In this fascinating application, low power radio expert Radiometrix, in close association with prosthetic limb specialist Blatchford Products, has supplied its BiM3 radio transceiver modules to enable the easy programming of modern prosthetics.
UK company Radiometrix, with its comprehensive technical support and R&D team comprising highly qualified RF design and applications engineers, has worked closely with Blatchford, UK, for a number of years on a range of programming systems for the company's prosthetic limbs. Radiometrix has a rare understanding of every aspect of modern applications for its low-power radio devices and it was this that helped provide Blatchford with exactly the right radio transceiver for the job.
The problem
A modern 'wooden leg' is actually anything but. For a start, it is not made of wood but modern engineering materials. It does have a 'knee' but this cannot twist through the angles that a real knee can. Nor do the wearer's muscles actuate and control any part below that attachment point to the 'stump' so, as the wearer's 'load-line' (essentially related to the centre of gravity) changes, as it will whilst walking, the prosthetic leg can either lock or fold (collapse). If these modes occur at the wrong time, such as slowing down, an accident may be inevitable. Even if there is not an embarrassing and painful fall, the wearer's gait can be very strange, for example, twisting or 'goose stepping'. Dynamic stability is key.
The key to reducing these unwanted effects as far as possible, and also making it easier to learn to walk, is in ensuring as much control as possible. Blatchford, with Radiometrix's help, has done just this with the control system of one of its products, the Adaptive Prosthesis.
The Adaptive Prosthesis
The main phases of walking that need to be considered and controlled as far as possible are 'stance' and 'swing'.
The Adaptive Prosthesis combines the best features of Blatchford's renowned earlier products, ie: a combined pneumatic/hydraulic cylinder - with common actuator rod - to provide 'swing' control and 'stance' control respectively. Previous models used either a pneumatic cylinder or a hydraulic, but not both. The new cylinder is expensively made in nickel-plated aluminium by Blatchford and its pneumatic end is - essentially - an air spring with good control over its rate. A tiny stepper motor drives the control valve. The cylinder is not a power source. The power derives from the wearer's own remaining musculature which is in the leg stump contained within an inner liner and outer socket which, in turn, is attached to the prosthetic leg. This cylinder gives the wearer what is termed 'spring assist'.
The hydraulic end of the cylinder provides the necessary 'stance control' with a DC motor-driven rotary valve controlling oil flow over each cycle. As the 'leg' extension increases, so does the resistance in this cylinder. It acts rather as an automotive suspension damper does, but with extra control, yet gives no appreciable resistance in the 'swing phase'.
The combined cylinder is mounted between a prosthetic knee and shin, coupled to each via pivots, so allowing controlled bending.
In addition, the Adaptive Prosthesis has a number of other walk modes not previously available in one unit: Stumble Detection; Ramp Mode; Standing Mode and Stairs and Descent Mode. The result for both Blatchford and the device's wearers is the 'best of both worlds' compared with earlier products.
The Adaptive Prosthesis is more akin to a modern Grand Prix racing car in its engineering and technology, with nickel-plated aluminium, carbon fibre, engineering plastics and advanced control and feedback systems!
Two-way radio communication
The Adaptive Prosthesis is operated by circuitry on a printed circuit board (PCB) which is screwed to the side of the combined cylinder. There are two sensor inputs: a GMR (giant magneto-resistive sensor) that measures the actuator rod stroke and a force sensor in a rubber pad on the knee. Essentially, the PCB and its processor control the motors and valves that adjust resistance and 'assist' to the wearer's advantage - ie, better, safer walking. Also mounted on the PCB is Radiometrix's BiM3 radio transceiver module.
Each prosthesis is set up to meet individual requirements. Previously, this had been carried out using a wire link to programme the control circuitry on the PCB. This had many obvious disadvantages for both programmer and wearer. More recently, Radiometrix helped out with a wireless one-way radio link. Though far better, there was still the problem that there was no feedback so that the programmer was just telling the prosthesis what to do, but not getting information on what it was actually doing.
Now, since the BiM3 was chosen by Andy Sykes, Blatchford's electronics expert and Adaptive Prosthesis project development engineer, two-way communication is simple. The proper feedback allows much easier setting up. When the prosthesis is correctly programmed, the BiM3 module is switched off until any later adjustments are necessary, so as to save power in the three lithium cells that provide energy.
A major reason for Sykes' choice of BiM3 is that it operates on both the 869,85 MHz or 914,5 MHz frequencies. It therefore meets the American FCC Regulations for low power radio devices too. This is important to Blatchford, as the company wishes to develop a larger share of the US market for prosthetic limbs. BiM3 is also compatible with other Radiometrix products that Blatchford also uses.
Radio module is super compact
The BiM3 radio transceiver module offers the advantages of high transmit power and data rates, greatly improved receiver interference rejection and a lower profile, all with reduced power consumption and antenna size. This makes the BiM3 ideal for use with battery-powered portable and handheld applications requiring the ultimate in miniaturisation. Two versions are available, covering the European 868-870 MHz band and the US 902-928 MHz band.
BiM3 features include the following: 869 MHz version to EN 300 220-3 and EN 301 489-3; 914 MHz version to FCC Part 15.249; SAW filter for enhanced transmit and receive performance; internal voltage regulator for increased stability; crystal-controlled PLL FM circuitry; usable range to 120 m; fully-screened; low profile and low power requirements.
The CE certified BiM3 is a compact 23 x 33 x 6 mm, operating frequency either 869,85 MHz or 914,5 MHz and the transmit power is 0 dBm (1 mW) nominal with a receive sensitivity of -100 dBm @ 1 ppm BER. Other salient performance features include a supply range of 2,7 V - 10 V, 8 mA transmit/10 mA receive with a data bit rate of 64 Kbps max. The receiver image rejection is >40 dB and the receiver local oscillator leakage is <-65 dBm.
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