모터의 종류와 원리 3 - QDD모터 - 이론

What is a QDD motor?

Generally, motors have high-speed, low-torque characteristics.
Therefore, a reduction gear is added to increase torque. When the motor rotates quickly with low force, the reduction gear lowers the speed accordingly, and the gear structure increases the torque.

A QDD motor stands for Quasi-Direct Drive, meaning it operates almost directly. It features a gear ratio of approximately 6:1 (compared to 7–12:1 in automobiles and 50–150:1 in robotic joints) to enhance
responsiveness, and its design allows torque to be estimated directly based on the current drawn by the motor.

The paper below indicates that a gear ratio of 100:1 is used for the Universal Robotics UR5e.

"Each joint of the robot includes a harmonic drive with a gear ratio equal to i = 100"

Garrido, S., et al. (2024). Electro-mechanical modeling and identification of the UR5e-series robot. Robotica. Cambridge University Press.

Why reduce the gear ratio?

This is because it is a method to improve responsiveness, an issue often cited regarding hardware in AI training. As the
gear ratio increases, it causes friction, backlash, and higher impedance, making the system nonlinear.
This results in reduced responsiveness to control inputs, longer training times, and increased control difficulty.

The QDD motor is a drive system proposed by MIT to improve the dynamic control performance of legged robots (Cheetah).

Features and Advantages of QDD Motors

1. Low Impedance & Reverse Drive Capability Due to
   the low reduction ratio, when an external force is applied to the motor, the joint does not encounter resistance from the reducer, allowing the motor to rotate in reverse easily.
   As a result, the robot naturally moves backward when subjected to external forces or impacts, making it well-suited for interacting with its environment.

2. Torque Predictability
   Since the gear ratio is low and the system is configured almost as a direct drive, there is minimal "backlash" caused by mechanical play, allowing torque
   to be predicted relatively accurately based on current compared to conventional motors.

3. Fast Response: Since it
   is less affected by the reducer, it is free from the inertial effects typical of conventional motors and offers faster response times.
   This allows for quicker response to control signals.

4. Size: The
   size of the reduction gear is reduced, resulting in a smaller motor.
   

Disadvantages of QDD Motors

1. Increased Design
   Complexity: Unlike BLDC motors, the gear ratio is small; therefore, to generate sufficient force, an increase in the performance and density of components such as windings, magnets, and capacitors is inevitable, leading to increased design complexity.

2. Heat Management:
   Although the size of the reduction gear is reduced, the density of components increases, necessitating appropriate heat dissipation measures.

3. Increased Costs: Costs rise
   due to increased design complexity, heat management requirements, and the need for more coils.

4. Torque Limitations:
   Due to the low reduction ratio, there is a limit to the absolute torque value compared to other motors under the same conditions.
   

QDD Motor Application Examples

1. MIT Cheetah

2. Unitree, Go1, 2