CN110554353B - Mobile robot absolute positioning method based on RFID system - Google Patents

Mobile robot absolute positioning method based on RFID system Download PDF

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CN110554353B
CN110554353B CN201910807094.6A CN201910807094A CN110554353B CN 110554353 B CN110554353 B CN 110554353B CN 201910807094 A CN201910807094 A CN 201910807094A CN 110554353 B CN110554353 B CN 110554353B
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pose
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陶波
吴海兵
黄飞虎
黎真龙
武翀
尹周平
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Huazhong University of Science and Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
    • G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
    • G01S5/0284—Relative positioning

Abstract

The invention belongs to the field of wireless positioning and discloses an absolute positioning method based on a mobile robot in an RFID system. The absolute positioning method comprises the following steps: (a) establishing an initial particle set, wherein the particles correspond to the mobile robot, and the pose of one particle corresponds to the pose of one mobile robot; (b) for the pose of each particle at the current t-1 moment, predicting the pose of each particle at the t moment by adopting a track prediction method; (c) constructing a relational expression of the total weight of the particles at the time t, the readability weight and the phase difference weight so as to obtain the total weight of each particle at the time t; (d) and calculating the pose of the mobile robot at the t moment by using the total weight and the pose of each particle, so as to realize the absolute positioning of the mobile robot. The invention realizes the absolute positioning of the mobile robot, and has simple calculation and high positioning precision.

Description

Mobile robot absolute positioning method based on RFID system
Technical Field
The invention belongs to the field of wireless positioning, and particularly relates to an absolute positioning method of a mobile robot based on an RFID system.
Background
The real-time positioning of the mobile robot can enable the real-time monitoring and scheduling of the mobile robot, improve the intelligent level of the manufacturing industry, and receive extensive attention of researchers and persons in the industry. The GPS positioning system is mainly used outdoors and fails in indoor environments such as warehousing and factories. Positioning methods based on sensors such as vision, laser, RFID, etc. have become a hotspot of research. The RFID is free from the interference of illumination and dust, is low in price, has an ID identification function, can robustly and efficiently complete the extraction and identification of the mark, and has outstanding advantages.
The existing positioning technology based on RFID is mainly based on the readability or RSSI of the label, and the positioning accuracy is limited due to the low readability and the position sensitivity of the RSSI. In addition, since this method needs to rely on dense reference tags, it brings about a large cost and a large workload of system installation, which limits the practicability of the method.
Disclosure of Invention
Aiming at the defects or improvement requirements of the prior art, the invention provides an absolute positioning method of a mobile robot based on an RFID system, which combines phase difference information with high sensitivity and readability, and realizes the absolute positioning of the mobile robot by combining a particle filtering positioning algorithm and an observation model based on phase difference through the provided readability particle pose judgment condition and a phase difference observation model, thereby solving the technical problem of the positioning of the mobile robot under the conditions of high precision and sparse label distribution.
In order to achieve the above object, according to the present invention, there is provided an absolute positioning method based on a mobile robot in an RFID system, where the RFID system includes a mobile robot, antennas and RFID reference tags, where the antennas are disposed on two sides of the mobile robot, the RFID reference tags are uniformly distributed on a surface of a motion area of the mobile robot, the mobile robot moves in the motion area, the RFID reference tags are identified by the antennas, and the absolute positioning of the mobile robot is implemented by combining a particle filter positioning algorithm and an observation model based on a phase difference, and the absolute positioning method specifically includes the following steps:
(a) establishing an initial particle set comprising a plurality of particles, and setting an initial pose of each particle, wherein the particles correspond to the mobile robot, and the pose of one particle corresponds to the pose of one mobile robot;
(b) for the pose of each particle at the current t-1 moment, predicting the pose of each particle at the t moment by adopting a track estimation method so as to obtain the pose of each particle at the t moment;
(c) setting a pose judgment condition for the pose of each particle at the time t, setting the readability weight of each particle at the time t according to the pose judgment condition, setting the phase difference weight of each particle at the time t according to the difference value of the observed phase of the RFID reference tag measured by the antenna on the mobile robot at the time tm and the observed phase of the RFID reference tag measured by the antenna on the mobile robot at the time t, and constructing a relational expression of the total weight of the particles at the time t, the readability weight and the phase difference weight so as to obtain the total weight of each particle at the time t, wherein tm is a time before the time t;
(d) and constructing a relational expression of the predicted pose, the total weight of each particle at the t moment in the new particle set and the pose of each particle, and calculating to obtain the predicted pose, wherein the predicted pose is the pose of the mobile robot at the t moment, so that the absolute positioning of the mobile robot is realized.
Further preferably, in step (a), the particle poses are set in the following manner:
Figure BDA0002183986060000021
wherein the content of the first and second substances,
Figure BDA0002183986060000022
is the state vector of the ith particle in the particle set at the initial instant. R1，R2And R3Is a random number between 0 and 1, Tj T1, T2, T3TjIs the x coordinate of the reference tag read at the initial time, r is the maximum read range of the reference tag, yTjIs the y coordinate of the reference tag read at the initial time.
Further preferably, in step (c), the particle pose determination condition is set in the following manner:
(a1) for each antenna, dividing an area I, an area II and an area III corresponding to the antenna, wherein the area I is an area which can be read by all the antennas, the area II is an area which can be recognized by the antenna in the area I, and the area III is an area which cannot be recognized by the antenna in the area I;
(a2) setting a pose determination condition of the particle, the pose determination condition being set to: under the pose, each antenna can meet the identification and judgment condition when identifying the RFID reference tag, wherein the identification and judgment condition is as follows: in the process of the RFID reference tag identified by the antenna, when the RFID reference tag can be identified, the RFID reference tag is fixed in the first area and is fixed out of the third area, and when the RFID reference tag can not be identified, the RFID reference tag is fixed out of the third area.
Further preferably, in step (c), the readability weight of each particle at the time t is set according to the pose determination condition, and the readability weight is set to 1 when the pose of the particle is combined with the pose determination condition, and is set to 0 otherwise.
Further preferably, in step (c), the phase difference weight is set as follows:
Figure BDA0002183986060000031
wherein, w2，i，tIs the phase difference weight, Δ θ, of the ith particle at time ttIs the difference in observed phase of the same RFID reference tag measured by the antenna at tm and at time t respectively,
Figure BDA0002183986060000032
is the predicted phase difference for the ith particle,
Figure BDA0002183986060000033
sigma is the standard deviation of the phase measurement noise,
Figure BDA0002183986060000034
is the difference in distance at time t,
Figure BDA0002183986060000035
d2 is the distance from the antenna to the RFID reference tag at time t, d1 is the distance from the antenna to the RFID reference tag at time tm, and λ represents the wavelength of the radio signal.
Further preferably, in step (c), the total weight is preferably performed according to the following expression:
wi，t＝wi-1，t-1*W1，i，t*w2，i，t
wherein, wi，tIs the total weight of the ith particle at time t, wi，t-1Is the total weight of the ith sub-particle at time t-1, w1，i，tIs the readability weight of the ith particle at time t, w2，i，tIs the phase difference weight of the ith particle at time t.
Further preferably, in step (d), the relation between the predicted pose and the total weight of each particle in the new set of particles at time i and the pose of each particle is preferably according to the following expression:
Figure BDA0002183986060000041
Figure BDA0002183986060000042
Figure BDA0002183986060000043
wherein x isfIs the abscissa of the predicted pose, yfIs the abscissa, th, of the predicted posefIs the attitude angle, x, of the predicted posei，tIs the x coordinate, y, of the ith particle at time t in the world coordinate systemi，tIs the y coordinate, th, of the ith particle in the world coordinate system at time ti，tAnd the attitude angle of the ith particle in the world coordinate system at the time t, wherein i is 1,2,3, …, and N is the total number of the particles in the particle set.
Further preferably, when the robot absolute positioning is performed next time, the proportion of the effective particles in the new particle set needs to be calculated, and the calculation is preferably performed according to the following expression:
Figure BDA0002183986060000044
when the effective particle ratio NeffEliminating or reserving by adopting a resampling algorithm when the set threshold value is not exceeded, so as to realize the updating of the particles, otherwise, the new particle set is not updated, wherein N iseffIs the proportion of effective particles.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
1. the invention uses particle filter positioning algorithm to make the pose of each particle correspond to the pose of a mobile robot, and uses the last time pose of a plurality of particle poses and the total weight of each particle pose to calculate and predict the pose of the particle at the current time, thus realizing absolute positioning of the mobile robot, simple calculation and high positioning precision;
2. the total weight of the particles is set to include readability weight and phase weight of an observation model based on phase difference, reasonability and phase of the particle pose are comprehensively considered, accuracy of the weight is improved, and accuracy of predicting the particle pose is further improved;
3. by adopting the absolute positioning method provided by the invention, the accurate positioning of the mobile robot position can be realized even under the condition that RFIF reference tags are sparsely distributed, and the problem of the positioning of the mobile robot under the condition of high precision and sparse tag distribution is further solved.
Drawings
FIG. 1 is a schematic diagram of the RFID system architecture constructed in accordance with a preferred embodiment of the present invention;
fig. 2 is a schematic illustration of the division of antenna areas constructed in accordance with a preferred embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
As shown in FIG. 1, the method installs a reader-writer and a reader-writer antenna on a mobile machine, and installs a passive ultrahigh frequency RFID tag as a reference tag on the ground. The EPCs of all the reference tags and the positions of the reference tags in the corresponding world coordinate system are used as known information and stored in the mobile robot, the mobile robot controls the RFID system to continuously read the reference tags in the continuous motion process, and positioning is carried out through the readability and phase related information of the tags extracted from the RFID system, and the method specifically comprises the following steps:
s1 establishes an initial particle set including a plurality of particles, and sets an initial pose of each particle. Considering that the purpose of the invention is to realize the positioning of the mobile robot, the state vector of the particle filter uses the pose x ═ of (x) of the mobile roboti，yi，thi) And i is 1,2,3, … N, wherein (x)i，yi) Is a world coordinate system lower position coordinate, th, of the mobile robotiIs the attitude angle of the mobile robot in the world coordinate system, and N is the number of particles in the particle set. The abscissa x of each particle in the set of particles satisfies the following relationship:
max(xTj)-r≤x≤min(xTj)+r
where Tj is T1, T2, T3, TM, Tj is a reference tag read during initialization, xTjThe x coordinate of the reference label read in the initialization stage, r is the maximum reading range of the reference label, the y coordinate of the particle set conforms to the same principle, the attitude angle of the particle set is 0-2 pi, and the random number is used.
The specific initialization formula is as follows:
Figure BDA0002183986060000061
wherein the content of the first and second substances,
Figure BDA0002183986060000062
is the state vector of the ith particle in the particle set at time 0. R1，R2And R3Is 0 toA random number between 1.
S2 predicts the state of particles by a track estimation method using an internal sensor of a mobile robot such as an odometer or a gyroscope. The track estimation method is a method in which the azimuth information of the mobile robot at the next time is obtained by recursive calculation based on the azimuth information of the mobile robot at the previous time, that is, the movement information of the robot is obtained from the information of the inertial sensor inside the mobile robot. Since the particle state vector in the present invention is directly expressed by the pose of the mobile robot, the relative motion information of the particles and the relative motion information of the mobile robot are matched in their respective coordinate systems, and the relative motion information of each particle in the particle set can be predicted from the relative motion information of the mobile robot. Considering factors such as measurement noise of a sensor, motion error of the mobile robot and the like, an additional small amount of gaussian noise needs to be added in state prediction.
S3 combines RFID readability and phase difference information to realize particle importance weight evaluation. Particle importance weight evaluation is first performed with readability: setting a particle pose judgment condition, judging the reasonability of the pose of the mobile robot represented by each particle in the particle set by using the particle pose judgment condition, and setting the readability weight of the particle subjected to reasonability detection as w1，i，t1, the readability weight of a particle that fails the rationality test is set to w1，i，t0, i represents the ith particle, and t represents the tth time;
the particle pose judgment conditions are set as follows: as shown in fig. 2, taking the antenna a as an example, three areas are defined by two circles and a rectangle, wherein the area one includes all readable spaces of the antenna, the tag in the area two can be read by the antenna, and the tag in the area three cannot be read by the antenna. Obviously, region two is a subset of region one, and considering the uncertainty of readability, region one is significantly larger than region two. The third area is a shadow area considering that the robot body can form label reading. Based on the three region rules, the designed particle pose judgment conditions are specifically as follows: if a tag can be read, then the tag must be within zone one and must be outside zone three; if a tag cannot be read, then the tag must be outside the area.
Second, particle importance weight evaluation is performed using the phase difference. Considering that Gaussian noise exists in phase measurement, a Gaussian probability density function based on phase difference is designed to calculate the particle weight, and the specific formula is as follows:
Figure BDA0002183986060000071
wherein, w2，i，tIs the phase difference weight, Δ θ, of the ith particle at time ttIs the difference in observed phase of the same RFID reference tag measured by the antenna at tm and at time t respectively,
Figure BDA0002183986060000072
is the predicted phase difference for the ith particle,
Figure BDA0002183986060000073
sigma is the standard deviation of the phase measurement noise,
Figure BDA0002183986060000074
is the difference in distance at time t,
Figure BDA0002183986060000075
d2 is the distance from the antenna to the RFID reference tag at time t, d1 is the distance from the antenna to the RFID reference tag at time tm, and λ represents the wavelength of the radio signal. The predicted distance difference can be obtained by the above-described track estimation method based on the internal sensor of the mobile robot such as a odometer and a gyroscope.
Finally, the final importance weight of the particle is the product of the weight of the particle in the last time step and the weight calculated by the readability and phase difference information respectively in the current time step:
wi，t＝wi，t-1*w1，i，t*w2，i，t
s4, calculating the pose of the mobile robot by estimating the weighted geometric center of the particle set, and realizing the positioning of the mobile robot:
Figure BDA0002183986060000081
Figure BDA0002183986060000082
Figure BDA0002183986060000083
s5 the weight of each particle in the above particle sets is different, some particles have a large weight and some particles have a small weight, so that the particles with small weights are removed by using a particle resampling algorithm, and the number of the particles with large weights is increased. The specific resampling method may be performed by using a plurality of existing methods, such as classification resampling, system resampling, and the like, which are the prior art and will not be described herein. It is worth pointing out that frequent resampling will reduce the robustness of the localization algorithm and accelerate particle depletion, so it is necessary to estimate the proportion of valid particles in the current set of particles:
Figure BDA0002183986060000084
if the effective particle ratio NeffAbove the set threshold, this indicates that resampling is now not required, otherwise resampling is required. After resampling, the weight of each particle in the set of particles may be changed to 1/N.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (8)

1. The method for absolute positioning of the mobile robot based on the RFID system is characterized in that the RFID system comprises the mobile robot, antennas and RFID reference tags, wherein the antennas are arranged on two sides of the mobile robot, the RFID reference tags are arranged on the surface of a motion area of the mobile robot, the mobile robot moves in the motion area of the mobile robot, the RFID reference tags are identified through the antennas, and the absolute positioning of the mobile robot is realized by combining a particle filter positioning algorithm, and specifically comprises the following steps:
(a) establishing an initial particle set comprising a plurality of particles, and setting an initial pose of each particle, wherein the particles correspond to the mobile robot, and the pose of one particle corresponds to the pose of one mobile robot;
(b) for the pose of each particle at the current t-1 moment, predicting the pose of each particle at the t moment by adopting a track estimation method so as to obtain the pose of each particle at the t moment;
(c) setting a particle pose judgment condition for the pose of each particle at the time t, setting the readability weight of each particle at the time t according to the pose judgment condition, setting the phase difference weight of each particle at the time t according to the difference value of the observed phase of the RFID reference label measured by the antenna on the mobile robot at the time tm and the observed phase of the RFID reference label measured by the antenna on the mobile robot at the time t, and constructing a relational expression of the total weight of the particles at the time t, the readability weight and the phase difference weight so as to obtain the total weight of each particle at the time t, wherein tm is a time before the time t;
(d) and constructing a relational expression of the predicted pose, the total weight of each particle at the t moment and the pose of each particle, and calculating to obtain the predicted pose, wherein the predicted pose is the pose of the mobile robot at the t moment, so that the absolute positioning of the mobile robot is realized.
2. The absolute positioning method based on the mobile robot in the RFID system according to claim 1, wherein in the step (a), the particle pose is set according to the following way:
Figure FDA0003316296730000021
wherein the content of the first and second substances,
Figure FDA0003316296730000022
is the state vector of the ith particle in the particle set at the initial time, R1，R2And R3Is a random number between 0 and 1, Tj T1, T2, T3TjIs the x coordinate of the RFID reference tag read at the initial time, r is the maximum read range of the RFID reference tag, yTjIs the y coordinate of the RFID reference tag read at the initial time.
3. The absolute positioning method based on mobile robot in RFID system as claimed in claim 1, wherein in step (c), the particle pose determination condition is set as follows:
(a1) for each antenna, dividing an area I, an area II and an area III corresponding to the antenna, wherein the area I is an area which can be read by all the antennas, the area II is an area which can be identified by the antenna in the area I by the RFID reference tag, and the area III is an area which cannot be identified by the antenna in the area I by the RFID reference tag;
(a2) setting a pose determination condition of the particle, the pose determination condition being set to: under the pose, each antenna can meet the identification and judgment condition when identifying the RFID reference tag, wherein the identification and judgment condition is as follows: in the process of the RFID reference tag identified by the antenna, when the RFID reference tag can be identified, the RFID reference tag is fixed in the first area and is fixed out of the third area, and when the RFID reference tag can not be identified, the RFID reference tag is fixed out of the third area.
4. The absolute positioning method based on mobile robot in RFID system according to claim 1 or 2, wherein in step (c), the readability weight of each particle at time t is set according to the pose determination condition, and the readability weight is set to 1 when the pose of the particle meets the pose determination condition, and is set to 0 otherwise.
5. The absolute positioning method of a mobile robot based on RFID system as claimed in claim 1 or 2, wherein in step (c), the phase difference weight is set as follows:
Figure FDA0003316296730000031
wherein, w2，i，tIs the phase difference weight, Δ θ, of the ith particle at time ttIs the difference in observed phase of the same RFID reference tag measured by the antenna at tm and at time t respectively,
Figure FDA0003316296730000032
is the predicted phase difference for the ith particle,
Figure FDA0003316296730000033
sigma is the standard deviation of the phase measurement noise,
Figure FDA0003316296730000034
is the difference in distance at time t,
Figure FDA0003316296730000035
d2 is the distance from the antenna to the RFID reference tag at time t, d1 is the distance from the antenna to the RFID reference tag at time tm, and λ represents the wavelength of the radio signal.
6. The method for absolute positioning of a mobile robot in an RFID-based system according to claim 1 or 2, wherein in the step (c), the total weight is calculated according to the following expression:
wi，t＝wi，t-1*w1，i，t*w2，i，t
wherein, wi，tIs the total weight of the ith particle at time t, wi，t-1Is the total weight of the ith particle at time t-1, w1，i，tIs the readability weight of the ith particle at time t, w2，i，tIs the phase difference weight of the ith particle at time t.
7. The method for absolute positioning of mobile robot based on RFID system of claim 6, wherein in step (d), the relation between the predicted pose and the total weight of each particle and pose of each particle at the time t is performed according to the following expression:
Figure FDA0003316296730000036
Figure FDA0003316296730000037
Figure FDA0003316296730000038
wherein x isfIs the x-axis coordinate, y, of the predicted posefIs the y-axis coordinate, th, of the predicted posefIs the attitude angle, x, of the predicted posei，tIs the x-axis coordinate, y, of the ith particle at time t in the world coordinate systemi，tIs the y-axis coordinate, th, of the ith particle at time t in the world coordinate systemi，tAnd the attitude angle of the ith particle in the world coordinate system at the time t, wherein i is 1,2,3, …, and N is the total number of the particles in the particle set.
8. The absolute positioning method based on the mobile robot in the RFID system according to claim 7, wherein the ratio of effective particles in the new set of particles is calculated next time the absolute positioning of the robot is performed, and the calculation is performed according to the following expression:
Figure FDA0003316296730000041
when the effective particle ratio NeffEliminating or reserving by adopting a resampling algorithm when the set threshold value is not exceeded, so as to realize the updating of the particles, otherwise, the new particle set is not updated, wherein N iseffIs the proportion of effective particles.
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