Some Arguments in Favour of Direct Electric Drive for an Artificial Elbow.

Experience with patients using artificial arms powered by compressed carbon dioxide indicates that reasonable requirements for a powered elbow are : 1) a Starting torque of about 80 pound inches; 2) an energy store of 4,800 foot pounds (100 per cent efficient conversion) carried on the body ; and 3) an energy store of maximum weight of 3 pounds. The figure of 4,800 foot pounds is arrived at by calculating the work done at full load by the present Hendon carbon dioxide motors and assuming a store of 1 50 grammes of carbon dioxide which is judged to be adequate as a day’s supply for an adult. The present motors use complete admission : that is, they work at a constant pressure, with very little expansion of the carbon dioxide in the working cylinder. The working pressure is pre-set to between 70 and 100 pounds per square inch. This means that the number of elbow movements is dependent only on the swept volume of the working cylinder and the mass of carbon dioxide carried in the store. With the Hendon motors assuming a store of 150 grammes ofcarbon dioxide, a working pressure of7O pounds per square inch and a working temperature of 40 degrees Fahrenheit, about 280 full elbow movements (flexion or extension) through 1 50 degrees are possible. This performance can be improved by having the downward pulling or extensor cylinder of smaller cross-sectional area and by increasing the working pressure. For example, if the diameter of the extensor cylinder is reduced to half (to about 06 inch), the number of full movements is increased to about 440 because of the smaller volume swept during extension. An increase in the working pressure allows a smaller flexor cylinder diameter, giving a smaller swept volume, but the density of the carbon dioxide in the cylinder increases, so that the advantage is not significant; it allows a smaller, lighter motor but the energy utilisation is not much improved. Some motors with small extensors have been made at Hendon and work quite satisfactorily, but control of the position of the forearm is more difficult because the two cylinders, having different working areas, require different pressures for equilibrium. An ideal motor, expanding 150 grammes of stored carbon dioxide isothermally from 840 pounds per square inch to fifteen pounds per square inch at 70 degrees Fahrenheit, would produce about 10,500 foot pounds. If this figure is taken as the maximum possible work available, the present motors (with flexor and extensor components of equal size) are about 42 per cent efficient if they are used at full load. The motors with unequal cylinders are about 72 per cent efficient. It is very unlikely that any other gas-powered motor-such as a small turbine-would have an efficiency which even approached 40 per cent, and it would probably be much more noisy. It must be concluded that the Hendon carbon dioxide systems are as good as they can be. In theory the system might be improved for an arm having two or more motors by cascading the motors: this would improve the energy utilisation but the increased complexity would probably rule it out. If longer use is required between bottle changes it is much easier to fit larger bottles. This last statement may seem na#{239}ve, but it is nevertheless worth making. The Hendon systems will soon use two bottles, each containing 75 grammes of gas, with one reducing valve and a change-over valve, weighing about 1 #{149}1 pounds (500 grammes). A child might object to a heavier store, but an adult should be able to carry 3 pounds (1,362 grammes), properly supported on the body, without undue inconvenience.