Abnormality detection device and position detector

Abstract

this abnormality detection device, in one embodiment, comprises a voltage detector for detecting a voltage corresponding to a resistance value of a thermistor. The abnormality detection device comprises: an acquisition unit that acquires a temperature-related variable on the basis of a signal from the voltage detector; and a determination unit that determines an abnormality in a thermistor circuit on the basis of a change in the variable. The acquisition unit acquires the variable at predetermined time intervals, and calculates, as a first measurement value, a slope of the variable with respect to time. The determination unit determines that an abnormality has occurred in the thermistor circuit if the first measurement value deviates from a predetermined first determination range.

Description

Anomaly detection device and position detector

[0001] The present disclosure relates to an abnormality detection device and a position detector.

[0002] It has been known for some time that a thermistor circuit containing a thermistor is used to measure the temperature of a device. A thermistor is a semiconductor element whose resistance value changes with temperature. A thermistor circuit can estimate the temperature based on the change in the thermistor's resistance value. For example, if the temperature of a device detected by the thermistor circuit becomes too high, the device's operation can be restricted or an alarm can be generated. Monitoring the device temperature with a thermistor circuit can prevent device failure. However, thermistor circuits can also fail. For example, a short circuit or broken wires can occur in the thermistor circuit.

[0003] As a method for detecting an abnormality in the thermistor circuit, if the temperature detected by the thermistor circuit is very high or very low, it can be determined that the thermistor circuit is abnormal. For example, if the temperature detected by the thermistor circuit is significantly outside the temperature range when the device is operating, it can be determined that the thermistor circuit is abnormal.

[0004] JP-A-4-95986 JP-A-2-104994

[0005] In the control for detecting an abnormality in the thermistor circuit, for example, a temperature judgment value lower than the temperature range when the device is operating can be set. Then, if the temperature detected by the thermistor circuit is below the temperature judgment value, it can be determined that the thermistor circuit is abnormal. Alternatively, a temperature judgment value higher than the temperature range when the device is operating can be set. Then, if the temperature detected by the thermistor circuit is higher than the temperature judgment value, it can be determined that the thermistor circuit is abnormal.

[0006] However, even if an abnormality occurs in the thermistor circuit, the detected temperature may still be within the operating temperature range of the device. For example, even if the actual temperature of the device is not high, an abnormality in the thermistor circuit may cause the detected temperature to be in the overheating range. Thus, when an abnormal temperature is detected by the thermistor circuit, it is difficult to determine whether the actual temperature of the device is abnormal or whether the thermistor circuit is abnormal.

[0007] Furthermore, even if the temperature detected by the thermistor circuit is within the operating temperature range of the device, an abnormality in the thermistor circuit may cause the detected temperature to be higher or lower than the actual temperature, which can result in the device being unable to properly manage its temperature.

[0008] A first aspect of the present disclosure is an abnormality detection device that detects an abnormality in a thermistor circuit. The abnormality detection device includes a voltage detector that detects a voltage corresponding to the resistance value of a thermistor. The abnormality detection device includes an acquisition unit that acquires a temperature-related variable based on a signal from the voltage detector, and a determination unit that determines an abnormality in the thermistor circuit based on a change in the variable. The acquisition unit acquires the variable at predetermined time intervals and calculates the slope of the variable with respect to time as a first measurement value. The determination unit determines that an abnormality has occurred in the thermistor circuit when the first measurement value deviates from a predetermined first determination range.

[0009] A second aspect of the present disclosure is an abnormality detection device that detects an abnormality in a thermistor circuit. The abnormality detection device includes a voltage detector that detects a voltage corresponding to the resistance value of a thermistor. The abnormality detection device includes an acquisition unit that acquires a temperature-related variable based on a signal from the voltage detector, and a determination unit that determines an abnormality in the thermistor circuit based on a change in the variable. The acquisition unit acquires the variable at predetermined time intervals and calculates a measured value by second-order differentiation of the variable with respect to time. The determination unit determines that an abnormality has occurred in the thermistor circuit when the measured value deviates from a predetermined determination range.

[0010] A third aspect of the present disclosure is a rotational position detector including the abnormality detection device described above and an angle calculation unit that detects the rotational position of a rotation shaft.

[0011] 1 is a block diagram of a machine tool according to an embodiment; FIG. 2 is a block diagram of a rotational position detector according to an embodiment; FIG. 3 is a thermistor circuit according to an embodiment; FIG. 4 is a graph of voltage detected in the thermistor circuit when overheating occurs in the device; FIG. 5 is a graph of voltage detected in the thermistor circuit when overheating of the device is resolved; FIG. 6 is a graph of voltage detected when cutting fluid adheres to the thermistor; FIG. 7 is a graph of voltage detected when a fixed resistor is damaged; FIG. 8 is a graph of voltage detected when the thermistor circuit is normal; FIG. 9 is a graph explaining control for determining an abnormality in the thermistor circuit when cutting fluid adheres to the thermistor according to the first embodiment; FIG. 10 is a graph explaining control for determining an abnormality in the thermistor circuit when a fixed resistor is damaged according to the first embodiment; FIG. 11 is a flowchart of control for detecting an abnormality in the thermistor circuit according to the first embodiment; FIG. 12 is a graph of voltage when cutting fluid adheres to the thermistor according to a comparative example; FIG. 13 is a graph of voltage when a fixed resistor is damaged according to a comparative example; and FIG. 14 is a graph showing the relationship between the temperature of the thermistor and detected voltage according to a second embodiment. 10 is a graph illustrating control for determining an abnormality in the thermistor circuit when cutting fluid adheres to the thermistor in the second embodiment. 11 is a graph illustrating control for determining an abnormality in the thermistor circuit when a fixed resistor is damaged in the second embodiment. 12 is a flowchart illustrating control for detecting an abnormality in the thermistor circuit in the second embodiment.

[0012] First Embodiment An abnormality detection device and a position detector according to a first embodiment will be described with reference to Figures 1 to 13. The abnormality detection device of this embodiment detects an abnormality in a thermistor circuit equipped with a thermistor. In this embodiment, a rotational position detector that detects the rotational position of a rotating shaft will be described as an example of a device equipped with a thermistor circuit. Furthermore, a machine tool will be described as an example of a device equipped with a rotational position detector.

[0013] 1 is a block diagram of a machine tool according to the present embodiment. Machine tool 1 is a device that cuts a workpiece based on a machining program 45 as an operating program. Machine tool 1 includes an electric motor 9 that drives the components of machine tool 1. Examples of electric motor 9 include a spindle motor that rotates a spindle that holds a tool, or a feed axis motor that moves a table that holds a workpiece or a spindle head including a spindle along a predetermined coordinate axis.

[0014] A rotational position detector 10 is attached to the electric motor 9 for detecting the rotational position or rotational speed of the output shaft. In this embodiment, the rotational position detector 10 is configured as an encoder. A rotational position signal output from the rotational position detector 10 is input to the machine control device 41. Note that the device equipped with the rotational position detector is not limited to the electric motor of a machine tool, and any device can be used.

[0015] The machine tool 1 of this embodiment is a numerically controlled device. The machine tool 1 is equipped with a machine control device 41 that controls the operations of its components. The machine control device 41 of this embodiment includes an arithmetic processing device (computer). The machine control device 41 includes a CPU (Central Processing Unit) as a processor. The machine control device 41 has a RAM (Random Access Memory), a ROM (Read Only Memory), and the like, which are connected to the CPU via a bus.

[0016] Machine tool 1 is driven based on command statements written in a machining program 45 created in advance. Machine control device 41 includes a memory unit 42 that stores information related to machine tool 1, and an operation control unit 43 that generates operation commands for electric motor 9 based on the machining program 45. Machine tool 1 includes a drive device 46 that includes an electric circuit that supplies electricity to electric motor 9 based on the operation commands generated by operation control unit 43. Electric motor 9 is driven by the electricity supplied by drive device 46.

[0017] The operation control unit 43 corresponds to a processor that operates in accordance with the machining program 45. The processor that functions as the operation control unit 43 is configured to be able to read information stored in the storage unit 42. The processor reads the machining program 45 stored in the storage unit 42 and performs the control defined in the machining program 45, thereby functioning as the operation control unit 43.

[0018] The storage unit 42 may be configured with a non-transitory storage medium capable of storing information, such as a volatile memory, a non-volatile memory, a magnetic storage medium, or an optical storage medium.

[0019] Machine control device 41 includes a display unit 44 that displays information related to machine tool 1. Display unit 44 can be configured with any display panel, such as a liquid crystal display panel or an organic EL (Electro Luminescence) display panel.

[0020] 2 shows a block diagram of the rotational position detector of this embodiment. The rotational position detector 10 can be attached to any rotating shaft, such as a drive shaft. In this embodiment, the rotational position detector 10 is attached to the output shaft of the electric motor 9.

[0021] The rotational position detector 10 of this embodiment is an optical detector. The rotational position detector 10 of this embodiment includes a light-emitting element 11 configured as an LED (Light Emitting Diode) and a light-receiving element 12 that receives light from the LED. The rotational position detector 10 also includes a rotating plate disposed between the light-emitting element 11 and the light-receiving element 12. The rotating plate has a plurality of small holes formed along its circumferential direction. The rotating plate is connected to the output shaft of the electric motor 9.

[0022] As the output shaft of the electric motor 9 and the rotating plate rotate, holes in the rotating plate allow or block light from the light-emitting element 11. The light-receiving element 12 receives the light that passes through the holes.

[0023] The rotational position detector 10 includes a circuit board 13 on which an electric circuit is formed. The rotational position detector 10 also includes a microcomputer disposed on the circuit board as an arithmetic processing device. The microcomputer of this embodiment includes a CPU as a processor. The processor is not limited to a CPU, and any element capable of arithmetic processing, such as an LSI (Large Scale Integration), an IC (Integrated Circuit), or an ASIC (Application Specific Integrated Circuit), can be used.

[0024] The microcomputer includes an angle calculation unit 21. The angle calculation unit 21 can calculate the rotational position (rotation angle) of the rotating plate based on the pattern of light received by the light receiving element 12. The angle calculation unit 21 can also detect the rotational speed based on the rotational position and time. The angle calculation unit 21 corresponds to a processor that performs predetermined processing. The processor performs predetermined control to function as the angle calculation unit 21.

[0025] The microcomputer includes a storage unit 14 that stores information related to the rotational position detector 10. The storage unit 14 can be configured with a non-transitory storage medium capable of storing information. The storage unit 14 can be configured with a storage medium such as a volatile memory, a non-volatile memory, a magnetic storage medium, or an optical storage medium. The processor stores information in the storage unit 14 and reads information stored in the storage unit 14. Note that the rotational position detector is not limited to the optical detector described above, and any other type, such as a magnetic type, can be used.

[0026] 3 shows the thermistor circuit of this embodiment. With reference to FIGS. 2 and 3, the rotational position detector 10 of this embodiment includes a thermistor circuit 19 that detects the temperature of the circuit board 13. The thermistor circuit 19 can be formed on the circuit board 13 of the rotational position detector 10. In this embodiment, the thermistor circuit 19 is arranged to detect the temperature of the circuit board 13.

[0027] In this embodiment, a thermistor circuit 19 including a negative temperature coefficient (NTC) thermistor whose resistance value decreases as the temperature increases will be described as an example. However, the abnormality detection device of this embodiment can be applied to a thermistor circuit including any thermistor. For example, control similar to that of this embodiment can be performed on a thermistor circuit including a positive temperature coefficient (PTC) thermistor whose resistance value increases as the temperature increases.

[0028] The thermistor circuit 19 includes a thermistor 15, which is a semiconductor whose resistance value changes with temperature. The thermistor circuit 19 also includes a fixed resistor 16 that maintains a predetermined resistance value. In the thermistor circuit 19 of this embodiment, the thermistor 15 and the fixed resistor 16 are connected in series. The fixed resistor 16 is grounded. A predetermined supply voltage Vcc is applied to the thermistor 15 by a power supply. The supply voltage Vcc is, for example, 5 V. The thermistor circuit 19 also includes a voltage detector 17 for detecting the voltage between the thermistor 15 and the fixed resistor 16.

[0029] The thermistor circuit 19 includes an acquisition unit 22 that acquires a variable related to temperature based on a signal from the voltage detector 17. The variable includes, for example, the voltage detected by the voltage detector 17, or the temperature of the thermistor 15 calculated from the voltage detected by the voltage detector 17. In this embodiment, the voltage detected by the voltage detector 17 is referred to as the "detected voltage."

[0030] The relationship between the detected voltage V detected by the voltage detector 17, the resistance value Rth of the thermistor 15, the resistance value R of the fixed resistor 16, and the supply voltage Vcc supplied by the power supply is expressed by the following equation (1).

[0031] V=R / (Rth+R)×Vcc…(1)

[0032] The resistance value Rth changes depending on the temperature of the thermistor 15. The acquisition unit 22 can calculate the resistance value Rth of the thermistor 15 from the detected voltage V detected by the voltage detector 17 based on the above formula (1).

[0033] The relationship between the resistance value Rth of the thermistor 15 and the temperature of the thermistor 15 can be determined in advance and stored in the storage unit 14. The acquisition unit 22 can then detect the temperature of the thermistor 15 based on the resistance value Rth of the thermistor 15.

[0034] The tendency of the resistance value to change with temperature may differ depending on the type of thermistor. However, the thermistor circuit shown in Figure 3 can be used for any thermistor. The temperature of the thermistor can be estimated based on the detected voltage.

[0035] The rotational position detector 10 of this embodiment includes an abnormality detection device that detects an abnormality in the thermistor circuit 19. The abnormality detection device includes a voltage detector 17 that detects a voltage corresponding to the resistance value of the thermistor 15. The abnormality detection device also includes an acquisition unit 22 that acquires a temperature-related variable based on a signal from the voltage detector 17. In this embodiment, the detected voltage detected by the voltage detector 17 will be used as an example of the variable acquired by the acquisition unit 22. The temperature-related variable is not limited to the detected voltage, and a temperature detected from the detected voltage may also be used. The abnormality detection device also includes a determination unit 23 that determines an abnormality in the thermistor circuit 19 based on a change in the variable acquired by the acquisition unit 22.

[0036] The abnormality detection device of this embodiment includes the above-mentioned microcomputer. The acquisition unit 22 and the determination unit 23 correspond to processors that perform predetermined processing. The processors perform predetermined control to function as the respective units.

[0037] 4 is a graph illustrating the change in detection voltage when the temperature of the circuit board increases. Referring to FIGS. 3 and 4, the resistance of thermistor 15 in this embodiment decreases as the temperature increases. As the temperature of thermistor 15 increases over the operating time of rotational position detector 10, the resistance of thermistor 15 decreases, and the detection voltage increases as indicated by arrow 61. In this example, at time t1, the detection voltage exceeds the alarm generation threshold at which an alarm is issued.

[0038] The alarm generation determination value in this embodiment is a determination value for determining whether the temperature of the circuit board 13 is too high. The alarm generation determination value can be set to a temperature higher than the normal temperature range of the circuit board 13. The alarm generation determination value is predetermined and stored in the storage unit 14. The determination unit 23 can determine that the temperature of the circuit board 13 is too high when the detected voltage exceeds the alarm generation determination value. In other words, the determination unit 23 can determine that the circuit board 13 is overheating.

[0039] The determination unit 23 notifies the machine control device 41 that the temperature of the circuit board 13 has exceeded the warning threshold. The display unit 44 can then display a warning that the temperature of the circuit board 13 has exceeded the warning threshold. Alternatively, the operation control unit 43 can limit the operation of the electric motor 9 to suppress the operation of the rotational position detector 10. Note that in addition to the threshold for issuing a high-temperature warning, a threshold for issuing a low-temperature warning may be set. In this case, the determination unit can determine that the temperature of the circuit board is too low when the temperature of the circuit board is lower than the low-temperature threshold.

[0040] Figure 5 shows a graph illustrating the change in the detected voltage over time when the alarm is canceled. In the graph in Figure 5, the temperature of the circuit board 13, which was in an overheated state, drops. As the temperature of the thermistor 15 drops, the detected voltage drops, as indicated by arrow 62. At time t2, the detected voltage becomes smaller than the alarm generation threshold.

[0041] The determination unit 23 notifies the machine control device 41 that the temperature of the circuit board 13 has fallen below the warning generation determination value. The display unit 44 can display that the temperature of the circuit board 13 has fallen within the normal temperature range. Alternatively, the operation control unit 43 can release the restriction on the operation of the electric motor 9.

[0042] In this way, the thermistor circuit 19 in this embodiment detects the temperature of the device to which the thermistor 15 is attached, and can issue an alarm when the detected temperature deviates from a predetermined normal temperature range.

[0043] However, if an abnormality occurs in the thermistor circuit 19, it may not be possible to accurately detect the temperature by the thermistor circuit 19. Next, an abnormality detection device for detecting an abnormality in the thermistor circuit 19 will be described.

[0044] 6 shows a graph of the detected voltage when liquid adheres to the thermistor of the thermistor circuit. In this example, cutting fluid adheres to thermistor 15 when a workpiece is machined by machine tool 1. At time t3, the resistance value of the thermistor decreases rapidly due to the adhesion of cutting fluid to the thermistor. The detected voltage rises rapidly, as indicated by arrow 63. In this example, the detected voltage rises above the alarm generation threshold.

[0045] The detected voltage increases when a liquid such as cutting fluid adheres to the thermistor, or when a liquid adheres to a fixed resistor. The detected voltage also increases when the solder pad to which the thermistor is fixed is short-circuited. The detected voltage also increases when a conductive foreign object such as metal comes into contact with the thermistor.

[0046] 7 shows a graph of the change in detection voltage over time when a crack occurs in the fixed resistor. At time t4, a crack occurs in the fixed resistor 16, causing the resistance value of the fixed resistor 16 to increase. The detection voltage then drops sharply, as indicated by arrow 64. In this example, the detection voltage, which was higher than the alarm generation threshold, drops to below the threshold. This phenomenon of a drop in detection voltage occurs, for example, when the lead wire in the thermistor circuit is damaged or the fixed resistor becomes detached from the solder pad.

[0047] Incidentally, referring to Fig. 6, even if the actual temperature of the circuit board is not rising, if the detected voltage rises and exceeds the alarm threshold, a circuit board temperature alarm is issued. Also, referring to Fig. 7, even if the actual temperature of the circuit board is not falling, if the detected voltage falls and becomes smaller than the alarm threshold, the circuit board temperature alarm is canceled. Thus, if the thermistor circuit is abnormal, an inaccurate determination of the device temperature may be made.

[0048] Alternatively, when the detection voltage changes from a state lower than the alarm generation threshold to a state higher than the alarm generation threshold, or when the detection voltage changes from a state higher than the alarm generation threshold to a state lower than the alarm generation threshold, it may be difficult to determine whether the temperature of the circuit board has changed or whether an abnormality has occurred in the thermistor circuit.

[0049] In the abnormality detection device of this embodiment, the acquisition unit 22 acquires a temperature-related variable based on a signal from the voltage detector 17 at predetermined time intervals. Here, the acquisition unit 22 acquires the detected voltage. The acquisition unit 22 calculates the slope of the detected voltage with respect to time as a first measurement value. Then, the determination unit 23 determines that an abnormality has occurred in the thermistor circuit if the first measurement value deviates from a predetermined first determination range. In other words, the abnormality detection device of this embodiment monitors the continuity of the variable value and determines that an abnormality has occurred in the thermistor circuit if the value of the variable changes suddenly.

[0050] FIG. 8 shows a graph of the detected voltage when the temperature of the circuit board is substantially constant within the normal temperature range and the thermistor circuit is normal. The acquisition unit 22 acquires the detected voltage at predetermined time intervals. The acquisition unit 22 can detect the detected voltage, for example, approximately every 1 msec. The acquisition unit 22 can then select a variable at an appropriate time interval from the detected variables. In the example shown in FIG. 8, the acquisition unit 22 acquires the detected voltage every 1 sec. The detected voltage detected by the voltage detector 17 is substantially constant. The determination unit 23 determines that the thermistor circuit is normal.

[0051] FIG. 9 shows a graph of the detected voltage when cutting fluid adheres to the thermistor. In addition to measurement points MP1 to MP6, the actual voltage is shown by a solid line. At time t3, cutting fluid adheres, and the detected voltage rises sharply, as indicated by arrow 63. In this example, the acquisition unit 22 acquires the detected voltages at measurement points MP1 to MP6 at one-second intervals. The actual detected voltage rises sharply between measurement points MP3 and MP4. The acquisition unit 22 calculates the slope of the detected voltage between measurement points MP1 to MP4 as the first measurement value.

[0052] In this example, the slope of the detected voltage per unit time is calculated as the first measurement value based on measurement points at predetermined time intervals. The acquisition unit 22 calculates the slope between adjacent measurement points as the first measurement value. For example, the acquisition unit 22 calculates the slope between measurement points MP1 and MP2, the slope between measurement points MP2 and MP3, and the slope between measurement points MP3 and MP4. A first determination range is predetermined for the first measurement value. The determination unit 23 determines that an abnormality has occurred in the thermistor circuit 19 when each first measurement value deviates from the first determination range.

[0053] Here, upper and lower limit values ​​of the slope are predefined as the first determination range. The upper limit value can be set to a large upward slope that cannot occur when the device is driven. The lower limit value can be set to a large downward slope with a large absolute value that cannot occur when the device is driven. For example, if the variable increases slowly, it can be determined that the thermistor circuit is normal and the temperature of the device is rising.

[0054] In this example, the slope from measurement point MP1 to measurement point MP2 and the slope from measurement point MP2 to measurement point MP3 are both nearly zero, falling within the first determination range. The determination unit 23 determines that the thermistor circuit 19 is normal. In contrast, the slope between measurement point MP3 and measurement point MP4, indicated by arrow 65, exceeds the upper limit of the first determination range. In this case, the determination unit 23 determines that an abnormality has occurred in the thermistor circuit 19. Furthermore, the slope from measurement point MP4 onward is nearly zero, falling within the first determination range. However, the determination unit 23 maintains its determination that an abnormality has occurred in the thermistor circuit 19 until the occurrence of the abnormality is reset by an operator or the like.

[0055] FIG. 10 shows a graph of the detected voltage when a crack occurs in the fixed resistor 16 and the resistor is damaged. At time t4, a crack occurs in the fixed resistor 16, causing the detected voltage to drop sharply, as indicated by arrow 64. The slope between the measurement points up to measurement point MP13 is nearly zero, which is within the first determination range. The determination unit 23 determines that the thermistor circuit 19 is normal. In contrast, the slope between measurement point MP13 and measurement point MP14, as indicated by arrow 66, is less than the lower limit of the first determination range. In this case, the determination unit 23 determines that an abnormality has occurred in the thermistor circuit 19. The slope from measurement point MP14 onward is within the first determination range. However, the determination unit 23 maintains its determination that an abnormality has occurred in the thermistor circuit 19 until the occurrence of the abnormality is reset by an operator or the like.

[0056] As described above, in this embodiment, it is possible to determine whether or not an abnormality has occurred in the thermistor circuit 19 based on the slope of the temperature-related variable and the first determination range. The storage unit 14 of the abnormality detection device in this embodiment can store information related to the abnormality in the thermistor circuit 19. When the determination unit 23 determines that an abnormality has occurred in the thermistor circuit 19, the storage unit 14 stores information related to the time when the abnormality occurred and the first measurement value. Examples of the information related to the first measurement value include the detected voltage at the measurement point where the abnormality occurred and the slope of the detected voltage.

[0057] Furthermore, when the determination unit 23 determines that an abnormality has occurred in the thermistor circuit 19, the display unit 44 can display a warning that an abnormality has occurred in the thermistor circuit 19. Alternatively, the display unit 44 may display information related to the first measurement value, such as the time, the detected voltage, and the slope of the detected voltage.

[0058] Furthermore, the determination unit 23 can determine whether the slope of the variable as the first measurement value is positive or negative. Referring to Fig. 9, when the first measurement value is a positive value, the determination unit 23 can estimate the cause of the abnormality, such as adhesion of cutting fluid to the thermistor or fixed resistor, contact with a conductive foreign object such as metal, or a short circuit between thermistor pads. Referring to Fig. 10, when the first measurement value is a negative value, the determination unit 23 can estimate the cause of the abnormality, such as damage to the fixed resistor, such as cracks in the fixed resistor, or damage to the lead wires of the thermistor circuit.

[0059] The storage unit 14 can store whether the first measurement value is a positive value or a negative value together with the time of occurrence of the abnormality. The storage unit 14 may also store the estimated cause of the abnormality. Furthermore, the display unit 44 can display whether the first measurement value is a positive value or a negative value together with the time. Furthermore, the display unit 44 may also display the estimated cause of the abnormality.

[0060] 11 shows a flowchart of control for detecting an abnormality in the thermistor circuit in this embodiment. In step 71, the acquisition unit 22 acquires the time and the detected voltage obtained from the voltage detector 17. Here, the detected voltage is acquired at predetermined time intervals. Next, in step 72, the storage unit 42 stores the time and the detected voltage.

[0061] In step 73, the acquisition unit 22 calculates the slope of the detected voltage as a first measurement value based on the previously acquired detected voltage and the currently acquired detected voltage. Here, the amount of change in the detected voltage per unit time is calculated as the slope of the detected voltage.

[0062] Next, in step 74, the determination unit 23 determines whether the slope of the detected voltage is within a first determination range. If the slope of the detected voltage is outside the first determination range in step 74, the determination unit 23 can determine that an abnormality has occurred in the thermistor circuit. In this case, control proceeds to step 76.

[0063] In step 76, the display unit 44 of the machine control device 41 displays an alarm indicating that an abnormality has occurred in the thermistor circuit 19. In step 77, the memory unit 14 of the rotational position detector 10 can store information related to the alarm. For example, the memory unit 14 can store information related to the first measurement value, such as the time the alarm occurred and the value of the detected voltage at the measurement point when the alarm occurred and the slope of the detected voltage. Then, the control for detecting the abnormality ends.

[0064] On the other hand, if the slope of the detected voltage is within the first determination range in step 74, control proceeds to step 75. In this case, the determination unit 23 can determine that the thermistor circuit is normal. In step 75, the abnormality detection device determines whether or not it has received an end signal for control to detect an abnormality in the thermistor circuit 19. The control to detect an abnormality ends, for example, when the electric motor 9 stops. If a signal to end abnormality monitoring has not been received, control returns to step 71, and the control from step 71 to step 75 is repeated. On the other hand, if a signal to end control to monitor an abnormality has been received in step 75, this control ends.

[0065] If the determination unit 23 determines in step 74 that an abnormality has occurred in the thermistor circuit, the machine control device 41 can perform any control. For example, the machine control device 41 can stop the machine tool 1 or restrict the operation of the electric motor 9.

[0066] As a comparative example, Figure 12 shows a graph of the detected voltage when cutting fluid adheres to the thermistor. In the example shown in Figure 12, cutting fluid adheres to the thermistor at time t5, and the detected voltage rises sharply, as indicated by arrow 63. However, although the detected voltage has risen, it has not yet reached the threshold for issuing an alarm. This makes it difficult for the operator to notice an abnormality in the thermistor circuit.

[0067] As a comparative example, Figure 13 shows a graph of the detected voltage when a crack occurs in the fixed resistor. In the example shown in Figure 13, a crack occurs in the fixed resistor at time t6, causing a sudden drop in the detected voltage, as indicated by arrow 64. However, because the detected voltage before the crack occurs in the fixed resistor is smaller than the alarm threshold, no alarm is issued. This makes it difficult for an operator to notice an abnormality in the thermistor circuit.

[0068] In addition to the possibility of false alarms being issued or cleared, as mentioned above, there are cases where an operator does not notice an abnormality in the thermistor circuit, resulting in control continuing at the temperature detected by the abnormal thermistor circuit.

[0069] In contrast, the anomaly detection device of this embodiment determines an anomaly in the thermistor circuit based on the gradient of the temperature-related variable, so it can detect an anomaly in the thermistor circuit without relying on the magnitude of the variable or the alarm generation threshold value. This allows for accurate detection of an anomaly in the thermistor circuit, improving the reliability of the temperature detected by the thermistor circuit.

[0070] The rotational position detector of this embodiment can be attached to the spindle motor of the spindle head of a machine tool. The spindle motor is placed inside a machining chamber where cutting fluid splashes, so there is a risk of cutting fluid adhering to the motor. Even in this case, the abnormality detection device of this embodiment can accurately detect an abnormality in the thermistor circuit.

[0071] Second Embodiment An abnormality detection device and a position detector in a second embodiment will be described with reference to Figures 14 to 17. The configuration of the machine tool and the configuration of the rotational position detector in this embodiment are the same as those in the first embodiment (see Figures 1 and 2). The abnormality detection device in this embodiment differs from the first embodiment in the control for detecting an abnormality in the thermistor circuit.

[0072] Figure 14 shows a graph of the detected voltage versus temperature of an NTC thermistor. The horizontal axis represents the thermistor temperature, and the vertical axis represents the detected voltage output from the thermistor circuit. In the low temperature range, the increase in the detected voltage relative to an increase in temperature is small. That is, on the low temperature side, the change in the detected voltage relative to a change in temperature is small. In contrast, in the high temperature range, the increase in the detected voltage relative to an increase in temperature is large.

[0073] For example, in the range below 80°C, the slope of the detection voltage with respect to temperature is small, but in the range above 80°C, the slope of the detection voltage with respect to temperature is large. For this reason, in the control for detecting an abnormality in the first embodiment, the detection sensitivity for an abnormality is high in the high temperature range, but in the low temperature range, the slope of the detection voltage with respect to temperature change is small, so the detection sensitivity is low. In other words, in the low temperature range, there is a risk that the accuracy of determining whether or not an abnormality has occurred in the thermistor circuit will decrease.

[0074] Referring to FIG. 2 , the acquisition unit 22 in this embodiment acquires a variable at predetermined time intervals. The acquisition unit 22 calculates a second measurement value by differentiating the variable twice with respect to time. The determination unit 23 then determines that an abnormality has occurred in the thermistor circuit when the second measurement value deviates from a predetermined second determination range. The second determination range can be set to have an upper limit set to a large value that cannot occur when the device is operated. Also, the second determination range can be set to have a lower limit set to a small value that cannot occur when the device is operated.

[0075] 15 shows a graph of the detected voltage when cutting fluid adheres to the thermistor. At time t3, cutting fluid adheres, and the detected voltage rises sharply, as indicated by arrow 63. The time intervals and detected voltages at measurement points MP1 to MP6 are the same as those in FIG. 9 for the first embodiment. In this example, the acquisition unit 22 acquires the detected voltage at one-second time intervals.

[0076] In this embodiment, the acquisition unit 22 selects three consecutive measurement points. Then, the acquisition unit 22 acquires the slope of the detected voltage from two consecutive measurement points. The acquisition unit 22 calculates a second measurement value by second-order differentiation of the detected voltage based on the slope of the adjacent detected voltages. Then, the determination unit 23 determines that an abnormality has occurred in the thermistor circuit if the second measurement value deviates from a predetermined second determination range.

[0077] For example, the acquisition unit 22 calculates a first slope from measurement point MP1 to measurement point MP2 and a second slope from measurement point MP2 to measurement point MP3. The acquisition unit 22 then subtracts the first slope from the second slope, divides the result by the time from measurement point MP1 to measurement point MP3, and calculates a second derivative. The second derivative corresponds to the second measurement value. In this example, the second measurement value is approximately zero. Because the second measurement value is within the second determination range, the determination unit 23 determines that the thermistor circuit 19 is normal.

[0078] Next, the acquisition unit 22 calculates the second-order differential value at measurement point MP3. The acquisition unit 22 acquires the detected voltages at measurement points MP2, MP3, and MP4. The acquisition unit 22 then calculates a first slope from measurement point MP2 to measurement point MP3, as indicated by arrow 67a, and a second slope from measurement point MP3 to measurement point MP4, as indicated by arrow 67b. The acquisition unit 22 then subtracts the first slope from the second slope, divides the result by the time ts1 from measurement point MP2 to measurement point MP4, and calculates the second differential value as the second measurement value. In this case, because the second measurement value exceeds the upper limit of the second determination range, the determination unit 23 determines that the thermistor circuit is abnormal.

[0079] After the determination unit 23 detects an abnormality in the thermistor circuit, control to detect the abnormality can be suspended until the abnormality alarm is reset. Alternatively, the acquisition unit 22 acquires the detected voltages at measurement points MP3, MP4, and MP5. In this case, the second measurement value is smaller than the lower limit of the second determination range. However, because the second measurement value has just exceeded the upper limit of the second determination range, the determination unit 23 may determine that the increase in the detected voltage has been completed.

[0080] FIG. 16 shows a graph of the detected voltage when a crack occurs in the fixed resistor 16. At time t4, a crack occurs in the fixed resistor 16, and the detected voltage suddenly decreases, as indicated by arrow 64. The acquisition unit 22 selects three consecutive measurement points. The acquisition unit 22 acquires the slope of the detected voltage from two consecutive measurement points. The acquisition unit 22 calculates the second-order derivative of the detected voltage with respect to time from the slope of the adjacent detected voltages, as a second measurement value.

[0081] The change in the slope of the detected voltage up to measurement point MP13 is small. For example, the second derivative of the detected voltage at measurement point MP12 is almost zero, which is within the second determination range. The determination unit 23 determines that the thermistor circuit 19 is normal.

[0082] Next, the acquisition unit 22 calculates the second-order differential value at measurement point MP13. The acquisition unit 22 subtracts the slope between measurement points MP12 and MP13, indicated by arrow 68a, from the slope between measurement points MP13 and MP14, indicated by arrow 68b, and divides this value by the time ts2 from measurement point MP12 to measurement point MP14 to calculate a second measurement value. Because the second measurement value is less than the lower limit of the second determination range, the determination unit 23 determines that an abnormality has occurred in the thermistor circuit 19. After the determination unit 23 determines that an abnormality has occurred in the thermistor circuit, control to detect the abnormality can be suspended until the abnormality alarm is reset.

[0083] In this way, if the value of the second derivative of the temperature-related variable falls outside the second determination range, it can be determined that an abnormality has occurred in the thermistor circuit. In other words, if the slope of the temperature-related variable changes suddenly, it can be determined that an abnormality has occurred in the thermistor circuit.

[0084] Furthermore, the determination unit 23 can determine whether the second measurement value is a positive value or a negative value. Then, the same control as in the first embodiment can be performed. Referring to FIG. 15 , when the second measurement value is a positive value, the determination unit 23 can estimate the cause in the same way as when the first measurement value is a positive value. For example, the determination unit 23 can estimate that cutting fluid has adhered to the thermistor or the fixed resistor. Referring to FIG. 16 , when the second measurement value is a negative value, the determination unit 23 can estimate the cause in the same way as when the first measurement value is a negative value. For example, the determination unit 23 can estimate that a crack has occurred in the fixed resistor.

[0085] As in the first embodiment, the display unit 44 may display the time of occurrence of the abnormality, the second measurement value, and an alarm, or may display whether the second measurement value is positive or negative, or may display the estimated cause of the abnormality. As in the first embodiment, the storage unit 42 may store the second measurement value, the alarm, and the estimated cause of the abnormality together with the time of occurrence of the abnormality.

[0086] 17 is a flowchart showing the control for detecting an abnormality in the thermistor circuit in this embodiment. Steps 71 and 72 are the same as steps 71 and 72 in the control in the first embodiment (see FIG. 11).

[0087] Next, in step 81, the acquisition unit 22 calculates a second measurement value by second-order differentiation of the detected voltage with respect to time based on the previously detected voltage and the currently detected voltage. In step 82, the determination unit 23 determines whether the second-order differentiation of the detected voltage is within a second determination range. If the second-order differentiation of the detected voltage is not within the second determination range in step 82, the determination unit 23 determines that an abnormality has occurred in the thermistor circuit. In this case, control proceeds to steps 84 and 85.

[0088] Steps 84 and 85 are similar to steps 76 and 77 in the control of the first embodiment (see FIG. 11). That is, the display unit 44 displays information about the alarm, and the storage unit 42 stores the information about the alarm. Then, this control ends.

[0089] On the other hand, if the second-order differential of the detected voltage is within the second determination range in step 82, the determination unit 23 determines that the thermistor circuit is normal. In this case, control proceeds to step 83.

[0090] The control of step 83 is the same as that of step 75 in the control of the first embodiment (see FIG. 11). That is, the controls of steps 71 to 83 are repeated until a signal to terminate the control for determining whether the thermistor circuit is normal is input.

[0091] The control for detecting an abnormality in the thermistor circuit according to this embodiment can accurately detect an abnormality in the thermistor circuit, regardless of the temperature of the thermistor. In particular, even when the change in the detection voltage relative to temperature is small (when the sensitivity of the detection voltage to temperature is low), an abnormality in the thermistor circuit can be accurately detected. For example, in the case of an NTC thermistor shown in FIG. 14, an abnormality in the thermistor circuit can be accurately detected even in the low-temperature range below 80°C.

[0092] Note that the control for detecting an abnormality in the first embodiment and the control for detecting an abnormality in the second embodiment may be performed simultaneously. For example, if it is determined that the thermistor circuit is abnormal in either the control in the first embodiment or the control in the second embodiment, it may ultimately be determined that the thermistor circuit is abnormal. Alternatively, the control for detecting an abnormality in the first embodiment and the control for detecting an abnormality in the second embodiment may be performed independently, and information related to the alarm may be displayed or stored separately.

[0093] According to at least one of the embodiments described above, it is possible to provide an abnormality detection device that can accurately detect an abnormality in a thermistor circuit.

[0094] Although the present disclosure has been described in detail, the present disclosure is not limited to the individual embodiments described above. Various additions, substitutions, modifications, partial deletions, etc. are possible in these embodiments without departing from the gist of the present disclosure or the spirit of the present disclosure derived from the content of the claims and their equivalents. These embodiments can also be implemented in combination. For example, in the above-described embodiments, the order of each operation and the order of each process are shown as examples and are not limited to these. The same applies when numerical values ​​or mathematical expressions are used in the description of the above-described embodiments.

[0095] The following supplementary notes are disclosed regarding the above-described embodiment and modifications.

[0096] (Supplementary Note 1) An abnormality detection device that detects an abnormality in a thermistor circuit, comprising: a voltage detector that detects a voltage corresponding to a resistance value of a thermistor; an acquisition unit that acquires a variable related to temperature based on a signal from the voltage detector; and a judgment unit that judges an abnormality in the thermistor circuit based on a change in the variable, wherein the acquisition unit acquires the variable at predetermined time intervals and calculates a slope of the variable with respect to time as a first measurement value, and the judgment unit judges that an abnormality has occurred in the thermistor circuit when the first measurement value deviates from a predetermined first judgment range.

[0097] (Supplementary Note 2) The abnormality detection device according to Supplementary Note 1, wherein the determination unit determines whether the first measurement value is a positive value or a negative value and displays the determination result on the display unit.

[0098] (Supplementary Note 3) An abnormality detection device according to Supplementary Note 1 or 2, further comprising a memory unit that stores information relating to an abnormality in the thermistor circuit, and when the determination unit determines that an abnormality has occurred in the thermistor circuit, the memory unit stores information relating to the time when the abnormality occurred and the first measurement value.

[0099] (Appendix 4) An abnormality detection device according to any one of Appendices 1 to 3, wherein the acquisition unit calculates a value obtained by second-order differentiation of the variable with respect to time as the second measurement value, and the determination unit determines that an abnormality has occurred in the thermistor circuit when the second measurement value deviates from a predetermined second determination range.

[0100] (Supplementary Note 5) A rotational position detector comprising: the abnormality detection device according to Supplementary Note 1; and an angle calculation unit that detects the rotational position of a rotation shaft.

[0101] (Supplementary Note 6) An abnormality detection device that detects an abnormality in a thermistor circuit, comprising: a voltage detector that detects a voltage corresponding to a resistance value of a thermistor; an acquisition unit that acquires a variable related to temperature based on a signal from the voltage detector; and a judgment unit that judges an abnormality in the thermistor circuit based on a change in the variable, wherein the acquisition unit acquires the variable at predetermined time intervals and calculates a value obtained by differentiating the variable twice with respect to time as a measured value, and the judgment unit judges that an abnormality has occurred in the thermistor circuit when the measured value deviates from a predetermined judgment range.

[0102] (Supplementary Note 7) The abnormality detection device according to Supplementary Note 6, wherein the determination unit determines whether the measurement value is a positive value or a negative value and displays the determination result on the display unit.

[0103] REFERENCE SIGNS LIST 10 Rotational position detector 13 Circuit board 14 Memory unit 17 Voltage detector 19 Thermistor circuit 22 Acquisition unit 23 Determination unit 41 Machine control device 42 Memory unit 44 Display unit

Claims

1. An abnormality detection device that detects an abnormality in a thermistor circuit, comprising: a voltage detector that detects a voltage corresponding to the resistance value of a thermistor; an acquisition unit that acquires a variable related to temperature based on a signal from the voltage detector; and a judgment unit that judges an abnormality in the thermistor circuit based on a change in the variable, wherein the acquisition unit acquires the variable at predetermined time intervals and calculates the slope of the variable with respect to time as a first measurement value, and the judgment unit judges that an abnormality has occurred in the thermistor circuit when the first measurement value deviates from a predetermined first judgment range.

2. The abnormality detection device according to claim 1, wherein the determination unit determines whether the first measurement value is a positive value or a negative value and displays the determination result on a display unit.

3. An abnormality detection device as described in claim 1 or 2, further comprising a memory unit that stores information regarding an abnormality in the thermistor circuit, and when the judgment unit judges that an abnormality has occurred in the thermistor circuit, the memory unit stores information regarding the time when the abnormality occurred and the first measurement value.

4. An abnormality detection device as described in any one of claims 1 to 3, wherein the acquisition unit calculates a value obtained by second-order differentiation of the variable with respect to time as a second measurement value, and the judgment unit judges that an abnormality has occurred in the thermistor circuit when the second measurement value deviates from a predetermined second judgment range.

5. A rotational position detector comprising the abnormality detection device according to claim 1 and an angle calculation unit that detects the rotational position of a rotation shaft.

6. An abnormality detection device for detecting an abnormality in a thermistor circuit, comprising: a voltage detector for detecting a voltage corresponding to the resistance value of a thermistor; an acquisition unit for acquiring a variable related to temperature based on a signal from the voltage detector; and a judgment unit for judging an abnormality in the thermistor circuit based on a change in the variable, wherein the acquisition unit acquires the variable at predetermined time intervals and calculates a measured value by second-order differentiation of the variable with respect to time, and the judgment unit judges that an abnormality has occurred in the thermistor circuit when the measured value deviates from a predetermined judgment range.

7. The abnormality detection device according to claim 6, wherein the determination unit determines whether the measurement value is a positive value or a negative value and displays the determination result on a display unit.

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