Open Hardware robot - Rosmo - Looking for volunteer(s)

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Sam_uk
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Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Sam_uk »

But they don't align with anything useful.
Hmm sorry about that, I believed they matched up with the part I've been referencing above. I'm not the author of that part. I'll get in touch with Rich to see if he can provide more info.

Can you see a way of making it work?
chrisb
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Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by chrisb »

Sam_uk wrote: Sat May 14, 2022 9:32 am I've installed https://github.com/JMG1/ExplodedAssembly but can't work out how to use it.
See https://forum.freecadweb.org/viewtopic.php?f=3&t=68681
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Sam_uk
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Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Sam_uk »

This is looking great now. Forgot to mention here is the https://raw.githubusercontent.com/rosmo ... _ESC.step (rosmo_esc.step)

For the encoder, I think we need something like this
Image

(excuse very crappy pic) It's a single part that you first screw to the motor (note countersunk holes) After you bolt to the motor you screw on the encoder.

The two badly drawn tabs protruding towards us are where the bolt holes are, whic bolt to the rest of the chassis. The part needs some extra material front and back (crappily drawn in) to make it long enough and strong enough. We may have to make the 'shaft' longer to accommodate this additional length.

* If both encoder cables could exit the top that would be ideal, I think we'd bash them pretty quickly with a bottom exiting one

* Screw holes/sockets in the PCB support struts will be needed, Or add those nut holders as seen on front-wheel axles?

* A little material needs to be removed between the spinning motor and chassis, just 1mm or whatever so it doesn't rub.

* We can tape the battery in for now, but slightly more holding would be a nice to have

Thanks for all your work, it's looking really cool.

I can't wait to print one!
Last edited by Sam_uk on Sun May 15, 2022 9:46 pm, edited 1 time in total.
Sam_uk
Posts: 39
Joined: Sat May 07, 2022 11:16 pm

Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Sam_uk »

Here's a first go at a Pololu rear wheel for the GB2208 motors https://github.com/rosmo-robot/Rosmo_3D ... r-v1.FCStd
Sam_uk
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Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Sam_uk »

A bit of further feedback from Rich who is developing the encoders:
I see! That’s pretty cool, they even designed tiny holes to fit the SMD LED and resistors on the top of the board…
TBH my feedback would be to make it less “tight” for the encoder. I have about 6 different encoder models in this shape, the AS5048A is just the first… but I think I’ll add the SC60228 soon, to give a cheaper (but 12bit) option, and the MT6701 to have a cheaper 14 bit option…
All these encoders have the same shape (outline) and holes, all have the SPI port in the same position. But the position of the LEDs and other SMD parts changes, the MT6701 has a switch to put it in ABI mode, etc…
So if the back of the encoder could be left more open, Rosmo users could choose different encoders based on price, performance and availability…

Points to consider:

general outline and screw holes - this is the easiest, and looks like it was done right.
screw holes for M screws should be the same size as the screw diameter if screwing directly into plastic. So M2 screws will screw well into 2mm holes.
The new encoder PCBs are 1mm thick - much thinner than ordinary PCBs.
The chip has a height of about 1mm, but other chips have other heights (e.g. the SC60228 is closer to 2mm)
The air-gap to the magnet should be about 0.5mm
The air-gap may not be too large or too small, or there will be weird failures
The backs of the sensors I make have the holes and the plug in the same location, but the rest of the PCB can vary - the location of the other SMD components, their number, etc… so it is better to keep the back free
Depending on the motor, the screw holes for the motor can be in different places - the sensor’s screw holes have to fit around these. Also the sensor should not directly touch the motor screws (risk of short circuits) so they need to be sufficiently recessed to ensure contact can’t happen
the Magnet with holder needs a hole of 11-12mm, and typically has a height (above the motor) of 5-7mm
depending on the motor, the magnet or magnet holder attach in different ways, with different heights that the magnet sticks out…
I think if we were able to go with my proposal above for amendments then this issue would be resolved?
Bance
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Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Bance »

Latest version, haven't checked against the newly posted comments.

I'm a bit busy tomorrow, but I'm sure i can do some stuff. :D

https://www.dropbox.com/s/0nn6jsv5yfklb ... FCStd?dl=0
Sam_uk
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Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Sam_uk »

Nice, thanks :)

I _think_ the only amendments are to

- add in some screw holes for the encoders.

- Remove material next to motor so it doesn't rub

Image

In fact, this part at the back could be removed completely?

Image

If it was possible to reduce the front overhang then we could try doing this trick: https://youtu.be/Jtwu7-T1a9o?t=108

Don't worry if it's too much hassle, very much a nice-to-have feature. I think it could be done by sliding the battery back above the motors?

Image

You could then re-shorten the part in front of the axle by sliding the PCB and supporting arms back?
Bance
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Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Bance »

Don't worry about shortening, its easy.

Have a look at the latest file, you will need to post photos of the actual motor so that we can design the magnet holder.

After that I can go through the file to rationalise and name key elements for easy customisation.
ScrewConfig.png
ScrewConfig.png (186.35 KiB) Viewed 575 times
https://www.dropbox.com/s/0nn6jsv5yfklb ... FCStd?dl=0
Sam_uk
Posts: 39
Joined: Sat May 07, 2022 11:16 pm

Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Sam_uk »

Looking good.

The motor is made by Tmotor. It is the GB2208: https://store.tmotor.com/goods.php?id=447

There is a bit of video of it here: https://youtu.be/O4NfNQkD_80?t=431
Sam_uk
Posts: 39
Joined: Sat May 07, 2022 11:16 pm

Re: Open Hardware robot - Rosmo - Looking for volunteer(s)

Post by Sam_uk »

One further observation: we don't have much ground clearance.

If someone felt motivated to add a few more teeth/ enlarge this part: https://github.com/rosmo-robot/Rosmo_3D ... r-v1.FCStd

Then we could reduce the overall length of the chassis to accommodate the reduced belt length?
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