Quentin Berl

Noémie Resseguier

Maria Katsogiannou

Franck Mauviel

Xavier Carcopino

Léon Boubli

Julie Blanc
email: julie.blanc@ap-hm.fr

Objective assessment of obstetrics residents' surgical skills in caesarean: Development and evaluation of a specific rating scale

Keywords:

Objective: To develop a modified version of Objective Structured Assessment of Technical Skill (OSATS) rating scale for evaluation of surgical skills specific to caesarean and to assess its relevance in documenting the residents' learning curve during their training. Secondarily, to verify the scale's stability to caesarean's level of difficulty and comparing self-assessment to hetero-assessment in order to propose a practical application of this rating scale during residency. Study Design: We conducted a multicentre observational prospective study, from May 2018 to November 2018. All residents at that time could participate and fill in the rating scale after caesarean. Senior surgeons had to fill in the same rating scale. We analysed correlation between self-assessments and hetero-assessments and sensitivity to change of the rating scale. Analysis of feature's relevance was performed by principal component analysis, factor analysis and reliability analysis. Results: In total, 234 rating scales were completed evaluating 18 residents. Our study demonstrated that our rating scale could be used to evaluate surgical skills of residents during caesarean and distinguish their year of residency (p < 0.001) with a high correlation between self and hetero-assessment (Intraclass Correlation coefficient for global score: 0.78; 95% CI 0.68À0.86). The principal component analysis revealed two dimensions corresponding to the two parts of the rating scale and the factorial analysis allowed us to confirm distribution of features according to these two dimensions. Cronbach's alpha allowed us to highlight the percentage of representation of the scale's features in relation to all potential theoretical features (0.93, 95% CI 0.82À0.95). Conclusion: Our rating scale could be used for self-assessment during residency and as a heteroassessment tool for validating defined stages of the internship.

Introduction

Learning the technique of surgery is central in every surgical residency program. To gain autonomy, the resident has to have theoretical knowledge, technical dexterity, communication skills, leadership and to be able to make decisions [START_REF] Moorthy | Objective assessment of technical skills in surgery[END_REF]. For a few years, new teaching and evaluation approaches have been developed [START_REF] Rodriguez-Paz | Beyond 'see one, do one, teach one': toward a different training paradigm[END_REF]. Assessment methods have to be objective, standardized, feasible, validated, reliable, practical and inexpensive [START_REF] Desender | Training with simulation versus operative room attendance[END_REF]. The senior surgeon have to maintain control by gradually reducing supervision as the resident gains surgical skills [START_REF] Müller | The learning curve in the context of the cesarean section[END_REF]. In 2005, the Royal College of Obstetricians and Gynaecologists (RCOG) defined six surgical procedures that need specific assessment procedures: caesarean section (CS), instrumental delivery, perineal repair, uterus revision, hysterectomy and exploratory laparoscopy [START_REF] Bisson | Assessing practical skills in obstetrics and gynaecology: educational issues and practical implications[END_REF].

CS is the most frequently performed conventional open surgical procedure in the world with 29.7 millions of births per year; it represented 20.4% of deliveries in France in 2016 [START_REF] Boerma | Global epidemiology of use of and disparities in caesarean sections[END_REF]. This surgery is performed in a repeated and precocious manner by obstetrics and gynaecology (OB/GYN) residents [START_REF] Rathat | Formation chirurgicale des internes, spécialistes en gynécologie-obstétrique : résultats d'une enquête électronique nationale[END_REF]. Several surgical skills can be acquired by simulation software [START_REF] Bouin | La simulation sur mannequin haute fidélité dans la formation initiale des sages-femmes[END_REF] or simulators [START_REF] Tassin | Simulateur d'amniocentèse : intérêts et développement d'un nouveau modèle reproductible et économique[END_REF][START_REF] Anastakis | Assessment of technical skills transfer from the bench training model to the human model[END_REF], educational videos [START_REF] Driscoll | Video assessment of basic surgical trainees' operative skills[END_REF], or even anatomic dissection [START_REF] Anastakis | Assessment of technical skills transfer from the bench training model to the human model[END_REF][START_REF] Sarker | Simulation and surgical training[END_REF] but all these methods are insufficient for residents training [START_REF] Alici | Objective Structured Assessment of Technical Skills (OSATS) evaluation of hysteroscopy training: a prospective study[END_REF]. In 2007, the RCOG officially used an educational tool called "objective structured assessment of technical skills" (OSATS) [START_REF] Martin | Objective structured assessment of technical skill (OSATS) for surgical residents[END_REF][START_REF] Reznick | Testing technical skill via an innovative "bench station" examination[END_REF]. This model, initially created for simulation, was modified and adapted to CS in 2005 by the RCOG [START_REF] Bisson | Assessing practical skills in obstetrics and gynaecology: educational issues and practical implications[END_REF]. It is composed of a first part when the assessor determines whether the resident needs help in a series of key steps of CS, called "task list". The second part evaluates the resident's overall surgical skills ("global rating scale") [START_REF] Swanson | Assessment of clinical skills with standardized patients: state of the art revisited[END_REF][START_REF] Goff | Assessment of resident surgical skills: is testing feasible?[END_REF]. This surgical learning assessment has shown excellent external validity in several studies [START_REF] Reznick | Testing technical skill via an innovative "bench station" examination[END_REF][START_REF] Goff | Assessment of resident surgical skills: is testing feasible?[END_REF][START_REF] Larsen | Objective assessment of surgical competence in gynaecological laparoscopy: development and validation of a procedure-specific rating scale[END_REF][START_REF] Van Hove | Objective assessment of technical surgical skills[END_REF]. The validity of OSATS scale adapted to caesarean has been studied only by reporting trainees' experiences [START_REF]Objective Structured Assessment of Technical Skill in assessing technical competence to carry out caesarean section with increasing seniority[END_REF].

In France, the resident's logbook is rarely used [START_REF] Rathat | Formation chirurgicale des internes, spécialistes en gynécologie-obstétrique : résultats d'une enquête électronique nationale[END_REF]. There is an actual request from residents for a formative assessment of theoretical knowledge and weighted practical assessments [START_REF] Rathat | Formation chirurgicale des internes, spécialistes en gynécologie-obstétrique : résultats d'une enquête électronique nationale[END_REF]. Unlike RCOG, there is no assessment scale of CS difficulty in France. Residents self-assessment is a current concern [START_REF] Gordon | A review of the validity and accuracy of self-assessments in health professions training[END_REF][START_REF] Ward | Measuring self-assessment: current state of the art[END_REF][START_REF] Bresson | Évaluation d'une formation de proximité à la mesure de la clarté nucale et à l'autoévaluation des clichés[END_REF][START_REF]Concordance auto évaluation et évaluation par un tiers pour les clichés biométriques d'échographie de dépistage du deuxième et troisième trimestre de la grossesse[END_REF].

Our primary objective was to develop an original surgical skills assessment scale adapted to CS and integrating a task list with a Likert scale to use by self and hetero-assessment and to evaluate its relevance in documenting the progression of resident's scores with the year of residency. Secondary objectives consisted in checking the scale's stability to caesarean section difficulty and comparing self-assessment to hetero-assessment in order to propose a practical application of this rating scale during internship.

Material and methods

Study design and data collection

This prospective observational multicentre study was conducted from May 2018 to November 2018 in three university hospitals where residents could perform CS with a senior surgeon from the first year of residency. All residents present at that time could participate and fill-in the rating scale right after CS whatever gestational ages (GA). Senior surgeons had to fill in the same rating scale. The ones that were not completed by both residents and senior doctors were excluded from our analysis.

The Aix-Marseille University Ethics Committee approved this study (N 2017-14-12-003).

The criteria related to CS difficulty and its characteristics were collected: time of procedure, GA, number of uterine scars, cervix dilation, body mass index (BMI), characteristics of CS (type of emergency code), total estimated blood loss, multiple pregnancy, clotting disorders, placentation abnormalities, general anaesthesia. Data were collected and anonymised. A composite end-point of preoperative characteristics was defined by: BMI ! 30 or full dilation or gestational age < 28 weeks or uterine scar ! 1 or placenta praevia or accreta or polymyomatous uterus or clotting disorders.

Rating scale

The rating scale included the OSATS scale used by RCOG for CS. Several elements of global rating scale (GRS) adapted to obstetrics according to recent publications were added to the rating scale. [START_REF] Reznick | Testing technical skill via an innovative "bench station" examination[END_REF][START_REF] Gurusamy | Virtual reality training for surgical trainees in laparoscopic surgery[END_REF][START_REF] Lentz | Testing surgical skills of obstetric and gynecologic residents in a bench laboratory setting: validity and reliability[END_REF] The relevance of the scale's content was verified and validated by three experts (JB, LB, XC) with more than 10 years of experience in OB/GYN. The scale was divided in two parts. The first part assessed resident autonomy for each surgery procedure step and the second part evaluated overall procedure of technical and non-technical skills [START_REF] Martin | Objective structured assessment of technical skill (OSATS) for surgical residents[END_REF][START_REF] Reznick | Testing technical skill via an innovative "bench station" examination[END_REF][START_REF] Lentz | Testing surgical skills of obstetric and gynecologic residents in a bench laboratory setting: validity and reliability[END_REF][START_REF] Qureshi | Assessment of competence for caesarean section with global rating scale[END_REF].

Autonomy scoresurgery procedure steps

The first part was worth maximum 70 points and reflected the resident's autonomy analysis during the different procedure steps with a five-point Likert scale [START_REF] Ofili-Yebovi | Deficient lower-segment Cesarean section scars: prevalence and risk factors[END_REF][START_REF] Regehr | Comparing the psychometric properties of checklists and global rating scales for assessing performance on an OSCE-format examination[END_REF]. Likert scale is more compelling than "pass/fail" [START_REF] Regehr | Comparing the psychometric properties of checklists and global rating scales for assessing performance on an OSCE-format examination[END_REF]. "0" was given in case the senior surgeon decided to perform the CS by himself. All Likert scores for each procedure step were then added together in order to have a global evaluation of autonomy. Features were the following ones: appropriate skin incision (e.g. length, position), safe entry of peritoneal cavity, careful management of bladder, appropriate uterine incision (e.g. length, position), safe and systematic delivery of baby, appropriate delivery of placenta, check uterine cavity (e.g. intact, empty, configuration), safe securing of uterine angles, check for ovarian pathology, appropriate closure of rectus sheath, attention to haemostasis, neatness of skin closure, appropriate management of deeply engaged head.

Technical and behavioural score

The second part assessed the resident's overall technical skills such as tissue respect, movement fluidity, long-term planning, suture technique, and management of operational assistance. Subjective aspects were also collected such as involvement in management, relational attitude with the patient and the surgical team, adaptation to emergency and/or to situation. Total score was worth a maximum of 35 points with a five-point Likert scale for each assessed item. Features are detailed below: (i) respect for tissue, (ii) time, motion and flow of operation and forward planning, (iii) suturing and knotting skills as appropriate for the procedure, (v) relations with patient and the surgical team, (vi) insight/attitude, (vii) documentation of procedures.

Global score

Global score (105 points maximum) was the sum of firstand second-part assessment scores.

Global subjective assessment of the procedure's difficulty was marked from 1 to 5. A final feature allowed the senior surgeon to give a global evaluation of the resident's behaviour (1-4 points); "simple supervision; passive assistance; active assistance; shows and explains" (Fig. 1).

Assessment scale validity

We analysed the distribution of global self-and heteroassessment scores according to year of residency (from 1st to 5th year) and to pre-and intraoperative characteristics: "English three-point score" (1=first caesarean or one scar with longitudinal incision, 2=twins and preterm before 28 weeks of gestation and two scars and obesity, 3=preterm before 28 weeks and placenta praevia and accreta), BMI ! 30, full dilation, CS urgency, postpartum haemorrhage (PPH) ! 500 mL, subjectivity level from 1 to 5, composite criterion of preoperative CS characteristics: BMI ! 30 or full dilation or GA < 28 weeks or uterine scar !1 or placenta praevia or placenta accreta or polymyomatous uterus or clotting disorders.

Statistical analysis

Statistical analysis was performed using RStudio V.1.0.44. We first carried out a descriptive analysis. Qualitative variables were expressed as absolute values and percentages. Quantitative variables were expressed as mean and standard deviation. In order to evaluate correlation between self-assessments and hetero-assessments, we calculated an intra-class correlation coefficient (ICC) with its 95% confidence interval (CI) for: autonomy score, technical and behaviour score, global score, subjective evaluation of CS difficulty and senior surgeon's behaviour. Sensitivity to change of the rating scale was analysed by comparing total scores according to: residents' year of residency, preoperative characteristics ("English score"), BMI ! 30, full dilation, occurrence of PPH, subjectivity level, composite criterion of caesarean section characteristics (BMI ! 30 or full dilation or GA < 28 weeks or uterine scar !1 or placenta praevia or placenta accreta or polymyomatous uterus or clotting disorders), emergency of CS defined by red code alert. Comparisons were done for binary variables with Student t test when appropriate, or with Mann-Whitney U test. For categorical variables, comparisons were done with the Kruskal-Wallis test. Analysis of feature's relevance was performed by principal component analysis for identification of the number of dimensions in the rating scale. Factorial analysis then allowed to present distribution of features according to the identified dimensions and to assess factor loadings in order to retain only the most relevant ones. Finally, according to the feature universe model, we estimated the Cronbach's alpha coefficient, and the 95% CI was obtained by bootstrap method. In order to evaluate the relevance of the "subjective difficulty of CS", we described and compared values of this quantitative variable according to the composite criterion of CS characteristics, occurrence of PPH and red code alert with a Mann-Whitney U test. All tests were twotailed. P < 0.05 was considered statistically significant.

Results

Study design and population

During the period of our study, 886 CS (of which 27.76% were elective) were performed in three maternity units where 25 residents were present. Among the 167 caesarean deliveries assessed, we excluded from our analysis 23 rating scales filled in only by residents and 12 filled in only by senior surgeons. Fifteen rating scales were also excluded because filled in by a resident who decide to change medical speciality from OB/GYN to psychiatry during the study time. A total of 234 assessments by residents and senior surgeons (117 CS) were analysed. The majority of the assessments, 92 (39.31%) were carried out by a 5th-year resident; three 4th-year residents carried out 58 assessments (24.78%); four 3rd-year and six 1st-year residents carried out respectively 36 assessments (15.38%); finally, four 2nd-year residents carried out only 12 assessments (5.13%). Among the 18 evaluated residents, 1 (5.55%) was a 5th-year resident, 3 (16.67 %) by a 4th-year resident, 4 (22.22%) by a 3rd-year resident, 4 (22.22%) by a 2nd-year resident and 6 (33/33%) by a 1st-year resident.

Among the 234 assessments, 60 (25.64%) and 68 (29.06%) were performed in two tertiary care level maternities, and 108 (46.15%) in one secondary care level maternity.

Among the 24 senior surgeons, one (4.17%) was a university professor, 14 (58.33%) were hospital practitioners and 9 (37.50%) were clinic head assistants.

Characteristics of evaluated caesarean deliveries

CS characteristics are presented in Table 1.

Analysis of self-assessment and hetero-assessment rating scales

Mean scores for different parts of the modified OSATS scale as well as total scores were calculated (Table 2). Global mean score of the two parts (on 105 points) was 95.08 (AE 12.30) for selfassessments and 97.48 (AE 11.01) for hetero-assessments. ICC evaluated correlation between residents and senior surgeons scores for the different assessed factors. We observed ICC > 0.5 for CS difficulty, senior surgeon behaviour, first and second parts scores as well as global score.

Evaluation according to year of residency

We observed progression of total self-and hetero-assessment scores with year of residency (Fig. 1). Indeed, 1st-year residents obtained lower assessment scores than 4th and 5th-year residents (Table 3). Our analysis showed a statistically significant difference (p < 0.001) among mean scores, which varied increasingly with the year of residency.

Assessment scores progression according to CS difficulty

We did not observe any significant difference in the mean assessment score according to the "English" score (p = 0.51), full dilation (p = 0.07), preoperative composite criterion (see Methods section) (p = 0.71) and CS code (p = 0.29) (Table 4). Impact of BMI on assessment score was on the borderline of significance (p = 0.05). Mean scores were quite close for patients with BMI ! 30 (97.32 AE 9.88) and those with BMI < 30 (96.55 AE 12.83). Occurrence of PPH during CS was statistically associated with assessment scores (p < 0.001). Finally, subjectivity level (based on senior surgeon's and resident's perception) was statistically associated to assessment scores (p = 0.002): the higher was the subjectivity level, the lower the assessment score was.

Subjective difficulty of CS

Subjective difficulty score was significantly associated with the composite criterion of preoperative characteristics (mean subjective score 2.8 AE 1.04 in case of composite criterion vs 1.89 AE 0.98 when non composite criterion, p < 0.001) and PPH (mean subjective score 3.6 AE 1.14 in case of PPH vs 2.38 AE 1.04 in absence of PPH, p < 0.001).

Rating scale features' relevance

The principal component analysis revealed two dimensions, two logical sets of features, corresponding to the two parts of the rating scale. Then, factorial analysis of the different stages of CS allowed us to confirm distribution of features according to these two dimensions and to sort them according to their factor loadings. Skin suture was the feature with the lowest factor loading (0.29). Several aspects of the factorial analysis obtained by senior surgeons' and residents' assessments were similar.

Cronbach's alpha allowed us to highlight the percentage of representation of the scale's features in relation to all potential theoretical features (0.93, 95% CI 0.82À0.95).

Discussion

Our study showed that this rating scale is a reliable tool that could be used to evaluate surgical skills of residents during CS according to their year of residency and with a high correlation between self and hetero-assessment. This suggests the relevance of our scale in assessing residents and according to the CS level of difficulty and its different procedure steps.

The difference in scores was more significant among first-year residents and 5th-year residents in agreement with other previously published studies. Unlike other studies [START_REF] Martin | Objective structured assessment of technical skill (OSATS) for surgical residents[END_REF][START_REF] Regehr | Comparing the psychometric properties of checklists and global rating scales for assessing performance on an OSCE-format examination[END_REF][START_REF] Regehr | Comparing the psychometric properties of checklists and global rating scales for assessing performance on an OSCE-format examination[END_REF][START_REF] Gosman | Innovative approaches to resident education[END_REF], we chose to integrate a task list by evaluating resident's autonomy during each procedure step of CS by using a Likert scale. We believe that in an educational objective, it is interesting to highlight the procedure steps that require special attention while pointing out the simple ones that can be carried out independently.

We revealed that the degree of emergency of CS was not statistically related to the total score unlike PPH. Indeed, in case of PPH, the senior surgeons usually give less autonomy to the residents since time is the key of the management of PPH. The composite criterion was not statistically related to the total score probably because lack of power. Some variable of the composite criterion are at limit of significance (BMI ! 30 and full dilatation). If demonstrated by a larger study, the features of the composite criterion (BMI ! 30 or full dilation or gestational age < 28 weeks or uterine scar ! 1 or placenta praevia or accreta or polymyomatous uterus or clotting disorders) could be considered to select CS for self-assessment or hetero-assessment.

Even though filling-in the rating scale took less than 3 min, it was not mandatory, and this could explain why all CS performed during the study period were not evaluated. The proportion of evaluated CS is low as previously reported in a study dealing with the assessment of instrumental deliveries [START_REF] Haloui | Instrumental extractions using Thierry's spatulas: evaluation of the learning curve[END_REF]. This could be explained by the busy activity in the three maternity units most of the time. Furthermore, in accordance with Gosman et al. [START_REF] Gosman | Focused assessment of surgical performance: difficulty with faculty compliance[END_REF], we believe that the lack of surgical assessment was not related to the resident's performance, but rather to the constraint that may represent, for some, the assessment process.

First limitation of our study is the duration of our study (6 months), not long enough to allow progression follow-up of residents over time. Secondly, we reported an unequal repartition of the residents depending on their year of residency. This could be explained by the distribution of the residents in the three obstetrics units during only one semester. This could be avoided by increasing the length of the study. Thirdly, it was impossible to conduct a blinded study. This bias could be avoided by an external independent assessor or by videos of surgical procedure sequences. Nevertheless, Goff et al. demonstrated that surgical skills could be assessed with equally strong validity and reliability regardless of whether administration of OSATS scales was blinded or not [START_REF] Goff | Surgical skills assessment: a blinded examination of obstetrics and gynecology residents[END_REF]. Several authors suggest that OSATS are biased [START_REF] Van Der Vleuten | The assessment of professional competence: building blocks for theory development[END_REF]. This can be avoided by increasing the sample size.

Our study presents several strengths, including its originality and its prospective and multicentre design. To our knowledge, no other study has been conducted for evaluation of OB/GYN residents on their overall surgical skills of CS, by self-and hetero-assessment. Similarities of the factorial analysis for the self and hetero-assessment are an additional proof of the conceptual validity of our tool. In addition, our study is the first to look at the progression of surgical evaluation scores according to CS clinical difficulty levels.

Direct observation of performance through specific assessment criteria was considered to be a valid and reliable method for evaluating the residents' surgical skills [START_REF] Martin | Objective structured assessment of technical skill (OSATS) for surgical residents[END_REF][START_REF] Reznick | Testing technical skill via an innovative "bench station" examination[END_REF][START_REF] Bodle | Value and face validity of objective structured assessment of technical skills (OSATS) for work based assessment of surgical skills in obstetrics and gynaecology[END_REF][START_REF] Chou | Evaluating the competency of gynecology residents in the operating room: validation of a new assessment tool[END_REF]. Self-assessment is generally approached in relation to the concept of autonomy as an aim of the educational process. Colthart et al. gave an operational definition to self-assessment as being a "personal evaluation of ones' characteristics and professional abilities in relation to perceived standards" [START_REF] Colthart | The effectiveness of self-assessment on the identification of learner needs, learner activity, and impact on clinical practice: BEME Guide no. 10[END_REF]. Several authors demonstrated high validity and reliability of the OSATS scale [START_REF] Reznick | Testing technical skill via an innovative "bench station" examination[END_REF][START_REF] Bodle | Value and face validity of objective structured assessment of technical skills (OSATS) for work based assessment of surgical skills in obstetrics and gynaecology[END_REF][START_REF] Adrales | Method of Laparoscopic Simulation Training and Competence Assessment 1, 2 1Presented at the 36th Annual Meeting of the Association for Academic Surgery. . p. 7-9 November 2Supported in part by an educational grant from Tyco/U.S. Surgical Corporation[END_REF][START_REF] Maithel | Construct and face validity of MIST-VR, Endotower, and CELTS: are we ready for skills assessment using simulators?[END_REF]. The correlation between self and hetero-assessment was high for the global score but the ICC was between 0.5 and 0.75 for the technical and behaviour score. This could be explained by the usual underestimation in case of self-assessment. The parallel use of our assessment scale by residents and senior surgeons may also serve as a basis for discussion and could facilitate the feedback given by the senior surgeon. By revising the different features of the rating scale together, senior surgeons could reveal potential overvaluation errors, leading to improvement of the resident's selfassessment ability [START_REF] Mandel | Self-assessment of resident surgical skills: is it feasible?[END_REF]. We think that this mode of assessment with Likert scales is a better way to respect educational process and accompany the residents than defining a threshold for each part to confirm the autonomy of the residents in performing a CS. However, further studies could evaluate the relevance of a threshold for the global score to confirm the autonomy of residents in performing a CS in pre-defined clinical situations at each stage of residency.

Our assessment scale could be used for self-assessment during the whole residency training. In addition, hetero-assessment could be validating at the end of first-year as well as the fourth and fifthyears of residency. Pre-and intraoperative criteria would allow to « select» the appropriate surgical situations to assess each level. These findings could be confirmed by evaluating this assessment scale is a larger sample of residents.

Funding

None

Fig. 1 .

 1 Fig. 1. Rating scale.

Fig. 1 .

 1 Fig. 1. (Continued)

Table 1

 1 Characteristics of evaluated caesareans.

	Characteristics	Results
	Time of procedure a (min)	36.14 AE
		16.63
	Total estimated blood loss a (mL)	297.43 AE
		265.70
	Post-Partum Haemorrhage b	10 (8.55%)
	Gestational age a (weeks)	37.48 AE 4.34
	Gestational age < 28 weeks b	7 (5.98%)
	Body Mass Index (BMI) a N=66	28.51 AE 6.15
	BMI ! 30 b	30 (45.45%)
	Number of uterine scars a	0.63 AE 0.91
	Cervix dilatation a (cm)	2.62 AE 3.10
	Full dilatation b	7 (5.98 %)
	Caesarean
	Elective b	35 (29.91%)
	Green code b	43 (36.75%)
	Orange code b	34 (29.06%)
	Red code b	5 (4.27%)

Post-Partum Haemorrhage defined by total estimated blood loss !.500 mL. Colour code defined by expected decision-birth duration: green > 30 min, orange <30 min, red < 15 min.

a Quantitative variables: Mean AE Standard Deviation. b Qualitative variables: n (%).

Table 2

 2 Correlation between self and hetero-assessment rating scales. Mean AE Standard Deviation.

	Parts of rating scale	Self-assessment scores N = 117	Hetero-assessment scores N = 117	ICC (95% CI)
	Subjective evaluation of caesarean difficulty	2.33 AE 1.08	2.63 AE 1.11	0.71 (0.61À0.80)
	Senior surgeon's behaviour	3.02 AE 1.01	3.12 AE 0.86	0.69 (0.52À0.80)
	First Part = Autonomy score	64.05 AE 8.91	64.84 AE 8.94	0.83 (0.71À0.91)
	Second Part = Technical and behaviour score	31.03 AE 4.42	32.64 AE 2.79	0.55 (0.46À0.64)
	Global score	95.08 AE 12.3	97.48 AE 11.01	0.78 (0.68À0.86)
	ICC, Intraclass Correlation Coefficient; CI, Confidence Interval.

Table 3

 3 Mean global scores depending on year of residency and caesarean characteristics. Partum Haemorrhage defined by estimated blood loss ! 500 mL. Colour code defined by expected decision-birth duration: green > 30 min, orange <30 min, red < 15 min.

	Year of residency	Mean global scores	p
	1	80.61 AE 17.44	<0.001
	2	91.92 AE 5.15
	3	95.69 AE 8.94
	4	98.66 AE 7.67
	5	101.71 AE 5.5
	Caesarean characteristics	Mean global scores	p
	« English » Score
	1	97.06 AE 9.50	0.51
	2	96.62 AE 11.66
	3	90.45 AE 18.93
	BMI
	< 30	96.55 AE 12.83	0.053
	! 30	97.32 AE 9.88
	Full dilatation
	No	96.39 AE 11.88	0.07
	Yes	94.57 AE 8.87
	PPH
	No	97.70 AE 10.08	<0.01
	Yes	81.10 AE 16.66
	Subjectivity level
	1	97.24 AE 9.66	<0.01
	2	99.45 AE 7.39
	3	94.89 AE 14.28
	4	91.68 AE 15.00
	5	89.20 AE 13.75
	Preoperative composite criterion
	No	96.48 AE 11.92	0.71
	Yes	96.17 AE 11.63
	Caesarean colour code
	Red	92.50 AE 12.12	0.29
	Orange	96.01 AE 10.77
	Green	96.63 AE 12.10
	Mean AE Standard Deviation.
	BMI: Body Mass Index.
	PPH: Post-

Acknowledgements

We thank Michelle Meunier for helping to correct the English.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Contributors to authorship

Details of ethics approval

This study was approved by the Aix-Marseille University Ethics Committee (N 2017À14-12À003).

Informed consent statement

Informed consent was obtained from all individual participants (residents and senior) included in the study.


C Schaeffer

R Interdonato

R Lancelot

M Roche

M Teisseire

Social Network Data and Epidemiological Intelligence: A Case Study of Avian Influenza

Methods & Materials: From ProMED-mail spanning 1994 to 2019, we established a unique dengue corpus using professionals' annotations which achieved near perfect agreement (90% Cohen's Kappa statistic). To generate ProMED-mail summaries, a dualchannel bidirectional long-short term memory with an attention mechanism that infuses latent-syntactic features was developed to identify key sentences from the alerts.

Results: Our method outperformed many well-known machine learning and neural network approaches in identifying important sentences, achieving a macro average F 1 -score performance of 93%. In addition to verifying the model, we also recruited 5 experts from related fields to conduct a satisfaction survey on the generated summaries, and 83.6% of the summaries received high satisfaction ratings.

Conclusion:

The proposed approach successfully fuses latentsyntactic features into a deep neural network to analyze the syntactic content information in the text. It then exploits the derived information to identify key sentences. When a new alert arrives, we can quickly identify the case-relevant-information that is essential for reference or further analysis.

Purpose: Event Based Surveillance (EBS) systems detect and monitor diseases by analysing articles from online newspapers and reports from health organizations (e.g. FAO, OIE, etc.). However, they partially integrate data from social networks, even though these data are present in large quantities on the web. The purpose of this study is to exploit social network data, such as Twitter and YouTube, to provide epidemiological and additional information for Avian Influenza surveillance.

Methods & Materials: In this context, we propose new textmining approaches combining lexical rules and statistical approaches, in order to normalise textual data from Social Network ('h5 n8' → 'H5N8') and to correct errors from YouTube transcriptions (e.g. 'birth flu' → 'bird flu'). Another challenge consists of extracting epidemiological events automatically by identifying spatial entities (Where?), thematic entities (What?), and temporal information (When?). For this, we extended Named Entity Recognition (NER) tools like spaCy.

Results: We collected 100 automatic transcripts of YouTube videos and 268 tweets, in English, dealing with avian influenza, thanks to dedicated API. We obtain encouraging results (i.e. accuracy around 0.6) in order to recognise automatically epidemiological information (e.g. hosts, symptoms etc.) in textual data contents. Extraction of spatial information obtains better results (i.e. accuracy around 0.8).

Conclusion:

The final objective of the study consists of linking social media data based on these entities with official information from health organisations, for the improvement of epidemiological monitoring.

CIRAD, Montpellier, France

Purpose: Epidemic intelligence (EI) has been adopted by several countries to reach fast detection of new and emerging infectious diseases. EI collects information from two types of sources: official sources (i.e. health reports from OIE or FAO) and unofficial sources (i.e. online media outlets, scientific publications, etc.). In France, the EI system PADI-web (Platform for Automated extraction of Disease Information from the Web) is used since 2014 to detect signals of animal health events with risk of introduction to France.

The objective of this work was to understand how health information (signal) is disseminated from a primary source (transmitter) to a final source (EI system) through quantitative and qualitative network analysis methods.

Methods & Materials: We analysed all English reports related to avian influenza detected by PADI-web between August 2018 and June 2019. We used the sources cited in the detected reports to trace the path of each signal. Signals were categorized as official and unofficial according to the source. We have built a directed network where the nodes represented the sources (characterized by type, location and geographical focus) and the edges represented the signal flow. To describe the network, we used network centrality measurements (degree, betweenness and eigenvector) to determine which nodes were important in the data dissemination. We also included the reactivity, calculated as the difference (in days) between the detection of an outbreak by PADI-web and its official notification by Empres-i (gold-standard) with a distinction between wild and domestic birds.

Results: PADI-web detected 202 official signals and 26 unofficial signals. The OIE occupies a central position in the PADI-web information network. National veterinary authorities were the major primary sources. Online news outlets followed by press agencies were the main secondary sources. A significant portion of the signals was detected early in wild birds (41%) and in domestic birds (13%).

Conclusion: This work showed PADI-web's capacity to detect early signals and can be used to define priority sources to improve this tool in terms of reactivity and data quality. In the future, similar work will be conducted on other diseases and EI systems to improve these systems.

PS21

  https://doi.org/10.1016/j.ijid.2021.12.234 1 , 2 , * , W. Zhou 1 , 2 , B.J. Cowling 1 , 2 , E.H. Lau1 , 2

04 (556) Understanding Outbreak Data Dissemination In Event Based Surveillance Systems. Application On Avian Influenza Using PADI-web B

  . Boudoua * , C. Hautefeuille , E. Arsevska , S. Valentin

https://doi.org/10.1016/j.ijid.2021.12.233 PS21.

School of Public Health, University of Hong Kong, Hong Kong, Hong Kong

Laboratory of Data Discovery for health, Hong Kong Science and Technology Park, Hong Kong, Hong Kong


Pierre Margerit

Tristan Gobin

Arthur Lebée

Jean-François Caron

The Robotized Laser Doppler Vibrometer: on the use of an industrial robot arm to perform 3D full-field velocity measurements

Keywords: Full-field measurements, Robotics, Laser Doppler Vibrometer

An original experimental setup, dedicated to the measurement of the dynamic response of structures, is presented. Called the Robotized Laser Doppler Vibrometer (RLDV), it consists in the assembly of a fixed point Laser Doppler Vibrometer (LDV) on a 6-axis industrial robot arm. This allows to measure the 3D velocity on the surface of objects with a wide range of shapes and sizes. In comparison with commercially available solutions, it represents an affordable and versatile tool. First, the motivations and the new challenges associated to the use of a robot arm to do such experiments are discussed. Second, the different strategies implemented to overcome these difficulties are developed. A software solution is proposed. By the use of a virtual model of the experimental setup, the measurement procedure can be designed and simulated in order to preview robot motion and avoid collisions. With a small number of parameters, the good proceeding of the measurement can be insured on a fine measurement mesh. The software is completed by an interface that allows for the setting of the signal acquisition parameters. At the end, all the procedure is automated and long measurements can be achieved with a reduced supervision. To illustrate the proposed setup, an experimental application is developed. The 3D modal shapes of a curved beam are measured and compared with numerical predictions, showing an excellent correlation.

Introduction

During the last few decades, a number of so-called full-field measurement techniques were developed: instead of being limited to single-point measurements, physical quantities can now be measured on fine meshes of points, providing insights on their spatial distribution. These techniques usually take advantage of the ability to replicate sensors (e.g cameras) or to move the measurement point location (e.g scanning sensors). Among these new tools, some allow a contactless measurement of the kinematic field of a structure submitted to a dynamic excitation. In the community of structural mechanics, the data redundancy offered by such measurement techniques encouraged the development of identification methods providing rich experimental data for material characterization or model validation [START_REF] Avril | Overview of identification methods of mechanical parameters based on full-field measurements[END_REF]. In dynamics, one can mention the Modal Analysis [START_REF] Ege | High-resolution modal analysis[END_REF], the Force Analysis Technique [START_REF] Leclère | Practical implementation of the corrected force analysis technique to identify the structural parameter and load distributions[END_REF], the Virtual Field Method [START_REF] Pierron | The Virtual Fields Method[END_REF] or the High-Resolution Wavevector Analysis [START_REF] Margerit | High Resolution Wavenumber Analysis (HRWA) for the Mechanical Characterisation of Viscoelastic Beams[END_REF][START_REF] Margerit | High-Resolution Wavevector Analysis for the characterization of the dynamic response of composite plates[END_REF].

A popular sensor used to perform dynamical measurements is the Laser Doppler Vibrometers (LDV). It allows the measurement of an object velocity at a fixed point, projected in the direction of the laser beam. In order to perform full-field measurements, the laser beam target position can be controlled with the help of galvo scanners. This is the principle of the Scanning Laser Doppler Vibrometer (SLDV), that is well established as a versatile tool to perform full-field measurement over a wide frequency range [START_REF] Rothberg | An international review of laser Doppler vibrometry : Making light work of vibration measurement[END_REF]. At each location along the mesh of measured points, the experiment is repeated so that the velocity field map can be reconstructed. By comparison to high-speed cameras, no data processing (i.e. optical flow computation) is needed and the proportionality between velocity and displacement magnitudes with frequency improves the signal to noise ratio the high frequency regime. As a counterpart, the use of the SLDV requires a particular attention to the experiment repeatability as the measured point changes. Moreover, when the structural response can be considered steady, Continuous-Scan Laser Doppler Vibrometry (CSLDV) can be implemented in order to speed up acquisition times [START_REF] Stanbridge | Measurement of total vibration at a point using a conical-scanning ldv[END_REF][START_REF] Halkon | Vibration measurements using continuous scanning laser doppler vibrometry: theoretical velocity sensitivity analysis with applications[END_REF][START_REF] Martarelli | Continuous scanning laser doppler vibrometry and speckle noise occurrence[END_REF].

Moreover, some identification methods need the measurement of the 3D kinematic field of the structure. As the LDV measures the instantaneous velocity in the direction of its laser beam, one has to perform the velocity measurement for a number of laser incidence angles in order to retrieve the three components of the velocity. As a consequence, either the SLDV head has to be moved [START_REF] Malley | Five-axis scanning laser vibrometry for three-dimensional measurements of nonplanar surfaces[END_REF][START_REF] Kim | 3-D vibration measurement using a single laser scanning vibrometer by moving to three different locations[END_REF] or three LDVs have to be used simultaneously [START_REF] Miyashita | Development of 3D vibration measurement system using laser doppler vibrometers[END_REF], thus leading to a high price of the overall setup. Moreover, these solutions requires a number of manual handling steps that limit the number of points that can be measured. In addition, uncertainties can arise from incorrect positioning of the laser head.

Recently, automated solutions were developed in order to overcome these limitations. Using a set of mirrors assembled on a rotary hollow shaft, it has been shown that it is possible to measure the three translations and local rotations (6 DOFs) of a point by implementing continuous scanning along circular paths [START_REF] Giuliani | Six degrees of freedom measurement with continuous scanning laser doppler vibrometer[END_REF][START_REF] Giuliani | Experimental validation of a newly designed 6 degrees of freedom scanning laser head: Application to threedimensional beam structure[END_REF]. Perhaps the most complete solution can be found in [START_REF] Oliver | Automated Robot-Based 3D Vibration Measurement System[END_REF], where three SLDVs are assembled on the head of an industrial robot arm. However, this commercially available solution is offered at a very high price.

The aim of the present work is to discuss a setup which is able to perform 3D velocity full-field measurements at a reduced price. The idea is to assemble a single point LDV on an industrial robot arm. The weight to be held being reduced, the robot arm can be smaller hence the price kept affordable. The present setup is consequently called the Robotized Laser Doppler Vibrometer (RLDV). A companion software solution is provided with the proposed setup [START_REF] Margerit | Code for the Robotized Laser Doppler Vibrometer[END_REF]. The first part, integrated in the Rhinoceros CAD software 1 by means of the Grasshopper plug-in, is used to build the virtual experimental setup, in which the mesh of measurement points can be defined. The preview and control of the robot motion is implemented by the use of the HAL 5.3 plug-in 2 . The second part of the proposed software solution is devoted to signal acquisition, processing and preview and is implemented in Matlab.

The work is organized as follows: first, a discussion is given about (i) the motivations of performing LDV measurements with an industrial robot arm and (ii) the difficulties that arise when using such device. Second, the proposed software solution is briefly described in order to give an overview of its operation. Third, an experimental application is presented to validate the setup. The 3D mode shapes and frequencies of a curved beam are 1. www.rhino3d.com/en 2. www.hal-robotics.com measured and compared to predicted mode shapes.

The Robotized Laser Doppler Vibrometer

A fixed point Laser Doppler Vibrometer measures the instantaneous velocity v k (t) in the direction n k of its laser beam (see Figure 1). By moving the LDV, the measurement of the velocity can be performed, at the same point, for K 3 distinct laser incidence vectors n k . As a consequence, the 3D instantaneous velocity u(t) = [u 1 (t), u 2 (t), u 3 (t)] of the point, expressed in the cartesian coordinates, can be retrieved:

u(t) = P † v(t) (2.1)
where P = [n 1 . . . n K ] is the transfer matrix, • † denotes the matrix pseudo inverse and

v(t) = [v 1 (t) . . . v K (t)]
contains the instantaneous velocity measured for each laser orientation n k . At this point, it can be noted that some LDV sensors include displacement decoder. In this case, retrieving the 3D instantaneous displacement would be straightforward, directly replacing the measured velocities v(t) by the obtained displacement in Eq. (2.1).

Motivations

Multiple axis robot arms, originally developed for industrial applications, can manipulate a wide range of tools of different weights and shapes, with speeds, position accuracy and holding loads which make them interesting for a wide number of applications. In particular, a number of reasons motivates the assembly of a LDV on a robot arm.

Accuracy. Even with a rough calibration, modern industrial robot arms can position the tool with a submillimeter accuracy. Consequently, the uncertainty in the position p of the measured point and the laser beam vectors n k can be reduced, thus lowering the uncertainty in the computed 3D velocity. Indeed, some processing methods based on full-field data make assumptions on the regularity of the measurement mesh [START_REF] Pierron | The Virtual Fields Method[END_REF][START_REF] Ablitzer | Identification of stiffness and damping properties of plates by using the local equation of motion[END_REF][START_REF] Margerit | High Resolution Wavenumber Analysis (HRWA) for the Mechanical Characterisation of Viscoelastic Beams[END_REF] that can lead to large bias in the results if not fulfilled.

Flexibility. In the procedure of retrieving the 3D components of the velocity (2.1), the choice of the positions of the LDV is critical. Indeed, when K > 3, the velocity in the cartesian coordinates is estimated in the least-squares sense. Hence the chosen incidence angles φ (see figure 1) have an influence on the conditioning of P, which is critical in the estimation of u(t) (equation (2.1)). In the case K = 3, the best strategy [START_REF] Khalil | Accuracy and noise analyses of 3D vibration measurements using laser Doppler vibrometer[END_REF] seems to measure the velocity along directions making an angle of 45 • with the object surface's normal at the considered point. Consequently, the laser incidence angles φ has to be optimized in order to ensure the good conditioning of the 3D velocity retrieval. In comparison to a Scanning Laser Doppler Vibrometers fixed on a tripod, a fixed Laser Doppler Vibrometer mounted on a 6-axis robot arm offer a lot of flexibility in the positioning of the tool and objects with a wide variety of shapes can be measured.

Automation. Being by definition a programmable manipulator, a robot arm can perform automatized measurements. As a consequence, full-field LDV measurements can be achieved on a great number of points with reduced supervision.

New challenges

As a counterpart of the motivations mentioned above, additional complexity arises with the use of a robot arm to perform such measurements. In particular, as the actuated robot arm joints are rotations, the motion of the robot is by far less intuitive than classical cartesian manipulators. Consequently, a number of elements have to be taken into account in the design of an experiment including the use of a robot arm.

Accessibility. When using a cartesian manipulator, one can infer the accessibility of one point, as every point on the volume given by the translation axes bounds is theoretically reachable. However, when using a robot arm with only pivot joints, the accessibility of a point cannot be guaranteed, even if it is in the sphere centered in the robot base position and which radius is the total arm length.

Motion interpolation. When moving between two robot configurations (the different LDV positions), one has to choose how to interpolate the positions. Based on cartesian manipulator experience, an intuitive way would be to linearly interpolate between two positions in the cartesian space. However, this type of motion can lead to very large joint rotations near the so-called robot arm singularities, thus leading to unwanted motion of the arm. In addition, even if both the start and the end positions are accessible, nothing can guarantee that every linearly interpolated positions in-between can be reached. An alternative choice could be to interpolate linearly between the two joint configurations, in the joint space (rotations). Hence the singularities would be avoided and the accessibility guaranteed. However, this type of motion often leads to large non-intuitive motion in the cartesian space. These so-called re-orientations has to be avoided as much as possible, as they can lead to collisions.

Collisions. During the robot motion, a number of collisions can occur: (i) the robot with itself; (ii) between the robot and the tool or (iii) with the experiment environment. As the consequences of such collisions can be dramatic for the experiment, any collision has to be avoided.

As a consequence of the added complexity, it is necessary to build a virtual model of the experiment, that allows to preview the robot motion, set and optimize the target parameters and perform collision tests. The main problem to be solved when designing a robot setup simulation is the inverse kinematic problem.

Inverse Kinematics. A robot setup can be described by a series of frames that define different coordinate systems. In the present application, the laser target is a frame F p (see Fig. 2), given by the laser beam vector n, the tar-

T 0 p (p(u, v), n, x m , )
geted point p and an additional angle ψ which describes the rotation of the frame around n (see Fig. 1). The target position p is given by the measured object position x m expressed in the (arbitrary) reference frame F 0 and the normalized coordinates (u, v) on the surface of the object. The robot is defined by its base frame F b , given by its position x b and orientation in the reference frame. Finally, the robot head frame F h , which coincides with the LDV base frame, is given by the 6 robot arm joint rotation angles j = [ j 1 , . . . , j 6 ]. From two subsequent frames can be defined geometrical transformations:

F j = T j i F i (2.
2)

The kinematic chain can finally be defined with the identity transformation I:

I = T 0 0 = T b 0 T h b T p h T 0 p (2.3)
Let us assume for now that each frame F i is given: the positions of the measured object and robot base are known and the transformation T p h is computed from the tool geometry and for a given laser beam length d. While the forward kinematic problem of computing the transformation T h b from the robot joint values j is straightforward, the inverse kinematic problem is more complex:

j = a ∈ R 6 | a i ∈ [ j - i , j + i ] , T h b (a) = T p h T 0 p T b 0 -1
(2.4) where the j - i and j + i are respectively the minimum and maximum allowed robot arm joint values. In particular, as the parameters j describe rotations, the solution is not unique. In order to simulate the robot motion, an inverse kinematic solver has to be implemented.

The controlling software

In order to be able to achieve measurements with the RLDV, a software solution has been developed for the present work [START_REF] Margerit | Code for the Robotized Laser Doppler Vibrometer[END_REF]. It is divided in two parts: the first part is devoted to the modelling of the geometries of the experiment and the control of the robot. The aim of the second part is to handle the setting of the signal acquisition parameters, the signal processing steps, the measurement preview and finally the export of the measurement. A block diagram of the software solution is schematized in Figure 3. Each part is detailed further in the two following subsections.

The virtual experiment model

The aim of the first part of the implemented software is to help the user of a RLDV setup to design the experiment and ensure the good proceeding of the measurement. It is based on the CAD software Rhinoceros3D 1 , with the Grasshopper plug-in which allows to process geometries via a graphical programming environment. This software configuration allows to model the different geometries of the experiment and process them with routines in order to create the RLDV targets, simulate the robot motion and write the code that is finally sent to the robot controller. The Grasshopper canvas associated to the proposed software [START_REF] Margerit | Code for the Robotized Laser Doppler Vibrometer[END_REF] implements the different strategies presented in the following paragraphs.

All the steps involving robotic specific tasks are achieved thanks to the HAL plug-in 2 . This tool is dedicated to the design of general robotic setups. It includes an inverse kinematic solver (problem (2.4)), a library of robot arm presets from robot manufacturers, a range of tools dedicated to robot target handling and motion simulation and finally a tool that allows to communicate with the robot controller.

Geometries of the experiment. The first step of the design of a RLDV experiment is to create a model of the geometries that will be involved in the measurement. Three types of geometries can be distinguished:

1. Measured objects, used to create the measurement mesh and the laser beam targets. 2. Reference objects, used to calibrate the measurement setup, for example to accurately determine the relative positions x r and x m of the robot and the measured object (see Figure 2). 3. Environment objects, included in the robot motion simulation in order to avoid collisions.

The refinement of the geometries has to be chosen carefully, as collision tests are computationally intense, with a burden increasing with the complexity of the involved geometries.

Measurement mesh. From the measured object geometry can be created the mesh of measurement points p. Basically, as the LDV measurement is performed on the surface of the object, the positions of the points are given by discrete values of the (u, v) parameters that describes the surface. A classical way of building the mesh is to choose points that lies on a regular grid of (u, v) values. However, a number of meshing strategies (e.g. Delaunay mesh) can be chosen since they are implemented in the Grasshopper plug-in. Laser targets. The next step is concerned by the definition of the laser beam basis P = [n 1 . . . n K ] (see expression (2.1) and Figure 1). By default, the targets are derived from the measured object surface. First, the normal of the surface of the measured object is evaluated at each measurement point p. Second, the K incident beam vectors are built with the angle shift to normal φ as parameter. These vectors are, by default, evenly distributed around the normal. Additionally, the overall target configuration can be rotated of an angle θ around the normal (see Figure 1). As not only a vector but a complete frame has to be given to fix the robot configuration, an angle ψ has to be set in order to define the rotation of the LDV around the laser beam axis. This target definition strategy is indicative and can be shortcut by the user. As a consequence, a wide variety of object can be measured, by adapting the target generation steps.

Robot motion preview. Once the targets have been built, the HAL plugin is used to preview the robot motion. At this step, a number of validity tests have to be performed in order to ensure the overall target accessibility. The software computes the inverse kinematics and returns information about the joint configuration of the arm, for each target. Invalid targets (joints out of bounds or unreachable positions) are made visible on the geometrical model. No collision test is performed at this step, as it would involve a high computational burden. The user then has to check for collisions visually.

Global target parametrization. In order to make all the targets accessible, the user has to adapt the parameters φ, θ and ψ represented on Figure 1. As a large number of points are usually measured with such a setup, setting these parameters for each individual target would represent a lot of work. In order to keep the target definition simple, a global parametrization is proposed. Inspired from finite element shape functions, each parameter is set as a function f of the (u(p), v(p)) parameters of the surface at each target point p:

f (p) = f (u(p), v(p)) = f 1 × (1 -u(p))(1 -v(p)) + f 2 × (1 + u(p))(1 -v(p)) + f 3 × (1 -u(p))(1 + v(p)) + f 4 × (1 + u(p))(1 + v(p)) (3.1)
with (u(p), v(p)) ∈ [-1; 1]. The user then has only to choose the 3 × 4 parameters {φ i , θ i , ψ i } with i = 1, . . . , 4, while keeping sufficient freedom to make all the targets accessible in an intuitive way. In addition, the laser focus distance d (distance from the LDV to the object surface, see Figure 1) can be tuned. As the Polytech PDV-100 LDV used in the present work has manual optics, d is set to be constant. Let us notice that some LDV have remotecontrolled optics, that could be used to even increase the freedom given to the user to optimize the target configuration. Once all set so that they are accessible, the targets can be sent to the second part of the software dedicated to signal acquisition, processing and preview. This second part (presented later) then becomes the master program, and the first part the slave program. When a robot motion is needed, the target to be reached is sent by the master software to the slave via the User Data Protocol (UDP). Before performing the robot motion, additional steps are needed.

Motion interpolation. Every motion is basically defined by the starting target and the end target. The right motion interpolation strategy (i.e. in the cartesian or the robot arm joints space) has to be chosen between them in order to ensure the feasibility of the motion task. A mixed approach is proposed. First, additional targets are added between the start and end targets. They are computed by a linear interpolation in the cartesian space. Then, between these targets the robot motion is interpolated in the joint space. As a consequence, both large motions of the arm due to linear interpolation in the joint space between two distant targets and singularities due to linear interpolation in the cartesian space are avoided.

Collision check. As the present steps are performed in the automated working mode, collisions have to be checked. Thanks to the chosen motion interpolation strategy, only a few arm positions has to be validated, the computational cost of such test is reduced. If a potential collision is detected for the scheduled motion, this motion is cancelled and the program returns an error.

Code generation. Once the trajectory of the robot has been generated and validated, the code necessary to communicate with the robot controller is written by the HAL plugin. Finally, the code is downloaded and the motion task performed directly.

Signal acquisition and processing

The second part of the software, implemented in Matlab, is devoted to signal acquisition, processing and preview. As it represents classical signal handling routines, only few details are given on these aspects here. A capture of the software interface is given in Figure 4.

A number of excitation signal types are available, both transient (e.g. burst) and stationary (e.g. white noise, blue noise, sweep). The signal processing routines allow to compute various indicators, as the transfer function between the velocity and the reference signals, the coherence, etc.

It has to be noticed that every signal processing step is performed before the 3D velocity determination procedure described in (2.1). Indeed, as this last operation can be badly conditioned, it is preferred to reduce the noise ratio as much as possible before. In addition, this last step induces correlation between the noise associated to each experiment realization, which effects on the measurement uncertainty can be hard to interpret.

Once the targets have been received from the virtual geometrical model, this second part of the software becomes the master program. It then allows to choose a specific target to perform some elementary checks (i.e. signal-tonoise ratio quantification) before starting the measurement. As the overall measurement procedure is automated, long measurements can be performed with a reduced supervision, allowing to achieve measurements on fine meshes of points.

Experimental validation case

In order to test the validity of the proposed setup, a simple application case study is presented. The modes of a curved beam are measured with the RLDV and the comparison with synthesized modes is performed. As the beam is curved, the mode shapes are expected to contain multiple velocity components. In order to be build with a laser cutting machine, the beam has been chosen so that its neutral axis lays in a plane. As a consequence, this plane is a plane of symmetry of the beam: the modes of the structure are either contained in this plane or with a dominant out-ofplane component.

The beam of b = 5 mm width is cut on a PMMA plate of h = 6 mm thickness (see Fig. 5). The Young and the shear modulus are respectively E = 4.5 GPa and G = 1.5 GPa.

The model

To validate the measured mode shapes, a curved beam model is implemented. Following the Timoshenko beam model, the kinematic field is postulated as:

u(s, z) = w(s) + α(s) ×       z 1 z 2 0       (4.1)
where s is the curvilinear coordinate and z = [z 1 z 2 ] the position in the beam section (see Fig. 5). By using finite elements, 2064 nodes with 6 degrees of freedom (3 translations w i , 3 angles θ i ) are defined. Assuming cantilever boundary conditions (see Figure 6), the first N numerical modal shapes q and frequencies ω are obtained so that:

K -ω 2 n M q n = 0 , n ∈ [[ 1, N ]] (4.2)

The measurement

Setup. The RLDV setup used for this experimental case is composed of a Polytec PDV-100 vibrometer assembled on an ABB IRB 120 industrial robot arm. An assembly plate is inserted between the LDV and the robot head in order to adapt the different screw holes. Both LDV and robot geometries are given by the respective manufacturers and used in the virtual experiment model. The real setup and its virtual model are represented in Figure 6.

For the signal acquisition, a National Instruments NiDAQ USB-6215 is interfaced with Matlab.

In order to be able to extract the modes of the beam, its free response have to be measured. As the experiment has to be fully automatized, the excitation of the beam has to be sufficiently repeatable. The solution chosen here is to use an electrodynamic shaker as an automatic hammer. At the beginning of each measurement realization, an impulsive electrical signal is sent to the shaker, which consequently hits the beam. This procedure has been found to produce reproducible impact loads with adjustable magnitude and length. In order to excite both in-plane and out-of-plane modes, the shaker is oriented at an oblique angle.

Target positioning. The velocity measurement is performed over the top surface of the beam, along the middle curve (see Fig. 5):

z = [0 h/2] (4.3)
From this curve are extracted P = 301 equally-spaced points that constitute the 1D measurement mesh. Then K = 3 laser targets are defined at each point with a constant incidence angle φ = 25 • . A 1D version of the shape function 3.1 is used: f 1D (s) = f (s, 0). Finally, only five parameters (θ 1 , θ 2 , ψ 1 , ψ 2 and d as defined in Fig. 1) need to be set in order to define all the targets. Signal acquisition settings. The maximum observed beam response duration is approximately 0.5 seconds, as the PMMA composing the beam has a high loss factor. Consequently, the experiment duration is set to 0.8 seconds, with a sampling frequency of 51.2 kHz, by far sufficient to avoid aliasing. The excitation signal is sent 10 ms after the beginning of each experiment realization. In order to enhance the signal to noise ratio and to check the experiment repeatability, M = 45 realizations of the experiment are performed for each laser position. Finally, a total of M × K × P = 45 × 3 × 301 = 40635 experiment realizations are performed, for an overall measurement duration of approximately 20 hours, including the time to move the robot between each target.

Averaging strategy When motions perpendicular to the laser beam vector occurs, the LDV sensor is source of speckle noise associated to the measured object's surface roughness. This phenomenon is due to the surface roughness which modifies the instantaneous laser diffraction pattern, thus the quality of the signal received by the LDV. While some Doppler Vibrometer sensors include tracking filters aiming at reducing this source of noise, the proposed solution implements averaging strategies using the ability to repeat the experiment automatically. However, because of the impulsive nature of the speckle noise, taking the mean over the M experiment realizations is not always relevant. Instead, it has been chosen here to compute, at each time sample, the median over the M available measurements. Hence realizations that contains high impulsive noise are discarded. Figure 7 illustrates the different strategies. It can be observed that the minimum and maximum measured values at each samples (gray envelope) have a very high magnitude compared to the signal amplitude. In addition, even with 45 realizations, the mean value (red line) remains sensitive to these variations. However, the median value (black line) reduces significantly the impulsive noise.

This noise reduction strategy is performed before the 3D velocity estimation procedure (2.1), as impulsive noise would introduce a large bias on this estimation.

As a result of the measurement, the mean absolute velocity over the points is represented in Figure 8. The three cartesian components of the velocity are plotted. From this results can be distinguished the two mode types: out-of plane modes have a dominant u y component (orange spectrum), while in-plane modes are composed of both u x and u z components (resp. blue and yellow spectra).

Modal analysis

In order to identify the natural frequencies and mode shapes of the beam, an High-Resolution Modal Analysis [START_REF] Ege | High-resolution modal analysis[END_REF][START_REF] Rébillat | Measurement of relevant elastic and damping material properties in sandwich thick plates[END_REF][START_REF] Margerit | Caractérisation large bande du comportement dynamique linéaire des structures hétérogènes viscoélastiques anisotropes[END_REF] is performed on the measured velocity 3D field.The modal analysis procedure is applied on the complete set of acquired data, in order to extract the modal frequencies, the modal dampings and the modal shapes simultaneously. The selection of the number of modes to keep is achieved using a stabilization diagram [START_REF] Reynders | Fully automated (operational) modal analysis[END_REF]. Only the 12 first modes of the beam are kept for the comparison, ranging from 10 Hz to 1 kHz. Identified modal frequencies, denoted as vertical gray lines, are labelled in the figure 8. It can be seen that the two first modes are close in frequency ( f a = 15.5 Hz and f b = 17.5 Hz). The separation of these two modes is made easier by their different displacement components: the first is an out-of-plane mode while the second is dominated by in-plane components of the motion. The corresponding mode shapes are represented in Figure 10.

Result comparison

To make the comparison between mode shapes obtained from the RLDV measurement and the predicted mode shapes, one has to define the observation matrix C that projects the nodal displacements q n computed with the model (4.2) on the measurement mesh, so that:

u n = Cq n (4.4)
where u n is the n th numerical modal vector evaluated on the measurement points. This observation matrix takes into account the contribution of the computed local beam rotation α in the velocity u of the measurement point on the top surface of the beam; from (4.1) and (4.3):

u(s) = w(s) + h 2       α 2 (s) -α 1 (s) 0       (4.5)
Defining a measured modal shape u i and the numerical mode u j , one can express the Modal Assurance Criterion matrix (MAC) as the normalized correlation coefficient between the two shapes:

MAC i j = * u i • u j u i u j (4.6)
The magnitude of the MAC matrix, computed from the 12 experimental modes ("a" to " ") extracted from the measurement and the 16 first modes computed with the beam model (1 to 16), is shown in Figure 9. The measured and the predicted modal shapes are very close, with a mean correlation coefficient of 99%. In Figure 10, the measured (round markers) and the predicted shapes (red lines) are superimposed, for each individual mode. The reference configuration is shown as a black dashed line. It can be seen that the shapes match closely, for both in-plane and out-of-plane modes.

Conclusion and Perspectives

An original experimental setup called the Robotized Laser Doppler Vibrometer (RLDV) has been presented. By assembling a single point Laser Doppler Vibrometer (LDV) on an industrial 6-axis robot arm, one is able to perform 3D velocity measurements in an automated way. In comparison to commercially available solutions, the setup is more affordable. By the use of a robot, the accuracy and flexibility of the LDV positioning is improved. Thus, objects with a wide variety of shapes and sizes can be measured. However, the use of a 6-axis robot introduces complexity, which was discussed in the second section of the work. To tackle potential issues associated to robot motion, collisions with surrounding objects or automated code generation, a virtual model of the experiment is built in order to simulate the robot motion and ensure the good proceeding of the experiment. An implementation of the proposed procedures is available freely [START_REF] Margerit | Code for the Robotized Laser Doppler Vibrometer[END_REF], using Matlab scripting for signal acquisition and processing and Rhino/Grasshopper/HAL for geometry definition and robotic-specific tasks. In order to illustrate an application of the setup, an experimental case was presented. The 3D mode shapes of a curved beam have been measured and compared to numerical predictions. It has been shown that measured and computed shapes are very close, with a correlation coefficient close to 1.

The experimental setup presented in this work has proven to be a versatile tool allowing the measurement of the dynamic response of a variety of objects [START_REF] Margerit | Caractérisation large bande du comportement dynamique linéaire des structures hétérogènes viscoélastiques anisotropes[END_REF], for a relatively affordable price. As for now, no investigation has been led on the uncertainties related to the use of a robot. As a perspective, two main uncertainty sources can be given:

Calibration. The robotized manipulator insures a relative positioning accuracy. However, a good absolute positioning accuracy can be hard to obtain, as the position of the robot and the measured object are given with some uncertainty. Until now, a visual check of the laser target position on several points over the measured object has proven to be sufficient to adjust and validate the positions of the different objects in the virtual setup. Nonetheless, a more accurate calibration procedure may be necessary for more demanding object geometries or experimental environments.

Vibration of the arm. The 6-axis robot arm cannot be considered infinitely stiff. A procedure could be designed in order to evaluate the contribution of the robot arm vibration in the measured velocity. In the present work, this contribution has been neglected. In addition, some robot arms have brakes that can be activated during the signal acquisition step, thus reducing the vibration due to the actuators at the joints.

In the present work, the focus was on LDV measurements. However, the assembly of any sensor on a robot arm could potentially benefit to a wide variety of measurements [START_REF] Nolan | Experimental characterization of the sound field in a reverberation room[END_REF].

Figure 1 :

 1 Figure 1: Measurement of the 3D velocity of a point. Laser beam vector basis {n i }, orientation of the basis θ, incidence angle φ and rotation of the LDV around the laser axis ψ.

Figure 3 :

 3 Figure 3: Block Diagram of the proposed RLDV software solution.

Figure 4 :

 4 Figure 4: A capture of the Matlab interface, the part of the RLDV software related to signal acquisition, processing and measurement preview.

2 Figure 5 :Figure 6 :

 256 Figure 5: Beam geometry and coordinate axis. h = 6 mm, b = 5 mm.

Figure 7 :

 7 Figure 7: Influence of the averaging steps on 45 realizations of the same measurement: range of the acquired velocities at each time sample (gray patch), mean over the realizations (red line) and median of the realizations (black line).

Figure 9 :

 9 Figure 9: Magnitude of the Modal Assurance Criterion matrix (%) between experimental and numerical modes.

Figure 10 :

 10 Figure 10: Comparison of experimental (black dots) and numerical mode shapes (red line).

Acknowledgments

This work is part of the ANR founded (french National Research Agency) project MAESSTRO (ModélisAtion Et Synthèse Sonore pour Tables d'haRmonie de pianO, ANR-14-CE07-0014).

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