Electricity: motive power systems – Positional servo systems
Patent
1996-12-12
1998-09-08
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
318364, 318569, 31856822, 31856818, G05B 1101
Patent
active
058049401
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for the numerical control of machines with several axes, in particular machine tools and robots, with compensation for the inaccuracies occurring when the axes are reversed.
BACKGROUND OF THE INVENTION
In modern industrial controls used, for example, in machine tools or robots, path inaccuracies often occur when the axes are reversed, basically caused by varying friction conditions, as well as slackness and torsional effects. When an axis is accelerated from a negative to a positive velocity or vice-versa, the motion can be disturbed during passage through zero due to the varying friction conditions and mechanical non-linearities. In the case of several axes interpolating with one another, this condition results in contour errors. This fact is especially obvious in the case of circular contours, where an axis moves with maximum translational speed when going from one quadrant to another, while the second axis changes the sign of its velocity. At such points of discontinuity with change of direction, but also at start from standstill, friction, slackness and torsion effects often cause the actual rotation speeds to deviate from the reference rotation speeds. In the case of "slackness effects," arising, for example, during the operation in the form of play between the gears of an associated gearbox, points of discontinuity result in a hysteresis-type machine operation. As a result, contour errors are often produced on the work piece being machined. Since it is desirable that such contour inaccuracies be avoided, a numerical machine control process is preferably designed so that such path inaccuracies occurring due to changing friction conditions, slackness effects, and torsional effects when the machine axes are reversed can be compensated for with the help of frictional precontrol through appropriate correction of the rotation speed reference value.
It is known that conventional processes for the numerical control of machines simply add or deduct a constant amount, the slackness, to compensate for slackness, usually in carriage drives, on the path to be traveled when an axis changes signs. This is normally done once in the computation process for each individual control data set (see Weck, M.: 1989,pp. 183 ff). Therefore, in this procedure, no error compensation is built into the control process. Since the path inaccuracies arising when the machine axes are reversed usually also depend on the acceleration, the addition or subtraction of a one-time constant amount provides no sufficient compensation for inaccuracies if the acceleration conditions vary. Instead, a process that is integrated in the main run of numerical control and compensates for path inaccuracies in real time is sought.
WO 90/07738 discloses a process for correcting non-linearities wherein correction signals are provided; however, varying acceleration conditions of the machine are not provided for generating such correction signals. In addition, such correction signals are predefined according to a fixed pattern and they cannot be optimized for the particular machine being used. Furthermore, extensive circuitry is required for obtaining a suitable injection point for such a correction signal.
From the publication IAS '93 Conference record of the 1993 IEEE Industry Applications Conference 28th IAS Annual Meeting (Cat. No. 93CH3366-2), Proceedings of IEEE Industry Application Society Annual Meeting, Toronto, Ontario, Canada, Oct. 2-8, 1993, Vol. 3, 1993, pp. 2027-2034, Seidl D. R. et al. there is known a self-learning knowledge-based system for slackness compensation, which, however, cannot be used for compensating other non-linearities such as, for example, varying friction conditions or torsional effects. In addition, the slackness effects are simulated in the form of characteristic curves, which is very computation-intensive, since the complete variation of the characteristic curves must be determined. This cannot be accomplished in real time with the required accuracy and
REFERENCES:
patent: 5274314 (1993-12-01), Maueira
Seidl et al, "Neural Network Compensation of Gear Backlash Hysteresis in Position-Controlled Mechanisms", IAS '93, Oct. 2-8, 1993, vol. 3 1993, pp. 2027-2034.
Erkens Friedrich
Maetschke Stefan
Quaschner Bernd
Lockett Kim
Shoop Jr. William M.
Siemens Aktiengesellschaft
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