Autopilot clutches

Good morning all. 

I've injured my back (it will be right in a few days) so there's a hiatus on flying powerful kites.  Although that has been good for some client work, the client work has not been good for my sanity... so I've diverted to building a couple of prototype clutches for autopilot lines.  

I have a prototype kite autopilot that might be able to, with more development, fly at the pilot point of a paraglider/kite (ie about 7m below the canopy) with a single line from there to the boat.  That version weighs about 2kg.  The gear motor, a 12v drill, is powerful enough to hold steering loads.  To go to something much lighter, perhaps of order 0.4kg, I'll need a clutch/brake to hold steering line positions.  The winch I'll use to move the steering lines will be powerful enough to move them at adequate speeds, but won't have the required static holding torque.  So I've made a couple of servo operated clutches - though I use that term rather loosely.

The one I think is very cool is a splice that is opened to allow the driven line to move then tensioned to lock it in both directions.  It needs a fair bit of tuning - but the video below proves the concept.  The holding line, which includes the 210mm length that encases the driven line, is about 330mm long and is anchored at each end, 310mm apart - so there is 20mm of slack in that line.  The servo has lines attached to each end of that middle 210mm length of the holding line. To allow the driven line to move, the servo pulls the ends of that 210mm section together which loosens the splice.  To lock the driven line, the servo releases the ends. The servo caries no load in either the open or closed positions and very little when moving.  The load, when the splice is closed, is carried by the holding line fixed at one end.  There is order 20mm of backlash with this setup.  I think that can be reduced to perhaps 10mm - eg the central section probably only needs to be 100mm rather than the 210mm I've used and should need only 10mm of relative movement to open and close rather than 20mm.

The other version is the kite autopilot equivalent of sticking a screw driver in the spokes of your friend's bike wheel.  The driven line runs 180° around a block. This block is necessary to keep the driven loop tensioned on the winch - so this clutch version would replace an existing piece of hardware.  The sheave has tapered sides and is crenallated at the bottom so the line cannot slip.  Accordingly, stopping that sheave, with a plunger (OK, rusty nail) connected to a very small servo, will lock the wheel. That 15mm of backlash could be reduced by putting more "spokes" on the sheave and reducing the diameter of the sheave.  This "design" suffers from the possibility of the plunger hitting a spoke (presently about 3% of the circumference). I'm happy with a fair bit of backlash and lower chance of "collision" - though some judicious tapering of spoke and plunger could reduce the collision risk/damage.  The servo is under no load in either position.  

Any thoughts you have, feel free to let me know.