Electricity: motive power systems – Positional servo systems – With compensating features
Patent
1991-07-15
1993-05-11
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
With compensating features
318561, 318569, 318601, 364175, 36447435, G05B 23275
Patent
active
052104788
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of correcting a change of position of a machine by which a change of position of a machine tool controlled by a numerical control device is corrected, and more particularly, to a method of correcting a change of position of a machine by which a change of position of a machine tool having control axes with different rigidities is corrected.
BACKGROUND ART
In a numerical control device (CNC), a workpiece is machined to a desired contour by moving a tool at a commanded speed along a commanded path by a machining program.
To obtain a satisfactorily machined surface corresponding faithfully to a command by a machine tool using such a numerical control device, it is essential to use a servo mechanism with a quick response that enables it to follow rapid changes in the command, and having a high stability by which a stable movement without vibration is ensured.
A speed detector and a position detector of a servomotor in the servo mechanism detect speed and position and a desired control is carried out by a feedback of the information obtained to a control circuit. The servo mechanism can be classified into three types; a semi-closed loop, a closed loop and a hybrid servo method, according to the method used to detect a position.
The best method among the above three servo mechanism methods is adopted for each machine tool, according to the accuracy and rigidity, etc. required by the machine tool.
Nevertheless, the mechanical rigidities of the axes of general machine tools are different from each other, and these differences are particularly noticeable among large-sized machine tools. Therefore, when a cutting is carried out by a machine tool having more than two axes, form errors are generated in the vicinity of a starting point and an end point of machining or at corners, and thus the accuracy of the cutting work becomes poor.
FIG. 6 shows a state of cutting with two axes according to a conventional method. In this Fig, it is assumed that machine rigidity is relatively weak at the X-axis and relatively strong at the Y-axis, and further, that the X-axis is moved by 0.100 mm and the Y-axis by 0.200 mm by these control axes.
If the machine rigidities of the X-axis and the Y-axis are the same, a cutting in a straight line, without error, as shown by the straight line A, is usually obtained, but if the machine rigidity of the X-axis is weak, the movement of the X-axis is too little, as shown by the curve B1, and even though the ratio of distribution pulses of the X-axis to the Y-axis is 1:2, the actual ratio of the machine positions becomes 1:4, as shown by the point C, which results in a form error.
DISCLOSURE OF THE INVENTION
The present invention has been made in view of the aforesaid drawbacks, and an object of the present invention is to provide a method of correcting a change of position of a machine, by which an abrupt machining error occurring due to difference in machine rigidities between locations at which control axes are provided can be minimized.
To achieve the above object, in accordance with the present invention, there is provided a method of correcting a change of position of a machine, in which a change of position of a machine tool having at least two control axes is corrected, characterized in that a correction pulse is added to a distribution pulse of respective axes so that a ratio of amounts of movement of the respective axes of the machine tool obtained from output values of position detectors used for detecting a position of the machine tool becomes equal to a ratio of predicted amounts of movement of the respective axes obtained from a predicted machine position based on the distribution pulses of the respective axes, assuming that a servo system has a first order lag.
The actual amounts of movement of the respective axes are derived from the output values of the position detectors used for detecting a position of the machine tool. Therefore, when the machine rigidity is the same at all axes, the ratio of the predicted amounts
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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