with the progress made on trashbot 6, i also wanted to have a compact, yet lightweight way of connecting to parts with two servos so that these two parts could move in two dimension: the shoulders and the feet. Continue reading →
I always love to obey to minimalism and I always love to construct stuff with a minimal amount of material and complexity. Of course, the question is then how replicable, durable and maintainable the result is.
In the present case it became obvious, that at some point in time, Trashbot would need to have an additional degree of freedom on his foot, namely an ankle that allows him to lean forward. The present prototype is acutally from August last year. But now I’m revisiting my designs as Trashbot recently got the long awaited additional hip servos and the logical next step is to add the ankle servos.
Last week I added three degrees more to Trashbot, two hip servos for forward movement (“kicking”) and one to the bone.
This week I found some time to do the first single servo movements and tests to check out the new geometry of the bot since the broader hips will affect the Center of Gravity etc. Here’s the first attempt to do what the normal gait would do: shift the body to one foot:
So, definetly software teaching me how to improve hardware… Next draft iteration: Continue reading →
I’ve been working on Trashbot for quite a while now, but the basic gait mechanism is still the same as in version one. The hips’ movements and the distance between the legs define the possible step length. This is annoying since the robot is rather tall and you’d expect that he’ll walk a bit faster than he actually does. However, moving faster induces stronger vibrations in the skeleton and makes him fall much easier.
this time it took really long to make more progress on trashbot but i had the feeling that there were many little improvements that made the bot better and somehow it felt like none of them were big enough to be worth a post.
now in the end i learn that quite a lot happened during the last 8 or so weeks and over easter i invested some time also into the cleanup of the software.
first of all, i added shins to the legs so that he wouldn’t swing too much when shifting his weight from one foot to the other. i used some screw / hooks that you use to hang wired lamps and have a tension on the wires:
recently, i had the idea to attach a laser pointer to trashbot’s neck and investigate the amplitudes of the laser on the ceiling. this time, the blog entry is shorter but the “how-to” is in the video:
from the results you can see much better the difference between active balancing:
vs no balancing (the spine is stiff, the upper body moves with the hip, thus larger amplitudes):
with this technique, i hope to be able to document further improvements on the movement, especially when i connect the gait pattern with the balancing mechanisms as scientific robots do.
i recently contrasted trashbot 3’s walk with the spine balancing or bein static and was not happy with the result of the video. one can have the impression that active vs non-active balancing is somewhat similar but in fact the real robot “feels” different as we’ve seen in the video above.
next, i’ll attach a little camera (gopro clone) to the trashbot’s chest to view the gait from “first person” perspective…
(if you can’t wait to see the end result, scroll down to the bottom watch the second last video )
after the lower body was walking and free from seizures when starting up the arduino, it was time to think about the rest of the robot. certainly, there’s still much to improve on the gait and most importantly the feet, but it’s my goal to build up the complete robot first before iterating the parts (the feet are under constant change as the weight distribution varies and also i’m constantly learning how to improve gait pattern.)
in contrast to many commercially available robots (like the bioloid or the lynxmotion pete), i wanted to have a machine that feels more natural and can move a bit more freely, especially with the head and the body trunk. usually, these two are rather stiff or absent (head). people invest dozens of servos into legs but not a single one into the spine. for me, the two servo legs are “good enough” for the moment, i’ll invest in the rest first.
on the other hand i’ve seen many gimbal mechanics mostly for quadcopters carrying the camera. some of them were even arduino controlled and here and there, you can see some code. interestingly, i wasn’t really able to find servo-driven automatic gimbals, most of those are driven by dc motors.
let’s view the goal first:
okay, here’s how i built it:
the base was taken from anold bike light:
and i added the first servo that can swing from left to right:
next up, was the second servo driving the chest allowing the bot to lean forward. it seemed to me that these are the two directions the biped needs to be stabilised in, since the feed generate a tilt to the sides and the hips rotate body on that tilted axis.
i wanted to have a possibility to update the mechanism easily and didn’t want to glue or solder anything, also bulky servo brackets seemed not the right way to go aesthetically. so i came up with a wire-based connection of the two made from a recycled wire that i had left from hanging lamps and some power connectors and a spring from the recycled typewriter:
it took a while to find the right parts from the type writer and re-arrange them so that i had a broad enough shoulder that could work as a “servo bracket” while being stable enough to support more hardware on top later on. and i recycled my first two letters “M” and “N”:
finally, connecting the two parts to this:
it was really tricky to find some parts to surround the upper servo so that i could atttach a second ankle on the other side of the servo. kind of proud to have it included in the wire fixation structure… also, you see a recycled air pump holder from an old bike that protects the bot’s chest servo should it fall backwards.
next i recycled a heat pipe from an old (TV?) pcb:
and shaped it to support the upper body on top of the hips and TADAA! here’s the assembled trashbot 3:
now it was time to develop the electronics and the software. (actually, i developed the software with the spine detached and assembled the whole beast when the autonomous gimbal was actually working).
standard setup with the gyro / accel MPU6050, most importantly with the interrupt attached to the arduino:
i tried a could of libraries for getting decent values (including the kalman filtering that can be done on the DSP of the MPU itself), but jeff rowberg’s is simply the best.
finally i wrote my own PID functions, mostly to understand how the maths work and how and whether i can fine tune something. PIDs are beasts as they are designed to self balance to a set value. now if the actuator and the sensor are attached to each other and controlled via a PID the whole system can swing like HELL. actually i burnt my first servo playing with the setup and was happy to learn that my humble mechanic design worked really well for maintainability…
i used mostly these links to understand how it works:
and lots of reading around this. the problem is that nobody can tell you how to tune your PID in your setup. so there’s a whole lot of trial and error with many strategies being published on how to find the right parameters. as you can see in the video, the parameters are sort of okay, but still the system is behaving a bit “choppy”. but if i react too often to the sensor values, the system starts to swing. so here’s some homework left and i hope that this blog post will spur some discussion in the forums. and there’s a nice blog post that obviously led to the PID library included in the arduino software distribution. i also want to learn from that.
let’s see the system in action:
finally, i also studied a bit how the bot walks with and without the gimbal active. the first part of the video shows it from front and top with the gimbal active, the second part shows it from top and side with a stiff upper body.
you can see that the upper body’s lateral amplitude is higher when switched off. this will be even more the case when we load the shoulders with a head and arms in the next iterations.
i will do another blog post investigating the gimbal effect more deeply. i just bough a tiny camera that i can attach to the neck and will tape trashbot from “first person perspective”
finally, i got decided to increase the trash share of the bot and get rid of the shiny new feet made from polystyrene and replace them with old PCBs. i also increased the length of the feet for better stability.
from the top it looks like i just added the switch and the relay, but i also added more “ports” for four more servos (intended for the upper body) as well as a “port” next to the switch for an UBEC. see the real wiring here:
as you can see the servos (left up) get their power via the “port” for the UBEC (bottom) and then via the switched part of the relay. the relay is powered directly by the arduino (red cable coming from top, arduino port d13).
the trick is that the relay is cuting off the servo power when not switched. the arduino then has to set d13 HIGH to power the relay’s coil and thus keep the power on. since the d13 can only be powered via software, the arduino can control the servos when it has booted up and during the boot up the servos don’t behave badly. (obviously the d13 could also be in a HIGH state when the arduino powers up but that seems to be just to short for the relay to react.)
alright, after all the construction mumbo jumbo, here’s the end result:
compare that to version 1:
i also used a new servo library VarSpeedServo which can move the servos at user defined speeds. not sure i’ll keep it because i want to experiment with more complext gait patterns but for now it’s a great solution.
during the work on trashbot v1 and rofi, i noticed that robots get seizures when powered up (see the second video in this post for examples). this is obviously because the arduino’s outputs are not in a defined state for a brief time and the servos also get power already leading to outputs sometimes firing until the arduino has loaded its bootloader and executes the user’s code.
here’s my first experiment, simply switching switching one led off and the other on. relays usually switch from one contact to another, meaning that even if the relay coil isn’t powered, there is a default state for both switches (one is on, one is off):
next i wanted to document how actually the servo moves into undefined positions when powered up, so that we see that this is a real problem to solve (yeah, i know that the 2s / 7.4v lipo may be too strong for the mg996r servo, will add a ubec soon):
okay, so finally, we add the two together and use one arduino output to control the relay coil, the relay coil then will switch the power to the servos. i will to this for the whole power rail feeding all the servos of the robot so that i only need one relay and one digital out of the arduino nano. let’s see how it works:
i will add the relay now to trashbot v2’s circuit and see whether there will be any more problems afterwards. i’m not sure whether i need something more (apart from the missing ubec to reduce the voltage for the servos) like diodes (coil loosing power gives it back to the arduino? or not strong enough?) or pull up/down resistors (have to teach myself what that’s all about next). but for now, i’m happy to see it working.