Marc Budinger

Jean-François Rouchon

Bertrand Nogarède

J-F Rouchon

B Nogarede

J-F Rouchon

A mason type analysis

Keywords: ultrasonic micromotor, TWUM, analytical modelling, equivalent circuit. I

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Piezoelectric actuators have many well-known advantages compared with other technologies. They provide a high torque-mass ratio, they can lock when the supply is switched off and they ensure increased comfort of use [START_REF] Sashida | An introduction to ultrasonic motors[END_REF] [START_REF] Ueha | Ultrasonic Motors[END_REF]. Cylindrical type ultrasonic motors have interesting characteristics for a good electromechanical energy conversion and have a very simple structure which is adapted to millimetric sizes. However there is litte in literature which deals with modelling and theoretical analysis. The objective of this paper is to supplements the kinematics analysis done by [START_REF] Lu | A kinematic analysis of cylindrical ultrasonic micromotors[END_REF]. The analytical modelling presented here is based on electric equivalent circuit and gives a global but simple model of the motor.

II. Modeling of the rotating-mode motor stator

Cylindrical ultrasonic micromotor has a cylindrical stator and two rotors. The stator consists of a titanium tube, a layer of PZT thin film and electrodes as shown in fig. n° 1. The poling direction of the PZT film is in the thickness direction. The fundamental bending vibration is generated by the piezoelectric length-extension effect.

Stator en titane

Film PZT (7-9 µm) Electrode V.sin(ωt) -V.cos(ωt)

V.cos(ωt) -V.sin(ωt) ∅ 2.4 mm ∅ 1.9 mm 10 mm

Figure n° 1 :

The structure of the stator transducer [START_REF] Morita | Design of a cylindrical ultrasonic micromotor to obtain mechanical output[END_REF] The stator of the rotating-mode motor presents a structure which resembles a Langevin type transducer. The difference comes from the level of the excited modes; modes of flexion for rotating-mode motor and longitudinal vibration modes for the Langevin type transducer.

Equivalent electric circuit modeling of Langevin type actuator

The Langevin type transducer is modeled in a classic way with Mason equivalent electric circuits [START_REF] Ikeda | Fundamentals of Piezoelectricity[END_REF]. From the transducer geometry, they make it possible to get a relatively precise model of the electro-mechanical conversion based on an analogy between longitudinal vibratory speed and current (the counterpart of effort in one section is then voltage).

According to this approach, a non piezoelectric rod in traction/compression can be modeled by the equivalent circuit of

F 0 U 0 U L Rod geometry 0 U & L U & ) 2 tan( L k jZ c ) 2 tan( L k jZ c ) sin( kL j Zc L o U U & & - 0 F L F Equivalent electric circuit

Figure n° 2 : Non piezoelectric rod modeling

Modeling by equivalent circuits of the stator

Following this approach, the model of the stator of a rotating-mode motor has been developed, which, in this case, lends itself to the calculation of the characteristic bending magnitudes and hence the angular vibratory speed and moment at a section.

Stator's equivalent diagram in flexion

A flexural vibration system can be modeled with a transfer matrix which connects vibratory efforts and speeds in these two ends [START_REF] Zhou | The performance and design of ultrasonic vibration system for flexural mode[END_REF]. It is also possible to represent this matrix by an electric circuit. For that, an elementary cross-section of a beam of section A and of moment of inertia I is isolated in Fig. n° 3.

x x+dx T+dT T u u+du M M+dM Ψ+dΨ Ψ

Figure n° 3 : Elementary cross-section of a beam in flexion

Displacement is orthogonal displacement (u) or rotation angle (Ψ). Effort is shearing (T) or moment (M). By applying the fundamental principle of dynamics to this elementary volume, we get:

x T t u A ∂ ∂ - = ∂ ∂ 2 2 ρ (1)
And

T x M t I - ∂ ∂ = ∂ Ψ ∂ 2 2 ρ (2)
The equations of the resistance of the materials give:

cI M x = ∂ ∂ψ (3)
And

x u AG a T T ∂ ∂ = - ψ (4)
The beams studied here are relatively short: the moment of inertia will not be neglected [START_REF] Timoshenko | Résistance des matériaux[END_REF]. We finally get:

I c I t u S t u aG I x u cI ∂ ∂ ∂       + = ∂ ∂ + ∂ ∂ + ∂ ∂ ρ ρ ρ ρ (5)
For steady-state, sinusoidal excitation and vibration the following complex notation can be adopted:

t j Ue u ω =
. The differential equation of propagation of the elastic wave then becomes:

0 ² 1 2 4 2 2 2 4 4 =         - + ∂ ∂         + + ∂ ∂ U S G a I x U G a c I x U cI T T ρ ω ρ ω ρ ω (6)
This has a solution of the form:

) ( ) ( ) cos( ) sin( ) ( 2 2 1 1 x k Dch x k Csh x k B x k A x U + + + = (7) with: cI G a c I k T 2 ² 1 1 ω ρ         + + ∆ = and: cI G a c I k T 2 ² 1 2 ω ρ         + - ∆ = where:         - +                 + = ∆ ² 1 ² 4 ² ² 1 ω ρ ω ρ ω ρ SG a I S cI G a c I T T
The structure of the motor is excited by ceramics, which only excite moments of flexion. It can be supposed that for a length L of non piezoelectric rod, only pure moments M 1 and M 2 are applied at the extreme sections S 1 and S 2 (shearing efforts T 1 and T 2 are then null).Starting from the general expression [START_REF] Timoshenko | Résistance des matériaux[END_REF] of the orthogonal displacement U, the expression of the moments is:

( ) ( ) ) ( ) ( ) cos( ) sin( ) ( 2 2 1 1 x k Dch x k Csh x k B x k A x M M M + + + = β α (8) with :         - = 2 1 ² k G a cI T M ρω α and :         + = 2 2 ² k G a cI T M ρω β
the expression of the angles is given by :

( ) ( ) ) ( ) ( ) sin( ) cos( ) ( 2 2 1 1 x k Dsh x k Cch x k B x k A x + + - = ψ ψ β α ψ (9)
with :

( )

² 1 ω ρ α α ψ I SG a SG a k T T M - + =
and :

( )

² 2 ω ρ β β ψ I SG a SG a k T T M - + =
and the expression of the efforts is written :

( ) ( ) ) ( ) ( ) sin( ) cos( ) ( 2 2 1 1 x k Dsh x k Cch x k B x k A x T T T + + - = β α (10) with :
( )

1 k SG a T T - = ψ α α and : ( ) 2 k SG a T T - = ψ β β
By applying boundary conditions at the end sections S 1 and S 2 (shearing efforts T 1 and T 2 null) coefficients A, B, C and D can be determined as functions of angles ψ 1 and ψ 2 . Once these coefficients are determined, the expression of the moments can be written:

( ) ( ) ( ) ( ) ( ) (
)

) 2 / ( cot ) 2 / ( cot 2 ) 2 / tanh( ) 2 / tan( 2 ) 2 ( 2 1 2 1 2 1 2 1 1 L k anh L k an L k L k L M M T M T M T T T M T M T T α β β α α β β α ψ ψ α β β α α β β α ψ ψ ψ ψ ψ ψ + - - + + - - + = - = (11) And ( ) ( ) ( ) ( ) ( ) (
)

) 2 / ( cot ) 2 / ( cot 2 ) 2 / tanh( ) 2 / tan( 2 ) 2 ( 2 1 2 1 2 1 2 1 2 L k anh L k an L k L k L M M T M T M T T T M T M T T α β β α α β β α ψ ψ α β β α α β β α ψ ψ ψ ψ ψ ψ + - - + + - - + - = + = 12)
They can also be rewritten in the form of:

( ) 2 1 1 1 ) 2 ( ψ ψ β ψ α - + = - = L M M (13)
And ( )

2 1 2 2 ) 2 ( ψ ψ β ψ α - + - = + = L M M (14)
With :

( )

T T T M T M j L k L k α β β α ω α β β α α ψ ψ - - = ) 2 / tanh( ) 2 / tan( ~2 1 and ( ) T T T M T M L k L k j α β β α ω α β β α β ψ ψ - + = ) sinh( ) sin( ~2 1
These equations can be put in the form of an electric equivalent circuit (Fig. n° 4) as for a Langevin type transducer. On the end surface of the stator, motion components in a cylindrical coordinate reference can be written as [START_REF] Lu | A kinematic analysis of cylindrical ultrasonic micromotors[END_REF] :

          + - + + =           ) cos( ) sin( ) cos( 0 0 0 θ ω ψ θ ω θ ω θ t R t U t U U U U x r
There is a travelling wave on the circumference of the stator. It is made up of two elliptic movements : a first, tangential and driving component, a second, radial and undesirable component. This undesirable component can be eliminated by inclining the contact zone (Fig. n° 5). The previous model is able to calculate this angle with geometrical parameters of the stator. To determine the value of ϕ, relation between ψ 0 and U 0 should be given. By taking again the equations described previously, one can show that:

2 2 ψ γ & & = U , with ( ) ( ) ( ) ( ) ( )
( ) ( ) ( )         + - - + - = 2 / cot 2 / cot / ~2 / tanh 2 / tan ) / 2 ) ( 2 1 2 1 2 1 L k anh L k an L k L k T T T T T T α β β α α β β α α β β α γ ψ ψ
And thus :

γ ϕ e R = ) tan(
For the geometry of stator of the Fig. n° 1, the angle of inclination calculated is nearly 24.5° (value close to the 25.5° calculated by finite elements [START_REF] Morita | Design of a cylindrical ultrasonic micromotor to obtain mechanical output[END_REF]).

Piezoelectric effect modeling

The ceramics used are a layer of PZT thin film. The poling direction of the PZT film is in the thickness direction and the bending vibration is generated by the length-extension effect. The electric field component E can be assumed constant along the full length of the ceramic. Stress and strain are thus expressed by :

III. Modeling of the stator/rotor contact

To obtain the complete characteristics of the motor, it is necessary to add to the stator model, which describes the electromechanical power conversion, an analytical modeling of rotor/stator interaction. This stage has already been carried out for travelling wave actuators [START_REF] Minotti | Moteur piezo-électrique à onde progressive : I. modélisation de la conversion d'énergie mécanique à l'interface stator/rotor[END_REF] [START_REF] Rouchon | The elastic contact area between a sinusoidal indentor and a layered solid : application to calculation of ultrasonic motors performances[END_REF].

Zone of contact

The vibrating stator is assumed to be equivalent to a portion of a cylinder and the rotor to a plane surface. Thus the rotor/stator contact is equivalent to a cylinder/plane contact of length 2a (Fig. n° 7). Hertz's theory provides the length of contact and the distribution of pressure p. Let x be the position on the external perimeter of the stator. The travelling wave deformation y has for formula at t=0:

) / cos( ) cos( 0 0 R x R R y ψ θ ψ = = (36)
The radius of the equivalent cylinder R eq takes then the expression:

0 2 2 / 1 ψ R x y R eq = ∂ ∂ = (37)
The length of contact takes as value:

2 / 1 * 2         = E R L a eq n π (38) with : c F L pres n =
, the linear load ;

rotor rotor stator stator E E E 2 2 * 1 1 1 γ γ - + - =
, the equivalent elasticity constant.

The distribution of pressure p is given by:

2 / 1 2 0 1               - = a x P p with a L P n π 2 0 = (39)
Calculation of the torque/speed curve for a given vibratory speed The rotor speed is deduced from the tangential part U θ of elliptic movement:

) / cos( 0 R x U U v ω θ = = & (40)
In Fig. n° 7, co-ordinate b corresponds to the point for which the local speed of the stator is equal at the tangential speed V r of the rotor :

) / cos( 0 R b U U V r ω θ = = & and ) / ( cos . 0 1 ω U V R b r - = (41)
The efforts transmitted to the rotor depend on relative speed :

• For v>V r or -b<x<+b : positive ;

• For v<V r or -a<x<-b ou +b<x<+a : negative.

By taking a law of friction of the type

dS V v sg p dF r ) ( . . - = µ
, torque C is calculated by:

        - - - =         - = ∫ ∫ ∫ ∫ a b b a b b dx a x dx a x cRP cdx x p cdx x p R C 2 0 2 0 0 ) / ( 1 ) / ( 1 2 ) ( ) ( 2 µ µ           -       - +       = 4

Expression of the mechano/mechanic conversion in the equivalent circuits

For the points corresponding to the b>a part of the characteristic (Figure 21.), the total power P θ developed in the contact for the tangential movement can be written in the following form :

dx R x a x P cU dx x v c x p P a a ∫ ∫             - = = 0 2 0 0 0 cos 1 2 ) ( . ). ( 2
ω µ µ θ (44) If 2 1       - a
x is approximated by

      a x 2 cos π , P θ becomes: dx R x a x P cU P a ∫             = 0 0 0 cos 2 cos 2 π ω µ θ , 2 2 0 0 2 cos       -             = R a a R a P cU P π π ω µ θ . (45)
Moreover, another expression of P θ is:

meca perte P P P + = θ θ with Ω = . max C P meca (46)
In equivalent circuits, the power P θ could be calculated by:

] [ ] [ 2 1 2 * 2 2 * 2 2 ψ ψ θ & & M Re M Re P = = (47)
This expression can be rewritten according to (35), and to :

P M Re U M Re γ ψ γ = = ] [ ] [ * 2 2 * 2 2 & & (48) Then: γ ] [ * 2 2 U M Re P & = (49) As 2 U & , 2
ψ& and 2

M are assumed to be co-phasal, the final expression of M 2 is :

2 2 0 0 2 2 cos       -             = = R a a R a P c U P M π π γ µ ω γ ( 50)
In the same way for the radial movement, the power P r developed can be written :

dx R x a x P cU P a r ∫             = 0 0 0 sin 2 cos 2 π ω µ 2 2 0 0 2 sin 2 2       -             - = π π ω µ R a R a R a a P cU P r (51)
As for power P θ , this power correspond to a tension M 2 given by expression :

2 2 0 2 2 sin 2 2       -             - = π π γ µ R a R a R a a P c M (52)
The equivalent circuit (Fig. n° 6) of the motor can thus be supplemented by adding :

A resistance R to take account of the mechanical losses within the stator (this resistance can be replaced by the use of complex elastic constant [15]) ;

• A voltage E θ to represent the power developed by tangential vibratory movement (Eq. (50));

• A voltage E r to represent the power developed by tangential radial movement (Eq. ( 52)).

IV. Numerical implementation of the model

Let us take the case of a micro motor made up of :

• A stator transducer (Fig. n° 1) ;

• Two rotors hold against the stator transducer at the top and the bottom by a spring.

The equivalent circuit of the motor is as Fig. n°9.

V. Conclusion

This article has presented a global model for cylindrical piezoelectric micromotor; it takes into account both electromechanical conversion in the stator and mechano-mechanical conversion between stator and rotor. The motor stator is represented here in a form very similar to a traditional Mason circuit. The differences occur in the particular configurations of movement, namely flexural instead of longitudinal vibrations. In the equivalent circuits, contact between rotor and stator is represented by voltages which are functions of vibratory speed. Useful results for design are obtained such as torque/speed curve, no load admittance, supply voltage, stress and amplitude of vibrations. Good agreements between model and numerical or experimental results are observed (on resonance frequency for example). This modeling is simple to implement and would also lend itself to optimisation by analytical methods [START_REF] Messine | Optimal design of electromechanical actuators : a new method based on global optimization[END_REF] because of the relatively small amount of computation required.

Notations

Fig. n° 2

 2 where voltage F and current U & represent respectively the effort and speed at the extremities of the rod. A transformer which expresses the electromechanical energy conversion is added to the circuit for a piezoelectric ceramic.

2 ψ&Figure n° 4 :

 24 Figure n° 4 : Electric form equivalent circuit

Figure n° 5 :

 5 Figure n° 5 : Contact zone geometry

Figure n° 6 :

 6 Figure n° 6 : Equivalent electric diagram of micromotor stator

Figure n° 7 :

 7 Figure n° 7 : Contact Stator/Rotor

Figure n° 8 :

 8 Figure n° 8 : Torque/speed characteristic

Figure n° 9 :

 9 Figure n° 9 : Equivalent electric diagram of micromotor

Figure n° 10 :

 10 Figure n° 10 : Equivalent RLC electric diagram of micromotor

Figure

  Figure n° 11 : Electromechanical caracteristics

& & &


Thierry Joffroy
email: thierry.joffroy@grenoble.archi.fr

PREVENTIVE CONSERVATION: A CONCEPT SUITED TO THE CONSERVATION OF EARTHEN-ARCHITECTURAL HERITAGE?

Keywords: traditional practices, risk, management

The concept of preventive conservation (PC) is relatively old, as the term was already in use as early as the end of the 19 th century. As the definition implies, the theory of preventive conservation was developed mainly in the context of work on movable heritage. Yet the concept appears to be equally applicable to built heritage, and more specifically to earthen-architectural heritage whose basic raw material is usually fragile by nature and in some circumstances, can decay relatively fast. Though earthen architecture is varied, one of its characteristics is that for each typology, there is always a specific way of ensuring durability or minimizing the risk of damage. This is achieved by implementing a variety of measures that depend on the physical, economic and social context of the site. What is interesting is that all of these measures are aimed at protecting and extending the life expectancy of the structures in question, thus linking them to the concept of preventive conservation. This leads to the conclusion that preventive conservation is by its very nature a concept well adapted to earthen structures. Taking into account climate change, which brings about unusual situations, reinforces the suitability of the concept, which allows us to anticipate natural disaster. This paper examines the suitability and limits of applying the concept of PC in the conservation of earthen architecture through theoretical analysis and practical examples. It concludes with recommendations for its adoption, taking into account intrinsic specificities, and both the tangible and intangible values of the heritage being considered for conservation.

INTRODUCTION

The concept of preventive conservation is relatively old, having been in use as early as the 19 th century. Yet it has not been widely utilized by practitioners. It was only in recent times, after the acceptance of the failure of the more commonly used methods of 'remedial conservation' that the concept reappeared in the 1970s, gained ground in the 1980s, and acquired recognition as a specific discipline in the early 1990s. The concept was widely disseminated, mainly through the efforts of ICCROM (International Centre for the Study of the Preservation and Restoration of Cultural Property ICCROM [START_REF] De Guichen | Preventive conservation: a mere fad or far-reaching change?[END_REF] posited that preventive conservation should be defined as follows: "The full range of actions designed to safeguard or increase the life expectancy of a collection or an object." As this definition implies, the theory of preventive conservation was mainly developed in the context of work on cultural material, primarily by ICCROM, but also by other organizations, such as the Association of Art, Archaeology Restorers with University Education (ARAAFU) or the International Institute for the Conservation of Historic and Artistic Works (IIC). However, at the beginning of the 1990s, North American professionals enlarged the field of application to historic buildings and housing artifacts, by adopting the New Orleans Charter (APTI/ AIC, 1990AIC, -1993)). This initiative did not move further, though the concept of 'risk management', which is nowadays quite widely considered, is similar, but in general is limited to disasters.

By the mid 1990s, a partnership with ICCROM (1) led CRAterre to explore the possibility of applying the concept of preventive conservation to the conservation of the Palais Royaux d'Abomey in Bénin (Figs. 1 and2). The results of this experience being quite promising, CRAterre decided to continue this exploration. This was the start of a series of field activities throughout the world in which preventive conservation was considered as a priority for the definition of conservation strategies and, further, for their implementation. The following is the result of this exploration, and the current state of our reflection on this question.

Figs. 1 and2. Preventive measures at the Royal Palaces of Abomey, 1996, Prema Project, ICCROM, CRAterre, DPC Benin (credits: Thierry Joffroy, 1996)

ADAPTATION OF PREVENTIVE CONSERVATION TO THE PRESERVATION OF EARTHEN-ARCHITECTURAL HERITAGE

When we started to consider preventive conservation, we quickly agreed with the idea that the concept could be applied to built heritage. However, if the problems of conservation and destructive phenomena were similar, it was also clear that the environmental factors, the scale of the work and the budgets involved were different and more complex. But adaptation was possible, and the old adage "prevention is better than to cure" is probably hard to question. The financial savings that this could bring was also evident, and this was a very important consideration for CRAterre, which works in the field of low-cost housing. The concept was also quite well expressed by what became a motto in the framework of the GAIA project (2): "Maximum understanding, minimum intervention" (A. Alva). In trying to adapt the definition of preventive conservation to immovable cultural heritage, it was quite natural to just complete the existing so that it could apply at the same time to both immovable and movable heritage. Thus, the definition was extended as follows: "Preventive conservation is the full range of actions designed to safeguard (or increase the life expectancy of) cultural heritage, artifacts or built structures". Looking at the specificity of immovable heritage, and more specifically, its related environmental and scale limitations (as compared to museum objects), we formulated an additional concept: "ensuring that the risk of damage is reduced to a minimum". The fragile nature of earth as a construction material is a factor that makes preventive conservation even more suitable. In certain circumstances, the destruction of structures built with earth develops quite fast. Hence, the importance of anticipating damage risks, and in many instances, the need to be able to intervene quickly so that the process can be arrested before pathologies develop to an irreversible stage. An observation or review of the various traditions used for building earthen structures makes it clear that durability is often aspired to and that it is achieved by implementing a whole variety of measures. These are often complementary with specific choices/uses that are adapted to the specificities of the physical, economic and social context of the site. What is interesting to note, however, is that all these measures are aimed at protecting and extending the life expectancy of the structures in question. Thus many of those can be associated with preventive-conservation measures, which leads on to the conclusion that preventive conservation is by its very nature a concept well adapted to earthen structures (Figs. 3 and4). Such traditional measures include:  the use of architectural shapes that generate minimal damage;  the carrying out of regular maintenance work on a larger or smaller scale;  the protection of earthen structures by other, more resistant, materials;  the physical-chemical stabilization of earth with natural products that improves its physical properties. It is, therefore, logical to assume that in order for preventive conservation to be successfully implemented at a specific heritage property, it is appropriate to first and foremost:  gain awareness of the range of these traditional measures in order to be able to identify those that are/have been used in the construction of the relevant structure(s);  have a good understanding of the circumstances and processes of degradation;  evaluate the validity of these solutions for the heritage in question;  look at the range of other possible preventive measures, including those that might be required in order to adapt to climatic change; and with specific regard to 'living' heritage:

 envisage the consequences of their implementation;  look for the adaptations needed to ensure integration, taking into account the evolution(s) of the cultural, social and economic environment. In this respect, it is important not to lose sight of the fact that in certain cases, the durability of the structures has not traditionally been the main objective. We need to recognize that in some cases, on the contrary, this relative durability linked with the possibility of recycling the raw materials presents endless opportunities for adapting or modifying structures, if not rebuilding them from scratch. This, therefore, allows the user to adapt his immediate environment as he pleases, as his needs and wishes evolve. These instances of traditions very much caught up in a process of evaluation raise what are specific conservation questions, because to a large extent the authenticity of these structures lies in this ongoing evolution. In some cultural areas, earth structures have gradually been reinforced, covered or partly replaced by other more resistant materials, just as a natural development process. In the same line it is important to recall here that conservation is to be value-oriented and that in addition to technical considerations, the values and the elements carrying them need to be well identified. In the field of earthen-architectural conservation, the issue of patina and its significance often leads to dilemma when taking conservation decisions.

IMPLEMENTATION OF PREVENTIVE CONSERVATION

The following presents a proposal for an intervention methodology constructed around 10 steps and points to be considered towards the application of the principles of preventive conservation to an earthen architectural heritage. The technical approach described does not exclude a participatory approach. On the contrary, working with stakeholders and sharing decisions with them is most of the time a plus towards an effective and successful implementation of preventive conservation.

Examination of the site:

 documentary research;  supplementary documentation of the building(s);  study of the physical, social and cultural context;  identification of the parties involvedindividuals and interest groups.

Identification of causes of damage (diagnosis):

 examination of the building in the light of its function (in connection with the way its durability is assured);  gathering of information (previous studies, works already completed);  repair of the 'disease' (effects of damage);  identification of definite causes (circumstances and processes);  identification of probable causes, and those still requiring verification.

Classification of causes according to risk level:

 damage;  progression of damage (active or passive);  risk of damaging effects following one after the other ('domino' effect);  elaboration of a risk hierarchy;  risk prioritization.

Urgent measures:

A first series of urgent measures can be implemented. These can include:  provisional repairs, consolidation, stabilization;  'permanent' (see point 7 below) repairs;  specific treatments (against insects, animals…);  other. Additional measures to minimize risk of damage:  protection against wind, fire, earthquakes, water;  vegetation controltrees, plants;  protection against insects, animals;  prohibition of visitor access and/or legal measures;  security (from vandalism);  other. Some of the above-listed measures may require a research-development phase before application, so as to ensure that the result is as close as possible to what is expected.

Implementation of regular inspection (monitoring):

 at least once a year and,  in the wake of specific events (exceptionally heavy rain, storms, fires). These inspections must cover most of the activities described in points 1 and 2 above. The assessments submitted will facilitate the elaboration of an annual conservation plan.

Regular maintenance:

It is desirable that maintenance should be the focus of a practical memorandum stipulating not merely what actions need to be taken but also within which time span, as well as the personnel, financial resources and equipment necessary. It shall include activities of various types, such as:  tidying up and de-weeding the territory surrounding the earthen architectural heritage;  repairing and maintaining surface drainage;  treating roof timbers;  re-plastering. Regular maintenance can also include ongoing work to eliminate secondary risks, which were not covered by the initial plan for urgent measures.

Repairs:

Despite well-advanced efforts to avoid all damage, repairs of varying magnitude (preventive conservation) can prove necessary as a result of:  accidents or vandalism;  certain materials wearing out;  or in order to make a structure suitable for use again (rehabilitation). In such cases it would be appropriate to proceed with repairs of a temporary or permanent nature (at the same time the concept 'permanent' should be used cautiously, since no materials last forever). "Permanent" repairs, of course, cannot be undertaken unless sufficient documentation (tangible and intangible, movable and immovable) is available to make it possible to proceed with due respect to the authenticity of the site. If doubts remain, repairs can still be carried out, as long as it is made clear where the repairs start and the original structure ends. In the case of earthen structures, it can prove difficult to mark that demarcation. It is, therefore, necessary to keep a thorough record of the intervention. Recent research has made it possible to perfect or apply technical solutions, which can prolong the life of earthen materials considerably. It is, however, appropriate to thoroughly evaluate:  their effectiveness in relation to the type(s) of earth available;  the cost of the envisaged repairs;  how easy they are to implement;  if the authentic nature of the site is being respected.

In many instances, research will be necessary to ensure that the treatment suggested is giving the expected results (physical, aesthetic properties).

Developing or nurturing expertise:

Preserving skills is essential in order to ensure that work will be carried out with due respect for the authenticity of the building(s). Initially, documentation relating to expertise should involve the following:  identification, selection and recruitment of skilled personnel;  research into documentation on the relevant expertise;  research and experiments, if the relevant expertise has been lost or new solutions are planned. If traditional channels for the transmission of the relevant expertise appear undermined, it would be appropriate to take steps to revive it by:  practical training (in the case of a large number of traditions);  on-site training;  specialist training;  academic training. Promotion and market research in connection with expertise are also a solution, which has to be considered, since facilitating practical experience and making it an integral part of modern life represents a major guarantee for the survival of expertise.

Natural resources

In some cases, the conservation of heritage and of the practices traditionally linked to it depends on the availability of the natural resources. These could include:  the earth itself: this may involve several qualities and, therefore, several quarries;  vegetal infusions, such as trees, plants that are used as additives;  animal infusions like hair, dung. Regarding the work itself, water is often very much required. Taking this into account, it is also important to verify that these resources can be available, both on the short and long term. For example, in some cases, the protection of a quarry can be paramount to ensure that the right building material is available. The improvement of access to water can also be a very important factor facilitating the continuation of some of the traditional conservation practices of regular rendering of historic structures. Besides that, the protection, and when necessary, the regeneration of tree species that are used for lintels, roofs, and beams can also be a paramount factor for ensuring that proper conservation work is implemented.

Equipment:

Many of the proposed interventions presented above can only be implemented if specific equipment is available. In some cases, the acquisition of modern equipment (mixer, crane) can be a good alternative for diminishing the load of work, and might be required when social practices have diminished. Equipment that can be useful includes:  the means for regular inspections, such as transport, cameras;  equipment required for condition surveys, like ladders, templates;  site-work equipment, such as transport, tools;  equipment for research and experimentation, like laboratory, templates, experimentation facilities;  organization of the documentation, such as hardware and software, files, shelves.

SPECIFITIES WITH REGARD TO ARCHAEOLOGICAL SITES

Archaeological sites have several specificities:  they are prone to plunder and to the impact of public works;  they do not have their traditional protection and so are often very much unstable, though, generally, they have deteriorated into a relatively stable shape (Tepa);

 when excavated for research, their relatively stable shape is changed and often renders the structures prone to quick decay. These considerations lead to the necessity to:  ensure that impact studies are undertaken before large public works are planned;  ensure that sensitive sites are protected/guarded;  ensure that conservation measures are planned together with the excavations, with the possible use of temporary shelters, backfilling, and/or work that ensures stability and renders possible the visibility of the revealed artifacts/structures. This last proposal is paramount, because poorly conducted excavations or those abandoned can lead to rising damp at the base of structures; poorly regulated surface drainage is extremely dangerous for earthen structures, and can result in complete destruction of the discovered structure (Figs. 5 and6).

FINANCE AND MANAGEMENT

Even though preventive conservation is an economical way of conserving heritage, it can only be put into practice if regular financial resources are available to enable the implementation of the full range of the proposed measures. In fact, it is still difficult to make a complete separation between the concept of preventive conservation from the broader one of 'heritage management', which involves legal, administrative and institutional issues, the exploitation and promotion of the site, permanent and/or temporary staff, operational partnerships, as well as technical and financial resource considerations. In this context, all efforts (activities) undertaken in connection with the development of the site can be regarded as preventive conservation, as soon as they are aimed directly or indirectly at generating resources and thus at making possible the provision of regular financial support, at a level adequate to ensure that the site concerned is monitored and regularly maintained. Paradoxically, this could extend as far as partial reconstruction (naturally based on thorough documentation) provided that it enables visitors and decision-makers to appreciate the site better. At some point, when you intervene at the site, those who pay for it are often interested to "see the difference". That might need to be addressed, at least partially. At the same time, such cases must remain the exception, knowing as we do that there are numerous other ways for promoting a site effectively, like guidebooks, publications, maps, panorama plinths, exhibitions, organization of cultural events. Considering that all these actions can prove to be effective and useful in relation to highquality conservation of the site, it is desirable to consider a preventive-conservation approach as part of the site's management plan. It is appropriate to remember the cultural significance and the attractive features of a site to be conserved, and that these are crucial for a correct identification of the conservation aims, the activities and measures adapted to the conservation of the site in question and their prioritization. Finally, over and above the elaboration of such a plan, only a dynamic vision of the future is effective. It is, therefore, appropriate to periodically revise the plan so as to adapt decisions and objectives in light of research being carried out and results obtained. If the site evolves, the interest in it will also rise. One also needs to take into account the evolution of the site's physical, social and cultural context.

CONCLUSION

As long as preventive measures are very common in the traditions of building and living in earthen buildings, preventive conservation appears to be naturally suited to the conservation of earthen architectural heritage. In our changing world, and more specifically, with regard to climatic changes, such a preventive approach also appears to be very compatible, as it can easily include the risks linked to it (risk management). In the context of a possible long-running economic crisis, its capacity to lead to low-cost solutions is also a very strong advantage. This paper has proposed a methodological approach for its implementation and provides a list of points that probably needs to be considered when trying to apply it. Through the reflections presented herein, one can realize that preventive conservation cannot be implemented without putting it into context of a larger management scope, which obviously also implies the involvement of stakeholders and, to a certain extent, sharing decisions with them. Furthermore, we also need to realize that in its basic concept, preventive conservation may lead us to freeze heritage in the state in which we found it. But as developed herein, the very nature of this heritage might evolve and freezing it might not be the right way to respect its authenticity. In the same vein, especially when dealing with living heritage (historic centers, cultural landscapes), it is also important to take into account the other recommendations that have recently been established by the conservation community (e.g. the Vienna Memorandum) that opens the reflection on the need for changes (acceptable changes) to ensure that living in heritage remains possible, a primordial condition for its conservation. Preventive conservation cannot be applied strictly in every situation, but it is a very useful concept for all those who wish to conserve earthen architecture but have limited means available to them. It also often leads to decisions that had naturally come to those who built and, later on, maintained these heritage structures, ensuring the authenticity of the interventions.

Fig. 3 .

 3 Fig. 3. Traditional rendering of the Askia tomb by the overall population of the city, Gao Mali (credits: Aldiouma Yattara, 2014)

Fig. 5 .

 5 Fig.5. Test walls at Loropeni, Burkina Faso, WMF project (credits: DavidGandreau, 2004)