Abstract:
A method for detecting a movement of a vehicle that has been shut down in a parked state, including: detecting a movement variable which describes a movement of the vehicle, integrating the movement variable, in a manner dependent on a movement direction of the vehicle, to obtain a movement travel, and, if the movement travel meets a predetermined condition, making a decision on the movement for detection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2015/078910 filed Dec. 8, 2015, which claims priority to German Patent Application No. 10 2014 225 831.6, filed Dec. 15, 2014, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for detecting a movement of a vehicle which has been shut down in a parked state, to a method for retensioning a parking brake and to a control device for carrying out at least one of the methods. 
       BACKGROUND OF THE INVENTION 
       [0003]    DE 10 2011 079 362 A1, which is incorporated by reference discloses a method for retensioning a parking brake of a vehicle, which parking brake keeps the vehicle in a stationary state when the vehicle is shut down in a parked state. 
       SUMMARY OF THE INVENTION 
       [0004]    An aspect of the present invention aims to improve the method for retensioning the parking brake. 
         [0005]    According to one aspect of the invention, a method for detecting a movement of a vehicle which is shut down in a parked state, comprises the steps:
       acquiring a movement variable which describes a movement of the vehicle,   integrating the movement variable as a function of a direction of movement of the vehicle with respect to a movement travel and   deciding on the movement to be detected if the movement travel satisfies a predetermined condition.       
 
         [0009]    The specified method is based on the consideration, that the retensioning of the parking brake which is specified at the beginning could be carried out on the basis of monitoring of rolling, in the scope of which a movement of the vehicle is detected when the vehicle is parked and therefore supposedly is assumed to be stationary. As a result, it would be possible to ensure directly that the application force of the parking brake is sufficiently high for the actual stationary state of the vehicle but not undesirably excessively high. 
         [0010]    However, it is problematic that a movement of the vehicle does not necessarily have to result from rolling of the vehicle. Since movements of the vehicle are basically possible owing to a non-infinite high rigidity of the parking brake, movements of the vehicle can occur when a person enters the vehicle, exits the vehicle or when the vehicle is being loaded or unloaded and these movements can then be undesirably interpreted as a rolling movement. However, in this case movements are comparatively small shaking movements of the vehicle which, however, cannot be avoided even with increased retensioning of the parking brake. 
         [0011]    If the parking brake is therefore to be retensioned on the basis of a rolling movement of the vehicle, the rolling movement should be clearly differentiated from the abovementioned small shaking movements. This is where the specified method comes in with the consideration that the small shaking movements are ultimately oscillating movements. An oscillation travel which results from the oscillating movements is therefore canceled out owing to its change of direction of movement over a predetermined consideration time period. This is not the case with a rolling movement. Although a rolling movement can basically also be an oscillating movement with a very high oscillation amplitude, this oscillating movement is large compared to the comparatively small shaking movements explained above, with the result that the rolling movement travel is not canceled out over the predetermined consideration time period. 
         [0012]    In the specified method, the predetermined condition for the movement travel can expediently be defined as a travel boundary which the movement travel should exceed within the previously explained consideration time period so that a rolling movement is decided on. The smaller the predetermined time period is selected to be here, the greater the extent to which the previously mentioned shaking movements are detected as a rolling movement. 
         [0013]    However, the use of a movement boundary and of a predetermined consideration time period as a predetermined condition are not to be considered to be restrictive. Rather, said predetermined condition can be defined as desired. For example, the predetermined condition could also be defined in a frequency range of the movement travel, wherein the movement to be detected can be decided on if the movement travel has sufficiently small (movement) frequency components. 
         [0014]    In order to determine the movement travel, a movement variable which describes the movement of the vehicle and which can be selected as desired is considered. In this way, the movement can be derived, for example from the acceleration of the vehicle or the speed of the vehicle. A further example which is preferred for implementation is described in the dependent claims. 
         [0015]    In one development of the specified method, the vehicle comprises wheels, each having one wheel rotational speed sensor for outputting rotational speed pulses which are dependent on a rotational speed of the respective wheel, wherein the movement variable is described by the rotational speed pulses. As a result of the use of the wheel rotational speed sensors which are present on the vehicle in any case, the specified method can be implemented in a cost-effective fashion on the vehicle because no new sensors have to be attached to the vehicle in order to acquire the measurement variables which are necessary to carry out the specified method. 
         [0016]    In an additional development of the specified method, a comparison of the rotational speed pulses from the wheel rotational speed sensor of a first wheel of the wheels of the vehicle and the rotational speed pulses from the wheel rotational speed sensor of a second wheel of the wheels of the vehicle is used to determine the direction of movement. On the basis of the comparison, the rotational speed pulses of the two wheel rotational speed sensors can be compared in order to infer a reversal of the direction of movement from, for example, the time profile of said rotational speed sensors. If the direction of movement is, for example, defined randomly at the beginning, it is therefore possible to infer the abovementioned oscillating movement over time. Whether the direction of movement which is initially defined randomly is correct or incorrect is irrelevant here because the specified method is ultimately intended to check only whether the movement to be detected is an oscillating movement or not and whether the resulting movement travel is canceled out or not. 
         [0017]    In an additional development of the specified method, the comparison comprises comparing whether a rotational speed pulse from the wheel rotational speed sensor of the first wheel of the vehicle is directly followed chronologically by two rotational speed pulses from the wheel rotational speed sensor of the other wheel of the vehicle. The development is based on the consideration that the abovementioned change of direction of movement gives rise to generation of the rotational speed pulses which are symmetrical in terms of time axes, with respect to the time of the change of the direction of movement. Since owing to the principle involved (for example owing to cornering etc.) the rotational speed sensors never output the rotational speed pulses in synchronism with one another, a change of direction of movement of the vehicle causes two rotational speed pulses of the first wheel to be output chronologically between two rotational speed pulses of the second wheel, which can be used as a criterion of the detection of the change of the direction of movement. 
         [0018]    With the developments described above, shaking movements of the vehicle can be differentiated cleanly from rolling movements of the vehicle at least in the longitudinal direction. However, if the vehicle is excited to shake, for example, when the door is closed in the transverse direction, the abovementioned rotational speed pulses occur completely randomly and are no longer predictable. 
         [0019]    This is where a different development of the specified method having the following steps comes in:
       acquiring a comparison movement variable which describes a comparison movement of the vehicle,   integrating the measured comparison movement variable as a function of a comparison movement direction with respect to a comparison movement travel, and   deciding on the movement to be detected if both the movement travel and the comparison movement travel satisfy the predetermined condition.       
 
         [0023]    The development is based on the consideration that in the case of an actual rolling movement the wheel rotational speed sensors of all the wheels generate the rotational speed pulses with the same pattern. The same applies if the movement variable is also acquired with different measuring pickups than with rotational speed sensors. The presence of this same pattern is checked with the comparison movement variable within the scope of the present development. Only if the movement which is to be detected is acquired on the basis of the movement variable and the comparison movement variable can a shaking movement of the vehicle actually be ruled out and a rolling movement can be inferred. 
         [0024]    In one particular development of the specified method, the comparison movement variable is described by means of the rotational speed pulses, and the comparison movement direction is determined on the basis of a comparison of the rotational speed pulses from the wheel rotational speed sensors of at least two wheels of the vehicle, at least one wheel of which is a third wheel of the wheels of the vehicle. In this way it is possible to avoid a situation in which the movement to be detected is evaluated twice on the same movement variable of the vehicle, and a rolling movement of the vehicle is therefore inferred erroneously. 
         [0025]    In yet another development of the specified method, a number of comparison movements based on the decision on the movement to be detected and/or the predetermined condition are dependent on a gradient of an underlying surface on which the vehicle is standing. Here, the decision should be configured all the more sensitively the steeper the underlying surface. 
         [0026]    In a further development, the specified method comprises the step of resetting the movement travel and/or the comparison movement travel if the movement travel and/or the comparison movement travel satisfy/satisfies a further predetermined condition which is in particular different from the predetermined condition. 
         [0027]    According to a further aspect of the invention, a method for retensioning a parking brake which keeps a vehicle which has been shut down in a parked state in a stationary state comprises the steps:
       detecting a movement of the vehicle which has been shut down in a parked state with a method as claimed in one of the preceding claims, and   retensioning of the parking brake on the basis of the detected movement.       
 
         [0030]    According to another aspect of the invention, a control device is configured to carry out one of the specified methods. 
         [0031]    In one development of the specified control device, the specified device has a memory and a processor. In this context, the specified method is stored in the form of a computer program in the memory, and the processor is provided for carrying out the method when the computer program is loaded into the processor from the memory. 
         [0032]    According to a further aspect of the invention, a computer program comprises program code means for carrying out all the steps of one of the specified methods when the computer program is executed on a computer or one of the specified devices. 
         [0033]    According to a further aspect of the invention, a computer program product contains a program code which is stored on a computer-readable data carrier and which, when executed on a data processing device, carries out one of the specified methods. 
         [0034]    According to a further aspect of the invention, a vehicle comprises one of the specified control devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The properties, features and advantages of this invention described above as well as the way in which they are achieved, become clearer and more readily understandable in conjunction with the following description of the exemplary embodiments which are explained in more detail in conjunction with the drawings, in which: 
           [0036]      FIG. 1  shows a schematic view of a vehicle which is parked on the road, 
           [0037]      FIG. 2  shows a schematic view of the vehicle from  FIG. 1 , 
           [0038]      FIG. 3  shows a schematic view of a closed-loop control circuit for retensioning a brake in the vehicle in  FIG. 2 , 
           [0039]      FIG. 4  shows a schematic view of a rotational speed sensor in the vehicle in  FIG. 2 , 
           [0040]      FIG. 5  shows an output signal from the rotational speed sensor in  FIG. 4 , 
           [0041]      FIG. 6  shows a device for processing the output signal from  FIG. 5 , 
           [0042]      FIG. 7  shows two output signals from two different rotational speed sensors in the vehicle in  FIG. 2  in a first movement situation of the vehicle in  FIG. 2 , 
           [0043]      FIG. 8  shows the two output signals in  FIG. 7  in a second movement situation of the vehicle in  FIG. 2 , and 
           [0044]      FIGS. 9 to 12  show alternatives for the device in  FIG. 6  for processing the output signal in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0045]    In the figures, the same technical elements are provided with the same reference symbols and are described only once. 
         [0046]    Reference is made to  FIG. 1  which shows a vehicle  2  which is parked on an underlying surface in the form of a road  1 , between a second vehicle  3  and a third vehicle  4 . The road  1  is inclined with a gradient  5 . 
         [0047]    The vehicle  2  comprises a chassis  7  which is supported in a moveable fashion on wheels  6 , and is located in a parking space  8  between the second vehicle  3  and the third vehicle  4 . So that the vehicle  2  does not roll down the road as a result of a downhill slope force  9  which is caused by the gradient  5 , and collides, for example, with the third vehicle  4 , it is kept in the stationary state in this parking space  8  by means of a parking brake  10  to be described below. 
         [0048]    In particular, the parking brake  10  holds the vehicle  2  with a holding force  11  which counteracts the downhill slope force  9 . More details will be given on this below with reference to  FIG. 2 . 
         [0049]    The parking brake  10  of the vehicle comprises a parking brake control device  12  which actuates, on the basis of a braking request  13 , brake actuators  15 , such as brake linings, which are mounted in a positionally fixed fashion with respect to the chassis  10 , in order to apply said brake lining. As a result of the application, brake engagement elements  16  such as brake drums or brake disks which are arranged in a positionally fixed fashion with respect to the wheels  6  are clamped by the brake actuators  15  with, in each case, a clamping force (not illustrated further), with the result that rotation of the wheels  6  with respect to the chassis  7  is blocked and a movement of the vehicle  2  is prevented. This generates the holding force  11  which is intended to keep the vehicle  2  in the stationary state. In this context, the brake control signals  14  can be transmitted in any desired fashion, for example hydraulically, pneumatically or with bowden cables. In vehicles which are embodied as passenger vehicles, the brake control signals  14  are frequently transmitted electrically, for which reason the term electronic parking brake is also used. 
         [0050]    The holding force  11  is primarily dependent here on the clamping forces with which the brake actuators  15  clamp the brake engagement elements  16 . The magnitude of the clamping forces is in turn defined by the brake control signals  14 . Therefore, if sufficiently large clamping forces are not predefined with the brake control signals  14 , the holding force  11  may not be sufficient to cancel out the downhill slope force  9  and keep the vehicle  2  in a stationary state. On the other hand, the clamping forces should, however, also not be unnecessarily large so that the mechanical loads in the parking brake  10  remain as low as possible, for example for the sake of a long service life. 
         [0051]    This is where the present embodiment comes in with the proposal to set the brake control signals  14  and therefore the holding force  11  as a function of a movement  17  of the vehicle  2  which is indicated in  FIG. 3 . The basis for the detection of the movement  17  of the vehicle  2  can basically be a measurement variable from any sensor in the vehicle  2 , which measurement variable contains a movement speed  18  of the vehicle  2 . 
         [0052]    This movement variable which contains the movement speed  18  should expediently be acquired with sensors which are already present in the vehicle  2 . For this purpose, for example rotational speed sensors  19  are appropriate which are usually present on each wheel  6  of the vehicle  2  in order to acquire a wheel rotational speed  20  of the respective wheel  6 . These wheel rotational speeds  20  are used in the vehicle  2  for various applications such as, for example, the vehicle movement dynamics control or parking assistant. Inter alia, the so-called ground speed of the vehicle  2  can also be derived from the wheel rotational speeds  20 , which ground speed could basically be used as a movement speed  18 . 
         [0053]    However, in order to detect a movement of the vehicle  2  as quickly as possible and therefore to keep the reaction times when setting the holding force  11  as short as possible, within the scope of the present embodiment is it proposed to detect the movement of the vehicle  2  directly from the wheel rotational speeds  19 . For this purpose, the wheel rotational speeds  20  are fed to a movement prevention device  21 . The movement prevention device  21  detects the movement  17  of the vehicle  2  with a movement detection device  27  in a manner to be described below. 
         [0054]    In the case of the detected movement  17 , the movement prevention device  21  outputs a brake request signal  22 , on the basis of which the parking brake control device  12  can then set the brake control signals  14  appropriately, in order to set the holding force  11  to a sufficiently large value by means of the clamping forces specified above. 
         [0055]    Viewed in abstract terms, the movement prevention device  21  constitutes part of a closed-loop control circuit  23  which is illustrated in  FIG. 3  and in which the detected movement  17  is used as an actual variable and is compared with a setpoint variable  24  before the movement  17  of zero. In a controller  25  in the movement prevention device  21 , a control difference  26  between the detected movement and the setpoint variable  24  is acquired, and the brake request signal  22  is set in such a way that the movement  17  is approximated to the setpoint variable  24  and therefore to a movement of zero. 
         [0056]    Before more details are given on the detection of the movement, firstly more details will be given on the acquisition of the measurement variable which contains the movement speed  18  and therefore on the wheel rotational speeds  20 . For this purpose, reference is made to  FIG. 4  which shows a schematic view of the wheel rotational speed sensor  19 . 
         [0057]    Each rotational speed sensor  19  is embodied in the present embodiment as an active rotational speed sensor which comprises an encoder element, mounted in a rotationally fixed fashion on the wheel  6 , in the form of an encoder disk  28  and a sensor circuit which is mounted in positionally fixed fashion with respect to the chassis  7  and is referred to below for the sake of simplicity as a reading head  29 . 
         [0058]    The encoder disk  28  is composed in the present embodiment of magnetic north poles  30  and magnetic south poles  31 , which are arranged in rows next to one another and which together generate a physical field in the form of an encoder magnetic field  32 . This encoder magnetic field  32  is indicated in  FIG. 3  with two field lines (illustrated by dashed lines) for the sake of clarity. If the encoder disk  28  which is mounted on the wheel  6  rotates with the latter in a rotational direction  33 , the encoder magnetic field  32  therefore rotates along with it. 
         [0059]    The reading head  29  which is positionally fixed with respect to the chassis  7  comprises in the present embodiment a measuring sensor  34  which senses the encoder magnetic field  32  of the encoder disk  28  which rotates with the wheel  6  and converts it into an encoder signal  35 . Owing to the principle involved, the encoder signal  35  is sinusoidal with a frequency which is directly dependent on the rotational speed  20 . In a signal evaluation circuit  36 , the sinusoidal encoder signal  35  is converted, for technical reasons, into a pulse signal  37  and output to the movement detection device  27  in the movement prevention device  21 . The frequency of the pulse signal  37  therefore remains the same as the frequency of the encoder signal  35 , and therefore the information about the rotational speed  20  is retained. Further background information on active wheel rotational speed sensors can be found in the relevant prior art, such as, for example, in DE 101 46 949 A1, which is incorporated by reference. 
         [0060]    More details are given below on a possible detection of the movement  17  in the movement detection device  27  on the basis of the wheel rotational speeds  20  with reference to  FIGS. 5 and 6  which show the pulse signal  37  in a pulse level  38 —time  39 —diagram. 
         [0061]    The pulse signal  37  indicates an accelerating movement  17  of the vehicle  2 . This can be detected from the fact that the encoder disk  28  rotates more and more quickly. Correspondingly, pulses  40  in the pulse signal  37 , which pulse to and fro between a first level value  41  and a second level value  42  become increasingly narrow over the time  39 . In other words, the number of pulses  40  increase over the time  39  with the increasing movement speed  18  of the vehicle  2 . The two level values  41 ,  42  are dependent on whether the encoder disk  28  is created with a magnetic north pole  30  or with a magnetic south pole  31  below the reading head  34 . 
         [0062]    A movement  17  of the vehicle  2  is understandably present when the vehicle  2  has detectedly traveled through a movement travel  43  indicated in  FIG. 1 . For this purpose, a predetermined condition can be defined for the movement travel  43 , for example, in the form of a travel boundary  44  indicated in  FIG. 1 . If the movement travel  43  exceeds the travel boundary  44 , the movement  17  of the vehicle  2  is decided on. The movement travel  44  results from integration of the movement speed  18  over the time  39 , with the result that the previously explained concept can be implemented extremely easily if the movement speed  18  is present directly. 
         [0063]    In the case of the present embodiment, the movement is to be detected in the movement detection device  27  from at least the wheel rotational speeds  20  from at least one of the wheel rotational speed sensors  19 . In order to integrate the movement speed  18  and therefore to acquire the movement travel  43 , the pulses  40  of the pulse signal  37  can be counted here in a counter  45 . A travel counting value  46  which is obtained in this way is directly dependent on the movement travel  44  and can be compared in a comparison element  47  with a travel counting boundary  49  which is dependent on the travel counting boundary  48  and can be stored in a memory  50 . If the travel counting value  46  exceeds the travel counting boundary  48 , the movement  17  is decided on with the comparison element  47 . 
         [0064]    On the basis of the movement  17  which is detected in this way it is therefore possible for the closed-loop control circuit  23  to correspondingly engage in the vehicle  2  in the manner described above. 
         [0065]    The movement  17  of the vehicle  2  which is to be detected is to be a rolling movement of the vehicle  2  here. In particular, the movement to be detected is not to include any shaking movements which arise when the vehicle  2  shakes to and fro as a result of an impact. Such shaking movements are not unusual because the brake actuators  15  on the vehicle  2  are arranged in a floating fashion with respect to the brake engagement elements  16  and therefore in the parked state the vehicle  2  is not held in an ideally roll-free fashion. As a rule, the encoder disk  28  has over ninety poles  30 ,  31 , with the result that even the smallest rolling movements of a few degrees can generate pulse signals  37  with pulses  40  which are multiplied when the vehicle shakes to and fro and therefore can undesirably give rise to a detected movement  17  and therefore undesirably to relatively strong application of the brake actuators  15 . 
         [0066]    In order to gate out these shaking movements during the detection of the movement of the vehicle  2 , in the present exemplary embodiment use is made of the realization that the shaking movement of the vehicle  2  is a diminishing movement with regular reversals  51  of the direction indicted in  FIG. 8 . These reversals  51  of direction are detected in the present exemplary embodiment and also taken into account in the integration of the movement travel  43  or of the travel counting value  46 . 
         [0067]    In order to detect the reversals  51  of direction in the present exemplary embodiment use is made of the realization that the pulse signals  37 ,  37 ′ generally do not have a time profile which is synchronous with one another because the wheels do not rotate with the same rotational speed  20  from time to time, for example during cornering. In this way, phase offsets  52  indicated in  FIG. 7  are introduced, said phase offsets  52  being indicated between the pulse signal  37  in  FIG. 5  and a further pulse signal  37 ′ from another wheel rotational speed sensor  19 . 
         [0068]    As indicated in  FIG. 8 , a reversal  51  of direction of the vehicle  2  has the effect, for example during the shaking movement which is to be gated out, that the individual pulse signals  37 ,  37 ′ from the individual wheel rotational speed sensors  19  have a mirror-symmetrical profile at the time of the respective reversal  51  of direction, with the effect that a pulse  40  generated in the pulse signal  37  is followed, before the reversal  51  of direction, provided with reference symbol  53  in  FIG. 8  for the sake of clarity, by a first pulse  40  and a second pulse  40  in the other pulse signal  37 ′, which are correspondingly provided with the reference symbols  54  and  55  in  FIG. 8  for the sake of clarity, before in the one pulse signal  37  with the pulse  53  it is followed by a further pulse  40  which is provided with the reference symbol  56  in  FIG. 8  for the sake of clarity. This means that in the case of a reversal  51  of direction at a pulse  53  in one of the pulse signals  37  two pulses  54 ,  55  are detected in the other pulse signal  37 ′ before a pulse is detected again in the pulse signal  37 . Such a profile of the pulses  40  in the pulse signals  37 ,  37 ′ cannot occur during a movement  17  in the form of a rolling movement of the vehicle  2  from the stationary state owing to the principle involved, with the result that this profile of the pulses  40  in the pulse signals  37 ,  37 ′ can be used as a detection criterion for the reversal  51  of direction. 
         [0069]    Therefore, within the scope of the present embodiment it is proposed to decide on the direction of reversal  51  on the basis of a comparison of two pulse signals  37 ,  37 ′ if the scenario illustrated in  FIG. 8  is detected. Then, for example instead of counting forward in the counter  54  in  FIG. 6  it is possible to count backward in order to take into account the profile of the movement travel  43  during the shaking movement of the vehicle  2 . 
         [0070]    The decision can be made with a double pulse detection device  57  which is shown in  FIG. 9  and which then actuates a switch  58  in order to conduct, if appropriate via a gating element  59 , one of the pulse signals  37  whose pulses  40  are counted to form the travel counting value  46 , so as to implement the abovementioned counting forward and counting backward. A corresponding technical capability of the counter  54  to count forward in the case of positive pulses  40  and to count backward in the case of negative pulses  40  is assumed here. 
         [0071]    However, in the case of a shaking movement transversely with respect to the rolling direction of the vehicle  2 , the pulses occur completely randomly at the wheels. In order to also separate as reliably as possible a rolling movement of the vehicle from a shaking movement in this situation, within the scope of the present embodiment it is proposed to carry out the abovementioned comparison of the pulse signals  37 ,  37 ′ not on the basis of two wheel rotational speed sensors  19  of the vehicle  2  but rather at least on the basis of three, preferably four, wheel rotational speed sensors  19  of the vehicle  2 . 
         [0072]    This is clarified with reference to  FIG. 10  in which the pulse signals  37 ,  37 ′ and  37 ″ from three different wheel rotational speed sensors  19  are evaluated by way of example. The three different pulse signals  37 ,  37 ′ and  37 ″ permit three comparison possibilities. Four different pulse signals  37  would correspondingly allow six comparison possibilities. Each comparison possibility for the detection of a double pulse as explained in  FIG. 8  is carried out with a separate double pulse detection device  57 , wherein each double pulse detection device  57  indicates, with a detection signal  60 , the presence of a double pulse. A counting device  61  counts the number of detected double pulses indicated by the individual detection signals  60 , at regular time intervals, for example once per software cycle. The counting device  61  now actuates the switch  58  only if the number of detected double pulses exceeds a predetermined double pulse counting threshold  62 . 
         [0073]    The counting device  61  ultimately ensures that a randomly occurring double pulse is not evaluated as the reversal  51  of direction. Only if ultimately all the wheels  6  of the vehicle  2  exhibit the same behavior can the shaking movement of the vehicle  2  be reliably inferred from a double pulse which occurs. 
         [0074]    Alternatively, as shown in  FIG. 11  it is also possible to detect a separate preliminary movement  17 ′ of the vehicle  2  on the basis of each comparison, wherein the counting device  61  indicates the movement  17  of the vehicle  2  when the number of preliminary movements exceeds the double pulse counting threshold  62 . 
         [0075]    In an embodiment indicated in  FIG. 12 , a single double pulse detection device  57  can be provided which receives all the pulse signals  37 ,  37 ′,  37 ″ and outputs the detection signal  60  as soon as two successive pulses  40  are sensed in one of the pulse signals  37 ,  37 ′,  37 ″ without a pulse  40  being sensed between them on one of the other pulse signals  37 ,  37 ′,  37 ″. In this case, the detection signal  60  directly contains the information about the reversal  51  of direction. 
         [0076]    In all the exemplary embodiments, the counters  54  can be reset if the travel counting value  46  of one of the counters  45  exceeds a resetting threshold which expediently should be higher than the travel counting boundary  49 . This fact is no longer illustrated graphically in the figures for the sake of clarity. 
         [0077]    Although the movement detection device  27  is used in the closed-loop control circuit  23  in  FIG. 3 , an alarm device, which, for example, warns the driver or other persons that the vehicle is moving and the parking brake  10  has to be pulled on more strongly, could alternatively be actuated on the basis of the detected movement. 
         [0078]    In addition, the double pulse counting threshold  62  can be selected as a function of the gradient  5 , in order, for example, to carry out the detection of the movement  17  more sensitively in the case of relatively large gradients.