Abstract:
The invention relates to a device with a magnetic position sensor comprising a field sensor ( 5 ) and analysis electronics ( 7 ) for linear displacements of a rod-shaped component ( 3 ), in particular the shaft of an actuator, with an element (M) generating a magnetic field ( 4 ) and a position sensor measuring the magnetic field strength angle of this field, the field angle signal determined by this sensor being used for displacement path determination. The element generating a magnetic field ( 4 ) is an axially magnetized magnet casing (M) which surrounds the rod-shaped component ( 3 ) of the actuator itself, is fixedly connected thereto and can rotate therewith about its displacement axis (z).

Description:
FIELD OF THE INVENTION 
     The invention relates to a device with a magnetic position sensor comprising a field sensor and analysis electronics for linear displacements of a rod-shaped component, in particular the shaft of an actuator, with an element generating a magnetic field and a position sensor measuring the magnetic field strength angle of this field, the field angle signal determined by this sensor being used for displacement path determination. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,570,015 discloses a magnetic position measurement device with which linear displacements of a rod-shaped component, for example a valve shaft, can be measured. A particular measurement unit, which consists of a shaft used for measurement purposes only and the position sensor, is placed on the rod-shaped component whose displacements are to be measured. Let into the shaft of the measurement unit is a rod-shaped magnet which generates a magnetic field in the longitudinal direction of the shaft. The position sensor is arranged fixedly in the area of the magnetic field. On relative displacements of the shaft in relation to its position, the position sensor measures by means of field sensors which, using analysis electronics, determine the magnetic field strength angle approximately proportionally to the displacement path. The path proportionality exists however only within a certain range. 
     Apart from the large space required, the drawback of this arrangement is that the shaft must not turn. If shielding is used, the device no longer works sufficiently linearly and is therefore no longer sufficiently precise. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a device with a magnetic position sensor which is simpler and smaller in structure, but in particular performs its measurements contactlessly and directly on the rod-shaped component of the actuator. 
     According to the invention, this object is achieved in that the element generating a magnetic field is an axially magnetized magnet casing which surrounds the rod-shaped component of the actuator itself, is fixedly connected thereto and can rotate therewith about its displacement axis. 
     If the element generating the magnetic field is attached directly to the shaft of the actuator, there is no need for a separate shaft of the measurement unit. Another advantage is that the shaft can rotate about its own axis without this affecting the measurement. This is very important because, in this case, a round actuator shaft does not require a twist-resistant guide for measurement. For faultless operation, it is advantageous if the actuator shaft consists of a non-ferromagnetic or only a weakly ferromagnetic material. The position sensor is also applicable in the case of inaccessible shaft ends. 
     With regard to the magnet casing length there is a long measurement range. This allows a compact construction with a non-temperature-sensitive measurement principle. There is also a high linearity of correlation between position and measurement signal. An additional analysis device, using a sensor curve to increase the precision, can be dispensed with in this case. 
     In a further embodiment of the invention, the field sensor comprises magnetoresistive elements, Hall effect sensors or field coils. 
     For the measurement accuracy it may be desirable to change the magnetic field—although not during operation but in the context of certain configurations—in relation to that of a single magnet casing. In a further embodiment of the invention, the magnet casing consists of a magnetic material of preferably axially joined ring discs. 
     The assembly of discs allows further variants. In a further embodiment of the invention, the ring discs consist of materials having a different remanence. 
     If an assembly of discs is not desired, the magnet casing in a further embodiment of the invention is magnetized axially differently when consisting of a homogeneous magnetic material. In these forms of magnet casings, the measurement accuracy is considerably improved. 
     In a further embodiment of the invention, the magnet casing, either consisting of a homogeneous material or assembled from ring discs has a ratio of diameter to length ranging from 2/3 to 3/2, preferably in the proximity of 1, so as to form a minimum field strength. 
     In a further embodiment of the invention, the field sensors are arranged on the inner side of a casing-shaped screen. This screen may have a round or a square cross-section. 
     In a further embodiment of the invention, the whole position sensor i.e. both the field sensors and the analysis electronics, are situated within the screen. This combination is extremely simple and flexible. 
     In a further embodiment of the invention, the magnet casing is let into a ring-shaped recess of the rod-shaped component and the inlet casing is surrounded by a non-ferromagnetic shaft casing of greater length. The shaft casing can also be let into the rod-shaped component. 
     In a further embodiment of the invention, the rod-shaped component is surrounded by a non-ferromagnetic linear guide casing, in the casing opening of which it can slide with the magnet casing, and the non-ferromagnetic linear guide casing is surrounded by a casing-shaped screen within which the field sensor is situated. 
     In a further embodiment of the invention, the analysis electronics are arranged on the outer side of the screen and carry the sensors within the linear guide casing by means of a carrier guided through the screen. 
     In a further embodiment of the invention, the rod-shaped component in the area of the measurement device consists of two parts connected together by means of a central pin arrangement, and the magnet casing and the shaft casing are pushed onto the rod-shaped component in the area of the pin arrangement. 
     Thus, a very simple and suitable and compact measurement device is obtained which can be used favorably, in particular in the automotive sector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be elucidated with reference to the drawings. 
     FIG. 1 is a diagram of a device with a magnetic position sensor according to the invention with a rod-shaped component, the axial displacements of which are determined by means of a magnet casing surrounding the component and fixedly connected thereto and a field sensor unit in the magnetic field of the magnet casing, 
     FIGS. 2 a  and  2   b  show curves of the measured angle on the path, 
     FIG. 3 shows a magnet casing composed of several ring bodies, 
     FIG. 4 is a cross-section of the measurement device with a screen, where the magnet casing consists of an axially homogeneous material or, as in FIG. 3, is assembled from several ring bodies, and where analysis electronics with field sensors are arranged within the screen, 
     FIG. 5 is a diagram of the angle/path curve in the arrangement in FIG. 4 with a comparison of the measurement errors occurring during use of the different magnet casings, 
     FIG. 6 shows a measurement device with a magnet casing integrated in the rod-shaped component and a very compact screened construction, 
     FIG. 7 is a section taken on the line VII—VII in the device shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a rod-shaped component, for example, the shaft  3  of a valve (not shown) which performs axial displacements. On the valve shaft  3  and fixedly connected thereto sits a magnet casing M which generates a magnetic field  4  made visible through the field lines  4   a . The field lines  4   a  are lines of equal vector potential. A field sensor  5 , which determines the magnetic field  4   a  prevailing around it, is arranged in the magnetic field  4 . Along the shaft axis z, the shaft performs z linear displacements, where it can also rotate about the shaft axis z. The magnet casing M has a length L. The external diameter of the magnet casing is given as D. r 0  is the distance of field sensor  5  from the shaft axis z. The magnetization direction of the magnet casing lies in the axial direction marked z. The field sensors  5  and analysis electronics (not shown in FIG. 1) together form a position sensor. 
     The angle to be measured is φ mes . This is the angle of a magnetic field strength H in relation to the z-axis. The field sensor  5  emits field strength signals to analysis electronics (not shown), from which it determines the field angle which in turn corresponds ideally to the displacement path. 
     FIG. 2 a  shows in a solid-line curve  6  the measured field angle on the displacement path. The broken line  6   a  below it is the ideal line. It should be noted that the curve is essentially rectilinear as a function of the displacement path in the selected range z mes  which corresponds to around 80% of the magnet length. FIG. 2 b  shows the measurement error in percent on the path with reference to a curve  7  showing that the measurement error in the range z mes  is very low from around −0.4 to +0.4 (approx. 80% of the magnet length). This diagram shows the percentage measurement error Δφ mes  on the displacement path z/L. 
     FIGS. 2 a  and  2   b  show the situation in which no screening is required and present when the arrangement has been carefully selected. The magnet casing consists of an axially homogeneous material. In practice, it is often not possible to work without a screening, in particular when several measurement devices are operated in the immediate vicinity. Here the working results deteriorate. This deterioration can be compensated with an axially structured magnet casing M described with reference to FIG.  3 . 
     FIG. 3 shows a magnet casing M comprising three ring bodies M 1 , M 2  and M 3  joined together. The structure is symmetrical i.e. the axial lengths L 1 , L 2  and L 3  are equal. Here, different dimensions may of course also be selected. There are various possibilities of structuring the ring bodies M 1 , M 2 , M 3 . One possibility is to make them of a fully magnetized material of different remanence. Another possibility would be to make the individual ring bodies M 1 , M 2 , M 3  of the same material and to magnetize them to different intensities. Another, preferable possibility is to use a magnet casing M of a uniform material and magnetize it to different intensities along the shaft axis z, using a suitable device. Further variants are also possible. Fewer than three ring bodies or more than three ring bodies are possible. 
     To achieve a casing length which is favorable with regard to measurement accuracy and achievable minimum field strength, the magnet casing, either consisting of a homogeneous material with axially modified magnetization or assembled from ring discs (M 1 , M 2 , M 3 ) has a ratio of diameter to length ranging from 2/3 to 3/2, preferably in the proximity of 1, so as to form a minimum field strength. 
     FIG. 4 shows how the measurement device can be screened from external fields. FIG. 4 shows the shaft  3  with shaft axis z. If a magnet casing M of an axially homogeneous material (version in which the broken separating lines M 4  must be ignored) sits on shaft  3 , then the broken-line measurement error curve shown in FIG. 5 applies, which although representative, is not desirable. If, however, an axially structured magnet casing M of the three different ring bodies M 1 , M 2 , M 3  as shown in FIG. 3 sits on shaft  3 , the measurement error can be compensated so that the solid-line error curve shown in FIG. 5 is obtained which particularly complies with the requirements. 
     A screening plate  26  of a ferromagnetic material extends around the arrangement. The shaft  3  consists of a non-ferromagnetic or only a weakly ferromagnetic material. The whole position sensor with analysis electronics  11  on which the field sensor  5  is situated, is arranged on the inner side  26   a  of the screening plate  26 . A connecting cable  8  transfers the position signals to a control device (not shown). The screening prevents interference fields from adjacent sensors or adjacent parts causing field distortions in the sensor area. 
     In contrast to FIG. 4, the shaft  3  may also have the structure shown in FIG. 6 with the magnet casing M let into the shaft  3 . It is evident from the construction that the shaft  3  is freely rotatable in relation to the sensor without causing measurement value changes. 
     FIG. 5 shows the diagram already indicated for percentage measurement errors. In the unscreened measurement device of FIG. 1, the broken-line error curve  9  is obtained. In the screened measurement device of FIG. 4, in which three magnetic ring bodies M 1 , M 2 , M 3  or an axially variable magnetization distribution are used, the solid-line error curve  10  is obtained. In the diagram, the percentage measurement error Δφ mes  shown on the displacement path z/L, where the path is marked z mes . In the configurations with an axially variable material or an axially variable magnetization, the measurement inaccuracy is reduced clearly quite considerably as compared with that shown in FIG. 2 a.    
     FIG. 6 shows another structure of the device in a longitudinal section. FIG. 7 is a cross-section of the device shown in FIG. 6, taken on the line VII—VII. The magnet casing M is let into a ring-shaped magnet recess  12  of the rod-shaped component  3 , for example, the valve shaft  3 . In this case, the magnet casing M may also consist of an axially homogeneous material, which is preferably structured axially magnetically, or it may consist of magnetic ring discs M 1 , M 2 , M 3 , where the number of three ring discs is merely an example. Similarly as in the other embodiments, only two or more than three ring discs may be used. The possible division into magnetic ring discs is indicated in FIG.  6 . The inlet magnet casing M is surrounded by a non-ferromagnetic shaft casing  13  of greater length. This shaft casing  13  is inserted on the magnet casing M into a cover recess  14  overlaying its magnet recess  12 . The outer wall  15  of the shaft casing  13  is preferably flush with the outer wall  16  of the valve shaft  3 . 
     A linear guide casing  17  surrounds the valve shaft  3  shown in FIG. 6, which is freely displaceable and rotatable in the casing opening  18 . The linear guide casing  17  consists of a non-ferromagnetic material. The linear guide casing  17  is surrounded by a casing-shaped screen  19  on which the analysis electronics  11  of the field sensor  5  are arranged. The screen  19  has an opening  21  extended as a blind hole  22  into the linear guide casing  17 . Arranged on the analysis electronics  11  is a carrier  23  which retains the field sensor  5  in the blind hole  22  of the linear guide casing  17 . 
     The valve shaft  3  within the shaft casing  13  has a central pin arrangement  24  which consists of a central pin  24   a  and a central recess  24   b  in which pin  24   a  engages. If the valve shaft  3  is extended, the magnet casing or casings M and the shaft casing  13  can easily be pushed on. 
     In all cases the field sensors  5  may be structured in known manner. It is possible to use paired magnetoresistive sensors, Hall effect sensors or field coils. 
     FIG. 7 is a section taken on the line VI—VI in the device of FIG.  6 . The valve shaft  3  in the center of non-ferromagnetic or weakly ferromagnetic material is surrounded by the magnet casing or casings M, the shaft casing  13  and the linear guide casing  17 . The linear guide casing  17  is surrounded by the screen  19 . The analysis electronics  11  carry the field sensor  5  by means of carrier  21 . The distance of the displacement axis z to the sensor  5  is given as r 0 . Di is the internal diameter of magnet casing M. D is the external diameter of the magnet casing. Ds is the internal diameter of the screen  19 . L is the axial length of the magnet casing. L 1 , L 2 , L 3  are the lengths of the individual ring magnets M 1 , M 2 , M 3  which together form the total axial length of the magnet casing M. 
     As an example of the device shown in FIGS. 6 and 7, the following dimensions can be given by way of example: 
     r 0 =0.64D 
     L=D 
     Ds=2×D 
     L 1 =L 2 =L 3   
     The magnetization of ring magnets M 1  and M 3  should be about 10% higher than the magnetization of M 2 .