OpenQDD V1 - 3D Printed Robotic Actuator

OpenQDD is a 3D-printed open-source robotic actuator that I designed to be a powerful, easy-to-build, and relatively inexpensive solution for creating dynamic robots. Robots like Spot, Atlas, and the MIT mini cheetah easily impress the eye with their agility, speed, and overall ability to mimic natural motion. With OpenQDD, creating high-performing actuators at an affordable price is possible.

• Details about OpenQDD
• Github (For CAD, Code, and BOM)

Tools

• needle nose pliers
• 2.5mm allen wrench
• soldering iron

Electronic Parts

• Eaglepower 8308 Brushless Motor 90KV
• ODrive S1 (w/Screw Terminals)

General Parts

• ⌀8 x 2.5mm Encoder Magnet
• 8x22x7mm Bearings
• 12x28x8mm Bearings (x3)
• 75x95x10mm Bearing
• M3 x 6mm Inserts (x19)
• M3 x 5mm Standoffs (x4)
• M3 x 6mm Screws (x6)
• M3 x 10mm Screws (x4)
• M3 x 14mm Screws (x3)
• M3 x 18mm Screws (x4)
• M4 Locknuts (x8)
• M4 x 10mm Screws (x4)
• M4 x 30mm Screws (x4)
• M4 x 45mm Screws (x4)

Total Cost of Parts: $247

Start by printing all 13 parts:

• BLDC Casing
• Output Bearing Casing
• Gearbox(64T)
• Gearbox Cover
• ODrive S1 Cover
• Magnet Holder
• 28T Helical Planet Gear (x3) (rafts are recommended – 0.3mm separation distance)
• 8T Helical Sun Gear
• Planet Carrier Top
• Planet Carrier Bottom
• Planet Carrier Bottom Cover

I printed all of these parts in PLA. I would recommend using the following printer settings:

• 20% infill
• 210° extruder
• 45° bed

Using a hot soldering iron, press M3 heated inserts into the following 3D-printed parts.

• BLDC Casing - x7 inserts
• Planet Carrier Top - x6 inserts
• Planet Carrier Bottom - x6 inserts

Using a hammer, lightly tap:

• The large 75mm x 95mm x 10mm deep groove ball bearing into the Output Bearing Casing.
• A 12mm x 28mm x 8mm bearing into each 28T Helical Planet Gear. 
• The 8mm x 22mm x 7mm bearing into the Planet Carrier Top.

• Press the encoder magnet into the Magnet Holder and then press the Magnet Holder onto the bottom of the brushless motor.
• Mount the 8T Helical Sun Gear onto the BLDC motor using x4 M3 x 18mm screws.
• Mount the brushless motor onto the BLDC Casing using x4 M4 x 10mm screws. 

Using x4 M3 standoffs and x4 M3 x 10mm screws, mount the ODrive S1 onto the bottom of the BLDC Casing. Using x3 M3 x 6mm screws, cover the S1 with the ODrive S1 Casing.

• Fit the 3D-printed Planet Carrier Bottom into the large deep grove bearing. Secure the placement of the Planet Carrier Bottom by screwing the Planet Carrier Bottom Cover onto the Planet Carrier Bottom with x3 M3 x 6mm screws.
• Stack the Output Bearing Casing onto the BLDC Casing and stack the Gearbox(64T) onto the Output Bearing Casing. Screw all three parts together using x4 M4 x 40mm screws and x4 M4 Locknuts. 

• Fit each 28T Helical Planet Gear onto the Planet Carrier Bottom. Enclose the Planet gears with the Planet Carrier Top using x3 M3 x 14mm screws. 
• Lubricate the gears. I used white lithium grease.
• Cover the gearbox with the Gearbox Cover and x4 M4 x 20mm screws. Since the output bearing is located on the inside of the actuator, you don't actually need the cover at all.

To control the actuator with the ODrive S1, first, configure the brushless motor in the ODrive Pro GUI. Here are the settings that I used to configure the 90KV Eagle Power brushless motor on the ODrive S1.

Motor Parameters

• Type: High Current
• Pole Pairs: 20
• Current limit: 36A 
• Motor Calib. Current: 18A
• Motor Calib. Voltage: 5V
• Lock-in Spin Current: 18A

To configure the other parameters on the ODrive, such as the power supply, refer to the ODrive Manual. Here are a couple of things to keep in mind while configuring the ODrive.

• Connect and configure the 2Ω break resistor if you are using a power supply and not a battery.
• Purchase a USB isolator to connect the ODrive to your computer in order to prevent ground looping.

To interface the ODrive with other devices, refer to the ODrive Manual for the communication protocols that can be used on the S1.