MIT researchers have developed an algorithm for bounding that they’ve successfully implemented in a robotic cheetah — a sleek, four-legged assemblage of gears, batteries, and electric motors that weighs about as much as its feline counterpart. The team recently took the robot for a test run on MIT’s Killian Court, where it bounded across the grass at a steady clip.The key to the bounding algorithm is in programming each of the robot's legs to exert a certain amount of force in the split second during which it hits the ground, in order to maintain a given speed: In general, the faster the desired speed, the more force must be applied to propel the robot forward.
In experiments on an indoor track, the robot sprinted up to 10 mph, even continuing to run after clearing a hurdle. The MIT researchers estimate that the current version of the robot may eventually reach speeds of up to 30 mph.
The MIT Cheetah 2 contains the custom electric motor designed by Jeffrey Lang, the Vitesse Professor of Electrical Engineering at MIT and the amplifier designed by David Otten, a principal research engineer in MIT’s Research Laboratory of Electronics.
This work was supported by the Defense Advanced Research Projects Agency.
MIT Cheetah Robot Bounds Off Tether, Outdoors - IEEE Spectrum
Home | MIT Biomimetics Robotics Lab
"Quadruped Bounding Control With Variable Duty Cycle via Vertical Impulse Scaling,"
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| Park, Hae-Won | Massachusetts Inst. of Tech |
| Chuah, Meng Yee (Michael) | Massachusetts Inst. of Tech |
| Kim, Sangbae | Massachusetts Inst. of Tech |
Abstract: This paper introduces a bounding gait control algorithm that allows a successful implementation of duty cycle modulation in the MIT Cheetah 2. Instead of controlling leg stiffness to emulate a `springy leg' inspired from the Spring-Loaded-Inverted-Pendulum (SLIP) model, the algorithm prescribes vertical impulse by generating scaled ground reaction forces at each step to achieve the desired stance and total stride duration. Therefore, we can control the duty cycle, the percent of stance phase of the total cycle. By prescribing the required vertical impulse of the ground reaction force at each step, the algorithm can adapt to variable duty cycles (the portion of time that leg stays on the ground) attributed to variations in running speed. Following linear momentum conservation law, in order to achieve a limit-cycle gait, the sum of all ground reaction forces must match vertical momentum loss due to gravity during a cycle.
In addition, we added a virtual compliance control in the vertical direction to enhance stability. The stiffness of the virtual compliance is selected based on the eigenvalue analysis of the linearized Poincare map and the chosen stiffness is 700 N/m, which corresponds to around 12% of the stiffness used in previous trotting experiments of MIT Cheetah, where the ground reaction forces are purely caused by the impedance controller with equilibrium point trajectories. This indicates that the virtual compliance control does not significantly contributes to generating ground reaction forces, but to stability. The experimental results show that the algorithm successfully prescribes the duty cycle for stable bounding gaits. This new approach can shed a light on variable speed running control algorithm.
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