%
%  spring_sens_rhs
%

  function dzc = spring_rhs(t,z,params)

  C = params(1);
  K = params(2);
  A = [0 1;-K -C];

  factor = sqrt(K - C^2/4);
  dz = -C*exp(-C*t/2).*cos(factor*t) - 2*exp(-C*t/2)*factor.*sin(factor*t);
  F = [0;dz];

  dzc = A*z - F;