#robotics Logs

Mar 24 2019

12:05 AM mun: rue_mohr i think i'll need about 30Nm torque
12:09 AM rue_mohr: 300kg, oof
12:09 AM zhanx: wow
12:09 AM zhanx: gonna need to lever that
12:09 AM rue_mohr: wait, did I go the right way?
12:09 AM zhanx: 30kg
12:09 AM mun: hmm 300kg-cm
12:09 AM rue_mohr: right
12:09 AM mun: i hope i got my number right though
12:09 AM rue_mohr: 30kg/1m
12:09 AM rue_mohr: no
12:09 AM rue_mohr: 3kg/1m?
12:09 AM rue_mohr: its x10 out
12:09 AM mun: yeah, about that
12:09 AM rue_mohr: (9.81)
12:09 AM mun: because the front wheel weights 3kg
12:09 AM rue_mohr: 3L of milk on a 1m stick
12:09 AM mun: and then i have a 8kg load
12:11 AM mun: each wheel on the rocker weights 3kg. that shouldn't be enough to back drive a servo capable of that much torque?
12:12 AM zhanx: that i can't tell you, you need to measure them
12:12 AM mun: i've tried even a hobby servo and that was pretty stiff
12:13 AM zhanx: with am arm or by hand? if you add the arm for a lever the same length that will change
12:13 AM mun: with an arm
12:14 AM zhanx: i know at 8" for the arm on a 485mg servo, 40grams would move it
12:16 AM mun: the one i tried was a 17g servo with a 3.5kg.cm torque
12:16 AM zhanx: mun i also have a digital scale etc
12:17 AM mun: actually, is there some sort of gearing mechanism that would allow me to do what i proposed?
12:18 AM rue_mohr: go for the lowest ratio servo you can
12:18 AM rue_mohr: aka if you find two servos, one with a 50:1 gearbox and one with a 10:1
12:18 AM zhanx: rue_mohr, got that pdf link for the gears still handy?
12:18 AM rue_mohr: go with the 10:1
12:18 AM rue_mohr: ? no
12:19 AM mun: rue_mohr yeah, i'd usually go with the lowest ratio servo
12:19 AM rue_mohr: mun, when you want to backdrive a servo, you want a servo with a large motor and a low ratio
12:19 AM zhanx: that one with gear ratio calculator etc
12:19 AM mun: but they're usually more expensive X)
12:19 AM rue_mohr: its a large system, its sounds doable
12:19 AM rue_mohr: you could make a servo
12:19 AM mun: make my servo.. hmmmmm
12:19 AM rue_mohr: your positioning requriements are frazy
12:19 AM rue_mohr: aren't crazy
12:20 AM mun: but could what i propose be achieved mechanically? like, maybe a planetary gear?
12:21 AM mun: hold the ring fixed to let the arm move passively?
12:22 AM zhanx: do you have your servos?
12:22 AM mun: zhanx well, not one that'd be capable of that much torque
12:23 AM zhanx: yea i was wondering the price on one if you did
12:23 AM mun: i know. it'd cost heaps, i imagine
12:24 AM zhanx: what about converting a stepper motor to a servo for it
12:24 AM mun: when i was doing my research i came by the iBot wheelchair that climbs over obstacles pretty well
12:24 AM mun: https://www.youtube.com/watch?v=9qYz2wsVKYE
12:24 AM mun: not sure what they use
12:25 AM zhanx: well converting a stepper to a servo requires a 3d printer also if you have one
12:25 AM zhanx: or these https://hackaday.com/2016/06/01/mechaduino-closed-loop-stepper-servos-for-everyone/
12:25 AM mun: zhanx but i still won't be able to make it backdrive easier than off the shelf servos, right?
12:26 AM zhanx: its a stepper it spins free with no power on it, just toggle it and read postion
12:26 AM mun: zhanx but a stepper without a gearbox for that much torque would cost heaps, i think :\
12:27 AM zhanx: well like i said you need a 3d printer for it, ie the gear box
12:27 AM zhanx: rue has one, i have one etc
12:29 AM mun: hmm hold on. if i use a (large) gear box, back driving would be much harder, isn't it?
12:29 AM mun: that's the issue i'm trying to overcome
12:29 AM zhanx: yes but lubed on a stepper less than a servo
12:29 AM mun: right, ok
12:29 AM zhanx: question, why passive
12:30 AM zhanx: why not a sensor and lift on command
12:30 AM mun: i'm planning to use it on my farm and the terrain is pretty rough
12:30 AM zhanx: the extra weight loss will help it then
12:30 AM rue_mohr: I dont know if you can get an off-the-shelf servo that strong....
12:30 AM rue_mohr: that would backdrive at all
12:30 AM mun: got rotten tree stumps everywhere
12:31 AM mun: i plan to let the rocker go 'freely' on bumps but actively over more vertical obstacles
12:31 AM zhanx: mun you need shocks then, aka linear servo to lift the tires on grub screws
12:31 AM mun: like a tree stump
12:32 AM mun: zhanx yeah, but i'm trying to keep the frame simple and the rocker bogie is a fairly good suspension
12:32 AM mun: my wheels are small too
12:32 AM zhanx: it is but
12:32 AM mun: well, relative to the stumps
12:32 AM mun: i'm just using hoverboard wheels. 6.5"
12:33 AM zhanx: i really want to see a layout of the bot when you can to help with it
12:33 AM zhanx: I offroad for fun
12:33 AM mun: zhanx well, at the moment it's like a standard rocker bogie
12:34 AM zhanx: which is done right is not controlled by a motor
12:34 AM mun: but it's just that i can't find a rocker bogie with an active rocker
12:34 AM zhanx: they normally arent
12:34 AM mun: not sure if it's meant to be infeasible
12:35 AM zhanx: i mean you could clock spring it and use that in the center to help
12:36 AM rue_mohr: I got my digital servo code working, its stepping a dc motor as we speak
12:36 AM rue_mohr: 90 degree steps
12:36 AM zhanx: nice
12:37 AM mun: oww
12:37 AM zhanx: i need to start collecting vacuum cleaners now
12:37 AM rue_mohr: slightly underdamped, but stable
12:39 AM mun: zhanx oh, didn't think of a clock spring. i was thinking of using a slip ring
12:40 AM zhanx: mun really at this point its tossing idea's at the wall and see what sticks
12:40 AM mun: yeah
12:40 AM zhanx: i am doing the same with this snake i want to make
12:41 AM mun: for stairs?
12:41 AM zhanx: for whatever
01:20 AM mun: about the passive/active link problem again, i wonder if a planetary gear can be used. is my understanding correct that, suppose the motor shaft is connected to the sun, if the ring gear is held fixed, the output will be solely controlled by the motor shaft
01:20 AM mun: and if the ring gear is let lose and the motor switched off, the output becomes free and passive?
03:06 AM mrdata_: mun, i love this video on planetary gears, https://www.youtube.com/watch?v=4HPtIXlno3w
03:07 AM mrdata_: i have practically no experience with them however
03:07 AM mun: same
04:38 PM SpeedEvil: https://www.amazon.com/Trailer-Valet-RVR-9-000lb/dp/B07BNB9B2Y Fun.
04:38 PM SpeedEvil: But not $6000 worth
08:56 PM ace4016: how much is an ATV? because i feel like you could get an ATV for that price and probably do the same
08:58 PM ace4016: i guess it's not that compact
08:59 PM zhanx: atv 400-10grand