Built-In Test (BIT) and production testing are two major test areas that must be considered from the start of the design effort. Otherwise, these and other testing considerations can negatively impact both manufacturing and life cycle costs. The contractor should develop and implement a corporate policy relative to an integrated design for testing effort.
Design for testing addresses the needs to: (1) collect data, during the developmental process, of particular performance characteristics; (2) enable efficient and economic production by providing ready access to and measurement of appropriate acceptance parameters; and (3) enable rapid and accurate assessment of the status of the product to the lowest repairable element when deployed. These objectives can all be achieved, but only if they are fully recognized from the beginning of the design process. It is natural for engineers to concentrate upon the functional design characteristics, measuring performance parameters during development to the detriment of the product's suitability for test and inspection of acceptance parameters during production. This trap can be avoided if a set of specific guidelines is promulgated before design is initiated. These guidelines are based upon a written corporate policy reflecting up-to-date manufacturing, test, and inspection processes and equipment. The design for testability is approached in a systematic fashion no different than other design aspects. Balanced integrated requirements are identified, a plan developed, audits and reviews conducted, and demonstrations performed.
Frequently the availability of a test asset is ignored and the capability built-in test is overlooked when designing for test. With very high-speed integrated circuitry, the only means of test is self-test. Testability and inspectability designed for automatic equipment facilitate production. When production problems are minimized (through proper design for test, fault isolation, and inspection) maintainability problems are minimized as well. A design that is easily and completely testable and inspect able without disassembly, adjustments, special environmental conditioning, or external equipment or stimuli for monitoring of responses, is amenable to economic production. When one major system element is required to stimulate another during test, or when there is little functional modularity within the design, complexities increase. So do production and support costs.
A testability mentality must be established within the design activity. Just as reliability must be designed in, so must testability and inspectability. All designs should be guided by a strong test philosophy. Test measurements, with tolerances, should be specified to ensure interchangeability of subassemblies. Test specifications should identify what to measure and the required results.
As an integral part of the design process, testability
design concepts should include: (1) physical and electrical partitioning, (2)
Unit Under Test and Automatic Test Equipment (ATE) compatibility, (3)
initialization, (4) test control and access, (5) parts selection, (6)
system-level and item-level BIT, and (7) distributed BIT.
ATE is necessary for both operational testing and
production acceptance testing. Therefore, the design tradeoffs between BIT,
ATE, and manual testing must be done early so that the ATE is selected and
designed concurrently with the prime equipment. The ATE should be tested with
the prime equipment during FSD for compatibility and should be available for
production testing. The Joint Service Automation Testing (AT) Acquisition
Planning Guide1 presents fundamentals of ATE design.
Involving knowledgeable manufacturing engineers familiar with the available and planned test and inspection processes and equipment is critical to designing prime hardware for economic production. This involvement must begin in the very early phase of design and is intimately associated with producibility. Proof Of Manufacturer (POM) models provide the basis for the evaluation of the maturity of manufacturing.
Testability and inspectability are such important aspects of manufacturing that it is absolutely essential to have POM systems/units available to verify production test and inspection equipment and methods. These same POM assets also are available to the manufacturing activity during the production effort and are an invaluable to "proofing" proposed changes in test or inspection before the formal commitment is made. Even if used only for the purpose of ensuring that the appropriate provisions have been made in the design for production test and inspection, POM models are cost-effective and will reduce life cycle costs.
Use of developmental test equipment for production generally perpetuates the desire to monitor performance parameters, continuing the data collection process. Data collection should be reserved for development and for troubleshooting. The production process should not be encumbered with making performance measurements on the product. Properly thought out acceptance criteria expressed in terms amenable to automatic test equipment should be the standard. Acceptance test requirements frequently are warmed-over performance requirement statements, thus carrying unnecessary measurements throughout the production run. Acceptance testing should be a check of the workmanship of the manufacturing process, not a proof of design or a requalification exercise.
A common trap is to assume that ingenuity in the design of manufacturing test and inspection equipment can compensate for deficiencies in the testability and inspectability of prime hardware. In fact, not much can be done to "add on" testability and inspectability if provisions for them were not in the original design. No amount of break out boxes, cables, and extender cards can compensate for poor testability design. The question of quantity is sometimes asked to determine whether the design should accommodate automatic testing and inspection. More frequently it is asked to guide manufacturing personnel in the type of equipment to be selected, rather than to influence prime hardware design.
A design that is fully maintainable is quite likely highly producible. If fault isolation needs are met, if access to signal flow is provided, if modularity of function is provided, if knowledge of proper performance can be determined without introducing external stimuli or monitoring requirements, and if alignments and adjustments are minimized, the maintainability needs regarding testing and inspection will have been met. These are the same objectives to be met in production.
1 Joint Service Automation Testing
(AT) Acquisition Planning Guide,
NAVMAT P9404/DARCOMP 700-19
AFSCP 800-38/ NAVMC 2719,