Machine tool displacement correcting apparatus

Details Machine tool displacement correcting apparatus Machine tool displacement correcting apparatus

1998-07-08

2002-01-22

Sheikh, Ayaz (Department: 2155)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Product assembly or manufacturing

C700S193000, C700S245000, C318S564000, C318S567000, C360S073050, C409S080000

Reexamination Certificate

active

06341245

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a displacement compensation apparatus for machine tools to compensate displacement resulting from centrifugal force and heat generation caused by spindle rotation in a machine tool.
BACKGROUND ART
Conventionally, an apparatus to compensate heat displacement of a spindle based on temperature changes on the spindle is known as a heat displacement compensation apparatus to compensate heat displacement resulting from heat generation caused by rotation of the spindle in the machine tool.
In the above displacement compensation apparatus, compensation a correlation between the displacement of the spindle and a difference in temperature between places where a heat displacement is most likely to occur and where change in temperature is relatively small is examined in advance, for instance, a difference in temperature between near the spindle and the bed. A compensation value corresponding to the difference in temperature is calculated from the correlation, and the heat displacement is compensated based on the compensation value. But the conventional compensation method of heat displacement has a disadvantage that a difference is often found between an actual heat displacement of a spindle and a compensation value derived from the compensation calculation based on the difference in temperature, when the spindle is rotated at a random rotation frequency.
It is clear from
FIG. 8
that a large deviation is found between a measured value and a compensation value obtained from the conventional compensation calculation, when the displacement of the spindle is measured after the spindle is rotated at rotation frequencies of, for instance, 1000 rpm, 3000 rpm, 0 (suspension), 2000 rpm, 3000 rpm, 1000 rpm for each 40 minutes and after leaving in suspension for 120 minutes.
When a spindle rotates, the spindle is expanded by centrifugal force. There is a disadvantage that an expansion of a spindle by centrifugal force makes the spindle shorten affected by the Poisson's effect, so that the position of the tool retreats to that extent from the product in process.
In the conventional heat displacement compensation method, as the compensation value is determined by the difference in temperature, it is impossible to compensate instant displacement caused by a change in the rotation frequency of the spindle.
The object of the present invention is to provide a displacement compensation apparatus for machine tools to precisely compensate displacement resulting from centrifugal force or heat generation caused by rotating spindle rotation even when the spindle is randomly rotated.
DISCLOSURE OF THE INVENTION
A displacement compensation apparatus for machine tools according to the present invention is to compensate a displacement resulting from centrifugal force and heat generation caused by rotation of a spindle in a machine tool. The apparatus is provided with; a historical data table storing a history of function values of rotation frequency of the spindle in each time period from present to past; a corrective coefficient table in which corrective coefficients are stored in advance relating the coefficients to the corresponding location in the historical data table and a compensation process means. The compensation process means reads out each location and each corrective coefficient which corresponds to the location from the corrective coefficient table and concurrently reads out a function value for the rotation frequency of the spindle at the corresponding location from the historical data table, calculates a compensation value from the function value at each location and the corresponding corrective coefficient, and compensates a commanded position of each shaft in the machine tool based on the compensation values.
The corrective coefficient is determined by collecting and storing measured data on displacement of the spindle, shaft rotation time data and rotation frequency data when the spindle is randomly rotated with changing the shaft rotation frequency and by processing a regression calculation based on these data.
The function value of the rotation frequency of the spindle is a value proportional to the square of the spindle rotation frequency, or a value based on the proportional value to the square of the spindle rotation frequency, with compensation by other value.
For instance, the function value is obtained from dividing a square of the rotation frequency of the spindle by 16384 and subtracting one tenth of the rotation frequency of the spindle. The number thus derived is used as the function value. The equation is expressed as follows.
 Functional equation=(rotation frequency×rotation frequency/16384) −0.1×rotation frequency  (1)
In another example, a function value is calculated as a square of the rotation frequency of the spindle is divided by 16384 and further divided by one plus a number of three times of inverse of the natural logarithm of the rotation frequency divided by 25000. The functional equation is expressed as follows.
Functional equation=(rotation frequency×rotation frequency/16384/(1.0+3.0/exp(rotation frequency/25000))  (2)
In general, as centrifugal force is proportional to a square of a rotation frequency of a spindle, a displacement which is proportional to centrifugal force is also proportional to a square of the rotation frequency of the spindle. And as long as a force acting on the spindle and a friction coefficient stay constant, the friction torque is in proportion to the rotation frequency of the spindle. As friction is a product of friction torque and rotation frequency, it is proportional to a square of the rotation frequency of the spindle. Heat generation is a result of consumed energy by friction, it is also proportional to a square of the rotation frequency of the spindle. Hence, when a value which is proportional to a square of a spindle rotation frequency is used as a function value of the spindle rotation frequency, the accuracy of compensation can be improved.
In actuality, as a force acting on the spindle tends to increase with the increase of the rotation frequency, beat generation changes more sharply than a proportion to a square of the rotation frequency of the spindle. Consequently, as a function value for the rotation frequency of the spindle, a number proportional to a square of the spindle rotation frequency as a main component and compensated by another number is used. Then the accuracy of compensation is much improved.
In compensating displacement, each stored location and corrective coefficient are read out from the corrective coefficient, concurrently the function value of the spindle rotation frequency in each location is read out from the historical data table. And the compensation value is calculated from the function value in each location and the corresponding corrective coefficient.
Now, each location which is read out from the corrective coefficient table is taken as k
00
, k
01
, k
02
, . . . k
m
, and corresponding each corrective coefficient as b
00
, b
01
, b
02
, . . . b
m
. And the function value of the spindle rotation frequency at each location read out from the historical data table is taken as R(k
00
), R(k
01
), R(k
02
), . . . R(k
m
), then the displacement amounts &dgr; is calculated from the following equation.
&dgr;=R(k
00
)×b
00
+R(k
01
)×b
01
+R(k
02
)×b
02
. . . +R(k
m
)×b
m
  (3)
The compensation value &egr; is obtained by changing the signs of the displacement amount &dgr;.
More specifically, as the corrective coefficient value obtained from the equation (3) is very small, 4194304 times of the corrective coefficient value is stored and the displacement amount &dgr; is obtained from dividing the calculated value by 4194304.
For instance, when compensating three axes X, Y and Z, the location read out from the corrective coefficient table are expressed as k
00
, k
01
, k
02
. . . k
m
the corrective coefficient in X axis corresponding to the

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