ITMI20012208A1 - PROCEDURE FOR THE COMPENSATION OF A SO-CALLED OFFSET-DRIFT OF AN ANGLE METER - Google Patents

Description

TEXT OF THE DESCRIPTION

State of the art

The invention relates to a method for compensating an offset-drift of an angle measurer according to the type of the main claim. Methods for the electrical measurement of an angle of rotation are already known. For this purpose, measurement methods are used, with which the sinusoidal and cosine signals of a sensor are processed. For example, magnetoresistive sensors are used such as AMR or GR sensors (anisotropy magnetoresistance, giant magnetoresistance), which, with the help of two complete bridges, respectively provide a sine and cosine signal. For a correct calculation in an angle function, the amplitudes of the sine and cosine signal must be equal and must not have any offset. While the same amplitude can be achieved by constructive measures, the offset values, which occur in particular through an influence of the temperature, cannot be predicted or predicted in advance. This means that during the operating time of such an angle measurer, the accuracy varies considerably, so that this measurer may no longer be usable. In the case of applications for determining the angle of rotation, for example in a motor vehicle, however, high demands are placed on accuracy and stability for the service life.

Advantages of the invention

The method according to the invention for the compensation of an offset drift of an angle measurer with the characterizing characteristics of the main claim has the advantage with respect to this that in the case of the angular determination a constantly corrected offset value is considered. In this way, the angle measurement becomes more reliable and remains constant in the predetermined precision range, so that the control processes based on it, for example, are reliably executable.

Through the measures set out in the dependent claims, advantageous improvements and improvements of the method indicated in the main claim are possible. Particularly advantageous is the fact that the off-set values are determined in a predetermined time interval. In particular, a temperature-dependent offset determination is thus obtained for the relevant temperatures in the motor vehicle, which always provides constant measurement results, regardless of the ambient or engine temperatures.

It is particularly advantageous that the detection of the offset or of the temperature coefficient takes place in the case of at least two predetermined temperatures, so that it is easily possible to calculate intermediate values by means of interpolation. In addition, this results in less testing effort during calibration.

It is also advantageous that the offset algorithm can be established by means of a non-linear temperature coefficient. For example, the temperature coefficient can be described by a third order polynomial. This polynomial offers a good approximation for the temperature behavior of a sensor.

In order to have easy access to the corresponding offset values, the storage of the offset values is preferably done in a table. The intermediate values can advantageously be determined by means of a simple interpolation.

It is also particularly advantageously provided that the long-term behavior is determined during the operation of the angle measurer, simply by averaging the offset values. The determination of at least one of the two bridge offsets (COS, SIN) during operation is therefore only possible, when the electrical angle covers an area of at least 180 °. For example, a mechanical angular field of at least 90 ° is sufficient in the case of AMR sensors, in which the electrical angle is double the mechanical angle.

By saving and averaging the detected offset values, it is possible to gradually adapt the offset to be corrected, thus avoiding jump functions. This increases the safety of the measurement.

Some advantageous applications are indicated in the case of some sensors operating according to the optical principle. Alternatively, the offset values can also be corrected and detected with magnetoresistive sensors.

It is also advantageous that the measuring accuracy of the angle sensor is monitored by means of a diagnostic function. Malfunctions can be easily reported. In particular, if a specified limit value for the offset is exceeded, an alarm signal can be issued, whereby electronic devices can respond accordingly.

The application of the method in an angle measuring device inside a motor vehicle, for example in correspondence with a transverse steering shaft, therefore appears particularly advantageous, since in this case high demands on precision and reliability of the locking device are required. measurement.

Drawings

The invention will be described in more detail below with reference to an executive example illustrated in the drawings, in which:

- fig. 1 illustrates an angle meter on a rotating shaft,

- fig. 2 illustrates a table,

- fig. 3 illustrates diagrams with angle signals,

- fig. 4 illustrates a further table,

- fig. 5 illustrates diagrams with offset curves,

- fig. 6 illustrates offset diagrams in the event of a long-term drift.

Description of the executive examples Fig. 1 illustrates an angle gauge 1 disposed in a suitable point of a shaft 3. The shaft 3 is for example a steering cross shaft or a lateral steering control rod of a motor vehicle. Alternatively, it is envisaged to arrange the angle sensor 1 also at the end of the shaft 3.

The angle sensor 1 essentially has a detector 2, which is for example made as a ring and can have one or more notches. These notches can be made optically, inductively, magnetically or in a commonly known way. Furthermore, a sensor 5 is provided which, on the basis of its arrangement and construction, scans the notches of the detector 2 and transmits corresponding signals to a processing unit 10, with which it is mounted on the same circuit board 4. To keep limited the constructive dimensions, in the circuit board 4 there is a cutout 8, in which the detector 2 is partially immersed.

A preferred embodiment has a magnetized detector 2, which is scanned by a magnetoresistive sensor 5. The sensor 5 is for example an AMR or GMR sensor element and supplies, by virtue of its mounting on separate outputs, a sinusoidal and cosine signal depending on the angle of rotation of the shaft 3.

In an alternative embodiment, the detector 2 is provided to be arranged at the end of the shaft 3, where for example the detector 2 is realized as a flat cylindrical magnet or with a rectangular parallelepiped shape. The manufacturing principle of the sensor 5 and of the angle measurement is known per se. The sensor 5 essentially has two complete AMR bridges rotated by 45 ° or alternatively two complete GMR bridges rotated by 90 °, which supply the sine or cosine signal. The angular determination then takes place through an arctangent operation, applied to the sine and cosine signal.

The operation of this arrangement will be described in more detail with reference to the following figures. For exact angular determination, the sine and cosine signals must have the same amplitude and cannot be offset. In particular, this must be maintained in the valid temperature range, for example in the field of motor vehicles. However, experience has shown that an offset drift occurs both with increasing temperature and with the useful life of the sensors 5 (long-term drift). Depending on the circumstances, this implies that the measurement accuracy of a sensor 5 calibrated at the ambient temperature can greatly deteriorate over the course of the stress time and in particular in the presence of high temperatures.

Fig. 2 shows in the upper part of the table first of all the formula for the calculation of the sine and cosine signal voltages, also considering the temperature-dependent offset and the offset which occurs following a lasting stress. In the lower part of the table of fig. 2 indicates some parameters, which can be measured with sensors commonly available on the market.

Fig. 3 illustrates in detail how the sine and cosine signals can vary in a temperature dependent manner. In the left part of fig. 3 shows the sine and cosine signal for ambient temperature with dashed lines and for increased temperature with solid lines. For this purpose a large surface magnetized detector was used, whereby a complete sine oscillation (period) corresponds to a rotation angle of 180 °. In the case of a complete rotation, two periods are then reproduced. In this case there is no offset drift. The two diagrams in the center of FIG. 2 instead show sine and cosine curves, which have a positive or negative offset. The resulting angular errors have been reported in the diagram on the right.

Fig. 4 illustrates a further table that reproduces the offset behavior over the long run. Initial values are shown on the left side of the table. The angular error in this case amounts to +/- 1.3 °. The two tables on the right side of fig. 4 instead show an incorrect angle with / -2.9 ° or / - 3.6 ° after 5000 hours of continuous stress. The observations were made for the operating temperature of 150 ° C respectively.

Fig. 5 illustrates three further diagrams with offset curves, which were taken in a temperature range from -50 ° to 150 °. The left-hand diagram of FIG. 5 with dotted characteristic line simultaneously shows measured values for the offset of the first bridge of sensor 5 (cosine bridge) and comparably the center diagram for the second bridge (sine bridge) of the same sensor 5. The solid curves were comparably calculate (Fit) and represent the approximation function obtained by means of a third order polynomial. The right-hand diagram of FIG. 5 shows the measurement angle error (error °) resulting from the arctangent formation of the sine and cosine signals, where their offset has been eliminated by means of the polynomial approximation functions.

The illustrated curves have been measured as an example and show the non-linear offset trend of the cosine bridge (left diagram) and the sine bridge (central diagram) of a single sensor 5 in relation to the temperature range of -50 ° C at 150 ° C generally present in a motor vehicle and chosen only as an example. As is evident from the solid lines, the non-linear temperature coefficient, i.e. the offset can easily be described relatively optimally by the third-order polynomial, effectively correcting the measured angle of rotation. As the diagram on the right shows, the resulting angular error is very small.

Two support values are sufficient for the calculation (Fit) and 3-4 are preferably used. Each sensor is heated for this purpose to the corresponding temperature values, the offset is determined and the approximation function is calculated.

The result is preferably recorded in a table and is therefore easily accessible. This is done in a calibration mode after sensor preparation. During subsequent operation, the resistance of the AMR elements, which are bridged, additionally serves as a temperature sensor. Its temperature course is sufficiently linear and reproducible, so that the temperature can also be measured directly with the AMR element.

It is necessary to determine the offset temperature coefficients of each sensor during production or during sensor assembly, as it is not possible to predetermine the exact offset trend. To determine the support values for the Fit, for example, a reference magnet is rotated through the sensor in the case of the set temperatures. The respective offset can then be calculated by adapting the detected sine and cosine signals or by averaging. As an alternative to the rotating reference magnet, an electromagnet can be used to determine the direction.

Fig. 6 schematically illustrates the temporal development of the offset in the case of a high temperature shift, with intermediate measurements being performed at predetermined time intervals. The left axis of the diagram shows the offset-drift, while the right axis reproduces the resulting wrong angle at room temperature (25 ° C) or at 150 ° C. The two illustrated curves differ from each other in that no preliminary thermal stress was performed in the case of the dotted curve. The solid characteristic line, on the other hand, indicates the offset behavior according to an executed temperature stress (burn-in), which has been inserted upstream of the permanent stress.

Experience has shown that each sensor behaves differently, so it is not possible to make a precise prediction on offset-drift. However, in order to be able to correct the offset drift in the long term, the following correction method is proposed according to the invention: During the steering process, the offset is determined from the signal amplitudes with the formation of the average. When, with the use of an AMR sensor, the steering angle covers at least 90 °, i.e. the electronic angle is twice the mechanical angle, i.e. 180 °, then it is possible to calculate by training of the average at least one of the two bridge offsets, since the two bridges of the AMR sensor are arranged at an angle of 45 ° to each other. The offset thus detected is stored and gradually corrected. This gradual correction, which is carried out for example from the circuit point of view by means of a low pass, prevents sudden changes in the offset. Such sudden variations would occur in an obligatory way, since the deviations between the actual trend of the offset temperature and the calculated one (Fit) would not be differentiated from the long-term offset-drift.

A further substantial aspect of the invention is also considered by the fact that the detected angle or offset values are monitored by means of a diagnostic function. When a predetermined limit is exceeded, it is then possible to issue a corresponding error message.

The use of a diagnosis function is important since, in the case of the Cordic algorithm generally used for arctangent calculation, the sum of the squares of the sine and cosine signals is automatically calculated. This value should be constant, corresponding to the known mathematical relationship: 1. Indeed, the value, depending on the magnitude of the temperature coefficient of the signal amplitudes and offset, is around a certain value range and therefore provides the relationship between an angle incorrect and the duration of intervention of the sensor. During the temperature calibration after sensor preparation, the value range of this diagnosis factor is determined. The maximum tolerable incorrect angle can be converted into a maximum diagnosis value. As soon as the sensor is approached during the next operation as a specified limit value or exceeds the value during the service life, the aforementioned alarm signal is issued. In this way it is possible to advantageously detect both too strong long-term drift, but also erroneous variations of the sensor elements.

Claims (14)

CLAIMS 1. Procedure for the compensation of an offset drift of an angle measurer (1), whose sensor element (5) scans a detector (2) arranged on a shaft (3), where the sensor element (5), corresponding to the angle of rotation of the shaft (3), supplies a sinusoidal or cosinusoidal signal to a processing unit (10), which from the received signals determines the rotation angle by forming an arctangent, characterized by the fact that the The processing unit (10) has an algorithm, with which, from the measured sine and cosine values or from the values derived from them, an offset value is detected and with which, depending on the temperature and / or time, they are corrected the sensor signals and therefore the rotation angle. Method according to claim 1, characterized in that the offset values are established within a predetermined temperature range. Method according to claim 2, characterized in that the temperature range for the relevant temperatures in the motor vehicle is predetermined. Method according to one of the preceding claims, characterized in that the determination of the offset values and / or a temperature coefficient takes place in a calibration step with at least two predetermined temperature levels. Method according to one of the preceding claims, characterized in that the algorithm has a non-linear temperature coefficient. Method according to claim 5, characterized in that the non-linear temperature coefficient is described by means of a third-order polynomial. Method according to one of the preceding claims, characterized in that the detected offset values are recorded in a table. Method according to claim 7, characterized in that intermediate values for the offset are determined by interpolation. Method according to one of the preceding claims, characterized in that during operation of the angle measuring device, a long-term offset value is calculated by means of averaging by repeated angle measurement, whereby the offset calculation in the long term it occurs when a rotation angle of 90 ° is exceeded. Method according to claim 9, characterized in that the detected offset value is stored and taken into account for the gradual correction. Method according to one of the preceding claims, characterized in that the offset corrections are performed on a sensor (5) and / or detector (2), which operates according to an optical principle. Method according to one of claims 1 to 10, characterized in that the offset corrections are performed on a sensor (5) and / or detector (2), which operates according to a magnetoresistive principle. Method according to one of the preceding claims, characterized in that an exceeding of a predetermined offset limit is signaled with the aid of a diagnostic function. Method according to one of the preceding claims, characterized in that the angle sensor (1) is arranged on a steering cross shaft (3) of a motor vehicle.