Q. My Instructor in Biomedical

20 IEEE CONTROL SYSTEMS MAGAZINE » AUGUST 2008 1066-033X/08/$25.00©2008IEEE BIPEDAL WALKING Q. My instructor in biomedical engineering mentioned that a lot of researchers are working on robots that can walk and run. Honda’s ASIMO seems to walk pretty well, but it doesn’t seem to run that well. How important is systems and control technology for getting robots to walk and run? Many of my friends walk and run quite well, and very few of them have had any control courses. So I don’t quite see what the challenges are. Eric: Thanks for asking your question. Legged locomotion is a fun research area with many challenging control problems. Since you mentioned ASIMO [5], which has two legs, I’ll restrict my response to two-legged (bipedal) robots. Most of my observations apply equally well to robots with a different number of legs, such as quadrupeds and hexapods. I suggest that readers who have never seen a robot walk or run watch a few Web videos of ASIMO walking or running before reading further. By doing so, you’ll better understand my response. You may be excited to know that some bipedal robots can walk stably without any feedback control. To be clear, “stably” is used here to mean “makes forward progress without falling down. ” By careful mechanical design and proper initialization, these passive-dynamic walkers [1], [4] can walk stably down shallow, inclined slopes. Unfortunately, a passivedynamic walker’s gaits are not robust, which means that the robot is easily toppled by small perturbations. Robustness can be improved with the addition of actuators and feedback control. This approach is the focus of the dynamic walking community (see http://www.dynamicwalking.org/). With regard to running, I do not know of any bipedal robot that can run stably without feedback control. So, the short answer to your question is: if you desire a bipedal robot that walks robustly or runs, control technology is critically important. But, the question remains: why is it that humans can walk without formal training in control, but the control of walking and running bipedal robots is so challenging? To get a sense of why humans can walk without formal training in control, observe that humans can perform many complex tasks without any training in control or understanding of the algorithms that they execute. For example, consider your reading of this response. You view the words, decode them, and understand their meaning all without needing to understand, let alone design, any of the algorithms involved, such as those that control the movement of your eyes and head. As another example, consider driving a car. You can learn to drive a car without understanding the algorithms that you use to steer the car or control its speed. I speculate that humans perform all of these tasks by exploiting innate algorithms that are refined through practice. I believe the same to be true of your ability to walk or run. You learned to walk and then run by using innate algorithms for stabilization and coordination that were refined through practice. On to the second part of the question: Why is the control of bipedal walking and running challenging? I assert that the challenges are primarily due to three reasons. First, there are many constraints that arise from physical limitations. For example, configuration constraints arise from limits on the joints’ ranges of motion, while joint velocity constraints arise from actuator saturation. One particular type of constraint—unilateral force constraints between the feet and ground—results in the robot being either underactuated or effectively underactuated. These force constraints are called “unilateral” since the ground can push on the leg but not pull on it. “Underactuated” means that the acceleration of all of the robot’s degrees of freedom cannot be independently specified. “Effectively underactuated” means that limited freedom is available for specifying the accelerations, that is, the robot is close to being underactuated. A bipedal robot is underactuated when the robot contacts the ground at only a point or In this issue of IEEE Control Systems Magazine, we invite Eric Westervelt of the Ohio State University to respond to a query on walking robots. Eric is a coauthor of the article on Rabbit, which was featured on the cover of the October 2003 issue. Readers are invited to submit technical questions, which will be directed to experts in the field. Please write to us about any topic, problem, or question relating to control system technology.