Abstract:
A method is disclosed for synchronizing network subscribers in an on-board network of a vehicle, the method comprising: receiving a message dependent on a first clock present in a first network subscriber by at least one second network subscriber if a predetermined condition has been satisfied, and synchronizing a second clock in the second network subscriber on the basis of the message dependent on the first timer if the predetermined condition has been satisfied.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. 10 2011 082 521.5, filed Sep. 12, 2011, and International Patent Application No. PCT/EP2012/067848, filed Sep. 12, 2012. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for synchronizing network subscribers in an onboard network in a vehicle, a control apparatus for performing the method and to a network subscriber having the control apparatus. 
       BACKGROUND OF THE INVENTION 
       [0003]    JP 11 115 627 A discloses the practice of connecting two network subscribers that are connected to a data bus to an additional dedicated line in order to synchronize the timings of said network subscribers to one another. 
         [0004]    It is an object of the invention to improve the timing synchronization of network subscribers connected to a data bus. 
       SUMMARY AND INTRODUCTORY DESCRIPTION OF THE INVENTION 
       [0005]    The object is achieved by the technical features disclosed herein and by way of the method disclosed herein. 
         [0006]    According to one aspect of the invention, a method for synchronizing network subscribers in an onboard network in a vehicle comprises the steps of: receiving a message that is dependent on a first timer that is existent in a first network subscriber by at least one second network subscriber when a predetermined condition is satisfied, and synchronizing a second timer in the second network subscriber on the basis of the message that is dependent on the first timer when the predetermined condition is satisfied. 
         [0007]    The method is based on the consideration that the dedicated line for synchronizing the timings of network subscribers that are existent in a vehicle network grows with the number of network subscribers, particularly because the individual network subscribers need to be connected to the dedicated line in star form, since the inevitable latencies would otherwise result in nonsynchronous reception of the relevant synchronization signal, which would render timing synchronization impossible. 
         [0008]    On the basis of this consideration, the specified method involves recognition of the fact that, although it would be possible to perform the synchronization via the onboard network of the vehicle, there would be need to guarantee that the latencies during reception of the synchronization signal do not result in all network subscribers ultimately having their timings synchronized at different times. 
         [0009]    The concept behind the specified method is to arrange with all network subscribers a particular time at which the synchronization needs to be performed. For this reason, the specified method proposes stipulating at least one predetermined condition that allows each network subscriber to recognize imminent synchronization and prompts said network subscriber to expect to receive the message that is suitable for synchronization. This predetermined condition may be arbitrary, which is part of the subject matter described herein. 
         [0010]    In one development, the specified method comprises the steps of: synchronizing the first timer on the basis of a reference time; and sending of the message to be received on the basis of the synchronized first timer. 
         [0011]    In an additional development of the specified method, the reference time is derived from a wirelessly received signal. This wirelessly received signal may, in principle, be any desired signal that is suitable for synchronizing the timings of the individual network subscribers. Thus, the wirelessly received signal may be a DCF77 signal, for example, which is used in Europe for synchronizing timers by radio, or the carrier frequency of a radio transmitter, the carrier frequency value of the latter being able to be derived on the basis of the radio transmitter itself, for example. 
         [0012]    With particular preference, the wirelessly received signal is a global navigation satellite system signal, GNSS signal for short, GNSS signals—for technical reasons—being signals with high-precision timing that allow network subscribers in the form of sensors in a vehicle to have their timings synchronized with a correspondingly high level of precision. Such synchronization is currently at the focus of current development pertaining to Car2X communication, that is to say data interchange from a vehicle to other vehicles or systems in the surroundings, such as traffic lights or other infrastructure components. The interchange of information about accidents and other hazard locations, about the condition of the road, road signs and much more makes it possible to attain an increase in safety and convenience. In many cases, this needs to involve the information being provided in real time. In order to ensure this real time, the information can be provided with a high-precision time stamp, for example, which is linked to the information by the respective network subscriber. This time stamp needs to have a correspondingly high level of precision, however, which is ensured by the synchronization on the basis of the GNSS signal. The GNSS signal used may be a global positioning system signal, GPS signal for short, a                                         signal, GLONASS signal for short, or a Galileo signal, for example. 
         [0013]    In one particular development of the specified method, the predetermined condition is time-triggered. The time trigger, that is to say the time release for the synchronization, may be of arbitrary embodiment. By way of example, it is thus possible for clocks to be provided within the individual network subscribers, which clocks run with a particular basic precision and permit sufficiently exact time release of the time-controlled synchronization. 
         [0014]    In one preferred development of the specified method, the time-triggered condition is the timing of a time slot in a time division multiple access data transmission method, TDMA data transmission method for short, in the network. Such a TDMA data transmission method is used by various network protocols, such as FlexRay, PSIS, LIN, etc. Since these protocols are known to a person skilled in the art, they will not be described in more detail. The development is based on the consideration that a TDMA data transmission method already requires very precise synchronization of the individual network subscribers among one another in order to be able to use data in time slots without great bandwidth losses. These bandwidth losses are essentially dependent on a lack of synchronization, that is to say on the uncertainty of when it is clear for all network subscribers when a time slot begins and when it ends. This base synchronization, which exists anyway for the operation of the TDMA method in the bus system, is now used in order to transmit the message that is dependent on the first timer with sufficient precision too. 
         [0015]    In an alterative particular development of the specified method, the predetermined condition is event-triggered. The event-dependent trigger, that is to say the event-dependent release for the synchronization, may likewise be of arbitrary embodiment. Whereas the time-dependent trigger operates deterministically from a point of view of timing, so that the synchronization takes place in a predictable manner, the event-oriented trigger operates stochastically, so that the synchronization takes place randomly from the point of view of timing, depending on when the event takes place. However, it would also be possible to enforce the random event by virtue of the onboard network of the vehicle being transferred to a particular state, for example, from which each network subscriber recognizes the imminent synchronization. Such an event-triggered method lends itself, by way of example, to all network protocols in which no temporally recurrent structures can be recognized on all network subscribers collectively, such as in the case of the CAN bus that is known to a person skilled in the art. 
         [0016]    In one preferred development of the specified method, the event-triggered condition is the output of a predetermined sensor signal. This sensor signal may be in any form. This development is possible for all network subscribers that are able to detect a common event, that is to say by means of sensors, such as acceleration, rotation rate, camera, etc. Each appliance involved in the synchronization logs the detection time and possibly the duration of detection for the predetermined sensor signal as an event-triggered condition, which may be a bollard or a particular curve, for example. The message that is dependent on the first timer is thus sent via the onboard network when the event-triggered condition has arisen and the relevant network subscribers expect to receive said message. 
         [0017]    With particular preference, the message that is dependent on the first timer contains a current time that the first timer outputs when the message is produced. 
         [0018]    According to a further aspect of the invention, a control apparatus is set up to perform one or more of the specified methods. 
         [0019]    In one development of the specified control apparatus, the specified apparatus has a memory and a processor. In this case, the specified method is stored in the memory in the form of a computer program and the processor is provided for executing the method when the computer program is loaded from the memory into the processor. 
         [0020]    According to a further aspect of the invention, a computer program comprises program code means in order to perform all the steps of one of the specified methods when the computer program is executed on a computer or one of the specified apparatuses. 
         [0021]    According to a further aspect of the invention, a computer program product contains a program code that is stored on a computer-readable data storage medium and that, when executed on a data processing device, performs one of the specific methods. 
         [0022]    According to a further aspect of the invention, a network subscriber for an onboard network in a vehicle comprises a specified control apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The properties, features and advantages described above for this invention and also the manner in which they are achieved become clearer and more distinctly comprehensible in connection with the description below of the exemplary embodiments, which are explained in more detail in connection with the figures of an exemplary embodiment, in which: 
           [0024]      FIG. 1  shows a basic illustration of an onboard network for a vehicle, and 
           [0025]      FIG. 2  shows a basic illustration of a TDMA transmission, and 
           [0026]      FIG. 3  shows a basic illustration of a transmission via a CAN bus. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In the figures, technical elements that are the same are provided with the same reference symbols and are described only once. 
         [0028]    Reference is made to  FIG. 1 , which shows a basic illustration of an onboard network  2  for a vehicle that is not shown further. 
         [0029]    The onboard network  2  comprises a data bus  4 , which may be in the form of a FlexRay bus that is known to a person skilled in the art or in the form of a CAN bus that is known to a person skilled in the art, for example. In the present embodiment, four network subscribers  6 ,  8 ,  10 ,  12  are connected to the data bus  4  bidirectionally. 
         [0030]    In the present embodiment, the first network subscriber  6  is in the form of a GNSS receiver  6  and has a GNSS antenna  14  that it can use to receive GNSS signals  16  from a GNSS satellite  18  in a manner that is known per se to a person skilled in the art. The GNSS signals  16  are used primarily for absolute location of the vehicle—not shown further—in the space. Alternatively, however, the GNSS receiver  6  may also be part in the first network subscriber  6 , which may be in the form of a navigation appliance, for example, or, as an integrated communication unit, combines a wide variety of communication and reception modules. 
         [0031]    In the present embodiment, the second network subscriber  8  and the third network subscriber are in the form of sensors. In the present case, the second network sensor  8  is meant to be in the form of an inertial sensor  8 , which, in a manner that is not shown further, senses accelerations and rotation rates for the vehicle that is not shown further and sends them to the data bus  4 . In the present embodiment, the second sensor  10  is meant to be in the form of a wheel speed sensor  10 , which, in a manner that is not shown further, senses the wheel speed of one or more wheels of the vehicle that is not shown further and sends it to the data bus  4 . The data bus  4  may have further sensors, such as a steering angle sensor, connected to it, which sense further variables, such as a steering angle for the vehicle that is not shown further, and sends them to the data bus  4 . 
         [0032]    In the present embodiment, the third network subscriber  12  is in the form of what is known as a fusion sensor  12 . The fusion sensor  12  is meant to connect conventional driving dynamics sensors, such as the output data from the inertial sensor  8  and the wheel speed sensor  10 , to the GNSS raw data from the GNSS receiver  6 . The “external anchor” comprising the GNSS raw data thus allows the fusion sensor  12  to improve the output data from the inertial sensor  8  and the wheel speed sensor  10  and to deliver them with greater reliability. The main advantage, however, is the possibility of delivering a very precise position for the vehicle that is not shown further. In this case, the position can, in principle, be delivered with lane precision. What is important in this case is the very exact timing synchronization of all data sources involved, that is to say of all sensors  8 ,  10 , the fusion sensor  12  and the GNSS receiver  6 , to a time base with an error of less than 100 μs. In this regard, each network subscriber  6  to  12  accordingly has a clock  22  to  26  that, by way of example, is respectively accommodated in a corresponding transmission device  28  to  34  for sending and receiving the synchronized data. In the present embodiment, the time base for synchronizing the clocks  22  to  26  comes from the GNSS receiver  6  in this case, since this provides an external reference that is of high precision on account of the system. The sensor  8  and  10  may also be part of the fusion sensor  12 . 
         [0033]    The synchronization of the clocks  20  to  26  is described below with reference to two methods. 
         [0034]    Reference is made to  FIG. 2 , which shows a basic illustration of synchronization over time  36  on the basis of a TDMA transmission. 
         [0035]    By way of example, the TDMA method is used in the FlexRay system cited further above, the performance of TDMA already requiring very precise synchronization between transmitter and receiver in order to be able to use the time slot method known to a person skilled in the art without great bandwidth losses. The reason is that the bandwidth losses consist in a pause in the length of the uncertainty of the synchronization being required at the start and end of every data transmission by a bus subscriber. The base synchronization that is existent anyway for operation of the bus system is now used in order to transmit the time pulse with sufficient precision too. 
         [0036]    In this regard, the GNSS receiver  6  can first of all produce a time pulse  40  at a reception time  38  on the basis of the reception of the GNSS signal  16 . 
         [0037]    First, the clock  20  of the transmission module  28  in the GNSS receiver  6  is synchronized on the basis of this time pulse  40  in a manner that is not shown. This can be accomplished either by hard setting of the clock  20  to the time pulse  40  or by adjusting the clock  20  to the time pulse  40 . The hard setting of the clock  20  can cause “skips” in time  36 , which are not shown further. If these are not wanted, adjustment to the time pulse  40  is more advantageous. In this case, a time difference between the time pulse  40  and the clock  20  over a prescribed length of time and possibly also a plurality of time pulses  40  is reduced to the extent that a threshold value for the time difference is observed. This can also be achieved by customizing PLLs or DLLs—which are known to a person skilled in the art—of the clock  20  of the transmission module  28 . Hard setting and adjustment can also be combined. 
         [0038]    In this case, the transmission module  28  in the GNSS receiver  6  has, for the synchronization of the clocks  20  to  26  of all network subscribers  6  to  12  on the data bus  4 , the role of a master that outputs a synchronization instruction that is followed by the remainder of the network subscribers  8  to  12 . 
         [0039]    This synchronization instruction is transmitted in a next time slot  42 —following the reception time—of the TDMA-based bus system as a message  44 —shown as a cross in FIG.  2 —that uses its timing  46  to convey the reception of the time pulse  40 , besides the unreduced data payload. Ideally, this message  44  additionally incorporates the latency  48  between the reception time  38  of the time pulse  40  and the start  46  of the time slot  42  used for transmitting the message  44 , in order to allow even more precise synchronization. 
         [0040]    Upon receiving the message  44  via the data bus  4 , the other network subscribers  8  to  12  synchronize their respective clock  20  to  26 , if possible with the correction using the additional information about the latency  48 . 
         [0041]    As an extension, it is also possible for one or more alternative messages, not shown in  FIG. 2 , that contain a high-precision time stamp to be sent between the messages  44  that contain the time pulse  40  or instead of messages  44  that contain the time pulse  40 . 
         [0042]    The method described within the framework of  FIG. 2  allows continuous synchronization of the clocks  20  to  26  of the individual network subscribers  6  to  12  in the data bus  4 , since the data bus  4  used as a TDMA bus has a temporally deterministic response. 
         [0043]    Reference is made to  FIG. 3 , which shows a basic illustration of synchronization over time  36  on the basis of a transmission via a CAN bus as data bus  4 . 
         [0044]    In the case of a data bus  4  that exhibits a nondeterministic timing response, for which reception of the time pulses  40  by the network subscribers  8  to  12  cannot be expected over time, such as in the case of the CAN bus, a characteristic event  50  that can be sensed by all network subscribers  8  to  12  that are to be synchronized can be used so that said subscribers can expect to receive the time pulses  40 . In a way, it is possible for reception to be signaled to said subscribers. 
         [0045]    In the present embodiment, a characteristic event  50  of this kind can be sensed by means of the sensor system of the network subscribers  6  to  12 , such as acceleration, rotation rate, camera, etc. Each network subscriber  6  to  12  involved in the synchronization logs the detection times and possibly the duration of detection for characteristic events  50 , which may be in the form of a bollard on in the form of a curve, for example. One of the network subscribers  6  to  12  must again act as a “master” in similar fashion to  FIG. 2 , ideally a network subscriber  6  to  12  that is already in sync with a very precise time by means of the GNSS signal  16 . In the present embodiment, the GNSS receiver  6  will again be treated as a master. If the GNSS receiver  6  recognizes a characteristic event  50  via a sensor system that has been connected to the GNSS receiver  6  especially or that is existent on the GNSS receiver  6  anyway, said event also being able to be detected by the respective other network subscribers  8  to  12 , then a message  44  that marks the time  38  of recognition of the characteristic event is sent via the data bus  6 . 
         [0046]    The message  44  may be a simple signal. Preferably, the message  44  additionally contains the delay  48  that has passed since the characteristic event  50  up to the time  46  of the message  44  being sent. This message  44  shall expediently be sent with very high priority for access to the data bus  4  so that no unknown delays are produced by access to the data bus  4 . All network subscribers  8  to  12  synchronize themselves to the received message and align their clocks  22  to  26  therewith. 
         [0047]    With particular preference, where the characteristic event is at an event end  52 , a second message  54  could be sent by the GNSS receiver  6  in the form of a master if a characteristic event  50  with a sufficiently long event duration  56  is involved. This second message  54  communicating the event end  52  may additionally contain the duration  54  of the characteristic event  50 . Hence, the other network subscribers  8  to  12  connected to the data bus  4  have three pieces of information—event start  38 , event end  52  and event duration  56 —relating to the characteristic event  50  and can accordingly customize their clocks  22  to  26  better. 
         [0048]    The method described allows distributed time stamping to take place, and not all data need to pass through a central system. 
         [0049]    While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation, and change without departing from the proper scope and fair meaning of the accompanying claims.