Design of Control Law for SMC

Hi,

My problem is as follows:

C1 = [C11 C12 C13 C14]' ;

X = [X1 X2 X3 X4]' ; % State Matrix

% Sliding Surface:

S(x) = C1'.X;

% Lyapunov Function:

V(x) = (1/2)[S(x)]^2;

% Input U is:

U = G[alpha.p + (1-alpha)q]; % Input Function

%{ where

p = X4 when X4(X4-X2) > 0

p = 0 when X4(X4 -X2) < 0

AND

q = X2 when -X2(X4-X2) > 0

q = 0 when -X2(X4-X2) < 0

%}

I want to design the control law for 'U' based on the conditions mentioned above so that the system states slide along the Surface S(x).

Thanks. Shilp