Autobench how to define a decreasing profil load

Question

I'm currently using autobench to test a personal web server. I want to define a scenario with 3 main steps:

Step 1: generate an increasing request load. For example from low_rate=10 to high_rate=100 with a rate=10 ==> Everything is OK here

Step 2: I want to be able to have a "fix" CPU load. For example if at the end of Step 1 my CPU load is around 50 I want my server to stay at a load of 50 for almost 3mn before decreasing ==> is it possible to do it with autobench? If yes, how can I do it? Else, can you propose some useful tools to active this step? Step 3: I want to be able do

Step 3: I want to be able to generate load from the current CPU load to 0. For example, configure autobench like this: low_rate=100 high_rate=10 and rate=-10 (negative value) ==> It is not working? Is it possible to do it with autobench? If yes how? Else, and you give me some advises to perform my experience? Sorry for my English!