Device and method for estimating train speed based on vibration signals

Abstract

The invention relates to a device and a method for estimating train speed. A vibration in a natural frequency band experienced by a train as it moves forward is sampled by means of first and second sensors at the same sampling frequency, to obtain a first set and a second set of sampling signals respectively; a first set and a second set of vibration signals are obtained on the basis of the first and second sets of sampling signals respectively, and the first and second sets of vibration signals are subjected to cross-correlation analysis, to obtain a target sampling difference; and a train speed is calculated on the basis of the target sampling difference.The train speed estimation device and method according to the present invention can accurately monitor train speed in real time without the use of any speed sensor or GNSS. Figure 1.

Description

Description Title of the invention: Device and method for estimating the speed of train based on vibration signals Technical field of the invention

[0001] The present invention relates to a train speed estimation, in particular a device and method for estimating train speed based on signals vibration. Prior art

[0002] In a railway train, in order to detect defects in components For rotating parts of the train, it is generally necessary to carry out precise monitoring of the real-time speed of the train. Currently, speed monitoring the train uses speed sensors or is based on the global positioning system satellite navigation (GNSS). However, speed sensors present a energy consumption and relatively high material costs, while the GNSS-based systems have very strict requirements in terms of signal conditions, and in scenarios presenting poor conditions of signals, such as tunnels, often mean that the real-time train speed is not cannot be obtained with precision. Summary of the invention

[0003] — In response to the above problem, the present invention relates to a device and a train speed estimation method, which estimates train speed in time real by sampling and analyzing a vibration signal synchronously.

[0004] According to one aspect of the present invention, a train speed estimation device is proposed, including: at least one pair of sensors, including a first sensor and a second sensor arranged in the direction of travel of a train, the the first sensor and the second sensor being configured to sample, at the same sampling frequency, a vibration within a band of natural frequencies undergone by the train as it moves forward, in order to obtain a first set of signals sampling and a second set of sampling signals respec- tively; and a processor, configured to obtain a first set of signals from vibration and a second set of vibration signals based on the first set of sampling signals and the second set of signals sampling respectively, and submit the first set of signals from vibration and the second set of vibration signals to a cor- analysis cross-relationship, to obtain a target sampling difference; the difference target sampling obtains a maximum cross-correlation between the first set of vibration signals and the second set of vibration signals, and the processor is configured to calculate a train speed based on the target sampling difference. According to another aspect of the present invention, a method for estimating train speed is proposed, comprising: sampling, at the same sampling frequency, a vibration in a natural frequency band experienced by the train as it moves forward by means of a first sensor and a second sensor in at least one pair of sensors, in order to obtain a first set of sampling signals and a second set of sampling signals respectively, in which the first sensor and the second sensor are arranged in the direction of advance of the train;obtaining a first set of vibration signals and a second set of vibration signals based on the first and second sets of sampling signals respectively, and subjecting the first and second sets of vibration signals to cross-correlation analysis to obtain a target sampling difference, in which the target sampling difference achieves maximum cross-correlation between the first and second sets of vibration signals; and calculating a train speed based on the target sampling difference. According to the principles of the present invention, real-time train speed can be accurately monitored without the need for any speed sensors or GNSS. Compared to the prior art, the train speed estimation device and method according to the present invention can reduce energy consumption during real-time train speed estimation, have higher accuracy than a GNSS-based train speed estimation method, and are particularly suitable for estimating underground train speed under non-GNSS signal conditions. Furthermore, the train speed estimation device and method according to the present invention are suitable for various types of railway vehicles, such as underground trains, multiple-unit trains, high-speed trains, and overhead trains, and are unaffected by factors such as train type and track type. Brief description of the figures The attributes and advantages of the present invention will become apparent from the detailed description below of illustrative embodiments included in conjunction with the drawings. In the drawings: [Fig.1] includes a Figure 1A schematically illustrating a side view of a part of a train in which a pair of sensors is provided according to an embodiment of the present invention, and a figure 1B schematically illustrating a bottom view of part of a train in which a pair of sensors is provided according to an embodiment of the present invention. [Fig.2] illustrates a functional diagram of a train speed estimation device according to an embodiment of the present invention. [Fig.3] illustrates a communication process by the sensor pair prior to sampling according to an embodiment of the present invention. [Fig.4] illustrates a flowchart of a train speed estimation method according to an embodiment of the present invention. [Fig. 5] illustrates a flowchart of communication via the sensor pair prior to sampling according to an embodiment of the present invention. Detailed description of the invention Illustrative embodiments of the present invention are clearly and completely described below in association with the drawings; obviously, the illustrative embodiments described are only some, but not all, of the embodiments of the present invention. An existing train speed estimation device monitors a train's speed using a speed sensor or a GNSS-based system, whereas the train speed estimation device according to the present invention synchronously samples a vibration signal by means of at least one pair of sensors arranged on the train, so as to estimate the train speed in real time. More precisely, in trains of various types such as underground trains, multiple-unit trains, high-speed trains, and overhead trains, there is a multibody dynamic system having a structure that is substantially symmetrical in the front-to-back direction; this multibody dynamic system may include, but is not limited to, structures such as train cars, car bogie connecting springs, and bogies.The multibody dynamic system, which is substantially symmetrical in the front-to-back direction, vibrates in a natural frequency band as the train moves forward; the vibration responses are similar to those at symmetrical front-to-back positions, and there is a time difference between vibration responses at front-to-back positions, this time difference being determined by the train's forward speed. In other words, the train's forward speed can be calculated from the time difference obtained by comparing vibration responses in the natural frequency band at symmetrical front-to-back positions in the system. Thus, in an embodiment according to the present invention, a vibration experienced by the train during the forward movement process is detected by means of [the system / device]. Multiple sensors arranged at different positions on the train in the direction of travel of the train, to obtain a time difference of vibration signals, and determine the speed of travel of the train according to the time difference. Figure 1A schematically illustrates a side view of a part of a train in which a pair of sensors is provided according to an embodiment of the present invention, and Figure 1B schematically illustrates a bottom view of a part of a train in which a pair of sensors is provided according to an embodiment of the present invention. As illustrated in Figures 1A and 1B, according to one embodiment of the present invention, the train comprises at least one car 101. At least one bogie 102 is provided beneath the car 101; a first wheel 103 and a second wheel 104 are provided at the lower part of the bogie 102. The train's multibody dynamic system (not shown) causes the train to vibrate in a band of natural frequencies during forward motion. A first sensor 105 and a second sensor 106 are provided at the axle centers of the first wheel 103 and the second wheel 104, respectively, so as to form a pair of sensors. Those skilled in the art should understand that the present invention is not limited to this specific arrangement of a train and sensors. For example, the pair of sensors can be arranged at any suitable position on the train in the direction of travel.A train can be a railway vehicle, a high-speed train, an underground train, an elevated train, or a freight train. Fig. 2 illustrates a functional diagram of a train speed estimation device according to an embodiment of the present invention. As shown in Figure 2, according to one embodiment of the present invention, a train speed estimation device 201 comprises at least one pair of sensors 202 and a processor 203. In Figure 2, the processor 203 is shown arranged separately from the pair of sensors 202, but the present invention is not limited to this. For example, the processor can be integrated with a sensor within the pair of sensors. The pair of sensors 202 comprises a first sensor 202-1 and a second sensor 202-2 arranged in the direction of travel of the train; the first sensor 202-1 and the second sensor 202-2 are configured to sample, at the same sampling frequency f, a vibration in the natural frequency band experienced by the train as it travels, in order to obtain a first set of sampling signals and a second set of sampling signals, respectively. Figure 3 illustrates a communication process by the pair of sensors prior to sampling according to an embodiment of the present invention. As illustrated in Figure 3, according to one embodiment of the present invention, before the first sensor 202-1 and the second sensor 202-2 perform sampling, the first sensor 202-1 sends standard initial pulses to P second sensor 202-2, where _ is a natural integer greater than or equal to 1. By P For example, p = 3. Each time the second sensor 202-2 receives a standard initial pulse from the first sensor 202-1, the second sensor 202-2 sends a return initial pulse to the first sensor 202-1. The first sensor 202-1 and the second sensor 202-2 can each record timestamps for the pulses they send and receive. According to one embodiment of the present invention, different pairs of sensors arranged in the train use different wireless channel frequency bands for communication, so as to avoid signal crosstalk. If the first sensor 202-1 receives initial return pulses from the P If the second sensor 202-2 receives a standard synchronization pulse, the first sensor 202-1 sends a standard synchronization pulse to the second sensor 202-2 and begins sampling. Otherwise, if the first sensor 202-1 does not receive p initial return pulses from the second sensor 202-2, then the first sensor 202-1 returns to the substep of sending the _ standard initial pulses. P If the second sensor 202-2 receives the standard synchronization pulse, that is to say a 1st pulse, coming from the first sensor 202-1, then The second sensor 202-2 starts sampling, and sends a return synchronization pulse to the first sensor 202-1. If the first sensor 202-1 receives the return synchronization pulse from the second sensor 202-2, then the first sensor 202-1 continues sampling. Otherwise, if the first sensor 202-1 does not receive the return synchronization pulse from the second sensor 202-2, then the first sensor 202-1 stops sampling and returns to the substep of sending the initial pulses. p standard. If the second sensor 202-2 receives a standard synchronization pulse from the first sensor 202-1 again, then the second sensor 202-2 stops sampling and then returns to the substep of starting sampling and sending a return synchronization pulse to the first sensor 202-1. sensor 202-1. According to one embodiment of the present invention, the above substeps can be repeated until the first sensor 202-1 and the second sensor 202-2 complete sampling. According to one embodiment of the present invention, after the first sensor 202-1 and the second sensor 202-2 have completed sampling, the first sensor 202-1 calculates a sampling start time difference a 7° between time points at which the first sensor 202-1 and the second sensor 202-2 begin sampling, based on a time difference between the sending of a standard initial pulse and the reception of a corresponding return initial pulse by the first sensor. For example, the difference in sampling start time (AT) is calculated using the following formula: Lg af, aTr= 55 Dies where AT is the time difference between the sending of the th initial standard pulse Ë by the first sensor 202-1 and the reception by the first sensor 202-1 of the th initial return pulse from the second sensor 202-2. Next, based on the average sampling start time difference A obtained, the first sensor 202-1 aligns the first set of sampling signals obtained by the first sensor 202-1 with the second set of sampling signals obtained by the second sensor 202-2. For example, this alignment is performed by removing sampling signals obtained by the first sensor 202-1 in the previous time period AT. According to one embodiment of the present invention, the processor 203 is configured to process the first set of sampling signals and the second set of sampling signals obtained by the first sensor 202-1 and the second sensor 202-2. More specifically, the processor 203 is configured to obtain a first set of vibration signals and a second set of vibration signals based on the first and second sets of sampling signals, and to subject the first and second sets of vibration signals to cross-correlation analysis to obtain a target sampling difference. The target sampling difference can achieve a maximum cross-correlation between the first set of vibration signals and the second set of vibration signals. The 203 processor is further configured to calculate a train speed based on the target sampling difference. According to one embodiment of the present invention, the operation of the processor 203, which obtains the first set of vibration signals and the second set of vibration signals based on the first and second sets of sampling signals respectively, may include the following step. The processor 203 subjects the first and second sets of sampling signals to spectral analysis so as to obtain a characteristic frequency band having an amplitude above a particular threshold, fluctuating in a low-frequency range, and sets f and fi as fixed parameters. For example, the low-frequency range is 200 Hz and below. Next, the processor 203 filters the first set of sampling signals and the second set of sampling signals in the characteristic frequency band, so as to obtain the first set of LA vibration signals &, © and the second set of x vibration signals Œ is the total number of vibration signals in each 4 where = 1, 2, 1 together. For example, the filtering could be bandpass filtering. According to one embodiment of the present invention, the operation of the processor 203, which subjects the first set of vibration signals and the second set of vibration signals to cross-correlation analysis, may include the following step. The processor 203 subjects the first set of vibration signals… and the second set of vibration signals (Aù % x, # x, ©) to a cross-correlation calculation, to obtain a cross-correlation (Rat) of the first set of vibration signals (x©) and the second set of vibration signals (i, x, 6), where = is a sampling difference between the first set of vibration signals (#\E) and the second set of vibration signals (p_i, x, (0)). Through cross-correlation analysis, the 203 processor obtains the target sampling difference, capable of achieving the maximum cross-correlation between the Tera first set of vibration signals x.) and the second set of signals To Do of vibration y For example, when the maximum absolute value X, (at) Y=t2 of is obtained. max|R (1 CR AT w According to one embodiment of the present invention, the operation of the processor 203, which calculates the train speed based on the target sampling difference, may include the following step. Based on the target sampling difference obtained, the processor 203 calculates a difference of Tikd time between the first set of vibration signals, and the second Érd x, set of vibration signals x For example, the time difference is 2, Enr calculated by the following formula: ! . Una = + Ty Next, based on the time difference t obtained, the processor 203 calculates 3 The train speed v. For example, the train speed v is calculated using the formula je ik next: LE Va we THE where L is the horizontal distance between the first sensor 202-1 and the second jk sensor 202-2. According to one embodiment of the present invention, the processor 203 determines whether the train speed v obtained is within a range of conventional train speeds. By ke For example, if the train speed v is within a certain reasonable range beyond je 20 km / h and including this speed, then the train speed calculation is determined 1 DR SIA M RE PS I ES EI MA ESS as being valid. If the train speed V is not within the speed range of 'I In a conventional train, the 203 processor ignores the train speed. For example, if the calculated train speed exceeds a specific range, this could generally be caused by vibration interference signals in the same frequency band. Thus, the 203 processor uses the train speed v, which is part of a Lu Al 'a normal range as estimated train speed. According to one embodiment of the present invention, based on the speed of Based on the estimated train and wheel circumference, the 203 processor calculates the wheel rotation speed. The train speed estimation device according to the present invention can accurately monitor train speed in real time without relying on any speed sensor or GNSS. Compared to the prior art, the train speed estimation device according to the present invention can reduce energy consumption during real-time train speed estimation, have higher accuracy than a GNSS-based train speed estimation method, and is particularly suitable for estimating the speed of underground trains under non-GNSS signal conditions. Furthermore, the train speed estimation device according to the present invention is suitable for various types of railway vehicles, such as underground trains, multiple-unit trains, high-speed trains, and overhead trains, and will not be affected by factors such as train type and track type. A train speed estimation method according to an embodiment of the present invention is described below with reference to Figures 4 and 5. The train speed estimation method is applied to a train having at least one pair of sensors provided on it, the pair of sensors comprising a first sensor and a second sensor arranged in the direction of travel of the train. The train speed estimation method corresponds to the train speed estimation device described above; therefore, certain details are not described again below. Figure 4 illustrates a flowchart of a train speed estimation method according to an embodiment of the present invention. Figure 5 illustrates a flowchart of communication via the sensor pair prior to sampling according to an embodiment of the present invention. As illustrated in Figure 4, according to one embodiment of the present invention, in step S401, a first sensor and a second sensor perform sampling, at the same sampling frequency f£, of a vibration in a band of s natural frequencies experienced by a train as it moves forward, in order to obtain a first set of sampling signals and a second set of sampling signals respectively. As illustrated in Figure 5, according to one embodiment of the present invention, before the first sensor and the second sensor perform the sampling standard initial pulses at the second sensor, S501, the first sensor sends; BP where ? is a natural number greater than or equal to 1. For example, p = 3. Each time the second sensor receives a standard initial pulse from the The first sensor receives a signal from the second sensor, and the second sensor sends an initial feedback pulse back to the first sensor. Both the first and second sensors can record time stamps for the pulses they send and receive. According to one embodiment of the present invention, different pairs of sensors arranged in the train use different wireless channel frequency bands for communication, so as to avoid signal crosstalk. In substep S502, if the first sensor receives initial pulses from P rctour cn provenance du deuxième sensor, alors le premier sensor envoic unc unc pulse de synchroniz standard au deuxième sensor, et commence le sondage. Otherwise, if the first sensor does not receive initial return pulses in p originating from the second sensor, then the first sensor returns to sub-step S501 sending the 7 standard initial pulses. In substep S503, if the second sensor receives the synchro- Standard nization, that is to say a G + 1)th impulse, coming from the first sensor, then the second sensor starts sampling, and sends a synchronization pulse back to the first sensor. In a substep S504, if the first sensor receives the return synchronization pulse from the second sensor, then the first sensor continues sampling. Otherwise, if the first sensor does not receive the return synchronization pulse from the second sensor, then the first sensor stops sampling and returns to substep S501 of sending the initial pulses. p standard. If the second sensor receives a standard synchronization pulse from the first sensor again, then the second sensor stops sampling, and then returns to substep S503 of starting sampling and sending a return synchronization pulse to the first sensor. According to one embodiment of the present invention, the above substeps can be repeated until the first sensor and the second sensor complete sampling. According to one embodiment of the present invention, after the first sensor and the second sensor have completed sampling, the first sensor calculates a at 7 between time points at which the first sensor and the second sensor begin sampling, based on a time difference between the sending of a standard initial pulse and the reception of a corresponding return initial pulse by the first sensor. Next, based on the average time difference (AT) obtained, the first sensor aligns the first set of sampling signals obtained by the first sensor with the second set of sampling signals obtained by the second sensor. For example, this alignment is performed by removing sampling signals obtained by the first sensor in the previous AT time period. Referring again to Figure 4, according to one embodiment of the present invention, in step S402, the first set of sampling signals and the second set of sampling signals are subjected to spectral analysis, so as to obtain a band of characteristic frequencies having a D fa) amplitude greater than a particular threshold in a low frequency range, and f and fi are set as fixed parameters. In step S403, the first set of sampling signals and the second set of sampling signals are filtered in the characteristic frequency band, so as to obtain a first set of vibration signals LA fl x, © and a second set of vibration signals % Oi= 12.7 where m is the total number of vibration signals in each set. In step S404, the first set of vibration signals x and the second set of vibration signals G (x,0) are subjected to a cross-correlation analysis k, to obtain a target sampling difference _ capable of obtaining the Trkd 8. Maximum cross-correlation between the first set of vibration signals xD; and the second set of vibration signals ee XD In step S405, based on the target sampling difference obtained, a $ ne T, ci Fed a time difference between the first set of vibration signals 7 Esnd x and the second set of vibration signals x,() is calculated. Fe. obtained, a speed of In step S406, based on the DNS time difference, I is calculated. train € Va obtained is in a a In step S407, it is determined whether the train speed v'ik conventional train speed range. If the train speed v is not within the range of 7 For conventional train speeds, the train speed is ignored. Thus, the train speed v falling within a normal range is used as the estimated train speed. AR In step S408, based on the estimated train speed and wheel circumference, a wheel rotation speed is calculated. The train speed estimation method according to the present invention can accurately monitor train speed in real time without the need for any speed sensors or GNSS. Compared to the prior art, the train speed estimation method according to the present invention can reduce energy consumption during real-time train speed estimation, have higher accuracy than a GNSS-based train speed estimation method, and is particularly suitable for estimating the speed of underground trains under non-GNSS signal conditions. Furthermore, the train speed estimation method according to the present invention is suitable for various types of railway vehicles, such as underground trains, multiple-unit trains, high-speed trains, and overhead trains, and will not be affected by factors such as train type and track type. The functions of the various elements illustrated in the drawings can be provided by using dedicated hardware and associated hardware with appropriate software capable of executing the software. When provided by a processor, functions can be performed by a single dedicated processor, a single shared processor, or multiple independent processors (some of which may be shared). Furthermore, the explicit use of the term "processor" or "controller" should not be interpreted as referring specifically to hardware capable of executing software, and may implicitly encompass, but not be limited to, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random-access memory (RAM), and non-volatile memory. This description has explained the principles of the present invention. Thus, it will be understood that a person skilled in the art will be able to conceive of various arrangements which, although not explicitly described or illustrated in this document, represent the principles of the present invention and are encompassed in its spirit and scope. All examples and conditional language set forth in the description are intended to be used for teaching purposes, to aid the reader in understanding the principles of the present invention and the concept proposed by the inventor to advance the art, and should be understood as not being limited to those examples and conditions set forth precisely. Furthermore, all statements in this document set forth the principles, aspects, and embodiments of the present invention, and particular examples thereof are intended to encompass structural and functional equivalents thereof. Moreover, such equivalents are intended to include both currently known equivalents and equivalents developed in the future—that is, any developed elements that perform the same function, regardless of their structure. A reference in the description to "an embodiment" or "embodyments" of the present invention and other variants thereof means that the attributes, structure, and specific features, etc., described in association with the embodiment are encompassed in at least one embodiment. Thus, the expression "in an embodiment" or "in embodiments" appearing in various places throughout the specification, and any other variant thereof, do not necessarily all refer to the same embodiment.

Claims

Demands

1. Train speed estimation device, comprising: at least one pair of sensors, comprising a first sensor and a second sensor arranged in the direction of travel of a train, the first sensor and second sensor being configured to exchange- to transcribe, at the same sampling frequency, a vibration in a band of natural frequencies experienced by the train as it moves forward, in order to to obtain a first set of sampling signals and a second set of sampling signals respectively; and a processor, configured to obtain a first set of signals vibration and a second set of vibration signals on the base of the first set of sampling signals and the second set of sampling signals respectively, and submit the first set of vibration signals and second set of vibration signals to a cross-correlation analysis, to obtain a target sampling difference; the sampling difference The target achieves a maximum cross-correlation between the first set vibration signals and the second set of signals vibration, and the processor is configured to calculate a speed of train based on the target sampling difference.

2. Train speed estimation device according to claim 1, in which the processor is further configured to obtain a bandwidth of characteristic frequencies of the first set of signals sampling and the second set of signals sampling having an amplitude greater than a threshold in a low-frequency range, and filter the first set of signals sampling and the second set of signals sampling in the characteristic frequency band, in order to to obtain the first set of vibration signals and the second set of vibration signals.

3. Train speed estimation device according to claim 1 or 2, in which, before the first sensor and the second sensor do not perform sampling; the first sensor is further configured to send _ standard initial pulses, where _ is an integer p - - # natural greater than or equal to 1, and the second sensor is further configured to send an initial return pulse each time that it receives the standard initial impulse; if the first sensor receives – let's say initial return pulses – then the P The first sensor is further configured to send a pulse of standard synchronization and start sampling; if the second sensor receives the standard synchronization pulse, then the second sensor is further configured to start sampling and sending a return synchronization pulse set if the first sensor receives the return synchronization pulse, then the first sensor is further configured to continue sampling.

4. Train speed estimation device according to claim 3, in which if the first sensor does not receive the synchronization pulse of back, then the first sensor is further configured to stop sampling, and return to the substep of sending the _ pulses p standard initials; and if the second sensor receives the standard synchronization pulse at new, then the second sensor is further configured to stop sampling, and then returning to the start substep of sampling and sending the synchronization pulse back.

5. Train speed estimation device according to claim 3 or 4, in which, after the first sensor and the second sensor have Once sampling is complete, the first sensor is further configured to calculate a difference in sampling start time based on of a time difference between the sending of the initial standard pulse and the reception of the corresponding initial return pulse by the first sensor, and align the first set of signals sampling on the second set of signals sampling based on the difference in start time sampling.

6. Train speed estimation device according to claim 1 or 3, in which different pairs of sensors in at least one pair of Sensors are configured to use different frequency bands. wireless channel for communication.

7. Train speed estimation device according to any one of the re- previous claims, in which the processor is further configured to determine if the calculated train speed is within a specific range, and ignore train speed if train speed is not not within the specific range.

8. A method for estimating train speed, comprising: sampling, at the same sampling frequency, of a vibration in a natural frequency band experienced by the train when it moves forward by means of a first sensor and a second sensor in at least one pair of sensors, in order to obtain a first a set of sampling signals and a second set of sampling signals respectively, the first sensor and the the second sensor being arranged in the direction of travel of the train obtaining a first set of vibration signals and a second set of vibration signals based on the first set of sampling signals and the second set of sampling signals respectively, and the act of submitting the first set of vibration signals and second set of vibration signals to a cross-correlation analysis, to obtain a target sampling difference, in which the difference target sampling obtains a maximum cross correlation between the first set of vibration signals and the second set of vibration signals; and calculating train speed based on the difference target sampling.

9. Method for estimating train speed according to claim 8, including, in addition: obtaining a band of frequencies characteristic of the first set of sampling signals and the second set of sampling signals having an amplitude greater than a threshold in a range of low frequencies; and filtering the first set of sampling signals and the second set of sampling signals in the band of characteristic frequencies, in order to obtain the first set of vibration signals and the second set of vibration signals.

10. Method for estimating train speed according to claim 8, including, in addition: before the first sensor and the second sensor perform the sampling, the sending by the first sensor of p standard initial pulses, where _ is a natural integer greater than or equal to ÿ 1, and the sending by the second sensor of an initial return pulse each time it receives the standard initial pulse; if the first sensor receives – let's say initial return pulses – then the D The first sensor sends a standard synchronization pulse and begins sampling; If the second sensor receives the standard synchronization pulse, then the second sensor begins sampling and sends a return synchronization pulse; and If the first sensor receives the return synchronization pulse, then the first sensor continues sampling.