[AR] Re: hovering rocket vertical position control
- From: Nathan Mogk <nm8911@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Tue, 17 Feb 2015 17:01:26 +0000
As I understand it, you typically have a transfer function which relates
your control inputs to your acceleration, similar to the transfer function
that relates instrument measurements to position/velocity determinations.
You seed it with reasonable initial values, and then let the controller
estimate the actual parameters as they change over time, using a Kalman
filter as Ben suggested.
Another slightly related question, what sort of control laws have people
used for their hovering vehicles? Has anyone used something similar to
http://arc.aiaa.org/doi/full/10.2514/1.58098? (Sorry if there is a paywall.
I am on a university network, and I can't tell). The control scheme uses
position and velocity waypoints to approximate the optimal trajectory for a
given time-to-go. That particular control scheme was used initially for
missile interceptors, but I think it would work well for hovering vehicles.
On Mon Feb 16 2015 at 9:07:11 PM Ben Brockert <wikkit@xxxxxxxxx> wrote:
> I think the Carmack approach was beyond option 3, to the point that
> the code didn't have a throttle valve map. It just went "my position
> (or velocity) is wrong, bang on this digital line until it has moved
> some distance" in a PID way.
>
> The model-based approach is to have a chamber pressure sensor. From
> that you know thrust, and from that (and static testing and first
> principles analysis) you have mdot. Start with a mass estimate and
> have it continue tweaking the mass estimate during flight, using the
> mdot as a known input and acceleration as a corrective factor. Then
> when you want a given acceleration you:
>
> (desired thrust)=(mass estimate)(desired accel)
> (desired throttle position)=f(desired thrust)
> (pwm signal)=f(desired throttle position, current throttle position)
>
> It is a more complex approach but can give much sharper results. The
> PID control for the final throttle valve control signal can be tuned
> on the ground without spending propellant or risking the vehicle. As
> can the acceleration-based mass correction, to a degree.
>
> Of course the likely candidate controller for a scenario when you
> think you know something but you're getting noisy input that suggests
> a difference is a Kalman filter.
>
> Ben
>
> On Mon, Feb 16, 2015 at 6:18 PM, <rsteinke@xxxxxxxxxxx> wrote:
> > I have a question about vertical position control for a hovering rocket.
> > One option is to do a PID control loop where the manipulated variable is
> > vertical acceleration and the controlled variable is vertical position.
> The
> > thing is, you aren't really manipulating vertical acceleration. You are
> > manipulating the throttle valve. You've got a few confounding factors in
> > the mapping from throttle valve position to thrust, and net thrust needs
> the
> > vehicle's weight subtracted out and weight is always changing as
> propellant
> > is consumed. I've thought of a few ideas to deal with this:
> >
> > 1) Do a lot of testing and analysis and get a really good educated guess
> of
> > thrust mapping and propellant usage. Any errors in those get handled by
> > integral gain in the vertical acceleration to vertical position
> controller.
> >
> > 2) Have another controller where the manipulated variable is throttle
> > position and the controlled variable is vertical acceleration.
> >
> > 3) Cut out the middle man and have a single controller from throttle
> > position to vertical position. I don't even know what the gains on such
> a
> > controller would look like without knowing where the "zero point" is of
> zero
> > net vertical thrust.
> >
> > What have other people done? Are there other ideas?
> >
> > Thanks
> > Bob
> >
>
>
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