Abstract:
A linear sensor is described with a casing ( 5 ), with a pin ( 7 ) which contains a permanent magnet ( 1 ) and which is mounted in the casing ( 5 ) in a linearly displaceable manner against the force of a spring ( 6 ), and with a magnetic field sensor ( 3 ) attached to said casing ( 5 ) for detecting of a displacement of the permanent magnet ( 1 ). The invention provides that the pin ( 7 ) is controlled by a first cylindrical guide ( 8 ) and a second cylindrical guide ( 8 ).

Description:
The invention relates to a linear sensor, comprising a sleeve, a pin which contains a permanent magnet and is supported in the sleeve so it can be linearly displaced against to the force of a spring, and a magnetic field sensor that is attached to the sleeve for detecting a displacement of the permanent magnet. 
     Such linear sensors are used in motor vehicle technology, for example, for controlling brake boosters, the activation of which is carried out as a function of the travel of a brake pedal. As the pedal travel increases, the brake pressure acting on a control member is increased, the travel of which can be determined by way of a linear sensor. Another example is boost-pressure controllers for exhaust gas turbochargers. In such applications, in general the pin of the linear sensor is pressed against a movable measurement object, such as a control member, by a spring force, so that the pin always rests against the measurement object and follows the movement thereof. 
     A linear sensor is known from DE 196 24 233 C1, wherein a cuboid permanent magnet is displaced relative to a magnetic field sensor disposed in a stationary manner on a housing. The known linear sensor, however, is less suited for applications in motor vehicle technology, because it is sensitive to vibrations which may develop due to engine operation, is prone to wear, and requires high manufacturing costs and frequent adjustments. 
     It is therefore an object of the invention to provide a way for creating a linear sensor which is suited for motor vehicle technology, is cost-effective to produce, and enables precise measurements, even if vibrations occur during driving operation. 
     SUMMARY OF THE INVENTION 
     In a linear sensor according to the invention, highly precise guidance of the pin is achieved using very simple means, preferably in that a constriction of the sleeve forms the first guide, which is supplemented by a second guide formed by a thickening of the pin. However, it is also possible to design the first and second guides each by a constriction of the sleeve. 
     Transverse movements of the pin resulting in measurement errors can be largely excluded with a linear sensor according to the invention. The position of a measurement object can therefore be precisely determined even if vibrations occur, such as those caused by operation of the engine, when using a linear sensor according to the invention. 
     Because the pin and the sleeve only come in contact with each other on the two guides, which is preferred, advantageously small friction surfaces are created. Small friction surfaces produce low friction forces, thereby enabling low-wear operation and high long-term stability. 
     The constriction is preferably located at an end of the sleeve, particularly at the end of the sleeve from which the pin protrudes. This measure has the advantage that a two-point guidance caused by the constriction of the sleeve and the thickening of the pin can take place over the largest distance possible and therefore with the highest precision possible. For the same reasons it is also preferred that the thickening is located at an end of the pin. 
     The magnetic field sensor is preferably attached to a section of the sleeve forming the constriction. This measure has the advantage that the distance between the magnetic field sensor and the pin, measured perpendicular to the direction of movement of the pin, remains constant, even under engine-related vibrations, and can therefore always be measured with high accuracy. 
     The pin preferably has a circular cross-section, which simplifies production and is favorable for low friction. It is particularly preferred if the pin can be rotated about the longitudinal axis thereof in the sleeve. During the operation of a vehicle, vibrations or movements of the measurement object may cause a torque to be applied on the pin. Because the pin can be rotated, such torques can relax without applying stress on the guidance of the pin. 
     According to an advantageous refinement of the invention, the pin and/or the sleeve are made of a graphite particle-containing plastic. In this way, abrasion, which is inevitable during operation, ensures a continuous supply of lubricant due to the release of graphite particles. Even after long operation, in this way low friction forces are always ensured, so that an accordingly designed linear sensor can be operated stably over very long periods. It is particularly preferred that the pin is made of a graphite particle-containing plastic, in particular that the sleeve is made of a graphite particle-free plastic. 
     According to another advantageous refinement of the invention, the sleeve and the pin are made of the same plastic, apart from an optionally present addition of graphite particles. In this way, the thermal coefficient of expansions between the sleeve and pin agree, resulting in consistently precise guidance over a very wide temperature range. Thermoset materials are the preferred materials for the sleeve and the pin. 
     According to a further advantageous refinement of the invention, the pin is rounded off at the end thereof protruding from the sleeve. This measure has the advantage that the risk of tilting the measurement object, which could impair the measuring accuracy, is reduced when pressing the pin against a measurement object. 
     At the end protruding from the sleeve, the pin preferably has a face, the diameter of which is 20% to 60% of the inside diameter of the sleeve at the constriction. A face that is reduced with respect to the pin diameter causes the transmission of transverse forces to be reduced in the event that the measurement object is tilted. As a result, the guidance of the pin in the sleeve is subjected to lower stress, and transverse movements impairing the measuring accuracy can be prevented more easily. A face having a diameter that is 20% to 60% the diameter of a cylindrical pin section supported in the sleeve is also large enough to limit the surface load of the contact surface resting against the measurement object to a degree that is suitable for low-wear operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the invention will be explained based on an exemplary embodiment with reference to the attached figure. The characteristics described can become subject matters of claims either individually or in combination with each other. 
         FIG. 1 : shows an embodiment of a linear sensor in a longitudinal section. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a linear sensor comprising a sleeve  5 , in which a cylindrical pin  7  is supported so it can be linearly displaced against the force of a spring  6 . The pin  7  comprises a permanent magnet  1 , which preferably is a bar magnet, and rests against a measurement object  2  with the end  4  thereof protruding from the sleeve  5 . If the distance of the measurement object  2  from the sleeve  5  increases, the pin  7  is pushed further out of the sleeve  5  by the spring  6 . As the measurement object  2  approaches the sleeve  5 , the pin  7  is pushed further into the sleeve  5  against the force of the spring  6 . 
     A magnetic field sensor  3 , preferably a Hall sensor, is attached to the sleeve  5 . A displacement of the pin  7  comprising the permanent magnet  1  disposed therein brings about a change in the magnetic field at the site of the magnetic field sensor  3 . By evaluating this change, the position of the permanent magnet  1  and hence that of the measurement object  2  can be determined. 
     The pin  7  comes in contact with the sleeve  5  at two guiding surfaces  8 . A first guiding surface  8  is formed by a constriction of the sleeve  5 , which is preferably located at the end  4  of the sleeve  5  with which the pin  7  protrudes from the sleeve  5 . A second guiding surface  8  is formed by a thickening of the pin  7 , preferably at the end thereof facing away from the measurement object  2 . In this way, the constriction of the sleeve  5  forms a first guide, which is supplemented by a second guide formed by the thickening of the pin  7 . In this way, very precise and low-friction guidance is achieved. 
     The pin  7  and the sleeve  5  come in contact with each other in a two-dimensional manner in the region of the guiding surfaces  8 , which further increases the precision of axial and radial guidance. For this purpose, the constriction is designed as a section of the sleeve  5  which has a reduced inside diameter, and the thickening of the pin is cylindrical, so that the guiding surfaces  8  in each case are cylindrical surfaces. It is advantageous to attach the magnetic field sensor  3  to the section of the sleeve  5  forming the constriction, because there the distance of the pin  7  from the magnetic field sensor  3  perpendicular to the direction of movement of the pin  7  remains constant with the highest accuracy, even if vibrations occur. 
     The spring  6  acts on an end of the pin  7 , this being the end having the thickening. In the embodiment illustrated, the thickening has a sleeve-like extension, into which the spiral coiled spring  6  protrudes. The spiral coiled spring  6  is supported on an abutment, which is not shown and which is preferably disposed outside of the sleeve  5 , but in principle can also be formed directly by the sleeve  5 . 
     At the end  4  protruding from the sleeve  5 , the pin  7  is rounded off and has a flat face, the diameter of which is 50% of the inside diameter of the sleeve  5  at the constriction thereof in the illustrated embodiment. Due to this geometric design, not only a sufficiently large contact surface for a low surface load is achieved, but also a low-wear operation. In addition, a reduced support surface minimizes friction, and the transmission of transverse forces during potential tilting of the measurement object  2  is prevented. In general, it is advantageous if the face has a diameter of approximately 20 to 60% of the inside diameter of the sleeve  5  at the constriction. 
     The pin  7  is produced by insert molding the permanent magnet  1  using a thermoset material. The sleeve  5  is also made of a thermoset material, preferably the same thermoset material. In this way, the pin  7  and the sleeve  5  have similar thermal expansion coefficients, whereby jamming and clearance are prevented in the region of the guiding surfaces  8  within a wide temperature range. The illustrated linear sensor can thus be installed in particular in the engine compartment of a motor vehicle and heat up to 170° C. during operation without impairment. 
     In order to improve the sliding properties, it is preferable to use a thermoset material having added graphite particles for the pin  7 . The added graphite particles reduce the hardness of the thermoset material. In this way, inevitable abrasion during operation will occur practically exclusively at the pin  7 , but not at the sleeve  5 . Because the abrasion comprises a graphite filler, the abrasive dust acts as a lubricant in the guiding surfaces  8 . This advantage can also be used in that the sleeve  5 , instead of the pin  7  or in addition to the pin  7 , is produced from graphite particle-containing plastic. 
     However, it is particularly advantageous if the thermoset material of the pin  7  comprises graphite particles. Abrasion occurring at the face  4  then reduces friction and therefore lowers the transversal forces, which can develop during tilting of the measurement object  2 . In principle, it is also possible to use graphite particle-containing plastic for both the sleeve  5  and the pin  7 . Due to the increased abrasion, however, this is not preferred. 
     REFERENCE NUMERALS 
     
         
           1  Permanent magnet 
           2  Measurement object 
           3  Magnetic field sensor 
           4  End 
           5  Sleeve 
           6  Spring 
           7  Pin 
           8  Guiding surface