Abstract:
The invention relates to an adaptive absolute steering sensor for the absolute determination of a turning angle, especially for determining the steering angle in a motor vehicle by way of a code applied across a 360° angular range for a determining the angle. The code and a sensor system are arranged in such a way that they are able to rotate in relation to each other. Absolute determination of the angle is carried out by reading the contrast information using a micro processor which determines both the angular position of the code and the fine resolution of the angles on the basis of the relative position of the recognized angle to the image on the photodetector system. At the same time the overall function of the system is verified and adjusted during each measurement. The code is determined using a photodetector system positioned in a single place, and used for determining the angle, whereby an unbroken segment of code track is shown on at least one photodetector line and at least one code word is detected to which corresponds a predetermined angle. The position of the code word in relation to the fixed position of the photodetector line is measured.

Description:
The invention relates to an adaptive absolute steering angle sensor for absolute determination of an angle of rotation, in particular for determining the steering angle in a motor vehicle by means of a code, provided over an angular range of 360°, for determining the angle. The code and a detector arrangement are arranged such that they can rotate relative to one another. Absolute determination of the angle is effected by the contrast information being read by means of a microprocessor which determines both the angular position of the code and the fine resolution of the angles as a result of the relative position of the identified code with respect to the image on the photodetector arrangement. At the same time, the overall function of the system is checked and adapted during each measurement. 
     The invention is based on the object of further improving the determination of the absolute angular position of a rotor, in particular of the steering wheel of a motor vehicle. 
     According to the invention, the code is determined by means of a photodetector arrangement provided at a single location and is used for determination of the angle. Furthermore, a contiguous segment of the code track is imaged onto at least one photodetector linear array, where at least one code word is detected and a predetermined angle corresponding to said code word and the position of the code word with regard to a fixed position of the photodetector linear array is measured. 
     For this purpose, the code is chosen in such a way that it is not repeated over the entire circumference in the observation region of the photodetector arrangement. The code is in a single track, unambiguous and closed. The sensor has the advantage that, compared with known methods, the angular resolution does not depend on he resolution of the code of the code track and does not depend on the number of code words, but rather only on the resolution of the sensors of the photodetector arrangement. That is to say that the angular resolution is independent of the code. Without the use of a reference mark, the angular resolution depends on the number of code words. If at least one code word of the code track is detected by the photodetector arrangement, an angular resolution of 1° would be obtained given 360 code words. 
     Since the angular resolution does not depend on the number of code words, the fewest possible code words should be used, in order to reduce the sensor sensitivity to environmental influences, such as soiling. This is achieved, e.g., by the use of 6-bit or 7-bit codes instead of 8-bit codes. 
     The measurement is performed by software in a microcontroller which, for this purpose, uses the image data of the photodetector arrangement. 
     In order to increase the resolution further, at least two different images of one or more circumferential codes may be imaged onto the photodetector arrangement. 
     The code track may be transilluminated with parallel light or may be illuminated from one side. 
     In a further refinement, it is provided that the code is imaged onto the photodetector linear array via an optical arrangement in such a way that, with a reading cycle of the linear array, not only is the absolute angle information determined but also the overall function of the system is checked and adapted. 
     In order to monitor the system functions, at least one reference shadow image may be projected onto the photodetector linear array. In the event of soiling in the region of optical components, e.g., the power of the light sources can then easily be adapted by increasing the control current. The failure of individual detectors of the photodetector arrangement is also noted and can be compensated for by computational methods. The reference shadow images can be generated by corresponding software either cyclically or by a computer-controlled monitoring device being switched on individually. 
     Furthermore, it is possible that for the compensation of optical and mechanical tolerances, the edge steepness and image size of the signals imaged on the detector linear array are evaluated. 
     The angular range is determined within 0° to 360° by way of the traveling speed of the vehicle. In order to detect the absolute steering angle even with vehicle systems switched off, he steering angle is determined by briefly switching on the steering angle sensor in time intervals in which rotation of preferably greater than 180° is not possible. 
     By virtue of the fact that a single photodetector arrangement can very rapidly evaluate the angle in the range of 360°, the system is suitable by simple co-registration of the 360° exceedance and hence for a plurality of revolutions. In order to ensure that not only for the traveling mode, the system must be switched on momentarily in each case in the standby mode, the switched-on intervals being chosen in such a way that steering wheel rotation of greater than 360° is not possible in this interval. Since the system, for the purpose of data transmission, has an interface with the vehicle computer, the vehicle speed can be accepted from there, in order to define the zero range of the steering angle, since, in normal vehicles, no steering angle over, e.g., +/−90° from the zero position can be traveled above a certain speed. 
     In an adaptive absolute angle sensor, moreover, provision is made of at least one light source for illuminating an angular range of the code, and provision is made of a photodetector arrangement for detecting the illuminated angular range of the code, where a microcontroller is assigned to the light source and to the photodetector arrangement. 
     In a first embodiment, as a light source, two light-emitting diodes are arranged symmetrically with respect to the optical axis, these being provided together with the photodetector arrangement and an optical arrangement on the same side of the circular ring. 
     In a second embodiment, a circular coding ring which is optically transparent at light locations of the code is provided. Furthermore, at least one light-emitting diode is arranged on one side of he coding ring and the photodetector arrangement is arranged on the other side of the coding ring. An optical arrangement is preferably provided on the side of the light-emitting diode. 
     In order to increase the reliability of the angle sensor and in order to detect code errors, a further refinement provides for the photodetector arrangement to have two sensors, which are arranged vertically, one above the other with regard to the course of the code track for the synchronous observation thereof, and for the images of the two sensors to be compared with one another. The comparison of the two images makes it possible to identify local soiling particles (effecting the code or optical arrangement) and also sensor errors. Two linear array sensors or two portions of an area sensor lying one above the other can be provided as the sensors. 
     In a further refinement, two sensors are arranged horizontally next to one another for synchronous registering of adjacent code words of the code track. 
     Charge-coupled elements (CCD) are preferably provided as the photodetectors. 
     In order further to reduce the sensor sensitivity to environmental influences, the light-dark lines of the code words should have the largest possible dimensions. The light-dark lines of the code words preferably have a width of from 2 to 3 mm. 
     In ore embodiment, it is provided that a matte plate is arranged between the light source and the transparent coding ring, and that the optical arrangement and the photodetector arrangement are provided on the other side of the coding ring. 
     In a further embodiment, a transparent coding ring has, in portions, cylindrical lenses lying next to one another and serving to generate the code and image the light sources on the photodetector arrangement. An additional optical arrangement is obviated in this embodiment. Stripes of different brightness are produced on the photodetector array as a result of the cylindrical lenses. 
     In a further refinement, it is provided that the coding ring has a prismatic cross section, there being arranged a light source which radiates in the axial direction of the coding ring. 
     In a further embodiment, there are arranged on a transparent coding ring, in portions, cylinders or lenses lying next to one another and serving to generate the code and image the light sources on the photodetector arrangement, where the cylinders extend in the axial direction of the coding ring and are fixed by one end area on the coding ring and are assigned to the light sources and are assigned with their other, exposed end areas [sic] to the photodetector arrangement. By means of regions with and without cylinders and configuration of the density and respectively with and without lenses, a code can be applied and correspondingly detected. The exposed end areas of the cylinders are expediently plane or are of lenticular design. 
     In one embodiment, at least one point light source is assigned to the transparent coding ring, which has optically transparent regions and optically opaque regions. In a first refinement, two point light sources are provided next to one another at a constant distance and a single photodetector arrangement is provided. This arrangement has the advantage that despite the change in the radial distance of the coding ring due to radial runout of the steering wheel or despite a change in the distance between the components due to mechanical or thermal influences, on account of the different shadow formation of the two light sources whose distance remains constant, the position and angular position of the code on the photodetector arrangement can be detected accurately. 
     The same advantageous effect can be obtained as follows: one point light source and two photodetector linear arrays, which are arranged one above the other at different distances from the light source, are provided. 
     A further refinement provides for at least one reference code to be assigned to the angle-determining code on the coding ring. In this case, the reference code may be arranged next to the angle-determining code, or be provided above and below the angle-determining code. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained using exemplary embodiments with reference to the drawings, in which: 
     FIG. 1 shows the fundamental design of a steering angle sensor for carrying out the method according to the invention in a reflected-light variant; 
     FIG. 1 a  shows the plan view of a portion of the code track; 
     FIG. 2 shows the assignment of the photosensitive cells of a CCD linear array to the voltage amplitudes; 
     FIG. 3 shows an embodiment of the steering angle sensor in which a shadow image is additionally projected onto a CCD linear array; 
     FIG. 4 shows a plan view of a code with a coarse region and a fine region; 
     FIG. 4 a  shows a section through the code arrangement with associated optical assemblies; 
     FIG. 5 shows a transmitted-light variant of the steering angle sensor; 
     FIG. 5 a  shows the plan view of a portion of the code track; 
     FIG. 6 shows an embodiment of the steering angle sensor with two CCD linear arrays lying vertically one above the other, a code track being assigned to said CCD linear arrays; 
     FIG. 6 a  shows a side view of the embodiment of FIG. 6; 
     FIG. 7 shows an embodiment of the steering angle sensor with two CCD linear arrays lying vertically one above the other, a respective code track being assigned to said CCD linear arrays; 
     FIG. 7 a  shows a side view of the embodiment of FIG. 7; 
     FIG. 8 shows an embodiment of the steering angle sensor with two CCD linear arrays lying horizontally next to one another; 
     FIG. 9 shows an embodiment with injection-molded codings in a plastic ring; 
     FIGS. 10 a,b  show an embodiment of the coding by means of cylindrical lenses; 
     FIG. 11 shows a coding ring with a prismatic cross section; 
     FIG. 12 a  shows a coding ring with cylinders; 
     FIG. 12 b  shows a coding ring with lenses arranged directly on it; 
     FIG. 13 shows a coding ring with a laser diode as the light source; 
     FIGS. 14 a,b  show a coding ring with two laser diodes as the light source; 
     FIGS. 15 a,b  show a coding ring with one laser diode and two CCD linear arrays arranged downstream; 
     FIG. 16 shows the combination of an angle-determining code with a reference code lying next to it; 
     FIG. 17 shows the combination of an angle-determining code with a reference code lying above it and below it; 
     FIG. 18 shows the assignment of the illumination and the CCD linear array to the combination according to FIG. 17; and 
     FIG. 19 shows the signal generated by the arrangement according to FIG.  19 . 
    
    
     DETAILED DESCRIPTION 
     A digital, single-track code  2  is applied on the circumference of a 360° rotatable circular ring  1  of a steering device. The code is configured in such a way that it is not repeated over the entire circumference or in an observation region  3 . It can be referred to as single-track, unambiguous and closed. This single-track code thus suffices for determining the absolute steering angle within 360°. 
     The observation region  3  is illuminated by light-emitting diodes  4  and  5  and imaged onto a photodetector linear array  7  via an optical arrangement  6 . The said photodetector linear array is embodied as a charge-coupled detector linear array (CCD linear array). The code  2  in the observation region  3  is embodied as a black-and-white code in accordance with FIG. 1 a . The code. is passed as a contrast difference from the photodetector linear array to a microcontroller  8 . The latter evaluates the contrast differences, decodes them and forwards the angle-of-rotation position to the vehicle  10  via an interface  9 . 
     The entire unit is supplied from the 12-volt vehicle electrical system  13  via a power supply  12 . 
     In order to detect the angular position, in this method a contiguous segment of the code track, namely the observation region  3 , is imaged onto the photodetector linear array  7 . Within 0 to 360°, the absolute steering angle can be unambiguously determined with a resolution that depends on the code chosen. The observation region is chosen in such a way that at least one code word  29  (FIG. 2) of the code track is detected by the CCD linear array  7 . Each code word corresponds to a steering angle, the angular resolution depending on the number of code words. A resolution of 1° is obtained given 360 code words. A coarse angle is determined in this way. 
     In this method, a high resolution, that is to say fine angle determination, is obtained independently of the resolution of the code of the code track and the number of code words. To that end, in accordance with FIG. 2, in the observation region  3 , the position of the start  25  and of the end  26  of a code word  29  is measured with regard to a fixed reference mark  28  of the stationary photodetector. The reference mark is provided at pixel No.  64  in this exemplary embodiment. The measurement is performed purely by software in the microcontroller  8 , which uses the image data of the photodetector linear array  7  for this purpose. 
     The position  27  of the code word with regard to the reference mark  28  of the photodetector linear array  7 , measured with the resolution of the photodetector, is obtained as a result. The position or the distance of the code word with respect to the reference mark and thus the angular resolution of the steering angle sensor is thus dependent only on the resolution with which the photodetector linear array resolves the observation region. In the exemplary embodiment, the photodetector linear array has 128 pixels, as a result of which steering angle resolutions of &lt;0.20 are achieved. The absolute angle is thus composed of the code word and the position. of the code word with respect to the photodetector linear array. 
     Since, presupposing that the observation region  3  detects at least one code word, the resolution of the steering angle sensor depends only on the resolution of the photodetector linear array, the light/dark lines of the code word can have large dimensions, e.g., 2-3 mm. It is expedient to use a code having the fewest possible code words. This is achieved, e.g., by the use of 6-bit or 7-bit codes instead of, e.g., 8-bit codes. The sensor sensitivity to environmental influences, e.g., soiling, is thereby reduced. 
     In FIG. 2, the linear assignment of the photosensitive cells of the CCD linear array in the observation region is represented on the x-axis  21  and the associated voltage amplitudes are represented on the y-axis  22 . If the code in the observation region  3  is imaged onto the photodetector linear array  7  very well, very distinct contrast differences with correspondingly sharp demarcations are produced. If the imaging is unsharp on account of an excessively large radial tolerance or owing to soiling, the photodetector linear array  7  produces a rough signal whose profile corresponds to the graph  23 . The code is reconstructed by known curve analysis and assessment in the microcontroller  8 , so that the signal train  24  is then present. As a result of the evaluation of the amplitudes as a function of the number of photodetectors of the photodetector linear array  7 , it is possible, in the event of progressive soiling or in the event of aging of the components, for the gain to be increased or for the luminance at the diodes  4  and  5  to be set in a correspondingly adaptive fashion. These settings can also be adapted over the circumference or the observation region. Axial tolerances are compensated for simply by way of the height of the code track  2 . 
     In order to increase the sensitivity further, the steering angle sensor can be configured further in accordance with FIGS. 4 and 4 a . The code on a circumference  41  is divided into an upper coarse region  42  for identification of the 0 to 360° and into a lower fine region  43 . The coarse code region  42  illuminated by means of a light-emitting diode  45  is imaged onto the photodetector linear array  7  via an optical arrangement  47  for the purpose of determining the coarse angle. Afterward, the fine code region  43  is illuminated by means of a light-emitting diode  46  and a smaller detail is imaged onto the photodetector linear array  7  via an optical arrangement  48 . This smaller detail may in turn comprise a code covering +/−10°. The two optical arrangements are separated by a diaphragm  44 . The imaging of a smaller detail enables the resolution and accuracy to be correspondingly increased. 
     A further embodiment of the steering angle sensor is illustrated in FIG.  3 . The arrangement corresponds to that of FIG. 1, but a monitoring and interface microprocessor  11  is provided for the purpose of testing the entire system, in which microprocessor software is installed which either in the event of switch-on and/or cyclically switches on, one or more light-emitting diodes  32  and  33  are imaged onto the photodetector linear array  7  via a mask  34 . As a result of the diodes  32  and  33  being switched on sequentially, a shadow image or a plurality of shadow images are generated successively on the photodetector linear array. As a result, the function of the entire arrangement can be tested via all the components. In the event of soiling in the region of the optical components, e.g., the optical power of the light-emitting diodes  4  and  5  can then easily be adapted by correspondingly increasing the control current. The failure of individual detectors of the photodetector linear array is also noted and can be compensated for by computational measures. 
     Throughout the operating time in which the measurements of the steering angle take place, it is possible, by evaluation of the rise and fall times of the signals in accordance with FIG. 2, of the amplitudes of the signals and also of the imaging of the code  2 , not only to monitor the entire system in the sense of diagnosis but also to compensate for tolerances and to attain the accuracy over virtually all operational influences. 
     Whereas FIG. 1 illustrates an embodiment of the steering angle sensor which operates with reflected light, FIG. 5 shows a sensor which operates with transmitted light. In this case, a circular ring  54  is optically transparent in the region of the light lines: of a code  53 . A light-emitting diode  52  and an optical arrangement  55  are arranged inside the ring. A photodetector linear array  51  is assigned to said diode and optical arrangement outside the circular ring. The code  53  in accordance with FIG. 5 a  corresponds to the code of FIG. 1 a . The code track is transilluminated with parallel light in this case. 
     In the exemplary embodiment of FIGS. 6 and 6 a , a provision is made of two linear array sensors  65  and  66 , which observe the same code track  62  at different points. Both are arranged vertically with regard to the course of the code track and detect the observation region  63  synchronously but at different positions  67 ,  68 . 
     As a result of the comparison of the two images, local soiling particles, e.g., on the code or optical arrangement, and also sensor errors can be identified. Instead of two single-row linear array sensors  65  and  66 , it is also possible to use an area sensor  61  for this function. 
     In the exemplary embodiment of FIGS. 7 and 7 a , a code  73  with two tracks  71 ,  72  is provided, as is evident from FIG. 7 a . Each of the two code tracks is observed by a linear array sensor  65 ,  66  or different linear arrays of an area sensor  61 . The code of the second code track may then be, for example, the inverse of that of the first code track, thereby resulting in simple control of the sensor input by simple subtraction of the measured values in the microcontroller  8 . 
     A further arrangement is illustrated in FIG.  8 . In order to increase the sensor reliability, a second CCD sensor  85  is positioned horizontally next to the first CCD sensor  84  in this exemplary embodiment. As a result of the horizontal arrangement different code words are registered at two different locations of the code track  82 . The difference between the measurement results of the two CCD sensors must produce the differential observation angle of the two photodetectors with respect to the code track. This angle is known on account of the position of the CCD sensors. 
     Instead of a second CCD sensor, it is also possible to use a correspondingly larger linear CCD sensor. Furthermore, it is possible to combine two photodetector linear arrays in an integrated circuit  81  with a common housing. 
     In the exemplary embodiment of FIG. 9, one or more LEDs  86  are imaged onto a photodetector arrangement  89  via an optical arrangement  88  directly or via a matte plate  87 . The coding consists of a ring  90  in which optically transparent angular ranges  91  and optically opaque angular ranges  92  are produced by injection-molded recesses or by correspondingly optically transparent and optically opaque plastics. 
     In the exemplary embodiment of FIGS. 10 a  and  b , one or more LEDs  86  are imaged on the photodetector array  89  directly or via the matte plate  87  or Fresnel lens by means of an arrangement of identical or differently configured cylindrical lenses  93  without a further optical arrangement. The cylindrical lenses  93  produce stripes of different brightness on the photodetector arrangement  89 . A corresponding code serving to sense the steering angle is generated by corresponding arrangement and distribution of the cylindrical lenses  93  on the coding ring  94 , which is produced from a transparent medium. The arrangement of the cylindrical lenses is shown in FIG. 10 b . Instead of the imaging of the LEDs onto a matte plate as in FIGS. 9 and 10, a coding ring  95  may be embodied in such a way that it comprises a prismatic arrangement in which one or more LEDs  86  irradiate the arrangement, e.g., axially (FIG.  11 ). The light beams are deflected at the hypotenuse  95   a  of the prism. The radial light emergence at the circumference of the coding ring  95  is configured in such a way that quasi-plane areal regions  96  and cylindrical lens arrangements  93  are situated there. The plane areas produce homogeneous light emergence onto the radially fitted photodetector arrangement  89 . The cylindrical lenses  93  produce regions of low and high luminance on the photodetector arrangement  89 . The coding for the purpose of steering angle determination is effected by corresponding distribution of the quasi-plane areas and by the identically or differently configured cylindrical lenses. 
     In a further embodiment according to FIG. 12 a , light is radiated in the axial direction by means of LEDs  86  into a transparent coding ring  97  and this coding ring has a number of cylinders  98  which, at one end, are connected directly to the coding ring and, at the other end, are configured either in a plane or lenticular manner. As a result, a corresponding luminance which can be detected by means of the photodetector arrangement  89  will occur at each of these cylinders. By means of regions with and without cylinders and configuration of the density, a code can be applied and correspondingly detected. A variant of this embodiment is shown by FIG. 12 b , in the case of which, instead of the cylinders  98 , lenses  99  are provided directly on the coding ring  97 . The effect corresponds to that of the arrangement with cylinders described above. 
     In addition to the embodiments which are shown in the figures and serve as an example of the design according to the invention, the arrangements can be configured in radial or axial form, where the direction of the passage of the light can be chosen in both possible directions. 
     In the exemplary embodiment of FIG. 13, a lens arrangement is not necessary. A virtually point light source, e.g., by the use of a laser diode  101  with a light-emitting area of, e.g., 2 μ×3 μwith the depletion layer, e.g., parallel to the axis, illuminates the coding ring  90 , which comprises the optically transparent regions  91  and the optically opaque regions  92 . Provided downstream of the coding ring  90  is a linear photodetector arrangement  89 , on which a luminance distribution is produced by the coding in the case of the optically transparent regions. This luminance distribution is evaluated. 
     The arrangement according to FIG. 13 can be improved by using two laser diodes  101  and  102  provided next to one another at a known distance, as is illustrated in FIGS. 14 a  and  b . The evaluation is effected in the manner described for the embodiment of FIG.  13 . However, if the radial distance of the coding ring  90  changes due to radial runout of the steering wheel or if the distance between the components changes due to mechanical or thermal influences, this being indicated by positions  1  and  2  of the photodetector arrangement in FIG. 14 a , then the position and angular position of the code can nevertheless be detected accurately on the photodetector arrangement  89  as a result of the different shadow formation of the two laser diodes  101  and  102  whose distance remains constant. In this case, the laser diodes  101  and  102  can be rapidly excited one after the other, the time being chosen to be short enough, e.g., 10 μs-100 μs, that an accuracy-limiting angular change does not occur at the steering wheel in this time. Instead of the laser diodes, it is also possible, of course, to use a monolithic LED in a double or triple arrangement, in which light-emitting areas with a very small extent in the axial direction are produced by means of corresponding masks. 
     The same effect is also obtained by using a single laser diode  101  or LED, in which shadow imaging is effected on two photodetector linear arrays  103  and  104  fitted at different distances from the code ring  90  (FIGS. 15 a  and  15   b ). The code can be determined accurately by virtue of the conditions of the shadow structure. The distance of the code ring in the event of axial runout is determined by means of the absolute extent of the code on the photodetector linear arrays. 
     The embodiment of FIG. 16 shows a combination of an angle-determining code  105  with a reference code  106 . By way of the projection of the code over a length which covers at least the angle code and the reference code, not only can the angle be evaluated, but also, on the one hand, the CCD or the photodetector arrangement can be tested by way of the reference code and, on the other hand, the distance and the exact angle can be determined by way of the known distances of the reference code. 
     The angle-determining code  105  can also be applied parallel to the reference code  106 , as is illustrated in FIG.  17 . If the reference code  106  is applied at the top and bottom on the edge of the angle codings, in the manner shown here, then an axial tolerance does not affect the imaging of the reference code  106 . 
     The two codes can be jointly imaged and illuminated by means of LEDs  86 , as is evident from FIG.  18 . As a result, at the photodetector arrangement  89 , an angle signal  107  and a reference signal  108  in accordance with FIG. 19 can be generated and evaluated.