Patent Publication Number: US-2020290674-A1

Title: Method and device for determining a sensor offset

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102019106568.2, filed Mar. 14, 2019, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for determining a sensor offset of a sensor in a device, in particular in a steering system for a motor vehicle. 
     BACKGROUND 
     A large number of different assistance systems, for example electric steering assistance systems, are used in modern motor vehicles. These steering assistance systems require a sensor for monitoring a plurality of components which are involved in the assistance system. It is therefore necessary, for example, to determine the position of the steering wheel in a motor vehicle so that the motion prescribed by the driver can be intensified in a desired fashion by the steering assistance system. 
     The sensors serve to determine the mechanical variables such as, for example, the length, the force or torque. However, sensors frequently have a design-induced constant offset which can falsify the measured variables. Therefore, methods for offset correction are necessary in order to be able to achieve sufficient measuring accuracy. 
     SUMMARY 
     The object of the present disclosure is to make available a simple method and a device for determining a sensor offset. 
     The object is achieved by a method for determining a sensor offset of a sensor in a device, in particular in a steering system for a motor vehicle, which has a first component, a second component which is movable with respect to the first component, a resetting element, an actuator and a sensor for determining the relative motion between the first component and the second component. Firstly, a predetermined relative motion between the first component and the second component is generated by the actuator. The resulting relative motion is measured by means of the sensor, and the sensor data obtained from the measurement are stored. A constant sensor offset of the sensor is then determined on the basis of the stored sensor data. 
     The method can correspondingly be applied in all situations in which two components which are movably arranged with respect to one another are present and can be monitored by means of a sensor. The relative motion can be here either a translation or a rotation. The two components must be able to be move freely so that the relative motion is controlled exclusively by the actuator. 
     Contributions by static friction between the components of the device, which can falsify the measurement or even make it impossible, are canceled out by the generation and measurement of a motion. Correspondingly, a measurement method is used in which exclusively dynamic friction components have to be taken into account. This significantly simplifies the measurement. 
     The method can also be carried out just once, for example even during the manufacture of a motor vehicle, since a constant sensor offset is determined. However, the measurement can also be repeated subsequently when necessary, in order to be able to determine changes in the sensor offset. 
     The resetting element can also be part of one of the components. In addition, the device can have a control unit which carries out the method for determining a sensor offset. In this case, the control unit is connected to the actuator and to the sensor, 
     The generated relative motion can be a periodic and symmetrical motion with maximum amplitudes. In this way, only symmetrical friction components occur in the dynamic state during the relative motion, as a result of which the evaluation of the sensor data is facilitated, since the contributions of the friction components cancel one another out. The relative motion can preferably be sinusoidal or can be sensed in the form of square-wave signals or triangular signals by the sensor. A sinusoidal relative motion can be generated, for example, by the actuator initiating an oscillation motion. 
     In one preferred embodiment, the relative motion is measured over a plurality of oscillation periods. This makes it possible to reduce measurement errors with respect to the determination of the maximum amplitudes. A periodic motion is preferably executed with a frequency of 10Hz over a total duration of one second. 
     The constant sensor offset can be determined as an arithmetic mean of the maximum amplitudes of the relative motion in the positive and negative directions within the at least one oscillation period from the stored sensor data. The maximum amplitudes in the positive and negative directions have, as a result of the symmetry of the excitation, the same value caused by the actuator, but with different signs. Since, in contrast to this, the constant sensor offset is included with the same sign in the positive and negative maximum amplitudes, said sensor offset can be determined by means of the arithmetic mean of the two maximum amplitudes. This makes the evaluation very easy. 
     The motion can preferably be generated if the device is otherwise unactivated, in particular if a steering wheel of the steering system is unactivated and/or is without contact with hands. Since the method initiates a relative motion between the first and second components via the actuator, the free motion of the second component is measured in relation to the first component. If the second component is additionally activated or arrested during the relative motion, the generated motion is modified and the result of the calibration is correspondingly falsified. For example when the method is used to calibrate a steering system of a motor vehicle it can therefore preferably be corned out at the end of the manufacture of the motor vehicle, 
     In a further preferred embodiment, a user of the device can be informed of a lack of activity by the generated motion. The method can therefore be earned out, for example, when a driver of a motor vehicle lets go of the steering wheel during travel. This can be detected by means of a hands-off detection system. The driver can then be informed, by a vibration of the steering wheel, that his hands are no longer on the steering wheel. This vibration can be used at the same time to calibrate the sensor. 
     The relative motion can be a rotational motion or a translator emotion between the first component and the second component, The type of motion used depends on the type of sensor to be calibrated and the first and second components which are present. It is therefore possible, for example, to calibrate a torque sensor by means of a relative motion of an upper steering train relative to a lower steering train of a motor vehicle. 
     In addition, a position-dependent deviation of the measured sensor data is determined on the basis of a maximum disruption, a disruption position in which the maximum disruption occurs, and the current position of the components with respect to one another, in particular where the maximum disruption and the disruption position are previously known. Disruption can occur, for example, owing to asymmetrical weight distributions at the first and/or second component, as a result of which additional position-dependent forces occur and affect the sensor signal, 
     The degree and the position-dependent change in the disruption can already be determined experimentally before the calibration or may be known on the basis of knowledge about the components involved. In the case of a rotational motion for determining the sensor offset, for example when a steering train rotates, the disruption position can be defined as a disruption angle, 
     In addition the object is achieved by a device, in particular a steering wheel system for a motor vehicle, having a first component, a second component which is movable with respect to the first component, a resetting element, an actuator for generating a relative motion between the first component and the second component, a sensor for determining the relative motion between the first component and the second component, and a control unit for controlling the actuator and the sensor, wherein the control unit is designed to execute the method described above. 
     The sensor can be here a force sensor, a torque sensor, a weight sensor, an acceleration sensor and/or a rotational acceleration sensor. All these sensors are based on the measurement of a mechanical variable which can be attributed to a relative motion of two elements with respect to one another. The sensor can be, for example, an optical sensor or a strain gauge. So that a sensor is suitable for the method, the sensor must be able to measure a displacement or rotation of one component relative to the other component. 
     In one preferred embodiment, the device is a steering system for a motor vehicle which has an upper steering train which constitutes the first component, and a lower steering train which constitutes the second component In addition, the resetting element can be a torsion rod which connects the upper steering train to the lower steering train in a torque-transmitting fashion, wherein the sensor is configured to measure a rotation of the torsion rod. The relative motion can be a rotational motion or a translator emotion between the first component and the second component. The type of motion used depends on the type of sensor to be calibrated and the first and second components which are present. It is therefore possible, for example, to calibrate a torque sensor by means of a relative motion of an upper steering train relative to a lower steering train of a motor vehicle. 
     Correspondingly, during the operation of the motor vehicle the sensor serves to monitor the position of the steering wheel. This can be utilized, for example, to indicate the direction desired by the driver of the motor vehicle to an electric steering assistance system. 
     Sufficiently high accuracy of the sensor can also be obtained in order to detect even small changes in angle of the steering wheel. These changes can occur, for example, when the driver of the motor vehicle lets go of the steering wheel during travel. Reliable detection in this case is of great importance, for example for a hands-off detection system. 
     It is possible that the steering wheel has an asymmetrical weight distribution so that the maximum disruption is dependent on the weight distribution and/or on the mounting angle of the steering wheel. Since the weight distribution of the steering wheel is known in advance, the influence of this disruption on the determination of the sensor offset can be correspondingly taken into account. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present disclosure can be found in the following description and the drawings, in which; 
         FIG. 1  shows a device according to the present disclosure for determining a sensor offset; 
         FIG. 2  shows a flow diagram of the method according to the present disclosure; and 
         FIG. 3  shows a measurement series recorded during the method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a device  10  for determining a sensor offset of a sensor  12 . In the embodiment shown the device  10  is a steering system for a motor vehicle (not shown) having a first component  14 , a second component  16 , a resetting element  18 , a control unit  28  and an actuator  30 . 
     The first component  14  is an upper steering train  20  which has a steering wheel  22 . The second component  16  is a lower steering train  24 . The upper steering train  20  is connected to the lower steering train  24  by means of the resetting element  18 . 
     In the described exemplary embodiment the resetting element  18  is a torsion rod  26 , by which the upper steering train  20  and the lower steering train  24  are connected in a torque-transmitting fashion. 
     The actuator  30 , which can set the lower steering train  24  in motion, is provided on the lower steering train  24 . 
     The sensor  12  is a position sensor which monitors the position or rotation of the torsion rod  26 , and can therefore detect the applied torque at the steering wheel  22 . 
     The sensor  12  is connected to the control unit  28  which can execute the method for determining the sensor offset. For this purpose, the control unit  28  is additionally connected to the actuator  30 . 
     In order to determine the sensor offset, the lower steering train  24  is set into a periodic motion (step S 1  in  FIG. 2 ) by the actuator  30  which is controlled by the control unit  28 . This motion can occur, for example in a sinusoidal fashion in order to initiate a periodic vibration. In this context, it is sufficient to generate a vibration with a frequency of 10 Hz for approximately one second. 
     The motion of the lower steering train  24  is transmitted to the upper steering train  20  by means of the torsion rod  26 , so that the upper steering train  20  and, for example, the steering rod  22  are also moved periodically. 
     As a result, a periodic rotation of the torsion rod  26  occurs, which is sensed by means of the sensor  12  and therefore the resulting relative motion of the lower steering train  24  and of the upper steering train  20  is measured (step S 2  in  FIG. 2 ).  FIG. 3  shows a measurement series of the resulting relative motion over a plurality of periods, 
     The sensor data obtained by the sensor  12  are stored in the control unit  28  and subsequently utilized in order to determine the constant sensor offset of the sensor  12 . 
     For this purpose, the maximum amplitude M is determined in the positive and negative directions during the vibration, wherein said vibration is tracked over a plurality of oscillation periods (step S 3  in  FIG. 2 ). The arithmetic mean of the maximum amplitude is then calculated in the positive and negative directions (step S 4  in  FIG. 2 ) and the sensor offset of the sensor  12  is therefore obtained, Said sensor offset can be stored in the control unit  28  so that all future obtained measured values of the sensor  12  can be corrected by the sensor offset. 
     During the determination of the sensor offset it must be ensured that the upper steering train  10  can move freely. Correspondingly, the steering wheel  22  must not be arrested. Since the sensor offset is constant, it is therefore preferably already determined at the end of the assembly of the motor vehicle, since a free motion has to be ensured there. 
     Alternatively and/or additionally, the sensor offset can be newly checked and, if appropriate, adapted at regular intervals. It is therefore possible, for example, for calibration to take place when the driver of the motor vehicle lets go of the steering wheel  22 . In this case, a hands-off detection system (not shown) ensures that the steering wheel  22  vibrates after a prescribed time period, in order to inform the driver that he no longer controls the steering wheel  22 . For this purpose, the actuator  30  can also be used. The hands-off detection system can be connected to the control unit  28  so that at the same time the vibration can be utilized for the calibration of the sensor offset, 
     If the steering wheel  22  has an asymmetrical weight distribution, corresponding angle-dependent correction terms can be stored in the control unit  28  and applied to the sensor data obtained by the sensor  12 . 
     In such a case, the measured sensor value can be determined according to the formula 
       v sensor (φ)=v+o+õ cos(φ−ϑ)
 
     where v sensor  (φ) is the measured sensor value which is dependent on the angle φ, v is the real value which should be sensed by the sensor, o is the sensor offset, õ is the maximum position-dependent deviation which occurs owing to the asymmetrical weight distribution, and  19  is the angle at which õ occurs. The measured sensor value changes correspondingly in an angle-dependent fashion in the form of circular function. 
     During the determination of the sensor offset, the corresponding contribution is extracted by calculation and subsequently included again in the measured sensor data during the operation of the motor vehicle.