Pauline Caille
email: caillepau@gmail.com

François Alexandre

Virginie Molinier

Nelly Heraud

The role of personality traits in inpatient pulmonary rehabilitation response in patients with chronic obstructive pulmonary disease

Keywords: Five Factor Model, Personality, Pulmonary rehabilitation, Respiratory Diseases, Response, Therapeutic Outcomes

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Introduction

Pulmonary rehabilitation (PR), the reference treatment for people with chronic respiratory diseases (CRDs), is a comprehensive and multidisciplinary intervention aiming to improve both the physical and psychological health of patients, as well as specific symptoms of the disease [START_REF] Spruit | An official American Thoracic Society/European Respiratory Society statement: key concepts and advances in pulmonary rehabilitation[END_REF]. Despite a large body of research demonstrating that PR generates numerous bio-psychosocial benefits [START_REF] Gordon | Effect of Pulmonary Rehabilitation on Symptoms of Anxiety and Depression in COPD: A Systematic Review and Meta-Analysis[END_REF][START_REF] Lacasse | Meta-analysis of respiratory rehabilitation in chronic obstructive pulmonary disease. A Cochrane systematic review[END_REF][START_REF] Mccarthy | Pulmonary rehabilitation for chronic obstructive pulmonary disease[END_REF], it reported a significant heterogeneity in PR response [START_REF] Garrod | Predictors of success and failure in pulmonary rehabilitation[END_REF][START_REF] Scott | Success in pulmonary rehabilitation in patients with chronic obstructive pulmonary disease[END_REF][START_REF] Spruit | Differential response to pulmonary rehabilitation in COPD: multidimensional profiling[END_REF][START_REF] Troosters | Exercise training in COPD: how to distinguish responders from nonresponders[END_REF]. Overall, these studies highlighted that between one-third to one-half of patients enrolled in a PR program exhibited no clinically significant improvement at the end of the PR program on various clinical outcomes including exercise capacity and quality of life [START_REF] Garrod | Predictors of success and failure in pulmonary rehabilitation[END_REF][START_REF] Troosters | Exercise training in COPD: how to distinguish responders from nonresponders[END_REF]. Given the key role of PR in CRDs management, the identification of factors associated with PR outcomes has been widely investigated in the literature [START_REF] Crisafulli | Role of comorbidities in a cohort of patients with COPD undergoing pulmonary rehabilitation[END_REF][START_REF] Ragaselvi | Predictors of response to pulmonary rehabilitation in stable chronic obstructive pulmonary disease patients: A prospective cohort study[END_REF][START_REF] Vagaggini | Clinical predictors of the efficacy of a pulmonary rehabilitation programme in patients with COPD[END_REF][START_REF] Walsh | Metabolic disease and participant age are independent predictors of response to pulmonary rehabilitation[END_REF]. However, the understanding of this PR response heterogeneity remains incomplete, particularly since most studies were essentially limited to investigating the influence of intrinsic parameters (e.g., forced expiratory volume in 1 second at the beginning of the program). Yet, extrinsic factors, such as therapeutic environment, and above all the way patients interact with it, could also notably influence PR effectiveness. This could be particularly relevant in the specific context of inpatient PR, characterized by i) a rupture with the patient's home environment lasting several weeks, ii) being confronted with new social rules and norms and iii) a new community life with other people suffering from CRD's and a variety of healthcare professionals. All patients, depending on their psychological predispositions, might adapt differently to a PR environment and thus, might not gain the same benefits from the therapeutic care management. For example, introvert patients could have greater difficulties progressing correctly in collective activities whereas, extroverts could benefit more from activities providing opportunities for social interaction. Thus, the heterogeneity observed in PR response could partly be the reflection of this adequation between PR environment and one's tendency to act, think and feel, i.e. personality traits. According to the Five Factor Model [START_REF] Digman | Personality structure: Emergence of the five-factor model[END_REF][START_REF] Mccrae | An introduction to the five-factor model and its applications[END_REF], personality is made of five dimensions present in varying degrees in individuals and influencing their behavior and life experiences [START_REF] Mccrae | Perspectives de la théorie des cinq facteurs (TCF): traits et culture[END_REF]. These five traits include Neuroticism (i.e., tendency to be emotionally unstable and stressed), Extraversion (i.e., tendency to be optimistic and dynamic), Openness to experience (i.e., tendency to look for new experiences), Agreeableness (i.e., tendency to be altruistic) and Conscientiousness (i.e., tendency to be self-disciplined). Previous studies showed that these personality traits were associated with some PR-specific environmental elements. For example, personality was associated with the development of therapeutic alliance [START_REF] Hirsh | The relationship between agreeableness and the development of the working alliance in patients with borderline personality disorder[END_REF], quantity and quality of social interpersonal interactions [START_REF] Nezlek | A cross-cultural study of relationships between daily social interaction and the five-factor model of personality[END_REF], respecting authority [START_REF] Bègue | Personality predicts obedience in a Milgram paradigm[END_REF] and assimilation of cultural social norms [START_REF] Gebauer | Cross-cultural variations in big five relationships with religiosity: a sociocultural motives perspective[END_REF]. Personality traits were also associated with treatment observance and compliance [START_REF] Axelsson | The influence of personality traits on reported adherence to medication in individuals with chronic disease: an epidemiological study in West Sweden[END_REF][START_REF] Axelsson | The function of medication beliefs as mediators between personality traits and adherence behavior in people with asthma[END_REF][START_REF] Hill | The role of adherence in the relationship between conscientiousness and perceived health[END_REF], effectiveness of non-pharmacological interventions [START_REF] Rouch | Personality Modulates the Efficacy of Art Intervention on Chronic Pain in a Population of Patients with Alzheimer's Disease[END_REF], and participation to rehabilitation activities [START_REF] Hilliard | Big Five Personality Characteristics and Adherence to Clinic-Based Rehabilitation Activities after ACL Surgery: A Prospective Analysis[END_REF]. Moreover, the association between personality traits and physical activity has been established [START_REF] Sutin | The Five-Factor Model of Personality and Physical Inactivity: A Meta-Analysis of 16 Samples[END_REF][START_REF] Wilson | Personality and physical activity: A systematic review and meta-analysis[END_REF]. Thus, these studies suggested that personality traits, given their relationships with PR elements, could influence the way patients adapt to a PR environment. In fact, personality could become an explanatory psychological factor for interindividual differences observed in PR response. This is why, we hypothesized that personality traits could be associated with the effectiveness of inpatient-PR. Therefore, the objective of the present study was to investigate associations between the five personality traits and PR response in persons with chronic obstructive pulmonary disease.

Material and methods

Ethics approval

The study protocol was approved by an independent French ethics committee (CPP Ouest IV Nantes, reference number: 2018-A03141-54, 03/04/2019). The present study abided by the principles of the Declaration of Helsinki for human experimentation of 1975, revised in 2000.

Data accessibility

Individual values for all supporting data are available upon request.

Participants and procedure

Patients were recruited during a 5-week inpatient PR program at the "Clinique du Souffle La

Pulmonary rehabilitation

The inpatient PR program was conducted following recommendations of the American Thoracic and European Respiratory societies [START_REF] Spruit | An official American Thoracic Society/European Respiratory Society statement: key concepts and advances in pulmonary rehabilitation[END_REF].

Measures

Personality traits

Personality traits of the Five-Factor model were measured at T1 using the French version of the Big Five Inventory (BFI-Fr) [START_REF] Plaisant | Validation par analyse factorielle du Big Five Inventory français (BFI-Fr). Analyse convergente avec le NEO-PI-R. In[END_REF]. Participants were asked whether they agreed or disagreed with 45 descriptive statements using a 5-point Likert scale, ranging from 1 (strongly disagree) to 5 (strongly agree). Eight items assessed the level of extraversion (e.g., "I see myself as someone who is sociable") and neuroticism (e.g., "I see myself as someone one who is easily anxious") in participants. Nine items measured the level of conscientiousness (e.g., "I see myself as someone who is effective in his work"). Finally, answers on ten items assessed the level of openness to experiences, (e.g., "I see myself as someone creative, full of original ideas"), as well as agreeableness (e.g., "I see myself as someone who is naturally forgiving"). A high score in a specific trait indicates a high level on that trait. In the sample, Cronbach alphas, measuring the internal consistency of the 5 scales, were .77 for neuroticism, .73 for extraversion, .85 for openness, .68 for agreeableness, and .65 for conscientiousness.

Sociodemographic, anthropometric and clinical characteristics

Collected demographics included self-reported age (in years), sex (coded 0 for men and 1 for women) and education (in years). Anthropometric data included height (cm), weight (kg), which led to calculating the body mass index (BMI) (kg.m -²). Deprivation was measured using the EPICES (Evaluation of Deprivation and Inequalities in Health Examination Centers) score [START_REF] Sass | Comparaison du score individuel de précarité des Centres d'examens de santé, EPICES, à la définition socio-administrative de la précarité[END_REF], calculated from weighted scoring of answers to 11 questions pertaining, for example, to All of these variables were assessed at T1.

Exercise capacity

Exercise capacity was assessed at T1 and at T2 with a 6-minute walking test (6MWT) at the start and at the end of PR, respectively. In accordance with good clinical practices and the guidelines of the American Thoracic Society [START_REF] Spruit | An official American Thoracic Society/European Respiratory Society statement: key concepts and advances in pulmonary rehabilitation[END_REF] patients were instructed to walk as far as possible in 6 minutes, in a 30-meter corridor.

Quality of life

Health-related quality of life was measured at T1 and at T2 with the short French self-reported questionnaire VQ11, specific to COPD [START_REF] Ninot | Le VQ11, un questionnaire de qualité de vie spécifique à la BPCO utilisable en clinique [The VQ11, a short health-related quality of life questionnaire for routine practice in COPD patients[END_REF]. This questionnaire includes 11 items related to feelings experienced in dealing with consequences of the disease. Each participant indicated the intensity experienced for each feeling on a scale ranging from 1 (not at all) to 5 (extremely).

This questionnaire enables the calculation of the overall health-related quality of life score and 3 sub-dimensions: functional, psychological and relational scores. The VQ11 score ranges from 11 to 55 and a low global score (< 22) indicates a better health-related quality of life.

Dyspnea

Dyspnea was assessed at T1 and at T2 using the French version of the "Multidimensional dyspnea profile" questionnaire (MDP) [START_REF] Banzett | Multidimensional Dyspnea Profile: an instrument for clinical and laboratory research[END_REF][START_REF] Meek | Reliability and validity of the multidimensional dyspnea profile[END_REF][START_REF] Morélot-Panzini | Real-life assessment of the multidimensional nature of dyspnoea in COPD outpatients[END_REF]. The MDP consists of 11 items evaluated on an analog scale ranging from 0 (not felt) to 10 (strongly felt). This questionnaire evaluates 3 subdomains of dyspnea. The first item (A1) assesses the immediate unpleasantness of the dyspnea. Then, five items (SQ) measure sensations linked to dyspnea, both in terms of quality and intensity. The last five items (A2) assess emotions arising from experiencing dyspnea, in terms of quality and intensity. From these scores, two components of dyspnea can be calculated:

the sensory dimension (MDP-SD) of dyspnea, corresponding to the sum of the 5 items from the SQ, and affective dimension (MDP-AD) of dyspnea calculated by adding A1 and A2. To complete the questionnaire, patients were instructed to focus on the worst episode of dyspnea experienced over the past two weeks.

Effect of pulmonary rehabilitation

Evolution of clinical indicators during PR

The effects of respiratory rehabilitation were studied via the evolution of the 6MWT distance and VQ11, MDP-SD and MDP-AD scores. The evolution of these clinical indicators was evidenced by the delta calculated by subtracting values at T1 from values at T2. In order to promote the readability of the results, all values were expressed in absolute values. Therefore, a positive score denotes an improvement of the parameters while a negative score represents a deterioration.

Categorization of COPD patients according to PR response • Responders vs. Non-Responders

Responders and non-responders were differentiated using the Minimal Clinically Important Difference (MCID) available for each indicator in the literature. These MCIDs and associated reference studies are listed in Table 1.

• Good-Responders vs. Poor-Responders

In order to best reflect the multidisciplinary objectives of the PR, a composite response was defined based on the previous distinctions from the assessment of physical and psychological indicators. Patients were therefore classified into 2 response profiles: the "poor responders" group, consisting of patients who only responded on 0, 1 or 2 parameters according to the respective MCIDs and the "good responders" group, were patients showed a clinically significant improvement on 3 or 4 indicators.

Data analyses

To test whether the five personality traits were associated with the evolution of clinical indicators (i.e., 6MWT, VQ11, MDP-SD, MDP-AD) during PR, multiple linear regression analyses were performed, adjusted for relevant demographic and clinical information: age (in years), sex (coded as 1 for men and 0 for women), educational attainment (in years), %FEV1, MoCA score, EPICES score and value at baseline. For these analyses, we used the lm() function of "stats" package in R studio software.

Furthermore, to test associations between the five personality traits and response on each clinical indicator, logistic regressions analyses were carried out, adjusted for age, sex, educational attainment, %FEV1, MoCA score, EPICES score and score at baseline. For each indicator, the responder group was recoded as 1 while the non-responder group was recoded as 0. For these analyses, we used the glm function of "stats" package in R studio software.

Moreover, independent sample t-tests and chi-squared analyses were performed to examine baseline differences between responders and non-responders on composite PR response regarding socio-demographics, anthropometrics, pulmonary function characteristics and clinical indicators. Additional logistic regression analyses were used to test relationships between PR composite response and the five personality traits, adjusted for relevant demographics and clinical elements: age, sex, education, %FEV1, MoCA score, and the EPICES score. For these analyses the "good response" group was recoded as 1 while the "poor response" group was recoded as 0. Taking into account the high number of statistical tests carried out, we chose to lower the significance level at p ≤.01, in order to limit the risk of type I error. Patients who did not complete the PR program or patients with incomplete data were excluded from the analyses. A flowchart is presented in figure 1. For all outcomes, personality scores were standardized and analyses were conducted separately for each trait and then with

Indicators

MCIDs References 6MWT + 30 meters [START_REF] Puhan | The minimal important difference of exercise tests in severe COPD[END_REF][START_REF] Singh | An official systematic review of the European Respiratory Society/American Thoracic Society: measurement properties of field walking tests in chronic respiratory disease[END_REF] VQ11 -2 points [START_REF] Coquart | Convergent Validity and Minimal Clinically Important Difference of the Maugeri Foundation Respiratory Failure Questionnaire (MRF-28) and the Chronic Obstructive Pulmonary Disease-Specific Health-Related Quality of Life questionnaire (VQ11)[END_REF] MDP-SD -3.02 points [START_REF] Ekström | Minimal Clinically Important Differences and Feasibility of Dyspnea-12 and the Multidimensional Dyspnea Profile in Cardiorespiratory Disease[END_REF] MDP all traits simultaneously within the same model. All statistical tests were carried out with R and R studio software. The R studio script is available upon request.

Figure 1. Flow Chart

Results

Patient characteristics

Among the 121 patients invited to participate in the present study, 102 met the inclusion criteria and 90 agreed to participate. Five patients did not complete the rehabilitation program and 11 patients did not complete all the evaluations. As such, the final sample included 74 persons with COPD (Figure 1). Baseline characteristics of participants are reported in

Evolution of clinical indicators during PR and associations with personality traits

The evolution of each clinical indicators during PR is presented in table 3. The results of multiple linear regression analyses performed to test the association between personality traits and the evolution of clinical indicators during PR are presented in Table 4. These analyses did not highlight any significant association between personality traits and the evolution of exercise capacity, quality of life, sensory and affective dyspnea dimensions. Furthermore, no significant association was found when all traits were simultaneously entered in the same model (Table S1).

Responders vs. Non-Responders and associations with personality traits

According to MCIDs of exercise capacity during the program, 42 (57%) patients were categorized as responders, and 32 (43%) as non-responders. For health-related quality of life, 59 (80%) COPD patients were classified as responders. For dyspnea, 50 (68%) and 58 (78%) patients were classified as responders on MDP-SD and MDP-AD respectively.

Logistic regression analyses revealed a significant association between openness and VQ11 response. Specifically, this result suggested that a high level of openness [OR = 0.36; 95% confidence interval (CI) = 0.15 -0.74] was associated with a lesser likelihood of responding on the VQ11 (Table 5). This relation did not remain significant when all traits were simultaneously entered into the adjusted logistic regression model (Table 6). No association between personality traits and 6MWT, MDP-SD and MDP-AD responses were found. Note. Adjusted for age, sex, %FEV1, MoCA score, EPICES score, Education and T1 score ns = not significant according to the significance level defined at p ≤.01 Δ : delta between T2 and T1 score No significant association was found when all traits were simultaneously entered in the same model.

Variables

Response to 6MWT

Response to VQ11

Response to MDP-SD

Response to MDP-AD

Good Responders vs. Poor Responders and associations with personality traits

Based on the composite response criteria defined in this study, 47 (62%) patients were classified as good responders and 27 (38%) as poor responders. Descriptive statistics for each group are presented in Table 7.

The independent sample t-tests and chi-squared performed to compare baseline characteristics between each group did not reveal any significant difference on sociodemographic, anthropometric, pulmonary function, and exercise capacity variables (Table 7).

Regarding psycho-cognitive variables, good responders had significantly higher scores on the MDP-AD [Mgoodresponders = 33 [21.50 -46.50] versus Mpoorresponders = 22 [14.50 -28.50]; W (72) = 405.5, p = <.01], indicating a poorer affective dimension of dyspnea at baseline (Table 7). Note. Coefficients are standardized regression coefficients. 95% confidence intervals are in parentheses. Adjusted for age, sex%,FEV1, MoCA score, EPICES score, Education and T1 score ns = not significant according to the significance level defined at p ≤.01 Logistic regressions did not unveil a significant association between personality traits and PR composite response (Table 8). No significant association was found when all traits were simultaneously entered in the same model (Table S2).

Discussion

The objective of the present study was to assess, for the first time, relationships between personality traits, as defined by the Five-Factor Model, and effectiveness of inpatient-PR on exercise tolerance, quality of life and dyspnea in persons with COPD. Consistent with our hypothesis, this study shows that personality traits are likely to modulate the effectiveness of the therapeutic intervention. In particular, a high level of openness to experience was associated with a lesser likelihood of responding to PR on quality of life. No association was found between the other traits and PR outcomes. In addition, none of the traits were associated with PR composite response. Thus, our study suggests that openness to experiences could be a risk factor for nonresponse to PR on quality of life. This result was unexpected because most studies that identified links between openness and therapeutic intervention effectiveness usually reported that this trait was a protective factor [START_REF] Bagby | Personality and differential treatment response in major depression: a randomized controlled trial comparing cognitive-behavioural therapy and pharmacotherapy[END_REF][START_REF] Johnson | Openness Personality Trait Associated With Benefit From a Nonpharmacological Breathlessness Intervention in People With Intrathoracic Cancer: An Exploratory Analysis[END_REF][START_REF] Marr | Investigating associations between personality and the efficacy of interventions for cognitive ageing: A systematic review[END_REF]. For example, a high level of openness increases the benefits from a nonpharmacological breathlessness intervention in persons with intrathoracic cancer [START_REF] Johnson | Openness Personality Trait Associated With Benefit From a Nonpharmacological Breathlessness Intervention in People With Intrathoracic Cancer: An Exploratory Analysis[END_REF]. In addition, it was positively associated with intervention effectiveness for cognitive impairment in older adults [START_REF] Marr | Investigating associations between personality and the efficacy of interventions for cognitive ageing: A systematic review[END_REF] and with decreased depression severity upon treatment completion [START_REF] Bagby | Personality and differential treatment response in major depression: a randomized controlled trial comparing cognitive-behavioural therapy and pharmacotherapy[END_REF]. Regarding quality of life, the few studies that investigated the relationship with openness to experiences following a clinical intervention reported inconsistent results. For example, while a high level of openness was significantly Note. Coefficients are standardized regression coefficients. 95% confidence intervals are in parentheses. Adjusted for age, sex, %FEV1, MoCA score, EPICES score and Education ns = not significant according to the significance level defined at p ≤.01 No significant association was found when all traits were simultaneously entered in the same model. associated with a poorer quality of life 6 months after breast reconstruction [START_REF] Juhl | Personality Traits as Predictors of Quality of Life and Body Image after Breast Reconstruction. Plastic and reconstructive surgery[END_REF], no association was evidenced in women with Breast Cancer 1 and 2 mutations, after risk-reducing salpingooophorectomy [START_REF] Stanisz | Can personality traits affect the quality of life of women with the BRCA1/BRCA2 mutations before and after prophylactic adnexectomy?[END_REF]. This lack of consensus might be explained by the fact that these associations could be related to contextual factors. Indeed, our results can partly by interpreted in light of the very specific context of inpatient-PR, which could interfere with the psychological needs of these individuals. For example, the highly supervised environment of inpatient PR imposes some restrictions (i.e., monitored outings and visits, strict daily schedule...), which do not align with the personal values of independence in thoughts and actions of individuals with a high level of openness [START_REF] Parks-Leduc | Personality traits and personal values: a meta-analysis. Personality and social psychology review : an official[END_REF]. Moreover, repeating the same activities during a 5-week period could create a routine, lacking stimulation for those individuals constantly looking for new and unusual experiences [START_REF] Mccrae | An introduction to the five-factor model and its applications[END_REF]. Therefore, the poor match between the openness trait and inpatient-PR environment specificities could interfere with the effectiveness of the therapeutic intervention. To test the validity of this hypothesis, it could be interesting to identify whether these associations between openness and PR response would be similar in the context of more independent therapeutic modalities such as outpatient or homebased program.

The lack of associations with the other traits of the Five-Factor Model in the present research is surprising since previous studies, investigating the role of personality in chronic disease [START_REF] Rouch | Personality Modulates the Efficacy of Art Intervention on Chronic Pain in a Population of Patients with Alzheimer's Disease[END_REF][START_REF] Franks | Five factor model personality factors moderated the effects of an intervention to enhance chronic disease management selfefficacy[END_REF] or mental health [START_REF] Bartolomei | Personality Factors and Attachment Styles as Predictors of the Therapeutic Efficacy of a Short-Term Ambulatory Intervention for Depression[END_REF][START_REF] Bucher | A meta-analytic review of personality traits and their associations with mental health treatment outcomes[END_REF][START_REF] Hengartner | Personality Traits and Psychopathology Over the Course of Six Months of Outpatient Psychotherapy: A Prospective Observational Study[END_REF] clinical outcomes, showed that neuroticism [START_REF] Rouch | Personality Modulates the Efficacy of Art Intervention on Chronic Pain in a Population of Patients with Alzheimer's Disease[END_REF][START_REF] Franks | Five factor model personality factors moderated the effects of an intervention to enhance chronic disease management selfefficacy[END_REF][START_REF] Bartolomei | Personality Factors and Attachment Styles as Predictors of the Therapeutic Efficacy of a Short-Term Ambulatory Intervention for Depression[END_REF][START_REF] Bucher | A meta-analytic review of personality traits and their associations with mental health treatment outcomes[END_REF][START_REF] Hengartner | Personality Traits and Psychopathology Over the Course of Six Months of Outpatient Psychotherapy: A Prospective Observational Study[END_REF], conscientiousness [START_REF] Franks | Five factor model personality factors moderated the effects of an intervention to enhance chronic disease management selfefficacy[END_REF][START_REF] Bucher | A meta-analytic review of personality traits and their associations with mental health treatment outcomes[END_REF][START_REF] Hengartner | Personality Traits and Psychopathology Over the Course of Six Months of Outpatient Psychotherapy: A Prospective Observational Study[END_REF], extraversion and agreeableness [START_REF] Franks | Five factor model personality factors moderated the effects of an intervention to enhance chronic disease management selfefficacy[END_REF][START_REF] Bucher | A meta-analytic review of personality traits and their associations with mental health treatment outcomes[END_REF] were likely to modulate the effect of therapeutic management. To date, few studies have investigated the mediating mechanisms underlying which traits could influence response to treatment [START_REF] Kushner | Therapeutic alliance mediates the association between personality and treatment outcome in patients with major depressive disorder[END_REF].

However, consistent associations between traits and behaviors that could influence a therapeutic management, such as adherence to treatment protocol [START_REF] Axelsson | The influence of personality traits on reported adherence to medication in individuals with chronic disease: an epidemiological study in West Sweden[END_REF][START_REF] Axelsson | The function of medication beliefs as mediators between personality traits and adherence behavior in people with asthma[END_REF][START_REF] Christensen | Personality and patient adherence: correlates of the five-factor model in renal dialysis[END_REF][START_REF] Molloy | Conscientiousness and medication adherence: a meta-analysis[END_REF][START_REF] Wheeler | Personality traits as predictors of adherence in adolescents with type I diabetes[END_REF] were identified. Nevertheless, the context of inpatient-PR, given these constraining characteristics (i.e., supervision by a multitude of healthcare professionals, care setting rules) offers few opportunities for patients to express their own individual behavioral patterns. Inpatient-PR could thus constitute a poor environment for the expression of these personality traits. In order to test this hypothesis, it might be interesting to identify associations between personality traits and more specific behaviors such as PR program dropout rates, level of engagement in therapeutic management or participation in sessions. Moreover, future studies should investigate the associations between personality traits and PR response in management settings where individual patient behaviors (e.g., therapeutic adherence) are central to the success of the program such as outpatient-PR or telerehabilitation.

In addition to the scientific relevance of improved understanding of psychological factors that may influence PR effectiveness [START_REF] Chevance | Implicit attitudes and the improvement of exercise capacity during pulmonary rehabilitation[END_REF][START_REF] Selzler | The importance of exercise self-efficacy for clinical outcomes in pulmonary rehabilitation[END_REF][START_REF] Zoeckler | Illness perceptions predict exercise capacity and psychological well-being after pulmonary rehabilitation in COPD patients[END_REF][START_REF] Arnold | Changes in personal control as a predictor of quality of life after pulmonary rehabilitation[END_REF], the results could also have practical clinical implications. Indeed, the association identified in this study represents the first basis for improving the response on the quality-of-life dimension, which is one of the most important domains given its association with mortality in persons with COPD [START_REF] Halpin | Identifying COPD patients at increased risk of mortality: predictive value of clinical study baseline data[END_REF][START_REF] Havlucu | Does one year change in quality of life predict the mortality in patients with chronic obstructive pulmonary disease?-Prospective cohort study[END_REF]. Assessing the openness to experience trait could help clinicians better target individuals less likely to benefit from the PR intervention on the quality-of-life and thus reinforce prevention for these patients.

With this objective in mind, additional research to determine cut-off scores for this trait, to detect the most vulnerable patients, would be highly relevant. Moreover, the differential effectiveness of a PR program according to one's personality emphasizes the importance to tailor non-pharmacological therapy to individual characteristics. In this perspective, future studies should help define which are the most suitable PR modalities and contents according to one's dispositions, including personality traits.

Strengths and methodological considerations

The present study has several strengths that include the investigation of PR effectiveness through both physiological and psychological indicators. Moreover, we analyzed PR effects using two complementary methodological approaches (evolution versus response). Finally, this study is one of the few to propose a composite indicator of PR response, which is more representative of the multiple objectives of this intervention. Some methodological considerations deserve some special attention. First, for the sake of statistical rigor, we chose to lower our significance threshold to 0.01. Nevertheless, results that did not express themselves according to the threshold we imposed may deserve more attention. Replication studies would be relevant to identify which results would be consistent. Moreover, the validity of MCIDs [START_REF] Ekström | Minimal Clinically Important Differences and Feasibility of Dyspnea-12 and the Multidimensional Dyspnea Profile in Cardiorespiratory Disease[END_REF] used to categorize responders and non-responders on dyspnea dimensions are questionable since they were not defined after a clinical intervention. However, they were the only MCIDs available and published to date. Finally, due to the lack of composite indicators in the literature, the methodology used to categorize good and poor responders, was defined in an arbitrary manner. Despite this limit, the resulting proportion of responders (62%) and non-responders (38%) is consistent with previous studies that reported a response rate of over 50% [START_REF] Garrod | Predictors of success and failure in pulmonary rehabilitation[END_REF][START_REF] Scott | Success in pulmonary rehabilitation in patients with chronic obstructive pulmonary disease[END_REF][START_REF] Troosters | Exercise training in COPD: how to distinguish responders from nonresponders[END_REF][START_REF] Ragaselvi | Predictors of response to pulmonary rehabilitation in stable chronic obstructive pulmonary disease patients: A prospective cohort study[END_REF]. Furthermore, findings pointing to good responders having lower affective dimension scores compared with poor responders is in line with previous studies that found a poorer health status at baseline in good responders, including dyspnea [START_REF] Spruit | Differential response to pulmonary rehabilitation in COPD: multidimensional profiling[END_REF][START_REF] Costi | Baseline Exercise Tolerance and Perceived Dyspnea to Identify the Ideal Candidate to Pulmonary Rehabilitation: A Risk Chart in COPD Patients[END_REF].

Conclusion

To sum up, identifying the determinants of PR effectiveness represents a major human, clinical and economic challenge. The present study shows that investigating personality traits as defined by the Five-Factor Model constitutes an interesting perspective to understand the heterogeneity observed in PR response. Specifically, our results indicate that high openness is associated with a lower likelihood of responding to PR on health-related quality of life. Thus, this study highlights the existence of new psychological factors that could be taken into account to design new strategies for individualized care pathways. However, additional research is needed to implement these findings in clinical practice.

  Vallonie" (Lodève, France), between April 2019 and January 2020. Patients were eligible for the study if they: 1) had a validated diagnosis of chronic obstructive pulmonary disease (COPD), 2) were older than 18 years old and 3) had healthcare coverage. Patients were not included in the study if they were unable to respond to paper-based questionnaires and/or if they had a mental disorder that could affect their judgment. Eligible participants were invited to participate in this study during an inclusion visit (T0) that took place during the first two days of the program. No intervention or incentive was provided to encourage study participation. In accordance with the regulations governing research displayed in 3 ° of article L.1121-1 of the French public health code concerning non interventional studies based only on questionnaires or interviews, a declaration of non-opposition to participating in the study was collected from all participants before inclusion. Patients were recruited on a voluntary basis, after medical advice and checking all criteria mentioned above. Patients enrolled in the study were assessed twice, during the first week (T1) and at the end of the last week of the PR program (T2). The mean time interval between T1 and T2 was 25 days (Standard deviation = 5 days).

  health insurance, home ownership, financial situation. The final EPICES score ranged from 0 to 100. Clinical characteristics included pulmonary and cognitive function assessments. Regarding pulmonary function assessment, forced expiratory volume at the first second of maximum expiration (FEV1), its predicted value (FEV1 % pred) and Tiffeneau index (FEV1 / FVC) were measured with a plethysmograph (V6200 Autobox, Sensormedics Corp., Yorba Linda, CA, USA). Cognitive impairment was assessed with the Montreal Cognitive Assessment (MoCA) test [29]. A score above or equal to 26 on a 0-30-point scale was considered normal.

  AUTHOR CONTRIBUTIONS P.C, N.H, V.M and F.A conceived and designed research. P.C performed experiments and analyzed data. P.C, N.H, V.M and F.A interpreted results of experiments. P.C prepared tables and drafted manuscript. N.H, F.A and V.M edited and revised manuscript. P.C, N.H, V.M and F.A approved final version of manuscript.

  The program was carried out over 5 weeks.

	Patients received a complete clinical evaluation (clinical assessment, respiratory function
	measures at rest by plethysmography, blood gas test, and body composition) and benefited from
	30 to 40 sessions of exercise training including endurance training (average of 7h/wk.) and
	resistance training both for upper and lower limbs (average of 2h/wk.). Exercises focused on
	balance, coordination, flexibility and posture. Furthermore, collective relaxation sessions (on
	average 4 hours / week) were also offered. In addition, participants had access to group
	therapeutic education sessions (e.g., pain and dyspnea self-management, smoking cessation,
	behavioral change, physical activity planning…) and nutrition, psychological and nursing
	support in individual sessions (average of 3 hours / wk.). Average PR duration in our sample
	was 35 days (SD=4 days).

Table 1 .

 1 Minimal Clinically Important Differences defined for 6MWT, VQ11, MDP-SD and MDP-AD.

Table 2 .

 2 Values are represented by the mean ± standard deviation in case of normal distribution (test with Q-Qplot, Skewness, Kurtosis), otherwise by the median [lower quartile (LQ)-upper quartile (UQ)].

Table 2 .

 2 Descriptive characteristics of the sample

	Variables	Mean / Median	Standard Deviation / [LQ-UQ]
	Demographic and anthropometric
	Sex n women (%)	29 (39%)	-
	Age (years)	68.39	8.64
	BMI (kg.m -²)	24.10	[20.61 -28.30]
	Education (years)	12.32	3.59
	EPICES	22.77	[13.61 -35.51]
	Pulmonary Function
	FEV1 (L)	0.95	[0.80 -1.18]
	FEV1 (% pred)	38.30	[31.00 -49.75]
	FEV1/FVC (%)	45.36	[37.93 -50.81]
	GOLD stage I n (%)	6 (8%)	-
	GOLD stage II n (%)	14 (19%)	-
	GOLD stage III n (%)	37 (50%)	-
	GOLD stage IV n (%)	17 (23%)	-
	Psycho-cognitive variables
	MOCA score	25	[23.00 -27.00]
		Score 26 < n = 50 (68%)
	VQ11	34.30	7.92
	MDP-SD	30.59	9.46
	MDP-AD	28.50	[19 -43.75]
	Exercise capacity
	6MWT (in meters)	370.39	85.78
	Personality traits
	Neuroticism	2.58	0.75
	Extraversion	3.12	0.68
	Openness	3.45	0.76
	Agreeableness	4.14	0.46
	Conscientiousness	3.84	0.52
	Note. Total N = 74, BMI: body mass index, FEV1: forced expiratory volume in one second; FVC: forced
	vital capacity; GOLD: global initiative for chronic obstructive lung disease

Table 3 .

 3 Evolution of clinical indicators during PR

Table 4 .

 4 Summary of multiple regression analyses predicting the evolution of clinical indicators during PR from personality traits in baseline

		T1	T2			Δ
	Variables	(Start of PR)	(End of PR)
			Mean ± Standard Deviation
			Median [LQ;UQ]
	6MWT	371.43 ± 85.02	409.99 ± 90.75	36.50 [10.50; 61.50]
	VQ11	34 [29.25;39.00]	29 [22.00; 34.00]		6 [2.00; 9.75]
	MDP-SD	31 [24.00; 38.00]	22 [13.25; 32.00]		8 [1.25; 17]
	MDP-AD	28.50 [19.00; 30.45]	17 [11.00; 25.00]		8 [-1.00; 23.50]
	Variables	Δ 6MWT	Δ VQ11	Δ MDP-SD	Δ MDP-AD
			β
			p-values
	Neuroticism	0.08ns	-0.01ns	-0.12ns		-0.12ns
		p=0.50	p=0.93	p=0.24		p=0.26
	Extraversion	-0.08ns	-0.06ns	0.23ns		0.11ns
		p=0.51	p=0.56	p=0.02		p=0.24
	Openness	-0.01ns	-0.16ns	0.23ns		0.16ns
		p=0.92	p=0.12	p=0.03		p=0.07
	Agreeableness	-0.15ns	-0.01ns	0.07ns		0.14ns
		p=0.24	p=0.93	p=0.52		p=0.15

Note. LQ: lower quartile, UQ: upper quartile Δ : relative difference between T2 and T1 score

Table 5 .

 5 Summary of logistic regression analyses predicting clinical indicators response from personality traits in baseline

	Conscientiousness	-0.04ns	0.06ns	0.00ns	-0.05ns
		p=0.74	p=0.54	p=0.96	p=0.55

Table 6 .

 6 Summary of logistic regression analyses predicting clinical indicators response from the five personality traits included in the same regression model

	Odd Ratios
	95% CI
	p-values

  Note. Coefficients are standardized regression coefficients. 95% confidence intervals are in parentheses. Adjusted for age, sex%,FEV1, MoCA score, EPICES score, Education and T1 score * p < .01, ns = not significant according to the significance level defined at p ≤.01

	Variables	Response to	Response to	Response to	Response to
		6MWT	VQ11	MDP-SD	MDP-AD
			Odd Ratios
			95% CI
			p-values
	Neuroticism	0.83ns	0.49ns	1.32ns	1.38ns
		(0.43-1.57)	(0.17-1.31)	(0.58-3.27)	(0.49-4.50)
		p=0.55	p=0.17	p=0.51	p=0.56
	Extraversion	0.49ns	0.91ns	1.75ns	1.48ns
		(0.24-0.91)	(0.39-2.11)	(0.80-4.16)	(0.55-4.61)
		p=0.03	p=0.82	p=0.18	p=0.46
	Openness	1.44ns	0.26ns	1.53ns	2.19ns
		(0.78-2.77)	(0.07-0.68)	(0.71-3.58)	(0.93-6.14)
		p=0.25	p=0.02	p=0.29	p=0.10
	Agreeableness	0.81ns	1.30ns	1.76ns	2.41ns
		(0.41-1.55)	(0.54-3.20)	(0.8-4.23)	(0.92-8.14)
		p=0.53	p=0.55	p=0.17	p=0.10
	Conscientiousness	0.75ns	0.60ns	1.35ns	1.19ns
		(0.42-1.30)	(0.28-1.22)	(0.67-2.85)	(0.53-2.75)
		p=0.31	p=0.16	p=0.41	p=0.67

Table 7 .

 7 Patient characteristics according to Poor or Good Responder Profile

Table 8 .

 8 Summary of logistic regression analyses predicting PR composite response from personality traits at baseline

	Variables	Poor responders n = 27	Good responders n = 47	p-values
	Demographic and anthropometric variables
	Sex n(% women)	10 (37%)	19 (40%)	.77ns
	Age (years)	69.11 ± 10.85	67.98 ± 7.18	.63ns
	BMI (kg.m -²)	24.31 [19.35-27.15]	23.89 [21.33-28.45]	.68ns
	Education (in years)	12.09 ± 3.90	12.45 ± 3.43	.70ns
	EPICES score	22.48 [13.61-29.97]	23.07 [13.61-39.64]	.48ns
		Pulmonary Function variables
	FEV1 (L/min)	0.95 [0.74-1.14]	0.95 [0.82 -1.92]	.78ns
	FEV1 (% pred)	37 [31-48.50]	40 [30 -50.02]	.78ns
	FEV1/FVC (%)	45.65 [35.36-51.75]	45.26 [39.51-50.31]	.84ns
	GOLD stage I n (%)	1 (2%)	3 (6%)	.42ns
	GOLD stage II n (%)	5 (26%)	19 (21%)	-
	GOLD stage III n (%)	16 (46%)	59 (45%)	-
	GOLD stage IV n (%)	5 (26%)	19 (28%)	-
		Psycho-cognitive variables
	MOCA score	25.19 ± 2.80	24.85 ± 3.02	.63ns
	VQ11	32.22 ± 7.95	35.49 ± 7.74	.09ns
	MDP-SD	28[23.50 -32.50]	34[26.50-39]	.03ns
	MDP-AD	22[14.50 -28.50]	33[21.50-46.50]	.01*
		Exercise capacity
	6MWT (in meters)	381.07 ± 71.24	365.89 ± 92.29	.43ns
	Note

. BMI: body

  mass index, FEV1: forced expiratory volume in one second; FVC: forced vital capacity; TLC: total lung capacity; GOLD: global initiative for chronic obstructive lung disease * p < .01, ns = not significant according to the significance level defined at p ≤.01

	Variables	Response to PR
		(Poor Responders = 0 and Good Responders = 1)
		Odds Ratios
		95% CI
		p-values
	Neuroticism	1.15ns
		(0.70-1.94)
		p=0.57

ACKNOWLEDGEMENTS

The authors would like to thank the experts of the Korian research committee for their precious advice. Moreover, the authors kindly thank all the volunteer participants who made this study possible. Finally, we also gratefully acknowledge the different clinical teams of the Clinique du Souffle La Vallonie for facilitating the implementation and the organization of this study within the pulmonary rehabilitation center. The authors also thank Bénédicte Clément (medical/science translator; EU agreement) for English editing.

FUNDING

The project was financially supported by the French Agency for Research and Technology [ANRT, CIFRE 2018/0198]. The sponsor had no influence on the conduct of this study, the analysis of the data, or the writing of this report.

DISCLOSURE

The authors declare no conflicts of interest financial or otherwise. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.


Catherine Da Cunha
email: catherine.da-cunha@ls2n.fr

Olivier Cardin
email: olivier.cardin@ls2n.fr

Guillaume Gallot
email: guillaume.gallot@capacites.fr-julien.viaud@capacites.fr

Julien Viaud

Designing the Digital Twins of Reconfigurable Manufacturing Systems: application on a smart factory

Keywords: Reconfigurable Manufacturing Systems, Modularity, Digital Twin, Manufacturing systems. 

The emergence of Digital Twins in manufacturing enables in-depth integration of the physical and informational worlds. Meanwhile, the concept of Reconfigurable Manufacturing Systems exhibits new opportunities and new constraints in the way the manufacturing systems are designed and managed. Among others, the notion of modularity is a prerequisite for both physical components and their informational counterparts. To provide the benefits of Digital Twin to Reconfigurable Manufacturing Systems, this paper introduces a modular design of the Digital Twin counterpart of the components. A prototype system is developed to demonstrate the modular design's benefits, with an application on a reconfigurable learning factory.

INTRODUCTION

The evolution of today's manufacturing systems is characterized by the parallel development of several key concepts meant to deeply transform the way the systems perform.

Among these concepts, Reconfigurable Manufacturing Systems (RMS) and Digital Twins (DT) are expected to be complementary and magnify their respective impact.

RMS are manufacturing systems whose physical and logical structures at all levels of the system's composition can be changed quickly and inexpensively to adjust production capacities and functionalities around a family of products responding to to sudden changes in the market [START_REF] Capawa Fotsoh | Towards a Reference Model for Configuration of Reconfigurable Manufacturing System (RMS)[END_REF]. The composition of RMS is generally based on physical modules and standard logics (cells, machines, material handling systems, storage equipment, controllers, tools, fasteners, etc.) that can be added, modified, rearranged, swapped or replaced quickly and reliably as needed to meet the changing requirements of the market requirements. The location of these modules can also be modified, at the workshop level, to achieve the configuration that best meets the production context requirements [START_REF] Capawa Fotsoh | Towards a Reference Model for Configuration of Reconfigurable Manufacturing System (RMS)[END_REF]. A configuration is defined as a state of the system that responds to a particular context [START_REF] Lameche | Modularity in the design of reconfigurable manufacturing systems[END_REF]. To define the configuration of the system, a set of system parameters must be defined that determine this configuration. The number of machines, of assembly lines, of loading stations, of AGVs, of operators, the positioning of these objects in the workshop, the rules for managing the AGV fleet, are some examples of configuration parameters.

The Digital Twin (DT) was first used in aerospace and aeronautic by NASA as a diagnostic and prognostic tool to plan the maintenance and increase the safety of an aircraft throughout its lifecycle. It used sensors data and near-realtime information to feed different simulators with the needed inputs to correctly mirror the behavior of the aircraft [START_REF] Shafto | DRAFT Modeling, Simulation, Information Technology & Processing Roadmap[END_REF][START_REF] Wang | The Use of High-Performance Fatigue Mechanics and the Extended Kalman/Particle Filters, for Diagnostics and Prognostics of Aircraft Structures[END_REF]. Since then, its usage has been largely developed, and the expectations in manufacturing are now important. The most convincing approach deals with predictive maintenance [START_REF] Vathoopan | Modular Fault Ascription and Corrective Maintenance Using a Digital Twin[END_REF][START_REF] Werner | Approach for a Holistic Predictive Maintenance Strategy by Incorporating a Digital Twin[END_REF], which generally focuses on a limited perimeter (often a machine or a group of machines). One of the main challenges is to apply this concept to large manufacturing systems [START_REF] Kong | Data Construction Method for the Applications of Workshop Digital Twin System[END_REF]). If SCADA systems, basic component of a DT, are already well established in industry, the other part, including the models, is not yet well established. Due to a lack of genericity in the approach, the setting up of a DT is time-consuming, and it has to be redone each time a modification in the manufacturing system occurs. When the enterprise strategy is based on rapid adaptations to market changes, methods have to be found to reduce DT's set-up time.

The adaptation between DT and RMS is therefore an important issue, and is facing a scientific deadlock that encompasses all the phases of the lifecycle of the DT, from its design to its usage in execution. This article focuses on the design phase of the DT, and presents a modular framework for DT design, that enables quick DT reconfiguration in execution.

Next section exhibits some related works about the currently developed applications of DT to RMS. The third section introduces the framework, based on the modularity feature of the RMS. The fourth section presents the FESTO smart factory that is used in the fifth section to experiment the creation of a DT based on the framework.

RELATED WORKS

Reconfigurable Manufacturing Systems

Since the 19 th century, mass production thrives thanks to dedicated manufacturing systems. They rely on technologies from the first and second industrial revolutions (mechanization and electrification).

From the 1990s, the evolution of markets and the increased competition between firms boosted portfolio evolution and customization. Those new production strategies required production systems that evolve and adapt rapidly.

Reconfigurable Manufacturing Systems (RMS) answer these needs to rapidly respond to abrupt changes and with cost control, building on the enabling technologies from the third and fourth industrial revolution (robotization and digitalization). This dynamic and evolving architecture combines the advantages of dedicated and flexible systems. RMS enables facing unpredictable, high-frequency market changes driven by market competition [START_REF] Koren | Reconfigurable Manufacturing Systems[END_REF].

RMS deeply rely on their capacity to analyze current performances of the system and identify room for improvements. This monitoring and managing aspect can be supported by digital twins.

Digital Twin for RMS

The issues currently facing the academic communities towards the Digital Twin concept applied to manufacturing systems are multiple. Among them, the design of Digital Twin in itself is a complex matter. Numerous generic architectures were proposed, with various focuses. For example, the architecture proposed by [START_REF] Răileanu | Integrating the Digital Twin of a Shop Floor Conveyor in the Manufacturing Control System[END_REF]) is especially focused on the DT's expected functionalities. It is composed of 4 layers (Fig. 1), corresponding to the data collection, data transmission, data aggregation, and decision making. The reconfiguration process deals with the third and fourth layers, where the DT models need to be reconfigured themselves in parallel with the actual reconfiguration process.

The architecture introduced by [START_REF] Redelinghuys | A six-layer architecture for the digital twin: a manufacturing case study implementation[END_REF] and denoted SLADTA (Fig. 2) is centered on the communication technologies enabling the connectivity and data transmission between the different layers of the architecture. It uses industrial OPC gateways to synchronize the data with the physical twins and interconnect those gateways to model the aggregation of the system. This architecture is centered on the DT's online phase, but the whole lifecycle of the DT is not dealt with, which is specifically relevant in the context of RMS. [START_REF] Redelinghuys | A six-layer architecture for the digital twin: a manufacturing case study implementation[END_REF] The framework introduced in [START_REF] Zhang | A Reconfigurable Modeling Approach for Digital Twin-based Manufacturing System[END_REF]) and presented in Fig. 3 is focused on the system's lifecycle and introduces the notion of reconfigurability. The five pillars of this framework are: the rules, the behaviors, the capabilities, the physics, and the geometry. Each element of the DT is considered as a unit with autonomous functions; it is equipped with each of the five models corresponding to the respective pillars. In this way, the structure of the DT inherits the modular aspect of the elements constituting the RMS.

Examining all these examples, a common feature of the considered RMS, that is also present in most of the RMS in literature, can be exhibited: the information is carried from one module to another of the RMS by the product (or its transporter), leading to consider each configuration of the RMS as a product-driven system [START_REF] Trentesaux | Product-Driven Control: A State of the Art and Future Trends[END_REF]. This work intends to introduce a behavior model able to take this specificity into account at the data aggregation layer level [START_REF] Răileanu | Integrating the Digital Twin of a Shop Floor Conveyor in the Manufacturing Control System[END_REF]. Fig. 3. Five-dimensional RDT model [START_REF] Zhang | A Reconfigurable Modeling Approach for Digital Twin-based Manufacturing System[END_REF]

DIGITAL TWIN MODULAR FRAMEWORK

The design of an RMS-adapted DT needs to be decomposed into three main steps. First, in coherence with the physical modules, the DT modules need to be defined to autonomously aggregate the data coming from the system (i.e. in any configuration of the final DT). Then, the products going through the modules need to be defined. Finally, the DT is meant to be designed by associating the modules. This section introduces some insights about those three steps in the following.

Definition of the modules

A module of RMS is here considered as designed with the same perimeter as a transformation station. The different transformations are either on the product itself (classical workstations), on the recipe of the product (e.g. control stations implying reworking), on the spatial behavior (e.g. divergences), or a composition of those transformations (e.g. disassembly stations composed by a transformation of the product followed by a spatial transformation). Each module is therefore designed as a succession of three elements: 1. One or several feeding areas, characterized by an entrance and a length (length can be equal to zero if the area does not exist in the physical world); 2. A transformation station, characterized by the parameters of the transformation;

3. One or several output areas, characterized by a length (that can be null) and an output.

Each module is equipped with the five models, as defined by [START_REF] Zhang | A Reconfigurable Modeling Approach for Digital Twin-based Manufacturing System[END_REF]):

1. The geometric model defines the length of the areas and the location of the station;

2. The physical model defines the technology used for transfers (e.g. conveyors and their associated speed); The way these different models are designed is out of the scope of this work. The remainder of this paper will focus on the definition of the design parameters.

Definition of the products

The product definition has to be independent of the chosen configuration of the RMS. In that sense, a given configuration of the RMS is in this work only seen as a Product-Driven System. In a Product-Driven System the products are transferred from one transformation station to another and are carrying their information.

Here products are considered as tokens that evolve in the modules and from one module to another (through their connections). The tokens are characterized by:  The identification of the related product;

 The information carried by the product (typically related to its recipe (BOM and routing), traceability data, or performance indicators);

 The product's location, expressed as the identification of the module and the position inside the module.

Connection of the modules

Once the modules are created, the last step is to connect them to fit the expected configuration. The definition of the configurations of DT needs to be recursive to consider the possibility of combining single modules into higher-level modules.

As the modules are defined, the interfaces are only depicted by the inputs and outputs of the transfer areas, and the product in itself handles the interoperability issues. 4. Adaptation to production requirements (Convertibility) is enabled by the diversity of operating cells that can be equipped on each production station.

5. Integrability of the system is possible thanks to the physical connection of the elements (system cable) and their informational connection (shared language)

6. The modularity aspect is enabled by the shared dimensions of elements and the transport systems (conveyors and AGV).

Case study line: As-Is configuration

This paper focuses on the DT of the "Inspection and finishing steps" modules. This line is composed of two linear stations and one branch station. They are equipped with operating cells, and conveyors the movement of products, see figure 4. Two cells (manual workplace and pick by light) are manual; they enable rectifications on identified non-conforming products.

We chose the physical station to aggregate the digital twin. Each cell can be represented through its own PLC, and each conveyor considered as two sub-conveyors separated by the operating cell. Figure 5 shows the as-is configuration using this filter.

Fig. 5. As-Is configuration

The as-is configuration of the "Inspection and finishing steps" line is not optimal. A non-conforming product will be detected by the camera and convey to the manual cells for the needed rectification, and only then completed by the top cover before exiting the line. 2 complete crossings of the line will then be necessary.

The path of this product will then be: Station 2 (entry), station 3 (only crossing), station 2 (detection), station 1 (only crossing), station 2(only crossing), station 3 (rectification), station 2 (detection), station 1 (positioning and press), station 2 (exiting) which correspond to 12 meters.

The layout could be modified to be more efficient

Case study line: To-Be configuration

The swap between stations 2 and 3 will decrease a faulty product's journey from 12 to 6 meters. This is a major productivity gain. The to-be configuration is represented in figure 6.

Fig. 6. To-Be configuration

On a RMS, such a change in layout is easy, yet the challenge is to transcribe these physical changes into the digital representation.

EXPERIMENTATIONS

General parameters

The static model of DT encompasses the definition of the modules (cf. section 3.1):

 CAD modeling of production elements (Fig. 7), operating cells, products (with different WIP status)



Connection with OPC/UA: this has to be done for stations equipped with operating cells and conveyors. Fig. 8 stresses data to be read/written in the OPC server of the "Press" operation cell. In our implementation, the product's definition (Cf. section 3.2) is done using a token. Each token has a unique identification (linked to its working order). It is modeled as a Json text file, Fig. 9. The As-Is configuration can be modeled with XML files, one for each twin station. In our As-Is configuration, there are 5 operating cells and 14 conveyors elements. Fig. 10 describes the DT press and its conveyors.

Fig. 11. DT press To-Be configuration

The xml files enable to set rapidly the design parameters of the new DT. Once the new DT is configured, data can be coherently collected in order to construct the associated behavior model.

CONCLUSIONS

Digital twins can be an important asset to manage production systems. Modification of the real production systems should therefore be translated rapidly into the DT. Using Reconfigurable manufacturing systems in the workshops implies frequent modifications of the systems. Therefore, the coupling between RMS and DT implies an initial design of the DT that enables a rapid reconfiguration of the DT together with the RMS. This article presents a design framework based on modularity and the coupling mechanisms of the different modules.

Future works will focus on the automation of this reconfiguration, using Product-Driven System characteristics, based on the configuration patterns defined in the Manufacturing Execution Systems' database of the RMS. Another lead is to extend the definition of the modules and configurations of RMS [START_REF] Capawa Fotsoh | Towards a Reference Model for Configuration of Reconfigurable Manufacturing System (RMS)[END_REF] to integrate the DT modules directly in the RMS modules definition, the same way the manufacturing control is.

Fig. 1 .

 1 Fig. 1. Digital Twin architecture[START_REF] Răileanu | Integrating the Digital Twin of a Shop Floor Conveyor in the Manufacturing Control System[END_REF]

3.

  The capability model defines the transformation performed by the station; 4. The rule model contains the rules of routing inside the modules; 5. Finally, the behavior model contains the set of data handled by the module and the real-time state of the module.

  des Sciences du Numérique de Nantes) has acquired a smart factory. It is composed of Festo CP Factory elements. The platform is an assembly line that meets the 6 RMS characteristics (as listed by[START_REF] Koren | Reconfigurable manufacturing systems: Principles, design, and future trends[END_REF]).1. Scalability is possible by the simple addition of new elements. The present platform possesses 12 elements (linear production stations, branch production stations, automatic storage systems, robotic assembly cells).2. Identification of problems (Diagnosability) is possible through the control console (of each element) and from the global MES 3. Flexibility within a product family (Customization) is possible. The platform permits to assemble mobile phone cases of different shapes and colors (Puviyarasu and da Cunha 2021).

Fig

  Fig. 4. Considered stations Three cells are automated:  Camera inspection: automated visual comparison of the actual product and customer order,  Positioning of the top cover of the product, from the components supply module,  Pressing of the top cover.

Fig. 7 .

 7 Fig. 7. CAD model (using 3Dx)

Fig. 9 

 9 Fig. 9 Product-token

Fig. 10 .

 10 Fig. 10. DT press As-Is configuration