acceleration) and the Y-axis normalized value (Y-axis acceleration) with a predetermined threshold value. In this case, the X-axis normalized value (X-axis acceleration) and the Y-axis normalized value (Y-axis acceleration) are examples of an "acceleration-based value" of the present invention.In the present embodiment, it is described that the initial value is set when both the X-axis acceleration and the Y-axis acceleration become equal to a value that can be regarded as a zero value, but the present invention is not limited thereto. The initial value may be set when either the X-axis acceleration or the Y-axis acceleration becomes equal to a value that can be regarded as a zero value. Instead of detecting that both the X-axis acceleration and the Y-axis acceleration become equal to a value that can be regarded as a zero value, the initial value may also be set by detecting that the acceleration in the Z-axis direction that is parallel to the gravity direction has reached 1G. Alternatively, the initial value may be set after the composite vector of the X-axis acceleration, the Y-axis acceleration, and the Z-axis acceleration in the direction of gravitational acceleration becomes 1G.In the present embodiment, it is described that the initial value for the X-axis acceleration (Y-axis acceleration) is set, but the present invention is not limited thereto. The initial value may also be set for the rotation angle of the nut 240 calculated from the X-axis acceleration (Y-axis acceleration).In the present embodiment, it is described that the X axis and the Y axis are orthogonal to each other, but the present invention is not limited thereto. The X-axis and the Y-axis need not be orthogonal to each other and may intersect each other.In the present embodiment, the plane in which the X axis and the Y axis are provided is described as being orthogonal to the rotation axis of the wheel rim 220, but the present invention is not limited thereto. The plane need not be orthogonal to the axis of rotation and may intersect the axis of rotation.In the present embodiment, it is described that the fastening state of the nut 240 is detected using the X-axis normalized value and the Y-axis normalized value, but the present invention is not limited thereto. When the X-axis and the Y-axis are orthogonal to each other, the fastening state of the nut 240 can be detected using an inverse triangulation function of the X-axis acceleration and the Y-axis acceleration. The inverse trigonometric function includes an arctangent function (arctan), an arcussinus function (arcsin), an arcussosinus function (arccos), an arcuscotange function (arccot), an arcuscane function (arccsc), and an arcussecan function (arcsec). Moreover, the fastening state of the nut 240 may be detected based on a relationship between the X-axis acceleration and the Y-axis acceleration.In the above embodiment, the number of sensor devices 100 for a wheel rim 220 may be changed accordingly as long as the number is one or more.The above-mentioned embodiments and the above-mentioned modifications can be combined appropriately as long as there is no technical inconsistency.The embodiments disclosed herein are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined not by the foregoing description but by the claims and is intended to include all modifications that correspond to the claims in meaning and scope.LIST OF REFERENCE CHARACTERS1: Acceleration sensor (sensor unit); 2: Signal processor (state detection unit); 100: Sensor device (detection device); 220: Wheel rim (rotating body); 240, 340, 440: Nut (fastening member); 250 a: Wheel hub (predetermined member) (Vehicle body).References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 2005-329907 [0002, 0003]

Claims

A detection device that detects a mounting state of a mounting member that mounts a predetermined member to a rotating body having a rotation axis intersecting a gravity direction, the detection device comprising: a sensor unit that detects acceleration in at least one axis along a plane intersecting the rotation axis of the rotating body when the predetermined member is mounted to the rotating body by the mounting member; and a state detection unit that detects the fastening state of the fastener based on a comparison result between a value based on the acceleration detected by the sensor unit and a value based on a reference acceleration defined as a reference value of the acceleration, wherein after an absolute value of the acceleration in the at least one axis becomes equal to a value that can be regarded as a zero value, the state detection unit defines a value based on the absolute value of the acceleration detected by the sensor unit and greater than the zero value as the reference acceleration.The detection device according to claim 1, wherein the state detection unit acquires the acceleration in the at least one axis of the fastener every predetermined period, and after the absolute value of the acceleration in the at least one axis becomes equal to a value that can be regarded as a zero value, the state detection unit defines an average value of values based on absolute values of accelerations acquired by the sensor unit for a plurality of times and greater than the zero value as a reference acceleration.The detection device according to claim 1 or 2, wherein the state detection unit acquires the acceleration in the at least one axis of the fastener in each predetermined period, and after the absolute value of the acceleration in the at least one axis becomes equal to a value that can be regarded as a zero value, the state detection unit increases the predetermined period.The detection device according to claim 3, wherein after the absolute value of the acceleration in the at least one axis becomes equal to a value that can be regarded as a zero value, the state detection unit gradually increases the predetermined period.The sensing device according to claim 1 or 2, wherein the sensor unit is provided in a nut that fixes a wheel rim to a vehicle body.The detection device according to claim 1 or 2, wherein the at least one axis includes a first axis and a second axis that intersect each other after an absolute value of the acceleration in each of the first axis and the second axis becomes equal to a value that can be regarded as a zero value, the state detection unit defines each of the acceleration in the first axis and the acceleration in the second axis as the reference acceleration when the absolute value of the acceleration in at least one of the first axis and the second axis becomes equal to or greater than a predetermined value that is greater than the zero value.A detection method for detecting a mounting state of a mounting member mounting a predetermined member to a rotating body having a rotation axis intersecting a gravity direction, the detection method comprising: detecting, by a sensor unit that detects the acceleration in the at least one axis along a plane intersecting the rotation axis of the rotating body, an acceleration in at least one axis; when the predetermined member is mounted to the rotating body by the mounting member; after an absolute value of the acceleration in the at least one axis becomes equal to a value that can be regarded as a zero value, defining, as a reference acceleration, a value based on the absolute value of the acceleration detected by the sensor unit and greater than the zero value; detecting the fastening state of the fastening element based on a comparison result between a value based on the acceleration detected by the sensor unit in the at least one axis and a value based on the reference acceleration.

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