Abstract:
A control unit of a vehicle brake system has a receive circuit for tapping and processing a digital signal provided by an active speed sensor for speed measurement on a vehicle wheel. The signal includes information concerning the speed of a pole wheel, which is arranged in front of the sensor, spaced therefrom by an air gap, and which rotates along with the wheel. The signal also includes an actual air gap value digitized in stages and comprising several bits. The control unit also has a comparator circuit for comparing the actual air gap value to a reference air gap value and for determining whether the actual air gap value exceeds the reference value by more than a defined tolerance value. An information circuit of the control circuit generates early warning information when the comparator circuit determines that the actual air gap value exceeds the reference value by more than the tolerance value.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a control unit for controlling a vehicle brake system. 
     BACKGROUND OF THE INVENTION 
     It is known to arrange a passive sensor in a drilled hole or in another securing opening in the region of a vehicle wheel in a clamping fashion in order to perform rotational speed sensing. Analog signals are transmitted from the passive rotational speed sensor to a control unit, which can additionally determine the respective air gap between the passive sensor and a pole wheel from these analog signals. The passive sensor can be displaced axially in the drilled hole and can therefore, under certain circumstances, assume an unacceptably large distance from the pole wheel, which can, however, be detected at the control unit. 
     Furthermore, if is known to arrange or to secure an active rotational speed sensor in an immovable fashion in the region of the pole wheel. The active rotational speed sensor therefore cannot be displaced unacceptably. When there is such an active rotational speed sensor, simple monitoring of the air gap is carried out by means of the rotational speed sensor itself. DE 199 11 774 A1 discloses detecting changes in the air gap by means of the rotational speed sensor in order to generate a corresponding status signal in good time before a possible sensor signal loss. However, in this context, only a binary distinction is made at the rotational speed sensor between an acceptable and an unacceptable air gap. 
     Furthermore, for other purposes, an analog signal, which is a measure of the air gap, is digitized with 3-bit coding and provided to a control unit. According to DE 102 03 483 A1, the word size with which, for example, the magnetic air gap field strength can be transmitted from the rotational speed sensor to the control unit can be 4 bits. The control unit can track the chronological profile of the air gap field strength with increased resolution and, if appropriate, evaluate it further. In particular, this procedure aims, according to DE 102 03 483 A1, to sense dynamic changes in the air gap and use them to determine driving states in a brake control system and/or vehicle dynamics control system. Furthermore, the observation of the current air gap according to DE 102 03 483 A1 can also be used as a measure of the lateral acceleration acting on the wheel or the wheel bearing temperature. Therefore, only changes in the air gap that are caused by the position of the pole wheel are evaluated at the control unit. On the other hand, the correct position of the rotational speed sensor is further monitored in a simple way by means of the speed sensor. 
     SUMMARY OF THE INVENTION 
     Generally speaking, it is an object of the present invention to improve the monitoring of an air gap between an active rotational speed sensor and a pole wheel or encoder, which is arranged on a wheel of the vehicle and rotates together with the wheel, when sensing rotational speed by means of the active rotational speed sensor. The rotational speed of the wheel can be inferred from the rotational speed of the pole wheel. 
     The control unit has a receiver circuit for tapping and processing the digital signal that is provided by the active rotational speed sensor for the rotational speed measurement at the wheel of the vehicle. This digital signal contains rotational speed information about the rotational speed of the pole wheel, which is arranged in front of the rotational speed sensor, spaced therefrom by an air gap, and which rotates along with the wheel. Furthermore, this digital signal comprises an actual air gap value that is digitized in a plurality of stages, comprises a plurality of bits and is a measure of the current air gap between the pole wheel and the rotational speed sensor. The actual air gap value can be directly a distance value but can alternatively also be another value, for example a field strength value, from which the air gap can be determined. At any rate, the actual air gap value is a measure of the current air gap between the pole wheel and the rotational speed sensor. The receiver circuit is also preferably designed to determine the rotational speed or the rotational frequency of the pole wheel from the rotational speed information. 
     Furthermore, the control unit has a comparator circuit designed to compare the actual air gap value with a setpoint air gap value. In particular, a computing device of the control unit can be designed such that it includes this comparator circuit and, if appropriate, further circuits indicated below. The comparator circuit is designed to determine whether the actual air gap value exceeds the setpoint air gap value by more than a defined tolerance value. Furthermore, the control unit has an information circuit for generating and providing pre-warning information if the comparator circuit has determined that the actual air gap value exceeds the setpoint air gap value by more than the tolerance value. In particular, the pre-warning information is generated and provided in response to a determination that the actual air gap value exceeds the setpoint air gap value by more than the tolerance value. According to a preferred embodiment, the defined tolerance value is 0, with the result that any known deviation of the actual air gap value from the setpoint air gap value generates pre-warning information. According to an alternative embodiment, the tolerance value is, however, a value other than 0, with the result that small deviations of the actual air gap value from the setpoint air gap value are tolerated without pre-warning information being generated. 
     Embodiments of the present invention permit multi-stage monitoring of the correct position of the rotational speed sensor. The control unit already detects slipping of the rotational speed sensor when the rotational speed sensor is still supplying correct values. For this reason, slipping of the rotational speed sensor can be detected and eliminated early. Inventive embodiments therefore permit an active rotational speed sensor to be clamped similarly or analogously to the known clamping of a passive or inductive rotational speed sensor. If the active rotational speed sensor shifts, for example, as a result of vibrations, this is signaled early and can be eliminated in a workshop, for example, by simply reprinting the rotational speed sensor. A complete failure of the rotational speed sensor can therefore be avoided in most cases, which increases safety during operation of the vehicle. 
     It should be appreciated that the control unit according to embodiments of the present invention can have application in a vehicle brake system, including in a rotational speed sensor arrangement. A method according to an embodiment of the present invention for sensing the rotational speed is effected by and corresponds to the purpose or the configuration of the control unit or of the brake system or of the rotational speed sensor arrangement. The inventive advantages thus correspond to the inventive advantages of the control unit. 
     According to an embodiment, the comparator circuit of the control unit is designed to compare the actual air gap value with a limiting air gap value. The comparator circuit can determine whether the actual air gap value reaches or exceeds the limiting air gap value. The limiting air gap value is a critical value, at which data or information from the corresponding rotational speed sensor should no longer be used. According to this embodiment, the information circuit is therefore designed to generate and provide fault information if the comparator circuit has determined that the actual air gap value reaches or exceeds the limiting air gap value. In particular, the information circuit is designed to generate and provide the fault information if the comparator circuit has determined that the actual air gap value exceeds the setpoint air gap value by more than the tolerance value and reaches or exceeds the limiting air gap value. 
     The comparator circuit preferably does not perform the comparison of the actual air gap value with the limiting air gap value until it has been detected that the actual air gap value exceeds the setpoint air gap value by more than the tolerance value or differs from the actual air gap value. A differentiation can be made between still tolerable and no longer tolerable deviations of the actual air gap value from the setpoint air gap value i.e., to generate pre-warning information and/or fault information as a function of the determined air gap between the rotational speed sensor and the pole wheel. 
     According to one variant of this embodiment, the limiting air gap value is set as a function of the setpoint air gap value, and therefore given by means of a set differential value similarly to the defined tolerance value. According to an alternative variant, the limiting air gap value is set independently of the setpoint air gap value. 
     According to another embodiment, the information circuit is designed to generate and provide installation information. This installation information comprises the information as to whether the installation of the rotational speed sensor has been determined as correct or incorrect. According to this embodiment, the comparator circuit is therefore designed to detect, by means of the comparison of the actual air gap value with a predefined air gap value or with an interval between predefined air gap values, the installation of the rotational speed sensor as correct if the actual air gap value corresponds to the predefined air gap value or is in the interval between the predefined air gap values, and otherwise to detect the installation as incorrect. 
     According to this embodiment, automatic detection of the correct installation position can be effected by means of the control unit during the fabrication of the vehicle or during or after assembly, for example at the end of the production line. The installation information therefore comprises assessment of the currently determined actual air gap value, which can, however, be contained additionally in the installation information. 
     According to a further embodiment of the present invention, the information circuit is alternatively or additionally designed to generate and provide further installation information that contains the actual air gap value. With this embodiment, during or after the fabrication of the vehicle, for example at the end of the production line, the digital measured value of the air gap, which has been determined and provided at the control unit, or the actual air gap value can be read out by means of a diagnostic device and, for example, displayed, with the result that it is possible to detect manually or by means of software on the diagnostic unit whether the rotational speed sensor is correctly installed. 
     According to another embodiment, the control unit has an initialization circuit for setting the setpoint air gap value to the current actual air gap value, in particular in response to installation of the rotational speed sensor, which has been detected as correct, and a memory circuit for storing the setpoint air gap value. With the initialization circuit, during fabrication or in response to a diagnosis at the end of the production line, the setpoint air gap value can be set, and with the memory circuit, can be stored, in particular, in a data memory. The comparator circuit therefore subsequently performs comparisons relative to the setpoint air gap value, which has been detected as correct. According to an alternative embodiment, the setpoint air gap value is set to a predefined value. This can be appropriate, in particular, when the defined tolerance value is a value other than zero, and during fabrication the only check has been whether the actual air gap value is in an interval between predefined air gap values. 
     A standardized data protocol, for example the AK protocol, is preferably used to transmit the rotational speed information and the actual air gap value. Rotational speed pulses are transmitted here with a first level, wherein the distance between the rotational speed pulses or the frequency thereof provides conclusive information about the rotational speed or rotational frequency of the pole wheel. After each rotational speed pulse, a data protocol is transmitted with a second level, which is reduced compared to the first level. The data protocol has a first bit sequence with information and a subsequent second bit sequence with additional information. This additional information preferably comprises the actual air gap value. The actual air gap value is preferably transmitted with word sizes of 3 or 4 bits in the digital signal or in the additional information. These 3 or 4 bits are free bits to which specific information can be freely assigned. In contrast, the first bit sequence is defined with the information for compatibility reasons. 
     In the data protocol, the data or bits, that is, the differentiation between 1 and 0, are defined either by the amplitude of the signal or preferably by the rise or fall in pulse edges. As an alternative to the AK protocol, the data are transmitted encoded in some other way, for example by means of pulse width modulation (PWM). 
     The receiver circuit is preferably designed to determine the rotational speed from the frequency of the arrival of rotational pulses, which are received with a first level, and to determine the actual air gap value from a data protocol, which is respectively received after a rotational speed pulse with a second level. In addition, the control unit is preferably designed to read out the actual air gap value from 3 or 4 bits in the digital signal. 
     The rotational speed sensor arrangement according to an embodiment of the present invention has, in addition to the inventive control unit, the active rotational speed sensor and a data interface tor transmitting the digital signal from the active rotational speed sensor to the control unit. The data interface is, for example, a two wire line, which is preferably twisted. The data interface is therefore similar to a data interface with a known passive rotational speed sensor. 
     According to one embodiment, the active rotational speed sensor has an active sensor element for actively sensing the rotation of the pole wheel, which is arranged in front of the rotational speed sensor, separated therefrom by the air gap, and rotates along with the edge. Furthermore, the active rotational speed sensor preferably has an air gap-detection circuit for determining an analog measured value, which is a measure of the current air gap between the pole wheel and the rotational speed sensor, and for multi-stage digitization of the analog measured value in order to generate the digital actual air gap value, which comprises a plurality of bits. In addition, the active rotational speed sensor has a transmitter circuit for providing the digital signal, which contains rotational speed information about the rotational speed of the pole wheel and the actual air gap value. 
     According to an embodiment, the rotational speed sensor for measuring the rotational speed is embodied such that it can be clamped, in particular by means of a clamping bushing, in any desired radial orientation and/or in an axially displaceable manner in a securing opening in front of the pole wheel, and/or is clamped in front of the clamping wheel and senses the rotational speed of the pole wheel there. The active sensor element is preferably embodied such that the active rotational speed sensor automatically detects its radial orientation. The active rotational speed sensor can therefore be clamped in the securing opening in a way that is analogous to a known inductive or passive rotational speed sensor. 
     Furthermore, according to an embodiment, the rotational speed sensor arrangement is embodied such that the rotational speed sensor can be supplied with electrical energy via the data interface. There is therefore no need to provide any additional line for supplying electrical energy beyond the data line, in particular the twisted two-wire line. 
     A brake system according to an embodiment of the present invention is, for example, a pneumatic brake system with brake cylinders that can be activated pneumatically in order to brake the wheels of the vehicle. The rotational speed information is used, for example, for automatic brake interventions in an anti-lock brake function or in an electronic stability program. 
     A vehicle according to an embodiment of the present invention, is, in particular, a motor vehicle with an engine. In addition, the vehicle is preferably a utility vehicle that can carry or tow a load. The vehicle has wheels that can be braked by means of a brake system, which can be the brake system according to an embodiment of the present invention. As an alternative to the brake system, the vehicle has the inventive control unit or the inventive rotational speed sensor arrangement. 
     Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
     The present invention accordingly comprises the features of construction, combination of elements, arrangement of parts, and the various steps and the relation of one or more of such steps with respect to each of the others, all as exemplified in the constructions herein set forth, and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is discussed in greater detail below on the basis of an exemplary embodiment illustrated in the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a vehicle, having a brake system that has a sensor arrangement with a control unit according to an exemplary embodiment of the present invention; 
         FIG. 2  is a block circuit diagram of the control unit of the exemplary embodiment according to  FIG. 1  with signal paths; and 
         FIG. 3  is a block circuit diagram of the rotational speed sensor of the exemplary embodiment according to  FIG. 1  with signal paths. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a simplified box illustration of a vehicle  1 , which is preferably a motor vehicle embodied as a utility vehicle. The vehicle  1  has a wheel  2  as well as further wheels and components. In particular, the vehicle  1  has, for example, an internal combustion engine for driving the wheel  2  and/or the further wheels and a brake system  4  whose system boundary is shown in  FIG. 1 . 
     By means of the brake system  4 , the vehicle  1  can be braked or arrested. For this purpose, a brake cylinder  6  is arranged on the wheel  2 . The brake cylinder can be activated pneumatically via a compressed air line  8 . The brake cylinder can also be activated hydraulically by a brake modulator  10 . The brake modulator  10  is activated by a control unit  14  via an electric control line  12 . The control unit  14  preferably performs the actuation of the brake modulator  10  as a function of an electric brake request signal, which is generated directly or indirectly by the activation of a brake pedal and provided to the control unit  14 . In addition, various sensor signals, for example of a sensed pressure in the compressed air line  8 , can be provided to the brake modulator  10 . The control unit  14  is connected to a rotational speed sensor  28  via a data interface  16 , which is preferably embodied as a twisted two-wire line. The control unit  14 , the data interface  16  and the rotational speed sensor  18  form, if appropriate together with further components, a sensor arrangement  20 . 
     The rotational speed sensor  18  is arranged in the region of the wheel  2  in front of a pole wheel  22 , which rotates along with the wheel  2 , but spaced apart from the pole wheel  22  by an air gap  24 . The pole wheel  22  is, for example, a steel gear wheel or a wheel with a permanent-magnetic structure, with the result that a signal voltage can be generated in the rotational speed sensor  18 . 
     The rotational speed sensor  18  is clamped in any desired radial orientation and in an axially displaceable fashion counter to a clamping force by means of a clamping bushing  26  in a securing opening  28 , which is provided by a securing device or by means of a drilled hole. For the mounting of the rotational speed sensor  18 , it is therefore sufficient to insert the clamping bushing  26  and subsequently the rotational speed sensor  18  into the securing opening  28 . 
     Rotational speed pulses and further data are transmitted to the control unit  14  via the data interface  16 . In addition, the rotational speed sensor  18  is supplied with electrical energy by the control unit  14  via the same data interface  16 . Corresponding arrangements can also be provided for the further wheels of the vehicle  1 . 
       FIG. 2  is a block circuit diagram of the control unit  14  of the exemplary embodiment according to  FIG. 1  showing internal signal paths between devices of the control unit  14 . According to  FIG. 1 , a digital signal  30  is provided to the control unit  14  via the data interface  16 . The digital signal  30  contains an actual air gap value  32 , which is generated by means of the rotational speed sensor  18  according to  FIG. 1  and is a measure of the air gap  24 . In addition, the digital signal  30  contains rotational speed information  34 . A receiver circuit  36  of the control unit  14  is configured to determine, from the rotational speed information  34  in the digital signal  30 , a rotational speed value  38  or the rotational speed of the pole wheel  22  and therefore that of the wheel  2 , and provide it to a brake controller  40 , which generates brake control signals  42 , if appropriate by taking into account the rotational speed value  38 , and can provide them to the brake modulator  10  via the electric control line  12  according to  FIG. 1 . 
     The receiver circuit  36  is also configured to extract the actual air gap value  32  from the digital signal  30  and to provide it both to a comparator circuit  44  of the control unit  14  and to an initialization circuit  46  of the control unit  14 . 
     Furthermore, the control unit  14  has a memory circuit  48  with a memory  50  and an information circuit  52 . The memory  50  is a data memory in which various predefined comparison values are stored and in which corresponding values can be stored by means of the memory circuit  48 . In the memory  50 , a predefined air gap value  54  is stored. The air gap value  54  is read out from the memory  50  by the comparator circuit  44  and compared with the actual air gap value  32  in order to check the correct position of the rotational speed sensor  18  in front of the pole wheel  22  after fabrication and before delivery of the vehicle  1 . In particular, an interval between predefined air gap values  54  is read out of the memory  50  by means of the comparator circuit  44 . If the actual air gap value  32  is within this interval, the installation position of the rotational speed sensor is detected as correct, with the result that the information circuit  52  generates installation information  56  according to which the position of the rotational speed sensor  18  has been detected as correct. This installation information  56  is also provided to the initialization circuit  46 , which, in response thereto, writes the actual air gap value  32  as a new setpoint air gap value  60  to the memory  50  by means of the memory circuit  48 . In addition, the measured air gap  24  is provided in the installation information  56  and/or in additionally provided further installation information  58 . 
     During normal operation of the vehicle  1  or of the brake system  4  or of the control unit  14 , the setpoint air gap value  60  is used by the comparator circuit  44  for comparison with the actual air gap value  32  in order to permit, in the event of a deviation above a tolerance value  62 , which is also read out from the memory  50 , the information circuit  52  to provide pre-warning information  64  if the comparator circuit  44  has determined that the actual air gap value  32  exceeds the setpoint air gap value  60  by more than the tolerance value  62 . As an alternative to the exemplary embodiment shown, the tolerance value  62  is not stored in the memory  50 . In particular, if the value 0 is stored as a tolerance value  62 , it is sufficient to determine a deviation of the actual air gap value from the setpoint air gap value  60  in order to generate the pre-warning information  64  in response thereto. 
     In response to the provision of the pre-warning information  64 , for example, a warning light in the driver&#39;s cab of the vehicle  1  is actuated in order to signal to a driver of the vehicle  1  that the vehicle should be taken to a mechanic. In the workshop, the rotational speed sensor  18  can be pressed again into a suitable position, which is checked, for example, by means of the first installation information  58 , which can preferably be read out by a diagnostic unit. The initialization circuit  46  preferably sets the setpoint air gap value  60  either automatically or subsequently by means of manual activation to the actual air gap value  32  at that time. 
     If the comparison of the actual air gap value  32  with the setpoint air gap value  60  has, however, produced a deviation greater than a limiting air gap value  66 , which is read out from the memory  50 , the information circuit  52  generates, alternatively or additionally to the pre-warning information  64 , fault information  68 , which is provided for actuating a warning light and the brake controller  40 . The brake controller  40  takes into account this fault information  68  during the decision as to whether the rotational speed value  38  can still be taken into account, or, in response to the fault information  68 , subsequently no longer takes into account the rotational speed value  38 . 
       FIG. 3  shows the rotational speed sensor  18  of the exemplary embodiment according to  FIG. 1  with its devices as a block circuit diagram in which internal signal paths are illustrated. 
     The rotational speed sensor  18  is an active rotational speed sensor, which has an active sensor element  70  with passive pickups. In order to sense the position or rotation of the pole wheel  22 , the active sensor element  70  requires an auxiliary voltage, which is provided indirectly via the data interface  16  according to  FIG. 1 . The active sensor element  70  comprises, for example, a Hall element or a magneto-resistive bridge. 
     The active sensor element  70  provides, in particular repeatedly, an analog measured value  72 , which is fed to an air gap-detection circuit  74  of the rotational speed sensor  18 . Alternatively, or additionally, the active sensor element  70  provides digital measured values directly. 
     The air gap-detection circuit  74  determines the actual air gap value  32 , preferably with a word size of 3 or 4 bits, from the analog measured value  72  or by means of multi-stage digitization. The actual air gap value  32  is provided in the digital signal  30  by means of a transmitter circuit  76  of the rotational speed sensor  18 , and the actual air gap value  32  can be fed to the control unit  14  via the data interface  16 . 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention that as a matter of language, might be said to fall there-between.