Coded data generation or conversion – Phase or time of phase change
Reexamination Certificate
1998-09-04
2001-12-04
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Phase or time of phase change
Reexamination Certificate
active
06326908
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to precision position measurement of mechanical motion.
BACKGROUND OF THE INVENTION
A variety of methods have been developed for measurement of mechanical motion. For example, synchros and resolvers are low impedance devices that are used to generate analog signals representative of motor position and are capable of driving a synchro-to-digital converter which converts the analog signal to a digital signal representative of position for use in a servo system. These devices require an alternating current (AC) reference excitation signal. The time-phase difference between the excitation signals in the rotor and stator of these devices may be used to develop a position signal proportional to the angular position of the rotor with respect to the stator. This position signal is representative of the spatial phase angle of the motor and is ideally independent of the frequency and amplitude of the excitation signal. Undesirable effects that decrease the accuracy and precision of synchro/resolver position measurement systems are harmonic distortion, time-phase shifts due to capacitance, cable length, etc., and nonlinearities due to mechanical imperfections in the magnetics and windings of the synchro/resolver. Because of these practical limitations to the accuracy and precision of synchro-resolver devices, other types of position encoders are used where greater accuracy and precision are required.
Rotary position encoders achieve higher accuracy than synchro/resolver systems and are relatively economical. Typically, rotary encoders are comprised of chrome on glass and are highly invariant with temperature changes. Even higher accuracy can be achieved by converting rotary motion to linear motion using precision-machined mechanical devices. However, such linear encoders are costly and unsuitable for many applications. Optical position encoders can also provide high accuracy. However, these encoder require tight alignment tolerances and clean environments, making them expensive and difficult to implement. Magnetic linear encoders are less sensitive to alignment tolerances but are as expensive as optical encoders and require additional mounting bracketry.
What is needed is a relatively inexpensive method for measurement of mechanical position with high precision and accuracy that is easy to implement.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide methods and apparatus for measurement of mechanical position with high precision and accuracy that are easy to implement and are inexpensive.
According to one aspect of the invention, a coarse position indicator generates one or more coarse position signals that are a function of mechanical position. A resolver system responsive to the coarse position signals generates a digital coarse position word that is representative of mechanical position. The resolver system can derive the digital coarse position word from coarse position signals that are functions only of position and are not carried by an AC reference excitation signal. The digital coarse position word derived from the resolver system addresses a memory location of a precision memory unit. The precision memory unit is calibrated so that each addressed memory location contains a position increment number which is used to form quadrature incremental position signals for use with dual channel position controls systems. The position increment number at each address indicates an incremental or null change in position corresponding to the digital coarse position signal which addresses the memory location wherein the increment number is stored.
A feature of the present invention is the ability to obtain precise mechanical incremental position measurements from a coarse indication of mechanical position. A coarse position indicator can be manufactured inexpensively and mounted without stringent alignment tolerances that are difficult to achieve, and without the necessity of expensive or sophisticated packaging to maintain a desired environment for correct operation. Thus, for example, a simple array of ordinary hall effect devices can be used to generate coarse position signals from which a precise position measurement can be obtained. Similarly a precise measurement can be obtained from coarse position signals obtained from low cost optical gratings, lightwave sources, and an array of lightwave sensors.
Another feature of the present invention is the ability to derive a digital position word from coarse position signals that are independent of any AC reference excitation signal. Thus, the coarse position signals can be non-time harmonic functions of position.
According to another aspect of the invention, the coarse position signals from an array of position sensors are combined to provide a more sinusoidal signal to enable a more precise position determination to be made. This is done by spatially arranging the sensors and combining their output signals to achieve substantial cancellation of harmonics, noise and DC offsets, as well as to substantially eliminate temperature drift effects.
These and additional features and advantages of the present invention will become further apparent and better understood with reference to the following detailed description and attached drawings.
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Data Device Inc. Product Sheet for RDC-19220 Series, Resolver-to-Digital Converters ©1990, 1999, pp. 1-20.*
Analog Devices, Inc. Resolver-to-Digital Converter AD2S90 product sheet Rev. D, 1999 pp. 1-12.
Beakley Bruce
Hoffman David
Stellitano Patrick
Williams Howard L.
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