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“Wearable Robotics” Replace Amputated Limbs

Frank Sup, a new faculty member in the Mechanical and Industrial Engineering Department, has spent the past five years developing a next generation lower-limb prosthesis. The device is an example of "wearable robotics," in which the knee and ankle joints are battery powered and guided by sensors that help the device adjust continually to terrains, slopes, and steps. It has been tested both in the field and in the lab by amputees for the past two and a half years.

“What I have been working on is actually to restore power to the lost lower limb, giving the prosthetic knee and ankle the capability of supplying torques and power,” explains Sup. “The device has the power to push off actively with each stride. We’ve made something that’s intelligent, light-weight, and powerful. We basically built a two-joint robot that’s rigidly attached to someone and provides mobility by sensing how he or she physically interacts with it.”

To create his prosthesis, Dr. Sup, who did his research while working as a graduate student and post-doctoral research assistant at Vanderbilt University, used technology gained by "mechatronics," which combines mechanical, electrical, and computer engineering principles. The term "mechatronics" was coined in 1969 by Tetsuro Mori, the senior engineer from a Japanese company named Yaskawa. An industrial robot is a prime example of a mechatronics system because it uses electronics, mechanics, and computing to do its day-to-day jobs.

Sup’s self-contained, battery powered knee and ankle prosthesis is intended to enhance the mobility of transfemoral amputees. Data about knee and ankle joint angle, torque, and power, taken during walking experiments at various speeds, has demonstrated the ability of the prosthesis to provide gait kinematics similar to a normal person.

“The hardware focuses on a compact mechanical drive system as well as a distributed electronic system that incorporates a microcontroller and servo amplifiers,” Dr. Sup explains. “Controller development focuses on extending the capabilities of the device to accommodate improved balance, variable speed walking, slopes, stairs, and recognizing the transitions between activities.”

Current lower-limb prostheses are passive devices, meaning that wearers must supply all the power. Someone who has lost one leg and wears a passive prosthesis must rely solely on the intact limb to climb stairs and must also compensate for loss of power when walking on level terrain. An additional disadvantage in current lower-limb prostheses is the difficulty in adjusting each stride to irregularities in terrain.  

This wearable robot’s system of sensors includes sensors on the heel and toe of the foot to measure the ground force, on the knee and ankle joints to measure position and moment, and on the main body to measure how the leg moves through space. So, during the gate cycle, the robot becomes an autonomous device that is continually looking for cues from these sensors, telling it how to adjust to the needs of amputees and to changing terrain as they walk.

What does this device allow lower-limb amputees to do now that they couldn’t do before? “Really the major goal of the device was just getting people up and walking,” Sup responds. “It supplies them with significant power, supplementing their own energy with every step. Without that power, they might have to expend up to 60 percent more energy. That’s a lot of energy, especially for an older person or someone who’s not in very good physical condition.”

As one result, the powered prosthesis increases a person’s walking speed at the same time as it decreases the cost of transport. In effect, it boosts the miles per gallon. And, with this added energy, the wearer gets a decreased sense of effort. The wearer doesn’t have to work as hard.

Yet another benefit is more ease at going up and down slopes. “Because the ankle is acting intelligently, automatically conforming to the terrain, the person has much better balance and stability and does not fall as easily,” says Sup. “Because you have this ankle conforming to the surface with continual adjustments going on, it gives the person much better control.”

Looking forward, Dr. Sup will be developing the Mechatronics and Robotics Research Laboratory at UMass Amherst. Here he will be expanding upon his earlier robotics work to better understand how people and robots can physically work together.

“The focus of my robotics research at UMass will be in wearable robotics and rehabilitation engineering, a field that looks at how to restore mobility and function due to debilitating conditions such as strokes.” The use of wearable robotics could allow for more controlled at-home therapies and the potential to allow for more independence during rehabilitation with the aid of the device. In cases where a full recovery is not possible, research will look at devices that can restore one’s ability to perform daily life activities.

What would these new devices look like and how would they behave? For an arm or leg, it would look like an exoskeleton, or the arm or leg from a plastic suit of armor that can be worn. The device would not only help power your movements, but give you feedback about its interactions with the surroundings and provide a collaborative human-machine experience. (October 2010)