JPH0815702B2 - Method for correcting thermal displacement of ball screw in NC machine tool - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for correcting a positioning error due to a thermal displacement of a ball screw for driving a feed which occurs during operation of an NC machine tool. More specifically, the present invention relates to a method for in-process correction by estimating the thermal displacement amount of a ball screw from the operating state by calculation every moment and giving the thermal displacement amount to the NC device as pitch error correction.

[0002]

2. Description of the Related Art A ball screw is one element of a machine tool and is used for moving a feed table, a table and the like. It has many features such as extremely high mechanical efficiency and is widely used as a driving or positioning element. However, in the ball screw, the frictional force of each part of the nut and the bearing and the heat generation of the servo motor are conducted, so that the temperature rises and thermal expansion occurs, causing thermal displacement. A semi-closed loop system is generally used in the current NC machine tools, but in the NC machine tool of this system, thermal displacement of the ball screw appears as a positioning error. Therefore, a method of applying pre-tension to the ball screw to absorb thermal expansion has been used as a countermeasure. However, recently, in addition to using a ball screw with a large diameter, the feed rate has become extremely high, so the amount of heat generated increases, and if a pre-tension method is used, a very large pulling force must be applied. However, there is a problem that the structure is deformed or the thrust bearing is seized by applying an excessive force.

Therefore, a method for correcting the thermal displacement of a ball screw has been developed which does not apply an excessive pretension to the ball screw and which does not require a special measuring device.
The method is proposed in No. 6 (hereinafter referred to as the already proposed method). This is because the temperature distribution of the ball screw is obtained from the heat generation amount of the ball screw, the thermal displacement amount of the ball screw is predicted moment by moment, and this thermal displacement amount is given to the NC device as a pitch error correction. This is the method of correction.

[0004]

By the way, in the above proposed method, the heat generation amount of the ball screw is specifically obtained from the product of the armature current and the voltage of the servomotor. Therefore, there are drawbacks as described below.

That is, the inventors of the present invention have shown in FIG.
The feeding pattern of the nut, that is, the table as shown in FIGS. 6A and 6B (hereinafter, FIG.
(B) is called a feed pattern B. ), The NC machine tool was operated, the correction amount of the ball screw based on the method proposed previously was calculated, and the actual elongation was measured. In addition, the feed pattern A is obtained by reciprocating the central region (200 mm) of the ball screw at a feed speed of 5 m / min, and the feed pattern B is obtained by reciprocating the same region at a feed speed of 6.5 m / min and a pitch of 20 mm. . Then, both patterns were operated for 30 minutes, the correction amount of the ball screw was calculated every 3 minutes, and the actual elongation was measured. The result is shown in FIG.
It is shown in (B). FIG. 7A is based on the feed pattern A, and FIG. 7B is based on the feed pattern B.

According to the experimental results, there is no great difference in the actual elongation of the ball screw between the feed patterns A and B. Nevertheless, the correction amount of the feed pattern B is larger than the correction amount of the feed pattern A and is approximately doubled. The reason for the difference in the correction amount is that a very large current flows when accelerating and decelerating the servo motor. Therefore, for the feed pattern B in which the number of acceleration and deceleration is large, the servo motor itself is accelerated and decelerated due to the heat generation amount. It is conceivable that a lot of energy will be included.

Therefore, in the proposed method, even the energy required for the acceleration / deceleration of the servo motor is calculated as the heat generation amount of the ball screw, so that the number of times of acceleration / deceleration of the servo motor is increased like the feed pattern B. However, there is a drawback that the correction amount becomes larger than the actual elongation. Therefore, the proposed method cannot be put to practical use when the number of times of acceleration / deceleration of the servo motor is large or the feed speed is high.

An object of the present invention is to provide a method for correcting thermal displacement of a ball screw in an NC machine tool, which makes it possible to approximate the correction amount to the actual elongation of the ball screw regardless of the number of acceleration / deceleration of the servo motor. It is in.

[0009]

Means and Actions for Solving the Problems
The following measures are taken to solve the above-mentioned problems.

The present invention provides a ball screw for an NC machine tool having a positioning ball screw, a servo motor for rotationally driving the ball screw, and speed control means for controlling the rotational speed of the servo motor. The thermal displacement correction method of No. 1 has the following steps.

A. Measuring the rotational speed of the servo motor b. Obtaining the amount of heat generated in the ball screw from the rotation speed c. Calculating the temperature distribution of the ball screw from the heat value d. Calculating the amount of thermal displacement of the ball screw from the temperature distribution e. Inputting the thermal displacement amount into the pitch error correction data memory area of the NC device to correct the pitch error correction amount of the ball screw.

Measurement of the rotation speed, calculation of the calorific value,
The calculation of the temperature distribution, the calculation of the thermal displacement amount, and the correction of the pitch error correction amount are repeated every predetermined time until the operation time of the NC machine tool ends.

The amount of contraction of the ball screw is calculated from the time when the power is turned off while the NC machine tool is stopped, and the pitch error of the ball screw is calculated by taking the amount of contraction into consideration when the operation of the NC machine tool is started. Initialize the correction amount of.

The amount of heat generation is obtained for each of the front bearing portion, the nut moving portion and the rear bearing portion of the ball screw, and the temperature distribution and the thermal displacement amount are calculated for each portion.

The nut moving section is divided into a plurality of sections,
The existence probability of the nut is obtained for each divided section, and the temperature distribution and the thermal displacement amount are calculated for each divided section from the existence probability and the heat generation amount of the entire nut moving unit.

The temperature distribution is calculated from a predetermined unsteady heat conduction equation.

[0017]

EXAMPLES In the examples shown below, the heat generation amount is obtained for each of the front bearing portion, the nut moving portion and the rear bearing portion of the ball screw, and the nut moving portion is further divided into a plurality of divisions. The calorific value is calculated for each. Then, the temperature distribution is calculated by solving the unsteady heat conduction equation for each part or each division, and the thermal displacement is calculated from this. First, a method of calculating the heat generation amount will be described.

[0018]Calculation of total heat  As shown in FIG. 3, the nut moving part has a predetermined length, for example.
It is divided into 5 sections having L / 8 and L / 4. At certain times
In each interval (50 ms), the nut exists in which section of FIG.
Whether or not there is a
Calculate the amount of heat generated and use the data
Store in the rear. The amount of heat generated is calculated by the following equation.

[0019]

[Equation 2] Here, Q: heat generation amount, F: feed speed (rotation speed), K,
T: A coefficient.

As shown in the following table, the data area has two sets of seven data areas for one ball screw axis.

[0021]

[Table 1] It should be noted that there are two sets of data areas for one axis.
This is because the amount of heat that is sequentially calculated is stored in the data area of another group while the correction amount is calculated by obtaining the temperature distribution from the data area of one group.

The amount of heat generated by nut movement in each section is
Every 50ms a fixed time (6400ms) or 12
It is calculated eight times, and these heat amounts are summed every 50 ms for each section and stored in the data areas (C) to (G) corresponding to the sections 1 to 5. Further, the total heat quantity of the heat quantities 1 to 5 of each of the sections 1 to 5 generated in 6400 ms and the total rotation speed of the rotation speed of each of the sections 1 to 5 are stored in the data areas (A) and (B), respectively.

The heat quantity of each section and the total heat quantity mentioned here include not only the net heat quantity generated only in the nut moving portion L of the ball screw but also the heat quantity generated in the front and rear bearing portions. Therefore, it is necessary to distribute the net heat generation amount of the nut moving portion and the front and rear bearing portions from the total heat amount.

[0024]Distribution of total heat  The distribution method of total heat shown below is the same as the proposed method.
The nut moving part and the front and rear bearing parts are
Is not thermally conductive to the minute and is approximately thermally independent
Can be regarded as
The heat source ratio is almost constant regardless of the feed rate.
It is based on the finding that there is.

That is, the total calorific value Q _O , nut moving part calorific value Q _N , front bearing calorific value Q _F , rear bearing calorific value Q _B
Then, the calorific value of each heat source is

[0026]

(Equation 3) Calculated from Here, the ratios η _N , η _F , η _B are constant according to the above findings, and Q _O , Q _N , Q _F , and
It suffices to measure Q _B and obtain the ratios η _N , η _F and η _B in advance.

[0027]Distribution of heat quantity to each division of nut moving part  In this embodiment, as described above, each division of the nut moving portion is performed.
The amount of heat for each category is also calculated. This is calculated as follows.
Can be That is, in the data areas (C) to (G)
The amount of heat stored is the total value calculated every 50 ms.
Yes, from this total value per each 50ms
Calculate the average heat value of.

From the average heat quantity and the total heat quantity, the existence probability X1, X2, ...
Find X5.

[0029]

[Equation 4] In this way, the existence probabilities X1, X2 for each section are
When X5 is obtained, the distributed heat amounts Q _N1 , Q _N2, ... Q _N5 for each section are calculated from the existence probability and the distributed heat amount Q _N for the nut moving portion by the following equation.

[0030]

[Equation 5] Calculation of temperature distribution  The amount of heat generated by each heat source was determined as described above.
Then, the temperature distribution is calculated from this heat quantity. Temperature distribution is already proposed
As in the proposed method, the following unsteady heat conduction equation,

[0031]

(Equation 6)

[0032]Calculation of thermal displacement  When the temperature distribution of each heat source of the ball screw is obtained,
From this, the amount of thermal displacement is calculated. The amount of thermal displacement is the proposed method
Similarly to

[0033]

(Equation 7) Required from. Here, ΔL: thermal displacement amount, β: linear expansion coefficient of ball screw material.

Next, a specific procedure of the thermal displacement correction method according to the present invention will be described. FIG. 1 is a functional block diagram for performing thermal displacement correction.

The interpolation control circuit 1 is a circuit for calculating the feed amount of the ball screw based on the processing data read into the NC tape 2. The pulse distributor 3 is for distributing a pulse signal corresponding to the feed amount of the ball screw to each axis and distributing the pulse signal to the speed control circuit 4.
This pulse signal is also given to the position register 5 to store the nut position data. The speed control circuit 4 controls the rotation speed of the servo motor 6 by a pulse signal. The rotation speed of the servo motor 6 is constantly detected by a speed detector 7 such as a rotary encoder, and the detection signal is input to the speed control circuit 4 and the temperature distribution calculation circuit 9.

The parameter memory 8 stores parameters relating to mechanical structures such as ball screw length and diameter, parameters relating to physical properties such as ball screw density and specific heat, and the heat distribution coefficients (ratio) η _N , η _F , η _B etc. is input. The temperature distribution calculation circuit 9 calculates the heat generation amount due to the movement of the nut in each section of the ball screw every 50 ms based on the rotation speed detection signal of the servo motor based on the equation (1), and after 6400 ms, calculates the heat generation amount from each heat source. Calculate the distribution of calorific value of each part.

Further, the temperature distribution calculation circuit 9 calculates the existence probability of the nut and calculates the heat generation amount of each section of the ball screw. Then, the temperature distribution calculation circuit 9 solves the equation (2) from these heat generation amounts and various data stored in the parameter memory 8 to calculate the temperature distribution of each heat source unit.

The correction data calculation circuit 10 calculates the correction amount from the temperature distribution calculated by the temperature distribution calculation circuit 9 based on the equation (3). The correction pulse generator 11 generates a correction pulse according to the correction amount calculated by the correction data calculation circuit 10 and supplies it to the speed control circuit 4.

FIG. 2 is a flow chart for performing thermal displacement correction. A specific procedure will be described according to this flowchart.

[0040]Initial setting  By finite element method from setting data such as parameters
Set the matrix required for calculation (S₁).

When the power is turned off by stopping the operation of the NC machine,
Since the ball screw shrinks over time, it is necessary to change the correction data when the power is turned on again, taking into account this shrinkage amount. For this reason, the time when the power is turned off is calculated (S ₂ ), data is calculated by setting the time as ΔT, the generated heat amount as 0, and the feeding speed as 0, and the temperature distribution is initialized (S ₃ ). Then, the shrinkage amount is calculated from the initially set temperature distribution and the temperature distribution before the power is turned off, and the correction data is changed (S ₄ ).

Since the amount of shrinkage is calculated in this way,
The temperature distribution is stored in an area that does not disappear when the power is turned off.

[0043]Real-time data input  Every 50 ms, the nut current position and feed speed data
Is input to the temperature distribution calculation circuit 9 (S_Five), Each category
The amount of heat generated is calculated based on the equation (1).

[0044]Data calculation  When a certain time (6400 ms) has passed (S₆),temperature
The probability of nut existence from the total calorific value by the distribution calculation circuit 9
And the temperature distribution of each heat source is calculated (S₇).

[0045]Change correction data  If the correction mode is ON (S₈), Correction data calculation
Based on the formula (3) from the temperature distribution by the circuit 10, the ball
The elongation of the edge is calculated as the correction amount and the correction data is changed.
(S₉). And the correction pulse corresponding to this correction data
Sent from the correction pulse generator 11 to the speed control circuit 4.
Be done.

As described above, after the power is turned on, that is, after the initial setting, the correction data is changed every fixed time (6400 ms).

[0047]Experimental example  FIG. 4 shows a thermal displacement correction amount and an actual elongation according to the method of the present invention.
Figure 3 shows an experimental setup for comparing pigs. In the experiment
The X axis of the vertical machining center was used. Servomotor
The ball screw 21 connected to 20 includes front and rear bearings 22,
It is supported by 23. The table 24 has a nut 25
And is attached to the ball screw 21.

For the extension of the ball screw 21, the dial gauge 27 is fixed to the spindle head 26 of the machining center, and the reference blocks 28 are provided at three points (both ends and the center) on the X axis of the table 24. It was measured by bringing 27 contacts into contact. The feed rate of the servo motor 20 was measured by the recorder 30 with a data memory by outputting feed rate monitor data inside the NC device 29 as a voltage.

The X-axis stroke is 760 mm, the servomotor rated output is AC 2.3 KW, max 3,000.
R. P. M, ball screw dimensions are length 1,140 mm, diameter 40 mmφ, pitch 12 mm, maximum rapid feed rate is 30
m / min.

With the above experimental apparatus, the machining center was operated with the feed patterns A and B shown in FIG. 6 as in the case of the proposed method, the correction amount was obtained, and the actual elongation was measured.

As in the case of the previously proposed method, the feed pattern A is the central region (200 mm) of the ball screw reciprocated at a feed speed of 5 m / min, and the feed pattern B is the same region at a feed speed of 6.5 m / min. It was reciprocated at a pitch of 20 mm for min. Then, both patterns were operated for 30 minutes, the correction amount of the ball screw was calculated every 3 minutes, and the actual elongation was measured. The result is shown in FIG.
It is shown in (B). 5A is based on the feed pattern A, and FIG. 5B is based on the feed pattern B.

The experimental results show that not only the feed pattern A but also the feed pattern B, the correction amount and the actual elongation are close to each other.

[0053]

As described in detail above, according to the present invention, the correction amount can be approximated to the actual elongation of the ball screw regardless of the frequency of acceleration / deceleration of the servomotor, and therefore the semi-closed loop system is used. It is possible to perform high-precision machining with the NC machine tool.

[Brief description of drawings]

FIG. 1 is a functional block diagram for carrying out the method of the present invention.

FIG. 2 is a flowchart for carrying out the present invention.

FIG. 3 is an explanatory diagram in a case where a ball screw is divided to obtain a heat amount.

FIG. 4 is a schematic diagram of an apparatus for experimenting with the method of the present invention.

FIG. 5 is a graph showing experimental results of the method of the present invention.

FIG. 6 is a diagram showing feed patterns selected for experimenting the method of the present invention and the method proposed in the prior art.

FIG. 7 is a graph showing an experimental result of the already proposed method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Kuwayama 1 Abiko, Abiko, Chiba Prefecture, Hitachi Seiki Co., Ltd. (72) Inventor, Hirokazu Miyazaki 1 Abiko, Abiko, Chiba, Ltd., Hitachi Seiki Co., Ltd. (56) References JP-A-63-256336 (JP, A)

Claims (6)

[Claims] Claim: What is claimed is: 1. A ball screw for a NC machine tool, comprising: a positioning ball screw; a servo motor for rotationally driving the ball screw; and speed control means for controlling the rotational speed of the servo motor. The displacement correction method has the following steps. a. Measuring the rotational speed of the servo motor b. Obtaining the amount of heat generated in the ball screw from the rotation speed c. Calculating the temperature distribution of the ball screw from the heat value d. Calculating the amount of thermal displacement of the ball screw from the temperature distribution e. Inputting the thermal displacement amount into the pitch error correction data memory area of the NC device to correct the pitch error correction amount of the ball screw. 2. The measurement of the rotation speed according to claim 1,
The calculation of the heat generation amount, the temperature distribution, the thermal displacement amount, and the correction of the pitch error correction amount are performed by the NC.
A method for correcting thermal displacement of a ball screw, which is repeated every predetermined time until the operating time of the machine tool ends. 3. The amount of shrinkage of the ball screw is calculated from the time when the power is turned off while the operation of the NC machine tool is stopped, and the amount of shrinkage is added when the operation of the NC machine tool is started. A method for correcting thermal displacement of a ball screw, wherein a correction amount of the pitch error of the ball screw is initialized. 4. The method according to any one of claims 1 to 3,
A thermal displacement correction of a ball screw, characterized in that the heat generation amount is obtained for each of the front bearing portion, the nut moving portion and the rear bearing portion of the ball screw, and the temperature distribution and the thermal displacement amount are calculated for each portion. Method. 5. The nut moving unit according to claim 4, wherein the nut moving unit is divided into a plurality of sections, the existence probability of the nut is obtained for each divided section, and each division is made from the existence probability and the heat generation amount of the entire nut moving unit. A thermal displacement correction method for a ball screw, characterized in that the temperature distribution and the thermal displacement amount are calculated for each section. 6. The method according to any one of claims 1 to 5,
The temperature distribution is calculated by the following unsteady heat conduction equation, A method for correcting thermal displacement of a ball screw, which is calculated from