Quadruped Robot Leg Using Capstan Drives

Implementation

1. Design

A. Mechanical - The design uses 2x BLDC motors for higher torque, and the shafts bolted to the motors are reeved in order to capture the rope. Dyneema rope is used since it has essentially zero creep until it breaks which ensures the rope is not stretched over time. Bearings are used to support motor driven shafts and the joint drums, and the motor drum to joint drum gear reduction is 10:1 which ensures the linkage is backdriveable. All parts were 3D printed except for the shaft bearings. The knee and hip joints are co-axial which compacts the overall design footprint and makes the inverse kinematics derivation simpler. 

B. Electrical - The design utilizes two brushless dc motor drivers powered by a 12V power supply, with the PWM control signal sent from an Arduino Uno microcontroller. Each motor has an encoder for positional feedback which is input into the Arduino for closed loop control of the motors. 

2. Brushless Motor Control

Positional control of the BLDC motors will be achieved using field oriented control (FOC) within a cascading PID controller. Field oriented control is a method by which three phase brushless motors' torque can be controlled using sinusoidal commutation with high efficiency (see references). Within the cascade controller, the position control loop determines the desired motor velocity, then the velocity control loop determines the target motor current, then the torque control loop (which implements FOC) determines the voltage commands to each phase of the BLDC motor. A control diagram is shown below.

3. Assembly 

The assembly of the quadruped leg is complete and pending electronics and controller integration and testing.

Next Steps

1. Implement BLDC FOC and inverse kinematics onto microcontroller

2. Conduct testing:

   1. integration

   2. range of motion

   3. speed

   4. load testing

   5. longevity

   6. controller performance for trajectory control of the end effector (foot pad)

References

1. https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.osti.gov/servlets/purl/1340266&ved=2ahUKEwillJy7hOCOAxVRmmoFHQp2CRYQFnoECBkQAQ&usg=AOvVaw0DoOl9F7V3BI__qcmw6XpG

2. https://docs.simplefoc.com/

3. https://www.mathworks.com/discovery/bldc-motor-control.html 

4. https://docs.simplefoc.com/foc_theory