Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2001-06-20
2003-05-20
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S575000, C318S600000, C318S625000, C318S632000
Reexamination Certificate
active
06566835
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of controlling a numerically controlled machine tool such as a milling machine, machining center or electric discharge machine having a plurality of feed shafts of three orthogonal axes of X, Y and Z or having a plurality of feed shafts of at least one of the rotary shafts of axes of A, B and C in addition to the three orthogonal axes of X, Y and Z. Further, the present invention relates to a 15 numerically controlled machine tool. In other words, the present invention relates to a new technique of a numerically controlled machine tool by which a workpiece can be machined with high accuracy even at a high feed speed.
DESCRIPTION OF THE PRIOR ART
Concerning a numerically controlled machine tool, it is required that a workpiece is accurately machined in a short period, that is, it is required that a workpiece is highly efficiently and accurately machined. In general, it is known that machining accuracy is deteriorated when the feed speed of a machine tool is raised. This deterioration in machining accuracy is caused by a lost motion of the feed shaft and a delay of servo-control of the numerically controlled machine tool. Therefore, in the case of a numerically controlled machine tool, in order to conduct machining with high accuracy even when the feed speed is raised to a high value, backlash of the feed shaft is corrected and further friction of the feed shaft is corrected, and furthermore speed adjusting control of the feed shaft is conducted according to the weight of a workpiece and the temperature of the feed shaft motor. For example, the following prior arts are provided.
The first prior art is disclosed in Japanese Patent Publication No. 2606773, which discloses an acceleration control method and device in a servo system. According to this prior art, lost motions of the feed shaft caused by backlash, elastic deformation and static friction in the case of inversion in the direction of movement of the feed shaft are corrected by conducting the most appropriate acceleration control corresponding to the respective characteristics so as to reduce the deterioration of machining accuracy. To accomplish the above object, the first, second and third acceleration for compensating the lost motions caused by backlash, elastic deformation and static friction in the feed system are added to the speed commands of the servo control unit, so that the delay caused by the lost motions can be immediately made up.
The second prior art is a servo motor control method disclosed in Japanese Patent Publication No. 2709969.
According to this method, for the object of conducting the most appropriate backlash correction even when the cutting condition fluctuates, the target value is set at a value, the sign of which is reverse to that, of the integrator of the speed control unit before the direction of movement is inverted, and a value obtained when the value of the integrator of the speed control unit is subtracted from the target value is multiplied by a constant, and the thus obtained value is made to be a value of backlash acceleration in the speed control unit, for example, a value obtained when a value proportional to the square root of a positional deviation at the moment when the direction of movement is inverted is multiplied, and the thus obtained value is made to be a value of backlash acceleration in the speed control unit.
The third prior art is a method and device of controlling acceleration and deceleration of a machine tool disclosed in Japanese Unexamined Patent Publication No. 11-90769. According to this prior art, for the object of ensuring high machining accuracy and shortening the machining time when the weights of moving things such as a tool and a workpiece are changed in the case of replacing them, the drive system is controlled by an acceleration corresponding to the rigidity of the machine tool, machining accuracy (allowable error) and weight of the workpiece. That is, there is disclosed a technique in which the acceleration is changed corresponding to the load inertia which has been previously set.
The fourth prior art is a speed control unit of a servo motor disclosed in Japanese Unexamined Patent Publication No. 6-274763. This patent publication describes a torque observer by which the load torque is estimated from the output torque of the feed shaft motor and the acceleration of an object to be driven.
According to this technique, a change in the estimated value of the load torque is detected, and the load inertia is estimated, and then the load inertia which has been set in the torque observer is renewed.
The fifth prior art is a method and device of controlling a numerically controlled device disclosed in Japanese Patent Publication No. 2853023. According to this technique, for the object of preventing the feed shaft motor from overheating even when the motor is continuously operated being frequently accelerated and decelerated because the feed shaft is quickly rotated, the temperature of the feed shaft motor is measured, and the thus measured temperature is compared with the predetermined temperature data allowed to the feed shaft motor. According to the result of comparison, the acceleration and the deceleration curve of the feed shaft are controlled being changed.
According to the first prior art, the acceleration is found, and the thus found acceleration is added to a speed command value of the servo control unit. In a numerically controlled machine tool, which is actually used, it is finally required that how high torque command value or how high electric current command value is outputted to the feed shaft motor drive means. Therefore, when the speed command value in the middle of servo control is changed like the first prior art, a delay is caused when the command value is converted into a torque command value or an electric current command value and arrives at the feed shaft motor drive means.
According to the second prior art, the backlash acceleration calculated according to the positional deviation is made to be a backlash acceleration in the speed control unit. Therefore, a delay still exists in the servo system composed of a positional feedback control means and speed feedback control means.
According to the third prior art, the load inertia is previously set at a predetermined value. Therefore, the acceleration is changed according to the weight of a workpiece. That is, when the weight of a workpiece is heavy, the acceleration is raised to an allowable limit, and when the weight of a workpiece is light, the acceleration is lowered. When the acceleration is lowered, the machining efficiency is deteriorated.
The fourth prior art relates to a torque observer for estimating the load torque of a common servo motor. The load torque is estimated according to the speed command value, and the load inertia is estimated according to the estimated load torque. Then, the estimated load inertia is sent to the transfer function of the mechanical system so as to conduct feed control. According to the aforementioned technique, since the load inertia is an estimated value, a delay is caused in the feed shaft of the device, and the machining accuracy is affected by the delay.
According to the fifth prior art, the time constant of acceleration and deceleration is controlled in accordance with the temperature of the feed shaft motor, so that the feed shaft motor is prevented from overheating without changing the command feed speed. When this technique is adopted, it is possible to prevent the feed shaft motor from overheating, however, the time constant of acceleration and deceleration is increased, and the machining accuracy is deteriorated.
Other than the above prior arts, there are provided conventional methods in which correction of backlash or correction of friction is conducted. However, according to these conventional methods, the same correction value is used without giving consideration to the speed and acceleration of a moving object. In the case of an actual machining operation, when an o
Teraoka Yoshikatsu
Yoshida Jun
Makino Milling Machine Co., Ltd.
Paul & Paul
Ro Bentsu
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