Uncrewed Systems Technology 046

22 In conversation | Professor Alan Wilson Wilson’s collaboration with Boston Dynamics included becoming involved in the Cheetah robot built for the US Defense Advanced Research Projects Agency, which ran at a record speed of 18 mph on a laboratory treadmill in 2012. While impressive for a legged robot, that speed is nothing like as fast as its feline namesake. Wilson says the major challenges with legged robots are in the areas of power consumption and control, which animals have had millions of years to evolve. In terms of power consumption, legged robots have a long way to go to match animals. Boston Dynamics’ Big Dog robot, for example, uses a 15 bhp (11 kW) two-stroke engine to drive the pump that provides the pressure for its hydraulic actuators, and as the concept of a 15 bhp dog suggests, it consumes about 20 times the power of an animal of the same size, Wilson notes. Elasticity and control “Muscles are good at generating lots of force in a single stroke,” he says. “You have a single actuator that can be powerful, damping and elastic, changing its properties in a fraction of a second. So you can have a lot of control and amelioration of energy in the system. “Animal and human legs are very elastic, and muscles are very good at storing energy. You can jump off a table, and run down a hill quite efficiently and quickly. You might hurt the next day, but that’s just showing how much energy you are dumping. “So biological systems have an advantage here, but robotics has always gone for classical control, which means control of position. So you make your robot rigid, as the last thing you want is for it to be elastic in any way, because then you can’t predict motion.” However, he adds that engineers can measure motion accurately with MEMS IMUs to enable corrective inputs. He compares the control problem to the one that faced SpaceX in the development of rocket boosters that can land vertically. Noting that each rocket motor is on the equivalent of a Stewart platform – which uses six hydraulic rams to generate realistic motion in aircraft simulators, for example – he explains that controlling them is non-trivial, because there are orientations the platform cannot take up owing to geometric limitations. “You cannot take it from one position and move it to another by whatever route you fancy; there are only a certain number of prescribed routes you can follow,” he says. “The animals I work on are designed particularly for running and walking, to oversimplify somewhat. A horse weighs about half a ton, yet it has skinny legs that are very strong but elastic. That matters, because if you take a straight strut of a leg and put a large load on it, it will buckle or break, while an elastic leg will stretch the mechanical impulse out over time. “Springy legs are inherently complex in some respects, but they also perform quite simple movements quite economically. It is a matter of versatility versus specificity in structural design that animals have had rather a long time to refine. “It’s something that the robotics world is coming to terms with, because it is a much harder system to control and it requires much more in terms of real-time control.” Kicking a dog Wilson comments that the really impressive thing about Big Dog in demonstration videos is when people kick it to try to knock it over. “You don’t know what your body motion is going to be until it happens, and then October/November 2022 | Uncrewed Systems Technology Hydraulics deliver remarkable amounts of energy and with exquisite control. The downside is you throw away a lot of energy in doing so Boston Dynamics’ Atlas is a whole-body mobility research platform with 28 joints powered by compact mobile hydraulics, which are still essential for legged robots (Courtesy of Boston Dynamics)