Introduction: Romulus M2 BLDC Actuator

By

The Romulus M2 Actuator was designed as the primary driver for my Romulus M2 robotic arm (coming soon). My goal was to create something simple, reliable, fast, and powerful. I believe this design accomplishes three of those four criteria quite well, though the "power" aspect is currently bottlenecked by the gearbox, which is rated for only 20 Nm. In destructive testing, I managed to push past this limit, reaching approximately 76 Nm before the gearbox failed. I am currently looking into similarly sized gearboxes from StepperOnline with higher torque ratings as a future upgrade. The actuator is designed for plug and play connections with 1” carbon fiber tube.


With a 50:1 reduction, this actuator is surprisingly back drivable and can be turned easily by hand. While it may not match the raw specs of some commercial motors, it benefits from the excellent reliability and documentation of the ODrive S1, and it is very simple to modify or repair. Given that the Eaglepower 90KV motor outputs approximately 2 Nm, a 50:1 gearbox multiplier has the potential to create a very fast and powerful actuator.


As it stands, the actuator is reliable and well-suited for a robotic arm. Additionally, the inherent strength and rigidity of the gearbox provide an excellent structural mounting point for connecting other robotic components.


Specifications

  1. 50:1 Planetary Gearbox
  2. ODrive S1 FOC Controller with an Onboard Encoder
  3. 90KV Eagle Power BLDC Motor
  4. x7 3D Printed Parts
  5. Nominal Output Torque: 20 Nm
  6. Moment Permissible Torque: 40 Nm
  7. Total Mass: 1480g (3.3 lbs)
  8. Total Cost: $263

Supplies

Tools

  1. Needle nose pliers
  2. Allen wrench set
  3. Screwdriver
  4. Soldering iron
  5. 3D Printer


Electronic Parts

  1. Eaglepower 8308 Brushless Motor 90KV
  2. ODrive S1 (w/Screw Terminals)


General Parts

  1. Gear Box MG Series Planetary Gearbox Gear Ratio 50:1
  2. Shaft Key 5x22x9mm
  3. Flange Coupling Connector 8mm
  4. Dowel Pin 8mm x 20mm Dowel Pin
  5. M3 x 5mm Standoffs (x4)
  6. ⌀8 x 2.5mm Encoder Magnet


Screws

  1. (4x) M4 10mm
  2. (4x) M4 15mm
  3. (4x) M4 20mm
  4. (4x) M4 60mm
  5. (4x) M3 12mm
  6. (1x) M4 10mm
  7. (4x) M3 60mm
  8. (4x) M3 10mm
  9. (1x) M3 5mm
  10. (4x) M4 12mm


Total Cost $263


Disclaimer: Buying the parts as listed is not necessarily the most economical way to achieve a high-torque actuator, but it may be cost-effective depending on your priorities and if you use alternative motor controllers and purchase hardware in bulk.

For example, the Robstride 03 actuator uses all metal parts and achieves 20 Nm working torque for $250.

Using an alternative motor controller like an ODESC V4.2 with AS5048A Magnetic Encoder and sourcing other inexpensive hardware could bring this actuator’s total cost down to around $170, which may offer a competitive value compared to commercial alternatives.

Step 1: 3D Print the Parts

Start by printing all 8 parts: (All of these parts were printed using Bambu PLA Basic on a Bambu A1 Mini printer.)


  1. Gear Box Housing (A,B)
  2. Motor Mounting Plate (x2)
  3. ODrive S1 Cover
  4. Input Shaft Coupler
  5. Encoder Magnet Mount
  6. Output Hub


Attachments

Step 2: Mount the Motor

  1. Press the encoder magnet into the Magnet Holder and then press the Magnet Holder onto the bottom of the brushless motor.
  2. Mount the brushless motor onto the Motor Mounting Plate using 4x M4 10mm screws.

Step 3: Mount the ODrive S1

  1. Using x4 M3 standoffs and x4 M3 x 10mm screws, mount the ODrive S1 onto the bottom of the BLDC Casing.
  2. Using x1 M3 x 6mm screw, cover the S1 with the ODrive S1 Casing.

Step 4: Input Coupling

  1. Attach the Input Shaft Coupler to the motor
  2. Attach the Flange Coupling Connector to the Input Shaft Coupler
  3. Attach the 8mm Dowel Pin to the Flange Coupling Connector

Step 5: Gear Box

  1. Attach the second Motor Mounting Plate to the bottom of the gearbox using 4x M4 10mm screws.
  2. Attach Gear Box Housing A and B to each other using 4x M3 60mm screws.
  3. Attach Gear Box Housing A and B to the gearbox using 4x M4 20mm from the top of the gear box

Step 6: Powertrain Assembly

  1. Slide the dowel connected to the motor into the gearbox input and secure using an allen key through the side port of the gearbox. Make sure to leave enough room so that the Flange Coupling screws are not rubbing on the top mounting plate.
  2. Connect the two Motor Mounting Plates to each other using 4x M4 60mm screws.

Step 7: Output Hub

  1. Place the Shaft Key into the output shaft of the gearbox
  2. Slide the Output Hub over the output shaft and secure from the top using 1x M4 10mm screw.

Step 8: Controlling the Actuator

To control the actuator with the ODrive S1, first, configure the brushless motor in the ODrive Pro GUI. Below are the settings used for the 90KV Eagle Power brushless motor on the ODrive S1:

Motor Parameters

  1. Type: High Current
  2. Pole Pairs: 20
  3. Current limit: 36A
  4. Motor Calib.Current: 18A
  5. Motor Calib.Voltage: 5V
  6. Lock-in Spin Current: 18A


Refer to the ODrive Manual for additional settings (e.g., power supply configuration).


Keep in mind:

  1. Connect and configure the 2Ω break resistor if you are using a power supply and not a battery.
  2. Purchase a USB isolator to connect the ODrive to your computer in order to prevent ground looping.
  3. To interface the ODrive with other devices, refer to the ODrive Manual for the communication protocols that can be used on the S1.