Researchers at MIT have developed a leg exoskeleton capable of carrying an 80-pound load without the use of motors. According to its developers, the prototype can support 80 percent of this weight while using less than one-thousandth of a percent of the power used by its motorized equivalents.
The aim of developing leg exoskeletons is to make it easier for people to carry heavy loads, says Hugh Herr, director of the Biomechatronics Group at MIT and leader of the research. By designing mechanical structures that transfer much of the load directly to the ground, rather than via the walker’s legs, it should be possible to enable soldiers and firefighters to carry heavier loads while reducing the risk of injury and the amount of metabolic effort they expend in doing so.
To date, most exoskeleton research has focused on using motors to carry the load. Not only is this expensive, requiring large power supplies and frequent refueling, but it also tends to be noisy, which can be a problem for military applications. Conor Walsh, a graduate student at MIT who also worked on the exoskeleton, says that the system “is much quieter than the powered exoskeletons” and only slightly noisier than normal human walking.
Working with Ken Endo, also an MIT graduate student, Herr and Walsh have taken a quasi-passive approach. Their mechanical system is specially designed to follow the movement of the wearer’s legs and mimic some of the energy-storage strategies that legs exploit to reduce muscle work.
When we walk, the muscle power required to swing our legs is minimal because of the pendulum-like exchange of gravitational potential energy and the kinetic energy of our limbs. Our muscles also provide a degree of elastic energy storage to help joints flex, which again reduces the amount of overall energy that walking requires.
The MIT exoskeleton works using similar principles. The payload worn on the user’s back is attached to two leglike mechanical structures that run parallel to the user’s legs. These structures have elastic energy-storage devices at the ankle and hip, and a damping device at the knee joint.
In simple terms, the springlike joints take advantage of the user’s motion and payload to store energy. For example, as the heel of one foot makes contact with the ground, the continued forward motion of the body will cause springs in that hip and ankle to be compressed. These springs help propel the leg forward at the next stride.
A variable damper in the knee joint lets the leg swing freely as it moves forward. Then, as the heel strikes the ground, the damping is increased to prevent the knee from buckling under the weight of the payload.
The exoskeleton is not entirely passive. A small amount of energy is required to control the dampers’ variability. (The dampers contain a fluid with tiny magnetic particles. When electricity is applied to the fluid, these particles change its viscosity.) But it is very efficient compared with other such systems. “Our exoskeleton only consumes two watts of electrical power during walking,” says Herr. This is nothing compared with the 3,000 watts consumed by a motorized exoskeleton.
But there is a catch. Tests of the exoskeleton revealed that although it lightens the load for the user, that person consumes 10 percent more oxygen than if he or she had simply carried the load without mechanical assistance. This higher metabolic rate is attributed to the fact that the device interferes with the natural gait of the walker. “Walking with the exoskeleton takes more energy than walking without,” says Michael Goldfarb, director of the Center for Intelligent Mechatronics at Vanderbilt University, in Nashville, TN.
Even so, it is a good effort, says Goldfarb. “I’m not aware of any exoskeleton–active or passive–that has been shown to effectively decrease metabolic energy expenditure,” he says. And even if more energy is burned, the exoskeleton still reduces the stress on the wearer’s back and legs.
The MIT group believes that by carefully selecting and angling the springs, it can reduce the amount of energy that a person needs to walk with the exoskeleton.
It would probably take about two years to commercialize this technology, says Herr. “But we have no plans at this time to move forward with commercialization,” he says.
Goldfarb still believes that there are hurdles to overcome. There are great advantages to using variable dampers and springs, not least that they are much lighter and less power hungry than motors and actuators, he says. But a device that requires less effort and is capable of covering a broad range of terrains, such as uneven surfaces and stairs, must have not just variable dampers but also springs of variable stiffness. This is a taller order, Goldfarb says.