Leaving aside the algorithm, if you want a dirt-simple rig for initial testing of purely your vertical position control, here's an (untested) thought: A simple compressed-air rocket, sliding on a vertical rod, fed by a long loop of flex-hose, might let you work out the initial control bugs cheaply.
Henry On 2/16/2015 4:18 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