Apparatus and method for thermal displacement correction for a machine tool

• US 6,456,896 B1
• Filed: 06/29/1998
• Issued: 09/24/2002
• Est. Priority Date: 11/11/1996
• Status: Expired due to Fees

• Current Assignee: Fanuc Ltd (Fanuc Corporation)
• Original Assignee: Fanuc Ltd (Fanuc Corporation)
• Inventors: Kojima, Kunio, Ishii, Hisao, Maekawa, Susumu, Takemoto, Masanobu, Ito, Susumu, Kamiguchi, Masao
• Primary Examiner(s): Wong, Peter
• Assistant Examiner(s): VO, TIM T
• Application Number:
  US09/101,022
• Time in Patent Office: 1,548 Days
• Field of Search: 700/193, 700/174, 700/192, 700/194
• US Class Current: 700/193
• CPC Class Codes: B23Q 3/15534 Magazines mounted on the spindle
• CPC Class Codes: G05B 19/404 characterised by control arrangements for compensation, e.g. for backlash, overshoot, tool offset, tool wear, temperature, machine construction errors, load, inertia G05B19/19, G05B19/41 take precedence

Abstract

The position of a feed shaft is monitored, the mean moving speed and moving frequency of the feed shaft are measured with every unit time for position correction, and a correction amount δn is determined from the speed and frequency according to an approximation formula and updated (c7 and c8). A position correction amount for a commanded position is determined from this correction amount, and the commanded position is corrected by the position correction amount (c1 to c6). The position correction amount for the commanded position is determined from this correction amount, and the commanded position is corrected by this position correction amount. Since the correction amount is determined according to the approximation formula, thermal displacement correction can be effected at all times without requiring any sensor. When the correction amount changes substantially (c10), the thermal displacement is measured by means of a sensor and used as the correction amount (c11 and c12), whereby accuracy is improved. The frequency of measurement by means of sensors is reduced, so that the machining time can be reduced.

Claims

1. A thermal displacement correction method for a machine tool controlled by a numerical control device, comprising:

storing in the numerical control device a first approximation formula for obtaining a correction amount against a thermal displacement from a mean moving speed and a moving frequency of a feed shaft;

storing in the numerical control device a second approximation formula for obtaining a position correction amount for a commanded position from the correction amount obtained according to the first approximation formula;

monitoring the position of the feed shaft;

determining the mean moving speed and moving frequency of the feed shaft;

determining the position correction amount for the commanded position of the feed shaft according to the first and second approximation formulas; and

correcting the commanded position by the position correction amount and outputting the corrected commanded position.

2. The thermal displacement correction method according to claim 1, further comprising

storing in the numerical control device a third approximation formula for determining a correction amount against a thermal displacement of a main spindle from a rotational speed of the main spindle;

monitoring the rotational speed of the main spindle;

determining a correction amount from the monitored rotational speed of the main spindle according to the third approximation formula; and

adding the correction amount from the monitored rotational speed of the main spindle to the position correction amount for the commanded position of the feed shaft for moving the main spindle, wherein the resulting amount is used as the correction amount for the feed shaft.

3. The thermal displacement correction method according to claim 1, further comprising

storing in the numerical control device a third approximation formula for determining a correction amount against a thermal displacement of an axis during a power-off time of the machine tool;

storing the time when the machine tool is disconnected from a power source and a correction amount for the time; and

determining a correction amount from the previously stored correction amount and the power-off time according to the third approximation formula when the machine tool is reconnected to the power source, wherein the determined correction amount is used as the correction amount for the time of reconnection to the power source.

4. The thermal displacement correction method according to claim 1, further comprising changing a coefficient of the first approximation formula for determining the correction amount depending on a varied shaft lubrication method.

5. The thermal displacement correction method according to claim 1, further comprising detecting a disturbance load torque on the axis, and changing the correction amount in accordance with the magnitude of the detected disturbance load torque.

6. The thermal displacement correction method according to claim 1, further comprising

measuring a position displacement or temperature through a sensor when a change of the correction amount determined according to the first approximation formula becomes equal to or higher than a set value;

and

determining a correction amount from the measured value.

7. The thermal displacement correction method according to claim 1, further comprising

inputting data on an axis and a position into the numerical control device;

determining a position correction amount for the inputted position according to the second approximation formula; and

displaying the position correction amount on a display unit.

8. A machine tool controlled by a numerical control device, comprising:

a feed shaft sensor that detects a position of a specific feed shaft of the machine tool with every preset sampling time Δ

T;

a memory; and

a processor that receives the position data from said feed shaft sensor, wherein said processor

determines a mean moving speed and a moving frequency with every preset correction unit time t(pc) (t(pc)=N*Δ

T;

N is an integer larger than

     1),

based upon the determined moving speed, the moving frequency, and the current position of the specific feed shaft, estimates and stores in said memory a thermal displacement of the specific feed shaft in the current position during the correction unit time, and

corrects a position command for the specific feed shaft against the estimated thermal displacement during the next correction unit time t(pc).

9. The machine tool according to claim 8, wherein the moving frequency is defined as the ratio (K/N) between a total number (N) of sampling times Δ

T in each correction unit time t(pc) and a number (K) of sampling times within which the detected moving speed of the specific feed shaft is not at zero.

10. The machine tool according to claim 8, wherein said processor

stores in said memory, when the operation of the machine tool is stopped, a time at which the operation of the machine tool is stopped and the estimated thermal displacement at the time when the operation of the machine is stopped, and

inputs, when the operation of the machine tool is resumed after the stoppage, a time at which the operation is resumed, and the stored estimated thermal displacement at the time when the operation is stopped,

wherein said processor estimates and stores in said memory the estimated thermal displacement at the time when the operation is resumed using the inputted values.

11. The machine tool controlled according to claim 8, wherein said processor

determines a correction amount against a thermal displacement of a main spindle from a detected rotational speed of the main spindle using a second approximation formula, and

further corrects the position command using the correction amount of the main spindle.

12. The machine tool according to claim 8, further comprising

a sensor that measures a position displacement of the specific feed shaft;

wherein said processor

corrects the position command using the measured position displacement when the estimated thermal displacement exceeds a set value.

13. The machine tool according to claim 8, wherein said processor

detects a disturbance load torque on the specific feed shaft;

and

corrects the position command in accordance with the detected disturbance load torque.

14. A computer readable medium encoded with processing instructions for implementing a thermal displacement correction method for a machine tool performed by a computer, the method comprising:

determining a mean moving speed and a moving frequency based upon received position data;

estimating a first correction amount for thermal displacement according to a first approximation formula using the mean moving speed and moving frequency;

estimating a position correction according to a second approximation formula using the first correction amount and a received commanded position; and

correcting the received commanded position according to the position correction.

15. The computer readable medium according to claim 14, further comprising

determining a second correction amount against a thermal displacement of a main spindle from a detected rotational speed of a main spindle using a third approximation formula; and

adding the second correction amount from the monitored rotational speed of the main spindle to the position correction amount for the commanded position of the feed shaft for moving the main spindle, wherein the resulting amount is used as first the correction amount for the feed shaft.

16. The computer readable medium according to claim 14, further comprising

storing a time when the machine tool is disconnected from a power source and a first correction amount for the time;

receiving a time of reconnection to the power source; and

determining a first correction amount for the time of reconnection using the previously stored first correction amount and the power-off time according to a third approximation formula when the machine tool is reconnected to the power source.

17. The computer readable medium according to claim 14, further comprising changing a coefficient of the first approximation formula for determining the first correction amount depending on a varied shaft lubrication method.

18. The computer readable medium according to claim 14, further comprising detecting a disturbance load torque on the specific feed shaft, and changing the first correction amount in accordance with the magnitude of the detected disturbance load torque.

19. The computer readable medium according to claim 14, further comprising, when the change of the first correction amount determined according to the first approximation formula becomes equal to or higher than a set value, receiving a measured position displacement and determining the first correction amount from the measured value.

20. The computer readable medium according to claim 14, further comprising receiving inputted data on an axis and a position, determining a position correction amount for the inputted position according to the second approximation formula, and outputting the position correction amount for display on a display unit.