Abstract
This paper presents an implementation of Extreme Learning Machine (ELM) in the Multi-Agent System (MAS). The proposed method is a trust measurement approach namely Certified Belief in Strength (CBS) for Extreme Learning Machine in Multi-Agent Systems (ELM-MAS-CBS). The CBS is applied on the individual agents of MAS, i.e., ELM neural network. The trust measurement is introduced to compute reputation and strength of the individual agents. Strong elements that are related to the ELM agents are assembled to form the trust management in which will be letting the CBS method to improve the performance in MAS. The efficacy of the ELM-MAS-CBS model is verified with several activation functions using benchmark datasets (i.e., Pima Indians Diabetes, Iris and Wine) and real world applications (i.e., circulating water systems and governor). The results show that the proposed ELM-MAS-CBS model is able to achieve better accuracy as compared with other approaches.
Keywords
Introduction
The Extreme Learning Machine (ELM) has proven to be an efficient learning algorithm over the years as compared to the traditional learning methods in the aspect of generalization and learning speed [1, 2, 3, 4, 5, 6]. ELM is capable of making universal approximation with random input weights and biases [7]. In other words, the hidden neurons are not required in neuron alike and the weights are the parameters that need to learn the connection between the hidden layer and the output layer.
Based on Huang et al. [8], ELM is extraordinarily efficient and lean towards to global optimum as compared with the traditional feedforward neural network (FNN) [8]. In addition, ELM can reach the best generalization bound of the traditional FNN where all the parameters are learned with commonly used activation functions [9]. In terms of efficiency and generalization, the performance of ELM is far better than traditional FNN and has been experimented in different kind of problems. The application of ELM has also been exemplified in different fields such as biomedical analysis [10, 11], chemical process [12], system modeling [13, 14], power systems [15], action recognition [16], hyperspectral images [17], etc.
There is some research works focusing on ensemble model to combine individual prediction of multiple ELMs to give a final output [18, 19, 20, 21, 22]. This strategy is also adopted in a Multi-Agent System (MAS). MAS allows the subproblems of a constraint satisfaction problem to be subcontracted to different problem solving agents with their own interests and goals. Thus, MAS has been applied to tackle problems in different fields successfully in the past decade. This is evidenced by a widespread application of MAS to different domains including e-Commerce [23], healthcare [24], military support [25], decision support [26], knowledge management [27], as well as control systems [28]. The general structure of a MAS is shown i n the Fig. 1 where the base platform is built by a group of ELMs that are deemed as individual agents. In this structure, the outcome of an individual ELM (individual agent) is sent to a parent agent, which is the decision combination module to make the final decision.
Generally, the average output in the decision combination module is based on the methods such as exact average [20], weighted average [7], confusion matrix [29], and voting [30]. Unfortunately, those approaches required additional algorithms to generate outcome in the decision combination module.
Recognition and rejection accuracy rates based trust measurement has been proposed [21]. In the model, two teams were used where the first team consists of three modified Fuzzy min-max (FMM) agents and the second team consists of three modified Fuzzy ARTMAP (FAM) agents. The model was presented with better performances as compared with other approaches mentioned in [21]. Another trust measurement strategy based on Bayesian formalism with FMM MAS was proposed in [31]. In this model, the FMM is used as a learning agent in MAS and followed by combining with Bayesian formalism to obtain a trust measurement. The results show that the model is able to yield the better performances as compared with other approaches mentioned in [31].
In the recent development of MAS model for trust measurement, a method namely Certified Belief in Strength (CBS), which based on strength and reputation of individual FMM based agent [31]. During the training process, trust is the strong elements that are related with the FMM agents which let the CBS method to improve the performance of the MAS. As a result, the CBS improved the performance of the MAS model by improving the accuracy rates of the individual agents.
Nowadays, an element that plays an important role in daily life is trust and trustworthiness. Especially occur in our social environment. Basically the element allows the consignment of duties and decisions to applicable persons, who can execute the duties [32]. The element had been developed in few areas, such as in e-business filed by Mui et al. [33] and in wireless sensor networks by Boukerche and Li [34].
These papers propose an extended CBS method using Extreme Learning Machine based Multi-Agent System (hereafter denoted as ELM-MAS-CBS). The difference is that MACS CBS used FMM which consists of multiple hyperboxes while the proposed approach employs a “team” concept which involves a group of individual ELM-based agents.
This paper is organized as follows. The algorithms of ELM-MAS-CBS are explained in Section 2. The flow chart of ELM-MAS-CBS is showed in Section 3. Section 4 showed the results and discussion of the benchmarking datasets. The applications of ELM-MAS-CBS in power generation are presented in Section 5. Lastly, Section 6 is the conclusion.
A general structure of MAS.
In this paper, the ELM-MAS-CBS model consists of three layers as shown in Fig. 2, i.e., the first layer consists of several individual ELM-based agents; the second layer consists of several teams of ELM-based and The CBS is implemented in the individual ELM-based agents. Then, the Manager will select the team with the highest CBS as the final decision as the output. In this paper, the number of teams is set as 3 (
Overview of ELM-MAS-CBS model.
The step-by-step training procedure is given as follows.
where
where
Once all the samples are trained using Step 1 to Step 10, the ELM-MAS-CBS can be used for prediction of a newly arrived and unknown input vector
The proposed sequence of ELM-MAS-CBS model is summarized in Fig. 3.
Sequence of algorithms of ELM-MAS-CBS.
Throughout this paper, there were three benchmark datasets (e.g. Pima Indians Diabetes (PID), Iris and Wine) were used to test the performance of ELM-MACS-CBS. For all experiments, the number of teams had set as 3 (
Specification of benchmark datasets [20]
Specification of benchmark datasets [20]
In the experiment, three benchmark datasets were evaluated using the adopted train-validation-test method. The 60% of the PID samples were used for training while the 20% were used to determine the most appropriate number of neurons (i.e.,
Testing accuracy rates of ELM-MAS-CBS using sigmoid activation function
There are two types of activation functions, i.e., Radial Basis Function (RBFun) and Sigmoid activation function (SigAct) are used in each benchmark dataset. Table 2 showed the accuracy rates based on sigmoid activation function for PID, Iris and Wine. In addition, Table 3 also shows the accuracy rates based on radial basis activation function for the three benchmark datasets. Among the Tables 2 and 3, the number of hidden neurons,
Testing accuracy rates of ELM-MAS-CBS using radial basis activation function
In the Tables 2 and 3, the increasing number of hidden neuron is not improved the accuracy rate. This is because that this situation in called overfitting, where the neural networks overestimate the complexity of the targeted problem. On the other hand, it also greatly degrades generalization capability, which leads to significant deviation in predictions. By doing this, allocating the proper number of hidden neurons to prevent overfitting is critical in function approximation using feedforward neural network.
Summary for test accuracy rates of ELM-MAS-CBS
Table 4 summarizes the results for using ELM-MAS-CBS in terms of the test accuracy and the number of hidden neurons for two types of activation function in the benchmark datasets. The results showed that the RBFun has the highest test accuracy rate as compared with the SigAct.
The proposed ELM-MAS-CBS is compared with other ELM. Table 5 showed that the test accuracy rates of ELM-MAS-CBS are comparable (if not superior) with MACS-TNC [13] and MACS-CBS [20].
Comparison with other approaches
The ELM-MAS-CBS is applied on the power generation in following section.
Circulation water systems
Circulating water systems.
An overview of Circulating Water System (CWS) is shown in Fig. 4 [36, 37]. This system consists of piping, turbine condensers and drum strainer between the inlet of the sea water and the outfall where the water will be returned into the sea. The major component in the CWS is the turbine condenser where it is used to remove the heat from the low pressure steam while attempting to maintain turbine backpressure at the lowest possible yet constant level.
A total number of 2500 data samples were collected and they were categorized into training, validation, and testing sets, as shown in Table 6 [38]. The proposed ELM-MAS-CBS was trained and validated to determine the optimal number of hidden neurons before it was tested. The results of test accuracy are listed in Table 7. The highest test accuracy of ELM-MAS-CBS is 96.92% and it was achieved by training ELM-MAS-CBS with a Radial Basis activation function. The proposed ELM-MAS-CBS trained using a Radial Basis activation function is compared with other classifiers, which include FAM [37] and SVM [38]. From Table 7, the test accuracy rate of ELM-MAS-CBS is comparable (if not superior) with FAM [37] and SVM [38].
Specification of Benchmark Dataset in CWS [38]
Governor model, GAST [40].
Comparison of CWS dataset
The GAST is one of the governor model [40]. It represents the principal dynamic characteristics of industrial gas turbines driving generators connected to electric power systems. Speed variations from nominal are expected to be small (approximately 5%). The model shown in Fig. 5 consists of a forward path with governor time constant,
The training data is collected on the output of the GAST block which is the mechanical power,
Summary of dataset in GAST
Summary of dataset in GAST
Details of the training, validation, and testing of the GAST dataset
Table 10 summarizes the results for using ELM-MAS-CBS in terms of the training time (seconds), test accuracy, and the number of hidden neurons for each activation function in GAST.As the results, the best test accuracy rate is 83.04% in Sigmoid activation function.
Test accuracy rates for two activation function in GAST
In this research, a new ELM-MAS-CBS model with three layers of ELMs has been developed. These papers propose an extended CBS method using Extreme Learning Machine based Multi-Agent System (hereafter denoted as ELM-MAS-CBS). The difference is that MACS CBS used FMM which consists of multiple hyperboxes while the proposed approach employs a “team” concept which involves a group of individual ELM-based agents.
The developed model is validated by using benchmark datasets which are Pima Indians Diabetes (PID), Iris and Wine. In the Table 5, the test accuracy for PID and Wine are higher when compare with other approaches but is lower in Iris. Therefore based on the outcome, the test accuracy rates of ELM-MAS-CBS are comparable (if not superior) with MACS-TNC [13] and MACS-CBS [20]. Not only that, the developed model also applied its application on the power generation which are circulating water systems and governor (GAST). The experimental results showed that the test accuracy rates of ELM-MAS-CBS for circulating water systems is comparable (if not superior) with other algorithms in which the proposed model is higher than FAM [37] and lower than SVM [38].
Although the results obtained from the benchmark studies on Pima Indians Diabetes (PID), Iris and Wine and applications in power generation (circulating water systems and governor, GAST) are encouraging, more studies using datasets from various application domains are required to validate the applicability of ELM-MAS-CBS in real world application. In addition, investigation of proposed model in nonstationary applications by replacing ELM of ELM-MAS-CBS with OSELM (online sequence version of ELM [7]) could be another research for further work.
The generalized activation functions are added to ‘future works’. For example, the generalized RBF (GRBF) activation function can continuously and smoothly reproduce different RBF by changing a real parameter. In addition, the generalized Gaussian distribution for GRBF can add a shape parameter to normal Gaussian distribution [41]. Therefore, a better corresponding between the shape of the kernel and the distribution of the distances.
