Encoder abnormality diagnosis device

Abstract

An encoder abnormality diagnosing apparatus configured to diagnose an abnormality in an encoder (1) that outputs a cosine waveform and a sine waveform in accordance with a rotation angle of an object, the apparatus including: a stop determination processing unit (15) configured to determine whether the object is rotating or stopping based on a change in a count value of a pulse signal obtained by pulsing each of the cosine waveform and the sine waveform; a frequency measuring unit (16) configured to measure a frequency of the pulse signal; and a frequency diagnosing unit (17) configured to, when the stop determination processing unit (15) determines that the object is stopping, compare the frequency measured by the frequency measuring unit with a preset upper limit frequency, and determine that an abnormality has occurred in the encoder when the measured frequency exceeds the upper limit frequency.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority from Japanese Patent Application No. 2020-176309 filed October 20, 2020, the entire contents of which (including the specification, claims, drawings and abstract) are incorporated herein by reference. TECHNICAL FIELD

[0003] Disclosed is an encoder abnormality diagnosing device configured to diagnose an abnormality in an encoder that outputs a cosine waveform and a sine waveform, based on a rotation angle of an object. BACKGROUND

[0004] For a machine tool spindle, international standards require a speed limit monitoring function. For example, for a spindle of a lathe, ISO 23125 requires speed limit monitoring in safety category 3.

[0005] As a detector for a machine tool spindle, a detector such as a magnetic encoder that outputs a sine waveform and a cosine waveform (hereinafter referred to as a sine / cosine signal output encoder) per rotation in N cycles according to a rotation angle is generally used.

[0006] A method of achieving speed monitoring in safety category 3 using a sine / cosine signal output encoder is disclosed, for example, in Apfeld, R., “Do safe drive controls also require safe position encoders?” ([online] Internet URL: http: / / www.dguv.de / medien / ifa / en / pub / rep / pdf / reports2013 / ifar0713e / safe_drive_controls.pdf (Non-Patent Literature 1)). In this Non-Patent Literature 1, a method performed using a single sine / cosine signal output encoder is disclosed. In the case of using a single sine / cosine signal output encoder, a high diagnosis rate is required for the encoder output signal. Therefore, it is necessary to diagnose the cosine waveform and the sine waveform based on sin 2 + cos 2 = 1, and this Non-Patent Literature 1 discloses that a diagnosis rate of ≥ 99% can be achieved by sin 2 + cos 2 = 1 diagnosis.

[0007] Figure 2 is a block diagram when speed is monitored in safety category 3 using a single sine / cosine signal output encoder. In the case of using a single sine / cosine signal output encoder, a high diagnosis rate is required for the encoder output signal. Therefore, it is necessary to diagnose the cosine waveform and the sine waveform based on sin Figure 2In the example shown, the encoder 1 outputs a sine / cosine waveform including a cosine waveform = A cos(N θ) and a sine waveform = A sin(N θ) in accordance with a rotation angle θ, where N indicates a frequency of the sine waveform and the cosine waveform output per rotation of the encoder. N is determined by the number of teeth of a gear in a magnetic encoder or the number of slits in an optical encoder. The level conversion circuit 2 converts the cosine waveform into a voltage level with an output of Vc that can be used by a subsequent circuit. The pulsing circuit 3 converts Vc into a pulse signal. Similarly, the level conversion circuit 4 converts the sine waveform into a voltage level with an output of Vs that can be used by a subsequent circuit, and the pulsing circuit 5 converts Vs into a pulse signal. The pulse signals output by the respective pulsing circuits 3 and 5 are pulse signals whose phases are offset by 90° from each other, and the counters 6 and 8 count the pulse multiplied by 4 in a known manner. The speed monitoring unit 7 calculates the rotation speed of the encoder based on the amount of change in the count value of the counter 6 and monitors the speed. In the case where the speed exceeds a speed limit, the speed monitoring unit 7 outputs an abnormality signal to the abnormality processing unit 14. Similarly, the speed monitoring unit 9 calculates the rotation speed of the encoder based on the amount of change in the count value of the counter 8 and monitors the speed. In the case where the speed exceeds a speed limit, the speed monitoring unit 9 outputs an abnormality signal to the abnormality processing unit 14. The speed monitoring units 7 and 9 monitor the respective speed calculation results and the respective speed monitoring results with each other, and in the case where a discrepancy arises between the calculation results of the speed monitoring units 7 and 9, the respective abnormality signals are output to the abnormality processing unit 14. The AD converters 10 and 11 perform AD conversion on the signals Vc and Vs output from the level conversion circuits 2 and 4, respectively, and output the respective AD-converted signals to the Lissajous radius calculation processing unit 12. The Lissajous radius calculation processing unit 12 performs various corrections including offset correction, amplitude ratio correction, phase correction, and the like on the AD conversion results of the signals Vc and Vs output by the AD converters 10 and 11, respectively, and then calculates the Lissajous radius. In the case where the Lissajous radius is outside a predetermined reference range, the diagnosis processing unit 13 outputs an abnormality signal to the abnormality processing unit 14. In the case of either of the speed monitoring units 7 and 9, the abnormality processing unit 14 performs an abnormality process such as shutting down the power supply to the motor, and the diagnosis processing unit 13 outputs an abnormality signal.

[0008] In Figure 3 In the case where the Lissajous radius 22 output by the Lissajous radius calculation processing unit 12 exceeds the upper threshold 23 or falls below the lower threshold 24 of the Lissajous radius, the diagnosis processing unit 13 outputs an abnormality signal to the abnormality processing unit 14. Hereinafter, the range defined by the upper threshold 23 and the lower threshold 24 will be referred to as the "reference range".

[0009] The Lissajous radii of the cosine waveform and the sine waveform output by the encoder fluctuate due to various factors. Examples of these fluctuation factors can include, in the case of a magnetic encoder, fluctuations in the gap between the gear and the encoder in the connected state, fluctuations in the rotation period of the encoder due to eccentricity of the gear, fluctuations in the ambient temperature, and fluctuations in the rotation frequency of the encoder. Therefore, it is necessary to set a threshold for diagnosing the Lissajous radius while reflecting such amounts of fluctuation; otherwise, over-detection can occur, for example, the radius is erroneously detected as abnormal even though it is normal.

[0010] However, depending on the failure mode of the encoder, a Lissajous waveform 21 as illustrated in Figure 4 In this case, the Lissajous waveform 21 reciprocates between the first and fourth quadrants, and the counter repeatedly counts up and down, whereby the object is determined to be in a stopped state even when the object is rotating. Furthermore, the Lissajous waveform 21 is always within the reference range, which makes it impossible to detect an abnormality. Needless to say, the Lissajous waveform 21 illustrated in Figure 4 can be detected as abnormal by narrowing the reference range, but narrowing the reference range makes it impossible to accept fluctuations in the Lissajous radius due to the above-mentioned temperature fluctuations and the like, whereby over-detection occurs.

[0011] Therefore, the present application discloses an encoder abnormality diagnosis device capable of more accurately detecting an abnormality in an encoder. SUMMARY

[0012] The encoder abnormality diagnosis device disclosed in the present application is an encoder abnormality diagnosis device configured to diagnose an abnormality in an encoder that outputs a cosine waveform and a sine waveform depending on a rotation angle of an object, the encoder abnormality diagnosis device including: a stop determination processing unit configured to determine whether the object is rotating or stopping based on a change in a count value of a pulse signal obtained by pulsing each of the cosine waveform and the sine waveform; a frequency measurement unit configured to measure a frequency of the pulse signal; and a frequency diagnosis unit configured to, when the stop determination processing unit determines that the object is stopping, compare the frequency measured by the frequency measurement unit with a preset upper limit frequency, and determine that an abnormality has occurred in the encoder when the measured frequency exceeds the upper limit frequency.

[0013] In this case, the upper limit frequency can be determined based on a control period of the object.

[0014] The upper limit frequency can be determined based on a maximum acceleration of the object and a hysteresis width for the pulsing.

[0015] According to the encoder abnormality diagnosing apparatus disclosed in the present application, when it is determined that the object is stopping, the presence or absence of rotation can also be checked based on the frequency of the pulse signal, whereby it is possible to more accurately detect an abnormality in the encoder. BRIEF DESCRIPTION OF DRAWINGS

[0016] One or more embodiments of the present disclosure will be described based on the following drawings, in which:

[0017] Figure 1 is a block diagram illustrating a configuration of an encoder abnormality diagnosing apparatus;

[0018] Figure 2 is a block diagram illustrating a configuration of a conventional encoder abnormality diagnosing apparatus;

[0019] Figure 3 is a view illustrating an example of a Lissajous waveform when the encoder is normal;

[0020] Figure 4 is a view illustrating an example of a Lissajous waveform when an abnormality occurs in the encoder;

[0021] Figure 5 is a flowchart illustrating a flow of stop determination processing;

[0022] Figure 6 is a view illustrating an example of a Lissajous waveform in an oscillation state; and

[0023] Figure 7 is a graph illustrating an operation of a pulsar circuit.

[0024] LIST OF REFERENCE NUMERALS

[0025] 1 encoder

[0026] 2, 4 level conversion circuit

[0027] 3, 5 pulsar circuit

[0028] 6, 8 counter

[0029] 7, 9 speed monitoring unit

[0030] 10, 11 AD converter

[0031] 12 Lissajous radius calculation processing unit

[0032] 13 diagnosis processing unit

[0033] 14 abnormality processing unit

[0034] 15, 18 stop determination processing unit

[0035] 16, 19 frequency measurement unit

[0036] 17, 20 frequency diagnosis unit

[0037] 21 lissajous waveform

[0038] 22 lissajous radius

[0039] 23 upper limit threshold value

[0040] 24 lower limit threshold value

[0041] 25 pulsed circuit input signal

[0042] 26 hysteresis width

[0043] 27 pulse signal DETAILED DESCRIPTION

[0044] The configuration of the encoder abnormality diagnosis device will be described by referring to Figure 1 , Figure 5 , Figure 6 and Figure 7 . Figure 1 is a block diagram of the encoder abnormality diagnosis device. The encoder 1 outputs a sine / cosine signal including a cosine waveform = A cos(N θ) and a sine waveform = A sin(N θ) according to a rotation angle θ, where N indicates the frequency of the sine waveform and the cosine waveform output per rotation of the encoder. N is determined by the number of teeth of a gear in a magnetic encoder or the number of slits in an optical encoder. The level conversion circuit 2 converts the cosine waveform into a voltage level whose output is Vc, which is used by a subsequent circuit. The pulsing circuit 3 converts Vc into a pulse signal. Similarly, the level conversion circuit 4 converts the sine waveform into a voltage level whose output is Vs, which is used by a subsequent circuit, and the pulsing circuit 5 converts Vs into a pulse signal. The pulse signals output by the respective pulsing circuits 3 and 5 are pulse signals whose phases are offset by 90° from each other, and the counters 6 and 8 count the pulse multiplied by 4 in a known manner. The speed monitoring unit 7 calculates the rotation speed of the encoder based on the amount of change in the count value of the counter 6 and monitors the speed. In the case where the speed exceeds a speed limit, the speed monitoring unit 7 outputs an abnormality signal to the abnormality processing unit 14. Similarly, the speed monitoring unit 9 calculates the rotation speed of the encoder based on the amount of change in the count value of the counter 8 and monitors the speed. In the case where the speed exceeds a speed limit, the speed monitoring unit 9 outputs an abnormality signal to the abnormality processing unit 14. The speed monitoring units 7 and 9 monitor the respective speed calculation results and the respective speed monitoring results with each other, and in the case where a difference arises between the calculation results of the speed monitoring units 7 and 9, an abnormality signal is output to the abnormality processing unit 14.

[0045] The AD converters 10 and 11 perform AD conversion on the signals Vc and Vs output from the level conversion circuits 2 and 4, respectively, and output the respective AD-converted signals to the Lissajous radius calculation processing unit 12. The Lissajous radius calculation processing unit 12 performs various corrections including offset correction, amplitude ratio correction, phase correction, and the like on the AD conversion results of the signals Vc and Vs output by the AD converters 10 and 11, respectively, and then calculates the Lissajous radius. In the case where the Lissajous radius is outside a predetermined reference range, the diagnosis processing unit 13 outputs an abnormality signal to the abnormality processing unit 14.

[0046] The stop determination processing unit 15 determines whether the object is rotating or stopped based on the count value output from the counter 6. The stop determination processing unit 15 outputs a stop flag in the activated state to the frequency diagnosis unit 17 when it is determined that the object is stopped, and outputs a stop flag in the inactivated state to the frequency diagnosis unit 17 when it is determined that the object is rotating. The frequency measurement unit 16 measures the frequency of the pulse signal output by the pulsing circuit 3, and outputs the measured frequency to the frequency diagnosis unit 17. The frequency diagnosis unit 17 compares the frequency of the pulse signal output by the frequency measurement unit 16 with an upper limit frequency when the stop flag output by the stop determination processing unit 15 is in the activated state, and outputs an abnormality signal to the abnormality processing unit 14 in the case where the frequency of the pulse signal exceeds the upper limit frequency.

[0047] Similarly, the stop determination processing unit 18 determines whether the object is rotating or stopped based on the count value output from the counter 8. The stop determination processing unit 18 outputs a stop flag in the activated state to the frequency diagnosis unit 20 when it is determined that the object is stopped, and outputs a stop flag in the inactivated state to the frequency diagnosis unit 20 when it is determined that the object is rotating. The frequency measurement unit 19 measures the frequency of the pulse signal output by the pulsing circuit 5, and outputs the measured frequency to the frequency diagnosis unit 20. The frequency diagnosis unit 20 compares the frequency of the pulse signal output by the frequency measurement unit 19 with an upper limit frequency when the stop flag output by the stop determination processing unit 18 is in the activated state, and outputs an abnormality signal to the abnormality processing unit 14 in the case where the frequency of the pulse signal exceeds the upper limit frequency.

[0048] In the case where any one of the speed monitoring units 7 and 9, the diagnosis processing unit 13, and the frequency diagnosis units 17 and 20 outputs an abnormality signal, the abnormality processing unit 14 performs an abnormality process such as shutting off the power supply to the motor.

[0049] Figure 5is a flowchart illustrating the flow of the processing to be performed by the stop determination processing unit 15. The stop determination processing unit 15 inputs the count value output from the counter 6, and calculates a value obtained by subtracting the previous count value from the current count value as a count difference value (S10). When the absolute value of the count difference value is equal to or greater than 2 (NO in S12), the stop determination processing unit 15 determines that the object is rotating, and turns off the stop flag (S14). On the other hand, when the absolute value of the count difference value is less than 2 (YES in S12), the stop determination processing unit 15 proceeds to step S16.

[0050] In step S16, the stop determination processing unit 15 checks the current state of the stop flag. As a result of the check, when the stop flag is off (NO in S16), the stop determination processing unit 15 sets the current count value as the value of the count stop value (S18), and proceeds to step S20. The count stop value is the count value when the stop flag is switched from off to on. On the other hand, when the stop flag is on (YES in S16), the stop determination processing unit 15 directly proceeds to step S20 without resetting the count stop value.

[0051] In step S20, the stop determination processing unit 15 calculates a count cumulative change value by subtracting the count stop value from the current count value. Then, when the absolute value of the count cumulative change value is equal to or greater than 2 (NO in S22), the stop determination processing unit 15 determines that the object is rotating, and turns off the stop flag (S14). On the other hand, when the absolute value of the count cumulative change value is less than 2 (YES in S22), the stop determination processing unit 15 determines that the object is stopping, and turns on the stop flag (S24). When the stop flag is set in step S14 or step S24, the stop determination processing unit 15 records the current count value as the previous count value (S26), and returns to step S10. Thereafter, the similar processing is repeated each time the count value is output.

[0052] As clearly described above, in this example, whether or not the object is rotating is determined based on the absolute value of the count difference value. Therefore, in the case of the Lissajous waveform 21 illustrated as an example, the counter repeatedly counts up and down. As a result, the stop determination processing unit 15 determines that the object is stopping even when the object is rotating, and turns on the stop flag. The same case applies to the determination processing to be performed by the stop determination processing unit 18. Figure 4

[0053] ​The determination result of the stop determination processing units 15, 18 is input to the frequency diagnosis units 17, 20 as described above. When the stop flag is activated, the frequency diagnosis units 17, 20 compare the frequency of the pulse signal output by the frequency measurement units 16, 19 with a preset upper limit frequency f max in order to determine whether an abnormality has occurred in the generation of the Lissajous waveform 21 as Figure 4 illustrated.

[0054] That is, in the case where the Lissajous waveform 21 as Figure 4 illustrated is generated, the stop determination processing units 15, 18 determine that the main shaft (measured object) is stopped even when it is rotating (i.e., the stop flag is activated), and thus an abnormality in the encoder cannot be detected. Then, in this example, in the case where the stop flag is activated, the frequency of the pulse signal is compared with the upper limit frequency f max , and in the case where the frequency of the pulse signal exceeds the upper limit frequency f max , it is determined that an abnormality has occurred.

[0055] The upper limit frequency f max is set to a value that can exclude the case where the main shaft is slightly oscillating. That is, the stop flag is activated not only when an abnormality occurs in the encoder but also when the main shaft is slightly oscillating. In the case where the main shaft is slightly oscillating, the frequency of the pulse signal is nonzero, but this state does not correspond to an abnormality in the encoder. Then, the upper limit frequency f max is set to a value that is sufficiently larger than the frequency of the pulse signal obtained when slight oscillation occurs, so that slight oscillation is prevented from being determined as an abnormality.

[0056] The setting of the upper limit frequency f max will be described with reference to Figure 6 and Figure 7 . Figure 6 is a view that illustrates an example of the Lissajous waveform 21 in an oscillation state. Figure 7 is a graph that illustrates the input / output relationship of the pulsar circuit 5. The pulsar circuit 5 generally includes a hysteresis comparator. Thus, in the case where the oscillation amplitude of the pulsar circuit input signal 25 is smaller than a certain hysteresis width 26, the pulsar circuit 5 does not output the pulse signal 27. In this case, the frequency output by the frequency measurement unit 19 becomes zero. Thus, in the case where the frequency measurement unit 19 outputs a nonzero frequency, it can be said that oscillation having an amplitude equal to or greater than the hysteresis width 26 has occurred.

[0057] Here, since the Lissajous waveform 21 as Figure 4The pulse signal 27 in the illustrated Lissajous waveform 21 has a very high frequency, making it impossible to generate during the normal oscillation of the spindle. In this example, the upper limit frequency allowed during the normal oscillation of the spindle is set to the upper limit frequency f. max For example, the upper limit frequency f is determined based on the spindle control cycle Tc. max When the spindle control cycle is Tc, or when the frequency is 1 / Tc, commands exceeding the Nyquist frequency (1 / (2·Tc)) cannot be output to the spindle. That is, (1 / (2·Tc)) can be considered the upper limit frequency allowed under normal spindle oscillation conditions. Therefore, the upper limit frequency is set to f. max = (1 / (2·Tc)), and it can be determined that the frequency of the pulse signal 27 exceeds f max An anomaly occurred when the expression = (1 / (2·Tc)).

[0058] As another mode, it can be based on the maximum allowable acceleration α of the principal axis. max Determine the upper limit frequency f max That is, as described above, when pulse signal 27 is obtained, the peak-to-peak transition of the sine or cosine waveform becomes equal to or greater than the hysteresis width 26 (hereinafter referred to as "hysteresis width ΔH"). Therefore, when the change in pulse signal 27 is at its minimum amplitude, Figure 6 In the case of the illustrated Lissajous waveform 21, the spindle angle θ oscillates with an amplitude of (ΔH / 2) in one tooth of the encoder. The spindle angle θ is represented by the following expression 1, where Z is the number of teeth of the encoder, t is time, and φ is the initial phase.

[0059] θ=(ΔH / 2)×(1 / Z)×sin(2πf·t+φ) Expression 1

[0060] The acceleration α is obtained by differentiating expression 1 twice, and the acceleration α is expressed by expression 2.

[0061] α=-(ΔH×2π 2 f 2 The expression 2 is: ) / Z×sin(2f·t+φ)

[0062] The acceleration α can be transformed into the maximum acceleration α. max The frequency f is set to the upper limit frequency f. max Therefore, by replacing α in expression 2 with α max To obtain the upper limit frequency f, solve the expression for the frequency f. max , and f max It is represented by the following expression 3.

[0063] f max= 1 / π x ((a max Z) / (2 ΔH)) 1 / 2 Expression 3

[0064] Therefore, when the upper limit frequency f max is determined based on the control period or the maximum acceleration, the slight oscillation and the anomaly in the encoder can be detected separately from each other, and the anomaly in the encoder can be detected more accurately. The method of determining the upper limit frequency f max illustrated herein is merely an example, and the upper limit frequency can be any value capable of distinguishing the slight oscillation and the anomaly in the encoder.

Claims

1. An encoder abnormality diagnosing apparatus configured to diagnose an abnormality in an encoder that outputs a cosine waveform and a sine waveform, according to a rotation angle of an object, the encoder abnormality diagnosing apparatus comprising: a stop determination processing unit configured to count a pulse signal with a counter and determine whether the object is rotating or stopping based on a variation amount in a count value of the counter, wherein the pulse signal is obtained by pulsing each of the cosine waveform and the sine waveform; a frequency measuring unit configured to measure a frequency of the pulse signal; and a frequency diagnosing unit configured to, when the stop determination processing unit determines that the object is stopping, compare the frequency measured by the frequency measuring unit with a preset upper limit frequency, and determine that an abnormality has occurred in the encoder when the measured frequency exceeds the upper limit frequency. The upper limit frequency is determined based on a control period of the object.

2. The encoder abnormality diagnosing apparatus according to claim 1, characterized by The upper limit frequency is determined based on a maximum acceleration of the object and a hysteresis width used for the pulsing.

3. The encoder abnormality diagnosing apparatus according to claim 1, characterized by 4. An encoder abnormality diagnosing method that diagnoses an abnormality in an encoder that outputs a cosine waveform and a sine waveform, according to a rotation angle of an object, the method comprising the steps of: counting a pulse signal with a counter and determining whether the object is rotating or stopping based on a variation amount in a count value of the counter, wherein the pulse signal is obtained by pulsing each of the cosine waveform and the sine waveform; measuring a frequency of the pulse signal; and when it is determined that the object is stopping, comparing the measured frequency of the pulse signal with a preset upper limit frequency, and determining that an abnormality has occurred in the encoder when the measured frequency exceeds the upper limit frequency. ​ ​

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