The variable speed drive has been put to many and varied uses, but thanks
to the efforts of an enterprising team of engineers at University College
London, its use to activate a 'bionic' bone must rank as one of the most
unusual. Les Hunt reports
Osteosarcoma, a form of bone cancer, affects over 100 youngsters a year
in the UK alone. The treatment can entail the removal of a section of
affected bone from the tibia, femur or humerus, which must be replaced by
an extensible bone replacement prosthesis. The problem is that the
prosthesis does not automatically grow to match the natural growth of the
patient so the implant must be capable of being mechanically extended as
the patient's leg or arm grows. In the past, this has meant successive
invasive procedures at intervals to access the prosthesis and lengthen
it. This, in turn, meant hospitalisation, pain and the risk of infection.
Engineers at University College London's Bio-medical Engineering faculty
based at the Royal National Orthopaedic Hospital (RNOH) have been working
on an implant that can be activated by non-invasive means.
Electromagnetic induction was deemed to be the most suitable method, and
three companies - whose names will be familiar to DPA readers - became
involved at various stages of the project.
The non-invasive procedure involves placing a small permanent magnetic
rotor - about the diameter of a penny and just 4mm thick - in the
patient's leg or arm. This drives the prosthetic implant (effectively a
worm geared power screw) via a tiny epicyclic gearbox, designed and
manufactured by Davall Gears. The prosthesis is manufactured from
non-magnetic titanium and cobalt chrome alloys, and the complete rotor
and gearbox assembly is embedded within the implant. Externally, the
driving force is provided by a three-phase stator winding, supplied by
ABB, which is enclosed in a resin-impregnated, doughnut-shaped housing.
The patient's leg or arm is simply placed inside the air core of the
stator during the extension process.
The stator windings are connected to a Danfoss VLT 2800 variable speed
drive. The VLT's variable torque control enables the rotor to produce
sufficient torque to drive the gearbox at between 40 and 60V (50Hz), and
it also provides accurate flux control in order to protect the gearbox.
To ensure smooth rotation at this low stator voltage, the drive's output
waveform is filtered before being applied to the stator. In this
application a single-phase to three-phase variable speed drive is
essential, as it allows the stator to be powered from a standard 240V
single-phase domestic supply.
The stator cores were supplied through ABB Motor Service Partner, EMR
Silverthorn based in Wembley. The UCL team initially used six air-cored
coils configured as a two-pole, three-phase winding. While this generated
sufficient torque to turn the rotor, it was found to be inefficient and
required oil cooling. Using a stator core based on ABB's standard 180
frame motor in a two-pole stack for 3,000rpm nominal speed, the UCL team
specified a series-wound stator with 552 turns of 1.06mm gauge wire, for
star connection. This configuration, which was custom made by EMR using
stator cores supplied free of charge by ABB, produced the best
performance during tests at the RNOH.
A vital component of this unusual drive train is the epicyclic gearbox.
The specification called for a very high reduction of 13,061:1,
sufficient to magnify the extremely low input torque and provide a
maximum output torque of 4Nm. The unit is essentially a series of
epicyclic gearboxes in a special configuration whose outside dimensions
measure just 21.5mm diameter by 18.5mm long. All parts of the gearbox are
manufactured from a non-magnetic stainless steel with a core strength of
100 ton/in2. Precision was vital at every stage of manufacture and
assembly in order to compl