Diagnostic methods for mechanical equipment and ball screw mechanisms

Abstract

To provide machine equipment capable of accurately diagnosing a ball screw mechanism while suppressing an increase in cost, and a method of diagnosing a ball screw mechanism.SOLUTION: Machine equipment 1 is equipped with a ball screw mechanism 2 having a ball screw shaft 21 and a ball screw nut 22, a servomotor 3 which generates torque to operate the ball screw mechanism 2, a servo amplifier 5 and a CNC 6 which control the servomotor 3, and a diagnostic module 46 which diagnoses the ball screw mechanism 2 by acquiring control information regarding the magnitude of torque generated by the servomotor 3. The diagnostic module 46 diagnoses the ball screw mechanism 2 on the basis of the amount of variation in the amount of control state obtained from the control information acquired in a constant-speed rotating region in which the servomotor 3 rotates at a constant speed.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to mechanical equipment having a ball screw mechanism and a diagnostic method for the ball screw mechanism.

Background Art

[0002] Conventionally, as devices and methods for predicting the remaining life of a ball screw mechanism, those described in Patent Documents 1 and 2 are known.

[0003] The one described in Patent Document 1 detects the total travel distance of the ball nut by integrating the change in the current position of the ball nut, and counts the number of passes through each stroke section for each time the ball nut passes through each preset stroke section, and detects the usage frequency of each part of the ball screw rod. Then, based on this detection result, the inspection timing and life of the ball screw mechanism are predicted.

[0004] The one described in Patent Document 2 detects the vibration generated by the rotation of a rotating part such as a ball screw in mechanical equipment by a vibration sensor, and compares the spectrum data obtained by filtering the waveform of the detected vibration with the bearing damage frequency calculated based on the rotation speed of the rotating part. Then, based on this comparison result, the remaining life of the abnormal part is predicted from the abnormal part, the degree of damage, and the operating environment of the rotating part.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] The method described in Patent Document 1 predicts the inspection timing and lifespan of the ball screw mechanism based solely on the total travel distance of the ball screw nut and the number of times each stroke section is passed. As a result, the prediction accuracy is low, and there is a risk of predicting an unnecessarily short remaining lifespan. On the other hand, the method described in Patent Document 2 can predict the remaining lifespan with relatively high accuracy if vibrations generated by the rotation of the rotating member can be accurately detected. However, in a factory with many pieces of machinery, it is difficult to accurately detect only the vibrations of the rotating member to be detected. Furthermore, the method described in Patent Document 2 requires a vibration sensor, which increases costs.

[0007] Therefore, the present invention aims to provide a machine and equipment capable of accurately diagnosing a ball screw mechanism while suppressing cost increases, and a method for diagnosing a ball screw mechanism. [Means for solving the problem]

[0008] To achieve the above objective, the present invention comprises a ball screw mechanism having a ball screw shaft and a ball screw nut, a servo motor that generates torque to operate the ball screw mechanism, a motor control device that controls the servo motor, and a diagnostic device that diagnoses the ball screw mechanism by acquiring control information regarding the magnitude of the torque generated by the servo motor from the motor control device at predetermined time intervals. The motor control device controls the servo motor in an operating pattern that includes an acceleration region that increases the rotational speed of the servo motor, a constant speed rotation region that keeps the rotational speed of the servo motor constant, and a deceleration region that decreases the rotational speed of the servo motor. The diagnostic device recognizes that it is in the constant speed rotation region based on the elapsed time after an operation command is sent to the motor control device or the position command value and speed command value of the servo motor, and in the constant speed rotation region, the control state amount obtained from the control information acquired from the motor control device The average value is calculated, the standard deviation as the amount of variation is determined by the difference between the control state quantity obtained from the control information acquired at the predetermined time interval and the average value, and based on a comparison of this standard deviation with a life threshold indicating that the remaining life of the ball screw mechanism is short, The present invention provides mechanical equipment for diagnosing the aforementioned ball screw mechanism.

[0009] Furthermore, in order to achieve the above objective, the present invention provides a method for diagnosing a ball screw mechanism having a ball screw shaft and a ball screw nut, and driven by a servo motor, wherein control information indicating the control state regarding the magnitude of torque generated by the servo motor is acquired from a motor control device that controls the servo motor at predetermined time intervals, and among the acceleration range in which the rotational speed of the servo motor is increased, the constant speed rotation range in which the rotational speed of the servo motor is kept constant, and the deceleration range in which the rotational speed of the servo motor is decreased, the method recognizes that it is in the constant speed rotation range based on the elapsed time after an operation command has been sent to the motor control device or the position command value or speed command value of the servo motor, and in the constant speed rotation range, the control state amount obtained from the control information acquired from the motor control device The average value is calculated, and the standard deviation as the amount of variation is determined by the difference between the control state quantity obtained from the control information acquired at the predetermined time interval and the average value. Based on a comparison of this standard deviation with a life threshold indicating that the remaining life of the ball screw mechanism is short, the ball screw mechanism is diagnosed, and if the standard deviation is greater than the life threshold, it is diagnosed that the remaining life of the ball screw mechanism is short. This document provides a diagnostic method for ball screw mechanisms. [Effects of the Invention]

[0010] According to the mechanical equipment and ball screw mechanism diagnostic method of the present invention, it is possible to accurately diagnose a ball screw mechanism while suppressing cost increases. [Brief explanation of the drawing]

[0011] [Figure 1] This is a block diagram showing an example of the configuration of a machine and equipment according to an embodiment of the present invention. [Figure 2] This is an explanatory diagram showing an example configuration of a ball screw mechanism and a servo motor, along with the machine body's slide table and guide rails, and the servo amplifier's control block. [Figure 3] This is a cross-sectional view showing the main part of the ball screw mechanism. [Figure 4] (a) and (b) are schematic graphs illustrating an example of the operating pattern of a servo motor controlled by a CNC and a servo amplifier. [Figure 5] (a) and (b) are graphs showing the torque generated by the servo motor in the first half of the operating pattern shown in Figures 4(a) and (b). [Figure 6](a) and (b) are enlarged graphs showing the torque waveform in the constant speed rotation range shown in Figures 5(a) and (b). [Modes for carrying out the invention]

[0012] [Embodiment] Embodiments of the present invention will be described with reference to Figures 1 to 6. The embodiments described below are presented as preferred specific examples for carrying out the present invention, and while some parts specifically illustrate various technically preferable technical matters, the technical scope of the present invention is not limited to these specific embodiments.

[0013] Figure 1 is a block diagram showing an example configuration of a mechanical device according to an embodiment of the present invention. This mechanical device 1 comprises a machine body 10 having a ball screw mechanism 2 and a servo motor 3 that generates torque to operate the ball screw mechanism 2; a programmable controller (hereinafter referred to as PLC) 4 that executes a sequence program describing the operating order of the machine body 10 and various interlocks; a servo amplifier 5 that supplies motor current to the servo motor 3; and a numerical control device (hereinafter referred to as CNC) 6 that sends position command value information to the servo amplifier 5.

[0014] The machine body 10 is equipped with multiple sensors 100, such as limit switches and proximity switches, for detecting the operating state of the machine body 10, and the detection signals from these multiple sensors 100 are input to the PLC 4. The PLC 4 sends an operation command to the CNC 6 by executing a sequence program in response to the detection signals from the multiple sensors 100. The CNC 6 executes an NC program in response to the operation command from the PLC 4 and controls the servo motor 3 via the servo amplifier 5. More specifically, the CNC 6 sends information of a position command value, which is the command value of the rotational position of the servo motor 3, to the servo amplifier 5. The servo amplifier 5 and CNC 6 are one embodiment of the motor control device of the present invention that controls the servo motor 3.

[0015] The ball screw mechanism 2 is driven by a servo motor 3 to linearly advance and retreat a moving object such as a processing device like a spindle or a grinding table, or a mold used for injection molding, or a workpiece (workpiece to be processed). In FIG. 1, one ball screw mechanism 2 and one servo motor 3 are shown, but the machine body 10 may have a plurality of ball screw mechanisms 2 and a plurality of servo motors 3. When the machine body 10 has a plurality of servo motors 3, a plurality of servo amplifiers 5 are connected to the CNC 6.

[0016] The PLC 4 is composed of a base unit 41, a power supply module 42, a CPU module 43, an input module 44 into which signals from a plurality of sensors 100 are input, a communication module 45 that communicates with the CNC 6, and a diagnostic module 46 having a diagnostic function for diagnosing the ball screw mechanism 2. The power supply module 42, the CPU module 43, the input module 44, the communication module 45, and the diagnostic module 46 are detachable from the base unit 41. The power supply module 42 converts an AC voltage of, for example, AC100V into a predetermined DC voltage and supplies it to each of the modules 43 to 46. The diagnostic module 46 is an aspect of the diagnostic device of the present invention for diagnosing the ball screw mechanism 2.

[0017] The CPU module 43 has a storage unit 431 that stores a sequence program and the like, and an execution unit 432 that executes the sequence program. The execution unit 432 acquires information indicating detection signals of a plurality of sensors 100 from the input module 44, executes the sequence program, and sends an operation command to the CNC 6 via the communication module 45. The diagnostic module 46 can acquire various types of information such as input signals, output signals, and data registers used in the sequence program from the CPU module 43.

[0018] FIG. 2 is an explanatory diagram showing a configuration example of the ball screw mechanism 2 and the servo motor 3 together with the slide table 11 and the guide rail 12 of the machine body 10. In addition, FIG. 2 shows a control block of the servo amplifier 5 for controlling the servo motor 3. FIG. 3 is a cross-sectional view showing a main part of the ball screw mechanism 2.

[0019] As shown in FIG. 3, the ball screw mechanism 2 has a shaft-like ball screw shaft 21, a cylindrical ball screw nut 22, and a plurality of balls 23. A screw groove 211 is formed spirally on the outer peripheral surface of the ball screw shaft 21. In the ball screw nut 22, a screw groove 221 is formed spirally on the inner peripheral surface, and a return path 222 for circulating a plurality of balls 23 is formed. The return path 222 opens at both ends of the screw groove 221 and returns the ball 23 that has reached one end of the screw groove 221 to the other end. The plurality of balls 23 circulate and roll along the screw groove 211 of the ball screw shaft 21 and the screw groove 221 of the ball screw nut 22 due to the relative rotation between the ball screw shaft 21 and the ball screw nut 22.

[0020] The slide table 11 has a table main body portion 111 on which a moving object is mounted and a connecting portion 112 for connecting to the ball screw nut 22. The table main body portion 111 is guided by the guide rail 12 and moves linearly. The machine body 10 has two guide rails 12 arranged parallel to the ball screw shaft 21, but only one of the guide rails 12 is shown in FIG. 2. The connecting portion 112 is fixed to the ball screw nut 22 by a plurality of bolts 113. The ball screw nut 22 is prevented from rotating with respect to the ball screw shaft 21 by the connecting portion 112.

[0021] The servo motor 3 is, for example, a three-phase AC motor and has a stator 31, a rotor 32, a motor shaft 33 fixed to the rotor 32, and a position detector 34 for detecting the rotational position of the rotor 32 with respect to the stator 31. The position detector 34 is, for example, a multi-rotation resolver and can detect the absolute position of the rotor 32 with respect to the stator 31.

[0022] One end of the ball screw shaft 21 is connected to the motor shaft 33 of the servo motor 3 via a coupling 13. The other end of the ball screw shaft 21 is supported by a bearing unit 14. The stator 31 generates a rotating magnetic field due to the motor current supplied from the servo amplifier 5, and the rotor 32 rotates together with the motor shaft 33 in accordance with this rotating magnetic field. As the ball screw shaft 21 rotates, the ball screw nut 22 moves along the axial direction of the ball screw shaft 21 together with the slide table 11.

[0023] The servo amplifier 5 includes a subtractor 51, a position control unit 52, a subtractor 53, a speed control unit 54, a current control unit 55, a differentiator 56, an inverter circuit 57, and a current sensor 58. The subtractor 51, position control unit 52, subtractor 53, speed control unit 54, current control unit 55, and differentiator 56 are implemented in software, for example, by a microprocessor executing a program. The inverter circuit 57 is configured by connecting switching elements such as IGBTs (isolated gate bipolar transistors) and FETs (field-effect transistors) in a three-phase bridge configuration.

[0024] The subtractor 51 subtracts the detected position of the servo motor 3 from the position command value sent from the CNC 6 and outputs the position deviation. The position control unit 52 calculates the speed command value based on this position deviation and outputs it to the subtractor 53. The subtractor 53 subtracts the detected speed (described later) from this speed command value and outputs the speed deviation. The speed control unit 54 calculates the current command value based on this speed deviation and outputs it to the current control unit 55. The current control unit 55 outputs a PWM (Pulse Width Modulation) signal to turn the switching elements of the inverter circuit 57 on / off so that the actual current value detected by the current sensor 58 matches the current command value.

[0025] The detected position, as detected by the position detector 34 of the servo motor 3, is subtracted from the position command value by the subtractor 51 and input to the differentiator 56. The differentiator 56 calculates the detected speed by differentiating the detected position. The detected speed calculated by the differentiator 56 is subtracted from the speed command value by the subtractor 53.

[0026] The above information regarding the detection position, detection speed, speed command value, current command value, and actual current value is sent from the servo amplifier 5 to the CNC 6. The CNC 6 determines the torque generated by the servo motor 3 based on the information sent from the servo amplifier 5 and outputs control information regarding the magnitude of this torque to the PLC 4. The diagnostic module 46 acquires this control information at predetermined time intervals and diagnoses the ball screw mechanism 2 based on the acquired control information.

[0027] The CNC 6 may output control information regarding the magnitude of torque generated by the servo motor 3 as a digital signal, or it may acquire it as an analog signal. When the CNC 6 outputs control information as a digital signal, the diagnostic module 46 acquires the control information, for example, via a communication module 45 that communicates with the CNC 6. On the other hand, when the CNC 6 outputs control information as an analog signal, the diagnostic module 46 can directly receive this analog signal from the CNC 6, perform AD conversion (analog-to-digital conversion), and acquire the control information. Furthermore, if the PLC 4 has a module equipped with an AD conversion function, the diagnostic module 46 may acquire the control information that has been AD converted by this module.

[0028] The diagnostic module 46 diagnoses the ball screw mechanism 2 based on the variation in control state variables obtained from control information acquired from the CNC 6 when the servo motor 3 rotates at a substantially constant rotational speed. The diagnostic module 46 may determine the variation using the acquired control information as is, or it may diagnose the ball screw mechanism 2 based on the variation in control state variables obtained by applying some kind of arithmetic processing to the control information, such as filtering or multiplying by a coefficient.

[0029] The following describes in more detail the diagnostic method for the ball screw mechanism 2 using the diagnostic module 46, based on a specific example. In this example, the CNC 6 outputs a torque value obtained based on the actual current value sent from the servo amplifier 5 as control information, the diagnostic module 46 uses the acquired control information as a control state variable to determine the amount of variation, and diagnoses the ball screw mechanism 2 based on a comparison of the obtained amount of variation with a predetermined threshold.

[0030] Figures 4(a) and 4(b) are schematic graphs illustrating an example of the operating pattern of the servo motor 3 controlled by the CNC 6 and the servo amplifier 5. In the graphs of Figures 4(a) and 4(b), the horizontal axis represents the time axis, the vertical axis of the graph in Figure 4(a) represents the rotational speed of the servo motor 3, and the vertical axis of the graph in Figure 4(b) represents the torque generated by the servo motor 3.

[0031] The servo amplifier 5 and CNC 6 control the servo motor 3 with an operating pattern that includes an acceleration zone that increases the rotational speed of the servo motor 3, a constant speed rotation zone that keeps the rotational speed of the servo motor 3 constant, a deceleration zone that decreases the rotational speed of the servo motor 3, and a stop zone that stops the rotation of the servo motor 3. In the constant speed rotation zone, the CNC 6 outputs a position command value that increases or decreases at a constant rate to the servo amplifier 5 at each control cycle. By controlling the servo motor 3 in the order of acceleration zone → constant speed rotation zone → deceleration zone, the slide table 11 and ball screw nut 22 move in the axial direction of the ball screw shaft 21 from the retracted end to the forward end, or from the forward end to the retracted end.

[0032] Figures 5(a) and (b) are graphs showing the torque generated by the servo motor 3 in the first half of the operating pattern shown in Figures 4(a) and (b). Figures 6(a) and (b) are graphs showing enlarged views of the torque waveform in the constant speed rotation range shown in Figures 5(a) and (b). Figures 5(a) and 6(a) show the torque generated by the servo motor 3 attached to the ball screw mechanism 2 in its initial state (new condition). Figures 5(b) and 6(b) show the torque generated by the servo motor 3 attached to the ball screw mechanism 2 in a state where its remaining lifespan has been shortened (deteriorated state). In Figures 5(a) and (b), the average torque in the constant speed rotation range is indicated by arrows A1 and A2, respectively. In Figures 6(a) and (b), the average torque in the constant speed rotation range is indicated by lines L1 and L2, respectively.

[0033] Furthermore, Figures 6(a) and (b) show the sampling periods S1 and S2, which are the time intervals at which the diagnostic module 46 acquires torque value information from the CNC 6, and the rotation periods R1 and R2, which are the time it takes for the rotor 32 of the servo motor 3 to complete one rotation in the constant speed rotation range. The diagnostic module 46 stores the torque value obtained by performing AD conversion on the analog signal from the CNC 6, for example, at sampling periods S1 and S2, in a buffer memory. The sampling periods S1 and S2 are set to be shorter than the rotation periods R1 and R2 of the servo motor 3 in the constant speed rotation range. It is desirable that the sampling periods S1 and S2 be one-tenth or less of the rotation periods R1 and R2.

[0034] The diagnostic module 46 diagnoses the ball screw mechanism 2 based on the amount of variation in torque values ​​acquired in the constant speed rotation range. Specifically, it calculates the average value of torque in the constant speed rotation range and determines the amount of variation by the difference between this average value and each torque value acquired at a predetermined sampling period. More specifically, it calculates the standard deviation s as the amount of variation using the following formula (1), and diagnoses the ball screw mechanism 2 based on a comparison of this standard deviation s with a predetermined threshold.

number

[0035] Furthermore, the diagnostic module 46 can recognize that the machine is in the constant speed rotation range based, for example, on the elapsed time after the PLC 4 sends an operation command to the CNC 6. Also, if the diagnostic module 46 can acquire information on position command values ​​and speed command values, for example, via the communication module 45, it can recognize that the machine is in the constant speed rotation range based on this information.

[0036] As is clear from the comparison between Figure 5(a) and Figure 5(b), and between Figure 6(b) and Figure 6(b), the torque value of the servo motor 3 attached to the deteriorated ball screw mechanism 2 has a larger variation in the average value, i.e., a larger standard deviation s, than the torque value of the servo motor 3 attached to the ball screw mechanism 2 in new condition. The cause of this is wear and rust on the ball 23 and screw grooves 211 and 221. When such wear and rust occur, the ball 23 does not roll smoothly, and a stick-slip phenomenon occurs in some areas, which increases the standard deviation s.

[0037] In this embodiment, a life threshold indicating a short remaining life of the ball screw mechanism 2 is used as a predetermined threshold for comparison with the standard deviation s. This life threshold can be set according to the standard deviation s before the replacement, for example, if there is a history of replacing the ball screw mechanism 2 due to wear in the machine equipment 1 or a similar machine in the past. Alternatively, the life threshold may be set by multiplying the standard deviation s when a new ball screw mechanism 2 is driven by the servo motor 3 by a coefficient.

[0038] When the ball screw mechanism 2 deteriorates, problems such as adverse effects on machining accuracy and increased vibration and noise can occur. Therefore, by detecting when the remaining lifespan of the ball screw mechanism 2 is short and replacing it with a new one before such problems occur, it is possible to prevent the occurrence of problems. In addition, if the ball screw mechanism 2 is a made-to-order product, ordering a replacement when a short remaining lifespan is detected can prevent failures that would lead to long-term shutdowns of the manufacturing line.

[0039] The diagnostic results of the ball screw mechanism 2 in the diagnostic module 46 may be read by, for example, a factory maintenance worker connecting a portable information device such as a laptop computer to the diagnostic module 46, or the results may be notified by illuminating an indicator light provided on the machine equipment 1.

[0040] (Effects of the embodiment) According to the embodiments of the present invention described above, the ball screw mechanism 2 can be diagnosed accurately while suppressing cost increases, without providing additional sensors for diagnosis, such as vibration sensors. In particular, in this embodiment, the ball screw mechanism 2 is diagnosed based on the amount of variation in the steady rotation range where no load is generated on the servo motor 3 for accelerating or decelerating the slide table 11 or the object to be moved, thereby improving the accuracy of the diagnosis of the ball screw mechanism 2. In other words, in this embodiment, the accuracy of the diagnosis of the ball screw mechanism 2 is improved by diagnosing the ball screw mechanism 2 based on the amount of variation in the steady rotation range where the rate of torque value fluctuation due to the failure of the multiple balls 23 to roll smoothly is relatively large.

[0041] In the above embodiment, the case in which the standard deviation s is used as the amount of variation was described, but this is not limited to this. For example, the amount of variation may be the sum of the differences between the average value of the torque in the steady rotation range and each sampled value, and the ball screw mechanism 2 may be diagnosed by comparing this amount of variation with a threshold value. In this case as well, the same effect as above can be obtained. Furthermore, the threshold value is not limited to the life threshold value described above, but may also be a threshold value that indicates when maintenance such as lubrication of the ball screw mechanism 2 is required.

[0042] (Note) The present invention has been described above based on embodiments, but these embodiments do not limit the invention as defined in the claims. It should also be noted that not all combinations of features described in the embodiments are necessarily essential for solving the problem of the invention. Furthermore, the present invention can be implemented by omitting some components, or by adding or substituting components, without departing from its spirit. [Explanation of Symbols]

[0043] 1... Mechanical equipment 2... Ball screw mechanism 21...Ball screw shaft 22...Ball screw nut 3…Servo motor 46…Diagnostic module (diagnostic device) 5…Servo amplifier (motor control device) 6…CNC (motor control device)

Claims

1. The system comprises a ball screw mechanism having a ball screw shaft and a ball screw nut, a servo motor that generates torque to operate the ball screw mechanism, a motor control device that controls the servo motor, and a diagnostic device that obtains control information regarding the magnitude of the torque generated by the servo motor from the motor control device at predetermined time intervals and diagnoses the ball screw mechanism. The motor control device controls the servo motor in an operating pattern that includes an acceleration range for increasing the rotational speed of the servo motor, a constant speed rotation range for keeping the rotational speed of the servo motor constant, and a deceleration range for decreasing the rotational speed of the servo motor. The diagnostic device recognizes that the motor is in the constant speed rotation range based on the elapsed time after an operation command has been sent to the motor control device or the position command value and speed command value of the servo motor, calculates the average value of the control state quantity obtained from the control information acquired from the motor control device in the constant speed rotation range, determines the standard deviation as the amount of variation based on the difference between the control state quantity obtained from the control information acquired at predetermined time intervals and the average value, and diagnoses the ball screw mechanism based on a comparison of this standard deviation with a life threshold indicating that the remaining life of the ball screw mechanism is short. Mechanical equipment.

2. The diagnostic device acquires the control information in the constant speed rotation range at time intervals shorter than the rotation period of the servo motor. The machinery and equipment according to claim 1.

3. A method for diagnosing a ball screw mechanism having a ball screw shaft and a ball screw nut, and driven by a servo motor, The motor control device that controls the servo motor acquires control information indicating the control state regarding the magnitude of the torque generated by the servo motor at predetermined time intervals. Among the acceleration range in which the rotational speed of the servo motor is increased, the constant speed range in which the rotational speed of the servo motor is kept constant, and the deceleration range in which the rotational speed of the servo motor is decreased, the constant speed range is recognized based on the elapsed time after an operation command is sent to the motor control device or the position command value or speed command value of the servo motor. The average value of the control state quantity obtained from the control information acquired from the motor control device in the constant speed range is calculated. The standard deviation as the amount of variation is determined by the difference between the control state quantity obtained from the control information acquired at predetermined time intervals and the average value. The ball screw mechanism is diagnosed based on a comparison of this standard deviation with a life threshold indicating that the remaining life of the ball screw mechanism is short. If the standard deviation is greater than the life threshold, it is diagnosed that the remaining life of the ball screw mechanism is short. A diagnostic method for ball screw mechanisms.

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