JP2006015461A - Thermal displacement estimating method for machine tool - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately estimate thermal displacement in all operating conditions from a transient state to a steady state by filling the time constant difference of temperature and thermal displacement. <P>SOLUTION: When rotating speed changes during the execution of thermal displacement correction including temperature measurement (step 3), a thermal displacement time constant at the rotating speed is computed (step 4), and a measured temperature time constant and the thermal displacement time constant are used to compute each response characteristic coefficient. The measured temperature and the temperature response characteristic coefficient are used to obtain a change of heating value (step 5). With this result as input, a thermal displacement equivalent temperature change is computed (step 6) to compute thermal displacement estimation of a spindle (step 7). Based on this value, the corresponding amount of correction is outputted to an NC device (step 8). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for estimating thermal displacement of a spindle or the like in a machine tool based on temperature.

  Generally, a machine tool has a heat source (for example, a rolling bearing of a main shaft) in each part due to the characteristics of the machine, and heat generated by the heat source is transmitted to each part of the machine, thereby causing thermal displacement of the machine body. Since the thermal displacement of the airframe greatly affects the machining accuracy, a method for estimating and correcting the thermal displacement from the airframe temperature information is widely adopted as a preventive measure.

As such a thermal displacement estimation method, a method is known in which the thermal displacement amount is estimated by Equation 1 using an immediate value of temperature rise.
Estimated thermal displacement δ = 5 × (temperature rise + 0.8) Equation 1

  However, in the transient state after the rotation speed of the spindle changes, the time response of temperature and thermal displacement is expressed by a first-order lag system with respect to the spindle rotation speed. A thermal displacement estimation error appears remarkably. FIG. 5B shows the relationship between the spindle thermal displacement of the machining center operated under the conditions of FIG. 5A and the temperature rise of the spindle bearing (relative value from the body temperature). FIG. 6 shows an estimation error between the estimated thermal displacement amount and the actual thermal displacement by this method.

  Therefore, what is disclosed in Patent Document 1 below is known as an attempt to accurately estimate the amount of thermal displacement even in a transient state. In the method of Patent Document 1, a temperature time constant shorter and longer than the thermal displacement time constant of the main spindle are measured, and a combination of them is measured to create a change having a time constant equivalent to the thermal displacement of the main spindle. A method is disclosed in which a time constant shorter than the thermal displacement time constant is measured and a change having a time constant equivalent to the thermal displacement of the spindle is created by performing a first-order lag process on the measurement result.

JP-A-8-174380

  In the thermal displacement estimation method in Patent Document 1 above, the former method of measuring a temperature time constant and a long temperature time constant and combining them to create a change having a time constant equivalent to the thermal displacement of the main shaft, It is difficult to determine the position at which two different types of temperature changes are measured, and the calculation processing for estimating the thermal displacement becomes complicated because it is necessary to synthesize temperature time constants. On the other hand, in the latter method, which produces a change with a time constant equivalent to the thermal displacement of the main spindle by performing a first-order lag process with a short time constant, the temperature measurement has a first-order lag characteristic. When the delay process is performed, the characteristic becomes a second order characteristic, and unlike the thermal displacement characteristic of the main axis of the first order lag characteristic, the thermal displacement estimation becomes inaccurate.

  Therefore, the invention described in claim 1 provides a method that can accurately estimate thermal displacement by simple arithmetic processing in all operating situations ranging from a steady state to a transient state by filling in the time constant difference between temperature and thermal displacement. It is intended to do.

  In order to achieve the above object, according to the first aspect of the present invention, the relationship between the thermal displacement time constant and the rotational speed related to the thermal displacement estimation target and the measured temperature time constant are determined in advance by experiment or numerical analysis. Measuring the temperature rise in the vicinity of the thermal displacement estimation target of the machine tool, and changing the calorific value using the response equation of the first-order lag system as an inverse problem from the measured temperature rise and the measured temperature time constant. Obtaining the calculated value, calculating the thermal displacement equivalent value from the thermal displacement time constant obtained from the rotational speed and the calculated value as a forward problem using a first-order lag response formula, and estimating the thermal displacement equivalent value And a step of converting to a thermal displacement amount.

  The invention described in claim 2 is characterized in that a difference obtained by subtracting a measured temperature of a structure serving as a reference for a machine tool from a measured temperature in the vicinity of the spindle is used as the measured temperature rise.

  According to the first aspect of the present invention, the first-order lag response processing calculation value is applied to calculate the thermal displacement amount with high accuracy, and the thermal displacement is estimated and calculated. Thus, there is an effect that the thermal displacement can be estimated accurately while reducing the processing capacity.

  According to the second aspect of the present invention, since the increase value of the main shaft relative to the airframe temperature is used, in addition to the above-described effect, the temperature change accompanying the change in the room temperature can be eliminated, and the effect that the thermal displacement of only the main shaft can be accurately estimated. There is.

Hereinafter, a spindle thermal displacement estimation method for a machine tool according to an embodiment of the present invention in which a thermal displacement estimation target is a spindle will be described. In general, the temperature change (temperature difference between the main shaft and the bed, that is, the room temperature) and the thermal displacement of the main shaft are proportional to each other in a steady state where the rotation speed of the main shaft is constant.
K: Thermal displacement conversion coefficient (μm / ° C)
It is well known that

Further, the change of the thermal displacement time constant T (N) (minute) with respect to the rotational speed N (min ⁻¹ ) can be obtained in advance by experiment or numerical analysis, and each characteristic coefficient A to C is adjusted to the expression 3 Expressed by fitting.
T (N) = A · e− ^{N / C} + B Equation 3
A: First characteristic coefficient B: Second characteristic coefficient C: Third characteristic coefficient

On the other hand, in a transient state after the rotation speed of the spindle changes, the time response of temperature and thermal displacement can be expressed by a first-order lag system with respect to the spindle rotation speed. Equation 4 expressing is preferably used. Incidentally, in this equation 4, if you give an input X _n, the output Y _n can be easily determined as a forward problem.
_{Y n = Y n-1 +} (X n -Y n-1) · α formula 4
X _{n: n-th} input Y _{n: n-th} of the first-order lag response output
α: Response characteristic coefficient

However, by modifying the equation 4, the response characteristic coefficient alpha by a temperature response characteristic coefficient alpha _T, the expression 5 is obtained. Incidentally, the equation 5, the measurement is easy temperature change TY _n, as an inverse problem of first-order lag system, it is possible to determine a change in TX _n calorific is input.
TX _n = TY _n-1 + (TY _n -TY _n-1 ) / α _T equation 5

Specifically, TX _n corresponding to a change in the amount of generated heat can be obtained by applying the measurement temperature to TY _n of Equation 5 and applying the previous measurement temperature to TY _n-1 . Here, the temperature response characteristic coefficient α _T is obtained from Equation 6 as a value corresponding to the temperature change from the measured temperature time constant T _T obtained in advance by experiment or numerical analysis and the temperature measurement interval Δt.
α _T = Δt / (Δt + T _T ) Equation 6

Further, since the temperature change and the main shaft thermal displacement indicate the response of the first-order lag system of the main shaft rotation speed, the main shaft characteristic is applied to the response characteristic coefficient α of Expression 4 by using the calculated value TX _n calculated by Expression 5 as an input. The thermal displacement response characteristic coefficient α _D is obtained and the equation 4 is transformed into the equation 7 to obtain a thermal displacement equivalent value DY _n corresponding to the main shaft thermal displacement, which is _expressed as the thermal displacement conversion coefficient K in the equation 2 By multiplying, it can be converted into an estimated thermal displacement amount of the main shaft.
_{DY n = DY n-1 +} (TX n -DY n-1) · α D type 7

Note that the thermal displacement response characteristic coefficient α _D here is obtained from Equation 8 as a thermal displacement estimation calculation from the thermal displacement time constant T (N) at the spindle rotational speed N at the time of temperature measurement. Further, the thermal displacement conversion coefficient K can be multiplied by the measured temperature when the measured temperature is obtained without being multiplied at the end.
α _D = Δt / (Δt + T (N)) Equation 8

  Next, an example in which the above embodiment is applied to a vertical machining center will be described based on the drawings as appropriate. FIG. 1 shows a thermal displacement correction system according to this embodiment. As is well known, the vertical machining center includes a spindle head 1, a column 2, a spindle 3, a bed 4 as a reference structure, a moving table 5, and the like.

  A first temperature sensor 6 for detecting the bearing heat generation temperature is attached to the main shaft 3, and a second temperature sensor 7 for detecting a reference temperature is attached to the bed 4. The temperature measuring device 8 converts an analog signal indicating the temperature from each of the temperature sensors 6 and 7 into a digital signal and digitizes it as a measured temperature, and calculates the measured temperature of the second temperature sensor 7 from the measured temperature of the first temperature sensor 6. Subtract the temperature rise value. The thermal displacement estimation calculator 9 estimates a correction amount of thermal displacement by applying a value such as a temperature change related to the measurement to a predetermined arithmetic expression based on a correction parameter stored in advance in the storage device 10. The NC device 11 performs position correction by a known method according to the correction amount.

  Next, with respect to the machine tool before position correction, an experiment or analysis for measuring the relationship between the thermal displacement time constant Tδ of the spindle and the spindle rotational speed N and the relationship between the measured temperature time constant and the spindle rotational speed N is performed. It is performed in advance, and the relational expression T (N) between the thermal displacement time constant Tδ and the spindle rotational speed N and the measurement temperature time constant are determined.

FIG. 2 shows the relationship between the rotational speed and the thermal displacement time constant with respect to the thermal displacement characteristics of the spindle shown in FIG. 1, and from this, the characteristic coefficients A to C are expressed as A = 5.25, B = 7.58, With C = 3000, the relational expression T (N) is identified by the following expression.
Thermal displacement time constant T (N) = 5.25 · e ^{−N / 3000} +7.58 Equation 9

Here, as a specific example, thermal displacement calculation at a spindle rotational speed of 2000 min ⁻¹ will be described. The measured temperature time constant determined by the experiment is 5.8 minutes, and the thermal displacement time constant from Equation 9 is T (2000) = 10.3 minutes. Furthermore, from the experiment and Equation 2, the thermal displacement conversion coefficient is determined to be K = 5 μm / ° C. In this case, assuming that the temperature measurement interval is 10 seconds, the temperature response characteristic coefficient α _T in Expression 5 corresponding to the temperature change is 0.028 from Expression 6, and the thermal displacement response characteristic coefficient in Expression 7 for estimating and calculating the thermal displacement. α _D becomes 0.016 from the equation 8.

  FIG. 3 is a flowchart according to the present embodiment. First, a temperature measurement value is acquired (Step 1), and thermal displacement correction including a temperature increase calculation is executed (Step 2). When the rotational speed changes again during this execution (Step 3), the thermal displacement time constant T (N) at the rotational speed is calculated by Equation 3 (Step 4).

Next, the change in the amount of heat generated is calculated by Equation 5 (Step 5). Based on this result, the thermal displacement equivalent temperature change DY _n is calculated using the thermal displacement response characteristic coefficient α _D of the thermal displacement time constant T (N) at the rotational speed according to Equation 7 (Step 6). By multiplying this result by the thermal displacement conversion coefficient K according to Equation 2, it is converted into the estimated thermal displacement amount of the main shaft (Step 7). Thereafter, a correction amount corresponding to this is output to the NC device 11, and the NC device 11 performs a thermal displacement correction process (Step 8). Then, the correction process is continued as appropriate (Step 9).

Although the thermal displacement amount conversion after calculating the thermal distortion corresponding temperature change DY _n in this process, with respect to the measured temperature (Step2), be subjected to a treatment by multiplying the pre-thermal displacement conversion coefficient K results Will be the same.

  FIG. 4 is a characteristic diagram showing an estimation error in the case of operating under the condition of FIG. 5A in the thermal displacement estimation according to the present embodiment. Compared with FIG. 6 related to the conventional example estimated by Equation 1 using the immediate temperature rise value, this embodiment can accurately estimate the thermal displacement even when the spindle changes to a rotational speed with a different time constant in a simple calculation. Is clear.

  Note that the present invention is not limited to the above-described embodiments, for example, it is applied to other machine tools such as a horizontal machining center, or the first-order delay system response processing is executed by another type of filter. Thermal displacement estimation can be performed for objects other than the main shaft.

It is the schematic which shows the thermal displacement correction | amendment system of the vertical machining center with which the method of this invention is implemented. It is a characteristic view which shows the example of the relationship between the thermal displacement time constant of a main axis | shaft, and rotational speed. It is a flowchart which shows one Example of the thermal displacement estimation method of this invention. It is a characteristic view which shows the error which estimated the thermal displacement by the method of this invention, aligning the horizontal axis of FIG. 5 and a time-dependent change. It is a characteristic diagram showing the rotational speed of the main shaft, (a) showing the rotational speed of the main shaft, and (b) showing the main shaft and the temperature rise. FIG. 6 is a characteristic diagram showing an error in estimating a thermal displacement based on an immediate value of temperature rise as a conventional example, with FIG. 5 aligned with the horizontal axis of change with time.

Explanation of symbols

3 Spindle 4 Bed (structure)
6 First temperature sensor 7 Second temperature sensor 8 Temperature measurement device 9 Thermal displacement estimation calculator 10 Storage device 11 NC device

Claims (2)

Steps for determining the relationship between the thermal displacement time constant and the rotational speed related to the thermal displacement estimation object and the measurement temperature time constant in advance by experiment or numerical analysis;
Measuring the temperature in the vicinity of the thermal displacement estimation target of the machine tool;
Obtaining a calculated value related to a change in calorific value using a response equation of a first-order lag system as an inverse problem from the measured temperature and the measured temperature time constant;
Calculating a temperature change corresponding to thermal displacement by a response equation of a first-order lag system as a forward problem from a thermal displacement time constant obtained from a rotation speed and the calculated value;
Converting the thermal displacement equivalent temperature change into an estimated thermal displacement amount;
A method of estimating thermal displacement of a machine tool, comprising:   2. The thermal displacement estimation method for a machine tool according to claim 1, wherein a temperature change obtained by subtracting a measured temperature of a structure serving as a reference for a machine tool from a measured temperature in the vicinity of the spindle is used as the measured temperature.

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