Upkie - Homemade wheeled biped robot

A homemade wheeled biped that can balance, crouch or turn around. It proudly stands on broomsticks and 3D printed parts.

Similar projects worth following
Upkie is a homemade wheeled biped robot that can balance, crouch, turn around, and more. It is designed to be 100% makeable at home, using only tools and components that can be ordered online (like mjbots actuators), and fully open source software that is maintained on GitHub.

To build Upkie from scratch, expect to spend around $2,500 in core components, 34 hours in 3D printing, and more hours of your work assembling and testing the beast. Here are the main links to get started:

What's next?

Here is a ranked list of ideas about what to do next to improve Upkie:

  1. Sit-down / stand-up controller
  2. Hollow leg, part 2: shin and ankle
  3. Whole-body control to show how the legs help negotiate uneven terrain

Chime in in the comments if you have suggestions, or if you want to try out (parts or all of) the hardware and/or software. Support will be provided to other makers, on GitHub for software and on Printables for hardware.

View all 19 components

  • Locomotion with a PS4 controller

    Tast's Robots11/13/2022 at 16:56 0 comments

    Upkie communicates with a regular PS4 controller. While this is not a key robotic feature, it is both cool (especially with kids 😃) and convenient to control the robot via Bluetooth. Before trying it out I was concerned about potential lags, or the Raspberry Pi loosing its connection to the controller, but the field summary after using it for months is: it just works.

    Here are my configuration-troubleshooting notes, turned project log 😉

    Pairing the PS4 controller to the Raspberry Pi

    The setup instructions are not specific to Upkie: we can connect a PS4 controller to any Raspberry Pi. Start with the Bluetooh command line:

    sudo bluetoothctl

    Enable the agent with the following four instructions:

    agent on
    discoverable on
    pairable on

    Then start scanning for devices. The terminal should start listing scan results with all the Bluetooth devices around you:

    scan on

    While the command tool is scanning, press and hold both the Share and PS buttons on your controller to switch it to pairing mode. Keep holding the buttons at the same time until it starts flashing white light. When it does, check the terminal output for a new line like this one:

    [NEW] Device AC:FD:93:14:25:D3 Wireless Controller

    Note down the controller MAC address (here AC:FD:93:14:25:D3). Check that the controller is still flashing and do:


    That should be it! If everything went well, you should see a "successful connection" message appear in the terminal, like so:

    [bluetooth]# connect AC:FD:93:FD:68:0F
    Attempting to connect to AC:FD:93:FD:68:0F
    [CHG] Device AC:FD:93:FD:68:0F Connected: yes
    [CHG] Device AC:FD:93:FD:68:0F UUIDs: 00001124-0000-1000-8000-00805f9b34fb
    [CHG] Device AC:FD:93:FD:68:0F UUIDs: 00001200-0000-1000-8000-00805f9b34fb
    [CHG] Device AC:FD:93:FD:68:0F ServicesResolved: yes
    [CHG] Device AC:FD:93:FD:68:0F Paired: yes
    Connection successful

    Finally, trust the controller so that it can connect after a reboot:

    [bluetooth]# trust AC:FD:93:FD:68:0F

    That's it! The controller should now appear as a regular Linux joystick device in /dev/input/js*. That's where the Joystick source in Vulp (Upkie's motion control software) will look for it.

    Troubleshooting a reconnection issue

    Sometimes, for instance if you pair the PS4 controller to another device (say, a PS4...), the connection may fail with an error like this:

    [bluetooth]# connect AC:FD:93:FD:68:0F
    Attempting to connect to AC:FD:93:FD:68:0F
    [CHG] Device AC:FD:93:FD:68:0F Connected: yes
    Failed to connect: org.bluez.Error.Failed
    [CHG] Device AC:FD:93:FD:68:0F Connected: no

    Watch out for errors in /var/log/syslog by running the following command while connecting:

    tail -f /var/log/syslog

    The error I faced was the following one:

    Apr 12 23:33:25 raspi bluetoothd[471]: Can't get HIDP connection info
    Apr 12 23:33:26 raspi bluetoothd[471]: connect error: Invalid exchange (52)

    A workaround for this is to unpair:

    [bluetooth]# paired-devices
    Device AC:FD:93:FD:68:0F Wireless Controller
    [bluetooth]# remove AC:FD:93:FD:68:0F
    [DEL] Device AC:FD:93:FD:68:0F Wireless Controller

    Then connect again:

    [bluetooth]# scan on
    Discovery started
    [CHG] Controller DC:A6:32:D6:AA:D7 Discovering: yes
    [NEW] Device AC:FD:93:FD:68:0F Wireless Controller
    [bluetooth]# connect AC:FD:93:FD:68:0F
    Attempting to connect to AC:FD:93:FD:68:0F
    [CHG] Device AC:FD:93:FD:68:0F Connected: yes
    [CHG] Device AC:FD:93:FD:68:0F UUIDs: 00001124-0000-1000-8000-00805f9b34fb
    [CHG] Device AC:FD:93:FD:68:0F UUIDs: 00001200-0000-1000-8000-00805f9b34fb
    [CHG] Device AC:FD:93:FD:68:0F ServicesResolved: yes
    [CHG] Device 76:63:59:CE:8E:69 RSSI: -90
    [CHG] Device AC:FD:93:FD:68:0F Paired: yes
    [CHG] Device FA:0D:AC:BF:8E:C9 RSSI: -97
    Connection successful

    Hoping this helps. Happy roaming!

  • Hollow leg, part 1

    Tast's Robots08/10/2022 at 16:18 0 comments

    Sawing broomsticks to get Upkie up and running quickly was great at the beginning of the project, but it came with a drawback that came to light while manipulating the robot: a cable can get on the wrong side of a leg and get snapped away if the human manipulating it is not paying attention. Here is a bad instance on a shin:

    Pulling a cable hasn't happened yet while balancing, as the corrugated pipes that hold power and communication cables are quite stiff by themselves and tend to stay away from the legs, but it has happened for sure while sitting and raising the beast. So, today's update is about routing cables inside the legs. (This is part 1, we'll see about the lower leg in part 2.)

    Our goals are:

    1. Ensure that cables don't collide with the limbs during motion.
    2. Keep the same range of motion for hip joints.

    The idea is to 3D print a cable guide inside the hip, femur and knee parts. The femur thus becomes 3D printed (bye wood 😢).

    Here is a short video summary of the update:

    More details on the successive iterations:

    1. Hollow cylinder: this was a first guess. It would have worked with pressure screws like before, but then it occurred to me that squares and hexagons are much better shapes to transmit torque. (See for example this video from Scilabus on screw heads.)
    2. Hexagonal femur: testing torque transmission on mock parts with an outer hexagon and inner cylindrical cavity. To get a nice fit when assembling 3D printed parts, we can set either the femur outer diameter or hip inner diameter to be slightly smaller/larger.
    3. Hip with a hollow hexagon: going for a slightly larger hip inner diameter with a +0.2 mm margin on each edge. One mistake I did was to test the fit with small chunks of the inner shape, which may go through whereas the complete part won't (due to friction or jamming). Better print the whole (or at least a reasonable chunk of the) part to test this.
    4. Cylindrical femur with hex ends: the hexagonal shapes on both femur ends are exactly the right length to match the hips, and the femur is otherwise cylindrical. This facilitate assembly. Also, the inner cylinder guides the cable out the middle of the part to meet the knee servo connectors.
    5. Fixes to the hip: 3D printing iteration, getting all dimensions just right.
    6. Knee with a hollow inner hexagon: adapting the knee stator, following the same approach as the hip.

    At this stage, the leg assembly looks like this:

    One further nice thing to add are small rivulets inside the hollow cable guides to hold the corrugated pipe in place during motion:

    There are still several questions on the table that this design does not answer. How about turning the knee servo 180° to avoid the cable outlet in the middle of the femur? Also, can we get corrugated pipe that bends more smoothly?

    Note that we are keeping some length of cable above the hips to maintain the range of motion of the hip joint, in particular so that the robot can sit down and up properly. Eventually we can redesign the side chassis plates (blue in the picture above) to hold the cable better with a hole of the matching size (as opposed to a wide rectangular hole right now).

    Those points are left to future work 👷

  • Blowing off some CPU steam

    Tast's Robots06/11/2022 at 20:07 0 comments

    When running the Pink balancer agent on Upkie, the four CPU cores of the Raspberry Pi are used as follows:

    ProcessesThreadsUsage (%)
    Spine (+ other system processes)
    Logger (+ other system threads)
    10 ± 5
    1SpineSpine loop
    10 ± 5
    2SpineCAN communications
    50 ± 20
    3AgentPython threads

    On average over all cores, the ARM processor is only used at about 50% of its capacity, but that (or the fact that one core is used at 100% all the time) is enough to drive the CPU temperature up to a problematic level:

    (If you were wondering why Vulp has a CPU temperature observer, now you now 😉) The issue is that to protect itself the Raspberry Pi throttles all computations if its CPU temperature hits 80 °C, which for Upkie results mostly in skipped control frames (wheels apply their commanded velocities for too long ⇒ balancing degrades) and in the worst case triggers Vulp's safety of stopping actuators when no action is sent for more than 100 ms.

    Luckily it's not hard to cool down a Pi. Having a standing fan blow air at Upkie from the sides, where its wide "seal ears" are, does the job just fine. But the robot is not designed to stay near a fan, so let's give him an "earpiece" so that it carries its own fan:

    There is already a second male XT-30 connector on the pi3hat in Upkie's head, so we can order a 24 V DC fan from online retail, solder a female XT-30 connector to it, and just plug it to the power bus. In this update, the fan has the following properties:

    • Dimensions: 60x60x15 mm
    • Speed: 4700 rpm
    • Noise: 32 dBA
    • Power: 1.8 W
    • Flow: 44 m³/h

    The mount is a temporary adapter to connect to the existing right plate. Once we have settled on fan dimensions, we can redesign the plate so that it plugs into it directly:

    The fan is effective immediately. It brings down the CPU temperature to an equilibrium around 43 °C while the balancer is running full steam:

    Now the issue has become noise 😅 The fan's continuous 32 dBA relentlessly drill into the ears of neighboring humans, whose thoughts are inevitably driven to the next revision: smaller fan, less dBA!

    🔊 → 🔉

  • February 2022 update

    Tast's Robots06/05/2022 at 11:10 0 comments

    Here comes a shot of the robot in its natural habitat: the living room!

    The main improvement in this update lies in the locomotion software, which performs smoother motions and allows the robot to crouch down further.

    There is also a hack, documented in the code as the non-minimum phase trick, to improve transitions from standing to driving with the simple PID balancer. Some pointers to dig further: this hack helps handle the non-minimum phase nature of balancing systems (the fact that to go one way one first needs to go "a little" the other way, like counter-steering in bikes) without going for full model predictive control.

    The small wheels (OD: 100 mm) on Upkie are nice because they are sleek, which fits well the assumption made by the PID balancer that controls the velocity of the contact points with the ground, unfortunately they are also a bit small which doesn't fit the ankle design so well. That, rather than knee torques, is actually what limits how much the robot can crouch before it starts scratching the hell out of the floor (^_^)

  • December 2021 update

    Tast's Robots06/05/2022 at 10:17 0 comments

    Let's start this log with a short tour of Upkie's very first "let's put things together" version:

    Cables are on their own, not attached to the skeleton (surely "skeleton" sounds better than "broomsticks" ;p) of the robot, and just float around thanks to the rigidity of the corrugated pipes that bundle them. This causes no problem for these first tests, but in future revisions we'll probably want to specify where they are and be sure Upkie can go through its full range of motion without pulling a cable out.

View all 5 project logs

  • 1
    Print and assemble the chassis

    Note that Upkie is standing upright, so its neutral orientation differs from that of the quadruped. The correspondence between plate names is given in the following table:

    headleft, rightbackbuttocksstiffeners
    • Screw the power dist board to the buttocks plate using four M2 screws
    • Screw the left and right plates to the buttocks plate using M3x8 screws
    • Screw the front plate to the left and right plates using M3x8 screws
    • Screw the two stiffeners to the front plate using M3x8 screws
    • (Leave out the internal covers for now)
  • 2
    Assemble the head plate
    • 3D print the head plate using its G-code (for Prusa i3 MK3S+) or 3MF project
    • Mount the Raspberry Pi to the plate using the four M2 hex spacers. USB ports should be on the side of the “USB” printed text.
    • Mount the pi3hat on top of the Pi using the four M2 screws that come with it.
    • Mount the head to the left and right plates so that the USB side is to the robot's right (eyes are front, battery is back).

    Pay extra care to the last point. If the head is mounted the opposite way, you will have (1) a hard time plugging in XT-30 power cables and (2) to update the IMU frame orientation in the robot's URDF, a.k.a., more trouble than necessary ;-)

    • Optionally, 3D print the handle and screw it to the head plate using four M3x8 screws (this can always be done at any later stage)
  • 3
    Print leg parts

    All parts are printed in PETG with a 0.2 mm layer height.

    Infill varies from 15% to 30%, but most of these values are guesstimates that don't come from observing parts breaking after robot falls. The only exception to this is the wheel hub, where there is an explicit infill modifier on the hex coupler connector.

View all 10 instructions

Enjoy this project?



Peabody1929 wrote 11/13/2022 at 19:44 point

For hollow legs, how about ABS plastic pipe used for sprinkler systems?  It comes in 1/2", 3/4" and 1" diameter.  

  Are you sure? yes | no

Tom Nardi wrote 08/12/2022 at 01:29 point

This is a phenomenal project, very excited to see it develop.

  Are you sure? yes | no

Tast's Robots wrote 11/13/2022 at 17:14 point

Thanks! In November-December it will be mostly software updates (pink and upkie_locomotion packages) on GitHub: Then back to hardware with the hollow shins!

  Are you sure? yes | no

Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates