CN111653105B - Vehicle speed measuring method and system based on passive technology on two sides of road - Google Patents
Vehicle speed measuring method and system based on passive technology on two sides of road Download PDFInfo
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- CN111653105B CN111653105B CN202010366209.5A CN202010366209A CN111653105B CN 111653105 B CN111653105 B CN 111653105B CN 202010366209 A CN202010366209 A CN 202010366209A CN 111653105 B CN111653105 B CN 111653105B
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Abstract
The invention discloses a vehicle speed measuring method and system based on passive technology on two sides of a road, which are characterized in that the RSS (received signal strength) value changes due to the fact that a vehicle enters and leaves a speed measuring area and shields two wireless communication links of an entrance boundary and an exit boundary, the length of a sliding window is dynamically selected according to the estimated speed of the vehicle, the RSS variances of the wireless communication links of the entrance boundary and the exit boundary of the vehicle are respectively calculated, the time of the vehicle just entering and leaving the speed measuring area is found through the mutation point of the variance, and the known length of the speed measuring area is used for calculating the speed of the vehicle; the wireless communication link RSS signal that the monitoring node in the district that tests the speed gathered can be applied to the multi-field vehicle and tests the speed, adopts multichannel scanning acquisition signal between the monitoring node simultaneously, even a certain channel produces great interference still can measure vehicle speed, helps improving the vehicle accuracy of testing the speed.
Description
Technical Field
The invention belongs to the field of vehicle speed measurement, and particularly relates to a vehicle speed measurement method and system based on passive technology on two sides of a road.
Background
The Radio Tomography (RTI) technology is a passive positioning technology based on a wireless sensor network, and realizes positioning and tracking of a target object by utilizing signal attenuation caused by shielding of wireless Radio frequency signals transmitted between nodes in the network by the target object. Compared with the traditional active positioning system, the passive positioning technology is adopted, only a plurality of wireless sensor nodes are deployed around the monitoring area to form a communication network, and the target can be positioned and tracked without carrying any electronic tag.
The vehicle speed measurement is widely applied in the fields of current road traffic, national defense land battlefields and the like. Common vehicle speed measurement modes include video speed measurement, induction coil speed measurement, radar speed measurement, ultrasonic speed measurement, infrared speed measurement, laser speed measurement and the like. The coil is tested the speed comparatively classic detection effect better, but the construction volume of burying induction coil underground on ground is big, and the road surface then needs to bury the coil again in case the change, and the maintenance work to the coil is huge in high latitude open freezing period and low latitude summer road surface and the place that the road surface quality is not good in addition. The video detection is not limited by the road condition, the road surface is not required to be damaged during installation, but the identification of the moving vehicle is difficult, and meanwhile, the detection method is influenced by light, weather and the like. The microwave radar speed measurement can detect fast moving vehicles, but is not suitable for complex intersection environments with multiple lanes, multiple vehicles and pedestrians. The ultrasonic speed measurement system has short service life and high cost in the extremely severe environment with dust. Although laser speed measurement is theoretically feasible, safety problems exist in the using process. The radar speed measurement is susceptible to environmental weather. Other wireless sensors test speed and are mostly based on active positioning technology, and need the vehicle itself to carry electronic tags, and it is difficult to obtain popularization and application actually.
The current vehicle speed measuring technology based on radar, laser and machine vision is not only with high costs, easily receives the unable all-weather work of environmental weather influence moreover, also can't see through the obstacle and measure the speed to the target. The invention designs a vehicle speed measurement algorithm based on a passive technology, which calculates the change of multichannel sliding variances of wireless communication links at an inlet and an outlet of a monitoring area by using an online sliding variance method, and calculates the vehicle speed. The method is free from the influence of the environment on the measurement, low in cost and high in measurement precision, and can be applied to the fields of battlefield vehicle perception, national defense safety monitoring, daily road traffic and the like.
Disclosure of Invention
The invention aims to provide a vehicle speed measuring method and system based on passive technology on two sides of a road, and provides a speed measuring method which is not influenced by environment, low in cost and high in measuring precision.
The technical solution for realizing the purpose of the invention is as follows:
a vehicle speed measuring method based on passive technology on two sides of a road comprises the following steps:
step 1, arranging wireless sensor nodes at an inlet and an outlet of a test area, and acquiring the distance between the wireless sensor nodes at the inlet and the outlet of the speed measurement area;
step 2, obtaining a variance value of RSS values of the wireless sensor nodes when no vehicle passes through the entrance boundary and the exit boundary of the speed measuring area;
step 3, calculating the time spent by the vehicle passing through the speed measuring area according to the time spent by the vehicle passing through the entrance boundary of the speed measuring area and the time spent by the vehicle reaching the exit boundary of the speed measuring area:
a sliding variance method is utilized, namely l sampling values are continuously taken according to the window length to be set into a queue, the length of the queue is fixed to be l, n groups of data are acquired in one scanning process and are arranged according to the time stamp sequence, and n-l +1 groups of variances are calculated; according to the first-in first-out principle, new data is sampled each time and put into the tail of the queue, one data at the head of the queue is deleted, variance operation is carried out on the one data in the queue, the signal intensity variances of the communication link at the entrance boundary and the exit boundary of the velocity measurement area are obtained respectively, and the variation relation between the variance and the calculation times is obtained; obtaining the time when the vehicle enters the speed measuring area and the time when the vehicle leaves the speed measuring area according to the variance and the slope k mutation of the calculation times, and calculating the time spent by the vehicle passing the speed measuring area;
and 4, calculating the speed of the vehicle by using the length of the speed measuring area and the time taken by the vehicle to pass through the speed measuring area.
A vehicle speed measuring system based on passive technology on two sides of a road comprises a detection network consisting of at least four wireless sensors and a processor for receiving sensor data and processing the received data; the processor is provided with a parameter input module, a variance value calculation module, a passing time calculation module and a vehicle speed calculation module;
the sensors are divided into two groups and are respectively arranged on two sides of an entrance road of the speed measuring area and two sides of an exit road of the speed measuring area;
the parameter input module is used for inputting the distance between the wireless sensor node at the inlet and the wireless sensor node at the outlet of the speed measuring area;
the variance value calculation module is used for calculating the variance value of RSS when no vehicle passes through the entrance boundary and the exit boundary of the speed measuring area;
the passing time calculation module is used for calculating the time spent by the vehicle passing through the speed measuring area by using a sliding variance method, namely l sampling values are continuously taken according to the window length to be set into a queue, the length of the queue is fixed to be l, n groups of data are collected in one scanning and are arranged according to the time stamp sequence, and n-l +1 groups of variances are calculated in total; according to the first-in first-out principle, new data is sampled each time and put into the tail of the queue, one data at the head of the queue is deleted, variance operation is carried out on the one data in the queue, the signal intensity variances of the communication link at the entrance boundary and the exit boundary of the velocity measurement area are obtained respectively, and the variation relation between the variance and the calculation times is obtained; obtaining the time when the vehicle enters the speed measuring area and the time when the vehicle leaves the speed measuring area according to the variance and the slope k mutation of the calculation times, and calculating the time spent by the vehicle passing the speed measuring area;
and the vehicle speed calculating module is used for calculating the vehicle speed according to the distance between the input sensor nodes and the time spent by the vehicle passing through the speed measuring area.
Compared with the prior art, the invention has the following remarkable advantages:
the method can measure the vehicle speed only by finding the time for increasing the RSS variance variation of the corresponding communication link without determining the vehicle position, adopts a passive speed measurement technology, does not need to carry any electronic equipment, reduces the vehicle speed measurement difficulty, has low node cost and small volume, adopts battery power supply, is convenient for quick hidden arrangement, and can be used in the fields of battlefield vehicle perception, national defense safety monitoring and the like; meanwhile, signals are acquired by scanning multiple channels among the monitoring nodes, and the vehicle speed can be measured even if a certain channel generates large interference, so that the vehicle speed measurement precision is improved.
Drawings
FIG. 1 is a flow chart of the method of the present invention.
Fig. 2 is a node deployment diagram of a velocity measurement area.
Detailed Description
The invention is further described with reference to the following figures and embodiments.
With reference to fig. 1, the method for measuring speed of a vehicle on both sides of a road based on passive technology of this embodiment includes the following steps:
step 1, arranging wireless sensor nodes in a test area, and acquiring the distance between the wireless sensor nodes at an inlet and an outlet of a speed measurement area, namely the effective distance L of a vehicle running in the speed measurement area;
with reference to fig. 2, two nodes are preset on two sides of a road respectively, and two nodes are ensured to be opposite to each other, namely a pair of nodes is arranged on two sides of an entrance road of a speed measuring area, and a pair of nodes is arranged on two sides of an exit road of the speed measuring area; the height of the nodes in the speed measurement area is selected to be not higher than the height of the vehicle, so that the horizontal communication link can be effectively shielded by the vehicle, and the height of each of the four selected nodes is 1 meter according to the general height of the vehicle. Two parallel communication links are respectively formed between two nodes at the inlet and two nodes at the outlet, and four crossed communication links are formed by crossing opposite nodes. And finally, the data acquired by the four wireless sensor nodes forming the speed measuring area are all sent to the base station node connected with the speed measuring algorithm operation terminal.
In order to ensure the effectiveness of vehicle speed measurement, a plurality of pairs of opposite equal-height nodes can be set on two sides of a road, and a speed measurement area can be formed by every four nodes, so that the condition that the vehicle speed cannot be measured due to the fault of a certain node is avoided; four wireless sensor nodes form a group of detection network, and every four nodes can form a speed measurement area to avoid that the vehicle speed cannot be measured due to the fault of a certain node.
A pair of switch nodes is arranged in front of an entrance of the speed measuring area at a certain distance; the wireless sensor with low power consumption can be selected by the pair of the nodes, the wireless sensor is in a normally open state, when a vehicle passes through the pair of the nodes, the system controls the speed measuring area node to work, when no vehicle passes through the road section, the speed measuring area node is in a standby state or a closed state, and the energy consumption of the wireless sensor of the speed measuring area node can be saved.
Step 2, obtaining a variance value of RSS values of the wireless sensor nodes when no vehicle passes through the entrance boundary and the exit boundary of the speed measuring area;
when no vehicle enters a speed measurement area, opening all wireless sensor nodes to collect Received Signal Strength (Received Signal Strength) RSS of all wireless communication links, and respectively calculating the variance values of the RSS when no vehicle passes through an entrance boundary and an exit boundary;
step 3, root ofAccording to the time t when the vehicle passes through the entrance boundary of the speed measuring area1And the time t of arriving at the exit boundary of the velocity measurement zone2And calculating the time taken by the vehicle to pass through the speed measuring area:
obtaining the time for the vehicle to enter and leave the area by using the variance of the RSS of a communication link at the boundary of the speed measuring area;
the vehicle entering the speed measuring area can cause the shielding of an entrance boundary communication link, and the vehicle leaving the speed measuring area can cause the shielding of an exit boundary communication link, so that the RSS of the boundary communication links at two sides is changed;
setting a sliding window, setting the length of the sliding window to be l, acquiring n groups of data in one scanning process, arranging the data according to the sequence of the time stamps, and calculating n-l +1 groups of variances;
the method of utilizing the sliding variance is characterized in that l sampling values are continuously taken according to the window length and set into a queue, the length of the queue is fixed to be l, a new communication link signal intensity value is sampled every time and put into the tail of the queue according to the first-in first-out principle, a signal link signal intensity value at the head of the queue is deleted, variance operation is carried out on l calculated communication link signal intensity values in the queue, the signal intensity variances of the communication link at the inlet boundary and the outlet boundary of the speed measuring area are respectively obtained, and the change relation between the variance and the calculation times is obtained;
when a vehicle enters a speed measurement area, the change of the RSS value of the communication link is increased due to the fact that the entrance boundary communication link is shielded, the variance value of the signal intensity of the boundary communication link calculated according to the sliding variance is increased, and the slope k of the variance and the calculation times is suddenly changed; if it is t1Sum of variance and t1The slope of 1 variance is a positive value and exceeds a preset threshold value, and the slope k at the moment is considered to be mutated, so that the n-l + t recorded in the node is considered1The n-l + t recorded by the system when the vehicle passes the boundary at the time of the time stamp1And the time corresponding to each timestamp is the time when the vehicle enters the speed measuring area.
When the vehicle leaves the speed measuring area, the RSS value of the communication link is increased due to the end of the shielding of the exit boundary communication link, and the boundary communication is calculated according to the sliding varianceThe trend of variance value increase of signal intensity of the signal link is ended, the slope k of variance and calculation times is suddenly changed, if t is2Sum of variance and t2The slope of 1 variance is a negative value and exceeds a preset threshold value, the slope k at the moment is considered to be mutated, and the n-l + t recorded in the node is considered to be2The n-l + t recorded by the system when the vehicle leaves the boundary at the time of the time stamp2And the time corresponding to each timestamp is the time when the vehicle leaves the speed measuring area.
And calculating the time taken by the vehicle to pass through the speed measuring area by combining the two acquired times.
The method for setting the length of the sliding window comprises the following steps:
since the signal strength is attenuated when the vehicle blocks the communication link, the variance is suddenly changed, in order to find the position where the signal strength of the communication link suddenly changes for the first time and the last time, namely the time when the vehicle starts to influence and finishes influencing the communication link, the window length is taken as 2 to calculate the sliding variance, when the variance suddenly changes, the vehicle starts to influence the communication link, and when the variance suddenly changes, the vehicle finishes influencing the communication link.
If the p-th group of variances is that the variance in the n-1 group of variances of the entrance boundary exceeds a preset threshold value for the first time, namely the variance in the group of variances is increased from the beginning, the vehicle can be considered to just enter a speed measuring area at the p + 1-th timestamp;
if the q group of variances exceeds a preset threshold value for the last time in the n-1 group of variances at the entrance boundary, namely the change is increased from the group of variances, the vehicle can be considered to completely enter the speed measuring area at the time of the q +1 timestamp; finding the time stamps of all variances with sudden changes; finding all timestamps between two timestamps with sudden change of the first variance and the last variance, and calculating the number of the timestamps with sudden change of the variance to be N;
determining the length of a long window and a short window according to a simulation experiment, so that the length of the windows can be reasonably selected in practice; calculating the vehicle speed, and setting the length of a long window to be 10 and the length of a short window to be 5;
if N is larger than the set threshold, the window length l is 10;
if N is smaller than the set threshold, the window length l is 5.
And 4, calculating the vehicle speed V by using the length of the speed measuring area and the time taken by the vehicle to pass through the speed measuring area.
V＝L/(t2-t1)
Based on the speed measuring method, the invention provides a vehicle speed measuring system based on passive technology on two sides of a road, which comprises a detection network consisting of at least four wireless sensors and a processor for receiving sensor data and processing the received data; the processor is provided with a parameter input module, a variance value calculation module, a passing time calculation module and a vehicle speed calculation module;
the four sensors are divided into two groups and are respectively arranged on two sides of an entrance road of the speed measuring area and two sides of an exit road of the speed measuring area; four wireless sensor nodes deployed in a speed measuring area can be used as a signal sending node and a signal collecting node, namely: for the ith node (i ═ 1,2,3,4), the node can send data to the other three nodes as a sending node, and can collect the data sent by the other three nodes as a collecting node. Each node arranged in the speed measuring area is provided with four communication channels, each communication link can obtain four groups of RSS data for processing and calculating the speed of the vehicle, the speed measuring precision of the vehicle can be improved conveniently, the RSS data of four different channels can be obtained by the same wireless communication link when the speed is measured, and the speed measuring precision is improved by utilizing RSS redundancy of different testing channels. All nodes are equal in height. And finally, the data acquired by the four wireless sensor nodes forming the speed measuring area are all sent to the base station node connected with the speed measuring algorithm operation terminal.
In order to ensure the effectiveness of vehicle speed measurement, a plurality of pairs of opposite equal-height nodes can be set on two sides of a road, and a speed measurement area can be formed by every four nodes, so that the condition that the vehicle speed cannot be measured due to the fault of a certain node is avoided;
further, a pair of switch nodes is arranged in front of the entrance of the speed measuring area at a certain distance; this wireless sensor to node selectable low-power consumption is in normally open state, and when the vehicle passed through this to the knuckle point, the work of system control district node that tests the speed, when no vehicle passed through this highway section, the district node that tests the speed was in standby or closed state, and the vehicle just opened work when passing through, can practice thrift the energy consumption that tests the speed and distinguish node wireless sensor. The speed measuring area node and the switch node can be fixed by selecting trees, light poles and the like on two sides of a road.
Furthermore, the wireless sensor nodes deployed in the speed measurement area select the omnidirectional antenna, so that the influence of node topology deployment on the communication link RSS is avoided.
The parameter input module is used for inputting the distance between the wireless sensor node at the inlet and the wireless sensor node at the outlet of the speed measuring area and setting a related threshold value of a sliding window; the distance between the two pairs of wireless sensor nodes can be manually measured before the vehicle passes through and input into the computing system of the processor for retrieval and use.
The variance value calculation module is used for calculating the variance value of RSS when no vehicle passes through the entrance boundary and the exit boundary of the speed measuring area, and the specific process is as follows:
when no vehicle enters a speed measuring area, opening all wireless sensor nodes to collect Received Signal Strength (Received Signal Strength) RSS of all wireless communication links, and respectively calculating the average value of RSS of two communication links at the entrance boundary and the average value of RSS values of two communication links at the exit boundary; calculating the variance value of RSS when no vehicle passes through the entrance boundary and the exit boundary of the speed measuring area;
the traffic time calculation module is used for calculating the time spent by the vehicle passing through the speed measuring area and obtaining the time spent by the vehicle entering and leaving the area by using the change of the variance of the signal intensity of the communication link at the boundary of the speed measuring area; the method comprises the steps of utilizing a sliding variance method, namely continuously taking l sampling values according to the window length to set the sampling values into a queue, fixing the length of the queue to be l, sampling new data to be put into the tail of the queue each time according to a first-in first-out principle, deleting data at the head of the queue, carrying out variance operation on the l data in the queue to respectively obtain the signal intensity variances of an inlet boundary and an outlet boundary of a speed measuring area, and obtaining the variation relation between the variance and the calculation times; and obtaining the time when the vehicle enters the speed measuring area and the time when the vehicle leaves the speed measuring area according to the variance and the sudden change of the slope k of the calculation times, and calculating the time spent by the vehicle passing the speed measuring area.
The passing time calculation module comprises a sliding window setting unit, an entrance boundary time acquisition unit, an exit boundary time acquisition unit and a passing time calculation unit;
the sliding window setting unit is used for setting a sliding window, setting the length of the sliding window to be l, and acquiring n groups of data in one scanning and arranging the data according to the time stamp sequence, so that n-l +1 groups of variances are calculated;
further, the method for setting the length of the sliding window in the sliding window setting unit is as follows:
calculating a sliding variance by taking the window length as 2, and finding out the positions where the signal intensity of the communication link changes suddenly for the first time and the last time;
if the p-th group of variances is that the variance in the n-1 group of variances of the entrance boundary exceeds a preset threshold value for the first time, namely the variance in the group of variances is increased from the beginning, the vehicle can be considered to just enter a speed measuring area at the p + 1-th timestamp;
if the q group of variances exceeds a preset threshold value for the last time in the n-1 group of variances at the entrance boundary, namely the change is increased from the group of variances, the vehicle can be considered to completely enter the speed measuring area at the time of the q +1 timestamp; finding the time stamps of all variances with sudden changes; finding all timestamps between two timestamps with sudden change of the first variance and the last variance, and calculating the number of the timestamps with sudden change of the variance to be N;
if the number N of the timestamps is larger than a set threshold value, namely the influence time of the vehicle on the entrance boundary communication link is longer, the vehicle speed is slower, the calculation of the vehicle speed by taking the larger length of the sliding window is more accurate, and l is 10;
if the number N of the timestamps is smaller than a set threshold value, namely the influence time of the vehicle on the entrance boundary communication link is short, the vehicle speed is high, the vehicle can be found to enter or leave by taking the length of a smaller sliding window, and l is 5.
The entrance boundary time obtaining unit is used for obtaining the time of the vehicle passing through the entrance boundary of the speed measuring area, and the specific process is as follows:
when the head is just rightWhen entering a speed measurement area, the change of the RSS value of the communication link is increased due to the fact that the entrance boundary communication link is shielded, the variance value of the boundary communication link signal strength calculated according to the sliding variance is increased, and the slope k of the variance and the calculation times is suddenly changed; if it is t1Sum of variance and t1The slope of 1 variance is a positive value and exceeds a preset threshold value, and the slope k at the moment is considered to be mutated, so that the n-l + t recorded in the node is considered1The n-l + t recorded by the system when the head just passes the boundary at each time stamp1And the time corresponding to each timestamp is the time when the vehicle enters the speed measuring area.
The exit boundary time obtaining unit is used for obtaining the time when the vehicle reaches the exit boundary of the speed measuring area, and the specific process is as follows:
when the vehicle head just leaves the speed measuring area, the change of the RSS value of the communication link is increased due to the fact that the vehicle head shields the exit boundary communication link, the variance value of the signal intensity of the boundary communication link calculated according to the sliding variance is increased, the variance and the slope k of the calculation times are suddenly changed, and if the t < th > is2Sum of variance and t2The slope of 1 variance is a positive value and exceeds a preset threshold value, and the slope k at the moment is considered to be mutated, so that the n-l + t recorded in the node is considered2The n-l + t recorded by the system when the head just passes the boundary at each time stamp2And the time corresponding to each timestamp is the time when the vehicle leaves the speed measuring area.
The passing time calculation unit is used for calculating the time spent by the vehicle passing through the speed measuring area according to the time spent by the vehicle passing through the entrance boundary of the speed measuring area and the time spent by the vehicle reaching the exit boundary of the speed measuring area.
And the vehicle speed calculating module is used for calculating the vehicle speed according to the distance between the input sensor nodes and the time spent by the vehicle passing through the speed measuring area.
The method can measure the vehicle speed only by finding the time for increasing the RSS variance variation of the corresponding communication link without determining the vehicle position, adopts a passive speed measurement technology, does not need to carry any electronic equipment, reduces the vehicle speed measurement difficulty, has low node cost and small volume, adopts battery power supply, is convenient for quick hidden arrangement, and can be used in the fields of battlefield vehicle perception, national defense safety monitoring and the like; meanwhile, signals are acquired by scanning multiple channels among the monitoring nodes, and the vehicle speed can be measured even if a certain channel generates large interference, so that the vehicle speed measurement precision is improved.
Claims (7)
1. A vehicle speed measuring method based on passive technology on two sides of a road is characterized by comprising the following steps:
step 1, arranging wireless sensor nodes at an inlet and an outlet of a test area, and acquiring the distance between the wireless sensor nodes at the inlet and the outlet of the speed measurement area;
step 2, obtaining a variance value of RSS values of the wireless sensor nodes when no vehicle passes through the entrance boundary and the exit boundary of the speed measuring area;
step 3, calculating the time spent by the vehicle passing through the speed measuring area according to the time spent by the vehicle passing through the entrance boundary of the speed measuring area and the time spent by the vehicle reaching the exit boundary of the speed measuring area:
a sliding variance method is utilized, namely l sampling values are continuously taken according to the window length to be set into a queue, the length of the queue is fixed to be l, n groups of data are acquired in one scanning process and are arranged according to the time stamp sequence, and n-l +1 groups of variances are calculated; according to the first-in first-out principle, sampling a new communication link signal intensity value at each time and putting the new communication link signal intensity value into the tail of the queue, deleting a signal link signal intensity value at the head of the queue, carrying out variance operation on the I data in the queue to respectively obtain the communication link signal intensity variances at the entrance boundary and the exit boundary of the velocity measurement area, and obtaining the variation relation between the variance and the calculation times; obtaining the time when the vehicle enters the speed measuring area and the time when the vehicle leaves the speed measuring area according to the variance and the slope k mutation of the calculation times, and calculating the time spent by the vehicle passing the speed measuring area; the sudden change is increased to the time when the vehicle enters the speed measuring area, and the sudden change is reduced to the time when the vehicle leaves the speed measuring area;
calculating the number of the timestamps with the variance mutation as N, and setting the window length:
calculating a sliding variance by taking the window length as 2, and finding out the positions where the signal intensity of the communication link changes suddenly for the first time and the last time;
if the p-th group of variances is that the variance in the n-1 group of variances of the entrance boundary exceeds a preset threshold value for the first time, namely the variance in the group of variances is increased from the beginning, the vehicle can be considered to just enter a speed measuring area at the p + 1-th timestamp;
if the q group of variances exceeds a preset threshold value for the last time in the n-1 group of variances at the entrance boundary, namely the change is increased from the group of variances, the vehicle can be considered to completely enter the speed measuring area at the time of the q +1 timestamp; finding the time stamps of all variances with sudden changes; finding all timestamps between two timestamps with sudden change of the first variance and the last variance, and calculating the number of the timestamps with sudden change of the variance to be N;
if N is larger than the set threshold, the length of a longer window is selected; if N is smaller than the set threshold, the length of a shorter window is taken;
and 4, calculating the speed of the vehicle by using the length of the speed measuring area and the time taken by the vehicle to pass through the speed measuring area.
2. The speed measuring method according to claim 1, wherein the sensor node arrangement mode in step 1 is as follows: the system comprises at least one group of detection networks, wherein each group of detection networks comprises four wireless sensors, a pair of nodes is arranged on two sides of an entrance road of a speed measuring area, and a pair of nodes is arranged on two sides of an exit road of the speed measuring area.
3. A method for measuring speed according to claim 2, wherein a pair of switch nodes are further provided in front of the entrance of the speed measuring area, and the pair of switch nodes are normally open for controlling the operation state of the speed measuring area nodes.
4. A vehicle speed measuring system based on passive technology on two sides of a road is characterized by comprising a detection network consisting of at least four wireless sensors and a processor for receiving sensor data and processing the received data; the processor is provided with a parameter input module, a variance value calculation module, a passing time calculation module and a vehicle speed calculation module;
the sensors are divided into two groups and are respectively arranged on two sides of an entrance road of the speed measuring area and two sides of an exit road of the speed measuring area;
the parameter input module is used for inputting the distance between the wireless sensor node at the inlet and the wireless sensor node at the outlet of the speed measuring area;
the variance value calculation module is used for calculating the variance value of RSS when no vehicle passes through the entrance boundary and the exit boundary of the speed measuring area;
the passing time calculation module is used for calculating the time spent by the vehicle passing through the speed measuring area by using a sliding variance method, namely l sampling values are continuously taken according to the window length to be set into a queue, the length of the queue is fixed to be l, n groups of data are collected in one scanning and are arranged according to the time stamp sequence, and n-l +1 groups of variances are calculated in total; according to the first-in first-out principle, a new signal link signal intensity value sampled every time is put at the tail of a queue, a signal link signal intensity value at the head of the queue is deleted, variance operation is carried out on the I data in the queue, the signal intensity variances of the communication links at the entrance boundary and the exit boundary of the velocity measurement area are obtained respectively, and the variation relation between the variance and the calculation times is obtained; obtaining the time when the vehicle enters the speed measuring area and the time when the vehicle leaves the speed measuring area according to the variance and the slope k mutation of the calculation times, and calculating the time spent by the vehicle passing the speed measuring area; the sudden change is increased to the time when the vehicle enters the speed measuring area, and the sudden change is reduced to the time when the vehicle leaves the speed measuring area;
the process of setting the length of the sliding window by the sliding window setting unit is as follows:
calculating a sliding variance by taking the window length as 2, and finding out the positions where the signal intensity of the communication link changes suddenly for the first time and the last time;
if the p-th group of variances is that the variance in the n-1 group of variances of the entrance boundary exceeds a preset threshold value for the first time, namely the variance in the group of variances is increased from the beginning, the vehicle can be considered to just enter a speed measuring area at the p + 1-th timestamp;
if the q group of variances exceeds a preset threshold value for the last time in the n-1 group of variances at the entrance boundary, namely the change is increased from the group of variances, the vehicle can be considered to completely enter the speed measuring area at the time of the q +1 timestamp; finding the time stamps of all variances with sudden changes; finding all timestamps between two timestamps with sudden change of the first variance and the last variance, and calculating the number of the timestamps with sudden change of the variance to be N;
if N is larger than the set threshold, the length of a longer window is selected; if N is smaller than the set threshold, the length of a shorter window is taken;
and the vehicle speed calculating module is used for calculating the vehicle speed according to the distance between the input sensor nodes and the time spent by the vehicle passing through the speed measuring area.
5. A tachometer system according to claim 4 wherein a pair of switch nodes are provided in front of the entrance to the tachometer area, the pair of switch nodes being normally open for controlling the operation of the tachometer area.
6. The system according to claim 4, wherein said transit time calculation module comprises a sliding window setting unit, an entrance boundary time acquisition unit, an exit boundary time acquisition unit, a transit time calculation unit;
the sliding window setting unit is used for setting a sliding window and setting the length of the sliding window;
the entrance boundary time acquisition unit is used for acquiring the time of the vehicle passing through the entrance boundary of the speed measuring area by using a sliding variance method;
the exit boundary time acquisition unit is used for acquiring the time of the vehicle reaching the exit boundary of the speed measuring area by using a sliding variance method;
the passing time calculation unit is used for calculating the time spent by the vehicle passing through the speed measuring area according to the time spent by the vehicle passing through the entrance boundary of the speed measuring area and the time spent by the vehicle reaching the exit boundary of the speed measuring area.
7. The system according to claim 4, wherein said sensor node selects an omni-directional antenna.
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