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Equipment reliability and the environment
Microelectronics and Reliability
Pergamon Press 1970. Vol. 9, pp. 145-156.
Printed in Great Britain
EQUIPMENT RELIABILITY AND THE ENVIRONMENT* J. HARRIS
Decca Radar Ltd., Croydon, Surrey 1. INTRODUCTION THE term "environment" is usually defined as
being a set of conditions under which a piece of equipment is expected to operate. The conditions described thus are invariably climatic or conditions of vibration and shock, etc. Indeed specifications for the design of new equipment inevitably state a required performance under given "environmental" conditions. The author believes that the definition of environment should be widened to include the method of supporting the equipment in the field during its production life. "Support" includes any reliability engineering programme from the pre-production stage onwards, provision of technical information service and maintenance, serviceability analysis, information feedback systems and the general post design engineering to eliminate repetitive faults. By this means, a manufacturer of commercial electronic equipments can create, to a degree, his environment and exercise a measure of control over the reliability of his equipment. This paper describes the problems of reliability in the marine market over the last 10-15 years and the efforts made by the author's company to improve the reliability of their product. The requirement for reliable commercial marine radar equipment at sea has been a growing problem for many years. The use of radar at sea grew enormously during the 1950s, but the reliability of the conventional valve equipment then in use did not improve at the same rate, if indeed it improved at all. In the larger complex equipments, it was unlikely, on a component population basis, that * Paper read at the Bath University of Technology, Centre for Adult Studies, Course in Electronics Reliability IV. 145
any manufacturer could produce an equipment which could operate over the periods of months without failure that the user so rightly demands. All manufacturers have recognized that reliability must be the prime consideration and have paid close attention over the years to the problem of improving their products. The authors' organization has paid particular attention to component specifications, use of preferred items, good inspection and quality control in production. In addition, this company's service division have made great efforts to investigate the causes of unreliability at sea and by the use of a punched card data processing system have produced analyses of equipment reliability of over 16,000 ships. These analyses have been issued on both monthly and 6-monthly basis for the past 5 years and show the failure rates of complete equipments, units and individual components. From these results, design changes to production equipment and retrospective modifications have been made. Continual improvement to existing and past designs have been possible and a number of improved components have been developed by close liaison with component manufacturers. Controlled sea trials, usually in trawlers, are the final acceptance test for these components. In the early 1960s, however, despite these efforts to co-ordinate design and production with the feedback of service information, it became very clear that more drastic changes were necessary to bring about the order of improved reliability that was really required. The transistor, by this time, was a feasible proposition and a development programme was laid down to produce a complete range of marine radars using transistors wherever possible. At the same time a study was made of the work carried out in the United States by the Advisory
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Group on Reliability of Electronic Equipment (A.G.R.E.E.) and similar work was undertaken by the Ministry of Aviation in the United Kingdom. Our studies led us to the decision to adapt the basic A.G.R.E.E. principles with the following very simple objectives in view: (a) to produce equipment which could meet the strictest environmental specification and had its inherent faults removed before production, rather than after 1 to 2 years at sea; (b) to ensure that the transition phase from development to production did not cause potential unreliability; (c) to maintain reliability standards throughout the production life, particularly guarding against latent defects due to changes in our own manufacturing techniques and in the manufacturing techniques of component suppliers. The A.G.R.E.E. report is a lengthy document which lays down procedures to be adopted from design to quantity production. The procedures form the basis of a disciplined programme aimed at improving reliability and the control of production quality. It is a basic philosophy which is really centred on carrying out environmental tests on numbers of equipments in three stages: (a) development prototypes; (b) pre-production models; (c) continued sample testing of factory production equipment during the whole of the production period.
2. THE D E S I G N PHASE
The implication of the A.G.R.E.E. philosophy in the design phase can best be summed up in the phrase "ensuring fitness for purpose". In essence this means to ensure that components and their methods of use are completely compatible with the operational environment that can be anticipated for the equipment. This work is continuous throughout the design and development of an equipment although there are peaks of special activity which occur at certain phases of the development. The initial work involves an extensive analysis of the experience recorded from our previous equipments and obtained from the records referred to in Section 1
of this paper. As an example the percentage of total failures attributed to the various types of components of a radar system are tabulated to show which areas offer the greatest opportunity for improvement. This analysis has to be approached with a very practical attitude because it is almost certain that the actual figures presented will have various and probably unknown weighting factors already incorporated, due perhaps to the class of use of the equipment and even to intangible factors such as a locally held prejudice at a particular service depot. With the guidance of this analysed experience, component tests are conducted both on existing types and on any alternatives which seem to show any promise. The tests attempt to accelerate failures so that comparisons can be made rapidly, as also can the effects of any modifications to the actual components introduced by the manufacturers following discussions with them. As the results of these tests become available the pattern for the new equipments and the lists of components that they will use becomes more firm. Interlocking with this period, detailed electrical and mechanical design proceeds with the engineers continually testing models. Again from past experience and also from the various type approval specifications, notably the British Board of Trade specification, overall shipboard conditions which a complete equipment must withstand are established. These include lower and upper temperature limits, high humidity conditions and vibration extending from very low frequencies to approximately 500 Hz at levels up to 1 g. The maximum rise above ambient temperature at any point in the equipment is limited to 15°C as it has been proved by bitter experience that localized hot spots can be the cause of many component failures. This implies that the actual components may be in an ambient temperature of 70°C when the equipment itself is in an ambient of 55°C. During the early stages of the design all components are tested to their full rating at temperatures of 70°C. This is an obvious and wellknown illustration of what is meant by the phrase quoted earlier that components and their methods of use must be proved to be compatible. A less well appreciated but comparable point arises in the mechanical design. Since it is not possible to design an equipment
E Q U I P M E N T R E L I A B I L I T Y AND T H E E N V I R O N M E N T to be infinitely stiff in all places there are bound to be mechanical resonances in certain parts of the equipment which have the effect of raising the effective vibration level above that which is applied to the complete equipment. If these resonances occur within the normal frequency spectrum and have a high mechanical gain (analogous to electrical Q) the actual vibration forces to which components are subjected can be extremely high resulting in high failure rate. To prevent this, resonant searches are conducted on models and prototypes at varying stages of the design to ensure that the mechanical Q is limited to a factor of 2 up to 50 Hz, to a factor of 5 between 50 and 100 Hz and a factor of 10 above 100 Hz. All components are then tested to levels of 2, 5 and 10 g in these frequency bands respectively. Again the components are checked to be compatible with the operational environment. This work of checking temperature rises and mechanical resonances, taking corrective action, producing another model incorporating the latest modifications and then retesting, continues right through the development. About half-way through the development period two or three complete equipments called "engineered models" are produced and these are then subjected to the complete specification test. Towards the end of the development period several evaluation equipments are made under laboratory control, but as nearly as possible in strict accordance with the production drawings, and are again submitted to the complete range of tests. All these models remain with the laboratory and are eventually completely written off and usually most of them are finally tested to destruction to determine the ultimate strengths or weaknesses in design. In the final phase of development pre-production models are made using factory tools and processes and these are submitted to the temperature cycling tests, which are described in more detail in the following section.
3. PRE-PRODUCTION A N D P R O D U C T I O N PHASE
The A.G.R.E.E. procedure calls for a number of pre-production prototypes to be made from production drawings and tools. These are subjected to severe environmental testing to ensure that the design is repeatable and that faults which
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exist at this stage can be eliminated from the design before full production starts. After the tests, these equipments are used for agents' conferences, sales introduction and for training purposes. It has been usual to allocate five equipments for pre-production testing, and in order to get in sufficient equipment hours of testing, these equipments are tested for a period of 2000 hr. This gives a total of 10,000 equipment hours in which to assess the potential reliability and prove the remedial action, which may have had to be taken during the test period. The monthly production sample consists of twenty equipments, taken at random, from finished unit stock and subjected to 500 hr of temperature cycling. This aggregates 5000 equipment hours of testing. The tests in both pre-production and production phases are identical. Two large cycling cabinets 21 ft × 16 ft high have been constructed and each equipped to accommodate and run ten complete marine radar sets. The temperature cycling rate is once per day starting at midnight with the equipment switched "off" and the temperature is dropped to -- 15°C. At 0900 hr the equipment is switched "on" with the input power supplies st at the specification lower limits (nominal --20 per cent) and given a performance check in the first hour. The equipment is left running and the cabinet temperature begins its run up to 55°C, hourly checks being made on equipment performance. The cabinet reaches 55°C after about 7 hr and at this point the power supplies are raised to the specification upper limits (nominal 10 per cent) for the latter part of the cycle. The equipments are given a final full performance check at about 2100 hr and then are switched "off". The cabinet is then vented to atmosphere for an hour and resealed ready for the next cycle of operations. The equipments are tested to the full factory test specification by inspection personnel both before and after the 500 hr of cycling. Short repairs and adjustments are carried out by engineers working in the test cabinet, but for larger repairs the equipment (which is trolley mounted) is withdrawn from the cabinet into a more comfortable atmosphere. The time spent by engineers in the test cabinet has to be controlled and the personnel involved are medically examined at regular intervals.
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4. D O C U M E N T A T I O N A N D C O - O R D I N A T I O N OF A.G.R.E.E. TEST W I T H POST D E S I G N FUNCTION
The A.G.R.E.E. production test programme is the responsibility of the Service Division who carry out the work in radar development laboratories. This ensures the closest possible liaison with R. & D. throughout the programme. Each equipment under test has a fault record card which is maintained daily. Weekly and monthly reports are written and submitted to the Monthly Serviceability Meeting to decide what action is necessary as a result of any trends shown up during testing, although faults which require urgent action are dealt with by the engineers on a day-to-day basis. The test reports give details of the faults occurring during the test period, sub-divided into those which would have caused a service call and other subsidiary faults, together with a general reason for failure, such as design, production, inspection or material defect. An example of this is given in Appendix 1. The post design function in service division is that of investigating causes of failures in equipment at sea; 10,000 marine radars are currently being studied and both monthly and 6-monthly reliability analyses are published within the company. These analyses show repetitive faults and trends in the reliability of equipments, units and components. This written information is presented monthly to the Serviceability Meeting where it is compared with A.G.R.E.E. test results and the monthly analysis of faults on production test presented by factory inspection department. This monthly meeting can obviously only discuss relatively long-term problems, but certain urgent problems have to be dealt with as they arise. They are brought to our attention by our depots and agents throughout the world by telephone, telex or letter and very often a fault which has only been reported occurring twice is investigated by an engineer visiting the ships involved. It is perhaps fortunate that the majority of ship movements and hence services to marine radars are in Europe and North America where communications and transport facilities are good. This enables engineers to be routed to the sources of trouble very quickly and very often it is reported to the monthly meeting that a fault has occurred, been investigated and a
change introduced into production already and no decisions required. It is very often necessary to introduce retrospective modifications to equipment in the field. This is accomplished by circulating modification instructions and materials to depots and agents whose job it becomes to carry out these changes. It is naturally difficult to do this, due to the movement of ships, and it is considered a successful exercise if 70 per cent of the equipments affected are actually modified. Little need be said about technical publications since it is only too obvious that a good technical manual is essential to ensure the maintainability of an equipment. This paper did not set out to discuss maintainability but the ability to maintain does have a secondary effect on the overall reliability of an equipment. Over the last 2 or 3 years it has become the practice to publish a bi-monthly Service Newsletter to inform agents of changes impending, new techniques in locating certain faults and in general to invite comment from service engineers all over the world. They have contributed generously and if nothing else the newsletter has become a means of communication between engineers who have probably never met or even heard of each other before.
5. GENERAL D I S C U S S I O N
This post design and A.G.R.E.E. procedure was introduced into the author's company in 1963 and has now been effected through three marinc radar devlopment-producfion programmes. It has meant large capital investment and heavy operating costs, which must continue to be incurred as long as this principle is used. However, the factors which effect the future of A.G.R.E.E. in what is strictly a commercial environment are obviously many and varied, but are divided largely into two groups, namely technical and financial. There must be a financial return on expenditure of this nature and it can be found in the increase of equipment sales due to better reliability and (long-term) lower maintenance costs, with an immediate reduction in the value of the guaranteed service account. Since the distribution of failures in the life of any equipment follows the "bath-tub" curve, the highest failure rate occurs in early life and largely in the 6-month's guarantee period. Therefore the costs to the company are high and a 20-25 per cent reduc-
E Q U I P M E N T R E L I A B I L I T Y AND T H E E N V I R O N M E N T tion in these costs alone can cover the A.G.R.E.E. operating costs. It is naturally difficult to find the advantage in producing better reliability by the A.G.R.E.E. method rather than by what might be termed "the natural evolution through better design", but after four years' operation certain facts are emerging which should be considered. The A.G.R.E.E. method of approach provides a continuous monitoring of the quality of equipment being produced, in a way that normal inspection and quality control methods cannot do. It is a monitor on all the functions--design, production and inspection--plus the advantage of bringing to light defects in the manufacture of components, at a time when corrective action can be taken and the inevitable failures in service minimized and in some cases avoided completely. There have been a few instances where components have failed during the 500 hr temperature cycling and have eventually failed in service after an estimated 2000 hr life. This is a good indication that component failures during A.G.R.E.E. test must be taken very seriously and investigated immediately, if field failures are to be reduced. The A.G.R.E.E. production testing with its documented reporting system is another link in the communication chain between design, production, inspection and the service engineer, the service division being regarded as representing the agents and customers. Because the A.G.R.E.E. procedure starts in the design stage, service division engineers have been attached to the design project teams to assist and advise where possible on reliability and designing for good accessibility and maintainability. The service engineer, having been involved in design problems and having carried out environmental test of the production equipments, is now far better equipped to go out in the field and investigate the basic causes of failure. It has given him a broader outlook towards field problems. T h e design laboratory, too, in its turn, has moved closer to the field problem by being more in contact with service division and its engineers, and through participation in contact with service division and its engineers and through participation in the environmental test in preproduction and production phases. We have, of course, to take stock of A.G.R.E.E.
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test results on production equipments and ask ourselves some basic questions. (1) What relationship is there between the mean time between failures obtained in the environmental test cabinet and the actual M T B F eventually achieved in service? We find the relationship between these two values of M T B F varies from equipment to equipment, but in general it could be said that the M T B F obtained in A.G.R.E.E. production testing is about one-fifth or one-sixth of that obtained in the field. The reason for this is that the equipment under environmental test is taken to the limits of the design specification every day for one month. This seeks out all the potential weaknesses in the system in a short period of time, whereas the equipments in the field are spread out geographically in widely differing environments which may or may not reach the limits of design specification on a few occasions per year, or in some cases in the life of the equipment. The difference in these results does not, in fact, matter. Even though the production test figure of M T B F seems low compared with the field figure, the important thing is to maintain the standard. If the M T B F in A.G.R.E.E. test is not maintained during the production phase then it must be expected that the field figure will also show a fall. A.G.R.E.E. philosophy and testing is a form of quality assurance and should be treated as such. We should not be constantly seeking for a definite indication of the expected field failure rate in the results of a rigidly controlled test. (2)The A.G.R.E.E. document states that a statistically calculated number of equipments should be used for pre-production test. If these equipments give the target M T B F figure then the production equipments will be ostensibly trouble free. Do we believe this? The author's answer is "No". We will discuss the target M T B F later, but the pre-production batch of equipment, whatever the number, is unlikely to eliminate all the design faults; this is largely due to the fact that preproduction equipments, whatever the number, are a relatively small batch and the purchasing of components and manufacture is done on that basis. Therefore in any pre-production batch, the full spread of the equipment manufacturers' tolerances and the component manufacturers' tolerances are
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not being experienced. It is not until the factory provision and manufacture 1000 or 1500 equipments that the full tolerances are seen. For this reason, in each of the three marine programmes we have run there have been one or two problems which did not appear until the first production batch. It has to be remembered that when a design laboratory is engaged on a new development project, the engineers involved are naturally striving to use the most advanced techniques they can, compatible with the time scale. A new commercial equipment cannot be held back indefinitely; it has to reach the market before it becomes obsolete. This means that some components are also in the design and pre-production stage running parallel to the equipment programme. When the components reach their production stage, tolerances and different techniques can bring delayed troubles.
Case-history
currents occurred. Fortunately this was the first production batch and no more equipments were issued. The capacitors were withdrawn and replaced with components which had been wound by a modified technique. Although this potential fault was discovered in production equipment rather than in the preproduction batch, without A.G.R.E.E. test to stress these components the fault would have occurred in the field when probably 1000 equipments had been delivered and we would have had to launch an expensive modification programme. This is one case in detail; there are others such as the cathode-ray tube paper label, where the adhesive dried off at high temperature allowing the label to fall across the terminals of a high voltage transformer and becoming ignited, causing a real fire hazard; and the resin-cast transformers which split open in low temperature because the manufacturer changed his method of assembly after the pre-production stage.
On a new design of marine radar equipment, it was desirable to use a new type high-voltage capacitor in the transmitter and care had to be taken in the choice of components--particularly as the magnetron coupling capacitors are passing pulse currents. During the prototype and pre-production testing no troubles were experienced and testing the components individually showed them to be remarkably good. However, when the first production batch of ten equipments were tested in the cabinet, several of these capacitors failed. The manufacturer investigated and found that all the early capacitors had been wound entirely by hand. When production started the machine operator would do the first few turns of foil and dielectric by hand. Then the machine was switched to power drive and this action created a ridge and trough across the full width of the winding. It was in this area of the winding that burning to the high peak pulse
We believe that the method of using a service organization to combine a general-liaison type post design engineering function with the so-called A.G.R.E.E. procedure is sound. It has produced improved reliability, particularly in the early life phase; it provides a good follow-up procedure throughout the production life of the equipment so that the problems can be solved quickly and efficiently. In terms of communication it has brought the design engineer and the customer closer together so that they each have a better understanding of the other's problems. Finally it has reduced the annual servicing cost to the customer and has reduced the company's guarantee account (per equipment) by an amount which more than pays for the operation.
6. CONCLUSION
EQUIPMENT
RELIABILITY
AND THE
ENVIRONMENT
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APPENDIX 1 A.G.R.E.E. testing J. HARRIS and D. W. SEARS Decca Radar Limited
Objectives A.G.R.E.E. testing of commercial marine radar was introduced some years ago in order to improve the reliability of equipment in service. This improvement in reliability is achieved by removing inherent weaknesses from the design during the prototype and pre-production stage and by monitoring new equipments from the factory for the whole period during which the equipment is in production.
Testing in the design and pre-production stages Very early in the design stage components, subassemblies and prototype units are subjected to various climatic and durability trials in the environmental testing laboratories, these tests include: (1) Vibration in the range 1-500 c/s. (2) Bump, electronic units 10,000 bumps at 5 g in three planes; mechanical units 4,000 bumps at 20 g in two planes. (3) Dry heat---storage at 70°C, operational temperatures of 55°C for Class B and 70°C for Class X equipment. (4) Damp heat---40°C with R.H. of 95 per cent. (5) Forty-two-day tropical life test (cycling between 35°C with R.H. greater than 90 per cent, to 20°C where saturation is allowed to occur). (6) Low temperature: --15°C for Class B and -- 25 °C for Class X equipment. (7) Driving rain and splash tests. (8) Corrosion tests. (9) Life tests and hot spot checks. (10) Radio and magnetic interference tests. On completion of the foregoing tests further models are made for laboratory reliability evaluation, sea trials and A.G.R.E.E. testing, these tests are intended to confirm the soundness of the design and to highlight any weaknesses. By the time that this work is completed, assembly of the pre-production models has started and a substantial percentage of these (30-50 per cent) are submitted to the A.G.R.E.E. temperature cycling procedure, thus by the time that the equipment is ready for production we have a good idea as to its capabilities, the likely spread of its parameters in production, and the reliability which it is likely to give in service.
The A.G.R.E.E. testing o/production equipment The first equipments off the production lines are passed to the A.G.R.E.E. temperature cycling chamber as they are completed and thereafter we continue to sample about 10 per cent of each month's output for as long as the equipment remains in production.
Our approach to the sampling and testing of production equipments differs from that given to the subject in the original A.G.R.E.E. document in that instead of specifying a contract M.T.B.F. and rejecting or accepting equipment on the basis of the results obtained, we measure the expected M T B F s of each month's production to determine whether or not the quality of production is being maintained. For the purpose of carrying out these tests we have two temperature cycling chambers, each of which can accommodate ten complete radar systems. Each complete radar is mounted on a trolley so as to facilitate its speedy removal from the chamber or to another test position. Each trolley is connected to a suitable power socket which may supply any required voltage from 12 V d.c. to 440 V 3-phase 60 c/s. A waveguide outlet leading to an active aerial on the roof of the building is available at each test position so that the overall equipment performance can be readily checked. Equipment types are tested in batches of ten and in order to ensure a constant flow of work for the engineering staff they are placed in the chambers at intervals over the month. Equipment is taken at random from the previous week's production. The work is carried out by members of the Service Division, who, since they are not involved in design, production and inspection, are able to approach the testing objectively. Before entering the temperature cycling chamber each radar is inspected mechanically and submitted to the full production electrical test specification, various parameters are recorded for comparison with the performance obtained when the temperature cycling is completed. During the actual A.G.R.E.E. test, which lasts for approximately 500 hr, the chambers in which the equipment is located are cycled daily between --15°C and +55°C as follows: 4 a.m.-
10 a.m.
Temperature
stabilized
at
--15°C
10 a.m.-
5 p.m.
5 p.m.-10.30 p.m. 10.30a.m.-12.30a.m.
12.30 a.m.--4 a.m.
Temperature rises from --15 to 55°C Temperature stabilized at 55°C Chamber vented to atmosphere, temperature falls to ambient Temperature fails from ambient to - - 15°C.
T h e units are switched on at 9 a.m. each day with the ambient temperature at --15°C and mains supply at minimum (e.g. --20 per cent below nominal on d.c.
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and D. W. SEARS
supplies), no space heaters or any preconditioning treatm e n t is allowed; the radar is then given a performance check in conjunction with an active serial. Hourly checks are carried out throughout the day to ensure that all units are functioning. D u r i n g the 2-hr period prior to 9 p.m. each equipment is again given a performance check with an active serial, at this time the temperature is at 55°C and the mains input voltages are set to maximum. At 9 p.m. the equipment is switched off and the working day is completed. H u m i d i t y was not initially intended to form part of the test, b u t in practice it has been found that when the temperature of the chamber is below the outside ambient, warm air which enters via the air intakes at the beginn i n g of the heating cycle and whenever the doors are opened, condenses on the cold metal surfaces so that for about 2 h r in the m o r n i n g the equipment is covered with moisture. Shortly after the A.G.R.E.E. test procedure was introduced a b u m p test was included for 15 radars in order to determine whether or not dry joints and intermittent faults could be m a d e to show up. Each unit received 1000 b u m p s at a level of 3 g and the results obtained were compare dwith those of 15 radars which were not bumped. T h e r e was no significant difference between the two batches and the test was stopped since it was very time c o n s u m i n g and was producing no positive results. We nevertheless retain the option to s u b m i t any e q u i p m e n t to any one or more of the environmental tests previously referred to, After 4 weeks in the chamber, the equipment is removed, re-submitted to the full production test specification, mechanically examined and refurbished as and where necessary. T h e test cards attached to the equipm e n t are stamped I~NVIRONMENTALLYTESTED before the equipment is passed to Finished U n i t Stock for ultimate despatch to a customer. It is interesting to note that some customers specifically request equipment which has been through the A.G.R.E.E. chambers.
Test records and reports A n y failures which occur during the 500-hr temperature cycling are recorded on a test sheet which is attached to each equipment; the date, time, temperature and action taken are also indicated. Failures are classified as (a) Primary defects which would probably institute a service. (b) Subsidiary defects which probably would not. Failures are also attributed to design, production, material or other causes, and persons responsible are notified of failures as they occur unless they are deemed to be of a random nature. A weekly report summarizes all the failures which occurred during the previous week. W h e n the last equipment of the batch has been cleared, a batch report is raised and circulated to directors and other interested parties within the company. T h i s report lists the serial n u m b e r s of all the units tested and records all failures which occurred before, during and after the temperature cycling, indicating
department, if any, is responsible for taking remedial action. Aggregate hours are plotted against total failures and an accept/reject wedge added to indicate the M T B F obtained as a result of the test. T h e boundaries of the wedge are determined from the formulae M T B F --
T T 0 . 8 1 F : - - 4 . 4 -- 0'81Fa-t-4'4
where F ! = n u m b e r of failures to permit a reject decision; Fa = n u m b e r of failures to permit an accepted decision; T = aggregate test hours. T h e above equations are based on a discrimination ratio of 1'5 and producers and consumers risks of 10 per cent. T h e front page of the report contains a s u m m a r y of the principal defects and indicates what action is being taken to eliminate them.
What we obtain for our effort T h e introduction of A.G.R.E.E. testing has necessitated the investment of large s u m s of money in plant and material and in addition to this the r u n n i n g costs are high since two shifts of engineers are required and rigorous safety precautions have to be taken. However, the investment and effort achieve the following results: (a) Since the equipment is covered by guarantee for a period of 6 months, the improvement in reliability results in a reduction in guarantee costs. (b) M a n y equipments in service are covered by rental maintenance agreements and the cost of maintaining these equipments is reduced. (c) Increased sales result from the improvement in reliability and in the fact that our customers can see that we are taking the matter seriously. F r o m time to time selected groups of customers are give the opportunity to see the testing programme in action. (d) If defects should slip through the design proving stage or if teething troubles occur when the equipm e n t is first p u t into production, a further chance is given for corrective action before large n u m b e r s of equipment get into service. (e) New and improved components or circuitry are independently evaluated before they go into service. We have ceased to experiment on our customers. (f) Any reduction in the standard of production equipm e n t is immediately brought to light so that if necessary all equipments on the factory floor or in Finished Unit Stock can be re-inspected. (g) W h e n a new type of equipment is first introduced into service, we have a nucleus of service engineers who are familiar with the e q u i p m e n t so that if necessary they can be sent to any part of the world to install or service the equipment and to train agents.
Pergamon Press 1970. Vol. 9, pp. 145-156.
Printed in Great Britain
EQUIPMENT RELIABILITY AND THE ENVIRONMENT* J. HARRIS
Decca Radar Ltd., Croydon, Surrey 1. INTRODUCTION THE term "environment" is usually defined as
being a set of conditions under which a piece of equipment is expected to operate. The conditions described thus are invariably climatic or conditions of vibration and shock, etc. Indeed specifications for the design of new equipment inevitably state a required performance under given "environmental" conditions. The author believes that the definition of environment should be widened to include the method of supporting the equipment in the field during its production life. "Support" includes any reliability engineering programme from the pre-production stage onwards, provision of technical information service and maintenance, serviceability analysis, information feedback systems and the general post design engineering to eliminate repetitive faults. By this means, a manufacturer of commercial electronic equipments can create, to a degree, his environment and exercise a measure of control over the reliability of his equipment. This paper describes the problems of reliability in the marine market over the last 10-15 years and the efforts made by the author's company to improve the reliability of their product. The requirement for reliable commercial marine radar equipment at sea has been a growing problem for many years. The use of radar at sea grew enormously during the 1950s, but the reliability of the conventional valve equipment then in use did not improve at the same rate, if indeed it improved at all. In the larger complex equipments, it was unlikely, on a component population basis, that * Paper read at the Bath University of Technology, Centre for Adult Studies, Course in Electronics Reliability IV. 145
any manufacturer could produce an equipment which could operate over the periods of months without failure that the user so rightly demands. All manufacturers have recognized that reliability must be the prime consideration and have paid close attention over the years to the problem of improving their products. The authors' organization has paid particular attention to component specifications, use of preferred items, good inspection and quality control in production. In addition, this company's service division have made great efforts to investigate the causes of unreliability at sea and by the use of a punched card data processing system have produced analyses of equipment reliability of over 16,000 ships. These analyses have been issued on both monthly and 6-monthly basis for the past 5 years and show the failure rates of complete equipments, units and individual components. From these results, design changes to production equipment and retrospective modifications have been made. Continual improvement to existing and past designs have been possible and a number of improved components have been developed by close liaison with component manufacturers. Controlled sea trials, usually in trawlers, are the final acceptance test for these components. In the early 1960s, however, despite these efforts to co-ordinate design and production with the feedback of service information, it became very clear that more drastic changes were necessary to bring about the order of improved reliability that was really required. The transistor, by this time, was a feasible proposition and a development programme was laid down to produce a complete range of marine radars using transistors wherever possible. At the same time a study was made of the work carried out in the United States by the Advisory
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Group on Reliability of Electronic Equipment (A.G.R.E.E.) and similar work was undertaken by the Ministry of Aviation in the United Kingdom. Our studies led us to the decision to adapt the basic A.G.R.E.E. principles with the following very simple objectives in view: (a) to produce equipment which could meet the strictest environmental specification and had its inherent faults removed before production, rather than after 1 to 2 years at sea; (b) to ensure that the transition phase from development to production did not cause potential unreliability; (c) to maintain reliability standards throughout the production life, particularly guarding against latent defects due to changes in our own manufacturing techniques and in the manufacturing techniques of component suppliers. The A.G.R.E.E. report is a lengthy document which lays down procedures to be adopted from design to quantity production. The procedures form the basis of a disciplined programme aimed at improving reliability and the control of production quality. It is a basic philosophy which is really centred on carrying out environmental tests on numbers of equipments in three stages: (a) development prototypes; (b) pre-production models; (c) continued sample testing of factory production equipment during the whole of the production period.
2. THE D E S I G N PHASE
The implication of the A.G.R.E.E. philosophy in the design phase can best be summed up in the phrase "ensuring fitness for purpose". In essence this means to ensure that components and their methods of use are completely compatible with the operational environment that can be anticipated for the equipment. This work is continuous throughout the design and development of an equipment although there are peaks of special activity which occur at certain phases of the development. The initial work involves an extensive analysis of the experience recorded from our previous equipments and obtained from the records referred to in Section 1
of this paper. As an example the percentage of total failures attributed to the various types of components of a radar system are tabulated to show which areas offer the greatest opportunity for improvement. This analysis has to be approached with a very practical attitude because it is almost certain that the actual figures presented will have various and probably unknown weighting factors already incorporated, due perhaps to the class of use of the equipment and even to intangible factors such as a locally held prejudice at a particular service depot. With the guidance of this analysed experience, component tests are conducted both on existing types and on any alternatives which seem to show any promise. The tests attempt to accelerate failures so that comparisons can be made rapidly, as also can the effects of any modifications to the actual components introduced by the manufacturers following discussions with them. As the results of these tests become available the pattern for the new equipments and the lists of components that they will use becomes more firm. Interlocking with this period, detailed electrical and mechanical design proceeds with the engineers continually testing models. Again from past experience and also from the various type approval specifications, notably the British Board of Trade specification, overall shipboard conditions which a complete equipment must withstand are established. These include lower and upper temperature limits, high humidity conditions and vibration extending from very low frequencies to approximately 500 Hz at levels up to 1 g. The maximum rise above ambient temperature at any point in the equipment is limited to 15°C as it has been proved by bitter experience that localized hot spots can be the cause of many component failures. This implies that the actual components may be in an ambient temperature of 70°C when the equipment itself is in an ambient of 55°C. During the early stages of the design all components are tested to their full rating at temperatures of 70°C. This is an obvious and wellknown illustration of what is meant by the phrase quoted earlier that components and their methods of use must be proved to be compatible. A less well appreciated but comparable point arises in the mechanical design. Since it is not possible to design an equipment
E Q U I P M E N T R E L I A B I L I T Y AND T H E E N V I R O N M E N T to be infinitely stiff in all places there are bound to be mechanical resonances in certain parts of the equipment which have the effect of raising the effective vibration level above that which is applied to the complete equipment. If these resonances occur within the normal frequency spectrum and have a high mechanical gain (analogous to electrical Q) the actual vibration forces to which components are subjected can be extremely high resulting in high failure rate. To prevent this, resonant searches are conducted on models and prototypes at varying stages of the design to ensure that the mechanical Q is limited to a factor of 2 up to 50 Hz, to a factor of 5 between 50 and 100 Hz and a factor of 10 above 100 Hz. All components are then tested to levels of 2, 5 and 10 g in these frequency bands respectively. Again the components are checked to be compatible with the operational environment. This work of checking temperature rises and mechanical resonances, taking corrective action, producing another model incorporating the latest modifications and then retesting, continues right through the development. About half-way through the development period two or three complete equipments called "engineered models" are produced and these are then subjected to the complete specification test. Towards the end of the development period several evaluation equipments are made under laboratory control, but as nearly as possible in strict accordance with the production drawings, and are again submitted to the complete range of tests. All these models remain with the laboratory and are eventually completely written off and usually most of them are finally tested to destruction to determine the ultimate strengths or weaknesses in design. In the final phase of development pre-production models are made using factory tools and processes and these are submitted to the temperature cycling tests, which are described in more detail in the following section.
3. PRE-PRODUCTION A N D P R O D U C T I O N PHASE
The A.G.R.E.E. procedure calls for a number of pre-production prototypes to be made from production drawings and tools. These are subjected to severe environmental testing to ensure that the design is repeatable and that faults which
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exist at this stage can be eliminated from the design before full production starts. After the tests, these equipments are used for agents' conferences, sales introduction and for training purposes. It has been usual to allocate five equipments for pre-production testing, and in order to get in sufficient equipment hours of testing, these equipments are tested for a period of 2000 hr. This gives a total of 10,000 equipment hours in which to assess the potential reliability and prove the remedial action, which may have had to be taken during the test period. The monthly production sample consists of twenty equipments, taken at random, from finished unit stock and subjected to 500 hr of temperature cycling. This aggregates 5000 equipment hours of testing. The tests in both pre-production and production phases are identical. Two large cycling cabinets 21 ft × 16 ft high have been constructed and each equipped to accommodate and run ten complete marine radar sets. The temperature cycling rate is once per day starting at midnight with the equipment switched "off" and the temperature is dropped to -- 15°C. At 0900 hr the equipment is switched "on" with the input power supplies st at the specification lower limits (nominal --20 per cent) and given a performance check in the first hour. The equipment is left running and the cabinet temperature begins its run up to 55°C, hourly checks being made on equipment performance. The cabinet reaches 55°C after about 7 hr and at this point the power supplies are raised to the specification upper limits (nominal 10 per cent) for the latter part of the cycle. The equipments are given a final full performance check at about 2100 hr and then are switched "off". The cabinet is then vented to atmosphere for an hour and resealed ready for the next cycle of operations. The equipments are tested to the full factory test specification by inspection personnel both before and after the 500 hr of cycling. Short repairs and adjustments are carried out by engineers working in the test cabinet, but for larger repairs the equipment (which is trolley mounted) is withdrawn from the cabinet into a more comfortable atmosphere. The time spent by engineers in the test cabinet has to be controlled and the personnel involved are medically examined at regular intervals.
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4. D O C U M E N T A T I O N A N D C O - O R D I N A T I O N OF A.G.R.E.E. TEST W I T H POST D E S I G N FUNCTION
The A.G.R.E.E. production test programme is the responsibility of the Service Division who carry out the work in radar development laboratories. This ensures the closest possible liaison with R. & D. throughout the programme. Each equipment under test has a fault record card which is maintained daily. Weekly and monthly reports are written and submitted to the Monthly Serviceability Meeting to decide what action is necessary as a result of any trends shown up during testing, although faults which require urgent action are dealt with by the engineers on a day-to-day basis. The test reports give details of the faults occurring during the test period, sub-divided into those which would have caused a service call and other subsidiary faults, together with a general reason for failure, such as design, production, inspection or material defect. An example of this is given in Appendix 1. The post design function in service division is that of investigating causes of failures in equipment at sea; 10,000 marine radars are currently being studied and both monthly and 6-monthly reliability analyses are published within the company. These analyses show repetitive faults and trends in the reliability of equipments, units and components. This written information is presented monthly to the Serviceability Meeting where it is compared with A.G.R.E.E. test results and the monthly analysis of faults on production test presented by factory inspection department. This monthly meeting can obviously only discuss relatively long-term problems, but certain urgent problems have to be dealt with as they arise. They are brought to our attention by our depots and agents throughout the world by telephone, telex or letter and very often a fault which has only been reported occurring twice is investigated by an engineer visiting the ships involved. It is perhaps fortunate that the majority of ship movements and hence services to marine radars are in Europe and North America where communications and transport facilities are good. This enables engineers to be routed to the sources of trouble very quickly and very often it is reported to the monthly meeting that a fault has occurred, been investigated and a
change introduced into production already and no decisions required. It is very often necessary to introduce retrospective modifications to equipment in the field. This is accomplished by circulating modification instructions and materials to depots and agents whose job it becomes to carry out these changes. It is naturally difficult to do this, due to the movement of ships, and it is considered a successful exercise if 70 per cent of the equipments affected are actually modified. Little need be said about technical publications since it is only too obvious that a good technical manual is essential to ensure the maintainability of an equipment. This paper did not set out to discuss maintainability but the ability to maintain does have a secondary effect on the overall reliability of an equipment. Over the last 2 or 3 years it has become the practice to publish a bi-monthly Service Newsletter to inform agents of changes impending, new techniques in locating certain faults and in general to invite comment from service engineers all over the world. They have contributed generously and if nothing else the newsletter has become a means of communication between engineers who have probably never met or even heard of each other before.
5. GENERAL D I S C U S S I O N
This post design and A.G.R.E.E. procedure was introduced into the author's company in 1963 and has now been effected through three marinc radar devlopment-producfion programmes. It has meant large capital investment and heavy operating costs, which must continue to be incurred as long as this principle is used. However, the factors which effect the future of A.G.R.E.E. in what is strictly a commercial environment are obviously many and varied, but are divided largely into two groups, namely technical and financial. There must be a financial return on expenditure of this nature and it can be found in the increase of equipment sales due to better reliability and (long-term) lower maintenance costs, with an immediate reduction in the value of the guaranteed service account. Since the distribution of failures in the life of any equipment follows the "bath-tub" curve, the highest failure rate occurs in early life and largely in the 6-month's guarantee period. Therefore the costs to the company are high and a 20-25 per cent reduc-
E Q U I P M E N T R E L I A B I L I T Y AND T H E E N V I R O N M E N T tion in these costs alone can cover the A.G.R.E.E. operating costs. It is naturally difficult to find the advantage in producing better reliability by the A.G.R.E.E. method rather than by what might be termed "the natural evolution through better design", but after four years' operation certain facts are emerging which should be considered. The A.G.R.E.E. method of approach provides a continuous monitoring of the quality of equipment being produced, in a way that normal inspection and quality control methods cannot do. It is a monitor on all the functions--design, production and inspection--plus the advantage of bringing to light defects in the manufacture of components, at a time when corrective action can be taken and the inevitable failures in service minimized and in some cases avoided completely. There have been a few instances where components have failed during the 500 hr temperature cycling and have eventually failed in service after an estimated 2000 hr life. This is a good indication that component failures during A.G.R.E.E. test must be taken very seriously and investigated immediately, if field failures are to be reduced. The A.G.R.E.E. production testing with its documented reporting system is another link in the communication chain between design, production, inspection and the service engineer, the service division being regarded as representing the agents and customers. Because the A.G.R.E.E. procedure starts in the design stage, service division engineers have been attached to the design project teams to assist and advise where possible on reliability and designing for good accessibility and maintainability. The service engineer, having been involved in design problems and having carried out environmental test of the production equipments, is now far better equipped to go out in the field and investigate the basic causes of failure. It has given him a broader outlook towards field problems. T h e design laboratory, too, in its turn, has moved closer to the field problem by being more in contact with service division and its engineers, and through participation in contact with service division and its engineers and through participation in the environmental test in preproduction and production phases. We have, of course, to take stock of A.G.R.E.E.
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test results on production equipments and ask ourselves some basic questions. (1) What relationship is there between the mean time between failures obtained in the environmental test cabinet and the actual M T B F eventually achieved in service? We find the relationship between these two values of M T B F varies from equipment to equipment, but in general it could be said that the M T B F obtained in A.G.R.E.E. production testing is about one-fifth or one-sixth of that obtained in the field. The reason for this is that the equipment under environmental test is taken to the limits of the design specification every day for one month. This seeks out all the potential weaknesses in the system in a short period of time, whereas the equipments in the field are spread out geographically in widely differing environments which may or may not reach the limits of design specification on a few occasions per year, or in some cases in the life of the equipment. The difference in these results does not, in fact, matter. Even though the production test figure of M T B F seems low compared with the field figure, the important thing is to maintain the standard. If the M T B F in A.G.R.E.E. test is not maintained during the production phase then it must be expected that the field figure will also show a fall. A.G.R.E.E. philosophy and testing is a form of quality assurance and should be treated as such. We should not be constantly seeking for a definite indication of the expected field failure rate in the results of a rigidly controlled test. (2)The A.G.R.E.E. document states that a statistically calculated number of equipments should be used for pre-production test. If these equipments give the target M T B F figure then the production equipments will be ostensibly trouble free. Do we believe this? The author's answer is "No". We will discuss the target M T B F later, but the pre-production batch of equipment, whatever the number, is unlikely to eliminate all the design faults; this is largely due to the fact that preproduction equipments, whatever the number, are a relatively small batch and the purchasing of components and manufacture is done on that basis. Therefore in any pre-production batch, the full spread of the equipment manufacturers' tolerances and the component manufacturers' tolerances are
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not being experienced. It is not until the factory provision and manufacture 1000 or 1500 equipments that the full tolerances are seen. For this reason, in each of the three marine programmes we have run there have been one or two problems which did not appear until the first production batch. It has to be remembered that when a design laboratory is engaged on a new development project, the engineers involved are naturally striving to use the most advanced techniques they can, compatible with the time scale. A new commercial equipment cannot be held back indefinitely; it has to reach the market before it becomes obsolete. This means that some components are also in the design and pre-production stage running parallel to the equipment programme. When the components reach their production stage, tolerances and different techniques can bring delayed troubles.
Case-history
currents occurred. Fortunately this was the first production batch and no more equipments were issued. The capacitors were withdrawn and replaced with components which had been wound by a modified technique. Although this potential fault was discovered in production equipment rather than in the preproduction batch, without A.G.R.E.E. test to stress these components the fault would have occurred in the field when probably 1000 equipments had been delivered and we would have had to launch an expensive modification programme. This is one case in detail; there are others such as the cathode-ray tube paper label, where the adhesive dried off at high temperature allowing the label to fall across the terminals of a high voltage transformer and becoming ignited, causing a real fire hazard; and the resin-cast transformers which split open in low temperature because the manufacturer changed his method of assembly after the pre-production stage.
On a new design of marine radar equipment, it was desirable to use a new type high-voltage capacitor in the transmitter and care had to be taken in the choice of components--particularly as the magnetron coupling capacitors are passing pulse currents. During the prototype and pre-production testing no troubles were experienced and testing the components individually showed them to be remarkably good. However, when the first production batch of ten equipments were tested in the cabinet, several of these capacitors failed. The manufacturer investigated and found that all the early capacitors had been wound entirely by hand. When production started the machine operator would do the first few turns of foil and dielectric by hand. Then the machine was switched to power drive and this action created a ridge and trough across the full width of the winding. It was in this area of the winding that burning to the high peak pulse
We believe that the method of using a service organization to combine a general-liaison type post design engineering function with the so-called A.G.R.E.E. procedure is sound. It has produced improved reliability, particularly in the early life phase; it provides a good follow-up procedure throughout the production life of the equipment so that the problems can be solved quickly and efficiently. In terms of communication it has brought the design engineer and the customer closer together so that they each have a better understanding of the other's problems. Finally it has reduced the annual servicing cost to the customer and has reduced the company's guarantee account (per equipment) by an amount which more than pays for the operation.
6. CONCLUSION
EQUIPMENT
RELIABILITY
AND THE
ENVIRONMENT
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APPENDIX 1 A.G.R.E.E. testing J. HARRIS and D. W. SEARS Decca Radar Limited
Objectives A.G.R.E.E. testing of commercial marine radar was introduced some years ago in order to improve the reliability of equipment in service. This improvement in reliability is achieved by removing inherent weaknesses from the design during the prototype and pre-production stage and by monitoring new equipments from the factory for the whole period during which the equipment is in production.
Testing in the design and pre-production stages Very early in the design stage components, subassemblies and prototype units are subjected to various climatic and durability trials in the environmental testing laboratories, these tests include: (1) Vibration in the range 1-500 c/s. (2) Bump, electronic units 10,000 bumps at 5 g in three planes; mechanical units 4,000 bumps at 20 g in two planes. (3) Dry heat---storage at 70°C, operational temperatures of 55°C for Class B and 70°C for Class X equipment. (4) Damp heat---40°C with R.H. of 95 per cent. (5) Forty-two-day tropical life test (cycling between 35°C with R.H. greater than 90 per cent, to 20°C where saturation is allowed to occur). (6) Low temperature: --15°C for Class B and -- 25 °C for Class X equipment. (7) Driving rain and splash tests. (8) Corrosion tests. (9) Life tests and hot spot checks. (10) Radio and magnetic interference tests. On completion of the foregoing tests further models are made for laboratory reliability evaluation, sea trials and A.G.R.E.E. testing, these tests are intended to confirm the soundness of the design and to highlight any weaknesses. By the time that this work is completed, assembly of the pre-production models has started and a substantial percentage of these (30-50 per cent) are submitted to the A.G.R.E.E. temperature cycling procedure, thus by the time that the equipment is ready for production we have a good idea as to its capabilities, the likely spread of its parameters in production, and the reliability which it is likely to give in service.
The A.G.R.E.E. testing o/production equipment The first equipments off the production lines are passed to the A.G.R.E.E. temperature cycling chamber as they are completed and thereafter we continue to sample about 10 per cent of each month's output for as long as the equipment remains in production.
Our approach to the sampling and testing of production equipments differs from that given to the subject in the original A.G.R.E.E. document in that instead of specifying a contract M.T.B.F. and rejecting or accepting equipment on the basis of the results obtained, we measure the expected M T B F s of each month's production to determine whether or not the quality of production is being maintained. For the purpose of carrying out these tests we have two temperature cycling chambers, each of which can accommodate ten complete radar systems. Each complete radar is mounted on a trolley so as to facilitate its speedy removal from the chamber or to another test position. Each trolley is connected to a suitable power socket which may supply any required voltage from 12 V d.c. to 440 V 3-phase 60 c/s. A waveguide outlet leading to an active aerial on the roof of the building is available at each test position so that the overall equipment performance can be readily checked. Equipment types are tested in batches of ten and in order to ensure a constant flow of work for the engineering staff they are placed in the chambers at intervals over the month. Equipment is taken at random from the previous week's production. The work is carried out by members of the Service Division, who, since they are not involved in design, production and inspection, are able to approach the testing objectively. Before entering the temperature cycling chamber each radar is inspected mechanically and submitted to the full production electrical test specification, various parameters are recorded for comparison with the performance obtained when the temperature cycling is completed. During the actual A.G.R.E.E. test, which lasts for approximately 500 hr, the chambers in which the equipment is located are cycled daily between --15°C and +55°C as follows: 4 a.m.-
10 a.m.
Temperature
stabilized
at
--15°C
10 a.m.-
5 p.m.
5 p.m.-10.30 p.m. 10.30a.m.-12.30a.m.
12.30 a.m.--4 a.m.
Temperature rises from --15 to 55°C Temperature stabilized at 55°C Chamber vented to atmosphere, temperature falls to ambient Temperature fails from ambient to - - 15°C.
T h e units are switched on at 9 a.m. each day with the ambient temperature at --15°C and mains supply at minimum (e.g. --20 per cent below nominal on d.c.
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and D. W. SEARS
supplies), no space heaters or any preconditioning treatm e n t is allowed; the radar is then given a performance check in conjunction with an active serial. Hourly checks are carried out throughout the day to ensure that all units are functioning. D u r i n g the 2-hr period prior to 9 p.m. each equipment is again given a performance check with an active serial, at this time the temperature is at 55°C and the mains input voltages are set to maximum. At 9 p.m. the equipment is switched off and the working day is completed. H u m i d i t y was not initially intended to form part of the test, b u t in practice it has been found that when the temperature of the chamber is below the outside ambient, warm air which enters via the air intakes at the beginn i n g of the heating cycle and whenever the doors are opened, condenses on the cold metal surfaces so that for about 2 h r in the m o r n i n g the equipment is covered with moisture. Shortly after the A.G.R.E.E. test procedure was introduced a b u m p test was included for 15 radars in order to determine whether or not dry joints and intermittent faults could be m a d e to show up. Each unit received 1000 b u m p s at a level of 3 g and the results obtained were compare dwith those of 15 radars which were not bumped. T h e r e was no significant difference between the two batches and the test was stopped since it was very time c o n s u m i n g and was producing no positive results. We nevertheless retain the option to s u b m i t any e q u i p m e n t to any one or more of the environmental tests previously referred to, After 4 weeks in the chamber, the equipment is removed, re-submitted to the full production test specification, mechanically examined and refurbished as and where necessary. T h e test cards attached to the equipm e n t are stamped I~NVIRONMENTALLYTESTED before the equipment is passed to Finished U n i t Stock for ultimate despatch to a customer. It is interesting to note that some customers specifically request equipment which has been through the A.G.R.E.E. chambers.
Test records and reports A n y failures which occur during the 500-hr temperature cycling are recorded on a test sheet which is attached to each equipment; the date, time, temperature and action taken are also indicated. Failures are classified as (a) Primary defects which would probably institute a service. (b) Subsidiary defects which probably would not. Failures are also attributed to design, production, material or other causes, and persons responsible are notified of failures as they occur unless they are deemed to be of a random nature. A weekly report summarizes all the failures which occurred during the previous week. W h e n the last equipment of the batch has been cleared, a batch report is raised and circulated to directors and other interested parties within the company. T h i s report lists the serial n u m b e r s of all the units tested and records all failures which occurred before, during and after the temperature cycling, indicating
department, if any, is responsible for taking remedial action. Aggregate hours are plotted against total failures and an accept/reject wedge added to indicate the M T B F obtained as a result of the test. T h e boundaries of the wedge are determined from the formulae M T B F --
T T 0 . 8 1 F : - - 4 . 4 -- 0'81Fa-t-4'4
where F ! = n u m b e r of failures to permit a reject decision; Fa = n u m b e r of failures to permit an accepted decision; T = aggregate test hours. T h e above equations are based on a discrimination ratio of 1'5 and producers and consumers risks of 10 per cent. T h e front page of the report contains a s u m m a r y of the principal defects and indicates what action is being taken to eliminate them.
What we obtain for our effort T h e introduction of A.G.R.E.E. testing has necessitated the investment of large s u m s of money in plant and material and in addition to this the r u n n i n g costs are high since two shifts of engineers are required and rigorous safety precautions have to be taken. However, the investment and effort achieve the following results: (a) Since the equipment is covered by guarantee for a period of 6 months, the improvement in reliability results in a reduction in guarantee costs. (b) M a n y equipments in service are covered by rental maintenance agreements and the cost of maintaining these equipments is reduced. (c) Increased sales result from the improvement in reliability and in the fact that our customers can see that we are taking the matter seriously. F r o m time to time selected groups of customers are give the opportunity to see the testing programme in action. (d) If defects should slip through the design proving stage or if teething troubles occur when the equipm e n t is first p u t into production, a further chance is given for corrective action before large n u m b e r s of equipment get into service. (e) New and improved components or circuitry are independently evaluated before they go into service. We have ceased to experiment on our customers. (f) Any reduction in the standard of production equipm e n t is immediately brought to light so that if necessary all equipments on the factory floor or in Finished Unit Stock can be re-inspected. (g) W h e n a new type of equipment is first introduced into service, we have a nucleus of service engineers who are familiar with the e q u i p m e n t so that if necessary they can be sent to any part of the world to install or service the equipment and to train agents.