4B Robot

A Baseball Size Quadruped Robot

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4B (Four-Ball) is a four-legged robot with 12DOF that folds into a 6cm ball! Although tiny it has three linkages on each leg and joints powered by micro servomotors. The overall weight of the robot had to be light enough for these tiny motors to have enough torque and lift it's on weight on at least three legs.

My plan is to also make it roll when folded. But first, 4B needs to take its first steps

4B is a hybrid robot that can be able to both walk and roll. It has three linkages (coxa, tibia and femuer) on each leg which folds into a 6cm diameter ball. That’s smaller than a baseball! To do so, every linkage had to perfectly align into the sphere while leaving space for the battery and pcb.

4B Robot

  • PCB Assembly Update

    Carl Bugeja07/21/2018 at 22:49 0 comments

    Today i have continued debugging the pcb for any soldering bugs and test all the circuitry. The pcb ended up weighing 6.5 grams. This includes the programming header. This can be eliminated by designing a programming jig which will definitely be handy to have. 

    For now, i have soldered a temporary battery header until i configure the sleep mode. 

    Unfortunately one of the programming pads got disconnected while doing rework on the BLE module 😐 this means that i have to start over with a new pcb. 

  • PCB Assembly

    Carl Bugeja07/19/2018 at 21:54 2 comments

    All the components have arrived and today i started soldering the pcb! 

    I’m quite happy with the result 🙂 The only issue i have found till know is with the USB socket. For some strange reason the slots for its alignment pins were not generated in the NC drill files. So i had to chop off these pins from the socket and soldered them with the pad.

  • PCBs have Arrived!

    Carl Bugeja06/14/2018 at 18:33 2 comments

    The tiny pcb for my robot project has just arrived! Only a few components remaining to arrive so that i can start soldering the whole board.

    It seems to fit perfectly in place!

  • PCB

    Carl Bugeja06/04/2018 at 23:08 0 comments

    These are the 3d renders of the 36*36mm 4-layer PCB board for the 4B robot. It contain the CYBLE-022001-00, a tiny ble module in a 10*10mm package. The BNO055 IMU which has an internal fusion block and the VCNL4040M3O as the proximity sensor. Servo connector are all placed around the edge, closer to the servo motors. It also has a vertical usb connector and the MCP73831T LiPo battery charger with a status rgb led in the middle.

  • Electronics System Architecture

    Carl Bugeja06/02/2018 at 16:03 3 comments

    So this is where the fun begins. Design all the electronics of 4B on a 36*36mm pcb. To do so I needed to narrow down the number of components to the bare minimum. And these are the main modules:

    • MCU integrated with BLE Module
    • 9-axis IMU
    • Proximity Sensor for z-axis control
    • Connectors for the servo-motors
    • 3.3V LDO voltage regulator
    • USB LiPo battery charger

    The MCU will be responsible:

    • Bluetooth communication
    • Reading data from the IMU and proximity sensor on the I2C bus.
    • Calculating the inverse kinematic equations to actuate the robot kinematic chain
    • Setting out 12 PWM waveforms to the servo-motors

    That's a bit too much to handle for one MCU, but given the available space on the board it has to work.

  • Assembly

    Carl Bugeja06/01/2018 at 05:13 1 comment

    This is the assembly video of the 4B robot:

    I'm quite happy with the way the design turned out. 

    All parts are 3d-printed with nylon plastic and their finish looks beautiful. Some of the linkages feel a little flexible but that can be easily fixed by making a thicker edge. The servo-motor hubs that are integrated with the linkages fit perfect inside the servo splines and the tiny SMF681X-ZZ bearings snaps firmly in place.

    The next step for this project is to start designing the electronics and pcb. Having a custom pcb in place will tidy the wiring mess so that I could test the robot more easily. 

  • 3D Printed Parts

    Carl Bugeja05/31/2018 at 20:09 0 comments

    All the 3D printed parts of 4B, printed from shapeways's strong & flexible nylon.

  • Servo Motors

    Carl Bugeja05/30/2018 at 21:32 2 comments

    This is not my first quadruped robot I've build ( One the most important things I've learned from this past project is that the selected servo-motors must be of very good quality. If one out servo is acting crazy, your twelve motor walking robot is doomed.

    After a lot of research I ended up narrowing the choice to the HK-5320 and HK-5330 for this project. Both of these servo-motors are under 2grams and can working with a 1S LiPo battery. The only difference between these servo motors is the dimensions and the torque. The HK-5320 measures 20x6.2x18mm and has a stall torque of 0.075kgcm, while the HK-5330 measures 20x6.2x23mm and has a stall torque of 0.17kgcm.

    So for the HK-5320 the robot's total weight can reach around 37.5grams (worst joint on 3legs), while for the HK-5330 a the robot's total weight can reach around 85grams.

    The mass of my robot should not exceed 50grams, so my aim is to use a combination of these servomotors, using the HK-5330 for the joints that require larger torque.

  • 3D Model

    Carl Bugeja05/30/2018 at 17:24 2 comments

    This is 4B's first 3d model prototype. The main parts consists from a base, 4 coxa linkages, 4 femuer linkages and 4 tibia linkages. The base and femuer linkages houses the HK-5320 micro servomotor.

    Most of the robot's weight is positioned at the centre. This includes the battery, pcb and four out of the twelve servo motors. This low centre of gravity will give the robot more stability.

  • Concept

    Carl Bugeja05/28/2018 at 19:22 0 comments

    My idea behind this project is to design a 12 degrees-of-freedom quadruped robot that folds into a 6cm diameter ball. This way the robot could both walk and roll. The trick is to divide the sphere into four and use these as the robot's tibia linkage. The rest of the leg's linkages need to fold into the same sphere.

    The challenge of this project is to fit all the mechanical parts, 12 servo-motors, the battery and the pcb into a very small space. 

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de∫hipu wrote 01/31/2019 at 10:47 point

Hi, are you still working on this? There hasn't been an update in a while, so maybe you moved with the project somewhere else? It's such a nice robot, it would be great to see it working!

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solano.luca wrote 06/19/2018 at 11:19 point

very beautiful. will you share the .stl?

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Carl Bugeja wrote 07/15/2018 at 21:45 point

Yes i will after testing the whole thing 

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JuPrgn wrote 09/18/2018 at 18:28 point

Hi do you have some results testing ? Do you still plan to share some files ? I would like to see more details 😉

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Chorcon wrote 06/07/2018 at 13:47 point


I clicked the project and thought to myself "It would be SO cool if he made it fold to a ball!". I then saw the animation in the description. I was positively surprised! :D

Thanks for sharing.

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Dan DWRobotics wrote 06/02/2018 at 21:23 point

This is looking awesome. Look forward to seeing it walking and rolling. That IMU is going to come in handy when it comes to getting the legs to gently nudge it around to make it roll.

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Carl Bugeja wrote 06/03/2018 at 06:30 point

Thanks :)

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Michael Graham wrote 06/02/2018 at 07:05 point

Very clever i love it! Reminds me of a star wars droidika

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Carl Bugeja wrote 06/03/2018 at 06:31 point

Thanks! yes it looks kind of similar

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de∫hipu wrote 05/30/2018 at 09:19 point

That looks great! I imagine it could be even able to roll when in the ball form!

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Carl Bugeja wrote 05/30/2018 at 16:43 point

Yes it should :)

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