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Method for calibrating an analog sensorRelated Patent Categories: Data Processing: Measuring, Calibrating, Or Testing, Measurement System, Speed, Rotational SpeedMethod for calibrating an analog sensor description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070073511, Method for calibrating an analog sensor. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] I. Field of the Invention [0002] The present invention relates generally to methods for determining the absolute angular position of a sensor which produces a repeating periodic analog sensor across a predetermined rotation range. [0003] II. Description of Related Art [0004] There are many different types of applications in industry where it is necessary to determine the absolute angular position of a rotatable actuator across a predetermined rotation range, e.g. 360 degrees. For example, in the automotive industry it is necessary to determine the position of a gearshift actuator and to accomplish automatic recovery of the position of the gearshift actuator upon energization or a power reset of the system. For example, such gearshift actuators are used for many vehicles to shift between two wheel and four wheel drive for the automotive vehicle. [0005] There have been a number of previously known angular position sensors which produce an output signal representative of the angular position of the actuator. For example, in one type of angular sensor, a wiper is secured to and rotates in unison with the actuator. This wiper, in turn, slides across a resistive member so that the resistance between the wiper and one end of the resistive member varies as a function of the rotational position of the wiper and thus the rotational position of the actuator. [0006] One disadvantage of this type of previously known position sensor, however, is that, due to the mechanical contact between the wiper and the resistive member, the sensor suffers mechanical wear, especially after protracted use. Such mechanical wear in the sensor can result in inaccurate and erratic readings from the sensor and even complete sensor failure. [0007] In order to eliminate the mechanical wear of these previously known angular sensors, there have been previously known noncontact sensors. In one type of noncontact sensor, a permanent magnet is mounted to the rotatable actuator so that the permanent magnet rotates in unison with the actuator. [0008] A sensor having a plurality of magnetic resistors is then mounted in alignment but not in contact with the permanent magnet attached to the actuator. These magnetic resistors are arranged in two bridges such that the magnetic resistors provide two sinusoidal output signals offset from each other by 45 degrees and which vary as a function of the angular position of the actuator. [0009] A disadvantage of these previously known sensors utilizing magnetic resistors, however, is that they produce a repeating periodic signal for every 180 degree rotation of the actuator. Consequently, it has not been possible to utilize these previously known sensors with magnetic resistors where the rotation range of the actuator exceeds 180 degrees. This, in turn, necessitated the use of two or more sensors which is prohibitively expensive for many applications. [0010] A still further disadvantage of these noncontact sensors which utilize magnetic resistors is that the sinusoidal output provided by the sensors was not true sinusoidal signals but, rather, approximations of sinusoidal signals. Consequently, even within the 180 degree angular range of the sensor, it was only possible to determine the position of the actuator with an accuracy of plus or minus a few degrees. In many applications, however, it is necessary to determine the absolute angular position of the actuator with greater accuracy, i.e. the absolute angle of the activation even though the absolute angle range of the actuator exceeds the periodic range of the sensor. Summary of the Present Invention [0011] The present invention provides a method for both calibrating a noncontact sensor which generates a repeating periodic analog output signal over a predetermined rotation range and for determining the absolute angular position of the actuator throughout that rotation range. [0012] In brief, the sensor utilizes magnetic resistors which produce two substantially sinusoidal output signals which are phase offset from each other by 45 degrees. Preferably, the sensor utilizes magnetic resistors and the output signals from the sensor have a period of 180 degrees. [0013] The sensor is first calibrated by connecting both the outputs from the sensor together and also the absolute angle information of the actuator from an absolute angle encoder as input signals to a processing circuit. The processing circuit is then programmed to divide the output signals from the sensor into a plurality of adjacent angular segments or areas and thereafter calculate a polynomial curve fit for the ratios of the sensor outputs for each angular segment. A relatively high degree of accuracy can be achieved by utilizing a third order polynomial curve fit and the constants for the coefficient of the polynomial curve fit for each angular segment are stored by the processing circuit in nonvolatile, i.e. persistent, memory. [0014] Following calibration of the sensor, the actuator is returned to a predetermined position and that position is also stored in nonvolatile memory by the processing circuit. [0015] Thereafter, in order to determine the absolute position of the actuator within the predetermined rotation range, the microprocessor utilizes the calibration data to determine the angular position of the actuator as a function of the sensor output. The processor also utilizes the previously stored position of the actuator as well as its direction of travel to determine the angular segment of the actuator and thus the absolute angle of the actuator. [0016] For example, assuming that the output signals from the sensor repeat every 180 degrees and that the rotation range for the sensor is 360 degrees, if the output sensors indicate an angular position of 40 degrees, the absolute position of the actuator as determined by the sensor may be either 40 degrees or 130 degrees. In this case, if the previous position of the sensor as stored in nonvolatile memory was 35 degrees, then the absolute position of the sensor would be 40 degrees. Conversely, if the previously stored value of the angular position of the actuator was 125 degrees, then the absolute angular position of the actuator would be 140 degrees. In either event, the newly determined absolute angular position of the actuator is then stored in nonvolatile memory and the entire process is reiteratively repeated. [0017] The present invention thus provides not only a highly accurate determination of the absolute angular position of the actuator but also extends the range of the angular sensor over two or even more periods of the sensor output. BRIEF DESCRIPTION OF THE DRAWING [0018] A better understanding of the present invention will be had upon reference to the following detailed description, when read in conjunction with the accompanying drawing, wherein like reference characters refer to like parts throughout the several views, and in which: [0019] FIG. 1 is a diagrammatic view illustrating an actuator with a position sensor; [0020] FIG. 2 is a top diagrammatic view of an exemplary sensor; [0021] FIG. 3 illustrates the waveform from the sensor in FIG. 2; Continue reading about Method for calibrating an analog sensor... 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