Olivier Chocron
email: chocron@enib.fr

Emmanuel Delaleau
email: delaleau@enib.fr

Trajectory-Based Synthesis of Propulsion Systems for Fixed-Thrusters AUVs

Keywords: AUV, Propulsion synthesis, Fixed-Thruster

This paper presents a synthesis method for the Autonomous Underwater Vehicle (AUV) propulsion system based on the features of a trajectory to follow. This method is based on solid/fluid dynamics analysis of a AUV performing the trajectory following task and gives actuation requirements to achieve it properly. This actuation is then used to generate a propulsion system under the form of a Thrust Configuration Matrix (TCM) that is compatible with the desired trajectory. From this matrix the number of thrusters, their position and direction can then be extracted to synthesis a fitted propulsion system. Thus, the propulsion capabilities of the robot will match the trajectory characteristics and it will be able to follow it. The objective of this work is to provide an evaluation as well as a design method to produce a controllable AUV for a given task. A second use of such an analysis is to provide an evaluation process allowing to perform AUV task-based design. The method is developed for generic fixed-thrusters AUVs on any trajectory, and is applied to an existing 4-thrusters AUV for a seabed scanning flat trajectory. The method shows why the initial AUV propulsion does not match the task and what is the solution solving the issue.

Introduction

Thanks to its autonomy, nowadays Underwater Autonomous Vehicles (AUVs) can perform long range surveillance missions and deep sea operations. This kind of vehicles are also used in off-shore installations, to map the sea floor, to secure harbors, to search for sea-mines, among other activities [START_REF] Antonelli | Underwater Robots: Motion and Force Control of Vehicle-Manipulator Systems[END_REF]. With enough effort in research, AUVs capabilities will be expanded allowing them to perform even more demanding tasks than the ones accomplished nowadays. Precisely, most AUVs are designed for specific applications as shown in [START_REF] Palmer | Analysis of the propulsion and manoeuvring characteristics of survey-style auvs and the development of a multi-purpose auv[END_REF]. In a long range type AUV, attention is focused on reducing drag forces and increasing thruster efficiency in order to reduce energy consumption and provide the AUV with more autonomy. Usually under-actuated, this type of vehicles are commonly propelled by a rear thruster and steered by control surfaces. The hull has a hydrodynamic design in order to reduce drag forces and to improve the autonomy capabilities of the AUV. Fully maneuverable AUVs have at least as many thrusters as their operational space has degrees of freedom (dof), usually six. Their design is focused on improving agility and multi-directional response (holonomy), which makes these vehicles energy-intensive ( [START_REF] Schultz | Autonomous underwater vehicle (AUV) propulsion system analysis and optimization[END_REF]).

Regarding the propulsion system, several architectures have been proposed. We can divide them into three categories: classical rear propeller and control surface architectures [START_REF] Vestgard | The HUGIN 3000 survey AUV[END_REF], bio-mimetic propulsion [START_REF] Yu | Construction and control of biomimetic robotic dolphin[END_REF][START_REF] Zhou | Design and locomotion control of a biomimetic underwater vehicle with fin propulsion[END_REF][START_REF] Khalil | Dynamic modeling and simulation of a 3-d serial eel-like robot[END_REF] and vector thrust [START_REF] Chocron | Reconfigurable magnetic-coupling thrusters for agile auvs[END_REF]. The latter can be achieved by different means: Some authors use a steerable rear propeller for propulsion, maneuvering and control [START_REF] Cavallo | Conceptual design of an auv equipped with a three degrees of freedom vectored thruster[END_REF][START_REF] Shea | Design and testing of the marport SQX-500 twin-pod auv[END_REF][START_REF] Sheng | Research on variable direction rotatable axis variable vector propeller of auv in vertical motion[END_REF][START_REF] Page | Hydrodynamics of an autonomous underwater vehicle equipped with a vectored thruster[END_REF][START_REF] Kopman | Masuv-1: A miniature underwater vehicle with multidirectional thrust vectoring for safe animal interactions[END_REF][START_REF] Nawrot | Conceptual design of a thrust-vectoring tailcone for underwater robotics[END_REF]. Some others make collective and cyclic pitch propellers by using a swash plate [START_REF] Nagashima | Development of a compact hybrid underwater vehicle using variable vector propeller[END_REF] or by using shape-memory alloys [START_REF] Wasylyszyn | Active control of underwater propulsor using shape memory alloys[END_REF]. Vector propulsion can be achieved by adding forces from fixed thrusters [START_REF] Steenson | Control of an AUV from thruster actuated hover to control surface actuated flight[END_REF][START_REF] Nh | Development of a new underwater disk robot[END_REF] or using re-orientable (or reconfigurable) propellers [START_REF] Lin | Development of a spherical underwater robot equipped with multiple vectored water-jet-based thrusters[END_REF][START_REF] Chocron | A validated feasibility prototype for auv reconfigurable magnetic coupling thruster[END_REF][START_REF] Vega | A flat design and validated model for auv reconfigurable magnetic coupling thruster[END_REF]. Using vector thrusters, it has been shown that a dynamic reconfiguration of the propulsion would allow to adapt the AUV to different in-mission tasks specifications [START_REF] Chocron | Evolutionary dynamic reconfiguration of auvs for underwater maintenance[END_REF]. Meanwhile, finding the fitted propulsion configuration for a given task remain a challenge.

This work is focused on the analysis and synthesis of fixed-thrusters propulsion architectures for AUV in order to achieve a trajectory following task. It provides an analysis of the needed wrench to move the robot throughout all the trajectory following and gives the method to build a compatible thrust configuration matrix (TCM). This fundamental matrix is then used to deduce the propulsion system configuration. In this way, we obtain the minimum number of thrusters, their possible positions and their orientation. In this way we obtain controllable AUVs for the trajectory. The method is first applied to analyze an ill-conditioned AUV that cannot achieve the task, then to analyze and design modified AUVs fit for the task.

Dynamic Model of AUV and Propulsion System

AUV Kinematic and Dynamic Models

To model the AUV, two orthogonal coordinate systems are used: R 0 (O 0 , x 0 y 0 , z 0 ) is the earth-fixed frame and R b (O b , x b , y b , z b ) is AUV body-fixed. In Fig 1 the AUV is depicted in its body-fixed coordinate system.

The vectors describing the motion of the AUV in 6 dof are:

η = (x y z φ θ ψ) t , ν = (u v w p q r) t
Here η is the vector of position and orientation in R 0 . The orientation in η is defined using an Euler ZY X (ψ, θ, φ) convention as described in [START_REF] Fossen | Guidance and Control of Ocean Vehicles[END_REF]. ν is the linear and angular absolute velocity vector in R b .

To change the AUV velocity vector from one to another coordinate system we use a velocity transformation matrix as given in [START_REF] Fossen | Guidance and Control of Ocean Vehicles[END_REF]:

Using this transformation matrix J(η), we can obtain the AUV absolute velocity vector expressed in R 0 from its expression in R b , see [START_REF] Fossen | Guidance and Control of Ocean Vehicles[END_REF]:

η = dη dt R0 = J(η) ν (1)
The nonlinear rigid-body dynamic equations of the underwater robot, can be formulated as in [START_REF] Antonelli | Underwater Robots: Motion and Force Control of Vehicle-Manipulator Systems[END_REF] and [START_REF] Fossen | Guidance and Control of Ocean Vehicles[END_REF]:

M ν + C ν + D ν + G = τ (2)
where M ∈ R 6×6 , C ∈ R 6×6 and D ∈ R 6×6 are the matrices of mass, Coriolis and centripetal terms, and damping respectively, including added mass terms. G is the vector of gravitational forces and moments. Lastly, τ is the wrench of external forces and moments (6 dof), it accounts for the propulsive wrench generated by the thrusters. τ is calculated as follows:

τ = B u ( 3
)
where B is the thrust control matrix (TCM), which depends on the propulsion system configuration (number, position and orientation of the actuators). u is the vector of the thrusters forces, i.e., the thrusts. Added mass effects, drag forces and gravitational/buoyancy efforts are included in matrices of Eqn. (2) as described in Fossen [START_REF] Fossen | Guidance and Control of Ocean Vehicles[END_REF] and Antonelli [START_REF] Antonelli | Underwater Robots: Motion and Force Control of Vehicle-Manipulator Systems[END_REF] and in our previous works [START_REF] Vega | Evaluation of auv fixed and vectorial propulsion systems with dynamic simulation and non-linear control[END_REF].

Fixed Propulsion System Model

AUVs with this type of propulsion are powered by several thrusters placed along different axis in order to combine their thrust and provide six (or less) actuated dof to the robot. This architecture uses a multi-directional propulsion based on force combination. Fig. 1 shows the "RSM" AUV which includes 4 thrusters: 2 horizontal thrusters are placed at the rear of the AUV and two vertical ones are centered symmetrically on the sides. This configuration provides actuation over the following DOF: Surge (linear motion along the x b -axis), Heave (linear motion along the z b -axis), Yaw (rotation along the y b -axis) and Roll (rotation along the x b -axis). Due to this configuration, 2 dof are not actuated: Pitch (rotation along y b -axis) and Sway (linear motion along y b -axis). Roll and Pitch motions are mechanically stabilized thanks to the relative position of the buoyancy and gravity centers. Conversely to the Pitch, the Sway is not controlled and not stabilized, thus free motion on y b -axis (drift) is expected to occur.

The thrusters position vectors with regard to R b are: The RSM AUV thrust control matrix can then be calculated as:

P 1 =   P 1x P 1y P 1z   , P 2 =   P 2x P 2y P 2z   , P 3 =   P 3x P 3y P 3z
B =         1 1 0 0 0 0 0 0 0 0 1 1 0 0 P 3y P 4y P 1z P 2z -P 3x -P 4x -P 1y -P 2y 0 0         with u =     u 1 u 2 u 3 u 4     (4)
where u 1 , u 2 , u 3 and u 4 are the thruster propulsion forces. If the AUV is controllable then B is invertible. In this case, it is possible to express the thrust control vector as: u = B -1 τ , where τ is the required propulsive wrench.

Model-Based Anticipation

To analyze the propulsion system w.r.t. the trajectory, we need to estimate the efforts needed to move the AUV during the trajectory tracking. We use the kinematic and dynamic models to compute the needed (or anticipation) torque τ a in order to follow the desired trajectory η d . The anticipation method we propose will simply "stick" the AUV on the trajectory such as η ≡ η d . The corresponding vector τ a can be understood as the "nominal control", steering ideally the AUV on the reference trajectory (See Pino and Delaleau [START_REF] Pino | Flatness-based computer-aided design of mechanical systems[END_REF] in the design spirit and Hagenmeyer and Delaleau [START_REF] Hagenmeyer | Exact feedforward linearization based on differential flatness[END_REF] in a control perspective).

To proceed, we calculate the inverse kinematic and dynamic models to obtain trajectory-dependent anticipation speed ν a and efforts (or wrench) τ a .

The point O e is an arbitrary point belonging to the AUV, with which we want to follow the trajectory. We can write the velocity of point O e in R b :

ν e =      1 0 0 0 z -y 0 1 0 -z 0 x 0 0 1 y -x 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1      T      u v w p q r      ν , r e =
ν a = T -1 J(η 2 ) -1 ηe (6)
Based on the dynamic model of the robot described before (Eq. 2), the following anticipation dynamic law is deduced (in R b ):

τ a = M νa + (C(ν a ) + D) ν a + G (7)
Fig. 2 depicts a pipeline following trajectory and Fig. 3 shows the wrench τ a we obtained by the anticipation method on the pipeline trajectory following task, applied on a generic AUV (6 dof). We can see that for the forces (first column), the propulsion is required on 3 axes, since surge, sway and heave are necessary to maintain the O e point on the trajectory. Inertia forces demand more efforts during the two turns. Considering the moments (second column), roll and pitch motions do not require any effort outside the turns (the trajectory being planar) while the turns generate small angular accelerations that should be compensated by mechanical stability. Finally, the yaw motion need an important moment during the turns. This can be summarized in the anticipated wrench τ a by writing: τ a = * * * 0 0 * t . Note that all stars ( * ) in a vector stand for independent non zero coefficients.

Synthesis of the Propulsion System

All the coordinates, forces and torques are expressed in the AUV frame R b . The robot is actuated by K thrusters located in points P k = (P x,k , P y,k , P z,k ) (see Fig. 1). The controlled force produced by thruster k is -→

F k = u k - → f k , where - → f k = 1 and |u k | = - → F k ; - → f k is the force direction and u k is the thrust (control variable). Thruster k also produce the torque -→ T k = ---→ O b P k ∧ -→ F k = u k ---→ O b P k ∧ - → f k -→ τ k
where -→ τ k is the "normalized" torque.

Each thruster is completely described by its position P k , its control variable (scalar) u k and its force direction

- → f k . The normalized wrench of thruster k is thus -→ ω k = ( - → f k , - → τ k ).
The B matrix is constituted by the normalize wrenches: each column of B is precisely the normalized wrench of one thruster.

Single Thrusters and Basic Configurations

One has seen that each thruster is potentially able to activate several dof (not independently of course). For the sake of simplicity of the design one seeks for simple situations in which the thrusters could only activate 1 or 2 degree of freedom. In case of 2 dof, this will be 1 translation and 1 rotation. The 9 configurations are listed and named in Table 1. One sees that a single thruster cannot activate a rotation without activating a translation.

In the sequel, the 3 basic thruster types that activate only translation are generically named as T • and the other 6 ones will be denoted T •• .

  Px 0 0     0 Py 0     0 0 Pz     0 Py Pz     0 Py Pz     Px 0 Pz     Px 0 Pz     Px Py 0     Px Py 0   Force direction   1 0 0     0 1 0     0 0 1     0 1 0     0 0 1     1 0 0     0 0 1     1 0 0     0 1 0   Produced Torque   0 0 0     0 0 0     0 0 0     -Pz 0 0     +Py 0 0     0 +Pz 0     0 -Px 0     0 0 -Py     0 0 +Px  

Thrusters Configuration Design

The aim of this paper is to find a thruster configuration which allows the robot to follow one or several given trajectories. Denotes by [t i , t f ] the time interval on which the trajectory τ a is defined: t ∈ [t i , t f ] -→ τ a (t) ∈ R 6 . Denotes by I(τ a ) the vector subspace of R 6 defined by:

I(τ a ) = span {τ a (t) | t ∈ [t i , t f ]}
The design procedure consists in finding a family of wrenches {ω 1 , . . . , ω K } among the T • 's and the T •• 's in Table 1 such that:

I(τ a ) ⊂ span {ω 1 , . . . , ω K }
The the family of thruster {ω 1 , . . . , ω K } is said to be minimal w.r.t. the trajectory τ a if the two following conditions 4 are satisfied: I(τ a ) ⊂ span {ω 1 , . . . , ω K } and I(τ a ) ⊂ span {ω 1 , . . . , ω K-1 } Design procedure5 :

• Step 1: Choose as many T •• thrusters as needed rotations6 .

• Step 2: Choose other T •• thrusters in order to cover all/most translations.

• Step 3: If needed, choose T • thrusters in order to cover missing translations.

An analysis is done on two already designed AUVs and the design procedure is illustrated to generate a new one that matches the task and is minimal.

Design Examples

RSM AUV: This existing AUV has 4 thrusters and must follow the pipeline trajectory:

τ a =         * * * 0 0 *         , B rsm =         1 1 0 0 0 0 0 0 0 0 1 1 0 0 Y 3 Y 4 0 0 0 0 -Y 1 -Y 2 0 0         , u rsm =     u 1 u 2 u 3 u 4     , τ rsm =         * 0 * * 0 *        
One sees that this design corresponds to the choice:

-2 thrusters T xz (one located at (X 1 Y 1 0) t and the other one at (X 2 Y 2 0) t ); -2 thrusters of type T zx ( located in (0

Y 3 Z 3 ) t and (0 Y 4 Z 4 ) t ).
Finally one has I(τ a ) ⊂ I(τ rsm ) and this AUV cannot perform the task properly. The second star in τ a (sway motion) is not matched in τ rsm due to the 0. Since this AUV is not suitable for task, we propose to add a fifth thruster to generate the missing surge actuation (see RSM2 below).

RSM2 AUV:

The adjunction of a "bow thruster" to RSM gives the RSM2 AUV that has a 5 thrusters and thus can follow the trajectory:

τ a =         * * * 0 0 *         , B rsm2 =         1 1 0 0 0 0 0 0 0 1 0 0 1 1 0 0 0 Y 3 Y 4 0 0 0 0 0 0 -Y 1 -Y 2 0 0 X 5         , u rsm2 =       u 1 u 2 u 3 u 4 u 5       , τ rsm2 =         * * * * 0 *        
On sees that RSM2 is obtained from RSM by adding one thruster of type T yz located at (X 5 Y 5 0) t . Here we have I(τ a ) ⊂ I(τ rsm ) and the AUV can achieve the task. However, the solution is not minimal as I(τ a ) = I(τ rsm ).

Here, the roll motion can be actuated whilst it is not a required DOF, which makes this configuration non-minimal w.r.t. the task and we will seek a 4-thrusters of the AUV achieving this task (see NEW AUV bellow).

NEW AUV: One seeks for a 4-dof AUV able to track τ a = [ * * * 00 * ] trajectories. We here apply the design procedure given in previous section:

• The two obtained configurations are:

{T xz , T xz , T yz , T z } , {T yz , T yz , T xz , T z }
Any other choice leads to more thrusters. Notice that same symbol appearing twice in the same configuration mean 2 thrusters of the same type at two different locations. In the next section, only the first case of this NEW AUV will be simulated. The corresponding TCM matrix B new is explicited below:

B new =         1 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 -Y 1 -Y 2 X 3 0         , u new =     u 1 u 2 u 3 u 4     , τ new ≡ τ a =         * * * 0 0 *        

Simulation and Results

The studied trajectory is depicted in Fig. 2. The 3 different configurations,namely "RSM", "RSM2" and "NEW", are considered and compared. Fig. 4 reports the propulsion efforts whilst Fig. 5 express the position and angular errors w.r.t. the desired reference trajectory. In the simulations, the control of the AUV is achieved by feedback linearization, however, the range of this work extends far beyond this method.

Conclusion

The main result of this work is to show that it is possible, knowing the dynamic model of the AUV to build an intrinsically controllable AUV thanks to a specifically designed propulsion system. We showed why some AUVs are unable to achieve a given familly of trajectory tasks and how to make it fit to do so. Moreover, we proposed a mathematical and programmable method to build a rational and controllable AUV, using the minimum number of thrusters to perform a trajectory following task. Simulations have been achieved to validate the design. Future works will deal with the generalization to other propulsion modes (not only thrusters) and will improve the control techniques based on nonlinear models. Real word experiments for trajectory following missions will then be done to validate the propulsion synthesis.

Fig. 1 :

 1 Fig. 1: RSM AUV: 4-fixed thrusters architecture

  O b O e with r e the position of point O e in R b and T the transformation matrix of velocity ν from O e to O b . Taking into account preceding equations leads to: ηe = J(η) ν e = J(η) T ν (5) This allows us to determine the relation between the velocity at point O e , namely ηe , and the robot anticipation velocity ν a at O b , expressed in R b :

Fig. 2 :

 2 Fig. 2: 3D representation of the trajectory

Step 1 : 2 : 3 :

 123 The needed rotation about z b require either a T xz or a T yz thruster.• Step At this step we choose either T xz , T yz or T yz , T xz • Step In the two cases, z b is missing we end-up with the choice of T z .

Fig. 4 :

 4 Fig. 4: AUV propulsion efforts : Ref -RSM --RSM2 .-NEW ..

Fig. 5 :

 5 Fig. 5: AUV trajectory errors : RSM -RSM2 .-NEW --

Table 1 :

 1 Wrenches associated to some thrusters.

	Configuration name	Tx	Ty	Tz	Tyx	Tzx	Txy	Tzy	Txz	Tyz
	Thruster
	position

The second condition is valid up to a renumbering of the ωi's.

The procedure favors the T•• are they are off-centered and more easily implementable than the T• ones.

Rotation thrusters T•• are chosen first as they generally induce translations.


Saad Aldoihi
email: saad.aldoihi@ensta-paristech.fr

Omar Hammami
email: hammami@ensta-paristech.fr

Evaluation of CT Scan Usability for Saudi Arabian Users

Keywords: Usability, CT scan Heuristic Evaluation, Human-computer Interaction

Like consumer electronic products, medical devices are becoming more complicated, with performance doubling every two years. With multiple commands and systems to negotiate, cognitive load can make it difficult for users to execute commands effectively. In the case of medical devices, which use advanced technology and require multidisciplinary inputs for design and development, cognitive workload is a significant factor. As these devices are very expensive and operators require specialized training, effective and economical methods are needed to evaluate the user experience. Heuristic evaluation is an effective method of identifying major usability problems and related issues. This study used heuristic evaluation to assess the usability of a CT scan and associated physical and mental loads for Saudi Arabian users. The findings indicate a gender difference in terms of consistency, flexibility, and document attributes, with a statistically significant gender difference in mental load.

I. INTRODUCTION

In performing a CT scan, it is important to ensure effective interaction between the radiologist and the CT scan device itself as well as the patient. In particular, the impeded speech command, which directs the patient to take, hold, and release their breath (available in many languages) is a key interaction between operator and machine. This fundamental task involves multiple disciplines, including Human Computer Interaction, Human Computer Design, and Usability. As much of the scan's accuracy depends on how fluently the operator can interact with the machine, designers are under pressure to extend the machine's capabilities and usability. According to the FDA database MAUDE [START_REF]MAUDE -Manufacturer and User Facility Device Experience[END_REF], 437 incidences of "User used incorrect product for intended use" were reported in the years 2016 and 2017, 11 of which resulted in death. For that reason, designers have a serious responsibility to ensure devices usability. This is not a straightforward matter like following a cooking recipe but depends on rules that emphasize goals rather than sets of actions [START_REF] Johnson | Designing with the Mind in Mind: Simple Guide to Understanding User Interface Design Guidelines[END_REF]. In most tasks, the interactions between operator and device are goal-oriented; once the user completes the desired task successfully and efficiently, the goal can be said to have been met [START_REF] Mackenzie | Human-Computer Interaction: An Empirical Research Perspective[END_REF]. In the case of a CT scan, the goal is achieved when the radiologist effectively completes successful testing of the patient.

In relation to usability, safety is a critical consideration when assessing the success of a CT scan. One study [START_REF]The surprising dangers of CT scans and X-rays[END_REF] showed that CT emits as much radiation as 200 chest X-rays, which for the average person is equivalent to more than seven years exposure in a natural setting. The same article reported that children commonly received adult doses of radiation. In her Testimony [START_REF]Improving the Safety of Medical Imaging[END_REF] before The United States House of Representatives Health Committee Subcommittee on Energy and Commerce, Rebecca Smith-Bindman MD stated that the most common type of CT scan emits a level of radiation equivalent to 1500 dental X-rays, and that in some CT scan models, the level is equivalent to 5,000 such X-rays. In most cases, whether on the basis of fact and reason or unfounded speculation, patients express concern about the possible effects of a CT scan on their health. As it is difficult to determine or evaluate current usability practices within the medical device industry, medical device usability issues need to be publicized, analyzed, and explained [START_REF] Campoe | Medical device usability analyses: An integrative review[END_REF]. Other industries such as air traffic control and nuclear energy have benefited immensely from human factors and usability practices to eliminate errors and improve safety [START_REF] Alper | A systematic review of safety violations in industry[END_REF]- [START_REF] Scanlon | The value of human factors to medication and patient safety in the ICU[END_REF]. Ultimately, whatever the industry, human factors and usability analyses are safety-driven [START_REF] Miller | Lessons from Another Industry: Aviation, Usability, and Medical Device Design[END_REF]. To the best of our knowledge, no existing study has applied heuristic evaluation specifically to CT scans. Any related studies have not measured CT scans as a direct product but rather as part of a picture archiving and communication system (PACS) or medical imaging software. To the best of our knowledge, this is the first paper to specifically apply heuristic evaluation to CT scans.

II. CONSOLIDATED USABILITY ATTRIBUTES

A. Baseline attributes

Nielsen and Molich [START_REF] Nielsen | Heuristic evaluation of user interfaces[END_REF] proposed a new method for evaluating usability, which they called "heuristics", and heuristic evaluation has since become a popular tool because of its effectiveness and low cost. Following its successful application in evaluating the user interface, heuristic evaluation has since been adopted in other domains [START_REF] Hermawati | Establishing usability heuristics for heuristics evaluation in a specific domain: Is there a consensus?[END_REF]. Nielsen [START_REF] Nielsen | 10 usability heuristics for user interface design[END_REF] introduced ten heuristics that serve as an evaluation guide for practitioners: 1) visibility of system status; 2) match between system and real world; 3) user control and freedom; 4) consistency and standards; 5) error prevention; 6) recognition rather than recall; 7) flexibility and efficiency of use; 8) aesthetic and minimalist design; 9) help for users to recognize, diagnose, and recover from error; and 10) help and documentation. As many authors in the usability field have sought to develop definitions and a holistic approach to usability, it may be inferred that usability cannot be consolidated as a single attribute, and many attempts have been made to compile a list of attributes. Similarly, Makoid [START_REF] Makoid | An Empirical Model for Usability Evaluation Based on the Dynamics of the Human-Computer Interface[END_REF] noted that there is no single agreed definition of usability; instead, different definitions may incorporate different parameters and attributes. Nevertheless, there is consensus on the importance of usability, and international organizations such as ISO have introduced usability attributes for the purposes of standardization. As noted by Shneiderman and Plaisant [START_REF] Shneiderman | Designing the User Interface: Strategies for Effective Human-Computer Interaction[END_REF], standardization accelerates industry adoption

B. Improved attributes

Shneiderman and Plaisant [START_REF] Shneiderman | Designing the User Interface: Strategies for Effective Human-Computer Interaction[END_REF] postulated that it is extremely difficult to designers to accomplish the final design without being forced to tradeoffs between attributes. In other word, increasing effectiveness of one attribute comes on the expenses of the another attributes. In order to achieve better yield of discovering usability problem, traditional heuristic evaluation has been modified, extended, and improved to suit a specific domain or task [START_REF] Ling | Extension of heuristic evaluation method: a review and reappraisal[END_REF] . To Ling and Salvendy [START_REF] Ling | Extension of heuristic evaluation method: a review and reappraisal[END_REF] heuristics evaluation categorized into three approaches: 1) alteration of the evaluation procedure, 2) expansion of the heuristics evaluation procedure, and 3) extending the HE method with a conformance rating scale. In the medical equipment domain, [START_REF] Zhang | Using usability heuristics to evaluate patient safety of medical devices[END_REF] developed heuristics evaluation which are extended modified version of Nielsen [START_REF] Nielsen | Usability Engineering[END_REF] and Shneiderman [START_REF] Shneiderman | Designing the user interface: strategies for effective human-computer interaction[END_REF]. Zhang et al. [START_REF] Zhang | Using usability heuristics to evaluate patient safety of medical devices[END_REF] combined Nielsen [START_REF] Nielsen | Usability Engineering[END_REF] and Shneiderman [START_REF] Shneiderman | Designing the user interface: strategies for effective human-computer interaction[END_REF] to constitute an extended and more fitted heuristics to medical devices.

III. METHOD

A. Participant

Careful sampling is a crucial element of successful research. In user research studies, recruiting participants who meet precisely determined criteria can prove very challenging [START_REF] Albert | Beyond the Usability Lab: Conducting Large-scale Online User Experience Studies[END_REF]. Cairns and Cox [START_REF] Cairns | Research Methods for Human-Computer Interaction[END_REF] insisted that recruiting people with specialist knowledge is essential to user study success. To ensure scientific integrity, the authors carefully specified a rigorous pre-qualification procedure to eliminate unwanted participants. To qualify as a participant, candidates had to meet three criteria: 1) their primary work involved handling and operating CT scans; 2) their job category was radiology (Radiologist or Technician); and 3) they were currently working in the Saudi public healthcare system. At the time of this study, there were 400 CT scan technicians working in the public sector, and there were about 191 CT scan devices (according to the Radiology Department in the Ministry of Health, November 2017). In total, there were 44 participants, ranging in age from 20 to 49 years (26 male and 18 female). Table 1 shows the participants' demographic data.

B. Instrument

The study instrument comprised two elements, the first of which was a two-part questionnaire. The first part gathered information about participant characteristics including gender, age, educational level, years of experience, job title, and nationality. The six measured variables included the following options: As shown in Fig. 1, the second element comprised two adapted questionnaires. The first section adapted the standard heuristic evaluation approach to identify major usability issues vis-à-vis CT scan, based on Zhang et al.'s [START_REF] Zhang | Using usability heuristics to evaluate patient safety of medical devices[END_REF] approach to heuristic evaluation. As shown in Table 2, heuristic evaluation tends to measure CT scan usability in terms of 14 attributes: Use users' language: The language should be always presented in a form understandable by the intended users.

Control

Users in control: Do not give users that impression that they are controlled by the systems. Document Help and documentation: Always provide help when needed, ideally context-sensitive help.

IV. RESULTS

A. Questionnaires reliability

The reliability of the heuristic questionnaire and NASA-TLX was measured using Cronbach's alpha. For the heuristic questionnaire, alpha was .98; for NASA-TLX, alpha was .79. As a rule of thumb, Leary [START_REF] Leary | Introduction to Behavioral Research Methods[END_REF] suggested that a Cronbach alpha in excess of .70 is generally adequate for newly developed questionnaires.

B. Usability attributes

A heuristic evaluation was conducted to identify usability issues faced by CT scan technicians. As shown in Fig. 2, the evaluation indicated a potentially catastrophic usability issue (i.e., leading to death) on all 14 tested usability attributes. The results in Fig. 3 show that technicians identified 529 issues in operating the CT scan, ranging in severity from Cosmetic to Catastrophe. Fig. 3 shows the combined severity for all 14 attributes, incorporating Cosmetic (88.2 cases), Minor (193.2 cases), Major (180.7 cases), and Catastrophe (66.9 cases). 2) Age, Level of Education, and Years of Experience The nonparametric Kruskal-Wallis test showed no differences between category groups.

C. NASA-TLX

NASA-TLX was used to identify the physical and mental loads associated with operating the CT scan. As shown in Fig. 4, a total of 3 cases registered high on physical load; 10 cases registered high on mental load; 2 cases registered high on frustration; and 5 cases registered high on discomfort. 2) Age and Level of Education The nonparametric Kruskal-Wallis test showed no difference between category groups.

3) Years of Experience A Kruskal-Wallis test showed a statistically significant difference by years of experience for at least one group on Frustration (x 2 = 10.9, p = .012) and Discomfort (x 2 = 9.2, p = .026). Dunn's pairwise test was performed for the six pairs of groups. There was strong evidence (p = .006, adjusted using the Bonferroni correction) of a difference in Frustration between 0-3 years and 8-11 years. The same pair also differed significantly (p = .036, adjusted using the Bonferroni correction) on Discomfort.

V. CONCLUSION

Advances in technology entail advances in systems, increasing the pressure on users to manage complexity safely and efficiently. These findings support the mounting evidence of physical load (high = 3, fairly high = 7, medium = 26) where users see themselves as contributing physically to operate a CT. As 86.3% of users believed that operating CT scan involves medium to high mental load, manufacturers should pursue designs that reduce both physical and mental loads. Across the 14 usability attributes, 66.9 cases of Catastrophic usability were recorded. These should be fixed immediately before allowing the product to go to market. To ensure that users are willing and able to operate within the confines of rigid safety and regulatory guidelines, manufacturers should devote more effort to CT scan usability.

These findings add to mounting evidence that users differ according to gender and years of experience. In the context of CT scan operation, males and females different on the Consistency attribute (Sequence of action, Color, Layout, Font, Terminology, and Standards). For that reason, it is recommended that designers should provide customizable options to suit end-user needs and requirements. In addition, as males and females differed significantly on mental load, designers should take account of these differences in the design process and should actively iterate to the end of the process, testing and comparing in order to accommodate the usability needs of both genders.

  (1) Gender (Male, Female); (2) Age (20-29, 30-39, 40-49); (3) Level of Education (Diploma, Bachelor, Master); (4) Years of Experience (0-3 years, 4-7 years, 8-11 years, 12+ years); (5) Radiology Job Title (Technician, Technician Specialist, Senior Technician Specialist, Consultant Technician, Registrar); (5) Nationality (Open).

( 1 )

 1 Consistency; (2) Visibility; (3) Match; (4) Minimalism; (5) Memory; (6) Feedback; (7) Flexibility; (8) Message; (9) Error; (10) Closure; (11) Undo; (12) Language; (13) Control; (14) Document. Each attribute was measured by items ranked on a five-point Likert scale (0 = No Problem, 1 = Cosmetic, 2 = Minor, 3 = Major, 4 = Usability Catastrophe). In a third step, NASA-TLX [22] was used to assess the physical and mental loads associated with operating a CT scan. The questionnaire measured the following variables: (1) Physical, (2) Mental, (3) Frustration, (4) Discomfort. Responses were again based on a five-point Likert scale (1 = Low, 2 = Fairly Low, 3 = Medium, 4 = Fairly High, 5 = High).

Fig. 1 .

 1 Fig. 1. Model Framework The second set was field observation. The first author conducted 16 hours of on-site observations at King Saud Medical City (KSMC) to investigate technicians' use of the CT scan in terms of usability and physical and mental loads. During that time, 8 hours were dedicated to CT scan operation in the Emergency Room (ER), and the other 8 hours were dedicated to CT scan operation in the Radiology Department. KSMC was chosen as the observation site after careful evaluation of several Riyadh hospitals in terms of throughtput and diversity of patients. The observed technicians were made aware of the study's purpose, and a consent form was distributed and obtained from each participant.

Fig. 2 .

 2 Fig. 2. Usability issues by attribute

Fig. 4 .

 4 Fig. 4. NASA-TLX severity 1) Gender A Mann Whitney test showed a difference between Male and Female on mental load (z = 3.23, p= .001, 2-sided).2) Age and Level of Education The nonparametric Kruskal-Wallis test showed no difference between category groups.3) Years of Experience A Kruskal-Wallis test showed a statistically significant difference by years of experience for at least one group on Frustration (x 2 = 10.9, p = .012) and Discomfort (x 2 = 9.2, p = .026). Dunn's pairwise test was performed for the six pairs of groups. There was strong evidence (p = .006, adjusted using the Bonferroni correction) of a difference in Frustration between 0-3 years and 8-11 years. The same pair also differed significantly (p = .036, adjusted using the Bonferroni correction) on Discomfort.

TABLE I

 I

			Male	26	59.10%
			Female	18	40.90%
			Total	44	100%
			Age
			20-29	14	31.80%
			30-39	23	52.30%
			40-49	7	15.90%
			Total	44	100%
			Level of Education
			Diploma	11	25.00%
			Bachelor	25	56.80%
			Master	8	18.20%
			Total	44	100%
			Years of Experience
			0-3 years	16	36.40%
			4-7 years	11	25.00%
			8-11 years	9	20.50%
			12+	8	18.20%
			Total	44	100%
			Radiology Job Title
			Consultant Technician	1	2.30%
			Registrar	3	6.80%
			Senior Technician Specialist	9	20.50%
			Technician	15	34.10%
			Technician Specialist	16	36.40%
			Total	44	100%
			Nationality
			Filipino	7	15.90%
			Indian	1	2.30%
			Pakistani	2	4.50%
			Saudi	33	75.00%
			Sudanese	1	2.30%
			Total	44	100%
	.	PARTICIPANT DEMOGRAPHIC
	Variable	Frequency	N %
	Gender

TABLE II .

 II Users should be informed about what is going on with the system through appropriate feedback and display of information. Match Match between system and world: The image of the system perceived by users should match the model the users have about the system. Minimalist Minimalist: Any extraneous information is a distraction and a slow-down.MemoryMinimize memory load: Users should not be required to memorize a lot of information to carry out tasks. Memory load reduces users' capacity to carry out the main tasks.FeedbackInformative feedback: Users should be given prompt and informative feedback about their actions. Flexibility Flexibility and efficiency: Users always learn and users are always different. Give users the flexibility of creating customization and shortcuts to accelerate their performance.MessageGood error messages: The messages should be informative enough such that users can understand the nature of errors, learn from errors, and recover from errors.

USABILITY ATTRIBUTES AS DEFINED BY ZHANG

[START_REF] Zhang | Using usability heuristics to evaluate patient safety of medical devices[END_REF]

Attribute Explanation Consistency Consistency and standards: Users should not have to wonder whether different words, situations, or actions mean the same thing. Standards and conventions in product design should be followed. Visibility Visibility of system state: Error Prevent errors: It is always better to design interfaces that prevent errors from happening in the first place. Closure Clear closure: Every task has a beginning and an end. Users should be clearly notified about the completion of a task. Undo Reversible actions: Users should be allowed to recover from errors. Reversible actions also encourage exploratory learning. Language

ACKNOWLEDGMENT

Saad Aldoihi thanks King Abdulaziz City Of Science and Technology for making this research possible by providing scholarship assistance. A thank you goes to Saudi Health Ministry for providing access to its hospitals and employees. A special thanks go to all participants who made the time and effort to participate in this study.


What I want to emphasize in this communication is that, first, because it cannot be accomplished without answering this question, maintenance actually participates in the very constitution of objects. And second, there is no single undisputed answer to the question "what counts as", and from this point of view, maintenance activities, in their very diversity, offer particularly fertile sites for exploring the variety of objects' ontologies. I'll proceed in six steps: First, I will briefly highlight the main analytical and empirical gestures of maintenance and repair studies. Then, I will explain what ontological enactments are, and why what Michael Lynch calls ontography is an interesting thing to do. Third, I will show that we can schematically distinguish two opposite horizons between which the maintenance ontological enactments are distributed. I will then emphasize the tensions and arrangements that occur in between these two regimes. Finally, I will highlight two important aspects of the dynamics of maintenance: the sociomaterial inquiries which it draws on, and the struggles it sometimes involves. (And… there will be no conclusion).

Throughout my presentation, I will refer to a set of great papers and fascinating investigations, but for obvious reasons, I won't be able to go into details. I apologize in advance and I hope it won't be too frustrating.

the rise of maintenance and repair studies

Recently, a series of papers and books have been published, which have progressively set the agenda of what could be marketed as Maintenance and Repair Studies. Among the scholars who inspired and initiate this stream of research are: Leigh Star, who wrote in 1991 discussing B. Latour, "there is no analytic reason to put aside maintenance" [START_REF] Star | Power, Technology and the Phenomenology of Conventions: on Being Allergic to Onions[END_REF], p. 42), Emanuel Schegloff (1992), Steve [START_REF] Brand | How buildings learn: What happens after they're built[END_REF]), Elizabeth Spelmann (2003), and David [START_REF] Edgerton | The Shock of the Old: Technology and Global History Since[END_REF]. The artist Mierle Laderman Ukeles, who published the Maintenance Art Manifesto in 1969, and dedicated her professional life to maintenance, is also a crucial inspiration. While coming from distinct disciplines and investigating very different objects, the researchers involved in this community of research cultivate common interests in the widespread though largely overlooked practices of mending, repairing, fixing, restoring, preserving, up-keeping… which they believe are at the center of the very constitution of society. What are, beyond their diversity, the main contributions of these Maintenance and Repair Studies? First, from David [START_REF] Edgerton | The Shock of the Old: Technology and Global History Since[END_REF] to Steve [START_REF] Jackson | Rethinking Repair[END_REF], almost every member of this community insists to distance themselves from the innovation-centric accounts that can be found in the press, but also in academic publications. Moreover, most of them study maintenance and repair as a means to bring to the light practices and people (generally workers and women) that are made invisible by these very accounts obsessed with innovation.

Second, these researchers seek to describe the ordinary life of objects in a more realistic way, as they deteriorate, wear out, no longer function perfectly, and need to be cared for. In so doing Maintenance and Repair Studies decenter the question of material agency, shifting the focus from the figures of enclosure, univocity and sturdiness, to take into consideration material fragility, which maintenance actually constitutes as a starting point [START_REF] Denis | Material Ordering and the Care of Things[END_REF]. In that sense, Maintenance and Repair Studies extend one of the main lessons of the first Science and Technology Studies, which states that every object is a precarious achievement whose strength and stability are anything but intrinsic.

Finally, in the vein of ethnomethodology, maintenance practices are investigated as processes of sociomaterial ordering, whose subtleties allow to broaden the traditional view of what comes into play in the daily and continuous production of order.

ontological enactments

These are ambitious lines of research, and you can understand after this short introduction that maintenance in a large sense has a lot to do with objects' modes of existence. But what, exactly? What can maintenance tell us about objects themselves? Such question resonates with recent debates in STS about ontology. To sum up very quickly the discussions about the so-called "ontological turn", let's recall that several researchers first stressed the importance of paying attention to the multiplicity of the world and of engaging into what AnneMarie Mol calls ontological politics [START_REF] Mol | Ontological politics. A word and some questions[END_REF]. What does it mean? It's pretty simple: it amounts to consider that there are no such things as simple, univocal, objects whose variations would depend only on external interpretations. For Mol and John Law (1994), reality is in itself multiple, and within this multiplicity various identities occur through different enactments.

A useful way to become sensitive to such multiplicity consists in interrupting as much as possible our own tendencies to define things from a so-called external, neutral, standpoint.

Most of all, as Michael Lynch put it, it is important to "keep the elevator to the ground" and practice what he calls ontography, that is, I quote, "investigating mundane, contingent enactments of particular orderings of similarity and difference" (Lynch, 2013, p. 458).

In numerous occasions, Maintenance and Repair

the what of maintenance: two regimes

Even if it's oversimplifying, one can say that recent research works on maintenance and repair reveal two distinct horizons, two regimes of maintenance that enact objects in very different ways. One is organized around the issues of stability and integrity, whereas the other is oriented towards functionality and flexibility.

stability and integrity: objects that stand

The first regime concerns very different objects, which are all maintained for one main reason: they are supposed to remain the same. And the problem of course, the reason why maintenance is needed in the first place, is that this sameness is nothing but granted. This is the case of the Paris Wayfinding system that David and I investigated [START_REF] Denis | Maintenance work and the performativity of urban inscriptions: the case of Paris subway signs[END_REF].

The maintainers of the subway signs work day after day to provide a stable, and thus reliable, graphic system to the riders. This is also the case of art restorers, who strive, as Fernando Domìnguez [START_REF] Rubio | On the discrepancy between objects and things[END_REF] showed about Mona Lisa, to perpetuate artworks in their original state. There are plenty of other examples, from the body of Lenin whose preservation has been meticulously investigated by Alexei [START_REF] Yurchak | Bodies of Lenin: The Hidden Science of Communist Sovereignty[END_REF], the Saint Anne Church in Manchester in Tim Edensor work [START_REF] Edensor | Entangled agencies, material networks and repair in a building assemblage: the mutable stone of St Ann's Church, Manchester[END_REF], or the dining room table carefully restored by a man called Ted, in Nicky Gregson and her colleagues' paper [START_REF] Gregson | Practices of Object Maintenance and Repair: How consumers attend to consumer objects within the home[END_REF].

Each of these objects are taken care of for the sake of their integrity. What is maintained here is their shape and more generally their physical properties.

Of course are neither these features standing alone, nor their definitions are floating in the air. They're nothing but self-evident characteristics. Actually, looking at these different cases, and following the maintainers during their interventions, we can identify two main resources on which maintainers rely to enact objects stability. In some cases, the integrity of objects is maintained through the use of standards that define in very precise and technical terms what these objects are, and how they should remain. This is the case of subway signs. This is also the case of an iPhone when it's repaired in an Apple Store. This standardized integrity generally amounts to enact always-pristine artifacts, "bright and shiny objects" as Steve Jackson (2014) calls them.

But, in other cases, the preservation of integrity goes with a less specific and more debatable thing called authenticity. I will come back to this challenging matter later, but let us keep in mind for the moment that for artworks for instance, or the dining room table I just mentioned, maintenance focuses on this question of authenticity.

functionality and flexibility: objects that work

But maintenance also deals with, and enacts, what seems to be very different artifacts. Some objects for instance are simply maintained in their "normal functioning", like the buildings whose concierge workers Ignaz [START_REF] Strebel | The living building: towards a geography of maintenance work[END_REF] shadowed. Others are subject to "normalization" at a larger scale such as the public transport system of Santiago [START_REF] Ureta | Normalizing Transantiago: On the challenges (and limits) of repairing infrastructures[END_REF] or a canal in northern Egypt [START_REF] Barnes | States of maintenance: Power, politics, and Egypts irrigation infrastructure[END_REF]. In none of these cases is strict material integrity at stake. In the space of "normal functioning" rigorous stability is not a sustainable goal. Here, it is the upholding of particular actions that comes first. What is maintained is the objects' ability to operate as they are supposed to.

In fact, numerous Maintenance and Repair Studies show that maintenance is far from always being a matter of immutability. This is not exactly for the sake of their integrity that mobile handsets are repaired in the small workshops of Kampala, where Lara Houston (2016) did her long-term ethnography. Likewise, if the Zimbabwe Bush Pump Marianne de Laet and Annemarie [START_REF] De Laet | The Zimbabwe Bush Pump: Mechanics of a Fluid Technology[END_REF] studied is long-lasting, it's not because the people who took care of it strived to keep it "the same", actually it's quite the opposite.

Sometimes maintenance relates to change itself. This is what Nicolas Nova shows for instance about updating and upgrading old phones. Or Marisa Cohn [START_REF] Cohn | Keeping Software Present: Software as a Timely Object for Digital STS[END_REF] who studied an ageing satellite software that is constantly adapted and patched up. In numerous cases what counts, thus, is less what the object intrinsically is or should be, but rather what it does. This is what Spelman note when she recalls Willie, the character of Douglas Harper's infamous book "Working Knowkedge". Willie is specialized in repairing Saab automobiles. But he doesn't care for authenticity. Willie mainly wants the cars to drive. Spelman writes a nice sentence to emphasize the kind of objects enacted through Willie's maintenance: "All that matters is whether you've still got something that performs in the ways cars are supposed to perform." (Spelman, 2003, p. 25) In addition to the verb perform, I think two terms are very important here in order to understand the ontological enactment at play. First, the word cars in plural. It is not this particular car, and not even this specific model of car, but cars in general. Second, the word something emphasizes the idea that we don't know exactly what is the outcome of such maintenance. This something is important because it highlights the fact that objects here are apprehended as mutable entities. Keeping them working implies that they are flexible. If their shape changes, it's not a problem. Sometimes this is even the very condition of their maintenance.

tensions and arrangements

Obviously, the description of these two regimes should not be seen as a call for rigidity and systematization. If the distinction is useful, it is simply because it shows the existence of very different normative horizons. To put it simply, it shows that, if they do not work, objects that are maintained to remain the same will not be considered maintained by those in the eyes of whom only functioning counts. Conversely, an old car that is still driving but whose door is not the right color will be a heresy in the eyes of collectors and preservationists.

But what is really interesting of course are the tensions these two normativities raise in almost every situation. The question of authenticity, for instance, is at the center of many debates and frictions. What exactly is authentic? What are the features of authenticity? Following these questions, we discover that the mere ideas of stability and immutability that we associated with the first regime of maintenance are relational notions.

Take the body of Lenin. [START_REF] Yurchak | Bodies of Lenin: The Hidden Science of Communist Sovereignty[END_REF] shows that for years its integrity has been enacted around two main properties: the state of the skin and the flexibility of joints. If that was preserved, anything else could be transformed. And actually all of what is inside the body, under the skin, has been replaced by different fluids and materials. And these various materials are sill being renewed on a regular basis.

Of course, it's very different for art preservation. As Domìnguez Rubio (2016) explains, in Northern Countries at least, authenticity lies in "the bond between author's intention and material form". May this bond break, and the object itself would disappear. It would become "something else". And many problems arise when the work of art in question is an installation with parts that are supposed to "work". Domìnguez [START_REF] Rubio | Preserving the unpreservable: docile and unruly objects at MoMA[END_REF] shows that, how to preserve these parts is, for the people in museums, a particularly tricky question.

There is another kind of tensions that can be at play when authenticity is at stake. For instance, if you want to have an authentic object that works, things get complicated. This is what [START_REF] Rosner | Antiquarian answers: Book restoration as a resource for design[END_REF] show about old books preservation practices that are torn between authenticity and usability (in this case readability) . 1 I don't have time to take other examples, but it's important to note that these tensions are numerous and that they are by no means peripheral. They are at the center of maintenance interventions and they generally lead to provisory arrangements in which neither one regime or the other is strictly "applied".

inquiries Another important aspect highlighted by Maintenance and Repair Studies is that these contrasted normativities do not represent a stabilized repertoire of actions that maintainers could simply deploy depending on the situation. Every scholar who has studied maintenance has shown that it goes through sociomaterial inquiries.

In the case of architecture, Albena [START_REF] Yaneva | How Buildings 'Surprise': The Renovation of the Alte Aula in Vienna[END_REF] shows for instance how a building regularly surprises the architect's team during the process of its renovation. Blánca Callen and Tomás Sanchez Criado (2015) describe the experimental trials that are conducted in mending practices around e-waste, and through which menders test objects vulnerability. Tim [START_REF] Dant | The 'Pragmatic' of Material Interaction[END_REF] who studied car repair for years depicts the pragmatics of material interaction at play while mechanics take care of an automobile and the sensual knowledge such interaction both develops and draws on. This is also striking about large infrastructures such as water networks, the maintenance of which I've been studying for a few months now with my colleague Daniel Florentin. And Yurshak (2015) shows, as well, that both researchers and authorities discovered many properties of Lenin's body over the years, progressively unfolding its changing materiality, regularly raising new fundamental questions and making decisions about the body's authenticity, which was not defined once and for all, prior to these explorations.

Actually, as Domìnguez Rubio shows about Mona Lisa, this is the case in most Arts institutions. In 1 Museum for instance, works of art have to be exhibited. This means that their preservation is never a matter of integrity only. Through their maintenance, artworks are enacted both as authentic stable objects and 'displayable' ones.

Maintenance ontological enactments are thus made through situated and more or less circumscribed inquiries that progressively unfold, refine and displace the "whatness of objects". In that sense, as Jackson and his colleagues [START_REF] Jackson | Learning, innovation, and sustainability among mobile phone repairers in Dhaka, Bangladesh[END_REF], but also Nicolas and Astrid in this conference, emphasized, repair and maintenance can be innovative, and above all, they constitute opportunities for learning.

struggles Finally, and this will work as my non-conclusion, it's also important to note that the dynamics of maintenance ontological enactments sometimes go through real struggles. For instance, an institution may promote certain criteria of authenticity that run counter to the definitions that other organizations advocate. As [START_REF] Edensor | Entangled agencies, material networks and repair in a building assemblage: the mutable stone of St Ann's Church, Manchester[END_REF] or [START_REF] Jones | Crafting authenticity: An ethnography of conservation practice[END_REF] show, it's a common thing in the domain of heritage conservation. Domìnguez Rubio (2014) also explains that the preservation of contemporary art sometimes witnesses great disputes between conservators, curators, experts and technicians.

Certainly the best known struggles nowadays are those that oppose the promoters of maintenance practices that enact objects as standardized always-pristine entities on one side and those who want to be able to repair these objects themselves without worrying about their integrity on the other side. This is what is at stake in the "Right to repair" movement in the US that grows against manufacturers such as Apple or John Deere. This is also what happened when Ford tried to standardize the maintenance of their cars, and eventually failed to do so [START_REF] Mcintyre | The failure of Fordism: reform of the automobile repair industry, 1913-1940[END_REF].

These struggles are of great interest, because while they show how controversial the question of the "whatness of things" can be, they also demonstrate that maintenance ontological enactments do not only concern a series of "internal" features that would define the inner and the contours of objects. These enactments notably involve a moral economy of work. They always come with a more or less explicit answer to the question, "who cares?" and more specifically who can, who is allowed, or who should take care of this particular object? More generally, these enactments imply specific infrastructures of maintenance [START_REF] Rosner | Designing for repair?: infrastructures and materialities of breakdown[END_REF]. Laws, tools, infrastructures… Using Haraway's vocabulary [START_REF] Haraway | Modest_Witness@Second_Millennium. FemaleMan©_Meets_OncoMouse™. Feminism and Technoscience[END_REF], one may say, therefore, that it's both the identity of the object and the contours of the world with which this object comes that are at stake in every maintenance operation.

  the maintenance of what? the contrasted ontologies of objects that last Jérôme Denis (with the invisible presence of David Pontille) Artisanal Electronics -Digital Culture of Repair Conference Geneva July, 6th 2018 Today, I would like to share with you some of the reflections my colleague David Pontille and I are trying to articulate for a few months now. Drawing on both the common interests and the diversity of the emerging field of Maintenance and Repair Studies, we would like to show that maintenance operations can be investigated as sites of what Steve Woolgar and Javier Lezaun (2013) call ontological enactments. Maintaining an object, we argue, always implies to define, more or less explicitly, what counts as, in, and around this object. What are the features that should remain or be restored for what is considered this object not to disappear or become something else? What counts as a car? As a church? As a phone? As a street or a road? What counts as Mona Lisa? As a clean wall in Paris? As a water pipe, a canal? What counts as the body of Lenin?

  Studies precisely show that maintainers face what Woolgar and Lezaun (2013) calls "the question of the whatness of things" on a daily basis. And, because they are directly confronted with the question of the durability of things, of their sustainability, maintenance activities provide a critical and intriguing site to do ontography, and discover various ontological enactments of objects. This is what I propose to explore next.