agp.cooperMPRT Project
Introduction
The MPRT project is both very interesting (for those who appreciate mechanical things at least and/or robotic) and useful. It is particularly useful for SCARA robots. This is my interest.
I have a Prototype SCARA project (https://hackaday.io/project/164066-prototype-scara) that uses direct drive stepper motors. I want to upgrade to geared drive. I have designs for belt drive but then the MPRT project came along.
Whats to Like?
Relatively simple, compact and high gearing ratios and potentially (to be demonstrated) low backlash. Ideally suited to a SCARA elbow.
Whats not to Like?
Not an off the self product, needs quite a bit of research, and trial and error to get it right.
Collaboration
My collaboration will be in part shared work with Daren, and parallel work focusing on a laser cut version (Daren is focusing on 3D printing) for my Prototype SCARA project:
Today
An Alternate Arrangement
As I am looking at low backlash (and understanding Daren concept better), I put forward an alternate arrangement:
The concept is not a full design so you have to use your imagination.
The concept here is a driven planetary ring (cyan circle and the three planet (pinion gears)) and driving the two separate centre rings.
Notice that the planetary gears are linked by the cyan ring and the planetary rings consists of two sandwiched and locked gears that drive the two centre (sun) separately (unlocked).
Although it looks completely different to Daren's concept is is basically just a rearrangement.
In operation, the planetary gears turn and rotate around the sun, but locked to where the sun gears align. As the Sun gears have different number of teeth they have to rotate at different speeds to meet the planetary gears.
Now let us look closer:
You should be able to see the blue and black gear pair, and the magenta and red pair, superimposed on each other.
You can see these gears are tightly meshed, no compromise.
Construction and design of the gears is very straight forward (no tricks required).
As an aside, I am using DeltaCAD, a cheap ($50) and really nice 2D CAD package. It comes with a macro language that I used to create my gears.
SCARA Elbow Design Concepts
Well this will be the topic form my next post.
AlanX
Discussions
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@Simon Merrett just how low is the backlash?
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Dunno, it's all potential at the moment. Daren will probably be the first one to test it.
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With the version using two different tooth pitch numbers for the output/stationary ring but one for the planets, printed a little loose, it is about 0.5 degrees right now. I am eyeballing it, but I'm also biasing upwards.
I would say with no changes other than printing the gears a bit more precisely, that could be as low as 0.15 degrees. With nylon 'ring' planets very slightly oversize it would be essentially 0, but the efficiency will go down then.
With the proposed change of using split planets to match both required gear pitch numbers, and using nylon 'ring' planets for their 'springy-ness', it can also be essentially 0. However now we would also get better efficiency and less wear, as we can maintain a true involute gear profile everywhere.
8-24 hours, and I should be able to prove/disprove that. Nice day here, so it's family time. I'm wearing a life preserver atm...
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@agp.cooper this is great! I can see that this would allow much easier axle and bearing arrangements than the solutions which work with teeth on the inside of a large ring. Flipping @Daren Schwenke 's concept to outside teeth is a creative and welcome variation. I understand that the suns will move at slightly different speeds but how do you envisage driving the planets? As far as I can see, one sun would effectively be static relative to your motor mount and the planets would have to be moving relative to the motor. Would we look to put teeth on the cyan planet carrier and drive that ring from the side?
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It was just easier to check the concept this way.
I have a version (that is like Daren's) that uses an Internal gear but I am having problems generating the gears. CodeBlocks is blowing up on me every couple of minutes.
The outer ring of planets are held with a cage and the cage is driven. Not ideal for me, the internal gear is better for my application.
I screwed my libraries trying to install the latest version of CodeBlocks so I have had to reinstall Mint. It will take me another day to get back to the gear code. I have a lot of software to reinstall. This time I am keeping a log.
Daren's version as far as I can see will have some backlash. My internal version will have no backlash (in theory!). Still credit to Daren for the original concept.
AlanX
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Your version should translate directly back to being a planetary system. I can work on converting it back to OpenSCAD for you as time permits. Post the values you used, and it should only take about 20 minutes to generate a new model with split planets/sun.
I also forgot to make a project for this on Github so we can share code... I should probably do that now before I get distracted again.
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@Daren Schwenke where / how do you see the drive being applied in the outer teeth / planet carrier version? When you say it "should translate directly back into being a planetary system" are you thinking purely in terms of being able to use a planetary gear generator for the sun/planet design generation or were you also thinking for the way it is driven (given that we want one sun to be relatively stationary in most cases)?
I'm coming at this from a traditional robot arm (not scara) more like #Sixi 2, so I can see that driving the planets from an off-(sun)-axis position could allow for really neat shoulder joints that can handle slew using standard bearings and are simpler because they don't have to accommodate an axially mounted / coupled drive. I hope @Dan Royer will take a look at this opportunity to revise the big joints in some of his designs with the potential for low backlash, stronger slew-handling joints and easier drive arrangements that #MPRT - Modified planetary robotics transmission offers.
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@Simon Merrett I'm saying I don't see any logical reason I can not turn what @agp.cooper has done here back into a proper planetary system. The planets and sun will both need to be split gears, but the tooth count and centerline for each of them is the same. The tooth pitch just varies between the top and bottom rings (and all the interfacing gears). Early on I theorized (and here I went to look for a link to this, and didn't find one... I swear I said this somewhere...) this could work, but then finding an acceptable gearset without splitting it made me lazy.
(edit) Found it: https://hackaday.io/project/164732-mprt-modified-planetary-robotics-transmission/discussion-123784
Now, we can have the best of both worlds, and I didn't have to do the math. :)
I could be wrong, but I don't think so. I should be able to try it out tomorrow night.
(edit) I don't know that just the virtue of turning it inside out has any defined advantage right now. The real strength of planetary systems in my mind is you get num_planets worth of tooth contact area transmitting the forces. Drive the system from off center, and you will be giving that up.
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Ah... you beat me to your introduction, and did it a whole lot better than I would have. :)
Welcome to the project AlanX. I like your split planet idea a whole lot and it will improve backlash to basically 'ideal' levels. Thank You.
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