Method for compensating for a delay in a data-transmission network based on the TDMA protocol

Abstract

The invention relates to a method for compensating for a delay in a bidirectional data-transmission network based on the TDMA protocol, between a master device (M1) and at least one slave device (E1), said delay being due to electronic data-transmission means internal to the master and slave devices. The method comprising the following iterative steps: - synchronising the slave device with master device by synchronisation on a synchronisation signal (S) transmitted by the master device; - measuring the internal delay introduced by electronic data-transmission means for each master device; and - compensating for the measured internal delay.

Description

[0001] DESCRIPTION

[0002] TITLE: Method for delay compensation in a data transmission network based on the TDMA protocol

[0003] Technical field

[0004] The present invention relates to data transmission networks, and relates more particularly to the time synchronization of exchanges of such data according to the time division multiple access protocol.

[0005] State of the prior art

[0006] Currently, aeronautical and avionics systems employ a very large number of sensors for different uses. These sensors can be grouped into three main application categories: regulation: these are the most critical sensors because the data measured by these sensors are used to regulate the mechanical and hydraulic systems of an aeronautical equipment in flight. They are used to control the system; monitoring: the data obtained by these sensors are used to monitor the behavior and health of a system or structure of an aircraft in flight; instrumentation: the data measured by these sensors are used for ground and flight test benches to validate and carry out maintenance of aeronautical equipment. The benches are complex because they employ a very large number of sensors.

[0007] Thus, each sensor used in an aircraft is independently connected by a cable to a computer that processes its data. This is called point-to-point topology. The cabling associated with each sensor then becomes particularly large, which poses problems of mass and size. As a result, adding new sensors is restrictive.

[0008] There are configurations that allow multiple sensors to communicate with a single computer via a single cable, such as AFDX (Avionics Full-Duplex Switched Ethernet).

[0009] For example, as illustrated in Figure 1, there are two types of devices implemented to ensure communication between the sensors and the computer, namely a device M1 configured as "master" and devices configured as "slaves" E1, E2, ..., EN. A master or slave device is defined as a set of electronic components providing the interface between the sensors and the computer and jointly ensuring communication between the sensors and the computer. The "slave" devices have the capacity to interface with multiple sensors with different applications, and receive the data measured by them. The "master" device M1, which is connected to an aeronautical computer, triggers the reception of data from each of the "slave" devices in order to transmit all of the data collected to the aeronautical computer.Thus, the “master” device manages communication on the sensor network.

[0010] Thus, the master device M1 transmits an instruction TR3 in the form of a sequence of data to each “slave” node E1, E2, ..., EN.

[0011] In the context of two-way communication, each “slave” device El, E2, ..., EN has programmable electronic means configured to transmit data TR2 to the “master” device M l, to indicate to it that the task has been carried out, for example.

[0012] The fact that a plurality of slave devices E1, E2, ..., EN exchange data with the master device M1 via the same link requires the data to be multiplexed. A protocol used for this type of link is a so-called time division multiplexing protocol, such as TDMA - time division multiple access protocol (for "Temporal Division Multiple Access" in English).

[0013] Thus, as illustrated in Figure 2, the data sequences TR3, transmitted by the master device M1 to each slave device E1, E2, ..., E2, comprise data frames TR1 and TR2. In this example, each sequence TR3 has a duration of the order of 1 ms.

[0014] The master device M1 has a master clock which clocks the transmission of data in each of the sequences TR3. Similarly, each slave device XI, X2, ..., XN has a slave clock which clocks the reception of data in each of the sequences TR3.

[0015] Each data frame TR I comprises a master signal M, comprising data relating to the master device M l such as sensor reading commands, or supervision and maintenance commands. Said frame TR I also comprises an analog synchronization signal S .

[0016] Each data frame TR2 has a plurality of time slots XI , X2, ... , XN intended for the transmission of data from each slave node E l , E2, ... , EN to the master device M l . In this example, we will take N=20 slave devices.

[0017] Each slave device El, E2, ..., EN then has the possibility of communicating the data TR2 with the master device M l during a time interval allocated to it and thus avoiding the risk of data interaction with the other slave devices.

[0018] To do this, it is necessary that the slave clock of each slave device El, E2, ..., EN, intended to clock the transmission and reception of data, is perfectly synchronized with the master clock of the master device M l .

[0019] In other words, the slave clock of each slave device El, E2, ..., EN must be clocked on the same reference clock.

[0020] Patent FR 3 108 817 describes the transmission of an analog synchronization signal S in each TRI data sequence. The synchronization signal S comprises an amplitude adjustment portion SI, which is in the form of a sine wave having a constant amplitude for a predetermined number of pulses and an optimized synchronization portion S2, which is in the form of a triangular-shaped amplitude modulation of the sine wave. The optimized synchronization portion S2 does not have a constant amplitude but a variable amplitude, thus the transition between the increasing phase and the decreasing phase is detectable quickly and precisely, which makes it possible to determine a very precise reference time TOP. It is thus possible to synchronize the master and slave clocks and to time-stamp very precisely time intervals of each TR3 sequence.The reference instant TOP thus makes it possible to define the start of the period at which data XI, X2, ..., XN are sent respectively by each of the slave devices El, E2, ..., EN to the master device M l.

[0021] However, the transmission and reception circuits of each device, whether master or slave, can lead, due to their electronic components, to the addition of a propagation delay of the synchronization signal S or the data TR2.

[0022] This delay also changes depending on the temperature at the master device and slave devices, which can become problematic in an environment with high thermal constraints.

[0023] This delay can also vary depending on the transmission time due in particular to a long distance between the master device and the slave devices.

[0024] To overcome these time lags, very "advanced" signal processing techniques are used. Although very effective, these techniques add complexity to the communicating system, either in terms of the computing / electronic processing power required, or because they increase the amount of information to be transmitted because the measured data is transmitted mutually between devices or on a specific channel. For example, when the transmission line is 30 meters long, the delay generated is of the order of 200 nanoseconds.

[0025] It is also possible to eliminate this delay by frequency compensation, for example by using the compensation technique known as "Least squares (LS)" or "Minimum Mean Square Error (MMSE)".

[0026] The challenge is therefore to overcome the problems mentioned above.

[0027] Statement of the invention

[0028] In view of the above, the invention allows each of the master / slave nodes to synchronize independently.

[0029] To do this, the subject of the invention is a method for compensating for delay in a bidirectional data transmission network based on the TDMA protocol, between a master device and at least one slave device, said delay being due to electronic data transmission means internal to the master and slave devices, the method comprising the following iterative steps:

[0030] - synchronization of the slave device on the master device, by synchronization on a synchronization signal emitted by the master device;

[0031] - the measurement of the internal delay introduced by electronic means of data transmission for each master and slave device; and

[0032] - compensation for the measured internal delay.

[0033] These steps are repeated in a loop.

[0034] Each node is thus able to carry out a measurement of its own electronics by "looping back", without affecting other nodes on the electrical cable. Each node then uses this measurement to change the time at which it will transmit on the electrical cable and center its listening window for the signal being received.

[0035] In this respect, each device autonomously measures the delay caused by the propagation of signals within its electronic transmission circuit, namely the transmission and reception of data.

[0036] In other words, there is no need for a slave device to know the delay generated by the master device.

[0037] Similarly, when a master or slave device is in a communicating group, it is not essential that it acquires the values ​​of the delays generated by other devices.

[0038] This frees up bandwidth and therefore increases the speed of data exchange between the master and slave devices. Indeed, no training frame is used for synchronization between the devices, which frees up bandwidth.

[0039] Advantageously, during the step of synchronizing the master device on the slave device, the slave device is synchronized to an analog synchronization signal emitted by the master device.

[0040] Furthermore, during the synchronization step of the slave device on the master device, a reference time defining the start of a period of sending data to each slave device is extracted from the data sent by the master device.

[0041] According to a feature of the invention, the internal delay is measured by looping a loop signal between the output and the input of each master and slave device and by measuring the propagation time of the looped signal between the output and the input of each master and slave device.

[0042] Advantageously, the loop signal is generated at regular time intervals.

[0043] The data is transmitted in the form of data sequences. For each data sequence, reception is delayed by a duration substantially equal to a first delay value relative to the master device, and data transmission is advanced by a duration substantially equal to a second delay value relative to the slave device.The invention also relates to a bidirectional data transmission system comprising a master device and at least one slave device, said system being capable of implementing bidirectional data communication between the master device and the slave device, the master device and the slave device comprising electronic means for transmitting data according to a time division multiple access protocol, the data transmission system comprising means for synchronizing the slave device on the master device, by synchronizing on a synchronization signal emitted by the master device, means for measuring the internal delay introduced by the electronic data transmission means for each master and slave device and means for compensating for the measured internal delay.

[0044] According to another characteristic of the transmission system according to the invention, the means for measuring the internal delay comprise means for looping a signal between the output and the input of each master and slave device and means for measuring the propagation time of the looped signal between the output and the input of each master and slave device.

[0045] The invention also relates to a turbomachine comprising at least one system as defined above.

[0046] It also relates to a sensor for aeronautical applications, in particular for regulation, monitoring and / or instrumentation comprising a system as defined above.

[0047] The invention also relates to an aircraft comprising a turbomachine as defined above.

[0048] The invention also relates to an aircraft comprising at least one system capable of implementing two-way data communication as defined above.

[0049] Brief description of the drawings

[0050] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0051] [Fig. l ], which has already been mentioned, schematically shows a master device communicating with a set of slave devices;

[0052] [Fig.2], previously mentioned, schematically represents a data frame transmitted by the master device to the slave device according to the invention;

[0053] [Fig.3] illustrates a data communication system comprising a master device and a slave device according to the prior art;

[0054] [Fig.4a] and

[0055] [Fig.4b] illustrate means for measuring a loopback delay between the data transmission and acquisition means of the master device and the slave device, respectively, according to one embodiment of the invention;

[0056] [Fig. 5a]and

[0057] [Fig.5b] illustrate the principle of delay compensation implemented by means of the method and the device according to the invention, and illustrate the transmission of a data sequence respectively without delay compensation, and with delay compensation; and

[0058] [Fig.6] presents a flowchart of a compensation method according to an embodiment of the invention.

[0059] Detailed description of at least one embodiment of the invention

[0060] In Figure 3 is shown an electronic system 1 which comprises the master electronic device M l as well as the slave electronic device E l .

[0061] For the sake of clarity, the slave devices E2, ..., EN are not illustrated in said figure. Reference will then be made to a single slave device El but this does not exclude the coupling of the master device M l with a plurality of slave devices having an electronic architecture similar to the slave device E l .

[0062] In order to implement bidirectional data communication between the master device M l and the slave device E l according to the time division multiple access protocol TDMA, the electronic system 1 comprises a transmission line 2 capable of propagating an analog electrical signal.

[0063] The transmission line 2 is then coupled to transmission means 4 and to acquisition means 5 of the master device 1 on the one hand, and to transmission means 6 and reception means 7 of the slave device E1, on the other hand.

[0064] More particularly, the transmission means 4 comprise a chain of electronic components capable of transforming a digital signal into an analog signal and then transmitting it to the slave device E1 via the transmission line 2.

[0065] As previously described with reference to Figure 2, each data sequence TR3 comprises a header frame TRI and a data frame TR2. The frame TRI comprises a master signal M, comprising data from the master device M1, transmitted by the master device M1 for the slave devices E1, E2, ..., EN, an analog synchronization signal S, transmitted by the master device M1 for the slave devices E1, E2, ..., EN. The signal S makes it possible in particular to time-stamp the time intervals of the frames of the data sequence TR3 very precisely. Each frame TR2 also comprises time intervals X1-X20 intended for the transmission of data from each slave device to the master device M1.

[0066] The analog synchronization signal S comprises an amplitude adjustment portion SI which is in the form of a sinusoidal wave having a constant amplitude for a predetermined number of pulses and a synchronization portion optimized so as to determine a reference instant TOP according to patent FR 3 108 817. The reference instant TOP makes it possible to define the start of the period at which data XI, X2, ..., XN are sent respectively by each of the slave devices to the master device. This reference instant TOP is known with great precision. In addition, it makes it possible to precisely determine the time period between two reference instants TOP, which is of the order of 1 ms, in order to deduce therefrom the frequency of the master clock and possibly correct the frequency of the slave clock.

[0067] In this respect, the reception means 7 comprise a chain of electronic components intended to filter the analog signal TR3 received by the transmission means 4 and to transform it into a digital signal to be processed.

[0068] As for the transmission means 6 of the slave device E1, their function is to transmit an analog signal to the acquisition means 5 capable of filtering it and then transforming it into a digital signal. The analog signal is ideally transmitted by the transmission means 6 during the time interval allocated to the slave device E1 and includes the data TR2 which are transmitted to the master device.

[0069] However, the electronic components of the transmission means 4 and acquisition means 5 of the master device M1 as well as those of the transmission means 6 and reception means 7 of the slave device E1 are likely to add a propagation delay of the synchronization signal S or of the data TRI and TR2.

[0070] This delay also changes depending on the temperature at the master device M l and the slave device E l and / or depending on the transmission time, particularly due to a long connection distance between the master device and the slave devices.

[0071] Thus, at a first observation point NI arranged between the transmission line 2 and the transmission means 4, the propagation delay DI of the analog signal TR I transmitted to the slave device E l is observed.

[0072] At a second observation point N2 located between the transmission line 2 and the reception means 7, the delay remains substantially equal to DI because the delay linked to the propagation of the signal through the transmission line 2 remains negligible. For example, when the transmission line 2 has a length of 30 meters, the delay generated is of the order of 200 nanoseconds at most.

[0073] It is also possible to eliminate this delay by MMSE (for “Minimum Mean Square Error”) frequency compensation, for example.

[0074] When the signal TR I is received by the reception means 7 and transformed into a digital signal, a delay D2 is added.

[0075] Likewise, the transformation of the digital signal into an analog signal TR2 capable of being transmitted by the transmission means 6, leads to the addition of a delay D3 measured at a third observation point N3.

[0076] Finally, the reception of the analog signal TR2 and its transformation into a digital signal by the acquisition means 5 is likely to extend the delay by a duration D4 measured at a fourth observation point N4.

[0077] It is therefore important to compensate for the propagation delays D l , D2, D3 and D4 when implementing data communication between the master device M l and the slave device E l according to the TDMA protocol.

[0078] Knowing the said delays D1, D2, D3 and D4 makes it possible to synchronize the windows for transmitting the signal XI -.. .XN and for listening to the signal XI -.. .XN during such a data exchange.

[0079] For this purpose, figure 4a illustrates the master device M1 which further comprises measuring means 8a coupled to the transmission means 4 and to the acquisition means 5.

[0080] Such measuring means 8a are capable of generating an analog loopback signal intended to propagate exclusively between the transmission means 4 and the data acquisition means 5 and undergoing a propagation delay due to the electronic transmission means of the master device. To do this, the master device M1 comprises a first switch 9a capable of coupling and decoupling the transmission means 4 and the acquisition means 5.

[0081] The master device M1 further comprises a second switch 10a capable of coupling and decoupling the transmission means 4 and the transmission line 2.

[0082] A third switch 11a is also arranged between the acquisition means 5 and the transmission line 2 so as to couple and decouple the reception means 5 and the transmission line 2.

[0083] Thus, the measuring means 8a can close the first switch 9a and open the switches 10a and 11a to circulate the loop signal only between the transmission means 4 and the data acquisition means 5.

[0084] The propagation delay of the loopback signal therefore corresponds to a first delay value Tl substantially equal to the sum of delays DI and D4.

[0085] As for the slave device El, with reference to figure 4b, this also comprises an electronic architecture similar to that of the master device M1 and likewise comprises measuring means 8b coupled to the transmission means 6 and to the reception means 7, as well as switches 9b, 10b, and 11b, so as to generate a loop signal intended to propagate exclusively between the reception means 7 and the transmission means 6 and undergoing a propagation delay due to the electronic transmission means of the slave device.

[0086] A second delay value T2 is thus measured and corresponds approximately to the sum of delays D2 and D3.

[0087] In other words, each device M l , E l autonomously measures the delay caused by the propagation of data within its transmission circuit and its data reception circuit.

[0088] As previously indicated, a delay is introduced at each point of view. The delays are therefore accumulated and added at each point, which causes a shift which becomes significant, especially at point of view N4. The listening window of the master device M1 of the signal XI, X2, XN must therefore be centered on the data in order to avoid these shifts. We can thus anticipate the delay of the electronics, via the circuit developed and previously described, and adapt the transmission of the data because the delay induced, in particular by the electronics, is known.

[0089] It should be noted that the delay values ​​Tl and T2 are measured periodically in order to update them according to the temperature evolution at the master device Ml and the slave device El. By periodic, we mean that the master and slave devices are available (i.e. they are not monopolized by sending data at this precise moment). This measurement is carried out once per cycle, each cycle preferably having a duration of 1 ms.

[0090] The loopback signals are therefore generated during the guard interval between two successive TR3 data sequences. A guard interval is understood to mean an interval in which the master or slave devices are free, i.e. they are neither transmitting nor receiving.

[0091] We refer to figures 5a and 5b which represent the electronic system 1 in which the delays T1 and T2 were measured before the transmission of a data sequence TR3, respectively before the compensation and after the compensation.

[0092] In these figures, observation points N 1 and N6 are observation points of the master device M l , observation points N2 and N5 are observation points of the transmission line 2 and observation points N3 and N4 are observation points of the slave device.

[0093] Likewise, in these figures:

[0094] The DI delay represents the delay of the transmission chain (Tx) of the master device: t Tx _ M =D 1 ;

[0095] The delay D2 represents the delay of the reception chain (Rx) of the slave device: at Tx Xn =D2 ;

[0096] The delay D3 represents the delay of the transmission reception chain (Tx) of the slave device: att Rx Xn = D3; The delay D4 represents the delay of the reception chain (Rx) of the master device: att Rx M =D4.

[0097] It will also be noted that the channel delay De represents the propagation time between the master device and the slave device, which is smaller than the delay due to the electronics compensated by means of the delay compensation method according to the invention, and which will be compensated by the channel compensation.

[0098] Thus, the delay due to the transmission chain (Tx) of the master device and the reception chain (Rx) of the slave device is: D l + D2.

[0099] The delay due to the slave device's transmission chain (Tx) and the device's reception chain (Rx) is: D3+D4.

[0100] Referring to Figure 5a, in the absence of compensation, the balance of delays at observation nodes NI to N6 is written:

[0101] Nl=0

[0102] N2=D 1

[0103] N3=D1+D2

[0104] N4=D1+D2

[0105] N5 = D 1 +D2+D3

[0106] N6= (D 1+D4) + (D2+D3).

[0107] For the slave device receiver (node ​​N3), the delay applies to both the TOP synchronization pulse and the data so that the time window is aligned with the allocated slot, without time shift.

[0108] Thus, the synchronization problem arises particularly in the return direction, i.e. when transmitting signals from the nodes to the master device.

[0109] Thus, with reference to figure 5b, the delay from the slave device E1 is compensated by a displacement of the time listening window of the slave device E1.

[0110] As for the acquisition means 5 of the master device M1, they are capable of starting the reception of the data TR2 according to the first delay value T1.

[0111] For this purpose, the acquisition means 5 are configured to delay the activation of the time listening window of the master device M1 by a duration substantially equal to the first measured delay value T1.

[0112] Thus, at the fourth observation point N4, the receiver of the slave node sends its signal in anticipation. The delay balance is thus DI + D2 - (D2 + D3) or DI - D3.

[0113] Similarly, the delay from the transmitter to device N5 is compensated by anticipation. At node N5, the delay balance is D 1 -D3+D3, or D l. Note that this delay is compensated by another method (MMSE for example).

[0114] At node N6, the balance of delays, due to compensation, is written D l + D4, this latter delay being compensated by the master device by delaying its listening window with its own measured delay.

[0115] We now refer to Figure 6 which illustrates a flowchart of a delay compensation method implemented during the bidirectional data communication between the master device M1 and the slave device E1 implemented by the electronic system 1 according to the invention. It will be noted that the steps of the method are implemented iteratively and are continuously looped.

[0116] The method begins with a step 100, during which the transmission means 4 transmit a data sequence TR3 to the slave device E1.

[0117] Since this is a data communication according to the TDMA protocol, the data sequence TR3 comprises data frames TR1 and TR2. During this first step 100, the reference TOP is extracted from the synchronization signal S, more particularly from the optimized synchronization portion S2, transmitted by the master device.

[0118] In step 200, the internal delay introduced by the electronics of each master and slave device is measured using the internal feedback system described previously.

[0119] The transmission means 7 of the slave advance the data transmission TR2 by a duration equal to the second delay value T2. During a transmission from the slave devices to the master device, the slave devices must compensate for their delay time. Each slave device will transmit earlier in its slot Xn and advance its transmission window by T2.

[0120] Finally, in step 300, the acquisition means 5 receive the data TR2 and delay their time listening window by a duration equal to the first known delay value T1, as described previously with reference to FIG. 5a, 5b.

[0121] When the internal electronics delay is known, the system uses the measured delay to compensate for delays in two stages:

[0122] Sending (transmitting) signals to the master device in advance;

[0123] Delay of the signal listening window in reception on the master device side

[0124] Thus, in step 300, the delays are compensated using the measurements obtained in step 200.

[0125] Thus, at each emission of a TR3 sequence, the propagation delays T1 and T2 are compensated.

[0126] Obviously, the invention is not limited to the embodiments and implementations described above and provided solely by way of example.

[0127] In particular, the TDMA protocol can be combined with the method of coding digital signals by orthogonal frequency division in the form of multiple OFDM subcarriers (for "Orthogonal Frequency Division Multiplex" according to the Anglo-Saxon term).

[0128] These techniques combined together make it possible to optimize spectrum utilization capacity. The total bandwidth is then shared into several sub-bands for a defined number of slave devices.

[0129] Alternatively, it is possible to perform single-carrier modulation by phase-shift keying (PSK).

Claims

CLAIMS 1. Method for compensating for delay in a bidirectional data transmission network based on the TDMA protocol, between a master device (M l ) and at least one slave device (E l ), ​​said delay being due to electronic data transmission means internal to the master and slave devices, the method comprising the following iterative steps: - synchronization of the slave device on the master device, by synchronization on a synchronization signal (S) emitted by the master device; - the measurement of the internal delay introduced by electronic means of data transmission for each master and slave device; and - compensation for the measured internal delay, and in which the internal delay is measured by looping a loop signal between the output and input of each master and slave device and measuring the propagation time of the looped signal between the output and input of each master and slave device.

2. Compensation method according to claim 1, in which, during the step of synchronizing the slave device on the master device, a time interval is extracted from the data transmitted by the master device comprising a synchronization signal (S) comprising an amplitude adjustment portion (SI) and a synchronization portion (S2) in the form of an amplitude modulation capable of determining a reference instant (TOP defining the start of a period of sending data to each slave device.

3. Compensation method according to one of claims 1 and 2, in which the loop signal is generated at regular time intervals.

4. Compensation method according to any one of claims 1 to 3, in which the data is transmitted in the form of data sequences (TR3), and in which, for each data sequence (TR3), the reception of the data (300) is delayed by a duration substantially equal to a first delay value (T1) relative to the master device, and the transmission (200) of the data (TR2) is advanced by a duration substantially equal to a second delay value (T2) relative to the slave device.

5. System (1) for bidirectional data transmission comprising a master device (M l ) and at least one slave device (E l ), ​​said system being capable of implementing bidirectional data communication between the master device (M l ) and the slave device (E l ), ​​the master device (M l ) and the slave device (E l ) comprising electronic means for transmitting data according to a time division multiple access protocol, the data transmission system comprising means for synchronizing the slave device on the master device by synchronizing on a synchronization signal (S) emitted by the master device, means for measuring the internal delay introduced by the electronic means for transmitting data for each master and slave device and means for compensating for the measured internal delay,and wherein the means for measuring the internal delay comprise means for looping a signal between the output and the input of each master and slave device and means for measuring the propagation time of the looped signal between the output and the input of each master and slave device., 6. Turbomachine comprising at least one system (1) according to claim 5.

7. Sensor for aeronautical application, in particular for regulation, monitoring and / or instrumentation, comprising a system (1) for bidirectional data transmission according to claim 5.

8. Aircraft comprising a two-way data transmission system (1) according to claim 5.

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