[sdiy] Drawbar and pedalboard

[brianw]

On Mar 26, 2025, at 4:12 PM, Donald Tillman wrote:
> On Mar 26, 2025, at 2:06 PM, brian wrote:
>> 
>> I designed a drawbar controller for a friend who plays various Hammond organs.
> 
> I always thought that modern drawbars would be a killer application for a "Rolamite".
> 
> "WTF is a..."
> 
> The Rolamite is a low-friction linear pulley arrangement.  It was heavily promoted in the old Edmund Scientific catalog as a new "Simple Machine", but I've never heard of many applications for it.  Apparently it's used as a crash detector for airbags.
> 
> A Rolamite might also be great for a sliding guitar pickup.
> 
>  -- Don

Is the Rolamite anything like the mechanism that Hal Chamberlin used for his synth keyboard controller where every key could slide like a fader? That design had concave grips for each key, so your finger could get a grip, so perhaps it wasn't frictionless.

In the world of commercial products, I believe that some of the newer versions of the Mackie Control Universal had something like the Rolamite instead of motorized faders. I think that the resistive element was completely omitted in favor of a 1-dimensional mouse-like encoder (plus motor) with 1024 steps per fader, to allow for more consistent reading of the position.

Looking at the Wikipedia article for the Rolamite, it seems that one (or both) of the rollers could have an encoder wheel attached, and the setup would make a very precise linear-to-rotary translator for positioning.

Brian

p.s. The encoder for a drawbar could be very low resolution, since only 9 positions are needed.