From the Apollo 8 Flight Journal: The FD/AI (Flight Director/Attitude Indicator) or "8-ball" is one of the most important instruments in the spacecraft. Designers had originally intended to give the crew three separate displays to show their attitude; one each for roll, pitch and yaw. Being pilots, the crews quickly threw out the three displays for a development of the artificial horizon familiar from aircraft instrument panels.
V2 will rely on three stepper motors to position the ball. Previously I put the motors more or less in line with their respective axes. Unfortunately, this introduced the challenges of dealing with rotating electrical connections, balance of the ball, motors turning the weight of other motors... Etc.
So, why not eliminate these issues at the cost of increased mechanical complexity! V2 sketch (please excuse the humanCAD)
My plan is to use gears (and belts potentially) to move the control axes to be colinear with the roll axis. All control axes will be concentric and will have its own shaft (two of which will be hollow). Basically like the three hands on a clock. This will allow all the motors to be outside the moving ball mechanism. The problem with this is it demands more performance of the motors.
For example: if I wish for the ball to pitch, I have to move the yaw motor in tandem with pitch to prevent the yaw bevel gear from moving relative to the pitching platform. Furthermore, if I want to move pitch and yaw at the same rate, the yaw motor must move twice as fast as the pitch motor. This issue is even worse if I want to roll, requiring compensation in both pitch and yaw axes.
I don't expect this to be a problem at low rates, but it severely limits the maximum rate of the ball. The pitch motor must be able to move at least twice as fast as the roll, even worse, the yaw must rotate 3x as fast as the roll motor. I'm not willing to sacrifice the accuracy of the instrument, so simply gearing the motors "faster" is not a solution.
I believe that an elegant mechanical solution exists, but I'm stumped. I think if I had a mechanism that behaves in a "I can turn you, but you can't turn me" manner might go a long way.