Grinding lead screw lead error online prediction method based on built-in grating ruler

Abstract

The invention discloses a grinding screw lead error online prediction method based on a built-in grating ruler, and the method comprises the steps: (1) inputting various parameters of a to-be-machined ball screw pair, including the type, nominal lead, effective stroke and grinding parameters, into CNC; (2) starting the numerical control thread grinding machine, and reading the rotating angle theta of a workpiece main shaft and the feeding displacement xa of a grinding wheel along the z axis by utilizing a circular encoder, a linear grating ruler and a data acquisition module while the thread grinding machine performs thread grinding; and (3) calculating a lead error curve of the ball screw. And (4) predicting each precision index of the ball screw. The invention aims to establish an efficient, convenient and precise thread grinder servo control performance evaluation method, namely an online prediction method for lead errors and precision indexes of a ball screw.

Description

Technical Field

[0001] The present invention relates to the fields of thread grinder servo control performance evaluation and ball screw lead error online measurement, and specifically to a thread grinder servo control performance evaluation method based on a built-in grating ruler, namely, an online prediction method for ball screw lead error. Background Art

[0002] With the advantages of high load, low damping, low energy consumption and high positioning accuracy, the linear drive solution of "servo motor + ball screw pair" has been widely used in automation fields such as precision CNC machine tools, robots, and injection molding machines. However, as the market's requirements for product quality continue to grow, higher standards have been put forward for the accuracy, stability and transmission efficiency of linear drive systems. As the core functional component of the linear drive solution of "servo motor + ball screw pair", the processing quality of the ball screw will greatly affect the transmission accuracy and transmission efficiency of the linear servo system.

[0003] In the current ball screw processing industry, offline detection technology has been relatively mature and widely used: for example, the thread profile of the ball screw and nut is measured with a profilometer, and further, the key parameters such as the thread diameter and contact angle are evaluated; the lead error of the ball screw is measured with a laser interferometer. However, these methods cannot perform real-time detection of the ball screw / nut thread being processed. At the same time, the offline measurement method also has the following disadvantages: first, it can only evaluate the processing accuracy of the ball screw and nut that have been processed. If it is found that the processed ball screw does not meet the tolerance requirements, the product will be scrapped at the worst, and secondary trimming will be performed at the least. However, the repeated disassembly and assembly of the ball screw during the trimming process will introduce clamping errors and the processing time will be lengthy; secondly, the processing process is still a "black box", which is not conducive to the traceability and compensation of processing errors; it can be seen that traditional offline measurement will lead to waste of raw materials and increase in production costs.

[0004] Realizing online measurement, accurate tracing and compensation control of machine tool processing errors through built-in or external sensors has become one of the most effective means to improve machine tool processing accuracy and control part quality, and is also a research hotspot in the current academic and industrial circles. In this context, the present invention proposes an online prediction method for the lead error of a grinding screw based on a built-in grating ruler, aiming to achieve real-time online prediction of the lead error of threaded parts such as ball screws and nuts, and based on this, complete accurate tracing of processing errors and compensation control of CNC machine tools. Summary of the invention

[0005] In order to overcome the shortcomings of the existing off-line ball screw accuracy measurement technology, the purpose of the present invention is to propose a thread grinder servo control performance evaluation method based on a built-in grating ruler, that is, an online prediction method for ball screw lead error.

[0006] The purpose of the present invention is achieved through the following technical solution: an online prediction method for the lead error of a grinding screw based on a built-in grating ruler, comprising the following steps:

[0007] Step 1: Input various parameters of the ball screw pair to be processed, including type, nominal lead, effective stroke and grinding parameters into the CNC.

[0008] Step 2: Start the CNC thread grinder. While the thread grinder is grinding the thread, use the circular encoder, linear grating ruler and data acquisition module to read the rotation angle θ (unit: rad or circle) of the workpiece spindle and the feed displacement x of the grinding wheel along the z-axis. a (Unit: mm or m), from which the actual lead curve of the ball screw can be drawn: x a (θ).

[0009] Step 3: Calculate the lead error curve of the ball screw. According to the definition of the lead P of the ball screw: the linear distance that the nut moves along its axial direction for each rotation of the ball screw; therefore, if the ball screw does not have a lead error after processing, the theoretical transmission curve of the ball screw can be drawn based on the rotation angle information θ of the workpiece spindle: n (θ), where x n (θ) is given by:

[0010]

[0011] Furthermore, the lead error curve e(θ) of the ball screw can be calculated:

[0012] e(θ)=x a (θ)-x n (θ)(2)

[0013] Note: In order to more intuitively show the accuracy level of the ball screw, in practice, That is, the effective stroke is used as the horizontal coordinate; therefore, e(θ), x a (θ) and x n (θ) can also be expressed as e(x), x a (x) and x n (x).

[0014] Step 4: According to the ISO 3408-3:2006 standard, the items to be inspected for different types of ball screw pairs are not exactly the same.

[0015] For positioning (P-type) ball screw pairs, it is necessary to predict the average travel deviation e within the effective travel of the ball screw. 0a , the allowable travel variation v within the effective travel up , the allowable travel variation v within 300mm travel300p and the allowable travel variation v within 2π (rad) 2πp .

[0016] For a transmission (T-type) ball screw pair, it is necessary to predict the average travel deviation e within the effective travel of the ball screw. 0a And the allowable travel variation v within 300mm travel 300p .

[0017] (1) For the positioning (P type) ball screw pair, the calculation methods for the inspection indicators are:

[0018] (a) Average travel deviation e within the effective travel 0a :

[0019] e 0a =e a -c(3)

[0020] Among them, e a Indicates the actual average travel deviation curve e m (x) (the least squares fitting straight line of the guide stroke error e(x) curve) is the maximum value within the effective stroke, and c represents the stroke compensation value specified by the user.

[0021] (b) Allowable travel variation within effective travel v up : refers to the residual fluctuation signal e v (x)

[0022] The maximum change within the effective stroke is calculated by the following formula:

[0023] v up =[max(e v (x))-min(e v (x))]| useful travel (4)

[0024] (c) Where, the residual fluctuation signal e v (x) is the residual curve obtained by subtracting the actual average stroke deviation curve from the actual lead error curve. Its calculation formula is e v (x)=

[0025] e(x)-e m (x).

[0026] (d) Allowable travel variation v within 300 mm travel 300p : refers to the residual fluctuation signal e v (x) The maximum change of any continuous 300mm within the effective stroke is calculated as follows:

[0027] v300p =max([max(e v (x))-min(e v (x))]| 300 )| useful travel (5)

[0028] (e) Allowable travel variation v within 2π (rad) 2πp : refers to the residual fluctuation signal e v (x)

[0029] Take the maximum variation of any arc (i.e. within the range of lead P) within the range of one rotation of the workpiece spindle. The specific calculation formula is as follows:

[0030] v 2πp =max([max(e v (x))-min(e v (x))]| P )| useful travel (6)

[0031] (2) For the transmission (T-type) ball screw pair, it is only necessary to check the average travel deviation e within the effective travel. 0a (c=0) and the allowable travel variation v within 300mm travel 300p The specific calculation method of the two indicators is the same as that of the above-mentioned positioning (P type) ball screw pair.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (i) The present invention can realize real-time online evaluation of the servo control performance of the thread grinder through the built-in encoder and grating ruler; based on this, the servo control error can be accurately traced and optimized to improve the lead accuracy of the thread.

[0034] (ii) The present invention can realize real-time online prediction of ball screw lead error through built-in encoder and grating ruler, can detect unqualified ball screws in advance, avoid the occurrence of large quantities of defective products, and easily realize full inspection of ball screw lead accuracy.

[0035] (III) The present invention utilizes the built-in sensor of the machine tool and performs on-machine measurement, so the technical cost is low, the measurement process is simple, and the scalability is high.

[0036] (iv) The on-machine measurement technology proposed in the present invention will greatly promote the realization of the secondary dressing technology of the ball screw, and can effectively avoid the time-consuming and difficult-to-guarantee repeated disassembly and assembly required for traditional offline detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a schematic diagram of the structure of the CNC external thread grinder in the present invention.

[0038] Figure 2 is the theoretical transmission curve x of the ball screw measured in this example n (θ) and the actual transmission curve x a (θ)

[0039] Figure 3 This is the lead error curve and accuracy index of the ball screw predicted in this example DETAILED DESCRIPTION

[0040] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0041] Example

[0042] Figure 1 It is a structural diagram of a CNC external thread grinder, which is mainly composed of a CNC system, a workpiece spindle, a Z-axis guide, an X-axis guide, a grinding wheel spindle, etc., and also includes a data acquisition and analysis module.

[0043] Preferably, the workpiece spindle is directly driven by an electric spindle, equipped with a high-precision circular encoder, and adopts a three-ring control structure to achieve precise rotation of the workpiece. In addition, a three-jaw chuck and a rotary ejector are configured to fix the ball screw to be processed.

[0044] Preferably, the Z axis and the X axis are installed perpendicular to each other, the Z axis is parallel to the workpiece spindle, and the X axis is perpendicular to the workpiece spindle; both include a drive system ("servo motor + precision ball screw pair" or linear motor) and a linear guide. A grating ruler is placed in the linear guide to provide actual position information of the Z axis and X axis slide.

[0045] Preferably, the grinding wheel spindle is directly driven by an electric spindle, adopts a speed closed-loop control mode, and is installed on a slide so that the grinding wheel can move linearly along the Z axis and the X axis; in addition, a grinding wheel swing head (A axis) is required to adjust the inclination angle of the grinding wheel; and a grinding wheel is used to grind the ball screw to be processed.

[0046] Preferably, the data acquisition and analysis module includes a data acquisition module and a data analysis module. The data acquisition module is used to receive the circular encoder angle signal of the workpiece spindle and the grating ruler position signal of the Z-axis guide; the data analysis module is used to process the encoder and grating ruler signals and calculate the lead error curve and various accuracy indicators of the ball screw after processing.

[0047] The servo control performance evaluation method of the CNC thread grinder, i.e., the online prediction method of the ball screw lead error, comprises the following steps:

[0048] (1) Input various parameters of the ball screw pair to be processed, including type, nominal lead, effective stroke and grinding parameters, into the CNC.

[0049] (2) Start the CNC thread grinder. While the thread grinder is grinding the thread, use the circular encoder, linear grating ruler and data acquisition module to read the rotation angle θ (unit: rad or circle) of the workpiece spindle and the feed displacement x of the grinding wheel along the z-axis. a (Unit: mm or m); From this, the actual transmission curve of the ball screw can be drawn: x a (θ), such as Figure 2 As shown by the dotted line in .

[0050] (3) Calculate the lead error curve of the ball screw. According to the definition of the lead P of the ball screw: the linear distance that the nut moves along its axial direction for each rotation of the screw; therefore, if the ball screw does not have a lead error after processing, the theoretical transmission curve of the ball screw can be drawn based on the rotation angle information of the workpiece spindle: n (θ), such as Figure 2 The solid line in the figure shows that x n (θ) is given by:

[0051]

[0052] Furthermore, the lead error curve e(θ) of the ball screw can be calculated:

[0053] e(θ)=x a (θ)-x n (θ)(2)

[0054] Note: In order to more intuitively show the accuracy level of the ball screw, in practice, That is, the effective stroke is used as the horizontal coordinate; therefore, e(θ), x a (θ) and x n (θ) can also be expressed as e(x), x a (x) and x n (x).

[0055] The lead error curve e(x) of the ball screw measured in this example is as follows: Figure 3 Shown by the solid line.

[0056] (4) For different types of ball screw pairs, predict their accuracy test indicators.

[0057] This example requires the processing of a positioning (P-type) ball screw pair, so four indicators need to be tested, namely: the average travel deviation e within the effective travel 0a , the allowable travel variation v within the effective travel up , the allowable travel variation v within 300mm travel300p and the allowable travel variation v within 2π (rad) 2πp , the specific results are as follows Figure 3 As shown, the calculation methods are:

[0058] (a) Average travel deviation e within the effective travel 0a :

[0059] e 0a =e a -c(3)

[0060] Among them, e a Indicates the actual average travel deviation curve e m (x) (the least squares fitting straight line of the guide stroke error curve e(x)) is the maximum value within the effective stroke, and c represents the stroke compensation value specified by the user.

[0061] (b) Allowable travel variation within effective travel v up : refers to the residual fluctuation signal e v (x)

[0062] The maximum change within the effective stroke is calculated by the following formula:

[0063] v up =[max(e v (x))-min(e v (x))]| useful travel (4)

[0064] Among them, the residual fluctuation signal e v (x) is the residual curve obtained by subtracting the actual average stroke deviation curve from the actual lead error curve. Its calculation formula is e v (x)=

[0065] e(x)-e m (x).

[0066] (c) Allowable travel variation v within 300 mm travel 300p : refers to the residual fluctuation signal e v (x) The maximum change of any continuous 300mm within the effective stroke is calculated as follows:

[0067] v 300p =max([max(e v (x))-min(e v (x))]| 300 )| useful travel (5)

[0068] (d) Allowable travel variation v within 2π (rad) 2πp : refers to the residual fluctuation signal e v (x)

[0069] Take the maximum variation of any arc (i.e. within the range of lead P) within the range of one rotation of the workpiece spindle. The specific calculation formula is as follows:

[0070] v 2πp =max([max(e v (x))-min(e v (x))]| P )| useful travel (6)

[0071] The above specific implementation modes are preferred embodiments of the present invention and cannot be used to limit the present invention. Any other changes or other equivalent replacement methods that do not deviate from the technical solution of the present invention are included in the protection scope of the present invention.

Claims

1. An online prediction method for the lead error of a grinding screw based on a built-in grating ruler, characterized in that: Includes steps: Input various parameters of the ball screw pair to be processed, including type, nominal lead, effective stroke and grinding parameters, into the CNC; Start the CNC thread grinder. While the thread grinder is grinding the thread, use the circular encoder, linear grating ruler and data acquisition module to read the rotation angle θ of the workpiece spindle and the feed displacement x of the grinding wheel along the z axis. a , from this, the actual transmission curve of the ball screw can be drawn: a (θ); Calculate the lead error curve of the ball screw; Predict the various accuracy indicators of the ball screw. The inspection indicators are the average travel deviation within the effective travel e 0a , the allowable travel variation v within the effective travel up 、Permissible travel variation v within 300mm travel 300p and the allowable travel variation v within 2π (rad) 2πp The specific inspection items depend on the type of ball screw pair to be processed.

2. The method for online prediction of lead error of a grinding screw based on a built-in grating ruler according to claim 1 is characterized in that: The data acquisition module reads the rotation angle θ of the workpiece spindle and converts it into the theoretical feed displacement x of the grinding wheel along the z-axis. n , the conversion relationship between the two is as follows: Where P represents the nominal lead of the ball screw.

3. The method for online prediction of lead error of a grinding screw based on a built-in grating ruler according to claim 1 is characterized in that: The calculation formula of the lead error curve e(θ) of the ball screw is: e(θ)=x a (θ)-x n (θ) (1) Note: In order to more intuitively display the accuracy level of the ball screw, in practice, That is, the effective stroke is used as the horizontal coordinate; therefore, e(θ), x a (θ) and x n (θ) can also be expressed as e(x), x a (x) and x n (x).

4. The method for online prediction of lead error of a grinding screw based on a built-in grating ruler according to claim 1 is characterized in that: (1) For the positioning (P type) ball screw pair, the calculation methods for the inspection indicators are: (a) Average travel deviation e within effective travel 0a : And 0a =and a -c (2) Among them, e a Indicates the actual average travel deviation curve e m (x) (the least squares fitting straight line of the guide stroke error curve e(x)) is the maximum value within the effective stroke, and c represents the stroke compensation value specified by the user. (b) Allowable travel variation within effective travel v up : refers to the residual fluctuation signal e v (x) The maximum change within the effective stroke, the specific calculation formula is as follows: v up =[max(e v (x))-min(e v (x))]| useful travel (3) Among them, the residual fluctuation signal e v (x) refers to the residual curve obtained by subtracting the actual average stroke deviation curve from the actual lead error curve. The calculation formula is: v (x) = e(x) - e m (x). (c) Allowable travel variation v within 300 mm travel 300p : refers to the residual fluctuation signal e v (x) The maximum change of any continuous 300mm within the effective stroke is calculated as follows: v 300p =max([max(e v (x))-min(e v (x))]| 300 )| useful travel (4) (d) Allowable travel variation v within 2π (rad) 2πp : refers to the residual fluctuation signal e v (x) Take the maximum variation of any arc (i.e. within the range of lead P) within the range of one revolution of the workpiece spindle. The specific calculation formula is as follows: v 2πp =max([max(e v (x))-min(e v (x))]| P )| useful travel (5)(2) For the transmission (T-type) ball screw pair, it is only necessary to check the average travel deviation r within the effective travel. 0a (c=0) and the allowable travel variation v within 300mm travel 300p The specific calculation method of the two indicators is the same as that of the above-mentioned positioning (P type) ball screw pair.

5. A CNC thread grinding machine for realizing claims 1-4, characterized in that: It consists of a CNC system, a workpiece spindle, a Z-axis guide, an X-axis guide, a grinding wheel spindle, etc. It also includes a data acquisition and analysis module. The workpiece spindle is directly driven by an electric spindle, equipped with a high-precision circular encoder, and adopts a three-ring control structure to achieve precise rotation of the workpiece. In addition, a three-jaw chuck and a rotary ejector are configured to fix the ball screw to be processed. The Z-axis and X-axis are installed perpendicular to each other, the Z-axis is parallel to the workpiece spindle, and the X-axis is perpendicular to the workpiece spindle; both include a drive system ("servo motor + precision ball screw pair" or linear motor) and a linear guide. A grating ruler is placed in the linear guide to provide actual position information of the Z-axis and X-axis slides. The grinding wheel spindle is directly driven by the electric spindle, adopts a speed closed-loop control mode, and is installed on a slide so that the grinding wheel can move linearly along the Z axis and the X axis. In addition, a grinding wheel swing head (A axis) is required to adjust the inclination angle of the grinding wheel, and a grinding wheel is used to grind the ball screw to be processed. The data acquisition and analysis module includes a data acquisition module and a data analysis module. The data acquisition module is used to receive the circular encoder angle signal of the workpiece spindle and the grating ruler position signal of the Z-axis guide; the data analysis module is used to process the circular encoder and grating ruler signals and calculate the lead error curve and various accuracy indicators of the ball screw after processing.

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