Droidbot 7 - Undefinable Whegged Robot

An ambitious mobile robotic platform for testing my skills and learning new ones. Inspired by robots real and sci-fi.

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I originally wanted to build my own R2-D2, but I also wanted to try out new things. I love the Astromech's "Swiss army knife" features but an R-Series droid was not looking like a suitable platform considering it's obviously a fixed design and that I also wanted something robust and able to operate in the field. So I started from scratch; what did I want?

I was inspired by a piece of concept art that I came across: RD7-06 (see links). Totally impossible (study it) but I loved the idea of a rugged, utility robot and had the form and function stuck in my head. Next came Handle. Got to have wheeled-legs; speed and efficiency of wheels with all-terrain suitability of legs.

I was aiming for something with retro-futuristic-industrial character, used and abused but still a strong platform for learning and maybe even some practical uses. Then the Avatar XPRIZE was announced as I was questioning my motivations (see log "I'm better than me").

Goals, Aspirations & Risks

This is an ambitious project with many challenges and risks. It is (unfortunately) just a hobby with the resource constraints that this imposes, but assuming I don't go bankrupt, go to jail or die; I'm am putting every reasonable effort into progressing this project. It is both a creative outlet and a tool for personal growth. Below is a non-exhaustive list of what I hope to achieve (both short and long term).

  • Sci-fi styling allowing novelty features where I want, rather than requiring a good reason. 70's and 80's childhood of Star Wars and Alien(s) having a big influence.
  • Large enough to operate outdoors and in limited human environments. Think R2-D2 size rather than table-top or carpet rover. This does mean costs will be higher though. 
  • Develop programming and electronics skills; motor control, control loops, sensing, user interfaces.
  • Develop mechanical skills; 3D modelling, 3D printing, gearboxes, levers and actuators.
  • Learn to use Linux & ROS (on-board Raspberry Pi(s) with Arduino ROSnodes, monitoring/control via laptop and custom remote).
  • Investigate higher level behaviour algorithms, machine vision, mapping, speech recognition, speech synthesis and maybe even AI.
  • Four stowable wheeled-legs allowing staged experimentation with modes of travel. With increasing difficulty:
    1. Compact 4-wheel slow manoeuvres. -- Fairly easy to do, just the physical size makes it more "interesting".
    2. Expanded (stable) 4-wheel manoeuvres. -- As above.
    3. Compact 2-wheel balance. -- I'm fairly confident I can get to here.
    4. Expanded 2-wheel balance? -- Unnecessary or transition to #5?
    5. Upright 2-wheel balance (think Boston Dynamics' Handle). -- The transition will be the difficult bit.
    6. Quadrupedal walk. -- Numerous gaits to try.
    7. Bipedal walk?
  • Investigate possible practical applications of a similar platform. I'm thinking of SAR and disaster relief with appropriate sensor and equipment load-outs. Power provision? Drinking water generation? Load carrying? Environmental monitoring and warning?

Foundation work

To date, I've been collecting lots of parts and doing some ground work. This includes:

  • 3D modelling with Fusion 360 (I do have some Solidworks experience).
  • Re-inventing the wheel trying to develop a scheme to network multiple Arduinos over UART, until I rediscovered
  • Discovering how alien installing anything on a Linux system is and particularly how error-prone the ROS install is (EDIT: I now think is due to my RPi. I thought it was a RPi2 when in fact it is an older B+).
  • Purchased a cheap router and made a wall-mounted fold-down router table to aid in chassis construction.
  • Evaluating various actuators (brushed DC gearmotors, BLDC, pneumatic and RC servo), sensors, NeoPixels and LCD/OLED displays.
  • I made a crude solar tracker (pan & tilt mechanism that automatically faces the brightest point visible to it) for possible solar PV development on the robot.
  • Working on how to best approach some of the more complex leg mechanisms and ensure the actuators will be powerful and fast enough with the range of motion for what I want to do. I do like Pneumatic Artificial Muscles, there's something about them...

Maplin XT75S UA3730 Electronic Code-Lock.pdf

A Maplin electronic code keypad kit from the 80's.

Adobe Portable Document Format - 4.92 MB - 12/20/2017 at 09:46


  • 1 × Raspberry Pi Have a RPi B+ and had trouble installing ROS. Will get the more powerful RPi3.
  • 2 × Arduino Mega Still WIP, so quantity may change.
  • 2 × 12mm plywood 1200x600mm approx. For main chassis. Cheap and easy to work with.
  • 1 × Generic 20x4 LCD
  • 1 × Analogue 3-axis joystick

View all 17 components

  • I'm better than me.

    Paul Croucha day ago 0 comments

    I've been thinking about the head and drive mechanism not being particularly robust and judging by discussions in the Astromech builders forums, the public can be brutal with displayed droids. I doubt I'd be attending the same (any) events but I want robust. I like robust.

    Frustrated with second-guessing myself and my inability to make decent progress I have to console myself with the adage; Fail Fast, Fail Often. Iterate. Prototype. Understanding that every change or "failure" is simply a step toward the new and improved version. I'm learning from everything. Nothing is wasted. Honest.

    Parallel to my own internal conflicts of self-worth is the feeling that I should be putting my efforts to better use. Granted, I want to build cool robots, I can't deny that passion but it bothers me that with everything going on in the world, I'm putting my efforts into what is essentially an expensive toy. I can do better than that.

    Coincidently, about this time, the Avatar XPRIZE is announced and I read the objectives, guidelines and example tasks. The goal for my robot has always been to have a tough, usable machine and the Avatar XPRIZE objectives just felt right! I'd compromised on my early goals, aimed lower and that grinds a little but I simply don't have the resources to be able to produce anything in any timescale close to that needed for the XPRIZE. I can adopt the worthy objectives though. I should at least try. And try using low cost components, materials and methods so something could be deployed for less than $1,000,000.

    Too hard? Too easy says Evan Ackerman (Contributing Editor for IEEE Spectrum's Automaton), suggesting some interesting additions:

    "So how could the competition be made better? Fundamentally, I think there are several different ways of looking at robotic avatars—you could consider them to be remote humans, with the goal of creating an experience for a user that’s as immersive and as much like “being there” as possible, or you could consider them to be one half of a robot-human team, where both the robot and the human augment each other’s strengths and while compensating for weaknesses."

    And is in-line with my thinking:

    "An avatar designed to be part of a more practical robot-human team might be much different, depending on what the system was designed to do. To use disaster relief as an example, you might not want the robotic system to be humanoid at all. You’d want the remote hardware to be capable of extreme mobility, to be very strong and durable, and to incorporate a suite of sensors that outclass anything a human has to offer, while relying on the human to make sense of the data and provide high level control of the robot. Some challenging tasks that would take advantage of this might be:
    • Perform a dexterous manipulation task in a smoke-filled environment.
    • Locate an unconscious human trapped under rubble, and rescue them.
    • Collaborate with other humans to unload heavy supplies out of a truck.


    To that end, I'm pivoting the project slightly. Aside from some redesign for robustness, the only real changes are more focus on making the arms more usable and the addition of some kind of stereoscopic vision system and remote HMI. I have no idea how far I'll get. Baby steps!

  • Head pitch, roll and yaw mechanism

    Paul Crouch03/08/2018 at 08:08 0 comments

    I've just assembled the new head mechanism and I'm quite pleased with it!

    The assembly will allow the head to pitch and roll in order to stay level in response to external disturbances (terrain mostly). I want a level head for mounting sensors and I think it'll look cool. There's a small slip-ring embedded in the center allowing 360° rotation via the gearhead motor clamped to the side. The whole thing sits on top of a linear actuator giving 150mm of vertical travel to provide clearance for pitch and roll but still allowing for a compact "stowed" mode.

    Head pitch, roll & yaw mech.

    Head pitch, roll & yaw mech.

    I was happy to finally get this assembled as progress had been slow recently with a few hurdles to get over. My cheap 3D printer has not been liking the cold either and spare time and money are of course major constraints, I guess they are for most people.

  • A different approach

    Paul Crouch02/27/2018 at 10:46 5 comments

    In order to help me decide on whether to proceed with PAMs or switch to motors, I made a crude model of a leg. It's difficult to see here but there's 4 PAMs for the hip (up/down & left/right) and 2 PAMs for the knee. The paracord "tendons" that attach to the muscles are not shown. The knee joint has a small drum built-in for the paracord to wrap around, to give (in theory) 180° of travel based on ~30mm of PAM stroke.

    PAM actuated leg concept - hip & knee
    Knee joint with drum for 2020 profile
    Ball joint for 2020 aluminium extrusion
    Training cue ball drilled, tapped and threaded onto M10 stud

    But I'm still not convinced. I've held off printing or buying any more parts until I can decide on which method I want to go with. I only buy enough of something to build a prototype and don't consider anything as wasted as I usually I learn something from it.

    In parallel to the PAM/motor dilemma, I began to dislike the direction I was going in with the overall design finding it is too restricting and, currently lacking some lower-body structure to work on, I felt I'd been caught in a loop with it; PAM actuation would mean one type of leg mounting configuration and motors require another. The head-drive felt like a bodge too (and subsequently gave substandard performance) with little room to improvements. My early "hash" chassis was becoming a hindrance. So I had a re-think.

    Below is crude model, just to get a feel for proportions, of an alternative chassis using 2020 profile. It will allow more freedom to adjust things and, more importantly, give me some lower-body structure to work on. The head will still be able to pitch, roll, yaw and move in the Z axis via the existing pitch/roll mechanism and linear actuator (not shown), though the yaw drive will probably move to the end of the linear actuator rather than turn the whole thing as I do currently.

    New approach to chassis using 2020 profile to give more space for legs and radically different appearance when unfolded.

    If using PAMs, the mounts for the hip ball-joints would be tilted back to allow the legs to fold against/into the body. They may have to be actuated too in order to achieve the desired range. PAMs aren't going to cut it if I have to add extra motors to work-around their shortcomings.

    I intend to have panels and the shape of the leg themselves give a more closed cylindrical look when folded up but a completely different look when opened out. I feel that I'll have more room for additional appendages too.


    The more I learn about BLDC motors the more I'm favouring them. I was unsure if I could easily get a high enough current ESC that has an equal reverse speed but it seems they are available, if I understand correctly it's the calibration/setup that determines min/max scaling for forward and reverse. I have also looked at VESC and ODrive but aside from availability issues, they are both quite an investment just to try, so I'll need to be sure.

    Looking at Hobbyking outrunner BLDCs:

    57g    £9.23    7A    75W      3S        KEDA 28-30 920KV Outrunner (purchased)

    153g  £16.28   28A  450W   3-4S     KEDA 36-48 1030Kv Outrunner 4S

    291g  £24.36  37A  600W   3-4S     KEDA 49-55 750Kv Outrunner 4S

    290g £19.84   65A  1300W  5-6S    KEDA 43-62 1650Kv Outrunner 6S

    390g £29.61   50A  1300W  5-6S    KEDA 49-64 330Kv Outrunner 6S

    500g £29.61   80A  1500W  5-6S    KEDA 56-63 195KV Outrunner 6S

    670g £36.48  90A  2000W             KEDA 63-64 190KV Outrunner 10S

    275g £36.45  58A  1260W   5-7S    Turnigy Aerodrive SK3-5045-450KV

    376g £37.49   65A ...

    Read more »

  • Am I fooling myself?

    Paul Crouch02/08/2018 at 18:49 0 comments

    Do PAMs really have much benefit over motors?

    Some (subjective) pros & cons of PAMs vs BLDC motors for this application:

    Pneumatic Artificial MusclesMotors (& gearbox)
    + Powerful for their weight/cost.
    + Fast.
    - Have to trade speed for torque.
    - Do I have the budget for big enough motors?
    + Negligible weight for muscle itself.
    - Weight of valves (400g each) & compressor (2.88kg). Reservoirs planned to be 2L soft drinks bottles (tested to 90psi).
    - Fittings, pressure sensors, regulator (all non-zero).
    - Weight of motors & gearboxes or lead-screws. Small KEDA 28-30 920KV Brushless Outrunner weighs in at 57g.
    Larger Turnigy Aerodrive SK3 - 5055-280KV Brushless Outrunner: 570g.

    + Inexpensive, even including cost of 3-way solenoid valves. ~£11 per DOF.
    + Single small compressor (85lpm) for ~£30.
    - Fittings & accessories should be considered (all non-zero).
    + Small BLDC motor ~£9
    - Larger (more torque) motors cost more. £40+ for bigger BLDCs.
    - Required ESCs can be costly too(?)
    + Compliant: built-in suspension for both rolling and walking. 
    + A degree of safety around those pesky humans.
    - Difficult to position and lack of rigidity would make balancing (future!) more difficult.
    - Limited unpowered support - collapses when not powered, so no static display or maintenance.
    + Easier to position.
    + Gearing that isn't back-driveable would automatically lock limbs for static support.
    - Could be too rigid? Transfer of shock. Not compliant to terrain.
    - Air demand could outstrip supply. Compressor choice & reservoir size have major impact.
    - Compressor current ~30A.
    Current of multiple BLDC motors?
    Small KEDA 28-30 920KV Outrunner max 7A.
    Turnigy Aerodrive SK3 - 5055-280KV max 60A.

    I'm concerned about the speed-torque trade-off with motors; I guess that's solved with bigger motors but that has weight and cost implications. PAMs seem to have plenty of power but I'm fairly sure I'd be shortening the life-span of the solenoid valves by operating them at high rates and it would be a huge PITA to have to switch to motors, because of reliability issues, once I'd committed to PAMs.

    Maybe a hybrid system? Best of both worlds or twice the weight, power consumption and complexity?

  • Successful PAM test!

    Paul Crouch02/07/2018 at 08:47 0 comments

    My first attempt at crude position control with pneumatic artificial muscles. Yes, there's overshoot and lag etc, but I'm pleased I achieved this with basically just a comparator with some deadband, no PID (yet).

    With better mechanical mounts for the PAMs and a different feedback sensor, I should be able to get tighter performance with software. But as a proof of concept; I'm happy! It may be a go for PAMs on DB-T7!

    Raw video below because I'm too busy/lazy to make it fancy. Enjoy!

  • Pneumatic Artificial Muscle (PAM) testing

    Paul Crouch01/28/2018 at 22:36 0 comments

    I’ve been playing with DIY air muscles on and off since 2007 and I'm contemplating using them on Droidbot-thingy, providing I'll be able to control them satisfactorily. I know they're strong and can be parallelled to further increase their pulling power but are they controllable enough for this application (super precision not needed)?

    BLDC motors and 3D printed gearboxes are the main alternative. Air muscles are cheap, easy, light, fast and powerful but are difficult to control and have limited stroke (difficult to get a limb to fold back on itself), whereas motors are great for positioning, easy(ish) to control and can have a limb fold up on itself with no problems. However in order to get enough torque, the gearbox must provide sufficient reduction but this comes at the cost of speed; I could have a super strong robot, a SLOW super strong robot. The gearbox is the complex bit and potentially costly. High torque gearhead motors are expensive and I need a lot of them. And I'm tight.

    My DIY PAM
    Quick test rig

    So I keep coming back to PAMs, at least until I convince myself there's a better way. The rig was just a quick lash-up to get a feel for the PAM performance as an opposing pair on a simple single-jointed limb. I know from experience that air muscles have a massive strength-to-weight ratio but I've never before got to the stage of testing for control, mainly because I had focused on the design of a cheap DIY proportional valve that would work at 100psi - it worked, but I didn't like it; too messy. Maybe now I could 3D print an improvement. Anyway, the muscles are directly fixed either side of the joint (containing a pair of skate bearings) without any additional pulleys (yet) and their mounts don't allow for proper articulation, but it'll do.

    I've popped plenty of air muscles in the past and today wasn't any different. I was trying a 3D printed end cap to see if it would have any advantages (save me having to process the nylon bolts) but I was seeing a lot of leaking, before the pop.

    End caps pop out if not clamped tightly enough
    Tweaking the design

    The second cap in the above photo was printed laying flat so build lines run top to bottom for mechanical strength, but it still leaked. I tried dipping it in ABS slurry to seal it, but the improvement was marginal. The third one was printed upright. It leaked. I tried the slurry sealing again. Nope. I tried and extra layer of rubber under the bladder... I had to admit it wasn't looking good.
    Just not up to it. Soapy water reveals air leaking from the cap itself

    I had been using plastic ratcheting hose clamps as they are lighter than the normal metal worm-drive type. Though I preferred the appearance and weight of the plastic clamps, they are pain to remove for repairs and I had to switch to standard ones in order to get the required clamping force and stop the braid and bladder pulling out under load. I was also hoping it would magically fix all the leaks.

    Of course I could print the cap at 100% infill but it would take over 3hrs with no guarantee it would be any better and that kind of defeated the point. I resolved to improve the existing mounts (I broke one with a high side load) with thicker M10 nylon bolts, but right now I wanted to test a pair on a limb. Raw vids below.

    Next I'll attempt to add some crude position control to help me with the PAM or motor and gearbox question.

    If anyone has any input, I'm all ears.

  • Head progress

    Paul Crouch01/25/2018 at 19:08 0 comments

    The head skeleton is pretty much done. But I'm not sure exactly which sensors I'll be using or how I'm going to mount them, so that's been delaying things a little bit.

    Still has an Astromech feel to it, I think.

    I need to adjust the final height of the linear actuator to match with the parked position of the head. It won't be fully extended is use; just enough to allow for pitch and roll. It will only be extended fully for access.

    A thin 3DP test panel that clips into place.

    All 3DP parts are in ABS so they can be solvent-welded together. Will obviously need filling and sanding before painting.  I have had a few problems with the recent cold ambient temperatures and the 3D printer, nothing that can't be managed.

    I've also setup Octoprint on a old Raspberry Pi with a webcam to keep an eye on prints, very handy!

    Tweaking the design again.

    I have already decided against this scheme, but I'm still torn between 3DP panels and shaped aluminium panels or possibly a mixture of both.

  • Where's your head at?

    Paul Crouch01/03/2018 at 19:27 0 comments

    Robot07 head design
    Latest head design (after at least a dozen concepts).
    Robot07 head design - support ribs
    First ribs for head, in need of tidying.
    Robot07 head design - old & new
    First segment (right) next to old design.

    My 3D printer stopped working just before Christmas and it took me a while to diagnose the root cause. It would get hot and extrude but not print when I tried a known-good file. A blocked nozzle was the original problem but very cold ambient temperatures and slipping feed drive had me disassembling the head numerous times. New nozzle and throat bolt (to be sure) and careful re-assembly had it back up and running by New Years Day (I know how to party!). Let the printing (re-)commence!

    Robot07 head design - 8 segments
    More parts awaiting prep.
    Robot07 head design - electronics to be installed
    Head will house some of the sensors and processors.

  • Getting in a spin

    Paul Crouch12/13/2017 at 23:24 0 comments

    I had to strip the whole thing down to cut additional access holes in the chassis so I could reach inside to mount the GT2 belt on the pulley. After re-assembling and touching up the paint, I set about re-motorising the head. I was using a length of open GT2 belt rather than the closed-loop type, because it's what I had available. Unfortunately the necessary tension needed to avoid belt skipping was too much for my DIY joint and the belt kept breaking. I reprinted the pulleys but for 5mm round belt (I have some for my mini lathe) and that set-up almost worked but I couldn't easily get enough tension for the not inconsiderable weight and drag of the bearing and assembly. Time for Plan C: a bodged variant of the original direct-drive but with a smaller drive wheel and rigid mounting. 

    I also abandoned trying to use the small bearings to maintain electrical continuity for the head lift motor (linear actuator) and resorted back to a mini-DC barrel plug and socket mounted in a bridge under the bottom bearing. The static contact resistance of the bearings were a few hundred Ohms and varied wildly when turned. The bearings were only cheap Chinese ones though, better quality bearings may perform better. The DC jack resistance was a lot lower and more consistent and seems to work perfectly, for now at least.


    I'm also trying out some new video editing software... and maybe I should get a microphone too.

  • Head actuation

    Paul Crouch10/18/2017 at 10:09 0 comments

    Ok, so I know what I'm doing for pitch and roll and I think I've got workable plan for lifting, but I'm still not sure how I'm now going to spin the whole assembly as I've moved away from the original drive (I was never 100% happy with it anyway).

    So the new pitch-roll head design sits on top of a linear actuator which will itself be clamped vertically in the centre of the main chassis able to turn with the head bearing. The bottom has its own bearing and will be supported from the bottom of the chassis.

    Pitch-Roll head
    Top collar
    Bottom collar

    One linear actuator, an old bearing from my motorbike headstock, some 20Kg/cm servos and some 3D printed parts...

    Head pitch, roll and lift mechanism

    I'm really quite keen to get some fundamental motion happening with this. Going to need a fairly beefy slip-ring first though. I'm probably going to using some small bearings...

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Enjoy this project?



Paul Crouch wrote 02/13/2018 at 08:27 point

Thanks, I'm glad someone finds it of interest.

Some compliance is useful for traversing uneven terrain. Imagine trying to push a rigid trolley across cobblestones? Not being locked rigid (via suspension, pneumatic tyres etc) smooths the ride. It's a trade-off; too much compliance makes it harder to control position.

  Are you sure? yes | no

malvasio.christophe wrote 02/12/2018 at 17:24 point

Hi nice work

what about water instead of air for PAM ?

  Are you sure? yes | no

Paul Crouch wrote 02/12/2018 at 18:46 point

Thank you.

I have thought about fluid filled muscles but dismissed it because I'd lose the springiness and gain weight. I think the valving may cause issues too, certainly couldn't use the cheap 3-way solenoid valves I've been testing with. However, it may make them a little more controllable as it would remove the spring effect and a closed system would negate the air capacity concern. I'm still on the fence as to which way to go, so thanks for the reminder!

  Are you sure? yes | no

malvasio.christophe wrote 02/13/2018 at 07:13 point

Thank you to put your efforts on this project ;)

it  reminded me about my wanted wheeling chair with legs and arms ...

why springiness is a need ?

  Are you sure? yes | no

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