1. Efficient Bipedal Robots Based on Passive-Dynamic Walkers
Collins, S., Ruina, A., Tedrake, R., & Wisse, M. (2005)
Greg Dachner
CLPS 1500

2. Introduction And they look a little clunky…
Engineers have been trying to build bipedal walking robots for the last few decades, with varied success.
But most of this work has focused on robots with precise joint control, akin to the motor programming view.
And they look a little clunky…
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3. Introduction
These robots require fine-tuned control of all their motors and are incredibly energy inefficient compared with natural human walking. Perhaps we can learn something from the dynamic system of the human body, and base a robot on natural human gait…

4. Passive Walkers
This line of research looks at robots called Passive Walkers, which do not control any joint angle at any time.
Instead, they rely on gravity and the coupling of their limbs and joints to allow them to walk.
This research began with fully Passive Walkers, with no motors at all. Instead, they just needed a slope:

5. Passive Walkers Advanced
Moving forward, the authors of this article wanted to show that Passive Walkers are not purely dependent upon gravity and slopes. At three institutions, three Passive Walkers were built, each with some form of motor to give the legs just enough push to keep the robot walking.
Cornell, Delft, MIT
The Cornell Biped
The Delft Biped
The MIT Learning Biped

6. Cornell and Delft Bipeds
While the control programs for standard walking robots are quite complicated, the programs for these two walkers are very simple.
Both of these robots have no sensors except one in each foot. They only detect contact with the ground.
When they step down, it briefly activates the motor in the other foot. This gives a small push off. Just enough to propel these walkers forward.
This is the only control within the system.

7. The MIT Learning Biped
This robot is designed to test how motor learning can be used in conjunction with dynamic walkers.
With each step, a small amount of noise is added to its lift trajectory. It then measures how this change influenced its ability to walk.
Over time, this feedback helps it find stable walking patterns, without the needs for any internal simulations. It arrives at this stable point usually within 10 minutes or about 600 steps.
This could give insight into how toddlers learn to walk.

8. How well do they perform?
In humans, the energy lost to walking on a flat surface is minimal. Most of it is lost when the foot hits the ground.
The problem with standard robots, that control each joint with a motor, is the energy cost is much higher. Especially when energy is used to actively brake motors.
Passive walker inspired robots escape these problems.

9. How well do they perform?
By mimicking the way humans walk, dynamic walkers perform very similar to humans and FAR better than standard walkers when it comes to energy usage.
Energy usage standardized for mechanical work as:
(energy used) / (weight x distance traveled)

10. Conclusion
As shown, robots based on Passive Walker designs are much more energy efficient than standard robots.
They could represent the next stage in human-like robotics.
As well, they can be used to explore our own dynamics for walking.

11. Thank You!
Collins, S., Ruina, A., Tedrake, R., & Wisse, M. (2005). Efficient bipedal robots based on passive-dynamic walkers. Science, 307(5712),

12. MIT Learning Curve