Technical background
In recent years, technology of Internet of things obtains very large development, Internet of Things, as its name suggests, be exactly " internet that the thing phase connects ", be by information sensing equipment such as radio-frequency (RF) identification (RFID), infrared inductor, GPS, laser scanners, agreement by agreement, any object is connected with the internet, carries out message exchange and communicate by letter, to realize a kind of network to intellectuality identification, location, tracking, monitoring and the management of object.
The location aware of Internet of Things, that the position of the object in the access network is located accurately, there have been at present some comparatively ripe location technologies to use, GPS is as present widely used location technology, be applied to various occasions, but accurately in the scope of line-of-sight propagation, GPS can't reach higher bearing accuracy, even inefficacy at the indoor satellite-signal that waits.The RFID technology is a kind of comparatively ripe recognition technology, also is one of gordian technique of Internet of Things, and it has noncontact, and is easy to carry, and data capacity is large, has the advantages such as ID of unique identification object.The RFID location technology is the hot topic of at present indoor among a small circle Position Research, the rfid system decipherment distance that works in uhf band can reach more than 10 meters, substantially meet the indoor positioning environment, therefore, adopting the RFID technology in the Indoor Location-aware of Internet of Things is a most suitable method.
The technology of RFID range finding mainly contains based on arriving signal intensity (RSSI), time of arrival (toa) (TOA), and signal arrival time difference (TDOA), signal arrives the methods such as phase differential (PDOA).It is larger that RSSI is affected by communication environments etc., and precision is low; The shorter TOA of indoor positioning propagation distance and TDOA are too high to hardware requirement, therefore are difficult to realize; Because reader is in the process of label transmitted signal, carrier wave charges to label, label reflects back carrier wave by the backscattering mode simultaneously, reader is compared with the phase place of transmitted signal carrier wave according to the backscattered signal carrier of label and is obtained a phase differential, just can obtain range information according to phase differential, even in complicated communication environments, can both obtain higher distance accuracy as long as can read label, therefore adopt the PDOA method that label is found range and be fit to the range finding of indoor environment.
(2) PDOA range measurement principle
Suppose that the signal that read write line sends out is s (t), in the situation of not considering tag modulates and noise effect, the signal that the tag reflection that read write line receives is returned can be written as:
The ρ amplitude relevant with propagation distance wherein,
It is the phase place of the signal that receives.So the distance between label and the reader can be expressed as:
If it is about 900MHz that system selects the frequency range of UHF, wavelength approximately is about 33cm, and for indoor localizing environment, generally is several meters to tens meters, therefore, and phase place
There is the phase ambiguity of 2n π, thereby causes dysmetria true,
Summary of the invention
The purpose of this invention is to provide a kind of method for sensing position of object in Internet of things, have the poor PDOA distance-finding method of dynamic frequency, can guarantee that the relative error of measuring system is stable, at actual range hour, reduce range error.
For achieving the above object, the present invention explains a kind of method for sensing position of object in Internet of things, and its key is to carry out according to the following steps:
Step 1: set up a three-dimensional coordinate system, position of object in Internet of things sensory perceptual system in location in the three-dimensional coordinate system, position of object in Internet of things sensory perceptual system are mainly by four UHF rfid interrogators, and electronic tag and total node form; Interconnect by the CAN bus between each UHF rfid interrogator, and all receive above total node, total node is connected with the internet, determines the three-dimensional coordinate position of each UHF rfid interrogator in the three-dimensional coordinate system;
Step 2: electronic tag is attached on the testee;
Step 3: first distance test, each UHF rfid interrogator send radio-frequency information to electronic tag, tentatively obtain each UHF rfid interrogator to the preliminary survey distance value d of electronic tag;
The first initial measurement frequency f 1 is set, and the ultimate range of setting initial measurement is d
Max,
Read write line is respectively with 2 frequencies: the first initial measurement frequency f
1With the second initial measurement frequency f
2Transmitted signal, the phase place of the signal that corresponding tag reflection is returned is respectively
With
It is d that electronic tag is in ultimate range
MaxThe time phase differential
Be 2 π, so can get:
C is light velocity value;
Thereby obtain the UHF rfid interrogator to the preliminary survey distance value d of electronic tag be:
Step 4: according to preliminary survey distance value d, reset ultimate range d '
Max=d+1.5m; Can obtain corresponding Δ f ' according to (4) formula
Max, the first initial measurement frequency f
1Constant, also just can determine the second double measurement frequency f '
2Write device respectively with 2 frequency f
1And f '
2Transmitted signal, the phase place of the signal that the measurement tag reflection is returned is respectively
With
Obtain
Can obtain accurate actual range d ' according to (4) formula;
Step 5: four UHF rfid interrogators send accurate actual range d ' to central controller by total node, central controller is tried to achieve the accurate location of electronic tag in the three-dimensional coordinate system in conjunction with the three-dimensional coordinate position of four UHF rfid interrogators in the three-dimensional coordinate system.
Described UHF rfid interrogator is by main control module, radio-frequency (RF) circuit module, and power amplifier module, the CAN communication module, circulator, phase differential acquisition module and antenna form;
Main control module is connected with the CAN communication module is two-way, main control module also is connected with radio-frequency (RF) circuit module is two-way, the unidirectional power amplifier module that is connected to of radio-frequency (RF) circuit module, the unidirectional input end that is connected to circulator of power amplifier module, circulator is connected with antenna, unidirectional radio circuit transceiver module and the phase differential acquisition module of being connected to of the output terminal of circulator, the output terminal of phase differential acquisition module connects main control module.
Described main control module: be used for the reading of label, and total node communication;
Described radio-frequency (RF) circuit module: be used for realizing encoding and decoding, the modulation /demodulation radio frequency signal generation adopts integrated chip AS3992 and corresponding peripheral circuit to realize;
But radio-frequency module power out is little, and power amplifier module increases the power of emission;
Described CAN communication module is comprised of independent CAN transceiver controller SJA1000 and CAN bus driver PCA82C250, is used for carrying out data communication with total node;
Described circulator is used for receiving and transmitting signal and separates;
Described phase differential acquisition module: be used for obtaining the direct phase differential of carrier wave that transmitted signal carrier wave and label backscattering are returned.
Described total Node configuration has the CAN module to be connected with described UHF rfid interrogator;
Described total Node configuration has the Internet communication module to be connected with the internet;
Described total Node configuration has central controller: central controller calculates the accurate location of electronic tag in the three-dimensional coordinate system.
Central controller is connected with the CAN module is two-way, and central controller also is connected with the Internet communication module is two-way, and central controller also is connected with human interface device.
Remarkable result of the present invention is: a kind of method for sensing position of object in Internet of things is provided, has had the poor PDOA distance-finding method of dynamic frequency, can guarantee that the relative error of measuring system is stable, at actual range hour, reduced range error.
Developing rapidly of Internet of Things, must cause the demand to accurate location-aware services, this paper proposes the Internet of Things location aware system based on UHF RFID scheme, be operated in 860~960MHz scope, in position perceived accuracy, the system expandability, the aspects such as networking all meet the needs of Internet of Things development, follow the tracks of in the hospital patient location, logistics management, there is larger using value in the fields such as robot control.Because the design of native system does not consider to have between label and the reader situation of barrier, if barrier is arranged, the distance that time records should be that electromagnetic wave signal passes through diffraction or diffraction arrives, thereby so that the bearing accuracy error is larger, therefore, will in next step work, analyze the method for the exact position perception of this situation.
Embodiment
The below is described in further detail the present invention with regard to specific embodiment.
Internet of Things location aware system is mainly by 4 UHF rfid interrogators, antenna, and electronic tag and total node form.Shown in Figure 1 is a system architecture in the zone, the UHF rfid interrogator, being placed in respectively the location is the suitable position of scope, interconnect by the CAN bus between each UHF rfid interrogator, and all receive above total node, total node is connected with the internet, before the location, set up a 3-D walls and floor, the coordinate of each read write line is known, label or the target of having carried label are when location aware regional, 4 UHFRFID read write lines get access to respectively the positional information of label, then with total node communication, with the positional information that gets access to, send to total node, total node calculates the position coordinates of label by corresponding algorithm, and the result that will obtain sends corresponding server to by the internet.Calculated the three-dimensional position of electronic tag by server.
Total Node configuration CAN module is connected with described UHF rfid interrogator;
Total Node configuration has the Internet communication module to be connected with the internet;
Total Node configuration has central controller.
Central controller is connected with the CAN module is two-way, and central controller also is connected with the Internet communication module is two-way, and central controller also is connected with human interface device.Also can calculate the accurate location of electronic tag in the three-dimensional coordinate system by central controller.
The UHF rfid interrogator is mainly by main control module, radio-frequency (RF) circuit module, and power amplifier module, the CAN communication module, circulator, phase differential acquisition module and antenna form.The structured flowchart of read write line as shown in Figure 2.
The CAN communication module mainly is comprised of independent CAN transceiver controller SJA1000 and CAN bus driver PCA82C250, be responsible for and sum up carry out data communication; Main control module mainly is responsible for reader to the reading of label, and the data processing function such as total node communication, and controller adopts the procedure stores amount larger, and processing speed and IO interface is the single-chip microcomputer PIC24FJ192GB106 of the relative PIC that meets design requirement all; Radio-frequency (RF) receiving and transmission module mainly realizes encoding and decoding, and the functions such as modulation /demodulation radio frequency signal generation adopt integrated chip AS3992 and corresponding peripheral circuit to realize; The passive RFID system that is operated in uhf band will be implemented in the certain area label is positioned, and need decipherment distance far away, but the power of radio-frequency module signal out is less, needs amplification module to increase the power of emission; The phase differential acquisition module is to obtain the direct phase differential of carrier wave that transmitted signal carrier wave and label backscattering are returned, and therefore adopts the integrated chip AD8302 that can obtain 2 road signal phase differences to realize; Read write line need to obtain the signal acquisition phase information that the label backscattering is returned, radio-frequency module also needs independently to receive signal, and system only has an antenna that transmitting-receiving is multiplexing, and the signal that is sent out for the signal that is unlikely to reflect disturbs, and adopts circulator that receiving and transmitting signal is separated.
The total major function of node in the sensory perceptual system of position is the work by the total line coordination read write line of CAN, the positional information that read write line is sent is by computing obtain the being positioned position coordinates of target, and will with the internet on some servers or terminal communicate, thereby the various location-aware applications in the realization Internet of Things, in order to use the convenience of installation, man-machine interface need to be set total node is arranged in addition.The structured flowchart of total node as shown in Figure 3.
The CAN communication module is the module that total node is communicated by letter with mounted each read write line among Fig. 3; Man-machine interface comprises button and demonstration; The Internet communication module is the module of total node and internet communication; Master controller is owing to need some calculating and control, but also needs network service, therefore selects the processor of the comparatively powerful ARM7 series of function.
System is mainly by the work of total each node of nodes coordinating, so the workflow of system is mainly divided the work of total node and read write line 2 parts.The program circuit of total node and read write line is respectively such as Fig. 4, and is shown in Figure 5.
Among Fig. 4, total node at first needs the parameter of system is configured, working environment such as system, the relevant information of location aware, the information such as the corresponding position coordinates of reader, then the information that reads label that whether has read write line to send on the poll CAN bus, when each reader can both read the label of same ID, then total node sends the order of measuring tag distances for each read write line by the CAN bus, returned the tag distances information of same ID when each read write line after, total node just begins to calculate the positional information of label, calculate after the positional information of label, just by internet interface and server communication, ID and the positional information of label are passed to server.
Among Fig. 5, read write line is at first done relevant initialization operation, then, just begin whether can obtain label in the poll localizing environment, when having label to enter read-write scope, just the id information with label sends to total node by the CAN bus, then wait for total node return command, if, during the always order of node restoring to normal position, read write line begins to measure the distance of label, and then the range information with label sends to total node by the CAN bus.
The UHF radio frequency band that native system adopts, mainly contain based on angle measurement with based on two kinds of technology of range finding at the RFID of this frequency range positioning principle at present, need the directional perception sensitivity of antenna just can reach bearing accuracy based on angle measurement, antenna complex and cost are high, therefore generally all adopt the method based on range finding.
To come target is positioned by distance in the three dimensions, need at least 4 readers just can solve unique coordinate figure, because always there is error in the measurement of distance, therefore do not adopt direct solving equations, and adopt gradient descent method to approach gradually the coordinate figure of target.Its principle is, at first in locating area, select an initial coordinate, then the distance between each read write line of calculated amount and initial coordinate points respectively, then constantly adjust selected coordinate figure, the method by iteration is gradually so that the distance of each read write line of coordinate distance is approached the actual distance that records.Adopt the location aware process of gradient descent method
Be that step 3, step 4 in this method can repeatedly recycle as required.
The selection of distance-finding method
The technology of RFID range finding mainly contains based on arriving signal intensity (RSSI), time of arrival (toa) (TOA), and signal arrival time difference (TDOA), signal arrives the methods such as phase differential (PDOA).
It is larger that RSSI is affected by communication environments etc., and precision is low; The shorter TOA of indoor positioning propagation distance and TDOA are too high to hardware requirement, therefore are difficult to realize; Because reader is in the process of label transmitted signal, carrier wave charges to label, label reflects back carrier wave by the backscattering mode simultaneously, reader is compared with the phase place of transmitted signal carrier wave according to the backscattered signal carrier of label and is obtained a phase differential, just can obtain range information according to phase differential, even in complicated communication environments, can both obtain higher distance accuracy as long as can read label, therefore adopt the PDOA method that label is found range and be fit to the range finding of indoor environment.
Suppose that the signal that read write line sends out is s (t), in the situation of not considering tag modulates and noise effect, the signal that the tag reflection that read write line receives is returned can be written as:
The ρ amplitude relevant with propagation distance wherein,
It is the phase place of the signal that receives.So the distance between label and the reader can be expressed as:
Because it is about 900MHz that native system is selected the frequency range of UHF, wavelength approximately is about 33cm, and for indoor localizing environment, generally is several meters to tens meters, therefore, and phase place
There is the phase ambiguity of 2n π, thereby causes dysmetria true, adopt bifrequency can eliminate phase ambiguity.
Vacation does not have, and read write line is respectively with 2 frequency f 1 and f2 transmitted signal, and the phase place of the signal that corresponding tag reflection is returned is respectively
With
If the cycle of the phase ambiguity that these 2 phase places produce is the same, with these 2 phase places just subtract each other can cancelling (2) phase fuzzy problem, so can get:
Thereby obtain apart from the value of d be:
In sum, adopt bifrequency PDOA range finding can effectively eliminate within the specific limits the problem of phase ambiguity, as long as the selected corresponding difference on the frequency of the ultimate range of measuring as required, just can be than the distance between Measurement accuracy label and the reader in without the situation of phase ambiguity.
The error analysis of PDOA range finding
In formula (4), when
Get maximal value 2 π the time, the relation of distance and the fuzzyyest difference on the frequency such as Fig. 6 institute are not.
As shown in Figure 6, distance to be measured is larger, and maximum unambiguous difference on the frequency is just less, and the method for selected difference on the frequency is usually, and the distance of the maximum demand of location aware environment range finding is determined a maximum frequency difference as required.For example, if the ultimate range that needs in the location aware environment to measure is 15m, can get the fuzzyyest frequency by formula (4) is 10MHz.
Always there is certain error in the phase place of system acquisition, if the error of the phase place of system acquisition is
So, getting the error of finally finding range by (4) formula is:
Can be learnt by formula (5), when
Value certain, the inversely proportional relation of difference on the frequency that the error of range finding and when range finding are selected.But because actual distance of not knowing between label and the read write line, if the distance of measuring according to the maximum demand of location aware environment is selected frequency, the difference on the frequency of then selecting can be less, thereby cause final error larger.
Like this, when distance to be measured is less, so that relative error is larger.Suppose that actual testing distance is d0, then relative error is expressed as:
Known by formula (6), when actual range hour, the relative error that obtains distance will be larger.
Therefore, if can select difference on the frequency according to the relation of the distance among Fig. 6 and difference on the frequency, just can reduce relative error, that is, at measuring distance hour, selected larger difference on the frequency, namely the Δ f in the formula (6) is larger, and relative error is also less.
By formula (5) and (6) as can be known, under the condition of identical phase-detection error, the used difference on the frequency that reduces to find range can reduce absolute error and relative error simultaneously.As shown in Figure 6, different distances is corresponding maximum frequency difference all, therefore, can adopt according to the difference of distance range that dynamic frequency is poor finds range, and crucially just is obtaining of this distance range.On the other hand, in the PDOA range finding, the maximum measure distance that direct chosen position perception needs is apart from the difference on the frequency Δ f of correspondence
mJust error is larger, so can use first Δ f
mMeasure and obtain one apart from d0, then can be by order in the formula (4) according to the d0 that obtains
Obtaining a new difference on the frequency is:
In the formula (7), owing to use Δ f
mThe distance of estimation has error, does not cause phase ambiguity in order to guarantee resulting difference on the frequency, need to add in the distance of premeasuring a maximum error E
D maxThe meaning of Min is to get the minimum meaning, because dmax needs the ultimate range of finding range in the location aware environment, owing to define in advance the maximum measure distance distance in the location aware environment, therefore, when if the value of d0+Edmax has surpassed dmax, should directly get the value of dmax.
Then the difference on the frequency that obtains according to formula (7) is adjusted the distance again and is measured, in whole location aware environment, difference according to the position range of label, changed dynamically the difference on the frequency of actual range finding by predictive algorithm, the value of difference on the frequency is dynamic change in whole perception environment, the poor range finding ratio juris of dynamic frequency that Here it is.
Because the location aware algorithm of native system has adopted gradient descent method, as long as the error of the distance between label and the read write line is little, the precision of location aware is just higher, and therefore, the accuracy of the measurement of the distance between label and the reader has determined the positioning performance of system.The below analyzes the range error of native system.
Suppose the maximum absolute error that the middle phase place of PDOA range finding is obtained
Can adopt dynamic frequency absolute error and relative error poor and that do not adopt dynamic frequency to differ from 2 kinds of methods range findings to distinguish as shown in Figure 7 and Figure 8 by the poor range measurement principle of dynamic frequency and formula (4)~(7).
From Fig. 7, can learn, always the absolute error than the generation of not using the poor distance-finding method of dynamic frequency is little to have used the absolute error of generation of the poor distance-finding method of dynamic frequency, only distance near maximum measure distance apart from the time just and do not use the absolute error of the poor method of dynamic frequency to move closer to.And can learn from Fig. 8, when distance is less, do not use the relative error error of the poor distance-finding method of dynamic frequency very large, maximum can reach 90%, the relative error of having used the poor distance-finding method of dynamic frequency then is no more than in 5% the less scope at one, and along with the range finding distance increase, very fast reduction.This shows that the error during the method for the poor range finding of dynamic frequency in this paper can effectively reduce to find range improves the precision of alliance perception.