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Soft computing
Lecture 7
Multi-Layer perceptrons
Why hidden layer is needed
Problem of XOR for simple perceptron
X2
X1
(0,1)
(0,0)(1,0)
(1,1)Class 1
Class 1
Class 2
Class 2
In this case it is not possible to draw descriminant line
Minimization of error
Main algorithm of training
Kinds of sigmoid used in perceptrons
Exponential
Rational
Hyperbolic tangent
Formulas for error back propagation algorithm
Modification of weights of synapsesof jth neuron connected with ith ones,xj – state of jth neuron (output)
For output layer
For hidden layersk – number of neuron in next layerconnected with jth neuron
(1)
(2)
(3)
(2), (1)
(1)
(3), (1)
(1)
Example of implementation TNN=Class(TObject) public State:integer; N,NR,NOut,NH:integer; a:real; Step:real; NL:integer; // ъюы-тю шЄхЁрЎшщ яЁш юсєўхэшш S1:array[1..10000] of integer; S2:array[1..200] of real; S3:array[1..5] of real; G3:array[1..5] of real; LX,LY:array[1..10000] of integer; W1:array[1..10000,1..200] of real; W2:array[1..200,1..5] of real; W1n:array[1..10000,1..200] of real; W2n:array[1..200,1..5] of real; SymOut:array[1..5] of string[32]; procedure FormStr; procedure Learn; procedure Work; procedure Neuron(i,j:integer); end;
Procedure of simulation of neuron; procedure TNN.Neuron(i,j:integer);var k:integer; Sum:real;begincase i of 1: begin if Form1.PaintBox1.Canvas.Pixels[LX[j],LY[j]]= clRed then S1[j]:=1 else S1[j]:=0; end; 2: begin Sum:=0.0; for k:=1 to NR do Sum:=Sum + S1[k]*W1[k,j]; if Sum> 0 then S2[j]:=Sum/(abs(Sum)+Net.a) else S2[j]:=0;end; 3: begin Sum:=0.0; for k:=1 to NH do Sum:=Sum + S2[k]*W2[k,j]; if Sum> 0 then S3[j]:=Sum/(abs(Sum)+Net.a) else S3[j]:=0; end; end;end;
Fragment of procedure of learning For i:=1 to NR do for j:=1 to NH do begin S:=0; for k:=1 to NOut do begin if (S3[k]>0) and (S3[k]<1) then D:=S3[k]*(1-S3[k]) else D:=1; W2n[j,k]:=W2[j,k]+Step*S2[j]*(G3[k]-S3[k])*D; S:=S+D*(G3[k]-S3[k])*W2[j,k] end; if (S2[j]>0) and (S2[j]<1) then D:=S2[j]*(1-S2[j]) else D:=1; S:=S*D; W1n[i,j]:=W1[i,j]+Step*S*S1[i]; end;end;
Generalization
Some of the test data are now misclassified. The problem is that the network, with two hidden units, now has too much freedom and has fitted a decision surface to the training data which follows its intricacies in pattern space without extracting the underlying trends.
Overfitting
Local minima
Two tasks solved by MLP
• Classification (recognition)– Usually binary outputs
• Regression (approximation)– Analog outputs
Theorem of Kolmogorov
“Any continuous function from input to output can be implemented in a three-layer net, given sufficient number of hidden units nH, proper nonlinearities, and weights.”
Advantages and disadvantages of MLP with back propagation
• Advantages:– Guarantee of possibility of solving of tasks
• Disadvantages:– Low speed of learning– Possibility of overfitting– Impossible to relearning– Selection of structure needed for solving of
concrete task is unknown
Increase of speed of learning
• Preliminary processing of features before getting to inputs of percepton
• Dynamical step of learning (in begin one is large, than one is decreasing)
• Using of second derivative in formulas for modification of weights
• Using hardware implementation
Fight against of overfitting
• Don’t select too small error for learning or too large number of iteration
Choice of structure
• Using of constructive learning algorithms– Deleting of nodes (neurons) and links
corresponding to one– Appending new neurons if it is needed
• Using of genetic algorithms for selection of suboptimal structure
Impossible to relearning
• Using of constructive learning algorithms– Deleting of nodes (neurons) and links
corresponding to one– Appending new neurons if it is needed