Step is plotting backwards

Im trying yo get the step response the transfunction:

                 244.2 s + 244.2
  -------------------------------------------------
  0.015 s^4 + 1.525 s^3 + 2.51 s^2 + 245.2 s + 1221

but the resulting plot is something kind of reversed:

Why is this happening? is there a way to solve it?

Also stepinfo give me wierd values:

RiseTime: NaN SettlingTime: NaN SettlingMin: NaN SettlingMax: NaN Overshoot: NaN Undershoot: NaN Peak: Inf PeakTime: Inf

Answer 1

[Star Strider]

The step function is correct.

Your system is unstable.

s = tf('s');
sys = (244.2*s + 244.2) / (0.015*s^4 + 1.525*s^3 + 2.51*s^2 + 245.2*s + 1221);
sys_poles = pole(sys)

sys_poles =
      -101.53 +          0i
       2.2371 +      12.99i
       2.2371 -      12.99i
      -4.6143 +          0i

You have a conjugate pair of poles in the RHP.

[Andres Yie]

Oh. I did not know the Step function ploted unstable systems that way.

Thanks!

[Star Strider]

My pleasure!

Answer 2

[Les Beckham]

This transfer function is unstable. It has a pair of complex (oscillatory) poles that are slightly in the right half plane. Thus, any disturbance (like a step) will cause the response to 'blow up'.

>> damp(sys)|

         Pole              Damping       Frequency      Time Constant  
                                       (rad/seconds)      (seconds)    

 -4.61e+00                 1.00e+00       4.61e+00         2.17e-01    
  2.24e+00 + 1.30e+01i    -1.70e-01       1.32e+01        -4.47e-01    
  2.24e+00 - 1.30e+01i    -1.70e-01       1.32e+01        -4.47e-01    
 -1.02e+02                 1.00e+00       1.02e+02         9.85e-03|