Position sensor system

Abstract: A magnetic position sensor system including a magnetic medium (20) divided into a plurality of areas (42, 44, 46) of magnetization for producing a magnetic field with a variable intensity. The magnetic medium (20) includes a first side, a second side and a longitudinal axis (25). A Hall cell (50) is used for detecting the intensity of the magnetic field produced by the magnetic medium (20) and for producing an electric signal which corresponds to the intensity detected. The magnetic medium (20) is movable with respect to the Hall cell (50) so that the Hall cell scans along the first side of the medium along the longitudinal axis (25) and detects the intensity of the magnetic field at any given point along the longitudinal axis of the medium.

Description: BACKGROUND OF THE INVENTION The invention relates to a position sensor system comprising: means for producing a magnetic field with an intensity that varies along a longitudinal axis of said means; sensing means for detecting the intensity of the magnetic field produced by the magnetic field producing means; means for producing an electric signal which corresponds to the intensity detected; and means for moving the magnetic field producing means with respect to the detecting means along the longitudinal axis so that the detecting means detects the intensity of magnetic field at any given point along the longitudinal axis of the magnetic field producing means; the magnetic field producing means comprising a magnetic medium having a plurality of longitudinally extending magnetic strips having respective geometries which vary with position along the longitudinal axis of the medium, adjacent ones of said magnetic strips having opposite polarities, the magnetic medium having a first side and a second side, the detecting means being arranged such that it scans along the first side of the medium. Position sensors of all types are commonly known in the art, especially in the automotive and industrial environments. Position sensors determine the location of a movable part with respect to a reference. Such sensors range from complex electrical devices to simple mechanical devices. The prior art includes position sensing devices adapted for many different applications as well as position sensors specifically adapted for automotive suspension systems. Adjustable suspension systems typically utilize one or more sensors for detecting the position of one or more parts of the suspension system with respect to either another part of the suspension system or another part of the chassis of the vehicle. There are many problems with the sensors available today. Sensors which provide only discrete signals can resolve a desired position into only a relatively small number of possibilities. Analog position sensing devices usually have a relatively short stroke or measuring distance. A position sensor of the kind set forth in the opening paragraph is disclosed in US-A-3 838 263. In the known sensor, the magnetic strips have respective, mutually opposing polarities and develop a composite magnetic field comprising respective component magnetic induction fields corresponding to these polarities. Depending on its position relative to the magnetic strips, the sensing means is penetrated in varying degrees by positive and negative induction. As a result of these penetrations, the sensing means produce an output signal that depends on the said relative position. Other position sensors are known from, for example, GB-A-2 056 692, GB-A-2 156 994, US-A-3 473 109 and DE-A-3 144 283. SUMMARY OF THE INVENTION It is an object of the invention to provide a simple, low cost position sensor that can be adapted for an automotive suspension system environment and in which the relation between the output signal of the sensing means and the position of the sensing means relative to the magnetic field producing means is particularly simple and well-defined. The position sensor system of claim 1 according to the present invention is characterized in that the number of magnetized strips is three, and in that the position sensor further comprises flux guides placed in close proximity to the sensing means for guiding magnetic flux of the magnetic field through the sensing means. In that sensor there is a central strip of one polarity that is flanked by two further strips having the opposite polarity. This novel geometry of the magnetized strips in combination with the provision of the flux guides has for its result that the magnetic lines of force at the location of the sensing means always follow the same, well-defined path, regardless of the relative position of the field producing means and the sensing means. Only the intensity of the magnetic field at the location of the sensing means varies monotonously as a function of this relative position. A preferred embodiment (claim 2) is characterized in that the variable intensity of the magnetic field along the longitudinal axis of the magnetic medium is linear. Thus, the present invention provides a linear output over a relatively long stroke. A linear output over a long stroke allows for instant error correction and position alliance. Thus, the present invention allows for instant computation of how much compensation is needed and in what direction in order to correct and arrive at an equilibrium position. The present invention is particularly suitable for an automotive suspension system. A automotive suspension sensor system for a vehicle can be constructed with the magnetic position sensor of the present invention. The magnetic medium is movable with respect to the sensing means. Thus, either the magnetic medium or the sensing means actually moves so that the sensing means scans along the longitudinal axis of the magnetic medium and detects the variable magnetic intensity produced by the magnetic medium. The signal representing the position of the magnetic medium relative to the magnetic field sensing means along the longitudinal axis of the magnetic medium is linear. An iron back up piece may be located behind the magnetic medium in order to enhance the magnetic field. The sensor system can be connected at one end to a stationary member of the suspension system and at another end to a movable part of the suspension system. The sensor is also connected to a central processing unit for calculations and error correction. The magnetic sensor of the present invention is simple and inexpensive to produce. It is adaptable for automotive suspension systems of most car models, especially inexpensive models. The present invention allows for smooth transitions for reactions to bumps and manoeuvring of a vehicle for most road and traffic conditions. The sensor according to the present invention is useful in providing suspension position information and ride height information for vehicle levelling systems or adjustable suspension systems. Other objects, features and advantages of the invention will become apparent from the following detailed description, appended claims and accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING The invention will now be described in detail with respect to several preferred embodiments with reference to the accompanying drawing, wherein; Fig. 1 is a plan view of a housing for a vehicle sensor system in accordance with the invention; Fig. 2 is a side view of the housing of the sensor system of Fig. 1; Fig. 3 is a plan view of the system shown in Fig. 1 with a movable part retracted within a stationary part shown in phantom; Fig. 4 is a plan view of the sensor system of Fig. 1 with the movable part extended; Fig. 5 is a cross-sectional side view of the movable part of the sensor system of Fig. 1; Fig. 6 is a perspective view of the Hall cell between the flux guides as arranged within the housing of the system of Fig. 1; Figs. 7a-7c are cross-sectional front views taken across the line 7-7 of Fig. 5 for different magnetic field intensities; Figs. 8a-8c are plan views of a magnetizing fixture at different angles in accordance with the invention; Fig. 9 is a circuit diagram for the sensor system of Fig. 1; and Fig. 10 is a chart that shows the linear output of the circuit diagram of Fig. 9 for the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a top view of a housing of a sensor system 10 in accordance with the present invention. The housing includes a stationary part 12 and a telescopically movable part 18. The telescopically movable part 18 is located in a first end 14 of the stationary part and is extendable as shown by the dotted lines of Fig. 1. Fig. 2 shows a side view of the housing of Fig. 1. One end of the movable part 18 has connection means 22 for connection to a movable part of a suspension system (not shown). One end 16 of the stationary part 12, shown opposite to the first end 14, has connection means 24 for connection to a stationary part of the suspension system. The positioning of the connection means in regard to the sensor system 10 and the suspension system can vary as long as the movable part 18 of the sensor system is connected to a movable part of the suspension system and the stationary part 12 of the sensor system is connected to a stationary part of the suspension system. A magnetic medium 20 is located on the movable part 18 as shown in Fig. 4 and by dotted lines in Fig. 3. The magnetic medium 20 can be a plastic bonded magnet adaptable for automotive applications. Fig. 3 shows the magnetic medium 20 and most of the movable part 18 retracted within the stationary part 12 of the housing and Fig. 4 shows the magnetic medium 20 and most of the movable part 18 extended outside of the housing. The movable part 18, with the magnetic medium 20, telescopically retracts in and extends out of the stationary part 12 of the housing. A sensing means 50 is located within the first end 14 of the stationary part 12 so that the magnetic medium 20 is scanned by the sensing means 50 as it travels in and out of the stationary part 12. The sensing means 50 is a linear Hall cell but can be any other suitable type of magnetic flux to voltage transducer component. The Hall cell 50 can be of a known type which provides a change in voltage output which varies in magnitude in accordance with variations of magnetic field strength or magnetic flux detected by the Hall cell 50. On two sides of the Hall cell 50 are flux guides 60, 65 to concentrate and guide the magnetic flux through the Hall cell 50. The flux guides 60, 65 are made of a ferrous material. Figs. 3 and 4 show the magnetic medium 20 divided into a plurality of areas of magnetization. Specifically, three areas of magnetization of particular geometric shapes form a magnetic field intensity that varies linearly along a longitudinal axis 25. The areas of magnetization may be different than the geometries and polarities shown in Figs. 3 and 4 as long as the magnetic field intensity varies so that positions are measurable over a relatively long stroke. A first area 42 of magnetization and a third area 46 of magnetization are south poles with triangular shaped geometries disposed oppositely to one another. Between the two south poles 42, 46 is an offset rectangular shaped north pole area 44. Alternatively, the pole areas could be arranged in any way in which a linearly varying output could be detected. Fig. 5 shows a cross-sectional view of the movable part 18 of the sensor system 10. A thin sheet of plastic material 72 is attached on top of the magnetic medium 20 as a protective coating. Alternatively, a magnet assembly could be injection molded into the movable part 18 using the same polymer as the movable part 18. An iron sheet 70 or other ferrous material is attached behind the magnetic medium 20 so that a concentrated magnetic return path is created. Fig. 6 shows the Hall cell 50 arranged between two flux guides 65,60. As the magnetic medium 20 is moved in and out of the stationary part 12 the magnetic field and flux around the Hall cell 50 varies. When the magnetic medium 20 is all the way extended the magnetic flux is as shown in Fig. 7a. When the magnetic medium 20 is pushed half way into the stationary part 12 the magnetic flux at the Hall cell 50 cancels itself and there is no net magnetic flux through the Hall cell 50 as shown by Fig. 7b. When the magnetic medium 20 is pushed all the way into the stationary part 12 the magnetic flux is as shown in Fig. 7c, which is the reverse of Fig. 7a. The Hall cell 50 provides an output signal which varies in magnitude in accordance with various magnetic fields or magnetic flux detected. The magnetic field intensity varies linearly along the longitudinal axis 25 because of the plurality of areas of magnetization arranged to have a linear output for a relatively long stroke. The area of magnetization can be polarized as shown in Figs. 8a-8c. A magnetizing fixture 75 is placed over the magnetic medium 20 at particular angles in order to create the desired pattern of magnetic areas. The magnetic fixture 75 is placed at an angle of 1 degree from the longitudinal axis 25 as shown in Fig. 8a, at 2 degrees in Fig. 8b, and at 4 degrees in Fig. 8c. The larger the angle with respect to the longitudinal axis 25, the steeper the slope of the linear output. Alternatively, the magnetizing fixture could be modified for functions other than linear, i.e., sine, cosine, exponential. Fig. 9 shows a circuit diagram for the sensor system 10 of the present invention. A constant 5 volts is supplied to the Hall cell 50 by a 5 volt line regulator. The Hall cell 50 outputs a control signal based upon the magnetic field across the Hall cell 50. The control signal is amplified and a power control signal is created to drive a 300 ohm load. Fig. 10 shows the output from the circuit of Fig. 9. The x-axis shows the position of the Hall cell with respect to the longitudinal axis in mm and the y-axis shows the corresponding output in milliamps. While the invention has been described and illustrated in connection with preferred embodiments, many variations and modifications as will be evident to those skilled in this art may be made therein without departing from the invention, and the invention as set forth in the appended claims is thus not to be limited to the precise details of construction set forth above as such variations and modifications are intended to be included within the scope of the appended claims.

Claims: A position sensor system comprising: means (20) for producing a magnetic field with an intensity that varies along a longitudinal axis of said means; sensing means (50) for detecting the intensity of the magnetic field produced by the magnetic field producing means; means for producing an electric signal which corresponds to the intensity detected; and means (18) for moving the magnetic field producing means with respect to the detecting means along the longitudinal axis so that the detecting means detects the intensity of magnetic field at any given point along the longitudinal axis of the magnetic field producing means; the magnetic field producing means (20) comprising a magnetic medium having a plurality of longitudinally extending magnetic strips (42,44,46) having respective geometries which vary with position along the longitudinal axis of the medium, adjacent ones of said magnetic strips having opposite polarities, the magnetic medium having a first side and a second side, the detecting means being arranged such that it scans along the first side of the medium, characterized in that the number of magnetized strips (42,44,46) is three, and in that the position sensor further comprises flux guides (65,60) placed in close proximity to the sensing means (50) for guiding magnetic flux of the magnetic field through the sensing means. A sensor system as claimed in Claim 1, characterized in that the variable intensity of the magnetic field along the longitudinal axis of the magnetic medium (20) is linear. A sensor system as claimed in any one of the preceding Claims characterized in that the magnetic medium (20) is a plastic bonded magnet. A sensor system as claimed in any one of the preceding Claims, characterized in that it further comprises an iron backup piece (70) extending along the second side of the magnetic medium (20) for creating a magnet return path.

Inventors: RIGGS WILLIAM DONALD, ROSSHIRT HERMANN, RIGGS, WILLIAM DONALD, ROSSHIRT, HERMANN, Riggs, William Donald, c/o INT. OCTROOIBUREAU B.V., Rosshirt, Hermann, c/o INT. OCTROOIBUREAU B.V.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: G01D5
IPC codes: G01D5, G01B7