Agile test planning

Inspection planning is defined in various sets of standards, including VDI Guideline 2619 and DIN 55350, as the planning of quality inspections. It is pointed out that quality inspections have different target objects, such as produced components, but also processes or prototypes, and can be carried out at different points in the product development process. Nevertheless, the understanding has been consolidated in the industry that inspection planning focuses on the planning and definition of product and process inspections in the production phase. Consequently, inspection planning is often equated with the design of quality assurance processes in production. However, this traditional understanding narrows the scope of consideration for efficient quality inspections too much, as will be shown in the following.

Challenges of test planning

Today's manufacturing companies face a variety of challenges that also influence testing activities to ensure product and process quality. For example, a continuing trend is that the complexity of products is constantly increasing. This is particularly true of mechatronic industries. These combine development results from different disciplines in their products. The complex interaction of individual assemblies or modules results in new sources of error that can impair product quality and whose risks must therefore be safeguarded within the scope of quality inspections.

However, test planning for mechatronic products is faced with the challenge that conventional tests in production often cannot safeguard various sources of error, or can do so only at great expense. Quality inspections in earlier phases of product development are a suitable solution to this problem. By planning an early assessment of product and process quality in the form of verification, for example, risks can be safeguarded before the production phase and thus costly tests in series production can be avoided.

Another challenge is the definition of a comprehensive testing strategy. This determines which testing activities are used and coordinated throughout the entire product development process in the company. The multitude of required or possible testing approaches in the life cycle of the product, such as verification during development, production-related quality assurance, requalification, product or process audits, requires an economic coordination of product and process testing, which is recorded in a company-specific testing strategy. Many companies do not yet use this approach for dovetailing testing activities along the product development process, or often use it only inadequately, partly due to the high level of complexity.

Need for action increases

The examples mentioned make it clear that the assurance of product quality by means of appropriate quality inspections and thus inspection planning is becoming increasingly important. At the same time, inspection planning that only focuses on the planning of quality inspections in the production phase provides too few starting points to be able to meet the challenges. For example, systematic planning of quality inspections, especially in the early phases of the product development process, remains largely unconsidered. The shifting of quality inspections to other phases of the product creation process reveals great potential for optimization. For example, cost-intensive tests in production can be avoided by appropriate functional validation of products in the product development phase.

The goal of modern and efficient inspection planning is therefore to make targeted use of various quality inspection options throughout the entire product creation process in order to minimize the overall effort required to ensure product and process quality. Here, various starting points can be identified, particularly in the early phases of product development, with the help of which an early quality inspection can be carried out.

Expanded understanding of inspection planning

In order to be able to uncover the potential of quality inspections in different phases of product creation, it is necessary to expand the understanding and the scope of inspection planning. It makes sense to transfer the scope of quality inspections from production to adjacent phases of the product life cycle. In this way, companies can identify further levers with which efficient quality inspections can be carried out along the entire product development process. This broadening of the scope is shown schematically in Figure 1.

Test planning in software development

The field of software development offers promising approaches and parallels in the expansion of the scope and the resulting broader definition of modern, holistic test planning. The starting point is the general procedure for software development according to the V-model, see Figure 2.

Thus, the product to be developed (in this case the software) is hierarchically structured into suitable structural levels and developed in individual units during the development process, based on the overall requirements. The product to be developed is defined at the individual levels by means of corresponding specifications, and the associated quality-relevant characteristics are determined. Suitable quality inspections can be defined for the individual levels to test and validate these specifications. These have the task of safeguarding the respective level during the integration of the product. In this way, test planning can be performed during development, in which the respective quality inspection can be considered separately from the individual phases in the product creation process. For example, verification tests in the development phase, examination criteria for the prototype phase or installation tests in the series phase can be defined for each level.

The procedure for developing products according to the V-model has already been adapted for the development of mechatronic products. The widely used VDI guideline 2206 presents a development methodology for mechatronic products, which also describes the maturity development of a product or process based on V-cycles. The planning of quality tests is carried out in the course of the level-specific property validation defined in the guideline. Thus, in the procedure, test planning is in principle detached from individual phases of the product development process.

Learning from software development

A methodical approach pursued in the Product Management Department at the Machine Tool Laboratory WZL of RWTH Aachen University provides for the transfer of concepts and procedures from the V-model of software development to the test planning of products from different industries. In particular, the aspect that test planning can be carried out during the development of the software, which allows the planning of quality tests in different phases of the development process, is to be used. Thus, a need for required quality tests can be identified for each level as well as for the integration of the product to higher levels. Subsequently, different possibilities of testing activities to secure the specifications in different phases (development, process planning, production) can be determined and a suitable combination of testing measures can be selected. Consequently, a broad decision-making basis for coordinated quality inspections along the entire product development process is created.

In order to transfer the test planning concepts from the V-Modell to product development in different industries, some prerequisites can be defined. First, a clear and strict hierarchical structuring of the product is required. On the one hand, level-specific quality inspections can be formulated to secure the product specifications for individual modules and components. On the other hand, associated risks can be clearly identified for the integration of a product level during assembly and secured with corresponding integration tests.

Furthermore, working with clear and testable product specifications is an essential prerequisite for the successful transfer of concepts. A characteristic criterion in software development is the requirement that specifications must be designed to be testable. Consequently, the product development process must take into account testability when defining product specifications. This is the only way to identify various levers that can be used to define different types of testing activities.

Companies that develop their products according to the V-Modell and consequently structure their product strictly hierarchically in the development process have the possibility to obtain a clear picture of the specifications to be secured and to define cross-phase measures for quality inspection, which can be coordinated with each other, especially from an economic point of view, in order to reduce the total amount of inspections in the product development process.

The Laboratory for Machine Tools and Production Engineering WZL at RWTH Aachen University is working on industry-related research projects in the field of test planning. If you as a company are interested in a cooperation in this field, we look forward to hearing from you.