Abstract:
A position-measuring device for fluidic cylinder-and-piston arrangements having at least one Hall sensor, preferably arranged in the area of the cylinder wall, especially in a cylinder wall, and a magnetic region, arranged in the piston. At least one Hall sensor array has at least two Hall sensors spaced one from the other in the direction of movement of the piston. One coil is provided whose magnetic field permits the switching points of the Hall sensors to be adjusted in response to the coil current.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a position-measuring device or sensor for cylinder-and-piston arrangements. 
   A position sensor of this kind, intended to indicate the position of a piston in the cylinder of a cylinder-and-piston arrangement in a braking system has been known, for example, from DE 100 53 995 A1. In the described arrangement, a hydraulic sensor cylinder is provided with a Hall element, the Hall sensor being inserted in the cylinder wall and a magnetic region being arranged in the cylinder-and-piston arrangement so that the movement of the piston in the cylinder will cause the Hall element to respond. A cylinder-and-piston arrangement wherein the position of the piston is sensed by means of Hall sensors has likewise been known From EP 0 536 926 B1. 
   A position-measuring device for fluidic cylinders comprising a long measuring coil, designed as an eddy-current long-distance sensor, has been disclosed in DE 202 05 822 U1. 
   It has further been known to detect the position of a piston by capacitive sensors. An actuator cylinder comprising a sensor for determining the position of the piston by means of capacitive sensors has been known from DE 44 10 363 A1. 
   Now, it is necessary in many cases to change or adjust the end or stop positions of fluidic cylinder-and-piston arrangements, for example during start-up or operation of the system. To this end, some of the known cylinder-and-piston arrangements have switching elements mounted in a groove in a cylinder profile, for example. In order to adjust a given position, the switching elements are displaced in the groove of the cylinder profile. 
   Apart from the necessity to effect the adjustment manually, a further disadvantage of an arrangement of this kind is seen in the fissured surface of the cylinder, resulting from the particular structure, which practically excludes the use of such an arrangement in areas where higher demands are placed on hygiene, for example in the foodstuff or pharmaceutical industries. 
   SUMMARY OF THE INVENTION 
   Thus, it is the object of the present invention to provide a position-measuring device for fluidic cylinder-and-piston arrangements which, in addition to permitting a technically simple way of adjusting the end or stop positions to be detected, can be easily integrated into a cylinder profile. 
   This object is achieved according to the invention by a position-measuring device for fluidic cylinder-and-piston arrangements. 
   By arranging a Hall sensor array comprising at least two Hall sensors, spaced one from the other in the direction of movement of the piston, and one coil whose magnetic field permits the switching points of the Hall sensors to be adjusted in response to the current that can be applied to the coil, it is rendered possible in a very advantageous way to not only select individual Hall sensors electronically, but also and especially to allow interpolation between the different Hall sensors. It is thus possible to keep the number of Hall sensors small while simultaneously guaranteeing very high position-sensing precision. 
   The Hall sensor array advantageously comprises a plurality of equally spaced Hall sensors preferably arranged on a common substrate. This arrangement improves the position-measuring accuracy. In addition, adjusting the system to the required switching position can be effected by suitably selecting, electronically, the respective Hall sensors in a manner that will be described hereafter in more detail. 
   Preferably, the Hall sensors are arranged linearly one behind the other and are surrounded by a frame-like coil of substantially rectangular shape (frame-type coil). With an electric current flowing through the frame-type coil, the coil produces a magnetic field which superposes on the magnetic field of the magnetic region of the cylinder and which works especially well with the Hall sensors. This provides the advantageous possibility to vary the magnetic flux density at the point of installation of the Hall sensors, as a function of the coil current, and to thereby achieve sort of an interpolation effect for the response behaviour between the different Hall sensors. 
   According to an especially favorable embodiment, the frame-type coil is arranged on one side of the substrate, whereas the Hall sensors are arranged on the other side thereof. This arrangement leads to a very compact structure of the Hall sensor array. 
   The Hall sensors may be selected in the most different ways. One advantageous embodiment provides that the Hall sensors are selected by means of a multiplexer and their control states are fed to a microcontroller. 
   Advantageously, the frame-type coil is supplied with a current, which is adjusted to a respective constant value by means of a current controller and which, in particular, is independent of the supply voltage and of the coil temperature. Preferably, the coil currents are predetermined by the microcontroller. 
   For determining the position of the piston that is to be measured, an advantageous embodiment of the invention provides that the position number of that Hall sensor, which is arranged the closest to the magnetic region of the piston, and the value of the respective coil current are stored in a memory of the microcontroller. 
   When the cylinder-and-piston arrangement is operated later, the microcontroller then determines the position number of the active Hall sensor as well as the relevant coil current, compares these values with the values stored in the memory of the microcontroller and, if the values are found to conform, emits an output signal. 
   A very essential advantage of this position-measuring device is seen in the fact that it can be adjusted electronically. Especially, no mechanical manipulations are required. Further, it is a considerable advantage that this position-measuring device, while its structure can be easily realized, allows the position of the piston of the cylinder-and-piston arrangement to be determined very precisely. 
   Further advantages and features of the invention will be apparent from the description that follows and from the representation of certain embodiments in the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a diagrammatic representation of a cylinder-and-piston arrangement with a position-measuring device according to the invention; 
       FIG. 2  shows a diagrammatic representation of the output signals of the Hall sensors of the Hall sensor array, across the cylinder movement; 
       FIG. 3  shows a diagrammatic representation of the structure of a Hall sensor according to the invention; 
       FIG. 4   a  shows a diagram representing the application of a current to the frame-type coil according to the invention; 
       FIG. 4   b  shows a diagram of the dependency of the current on the position of the piston; and 
       FIG. 5  shows a diagrammatic representation of a circuit arrangement used for defining the position to be detected and for detecting that position in operation of the cylinder-and-piston arrangement. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A cylinder-and-piston arrangement comprising a cylinder  10  and a piston  20  is represented diagrammatically in  FIG. 1 . A Hall sensor array, indicated by reference numeral  30 , which comprises a plurality of Hall sensors  32  arranged on a circuit board  31 , is provided in a wall of the cylinder. In the case of the cylinder-and-piston arrangement represented in  FIG. 1 , the Hall sensor array  30  is arranged in a cylinder wall. It is, however, understood that the invention is not limited to that arrangement, but that instead the Hall sensor array may also be arranged outside the cylinder wall or at a spacing from the latter, for example. 
   The Hall sensors  32  are positioned on the circuit board  31  linearly one behind the other and at substantially equal spacings one from the other along the direction of movement of the cylinder  20 . The cylinder head  20  comprises a magnetic region, for example in the form of a permanent magnet  21 . The Hall sensors  32  may be configured, especially, as Hall-effect switches that directly initiate a switching action every time a permanent magnet  21  occupies the position opposite a Hall-effect switch  32 . 
     FIG. 2  illustrates the transfer function of the Hall-effect switches  32  as a function of the position of the piston and of the permanent magnet  21  connected to it. Every time the permanent magnet  21  passes a Hall-effect switch  321 ,  322  . . .  32   n  a substantially rectangular signal pulse is released. The resolution achievable by the arrangement depends in this case on the number n of Hall-effect switches  32 . 
   In order to permit the number of Hall-effect switches  32  to be kept small, a substantially rectangular coil  40 , for example, is provided which encloses the Hall-effect switches  32  in the form of a frame, as illustrated in  FIG. 3 . The frame-type coil  40  may be formed, for example, from enameled copper wire arranged on one side of the substrate  31 , whereas the Hall-effect switches are arranged on the other side of the substrate  31 . When electric current flows through the coil  40 , it produces a magnetic field that superposes on the magnetic field of the permanent magnet  21  of the piston  20 . This provides the possibility to vary the magnetic flux density at the point of installation of the Hall-effect switches  32  via the value of the coil current, which in turn permits the response behaviour between the different Hall-effect switches  32  to be sort of interpolated and the resolution of the position-measuring effect of the piston  20  to be improved. 
     FIG. 4   a  shows a diagrammatic representation of the arrangement of the piston  20  and the frame-type coil  40 , to which a current I coil  is applied. When the permanent magnet  21  of the piston  20  is positioned at a distance x, for example, from the Hall-effect switch  32 , a substantially linear curve is obtained over that distance x for the current at which the Hall-effect switch  32  responds (illustrated diagrammatically in  FIG. 4   b ). 
   A measuring process will now be described with reference to  FIG. 5 . 
   When the piston  20  and, together with it, the permanent magnet  21  have been displaced to a position which is to be detected later, the coil current I coil  is increased by steps, starting at zero, until one of the Hall-effect switches  32  responds. The number of the Hall-effect switch, indicated in  FIG. 5  by 1 to n, and the relevant coil current I coil  then clearly represent the position of the cylinder piston  20 . 
   The entire measuring process is controlled by a microcontroller  50 . The microcontroller  50  comprises, in addition to a program storage  51 , a data storage  52  and a non-volatile data storage medium  53 , for example an EEPROM. 
   The Hall-effect switches  32  are selected through a multiplexer  60  controlled by the microcontroller  50 , and their control states are read in sequentially. 
   A current controller  70  is provided for supplying the frame-type coil or interpolation coil  40 , which controller keeps the coil current I coil  constant irrespective of the supply voltage and the coil temperature. The value of the coil current I coil  is selected via a pulse width-modulated output of the microcontroller  50 , by initially integrating a PWM signal (pulse width-modulated signal) via a low-pass filter  80  comprising a capacitor  81  and a resistor  82 . The microcontroller  50  as such is supplied with an operating voltage from a power supply unit  55 . 
   An operator interface  90 , comprising an operating key  91  and two display means, for example light-emitting diodes  92  and  93 , is provided as means for operating the arrangement. For reporting or outputting the position of the piston  20  two transistor switching outputs  101 ,  102  are provided, for example. 
   The program stored in the program storage  51  of the microcontroller  50  comprises essentially two main blocks. The first main block serves to set the switching points. The corresponding adjustment is made as follows: Once an operator has pressed the operating key  91 , the microcontroller  50  enters a setting mode. The piston rod  20  and, together with it, the permanent magnet  21  mounted thereon must then be displaced to the position for which the respective switching output is to emit a signal. The microcontroller  50  then determines the number n of the active Hall-effect switch  32  and the value of the respective coil current I coil . The two values are stored in the non-volatile data storage, the EEPROM  53  of the microcontroller  50 . 
   The second main block is devoted to the measuring operation which proceeds as follows: The microcontroller  50  continuously determines both the number n of the active Hall-effect switch  32  and the relevant coil current I coil  and compares the values so determined with the values stored in the EEPROM  53 . If the values are found to conform, the output transistor  101  or  102  associated to the respective switching point is connected through. 
   The compact design of the Hall sensor array  30  permits the latter to be installed as a single unit in a wall or in the profile of the cylinder  10 . The position can be determined without any mechanical setting operations, for example without any displacement of the Hall-effect switches, or the like. As a result, the cylinder  10  may have an absolutely smooth outer surface, which makes it suited, for example, for use in the foodstuff and pharmaceutical industries. In addition, start-up times can be clearly reduced due to the electronic starting-up process and the particular way of setting the switching points which does not require any mechanical setting operation. 
   A particular advantage lies in the fact that the Hall sensor array  30  described above requires only a limited small number of Hall-effect switches  32 . This is so because the switching points of the Hall-effect switches  32  can be adjusted through the frame-type coil or interpolation coil  40 , via the coil current I coil , so that, theoretically, infinite resolution can be realized. The only action necessary consists in increasing the coil current I coil  until the Hall-effect switch  32  responds. 
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.