20-01-2012, 01:22 PM
An automated insulin pump
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Injection scenarios
Level of sugar is in the unsafe band
Do not inject insulin;
Initiate warning for the sufferer.
Level of sugar is falling
Do not inject insulin if in safe band. Inject insulin if rate of change of level is decreasing.
Level of sugar is stable
Do not inject insulin if level is in the safe band;
Inject insulin if level is in the undesirable band to bring down glucose level;
Amount injected should be proportionate to the degree of undesirability ie inject more if level is 20 rather than 10.
Functional requirements
-If the reading is below the safe minimum, no insulin shall be delivered.
-If the reading is within the safe zone, then insulin is only delivered if the level of sugar is rising and the rate of increase of sugar level is increasing.
-If the reading is above the recommended level, insulin is delivered unless the level of blood sugar is falling and the rate of decrease of the blood sugar level is increasing.
System failures
Transient failures
can be repaired by user actions such as resetting or recalibrating the machine. For these types of failure, a relatively low value of POFOD (say 0.002) may be acceptable. This means that one failure may occur in every 500 demands made on the machine. This is approximately once every 3.5 days.
Permanent failures
require the machine to be repaired by the manufacturer. The probability of this type of failure should be much lower. Roughly once a year is the minimum figure so POFOD should be no more than 0.00002.
