Getting a grip

4 min read

Development of fully functioning prosthetic hand would not have been possible without some pioneering work with the motors and actuators. Tim Fryer reports

Prostheses have been recorded way back in Egyptian times, but it is only very recently that technology has advanced enough to turn them from being visually acceptable, into something that is genuinely useful for amputees.

RSL Sleeper has developed the bebionic3, which they claim to be 'the world's most lifelike, affordable, functional and easy to use myoelectric hand commercially available today'. Myolectric means it takes the electrical impulses from the arm muscles and translates them into movements in the robotic hand.

The first hands of this type were designed for the US military and was restricted to large and medium hands due to the motor technology at the time. Ted Varley, technical director of RSL Sleeper, said: "Small hands would have been too weak to be functional five years ago." And this left a substantial gap in the market. While a prosthetic hand will obviously never be able to fully replicate the real thing, there was still big leaps to be made in terms of functionally, and there was not a size appropriate option for smaller people.

Varley, who joined the company five years ago at the inception of the project, described the objectives: "The bebionic was designed as a multi-articulated hand for general patients using standard myo architecture. Hands prior to this had a very strong pinch but low contact area. The ilimb, which came slightly before bebionic, proved that people would accept a hand with a lower grip force, but with a compliant grip to give more control. But prosthesis can always be improved, the human hand is an amazing thing. Weight, power and sensory feedback would be a great improvement."

Moreover, it had to have mass market capability - it had to be cost comparable with an existing myo with simple controls in order to get lots of people using them.

Version one was introduced in 2010 and was improved on for the second version, which was faster and had the opportunity for medium sized hands. The recent introduction, bebionic3, looks better and is much stronger, making extensive use of aluminium and stainless steel. More particularly this is the 'small hand', meaning that amputees of any stature now have a bebionic option.

The main problem in developing the small hand was in packaging – trying to match the performance of the medium hand in an extremely small space. Varley said: "We were benchmarking with a much larger motor, so we had a number of parameters to work to. I spent a great deal of time in Switzerland going though the motor and actuator design."

Motoring on

Dr Fritz Faulhaber, the high performance motor manufacturer with headquarters in Germany and precision micro drives facility in Switzerland, offered a potential solution to RSL Steeper's miniaturisation problems with high power motors and versatility in customisation. The link between the two companies was made by Faulhaber's UK distributor Electro Mechanical Systems, whose sales and marketing director, Stewart Goulding, commented: "Together we invested a huge amount of design resource into this project. A dedicated project team was assigned in Switzerland and a large amount of in-house testing of materials and components took place as well as regular meetings together with RSL in both the UK and at the Swiss plant."

The motor type used is a brand new series 1024M006SR, which is the strongest motor in the frame size – 10mm diameter and 24mm long. It features high stall torque of 4.6mNm. A new end cap was designed providing easier assembly with the Faulhaber range of optical and magnetic encoders. A new cover and material was used with more accurate brush positioning for increased performance even at higher temperature range and new commutator and brushes were integrated with more accurate segment geometry for a cleaner commutation signal.

Goulding said: "A totally new coil construction was developed with 60% more copper than its predecessor and in combination with a new Neodymium rare earth magnet, exceptionally high performance was guaranteed. Improved tolerance on the shaft for better alignment and higher efficiency, improved alignment of the bearings and their positioning reduces contamination from the brushes resulting in even higher system efficiency and longer lifetime. The advances in this motor design resulted in a huge step change and best in class performance when tested in the application and compared with other motors."

The 1024M006SR delivered consistently high torque even in repeat grip patterns driving up to stall condition. Other motors showed signs of reaching their thermal limits and not delivering consistent performance under the same operating conditions. One of the key factors of this motor is the flat speed/torque curve resulting in a strong consistent torque across the speed range. The addition of a robust planetary gearhead and 3-channel magnetic encoder with all flying leads including the motor power supply exiting from the same point together with special loom and tiny JST connectors made this combination much easier to package compared to others, according to Varley. The 3-channel 32 line encoder gave them all the positional and speed feedback data required but added just 8.5mm to the overall length.

This was only half the story though as the requirement for a thumb actuator was altogether more challenging. There was insufficient space in the hand to mount the thumb actuator so this needed to be integrated within the thumb itself. Based on the same motor technology, RSL was looking for a complete in-line linear actuator in the smallest dimension but capable of delivering high force at the tip of the thumb.

Faulhaber was able to develop a complete actuator solution calling on the competences and synergies of other Faulhaber Group companies. Project managed by an experienced team in Switzerland, Faulhaber were able to integrate a high precision 4-point contact bearing from their Group company Micro Precision Systems, enabling them to develop the most powerful and importantly smallest linear actuator capable of sustaining axial forces up to 300N.

The unique characteristic of the high precision 4-point (shorter than conventional bearings) contact bearing and high axial force capability meant that a considerably shorter overall actuator length could be achieved. In combination with a precision lead-screw, Faulhaber tested five combinations of nut material and lubrication before reaching the optimal performance from the actuator using a PEEK (Polyether ether ketone) material. A precision made customised enclosure with integrated pivot points completed the system.

Technology integrators

The motors and actuators are typical of Varley's strategy – while everything down to the bolts are custom made, the actual technology behind then tend to be more established. He commented: "We are looking at a number of developments, like application of ceramics and compact actuator systems. But what we are really doing is taking today's technology and bringing it together. If we were to develop the tech as well the project would have too much risk."