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Artificial_intelligence_coursework_2

Artificial Inteligence, Computer Science, Middlesex University, London

Overview

In this project we should choose and implement an algorithm for simple text recognition from pictures. There were a set of standardized pictures of numbers.

Multi layer perceptron

Description

Single-layer perceptron (or just Perceptron) was one of the first attempts at simulating brain functions. The basic idea of perceptron algorithm is simple: the perceptron consists of n-number of inputs with n-number of weights and a transfer function. The weighted inputs are summed up, and the sum is passed through a transport function which returns one result of the perceptron. The weights indicate an importance of the particular input. There are many kinds of transfer functions: step function, a sigmoid function, linear function, etc. The only one perceptron is a linear classifier, thus it would not be efficient for our problem, so I use the extended version which is multi-layer perceptron. In that case, many perceptrons are connected into neural network. A typical neural network based on multi-layer perceptron has one input layer, one or more hidden layers and one output layer. All the perceptrons in one layer receive a signal from all the perceptrons from the previous layer, and they are connected with all perceptrons in subsequent layer Supervised learning is usually implemented using back-propagation algorithm which consists of two steps. Firstly there is a forward pass – the given inputs are evaluated and compared with predicted outputs. Secondly, in backward pass, the weights are updated to get the required output. I have used backpropagation algorithm described here https://www.root.cz/clanky/biologicke-algoritmy-5-neuronove-site/?ic=serial-box&icc=text-title (sorry, it is in the Czech language).

Training

The network initializes weights in the beginning to random double number in the range <-1.0; 1.0>, the threshold is set to be 1, learning rate 0.05. The network trains at maximum 2000 iterations or 2 minutes or the training result reaches defined value (depends what comes first). If the network stuck, mutation is applied.

Mutation

Sometimes it happens, that while learning the system is stuck in a local maximum. To prevent this situation, I monitor if the learning result getting better, if not a mutation is applied. It means that defined number of weights are set to random values.

Defining parameters

Quality of the networks depends on many parameters such as a number of hidden layers and number of neurons in each layer, γ, etc. To try to find the best combination of parameters I have used a genetic algorithm. There were 50 individuals in each generation and the program run for several hours. I stored configuration of the networks (individuals) and their results to text file so that I could analyse it later. The network can return results with accuracy about 96 % and these do not depend too much on the number of hidden layers (I tried it on range <1; 6> layers). For reasonable result ( > 90 %) it seems to be necessary to have more than 10 neurons in two hidden layers or at least 17 neurons in one hidden layer.

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A uni project for text recognition using MLP

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