Many infantry operations in urban environments, such as building clearing, are extremely dangerous and difficult and
often result in high casualty rates. Despite the fast pace of technological progress in many other areas, the tactics and
technology deployed for many of these dangerous urban operation have not changed much in the last 50 years. While
robots have been extremely useful for improvised explosive device (IED) detonation, under-vehicle inspection,
surveillance, and cave exploration, there is still no fieldable robot that can operate effectively in cluttered streets and
Developing a fieldable robot that can maneuver in complex urban environments is challenging due to narrow corridors,
stairs, rubble, doors and cluttered doorways, and other obstacles. Typical wheeled and tracked robots have trouble
getting through most of these obstacles. A bipedal humanoid is ideally shaped for many of these obstacles because its
legs are long and skinny. Therefore it has the potential to step over large barriers, gaps, rocks, and steps, yet squeeze
through narrow passageways, and through narrow doorways. By being able to walk with one foot directly in front of the
other, humanoids also have the potential to walk over narrow "balance beam" style objects and can cross a narrow row
of stepping stones.
We describe some recent advances in humanoid robots, particularly recovery from disturbances, such as pushes and
walking over rough terrain. Our disturbance recovery algorithms are based on the concept of Capture Points. An N-Step
Capture Point is a point on the ground in which a legged robot can step to in order to stop in N steps. The N-Step
Capture Region is the set of all N-Step Capture Points. In order to walk without falling, a legged robot must step
somewhere in the intersection between an N-Step Capture Region and the available footholds on the ground. We present
results of push recovery using Capture Points on our humanoid robot M2V2.
We present the mechanical design of a bipedal walking robot named M2V2, as well as control strategies to be
implemented for walking and balance recovery. M2V2 has 12 actuated degrees of freedom in the lower body: three at
each hip, one at each knee, and two at each ankle. Each degree of freedom is powered by a force controllable Series
Elastic Actuator. These actuators provide high force fidelity and low impedance, allowing for control techniques that
exploit the natural dynamics of the robot. The walking and balance recovery controllers will use the concepts of Capture
Points and the Capture Region in order to decide where to step. A Capture Point is a point on the ground in which a
biped can step to in order to stop, and the Capture Region is the locus of such points.
We present the design and initial results of a power-autonomous planar monopedal robot. The robot is a gasoline powered, two degree of freedom robot that runs in a circle, constrained by a boom. The robot uses hydraulic Series Elastic Actuators, force-controllable actuators which provide high force fidelity, moderate bandwidth, and low impedance. The actuators are mounted in the body of the robot, with cable drives transmitting power to the hip and knee joints of the leg. A two-stroke, gasoline engine drives a constant displacement pump which pressurizes an accumulator. Absolute position and spring deflection of each of the Series Elastic Actuators are measured using linear encoders. The spring deflection is translated into force output and compared to desired force in a closed loop force-control algorithm implemented in software. The output signal of each force controller drives high performance servo valves which control flow to each of the pistons of the actuators.
In designing the robot, we used a simulation-based iterative design approach. Preliminary estimates of the robot's physical parameters were based on past experience and used to create a physically realistic simulation model of the robot. Next, a control algorithm was implemented in simulation to produce planar hopping. Using the joint power requirements and range of motions from simulation, we worked backward specifying pulley diameter, piston diameter and stroke, hydraulic pressure and flow, servo valve flow and bandwidth, gear pump flow, and engine power requirements. Components that meet or exceed these specifications were chosen and integrated into the robot design. Using CAD software, we calculated the physical parameters of the robot design, replaced the original estimates with the CAD estimates, and produced new joint power requirements. We iterated on this process, resulting in a design which was prototyped and tested.
The Monopod currently runs at approximately 1.2 m/s with the weight of all the power generating components, but powered from an off-board pump. On a test stand, the eventual on-board power system generates enough pressure and flow to meet the requirements of these runs and we are currently integrating the power system into the real robot. When operated from an off-board system without carrying the weight of the power generating components, the robot currently runs at approximately 2.25 m/s. Ongoing work is focused on integrating the power system into the robot, improving the control algorithm, and investigating methods for improving efficiency.
Series Elastic Actuators provide many benefits in force control of robots in unconstrained environments. These benefits include high force fidelity, extremely low impedance, low friction, and good force control bandwidth. Series Elastic Actuators employ a novel mechanical design architecture which goes against the common machine design principal of "stiffer is better." A compliant element is placed between the gear train and driven load to intentionally reduce the stiffness of the actuator. A position sensor measures the deflection, and the force output is accurately calculated using Hooke’s Law (F=Kx). A control loop then servos the actuator to the desired output force. The resulting actuator has inherent shock tolerance, high force fidelity and extremely low impedance. These characteristics are desirable in many applications including legged robots, exoskeletons for human performance amplification, robotic arms, haptic interfaces, and adaptive suspensions. We describe several variations of Series Elastic Actuators that have been developed using both electric and hydraulic components.