Electrics test rig - for checking servos/wiring/controls

In the been-there-did-that-found-out-far-to-late category of things.....  

The checking that the electrics work in the context of a partly built model is often a messy business. The confusing business of exploring TX controls and servo linkages and mixes is taking brain time at the same time as wondering how the wiring is going to thread through the ribs and fuse and potentially dragging the whole lot off the table.

In order to focus on the electrics and controls I decided to create a test rig that would host various control surfaces and control horn locations as well as providing mounting slots for servos in a variety of orientations and also host a variety of motor mounts.  

I began with the multi-purpose motor mount that MagPi was posting about recently (all that hard fought shouting at FreeCAD).  I measured my various motor supports 20mm, 31.5mm and 34mm beasties and then sketched and created a mounting block. The initial version was simple - I made a sketch of the layout and extruded it,
 

I later revised it with those hex retainers and bumped up the width and added a couple of pass-throughs for motor wiring.

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I measured a couple of typical servos and created some simple air-foil shapes for the control surface. I then designed two vertical slabs with cut-outs for likely servo positions and slots to take a left and right harness for control surfaces and at the aft holders for vertical stab and for an elevator.  Below is one of the side pieces along with control surfaces with cut-outs for control horns in various locations.

 

A few iterations were involved but here are some images of the beastie with servos and linkages mounted.  I will have more images later....

 

 

 

I used prototype of servos to plan the rig.

 

A bit more work to do but I can make the CAD and STL files available for folk....



 

What minimum size holes is it practical to print (for the hinges)?
What thread is printable? eg M3 has 0.5mm pitch
Are the servo screws self tappers into the plastic?
Is the motor plate screwed on into plastic or is there a nut at the back?

I am using 1mm piano wire for the pins holding the control surfaces.  The radius of the hole I ended up using for the holes I embedded in the various bodies is 0.7mm radius. Most of these resulted in being able to finger-push in the pins. I used the same hole size in the control surfaces. In a few cases I used a small hand drill (thanks LIDL) to tidy up the hole.

I did include holes for screws to hold in the servos - those were also 0.7mm radius and seemed to work well with the screws that come with servos. In a couple of cases where I forgot to place a screw hole for the servo I used an awl to make a pilot start and just let the servo screw cut its own thread.  A choice. What was new to me in this case was that I took the time to make prototype servos and placed them in the bodies I intended to print in order to line up the screw holes.  

   

For the bolts holding the motor plate I used 1mm and then reamed it out a bit with a drill bit.  Depends if you want to have the machine threads cut into the plastic.  I placed a 4mm radius hole cross-ways to take a retaining nut. The bolt held fine without the bolt.  The alternative is to go with a slightly bigger diameter hole so the bolt goes in freely and totally rely on the nut to do the heavy lifting.

 

 

As a guy that worked in a University for quite a few decades, yes perhaps it is an academic approach.  Then again I chose to be a researcher the whole time rather than be a lecturer. I also chose to do consulting work to fund the research I wanted to do rather than spend hundreds of hours doing grant proposals.  Back to the test rig....

I could have put a test rig together with foam.  Certainly that would be an option.  And, looking back on it that might have been a way to fast-forward the printed design.

One of the reasons I wanted to build a test rig was to be able to play around with control ideas at the VERY early stages of a model design. What length of control horn and servo arm? How much space will be needed later on for the servo arm if I want to allow for mechanical differential?  Can I find Tx settings are close to the movement I get from a mechanical differential?  What Tx settings can work with a mechanical differential?  Does orientation of the servo impact the control surface behaviour?  So I designed in slots for servos in multiple locations to be able to check that out, I made slots for control surface horns in various locations, the horns have a dozen attachment points so there is a matrix of adjustments possible. All feed-in information for planning models. 

Also I just got a Rx with a gyro.  New territory for me.  I wanted a safe way to explore it and the Tx options in order to gain some confidence.  Doing that on a full sized model is an option but being able to see if the control surfaces are moving as expected does not take a full size plane. And because the man-cave is often uncomfortable I wanted something small I could work with inside.

If you want the parts to print yourself just let me know....