diff(x2(t), t) = k5*x4(t) - k3*x2(t) - k6*x2(t) + k1*x1(t) - k7*x2(t),
diff(x1(t), t) = u1(t) - k2*x1(t) - k1*x1(t),
diff(x3(t), t) = k3*x2(t) - k4*x3(t) + k2*x1(t),
diff(x4(t), t) = -k5*x4(t) + k6*x2(t),
y2(t) = s3*x3(t),
y1(t) = s2*x2(t)
