Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1991-07-02
1993-01-05
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
318632, 36447428, 36447435, G05B 1925
Patent
active
051774216
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of correcting a change of position of a machine tool controlled by a numerical control device, and more particularly, to a position correction method in which a change of position of a machine tool with control axes having different rigidities is corrected.
2. Description of the Related Art
In a numerical control device (CNC), a workpiece is machined to a desired contour by moving a tool along a commanded path at a commanded speed in accordance with a machining program.
To obtain a satisfactorily machined surface produced in accordance with commands given to a machine tool, using such a numerical control device, it is essential to provide a servo mechanism having a responsiveness such that it can faithfully follow rapid changes of commands and having a high stability enabling the maintaining of a stable movement without vibration.
The speed and position of a servomotor in the servo mechanism are detected by a speed detector and a position detector, and the data thus detected is fed back to a control circuit for control. The servo mechanism can be classified into three types according to the method used for the position detection; i.e., a semi-closed-loop type, a closed-loop type, and a hybrid servo type.
Among these three methods used for the servo mechanism, a method best suited to each machine tool is employed according to a required accuracy and rigidity, etc. of the machine tool.
In ordinary machine tools, however, the individual axes have different mechanical rigidities, and this difference becomes particularly large in large-sized machine tools. Therefore, when a cutting is effected by a machine tool with more than two axes, machining errors occur in the vicinity of the machining start and end points or at corners, and thus the cutting cannot be effected with a high accuracy.
FIG. 4 shows a cutting with two axes according to the prior art. In this example, it is assumed that the X-axis has a relatively weak mechanical rigidity and the Y-axis has a relatively strong mechanical rigidity, and with these control axes, when there is movement along the X-axis 0.100 mm, of the movement along Y-axis is 0.200 mm.
If the mechanical rigidities of the X- and Y-axes are the same, a linear cutting with no error is usually effected as indicated by the straight line A, but if mechanical rigidity of the X-axis is weak, a required movement along the X-axis is not achieved, as indicated by the curve B1, and even though the ratio between the distribution pulses for the X-axis and Y-axis is 1:2, an actual ratio of machine position becomes 1:4, as shown at the point C, thus causing a machining error.
SUMMARY OF THE INVENTION
The present invention has been made in view of the aforesaid drawbacks, and an object thereof is to provide a method of correcting a machine position change in which an abrupt machining error caused by a difference of the mechanical rigidities of machine parts in the direction of control axes can be suppressed.
To achieve the above object, the present invention provides a method of correcting a change of position of a machine tool having at least two control axes, characterized in that a correction pulse is added to a distribution pulse for individual axes, such that the ratio of actual amounts of movement of the respective axes of the machine tool, obtained from output values of a position detector for detecting a position of the machine tool, becomes equal to the ratio of the distribution pulses for the respective axes.
The actual amounts of movement along the respective axes are obtained from the output values of the position detector for detecting the position of the machine tool, and based on these movement amounts, values by which the movement has not been achieved by the distribution pulses are derived. Therefore, when the mechanical rigidities of all of the axes are the same, the ratio of the distribution pulses for the respective axes is usually equal to the ratio of the amounts of movement of
REFERENCES:
patent: 4502108 (1985-02-01), Nozawa et al.
patent: 4514813 (1985-04-01), Nozawa et al.
patent: 4659265 (1987-04-01), Kishi et al.
Fujibayashi Kentaro
Haga Makoto
Sasaki Takao
Fanuc Ltd.
Martin David
Shoop Jr. William M.
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