The reason I am still pushing for the hemisphere because I think its elegant, simple and more intuitive. Its our combined brainchild, and as Joshua said, its cool.
Actually, here is what Joshua had to say about the hemisphere
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The spherical drive is really cool - but I prefer the first design (attached) from Daniel (because of the anisotropic nature of samples - we would still need to reposition contacts/micromanipulators - there is no escaping it) -- the added complexity for 3d printing and cost of 3 motors/controllers does not seem worth it. File that away though - might make an unreal BRDF system."
Lets look at the points Joshua raises one by one,
" the anisotropic nature of samples - we would still need to reposition contacts/micromanipulators - there is no escaping it"
The hemisphere with the third wheel can spin like a disc on a turntable. This enables the stage to reposition using software and a motor, instead of doing it by hand. That is an advantage.
"the added complexity for 3d printing and cost of 3 motors/controllers does not seem worth it"
I would concede the point about complexity in 3d printing. However, the sphere does not have to be 3d printed. The main reason for 3d printing was to have the system accesable to developing countries or smaller labs that can not afford prooprietry systems. The hemishpehere can be made out of regular objects that can be easily found. Glass flasks used in chemistry labs, globes, or any sphere item of the right size. The system i have proposed does not use any special omni wheels. so apart from the stepper motors, there is nothing particulary special about it.
As far as the complexity of controlling 3 motors is concerened, that I think is the major reason Joshua feels we should lay off the grass for a while :)
Using 3 motors with wheels working in tandem at diffrent speeds to move the hemisphere, is a very complex design. As i explained, we do not need it. The projects we looked at ( Robot navigator and the motorcycle ) require speed of movement. We require precision. For their systems, getting in the complex geometry of 3d movements and controlling the motors using the spheres movements is neccasry. For us, we can just use simple movements in x and y axis at low speeds.
The system we have shown to Joshua is very complex to control. That is the major reason he thinks its not feasible. But with reduced complexity of movement and resultant ease of control, I think Joshua would be ok with the system.
Another major concern for me is what Joshua wrote in his last email.
The Fig on pg 11 and the design on pg 20 looks like the geometrical mistake from before - I dont see how it gets all of the angles of incidence
I belive he is pointing to the worm gear underneath the stage. In its current orientation, it will spin the stage like a disc on a turntable but it wont tilt it. That is my understanding as well. i think we need to have a look at it again.
" I am agnostic on the worm drive - if the team thinks it has an advantage over the direct drive - lets do it -- but I worry about wear on the gears messing with precision down the line."
"
The worm drive will have the issue of gear teeth wearing out. Specially if we are doing 3d printed ones. If we do metal ones, it raises the same issue as Joshua had with sourcing a hemisphere. its not as readily available and not 3d printed.
The last thing i want to comment about is the Torque requirement. If we go with the model Joshua is recommending, our torque requirements are still not fulfilled according to calculations Tibi presented. We were require 10 times more torque than the motors we selected output. The fundamental reason why the hemisphere idea came in to being was that the motors could not produce enough torque. If we go with the older design, we still have the same issue.
Hope I made some sense.
Best,
Abran