Abstract:
A steering device which includes coded microstructures which are provided on the steering shaft and/or on a device that is connected to the steering shaft in a non-positive manner; a sensor which detects the microstructures and outputs associated measuring signals; and an electronic circuit to which the measuring signals of the sensor are fed, and which outputs electronic signals to control the steering is disclosed.

Description:
RELATED CASES 
     This application is a continuation of International Application PCT/EP00/02839 which was internationally filed on Mar. 30, 2000 and has a foreign priority date of Apr. 1, 1999 and which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present application is directed to a steering device for vehicles, and in particular to a steering device comprising a steering shaft, a sensor for determining the movement of the steering shaft, and a circuit for evaluating the measuring signals of the sensor. 
     2. Background of Related Art 
     Vehicle steering mechanisms may take different forms. Rack and pinion steerage is used particularly often. With rack steerage, a driver exerts a torque on a steering column via a steering wheel. Direct power transmission then continues via a pinion, i.e. a gear wheel, to a rack. Longitudinal movement of the rack results in longitudinal movement of a steering shaft in, or on which, the rack is mounted. The steering shaft in turn moves the steering gear, with the vehicle wheels arranged on it, and is steered in this manner. 
     To assist the direct power transmission by the driver it is also known to use hydraulic power-assisted steering mechanisms, in which a pressure chamber runs a piston fixed to the steering shaft. By controlling the pressure in the chamber filled with hydraulic oil the piston can be moved, thereby assisting the steering gear in addition to the power transmission by the driver. Alternatively, the pinion drive may be assisted by an electric motor. 
     In order to provide these various forms of assistance it is naturally desirable to have a measuring signal available which correlates with the state of the steerage. The signal could then take over appropriate control to boost the steering, for power-assisted steering and similar purposes, and could also allow for self-regulating systems. Over and above the control of the servo mechanism, allowance should also be made for boosting measures to optimise the steering and attenuation action of motor vehicles or simultaneous control of all four wheels and other intelligent steering systems. 
     Various proposals have already been made for obtaining a signal which correlates with the state of the steerage. 
     Thus, it is proposed in DE 40 29 764 A1 to arrange length measuring means between the steering wheel and the front axle, responding to displacement of the steering rack. Inductive or ohmic devices are proposed for these means. A design with two magneto-resistive sensors is known from EP 0 410 583 B1. Here, the magnetic coupling is changed on movement of the steering shaft, thus enabling the position to be determined. However, this involves changing the geometry of the steering shaft and also providing it with a groove, which apart from the expense, gives it a certain susceptibility to trouble. EP 0 376 456 B1 also operates with a magnet which is arranged on the steering shaft and surrounded by an induction coil. A change in induction can be associated with a change in displacement. 
     Steering angle sensors operating with magnetic field sensors, so-called Hall sensors, are known from DE 197 03 903 A1 and DE 197 52 346 A1. 
     These known proposals have the drawback that measurement only allows restricted accuracy. Another problematic feature is that the measurements are relative, so that measuring errors add up over time. The proposals are not, therefore, practicable for use in intelligent steering systems. 
     It is known from DE 37 03 591 C2, in a rack steering mechanism at the end of the steering column, to measure the rotary angle of the column by appropriately acting on an induction coil or a piezo power-measuring cell. However, the end of the steering column also carries the power transmission to the steering rack and is both structurally confined and unfavourable for measurements, particularly as a great deal of malfunctioning may take place there. 
     There is, therefore, needed in the art a steering device in which it is possible to pick up a signal correlating with the state of the steering mechanism and more suitable for controlling intelligent steering systems of that type. 
     SUMMARY 
     The present invention is directed to a steering device which includes coded microstructures which are provided on the steering shaft and/or on a device that is connected to the steering shaft in a non-positive manner; a sensor which detects the microstructures and outputs associated measuring signals; and an electronic circuit to which the measuring signals of the sensor are fed, and which outputs electronic signals to control the steering. 
     The invention proposes a steering device for vehicles which allows absolute measurements of position. Therefore, the disadvantages associated with the state of the art no longer exist. The steering device according to the invention is more accurate and supplies reproducible measuring signals. Regulation and/or control of the movement of the steering shaft becomes possible, particularly for intelligent steering systems. 
     Advanced surface techniques with processes indicating the microstructure are thus combined with a high-resolution sensor, i.e. a detection system, with an appropriate electronic circuit. The term “microstructures” refers here to structures with dimensions in the micrometer range. 
     The term “detect” refers particularly to processes where contact-free recognition takes place, preferably optically or magnetically. However, other detection methods which read, sense, feel or otherwise recognise also come into consideration. 
     The invention allows absolute determination of the position of the steering shaft in a rapid, high-resolution and reliable manner, with resolution in the low micrometer range. Falsification or trouble from electromagnetic fields or in the region of the steering mechanism either does not take place or is negligible. 
     The invention may be applied successfully in particular to advanced, so-called intelligent steering systems. 
     It is possible to equip the actual steering shaft with microstructures. The disadvantage of doing so would be the difficulty of manipulating the whole shaft during the fitting process. In order to avoid this, smaller, interchangeable elements which can be non-positively connected to the steering shaft, e.g. in bar form, may be appropriately equipped, then inserted. 
     The microstructures are advantageously formed so that they contain suitable coding, allowing the position of the steering shaft to be determined accurately. 
     The microstructures are preferably detected by optical scanning methods, particularly using elements from microsystem technology. Microsystem technology is understood here as the fields of microstructure technology, micro-optics and fibre optics. Microlenses with diameters down to about 10 μm and focal lengths of the same order of magnitude may be used. If glass or other fibres and very small diameters are used, the microlenses can be fixed directly on the end face of the fibres. The entire system may have Y branches and is integrated with individual modules to form a compact microsystem. The modules may, if appropriate, be spatially offset over the optical fibres—for example to allow optoelectronic components and the evaluating electronic means to be operated optimally within low-temperature ranges. 
     Tribologically suitable film systems are advantageously applied to the steering shaft or to a linear means connected thereto without play, described as a device or measuring device. This may be done by thin film processes which have proved successful in other industrial fields. Special microstructures are produced by high-resolution structuring and etching processes. The microstructures are constituted so that they can be read by the sensors. 
     The optical contrast, i.e. the difference in reflectivity, of the microstructures to the steering shaft surface below them may for example be modified, so that the pattern can be optically recognised by means of miniaturised fibre optical systems. Another example is to make the microstructures in the form of a reflection hologram, with coding as in the previous example (segment-wise) and with reading effected by a suitable miniaturised optical system. The functional layer may be crystalline or amorphous and the hologram may be written in a phase or angle code. The hologram may function in one frequency range (monochromatic) or more than one (coloured), and the information may be written (to the hologram) by a digital or analog process. 
     Other physical methods may be employed instead of, or as well as, optical sensors or optically detectable microstructures. Thus, microstructures may also be formed in magnetic films, e.g. CoSm or NdFeB. The sensors could then in particular be magnetic sensors, otherwise used in data storage technology. 
     Microstructures are produced on the steering shaft or on the device non-positively connected thereto in the form of incremental markings. Tribologically optimised layer systems are preferred, using high-resolution lithographic or laser technology methods suitable for three-dimensional applications. The lithographic methods considered are of the photo, electronic, X ray and/or ionic type. 
     Multiple-layer or composite structures may equally be employed. 
     The patterns formed are preferably dimensioned in micrometers. The layer systems, combined with an appropriate sensory recognition system, enable the current position to be determined absolutely, to an accuracy of only a few micrometers. 
     In an advantageous embodiment of the invention two complementary, parallel patterns are provided with suitable coding, e.g. bit coding. In one embodiment the marking structure comprises strips which are optically distinguishable by reflection, the strip patterns containing binary L/O coding. 
     In this way the displacement-measuring system, which may be fully integrated into the steering mechanism, can recognise the current absolute position of the steerage in every operating phase by means of the bit coding. 
     Various patterns are possible. For example a dual code, a Gray code or even stepped codes known per se from relevant mathematical processes may be used. 
     It is particularly preferable to use optical sensors, especially fibre-optical double sensors, for scanning the markings and microstructures. Multiple sensors are also possible, especially in array form. 
     In a preferred method the microstructures are produced by applying thin film techniques. These techniques are advantageously PVD (physical vapour deposition) and/or CVD (chemical vapour deposition). As already mentioned, structuring is effected by lithographical processes. 
     The microstructures can also be formed by dry etching and/or wet chemical etching. 
     Alternatively, they may be made by laser beam techniques, e.g. direct-writing laser ablation processes and/or laser-lithographic processes and/or direct-action mask-related laser structuring methods. 
     The microstructures are preferably built up from tribological hard-material layered systems. Single or multi-layer films may be used. They are preferably made of titanium nitride (TiN) and/or titanium aluminium nitride (TiAlN) and/or titanium carbonitride (TiCN) films and/or aluminium oxide films and/or amorphous diamantine hydrocarbon films with or without metal doping and/or amorphous diamantine carbon films with or without metal doping and/or amorphous CN films and/or cubic boron nitride films and/or diamond films. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       It should be understood that the drawings are provided for the purpose of illustration only and are not intended to define the limits of the invention. The foregoing and other objects and advantages of the embodiments described herein will become apparent with reference to the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagrammatic section through elements of an embodiment of a steering device according to the invention; 
         FIG. 2  is an alternative embodiment to  FIG. 1 ; 
         FIG. 3  is a diagrammatic representation of a microsystem-type sensor system for an embodiment of the steering device according to the invention; 
         FIG. 4  is a detailed representation of a member from  FIG. 3 ; 
         FIG. 5  is a detailed representation of an alternative embodiment of that member from  FIG. 3 ; 
         FIG. 6  is a detailed representation of another member from  FIG. 3 ; 
         FIG. 7  shows an example of a microstructure; 
         FIG. 8  shows an alternative embodiment of  FIG. 7 ; 
         FIG. 9  shows another alternative embodiment of  FIG. 7 ; 
         FIG. 10  is a diagrammatic section through a microstructure; 
         FIG. 11  shows the  FIG. 10  embodiment after a possible further processing step; 
         FIG. 12  is a diagrammatic section through another embodiment similar to  FIG. 10 ; 
         FIG. 13  is a diagrammatic section through a third embodiment similar to  FIG. 10 ; 
         FIG. 14  shows the  FIG. 13  embodiment after a possible further processing step; and 
         FIG. 15  is a diagrammatic representation of an embodiment of a sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a steering device according to the invention is shown in  FIG. 1 , and includes a mounting block  10 , inside which there is a pressure chamber  11  containing hydraulic oil  12 , the chamber  11  being nearly full of oil  12  as shown. The oil  12  is under a pressure p. In  FIG. 1  the mounting block  10  is represented purely diagrammatically; it is substantially cylindrical here, with considerable proportions of the block extending out of  FIG. 1 . 
     The steering shaft  20  runs approximately along the cylinder axis of the mounting block  10 . It thus extends through the pressure chamber  11  with the hydraulic oil  12 . The shaft  20  is provided with a steering rack  21 , indicated here in  FIG. 1  by corresponding tooth signs. The rack  21  is driven by a pinion  22 . The pinion is coupled to the steering mechanism of a vehicle (not shown). When the steering wheel e.g. of a passenger car is turned the corresponding torque is transmitted through the pinion  22  to the rack  21  and displaces the whole steering shaft  20  with it along the axis through the mounting block  10 . 
     A piston  23  is also seated on the steering shaft  20  with a non-positive connection thereto. It is arranged inside the pressure chamber  11  and thus in the hydraulic oil  12 , whereas the pinion  22  and rack  21  are located outside the chamber  11 . 
     The steering shaft  20  thus passes through the wall of the pressure chamber  11  in two places. Both places are sealed by seals  24 , preferably Viton seals. The piston  23  moves along with the shaft  20  by virtue of its non-positive connection thereto. It fills the entire cross-section of the chamber  11 . The piston  23 , and thus the steering shaft  20 , can consequently be moved by changes in the pressure of the hydraulic oil  12 . This is a common method of strengthening the forces exerted by the user of the vehicle through the pinion  22 . 
     Suitable diameters for steering shafts  20  are about 20 to 40 mm, suitable diameters for pressure chambers  11  about 40 to 70 mm, steering shafts  20  may e.g. have lengths of the order of 800 mm, and the length of the pressure chamber  11  may e.g. be 200 to 400 mm. Quite different dimensions may of course be appropriate according to the requirements for the steering device, as would be known to those of skill in the art. 
     A mounting bore  13  is formed in the mounting block  10  outside the pressure chamber  11 . It extends from the outer wall of the block  10  to the through bore in which the steering shaft  20  is located. The mounting bore  13  contains a sensor  35  which may for example comprise the ends of a fibreglass sensory mechanism. 
     In this particular region the outside of the shaft  20  is provided with marking  30 . The marking  30  comprises microstructures  31  arranged on top of the shaft  20 . These are coded axially of the shaft  20  so that different bit patterns pass below the sensor  35  when the shaft  20  moves longitudinally relative to the mounting block  10 . The signals from the sensor  35  are passed to an electronic circuit  40  (not specifically shown in  FIG. 1 ). The circuit  40  can then determine and transmit the position of the shaft  20  relative to the block  10  from the readings of the sensor  35 . 
     Apart from the longitudinal movement of the shaft  20  other movements of the shaft are not important for the steering mechanism. Hence nothing concerning any rotation of the shaft  20  is shown in  FIG. 1 . Any versions which ensure that the pinion  22  runs appropriately over the steering rack  21  are possible here. 
     Another, alternative embodiment is shown in  FIG. 2  in a view similar to  FIG. 1 . 
     In  FIG. 2  the mounting block  10  will again be recognized, along with the pressure chamber  11  and hydraulic oil  12 . The steering shaft  20  with the rack  21  again passes through the block  10  and chamber  11 . Here too, the pinion  22  drives the rack  21 . A piston  23  which can move inside the pressure chamber  11  is also seated on the shaft  20 . 
     In contrast with  FIG. 1 , a mounting bore  13  is not only provided, but another mounting bore  14  is also provided outside the pressure chamber  11 . 
     This difference enables two sensors  35  and  36  to be provided. Redundant or complementary microstructures  31  of the marking  30  or microstructures double-coded in another form can, therefore, be read out. The sensors  35  and  36  are preferably fibre optic reflection ones. The light source for the reflection sensors is formed by light-emitting diodes (LEDs), which are spectrally adapted to the hydraulic oil  12  used in the pressure chamber  11 . Pentosin may preferably be employed as the hydraulic oil  12 . 
     The pressure p of the hydraulic oil  12  in the pressure chamber  11  is regulated by valves in a valve control housing (not shown). 
     The steering shaft  20  is sealed at the openings where it passes into and out of the pressure chamber  11  by seals  24 , for example Viton seals. It thus has a central position corresponding to the steering angle 0°. This is indicated as central position X 0  in  FIG. 2 . Movement respectively to the right and left then takes place in the direction of steering shaft position +X (right) and in direction −X (left). These respective end positions correspond to a linear stroke which may typically be ±75 mm. It results in different stop angles of the steering mechanism according to the type of vehicle. The linear stroke may also be smaller, e.g. ±50 mm in individual cases, according to the type of vehicle. 
     In  FIG. 2 , the two mounting bores  13  and  14  are arranged outside the pressure chamber  11 , so the two individual sensors  35  and  36  are also arranged outside it. It is also possible to provide an integrated pair of sensors. 
     In another embodiment, the sensor or sensors  35  and  36  may be positioned inside the pressure chamber  11 . The sensor or sensors may then, for example, be spaced from the steering shaft  20  and pick up the steering shaft data as an optical sensor through the hydraulic oil  12 . 
     This enables the sensor to provide information about the turbidity of the hydraulic oil  12  in the chamber  11 , as well as reading the microstructures  31  of the marking  30  on the steering shaft  20 . The information can be used as a criterion for changing the oil  12 . A suitable transmitting wavelength for the optical sensor  35  is selected according to the turbidity and spectral absorption of the oil  12 . A system of this type operates even when dirty with abraded particles or an oil film, and preferably has suitable redundancy, fault tolerance and azimuthal tolerance for safety reasons. 
     The sensors may be fibre optic sensors with two individual fibres. As indicated in  FIG. 2 , the fibres may be parallel or inclined to each other to absorb incoming and reflected light (not shown). However, it is also possible to use fibre optic reflection sensors in a Y structure or to take into account arrangements with fibre lines or fibre bunches. 
     The sensors  35  and  36  or a sensor system  37  (see  FIG. 3  for such a system) are employed as transmitters or receivers and may be coupled direct to the fibres by a particularly temperature-resistant installation and connection method. Alternatively, they may be arranged over a feed fibre located in a lower-temperature region. In another embodiment, the sensor module is fabricated as a compact, miniaturised (microtechnical) module and mounted in the system in order to simplify assembly. 
     In another embodiment (not illustrated) designed to increase reliability and avoid malfunctioning, two sensors  35  are juxtaposed azimuthally. These then sense two complementary bit patterns, both in the form of individual markings  30  applied by the thin film method and arranged parallel, with corresponding microstructures  31 . 
     An embodiment of marking  30  with microstructures  31  is shown diagrammatically in  FIG. 3 . Here, the steering shaft  20  is reproduced purely diagrammatically as a cut-out; it extends parallel with the x-direction indicated. 
     A sensor system  37  with an array of fibre optical Y branches  38  can further be seen. It has a module “A” for generating and coupling the light  51  into the input or coupling-in fibres  39  of the fibre optical Y branching element  38 . 
     A module “B” is also provided, with an array arranged in the y-direction of lenses  52 , particularly microlenses, for generating parallel output beam pencils. The output beam pencils  53  fall onto the microstructures  31  of the marking  30  on the steering shaft  20 . These microstructures  31  form a succession of sequences. Position-specific selective retroflection takes place. The retroflected light passes back through the lenses  52  into the fibres of module B and thence to a module C for uncoupling and detecting the light  55  retroflected and leaving the fibre optical Y branching element  38 . 
     Moreover in  FIG. 3 :
     ±x is the axial direction, i.e. the direction of movement of the steering shaft;   ±y is the azimuthal direction, i.e. the direction in which the position-specific bit pattern is arranged; and   z is the direction in which the sensor system is installed.   

     Coordinates x and z are orthogonal to each other; coordinate z points in the direction of the tangent to the surface of the steering shaft  20  which is orthogonal to x and z. 
       FIG. 4  shows a detail from  FIG. 3 , namely a first version of a transmitting and coupling-in module “A” with a single source  51 , a single lens  52  and a plurality of coupling fibres  39  of the Y branching element  38 . 
       FIG. 5  shows an alternative to  FIG. 4 , a different version of a transmitting and coupling-in module “A” with an array of lenses  52 . The fibres are bunched then separated again as coupling fibres  39  of the Y branching element  38 . 
       FIG. 6  shows another detail from  FIG. 3 , namely an embodiment of an uncoupling, reception and assessment module “C” with uncoupling fibres  54  bunched along a certain length, an array of lenses  52 , a line of detectors  56 , the electronic circuit  40  with the electronic assessment means and the output signal  60  with the “position of the steering shaft”. 
       FIG. 7  shows 8-bit coding in a radial direction and periodic displacement marks in an axial direction. 
       FIG. 8  shows an example of an arrangement of blocks with individual coding. 
       FIG. 9  shows an example of an arrangement of different structure sequences and a guide structure with periodic division for tracking with azimuthal displacement. 
       FIGS. 10-14  show embodiments of possible methods of producing the microstructures  31 . A coded pattern is produced on a basic member  81 , which may also be the steering shaft  20  or another device non-positively coupled thereto. For a version where detection is to take place by optical blanking of the patterns the basic member  81  is surface-treated with a focused laser beam, so that laser-ablative processes at the point of action cause stripping and thus lasting marking (cf.  FIG. 10 ). 
     Eximer lasers are preferably used for this purpose, owing to their high resolution. The pattern thus produced can then be covered with a friction and wear-reducing film  82 , as shown in  FIG. 11 . A metal-doped amorphous hydrocarbon film is well suited as such a covering film in the region of the steering shaft, and is preferably applied in a thickness of 0.5 to 5 μm by known plasma-supported PACVD processes (magnetron sputtering processes with a substrate bias and a hydrocarbon gas, preferably C 2 H 2 ). Titanium or tungsten is preferably employed as the doping metal for this application. The metal-doped amorphous hydrocarbon layer may, for example, be produced using a Leybold large capacity sputtering plant, model Tritec 1000 with two tungsten targets installed. The plant has a rotary holder which can accommodate up to 20 steering shafts according to the equipment. After the normal pumping process whereby the chamber is pumped out to about 10 −5  hPa, argon is admitted up to a pressure of 3×10 −3  hPa and the substrate is surface-cleaned by ion bombardment at a bias potential of 100 to 300 V. The targets are pre-sputtered at about 6 KW in the process. A graded film of tungsten-doped hydrocarbon is formed without interrupting the plasma, by opening the target covers and successively adding C 2 H 2  to the process. A few minutes later the C 2 H 2  gas flow is adjusted to bring the ratio of tungsten to carbon in the layer to 5-10%. During the production of the metal-doped amorphous hydrocarbon film the substrates are coupled with a bias potential of from about 100 to 300 V, preferably 200 V. Under these conditions a film thickness of about 1 μm is applied in half an hour. 
     Other solutions explaining the use of a structured film are shown in  FIGS. 12-14 . The film structure may be utilized for different sensing principles. In the case of optical detection, film structures may e.g. have an appropriate contrast (surface or edge contrast) with the surrounding surface. The film structure may, however, be produced from a magnetic material and read by means of a magnetic sensor or a magnetic sensor matrix. In such a case a magnetic film is used, preferably a film of CoSm or FeSi or NdFeB, with or without additives. 
     The steering shaft  20  or basic element  81  is coated in a vacuum process, in this case with two films  83 ,  84 , the lower film  83  respectively being a metal-doped amorphous hydrocarbon film onto which a TiN film is deposited. The thickness of the upper film  84  is approximately 0.5 μm. TiN is preferably used in combination with a Ti-doped hydrocarbon film. The ethine is merely substituted by nitrogen, again without interrupting the plasma. The film  84  is structured by photo-lithography, by coating the coated steering shaft  20  with a photosensitive resist. It is approximately 2.5 μm thick. The patterns are then produced over a large area on the shaft by means of a mask. 
     When the resist pattern has developed, the TiN film  84  is removed from places where there are no photosensitive resist patterns, by wet-chemical etching using known etching agents. 
     Patterns may also be made countersunk, i.e. planarized, as shown in  FIG. 13 . In such a case, the steering shaft  20  is coated with, for example, a W-doped amorphous hydrocarbon film  85 , after which a photoresist pattern is formed on it. By means of photoresist masking a 0.2-1.0 μm depression is then etched in the W-doped amorphous hydrocarbon film in a reactively conducted plasma etching process (etching gases Ar/SF 6 ). The photoresist mask is maintained and the depression is then refilled by sputtering e.g. TiN. This makes the surface even microscopically smooth. 
     A further embodiment is illustrated in  FIG. 14 , where a tribologically optimised film  86  for the previously described substructure is applied. In this case, even film materials which do not necessarily have good tribological properties may be used to form the pattern. 
     An embodiment of a sensor  35  is shown in  FIG. 15 . This is a magnetic sensor. It comprises a linear arrangement of magnetic sensors which can read a magnetic structure e.g. in an 8-bit code. The polar structures of the reading head are shown; operating safety is improved and the number of codings increased by using a second line. The sensor  35  may, for example, be made from known magnetoresistive or inductive single sensors produced by similarly known thin film methods. To minimize the spacing from the magnetic microstructures on the steering shaft  20 , the polar structures of the reading sensors are arranged on an arc matching the diameter of the shaft. 
     It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting, but merely as exemplifications of a preferred embodiment(s). Those skilled in the art will envision other modifications within the scope and spirit of the invention.