APM_Control: AP_RollContoroller: move rate limit

[IamPete1]

this moves the rate limiting from _get_rate_out up to get_servo_out. This brings it inline with the pitch controller and removes the rate limit with using get_rate_out in acro mode.

fix for: #9192

[WickedShell]

I'm not a control guy, but doesn't this open you up to large spikes being driven into the integrator that can take quite awhile to remove later?

[IamPete1]

I'm not sure I follow, functionality should be the same, except when get_rate_out is used directly in acro mode. I that case the max rate is set by the acro parameter. There were complaints that acro mode was still limited by the max rate and the acro rate param was ignored.

[WickedShell]

This is removing the limit on the error calculation, which is used to update the integrator. This means a single large roll change (say large roll left to large roll right) can create a very large error that gets put into the integrator, and doesn't have an effect on the actual vehicle control, as the P or FF term may be sufficient to saturate it). As I said though I'm not a controls person, so I may be misunderstanding the problem, or the behavior I've described is actually good.

[IamPete1]

it moves the limit from _get_rate_out to get_servo_out all modes except acro use get_servo_out so should be unaffected.

[WickedShell]

... I hadn't opened quite enough context on get_servo_out and had forgotten that it is just piping to get_rate_out sorry, my objections make no sense :)

[tridge]

I'm not convinced that disabling the rate limiter in ACRO mode is the right thing to do. I would support having a way to set the acro rate limit separately though. Maybe we should have an optional parameter which is a rate limit for get_rate_out()?

[tridge]

hmm, I guess the acro rate is already a parameter ...
ok, I'm convinced!