2.1. Understanding sustainable product design

The concept of sustainability has become a central topic in the political discourse (Reference HauffHauff, 2021). In addition, companies have made sustainability a key focus in product development, presenting them with ecological, economic, and social challenges that must be balanced. When resources are scarce and consumers are increasingly seeking environmentally friendly solutions, it is of paramount importance that products are designed in a sustainable manner throughout their entire life cycle (Reference Schuh, Kuhn, Keuper, Patzwald, Schenk, Guo, Feucht, Kantelberg, Rossmair, Schroth, Viethen, Zeller, Schmitt, Bergs, Brecher and SchuhSchuh et al., 2023). The product life cycle is a conceptual framework that describes the various phases a product undergoes from its initial business idea to its disposal. The lifespan of products is limited (Reference LenkLenk, 2000) and has been observed to undergo a constant shortening in recent years (Appenzeller, Reference Appenzeller and Appenzeller2021; Braun et al., Reference Braun, Künnemann, Rich, Mißler-Behr, Woll, Woll and Goldmann2022; Huisman et al., Reference Braun, Künnemann, Rich, Mißler-Behr, Woll, Woll and Goldmann2012). There are multiple descriptions of the product life cycle, which vary in terms of their level of detail and their phasing. Nevertheless, the product development process (PDP) represents a pivotal point within the product life cycle representing the initial phase of a product life cycle (Reference Albers, Gausemeier, Anderl, Eigner, Sendler and StarkAlbers & Gausemeier, 2012). It encompasses strategic planning, product development, and production system development. Despite the categorical delineation of these areas of responsibility, the chronological sequence of these phases is not necessarily strictly determined and may well overlap (Reference Albers, Gausemeier, Anderl, Eigner, Sendler and StarkAlbers & Gausemeier, 2012). The product development phase offers significant potential for enhancing sustainability. In this context, it is essential to consider the sustainability-oriented concepts of eco-efficiency and eco-effectiveness. While the latter is initially concerned with the question of whether the appropriate goals and strategies are being pursued, the concept of eco-efficiency focuses on the effective utilization of resources to achieve these goals (Reference Stahlmann and ClausenStahlmann & Clausen, 1999). Accordingly, a product may be defined as eco-efficient if it yields a favourable impact on the three fundamental dimensions of sustainability, namely ecology, economy, and social issues (Reference HauffHauff, 2021). Becker (Reference Becker, Scholz, Pastoors, Becker, Hofmann and van Dun2018) defines an eco-efficient product as one that is economically competitive, improves quality of life by satisfying human needs, and consumes fewer resources than nature can regenerate in the same period of time. There are already various approaches to doing this, e.g., R-strategies. The name is derived from the same initial letter “R” of the various approaches. Examples include reuse, repair, refurbishing and remanufacturing. The objective of these approaches is to extend the lifespan of products and facilitate multiple subsequent life cycles, thereby fostering a circular economy (Reference Acerbi, Sassanelli, Terzi, Taisch, Sassanelli and TerziAcerbi et al., 2021; Reference Reike, Vermeulen, Witjes, Babbitt, Gaustad, Fisher, Liu and ChenReike et al., 2018). In addition, it is also possible to achieve these effects through upgrades. Upgrades facilitate the extension of product life cycles, significantly prolonging their use phase. Consequently, upgradeable products exhibit greater resource efficiency and are more sustainable than conventional alternatives (Reference Schuh, Kuhn, Keuper, Patzwald, Schenk, Guo, Feucht, Kantelberg, Rossmair, Schroth, Viethen, Zeller, Schmitt, Bergs, Brecher and SchuhSchuh et al., 2023).

2.2. Designing upgradeable mechatronic systems - preliminary studies

It thus follows that product development should be linked to upgrades in terms of their potential to increase sustainability. Kuebler et al. (Reference Kuebler, Beck, Glasmacher, Rapp and Albers2023) identified success factors from literature that are relevant to the integration of hardware upgrades into the product life cycle. A two-stage design, comprising a systematic literature review and a preceding interview study, facilitated the illumination of the topic of future-robust product design. Upgrades and the relevance of the design of upgradeable mechatronic systems were identified as particularly salient areas of influence. From the market perspective, the fulfilment of customer needs and requirements was identified as a key influencing success factor for upgrades, particularly in the identification of upgrades. Regarding the design of upgrades, the design of upgrades for changeability was identified as a critical success factor at the system level (Reference Kuebler, Beck, Glasmacher, Rapp and AlbersKuebler et al., 2023). In order to implement this technically, a modular product architecture and standardization of interfaces are required. Standardized interfaces coupled with uncertain future conditions and changing customer requirements, represent a significant challenge. In particular, components that are subject to constant change can be designed variably through a modular architecture (Reference Kuebler, Lange, Glasmacher, Düser and AlbersKuebler et al., 2024).

2.3. Understanding upgrades

In the field of linguistics, an upgrade is defined as “improving the quality or usefulness of a computer or machine” (Cambridge Dictionary, 2024). In the context of mechatronic systems, Albers et al. (Reference Albers, Düser, Kuebler, Schwarz, Lickefett, Pfaff and Thümmel2023), define an upgrade as a modification of a mechatronic system with the objective of enhancing user-friendliness and performance within its use phase, thereby extending its useful life while simultaneously addressing the evolving needs of supplier, user and customer. The implementation of the upgrades can be carried out by expert personnel, the manufacturer, or the user themselves, depending on the specific type of upgrade (Reference Khan and WuestKhan & Wuest, 2019). For this to be achieved, the product must be designed to be upgradeable from the outset. As upgrades interfere with the idea of developing and launching products at distinct times, the model of system generation engineering (SGE) allows to understand the interactions of upgrades between new system generations (see Fig. 1) (Reference Albers, Düser, Kuebler, Schwarz, Lickefett, Pfaff and ThümmelAlbers et al., 2023).

Figure 1.

Interdependencies between different product generations when upgrading (Reference Albers, Düser, Kuebler, Schwarz, Lickefett, Pfaff and ThümmelAlbers et al., 2023)

In the model of SGE, a new generation is developed by varying elements of an associated reference system in accordance with three variation principles (Reference Albers, Rapp, Krause and HeydenAlbers & Rapp, 2022). Referring to upgrades, only individual subsystems are added or modified. Conversely, the reference system of a new upgrade variant is primarily comprised of elements from the previous generation. In addition, upgrades may be implemented at different time horizons. They are integrated throughout the entire use phase, and also continuously planned and designed in the product engineering process. Figure 1 illustrates that upgrade variants pertain to systems whose utilization is extended. In accordance with the definition of variants, product generation G_i=n is modified to create upgrade generations g=1 and subsequently g=2, a process that extends to subsequent product generations. To do so, upgrades adapt a product to uncertain environmental changes and to benefit from new inventions (Reference Albers, Düser, Kuebler, Schwarz, Lickefett, Pfaff and ThümmelAlbers et al., 2023). This adaptability allows for the continuous improvement of the product and its maintenance without the necessity of developing and launching entirely new product generations. Therefore, the products’ resource efficiency increases, as less material and energy are required to manufacture new products. Furthermore, the dissemination of innovations can be accelerated as existing products can be upgraded quickly. As a result, customers and users benefit from a more affordable integration of new technologies, as they do not need to purchase a new product in order to benefit from new technology (Reference Khan and WuestKhan & Wuest, 2019).

4.1. Development of the study goals and preparation of the interviews

Building upon the findings of Kuebler et al. (Reference Kuebler, Beck, Glasmacher, Rapp and Albers2023) (see 2.2) describing the phenomenon of upgrades rather than the embedding of these within a structured design process, these factors were clustered for the development of a design support testbed in a structured development process. As no implication should be made regarding a specific focus of the correlated product development process, the different interview findings need to be comparable. As product development can be viewed as a problem-solving process (Reference AlbersAlbers, 2010), the preliminary work (see 2.2) was clustered according to the SPALTEN problem-solving approach outlined by Albers et al. (Reference Albers, Reiß, Bursac, Breitschuh and Boks2016). SPALTEN, a German acronym, is a structured seven-step approach with two recurring activities: Situation analysis, Problem containment, Alternative solutions, L – Selection of solutions, T – Consequence analysis, E – Make Decision and realization, and N –Recapitulate and Learn. The recurring activities adapt problem-solving team (PST) and Information Check, ensuring team refinement and sufficiency checks after each step. (Reference Albers, Reiß, Bursac, Breitschuh and BoksAlbers et al., 2016) After mapping the factors, the understandability of the categories was enhanced for the interviews by referring to verbs describing the corresponding development activity: Identification, Planning, Design and process communication and control (see Fig. 3).

Figure 3.

Upgrade influences according to Kuebler et al. (Reference Kuebler, Beck, Glasmacher, Rapp and Albers2023) categorized into product development activities embodied as a problem solving process

Based on this framework, a six-item interview guideline was developed to systematically gather relevant insights. The first item was implemented to ensure that the interviewees met the predefined sample criteria. To do so, their experience in the relevant areas of product development and strategic foresight, as well as their current employment, were assessed (I). To capture the initial perceptions of the interviewees without bias, the second item engaged them in a role-playing exercise. They were asked to describe their approach as lead of a development team in extending a products’ life cycles. Building on this, they were then placed in a scenario where they had to guide the team in developing upgrades (II). Subsequently, the four categories outlined in Figure 3 - identification (III), planning (IV), design (V), and process communication and control (VI) - were used as a basis for exploring specific activities, applicable tools, and processes relevant to each category. The questions in the semi-structured interview blocks were based on the associated factors (see Fig. 3). To efficiently utilize the interviewees’ time, two persons were working together to conduct the interviews with roles remaining consistent throughout the whole interview. One person marked already addressed influences or questions to keep track of open questions in the background, while the other remained the responsible interviewer. Therefore, a structured yet open-ended approach to capturing diverse insights on developing upgrades was ensured.

4.2. Characterization of the sample

To identify factors for developing a design support on upgradable mechatronic systems, eight semi-structured interviews were conducted via Microsoft Teams between June 23 and August 7, 2023, averaging 49 minutes each. Following a selective sampling approach, participants were chosen based on predefined criteria to ensure expertise in product development and strategic foresight (see Fig. 4).

Figure 4.

Demographics of interviewees with regard to experience on researched fields

The target group was deliberately narrow, prioritizing relevant knowledge over represent-ativeness. Invitations were sent individually to maintain confidentiality among participants. As a result of this procedure and the subsequent anonymization of the data during transcription, the impartiality of the responses provided by the interviewees was guaranteed, and the objectivity of the verbal data collected was enhanced (Reference Doppler and SteffenDoppler & Steffen, 2019). To prevent any inferences being drawn about the individuals based on the data collected, the names of the participants were anonymized. Only the position and branch in which the interviewee was employed at the time of the interview were provided (see Fig. 4). The eight interview participants can be assigned to four different industries. Although potential participants of both genders were requested, only male participants are represented in the final sample due to a lack of responses. With regards to strategic foresight, the participants have an average of around 8.25 years of experience, whereas the average experience in product development is almost 13 years. Overall, the majority of the participants' expertise can be found in product development. With a share of almost 61% of all years of experience, this area has the most experience.

4.3. Results of the interview study

The expert study was evaluated by using the qualitative content analysis proposed by Kuckartz and Rädiker (Reference Kuckartz and Rädiker2022). In accordance with their process, the second step entails identifying deductive main categories that will serve as the foundation for the initial coding process. This entails utilizing the categories outlined in the guideline (Reference Kuckartz and RädikerKuckartz & Rädiker, 2022). The transcripts of the interviews were subjected to two rounds of review, with a total of 414 text passages being coded in the MAXQDA 2020 software. In the initial coding phase, the text passages were allocated to the designated categories. Subsequently, multi-layered subcategories were formed within each main category, on the basis of which the second coding process was then carried out. In the event that an individual text passage had been previously coded in a manner that was deemed to be unsuitable, it was moved to a more suitable category. The 414 text passages were assigned to 78 categories in total. The four categories that had been derived from the guidelines, in addition to a category for the recording of demographic participant data, constituted the deductive category system. Based on the content of the aforementioned deductive categories, multi-layered subcategories were then formed that were thematically relevant. Figure 5 shows the deductive main categories and the inductively determined 17 first-order subcategories.

Figure 5.

Deductive categories and formed inductive first-order subcategories

Regarding the identification of upgrades, twelve influencing factors were identified. The largest share, comprising six factors, was attributed to the consideration of future areas of influence. Four factors were attributed to the methodical handling of uncertainties. One factor could be derived from the statements regarding the company's knowledge of its own product and its own requirements. In particular, assistance in identifying prospective customer requirements pertinent to future products represents a pivotal influencing factor with regard to upgradeability (I6,8). I1, I2, I6, I7, and I8 explicitly highlight the importance of meeting customer needs. Consequently, an upgrade is only justifiable if the customer perceives a tangible benefit and if the necessity of the upgrade is evident (I1,2,3,6,7,8).

In regard to the planning of upgrades, seven factors were identified. I1 and I5 underscore the significance of roadmapping, particularly the formulation of system roadmaps to clarify interdependencies and create release schedules. Consequently, the extent to which existing tools can be integrated should be included as a factor. Furthermore, the interview study identified three additional criteria for planning. It is necessary to examine the relevance of the upgrade in the planning process. This entails identifying critical failure points associated with non-implementation (I3) and anticipating technological advancements to ensure the product is designed for upgradeability (I6). Furthermore, it is essential to be able to accurately estimate the cost of the modifications to the system that are necessary for an upgrade at an early stage (I2,3). The technical implementation of the system, and the resulting interactions and dependencies, determine the extent of the difficulties associated with implementing an upgrade (I5). In cases where there are numerous interdependencies between two subsystems, the feasibility of upgrading both of them separately is significantly reduced (I1,5).

With respect to the design of upgrades, eight factors were identified. The capacity to abstract and modularize discrete levels or subsystems of a system in a targeted manner (I1,2,6) without introducing new dependencies was identified as a critical factor for the design of the product architecture. Accordingly, the boundaries of subsystems must be explicitly delineated and defined (I1,2,5,6,7). It is essential that interfaces are designed future-robust and compatibility (I2,4,5,6,7,8). Moreover, it is preferable to fulfil the requirements of users through the utilization of existing components and subsystems (I1). This is also beneficial for the commercialization of an upgrade, as it means less principal variation in the remaining subsystem (I3,4). Consequently, as the proportion of new development shrinks, the easier upgrades can be offered to the customer (I2). For this to be achieved, it is essential that the production system is compatible with the upgrade, without necessitating a significant adaptation effort (I3). The feasibility of utilizing existing resources is, therefore, another factor to be considered. Ultimately, eleven factors were identified with respect to process communication and control. In particular, four factors were identified with regard to support through methodology. The tools, methods, and processes must be adapted to align with the requirements of the upgradeable product (I5). Methodological approaches that are currently in use are not employed due to the necessity of modifying them to a significant extent. Additionally, the utilization of unsuitable alternatives is observed due to the unavailability of suitable options. Accordingly, adaptability to the specific needs and circumstances is of paramount importance, as is the assurance of a continuous support process. Consequently, it is imperative that the individual steps be precisely coordinated (I5) and that an efficacious flow of information be ensured between the activities. A consistent approach that explicitly delineates the requisite activity at all times and ensures access to knowledge commensurate with the circumstances is of paramount importance (I1,2).