Abstract:
A system for a transmission of several additional pieces of information by a single modification of a speed signal. In addition to the modification of the speed signal in the area near the wheel (modified speed sensor), the system provides the special evaluation of the speed signal, modified, at a distance from the wheel (controller). In addition, the system naturally also includes the combination of the special speed sensor and the controller.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a system for changing and/or evaluating a signal representing the rotation speed of at least one with the features set forth in the preambles of the independent claims. 
   BACKGROUND INFORMATION 
   Measuring the speeds of rotation of the vehicle wheels for control of the braking force, drive force and/or driving dynamics of a motor vehicle in open or closed loop is known. To do this in conventional manner, various methods (e.g. Hall or magneto-resistive sensors) are used. In addition, measuring the wear of the brake pad of a vehicle is known in that, for example, contact pins are embedded at a specific depth of the brake pads, which trigger a contact upon actuation of the brake when the brake pad is worn to this depth. 
   For example, the article “Integrierte Hall-Effekt-Sensoren zur Positions-und Drehzahlerkennung” (Integrated Hall Effect Sensors for Position and Speed Recognition) of the journal “elektronik industries,” 7-1995, pp. 29-31, describes active sensors for use in the motor vehicle for anti-lock braking, traction control, engine and transmission open-loop and closed-loop control systems. Such sensors supply two current levels in a two-wire circuit which are converted into two voltage levels by a measuring resistor in an appropriate controller. 
   In addition to the Hall effect sensors, the use of magneto-resistive sensors is also known for speed recording, e.g., from the article “Neue, alternative Lösungen für Drehzahlsensoren im Kraftfahrzeug auf magnetoresisitiver Basis,” (New Alternative Solutions for Speed Sensors based on Magnetoresistance of the Motor Vehicle), VDI Reports No. 509, 1984. 
   German Patent No. 26 06 012 (U.S. Pat. No. 4,076,330) describes a special common arrangement for detecting the wear on a brake pad and for detecting the wheel speed. To do this, the brake pad wear detected and the wheel speed detected by an inductively operating sensor are supplied via a common signal line to an analyzer. This is achieved in that the wheel speed sensor is completely or partially short-circuited in response to a detected brake pad wear. 
   Other systems as described, for example, in German Patent No. 43 22 440, require at least two signal lines between one wheel unit and the analyzer for detecting the speed and the brake pad wear on a wheel and a wheel brake, respectively. 
   In the speed detection method mentioned above, it is known that the air gap between the rotating ring gear and the actual sensor element has a considerable influence on the quality of the speed signal. Reference is made in this respect to e.g., German Patent Application No. 32 01 811. 
   The above-mentioned information (for example, brake pad wear and air gap/signal quality) is generally detected near the wheel and evaluated in a control unit mounted at a distance from the wheel. To do this, the information must be transmitted to the controller. 
   German Patent Application No. 196 18867.9 describes how to modify a rotational-speed signal in various specifiable ways for transmitting additional information (excessive brake pad wear, air gap that is too large/defective signal quality). The modification is carried out in different ways depending on the additional information to be transmitted. Making the different modifications of the speed signal requires a certain amount of effort. 
   The object of the present invention is to implement a very simple and reliable transmission of the speed signal and other information. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a transmission of several pieces of additional information by a single modification of a rotation speed signal. In addition to the modification of the speed signal according to the present invention in the area near the wheel (modified speed sensor), the system according to the present invention provides the special evaluation of the speed signal modified according to the present invention in the area at a distance from the wheel (controller). In addition, the present system according to the present invention also includes the combination of the special speed sensor and the controller. 
   During the modification of a signal representing a rotary movement of a vehicle wheel, the system according to the present invention includes first means for generating a first signal representing the rotary movement and second means for generating at least two further signals, in each case one of the additional signals representing various operating conditions of at least two different devices as additional information. Such devices can be, for example, the first means (e.g., speed sensor) itself or the brake pad of a wheel brake present at the vehicle wheel. In addition, third means are provided with which the first signal can be modified in a specifiable manner as a function of the further signals. 
   According to the present invention, the third means are structured in such a way that the modification is specified in a single way, and this modification is carried out as a function of at least one of the further signals. 
   The modification according to the present invention of the speed signal has the advantage that the additional information (for example, about the air gap/signal quality and/or the brake-pad wear mentioned at the outset) can be transmitted in a simple and reliable manner via the speed-sensor output line. This eliminates, for example, the second signal line mentioned at the outset provided exclusively for the transmission of the additional information. In particular, the present invention exhibits, in comparison to German Patent Application No. 196 18867.9, the advantage that at least two different pieces of additional information (e.g., excessive brake pad wear, defective signal quality/excessively large air gap) can be transmitted by only a single possible change of the speed signal. In the subject matter of 196 18867.9, either only one single additional piece of information is superimposed on the speed signal or, in the case of several pieces of additional information, this additional information is superimposed on the speed signal in a coded manner which requires a certain amount of effort in circuit and/or programming technology. According to the present invention, at least two additional pieces of information are transmitted simultaneously by a single modification of the speed signal. If it is assumed that only one single speed modification is possible, according to the present invention all the additional pieces of information lead to the possible modification of the speed signal and are thus output, instead of an alternative decision being made for one of several additional pieces of information. In this context, the speed sensor and the detection of the above-mentioned additional information form one compact unit. 
   In evaluating a signal representing a rotary movement of a vehicle wheel, the invention assumes that the vehicle wheel has a wheel brake and the signal for transmitting additional pieces of information, e.g., wear of the brake pad of the wheel brake or quality of the signal, is changed in a manner that can be specified. 
   According to another embodiment of the present invention, mean are provided for generating at least one signal representing an actuation of the wheel brake. In addition, the system according to the present invention has evaluation means, by which the signal or the modified signal is (gated) combined at least with the generated signal representing a brake actuation. At least two signals representing the additional pieces of information are then formed as a function of this combination. 
   The evaluation of the speed signal or of the modified speed signal according to the invention has the advantage that additional pieces of information, e.g., regarding the air gap/signal quality and/or the brake-pad wear mentioned at the outset, can be transmitted via the sensor output line in a simple and reliable manner. The evaluation of the speed signal according to the present invention falls back upon signals that are generally present in the controller anyway. In so doing, use can be made of a brake-lights-switch signal and/or a signal representing the brake pressure as information regarding brake actuation. 
   The present invention also relates to the overall system that is based on a system for changing and evaluating a signal representing a rotary movement, which has first means for generating a signal representing the rotary movement and second means for generating at least two additional signals. In this context, in each case, one of the additional signals represents different operating conditions of at least two other devices as additional information. Such devices may be the speed sensor itself or the brake pad. In addition, third means are present by which the first signal can be modified as a function of the further signals in a manner that can be specified. By using fourth means, the first or the modified first signal is evaluated, at least one signal being generated as a function of this evaluation, the signal representing the various operating conditions of at least two different devices. 
   According to another embodiment of the present invention, fifth means for generating at least one signal representing a brake actuation are provided, and the third means are structured in such a way that the change can be specified in a single manner. This change is made as a function of at least one of the further signals. The fourth means are structured in such a way that the first or the modified first signal is combined (gated) with at least the generated signal representing one brake actuation. As a function of this combination, at least two signals are then formed representing the additional pieces of information. 
   The entire system naturally combines the above-mentioned advantages of the individual systems. 
   Another embodiment of the present invention provides that at the end of the vehicle production (end of the assembly line) a relatively simple test for correct installation of the speed sensors can be carried out. Since at this point at the end of the line, the brake pad is new, a modification according to the present invention of the speed signal can only result from an incorrectly installed speed sensor. 
   In another embodiment of the present invention, it is provided that the first means are structured such that the first signal assumes at least two initial current values and/or at least two initial voltage values. The third means are then structured in such a way that to change the first signal in a single manner that can be specified, at least one of the initial current values and/or at least one of the initial voltage values can be changed to a second current value and/or a second voltage value for at least a specific time as a function of the second signal. This embodiment assumes in particular that the first means are designed as an active speed sensor known in and of itself. 
   The generating means, or the fifth means, can additionally be designed to generate at least one of the signals representing the vehicle velocity. 
   In addition, the gating in the evaluating means, or in the fourth means, can be advantageously designed so that the signals representing the additional information are formed from the time correlation of the signal representing the wheel brake actuation to the specifiable change of the signal representing the rotary movement of a vehicle wheel. 
   The second means are advantageously designed to generate a signal representing brake-pad wear on at least one vehicle wheel brake and/or to generate a signal representing the amplitude of a signal joined to the first signal (speed signal). 
   In particular, the first, second and third means are near the wheel and/or the fourth and fifth means, or the evaluation means, are mounted at a distance from the wheel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic block diagram of a conventional system. 
       FIG. 2  shows a diagram of a combination of an active rotation speed sensor with a brake pad wear detector. 
       FIG. 3   a  shows a first embodiment of a circuit arrangement of a speed signal modification system according to the present invention. 
       FIG. 3   b  shows a second embodiment of the circuit arrangement of the speed signal modification system according to the present invention. 
       FIG. 3   c  shows a block diagram of an evaluation arrangement according to the present invention. 
       FIG. 4  shows a first graphical representation of output signal curves for the system illustrated in  FIGS. 3   a  and  3   b.    
       FIG. 5   a  shows a second graphical representation of the output signal curves for the system illustrated in  FIGS. 3   a  and  3   b.    
       FIG. 5   b  shows a third graphical representation of the output signal curves for the system illustrated in  FIGS. 3   a  and  3   b.    
       FIG. 6  shows an exemplary arrangement for detecting an excessive air gap. 
       FIG. 7  shows a graphic representation of output signal generated using the arrangement illustrated in FIG.  6 . 
       FIG. 8  shows a flow chart diagram of a block illustrated in  FIG. 3   c.    
   

   DETAILED DESCRIPTION 
     FIG. 1  shows, as a schematic block diagram, a system for determining brake pad wear and wheel speeds in a motor vehicle. 
   The wheel units of a motor vehicle are designated with reference numbers  11   a-d . These wheel units include, in particular, the wheels, the rotation speeds of which (wheel speeds) will be measured and the brake system (friction brake) allocated to each wheel unit. The speed sensors and brake-pad-wear sensors allocated to each wheel are indicated with reference symbols  102   a-d , and will be described in more detail using  FIGS. 2 ,  3   a ,  3   b  and /or  3   c  in so far as they concern the invention. Reference is made explicitly to the related art mentioned above with regard to the structure of these sensors, which is beyond the scope of the present invention. 
   The output signals of speed sensors and brake-pad-wear sensors  102   a-d  are put through to controller  103 , the transmission lines being represented by  105   a-d . The information transmitted by transmission lines  105   a-d  is then evaluated centrally for all wheel units in controller  103 . The condition of the brake pads is supplied as evaluation result by controller  103  to display instrument  110  by way of lines  18   a-d . Generally the driver is given appropriate information in the event of a certain degree of wear of one or more brake pads. 
   For the sake of completeness, the brake systems of individual wheel units  11   a-d  which can be controlled from controller  103  are sketched with reference characters  14   a-d.    
     FIGS. 2 ,  3   a  and  3   b  show various embodiments using a single wheel unit as an example. 
     FIG. 2  shows a combination of an active speed sensor with a brake pad wear detector. As described above, a known Hall speed sensor or a known magneto-resistive speed sensor can be provided as “active” speed sensor  102 .  FIG. 2  shows schematically that a sensor element  1021  scans a passive-magnetic type incremental rotor  101 . As a function of the scanned increments of rotor  101 , sensor element  1021  sets two current levels i 1  and i 2 . This is shown in  FIG. 2  with two power sources  1022  and  1023  being switched on and off. 
   Speed sensor  102  is connected to controller  103  via lines  105  using plug connectors  1021   a  and  b  and  1031   a  and  b . Input amplifier  1036  detects, with the help of input resistor R, the voltage values corresponding to the current levels of speed sensor  102 
 
 U   Low   =R*i   1 
 
 U   High   =R *( i   1   +i   2 )
 
     FIG. 4  shows a typical curve with wheel speed that is basically constant in lower signal line  301 . The desired wheel speed is obtained by evaluation of the frequency of this signal. 
   The bottom part of  FIG. 2  shows schematically a conventional brake-pad-wear detector  104  on a wheel brake. As described above, the brake-pad-wear sensor, known as such from the related art, determines the wear on the brake pad of a vehicle brake in that e.g., contact pins are embedded at a specific depth of the brake pads and trigger a contact upon actuation of the brakes (the brake pad is pressed onto the brake disc) when the brake pad is worn to this depth. This contact is indicated in  FIG. 2  with switch  1041 . In normal cases, (no brake pad wear requiring display) switch  1041  is open, voltage U+ not being grounded. If the brake pad reaches a certain degree of wear, switch  1041  is closed, which is detected because of grounding through connection  106  or plug connectors  1012  and  1031  in evaluation circuit  1037 . 
   As can be seen in the embodiment shown in  FIG. 2 , separate signal lines  105  and  106  are necessary in each case for transmission of wheel speed information and information about brake pad condition. 
   The system according to the present invention will now be explained using  FIGS. 3   a  and  b . In this embodiment, speed sensor  102  described in  FIG. 2  was supplemented with additional current source i 3 , which is arranged in parallel to the speed sensor shown in FIG.  2 . 
   In  FIG. 3   a , additional power source i 3  can be connected via transistor  1029  into the power circuit between the speed sensor and the evaluation unit if transistor  5032  is switched to transmission (forward). 
   Transistor  1029  is controlled by logical OR gate  1028 . Signal S or BBV coming from switch  1041  already described using  FIG. 2 , and signal LS coming from block  5102  are applied to OR gate  1028 . As described above, switch  1041  changes its switching status if during an actuation of the brakes a specific brake pad wear is recognized. Generation of signal LS and the function of sensor element  5030  and comparator  5031  will be described below using  FIGS. 6 and 7 . 
     FIG. 6  shows, as an example, sensor element  5030  and the detection of excess distance of a Hall or magneto-resistive sensor from the ring gear of the vehicle wheel that has already been described, whose speed of rotation will be detected. Sensor element  5030  is the sensor element indicated with the same reference numbers in  FIGS. 3   a  and  3   b . Sensor element  5030  is a known Wheatstone bridge with a typical ring-shaped arrangement of resistors. As the individual segments of the ring gear that is not shown ( 101 / FIG. 3   a ) pass by, bridge voltage U B  is generated in this Wheatstone bridge and is supplied to comparators  5031  and  5101 . Comparator  5031  corresponds to the comparator in  FIGS. 3   a  and  3   b  with the same reference symbols and is used to evaluate the wheel speed. Another evaluation of the bridge voltage takes place in comparator  5101  in such a way that this bridge voltage is compared to a relatively high threshold value U H . More details will be given on the background of the two threshold comparisons in the following using FIG.  7 . 
     FIG. 7  shows a typical signal curve of the bridge voltage over time. The bridge voltage periodically increases and periodically decreases depending on the speed of passage of the individual ring gear segments ( 101 / FIG. 3   a ). If the distance, the air gap, between the ring gear and Wheatstone bridge  5030  remains constant, the bridge voltage has a constant amplitude. However, if this distance becomes too great, the bridge voltage amplitude decreases. This case is shown in FIG.  7 . 
   A first threshold comparison in comparator  5031  compares the bridge voltage signal to a relatively low threshold value, e.g., 40 mV. On the output side, comparator  5031  then supplies the triggering signal, shown in bottom signal curve K 1  in  FIG. 7 , for current sources i 1  and i 2  (see  FIGS. 5   a  and  5   b ). Therefore, signal K 1  represents the wheel speed, even given an increasing air gap. Comparator  5101  checks the bridge voltage signal amplitude, in that a relatively high threshold of e.g., 60 mV is set in this comparator. If the distance between the ring gear and the Wheatstone bridge, the air gap, is sufficiently small, the amplitude of the bridge voltage signal is greater than the threshold of comparator  5101 . The output signal of comparator  5101  is shown, as can be seen in lower signal curve K 2  in  FIG. 7 , in regular operation with a time delay of signal K 1  compared to signal K 1 . However, if comparator signal K 2  fails to appear, the bridge-voltage signal amplitude decreases, which indicates an excessive air gap. 
   The absence of signal K 2  is detected in unit (e.g., detector)  5102  and results in generation of signal LS. Unit  5102  is indicated in  FIGS. 3   a  and  3   b  with the same reference numbers. 
   In summary regarding air gap recognition, it can be stated that the speed signals of a wheel are detected by using an active sensor, e.g., Hall sensor or magneto-resistive sensor. The sensors have a Wheatstone bridge that is unbalanced by a changing magnetic field. The speed signal is obtained from this unbalance. The amount of unbalance has a fixed relationship to the magnitude of the magnetic-field difference between the two halves of the bridge. Among other things, the magnetic-field difference is a function of the distance of the sensor from the magnet wheel. If the amount of bridge unbalance is evaluated, a statement can be made on the air gap between sensor and magnet wheel, and thus on the signal quality of the speed signal. This evaluation can be carried out with comparator  5101 , which has a greater hysteresis (U H =60 mV) than the normal useful signal comparator  5031  (U H =40 mV). If the air gap is small, both the comparators switch, but if the air gap is too large only the useful signal comparator  5031  switches. This provides an early warning system for an air gap that is too large without simultaneously losing the wheel speed information. This information can be used, for example, as an end-of-the-line check during vehicle manufacturing, in the shop or while driving. 
   As  FIG. 3   a  shows, transistor  5032  is triggered as a function of comparator  5031  described in  FIGS. 6 and 7 . If transistor  1029  is blocked, current level i 1  (low level) and [i 1 +i 2 ] (high level), whose frequency indicates the wheel speed, are periodically present at output  105 ′ of sensor unit  102 ′. 
   By triggering transistor  1029 , current source i 3  is superimposed on current level [i 1 +i 2 ] if either signal LS (unit  5102 / FIG. 6 ) represents an air gap that has to be displayed “or” signal BBV represents a brake pad wear that has to be displayed. The logical “or” operation occurs in logical OR gate  1028 . If transistor  1029  is switched to transmission (forward), the high level of the speed signal increases at output  105 ′ to the level [i 1 +i 2 +i 3 ] (high level′). Output  105 ′ is connected to input  1031   b  of the controller i.e., of evaluation unit  103 ′. 
   Depending on the switching status of transistor  1029  and as a function of signal BBV “or” signal LS, input amplifier  1036 ′, with the help of input resistor R, detects the voltage values corresponding to the above-mentioned current levels
 
 U   Low   =R*i   1 
 
 U   High   =R *( i   1   +i   2 )
 
or
 
 U   High   ′=R *( i   1   +i   2   +i   3 )],
 
depending on whether a brake pad wear that needs to be displayed or an air gap that needs to be displayed has been recognized (U High ′) or not (U High ).
 
   In addition to typical curve  301  already described with additional power source  1014  switched off, upper signal line  302  in  FIG. 4  shows the signal curve with power source i 3  switched on. The upper signal level (high′-level) is thus shifted by offset (R*i 3 ) compared to lower signal level  301  (high-level). 
   Desired wheel speed N is obtained by evaluating the frequency of these signals shown in signal line  301  or  302  in block  1034  of  FIG. 3   c . Speed N can then be supplied to the actual brake-, drive- or other closed loop/open loop control  1035 . In the case of brake or drive closed loop/open loop control, wheel brakes  11   a-d  are driven (signals  14   a-d ) as a function of the speeds detected. Frequency evaluation  1034  is designed in such a way that the frequency of signal lines  301  and  302  is determined independently of the offset caused by the position of switch  1041  mentioned above. In this manner, speed detection is always ensured independently of recognized brake pad wear that is too great or a recognized air gap that is too large. This is important for system availability. 
   In addition to evaluation  1034  mentioned above regarding wheel speeds, signals  301  and/or  302  are supplied to threshold comparator  1032 . This threshold comparator  1032  recognizes whether the offset caused by switch  1029  (R*i 3 ) is present at the high level or not. The threshold in unit  1032  lies between levels [R*(i 1 +i 2 )] and [R*(i 1 +i 2 +i 3 )]. 
   Therefore, on the output side of threshold comparator  1032 , a signal M on/off  is present which gives information on whether either a brake pad wear that needs to be displayed and/or an air gap that needs to be displayed are present (signal value M on ) or not (signal value M off ). Signal M with the signal value M on  or M off  is supplied to block  1033 , the function of which will be described in more detail using FIG.  8 . In addition, output signal BLS of one brake light switch  1037  and signal V (block  1036 ) representing the longitudinal vehicle velocity are supplied to block  1033 . 
   Block  1037  represents a switch that, in a known manner, senses an actuation of the brakes in such a way that the switch is connected to the brake pedal that can be actuated by the driver. Such a switch (brake light switch) is generally present on the vehicle for actuation of the brake light. Signal BLS can naturally also be generated as an alternative or as a supplement to the brake light switch in block  1037  as a function of the momentary brake pressure. A signal representing the momentary brake pressure is available in many braking systems (anti-lock braking systems, traction control systems or driving dynamics systems) in a known way in the corresponding controller. 
   Signal V representing the longitudinal vehicle velocity can be formed in a known manner from the wheel movements of one or several wheels and is also generally present as a reference speed, as it is called, in many braking systems (anti-lock braking systems, traction control systems or driving dynamics systems) (dotted line to brake, drive or other closed loop/open loop control  1035 ). 
   In  FIG. 8 , after start step  801 , signal value M on/off  that is currently present at block  1033  and the current value of signal BLS on/off  and V are input in step  802 . There is an inquiry in step  803  of whether signal M has the value M on , value M on  being output by block  1032  if the speed signal high level is increased. 
   If there is no increase in the high level of the speed signal, value M off  is output, which means that switch  1029  ( FIG. 3   a ) is open and consequently neither an air gap that needs to be displayed (signal LS,  FIG. 3   a ), nor a brake pad wear that needs to be displayed (signal BBV,  FIG. 3   a ) is present. In this case, processing moves on immediately to final step  807 . 
   If there is an increase in the speed signal high level, after step  803  the processing goes over to step  804  in which a determination is made of whether signal M on  is correlated in time with brake actuation signal BLS on . This can mean there is a determination of whether signal value M on  only occurs if a brake actuation is simultaneously displayed due to signal BLS on . Such a correlation can occur due to the one-time simultaneous occurrence of values M on  and BLS on , but it can also be set so that determination occurs only after a predefinable repetition frequency of such a correlation. 
   If in step  804  a correlation is found between the occurrence of signal values M on  and BLS on , this means that a change in the speed signal occurs through switching on the power source i 3  whenever a brake actuation occurs. As described previously, excessive brake pad wear is detected only by contact with the brake disc of the contact pin embedded in the brake pad, i.e., only during a brake actuation. A possible air gap that is too large between sensor element  5030  ( FIG. 3   a ) and ring gear  101  ( FIG. 3   a ) is, on the other hand, independent of brake actuation. A correlation in time between the occurrence of signal values M on  and BLS on  thus means that excessive brake pad wear is present. In step  805 , this brake pad wear is displayed in display  110   a  by outputting signal  18   a.    
   If in step  804  no correlation is determined between the occurrence of signal values M on  and BLS on , this means that a change in the speed signal by switching on power source i 3  is present, independently of brake actuation. This indicates an air gap that is too large (defective quality of the speed signal) between sensor element  5030  ( FIG. 3   a ) and ring gear  101  ( FIG. 3   a ). If there is now another (optional) inquiry in step  808  of whether the vehicle longitudinal speed exceeds a predefinable threshold value SW, it means that if a threshold value is exceeded, an excessive air gap is present. In step  805 , this defective signal quality is displayed in display  110   b  by outputting signal  18   b . If the vehicle is standing or only moving slowly, end step  807  will be triggered immediately. 
   While the embodiment shown in  FIG. 3   c  has separate displays  110   a  and  110   b  for displaying excessive brake pad wear and defective quality of the speed signal, respectively, a single display can also be provided since both errors can be rated equivalent in severity in driving operation and require immediate shop service. The cause of such a display being activated can be clearly diagnosed using appropriate service instructions. 
   In the embodiment shown in  FIG. 3   a , in the presence of excessive brake pad wear and/or an excessively large air gap, each speed-signal high level is increased. In the following variation, on the other hand, only every nth high level is increased, in the concrete example, every fourth high level. This minimizes the loss of power caused by the offset. In addition, this version of the invention has the advantage during transmission of the brake pad wear that possible bounce in the brake pad wear switch will not result in incorrect display, since the offset is only initiated after the occurrence of n high levels. 
     FIG. 3   b  shows this second embodiment variation of the present invention. In it, reference number  502  designates a unit which, similar to unit  102 ′ described above ( FIG. 3   a ), combines the actual speed detection and parts of the brake pad wear detection. Unit  502  is connected by connections  5051  and  5052  to inputs  1031   a  and  1031   b  of a controller not shown in  FIG. 3   b . This controller corresponds basically to unit  103 ′ explained in  FIG. 3   c.    
   In addition, unit  502  is connected by connections  5053  and  5054  to brake pad switch S 1  (corresponds to switch  1041  in  FIGS. 2 and 3   a ). Switch S 1  is closed in the normal case in this embodiment (no brake pad wear needing to be displayed). In addition,  FIG. 3   b  shows block  5102  that generates signal LS (air gap/signal quality), which was already described using  FIGS. 6 and 7 . 
   The actual speed detection is carried out analogously to the manner described using FIGS.  2  and/or  3   a.    
   If the brake pad reaches a specific degree of wear, switch S 1  is opened. Because of the open position of switch S 1 , the upper input of logical OR gate  5055  shown in  FIG. 3   b  will be at low level; with switch S 1  closed, the corresponding input of logical OR gate  5055  will be at high level. If an excessively large air gap is found in block  5102 , OR gate  5055  at the corresponding input will be assigned a low level. Therefore on the output side of OR gate  5055 , high level is always present if neither a brake pad wear that needs to be displayed nor an excessively large air gap is detected. Otherwise there is a low signal present at the output side of OR gate  5055 . 
   The triggering signal of transistor  5032  is supplied, inverted, to the lower input of logical AND gate  5021 . This means that a triggering of transistor  5032  (power source i 2  switched on, speed signal at high level) is present as low level (inverted) at the logical AND gate  5021 . When power source i 2  is switched off by the transistor (low level at transistor  5032 ) it results, because of the inversion, in the presence of a high level at the lower input of AND gate  5021 . On the output side, a high level is present at AND gate  5021  if neither brake pad wear that needs to be displayed (switch S 1  closed, upper input of OR gate  5055  at high level) nor an air gap that needs to be displayed (lower input of OR gate at high level) is present and at the same time power source i 2  is switched off. Otherwise, the AND gate output is at low level. 
   The output of AND gate  5021  is applied to the input of logical OR gate  5022 . In addition, comparators K 1  and K 2  are connected to the other two inputs of OR gate  5022 . 
   Comparator K 1  compares input voltage VCC of sensor unit  502  with a predefinable threshold value REF.K 1 . This is done by detecting low voltages, which can impair proper operation of unit  502 . If a low voltage such as this occurs, thus if VCC is lower than REF.K 1 , a high level will be present at the upper input of OR gate  5022 . Otherwise, this input is at low level. 
   Comparator K 2  compares the temperature detected by temperature sensor  5025  with predefinable threshold value REF.K 2 . This means temperature sensor  5025  measures the temperature to which sensor unit  502  is subject. In this context, temperature sensor  5025  is integrated directly in a known manner into the integrated circuit (IC) of sensor unit  502 , e.g., as a diode, whose temperature-dependent flux voltage is measured. The basis of temperature measurement is that sensor unit  502  is generally near the wheel, i.e., also installed in the proximity of the brake discs. The heat coming from the brake discs can heat sensor unit  502  in such a way that proper operation of unit  502  is impaired. If overheating of this type occurs, thus if the temperature measured is greater than REF.K 2 , a high level will be present at the lower input of OR gate  5022 . Otherwise, this input is at low level. 
   Therefore, a high signal is present at the output side of OR gate  5022  if at least one of the three inputs is at high level, thus if
         either overheating of sensor unit  502  or   low voltage or   no brake pad wear that needs to be displayed and no air gap that needs to be displayed are present and, at the same time, power source i 2  is switched off.       

   Otherwise, the OR gate output is at low level. 
   The output of OR gate  5022  is connected to reset input R of counter  5023 . Counter  5023  is reset when there is a high signal at input R. Clock input C of counter  5023  is connected to the control signal for transistor  5032 . Input C thus receives a high level if power source i 2  is switched on and a low level if power source i 2  is switched off. Counter  5023 , designed in a known way as a flip-flop switch, is therefore always switched when power source i 2  is switched on or off. Counter  5023  has three outputs, which are at high level when the level present at clock input C has changed from low to high the first, second and fourth time. This means that three high levels are thus present at AND gate  5024 , to which the outputs of counter  5023  are supplied, when power source i 2  is switched on for the fourth time. In this case (all three inputs of AND gate  5024  are at high), the AND gate supplies a high level at its output side, after which third power source i 3  is switched on. Current i 3  from power source i 3  is then superimposed on the current that is present at this time (i 1 +i 2 ), which leads to a total current (i 1 +i 2 +i 3 ) at output  5052 . Power source i 3  can be switched on by a transistor that is not shown in  FIG. 3   b  which is connected in series to this power source i 3 . This would then occur similarly to switching power source i 3  on and off with transistor  1029  shown in  FIG. 3   a.    
     FIG. 5   a  shows the signal present at output  5052  if switch S 1  is closed (no brake pad wear that needs to be displayed) and no air gap that needs to be displayed are present. The upper input of AND gate  5052  shown in the lower signal line of  FIG. 5   a  is then set high. Counter  5023  (input R) is always reset by OR gate  5022  if power source i 2  is switched off. This ensures that third power source i 3  remains switched off if no brake pad wear that needs to be displayed and no air gap that needs to be displayed are present. In controller  103 ′ (input  1031   b ), the signal present at output  5052  is then converted via resistor R into a voltage, whereupon wheel speed N is determined by frequency analysis  1034  already described. 
     FIG. 5   b  shows the curve of the signal present at output  5052  when switch S 1  is open (brake pad wear that needs to be displayed) and/or an air gap that needs to be displayed is present. The upper input of AND gate  5052  that is shown in lower signal line of  FIG. 5   b  is then set low. Counter  5023  (input R) is only reset by OR gate  5022  if a low voltage (comparator K 1 ) or excess temperature (comparator K 2 ) is present. In the normal case (neither over-voltage nor excess temperature) input R of counter  5023  is at low, whereupon power source i 3  is switched on each fourth time power source i 2  is switched on. This results in the speed signal curve shown in the upper part of  FIG. 5   b.    
   As already described using  FIG. 3   c , the signal present at output  5052  is converted into a voltage via resistor R in controller  103 ′ (input  1031   b ), whereupon wheel speed N is determined by frequency analysis  1034  already described. In addition, threshold value comparison  1032  recognizes whether level R*(i 1 +i 2 ) has been exceeded. In the case of a brake pad wear that needs to be displayed or an air gap that needs to be displayed, this is given by the increase of the fourth high level of the speed signal and is then evaluated by forming signal M on  in unit  1022  as already described.