METHOD FOR CORRECTING MEASUREMENT ERRORS OF AN ANGULAR POSITION USING AN ANGULAR POSITION SENSOR

Abstract

The invention relates to a method for correcting low- and high-frequency errors generated by an angular position sensor, characterized in that it comprises at least the following steps: a) measuring a first known angular position (θ1), said measurement being provided by an angular position sensor (C) of an object using a set of at least two photodetectors (P, P1) whose number is a multiple of two; b) measuring at least a second known angular position (θ2), said measurement being provided by the same sensor (C) as in step a) using the same set of photodetectors (P, P1) placed in the same location as in step a) and whose number is the same multiple of two as in step a); c) obtaining, for each of the measurements of the first (θ1) and second (θ2) angular positions obtained in steps a) and b), a first (θ'1) and second (θ'2) angular position corrected by a correction (E1, E2) of the measurements of each angular position (θ1,θ2) by comparison with the known values ​​of the first and second angular positions respectively, and - d) calculate the average (θ3) of the first and second angular positions obtained (θ'1, θ'2) after correction in step c). Figure to be published with the abbreviation: Fig. 5,

Description

Title of the invention: METHOD FOR CORRECTING MEASUREMENT ERRORS OF AN ANGULAR POSITION BY AN ANGULAR POSITION SENSOR

[0001] The invention relates to a method for correcting an error in measuring an angular position by an angular position sensor.

[0002] Here, the expression "angular position sensor" designates both a magnetic angular position sensor called a magnetic encoder and an optical angular position sensor called an optical encoder. These different names will be used subsequently to designate magnetic or optical angular position sensors.

[0003] An angular position sensor is a device for determining, without contact, the angular position of an object. There are two types of encoders for determining an angular position of an object: Optical encoders and magnetic encoders. In an optical encoder, a disk comprising opaque zones and transparent zones is rotated by the rotation shaft of the encoder. An optical beam emitted by infrared diodes passes through the transparent zones of the rotating disk and strikes photo-detectors, creating an analog signal which is amplified and transformed into a digital signal, the latter being transmitted to a data processing means which makes it possible to provide the angular position. In the case of a magnetic encoder, a permanent magnet, at least bipolar, is fixed to a rotation shaft and generates a magnetic field. This magnetic field varies according to the displacement of the magnet, therefore according to the rotation of the latter.The magnetic field is detected by a disc-shaped sensor which produces two sinusoidal curves, each 90° out of phase and corresponding to a sine and a cosine. By calculating the arc tangent, the measurement of an angle is obtained.

[0004] Note that in both cases, we obtain a curve providing the position of the object as a function of the angle. Such a curve is, in theory, linear between 0° and 360°. It turns out that in reality this curve is not a straight line but has a double undulation around the theoretical line. A first undulation of a given amplitude and frequency on which is superimposed a second undulation of lower amplitude and frequency. We thus obtain an irregular sinusoidal curve, with a jagged appearance. The first undulation corresponds to a so-called low-frequency error and the second to a so-called high-frequency error. Subsequently, the expressions "low-frequency errors" and "high-frequency errors" will also be used. Low-frequency errors are errors due to a centering defect of the optical disc or magnetic track. These are regular errors and repetitive for the same mounting conditions of the same encoder. They are relatively frequent because a small error can induce a large measurement deviation. For example, a disc offset of an optical encoder of 10 pm, with a reading diameter of 35 mm, corresponds to an error of approximately one minute of angle, peak to peak. This type of error varies if the mounting conditions of the encoder vary. It is therefore an error essentially of mechanical origin.

[0005] High-frequency errors are the result of other types of errors that may be encountered. Non-limiting examples include an optical disk engraving error, an error on the magnetic track, an error on the engraving pitch, an error on the positioning of the photodetectors, a phase shift error of the sine and cosine signals for a magnetic encoder. Because of these two types of errors, known solutions are used, particularly by the manufacturer, to check the encoders, optical or magnetic, before they are marketed to the customer. The manufacturer has a standard curve for a given angle which makes it possible to correct certain errors, particularly those at low frequencies.Since the final digital signal corresponds to a sine and a cosine, we can verify that there is an error at high frequencies by checking whether or not there is a phase shift and / or a difference in the shapes of the curves for two different and known angles, between the sine and cosine curves. By realigning the curves between the two angles, we correct, or at least limit, the errors at high frequencies. If such a solution is applicable and relatively satisfactory when checking angular position encoders at the manufacturer, it turns out that at the end user the constraints of mounting the encoders are different. As a result, error correction can be ineffective or even increase the errors because an additional error is generated at low frequencies by inducing a phase opposition once the encoder is mounted at the user's. It thus happens that the initial error is amplified up to double.In other words, the error correction file is fixed and only valid at the manufacturer but disconnected from reality once the encoder is mounted at the user's premises and ready for use. Such a solution, although easy to implement at the manufacturer's premises, is not satisfactory because it can prove to be totally ineffective once the encoder is in the operating configuration. This is a so-called calibration solution which cannot be reproduced as is at the user's premises.

[0006] To remedy this defect, another solution is known, particularly for optical encoders, which consists of correcting the eccentricities, therefore the defects mainly at the origin of errors at low frequencies, not with values ​​determined as previously but by a calculation. When measuring two different angles, two photo-detectors are used, placed opposite each other, therefore diametrically opposed with respect to the disk. The relative phase of the signals from the two photos detectors evolves sinusoidally over one revolution. We then obtain for each angle two curves in phase opposition, the photo-detectors ensuring measurements of angles at 180° to each other. By averaging the curves, we obtain an average value of the angle, corrected for low-frequency errors. Once this correction is made, the error at high frequencies is still present. If we take the difference between the angle resulting from the average and the standard value to correct the error at high frequencies, this implies that the photo-detectors which are diametrically opposed read exactly the same thing. However, the disc is still off-center, so the two photo-detectors do not read the same thing. In fact, the error at high frequencies is not only still present but amplified.Also known from US-A-2021 278 260 is a sensor comprising a transmitter coil designed to be powered by a high-frequency current source and at least two receiver coils. One of the receiver coils generates a sine-type signal and the other generates a cosine-type signal during rotation of the coupler. One module compensates for the non-sinusoidal output signals. Another module processes the non-sinusoidal signals from the receiver coils, determines an error in these non-sinusoidal signals, mathematically compensates the assembly to eliminate the error and generates an output signal representative of the rotational position of the coupler. US-A-2020 240 811 describes a filter which reduces the high-frequency content of the output signal and generates an offset correction signal.

[0007] Most of the solutions envisaged favor the correction of one type of errors, low or high frequencies, to the detriment of the other type of errors. Thus there is a need for error correction of encoders, optical or magnetic, in order to improve their performance to develop their uses in many fields of application.

[0008] The object of the invention is to propose a method making it possible to optimally correct the two types of errors, without favoring the correction of one type of error, low or high frequencies, over the other.

[0009] To this end, the invention relates to a method for correcting so-called low and high frequency errors generated by an angular position sensor when measuring an angular position, characterized in that it comprises at least the following steps:

[0010] - a) performing a measurement of a first known angular position, said measurement being provided by an angular position sensor of an object using a set of at least two photo-detectors, the number of which is a multiple of two,

[0011] - b) carrying out a measurement of at least one second known angular position, said measurement being provided by the same sensor as in step a) using the same set of photo-detectors placed in the same place as in step a) and the number of which is the same multiple of two than in step a),

[0012] - c) obtaining, for each of the measurements of the first and second angular positions obtained in steps a) and b) a first and second angular positions corrected by a correction of the measurements of each angular position by comparison with respectively the values ​​of the first and second known angular positions and

[0013] - d) averaging the first and second angular positions obtained after correction in step c).

[0014] Thanks to the invention, without structural modification of the same angular position sensor, with a set of at least two photo-detectors, two measurements of two distinct and known angular positions are carried out. The angular position sensor being the same, the so-called high-frequency errors which are due to etching errors, the etching pitch, the positioning of the photo-detectors are the same, this type of error being linked to the very structure of the angular position sensor and the photo-detectors. Step b) thus makes it possible to correct, as much as possible, this type of error. Step d) makes it possible to correct as much as possible the so-called low-frequency errors due to the off-centering of the disc constituting the angular position sensor. Unlike known solutions, averaging the values ​​does not affect the high-frequency errors because here a given corrected value specific to each angular position is used.In this way, the high-frequency error is identical, or at least similar, for both angular position measurements.

[0015] According to advantageous but not obligatory aspects, such a method may comprise one or more of the following steps:

[0016] The photo-detectors of the same set of photo-detectors of the angular position sensor used in steps a) and b) are placed diametrically opposite each other on the angular position sensor, two by two.

[0017] If during step b) several measurements of known and different angular positions are carried out, the number of known and different angular positions must be even.

[0018] The invention will be better understood and other advantages thereof will appear more clearly on reading the description which follows, given solely by way of non-limiting example and with reference to the appended drawings in which:

[0019] [Fig-1] is a theoretical curve, without error, of the angular positions between 0° and 360° when rotating an angular position sensor one and a half turns,

[0020] [Fig.2] is a curve similar to [Fig.l], of actual measured angular positions between 0° and 360° during rotation of an angular position sensor over one revolution,

[0021] [Fig.3] is a curve illustrating the errors observed between 0° and 360° with respect to the theoretical value, from the data in [Fig.2],

[0022] [Fig.4] is an enlarged view of detail IV in [Fig.3] and

[0023] [Fig.5] is a simplified diagram of the error correction method according to a mode of realization of the invention.

[0024] Hereinafter, the terms sensor and encoder will be used interchangeably to designate an angular position sensor, whether optical or magnetic. [Fig.l] is a so-called theoretical curve, i.e. representing the angular position of a rotating object as a function of an angle of rotation. Thus, during a complete rotation of the object on itself between 0 degrees and 360 degrees, the angle increases as well as the angular position of the object. Once the complete rotation has been completed, the angular position of the object returns to zero and the curve starts again as illustrated in [Fig.l]. In such a theoretical case where there is no error between the angular position and the angle, this curve is linear. It is a straight line with a slope generally at 45°, i.e. the angular position of the rotating object is directly dependent on the angle. Of course, in reality this is not the case.Indeed, the rotation of the angular position sensor, in particular the disc or magnetic track constituting the sensor, is not necessarily perfect because there may be an offset of the disc or magnetic track, depending on whether the position sensor is of the optical or magnetic type. It is recalled here that an offset of the disc or magnetic track of 10 microns for a reading diameter of 35 mm corresponds to approximately one minute of peak-to-peak angle, or 60 seconds of arc in the case of a magnetic angular position sensor. The eccentricity of the optical disk or the magnetic track is a regular and repetitive error of a mechanical type as long as the same mounting conditions for the same angular position sensor are used.

[0025] Other types of errors can be added to this error. These are errors that can be encountered during the engraving of the disc, on variations in the engraving pitch, on the positioning of the photo-detectors or other. As appears in [Fig.2] such errors induce a linearity defect, that is to say generate an undulation of the curve with respect to the theoretical straight line. This undulation is mainly regular since the main error is a repetitive error which is induced by a centering defect of the optical disc or the magnetic track. In [Fig.2] such an undulation of the curve is at low frequencies. Other errors, generally of lesser amplitudes, irregular and variable, also affect the measurement.

[0026] Thus, when we represent the errors in degrees from a numerical point of view with respect to the angle concerned, as illustrated in [Fig.3], we obtain a curve which presents a double undulation: the first undulation is that mentioned above and concerns the so-called low-frequency errors. This is the undulation which has the most significant amplitude. So-called high-frequency errors with lower amplitudes generate a second undulation of the curve. If the low-frequency errors remain the most significant type of errors, it is nonetheless true that the high-frequency errors are present and affect the measurement, in particular when we seeks significant precision of the order of an angular second. As indicated in the preamble, there are known solutions for correcting repetitive errors at low frequencies. To do this, we compare, using a pre-established file, the angular position measurements made with the magnetic or optical sensor with known data on the angular position of a standard object. The difference between the measured angular position and the theoretical one quantifies the deviation due to the decentering, therefore the value of the error, which makes it possible to correct the measured signal so that the displayed signal provided by the sensor takes into account the decentering error and is therefore representative of reality. Such a solution is satisfactory with regard to the correction of errors at low frequencies provided that the mounting conditions of the angular position sensor are identical and this permanently.However, this correction is carried out in the factory by the manufacturer and the optical or magnetic encoder, once mounted at the customer's site, rarely has assembly conditions identical to those encountered in the factory. As a result, not only is the error not corrected but it is potentially amplified at the customer's site since the correction itself becomes an error which is added to the initial error. The erroneous value may be such that in some cases a doubling of the error between the factory and the customer has been observed. As a result, manufacturers tend to carry out, for encoders incorporating their own bearings, low-frequency error corrections at a minimum in the factory so that once the sensor is mounted at the customer's site, low-frequency errors are limited.

[0027] Errors of the other type, called high-frequency errors, are due to defects other than off-centering and are, admittedly, of smaller amplitudes but are variable and irregular, not only in amplitude but also in frequency. For the record, this type of error is due, for example, to etching defects, the etching pitch. Existing solutions are relatively ineffective in correcting this type of error. Indeed, even with solutions based on calculation and using several photo-detectors, these do not detect exactly the same thing since high-frequency errors have variable amplitudes and frequencies, which means that they are not necessarily identical between the measurements made by the different photo-detectors, depending on their position.

[0028] The solution proposed by the invention makes it possible to correct, or at least to minimize in a simple manner not only the low frequency errors but also the high frequency errors. For this, as illustrated in [Fig. 5], two photo-detectors P, PI are used which are placed so as to be diametrically opposed above the disc or the circular magnetic track, referenced by the letter C, constituting the angular position sensor. The photo-detectors P, PI will each carry out, with an offset corresponding to an angle of 180°, a separate measurement of a given angular position. The expression "angular position" here designates the position angular of the sensor axis, therefore the theoretical axis of the circular track C. According to the invention, each angular position measurement is predefined and of a known value. In this way, it is possible to associate with each measurement carried out by each of the photo-detectors P, PI a file containing the exact theoretical values ​​of the known and given angular position that the photo-detector P or PI must measure. Associated with each measurement of an angular position 01 or 02 carried out by, respectively, the photo-detector P, PI is a corrected value 0' 1 or 0'2 of each given and known angular position. The correction is referenced El, E2 for respectively the angular positions 01, 02 and is carried out by association, that is to say by modifying the measured angle by a suitable value to obtain the given and known value.In this way, on the one hand, the low-frequency error is corrected in a known manner, knowing that this error is equal in absolute value and opposite for the two measurements carried out with a shift of 180° by the photo-detectors P, PI and, on the other hand, the high-frequency errors are also corrected. Indeed, each photo-detector P, PI having a given control file, for a defined and known angular position, because the photo-detectors P, PI are placed at 180° relative to each other, they are therefore positioned so as to generate two angle values ​​detected in phase opposition. With such an arrangement, the low-frequency errors detected by each photo-detector P, PI are identical and cancel each other out because in phase opposition, only the high-frequency errors are different and taken into account and processed individually during the correction of each value 01 or 02 measured by the photo-detectors P, PI.In another step, it is sufficient to take the average of the angular positions corrected according to the relation (0' 1 and 0 '2) / 2 to obtain a value 03 corrected for the angular position, as much with regard to the low frequency errors as high frequency errors. This angular position 03 obtained by calculation is corrected, by a given factor, with respect to the given and known angular position since the so-called high frequency errors have been taken into account during each of the measurements carried out by each photodetector P, PI.

[0029] In order to improve the consideration of high-frequency errors, and therefore de facto to achieve a finer correction of this type of error, it is possible to use a number of photo-detectors greater than two, provided that this number is necessarily a multiple of two and that the photo-detectors are always diametrically positioned two by two on the sensor. In addition, each photo-detector P, PI must be associated with a single given and known angular position in order to generate a corrective file per predefined angular position. In such a case, the last step performs an average between all the corrected values ​​of measured angular positions.

[0030] Such a solution for correcting so-called low and high frequency errors is independent depending on the sensor mounting conditions and can therefore be applied without difficulty at the customer's premises when the latter has carried out the sensor mounting.

[0031] This correction of the so-called low and high frequency errors is carried out by calculation in the factory, therefore in a hidden manner for the customer who receives a sensor already including the correction of the angular position. Thanks to the invention, the final assembly conditions encountered at the customer's premises have little or no effect on the error corrections carried out in the factory. Even if the low and high frequency errors are not completely corrected, they are brought back to values ​​acceptable to the customer when using the sensor. In addition, such a correction is carried out solely by calculation, therefore embedded in the sensor's electronics, which means that it is present and pre-established in the factory, the customer having no steps to take to correct his sensor and not being able to intervene on this correction.

Claims

Claims

1. Method for correcting so-called low and high frequency errors generated by an angular position sensor when measuring an angular position, characterized in that it comprises at least the following steps: - a) carrying out a measurement (01) of a first known angular position, said measurement being provided by a sensor (C) of the angular position of an object using a set of at least two photo-detectors (P, PI) the number of which is a multiple of two, the photo-detectors (P, PI) of the same set of photo-detectors used being diametrically opposed on the sensor (C), two by two, - b) carrying out a measurement (02) of at least a second known angular position, said measurement being provided by the same sensor (C) as in step a) using the same set of photo-detectors (P, PI) placed at the same location as in step a) and the number of which is the same multiple of two as in step a), - c) associate with each measurement (01, 02) carried out in steps a) and b) a file containing the exact theoretical values ​​of each known angular position (01, 02) that each photo-detector (P, PD, - d) obtaining, for each of the measurements (01, 02) of the first and second angular positions obtained in steps a) and b) a first (0' 1) and second (0'2) angular positions corrected by a correction (El, E2) of the measurements (01, 02), of each angular position, each correction (El, E2) being carried out by modifying the measured angle (01, 02) by a value adapted to obtain respectively the values ​​of the first and second known angular positions defined in step c) and - e) taking the average (03) of the first and second angular positions obtained (0'1, 0'2) after correction in step c).

2. Method according to claim 1, characterized in that if during step b) several measurements of known and different angular positions are carried out, the number of known and different angular positions must be even.

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