Abstract:
The invention relates to a device for detecting movements and/or positions of an object with the aid of voltage pulses induced by the field of adjacent magnets ( 6, 7 ) in a coil ( 4 ). In order to generate a sufficiently strong signal also at small relative speeds between the magnets ( 6, 7 ) and the coil ( 4 ), the coil ( 4 ) encloses a spring ( 5 ) in which a quick fore-and-aft movement can be induced by the magnetic fields of the magnets ( 6, 7 ), the magnetic poles of the spring ( 5 ) being reversed during said fore-and-aft movement.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to an apparatus for the detection of movements and/or positions of an object, in which a voltage pulse is in each case produced as a function of the movements and/or positions of the object in at least one coil, by means of at least two magnets which carry out a relative movement with respect to the coil and act on it with their fields, with the north-south axes of the respective two adjacent and opposite-polarity magnets which form a magnet pair being oriented, in the same way as the longitudinal axis of the coil, essentially at right angles to the movement direction of the object.  
       PRIOR ART  
       [0002]     An apparatus of the above type in the form of a tachometer is known from DE 102 19 303 C1. In the known apparatus, the moving object is formed by a hollow shaft on which a magnet supporting arm is mounted such that it can pivot and is fitted with two magnets of opposite polarity which are offset with respect to one another in the direction of the longitudinal axis of the hollow shaft. Coils are arranged at a distance from the shaft and over the circumference of the shaft and supply voltage pulses to an evaluation unit under the influence of the fields of the magnets passing them. The coils are held by webs which form coil cores and connect magnetically permeable ring segments to an outer ring which surrounds the ring segments and the coils and is likewise magnetically permeable. Further ring segments are arranged between the ring segments to which the coils are fitted, are separated from them by a gap, and are likewise connected to the outer ring. The widths of the gaps between the ring segments in this case correspond essentially to the width of the magnets. The described configuration of the known apparatus means that a rapid pivoting movement of the magnet support arm takes place in the area of the gaps between the ring segments when the shaft is rotating at slow speeds and, as a consequence, this leads to production of a strong voltage pulse in the coils. This results in a sufficiently strong signal for electronic evaluation even at low rotation speeds.  
         [0003]     The known apparatus is not regarded as being completely satisfactory because the use of a magnet supporting arm which can pivot and its bearing on the shaft of the tachometer are associated with a comparatively high degree of complexity.  
         [0004]     DE 43 42 069 A1 discloses an apparatus with two opposite-polarity magnets, the first of which carries out a movement in synchronism with the object. A second magnet of opposite polarity is associated with this first magnet and is located at one end of a leaf spring, whose other end is attached to a fixed holder. The second magnet is arranged above the movement path of the first magnet. When the first magnet approaches the second magnet during slow movements, the latter is repelled because of its opposite polarity and the leaf spring is deformed, leading to an accumulation of potential spring energy in it. Since the restoring force of the leaf spring increases as the deflection increases, a deflection position is reached, as the movement of the first magnet progresses, at which the restoring force of the leaf spring is greater than the repulsion forces between the first and the second magnet, resulting in the leaf spring springing back, dissipating the potential energy accumulated in it. In order to convert the kinetic energy released as the leaf spring springs back to an electrical pulse, a coil which has an iron core is provided on the side of the second magnet facing away from the path of the first magnet. The magnetic flux which passes through the iron core of this coil is changed in a very short time during the backward movement of the second magnet, so that a voltage pulse is induced in the coil. In order to produce pulses in this apparatus which are sufficiently large, comparatively large magnets are required and a likewise comparatively large leaf spring, in order to store a sufficiently large amount of energy. In other words, the space requirement for the magnets and the leaf spring is comparatively large.  
       DESCRIPTION OF THE INVENTION  
       [0005]     The invention likewise has the aim of inducing sufficiently large voltage pulses in a coil in particular when an object is moving slowly and to achieve this, in contrast to the known solutions, using simpler means.  
         [0006]     The stated object is achieved in a first embodiment of the invention in that the magnets are arranged one behind the other in the direction of movement of the object, and the coil at least partially surrounds a spring which is composed of magnetically permeable material and whose end facing the magnets carries out a reciprocating movement as a result of the reluctance effect under the influence of the fields of the magnets of the magnet pair, which reciprocating movement causes a polarity change, which produces the respective voltage pulse, in the magnetic field in the coil.  
         [0007]     The apparatus according to the invention is distinguished by a surprisingly simple, compact and maintenance-free design. In this apparatus, the respective leading magnet drives the free end of the spring with it, since the lines of force of this magnet search for the path of least magnetic reluctance, based on the reluctance principle. When the magnet is moved further, then a state is reached in which the restoring force of the spring overcomes the magnetic driving force and the spring moves into the area of the lagging magnet, of opposite polarity, by which it is absorbed with additional acceleration. This means that the spring suddenly reverses the polarity of the magnetic field acting in the coil thus resulting in a strong voltage being produced in the coil surrounding the spring. In general, this remagnetization also takes place at higher speeds of the object when the two adjacent opposite-polarity magnets which form a magnet pair move quickly past the spring, without the spring being able to carry out any significant movements in this case, because of its inertia.  
         [0008]     A second solution to the stated problem consists in that the magnets are arranged alongside one another when considered in the direction of their relative movement with respect to the coil and the coil at least partially surrounds a spring which is composed of magnetically permeable material and whose end facing the magnets carries out a reciprocating movement on the basis of the reluctance effect under the influence of the fields of the magnets of the magnet pair, which reciprocating movement causes a polarity change, which produces the respective voltage pulse, in the magnetic field in the coil, with both the pole surfaces of the magnets and that end surface of the leaf spring which faces the pole surfaces of the magnets being essentially in the form of rectangles, whose longitudinal axes include an angle α of less than 60° with the tangent to the movement path of the magnets.  
         [0009]     The alternative solution has a further advantage in that force or torque pulsations are reduced to a negligible level. Pulsations such as these have a disturbing effect in the case of the first embodiment of apparatuses in the form of tachometers when the aim is to monitor the rotation speed of relatively small drives.  
         [0010]     Further features and details of the invention will become evident from the dependent claims, from the attached, schematic drawings, and from their description in the following text. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     In the figures:  
         [0012]      FIG. 1  shows a section through a first apparatus in the form of a tachometer,  
         [0013]      FIG. 2  shows a section along the line II-II in  FIG. 1 ,  
         [0014]      FIGS. 3   a - d  show, highly schematically, the interaction between those parts of the apparatus as shown in  FIGS. 1 and 2  which are used to generate a voltage pulse,  
         [0015]      FIG. 4  shows voltage pulses which are produced when the shaft of the tachometer is rotating at low speed, and their dependency on the rotation direction,  
         [0016]      FIG. 5  shows the voltage pulses which are produced when the tachometer shaft is rotating at high speed, and their dependence on the rotation direction,  
         [0017]      FIG. 6  shows a section, corresponding to that in  FIG. 1 , through a slightly modified apparatus,  
         [0018]      FIG. 7  shows the electrical circuit by means of which the voltages which are produced in the coils of the apparatus shown in  FIG. 6  are rectified and are supplied to an electronic counting circuit,  
         [0019]      FIG. 8  shows a section, corresponding to that in  FIG. 6 , through a tachometer in the form of a so-called external rotor,  
         [0020]      FIG. 9  shows a section along the line IX-IX in  FIG. 8 ,  
         [0021]      FIG. 10  shows a side view of the major parts of an apparatus which is used in conjunction with a machine tool,  
         [0022]      FIG. 11  shows a plan view of the parts illustrated in  FIG. 10 ,  
         [0023]      FIG. 12  shows an end view of the parts illustrated in  FIG. 10 ,  
         [0024]      FIG. 13  shows a section along the line XIII-XIII in  FIG. 11 ,  
         [0025]      FIGS. 14   a - d  show an illustration, corresponding to  FIGS. 3   a  to  3   d , of the interaction between the magnets and the leaf spring in the apparatus according to the alternative solution,  
         [0026]      FIG. 15  shows the position of the magnets, of the leaf spring and of a coil which partially surrounds the leaf spring in a tachometer according to the alternative solution,  
         [0027]      FIG. 16  shows a section along the line XVI-XVI in  FIG. 15 , on an enlarged scale,  
         [0028]      FIG. 17  shows a section, corresponding to  FIG. 16 , through a first modified embodiment,  
         [0029]      FIG. 18  shows a section, corresponding to  FIG. 16 , through a second modified embodiment,  
         [0030]      FIG. 19  shows the voltage pulses which are produced when the shaft of the tachometer as shown in  FIG. 15  is rotated at low speed, as a function of the rotation direction,  
         [0031]      FIG. 20  shows the electrical circuit by means of which the voltages which are produced in the coil of the apparatus shown in  FIG. 15  are rectified and are supplied to an electronic counting circuit,  
         [0032]      FIG. 21  shows the arrangement of the magnets and of the leaf spring on which they act in a so-called multiturn transmitter,  
         [0033]      FIG. 22  shows a circuit diagram of the apparatus shown in  FIG. 21 ,  
         [0034]      FIG. 23  shows an apparatus in which the leaf spring and the coil are used only for supplying voltage to a microprocessor circuit and for checking sensors which are used for counting purposes, and  
         [0035]      FIG. 24  shows a schematic circuit diagram of the apparatus shown in  FIG. 23 . 
     
    
     APPROACHES TO IMPLEMENTATION OF THE INVENTION  
       [0036]     In  FIGS. 1 and 2 ,  1  is the hollow shaft of a tachometer which can be connected to a shaft, which is not illustrated but whose revolutions are intended to be counted, such that they rotate together. A coil  4  which surrounds a magnetically permeable spring  5  is mounted on an annular printed circuit board  3  which is connected to the outer wall  2  of the housing of the tachometer. One end of the spring  5 , which is in the form of a leaf spring, projects slightly beyond the lower end of the coil  4 , and its other end is firmly clamped in at the upper end of the coil  4 . That part of the spring  5  which is not clamped in can carry out reciprocating movements in the cavity in the coil  4 , as is also described in  FIGS. 3   a  to  3   d.    
         [0037]     Two magnets  6  and  7  of opposite polarity which form a magnet pair are arranged one behind the other on the circumference of the hollow shaft  1  of the tachometer in the rotation direction of the hollow shaft  1 , and their north-south axes run at right angles to the movement direction of the hollow shaft  1 , in the same way as the longitudinal axes of the coil  4 . In other words, the magnetic fields of the magnets  6 ,  7  are oriented radially. Owing to the different polarity of the magnets  6 ,  7 , the lines of force  8 ,  9  run in opposite directions.  
         [0038]      FIGS. 3   a  to  3   d —in which the coil  4  has been omitted—show how the spring  5  behaves under the influence of the fields of the magnets  6  and  7  when the magnets  6  and  7  pass the spring&#39;s free end.  
         [0039]      FIG. 3   a  shows the spring  5  in the stretched rest state. When the hollow shaft  1  rotates in the direction of the arrow  10 , that is to say clockwise, then—as shown in  FIG. 3   b —the magnet  6  first of all moves under the free end of the spring  5 , and the lines of force  8  of the magnet  6  are concentrated on the surface of the spring  5 . When the hollow shaft  1  moves further to the position illustrated in  FIG. 3   c , then the freely moving part of the spring  5  is driven with it, since the lines of force  8  of the magnet  6  search for the path of least magnetic reluctance that is offered to them by the spring  5 , on the basis of the reluctance principle. When the hollow shaft  1  moves further in the rotation direction, then the point is reached at which the restoring force of the spring  5  overcomes the driving force of the magnet  6 , and the spring  5  snaps back. During this process, it moves into the influence area of the magnet  7 , whose lines of force  9  run in the opposite direction to the lines of force  8  of the magnet  6 . When the spring  5  changes between the positions illustrated in  FIGS. 3   c  and  3   d , this results in sudden remagnetization of the spring  5 , which produces a strong voltage pulse in the coil  4  surrounding the spring  5 , as is illustrated in  FIG. 4 .  FIG. 4  also shows that the polarity of the induced voltage pulses  11 ,  12  depends on the rotation direction as indicated by the arrows  13  and  14 , that is to say on whether the spring  5  is acted on by the magnet  6  first of all and then by the magnet  7  or, conversely, by the magnet  7  first and then by the magnet  6 . Voltage pulses  11 ,  12  are obviously likewise produced when a linear movement takes place in the direction of the arrows  15 ,  16 .  
         [0040]      FIG. 5  shows the relationships for the situation in which the hollow shaft  1  is rotating at higher rotation speeds. Because of the increased speed with which the magnets  6 ,  7  pass the spring  5 , sufficiently large voltage pulses  17 ,  18  are generated just on the basis of the rapid magnetic polarity change, irrespective of the movement of the spring  5 . The initial and subsequent pulses  19 ,  20  and  21 ,  22 , respectively illustrated in  FIG. 5  result simply from the fact that a small voltage pulse is produced when one of the two magnets  6  or  7  moves under the spring  5 , with this small voltage pulse being considerably less than the main voltage pulse which is produced by the magnetic pole change.  
         [0041]     The described apparatus makes it possible to unambiguously determine the number of revolutions of the hollow shaft  1  and its rotation direction using only one spring  5  and one coil  4 . If the tachometer is intended to be connected to a so-called single-turn absolute transmitter which, as the name says, detects the absolute position within one revolution, while the tachometer counts the number of complete revolutions carried out, then, as is shown in  FIG. 6 , at least two coils  4  equipped with springs  5  should be provided. In order in this case to ensure unambiguous synchronization between the single-turn part and the multiturn part, the single-turn transmitter is positioned on the hollow shaft  1  such that its transition from “complete revolution” (=360°) to “start of the revolution” (=0°) coincides with the axis of symmetry  23 .  
         [0042]      FIG. 7  shows how the voltages which are produced in the coils  4  (which are connected electrically back-to-back in series) in the arrangement as shown in  FIG. 6  can be rectified by diodes and can be used to supply an electronic counting circuit, which is not illustrated, and how up and down signals can be obtained in a simple manner.  
         [0043]     While the embodiments described so far relate to tachometers that are referred to as so-called internal rotors,  FIGS. 8 and 9  illustrate a tachometer of the so-called external-rotor type. In this type, the hollow shaft  24  is surrounded by a collar  25  which at least partially surrounds it and on whose inner wall  26  facing the hollow shaft  24  the magnets  6  and  7 , which revolve together with the hollow shaft  24 , are mounted. This solution not only has the advantage that the centrifugal forces which are exerted on the magnets  6 ,  7  at relatively high rotation speeds are recorded well, but that, furthermore, magnetic interference fields are shielded by the annular collar  25  and the flange  27  which connects this collar  25  to the hollow shaft  24 , from the coils  4  and from the electronic circuit  28  which is accommodated on the printed circuit board  3 .  
         [0044]     FIGS.  10  to  12  show a further possible way to use the apparatus carrying out the function of a voltage generator. In the figures,  29  denotes a linear scale which is provided with a coding  30 , which is known and will therefore not be explained in any more detail, and is indicated by crossed lines, in order to allow an absolute measurement. The linear scale, which is fitted to the bed  31  of a machine tool, comprises a plurality of sections  29   a ,  29   b  and  29   c , which allow it to be produced at low cost. In addition to the scale  29 , magnet pairs which each comprise two magnets  6  and  7  are arranged in the area of the abutment points between the abovementioned sections, with the distance between centers P of the magnet pairs corresponding to the length of in each case one section  29   a ,  29   b  or  29   c  of the scale  29 . The object which carries out a movement with respect to the magnet pairs  6 ,  7  is in the illustrated case formed by a carriage  32  which can be moved backwards and forwards along guide tracks  33 , only one of which is shown. A sensor head  34  is mounted on the carriage  32  and—as can be seen in  FIG. 13 —has scanning electronics  35 , which read the coding, and two coils  4 , each of which in turn surrounds one spring  5 .  
         [0045]     When the sensor head  34  passes the magnet pair  6 ,  7 , as shown on the left in  FIGS. 10 and 11 , from left to right, then the coils  4 ,  4  signal to a non-volatile memory for the evaluation electronics that the separating line  36  between the sections  29   a  and  29   b  has been crossed over, and that the sensor head  34  is in the area of the section  29   b . There, the scanning electronics  35  determine an absolute position which occurs only once on the section  29   b  within that section, that is to say in the region of the distance P. The absolute position of the carriage  32  on the bed  31  of the machine tool can be determined from the value which is stored in the non-volatile memory and was obtained with the aid of the two coils  4 , and from the value currently being read by the scanning electronics  35 .  
         [0046]     The use of two position measurement systems each comprising one spring  5  and one coil  4  has been found to be worthwhile to ensure that any position errors of the magnets  6 ,  7  resulting from the physical distance between the springs  5  and within the range of permissible installation tolerances do not lead to errors in the identification of the sections a-c of the linear scale  29 . The described solution allows long and in consequence expensive linear scales to be replaced by a plurality of short linear scales, which can be produced with considerably less effort.  
         [0047]     When the magnet  6  in the case of  FIGS. 3   a  to  3   d  approaches the leaf spring  5 , then the force of attraction between the magnet  6  and the leaf spring  5  leads to a certain amount of acceleration of the drive shaft of the tachometer. The acceleration is followed by a deceleration as soon as the leaf spring  5  is driven by the magnet  6  and, as is illustrated in  FIG. 3   c , is deflected. The deceleration is finished suddenly, as soon as the free end of the leaf spring  5  jumps over to the magnet  7 , so to speak. The cessation in the deceleration extends over only a short instant since the magnet  7  then once again deflects the leaf spring  5  which once again leads to a deceleration in the movement of the shaft of the tachometer. This deceleration ceases as soon as the restoring force of the leaf spring  5  exceeds the force of attraction between it and the magnet  7 . As already mentioned in the introduction, the torque pulsations which result from the relationships described above, particularly in the case of tachometers with which relatively small drives are equipped, have a disturbing effect, quite apart from the fact that the leaf spring is caused to oscillate under the influence of the pulsations, which can lead to noise being developed.  
         [0048]      FIGS. 14   a  to  14   d  illustrate, schematically, how the described torque pulsations can be attenuated in the sense of the invention in such a way that they no longer have a disturbing effect. In contrast to the situation in  FIGS. 3   a  to  3   d , both the longitudinal axes of the essentially rectangular pole surfaces of the revolving permanent magnets  44 ,  45  of opposite polarity and the end surfaces of the fixed leaf spring  46  assume an angle α to the tangent  47  to the movement path  48  (which in this case is in the form of a circle) of the magnets  44 ,  45  in  FIGS. 14   a  to  14   d . The angle α is considerably less than 90°, and in practice is 20 to 30°. When the magnets  44 ,  45  move towards the leaf spring  46  in the illustrated case, then this results in only a minor accelerating torque, since the leaf spring  46  faces the magnet  44  only on its narrow face. When, as is shown in  FIG. 14   b , the leaf spring  46  moves into the area of the pole surface of the magnet  44 , then the magnet  44  tries to keep the leaf spring  46  on its center axis as far as possible, on the basis of the reluctance effect. As the movement of the magnet  44  progresses, the leaf spring  46  is in consequence forced inwards by a specific amount from its initial position, that is to say in the direction of the longitudinal axis  49  of the tachometer. While the restoring force of the leaf spring  46  exceeds the reluctance force, then the leaf spring  46  jumps from the position shown by a dashed line in  FIG. 14   c  to the position illustrated as a solid line. In this case, a sudden magnetic flux change takes place, which is also used to produce a voltage pulse in the case of the already described apparatuses. When the magnets  44 ,  45  which are arranged alongside one another move further—as indicated in  FIG. 14   d —then the magnet  45  releases the leaf spring  46  again. Since the narrow face of the leaf spring  46  in this case moves out of the area of the influence of the magnet  45 —and this is in any case in a position which corresponds to its initial position—the braking torque which occurs before it emerges remains small.  
         [0049]     When the movement of the permanent magnets  44 ,  45  is reversed, the leaf spring  46  is attracted by the magnet  45  and is deflected outwards by it. It then jumps over to the magnet  44 , producing a voltage pulse, from which magnet  44  it is finally released again in the position shown in  FIG. 14   a.    
         [0050]     Both the pole surfaces of the magnets  44 ,  45  and that end surface of the leaf spring  46  which faces the pole surfaces when voltage is produced are—as already mentioned—essentially rectangular, although the longitudinal sides of the rectangles may be curved. The latter applies in particular for the touching longitudinal faces of the magnets  44 ,  45  which are arranged alongside one another. It has been found to be advantageous for the magnets  44 ,  45  to have rectangular cross sections whose length is two to three times greater than the length of the leaf spring  46 .  
         [0051]      FIG. 15  shows, in a highly schematic form, an end view of those parts of a tachometer having a hollow shaft  50  which are significant to the present invention, in which case a mount  51 , to which the permanent magnets  44 ,  45  are fitted and which is connected to the hollow shaft  50 , is illustrated in  FIG. 16  and is in the form of an annular disk, has been omitted for clarity reasons.  
         [0052]     Details of the mounting of the leaf spring  46  and of the arrangement of a coil  52  which partially surrounds it can be found in  FIG. 16 . As can be seen, the lower end of the leaf spring  46  is clamped in between two plates  54 ,  55 , which are preferably composed of plastic, with the interposition of an oscillation-damping material  53 , with these plates  54 ,  55  being mounted in a recess  56  in one wall  57  of the housing of the tachometer. The free end of the leaf spring  46  projects through an opening  58  in a printed circuit board  59  which is fitted, inter alia, with the coil  52 .  
         [0053]      FIGS. 17 and 18  show possible modifications of the arrangement shown in  FIG. 16 . In  FIG. 17 , the coil  52  is mounted on the end surfaces of the lengthened plates  54 ,  55 . In  FIG. 18 , the free end of the leaf spring  46  is used as a holder for the coil  52 .  
         [0054]     In  FIG. 19 , the voltage pulses  60 ,  61  which are induced in the coil  52  at slow rotation speeds are shown as a function of the rotation directions indicated by arrows  62 ,  63 . Corresponding voltage pulses are obviously also produced in the event of linear movements in the direction of the arrows  64 ,  65 .  
         [0055]      FIG. 20  shows a circuit using which the voltage pulses  60  and  61 , respectively, produced in the coil  52  are rectified by means of respective diodes D 1  and D 4 , and are supplied as a supply voltage to an electronic counting circuit E. The counting circuit, which is in the form of a microprocessor circuit, is additionally supplied with an up signal and a down signal, respectively, via the diodes D 5  and D 6 , as a function of the polarity of the voltage pulses which are produced.  
         [0056]     The described apparatus allows the number of revolutions of the hollow shaft  50  and its rotation direction to be unambiguously determined using only one spring  46  and one coil  52 . If the tachometer is intended to be connected to a so-called single-turn absolute transmitter which, as the name says, detects the absolute position within one revolution, while the tachometer counts the number of complete revolutions carried out then, as is shown in  FIG. 21 , at least two coils  52  and  52 ′ equipped with springs  46  should once again be provided. In order to ensure unambiguous synchronization between the single-turn part and the tachometer in this case, the single-turn part is also in this case positioned on the hollow shaft  50  such that its transition from “full revolution” (=360°) to “start of the revolution” (=0°) coincides with the axis of symmetry  66 .  
         [0057]      FIG. 22  shows how the voltages which are produced in the coils  52  (which are connected electrically back to back in parallel) in the arrangement shown in  FIG. 21  can be rectified by diodes D 1 -D 4  and can be used to supply an electronic counting circuit E, and how up and down signals can be obtained in a simple manner via the diodes D 1 -D 4 .  
         [0058]     While, in the case of the embodiments described above, the voltage pulses which are produced in the coils  52  have been used both to supply power to the counting circuit E and for revolution counting,  FIG. 23  illustrates a solution in which the coil  52  which partially surrounds the leaf spring  46 , and three permanent magnet pairs  44 ,  45 ;  44 ′,  45 ′ and  44 ″,  45 ″, are used only to supply power to a counting circuit E. In contrast, the rotation speed is detected by means of an actuator  67  and three fixed sensors  68 ,  69  and  70 , which can be driven by it. When, in the case of the apparatus shown in  FIG. 23 , the magnets  44 ,  45  move through an angle β in the clockwise direction, then they reach the coil  52  and the leaf spring  46  at a time at which the actuator  67  acts on the sensor  68  after carrying out a movement through the same angle β. When the mount (which has also been omitted in  FIG. 23  for clarity reasons but in practice rotates above the plane of the drawing) for the magnet pairs  44 ,  45 ;  44 ′,  45 ′ and  44 ″,  45 ″ and the actuator  67  rotate further through an angle γ of 120°, then the magnets  44 ′,  45 ′ now operate the leaf spring  46 , while the actuator  67  at the same time activates the sensor  69 . When the mount rotates further through an angle γ, the magnet pair  44 ″,  45 ″ act on the spring  46 , and the actuator  67  activates the sensor  70 .  
         [0059]     In the illustrated situation, although this is not absolutely essential, both the magnet pairs  44 ,  45 ;  44 ′,  45 ′ and  44 ″,  45 ″ which rotate about the axis  49  of the tachometer and the fixed sensors  68 ,  69 ,  70  are in each case offset through 120° with respect to one another, with the sensor  70  being diametrically opposite the coil  52 . In consequence, at low rotation speeds, the spring  46  is triggered three times per revolution of the hollow shaft  50  of the tachometer. Whenever the spring  46  and the coil  52  are triggered, one of the sensors  68 ,  69 ,  70  is activated at the same time, and the microprocessor circuit E that is illustrated in  FIG. 24  checks its status. A microprocessor uses the sequence of the sensors  68 ,  69  and  70  which are activated by the actuator  67  to identify the rotation direction in a known manner, and derives up or down counting pulses from this. Interference pulses, which can occur for example on operation of a motor parking brake in the coil  52 , do not lead to incorrect counting, since a counting process takes place only when the coil  52  produces a voltage pulse and one of the sensors  68 ,  69  and  70  is driven at the same time.  
         [0060]     The actuator  67  may be a magnet which, for example, drives MR or Hall sensors, or else Reed contacts. However, it is also possible to use photosensitive sensors. In this case, an opening is provided in the mount for the magnet pairs, instead of the actuator  67 , through which a light beam can pass from light-emitting diodes which are arranged in a fixed position with respect to the sensors, like them.