Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2000-04-07
2001-09-18
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S567000, C318S568110, C700S108000
Reexamination Certificate
active
06291959
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a method and an apparatus for controlling a numerically controlled machine tool or, in particular, to a method and an apparatus for controlling a numerically controlled machine tool wherein the machining operation is performed by numerically controlling the feed operation of a plurality of feed axes while preventing an alarm which otherwise might be issued due to the overheating of the drive means of each feed axis, thus making it possible to carry out the machining operation at as high a speed as possible to realize an improved machining efficiency. The above-described machine tool is not limited to a machine tool used only for cutting or grinding, but includes a punch press machine having a feed axis mechanism with a feed motor and a laser beam machine for machining a work with a high energy laser beam. The present invention is widely applicable to machine tools including these machines.
PRIOR ART
Consideration will be given to a typical machine tool for performing the cutting or grinding operation. In the case where the machining operation of the machine tool is numerically controlled, as is well known, a feed motor for each of a plurality of feed axes is controlled, with the rotation of the main spindle of the machine tool, in accordance with a numerical control program (NC program) which is input to a numerical control unit (NC unit) in advance, and the relative machining operation between the work and the machining tool (hereinafter referred to simply as the tool) is controlled so that the desired machining operation is performed on the work based on the machining program.
In this type of numerically controlled machine tool, it is indispensable for securing the machining precision to accurately control the operation of each feed motor for driving the feed operation along each feed axis. At the same time, it is crucial to prevent overheating of the respective feed motors, thereby protecting the machine from breakage or malfunction, in order to achieve a high efficiency in the machining operation.
A basic configuration of a numerically controlled machine tool will be considered hereinbelow. Generally, the numerically controlled machine is provided with a machine tool, a numerical control unit, a machine control unit and an electric power unit. The numerical control unit includes a program read-interpret unit for reading and interpreting the NC program, an interpreted program storage unit for temporarily storing an interpreted program, a program execution command unit for fetching the stored program from the interpreted program storage unit whenever necessary and issuing execution program data, an interpolation and acceleration/deceleration control unit for computing by interpolation the travel data of the operation designated by the NC program, distributing the travel data to each feed axis and controlling the acceleration and deceleration of each feed axis, and a servo control unit for issuing a position command and for correcting the position command and the speed command in accordance with a feedback signal. The servo control unit, in turn, includes a control unit for outputting a motor command signal and a servo amplifier for amplifying the output of the control unit and generating a motor drive current, wherein the current generated by the servo amplifier is used for controlling and driving each of the feed motors of the machine tool.
Also, the machine body of the machine tool includes a main spindle having a tool or the like mounted thereon and rotationally driven by the spindle motor, a spindle head for rotatably supporting the main spindle, a feed motor configured with a servo motor for driving the feeding operation of each of a plurality of feed axes described above, a position detector provided for each feed axis to detect the current position data with the feeding operation of each feed axis, and a worktable having appropriately mounted thereon a workpiece to be machined and adapted to travel along a plurality of feed axes with respect to the base of the machine body. The main spindle motor, each feed motor and the position detector are connected to the numerical control unit, the machine control unit and the electric power unit through a signal line and a power line. The servo amplifier for the numerical control unit described above supplies a desired amount of electric drive current to each feed motor of the machine body of the machine tool, based on the power introduced from the electric power unit, and each feed motor is driven so as to set the tool at a desired machining position. Also, the tool and the workpiece are moved relatively along the feed axes so that the machining operation such as cutting or grinding of the workpiece is performed. In the present invention, all elements for drive use, including the servo amplifier of the servo control unit provided for the numerical control unit and the feed motor supplied with the electric drive current from the servo amplifier, are collectively called a drive means for a feed axis or feed axes.
Now, the drive means of a feed axis, which includes a servo amplifier and a feed motor provided for a numerically controlled machine tool, is basically designed to be capable of exhibiting a sufficient machining performance at a rated current value. As long as the machine tool is operated or driven at or below the rated current value, continuous operation is possible, but in a case where the rated current is exceeded, a discontinuous operation may be permitted. When the operation exceeding the rated current occur frequently, the heat generation by the feed motor increases, so that the motor-temperature MT of the feed motor for the feed axis exceeds a tolerable motor-temperature MT
a
, determined to maintain the desired motor performance. In such a case, therefore, an alarm signal indicating overheating is generated to stop the operation of the feed motor.
FIG. 11
, in which the abscissa represents the time T and the ordinate the motor temperature MT, shows temperature curves for three cases including (I) the case in which the machine tool is operated without controlling the heat generation amount of the feed motor, (II) the case in which the machine tool is operated by maintaining a rated heat generation amount determined from the design, and (III) the case in which the machine tool is operated while controlling the tolerable heat generation amount in accordance with the motor temperature of the feed motor. In the case (I) where the heat generation amount is not controlled, the motor temperature MT quickly exceeds the tolerable motor-temperature MT
a
. In the case (II) where the machine tool is operated to maintain a rated amount of heat generation or in the case (III) where the machine tool is operated to control the tolerable amount of heat generation in accordance with the motor temperature, however, it is found that the operation can be continued without exceeding the tolerable motor-temperature MT
a.
The feeding operation performed by each of the feed motors, on the other hand, includes a machining-feed in which the tool engaging a workpiece is fed to perform the machining operation, and a rapid-feed for quickly accomplishing the positioning operation for determining the relative positions of the tool and the workpiece along each of the feed axes. The rapid-feed is a non-machining operation, and therefore desirably contributes to the improvement in the machining efficiency by increasing the speed thereof as far as possible. In fact, the acceleration or deceleration of the rapid-feed causes an electric current flow three or four times as large as the rated current in the feed motor. The drive means is, however, so configured that unless the electric current value averaged for a predetermined elapsed time exceeds the rated current value, the temperature of the feed motor does not increase beyond the tolerable motor-temperature MT
a
to such a level as to cause an overheating.
The high-speed machines currently provided are intended to increase the speed of r
Mori Norio
Teraoka Yoshikatsu
Yoshida Jun
Leykin Rita
Makino Milling Machine Co., Ltd.
Nappi Robert E.
Paul & Paul
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