Data processing: measuring – calibrating – or testing – Calibration or correction system – Error due to component compatibility
Reexamination Certificate
2002-06-24
2004-07-27
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Error due to component compatibility
Reexamination Certificate
active
06768956
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high precision position detecting apparatus capable of removing error caused by an offset in a signal from a position sensor outputting two signals which periodically vary in correspondence with measured displacement and differ 90° in phase, or by an amplitude difference or a phase difference between the two signals.
2. Description of the Related Art
Generally, when a movable unit or the like of a machine tool is moved, not only is positional control based on position information detected by a position detecting apparatus performed, but speed feedback control of an electric motor based on speed information derived from a difference in the position information is also performed. There has especially been a trend to increase a loop gain of the speed feedback in order to improve the response characteristics of a machine. The above-described conventional position detecting apparatus can remove an offset or an amplitude difference contained in output of a position sensor immediately after manufacture, but error caused by a change of an offset or an amplitude difference with the passage of a time after manufacture or by the installation environment of the position sensor cannot be removed. In other words, after the position detecting apparatus is manufactured, the offset or the amplitude difference generates a delicate change due to variations in the ambient temperature, changes in the components of the position sensor, leakage magnetic flux generated by an electric motor having the position detecting apparatus incorporated therein, and the like.
Furthermore, in the conventional position detecting apparatus, a generally used method of measuring the offset value or the amplitude correction value at the time of manufacture is to measure the maximum value and the minimum value of the output signal from the position sensor and derive the offset value and the amplitude value from the average thereof and the difference therebetween. Alternatively, as disclosed in Japanese Patent Laid-Open Publication No. Hei 5-256638, the offset value and the amplitude value can be derived from position sensor output signal values from a plurality of positions satisfying a specific condition. Because values of the position sensor output signal at a specific position are used in these methods, the methods are likely to be influenced by noise, waveform distortion, or the like, and there remains a problem that the offset and amplitude correction values cannot be precisely measured at the time of manufacture of the position detecting apparatus.
Ordinarily, influence exerted on the machining accuracy of a machine tool by a change of the offset, the amplitude difference, or the phase difference over time, or by measurement error at the time of shipment is, for practical purposes, negligibly small. However, because the position detecting error caused by such error becomes an error having a cycle equal to or twice that of the error of output signal of the position sensor, a frequency of speed ripple caused by the error and a machinery resonance frequency may, depending on the moving speed of the movable unit, coincide with each other and generate an irregular sound at the resonance position. In particular, because the irregular sound increases in proportion to the loop gain of the speed feedback and loop gain therefore cannot be increased, mechanical performance deteriorates. This problem is common also with optical type or magnetic type encoders in addition to a conventionally used resolver, whenever a position is derived by interpolation processing from two signals which periodically vary in correspondence with measured displacement and which differ 90° in phase.
SUMMARY OF THE INVENTION
The present invention was conceived to solve the above-described problems, and an object thereof is to provide a high precision position detecting apparatus capable of reducing periodic position detecting error caused by change in an offset, an amplitude difference or a phase difference over the passage of time or by installation environment, which is contained in an output signal of a position sensor having, as output, a plurality of signals which periodically vary in correspondence with measured displacement and respectively differ in phase, for improvement of the response characteristics of a movable unit of a machine tool or the like.
In the following,
FIG. 2
is a block diagram showing an example of a position detecting apparatus. In
FIG. 2
, a position sensor
1
is a one-phase excitation two-phase output type resolver, which outputs a multiple of 100 times a rotational angle &thgr; of the input shaft of the position sensor. Namely, when an input shaft is rotated, an excitation signal is amplitude modulated to a sine value and cosine value of a rotation quantity by the resolver, and signals AS and AC amplified by amplifiers
2
and
3
are output. In the example shown in
FIG. 2
, frequency of the excitation signal is 50 KHz, and, if the rotation angle of the input shaft of the position sensor
1
is set as &thgr; and the amplitude of the output signal is set as G, the signals AS and AC can be expressed by the following formulas 1 and 2:
AS=G
·Sin (100&thgr;) Sin (2·&pgr;·50000
·t
. . . (1)
AC=G
·Cos (100&thgr;) Sin (2·&pgr;·50000
·t
. . . (2)
These signals AS and AC are sampled and digitized by a pulse signal TIM of a period of 20 &mgr;S output in synchronism with the excitation signal from a timing controller
13
at a timing of Sin (100000&pgr;t)=1 by respective AD converters
4
and
5
, and converted into values DS and DC, respectively. Consequently, the values DS and DC can be expressed by formulas 3 and 4 below, respectively, and the output of the position sensor can be regarded as two signals which periodically vary in correspondence with the measured displacement &thgr; and differ 90° in phase each other, the formulas being:
DS=G
·Sin (100&thgr;) . . . (3)
DC=G
·Cos (100&thgr;) . . . (4)
Because in actual practice the above-described digitized two values DS and DC contain an offset voltage and an amplitude difference due to product dispersion of the position sensor, or a signal amplifier, the above-described formulas 3 and 4 are more exactly expressed by the following formulas 5 and 6:
DS=B·G
·Sin (100&thgr;)+SOF . . . (5)
DC=G
·Cos (100&thgr;)+COF . . . (6)
These offset values SOF, COF and an amplitude correction value BAJ (=1/B) representing the amplitude ratio between two signals contained in these values DS and DC are measured at the time of manufacture of the position detecting apparatus, stored in a nonvolatile memory or the like in the position detecting apparatus, and set in respective memories
6
,
7
, and
10
prior to performance of position detection. In subtraction devices
8
and
9
, the offset values SOF and COF stored in the memories
6
and
7
are subtracted from the values DS and DC, respectively, to create values DSA and DCA. The value DSA is further multiplied by a multiplier
11
with the amplitude correction value BAJ stored in the memory
10
to become a value DSB of an amplitude which is approximately equal to that of the value DCA.
The values DSB and DCA are subjected to inverse tangent calculation with two variables as input by an interpolation calculator
12
and converted into a position signal POS representing a rotation quantity within 1/100
th
of one rotation of the input shaft of the position sensor
1
. Then, although not shown in the drawing, in an actual position detecting apparatus, position data of at least more than one rotation of the input shaft of the position sensor
1
is derived by incremental processing or the like based on the change of the position signal POS.
It should again be noted that, even after the manufacture of the position detecting apparatus, offset and amplitude differences are generated as a result of variation of ambient
Barlow John
Lau Tung S.
Okuma Corporation
Oliff & Berridg,e PLC
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