Keith,
After a lot of research, I started by choosing the particular actuator
because it contained both A) internal limit switches, eliminating the need
for potentially fragile external microswitches, more wiring, etc., and B) an
internal position-sensing potentiometer, to support external programming of
more or less precise stepped positioning. Given those two primary
requirements, this particular model had the appropriate range, speed, torque
and load specs.
The actuator's end-to-end travel is 2". The point of connection (mixer arm
forward hole) required ~1-5/8" vertical motion to cover all but a smidgen of
the motion between the mixer arm stops, so I had to reduce ("gear down") the
actuator's limit-to-limit 2" travel to 1-5/8" of movement at the mixer arm
forward hole connection point. I could have done that by approaching the
mixer arm "directly" from above as you suggest, but with the actuator
"leaning" backwards at an acute angle to the mixer arm -- enough to reduce
the motion at the mixer arm from 2" produced by a perpendicular approach to
the mixer arm. However, leaning the actuator back would have put it in the
midst of a popular window location, and required a fair amount of new
support structure there. One motivation for the electric drive in the first
place was to reduce weight over the Teleflex cable, so making elaborate new
structure where I wanted a window didn't seem right. Instead, I chose to
"gear down" the range of travel by (painstakingly) calibrating lobes of a
bell crank. Even though I started with a mock up of the pivot geometry on a
piece of plywood, it took several bell cranks before I got the travel
reduction just right. Six attempts, actually, but in the end, the mixer arm
moves through all but about 1/16" of its total travel available at the mixer
arm stops, and stops just before those via the actuator's internal limit
switches.
To summarize, I started with the actuator I wanted, then designed the
mounting so that actuator would work without impeding a future window. This
did result in some tight clearance issues between the actuator, bell crank,
and vertical push-pull tubes, and all those things and the bell crank
dimensions and location took quite awhile to resolve with the right
hardware. {Ed.: Hey, that's a gross understatement!}
As noted, I haven't flown it yet, but I'm happy with how the nitty-gritty
electro-mechanical operation turned out and have no reason to think it won't
work well. In the end, I probably didn't save as much weight as I'd hoped,
but I'm optimistic I'll get tight, repeatable step positioning. I suppose
the system may develop some slush/slop/play over time, as Curt has
experienced with his but if I too can get 499 hrs. of tight, repeatable step
positoning under various loads before needing an overhaul... I'll be happy
with that. :-)
Ron
On Thu, Nov 6, 2008 at 9:31 PM, <
kpierson@dcsol.com> wrote:
Ron: Thank you for answering my post. After looking at the picture
of your actuator, I am curious as to why you didn't mount it
vertical so it could direct drive.
Keith
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