Method of constructing navigation map by robot using mouse hippocampal place cell model

Abstract

A robot constructs a navigation map based on the cognitive mechanism of rat hippocampus. The robot collects current self-motion cues and color depth map information through exploring the environment; self-motion cues form spatial environment codes gradually through path integral and feature extraction of spatial cells in hippocampus, place field of place cells is gradually formed during exploring the process and covers the whole environment to form a cognitive map. Further, Kinect collects scene view and color depth map information of the current position in right ahead direction as an absolute reference, proceeding path closed-loop detection to correct the errors of the path integral. At a close-loop point, the system proceeds reset of spatial cells discharging activity to correct the errors of the path integral. The final point in navigation map includes coding information of place cells series, corresponding visual cues and position topological relationship.

Description

TECHNICAL FIELD

The present disclosure involves a navigation map building algorithm based on the cognitive mechanism of rat hippocampus. Based on the navigation character of spatial cell in the hippocampal structure of the mammal, the robot moves automatically, and navigation map is built automatically. It is used for robot navigation map building under multiscale and complex environments.

BACKGROUND

Most currently used navigation map building algorithm is SLAM algorithm, such as grid map algorithm and topological map algorithm, the environmental feature point is collected by the wide-angle lens to build navigation map, it demands high quality for visual signals, image processing algorithms, and hardware performance. However, grid division and topological point collection are mainly finished by man-made setting, it only suitable for the specific static environment and hard to an extension. With the continuous complex of current motion environment, especially in dynamic condition, single navigation method is not satisfied with actual requirements, multi-navigation strategy will complement advantages of kinds of navigation methods to get better effect, the usually used data fusion mode is Kalman filtering algorithm, however, Kalman filtering algorithm needs to build motion model and observation model of system precisely, complex motion environment modeling is very complex in calculation, it restricts the application of Kalman filtering.

According to the mechanism of the biocognitive environment to make robot build environment cognitive map more intelligent, many drawbacks of currently used environment map building algorithm can be solved. Mammals (such as rat and human) need to encode for the environment when they move in it purposefully; it needs inner fusion for various sensor information, the inner neural expression is formed, this inner neural expression called cognitive map. Spatial navigation cell relates to environment cognition, and cognitive map building in the hippocampal structure are head direction cell, stripe cell, grid cell, border cell and spatial cell.

Hippocampus is the key encephalic region for animal environment cognition. In 1971, O'Keefe and Dostrovsky found that pyramidal neurons of region CA1 and CA2 in hippocampus only discharge in specific spatial position, when an animal is in a specific position, pyramidal neurons has the highest discharge frequency, when it goes away from this position, discharge frequency is decreased, these neurons is called place cell, animal activity range in environment relates to its discharge activity is called place field. Reflection relationship between brain and external environment is formed since selective discharge of place cell; it is a key factor for animal self-orientation and spatial environment cognition. More precisely, place cell has following properties:

Place field of place cell is generated swiftly when animal turns into a new environment; it will cover the whole environment through the traversal of environment;

Same position cell may issue in different environment and has different place field;

Different with neurocyte on visual cortex, specific position of place cell in brain has no relationship with its corresponding place field, in another word, place field of specific geographic position corresponding to two adjacent place cells may be not adjacent;

Exogenous information (such as visual, smell) and endogenous information (such as vestibule and body) can lead place cell to issue, and static place field is formed, place cell can also issue and form static place field without exogenous information (such as dark environment).

In 1990, Taube found a kind of head direction orientation neuron in back subicular, when animal's head heads to a specific direction, this kind of neuron has maximum discharge, it is called head direction cell, it is a kind of dependent head direction neuron, its discharge activity only relates to head direction in horizontal plane, it has no relation to the position, posture and actions if animal. Each head direction cell has only one optimum direction, in a fixed environment, head direction cell can contain steady state in a long time. It can be expressed as a gauss model.

In 2005, Hafting found grid cell which discharges strongly to specific spatial position through changing the shape and size of test chamber, when rat moves in two-dimensional space, regularly repeatable discharge of grid cell occurs in specific position, this spatial range is called grid field, triangle evoked set which is formed by connecting multiple grid cells discharge region, it covers whole spatial environment that the rat has passed. Spacing between two grid nodes is about 39-78 cm, spacing of grid firing field on dorsoventral side-axis along the entorhinal cortex is increasing gradually, usually, evoked set of same grid cell in different environment is different, when the rat is in dark environment, grid set is steady; there are 4 basic feature of grid formed by grid cell: {circle around (1)} spacing: distance between the centers of firing fields; {circle around (2)} orientation: angularity versus external reference coordinate; {circle around (3)} phase: displacement in x-axis and y-axis versus external reference point; {circle around (4)} size of firing field: spatial range of grid cell discharging. These 4 elements are a spatial parameter of grid map.

In 2012, O'Keefe proved that there exist periodical stripe firing field in parasubicular cortex and entorhinal cortex, it is called stripe cells, its' firing field covers the whole spatial environment in a parallel stripe shape. Stripe cells is considered as basic mechanism to finish path integral, its' firing activity can be characterized by four characters: {circle around (1)} stripe spacing: central distance between two stripe firing field; {circle around (2)} stripe field width: spatial horizontal discharge range of firing field; {circle around (3)} stripe phase: displacement verse external reference point; {circle around (4)} preferred direction: angularity of stripe field verse external reference coordinate system;

Activation rate of these five kinds of the spatial cell is shown in FIG. 2.

Head direction cell is located in presubiculum to encode head direction information, it projects on stripe cell in shallow cortex of entorhinal cortex, stripe cells in entorhinal cortex is worked as the input of grid cells to encode linear velocity information, grid cells proceeds path integral for inputting information of stripe cells, specific position codes and place field of place cells are formed through feature extraction. Place field is the key factors to build the cognitive map. Hippocampus has been considered as cognitive map structure of brain spatial environment representation, its' place cells, grid cells, and border cells in the entorhinal cortex, head direction cells in multiple encephalic regions, newly discovered stripe cells and various kinds of the sensory system which are formed the spatial navigation system inner brain.

Grid cells discharging and path integral of inner-source information is simulated by continuous attractor model, activity of spatial cells is derived from collective behavior of attractor neural network, the final state of network activity is: continuous steady state in low-dimensional flow pattern, these final steady state is attractor, position adjustment and update in flow pattern plate is derived from response of velocity of rat motion.

The present disclosure builds robot navigation map through combining hippocampus spatial cells of rat brain and color depth map collected by Kinect, attractor model is used as calculation model of spatial cells. Comparing with traditional SLAM navigation method, it requires low performance of hardware and sensor, it has better expandability and adaptability, the data fusion processing method used in the present disclosure is cost-effective compared with traditional Kalman filtering method, it builds a cognitive map of indoor and outdoor environment effectively and precisely.

SUMMARY

The present According to anatomical structures of the animal hippocampus and neurophysiological characteristics of spatial navigation cells, the present disclosure discloses an environment navigation map building method based on spatial cells action mechanism. Based on information transfer loop of spatial navigation cell in mammal hippocampus, as is shown in FIG. 3, robot collects current self-motion cues and color depth map information through exploring the environment, self-motion cues form spatial environment codes gradually through path integral and feature extraction of spatial cells in hippocampus, place field of place cells is gradually formed during exploring process and covers the whole environment to form a cognitive map. While at the same time, Kinect collects scene view and color depth map information of the current position in right ahead direction as an absolute reference, proceeding path closed-loop detection to correct the errors of the path integral. At a close-loop point, the system proceeds reset of spatial cells discharging activity to correct the errors of the path integral. The final point in navigation map including coding information of place cells series, corresponding visual cues and position topological relationship, a schematic diagram is shown in FIG. 1.

The present disclosure uses following technical scheme to reach the requirements mentioned above:

A navigation map building algorithm based on the cognitive mechanism of rat hippocampus, hardware schematic diagram is shown in FIG. 4.

The main part of data input of the algorithm is self-motion cues and visual information of image; it is realized by photoelectric encoder, gyroscope, and RGB-D sensor; photoelectric encoder collect velocity signals, gyroscope collects angle signals, RGB-D sensor collects color and depth images; CPU is used for calculating and managing the algorithm created by cognitive map, electrical control signal is created through electrical machinery to control the movement of robot.

Robot navigation map building algorithm based on hippocampus spatial cells, the whole process of the algorithm is shown below.

Step (1): at the beginning, set the head direction of the static robot as 0, collecting its transient velocity and angle information through its self-contained photoelectric encoder and gyroscope, while at the same time Kinetic collects image with the velocity of 2 FPS.

Step (2): linear velocity integral based on stripe cells;

Schematic diagram of single-dimensional stripe cells cyclic attractor model is shown in FIG. 5 (a). Robot collects its motion linear velocity and angle information through its self-contained photoelectric encoder and gyroscope, a moment of “t,” the velocity in φ direction φ(t) is v(t) so that the velocity and displacement in ν_θ(t) and D_θ(t) in a direction are:ν_θ(t)=cos(θ−φ(t))ν(t)  (1)D_θ(t)=∫₀^tν_θ(τ)dt  (2)

direction displacements are transformed into stripe cells to move in preferred direction θ, x_θα is discharging rate of stripe cells, it means in θ direction, the phase of stripe is α; discharging period of stripe cells is 1; stripe cells have the highest discharging rate at periodical position nl+α (n is integer) in preferred direction, thus, distance of stripe cells periodical discharge reset represents ω_θα(t) is space phase difference between D_θ and α:ω_θα=(d_θ(t)−α)mod l  (3)

Discharging rate of stripe cells can be expressed as:

$\begin{array}{cc}{x}_{\theta \alpha }\left(t\right)=e^{\left(-\frac{{\left({\omega }_{\theta \alpha }\left(t\right)-\frac{l}{2}\right)}^2}{2{\sigma }_s^2}\right)}& \left(4\right)\end{array}$

Standard deviation is expressed as σ_s, it describes the width of stripe cells in preferred direction.

Stripe cells across periodical position and its multiple positions are excited at the same time, displacement in space-specific direction is encoded as prior input information of grid cells and drive attractor of grid cells move on the flat surface.

Step (3): periodical position encoding of grid cells attractor model under two-dimensional continuous space.

There exists recursion connection between grid cells, interactions between weight values create grid cells attractors in space-specific position, the attractors is distributed in a hexagon; meanwhile, prior input from stripe cells drives attractor of grid cells to move on a flat surface to proceed periodical specific encoding for the whole surface; attractor surface is also called neural plate. Schematic diagram of two-dimensional grid cells cyclic attractor model is shown in FIG. 5 (b).

The kinetic formula of grid cells discharging rate is:

$\begin{array}{cc}\tau \frac{{\mathrm{ds}}_i}{\mathrm{dt}}=f\left[\sum _j{w}_{\mathrm{ij}}^g{s}_j+B_i\right]-s_i& \left(5\right)\end{array}$

In the formula, τ represents corresponding time constant of neural, neural transferring function ƒ[⋅] is a simple nonlinear rectification function, x>0, ƒ(x)=x, x≤0, ƒ(x)=0; neural discharging rate of neure i in current position is s_i, w_ij^g represents connection weight value from neure i to neure j in neural plate, Σ_jw_ij^gs_j represents inhibitive recursive input which is projected on neure i, B_i represents prior excited input from stripe cells; each gird cell nerue i has a preferred direction θ_i, this preferred direction information is determined by projection of stripe cells.

Grid cells receive a prior projection from stripe cells, preferred direction information in prior projection is used for ensuring the direction changing of output weight value, and ensuring velocity input information received by a robot.B_i=ω_θα(t)  (6)

There exists recursion connection between grid cells, its recursion connection weight value is:w_ij^g=w₀−−ω_θα(t)  (7) and,w₀=ae^{−γ|{right arrow over (x)}|2}−e^{−β|{right arrow over (x)}|2}  (8)

The shape of weight matrix likes a sombrero, high in the mid and low on both sides, its mid position is −s_θα(t), of all parameter setting, γ=1.05×β, β=3/λ_net², λ_net is period of grid which is formed in nerual plate, which is determined by period 1 of stripe cells; a=1, all connection is inhibitory, partly inhibitive connection is enough to generate response of grid cells.

Step (4): coding generation in specific spatial position through the competitive neural network from grid cells to place cell.

Grid cells are input source of place cell, discharging of place cell is the output of path integral system; the distribution of weight value from grid cell series to position synapsis is studied to generate unimodal firing field of place cell series, determining the active proportion of grid cell with overlapping activity package in each position.

Using competitive Heb study algorithm to find the subset of grid cell series activities, calculating place cell series activities:

$\begin{array}{cc}\frac{{\mathrm{dw}}_{\mathrm{ij}}^{\mathrm{eh}}}{\mathrm{dt}}={\mathrm{kp}}_i\left(s_j-\langle \mathrm{sj}\rangle \right)& \left(9\right)\end{array}$

k represents study rate, p_i is discharging rate of place cell, s_j represents grid cell discharging rate, ⋅ represents average discharging rate of grid cell, the right side of equation (9) determines change direction of weight: if current grid cell active rate is higher than average active rate which is input, synaptic connections is enhanced; otherwise, synaptic connections is inhibited; set w_ij^eh≥0 to ensure weight value is not a negative value, therefore, weight value through equation (9) is inhibited;

place cell space selectivity to a given place is derived from: to this given place, selective binding of activities of grid cell with many spatial phases and spatial spacing; therefore, grid cell with much spatial spacing and directions is needed, stripe cell with much spatial spacing and directions is also needed; each neural plate represents a grid cell series activity, the present disclosure generates many grid cell series with different size through average collecting a and λ_net; equation (9) detects grid cell series activity form spatial overlapping of multi-layer neural plate.

Place cell series activity is derived from projecting information of grid cell;p_i(r)=A·ƒ[Σ_j=1^Mw_ij^ehs_j(r)−C_inh]  (10)

A and C_inh are gain constant and inhibitive constant of place cell network, M represents number of layers of grid cell neural plate, w_ij^eh represents synaptic connection weight value from grid cell series “j” to place cell series “i”, “r” represents current position of the animal; C_inh is used to control the number of firing field of place cell, it is determined by B*max(p_i(r)); it is ensured that there are few subsets of grid cell series is selected as single firing field of place cell through competitive algorithm mentioned above.

Step (5): iteration and update of place cell path integral

Measurement model at relative location of the actual external environment is built by place cell attractor model; its attractor model neural plate is shown in FIG. 5 (c). Two-dimensional continuous attractor model is on a neural plate through partial excitatory, partial inhibitive connection and overall inhibitive connection to form a random activity package; this attractor is driven by spatial cell path integral system;

Step (5-1) obtaining relative location of current points;

excitatory weight value connection matrix Σ_m,n of place cell is built by a two dimensional Gaussian distribution, “m, n” represents the distance between abscissas of belonging units in coordinates X and Y; the distribution of weight value can be expressed as:ε_m,n=e^{−(m2+n2)/kp}  (11)

k_p represents width constant of position distribution;

A variable quantity of place cell activity resulted from partial excitatory connection can be expressed as:Δp_EX,EY=Σ_i=0^(nX−1)Σ_j=0^(nY−1)p_i,jε_m,n  (12)

n_X, n_Y represents the scale of two dimensional matrix of place cell in (X,Y) space, it represents the activity range of attractor on neural plate; since place cell is borderless in network which is shown in FIG. 5(c). Place cell which located at the border of neural plate will connect with place cell at the border of another neural plate to generate excitatory connection; the precondition of place cell iteration and visual template matching is: finding relative position of place cell attractor in neural plate, coordinate of this relative position can be expressed by the subscript of weight value matrix:m=(X−i)(mod n_X)  (13a)n=(Y−j)(mod n_Y)  (13b)

Each place cell also receives overall inhibitive signal from the whole network; It is different from grid cell sombrero attractor model, inhibitive signal of place cell occurs after partial excitatory connection instead of working at the same time; symmetry between excitatory and inhibitive connection matrix guarantees suitable neural network kinetics, it ensures attractor in space cannot be excited without limitation; variable quantity of place cell activity resulted from inhibiting connection can be expressed as:Δp_IX,IY=Σ_i=0^nXΣ_j=0^nYp_i,jψ_m,n−φ  (14)

ψ_m,n represents weight value of inhibitive connection, level of overall inhibition is controlled by φ;

To ensure all place cell activity is no less than zero at the moment of “t+1”, compare place cell activation rate with 0:p_X,Y^t+1=max{p_X,Y^t+Δp_EX,EY+Δp_IX,IY,0}  (15)

then, proceed normalization for place cell activation rate:

$\begin{array}{cc}{p}_{X,Y}^{t+1}=\frac{p_{X,Y}^t}{\sum _{i=0}^{n_X}\sum _{j=0}^{n_Y}{p}_{i,j}^t}& \left(16\right)\end{array}$

Step (5-2) place cell path integral

In the present disclosure, movement of place cell attractor is derived from path integral of spacing cell over self-motion cues, displacement in specific direction which drives grid cell attractor to move is encoded by stripe cell, grid attractor encodes two-dimensional space in specific direction to excite different grid cell series; movement of place attractor is determined by the subset of different grid cell series activity. Schematic diagram of this process is shown in FIG. 6.

Place cell discharging rate p_X,Y^t+1 after path integral can be expressed as:p_X,Y^t+1=Σ_m=δX0^δX0+1Σ_n=δY0^δY0+1α_mnp_(m+X)(n+Y)^t  (17)

δX₀, δY₀ is rounded-down shift in X-Y the coordinate system, this shift value is determined by velocity and direction information:

$\begin{array}{cc}\left[\begin{array}{c}\delta X_0\\ \delta Y_0\end{array}\right]=\left[\begin{array}{c}\lfloor k_m{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\cos \theta \rfloor \\ \lfloor k_n{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\sin \theta \rfloor \end{array}\right]& \left(18\right)\end{array}$

[⋅] represents rounded-down, k_m, k_n are path integral constant, _θi is unit vector headed to θ_i, θ represents current head direction, place cell activation rate in next moment is obtained by current place cell activation rate times residual value α_mn, residual value is obtained by quantization of place cell activity package diffusion, this diffusion can be expressed by residual shift, it is derived from path integral of grid cell, path integral of grid cell is the derived from forward driving of stripe cell; so that residual shift is:

$\begin{array}{cc}\left[\begin{array}{c}\delta X_f\\ \delta Y_f\end{array}\right]=\left[\begin{array}{c}{k}_m{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\cos \theta -\delta X_0\\ k_n{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\sin \theta -\delta Y_0\end{array}\right]& \left(19\right)\end{array}$

Residual value is the piecewise function of residual shift:

$\begin{array}{cc}{\alpha }_{\mathrm{mn}}=g\left(\delta X_f,m-\delta X_0\right)g\left(\delta Y_f,n-\delta Y_0\right)& \left(20\right)\\ g\left(p,q\right)=\{\begin{array}{cc}1-p,& q=0\\ p,& q=1\end{array}& \left(21\right)\end{array}$

Step (5-3) matching of view template

Cognitive map generated by path integral has large error in wild area, it can not generate precise cognitive map, the present disclosure uses kinetic to collect RGB map and depth figure, proceeds closed-loop detection, when closed-loop is detected, RGB-D map as visual cues is used to correct the error of path integral and reset navigation cell series activity; when illumination changes, RGB map will be influenced, but depth map will not be influenced by illumination, closed-loop detection and new environment cognitive can be finished by comparing depth map and RGB map.

Visual template matching algorithm uses scan line intensity distribution of color and depth map; scan line intensity distribution is a one-dimensional vector, it is normalization of the sum of each line of the grey-scale map. Scanline intensity distribution map of an image is shown in FIG. 7; a represents the color map, b represents depth map. Scanline intensity distribution of figure which is collected by robot during detection process is stored as partial visual template, compare scan line intensity distribution of current figure with partial visual template which is stored before, if it matches successfully, so that we consider a closed-loop is found, it will be considered as a new visual template to store if it not matches.

Using average intensity absolute difference function to compare figure distribution and visual template; average intensity absolute difference between two scan line intensity distributions also called intensity shift, it is expressed as g(c):

$\begin{array}{cc}g\left(c,I^j,I^k\right)=\frac{1}{b-c}\left(\sum _{i=1}^{b-c}I_{i+\max \left(c,0\right)}^j-I_{i-\min \left(c,0\right)}^j\right)& \left(22\right)\end{array}$

I^j, I^k is scan line intensity distribution of figures which are compared, c is profile shift, b represents width of map;

Since map matching is highly influenced by illumination, lower the illumination influence to improve matching accuracy at different environment, color map and depth map matching are used simultaneously to ensure absolute position, since illumination intensity is different at different time in actual environment, different weight value is set to shift difference between color map and depth map, map fitness metric G can be obtained:G=μ_R|g_iR(c)−g(c)|+μ_D|μg_iD(c)−g(c)|  (23)

μ_R and μ_D are weight value of color map and depth map, μ_R+μ_D=1, in continuous map, the minimum shift value of I^j and I^k is c_m, it equals to the minimum G value of two maps;c_m=min_{cϵ[ρ-b,b-ρ]}(G)  (24)

Shift ρ ensures there is an overlapping between two maps; set map compare threshold is c_t, when c_m≤c_t, current map is a new map, it is stored in visual template set {Vⁱ}, when c_m≥c_t, it is considered as a repeating scene.

Step (6) construction and correction of cognitive map

Topological relationships between place cell discharging activities is established by cognitive map built in the present disclosure; it includes topological experimental point “e,” topological relation between experimental points is expressed as t_ij; each experimental point includes: place cell discharging activity pⁱ of current position, visual template Vⁱ; position of single experimental point is expressed as pⁱ; so that single experimental point is defined as:e_i={pⁱ,Vⁱ,dⁱ}  (25)

Step (6-1) empirical topological iteration

Set empirical threshold is S_th, a position metric D is obtained by comparing current position and position in experimental point:D=|pⁱ−p|  (26)

When position metric of current experimental point is higher than empirical threshold or new visual template is found, a new experimental point is built;

transfer quantity t_ij stores position change quantity which is calculated by path integral:t_ij={Δd^ij}  (27)

t_ij forms a new connection relationship between current experimental point and former experimental point, e_j={p^j,V^j,dⁱ+Δd^ij}, experimental point will not change during empirical iteration, it will only change when closed-loop is deceted;

Step (6-2) updating of empirical map in closed-loop position

When the actual closed-loop point is detected by the visual template, the robot returns to the same position, however, new experimental point generated by the accumulation of location variable in a closed-loop position not matches with this same position, to match them, all experimental point in the closed-loop position should be updated:Δdⁱ=θ[Σ_j=1^Nƒ(d^j−dⁱ−Δd^ij)+Σ_k=1^Nt(d^k−dⁱ−Δd^ki)]  (28)

θ is a correcting rate constant, N_ƒ represents the transfer number from experimental point e_i to another experimental point, N_t represents the transfer number from another experimental point to experimental point e_i; in actual experiment, set θ=0.5, higher value will lead to unstable of the whole map, update process of the whole map is continuously, but it is the most significant in closed-loop position;

Step (6-3) spatial cell discharging reset

When the closed-loop point is detected by the robot through visual template matching, discharging rate of the spatial cell is reset to its previous active state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an algorithm and schematic diagram of the present disclosure.

FIG. 2 is discharging rate of spatial cells involves in the present disclosure. Left diagram in A, C, D are discharging rate diagrams, the left diagram in B is polar coordinates of direction cells, this direction cell has the highest discharging rate in the southeast; Right diagrams are discharging rate diagrams of the locus.

FIG. 3 is a schematic diagram of information transferring and models involves in the present disclosure.

FIG. 4 is hardware structure in the present disclosure.

FIG. 5 is a schematic diagram of spatial cells attractor model; a is stripe cells single-dimensional cyclic attractor model, b is grid cells two-dimensional cyclic attractor model, c is place cells cyclic attractor plate.

FIG. 6 is a schematic diagram of place cells path integral. Black point is the central position of place cells attractor, the path integral of place cells attractor proceeds with the moving of grid cells.

FIG. 7 is a schematic diagram of visual template and scan line intensity distribution; a is color map collected by Kinect, b is corresponding depth map, c is scan line intensity distribution corresponding to a color map.

FIG. 8 is the robotic platform used in the present disclosure.

FIG. 9 is algorithm flow chart of the present disclosure.

FIG. 10 is a 2 m*2 m experimental environment in embodiment 1. Redline is real motion track of robot.

FIG. 11 is path integral map and place cells expression map at last in embodiment 1, red point in 10b is discharging rate point of place cells.

FIG. 12 color diagrams collected in a 3 m*10 m experimental environment crowded by many similar color diagrams in embodiment 2.

FIG. 13 is scanning top-view of the locus in a 3 m*10 m experimental environment in embodiment 2; blue line is the real track of robot.

FIG. 14 is map construction process in the present disclosure.

FIG. 15 is reset process of closed-loop detection and spatial cells discharging.

FIG. 16 is scanning top-view of the cyclic building which radius is 35 m; blue line is the real track of robot.

FIG. 17 is a cognitive map and elemental maps formed by robot at 1050 s in the present disclosure.

FIG. 18 is comparison diagram between path integral map and cognitive map.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present disclosure discloses a navigation map building algorithm based on cognitive mechanism of rat hippocampus, it uses a few sensors to build precise and universal robot navigation map through bionic ways, some problems of SLAM algorithm to sensor is solved, such as high requirements of hardware, high calculation complexity, limit precision and low adaptability. Next, the present disclosure will be described in more details accompanied with preferred embodiments.

Robot in FIG. 8 is used to build a map for all embodiments. Moving device includes two front-wheel and one back-wheel, back-wheel is a small steering wheel, it is convenient for steady support and turning of the robot. Front-wheel equips photoelectric encoder to record and collect the robot moving velocity. Moving direction of the robot is collected by the built-in gyroscope.

Kinect is placed on the surface of the platform; it uses the inverter for electricity supply when the robot is moving, the RGB-D image is collected. Kinect connects PC directly and collects an RGB and a depth image with 2 FPS. The whole platform communicates with the robot through USB interface. Set the maximum velocity of moving robot platform is 0.5 m/s.

Spatial cells network initialization. Set the number of head direction cell is 360, λ_net represents uniformly distributed sampling with an interval of 1 from 12-52. w₀ represents adjustment vector a=1.1, k represents study rate is 0.0005, inhibited coefficient B=0.5. Set stripe head direction of stripe cells as θ_i=[0° 90° 180° 270°], to obtain hexagonal grid field, recursive weights connection of grid cells needs to be initialized, cells at the border of neural plate connects with cells at contrary border which is shown in FIG. 5, neural forms a cyclic surface, hexagonal grid field is formed by cyclic surface distortion. Based on the equation w_ij^g=w₀−−s_θα(t), position of neure “i” is =x_ix, x_iy, x_ix=(i_x−0.5)/N, x_iy=(i_y−0.5)/N, i_x, i_xϵ{1 . . . , N}. Distance between nerues on grid cells neural plate is called the induced metric, it is expressed as dist(.,.), it is an Euler paradigm in two-dimensional manifold, it can be expressed as:dist(,):=∥−∥_tri=_j=1⁹ min∥−+offset_j∥  (29)

offset_j represents preset offset to realize cyclic distortion, it occurrence is: ofset₁:=(0, 0); offset₂:=(−1, 0); offset₃:=(1, 0); offset₄:=(0, 1); offset₅:=(0, 1); offset₆:=(i, 1); offset₇:=(i, 1) offset₈:=(i, −1); offset₉:=(−1, −1); ∥⋅∥ represents Euler paradigm.

Weight initialization from grid cells to place cells is based on the number of neural plate of grid cells, set number M=80 in simulation experiment,

weight initialization of each layer is 1/M.

Parameter setting of place cells and visual image matching are shown in table 1. Table 1 is preferences of the present disclosure.

Mode of execution below is used in all embodiments

Step 1, data collecting, make the robot move in the environment, collects image information of velocity, direction, and position. Collection period is 500 ms.

Step 2, path integral of spatial cells. Three kinds of spatial cells attractor models is shown in FIG. 5, at moment “t,” velocity of robot in direction φ(t) is ν(t), so that ν_θ(t) and displacement D_θ(t) in direction θ is:ν_θ(t)=cos(θ−φ(t))ν(t)D_θ(t)=∫₀^tν_θ(τ)dt

Direction displacements are transformed into stripe cells to move in preferred direction θ, x_θα is discharging rate of stripe cells, it means in θ direction, the phase of stripe is α; discharging period of stripe cells is 1; stripe cells have the highest discharging rate at periodical position nl+α (n is integer) in preferred direction, its′discharging rate can be expressed as:

$x_{\theta \alpha }\left(t\right)=e^{\left(-\frac{{\left({\omega }_{\theta \alpha }\left(t\right)-\frac{l}{2}\right)}^2}{2{\sigma }_s^2}\right)}$

Step 3, grid cells attractor proceeds periodical environment encoding driven by front-projection of stripe cells discharging rate.

Weight connection between grid cells forms grid cells attractor,w_ij^g=w₀−−ω_θα(t)Initial weight is set as:w₀=ae^{−γ|{right arrow over (x)}|2}−e^{−β|{right arrow over (x)}|2}

Grid cells discharging rate is determined by recursive connection and front-projection:

$\tau \frac{{\mathrm{ds}}_i}{\mathrm{dt}}=f\left[{\sum }_j{w}_{\mathrm{ij}}^g{s}_j+x_{\mathrm{\theta \alpha }}\left(t\right)\right]-s_i$

Step 4, grid cells generates place cells codes verse specific spatial position through competitive Heb study network.

A subset of grid cells series activity of generated place cells firing field is determined by using competitive Heb study.

$\frac{{\mathrm{dw}}_{\mathrm{ij}}^{\mathrm{eh}}}{\mathrm{dt}}={\mathrm{kp}}_i\left(s_j-\langle \mathrm{sj}\rangle \right)$

s_j represents grid cell discharging rate, ⋅ represents average discharging rate of grid cell, the right side of above equation determines change direction of weight: if current grid cell active rate is higher than average active rate which is input, synaptic connections is enhanced; otherwise, synaptic connections is inhibited. Through this kind of competitive study, grid cells series which exceeds average activation rate is determined as place cells discharging rate.

Place cells series activity is derived from projecting information of grid cells:p_i(r)=A·ƒ[Σ_j=1^Mw_ij^ehs_j(r)−C_inh]

A and C_inh are gain constant and inhibitive constant of place cell network, M represents number of layers of grid cell neural plate, w_ij^eh represents synaptic connection weight value from grid cell series “j” to place cell series “i”, “r” represents current position of the animal; C_inh is used to control the number of firing field of place cell, it is determined by B*maxp_i(r); it is ensured that there are few subsets of grid cell series is selected as single firing field of place cell through competitive algorithm mentioned above.

Step 5, discharging rate and discharging position determination in attractor surface of place cells.

measurement model at relative location of the actual external environment is built by place cell attractor model; two-dimensional continuous attractor model is on a neural plate through partial excitatory, partial inhibitive connection and overall inhibitive connection to form a random activity bump. This attractor is driven by the spatial cell path integral system; it is reset by image information of current position. Activity bump is shown as a grey neuron in FIG. 5; it is similar to cyclic grid cells attractor model. Place cells at the border of network connect with place cells at another border to become cyclic.

Excitatory weight value connection matrix ε_m,n of place cell is built by a two-dimensional Gaussian distribution, “m, n” represents the distance between abscissas of belonging units in coordinates X and Y; the distribution of weight value can be expressed as:ε_m,n=e^{−(m2+n2)/kp}

k_p represents width constant of position distribution;

A variable quantity of place cell activity resulted from partial excitatory connection can be expressed as:Δp_EX,EY=Σ_i=0^(nX−1)Σ_j=0^(nY−1)p_i,jε_m,n

n_X, n_Y represents the scale of two dimensional matrix of place cell in (X,Y) space, it represents the activity range of attractor on neural plate; since place cell is borderless in network which is shown in FIG. 5c, place cell which located at the border of neural plate will connect with place cell at the border of another neural plate to generate excitatory connection; the precondition of place cell iteration and visual template matching is: finding relative position of place cell attractor in neural plate, coordinate of this relative position can be expressed by the subscript of weight value matrix:m=(X−i)(mod n_X)  (13a)n=(Y−j)(mod n_Y)  (13b)

Each place cell also receives an overall inhibitive signal from the whole network; it is different from the grid cell sombrero attractor model; an inhibitive signal of place cell occurs after partial excitatory connection instead of working at the same time; symmetry between excitatory and inhibitive connection matrix guarantees suitable neural network kinetics. It ensures attractor in space cannot be excited without limitation. A variable quantity of place cell activity resulted from inhibiting connection can be expressed as:Δp_X,Y=Σ_i=0^nXΣ_j+0^nYp_i,jψ_m,n−φ

ψ_m,n represents weight value of inhibitive connection, level of overall inhibition is controlled by φ;

To ensure all place cell activity is no less than zero at the moment of “t+1”, compare place cell activation rate with 0:p_X,Y^t+1=max{p_X,Y^t+Δp_EX,EY+Δp_IX,IY,0}

Then, proceed normalization for place cell activation rate:

$p_{X,Y}^{t+1}=\frac{p_{X,Y}^t}{\sum _{i=0}^{n_X}\sum _{j=0}^{n_Y}{p}_{i,j}^t}$

Step 6, place cells path integral

Place update of place cells is driven by upstream cortex spatial cells path integral, discharging rate of path integral attractor at the previous moment is determined by offset and current discharging rate of place cells attractor, this process is shown in FIG. 6. So that discharging rate of place cells at next moment can be expressed as:p_X,Y^t+1=Σ_m=δX0^δX0+1Σ_nδY0^δY0+1α_mnp_(m+X)(n+Y)^t

δX₀, δY₀ is rounded-down shift in X-Y the coordinate system, this shift value is determined by velocity and direction information:

$\left[\begin{array}{c}\delta X_0\\ \delta Y_0\end{array}\right]=\left[\begin{array}{c}\lfloor k_m{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\cos \theta \rfloor \\ \lfloor k_n{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\sin \theta \rfloor \end{array}\right]$

[⋅] represents rounded-down, k_m, k_ni are path integral constant, {right arrow over (e)}_θi is unit vector headed to θ_i, θ represents current head direction, place cell activation rate in next moment is obtained by current place cell activation rate times residual value α_mn, residual value is obtained by quantization of place cell activity package diffusion, this diffusion can be expressed by residual shift, it is derived from path integral of grid cell, path integral of grid cell is derived from forward driving of stripe cell. So that residual shift is:

$\begin{array}{cc}\left[\begin{array}{c}\delta X_f\\ \delta Y_f\end{array}\right]=\left[\begin{array}{c}{k}_m{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\cos \theta -\delta X_0\\ k_n{\stackrel{\rightharpoonup }{e}}_{{\theta }_i}v·\sin \theta -\delta Y_0\end{array}\right]& \left(19\right)\end{array}$

Residual value is the piecewise function of residual shift:

${\alpha }_{\mathrm{mn}}=g\left(\delta X_f,m-\delta X_0\right)g\left(\delta Y_f,n-\delta Y_0\right)$ $g\left(p,q\right)=\{\begin{array}{cc}1-p,& q=0\\ p,& q=1\end{array}$

Step 7, matching and construction of visual template of the current point.

Robot collects color and depth map in current point through Kinect, calculating image scan line intensity distribution of current point, first, transfer the color image into the gray image, then, sum all intensity in each line of gray level image and normalization. A scan line intensity distribution image is shown in FIG. 7, 7a is a color map, 7b is depth map.

Image scan line intensity distribution which is collected by a robot in the whole process is stored as a partial visual template, compare scan line intensity distribution of a current image with a partial visual template which is previously stored to ensure whether the robot reaches visited position.

Using average intensity absolute difference function to compare figure distribution and visual template; average intensity absolute difference between two scan line intensity distributions also called intensity shift, it is expressed as g(c):

$\begin{array}{cc}g\left(\begin{array}{ccc}c,& I^j,& I^k\end{array}\right)=\frac{1}{b-c}\left(\sum _{i=1}^{b-c}I_{i+\max \left(c,0\right)}^j-I_{i-\min \left(c,0\right)}^j\right)& \left(3\text{-}5\right)\end{array}$

I^j, I^k is scan line intensity distribution of figures which are compared, c is profile shift, b represents width of map.

Since map matching is highly influenced by illumination, lower the illumination influence to improve matching accuracy at different environment, color map and depth map matching are used simultaneously to ensure absolute position, since illumination intensity is different at different time in actual environment, different weight value is set to shift difference between color map and depth map, map fitness metric G can be obtained:G=μ_R|g_iR(c)−g(c)|+μ_D∥g_iD(c)−g(c)|

μ_R and μ_D are weight value of color map and depth map, μ_R+μ_D=1, in continuous map, the minimum shift value of I^j and I^k is c_m, it equals to the minimum G value of two maps;c_m=min_{cϵ[ρ-b,b-ρ]}(G)

Shift ρ ensures there is an overlapping between two maps. Set map compare threshold is c_t, when c_m<c_t, image is not matching, current map is a new map, it is stored in visual template set {Vⁱ}, when c_m≥c_t, it is considered as a repeating scene.

Step 8, construction of cognitive map

Topological relationships between place cell discharging activities are established by cognitive map built in the present disclosure; it includes topological experimental point “e.” Topological relation between experimental points is expressed as t_ij. Each experimental point includes: place cell discharging activity pⁱ of current position, visual template Vⁱ; position of single experimental point is expressed as pⁱ; so that single experimental point is defined as:e_i={pⁱ,Vⁱ,dⁱ}

Set empirical threshold is S_th, a position metric D is obtained by comparing current position and position in experimental point:D=|pⁱ−p|

When position metric of the current experimental point is higher than an empirical threshold or new visual template is found, a new experimental point is built. Environment experimental point is built gradually during robot exploring the environment.

Transfer quantity t_ij stores position change quantity which is calculated by path integral:t_ij={Δd^ij}

t_ij forms a new connection relationship between current experimental point and former experimental point, e_j={p^j,V^j,dⁱ+Δd^ij}, experimental point will not change during empirical iteration, it will only change when closed-loop is deceted.

Step 9, updating of empirical map in closed-loop position

When the actual closed-loop point is detected by the visual template, the robot returns to the same position, however, new experimental point generated by the accumulation of location variable in a closed-loop position not matches with this same position, to match them, all experimental point in the closed-loop position should be updated:Δdⁱ=θ[Σ_j=1^Nƒ(d^j−dⁱ−Δd^ij)+Σ_k=1^Nt(d^k−dⁱ−Δd^ki)]  (28)

θ is a correcting rate constant, N_ƒ represents the transfer number from experimental point e_i to another experimental point, N_t represents the transfer number from another experimental point to experimental point e_i; in actual experiment, set θ=0.5, higher value will lead to unstable of the whole map, update process of the whole map is continuously, but it is the most significant in closed-loop position.

Embodiment 1

The robot moves in the shape of Arabic numerals “8” for 399 s in a 2 m*2 m experimental environment which is shown in FIG. 10.

Final generated path integral map and place cells discharging rate expressing map is shown in FIG. 11. It is obviously that path integral map cannot express the passing environment correctly. However, place cells discharging rate map can express the passing environment perfectly.

Embodiment 2

3 m*10 m experimental environment crowded by many similar color diagrams, the robot moves around the environment until finish exploring from 16 s to 170 s. The experimental environment of embodiment 2 is shown in FIG. 13. Effective diagram of a cognitive map built by this model is shown in line 2, row 3 of FIG. 13.

It can be seen, at last, a precisely cognitive map for the whole environment is built on the model. However, original mileage nomograph has a large deviation with the environment. The construction process of the map is shown in FIG. 14. The first line is mileage nomograph map collected by a robot; the second line is construction process of cognitive map based on the present disclosure. As is seen in the figure, when there is no closed-loop point is detected, there is no difference between mileage nomograph map and cognitive map, at 89 s, environment close-loop point is detected, cognitive map adjusts itself at 89.5 s, as time goes on, deviations of mileage nomograph map becomes larger and larger. Since there is closed-loop detection during the construction process of cognitive map, it is closer to actual motion track.

Process of closed-loop detection and reset of spatial cells discharging is shown in FIG. 15, the first line is cognitive map and place cells discharging rate at 7.5 s, at 89 s, environment close-loop point is detected (in line 2), the model adjusts itself at 89.5 s (in line 3) and proceed reset of discharging rate. It is remarkable that place cells series activity in FIG. 15 is not exactly the same with cognitive map point (cyclic in the first row), it because lace cells series activity express the relative position of the robot in the environment. The present disclosure is effective in the easily-confused environment.

Embodiment 3

Exploring a cyclic building (radius=35 m) with the method mentioned in the present disclosure to prove it is reliable in cognitive map building in the huge and complex environment, the front view of the cyclic building is shown in FIG. 16.

Robot in FIG. 8 and method in FIG. 9 is used to explore cyclic building with 1050 s. Exploring track is the blue line shown in FIG. 16. As is shown in FIG. 17, after comparing path integral map and final cognitive map, path integral map cannot describe current environment map precisely, but cognitive map describes current environment map precisely.

FIG. 18 describes evolutionary process of path integral map and cognitive map. The evolutionary process of the cognitive map with the time goes on, 18A is origin mileage map, 18B is cognitive map disclosed by the present disclosure. As we can see, before going through the first closed-loop, the cognitive map is same with origin map, however at t=280 s, origin map which is generated by path integral using origin mileage appears position ambiguity, it cannot cognize that the robot has been already back to the close-loop point. After correcting errors by using visual images, the actual cognitive map will describe current position precisely. It shows that reset of close-loop by using the RGB-D image is effective. Only when the close-loop point is detected, cognitive map in actual position will correct and update for origin map. In the end, a precise cognitive map for the actual indoor environment (shown in FIG. 16) is built on the model disclosed in the present disclosure; this map has been encoded metric and topology information of spatial position.

Embodiment 1, 2 and 3 have been proved that the present disclosure has good universality and validity. Embodiments herein can precisely generate cognitive maps for different environments.

Claims

1. A robot for constructing navigation maps using a mouse hippocampal place cell model, the robot comprising: a photoelectric encoder configured to collect velocity signals;a gyroscope configured to collect angle signals;an RGB-D sensor configured to collects color and depth images; anda CPU configured to perform acts to construct the navigation maps, the acts comprising: step (1) setting a head direction of the robot as 0 when the robot is static, collecting a transient velocity and angle information through the photoelectric encoder and the gyroscope, wherein images with the velocity of 2 frames per second are collected using a motion sensing input device,step (2) based on a linear velocity integral of stripe cells associated with the mouse hippocampal place cell model, collecting a motion linear velocity of the robot and angle information through the photoelectric encoder and the gyroscope, wherein at moment of “t”, the velocity in φ direction φ(t) is v(t) so that the velocity and displacement in νθ(t) and Dθ(t) in θ direction meet equation (1) and (2), respectively: νθ(t)=cos(θ−φ(t))ν(t)  (1)Dθ(t)=∫0tνθ(τ)dt  (2), anddirection displacements are transformed into the stripe cells to move in preferred direction θ, xθα is discharging rate of the stripe cells in θ direction, the phase of the stripe cells is α; a discharging period of the stripe cells is 1, the stripe cells have the highest discharging rate at a periodical position nl+α in the preferred direction such that a distance of periodical discharge resetting of the stripe cells represents ωθα(t), which is space phase difference between Dθ and α and is calculated using equation (3): ωθα(t)=(dθ(t)−α)mod l  (3)wherein a discharging rate of the stripe cells is calculated using equation (4):