dARM is a 3d printed 6 DOF robotic arm. Initially, I wanted to build a 3d printed arm as a test of my new Bambu X1C. I found most existing projects used stepper motors. I wanted a better power density.
arm.mp4
So the dARM journey began. The goal was to design a cheap, powerful, robust, modular, and easily maintained arm from scratch using ODrive S1 BLDC controllers. It stands .975 meters tall and can easily hold 5 lbs in the horizontal position.
penguin.mp4
After iterating through various designs, modularity and maintenance became top priorities. If any individual component fails, its modular section can be removed from the larger arm for easy replacement.
Version numbers are determined purely by emotion. Don't ask what SDE stands for.
| Version | Date | Notes |
|---|---|---|
| 0.35 SDE | 12/28/2024 | 6 axis, 7 motor design |
| 0.45 SDE | 1/17/2025 | Gripper v2 added |
| 0.55 SDE | 1/28/2025 | Power box added |
| 0.56 SDE | 3/15/2025 | New gripper motor magnet holder |
| 0.59 SDE | 3/24/2025 | Dock and finger v4 added |
| 0.60 SDE | 4/5/2025 | Gripper gear/magenet holder integration |
| 0.62 SDE | 5/2/2025 | ROS2 pkg: published in submodule |
| 0.70 SDE | 6/22/2025 | ROS2 pkg: teleop node and refined URDF |
Without the below projects and youtube channels the dARM project wouldn't exist. I can't thank them enough for the inspiration and advice they've provided.
What can I say about James. I've been watching his channel for years. Initially, because he did interesting things with 3d printed parts, but shortly after that because the things he did were so consistently interesting. James introduced me to ODrives as a control mechansim for BLDC motors.
Another great youtuber. He focuses on robotic arms, mostly 3d printed robotic arms. He covers specifics of design, assembly, and control for several different robot arms. He also covers ODrives in great detail, including a video that uses a custom modified firmware to increase communication speed for specific queries over CAN.
The basis for the primary actuators are the OpenQDD by Aaed Musa. Aaed also has an excellent youtube channel. We use the same basic layout of an 8308 motor, an ODrive S1, and a planetary gearbox. Our actuators have been redesigned from scatch to increase strength and rigidity, tighten tolerances, print easier, and to have more robust mounting capabilities.
The basis for the gripper is the SSG48 project. We use a different motor, controller, and mounting dimensions, but we keep the basic motor housing layout, MGN7C rail/carriers, and a rack and pinion to move the fingers.
Gary is an excellent electrical engineer, former owner of GCM Computers, and my former boss. I want to thank Gary for the learning environment he provided me for nearly a decade under his employee. I also want to thank him for the advice on the power system for the dARM.
The dARM is controlled with a Raspberry Pi 4b communicating to eight ODrive S1s via CAN bus. This is achieved with the help of an RS485 CAN Hat on the Pi. The ODrives are using the built in encoder with encoder magnets attached to each motor.
In a CAN network, the controller does not require a node ID, but each device on the network must have one. From this point forward, assume that the term "node" refers specifically to an ODrive and that node IDs are zero-indexed.
The Pi is currently running Ubuntu 22.04 Server, but everything should still work with newer versions. Instructions for installing Ubuntu on a Pi can be found here. Set the hostname to pidarm and the username to darm. It is also recommened to set a static IP address so it can be easily accessed via SSH.
The CAN Hat communicates to our ODrives, but it must also communicate to the Pi. This is done through a Serial Peripheral Interface (SPI). To enable the interface edit /boot/firmware/config.txt and add the following at the bottom of the file:
dtparam=spi=on
dtoverlay=mcp2515-can0,oscillator=12000000,interrupt=25
dtoverlay=spi0-hw-cs
Reboot and run dmesg | grep MCP2515 to verify the CAN Hat is now recognized.
Next we need to bring up the CAN interface with the following:
sudo ip link set can0 up type can bitrate 1000000
Then run ip a to veryify the interface is up.
Finally, install some handy CAN troubleshooting tools:
sudo apt-get install can-utils <---- includes candump which can be used to see heartbeats from ODrives
sudo apt install python3-can <---- includes CAN viewer script which can view all CAN traffic
A systemd service can be used to bring up the CAN interface on boot. Create file /etc/systemd/system/can0-setup.service with the following contents:
[Unit]
Description=Set up CAN0 interface
After=network.target
[Service]
Type=oneshot
ExecStart=/sbin/ip link set can0 up type can bitrate 1000000
[Install]
WantedBy=multi-user.target
Then run the following commands:
sudo systemctl enable can0-setup.service
sudo systemctl start can0-setup.service
sudo systemctl status can0-setup.service
sudo reboot
Finally, run ip a to verify the CAN interface started on boot.
cd to an appropriate directory and run git clone https://github.com/JesseDarr/dARM.git to clone the repo. You will find the test scripts in the odrive_tools folder.
💡 Note: This folder is actually a git sub module that links to this repo:
https://github.com/JesseDarr/dARM_odrive_tools.git
If you want to buy a cheap bluetooth dongle you can use an XBox One, XBox Series X, PS4 or PS5 controller with the gamecontroller.py script to control the dARM. You will need to install the appropriate drives and use the bluetoothctl command to pair/connect the controller. Run ls /dev/input and check for js0 to ensure the controller is ready to work with the script.
Bluetooth on Linux is sketchy...bluetooth is sketchy...to combat this reset_interfaces.sh will reset the CAN and Bluetooth interfaces. It can save a few reboots from time to time.
If using a Playstation Controller you also want to ensure the Sony module loads on boot. Edit /etc/modules and add hid-sony to the bottom line.
CAN wiring starts from the CAN Hat on the Pi. A single twisted pair cable connects it to ODrive 0, which is then connected to ODrive 1, and so on with each ODrive daisy chained from the last like this:
graph LR
subgraph "Raspberry Pi 4b"
CAN_Hat[CAN Hat]
end
CAN_Hat <--> ODrive0[ODrive 0]
ODrive0 <--> ODrive1[ODrive 1]
ODrive1 <--> ODrive27[ODrive 2-7]
It's not well documented, but the ODrive S1 includes two 4-pin JST-GH ports. Each ODrive has 1 cable connected to the previous node and 1 cable connected to the next node. It does not matter which port is used for which cable. This allows some freedom when building custom length CAN cables.
Be sure to enabled the 120ohm resistor on ODrive 7 by flipping the DIP Switch to 120R. All other ODrives should have this DIP Switch set to No R.
The BOM lists sacrifical 4pin JST-GH wires. You will need to cut them in half and solder them into twisted pair. You need to wire the twisted pair in a roll over fashion such that PIN 2 is wired to PIN 2, and PIN 3 is wired to PIN 3. It is recommend to solder 1 end connector onto the tiwsted pair, attach it to an ODrive, and then measure the required length of that cable.
The cable that connects the PI to ODrive 0 is a special case: it must also include PIN 4 for ground. It should be wired into the CAN Hat like this:
Power is provided by a 1200W 48V switching power supply. While this voltage exceeds the motors’ nominal ratings, each motor typically draws only 2–3 amps and operates intermittently rather than under continuous load.
The switching power supply along with a power monitor and power switch are packaged up inside our power box. AC power goes in one side and DC power comes out the other.
The inside looks like this:
Wiring diagram:
Power wiring is all 14 AWG except for the gripper motor which is powerd with 18 AWG. It is likely that the forearm motors could also be powered with 18 AWG.
Wiring a robot arm is difficult. Wiring is all custom length and is measured after the robot has been assembled. This allows a wire to be screwed into an ODrive, the respective joint moved to its most distant position, and an appropriate length of cable cut.
Wiring is also facilitated by T Tap Connectors. These provide a T split at 90° and makes wiring much easier. You can find them in the BOM. Zip tie 2 of them bottom to bottom to handle both the positive and negative wires.
Here is an overall power wiring diagram.
%%{ init: { "flowchart": { "defaultRenderer": "elk" } } }%%
graph TD
PowerBox[Power Box] --> XT90[XT90]
XT90 --> T0( )
XT90 --> ODrive0[ODrive 0]
T0 --> T1( )
T0 --> T2( )
T1 --> ODrive1[ODrive 1]
T1 --> ODrive2[ODrive 2]
T2 --> ODrive3[ODrive 3]
T2 --> T3( )
T3 --> ODrive4[ODrive 4]
T3 --> ODrive5[ODrive 5]
ODrive5 --> ODrive6[ODrive 6]
ODrive6 --> ODrive7[ODrive 7]
💡 Note: Each line in the diagram represents both postive and negative wires, and each empty square represents a
T Tap Connector.
The dARM sits on top of a base made of 6 legs each holding a 5lb weight to provide a solid base.
Joints 0–3 provide a single axis of rotation each and are powered by the primary actuators. Joint 1 carries the highest load and uses a dual-motor setup to provide the necessary torque.
The forearm uses two motors connected through a two-stage belt system to drive a differential, enabling two independent axes of rotation.
The gripper uses a single motor to drive a pair of Compliant TPU fingers.
These are the modular building blocks of joints 0-3. Each actuator use an Eagle Power 8308 BLDC motor mated to a 9:1 planetary gearbox.
The gear box uses a helical angle of 15°, a pressure angle of 15°, and a module of 1.4 mm. Here are the tooth counts:
| Gear | Count |
|---|---|
| Sun | 8 |
| Planet | 28 |
| Ring | 64 |
The primary actuators come in 2 flavors: single bearing and double bearing.
Double bearing actuators handle load perpendicular to the rotation axis better. They are used in joints that rotates on the vertical axis.
Single bearing actuators are used in joints that rotate on the horizontal axis.
💡 Note: Joint 1 uses 2 single bearing actuators for increased power. This also eliminates the need for a double bearing actuator.
Below is an explode animation of each flavor. They should give you a good idea of the sub components involved and are also a good assembly reference.
Actuator_Single_Bearing.mp4
Actuator_Double_Bearing.mp4
The forearm uses two GB36-2 BLDC motors connected through a pulley system to drive a differential.
The pulley system consists of two-stages. Each uses 20 tooth and 72 toothpulleys connected via a 300 mm GT2 belt. This provides a ratio of 3.6:1 per stage and a final drive ratio of 12.96:1
The motors oppose each other. If they are driven in opposite directions then the differential will bend along the X axis. If they are driven in the same direction then the differential will twist along the Z axis.
Interestingly, this means both motors are in use for every movement the differential makes. So the final drive ratio approaches a potential maximum of approximately 26:1.
The differential uses Zerol Bevel Gears made with Bevel Gear Maker. They pinion and wheel gears each have 38 teeth for a final ratio of 1:1. Other measurements needed to reproduce the gears:
| Parameter | Value |
|---|---|
| Module | 1.6 mm |
| Pressure Angle | 25° |
| Face Width | 14 mm |
| Clearance Factor | 0.3 mm |
Below is an explode animation for the forearm:
Forearm.mp4
The gripper uses a GM5208-24 BLDC motor to drive a rack and pinion to articulate the Compliant TPU Fingers which ride along a MGN7C rail.
The compliant nature of the fingers as well as the rubbery TPU allow the fingers to change shape as they come in contact with an object. They form around the object to provide a better grip. The gripping surface is printed with fuzzy skin to provide additional friction.
The rack and pinion system has the following parameters:
| Parameter | Value |
|---|---|
| Module | 1.4 mm |
| Pressure Angle | 30° |
| Gear Teeth | 8 |
| Rack Teeth | 10 |
| Gear Height | 11.2 mm |
| Rack Heigh | 7.7 mm |
Below is an explode animation of the gripper:
Gripper.mp4
Pillars are everywhere. They are used to connect the legs to joint 0, they are used in all 5 of the primary actuators, and they are used again in the gripper.
They provide open space around the motors for improved air flow. A pair of them can be replaced with an adapter to provide sturdy connection point to other parts of the robot.
The strength of the pillars is a function of their cross sectional area. This is used to our advantage by offsetting the bolt holes in the pillars in the primary actuators. This allows for the total actuator diameter to be reduced by a few milimeters.
This section is a collection of tips for assembling the robot.
Some sanding is required, specifically anywhere a bearing will touch a printed part. All bearings should be difficult to press fit into its coresponding part, but not too difficult as to deform the geometry of the part. To achieve this type of fit sanding with 150 grit sand paper is necessary. Be paitent. Sand, test fit, sand, test fit, sand, test fit, over and over.
Additionally if there are any blemishes on the mating surface of 2 printed parts they should be sanded flat.
CA glue, otherwise known as Super glue, works wonders with PLA. There is a nice little write up of the subject here.
It is recommended to glue in all nuts that are slotted in. Not only does it ensure they won't fall out during assembly or maintenance, it also makes it much more difficult for a nut to "strip" the surounding PLA.
The modular units of the dARM should each be fully assembled first. These units are:
- Legs
- Joint 0
- Joint 1
- Joint 2
- Joint 3
- Forearm
- Gripper
Once these units are assembled piece them together from the bottom up. It is recommended to pause building and to test all joints after each new joint is added to the robot. console.py is perfect for this as it will dynamically add additional control sliders to the interface as additional ODrives are added to the network.
The single bearing primary actuators should be assembled in this order:
- Mount the
motorto themotor plate(don't forget the magenet holder) - Attach the
sun gearto themotor - Insert the
large bearinginto thebearing holder - Build the
carrier basearound thelarge bearing - Attach the
bearing holderto themotor platevia thepillars - Place the
ring gearon top of thebearing holder - Insert the
planet gearsonto the posts of thebearing holder - Attach the
cover plateand bolt the entire assembly together - Attach the
carrier toporcarrier adapterdepending on the joint
The double bearing primary actuators are the same as above save the additional step of placing the second bearing holder on top of the ring gear before attaching the cover plate.
There are several different motor plates. They look mostly identical, but they are not. There are small 1mm tall arcs used to guide our pillars into place since they have an offset bolt hole in them. These arcs differ from actuator to actuator depending on a few factors.
The dARM.3mf file has the different motor plates clearly labeled. Be sure to keep track of them or it will come at the price of disassembly/reassembly.
Although they are not seen in any fo the CAD, all 5 primary actuators uses a 2 ohm brake resistor. Wiring instructions can be found on in the ODrive documentation. Zip tie them to the standoffs holding the ODrive on the respective actuator.
Below is a list of useful links: