Abstract:
Apparatus and method for measuring displacements of a rotary or linearly-displaceable member by counting periods of displacement thereof along a predetermined displacement path, including a pulse generator located at a pulse-generation station proximate to the displacement path; and an electrical circuit controlled by the pulse generator to actuate a first sensor to sense the status of the displaceable member at the particular instant one of said second machine-sensible elements passes through the pulse-generation station, and to increment a counter in accordance with said status determination. In the described preferred embodiments, the second machine-sensible elements are magnetic elements; and the pulse generator includes a coil, a magnetic core magnetically coupled to the coil, and a spring-mounting for the magnetic core causing the core to move from an initial position in one direction with respect to the coil when aligned with one of the second machine-sensible elements, and to be returned in the opposite direction by the spring to its initial position, whereby the coil generates pulses during such movements of the magnetic core.

Description:
RELATED APPLICATIONS 
     This Application is a National Phase of PCT Patent Application No. PCT/IL2008/001190 having International filing date of Sep. 3, 2008, which claims the benefit of U.S. Provisional Patent Application No. 60/935,857 filed on Sep. 4, 2007. The contents of the above Applications are all incorporated herein by reference. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus and methods for measuring displacements of a displaceable member along a predetermined displacement path. The invention is described below particularly with respect to rotary displaceable members such as in rotary encoders, but may also be used with respect to linearly-displaceable members, such as in linear encoders. 
     The displacement of rotary members is usually measured by counting the number of turns or revolutions experienced by the rotary member as well as fractions thereof, which fractions determine the resolution of the measurement apparatus. Turn counting is needed for example in absolute encoders mounted on motors, e.g., as explained in U.S. Pat. No. 6,628,741 by Netzer. 
     Turn counting systems are used in rotary encoders to provide absolute position information of high precision even when there is an interruption of the power supply to the system and the shaft to be monitored has been turning during these power supply interruptions. Many absolute encoders use batteries in order to monitor and record the turn counting while external power is interrupted. However the use of battery has a number of drawbacks. Thus, batteries have a limited life time. Moreover, replacing a battery without losing the recorded turn counting requires special circuitry, for example a large capacitor, to back up the recorded data during the battery replacement, which circuitry results in additional cost of the encoder. Further, batteries tolerate a limited range of temperatures, and therefore where the encoders are to be used in high temperature environments, the battery cannot be placed inside the encoder. 
     U.S. Pat. No. 5,565,769 and U.S. Pat. No. 5,714,882 describe systems that are able to count and register the number of turns of a shaft without a battery; however, these systems are sensitive to vibrations. U.S. Pat. No. 6,628,741B1 describes apparatus to implement a turns counter without a battery by using a reed relay; however reed relays are sensitive to vibrations, have a limited life time, and can be damaged or destroyed in case of high accelerations. Another drawback is that the amount of energy produced in order to count and store the number of turns is very small, which limits the system to the use of ferroelectric memories; unfortunately, these memories are not available in small sizes, and this again limits their application in encoders. 
     Similar problems are involved in linear encoders for measuring linear displacements of a displaceable member. 
     OBJECT AND BRIEF SUMMARY OF THE PRESENT INVENTION 
     An object of the present invention is to provide apparatus, and also a method, for measuring displacements of a displaceable member having advantages in one or more of the above respects. 
     According to one aspect of the present invention, there is provided apparatus for measuring displacements of a displaceable member along a predetermined displacement path by counting periods of displacement thereof along said predetermined displacement path, said apparatus, comprising: 
     a first machine-sensible element carried by the displaceable member and occupying a length thereof defining only a portion of a period of displacement of the displaceable member; 
     a first sensor located at a sensing station proximate to the displacement path so as to be capable of sensing the presence or absence of the first machine-sensible element in the sensing station, and thereby of determining the displacement state of the displaceable member at any particular instant during the displacement of said displaceable member; 
     a pulse generator located at a pulse-generation station proximate to said displacement path; 
     a plurality of second machine-sensible elements carried by the displaceable member at spaced intervals along each displacement period of the displacement member, each of the second machine-sensible elements being able to actuate said pulse generator when moving through the pulse-generation station; and 
     an electrical counter for counting the displacement periods of the displaceable member; 
     said second machine-sensible elements being separate and distinct from said first machine-sensible element and dividing the predetermined displacement path into a plurality of fractions each including one of said second machine-sensible elements; 
     said apparatus further comprising an electrical circuit controlled by the pulse generator to actuate the first sensor to sense the displacement state of the displaceable member at the particular instant one of said second machine-sensible elements passes through the pulse-generation station, and to increment the counter only when a transition from one displacement state to another is sensed at the instant. 
     According to further features in the described preferred embodiments, the first and second machine-sensible elements are magnetic elements, and the pulse generator includes a coil, a magnetic core magnetically coupled to the coil, and a spring-mounting for the magnetic core causing the core to move from an initial position in one direction with respect to the coil when aligned with one of the second machine-sensible elements, and to be returned in the opposite direction by the spring to its initial position, whereby the coil generates a pulse during each such movement of the magnetic core. Such a construction obviates the need for a separate battery supply. 
     Several embodiments are described wherein the displaceable member is a rotary member, and the counter counts the number of periods of rotation and fractions thereof of the rotary member. Another embodiment is described wherein the displaceable member is a linearly-displaceable member, and the counter counts the number of periods of linear displacements and fractions thereof experienced by the displaceable member. 
     According to another aspect of the present invention, there is provided a method for measuring displacements of a displaceable member along a predetermined displacement path, by counting periods of displacement thereof along said predetermined displacement path, said apparatus comprising: 
     applying a first machine-sensible element to the displaceable member to occupy a length thereof defining only a portion of a period of displacement of the displaceable member; 
     providing a first sensor at a sensing station proximate to the displacement path so as to be capable of sensing the presence or absence of the first machine-sensible element in the sensing station, and thereby of determining the status of the displacement state of the displaceable member, at any particular instant during the displacement of the displaceable member; 
     providing a pulse generator at a pulse-generation station proximate to said displacement path, and an electrical counter controlled by said pulse generator; and 
     applying a plurality of second machine-sensible elements to the displaceable member at spaced intervals along each displacement period of the displacement member, each of the second machine-sensible elements being able to actuate said pulse generator when moving through the pulse-generation station; said second machine-sensible elements being separate and distinct from said first machine-sensible element and dividing the predetermined displacement path into a plurity of fractions each including one of said second machine-sensible elements; 
     controlling the pulse generator to actuate the first sensor to sense the displacement state of the displaceable member at the particular instant one of the second machine-sensible elements passes through the pulse-generation station, and to increment the electrical counter only when a transition from one displacement state to another is sensed at that instant. 
     As will be described more particularly below, the apparatus and method of the present invention as briefly described above enable measuring the displacement of a displaceable member in a manner which does not require a battery, which uses standard electronic devices, and which is relatively insensitive to vibrations. 
     Further features and advantages of the invention will be apparent from the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  schematically illustrates one form of apparatus for measuring displacements of a rotary member in accordance with the present invention; 
         FIG. 2  is a diagram helpful in explaining the operation of the apparatus of  FIG. 1 ; 
         FIG. 3  schematically illustrates another apparatus for measuring displacements of a rotary shaft in accordance with the present invention providing higher resolution than the apparatus of  FIG. 1 ; 
       and  FIG. 4  schematically illustrates one form of apparatus for measuring displacements of a linear displaceable member in accordance with the present invention. 
     
    
    
     It is to be understood that the foregoing drawings, and the description below, are provided primarily for purposes of facilitating understanding the conceptual aspects of the invention and possible embodiments thereof, including what is presently considered to be a preferred embodiment. In the interest of clarity and brevity, no attempt is made to provide more details than necessary to enable one skilled in the art, using routine skill and design, to understand and practice the described invention. It is to be further understood that the embodiments described are for purposes of example only, and that the invention is capable of being embodied in other forms and applications than described herein. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is first made to  FIG. 1  schematically illustrating one form of measuring apparatus constructed in accordance with the present invention for measuring the turns or rotations of a shaft  10  about a rotary axis  11 . The apparatus illustrated in  FIG. 1  may be in the form of a stand-alone turns counter, or a one-turn absolute encoder providing a precise measurement of the rotation angle of the shaft. As will be shown below, the apparatus illustrated in  FIG. 1  is designed for recording the number of turns and/or fractions of a turn, without the need for external power, since the required power is received from the rotating shaft by means of magnetic induction. 
     Thus, as shown in  FIG. 1 , the rotary shaft  10  itself, whose rotations are to be counted, or a separate disc fixed to that shaft, includes a first machine-sensible element  12  extending around the outer circumference of the shaft for a length defining one-half of a period of displacement (one rotation) of the shaft. Thus, as seen in  FIG. 1 , machine-sensible element  12  covers one-half the circumference of shaft  10 , leaving the other half uncovered. Accordingly, each full rotation or turn of shaft  10  is constituted of a single period, one-half of which is occupied by machine-sensible element  12 , whereas the other half is not occupied by that element. 
     The apparatus illustrated in  FIG. 1  further includes at least one sensor, preferably two sensors,  13   a ,  13   b , spaced from each other. Sensors  13   a ,  13   b  are located at sensing stations proximate to the displacement path of rotary shaft  10  so as to be capable of sensing the presence or absence of machine-sensible element  12  in the sensing station, and thereby of determining the status of the shaft  10  at any particular instant during the rotation of the shaft. In the described preferred embodiment, machine-sensible element  12  is a magnetic element, and the two sensors  13   a ,  13   b  are Hall sensors spaced 90° from each other around the outer surface of shaft  10 . 
     The apparatus illustrated in  FIG. 1  further includes a pulse generator, generally designated  14 , fixed at another location, called a pulse generation station, proximate to the rotary shaft  10 . Pulse generator  14  includes a magnetic core  14   a  mounted in cantilever fashion at one end of an elastic arm  14   b , whose opposite end is fixed at  14   c , and is movable with respect to a coil  14   d  to generate a pulse therein upon each movement of the core with respect to the coil. 
     Shaft  10  further carries a plurality of second machine-sensible elements  15   a - 15   d  equally spaced in a circular array around the axis of rotation  11  of the shaft. In the illustrated example, there are four of such machine-sensible elements  15   a - 15   d ; two ( 15   a ,  15   b ) are located in the sector of shaft  10  occupied by the first machine-sensible element  12 , and the other two ( 15   c ,  15   d ) are located in the sector of shaft  10  not occupied by machine-sensible element  12 . Machine-sensible elements  15   a - 15   d  are also magnets, so as to attract magnetic core  14   a  of pulse generator  14  to generate in coil  14   d  a pulse each time a magnetic element  15   a - 15   d  moves proximately to, and away from, core  14   a  of the pulse generator. 
     Each of the two sensors  13   a ,  13   b  is connected by leads  16   a ,  16   b , and coil  14   d  of pulse generator  14  is connected by leads  17 , to an electrical control circuit  18 , which circuit controls a turns counter  19 . 
     The system illustrated in  FIG. 1  operates as follows: 
     When shaft  10  rotates counter-clockwise, the four magnets  15   a - 15   d  also rotate around rotary axis  11 , such that each of the four magnets  15   a - 15   d  move towards, and then away from, core  14   a  of pulse generator  14 . With each such movement of a magnet  15   a - 15   d  with respect to core  14   a , the latter core is abruptly moved towards or away from coil  14   d , to thereby generate a pulse within the coil with each such movement. These pulses are fed via lead  17  to the electrical control circuit  18 . Each such pulse, generated in coil  14   d  and applied to electrical control circuit  18  via lead  17 , activates the electrical circuit for a short period of time during which the electrical circuit enables sensors  13   a ,  13   b  to register the rotary position of shaft  10  at that instant. Sensors  13   a ,  13   b  thus serve as state sensors, determining the displacement state of the shaft at any particular instant, namely the instant at which electrical circuit  18  receives a pulse from pulse generator  14 . Electrical circuit  18  also controls a turns counter  19 , which accumulates, in a non-volatile memory, the absolute position of the shaft by counting the number of periods (rotations) and fractions thereof traversed by the shaft. 
     It will be seen that since magnets  15   a - 15   d  are spaced 90° from each other, the rotary position of the shaft can be determined with a resolution of one-quarter of a turn. Also, since there are two displacement state sensors  13   a ,  13   b , located 90° with respect to each other, the measuring system is able to distinguish the direction of the respective increment of rotation, i.e., whether in the forward direction or in the reverse direction. 
     It will be further seen that a pulse is outputted by the pulse generator  14  each time a magnet  15   a - 15   d  moves towards core  14   a  and then away from the core, and that each such pulse produced in core  14   d  is not linear, but sharply increases as the magnet moves towards the core, and sharply decreases as the magnet moves away from the core. It will be further seen that these movements of the core will generate energy to activate the turns counter  19 . Thus, it is a particular advantage of the illustrated apparatus that the amount of energy available from the pulse generator  14  is large enough to allow the storage of the shaft position in a non-volatile memory, like a ferro-electric memory, a flash memory, or an EPROM. 
       FIG. 2  illustrates the sequence of states involved in the apparatus illustrated in  FIG. 1 , wherein one turn to be counted is constituted of one period since there is only one magnet  12  extending for one-half the circumference of the shaft. Following is an Increment Table used to update the number of turns of the shaft in accordance with the sequence of states in  FIG. 2 . 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Increment Table 
               
             
          
           
               
                   
                   
                   
                 Previous 
                 Position 
               
               
                 Sensor 1 (13a) 
                 Sensor 2 (13b) 
                 Sector 
                 Sector 
                 Increment 
               
               
                   
               
             
          
           
               
                 0 
                 0 
                 s0 
                 s0 
                 0 
               
               
                 0 
                 0 
                 s0 
                 s1 
                 −0.25 
               
               
                 0 
                 0 
                 s0 
                 s2 
                 n.a. 
               
               
                 0 
                 0 
                 s0 
                 s3 
                 +0.25 
               
               
                 1 
                 0 
                 s1 
                 s0 
                 +0.25 
               
               
                 1 
                 0 
                 s1 
                 s1 
                 0 
               
               
                 1 
                 0 
                 s1 
                 s2 
                 −0.25 
               
               
                 1 
                 0 
                 s2 
                 s3 
                 n.a. 
               
               
                 1 
                 1 
                 s2 
                 s0 
                 n.a. 
               
               
                 1 
                 1 
                 s2 
                 s1 
                 +0.25 
               
               
                 1 
                 1 
                 s2 
                 s2 
                 0 
               
               
                 1 
                 1 
                 s2 
                 s3 
                 −0.25 
               
               
                 0 
                 1 
                 s3 
                 s0 
                 −0.25 
               
               
                 0 
                 1 
                 s3 
                 s1 
                 n.a. 
               
               
                 0 
                 1 
                 s3 
                 s2 
                 +0.25 
               
               
                 0 
                 1 
                 s3 
                 s3 
                 0 
               
               
                   
               
             
          
         
       
     
     Thus, the sequence of the states  21  and  22  of sensor  13   a ,  13   b  is shown in  FIG. 2  in relation to the rotation angle. The states of each sensor is represented by two values indicating whether the sensor is close to the sensible element or not. Whenever the electronic circuit  18  is activated, then each sensor state is sensed by the electronic circuit. 
       FIG. 2  also shows, as grey areas  23 , the range of angles when the pulse generator  14  outputs pulses to cause the electrical circuit  18  to activate the turns counters  19 . A sector can be defined as a range of angles for which the state of sensors  13   a ,  13   b  is constant. As shown in  FIG. 2 , four sector s 0 -s 3  are defined in one turn  24 , and the pulse generator outputs one pulse of energy between two transitions of the states  21  or  22 . 
     As explained above, the electric circuit  18  and turns counter  19  are activated at least once each time shaft  10  moves at least one quarter of a turn, such that there will always be at least one update of the position measurement for each one quarter of a turn. In the preferred embodiment, Hall sensors are used because they provide sensing with minimum power consumption. However, other types of sensors can be used, such as reed relays, proximity sensors, or other types of sensors. 
     It will also be appreciated that in the described preferred embodiment of  FIGS. 1 and 2 , as well as in the other embodiments described below, the machine-sensible elements  12  and  15   a - 15   d  are magnetic elements which generate the required electricity and therefore do not need a battery. However, in some applications where a battery can be used, such elements could be optical elements, rather than magnetic elements, whereupon the status sensors  13   a ,  13   b , as well the pulse generator  14 , would be optically activated rather than magnetically activated. 
     In addition, whereas in the preferred embodiment illustrated in  FIGS. 1 and 2 , two status sensors  13   a ,  13   b  are used, so that the rotation of the shaft in both directions can be registered, there may be applications where the shaft is susceptible of moving only in one direction, in which case only one displacement state sensor would be needed to count half-turns. 
     In addition, sectors in the preferred embodiment are shown covering a 90 degrees range of angle; however sectors can be of different sizes, so long as that there is at least one activation of the pulse generator in the range of each sensor. 
     A particular advantage of the apparatus illustrated is that it does not count the number of pulses outputted by the pulse generator, but rather such pulses are used to provide energy to a separate turns counter  19 . As a result, the described system is not sensitive to vibrations. If vibrations occur, and these vibrations result in a movement of the moving core  14   a  of the pulse generator  14 , and a pulse of energy is outputted more than once in a quarter of a turn, then the increment by one quarter of a turn will be done only for the first pulse; the following pulse will result in a zero increment value. This is clearly shown in the above Increment Table, wherein the position increment is given as a function of the present states of the sensors and the previous states as sensed by the electronic circuit  18  and the turns counter  19 . Whenever activated, the electronic circuit  18  checks, at a high rate, the states of the sensors, and updates the shaft position according to the table. The checking cycle is short enough so that the shaft position will be updated even if the shaft has a high rotational speed. 
     Also in the above Increment Table, a sector is defined as a range of angles for which the state of sensors  13   a  and  13   b  remains constant. 
     It must be understood that the preferred embodiment illustrated shows sectors of exactly one quarter of a turn; however the sectors may be of different sizes, as long as there is at least one activation of the pulse generator within one sector, i.e. at least one of the second sensible member activates the pulse generator within the sector range. 
     Another advantage of the illustrated apparatus is that the turn counting resolution is one-fourth of a turn, i.e., one-fourth of a period, using only two sensors. Another advantage of the illustrated apparatus is that only one magnetic energy generating element, i.e., pulse generator  14 , is used for a bi-directional turn counter. This is to be sharply distinguished from the systems illustrated in the above-cited US patents, which need at least three magnetic energy generating elements in order to count in both directions. 
     In the embodiment illustrated in  FIGS. 1 and 2 , the apparatus is used to count the number of turns of a shaft, each turn representing a period of displacement, with one-quarter of a turn resolution. It will be appreciated that the same apparatus can include a counting system having a different resolution than one-quarter turn by providing a different number of magnets  12  (or other machine-sensible elements), to thereby define a different number of sectors of the shaft to produce at least one pulse of energy per sector. 
       FIG. 3  illustrates an apparatus wherein the shaft  30  is divided into eight sectors by eight magnets  35   a - 35   h  equally arranged in a circular array around the rotary axis  31  of the shaft. In the apparatus illustrated in  FIG. 3 , there are two outer magnets  32   a ,  32   b  each extending for one-half the circumference of the rotary shaft, so as to define two periods to be counted, for each rotation, with each period divided into four sectors defined by the state of two displacement state sensors  33   a ,  33   b , each being at one-fourth of a period, i.e., one-eighth of an angular turn. For each sector range, an inner magnet ( 35   a - 35   d ) is positioned to activate the pulse sensor. 
     The remainder of the system illustrated in  FIG. 3  is constructed and operates in substantially the same manner as described above with respect to  FIGS. 1 and 2 . Thus, the apparatus illustrated in  FIG. 3  includes a pulse generator, generally designated  34 , including a movable core  34   a  secured to one end of an elastic member  34   b  whose opposite end  34   c  is fixed, and movable within a coil  34   d  when each of the inner magnets  35   a - 35   h  passes into and out of alignment with the magnetic core  34   a . The pulses generated by coil  34   d  are applied to electrical circuit  38  via leads  37 ; and the status of each of the status sensors  33   a ,  33   b , with respect to the outer magnets  32   a ,  32   b , is fed to electrical circuit  38  via leads  36   a  and  36   b  from the two status sensors  33   a ,  33   b . Electrical circuit  38  thus increments (or decrements) turns counter  39  according to the sensed displacement state, as described above with respect to  FIGS. 1 and 2 . 
     It will thus be seen that the apparatus illustrated in  FIG. 3  has a resolution of one-eighth of a turn, rather than one-quarter of a turn as in  FIGS. 1 and 2 . 
     It will be appreciated that a displacement measuring system constructed in accordance with the present invention may be based on more than one or two periods for each turn by providing the rotary shaft with the appropriate number of outer magnets ( 12 ), namely one for each such period and extending for one-half the distance of the respective period. It will also be appreciated that the apparatus may be constructed to provide a different number of sectors, and thereby a different resolution, by providing the appropriate number of inner magnets to actuate the pulse generator at least once for each sector during each rotation. 
       FIG. 4  schematically illustrates the invention implemented in apparatus for measuring linear displacements in the form of linear displacement periods and fractions thereof along a linear displacement path, rather than rotary displacements as in  FIGS. 1-3 . Thus, the apparatus illustrated in  FIG. 4  includes a linearly-displaceable member, generally designated  40 , displaceable in a linear path as indicated by arrow  41 . Displaceable member  40  includes, on one side, a plurality of machine-sensible elements, namely magnets  42   a - 42   g , one for each period of displacement of member  40 , with each such magnet covering one-half the period. The respective side of displaceable member  40  further includes two displacement state sensors  43   a ,  43   b.    
     The opposite side of displaceable member  40  is provided with a pulse generator  44  including a movable core  44   a  carried at one end of an elastic arm  44   b , with the opposite end of the elastic arm  44   c  fixedly mounted, and with the core  44   a  movable with respect to a coil  44   d  to generate an electrical pulse with each movement of the coil. In this case, however, pulse generator  44  is actuated by a plurality of magnets  45   a - 45   n , corresponding to the number of periods defined by magnets  42   a - 42   g  and the resolution desired in the measuring apparatus. In the example illustrated in  FIG. 4 , magnets  42   a - 42   g  divide the length of the displaceable member into seven periods  46 , and the magnets  45   a - 45   n  divide each period  46  into four fractions, such that the measuring apparatus has a resolution of one-fourth period. 
     The apparatus illustrated in  FIG. 4  is otherwise constructed and operates in the same manner as described above with respect to  FIGS. 1-3 , to measure the linear displacement of member  40  in terms of periods  46 , with a resolution of one-fourth period. It will be appreciated that the distance of each period  46  is precisely known, so that the apparatus illustrated in  FIG. 4  measures displacement in terms of absolute values of displacement. 
     While the invention has been described with respect to several preferred embodiments, it will be appreciated that many variations may be made. For example, one or both of the types of machine-sensible elements could be optical elements or capacitive-type elements, rather than magnetic elements, and the pulse generator could be an optically-actuated one, rather than a magnetically-actuated one. In addition, the resolution of the measuring apparatus could be increased (or decreased) providing the appropriate number of magnets for actuating the pulse generator, and the appropriate spacing of the status sensors cooperable with the status magnets. 
     Many other variations, modifications and applications of the invention will be apparent.