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Date: Mon, 24 May 2010 17:54:52 -0400
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Subject: Re: [LML] Re: Legacy Trim Tab Servo Speed Control
From: paul miller <paul@tbm700.com>
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References: <list-4324353@logan.com>

I concur with Chris about reducing voltage.  The right way to do this =
would be with the full voltage and PWM (pulse width modulation) for the =
speed.  That way full torque is available but speed will vary.   In my =
past, I've done troubleshooting work on repairing the circuit boards on =
Cessna autopilots and that is exactly how they operate.  More =
importantly, if anyone uses such a device at altitude, the full voltage =
will prevent motor stall from the air gap thinning out.   Many of the =
older trim systems with dirty brushes and rotors fail at altitude but =
work perfectly fine on the ground for that reason.  If someone builds =
it, I would add it or try it or buy it.  Some additional comments on =
trim from the Cessna autopilot:
1) Trim is set to run faster up than down by about 20% for runaway and =
autopilot purposes
2) Airspeed controls the dual speed setting
3) on the C414A it took 32 seconds for full up to full down but that =
number has a lot to do with control if the trim starts to runaway.  My =
Legacy is 10 seconds and that is too fast.

If anyone wants the 28v circuit for such a system, I have the old =
manuals somewhere and the parts are simple discreet components with a =
1000 Hz oscillator for PWM frequency.   =20

Great thread, hope something comes of it.

Paul Miller
L2K