Who cites non-English-language pharmaceutical articles?

Keywords: Authorship, Language, Periodicals as Topic, statistics & numerical data, Pharmacists English language, publication, pharmacy

The objective was to determine a link between the number of non-English language references in the bibliographies of publications in international pharmaceutical journals and the geographic origin of these publications.

METHODS

A systematic prospective analysis of 7 international pharmaceutical journals in 2005 2006. All research articles whom corresponding author was a pharmacist were included. For each article, were recorded

INTRODUCTION

By writing a scientific paper, a practical but crucial question is to realize an almost comprehensive analysis of the literature (this point is even more relevant for review articles and meta-analysis) (

). But this approach is not easy because the various databases do not 1 -3 cover the totality of the scientific production on a specific topic ( ). Furthermore, even if English seems to become the quasi-universal 4 language of science ( ), many high-quality papers are published in others languages. It can be questioned if pharmacists of various 5 -10 countries actually realize as authors a comprehensive bibliographic analysis of the topic of their manuscripts ( , ) by searching also 1 11 -13 non-English language articles. Until yet, there was no evidence to conclude.

OBJECTIVE

The objective of the study was to determine a link between the number of non-English language references in the bibliographies of publications in international pharmaceutical journals and the geographic origin of these publications.

METHODS

A systematic prospective analysis of 7 international pharmaceutical journals (Cf ) published in 2005 2006 was performed. All table I research articles whom corresponding author was a pharmacist were included. The corresponding author was determined as a pharmacist if his/her title was mentioned, or if his/her professional affiliation or e-mail address mentioned a pharmaceutical entity (i.e. pharmacy department of a medical center, college of pharmacy of a university ). Every issue of the journals was analyzed, except the supplements.

…

The articles where the whole title of the bibliographic references was not mentioned were not included.

For each article included, were recorded: the geographic origin of the corresponding author (classified in: North America, Latin America, Oceania, Europe, Asia, Africa); the country of the corresponding author; the title of the journal; the number of non-English language references in the bibliography (classified in: Spanish, German, French, Portuguese, Dutch, Russian, Japanese, Chinese, Italian, others).

The main outcome was the percentage of non-English language references in the bibliographies of publications for each geographic origin of the corresponding author. Secondary outcomes were: percentage of non-English language references in the bibliographies of publications for each country of the corresponding author, for each journal. Significant differences were searched for each criterion by Chi-squared test.

RESULTS

1,568 articles were included (

), corresponding to 45,949 bibliographic references, of whom 542 where non-English references Table 1 (1.18 ).

%

The percentage of non-English language references in the bibliographies of publications for each geographic origin of the corresponding author (main outcome) is shown in : the homogeneity Chi-squared test is very significant (p < 0.0001). North Table 2 America is the geographic zone of the world with the lowest rate of non-English language references in bibliographies of published articles; significant differences appear between North America and Europe, Latin America and Asia.

The percentage of non-English language references in the bibliographies of publications for each nationality of the corresponding author is shown in (only the 15 countries with the largest number of articles appear): the homogeneity Chi-squared test is very Table 3 significant (p < 0.0001). For Australia, all references without exceptions were in English. United States, United Kingdom and China present also a specific low rate of non-English language references. Significant differences appear between USA and almost all countries except United Kingdom, Australia and China.

The percentage of non-English language references in the bibliographies of publications for each journal is shown in : the Table 4 homogeneity Chi-squared test is very significant (p < 0.0001). The two journals with the lowest rate of non-English language references in bibliographies of published articles are edited in the USA (Am J Health-System Pharm, J Pharm Sc); in opposition, the two journals with the highest rate of non-English language references in bibliographies of published articles are edited in Europe (Pharm World Sci, Eur J Hosp Pharm).

DISCUSSION

Study limitations

First is the role of self-citation ( ). It is a commonplace that authors are likely to cite their own previous papers: thus, it is not 14 illogical that -let s say: a Spanish pharmacist writing in English in international journals could easily cite its own previous articles ' published in Spanish. So, a greater frequency of non-English references in articles written by non-English native language pharmacists is not surprising.

The citation of regulatory texts of the country where the study was performed is not unusual and pharmacists often work in their own country. This could also explain this greater frequency of non-English references in articles written by non-English native language pharmacists.

A third limitation is linked to the role of the co-authors. As we only recorded the geographic origin of the corresponding author, it cannot be excluded that the bibliographic analysis was performed by co-authors of this paper, who could be of other native languages than the corresponding author.

Fourth, the panel of journals that we choose was not proved to be representative of pharmaceutical international journals; we selected journals on a practical considerations basis (availability in our University library). Even, some issues of Am J Health-System Pharm were not included because they vanished from the library before being examined.

A potential bias is the lack or the low number of pharmaceutical colleges in universities of some countries (i.e.: Belgium, Luxemburg, Russia ), which could draw pharmacists researchers to work in medical or scientific universities. This could impeach such people to be … determined as pharmacists in our study. In fact, we do not identify a single Russian author in our study. Thus, we cannot exclude that our study is not comprehensive. Papers from Latin America, from Africa and, in lower manner, from Oceania, were in such low number that a valid statistical analysis could not be drawn.

Moreover is the case of non-Latin alphabets (i.e.: Cyrillic, Japanese, Korean, Chinese ). Bibliographic references in languages using … such alphabets may be translated in English without mention of the original publication language; thus, it could prevent us to identify, for example, non-English references in Chinese manuscripts.

We initially planned to analyze the native language of the authors as a factor of citation of non-English language references, but we were not able to; indeed, if we could assume that, for instance, the native language of a German or Austrian author will be German, the situation of Canadian, Belgian or Swiss pharmacists could not be easily solved ( ). 5

We only include articles written by pharmacists; our results cannot be generalized to others types of health professionals.

Rate of non-English language references: role of English native language?

Globally, the rate of non-English language references that we observed (1.18 ) is very lower than the rates which may be noted in % non-English language journals (up to 7.9 ) ( , ). % 15 16

Our study shows slight differences between geographic zones ( ). Pharmacists from Latin America, Europe and Asia are more table II likely to reference non-English language articles in their publications than North Americans. A sub-analysis on major-publishing countries (

) revealed that pharmacists from only-English-speaking countries (USA, United Kingdom, Australia) were particularly refractory table III to include non-English language references in the bibliographies of their publications (rate of non-English language references from 0.00% to 0.26 ). On the contrary, pharmacists from countries where English is not the only language (continental Europe, Canada, Japan) cite % relatively often (1 to 6 ) non-English language articles; in fact, they cite often works written in their own language: for example, in our % study, 100 of non-English languages references cited by Japanese authors are in Japanese; 95 of non-English languages references % %

cited by Spanish authors are in Spanish; 91 of non-English languages references cited by German (respectively French) authors are in % German (respectively in French) (data not shown). The role of English native language is partly supported by the low rate of non-English language references in articles written by pharmacists from various countries where English is the official language or is dominant, but even in this group, slight differences between geographic zones remain: in our study, the rate is 1.23 in Irish articles, 1.04 in Nigerian % % articles, and 0.00 in articles from Hong-Kong, Singapore, New-Zealand, South Africa and India (data not shown). At the opposite, China % has a very low rate of non-English language references, but it has been reported that Far-East countries were supposed to accept easily linguistic hurdles ( ). 7

Rate of non-English language references: hypotheses

It may be questioned why some pharmacists do not include non-English language references in the bibliographies of their published articles:

they did not read these articles (they are not able to read others languages than English/they are not able to obtain non-English languages journals); they read these articles, but they considered that their quality was too poor to cite them; they initially included non-English language references in the bibliographies of their manuscripts, but the reviewers or editors suggested that these citations would be retracted ( , , ). 6 17 18 While English is widely spoken by scientists all around the world ( , ), it is possible on the contrary that only few English native 5 6 language pharmacists can read or even scan a foreign language article ( ): as 30 of US adult residents have not taken any foreign 19 % language instruction in schools ( ) and 64 of community pharmacies in Milwaukee County (USA) are not able to communicate 20 % verbally with patients in non-English languages ( ), it cannot be excluded that most US pharmacist-researchers only read English. 21

We may also speculate about a low diffusion of foreign pharmaceutical journals in North-American medical centers or universities ( 22,

), but without serious basis; on the contrary, the low number of non-English languages journals in international databases could prevent English native language pharmacists to identify relevant articles written in others languages ( , , ). The poor quality of 4 14 19 non-English language papers could be a speculative reason of their exclusion of bibliographic references, in spite several studies have shown that quality of papers and language were not correlated ( , , ). But this is still widely spread: in a Spanish study 12 24 25 id e re ue é ç about nursing journals, the low citation of English documents was presented as a marker of scientific insularity ( ). 16

Concerning the role of reviewers or editors, Ross . ( ) provided evidence of bias in peer-review of cardiology papers, favoring et al 26 authors from English-speaking countries. Man . ( ) showed that English proficiency was strongly associated with publication output et al 27 in the highest ranked medical journals. Tortosa-Serrano . ( ) observed in a Spanish critical care journal a larger diversity of origins et al 28 of bibliographic references than our series.

Last, the hypothesis that English native language pharmacists could concentrate on English-language references because it is easier to do is not to be rejected.

CONCLUSION

We showed that non-English language works are seldom cited as bibliographic references in articles written by North-American pharmacists. This attitude is also frequent in articles written by British and Australian pharmacists. The role of native English language is probable, but the fundamental reasons of this restriction are not clear. However, it may lead in such cases to a partial bibliographic analysis and produce results different from those which have been obtained without restriction ( ): this fact cannot be admitted by scientists ( , , 3 1 2 , ). The assistance of multi-lingual documentalists to only-English-speaking pharmacists to access to international literature should be 6 13 considered.

References:

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Table 1

 1 Journals analyzed

	Title of journals

Table 2

 2 Percentage of non-English language references and geographic origin of author

Origin Number of articles Number of references Number of non-English language references Percentage of non-English language references χ 2 test (to North America)

	North America 866	26,070	78	0.30%	/
	Latin America 10	261	20	7.66%	p < 0.0001
	Oceania	34	995	1	0.10%	NA
	Europe	479	13,676	412	3.01%	p < 0.0001
	Asia	166	4,631	27	0.58%	p < 0.01
	Africa	13	316	4	1.27%	NA
	NA: not applicable

Table 3

 3 Percentage of non-English language references and country of author

Country Number of articles Number of references Number of non-English language references Percentage of non-English language references

						χ 2 test (to USA)
	USA	816	24,354	49	0.20%	/
	United Kingdom	93	2,649	7	0.26%	NS
	Japan	54	1,675	18	1.07%	p < 0.0001
	Netherlands	53	1,523	71	4.66%	p < 0.0001
	Canada	50	1,716	29	1.69%	p < 0.0001
	Spain	48	1,164	84	7.22%	p < 0.0001
	Italy	42	1,305	10	0.77%	p < 0.0001
	France	40	965	46	4.77%	p < 0.0001
	Germany	31	1,239	75	6.05%	p < 0.0001
	Denmark	27	805	13	1.61%	p < 0.0001
	Australia	24	719	0	0.00%	NA
	Sweden	24	733	27	3.68%	p < 0.0001
	China	22	444	1	0.23%	NA
	Belgium	21	539	14	2.60%	p < 0.0001
	Switzerland	21	475	12	2.53%	p < 0.0001
	NA: not applicable; NS: not significant

Table 4

 4 Percentage of non-English language references and journal of publication

	Journal	Number of articles	Number of references	Number of non-English language references	Percentage of non-English language references
	Am J Health-Syst Pharm	315	7,877	45	0.45%
	Ann Pharmacother	467	13,132	218	1.66%
	Drug Delivery Sc Technol	110	2,834	19	0.67%
	Eur J Hosp Pharm	36	659	21	3.19%
	Eur J Pharm Sc	204	7,548	75	0.99%
	J Pharm Sc	337	11,572	50	0.43%
	Pharm World Sci	99	2,295	111	4.84%


This is a major step towards open access, but the implementation of such a commitment in such a short time frame can only be done -for the sake of simplicity -by initially following the two already existing and non-exclusive paths :

1. The first is to publish journals and books in open access ("Gold Open Access"), which implies either institutional funding of platforms or journals operated by institutions or scientific communities ("Diamond Open Access" model), or the payment of publication fees for which business models compliant with the principles of "Fair Open Access" should be implemented.

2. The second is to make it at least possible, if not mandatory, to deposit and make the publication available without embargo in a reliable open access repository ("Green Open Access").

It is essential, while building on what already exists, to also consider the development of other, more innovative routes. Indeed, Plan S, while expressing the need to establish strict compliance criteria for the services of eligible journals and platforms, seems to focus on the "APC" hypothesis (who will pay for them? will they be capped?) without explicitly considering alternative business models such as "Diamond" and by reducing the potential for editorial innovation in open access repositories. Consequently, the ANR, which will have to implement Plan S, must take into account in its roadmap open access journals free of charge for authors, alternative business models such as "Diamond" and open access repositories, thus respecting the diversity of models advocated by the Jussieu Call for Open Science and Bibliodiversity signed by the majority of French research institutions.

It is along these lines that the specialized groups of the French Open Science Committee (Group "Open Scientific Publishing" and Project Group "Building Bibliodiversity"), made up of researchers from various disciplines and professionals in scientific and technical information (IST), have worked out the

following recommendations for an implementation of Plan S in phase with our European colleagues.

General recommendations are first presented, then the 10 points of Plan S are addressed in order.

The Open Science Committee acknowledges the clarifications made to Plan S by the Implementation Guidance published on 27th of November 2018. These clarifications eliminate some ambiguities and are for most of them in line with the recommendations made by the Committee for the implementation of Plan S by the ANR. However, we are writing a short memorandum on "critical watchpoints" in response to the online consultation of cOAlition S, to highlight perhaps questionable points, or from our point of view, still unclear in the implementation guidance.

We recommend that, when Plan S is implemented by the ANR, a very instructional note be issued to explain to researchers the new publishing terms and conditions that are now compatible with ANR grants.

General recommendations for a roadmap

Establish the bibliodiversity / no-single-model principle. Open science must entail support for the diversity of business and publishing models. The principles set out in the Jussieu Call for Open Science and Bibliodiversity should ground the roadmaps to be developed by the signatory agencies of Plan S.

In this respect, point 1 of the Plan S implementation guidance provides that it is necessary not to favour any economic model or any method of implementing Open Access.

Respect the specificities of the disciplines. Particular attention should be paid to the practices and conditions of scientific communication, publication and evaluation of the different disciplines. In particular, dissimilarities between HSS and STM disciplines must be taken into account, including the evaluation of researchers. -Open archiving, without exclusive assignment of rights to publishers and without embargo, must be explicitly mentioned as one of the solutions to be promoted or even preferred (see point 8).

-Support for open access repositories (HAL, national and international, institutional and / or disciplinary archives) must be systematic, their interoperability being a compliance criterion.

Here again, the implementation guidance states in point 2 that depositing in an open access repository materialises compliance with Plan S.

Implement innovation support mechanisms for national and international infrastructures, university presses and editorial structures, content or referencing platforms, journals and services offering original editorial and business models that meet the criteria of exemplarity as defined by the Open Science Committee (CoSO) by the first half of 2019. In particular, the dynamic development of research journals operated or supported by public institutions towards open models should be encouraged (see points 2 and 3).

In this sense, the statements in point 4 of the Plan S implementation guidance are in line with CoSO's proposals.

Require the implementation of principles of transparency and measurement of the costs of open

access. This is a key point in the implementation and evolution of open access publishing mechanisms.

-The price of the various publication operations -peer review, editorial tasks, and disseminationshould be made public by the publisher of a journal or a book, to enable the assessment of the adequacy between the actual prices of the services provided and the costs of open access publication.

-When fees are paid as APCs or BPCs by authors or their institution to publish in open access, these must be identified and made public by institutions and / or research funding agencies in compliance with the Open APC initiative.

Points 3 and 9 of the Implementation Guidance provide that transparency of APC costs will be a criterion for compliance with Plan S.

Comments and recommendations on the 10 points of Plan S 1. Authors retain copyright of their publication with no restrictions. All publications must be published under an open license, preferably the Creative Commons Attribution Licence CC BY. In all cases, the license applied should fulfil the requirements defined by the Berlin Declaration. This is a crucial point because it entails that authors are free to deposit and distribute, whenever they wish (i.e. without taking any embargo into account) and wherever they wish, the final version of the manuscript accepted for publication (or the final published version if authorised by the publisher).

Ideally, the open-licensed distribution of the version of the author manuscript accepted for publication (at least) under a CC-BY type of licence is to be preferred (or CC-BY-SA if appropriate), ensuring that this is not restricted or prohibited by an assignment of rights in a publishing contract (see point 8).

Elements of implementation -Provide model addenda to amend contracts to allow an open archive deposit without embargo for the final version of the manuscript accepted for publication (or the published version if authorized the publisher).

The terms of these amendments will have to be taken up and integrated into the consortium negotiations with publishers at national level by the Couperin consortium and by cOAlition S at European level.

-Inform researchers and project leaders about open licenses and the implementation of amendments.

2.

The Funders will ensure jointly the establishment of robust criteria and requirements for the services that compliant high quality Open Access journals and Open Access platforms must provide.

Any publication resulting from research funded by the ANR and other agencies involved in cOAlition S must meet both following requirements at the same time:

-Quality peer reviewing and evaluation process;

-Open and permanent dissemination of its content.

These requirements can be met by publishing in a journal or a book, by depositing in an open access repository, by using a peer review platform, or by a combination of these, e.g. publishing in a subscription journal and depositing in HAL; publishing in an open access journal; peer review using a dedicated platform and publication in a journal.

To be able to choose these mechanisms, the researchers must be able to simply and easily check for the compliance criteria when choosing the journal/dissemination platform in which they wish to publish.

Elements of implementation

For open access journals

A first minimum level of compliance should be provided for, as well as a second level meeting higher opening requirements.

The minimum level of compliance includes the following elements:

-No requirement for exclusive assignment of rights by the author: this is in compliance with the provisions of Plan S point 1.

-Scientific quality of the journal. This element is not specific to open access journals, but it is fundamental and must be emphasised. The scientific quality of the journal is assessed at the same time by:

the quality of the proofreading and peer review process, the recognition of the journal in the discipline (regardless of its impact factor), its actual and substantial editorial added value (material and intellectual).

-Technical and editorial quality of the journal: this criterion can be verified, for example, by its indexing in the DOAJ (Directory of Open Access Journals). However, it is important to ensure not to discriminate new journals that have not yet been referenced in the DOAJ.

-When publishing charges are paid, the cost breakdown of these charges must be transparent (nature and quality of services -clear and accessible display -servicing scale).

The desirable level also includes the following criteria and services:

-Openness (total or partial) of the journal and peer review process ("Open Peer-review").

-Open access policy to the scientific data on which the article is based (link between the data and the publication).

For open access platforms Open access repositories

In the case of a deposit of the publication (last version of the author manuscript accepted for publication at least) in an open repository, the latter must provide the following services:

immediate and free access to the text without prior registration by the reader, a permanent and legal archiving of the content, a technical configuration enabling open and interoperable data and metadata.

In this respect, dissemination on social networks such as ResearchGate, Academia or MyScienceWork is not enough to be Plan S compliant.

Peer review platforms

A complete and high quality peer review can be carried out outside the framework of a scientific journal by a recognized and organized community based on an open access repository for publication. This distribution method is compliant with Plan S.

3.

In case such high quality Open Access journals or platforms do not yet exist, the Funders will, in a coordinated way, provide incentives to establish and support them when appropriate; support will also

4.

Where applicable, Open Access publication fees are covered by the Funders or universities, not by individual researchers; it is acknowledged that all scientists should be able to publish their work Open Access even if their institutions have limited means.

This provision contains the risk of drifting towards a model entirely based on fee-based publication, the fees of which will be difficult to control if not associated with services whose charges are justified by the publisher (see above: "transparency").

However, "virtuous" publication fees, which are transparent and related to the services provided, are acceptable because the editorial process and publishing have a cost.

The solution we uphold and which is advocated by the Jussieu Call is to encourage the collective financing of platforms and journals that do not require author payment.

When Open Access publication fees are applied, their funding is standardised and capped (across Europe).

There is probably a risk of prices being aligned on such capping. Consequently, price caps should be adjusted to the lowest level, while allowing sustainable models to be built with publishers.

Moreover, an average cap makes no sense in disciplinary terms and work out actual publishing costs as closely as possible to disciplinary and national specificities is required.

Capping, from our point of view, can therefore only be disciplinary and it seems preferable for the ANR to set the amount for each discipline in its national rules or delegate this charge to its disciplinary sections in the context of the examination of the applications.

Evaluators should be provided with evidence to assess the models and pricing of publishing services.

When capped fees -which we cannot conceive but low -have to be introduced, it will be necessary to take into account the case of open access journals that do not require publication fees and which are currently a majority in certain discipliness

Elements of implementation

-To avoid levelling up, we recommend that the money not spent on the "publications" budget line of a project, can be used on the other lines and not the other way around (principle of asymmetric fungibility).

-It appears essential to require the transparency of publishing prices in order to relate the amounts requested to the services actually provided.

-For transparency to make sense, it is necessary to construct a scale of actual rates for services by discipline comprising evaluation processes, material editing and publishing costs acceptable to all. This scale must be developed with public publishers, learned societies and other stakeholders in open access publishing. The CoSO is willing to contribute to this collective task.

-We recommend that the work on price transparency be coordinated at the level of the cOAlition S stakeholders and take the reality of the negotiations into account.

6. The Funders will ask universities, research organisations, and libraries to align their policies and strategies, notably to ensure transparency.

If this is an alignment being sought at a European level, care should be taken not to move towards a single open access developing model but to support bibliodiversity while respecting disciplinary specificities.

We also recommend to ensure:

shared principles of transparency as defined in the general recommendations and mainly in points 2 and 5;

common and harmonized principles for transferring subscription savings to support for editorial production;

convergence towards innovative principles and a harmonisation of research evaluation methods and practices in line with DORA's recommendations;

that consortia negotiations with publishers promote the implementation of Plan S for authors from the institutions or country concerned.

7.

The above principles shall apply to all types of scholarly publications, but it is understood that the timeline to achieve Open Access for monographs and books may exceed 1st of January 2020.

The date of 1st of January 2020 seems unrealistic considering the production times for monographs.

In addition, in the absence of a reasoned and quantified approach to book publishing practices, there is an urgent need for a study on the economy, publishing methods and costs of open access monograph publication, an outcome of which could be to consider setting a date and mechanisms for the open access dissemination of ANR-funded research monographs. Particular attention should be paid to innovative business models.

However, we believe that the case of collective works can already be equated to that of journal articles with the possibility of depositing chapters in open access repositories, on the model of articles.

Elements of implementation

-Carry out a study on the economy, publishing methods and open access publication costs of monographs, in line with the principles set out in the "General Recommendations" and point 5 for establishing cost transparency. This study should be conducted with public publishers, learned societies and other stakeholders in open access publishing.

-We also recommend that we approach university presses to consider, by relying on their experience, the implementation of mechanisms for publishing books natively in open access and in a capacity of using alternative business models as defined by Operas (white paper: Open Access Business Models), such as the Freemium model, collaborative library funding, etc.

-Carry out a pilot project on books.

8.

The importance of open archives and repositories for hosting research outputs is acknowledged because of their long-term archiving function and their potential for editorial innovation.

Open access repositories must become the venue of choice for the distribution of publications financed by the ANR and all members of cOAlition S.

In this sense, it is appropriate for the ANR to contractually require a systematic deposit or automatic transfer into HAL from another archive or any other source.

We therefore recommend, for articles accepted for publication that, at a minimum, the full text of the final version of the manuscript accepted for publication or, ideally, the "publisher" version (in the same way as in the virtuous examples of EDP Sciences and the American Physical Society) be made available in an open access repository immediately after publication (without embargo).

The importance of open repositories is far more significant than long-term archiving function of published articles. It is particularly meaningful as:

the fastest way to disseminate results, possibly before acceptance or publication;

the breeding ground for many innovative projects, for example, in the field of overlay journals (like Episciences, etc.), evaluation methods (Peer Community In, etc.) and others to come (see the Jussieu Call and its implementation) ;

the promise -through the implementation of interoperability -of a corpus that transcends catalogue boundaries and thus facilitates access to documentation for all and text mining;

an institutional showcase to bring together the scientific production of an institution in one single place;

a tool for researchers to store and distribute their own research output and build their reports.

9.

The "hybrid" publishing model does not comply with the above principles.

As a shift to open access, this very opaque model amounts to a double payment that cannot be acceptable.

However, a large number of journals using this business model are, for the time being, the flagship journals of entire scientific communities and have a high reputation, and therefore a strong appeal.

Without, at least, a rapid and radical evolution of research evaluation procedures, it is therefore unrealistic to think that this situation can be changed by 1st of January 2020.

Elements of implementation

It therefore seems more realistic in the immediate future to:

communicate and inform researchers about the existence and functioning of this hybrid economic model which most of them are not aware of;

remind researchers that open archiving and dissemination is fully compliant with Plan S and does not require any payment of publication fees for open access;

remind researchers that publishing in a hybrid journal without paying the open access publication fees, but depositing in an open archive repository is compliant with Plan S; but also to:

stress that these journals must be subject to the same level of quality requirements as the open access journals mentioned in point 2;

maintain the requirement for a reduction in subscription fees and / or publication charges for these journals, taking into account their double financing and in relation to the actual service provided (see "General recommendations": pricing transparency);

drive the publishers of hybrid journals to carry out, with specific deadlines, their shift into fully open access journals.

10. The Funders will monitor compliance and sanction non-compliance.

A monitoring system, with possible sanctions, can only support the implementation of Plan S. The monitoring arrangements and the form of these sanctions can only be determined by the funders themselves.

We recommend that the ANR verify when reviewing projects, and if necessary when evaluating them (through deliverables related to the publication of sources for example), that the publications are compliant and the corresponding costs acceptable.

Consideration could be given to opening the expected budget line earmarked for publications only after publication and subject to a verification of compliance.

The ANR could, as is the case for European funds, create a pooled fund to finance publications, which are often published after the term of a project.

The ANR and the other members of cOAlition S may also make global audits of the implementation of the plan, for example annually, and based on that take adjustment measures.

The most important thing at this stage seems to us to keep an explanatory attitude and provide support, to clearly tell researchers how they can comply with Plan S.

  Plan a phased deployment. While the 2020 objective can be realistically achieved by relying mainly on the dissemination potential of open access repositories, only the funding of innovative infrastructures, platforms and journals, under the control of the scientific community, will enable the necessary systemic change in conjunction with open access repositories. Assert and strengthen the essential role of open access repositories.

  be provided for Open Access infrastructures where necessary. Supporting open science infrastructures, platforms and journals financially is essential and we encourage the ANR as a cOAlition S funder to do so. It is up to the Commission to determine whether this support is provided at project level (mechanisms to be determined, see point 5), assumed by the funder on a specific line or by a combination of both. This support should be provided both to existing infrastructures, platforms or journals and to emerging mechanisms. Particular attention should be paid to supporting journal flipping, for journals operated by public institutions or a scientific community not yet open access but aiming to become so, regardless of their initial business model (subscription-based, subscription-based with a moving wall, hybrid, etc.) Elements of implementation Whether to promote the creation of new infrastructures, platforms and journals, to encourage their conversion to an open model or to support existing ones, the ANR, as a member of cOAlition S, should care to fulfil the exemplary criteria currently being prepared by the CoSO for the first half of 2019, and established along the following lines: control by the scientific community, guarantees of transparency in terms of governance, organisation, costs and management, promoting quality and sustainability, registration in a legal form protecting them from a possible takeover, the preferential use of free software, the compliance with open standards and norms for data and metadata and the provision of open APIs.

Recommendations for Plan S implementation by the ANR A Contribution of the French Open Science Committee Project Group "Building Bibliodiversity" to Plan S, the ANR undertakes to ensure that scientific


of documents and productions developed within the entire research cycle and their modes of open dissemination

Many stakeholders agree that the implementation of open science requires all research processes and activities to be taken into account in evaluation. This concerns both publications and research projects, individuals, collectives or institutions, and their interactions with society. The National Open Science Plan emphasizes in particular the importance of taking into account a qualitative rather than a quantitative approach and cites the Leiden manifesto (Hicks & al., 2015) and the San Francisco Declaration on Research Evaluation (DORA, 2012). Even if similar approaches existed before the open science movement, it aims to make them visible, legitimate and sustainable.

These recommendations of the Open Science Committee are based on those of the Open Science Platform Policy (OSPP), which unites the efforts and strategies of the European Union around 8 priority pillars, two of which are specific to evaluation issues: encouraging open practices and their recognition; and the deployment of a new generation of research indicators. In these recommendations, intended for all institutions piloting evaluation systems, we have identified two essential dimensions that are at the heart of open science.

1/ The Open Science Committee encourages the promotion of good practices in open science, in particular those that push for the availability of all types of documents and more broadly of research products, including but not limited to data and publications. Indeed, this availability will facilitate, on the one hand, the full recognition of the diversity of individual and collective contributions to research and, on the other hand, will encourage people to discover the content of these productions and to evaluate their quality. 2/ The Open Science Committee supports the recognition of the full range of research activities, in the plurality of disciplinary fields, as opposed to mechanisms that would focus only on scientific publications, or even on a limited part of them or on resulting indicators (McKiernan & al., 2019).

Similarly, the recognition of new configurations of multidisciplinary forms of knowledge, and in particular of citizen science, must be strengthened.

To do this, the following are distinguished:

.

In compliance with the legal and regulatory principles governing the opening and dissemination of research results and data, we recommend the acknowledged principle of "as open as possible, as closed as necessary" be followed.

The documents and productions referred to in Table 1 may be considered fully open only if they are made public under the conditions of a so-called free or open licence. Indeed, an object distributed without a licence can certainly be consulted by third parties, which constitutes a minimum form of opening, but its methods of reuse are not known. The final choice of licence is made by those who make their production public, in accordance with Decree No. 2017-638 (Prime Minister, 2017).

Beyond a licence, the systems for publishing, hosting and archiving documents and productions are intended to comply with the exemplary criteria defined by the Open Science Committee (2019).

The Open Science Committee recommends that all bodies and institutions involved in evaluation activities discuss, disseminate and adapt these good practices to their context. This includes, in particular, research funding agencies, publication and evaluation bodies. Communities and institutions remain sovereign in their adaptation of these recommendations, particularly in the aggregation, qualification and prioritization of types of productions and documents. The Open Science Committee expects transparency from these operations, including the justification of the reasons why certain productions or activities would not be taken into account in their evaluations.

open-science-fund/ European Commission (2017/or recognition for researchers practicing Open Science, 2017. research/openscience/pdf/ospp_rewards_wg03112017.pdf Research Assessment. Available at: https://sfdora.org/read/ HCERES, Guides des produits de la recherche, 2019. Available at: https://www.hceres.fr/fr/guides-des-produits-de-la-recherche-et-activites-de-recherche-0

Data

Any type of data produced in the course of research, defined as factual records that are used as primary sources for scientific research and are generally recognized by the scientific community as necessary to validate research results.


Nicolas Huloux

Jocelyn Monnoyer
email: jocelyn.monnoyer@etu.univ-amu.frmarc.boyron

Marc Boyron

Michaël Wiertlewski
email: michael.wiertlewski@univ-amu.fr

Overcoming the Variability of Fingertip Friction with Surface-Haptic Force-Feedback

Keywords: Surface Haptics, Feedback control, Squeeze film, Ultrasonic friction modulation

come

Introduction

Surface-haptic promises to restore the tangibility of virtual interfaces while users are interacting with flat and featureless touchscreens. Virtual bumps, texture, clicks and scrolling effects produce sensations that guide the users' motion and provide them with feedback about their actions. One of the most promising approaches developed to date for generating rich perceptual sensations consists in modulating the friction between the user's finger and the plate via either electrostatic adhesion [START_REF] Shultz | The application of tactile, audible, and ultrasonic forces to human fingertips using broadband electroadhesion[END_REF] or ultrasonic squeeze-film levitation [START_REF] Biet | Squeeze film effect for the design of an ultrasonic tactile plate[END_REF][START_REF] Winfield | T-pad: Tactile pattern display through variable friction reduction[END_REF]. The main advantage of these methods over vibrotactile stimulation is that they produce stimuli which not only include transient events -i.e. texture, impacts or vibrations-but affects continuous forces exerted by a sliding fingertip. Modulating these forces makes it possible to induce of tactile illusions of shape and large features, such as bumps and holes, similar to what could be achieved with robotic force feedback devices [START_REF] Minsky | Feeling and seeing: issues in force display[END_REF][START_REF] Robles-De-La-Torre | Force can overcome object geometry in the perception of shape through active touch[END_REF].

In spite of their advantages, friction modulation methods tend to produce signals that are distorted and attenuated [START_REF] Sednaoui | Experimental evaluation of friction reduction in ultrasonic devices[END_REF][START_REF] Monnoyer | Optimal skin impedance promotes perception of ultrasonic switches[END_REF]. The main factor responsible for the signal variance is the complexity of the frictional behavior of a finger sliding over a glass plate. The angle of the finger, the pressure force, and the moisture of the skin all affect the friction force [START_REF] Tomlinson | The effect of normal force and roughness on friction in human finger contact[END_REF]. Worst still, even if the exploration conditions are accounted for, sweating and the subsequent softening of the stratum corneum induce large changes in the friction with time [START_REF] Pasumarty | Friction of the human finger pad: influence of moisture, occlusion and velocity[END_REF].

In addition the fact that they both rely on friction to produce forces to the fingertip, each of the above two technologies has its own drawbacks. Despite the advantages of electro-adhesion methods, such as their large functional bandwidth and their compatibility with strong forces, the occurrence of non-linearities results in the distortion of the signal [START_REF] Shultz | The application of tactile, audible, and ultrasonic forces to human fingertips using broadband electroadhesion[END_REF]. Squeeze-film levitation methods based on ultrasonic standing waves make it possible to modulate the friction force in a wider dynamic range, but have disadvantages such as poor frequency responses [START_REF] Meyer | Dynamics of ultrasonic and electrostatic friction modulation for rendering texture on haptic surfaces[END_REF] and large static non-linearities [START_REF] Wiertlewski | Partial squeeze film levitation modulates fingertip friction[END_REF]. It has been proposed to combine both methods in order to increase the dynamic range of possible signals [START_REF] Vezzoli | Physical and perceptual independence of ultrasonic vibration and electrovibration for friction modulation[END_REF][START_REF] Smith | Hue: A hybrid ultrasonic and electrostatic variable friction touchscreen[END_REF], but this does not solve the variability inherent to friction force modulation processes.

Feedforward filters can be used to counteract the attenuation inherent to ultrasonic friction modulation and provide users with fast and clear signals. Dynamics is improved by temporarily overdriving the actuators in order to quickly reach a given friction force [START_REF] Wiertlewski | A highfidelity surface-haptic device for texture rendering on bare finger[END_REF]. Distortions due to static non-linearities can also be corrected using a look-up table [START_REF] Vezzoli | Texture rendering strategies with a high fidelity-capacitive visual-haptic friction control device[END_REF]. Although these feedforward model-based approaches give better performances than those of open-loop systems, the inherent variability prevents to precisely regulate a given force. Ben Messaoud et al. recently proposed to tackle these drawbacks by introducing real-time measurements of the friction force that are fed to a closed-loop feedback control able to reject unknown disturbances [START_REF] Messaoud | High fidelity closed loop controlled friction in smarttac tactile stimulator[END_REF]. The control strategy relies on a sliding mode strategy to robustly remove static error and improve the dynamic response.

However, like any closed-loop control system, noise from the force sensor was re-injected into the actuators, which resulted in noisy stimulus. In order to overcome the limitations of this approach, we developed a custom built capacitive sensor which is able to resolve the force with a dynamic range ratio of more than 1:50,000. This sensor is used as the backbone of a closed-loop force feedback scheme which is able to produce low noise signal, yet accurately control of the friction force by means of a state-of-the-art proportional-integral controller.

Simulations

Linearized dynamic model of ultrasonic friction modulation

In order to account for the stochastic nature of ultrasonic friction modulation and design the most appropriate controller, the relationship between the amplitude set on the plate and the frictional force that is actually produced has to be modeled. We recorded the force and amplitude data of 26 participants while they were sliding there finger on a glass plate excited with a 30kHz ultrasonic carrier of 2.5µm amplitude modulated by a 1Hz sine wave. Participants were free to explore the surface which resulted in a dataset that contained various exploration speed, forces and finger posture. Datapoints showing force as a function of the vibration amplitude are presented in Fig. 1a along with statistics across trials on binned data. Although the individual trials often showed the existence of a clearly negative correlation between the vibration amplitude and the friction force, the relationships observed from trial to trial were not consistent. To account for this behavior in simulations, the non-linear Gaussian model described in [START_REF] Wiertlewski | Partial squeeze film levitation modulates fingertip friction[END_REF] was extended by adding an unknown perturbation d, which was bounded so as to prevent the occurrence of negative friction force values. The modified friction force model was therefore:

f = f 0 exp - a 2 2τ 2 + d = g(a) + d (1)
where f 0 = 0.8N is the nominal friction force, a is the vibration amplitude, and τ = 1.32µm is the susceptibility to ultrasonic levitation. The deterministic behavior is denoted by the function g(a), and d is taken to be a 1Hz sine wave with an amplitude of 0.8N and an offset of 0.4N.

Control strategy

The block diagram of the control strategy implemented in this study is shown in Fig. 1b. The friction modulation process, in which the amplitude is the input, delivers a friction force. This process is modeled by means of a linear timeinvariant function serving as a first order low pass filter with a cutoff frequency of 100Hz, in line with the attenuation reported in [START_REF] Meyer | Dynamics of ultrasonic and electrostatic friction modulation for rendering texture on haptic surfaces[END_REF]. The filter is followed by the non-linear relationship described by equation 1 to complete the model for the friction modulation process.

A lookup table is implemented in the controller to compensate for the nonlinearity of the ultrasonic friction modulation captured by equation 1. The lookup table and the friction modulation process combines into a single linear time-invariant transfer function called P (s), to which a saturation function is added, where the amplitude of the ultrasonic wave is bounded between 0 and 2.5µm. The controller C(s) therefore has to compensate for the corrected process P (s), which acts as a first-order low-pass filter, and for a slowly evolving unknown disturbance, d.

A force sensor measures the friction force applied to the finger along with some undesirable noise n. This parameter typically fits a zero-mean random Gaussian process. The dynamics of the sensor are modeled by means of the transfer function S(s), which, to simplify the simulation, is taken to be a low-pass filter with a cutoff frequency of 1kHz. The force measured f m is then subtracted from the set-point force f r in order to compute the error e, which is subsequently fed to the controller.

Precision and accuracy trade-off

The ideal control scheme would have a fast, effective means of disturbance rejection without having to add any noise to the friction force measured by the system. In practice, these two objectives impose opposite constraints on the controller. In the current implementation, a proportional-integral controller was adopted so that C(s) = K p + K i /s. A larger set of gains K p and K i ensures fast convergence, but results in the amplification of the noise introduced by the sensor. This noise is detrimental to the user's tactile experience because it adds fluctuations that are not part of the original signal. This trade-off between the convergence to a specific value (the accuracy) and the noise injected (the precision) in the closed loop has to be evaluated, giving, the maximum noise that the sensor can generate.

From the block diagram described in Fig. 1, taking only the linear behavior into account, the expression for the force perceived by the finger in the Laplace domain can be expressed as follows:

f a = d 1 1 + G + f r CP 1 + G -n G 1 + G (2)
where G(s) = C(s)P (s)S(s). To study the effects of the noise n on the variance of the friction force σ 2 f , the variance of the setpoint and the disturbance can be taken to be null in the steady state. Error propagation analysis therefore gives the following equation:

σ 2 f = G 1 + G 2 σ 2 n (3)
This equation shows that the noise perceived by the user, σ 2 f is affected by the gain of the controller. A lower gain attenuates the noise originating from the sensors. However, since the disturbance is affected by the sensitivity function 1 1+G , which favors high gains, an optimum can be found. Fig. 2a shows the results of a set of simulations, where the controller's parameters K p and K i were varied and the signal fidelity of the control was measured. The fidelity of the system is reflected in the goodness-of-fit R 2 between the input and the output of the simulation in comparison with an ideal transfer function in which f a = f r . Both the disturbance rejection and the noise attenuation have a positive impact on the fidelity. In this simulation, the setpoint signal was a logarithmically-swept sinusoid ranging from 1Hz to 100Hz with an amplitude of 1.2N, the disturbance was a 1Hz sine wave with an amplitude of 0.4N and the noise was a white Gaussian noise with an amplitude of 10 -5 N. As shown by the graph in Fig. 2, proportional gains K p ranging between 5 and 20 provide an efficient disturbance rejection while preventing any significant feedback noise. The integral term K i is responsible for rejecting the static error, but as long as it is above the value of 2, its tuning has fairly little influence. The input-output relationship with three different proportional gains K p = [40, 10, 1] is shown in Fig. 2b,c andd, respectively. A high gain, as in Fig. 2b, results in a noisy and saturated command, whereas a low gain, as in Fig. 2d, results in a low precision controller. The optimum efficiency was found to occur at values of K p = 10 and K i = 2, as shown in Fig. 2c.

The influence of the noise on the fidelity and the saturation of the actuation is shown in Fig. 2e,f. The saturation index was calculated from the ratio between the number of data points having a saturated input and the total number of data points. Noise levels above 10 -4 N in the force sensor will significantly decrease the precision and increase the likelihood of saturating the output. We therefore adopted a proportional gain of K p = 10.

Based on the results of the simulation, it is clear that the effectiveness of the controller depends on the noise added by the sensor, and that low noise results in a sharp, untainted tactile stimulation. It has been estimated that the lowest force perceivable by the human somatosensory system is in the 5.10 -4 N range [START_REF] Wiertlewski | A highfidelity surface-haptic device for texture rendering on bare finger[END_REF]. The output noise produced by a sensor with a noise level of 5.10 -5 N remains subliminal.

Friction force sensor design

The output of the simulation reveals that the force sensor is the centerpiece of the control strategy, and proper care must be taken in the choice of technology and design to achieve high-fidelity tactile renderings on surface-haptic devices.

Performance requirements

To ensure that the force perceived by the user will remain untainted by the noise, while being able to support forces as high as 2.5N, the sensor noise must be less than 5.10 -5 N. These requirements lead to a dynamic range figure of 1:50,000 or about 95 dB. The stiffness and frequency bandwidth are also crucial factors, since they directly affect the sensor's response via the transfer function S(s). A softer sensor might have a high signal to noise ratio, but this will be at the expense of a low frequency bandwidth. The glass plate typically weights approximately m p = 400g, and the sensor's first resonance frequency is prescribed by f 0 = 1/2π k/m p = 250Hz in the case of a sensor stiffness of 1N.µm. This means that in order to resolve 2N, the sensor will be displaced by 2µm. Since force sensors are based on the measured displacement of a known elastic structure, the underlying displacement sensor should have a minimum noise of 0.5pm.

The sensitivity requirements rule out the use of metal and semiconductor strain-gauge sensors, which usually have signal-to-noise ratios of the order of 1:1,000 in rigid load-cells. Piezoelectric sensors, while having an exceptionally high signal-to-noise ratios and high stiffness, are not suitable for closed-loop feedback applications because of the low-frequency drift, which would require frequent resetting of the control loop. All these constraints are within the range of capacitive sensors, which are capable of a dynamic range within the present requirements while remaining impervious to drift [START_REF] Lotters | A sensitive differential capacitance to voltage converter for sensor applications[END_REF].

Implementation

The capacitive force sensor is based on the architecture illustrated in Fig. 3. The ultrasonic plate is fixed to a frame that is suspended by four curved leaf springs grounded to the rest of the device. The curved leaf spring have a relatively high Fig. 3: Mechanical and electrical schematics of the capacitive force sensor, illustrated on one axis mobility in the plane of the surface, with a lateral stiffness of 6.10 6 N/m, while maintain rigidity three order of magnitude higher along the normal axis which keep it stiff. This structure constrains the motion of the inner frame to two translations collinear with the friction force. The deformation of the leaf spring is determined by four non-contact sensors, each of which measures the distance of each edge of the suspended frame to the grounded frame. The measurements are differential in each dimension, with one sensor experiencing a reduction in the gap, while the one on the opposite side detects an increase. This differential amplification method counteracts the thermal effects and unwanted electromagnetic perturbations. Each sensor is composed of one rectangular active electrode, surrounded by a single trace connected to ground that shields the active electrode from external perturbation. These electrodes are made out of 75µm-thick copper coated using an electroless nickel immersion gold process which provides a smooth finish and immunity to corrosion (pcb-pool, Aarbergen, Germany).

The copper trace is mounted on a FR4 substrate for a total active area of 40 × 4.5mm 2 . An intermediate part maintains these electrodes parallel to the inner frame, which is conductive and excited by a 120 kHz sinusoidal signal V , polarizing the sensing electrode. Charges q ± are proportional to the voltage V and inversely proportional to the distance e ± such that q ± = V 0 A/e ± , where 0 is the permittivity of the air and A is the active area of the sensors.

The capacitive measurement circuit is adapted from the low-noise topology described in [START_REF] Lotters | A sensitive differential capacitance to voltage converter for sensor applications[END_REF]. The charges from the sensing electrodes are transformed into a voltage via a charge amplifier and the signal is then passed through a selective filter with a center frequency matching the excitation of 120 kHz. After the filter, there is no artifacts left in the signal. At that stage, the envelope of the signal, which depends on the distance between the electrode and the outer frame, is recovered using synchronous demodulation followed by a low pass filter with 1 kHz cutoff frequency. The last operation takes the difference between the signal coming from two electrodes to recover the signed voltage V s that reflects the displacement of the inner frame and therefore the force that is applied by the fingertip.

Sensor characterization

The sensor was calibrated under quasi-static conditions, using a set of standard weights which applied to the frame a known gravitational force via a string and pulley system. The results are presented in Fig. 4a, and the linear regression showed a goodness of fit of R 2 = 0.95. The frequency bandwidth measurements were performed using an impact hammer. The signal was then normalized and converted into the maximum sensing value. The frequency response showed the occurrence of an initial resonance at 145 Hz owing to the large glass plate. The noise of the sensor studied using a 10 s sample without any external perturbations show the existence of a floor at 5.10 -5 N in the low frequency and an attenuation above 300 Hz, due to a first order filter. The frequency response and noise spectrum are shown in Fig. 4b. The dynamic range of the sensor before its first resonance is 1:50,000, or about 93dB. A picture of the final device is shown in Fig. 4c. A glass plate measuring 255× 140 ×3 mm 3 vibrating at a frequency of 46kHz in a 16×0 mode provided the friction modulation. Four piezoelectric actuators are bonded to the plate and provide a maximum displacement of 2.5µm. The plate is clamped to the inner frame at the end of 3 nodal lines on each side. The inner and outer frame and curved leaf springs were milled out of a block of 2024-grade aluminum alloy.

In addition to the lateral force sensor, the system is equipped with an optical position sensor based on [START_REF] Wiertlewski | A highfidelity surface-haptic device for texture rendering on bare finger[END_REF], not used in this study, and 4 normal force sensors at each corner to measure the user's finger normal force. Finger position and normal force sensors can drive the production of complex virtual environments. The realtime control is ensured by a low-latency 5 kHz timer implemented on a microcontroller (Teensy 3.1, PRJC, Portland, or, USA). Every cycle the sensors values are acquired, the PI control and the look-up table are computed and the amplitude of the ultrasonic carrier is modulated. The amplitude modulated ultrasonic carrier is then filtered with a 10 to 50 kHz bandpass filter and amplified before being sent to the piezoelectric actuators. The controller parameters were set at K p = 10 and K i = 2 in accordance with section 2.3.

Closed-loop performance

Comparisons were made between the open-loop and closed-loop modes of operation of the friction modulation using a commanded force f r matching a 1 Hz sine wave between 0.3 and 0.9 N, which corresponds to a range of reliably achievable forces. This friction range was implemented using three 1 Hz sine waves 0.1 N in amplitude with a mean value of [0.4 0.6 0.8] N as the setpoint to minimize the variations in the friction force. One participant slid his finger along the interface at 20mm.s -1 5 times with each force signal and 50 periods of the modulation were recorded. As shown in Fig. 5a, the open-loop response poorly matched the commanded force since the linear regression showed a goodness of fit of only R 2 = 0.22. The closed loop condition gave satisfactory tracking of the setpoint, since the linear regression yielded a goodness of fit of R 2 = 0.98. Low friction values caused most of the errors, partly because the large amplitudes required could not be reached with the present ultrasonic plate. The step responses recorded are shown in Fig. 5b. The frictional force was tracked with a response time of 37 ± 20ms (SD) with a decreasing friction step and 32 ± 16 ms with an increasing friction step, with an exploration speed of approximately 20mm.s -1 in both cases. The results shown in this figure can be improved with larger gains in the closed loop at the expense of greater noise. The traces in the figure show that the steady state friction was not as noise-free as one might have expected in view of the signal to noise ratio of the sensor. It seems likely that the additional fluctuations may have been due to friction noise and tremors of the user's hand. Fig. 5c gives a close-up view of the control strategy in a single step. In order to achieve fast responses, the controller overshoots in amplitude before converging back to a lower steady-state value to stabilize the output.

Discussion

The disturbance rejection of the closed-loop force feedback is on a par with the results obtained by [START_REF] Messaoud | High fidelity closed loop controlled friction in smarttac tactile stimulator[END_REF], who used a strain gauge sensor and a sliding mode controller. The steady state friction is kept within 20mN of the set point, which is a substantial improvement over the 0.25 N variations which occur when ultrasonic friction modulation is operating in the open-loop mode. The custom-built force sensor has a noise floor of less than 10 -4 N, and the gain of the controller is set so as to limit the noise fed back to the actuator, which results in the production of tactile stimuli that are sharp and devoid of artifacts. Yet the traces in Fig. 5b show that the actual friction force undergoes fluctuations in the order of 20mN. These fluctuations are also present in open loop control [START_REF] Meyer | Dynamics of ultrasonic and electrostatic friction modulation for rendering texture on haptic surfaces[END_REF] and in closed loop control that uses strain-gauge load cells [START_REF] Messaoud | High fidelity closed loop controlled friction in smarttac tactile stimulator[END_REF]. The remaining fluctuations are probably due to physiological and frictional noise [START_REF] Wiertlewski | On the 1/f noise and non-integer harmonic decay of the interaction of a finger sliding on flat and sinusoidal surfaces[END_REF], which the feedback system struggles to cancel. A frequency synthesis of the control taking into account the friction noise might help to develop a higher order controller with which these fluctuations could be ironed out.

It is worth noting that the present simulation of the entire system revealed that in order to obtain a specific noise level in the output force, the sensor noise has to be one order of magnitude lower. This particular ratio, which was not predicted by the noise sensitivity function based on linear control theory, may have resulted from the non linearity present in the real-life system. The response time is also reduced from 50ms to 20ms with respect to open-loop condition, at slow sliding speeds. These results could be further improved by using a more powerful actuator, since the current dynamics were limited only by the maximum amplitude that the plate can reach.

Conclusion

In this paper, we report on the simulation and implementation of a surfacehaptic device which can be used to deliver precise and accurate stimuli to a user's sliding finger in the dynamic range of 0.3 to 0.9N. In particular, it was established that the combination of a force sensor with a wide dynamic range (1:50,000) and a PI controller sufficed to considerably reduce the variability of the stimuli produced by the ultrasonic friction modulation.

The performances of the current device were limited by the power of the ultrasonic plate, and ongoing engineering studies are now focusing on building a more powerful actuator which is compatible with fast, large changes in the amplitude. Thanks to the online regulation of the friction force, frictional stimuli with great precision and accuracy are delivered, and this interface therefore provides a valuable tool for use in psychophysical experiments requiring finely controlled stimuli [START_REF] Bernard | Harmonious textures: The perceptual dimensions of synthetic sinusoidal gratings[END_REF]. The improved sharpness and fidelity are also key assets for simulating unambiguous virtual environments on user interfaces.

Fig. 1 :

 1 Fig. 1: a Friction force and amplitude data points recorded in 26 sliding trials are shown in gray. Means and standard deviations are presented in black. b. Block diagram of the control scheme.

Fig. 2 :

 2 Fig. 2: a. The fidelity of the control given by the R 2 parameter in comparison with f a = f r in the case of a set of K i and K p coefficients. b. K i = 2 and K p = 40. Over-corrected signal results in noisy signals c. K i = 2 and K p = 10. The right balance between a high gain and fast convergence. d. K i = 2 and K p = 1. Low gains give unsatisfactory disturbance rejection. e. Effects of greater sensor noise on the precision of the control. f. Greater noise results in significant saturation of the controller.

Fig. 4 :

 4 Fig. 4: a. Linearity of the lateral force sensor b. Frequency spectrum of the noise and impulse response c. Picture of the device

Fig. 5 :

 5 Fig. 5: Friction force feedback control. a. The closed loop limits the variability and compensate for the static non-linearity b The system response to successive unit steps shows a response time of around 30ms. During a unit step, the commanded amplitude of the ultrasonic waves overshoots.

Acknowledgments

The authors would like to thank Julien Diperi for his assistance with the mechanical engineering and Stéphane Viollet for his thoughtful comments on the design of the controller. This research was part of the French Research Agency project iota (ANR-16-CE33-0002), with some additional support from the Openlab PSA-AMU "Automotive Motion Lab" project.