function [ dy ] = derivative( y )
% derivative. Assume sampling rate = 10ms

dy = 0*y;
n = length(y);
for i=2:n-1
  dy(:,i) = ( y(:,i+1) - y(:,i-1) ) / 0.02;
end

dy(:,1) = 2*dy(:,2) - dy(:,3);
dy(:,n) = 2*dy(:,n-1) - dy(:,n-2);

end