This episode deals with design of the spice jar platter and an introduction to the drive mechanism. It will also include some notes on servo motor signals.
The circular platter holds 24 spice jars in cylindrical cavities so that when a jar is pushed up from below, it slides up its cylinder without toppling over. It can accommodate an extra jar in the center.
The platter is about 30 cm in diameter, which is so big I have to assemble a larger 3-D printer just to build it. (Which is why it isn't printed yet).
The platter will be off the counter top by a few inches to accommodate the drive mechanism. It will be mounted to a sturdy 8mm shaft which rotates on ball bearing flanges.
The shaft has a toothed pulley driven by a 360 degree servo motor via a rubber belt. Some of these components are shown here:
The shaft will be cut down when the height of the system is finallized.
It will be tricky to mount the second pulley on the servo - I'm hoping a 3-D printed adapter will work because they need to be concentric.
This servo motor rotates 360 degrees given a normal servo signal.
At zero degrees of rotation the signal will be a 60 Hz rectangular wave which is high for 1 ms (pulse width) and low for the rest of each cycle.
At 360 degrees of rotation the signal will still be a 60 Hz rectangular wave but it will be high for 2 ms and low for the rest of each cycle.
There are 16 jar positions which means each position will be 22.5 degrees from its neighbors. It also means that each position will see the signal pulse width change by 0.0625 ms or 62.5 microseconds. These numbers may change slightly depending on the actual servo characteristics.
The raising piston is 20 mm in diameter and the opening under each jar is 38 mm which allows the position to be off by ± 9 mm without having the piston hit the platter. This is about ±2.3% or ±8.2 degrees or a pulse width deviation of ±22 microseconds. (excluding servo motor accuracy and drive train backlash)
It sounds a bit tight, but I think servo motors can generally handle 16 unique positions. Note that the inner ring of jars has a much wider tolerance for misalignment. (±4%)
Here is a quick look at the outside and the inside of the CAD model of the main platter:
Next I need to get a 3-D printer working that can print this platter.....but there is also lots of other work to do to design the gear train geometry and mounting systems.
Design Challenge Links:
Project Links:
Blog Glenn 1 - AIY Voice Kit Unboxing
Blog Doug 2 - The Block Diagram and Bill of Materials
Blog Doug 3 - Spice Jar Lift Mechanism
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