— An array of sensors lets the artificial handle manipulate objects without crushing them or letting them slip from its grasp (Image: Southampton University)
Piezoelectric crystals and screen printed circuitry sense pressure, temperature and vibrations (Image: Southampton University)
An artificial hand built in the UK has fingertip sensors that let it grasp delicate objects without crushing or dropping them.
A previous prototype has proved itself capable of grappling with door keys and twisting the lid off a jar (see New robot hand is even more human). The latest incarnation not only moves more like a real hand but also has improved sense of touch (990KB, Windows Media Player format).
"We've added new arrays of sensors that allow it to sense temperature, grip-force and whether an object is slipping," says Neil White, an electronic engineer at Southampton University who developed the hand with colleagues Paul Chappell, Andy Cranny and Darryl Cotton.
Its developers hope that the robotic hand could eventually give amputees greater dexterity and deftness of touch via a prosthetic limb. Like some existing mechanical prosthetics, it could be controlled by connecting its motors to nerves in an amputee's arm, shoulder or chest.
Pressure sensors in each fingertip connect to a control system that maintains the hand's grip. "If a hand without them held a polystyrene cup it would just crush it," White explains. By contrast, the new hand uses feedback from its sensors to prevent each finger from closing further, once an object is gripped.
Gripping an object too lightly can be a problem with existing artificial hands. "The slip sensors prevent that by detecting the vibration as an object slips through the fingers," says White.
Other slip-detectors use microphones to pick up the sound caused when an object starts slipping, he explains: "Using vibration is more robust because there can be no interference in noisy environments. Some hands that use sound will close just when you whistle at them."
The hand's sensors consist of patches of piezoelectric crystals surrounded by circuitry, all screen printed directly onto the each fingertip through a technique called "thick-film fabrication". The piezoelectric crystals create voltages when their shape changes, and can detect changes in temperature, vibration and strain.
Thick-film fabrication is cheaper than using conventional silicon, says White. This could be important for prosthetic devices, he adds, as they will only be manufactured in small numbers, preventing the development on an economy of scale.
Giving prosthetic hands the ability to "feel" objects is important, says Göran Lundborg at Lund University in Sweden. "If people are to use them in place of real hands they need to have similar abilities," he told New Scientist.
Lundborg adds that the ultimate goal is to find a way to let a person's brain control the feedback loop between an artificial hand's sensors and motors. In future, this might be achieved by connecting the sensor output directly to a patient's brain or nerves, he suggests.
But, in the meantime, there may be simpler ways to do it. "We have experimented with feeding the output from small microphones in a glove into earphones," Lundborg says.
With training, subjects involved in the experiment were able to distinguish between the sounds produced by grasping different types of objects with the glove. MRI scans also revealed that they processed information from the earphones using the area of the brain that normally deals with touch.