CN108414972A - A kind of mobile robot RFID localization methods based on phase property - Google Patents

Abstract

The invention belongs to the field of wireless positioning, and specifically discloses a mobile robot RFID positioning method based on phase characteristics, including the following steps: when the mobile robot moves, the S1 RFID reader-writer forms a radio frequency loop with the RFID tag installed on the target object, and the RFID The reader measures the phase information and signal wavelength information of the RFID radio frequency loop; S2 converts the periodic non-continuous phase information into continuous non-periodic phase information, and obtains the position information of the RFID reader antenna; S3 constructs the unwrapped phase-position Model, the signal wavelength information, continuous aperiodic phase information and antenna position information are substituted into the unwrapped phase-position model, and the position information of the RFID tag relative to the mobile robot is calculated to achieve the positioning of the target. The invention has the advantages of high positioning accuracy, strong anti-interference ability, small calculation amount, simple system and the like.

Description

A RFID positioning method for mobile robots based on phase features

technical field

The invention belongs to the field of wireless positioning, and more particularly relates to an RFID positioning method for a mobile robot based on phase characteristics.

Background technique

RFID (Radio Frequency Identification), that is, radio frequency identification technology, is a wireless communication technology for building the Internet of Things. Passive UHF RFID has been widely used due to its unique ID, low cost, long distance, and no need for batteries. warehouse management, etc. RFID-based positioning technology can greatly improve the efficiency of item management. In the past ten years, RFID-based indoor positioning systems and methods have been continuously proposed. Among them, signal strength (RSSI) and phase are the two most important radio frequency information in RFID positioning.

Due to the existence of environmental interference such as multipath effects, the attenuation of signal strength and the mapping relationship between the distance between the antenna and the tag are not reliable, and the distance resolution of the signal strength is relatively rough, so the method based on signal strength often uses reference tags. Arrange the reference tag system in the environment in advance, and calibrate the position information of the reference tag, the system is complex.

The essence of phase information is a kind of signal propagation delay information, which is highly sensitive to distance, up to 2°/mm. Angle of Arrival (AOA) and Synthetic Aperture (SAR) are the two main phase-based positioning methods. Among them, the AOA method uses the phase difference information between the antenna arrays to calculate the angle between the tag and the antenna. Multiple antenna arrays The position of the tag can be obtained by calculating the intersection point of each direction angle. The AOA method requires the distance between the antennas to be less than half a wavelength, but the size of the directional antenna itself is large, and this requirement is often difficult to meet; while the SAR method uses a moving antenna to form a target for the tag. Synthetic aperture observation, this method can observe tags from all directions, the influence of environmental interference on positioning can be reduced, and the positioning accuracy is high. With a large calculation load, the real-time performance is limited.

Contents of the invention

In view of the above defects or improvement needs of the prior art, the present invention provides a mobile robot RFID positioning method based on phase characteristics, which utilizes a mobile robot carrying an RFID reader to calculate the relative position of a target object equipped with an RFID tag relative to the mobile robot. It has the advantages of high positioning accuracy, strong anti-interference ability, small calculation amount, and simple system.

In order to achieve the above object, the present invention proposes a mobile robot RFID positioning method based on phase features, the method comprises the following steps:

S1 The RFID reader installed on the mobile robot forms a radio frequency loop with the RFID tag installed on the target when the mobile robot is moving. The RFID reader continuously measures the phase information and signal wavelength information of the RFID radio frequency loop. The phase information is Periodic discontinuous information;

S2 converts the periodic non-continuous phase information into continuous non-periodic phase information, and obtains the position information of the RFID reader antenna;

S3 constructs the unwrapped phase-position model, substitutes the signal wavelength information, continuous non-periodic phase information and the position information of the RFID reader antenna into the unwrapped phase-position model, and calculates the position of the RFID tag relative to the mobile robot Position information to realize the positioning of the target.

As a further preference, in step S2, using the phase unwrapping algorithm to convert periodic non-continuous phase information into continuous non-periodic phase information is specifically: Let γ ⁽¹⁾ = θ ⁽¹⁾ , and start the following loop until all the periodic The discontinuous phase information is transformed into continuous aperiodic phase information:

When θ ⁽ⁱ⁾ -θ ^(i-1) >Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) + (θ ⁽ⁱ⁾ -θ ^(i-1) )-2π;

When θ ⁽ⁱ⁾ -θ ^(i-1) <-Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) +(θ ⁽ⁱ⁾ -θ ^(i-1) )+2π;

When -Ψ≤θ ⁽ⁱ⁾ -θ ^(i-1) ≤Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) + (θ ⁽ⁱ⁾ -θ ^(i-1) );

Among them, θ ⁽ⁱ⁾ is the phase measured at the i-th phase measurement moment, γ ⁽ⁱ⁾ is the actual unwrapped phase corresponding to the phase measured at the i-th phase measurement moment, Ψ is any value from 0 to 2π, i=2˜N, N is the number of phase information.

As a further preference, in step S2, the position information of the antenna is obtained by using the dead reckoning method and the rotation-translation transformation, specifically: according to the orientation information of the mobile robot at the last moment i-1 and the inertial sensor information inside the mobile robot, through recursion The position information of the mobile robot at the current moment i is calculated; according to the position information of the mobile robot at the current time i, the position information of the RFID reader antenna at the current moment is obtained through the rotation-translation transformation equation.

As a further preference, the rotation-translation transformation equation is specifically:

Among them, (x ⁽ⁱ⁾ , y ⁽ⁱ⁾ , th ⁽ⁱ⁾ ) is the orientation of the mobile robot corresponding to the i-th phase measurement moment, z is the z coordinate of the RFID reader antenna in the mobile robot coordinate system, (ρ , α) is the polar coordinate of the RFID reader antenna in the mobile robot coordinate system, is the position coordinate of the RFID reader antenna corresponding to the i-th phase measurement moment.

As a further preference, the unwrapped phase-position model is specifically:

Among them, δ ⁽ⁱ⁾ is the theoretical unwrapped phase corresponding to the phase measured at the ith phase measurement moment, λ ⁽ⁱ⁾ is the signal wavelength corresponding to the ith phase measurement moment, is the position coordinate of the RFID reader antenna corresponding to the i-th phase measurement moment, (x, y, z) is the coordinate of the RFID tag to be located on the target, and β is the fuzzy factor.

As a further preference, in step S3, the position information of the RFID tag relative to the mobile robot is obtained through numerical iterative optimization algorithm calculation, specifically:

S31 establishes the cost function as follows:

Among them, N is the number of phase information;

S32 Solve the minimum value of the cost function, so that the corresponding (x, y, z) when the cost function obtains the minimum value is the position information of the RFID tag relative to the mobile robot.

As a further preference, said N≥4.

Generally speaking, compared with the prior art, the above technical solution conceived by the present invention mainly has the following technical advantages:

1. The method of the present invention combines the mobile robot with the RFID system, which can read the tags from different directions, effectively reducing the impact of environmental interference on positioning, and with the help of phase information with high distance resolution, higher positioning can be obtained precision.

2. The present invention builds an unwrapped phase-position model through the designed phase unwrapping algorithm, and can realize the solution of the tag position through the conventional numerical iterative optimization algorithm, and the calculation load is small.

3. The positioning method proposed by the present invention does not need a reference tag, the system is simple, the cost is low, and it has the advantages of accurate positioning, strong anti-interference ability, and small calculation amount.

Description of drawings

Fig. 1 is a schematic diagram of the RFID positioning system for realizing the positioning method of the present invention;

Fig. 2 is the flowchart of the mobile robot RFID location method based on phase feature of the present invention;

Fig. 3 is a schematic diagram of the results of the phase unwrapping algorithm of the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not constitute a conflict with each other.

As shown in Figure 1, the present invention combines the RFID system with the mobile robot, uses the mobile robot carrying the RFID reader to solve the position information of the target object with the RFID tag relative to the mobile robot, and the reader communicates with the tag During the process, phase information and channel wavelength information can be measured, and inertial sensors are installed on the mobile robot.

As shown in Figure 2, a mobile robot RFID positioning method based on phase characteristics provided by an embodiment of the present invention uses a mobile robot carrying an RFID reader to calculate the position information of the target object with the RFID tag relative to the mobile robot, The method specifically includes the following steps:

S1 The RFID reader installed on the mobile robot forms a radio frequency loop with the RFID tag installed on the target when it moves with the mobile robot. The RFID reader continuously measures the phase information and signal wavelength information of the RFID radio frequency loop. The phase information is periodic discontinuous information;

S2 converts the periodic non-continuous phase information into continuous non-periodic phase information, and obtains the position information of the RFID reader antenna;

For converting periodic non-continuous phase information into continuous non-periodic phase information, the phase unwrapping algorithm is specifically used to convert periodic non-continuous phase information into continuous non-periodic phase information, where the phase unwrapping algorithm is specifically: Let γ ⁽¹⁾ ＝θ ⁽¹⁾ , and start the cycle as follows until all periodic non-continuous phase information is transformed into continuous non-periodic phase information:

When θ ⁽ⁱ⁾ -θ ^(i-1) >Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) + (θ ⁽ⁱ⁾ -θ ^(i-1) )-2π;

When θ ⁽ⁱ⁾ -θ ^(i-1) <-Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) +(θ ⁽ⁱ⁾ -θ ^(i-1) )+2π;

When -Ψ≤θ ⁽ⁱ⁾ -θ ^(i-1) ≤Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) + (θ ⁽ⁱ⁾ -θ ^(i-1) );

Among them, θ ⁽ⁱ⁾ is the phase measured at the i-th phase measurement moment, γ ⁽ⁱ⁾ is the actual unwrapped phase corresponding to the phase measured at the i-th phase measurement moment, Ψ is any value from 0 to 2π, i=2˜N, N is the number of phase information.

The phase values before and after unwrapping are shown in Figure 3. The phase before unwrapping is periodic and non-continuous, and its size is limited between 0-2π. The phase after unwrapping is continuous and non-periodic, and its size is no longer limited to 0-2π.

For the position information of the RFID reader antenna, the position information of the antenna is obtained by using the dead reckoning method and the rotation-translation transformation. The inertial sensor information of the mobile robot at the current moment (i-th moment) is obtained by recursive calculation (x ⁽ⁱ⁾ , y ⁽ⁱ⁾ , th ⁽ⁱ⁾ ); Transform the equation to get the position information of the RFID reader antenna at the current moment

For the dead reckoning method, it uses the information of the inertial sensors (such as odometer, gyroscope, accelerometer, magnetometer, etc.) The orientation information of the mobile robot at one moment, that is, the movement information of the robot can be obtained according to the information of the inertial sensor inside the mobile robot, and then the orientation of the mobile robot at the current moment can be obtained by superimposing the movement information on the orientation information of the mobile robot at the previous moment information, and the orientation information of the mobile robot at the last moment can be obtained by recursive calculation in the same way. For example, for a two-wheel differential mobile robot, the odometer sensors installed in the two wheels can respectively perceive the distances of the two wheels, and then through its differential kinematics model, the orientation information of the mobile robot at the previous moment can be transmitted Push to get the location information of the mobile robot at the current moment. It should be noted that the dead reckoning method can only obtain relative azimuth information, but not absolute azimuth information. Since the positioning method proposed by the present invention finally obtains the position coordinates of the tag relative to the current position of the mobile carrier, only its relative position information is needed, so the dead reckoning method can meet the requirements of the present invention.

Specifically, the rotation-translation transformation equation is specifically:

Among them, (x ⁽ⁱ⁾ , y ⁽ⁱ⁾ , th ⁽ⁱ⁾ ) is the orientation of the mobile robot corresponding to the i-th phase measurement moment (position + direction, orientation in the robot coordinate system), x ⁽ⁱ⁾ , y ⁽ⁱ⁾ is the position of the mobile robot, th ⁽ⁱ⁾ is the direction of the mobile robot, which can be obtained by dead reckoning, and z is the z coordinate of the RFID reader antenna in the mobile robot coordinate system (it is a known parameter, That is, the height of the RFID reader antenna in the Z-axis direction of the mobile robot coordinate system), (ρ, α) is the polar coordinate (which is a known parameter) of the RFID reader antenna in the mobile robot coordinate system, is the position coordinate of the reader antenna corresponding to the i-th phase measurement moment (in the robot coordinate system).

S3 establishes the unwrapping phase-position model, and substitutes the signal wavelength information obtained in step S1 and the continuous aperiodic phase information obtained in step S2 (that is, the phase information after unwrapping processing) and the position information of the antenna into the unwrapping phase-position model , calculate and obtain the position information of the RFID tag relative to the mobile robot, so as to realize the positioning of the target.

For the unwrapped phase-position model, it is specifically:

Wherein, δ ⁽ⁱ⁾ is the theoretical unwrapped phase corresponding to the phase measured at the i-th phase measurement moment, and λ ⁽ⁱ⁾ is the signal wavelength corresponding to the i-th phase measurement moment, which is obtained by step S1; is the position coordinates of the reader antenna corresponding to the i-th phase measurement moment, which is obtained by step S2; (x, y, z) is the coordinates of the RFID tag to be positioned on the target (the RFID tag in the mobile robot coordinate system Coordinates), β is the ambiguity factor, which is an additional parameter of the model, representing the ambiguity of the unwrapped phase, which can be set as required. In this model, there are four unknowns, which are x, y, z, and β. By solving the equation, the positioning of the label can be realized, where x represents the x coordinate of the label, y represents the y coordinate of the label, and z represents the z coordinate of the label , β represents the fuzzy factor.

Specifically, after substituting the corresponding signal wavelength information, continuous aperiodic phase information and antenna position information obtained at each moment into the unwrapped phase-position model, the following equations can be obtained:

Among them, N is the number of phase information, that is, the number of phase measurement moments. Since there are 4 unknowns in the unwrapped phase-position model, in order to ensure a certain accuracy, the number of equations should be no less than 4, that is, at least four different position to achieve phase observation.

Since the equations obtained above are generally overdetermined and nonlinear, it is difficult to obtain an analytical solution, so a numerical iterative optimization algorithm is needed to solve them. Since the observation noise of the system information presents a Gaussian distribution, the present invention establishes a cost function according to the least squares criterion:

Among them, γ ⁽ⁱ⁾ is the unwrapped phase corresponding to the phase measured at the i-th phase measurement moment, which is obtained in step S2. The essence of the tag position solution is to solve the minimum value of the cost function. The specific solution method Many existing methods can be selected, such as Newton's method, Levenberg-Marquardt, trust region algorithm, etc., which are existing technologies and will not be repeated here. This type of optimization algorithm generally needs to set an iterative initial point, which is limited according to actual needs, and then according to the iterative rule, the cost function is gradually reduced, and finally the minimum value of the function is found to realize the solution of the label position, that is, to solve The minimum value of the cost function, so that the corresponding (x, y, z) when the cost function obtains the minimum value is the position information of the RFID tag relative to the mobile robot (that is, the coordinates of the RFID tag in the mobile robot coordinate system).

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (7)

1. a mobile robot RFID location method based on phase characteristics, it is characterized in that, the method comprises the steps: S1 The RFID reader installed on the mobile robot forms a radio frequency loop with the RFID tag installed on the target when the mobile robot is moving. The RFID reader continuously measures the phase information and signal wavelength information of the RFID radio frequency loop. The phase information is Periodic discontinuous information; S2 converts the periodic non-continuous phase information into continuous non-periodic phase information, and obtains the position information of the RFID reader antenna; S3 constructs the unwrapped phase-position model, substitutes the signal wavelength information, continuous non-periodic phase information and the position information of the RFID reader antenna into the unwrapped phase-position model, and calculates the position of the RFID tag relative to the mobile robot Position information to realize the positioning of the target. 2. The mobile robot RFID positioning method based on phase features as claimed in claim 1, wherein, in step S2, the phase unwrapping algorithm is used to convert the periodic non-continuous phase information into continuous non-periodic phase information as follows: make γ ⁽¹⁾ ＝θ ⁽¹⁾ , and start the cycle as follows until all periodic non-continuous phase information is transformed into continuous non-periodic phase information: When θ ⁽ⁱ⁾ -θ ^(i-1) >Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) + (θ ⁽ⁱ⁾ -θ ^(i-1) )-2π; When θ ⁽ⁱ⁾ -θ ^(i-1) <-Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) +(θ ⁽ⁱ⁾ -θ ^(i-1) )+2π; When -Ψ≤θ ⁽ⁱ⁾ -θ ^(i-1) ≤Ψ, let γ ⁽ⁱ⁾ = γ ^(i-1) + (θ ⁽ⁱ⁾ -θ ^(i-1) ); Among them, θ ⁽ⁱ⁾ is the phase measured at the i-th phase measurement moment, γ ⁽ⁱ⁾ is the actual unwrapped phase corresponding to the phase measured at the i-th phase measurement moment, Ψ is any value from 0 to 2π, i=2˜N, N is the number of phase information. 3. The mobile robot RFID positioning method based on phase features as claimed in claim 1 or 2, wherein, in step S2, the position information of the antenna is obtained by using the dead reckoning method and the rotation and translation transformation, specifically: according to the last moment The orientation information of the mobile robot at i-1 and the inertial sensor information inside the mobile robot are calculated by recursive calculation to obtain the orientation information of the mobile robot at the current moment i; according to the orientation information of the mobile robot at the current moment i, the RFID reading at the current moment is obtained through the rotation-translation transformation equation The location information of the writer antenna. 4. the mobile robot RFID positioning method based on phase feature as claimed in claim 3, is characterized in that, described rotation-translation transformation equation is specifically: Among them, (x ⁽ⁱ⁾ , y ⁽ⁱ⁾ , th ⁽ⁱ⁾ ) is the orientation of the mobile robot corresponding to the i-th phase measurement moment, z is the z coordinate of the RFID reader antenna in the mobile robot coordinate system, (ρ , α) is the polar coordinate of the RFID reader antenna in the mobile robot coordinate system, is the position coordinate of the RFID reader antenna corresponding to the i-th phase measurement moment. 5. the mobile robot RFID location method based on phase feature as claimed in claim 1, is characterized in that, described unwrapping phase-position model is specifically: Among them, δ ⁽ⁱ⁾ is the theoretical unwrapped phase corresponding to the phase measured at the ith phase measurement moment, λ ⁽ⁱ⁾ is the signal wavelength corresponding to the ith phase measurement moment, is the position coordinate of the RFID reader antenna corresponding to the i-th phase measurement moment, (x, y, z) is the coordinate of the RFID tag to be located on the target, and β is the fuzzy factor. 6. the mobile robot RFID localization method based on phase feature as claimed in claim 1, is characterized in that, in the step S3, calculates and obtains the positional information of RFID tag relative to mobile robot by numerical iterative optimization algorithm, specifically: S31 establishes the cost function as follows: Among them, N is the number of phase information; S32 Solve the minimum value of the cost function, so that the corresponding (x, y, z) when the cost function obtains the minimum value is the position information of the RFID tag relative to the mobile robot. 7. The RFID positioning method for mobile robots based on phase features according to any one of claims 1-6, wherein said N≥4.