Example 6.13   Level control  p.119

para1 = {kp ->  1, τ ->  1, r0 -> 1, 
        d0 ->  2}

para2 = {gr -> kp + ki/s, gp -> 1/(τ s + 1), rs -> r0/s, 
        ds -> d0/s}

hs = 
      Simplify[go/(1 + go) rs + gp/(1 + go) ds /. 
            go ->  gr gp /. para2]

ht1 = 
      InverseLaplaceTransform[hs /. ki ->  0 /. para1, s, 
        t]

ht2 = 
      InverseLaplaceTransform[hs /. ki ->  1 /. para1, s, 
        t]

Plot[{ht1, ht2, 1}, {t, 0, 10}, PlotRange ->  {0, 2}, 
      PlotStyle ->  {Hue[0.9], Hue[0.7], Hue[0.5]}]