Electricity: motive power systems – Positional servo systems
Reexamination Certificate
2002-08-05
2003-11-04
Masih, Karen (Department: 2837)
Electricity: motive power systems
Positional servo systems
C318S569000, C318S567000
Reexamination Certificate
active
06642684
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to a system and a method for controlling a positioning device, in particular actuators pertaining to machine tools, with an incremental position measuring device with output signals of the form sin &phgr;
1
, sin (&phgr;
1
+p) and a control unit converting the output signals into control signals and consequently controlling the positioning device. In this case, p is an angle by which the two output signals are phase-shifted.
The invention also relates to the use of a device, which can be used with said system or when carrying out said method, as a phase mixing device.
BACKGROUND OF THE INVENTION
In the case of machine tools, processing machines and production machines, but also in the case of various measuring and testing devices, it is necessary to position a tool or a measuring sensor automatically in relation to a workpiece or a part to be tested. For reasons of simplicity, the invention is always described below with reference to the example of positioning a tool of a machine tool in relation to a workpiece, without however restricting the invention to this specific application.
For the positioning of tool and workpiece, the tools on modern machine tools can generally be moved and swiveled or rotated about five or more axes. The moving and swiveling or rotating movements are controlled by a central control unit, generally what is known as numerical control (NC), the control unit continually receiving information from a position measuring device about the positional changes made during the moving, swiveling or rotating.
Among the position measuring devices, what are known as incremental (counting) position measuring devices have proven successful in particular. Depending on the type of output signals, a distinction is drawn here between measuring devices with rectangular (and consequently quasi digitized) output signals (the signals generally being transmitted in the form of a voltage pulse) and those with sinusoidal (that is analog) output signals. The devices with rectangular output signals are distinguished by relatively simple signal processing, the positional deviations generally amounting to ±3% of the signal period.
If traversing rates of at least 0.01 m/min are to be realized with a relatively short sampling time of, for example, 250 &mgr;s, and if, for reasons of accuracy, a change of at least one measuring step per sampling cycle is to take place, a measuring step of only 0.04 &mgr;m is required. With this measuring step, at 60 m/min for example, a step frequency of 60 m/(60s*0.04 &mgr;m)=25 MHz is obtained. To keep down the complexity of the circuitry in the downstream electronics, input frequencies of less than 1 MHz are desirable. For such high traversing rates and very small measuring steps, incremental position measuring devices with sinusoidal output signals have proven to be highly successful, a, relative positional deviation within a signal period for drives with digital speed control of less than 1% of the respective signal period of the measuring device making said devices even more accurate than measuring devices with rectangular output signals.
However, there is the problem that, on account of. Nonlinearities, thermal expansions, vibrations etc., the workpiece and tool are in fact in a position in relation to each other which deviates from the position which the control unit has calculated on the basis of the signals transmitted to it by the measuring device. Although such deviations are often only very small, in cases in which extremely small production tolerances are prescribed for the machining of a workpiece they can lead to the complete workpiece becoming unusable. In this case, the deviations occurring under certain conditions and/or in certain positions can often be calculated or measured in advance.
DE 32 01 005 C2 discloses a system for controlling a positioning device, in particular actuators pertaining to machine tools, which comprises an incremental position measuring device with output signals of the form sin &phgr;
1
, sin (&phgr;
1
±p), p being an angle by which the two output signals are phase-shifted, a control unit converting the output signals into control signals and consequently controlling a positioning device, and means for mixing a correction angle &phgr;
2
into the output signals, the means being connected between the position measuring device and the control unit in such a way that the output signals obtain the form sin (&phgr;
1
±&phgr;
2
), sin (&phgr;
1
±&phgr;
2
±p) before they are converted n control signals by the control unit.
However, this system does not allow arbitrary correction angles to be taken into account and consequently cannot be used for controlling the positioning device.
DE 32 01 005 C2 stipulates that it is absolutely necessary to provide correction tracks alongside a scale, which have to be scanned for the acquisition of correction signals. According to the teaching of this document, it is consequently always absolutely necessary, for at least two “scales” to be scanned, that is to say firstly the actual scale and the associated correction track. The correction angles are consequently not arbitrary, but result exclusively from the scanned values of the correction track.
GB 2 335 987 A1 discloses a method for calibrating and compensating errors which are in the measuring system itself. The aim here is to eliminate “short range errors”, in also being possible for these errors to be “phase errors”, which have to be initially determined by means of FFT analysis and then used for correction. However, a machine in the sense of the present application cannot be controlled by calibrating or compensating the errors.
WO 98/00921 discloses an angle encoder for throttle valves. In the evaluation unit belonging to the encoder, the angle of the encoder is determined by mixing and multiplying harmonics of the excitation signal fed in. However, this evaluation unit is not a phase mixer in the sense of the present application.
The document U.S. Pat. No. 5,414,516 discloses a method for the high-resolving evaluation of the angle of two sinusoidal measuring signals, which are respectively offset by 90°. This document does not teach the active control of a positioning device by mixing in a correction angle.
The document EP 0 652 419 A1 discloses a method for the compensation of phase errors in measuring systems, it being possible to take into account angle errors determined in advance or else online, independently of the angle of the measuring system determined at the particular time, in the further processing by angle addition. However, the method does not allow arbitrary angles to be mixed into the measuring system. Rather, only errors within a graduation period are corrected here.
The document U.S. Pat. No. 5,375,066 discloses an apparatus for mixing in correction angles, which can be used only in the case of digital measuring systems or measuring systems with TTL level and not in the case of analog, sinusoidal measuring system signals which are phase-shifted.
SUMMARY OF THE INVENTION
Against this background, the invention is based on the object of specifying a device and a method for controlling a positioning device in which certain positional deviations can be automatically taken into account, without changes having to be made to the control units which, in themselves, have proven successful, or their mode of operation. In particular, it is intended that arbitrary correction angles can be taken into account, so that they can theoretically also be used for controlling the positioning device.
The object is achieved by a system for controlling a positioning device, in particular actuators pertaining to machine tools, with an incremental position measuring device with output signals of the form sine, sin (&phgr;
1
±p), p being an angle by which the two output signals are phase-shifted, a control unit converting the output signals into control signals and consequently controlling a positioning device, and means for
DS Technologie Werkzeugmaschinenbau GmbH
Masih Karen
Ostrolenk Faber Gerb & Soffen, LLP
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