project specification

M2V2 Robot

A twelve degree of freedom lower-body humanoid robot with force controllable series elastic actuators at each degree of freedom. The robot has 3 degrees of freedom in each hip, one in each knee, and two in each ankle. Linear Series Elastic Actuators with brushless DC motors are used at each joint. These actuators allow for high fidelity force control, which enables low impedance control methods. The robot does not have any upper-body degrees of freedom as its main role is in bipedal walking research.

Specifications

Degrees of freedom12
Total weight45 kg
Sensors29
12 actuator sensors measuring actuator force,
12 actuator sensors measuring actuator position
1 inertial sensor measuring body roll, pitch and yaw
4 oot switches monitoring the condition of the foot
Computer systemPC104

Overview

The robot has 3 degrees of freedom in each hip, one in each knee, and two in each ankle. Linear Series Elastic Actuators with brushless DC motors are used at each joint. These actuators allow for high fidelity force control, which enables low impedance control methods. The robot does not have any upper-body degrees of freedom as its main role is in bipedal walking research. 

The legs are made of carbon fiber tubes permanently bonded to machined aluminum components, which define the joint ranges of motion. The body of the robot is made of seven carbon fiber plates mounted orthogonally to one another. These plates provide a stiff structure for actuator mounting as well as a protective housing for the computer, motor amplifiers and batteries. 

Actuators

The robot's actuators include integrated force and position sensing with quadrature encoders as well as a differential driver to reduce noise on the digital lines.  Mechanically, the actuators have been redesigned with floating linear bushings. The floating design prevents binding, especially at the ends of travel where manufacturing tolerances between adjacent parts becomes critical for proper alignment. The floating bushings have reduced assembly time of the actuators, as well as greatly improved the performance. 

The dynamic range is the ratio of the maximum output force and the lowest resolvable force. Series Elastic Actuators typically have dynamic ranges exceeding 300:1, and therefore enable high force-fidelity applications such as the proposed robot. 

Control

Virtual Model Control is used to control the orientation and height of the robot. Speed is controlled through the center of pressure on the support feet and through foot placement. To determine the most appropriate place to step, the robot uses the concept of capture points and capture regions. 

A capture point is a point on the ground in which the center of pressure can be placed in order to stop a robot. The capture region is the set of capture points. It uses a linear inverted pendulum plus feet model for determining the location of the capture region. With this model the capture point dynamics are linear and first order, allowing for easy determination of where to step.

References

Describes the robot's design, anatomy, actuators, and computer and development interface. Goes into the controls.

Jerry E. Pratt, John Rebula, Twan Koolen, et al. 2008

Describes the robot, its actuators, and future work.

Jerry E. Pratt, John Rebula, Twan Koolen, et al. 2009

Step by step instructions on How to rebuild a Series Elastic Actuator for the M2V2 Humanoid Robot.

https://www.ihmc.us/

Short paper discussing balance, walking, and push recovery control of the Yobotics-IHMC biped.

Jerry E. Pratt, John Rebula, Twan Koolen, et al.

Wevolver 2022