For the motor & electronics, I’ve chosen to use an enclosure made from die cast aluminium. The Hammond 1590P1, which is approximately 5.5″ long x 2.5″ wide x 2″ high.
I wanted an aluminium rather than plastic enclosure for a couple of reasons, firstly to resist flexing under the cable tension, and secondly so that I could drill and tap the hole for the cable barrel adjuster. (I am expecting the use of a metal enclosure to become a headache if I move to a wireless handlebar controller, but will cross that bridge as & when I come to it).
One thing worth noting with this enclosure is that the walls are not quite perpendicular to the base, so the hole for the cable entry will need to be drilled at right angles to the wall, not parallel to the base. Not a problem in itself, just something to bear in mind when it comes to clamping it in the drill press.
I’ve also decided to to try a wheel / pulley on the motor instead of arm. There are pros and cons to this; On the positive side, it will simplify the code by not having to deal with variations in the angular to linear travel, and will keep the cable pull in a single direction (whereas it currently goes a bit off axis as the arm moves through it’s travel). On the negative side, it requires making the pulley itself (unless I find something off the shelf), and the standard cable is too thick (I believe) for the bend radius required for my ideal direct attachment.
My thinking is along the same lines as a motorcycle twist grip, with the cable nipple held captive, and the cable running in a channel around the outside of the pulley. See crude diagram below.
The bend radius will of course depend on the pulley size. To maximise the leverage against the cable tension, and to minimise the effect of any positioning inaccuracy in the servo, I’d ideally use the full range of servo rotation (say 180 degrees) for the 30mm of cable travel. This would mean a minimum diameter of approximately 20mm for the pulley (giving a 63mm circumference).
However, I think the standard cable will kink/deform if bent this tightly, which leaves me three options; (1) Increase the size of the pulley, with a trade-off against leverage/accuracy. (2) Add a flexible leader of some description to the cable. (3) Replace the standard cable with something more flexible.
Am a bit unsure about replacing the cable, will certainly be the tidiest and most compact option – but I’ll need to cut the original cable to get it off the resistance unit. It’s not that I’m likely to ever want to put the original handlebar controller back on, but nonetheless seems a bit permanent.
The other mechanical task will be to make up a motor bracket. Am planning to orient the motor on it’s side, so will probably just need a section of right angle aluminium, cut & drilled on one side for the servo mounting, and riveted to the base of the box on the other.
More updates soon…