Dr Mike J Smith
Kingston University

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Continuous Rotation Servos

Fri, 20 Jul 2007

The rig designs I have used for flying my cameras (from kites), all use model aircraft servos (e.g. Hitec, Futaba) to drive the rotation (horizontally) of the vertically pointing camera. For photogrammetric applications, vertical photos are the only ones worth taking and therefore the vertical axis is the only one about which we need to move the camera.

This creates a problem because the servos are designed for model aircraft which require movement of up to ~90 degrees for controls such as the rudder and flaps. With rotation around the vertical axis, we ideally want continuous 360 degree rotation. So what do we do?? Well the e-magazine KAPER lists six different ways of achieving full 360 degree rotation. These employ both external modifications (gearing) and internal modifications (extra electrics). Probably the simplest, and most effective, modification is method 4 which involves opening up the servo and cutting away the bump stops and grinding off the top of the final drive gear (ideally using a Dremel).

The first rig we built uses this method, unfortunately minor fluctations in the RC transmitter means that the servo occasionally stutters or will rotate ("creep") ever so slowly when its meant to be in a neutral position. James Gentles has produced a little bit of kit which solves this problem (and is sold at Brooxes and KAPShop). The digital Futaba RC controller I use allows micro-adjustments to the controls which actually stops creep entirely, so I didn't need this solution.

However next on the shopping list is the Hitec HSR–1422CR which purports to allow continuous rotation out-of-the-box. It's on order so it'll be interesting to see if this solves the problem

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