wolfram mathematica - Manipulate does not accept constant (i.e. just number) as upper limit for variable -

In the code below, if I Senjejh (4 / ([Zeta] [omega])), 20 , Nothing has been plotted. Why? If I cast the two sliders at the beginning, then nothing has been done Plot

  ClearAll [\ [zeta], \ [Omega]] {slider [Dynamic [\ [zeta]] {0.1, 1.4, dynamic [\ [Omega]]} TF [\ [Omega] _ \ [Omega]], dynamic [\ [Zeta]] {Sleder [dynamic [\ [Omega]], {1, 5 0.1} [Zeta] _] = Hstantrnfnkshn model [\ [Omega] ^ 2 / (s ^ 2 + 2 \ [Zeta] \ [omega] s + [omega] ^ 2), S] F [Ti_] = Output Response [TF [\ omega], \ [jeta]], unitstep [t], t]; Manipulation [Plot [F], {T, 0, (4 / ([[Zeta] * [Omega]), PlotRange - & gt; {{0, (4 / (\ [Zeta] [[omega]))}, {0, 2}}], {{[Zeta], 0.2}, 0.1, 1.4}, {{\ [Omega] 1}, 0.5, 4}]    
 
 
  TF [o_, z_] = transfer function model [o ^ 2 / (s ^ 2 + 2 Jhojh + o ^ 2), S] F [Ti_, O_, Z_] = output response [TF [O, Z], Unitstep [t], t ]; Manipulation [Plot [F [T, O, Z], {T, 0, 20}, PlotRange - & gt; {0, 2}], {{z, 0.2}, 0.1, 1.4}, {{o, 1}, 0.5, 4}]