3.3.1. Exploration and exploitation
Exploration and exploitation are two basic strategies in the process of search optimization [
26,
30]. Among them, exploration refers to the searching strategy aimed at obtaining the information of objective function from the perspective of breadth in the searching process. On the other hand, exploitation refers to the searching strategy based on the function information obtained by exploration, which aims to find the optimal solution at the depth level.
In nature, animals homing in unknown environments through the exploration of unknown environment and the exploitation of historical information. The source seeking process can be described as follows: in the initial stage, animals may explore the environment through their own movement to obtain the distribution information of features to make up for the lack of environmental cognition; then use the accessed information, search for the source path, and respond to the change of the environmental features of distribution, search behaviors to maintain equilibrium between the exploration and exploitation, in order to obtain maximum profits (here, the benefits including the information of the cognition of environmental benefits and the optimization of target point of convergence), thus guide individuals to arrive at the target point.
Inspired by the homing behaviors of animals, we propose a balanced searching strategy, which focuses on the exploration and exploitation of environmental information, and focuses on the dynamic balanced of information returns and optimization returns, then combines different stages of searching to carry out equilibrium searching. In the early stage of searching, the environment information is explored through random roaming, and the trend movement is gradually formed. In the stage of source searching, the searching bias is dynamically adjusted between exploration and exploitation for the purpose of maximizing revenue.
3.3.2. The algorithm of source seeking
(1) Search behaviors design based on evolutionary algorithm
In the following, a source seeking algorithm is proposed based on the balanced searching strategy.
The searching process depends on the movement of the carrier, and its searching behaviors can be characterized by the motion parameter u. Drawing on the idea of evolutionary algorithm, the evolutionary population is constructed by taking the feasible searching behaviors as the sample of evolutionary population. So, the j-th sample individual can be defined as follow:
Where, is a random number; is searching space compression ratio; stands for population size; usually, set as ).
The carrier's searching for the target features of environment depends on the movement in space. Multi-target searching and the movement of the source space have the temporal characteristics as shown in the following
Figure 4.
Combined with the
Figure 4, the source seeking process can be described as follow. At the
k-time, select a certain
from the evolutionary population Pop(
k) with medium probability as the motion parameter of the carrier, and the carrier will obtain the motion displacement of
by executing
. The
and
at positions
and
were measured by carrier movement, then substitute it into Equation 5 for multi-objective solution and calculate multi-objective function
F. According to the convergence state of the multi-objective function
F, the source seeking performance of the executed samples is evaluated, and the breeding or elimination operation is used to increase or decrease the proportion of such samples. Through the mutation operation to improve the population diversity, a new population Pop(
k+1) is obtained, and the next round of searching process is re-entered. Through repeated iterations, the multi-objective function converges to the minimum and the source search task is realized.
(2) Search bias measure
The bias of search behaviors can be measured by the diversity of the evolving population. The higher population diversity, the more scattered the sample distribution in the population, resulting in greater randomness of the search behaviors, and the search behaviors is biased towards exploration at this moment; the lower population diversity, the more concentrated the distribution of samples in the population, resulting in less randomness in the search behaviors, which is biased toward development at this time. For measuring the populations diversity, the concept of distributed entropy is introduced here.
Definition 1: Distributed entropy: The sample type is class
N and the sample individual can be represented as
. At some point in the evolution of the group, the proportion of sample individuals in the group is
respectively, and satisfied
,then the distribution entropy is:
The distribution entropy is nonnegative, symmetric, and additive in the distribution space, and which is a strictly concave function. When all samples are uniformly distributed, H has a unique maximum value; When one type of individual dominates the population, that is , there are:
Distribution entropy is lowest currently.
The distribution entropy quantifies the population diversity and reflects the searching bias of the population. Exactly, the larger the entropy value is, the stronger the global exploration behaviors of the population will be. The smaller the entropy value is, the stronger the local exploitation ability of the population will be.
(3) The strategy of balance
Combined with the analysis of the source seeking path of implicit information field in
Section 3.1, the source seeking path will change with the spatial position. It is not conducive to the tracking of the source path when distribution entropy of the evolutionary population is too large or too small in the source seeking process. Therefore, we combine the structural characteristics of evolutionary algorithms and the distribution characteristics of source paths to give a specific algorithm of balanced searching strategy. We divided the source searching process into three stages (see Figure5).
Figure 5.
Schematic diagram of decomposition in source searching stage.
Figure 5.
Schematic diagram of decomposition in source searching stage.
In the first stage, the initial stage of source seeking, the searching behaviors is mainly exploration. When the population sample species at time k is greater than 1 time, it will enter the source seeking stage.
In the second stage, the process of source seeking, the searching behaviors changes dynamically between exploration and exploitation.
When the distribution entropy H is less than or equal to the highest threshold entropy , the searching process is exploitation-oriented to avoid too much random motion leading to the failure of the search. When the distribution entropy H is greater than or equal to the lowest threshold entropy , the process of source seeking enters the searching process dominated by exploration to avoid the premature population problem caused by low population diversity. Other time, the carrier carries out the search task according to the results of population evolution.
In the third stage, the end of source seeking. When the parameter space reaches and the real space reaches in Equation 3, the source seeking task can be completed.
So far, we give the source seeking method based on balanced searching strategy, and then combine mathematical analysis and experimental simulation to verify the effectiveness and rationality of the algorithm.