V 5.1

A project log for Open HD

A high torque, high precision, Strain Wave Gear (harmonic drive) based 3D printable closed loop servo actuator for use in robotic arms

Sam ArmstrongSam Armstrong 09/17/2018 at 01:476 Comments

V 5.1 is a general refinement sweep 

That brings a revised wave generator, fixed spline,  and output shaft assembly. There are now 3mm dowel pins of various lengths (40mm, 14mm, 8mm) placed throughout the fixed spline as well as output shaft assembly for precise mating and added structural support. 

Changes to the wave generator were primarily processes focused to make their manufacturing simpler to automate. List of component changes:

Changes to the fixed spline consisted only of two component changes and some slight procces refinement.

Changes to the (upper) output shaft assembly

Integration of the still unfinished encoder head also came with the V 5.1. Currently the encoder head is using a transmissive photointerrupter (link). I will be switching to a Broadcom reflective sensor in the future, but for now I have some stock of the EE-SX1108 and they work fine as a placeholder till then.


Soren Andersen wrote 06/10/2019 at 08:50 point

It looks very good. What are you using to run the motor? Can something like an odrive be used? (

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Sam Armstrong wrote 06/11/2019 at 19:20 point

haha of course! I’ve been using odrive the entire time. I don’t plan on including a motor controller when it’s complete. Just temp sensor, encoder, and motor 

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Soren Andersen wrote 06/12/2019 at 07:43 point

Great. Does it have the encoder on the input or output axis? I think odrive needs the encoder on the input axis. 

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Sam Armstrong wrote 06/20/2019 at 22:41 point

Yeah, so the plan right now is to have the motors made with integrated hall sensors for the input side position control and have the high resolution encoder on the output to compensate for any torsional flex in the flexspline. This seems like the least expensive option while not sacrificing any meaningful precision. But I'm definitely open to suggestions. 

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Michael G wrote 05/25/2019 at 01:20 point

Really clever design -- have you considered getting components made using SLS or Multi-jet Fusion? Much stronger and especially MJF starts to get both cheap and quick when you get into reasonable quantities. And of course no layer lines... Just an idea though

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Sam Armstrong wrote 06/11/2019 at 19:30 point

Hey thanks! I have one version where the fixed spline was “jet fusionend”. Unfortunately I don’t have direct access to one or the capital to buy one haha. So for my purposes, at least at preset, keeping everything coming off Ultimakers works best. 

It is a good idea though, and perhaps future versions will end up having parts or all parts made using a more scalable printing method. My biggest goal though for this version is to keep it manufacturable  for the consumer. 

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