Thanks Jim, that makes sense. If we reduced the load's inertia (halving rudder weight), then the required res. frequency should increase. It assumes that the motor's inertia doesn't change. Or conversely, if the motor's inertia was increased (cored motor rather than coreless), the effect would be similar. In parallel, if K were increased by using a stiffer transmission set-up, resonant frequency should increase, hence the comments whether or not any one used a carbon push rod for rudder
I didn't measure the oscillation frequency on mine but if I could hazard a guess I'd say it was around 10 cycles per second (10 Hz)
MattK
ORIGINAL: OhD
ORIGINAL: MTK
Jim O,
I have had brand new 8411's behave similarly to what Eric Henderson described above. Very rapid and extreme oscillation at neutral, on other surfaces besides rudder, on pattern and larger planes alike. I sent them back and was told that the dead band at neutral needed to be broadened. Yet some (most?) 8411's do not display that oscillation so maybe there is something to that. As an old Radio man, you probably know better than anyone. I would really like to know what the answer is.
MattK
ORIGINAL: OhD
Eric,
Has anyone tried replacing the cables in the pull pull with carbon fiber rods as used in the elevator set-up? Seems like we don't want any thing that works like a spring in our control system. We need to get the natural frequency of the mechanical system up higher than that of the servo. I'm using two solid pushrods on the rudder of my 2.6M Extra (to two servos) and there is no sign of oscillation and there is zero slop. Just a thought. I bought 8411s for my Impact but haven't decided how I'm going to install it.
Jim O
Matt,
I don't understand everything I know about servomechanisms but here are some basics:
To eliminate oscillations, the mechanical resonant frequency of the system should be ten times the servo loop bandwidth. That says we want the resonant frequency as high as possible. It is defined by
F= {the square root of [K(Jload+Jmotor)/Jload*Jmotor]}/2pi
F= resonant frequency
J= inertia
K=transmission stiffness
So we want K to be as stiff as possible and the ratio of load inertia to motor inertia to be low. The gear train makes the reflected load inertia look lower as would mechanical advantage in the linkage but we don't have much control over that in a pull pull system so we need to keep the inertia of the rudder low.
By the way we want the servo bandwidth high for good response. I suspect the band width is less than 3 HZ so we'd like the resonant frequency up above 30 HZ. Did anyone measure or estimate the frequency of the oscillations?
Our RC servo systems are non-linear from a servo analysis stand point so they are very difficult to analyze, but I bet we have some young engineers out there that could do it. I've got to go build my Impact.
Jim O