Скачать презентацию An automated insulin pump Ian Sommerville 2004 Software Скачать презентацию An automated insulin pump Ian Sommerville 2004 Software

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An automated insulin pump ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump An automated insulin pump ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 1

Concept of operation l l l Using readings from an embedded sensor, the system Concept of operation l l l Using readings from an embedded sensor, the system automatically measures the level of glucose in the sufferer’s body Consecutive readings are compared and, if they indicate that the level of glucose is rising (see next slide) then insulin is injected to counteract this rise The ideal situation is a consistent level of sugar that is within some ‘safe’ band ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 2

Sugar levels l Unsafe • l Safe • l A very low level of Sugar levels l Unsafe • l Safe • l A very low level of sugar (arbitrarily, we will call this 3 units) is dangerous and can result in hypoglaecemia which can result in a diabetic coma and ultimately death. Between 3 units and about 7 units, the levels of sugar are ‘safe’ and are comparable to those in people without diabetes. This is the ideal band. Undesirable • Above 7 units of insulin is undesirable but high levels are not dangerous in the short-term. Continuous high-levels however can result in long-term side-effects. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 3

Insulin injection l l l The decision when to apply insulin does NOT depend Insulin injection l l l The decision when to apply insulin does NOT depend on the absolute level of glucose that is measured in the sufferer’s blood. The reason for this is that insulin does not act instantaneously and the change in sugar level does not simply depend on a single injection but also on previous injections. A more complex decision based on previous levels and rate of change of sugar level is used. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 4

Injection scenarios l Level of sugar is in the unsafe band • • l Injection scenarios l Level of sugar is in the unsafe band • • l Level of sugar is falling • l Do not inject insulin; Initiate warning for the sufferer. 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 5

Injection scenarios l Level of sugar is increasing • Reading in unsafe band • Injection scenarios l Level of sugar is increasing • Reading in unsafe band • No injection. • Reading in safe band • Inject only if the rate of increase is constant or increasing. If constant, inject standard amount; if increasing, compute amount based on increase. • Reading in unsafe band • Inject constant amount if rate of increase is constant or decreasing. • Inject computed amount if rate of increase is increasing. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 6

Glucose measurements Sugar level Inject Undesirable area Inject Do not inject Safe area Unsafe Glucose measurements Sugar level Inject Undesirable area Inject Do not inject Safe area Unsafe area t 1 ©Ian Sommerville 2004 t 2 t 3 Software Engineering, 7 th edition. Insulin Pump Time Slide 7

System specification l Functional specification • l How to carry out the computation to System specification l Functional specification • l How to carry out the computation to determine if insulin should be administered Dependability specification • Requirements to ensure safe operation of the pump ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 8

Functional requirements l l l If the reading is below the safe minimum, no Functional requirements l l l 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 9

Formal specification l l l Because of the complexity of the functional specification, there Formal specification l l l Because of the complexity of the functional specification, there is considerable scope for misinterpretation This system is an example where formal specification can be used to define the insulin to be delivered in each case The formal specification of the insulin pump is covered in the following lecture ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 10

Dependability specification l Availability • l Reliability • l Intermittent demands for service are Dependability specification l Availability • l Reliability • l Intermittent demands for service are made on the system Safety • l The pump should have a high level of availability but the nature of diabetes is such that continuous availability is unnecessary The key safety requirements are that the operation of the system should never result in a very low level of blood sugar. A fail-safe position is for no insulin to be delivered Security • Not really applicable in this case ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 11

System availability l In specifying the availability, issues that must be considered are: • System availability l In specifying the availability, issues that must be considered are: • • The machine does not have to be continuously available as failure to deliver insulin on a single occasion (say) is not a problem However, no insulin delivery over a few hours would have an effect on the patient’s health The machine software can be reset by switching it on and off hence recovery from software errors is possible without compromising the usefulness of the system Hardware failures can only be repaired by return to the manufacturer. This means, in practice, a loss of availability of at least 3 days ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 12

Availability l A general specification of availability suggests that the machine should not have Availability l A general specification of availability suggests that the machine should not have to be returned to the manufacturer more than once every years (this repair time dominates everything else) so • l System availability = 727/730 *100 = 0. 99 It is much harder to specify the software availability as the demands are intermittent. In this case, you would subsume availability under reliability ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 13

Reliability metric l l l Demands on the system are intermittent (several times per Reliability metric l l l Demands on the system are intermittent (several times per hour) and the system must be able to respond to these demands In this case, the most appropriate metric is therefore Probability of Failure on Demand Other metrics • • Short transactions so MTTF not appropriate Insufficient number of demands for ROCOF to be appropriate ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 14

System failures l Transient failures • l can be repaired by user actions such System failures l Transient failures • l 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. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 15

System hazard analysis l Physical hazards • l Electrical hazards • l Hazards that System hazard analysis l Physical hazards • l Electrical hazards • l Hazards that result from some physical failure of the system Hazards that result from some electrical failure of the system Biological hazards • Hazards that result from some system failure that interferes with biological processes ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 16

Insulin system hazards l l l insulin overdose or underdose (biological) power failure (electrical) Insulin system hazards l l l insulin overdose or underdose (biological) power failure (electrical) machine interferes electrically with other medical equipment such as a heart pacemaker (electrical) parts of machine break off in patient’s body(physical) infection caused by introduction of machine (biol. ) allergic reaction to the materials or insulin used in the machine (biol). ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 17

Risk analysis example ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide Risk analysis example ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 18

Software-related hazards l l l Only insulin overdose and insulin underdose are software related Software-related hazards l l l Only insulin overdose and insulin underdose are software related hazards The other hazards are related to the hardware and physical design of the machine Insulin underdose and insulin overdose can be the result of errors made by the software in computing the dose required ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 19

Software problems l Arithmetic error • • l Some arithmetic computation causes a representation Software problems l Arithmetic error • • l Some arithmetic computation causes a representation failure (overflow or underflow) Specification may state that arithmetic error must be detected an exception handler included for each arithmetic error. The action to be taken for these errors should be defined Algorithmic error • • Difficult to detect anomalous situation May use ‘realism’ checks on the computed dose of insulin ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 20

Insulin pump fault tree ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Insulin pump fault tree ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 21

General dependability requirements • • • SR 1: The system shall not deliver a General dependability requirements • • • SR 1: The system shall not deliver a single dose of insulin that is greater than a specified maximum dose for a system user. SR 2: The system shall not deliver a daily cumulative dose of insulin that is greater than a specified maximum for a system user. SR 3: The system shall include a hardware diagnostic facility that should be executed at least 4 times per hour. SR 4: The system shall include an exception handler for all of the exceptions that are identified in Table 3. SR 5: The audible alarm shall be sounded when any hardware anomaly is discovered and a diagnostic message as defined in Table 4 should be displayed. ©Ian Sommerville 2004 Software Engineering, 7 th edition. Insulin Pump Slide 22