Abstract:
The invention relates to a method for monitoring air pressure in vehicle tires using wheel electronics, which comprise a pressure sensor for measuring the air pressure present in the tire, a transmitter for transmitting pressure information, and a receiver for receiving interrogation signals, wherein a transmission activity of the wheel electronics is triggered by the reception of an interrogation signal, which is transmitted as a pulse train comprising a command section that encodes a transmission instruction for the wheel electronics. According to the invention, the pulse train comprises an adjustment section having several pulses before the command section.

Description:
[0001]    In order to monitor the tire pressure in motor vehicles, devices are used which comprise a sensor for measuring the pressure and a transmitter for wirelessly transmitting pressure information. These devices are generally referred to as wheel electronics and are disposed in the pneumatic tires. Wheel electronics can be attached to a valve base or to the rim, for example. 
         [0002]    The pressure information transmitted by the wheel electronics can be received by a receiver mounted in or on the vehicle. Such tire pressure monitoring systems can indicate insufficient air pressure in a vehicle tire or a dangerous drop in pressure to a driver. Wheel electronics in which the transmission activity can be triggered by the reception of an interrogation signal are particularly advantageous. In such wheel electronics, a central unit disposed on the vehicle can obtain information, when needed, about the individual vehicle tires by interrogating the wheel electronics. 
         [0003]    Regardless of whether wheel electronics cover the energy demand thereof from a battery or a mechanical-electric or thermo-electric converter, achieving the most economical energy consumption possible is a constant objective that is pursued in the development and improvement of wheel electrics. 
         [0004]    It is therefore an object of the present invention to identify a way as to how the energy consumption of wheel electronics can be reduced. 
         [0005]    This object is achieved by a method having the features listed in claim  1  and by a tire pressure monitoring system according to claim  15 . Advantageous refinements of the invention are the subject matter of the dependent claims. 
       SUMMARY OF THE INVENTION 
       [0006]    In a method according to the invention, a pulse train, which comprises an adjustment section having several pulses before a command section encoding a transmission instruction, is used as an interrogation signal. In this way, an amplification factor, which is used to amplify the pulses of the interrogation signal, can be adjusted in the wheel electronics to a degree that is required for receiving and evaluating the command section. The reception conditions for the interrogation signal depend on a number of factors, including the vehicle-specific geometry of the wheel house, and are therefore subject to considerable fluctuations. With the method according to the invention, the amplification can be reduced to the degree necessary for the reception conditions in the specific case. This enables a more economical operation of the wheel electronics. 
         [0007]    The adjustment section preferably comprises three to ten pulses, with three to seven pulses being particularly preferred. This measure has the advantage that not only a sufficient number of pulses is available in order to reliably determine the necessary amplification factor, but also that an unnecessarily long reception activity can be avoided. 
         [0008]    The reception conditions, and therefore the necessary amplification, can be determined particularly well at the edges of pulses. As a result, short pulses are advantageous for the adjustment section. The pulses of the adjustment section preferably have equal lengths within the context of system accuracy. For example, the pulses of the adjustment section can all have one length, which encodes the value zero in the bit string present in the command section. 
         [0009]    The command section preferably comprises pulses which are longer than the pulses of the adjustment section, in particular pulses which are at least twice as long as the pulses of the adjustment section. Thereby the command section may also comprise pulses which are as long as, or even shorter than, the pulses of the adjustment section. The command section, for example, can be a bit string, wherein long pulses encode the value 1 and short pulses the value zero. In this case, the short pulses preferably have the same lengths as the pulses of the adjustment section. 
         [0010]    According to an advantageous refinement of the invention, between the adjustment section and the command section, the pulse train comprises a start pulse, which is at least twice as long, preferably at least three times as long, in particular at least four times as long, as the pulses present in the adjustment section. The start pulse is preferably also longer than the pulses present in the command section. In this way, it is possible to prevent the last pulse of the adjustment section from being accidentally interpreted as a pulse of the command section. For example, the start pulse can be longer than a pulse encoding a 1, and also longer than a pulse encoding a 0. The start pulse is preferably at least as long as the sum of the pulse durations of a pulse of the adjustment section and a pulse of the command section. For example, the start pulse can be equal in length, preferably longer, than the sum of a pulse describing a 0 and a pulse describing a 1 of the command section. 
         [0011]    According to a further advantageous refinement of the invention, the pulse train comprises a wake-up pulse before the adjustment section. The wake-up pulse is preferably longer than the adjustment section. The wake-up pulse can advantageously be used to keep wheel electronics from being continuously ready to receive. Because the pulse train forming an interrogation signal comprises a wake-up pulse at the beginning, it suffices if wheel electronics check at regular intervals whether an intensity that is characteristic of a pulse is present at the receiver.  1   f  this is the case, the receiver remains ready to receive in order to receive the subsequent sections of the interrogation signal.  1   f  this is not the case, the wheel electronics can return into an energy-saving state. In a particularly preferred embodiment, the duration of the wake-up pulse accounts for more than half, such as at least two thirds, of the duration of the pulse train. 
         [0012]    The transmitter present in the wheel electronics is preferably an HF transmitter, this being a radio-frequency transmitter. The wheel electronics thus transmit pressure information in the form of radio-frequency signals. The transmission is preferably carried out at more than 100 MHz, such as 433 MHz, referred to as the ISM band, or in the 315 MHz region or in the 868 MHz region. For the interrogation signals, the low-frequency range (LF) is preferred. The range between 50 kHz and 300 kHz is particularly suited. The pulse train of the interrogation signal can be generated in that a carrier wave is pulse width modulated in the above-mentioned frequency range, such as by switching the transmitter on and off. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Further details and advantages of the invention will be described in an exemplary embodiment with reference to the attached drawing. Shown are: 
           [0014]      FIG. 1  is a schematic illustration of a tire pressure monitoring system, and 
           [0015]      FIG. 2  is a schematic illustration of the pulse train of an interrogation signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The system illustrated in  FIG. 1  for monitoring the tire pressure in vehicles comprises a plurality of wheel electronics  1 , which in each case are disposed in one of the pneumatic tires of a vehicle, and a pressure sensor  1   a , a temperature sensor  1   b , a control unit  1   c , a power source  1   d , such as a battery or a generator, an HF transmitter  1   e , and an LF receiver  1   f . The system furthermore comprises a central unit  2 , which is disposed on the vehicle in a stationary manner and to which the interrogation transmitter  3  and at least one HF receiver  4  are connected. 
         [0017]    Since the interrogation transmitters  3  emit LF signals having a carrier frequency of 50 kHz to 300 kHz, for example, in particular of 100 kHz to 150 kHz, the interrogation signals  10  have a relatively short range. Each end of a vehicle axle is therefore preferably associated with an interrogation transmitter  3  in order to interrogate the wheel electronics  1  of the wheels disposed at the particular ends of the vehicle axles. 
         [0018]    In contrast, the wheel electronics emit pressure information in the form of HF signals. The HF carrier frequency used is preferably at least 100 MHz. For example, 433 MHz, 315 MHz or 868 MHz are common for wheel electronics  1 . The signals emitted by the wheel electronics  1  therefore have a larger range, so that generally a single receiver  4  suffices, which is connected to the central unit  2 . However, it is also possible to connect a plurality of receivers  4  to the central unit  2  and, for example, to associate a dedicated receiver  4  with each axle or even each wheel position. 
         [0019]    A transmission activity of the wheel electronics  1  is triggered by the reception of an interrogation signal  10 . The interrogation signal  10  is transmitted in the form of a pulse train, which is illustrated schematically in  FIG. 2 . 
         [0020]    The interrogation signal  10  starts with a wake-up pulse  11 . The wake-up pulse  11  is followed by an adjustment section  12 , which comprises several, for example five, pulses  12   a . The adjustment section is followed by a start pulse  13 , which is followed by a command section  14  encoding a transmission instruction for the wheel electronics  1 . The command section  14 , for example, can prompt the wheel electronics  1  to emit current pressure information. Another possibility is that the command section  14  prompts the wheel electronics  1  to emit type information, such as the version of the software it contains. 
         [0021]    In the illustrated embodiment, the wake-up pulse  11  accounts for more than half the duration of the pulse train  10 . The wake-up pulse  11  is longer than the adjustment section  12 , and it is also longer than the command section  14 . The wake-up pulse  11  is preferably longer than the sum of the duration of the command section  14  and of the duration of the adjustment section  12 . The longer the wake-up pulse  11 , the rougher the sampling can be in the wheel electronics. As a result, the longer the wake-up pulse, the lower is the energy requirement and also the susceptibility of the wheel electronics to interference. An upper limit for the maximum length of the wake-up pulse is defined by the desired response time of the system. 
         [0022]    The wheel electronics  1  periodically activate the receiver  1   f  in order to check whether an interrogation signal  10  is active. By selecting the time intervals during which the receiver  1   f  of the wheel electronics  1  is activated shorter than the wake-up pulse  11 , it takes little energy expenditure for the wheel electronics  1  to be ready to receive during the adjustment section  12  of the interrogation signal  10 . 
         [0023]    In order to determine whether a wake-up pulse  11  is active, it suffices to activate the receiver  1   f  of the wheel electronics  1  for a very short time, such as for ½ ms or less. The energy this requires is very low, since the receiver if of the wheel electronics  1  is shut off the majority of the time. The receiver  1   f  of the wheel electronics  1  is preferably activated at time intervals amounting to at least ⅔, preferably at least ¾, of the duration of the wake-up pulse  11 , but less than the duration of the wake-up pulse  11 . 
         [0024]    In the illustrated embodiment, the receiver is activated every 32 ms for less than ½ ms. In the illustrated embodiment, the duration of the wake-up pulse  11  is 42 ms. The duration of the wake-up pulse  11  can also be selected longer or shorter without difficulty. However, it must be taken into account that the wheel electronics  1  require more frequent activation the shorter the wake-up pulse  11  is. A longer wake-up pulse  11  has the advantage of a more economical operation of the wheel electronics  1 , however means that accordingly more time is required to interrogate the wheel electronics  1  with respect to pressure information. A particularly advantageous range for the duration of the wake-up pulse  11  is between 30 and 200 ms, particularly 35 to 100 ms. 
         [0025]    The adjustment section  12  follows the wake-up pulse  11  and has several pulses  12   a . The adjustment section  12  preferably comprises three to ten pulses  12   a , with three to seven being particularly preferred. The pulses  12   a  present in the adjustment section  12  are used to adapt the signal amplification to the prevailing reception conditions, so that the wheel electronics  1  can understand and evaluate the command section  14  of the interrogation signal  10  without error. By adjusting the amplification factor of an amplifier present in the receiver  1   f  of the wheel electronics  1  to the necessary extent, a particularly energy-saving operation of the wheel electronics  1  can be implemented, because unnecessarily high signal amplification can be avoided. 
         [0026]    The pulses  12   a  of the adjustment section  12  preferably have equal lengths within the context of system accuracy. For example, the pulses  12   a  of the adjustment section can have a length which is used in the command section  14  to describe a zero. 
         [0027]    Between the adjustment section  12  and the command section  14 , the pulse train comprises the start pulse  13 . The start pulse  13  prevents the wheel electronics  1  from interpreting pulses  12   a  of the adjustment section  12  as part of the command section  14 . The start pulse  13  is at least twice as long, preferably three times as long, in particular at least four times as long, as pulses  12   a  present in the adjustment section  12 . In the illustrated embodiment, the start pulse  13  is five times as long as the pulses  12   a  of the adjustment section  12 . The start pulse  13  is preferably also longer than the pulses  14   a ,  14   b  present in the command section  14 . It is particularly preferred when the start pulse  13  is at least as long as the sum of the pulse durations of a pulse  12   a  of the adjustment section  12  and a pulse  14   a ,  14   b  of the command section  14 . In general, it is advantageous when the start pulse  13  is three to ten times as long as a pulse  12   a  of the adjustment section  12 , particularly four to six times as long as a pulse  12   a  of the adjustment section  12 . 
         [0028]    The command section  14  follows the start pulse  13 . The command section  14  preferably comprises a bit string in the form of short pulses  14   a  and long pulses  14   b . To this end, short pulses  14   a  can encode a zero and long pulses  14   b  can encode a one. As was already mentioned, the lengths of the short pulses  14   a  of the command section  14  can correspond to those of the pulses  12   a  of the adjustment section  12 . 
         [0029]    The wheel electronics  1  described above may comprise a roller switch, which is closed by acceleration forces developing while driving. Advantageously, the wheel electronics  1  can then automatically perform pressure measurements at specified time intervals during travel. When the wheel electronics  1  find a dangerous drop in pressure, corresponding pressure information can be sent even without a request by an interrogation signal. Wheel electronics  1  which not only can be prompted into transmission by an interrogation signal  10 , but are also able to automatically transmit, are known from DE 102 17 239 A1, for example. 
       REFERENCE NUMERALS 
       [0000]    
       
           1  Wheel electronics 
           1   a  Pressure sensor 
           1   b  Temperature sensor 
           1   c  Control unit 
           1   d  Power source 
           1   e  HF transmitter 
           1   f  LF receiver 
           2  Central unit 
           3  Interrogation transmitter 
           4  HF receiver 
           5  Adjustment section 
           10  Interrogation signal 
           11  Wake-up pulse 
           12  Adjustment section 
           12   a  Pulse 
           13  Start pulse 
           14  Command section 
           14   a  Short pulse 
           14   b  Long pulse