I have plotted a unit step response to G(s) which is a transfer function representing an remotely operated vehicle (ROV).
The issue is that I'm trying to interpret the result. I would have expected the line to drop, showing the ROV dropping due to gravity etc. It doesn't, it climbs?

G(s)= -0.125(s+0.452)/(s+1.25)(s^2+0.234s+0.0163)
Unit Step Response : https://app.box.com/s/sxpd9boxnqm3qhokptrl

Could anyone explain why the plot rises the way it does?

 

your mathmatical expression does indeed return negative values.

could the problem be in the graphing applications interpretation of the expression. (are you sure it is entered the way it should be for that application)

 

I'm using Simulink. I used a step input block, entered G(s) as a transfer function block & then put the output to scope?

 

not familar with simulink...whatever it is doing, it is not performing the calculations correctly. try simplifying your function just to see the output is correct then add more elements until you get the problem portion isolated

 

This is the unit step response I obtained....

No issues as far as I can see. Kermit has probably indicated your likely source of error.

 

Yes I'd used wxMaxima to simplify the TF before entering it. It missed a '-ve'. Thanks for your input...
Step Response: https://app.box.com/s/0hz0i3fxel1knjzhl4az