The Dextrus hand is designed with the user in mind. All of the motors and electronics will fit inside the palm of the hand, meaning it can fit a range of different amputees.
The hand will connect to an existing fitted prosthesis using standard connectors, which means anyone can use it without requiring a custom fitting. An intuitive and simple control system will be implemented with two EMG sensors that can be placed on any muscles. It will also have a documented serial communication interface so users can create their own custom control hardware.
One of the most expensive aspects of leading robotic prosthetic hands is the materials that are used to create them. Titanium and Carbon fibre enable these devices to be used all day and not wear out after a few months, but when designing on a budget these luxuries aren’t available. ABS plastic will be used to create the majority of the parts in the Dextrus hand. That’s the same tough, durable material that Lego is made from.
The plastic parts are created using 3D printers. In the past couple of years there has been a revolution in 3D printing, a worldwide community of enthusiasts has brought the price of these devices down. With this collective experience they’ve created accurate, high resolution 3D printers that are capable of making the intricate parts required to build a device like this at an affordable cost. This means that if a part does wear or break, a replacement can simply be printed and it can be replaced. The parts are so cheap that you can even have a handful of spare fingers just in case. The Dextrus hand is designed to make replacing parts quick and easy.
When you grasp an object, your fingers mould themselves around the shape of the object to gain a firm grip. In some prosthetic hands, the fingers are actuated in a synchronous way where all of the joints of an individual finger move in sync. In the Dextrus hand, each finger is actuated by a single tendon that runs through all of the joints to the tip. This means it will grasp just like a human hand, adapting to fit any form that’s placed in it.
Marine grade stainless steel tendons are used in the fingers; these have a minimum breaking load of 18Kg (per finger) and a nylon coating to make sure they move smoothly through the joints. To ensure flexibility a 7×7 tendon is used; this means the cable has 7 cores, each made up of 7 strands of stainless steel wire.
If you’re using the hand all day the changes in tension on the tendons and joints causes wear that will eventually result in failure of a part. To maximise the lifetime of the joints in the hand, the tendons are held taught by a compression spring tensioner assembly. This means that if you bump a finger by accident, instead of the tendon breaking, the compression spring will flex and the finger will return to its original position.
When someone grabs an object they don’t have to think about moving each finger to the exact position it needs to be in to grasp the object. Instead we use feedback and the sense of touch, so we can simply think “close the fingers until they come into contact with something, and then stop”. The Dextrus hand works in the exact same way, by using feedback sensors as the fingers close, it knows when it is gripping an object and how hard it’s gripping the object. You just have to tell it to when to close and open.
The Dextrus hand can articulate each finger and thumb individually, enabling it to grasp all sorts of different shapes and sizes with ease. Each finger has it’s own feedback sensor, meaning they individually know when they’ve come into contact with an object, and can stop closing and just grip the object in place.
Miniature, epicyclic geared motors are used to achieve the power necessary to grip household objects. These are highly efficient and offer a fine enough grip to grasp delicate objects, but enough force to hold heavier household objects firmly.
Preliminary tests and calculations suggest that the Dextrus hand will be able to operate for around 8-12 hours on a single charge with lithium ion batteries. There will be expansion capabilities for additional batteries.