function [theta, s] = mass(si, dt, C, M, L, P, Td, dTheta)
s = si;
if ~isstruct(si)
	alpha = 1.0;
	if Td > 0
		alpha = dt/Td;
	end
	s = struct('e', 0, 'a', alpha, 'x', 0, 'gain', 0.8);
end

s.e = s.e * (1-((L*(0+dTheta))*dt)/(M*C));
s.x = (1-s.a)*s.x + s.gain*s.a*P*dt;
s.e = s.e + s.x;
theta = s.e/(M*C);
endfunction