Patent Publication Number: US-2011062949-A1

Title: Shielded position sensor for translationally moving parts

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 11/806,414 filed on May 31, 2007, which claims priority under 35 U.S.C. §119 to Swiss Application 00882/06 filed in Switzerland on Jun. 1, 2006, the entire contents of which are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to a sensor for monitoring a part which can move translationally relative to a sensor with reference to its position on the axis of motion, and to a cylinder with such a sensor for monitoring the position of the piston with respect to its axis of motion. The disclosure relates especially to a safety position sensor for a pneumatic or hydraulic cylinder and such a cylinder with the position sensor. 
     BACKGROUND INFORMATION 
     Monitoring the position of a piston which is provided with a magnet using a reed contact which can be influenced by the magnet is known. This is often achieved with magnetic rings around the piston and one or two reed switches which act in parallel on the cylinder. The cylinder consists of nonmagnetic material for this purpose. 
     These reed contacts can moreover be caused to close by magnets attached from the outside when they should be open due to the position of the cylinder. This should be avoided for safety reasons. Moreover unintended movement of the sensor along the axis of motion of the cylinder should be prevented to allow calibration of the position of the sensor. 
     SUMMARY 
     A safety sensor is disclosed which indicates a safe position of a movable part only when this part is in the safe position. It should not be able to be influenced by external magnet fields to indicate a safe position of the cylinder when it is in an unsafe position. Thereby, the sensor can be positioned and safe fixing of the sensor is possible for example on a cylinder in this position using technically simple means. 
     A sensor for monitoring a part which can move translationally relative to a sensor with reference to its position on the axis of motion can be actuated by means of a magnet which is oriented in the direction of motion with respect to the north and south pole and which is located on the movable part. An exemplary embodiment has three reed contacts which are arranged in parallel and of which the first and the second reed contact are located in one plane. They are located parallel next to one another with their lengthwise axes. By this arrangement they can be actuated almost at the same time by the magnetic field of the magnet which is located on the movable part. The third reed contact is located at a distance to the plane of the first and the second reed contact. This sensor, with the third reed contact located at a greater distance to the actuating magnet than the other two reed contacts, is safe against manipulation by external magnetic fields. An external magnetic field applied from the outside will always first actuate the third reed switch which is nearer the magnet which has been applied from outside. It is almost impossible not to actuate the third reed contact located near the first two reed contacts when the first and second reed contact are actuated with an outside magnetic field. In order to actuate the first two reed contacts without actuating the third reed contact, it is necessary to achieve a drop in the magnetic field as large as possible between the position of the first two and the position of the third reed contact. This is essentially only possible when the field magnet is near the first two reed contacts, and therefore at the position which is already intended for the field magnet. Moreover fine tuning of the sensitivities of the reed contacts and of the strength of the magnetic field which has been generated by the magnet on the movable part is necessary. 
     This sensor is especially suited to detecting the position of a piston in a cylinder. In this connection the sensor is located on the cylinder which consists of nonmagnetic material, for example aluminum, and the piston bears a magnet, e.g., a magnetic ring around the piston. 
     The third reed contact is advantageously at most as sensitive as the two identically switching reed contacts which are arranged in parallel, so that it does not respond only at a stronger magnetic field than this. Thus the distance to the field magnet is the deciding factor. 
     In order to make the range for a sensor signal more precise and to make the boundary region between the position of the magnet which excites the sensor signal and the position which does not excite a sensor signal as short as possible, on one end of the two identically switching reed contacts, e.g., on both ends of the reed contacts, the sensor is provided with a shield, especially an iron part. This shield distorts the magnetic field of the field magnet so that the magnetic field is deflected by the material of the shield away from the region of the reed contacts. Starting from a certain position of the magnet the magnetic field jumps with one pole out of the shield and with the other pole into the shield, i.e. when mainly the field lines in the north have been deformed by the shield until then and the field lines in the south have remained essentially undeformed, the deformation jumps suddenly onto the field lines of the south pole, while the field lines of the north pole remain largely undeformed. In this way a more distinct signal is achieved. 
     This jumping can also be achieved with bilateral shielding, all the field lines being deformed. The field lines which have been acquired by the first shield are however suddenly acquired by the other shield, the first shield immediately acquiring the field lines of the other pole. 
     The bilateral shielding has especially the advantage that backward installation of the sensor is precluded. Alternatively there can be unambiguous identification of the sensor and the position of the shield. 
     For simple installation of the sensors which cannot be moved by vibrations and unintentional manipulation, the sensor is provided on opposing ends with a mounting tail. This mounting tail prevents movement of the sensor away from the end of the tail so that with two opposing mounting tails movement in both directions is prevented. 
     In order to easily attach the mounting tails there are advantageously two catch heads. The mounting tail is provided with teeth and the catch head is provided with a catch mechanism which engages the teeth. The teeth and the catch mechanism in interaction allow the catch heads to be slipped on, but prevent the catch head from being pulled off once it has been slipped on. These teeth and catch mechanisms are known from cable links. 
     It can be provided that the catch mechanism can be released from active connection with the mounting tail by means of a tool. 
     Simple and reliably immovable positioning of the sensor for monitoring a part which can move translationally relative to the sensor with reference to its position on the axis of motion which can be actuated by means of a magnet located on the movable part can be achieved in that the sensor is provided on the opposing ends with a mounting tail and that there are two catch heads. Each mounting tail is provided with teeth and each catch head is provided with a catch mechanism which engages the teeth, which in interaction allow the catch heads to be slipped on the mounting tail, but prevent the catch heads from being pulled off once a catch head has been slipped on. The catch heads have a stop which after mounting of the sensor, for example on a cylinder, presses against the stop surface of the cylinder so that the sensor is prevented by one mounting tail from moving in one direction, and with the other mounting tail from moving in the opposite direction. 
     For fine calibration, a stop of the catch heads can be connected via a thread to the catch mechanism of the catch heads, which thread extends in the lengthwise direction of the mounting tail. For the sake of safety the stops can be captively connected to the catch mechanism. Twisting of the stops around the thread axis should not be able to lead to release of the catch head from the mounting tail. 
     Alternatively to the version with two mounting tails on the sensor and two catch heads which interact with them, the sensor can also be made with two catch mechanisms which engage oppositely, and fixing of the sensor can take place by insertion of the mounting tails which are provided with stops and teeth. They are inserted into the two catch mechanisms from both sides. Advantageously the mounting tails are made such that they have room next to one another in a groove which is made for example on the cylinder and can therefore overlap in the lengthwise direction. 
     Formulated somewhat more generally, the sensor is characterized by a fastening means which is made interacting with the sensor or connected to the latter. It prevents shifting of the sensor, especially in the direction of motion of the piston. The fastening means connects the sensor via, e.g., a detachable form-fit to a carrier for the sensor. The carrier is mainly the wall of the cylinder. There can be a spring means for maintaining the form-fit. The spring means are captive and reliably maintain the form-fit, for example between the mounting tail and catch lip, or between a tooth and a recess which has been pressed into the surface of the carrier. 
     The sensor can be provided with a fastening means which has at least one tooth which can be pressed into the surface of a groove which holds the sensor. A depression for the form-fit with the tooth can be achieved by this pressing in. But alternatively there can also be for example edging in the surface of the carrier which has a series of recesses. In this case the tooth engages one of these recesses of the edging. 
     The fastening means advantageously encompasses the sensor body. In this way it is likewise positively connected to the fastening means. 
     The fastening means can thus be a spring element which has been detached from the sensor. This has the advantage that the fastening means can be produced in one piece from for example a spring steel strip which is made to surround the sensor body. Advantageously there are at least two teeth which can be caused to engage the surface of the groove in the carrier. On this spring element moreover a flange  69  is advantageously formed and fits into the undercut of a groove which has been undercut in a T-shape. Thus the spring element simply together with the sensor can be pushed in the lengthwise direction into and out of the groove. 
     The disclosure also relates to a cylinder with a piston in which on the piston there is a magnet which is pointed with the polarity in the direction of motion of the piston. On the cylinder there is a sensor for monitoring the position of the piston in the cylinder. The sensor can make the monitoring of the piston position reliable. It ensures safety in that the two reed contacts have to respond at the same time to the magnetic field of the magnet and in that the third reed contact should not respond. The third reed contact due to its greater distance from the magnet and piston is in a closer position to the external magnetic field acting from the outside. It therefore always switches before the two inner reed contacts when an external magnetic field which is acting from the outside is strong enough to influence the sensor. Since the third reed contact opens when the other two close (or vice versa), a safe position of the piston cannot be simulated by the external magnetic field. 
     On the cylinder a groove is advantageously formed in which the sensor is movably supported in the direction of motion of the piston. The sensor can be inserted into this groove from the outside and can be moved into the desired position along the direction of displacement which is possible in it. 
     The sensor is advantageously made as a cylindrical part (but a non-axially symmetrical and therefore non-circularly cylindrical part, but can be as a right parallelepiped which is made symmetrical to the mirror plane), with a cylinder axis which is parallel to the lengthwise direction of the reed contacts. 
     Due to the deviation from axial symmetry the cross section of the sensor perpendicular to its cylinder axis is made such that the sensor can be pushed into the groove in the cylinder only in a single rotary position with respect to its cylinder axis. Thus insertion of the cylinder in an incorrectly aligned position can be made impossible. Alternatively (or additionally) marking of the sensor is possible which makes incorrect insertion of the sensor into the groove obviously detectable. 
     In the known manner a magnetic ring is formed around the piston so that the position always has the same effect on the sensor regardless of its rotary position around the piston axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is detailed below using the examples shown in the figures. 
         FIG. 1  shows in a perspective and schematic the basic module of an exemplary sensor, 
         FIG. 2  shows a view of a sensor which has been potted in a sensor body, 
         FIG. 3  shows a perspective sketch of a first exemplary embodiment of the sensor with two mounting tails, 
         FIG. 4  shows a perspective sketch of a catch head for locking interaction with a mounting tail, 
         FIG. 5  shows a lengthwise section through a cylinder with an exemplary sensor and a field magnet on the piston, in which the piston is in an unsafe position, 
         FIG. 6  shows a cross section through the cylinder as shown in  FIG. 5 , 
         FIG. 7  shows a lengthwise section through the cylinder in which the piston is in a safe position, 
         FIG. 8  shows an extract from a lengthwise section through a cylinder with a second exemplary embodiment of a sensor without a mounting tail, with the piston in a safe and in an unsafe position, one field line at a time being shown which is deformed by the shielding which is present in the sensor. 
         FIG. 9  shows a view of a cylinder in the direction of the cylinder axis of the sensor and the axis of motion of the piston, with a T-shaped groove and an exemplary sensor in it. 
         FIG. 10  shows an extract from  FIG. 9  with the sensor provided with a locking means in the groove. 
         FIG. 11  shows a perspective sketch of a sensor and a spring element for attaching the sensor in a groove which has been undercut in a T-shape. 
         FIG. 12  shows a top view of another exemplary embodiment of such a spring element. 
         FIG. 13  shows four circuit diagrams which enable detection of the sensor state with two electrical lines. 
     
    
    
     DETAILED DESCRIPTION 
     The heart of the sensor  11  shown in  FIG. 1  is comprised of a printed board  13  and the first two reed contacts  15 ,  16  and a third reed contact  17 . The first two reed contacts  15  and  16  are make contacts which are arranged in parallel. The third reed contact  17  is connected in series to the first reed contact  15  and is a break contact. 
     The first two reed contacts are located next to one another underneath the printed board  13  in a common plane such that the terminal points for the reed contacts are arranged in a rectangle. The third reed contact  17  is located above the printed board  13  and parallel to it. Its terminal points lie on the two sides of the rectangle formed by the terminal points of the first two reed contacts  15 ,  16 . 
     In the plane of the first two reed contacts  15  and  16  there is a metal rod as magnetic shielding  19 . The three-dimensional execution of the metal rod in the example is aligned perpendicular to the lengthwise direction of the reed contacts  15  and  16 . The width of the metal rod is pointed in the lengthwise direction of the reed contacts  15  and  16 . In this way the shielding is located as much as possible at a distance to the third reed contact and effectively shields the first two reed contacts. 
     These aforementioned parts are potted in the plastic sensor body  21  which is shown in  FIG. 2 . This sensor body  21  has a T-shaped cross section. Within the upright of the T is the third reed contact  17 , in the crosspiece of the T are the printed board and the first two reed contacts  15  and  16 , and if present, the shielding  19 . The sensor body  21  is matched in its shape to the receiving means which is designed to hold the sensor. The illustrated cylindrical shape with the T-shaped cross section is suited to arranging the sensor to be able to move lengthwise in a groove which is made undercut in a T shape. 
     The sensor  11  which is shown in  FIG. 3  with two of the catch heads  23  shown in  FIG. 4  on the two mounting tails  25  can be fixed in different positions in this groove which has been undercut in a T shape. The sensor  11  on the central sensor body  21  has one mounting tail  25  each on the opposing T-shaped end sides which lie perpendicular to the lengthwise direction of the reed contacts. They are cast in one piece with the sensor body  21 . Connecting cables  27  for the sensor emerge from one of these end sides. These cables can also emerge on the two end sides, in contrast to the figures. The connecting cables  27  can be routed through the openings  29  in the catch head  23 . 
     On the catch head  23  a catch opening  31  for routing through the mounting tail  25  is formed. In this catch opening  21  there is a catch lip  33  which can engage the teeth  35  on the mounting tails  25 . On the catch head a stop surface  37  is formed which can interact with one end side of the wall which forms the groove. 
     In  FIG. 5  this sensor  11  is located in a groove  41  in the wall of a cylinder  43 . The piston  45  is supported to be able to move linearly in the cylinder space  47 . It can be pushed back and forth by air, hydraulic fluid, or another medium, or it moves this medium by its position change which is caused for example by a motor. For entry and exit of the medium in the piston wall there are openings  46 . This motion of the piston is transferred with a plunger  48  from the motor to the piston or from the piston to a tool, for example. On the periphery of the piston  45  there is a ring magnet  49 . A field line  51  represents the local magnetic field of the magnet  49  schematically simplified. 
     In  FIG. 5  the piston is in the unsafe region B. In  FIG. 7  the piston  45  is shown in the safe region A. When the piston  45  is in the unsafe region B, the magnetic field of the magnet  49  is not strong enough to switch the reed contacts in the sensor  11 . Only in the position in which the field lines extend through the reed contacts can the magnet switch them. The first two reed contacts  15  and  16  are nearer the magnet. They therefore shield the outer, third reed contact in addition. The magnet lines extend, as soon as they reach the region of the contact elements of the reed switch, concentrated through them and thus excite the force which closes the contact in them. 
     In  FIG. 8  the same situations as in  FIGS. 5 and 7  are shown. But the sensor is provided with two shields  19 . The field lines  51  are therefore distorted in both positions of the piston  45 . Therefore in the region of the sensor  11  there is a safe position A. Outside of this region the piston  45  is in an unsafe position. The transition region between these regions with the shield  19  is more precise than without this shield. 
     Furthermore in  FIG. 8 , in contrast to  FIGS. 1 to 7 , the sensor  11  is not made with two mounting tails  25 , but with two catch mechanisms, especially two catch lips  33 , for locking engagement with the teeth  35  of the fixing parts  53 . These fixing parts  53  have a mounting tail  25  with teeth  35  and a stop head  55 . The teeth of the mounting tail  25  engage the catch lips on the sensor body  21 . Since the mounting tails which engage them can no longer be pulled back, in this way the sensor body is fixed in its position as soon as the two stop heads of the fixing parts  53  are in contact with the cylinder  43 . So that the stop heads do not project over the length of the cylinder  43 , the groove on its end is widened and the stop heads  55  fit into the widened groove. Advantageously the two lateral flanges  59  of the groove wall which form the undercut are cut out on their end. 
     The engagement can be releasable with a tool. For this purpose either the catch lip  33  can be pressed away from the mounting tail  25 , or the mounting tail  25  can be raised off the catch lip  33 . In the former case the catch lip  33  is elastically connected to the sensor body  21  or is made on a part which is elastically connected to the sensor body  21 . In the latter case the mounting tails  25  are elastically supported against the groove  41 . In any case the engagement is maintained by spring means. 
     Another example for safe attachment of the sensor  11  in the groove  41  of a cylinder  43  is shown in  FIGS. 9 and 10 . On the sensor  11  a fastening disk  61  is attached which consists of a harder material than the jacket of the cylinder in which the groove  41  which has been undercut in a T-shape is made. This disk  61  is provided with a thread into which a screw  65  is screwed. On the edge or on the four corners of the disk  61  teeth  63  are formed. By tightening the screw  65  these teeth can be pressed into the material of the flange  59  which forms the undercut of the groove  41 . The teeth are thus engaged with the flange  59 . In this way a form-fit is established. This disk is held by a spring  67  in this engagement position. The spring  67  can, as shown, be a helical spring, or also a leaf spring. There can be resilience between the sensor with the fastening disk and the groove, or between the sensor body  21  and the fastening disk  61 . By turning back the screw  65  and pressing the screw in against the spring force of the spring  67  the disk  61  can be disengaged from the groove wall so that the sensor can be moved. The sensor however is not unintentionally moved since engagement is ensured by the spring force. 
     Instead of the disk  61  a leaf spring can also be provided with teeth  63  and a thread and therefore can assume the function of the disk and spring at the same time. To press the teeth into the groove wall a tool which is independent of the sensor can also be used instead of the screw  65 . 
     The spring element which is shown in the  FIG. 11  for attaching the sensor  11  in a groove  41  which has been undercut in a T shape is produced in one piece from spring steel. It has a part which surrounds the sensor body  21  and a spring part with two teeth  63  which can be caused to engage a recess in the side wall of the groove, especially of the narrower and outer part of the T-shaped groove. These recesses can be attained by pressing a tooth  63  into the aluminum of the piston jacket. 
     The parts which surround the sensor body  21  in the mounted position are between the sensor body and the flange  59 . They can additionally apply a spring force to the sensor which presses it against the base of the groove. The teeth  63  in the elastic part are pressed to the outside by the spring force of this part. In the relieved state of the spring element they are at a greater distance from one another than the width of the groove. In this way when the spring element is inserted into the groove with deformation of the spring parts against one another, they must be caused to approach. With a screwdriver or a special tool the teeth  63  which have been folded and punched out of spring steel can be pressed directly against one another into the aluminum of the flange  59  and therefore hold positively. 
     One version of this spring element is shown in  FIG. 12 . It likewise has flanges  69 , a part which surrounds the sensor body and two spring parts with teeth  63 . But here they are in front of and behind the sensor. 
       FIG. 13  shows four circuit diagrams in which the three reed contacts  15 ,  16 ,  17  are combined into two printed conductors and can have three distinguishable switching patterns. Aside from these circuit diagrams, it is also possible to tap the three reed contacts individually and to analyze the operating state of the sensor with downstream logic. 
     In  FIGS. 13   a  and  13   b  the first reed contact  15  and the third reed contact  17  are connected in series. In the series connection it is necessary for the first reed contact to be a make contact and for the third to be a break contact in order to obtain a distinct sensor signal. The second reed contact can be a break contact ( FIG. 13   b ) or a make contact ( FIG. 13   a ). 
     In  FIGS. 13   c  and  13   d  the first reed contact  15  and the third reed contact  17  are connected in parallel. This requires that the first reed contact  15  is a break contact and the third reed contact  17  is a make contact so that distinct sensor signals are generated. The second reed contact can again be a break contact or a make contact. 
     The state of the reed contacts is analyzed with a logic circuit (e.g., with an electronic component). The following applies for the cited circuits: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Diagram 
               
            
           
           
               
               
               
               
               
            
               
                   
                 13a 
                 13b 
                 13c 
                 13d 
               
            
           
           
               
               
            
               
                   
                 Reed contacts 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 15/17 
                 16 
                 15/17 
                 16 
                 15/17 
                 16 
                 15/17 
                 16 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Activated by the 
                 1 
                 1 
                 1 
                 0 
                 0 
                 1 
                 0 
                 0 
               
               
                 exciter part 
               
               
                 Not activated 
                 0 
                 0 
                 0 
                 1 
                 1 
                 0 
                 1 
                 1 
               
               
                 Activated by the external 
                 0 
                 1 
                 0 
                 0 
                 1 
                 1 
                 1 
                 0 
               
               
                 magnetic field 
               
               
                   
               
            
           
         
       
     
     If the reed contacts are tapped individually, each reed contact independently of one another can be a make contact or a break contact. The logic circuit can be configured accordingly such that the open position and closed position of the individual reed contacts are correctly interpreted. 
     It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.