I'm working on an multi-finned underwater robotic system and have come across a bit of a snag. Each fin is made up of a series of fin rays with each ray actuated by one servomotor via tensioned cables (pull-pull). One of the troubles I was having is that, due to the flexible body and space constraints inside of the robot, the cable tracks don't maintain a constant length when in motion so the cables lose tension.
I found a possible solution while riding my bike to campus one morning. The brake and derailleur cables on a bike are actuated from levers on the handlebars. When the handlebars move, the cable tension would change were it not for the cable housing that runs from the brake levers to a stationary point on the frame. So I want to use the same kind of "bowden cable" approach (a brand name, I know, but it's widely used). This will allow me more flexibility in placing the servos and also make it easier to deal with adjusting cable tension using barrel adjusters.
The cable housing is essentially a flexible conduit that does not compress significantly along it's axis. And because there's a lot of demand for it in both the biking and machine controls industry there's no shortage of suppliers. However, I haven't been able to source cable housing that's less than 4mm OD (ideal size would be 3mm or less). I was hoping I could get suggestions of alternatives; either sources or materials or a redesign.
Thanks in advance,
Some additional information:
Overall robot's size (restricted by tank size): 24"x12"x12"
Servo size: 20 x 22 x 10 mm (.79"x.87"x.40")
Number of servos per fin: 5+
Number of total fins: 5+ (starting with 2)