Cosmetic gloves have a parasitic positive stiffness that works against the flexion of finger joints, requiring more control effort of the user. Gerwin Smit, PhD, researcher at the Delft University of Technology, has developed an adjustable compensation mechanism that reduces this stiffness and reduces required input torque in half.
“We discovered that current hands need a lot of activation force…you need a lot of force to pull the cable of the prosthesis,” Smit told O&P Business News. “We wanted to make it easier to operate the device.”
Cosmetic gloves
Prosthetic hands are typically covered by cosmetic gloves that offer aesthetic benefits, while protecting against dirt and water damage. However, they also include properties that can negatively affect user experience, according to Smit.
“The cosmetic glove makes it look nice, cosmetically appealing and humanlike,” he said. “However, from a mechanical point of view, it does not have a positive contribution because it resists movement.”
The stiffness of the material counteracts movement of the hand, increases user control effort in body-powered prostheses and decreases battery life in electric-powered prostheses.
Compensation mechanism
Researchers launched an experimental test bench study aimed to design and evaluate a mechanism that compensates stiffness and reduces the required input torque to control the fingers on a prosthetic hand.
Stiffness and required torque were measured for both a silicone and polyvinylchloride (PVC) glove, by flexing the metacarpophalangeal joint of the index finger joint at 90°.
Findings showed the measured stiffness could be reduced by using a tension spring that causes a negative stiffness.
Gerwin Smit
“We already knew that the cosmetic glove…counteracts movement,” Smit said. “If it acts like a spring that works in one way, we can add another spring that acts in the opposite way, and counteract the force of the glove by the force of the spring.”
Researchers placed a frame consisting of two bars connected with a joint, inside the finger slot. This frame was used as the compensation mechanism by attaching a helical tension spring to one of 12 points above and below the joint.
A cable, running over a pulley, controlled the joint angle, and ball bearings were added to minimize friction. One end was attached to the measurement set up, which measured force, displacement and activated the cable. The other end was attached to a counter mass, which returned the finger joint to its initial position.
The extension force, applied by the counter mass, was subtracted from the cable force, and the resulting force was multiplied by the pulley radius to obtain the activation torque. This was recorded for each joint angle. Stiffness, dimensions and alignment of the spring were tested in 144 different configurations. Researchers examined each arrangement to find which allowed the least amount of input force.
“Our model predicted an optimal configuration, then we did measurements with that configuration, and it turned out that we significantly could reduce the force,” Smit said.
Required torque was reduced by 58% for the PVC glove and 52% for the silicone glove. The mechanism successfully reduced input force, energy and overall control effort of the user.
Benefits
One advantage of the mechanism is that it can be used in electric-powered hands. In this type of prosthesis, it can help reduce overall power consumption.
The mechanism also offers advantages in body-powered prostheses, as they typically require high energy input. The device can help reduce total force required, resulting in faster and more efficient operation.
Smit said the biggest impact of the compensation mechanism may be its simplicity, as it operates using only a spring, the attachment points of the joint and a small build-in space.
“We did not need a complex mechanism,” he said. “It was doable with just a simple helical spring, and everything fits inside the finger. We were surprised that it turned out to be so easy.”
The compensatory mechanism can fit into any standard cosmetic glove, work with individual fingers and compensate each joint that has parasitic positive stiffness.
Future considerations
Smit said some aspects of the mechanism and cosmetic glove could benefit from further research. For instance, there is limited build-in space inside the finger of a prosthetic hand. Also, although the mechanism was able to reduce required torque, it was not possible to accomplish a perfect compensation. The glove characteristic was not entirely linear, had a substantial amount of hysteresis and the device itself added a small amount of extra friction. As a result, the system constantly needed an input torque and continuously dissipated energy during motion.
Smit said the mechanical properties of cosmetic gloves should be more closely analyzed during production, and maintain a consistent design so the mechanism does not need modification.
“There are differences between gloves…the thicknesses can vary,” he said. “It would be preferable if you have would have cosmetic gloves with constant properties, and then you could have one mechanism that you do not need to adjust.”
Despite minor drawbacks, the compensation mechanism can have an immediate impact on the use of prosthetic hands, enhancing the overall experience for the user, according to Smit. — by Shawn M. Carter
Disclosure: Smit has no relevant financial disclosures.
