Life Cycle Model

The Life Cycle Model is one of two methodical concepts that build the basis for Systems Engineering. Next to the Problem-Solving concept, which deals with the challenge of developing solutions for project management challenges, the Life Cycle Model aims to structure the life of an engineering system by a number of subsequent phases. Commonly those include development, realisation, utilisation and disposal. The Life Cycle Model can be understood as an overall framework that defines criteria and expected results for each phase. This allows for the evaluation of technical systems according to their current position within the life cycle. Each phase can be supported by a variety of tools and methods, which are relevant to the project and its content. Deviations on the denominations, quantity and content of those phases do exist and are discussed in the chapter Variations of the Life Cycle Model. This article is primarily based on the work of Rainer Züst and Peter Troxler, who propose the use of the Life Cycle Model in a Systems Engineering context. The article follows the Life Cycle Model phases chronologically, providing a theoretic backbone, which is illustrated by case examples of the fictional company A.

Background

The here introduced Life Cycle Model aims to adapt the holistic view derived from Systems Engineering. The model does not only apply to a specific product, but includes the context of its use and its interaction with other systems. System Engineering as used in this article is understood as:

Systems engineering is an interdisciplinary approach and means to enable the realisation of successful systems. It focuses on defining customer needs and required functionality early in the development cycle, documenting requirements, and then proceeding with design synthesis and system validation while considering the complete problem: operations, cost and schedule, performance, training and support, test, manufacturing, and disposal. SE considers both the business and the technical needs of all customers with the goal of providing a quality product that meets the user needs.

The origin of the Life Cycle Model can not be clearly attributed to a single author. A range of similar models have been presented by the guide Project Management Book of Knowledge and the Project Management Handbook. Life Cycle Models in Systems Engineering context have been described in the INCOSE Systems Engineering Handbook and the publication Systems Engineering Practice.

The Life Cycle Phases

Figure 1: Life cycle phases and decision gates

Most of the literature divides the life cycle of an engineering system in-between four to seven stages. While the content of the model and its phases is depended on the targeted system, there are different definitions of each stage. INCOSE for example splits the development stage into three substages which encompass exploratory research, concept and development phase, whereas Züst et al. suggest to include all three stages in one development phase. Further differences include whether the support/maintenance phase is excluded of the utilisation phase or, as suggested here, exists as an activity cycle parallel to the utilisation phase (Figure 1). A definition out of the ISO 15288 standards states in this context:

6.2.1.3 (a) (5) NOTE The life cycle model comprises one or more stage models, as needed. It is assembled as a sequence of stages that may overlap and/or iterate, as appropriate for the system-of-interest's scope, magnitude, complexity, changing needs and opportunities.

The statement puts the focus correctly on the important fact that no matter which Life Cycle Model is chosen, the user will have to adapt it to the engineering system he/she attempts to structure.

A common feature of any structure in the Life Cycle Model are the decisions gates, which mark the end of one phase and the beginning of another. Primarily the focus is on the quality of the results of a particular phase and the decision wether those results allow to progress forward or if adjustments or even an additional iteration are required. Any project progression without reaching the previous' phases goals may entail increasing risk as the project develops. Decision options include whether or to continue to the next phase and how shortcomings of the previous phase are addressed (Figure 1).

Development Phase

Figure 2: Influence vs knowledge of engineering system

This early phase in the life cycle is crucial for the development of any Engineering System. Here the need for change is established and the decision whether or not to act are taken. Decisions which are taken do need careful consideration as they will typically influence the whole life cycle. The further the development phase progresses, the less influence can be taken on the system (Figure 2). The development phase consists of four steps:

• Proposal for systems design

The proposal for systems design originates at the recognition of a need for change. The change may require an improvement of an Engineering System or its new development. A commonly used tool in this stage is the SWOT analysis in order to pinpoint weaknesses or opportunities. Based on an assessment of the need, the decision whether or not to solve the challenge is taken.

• Preliminary study

The preliminary study aims to provide a broad picture of the challenge. It includes elements like Stakeholder Analysis and Situation analysis. The target of the preliminary study is a clear problem description and a set of objectives which shall be addressed by a potential solution. Furthermore an overview of risk and uncertainties is established, which for example can be addressed by the application of Risk management strategy. When dealing with complex projects the Causal Loop Diagram might support efforts to achieve further structure within the preliminary study. Concept proposals are developed which are concretised during the main study.

• Main study

The aim of the main study is to specify the concepts in detail. The focus shifts from a broad perspective to the engineering system. The concepts are evaluated against the objectives in order to establish their suitability towards the problem solution and investigates whether issues like stakeholder involvement or critical components within the system are known and dealt with sufficiently.

• Detailed study

This phase details the studies of the subsystems and their interrelations. The focus shifts to specific aspects of the solution, which might be critical in relation to e.g. risk, uncertainty or unexplored issues, which previously have been neglected. The output of this phase is accurate information about each subsystem and gives advice towards the implementation of the Engineering System.

Case Example Development Phase

The case company A, experiences unpleasant feedback from their customers on one of their products. The need for change is recognised and the decision to improve the situation is taken (proposal to system design). The company asses the extend of the problem. Internal as well as external factors are included in order to pinpoint the origin of the problems. The key stakeholders are identified and the area of solution defined. The search for solutions through creative methods leads to the development of concepts, which may be evaluated by using tools such as Cost-Benefit Analysis or an evaluation matrix with a specific set of criteria (preliminary study). The preferred concept is specified and analysed in detail to establish the effect that can be expected to influence the system (main study). Finally each of the systems parts are studied in detail in order to ensure that the concept addresses all issues raised in the first step in a high quality.

Realisation Phase

The realisation phase covers the transition in between the development and utilisation phases. It can be divided in two major steps, which are system realisation and system installation. The system realisation includes tasks which transform the concept into a tangible system. Examples are the production of machinery or, in case of IT and service systems, the full documentation of the system. At this point, the system is ready to be implemented. The second phase describes the implementation itself. The system is rolled out, which includes the system's installation and the instruction of the customer/end-user.

Case Example Realisation Phase

Company A has developed a concrete concept to deal with its unsatisfied customer and chooses to redesign their existing product. The new product is developed in accordance to the established objectives, defined during the previous phase. Documentation for the new product needs to be released and customers, who are in possession of the previous product get informed about the possibility to upgrade.

Utilisation Phase

The System is in operation and its performance monitored by a suitable system. Deviations from the expected performance can be grouped into an unintended use of the system by the user or insufficient planning throughout development and realisation phase. Depending on the systems requirements, the use phase is interrupted by activity cycles in order to improve life time and performance of the system.

Case Example Utilisation Phase

Company A has implemented the redesigned product within the system. The monitoring is based on customer feedback. Depending on the significance of the feedback the company can decide whether a third iteration and further improvement of the system is necessary. Smaller changes do not require a whole new Systems Engineering process.

Disposal Phase

The disposal phase describes the decommissioning of a system. The result of this phase is either a complete removal or a radical change of the original system. Ideally the disposal phase is considered during the development phase in order to allow a smooth removal of the system.

Case Example Disposal Phase

Company A continues to receives poor feedback on the redesigned product and decides to upgrade the product one more time. The development of the third generation of the system is interrelated with the disposal of the previous one, as it acts as a successor. Depending on the scale of the upgrade it could either be defined as activity cycle in order to enhance the product or as a complete new life cycle of an engineering system.

Activity Cycles within Individual Life Cycle Phases

Throughout its life cycle any engineering system requires frequent maintenance and updating. Activity cycles are complementary to the four regular phases of the model and target the increase of the life time/product use phase. An example hereof is the servicing or repair of a product: After a certain time in use the product is examined and weak parts exchanged, after which the product re-enters the use phase (e.g. machinery). Another example is the upgrade of a system/product in order to respond to new market developments (e.g. IT software). The activity cycles are iterative and can be repeated as often as required. Usually the activity cycles are organised in separate development and realisation phases. A spare part, for example, does require the same amount of attention like the main system, in oder to ensure that the part will fit in seamlessly. Once the part is integrated in the system, the life cycle of both system and part merge and they usually continue to co-exist throughout the use and disposal phase.

Life Cycle Model Approaches and Reflection

In practice, four different approaches to the Life Cycle Model exist:

• The plan-driven method
• The incremental & Iterative development
• Lean development
• Agile development

The Life Cycle Model in a Systems Engineering perspective is a generic model with broad area of application. However, depending on the use-context it does need refinement by the user in order to achieve the best possible result. As focus areas vary in every project, so does the Life Cycle Model need adjustment of the tools and methods, which are used during the four phases of the life cycle to ensure a relevance to the issues that shall be addressed. As noted above a variety of Life Cycle Models have been developed already and every single one does differ by the targeted application. As Systems Engineering itself encompasses a huge variety of products, technical and other systems, it might be reasonable to clarify, which tools can be utilised during which phase and how they can contribute to the specific challenges.

Variations of the Life Cycle Model

Life Cycle Models are usually featuring similar phases as described above, but as their use-context and perspectives differ, the content of each phase might be subject to change.

• Relevant for Systems Engineering:
  • The Agile Project Management model; structures the life of a project in the five stages of envisioning, speculation, exploration, adaptation and closing.
  • The Vee-model; illustrates Systems Engineering activities with a focus on concept and development phase.
  • The Waterfall model; originally used for hardware development was adapted to the creation of software.
  • The Spiral model; developed for software projects.
  • Bonnal, Gourc & Lacoste (2002) define five project management related Life Cycle Models with differing focus: Strait forward, fractal, risk, quality or control oriented.
• Other applications:
  • Life Cycle Model in the context of sustainability. It creates the basis for Life Cycle Sustainability Assessments and Life Cycle Management. Does include the stage of material extraction and is focussed on environment, business and social impacts of a product system.
  • Product Life Cycle. The life cycle from a business perspective. It may include issues such as innovation diffusion, maturity of product/market or revenue over time.