FR3047569A1 - HOMOGENEOUS LIQUID JET OPTICAL DEVICE WITHIN A HETEROGENEOUS AIR ENVIRONMENT - Google Patents

Abstract

The invention relates to an optical device implementing a homogeneous liquid jet within a heterogeneous air medium. The invention uses a generator (20) of a liquid jet, comprising a convergent nozzle (201) and a lock (202) communicating with said convergent nozzle. A housing (101) of an optical apparatus (10) is inserted into the airlock (202). An at least partially transparent liquid is conveyed from a source (204) of said liquid to the lock (202) and is ejected via the convergent nozzle (201). A subject (40) located sufficiently close to the convergent nozzle (201) is impacted at least partially by a homogeneous and at least partially transparent liquid jet (30). The homogeneous and at least partially transparent liquid jet (30) connecting the subject (40) to the housing (101) makes it possible for the light of the housing (101) to propagate towards the subject (40) or said subject towards said housing, without refraction light. It is thus possible to make quality shots using a camera and or distance measurements using a laser range finder for example. The device according to the invention is particularly suitable for control operations during waterjet cutting processes.

Description

HOMOGENEOUS LIQUID JET OPTICAL DEVICE WITHIN A MEDIUM

AERIEN HETEROGENE

TECHNICAL AREA

The present invention belongs to the field of optics.

More particularly, the invention belongs to the field of the implementation of optical devices in the presence of liquid on the surface of a subject of interest placed in an air environment whose transparency is possibly affected by the presence of aerosols.

More particularly, the invention belongs to the field of the implementation of optical devices for observation and or measurement in the context of industrial processes.

STATE OF THE ART

It often happens that optical devices are used for control and / or servo operations. The optical apparatus may be for example: an observation apparatus, for example a camera used to know the state of an object difficult to access from an underwater foundation; - A measuring instrument, for example a laser rangefinder for performing a distance measurement.

In certain situations, the use of the optical apparatus is limited by a heterogeneity of the ambient environment, in particular smoky or dusty air. The heterogeneity of the medium renders it totally or partially opaque and limits or prevents the use of optical devices.

The subject of interest may also have on the surface polluting elements, such as dusts or drops of water which reduce the visibility of the surface of the element.

The issues mentioned above appear in particular during shooting operations in an industrial environment. It happens that the machines used in industrial processes are generating aerosols or opaque elements interposed between the subject to be observed and the shooting device.

In these cases, a suction system such as a hood can be implemented, but such a system, when it can be used, can be complex or constraining to implement, for questionable efficiency.

Another solution consists in performing the shooting operations after cleaning and drying the observation scene. The disadvantage of this solution is that it does not allow real-time observation.

Some devices exist to solve, at least partially, problems of transparency in water, such as that disclosed in European Patent DE 103 61 935 B4.

The device presented in this document comprises a flexible bag placed between a subject to be observed and a video camera. The flexible bag is filled with clear water and attached to the lens of the camera. The whole is immersed in the turbid aqueous medium in which the shots of an object must be made. The bag is brought into contact with the surface of the subject to be observed so that the space thus created between the subject and the camera makes it possible to take a good quality shot despite a potentially opaque environment in which the observed object is immersed . The flexibility of the bag allows to marry the surface of a subject that can have a complex shape.

This device has various drawbacks: it must be used immersed so that the weight of the flexible bag filled with water is compensated by the buoyancy of Archimedes; - the flexible bag must be in direct contact with the subject, otherwise the opaque environment would remain between the bag and the subject, and obstruct the shooting, it is not possible to work away from the subject or with a moving object, and a measure of distance from the device to the subject is not relevant in the context of this invention; - The direct contact between the device and the subject is likely to cause rapid wear of said device; - If the subject has one or more polluting elements on its surface, for example sand or mud, these pollutants will be maintained between the bag and the subject and the images will not be exploitable.

SUMMARY OF THE INVENTION

The device according to the invention provides a solution to overcome the shortcomings of the prior art in the field of the implementation of optical devices in heterogeneous air media.

The optical device comprises an optical observation and / or measuring device arranged so that a line of sight of said optical apparatus, along or near the line of sight, propagates light rays through a homogeneous and transparent liquid medium. .

The liquid medium is confined in an air stream produced by a jet generator. The jet has a longitudinal axis substantially coincident with the line of sight of the optical apparatus.

Furthermore, the jet is produced with a velocity Vjet of the liquid in the jet sufficient to produce a homogeneous and transparent jet over a non-zero Lbu failure length in the direction of the longitudinal axis of the jet.

The break length Lbu is the distance, measured from the output of the jet generator, at which the first discontinuity of the jet appears, for example an air bubble. This length is defined and not zero as soon as one leaves a drip diet.

The jet is no longer homogeneous beyond this distance, the light rays undergo refractions and the observation and or measurement can be affected. The Lbu break length thus determines a maximum distance of observation and / or measurement. Beyond, observation and or measurement can be degraded.

In one embodiment, the jet generator comprises: - a convergent nozzle, having an inlet section and an outlet section and delimiting a hollow volume; - an airlock ; a source delivering the liquid in the hollow volume under a pressure Pa of admission and with a flow rate adapted to obtain the velocity Vjet of liquid in the desired jet at the outlet of the convergent nozzle; - At least one conduit for conveying the liquid from the source to the airlock.

The airlock and the convergent nozzle are assembled together so as to ensure a seal between the hollow volume and an external environment.

In one embodiment, the airlock comprises at least: a central hole passing through said airlock; at least one eccentric hole for admission of the liquid. The at least one conduit carrying the liquid is connected to the at least one eccentric hole, thereby allowing the liquid to be admitted into the hollow space through the at least one eccentric hole.

In this embodiment, all or part of the optical apparatus is held in the central hole. A seal is implemented between the housing and the convergent nozzle to prevent liquid leakage from the hollow volume of the converging nozzle to a rear portion of the lock.

One end of the housing is thus immersed in the hollow volume of the convergent nozzle. The distribution of the at least one eccentric hole is such that the flow around the immersed end of the housing is homogeneous, in order to limit the turbulence in the hollow volume 201c that may create adverse effects for the stability of the jet, for example cavitation.

In one embodiment, the optical apparatus is a camera. The line of sight of the camera is substantially aligned with the axis of the convergent nozzle and the jet, so that light rays can propagate from a subject wet by the jet, located at a distance from the bottom convergent nozzle outlet. at Lbu break length, to reach a camera sensor and thus achieve a shot.

In one embodiment, the optical apparatus is a time-of-flight laser rangefinder. A laser beam is emitted from the rangefinder, said laser beam propagating in the jet to a jet-wet subject located at a distance from the converging nozzle outlet less than the Lbu break length. The ray reflected by the subject is propagated in the jet to the laser range finder, thus allowing distance measurement.

In one embodiment, the optical apparatus is a triangulation laser range finder.

In one embodiment, the airlock and the convergent nozzle are of substantially circular section. The jet thus formed will also be of circular section.

In one embodiment, the airlock and the convergent nozzle are of substantially rectangular section. The jet formed will therefore also of substantially rectangular section.

In one embodiment, the optical apparatus has dimmable illumination according to the scene to be observed.

In one embodiment, the liquid used is water. The choice of this liquid can be particularly advantageous during industrial processes using machining devices.

These machining devices can generate aerosols proving troublesome during the control of the quality of the machining process.

The device according to the invention can be associated with a machining device to enable the observation of a working zone of the machining device, a position of which is defined relative to a working zone of the machining device.

In the same way, the device according to the invention can be associated with a machining device for restoring a distance measurement of a reference point of an effector of the machining device with respect to a part in progress. machining or control, when the machining or control device is in operation.

Advantageously, the assembly comprises means for controlling the position of the effector in order to maintain the measured distance at a constant value predefined within a predefined tolerance. The invention thus makes it possible to use an optical apparatus in a heterogeneous environment, for example polluted by smoke or dust, by recreating a homogeneous volume in which light can propagate without disturbance. The existence of a rupture length Lbu makes it possible to obtain a uniform jet over a certain distance and thus to work away from the subject, provided that said subject is located at a distance less than said break length Lbu.

The fact of being able to work away from the subject makes it possible to avoid the problems of wear likely to appear with current devices in direct contact with said subject.

Furthermore, the Lbu rupture length being defined for any sufficiently large jet flow, that is to say, as far as the droplet regime has been left, the device is versatile and can be used to various applications.

The device according to the invention is particularly intended for industrial operations, for example observation of the cutting line during a jet cutting operation.

BRIEF DESCRIPTION OF THE FIGURES The invention will be better understood on reading the description which follows and on examining the figures that accompany it. These are presented only as an indication and in no way limitative of the invention.

Figure 1 is a schematic illustration of an embodiment of the device of the invention in which the convergent nozzle and the lock chamber have circular sections, illustrated in operational condition;

Figure 2 shows detail rear views (a) and detail side views (b) of the convergent nozzle and the lock chamber assembled together in the embodiment of Figure 1;

FIG. 3a is a vertical sectional view of the convergent nozzle and the airlock assembled together with the housing of the optical apparatus inserted in the airlock in the embodiment of FIG. 1, in operational condition when the optical apparatus is an observation apparatus;

3b is a vertical sectional view of the convergent nozzle and the airlock assembled together and in the presence of the housing of the optical apparatus inserted in the airlock in the embodiment of FIG. 1, in operational condition when the apparatus Optical is a time-of-flight rangefinder;

Figure 4 shows rear views, detail (a), detail side (b), and front, detail (c), of the convergent nozzle and the lock chamber assembled together in a second embodiment in which the airlock and the convergent nozzle are of substantially rectangular sections with rounded corners;

Figure 5a is a horizontal sectional view of the converging nozzle and airlock assembled together in the presence of the casing inserted into the airlock in the embodiment of Figure 4 and wherein the optical apparatus is a laser triangulation range finder;

Figure 5b is a vertical sectional view of the convergent nozzle and airlock assembled together in the presence of the casing inserted in the airlock in the embodiment of Figure 4 and wherein the optical apparatus is a laser triangulation range finder;

Figure 6 is a schematic illustration of the concept of Lbu rupture length of a liquid cylindrical jet produced by a convergent nozzle;

Figure 7 shows the stability curve of a liquid cylindrical jet.

In the drawings, similar elements, even of different shape, bear the same mark.

EXAMPLES OF EMBODIMENTS

The optical device according to the invention illustrated schematically in FIG. 1 comprises: an optical apparatus 10; a jet generator 20 of a liquid.

All or part of the optical apparatus 10 is arranged in the generator 20.

In the exemplary embodiment illustrated in FIG. 1, the optical apparatus 10 comprises a box 101, of circular section in the illustrated example, connected via a link 102 to a terminal 103 enabling data exchanges between said housing and said terminal, or a power supply of said housing by said terminal.

The terminal 103 is implemented in the device: as a display terminal on which data from the box 101 are received; and or as a control terminal for acting on the operation of said housing.

The link 102 may be of any known type for the transmission of data in digital or analog form, for example electrical or optical wired, for example radioelectric.

The housing 101 comprises: a body 101a; an optical window 101 b.

In the exemplary embodiment illustrated in FIG. 1, the generator 20 comprises: a convergent nozzle 201, illustrated in FIG. 2, having an inlet section 201a greater than an outlet section 201b and delimiting a hollow volume 201c; a lock 202 in direct communication with the inlet section 201a of the convergent nozzle 201; three ducts 203, in the illustrated example; a source 204 connected to said airlock via said three ducts; adapted to deliver a liquid under a pressure Pa admission into the hollow volume 201c.

The convergent nozzle 201 is substantially conical, of circular section. The convergent nozzle 201 is shaped to produce a cylindrical jet during the implementation of the device, an axis of which corresponds to a longitudinal axis of said convergent nozzle.

The airlock 202 is a partially solid substantially tubular block of circular section, and has a longitudinal axis substantially coincident with the longitudinal axis of the convergent nozzle 201.

The inlet section 201a of the convergent nozzle 201 is substantially identical to the section of the lock 202. They are for example integrally made or are separate and assembled together so as to ensure a seal between the hollow volume 201 this a medium outside.

Advantageously, the convergent nozzle 201 and the lock 202 are made of a stainless material and assembled together for example by welding, gluing, crimping.

In the example illustrated in FIG. 1, the airlock 202 is traversed right through, along axes parallel to the longitudinal axis of said airlock 202, by a set of four circular holes: a central hole 202a; three eccentric holes 202b, of substantially identical diameters, distributed regularly around said central hole 202a in order to create a homogeneous flow and to limit the turbulences in the hollow volume 201c that would be likely to create detrimental effects for the stability of the jet, for example cavitation.

In a rear view of the airlock, as shown in detail (a) of FIG. 2, the centers of the circles corresponding to the three eccentric holes 202b form an isosceles triangle circumscribing in a circle of center the center of the circle corresponding to a section of the central hole 202a.

The housing 101 of the optical apparatus is of substantially circular cross section and of a diameter adapted to that of said central hole 202a so that the housing 101 can be introduced and maintained without substantial play in said central hole 202a.

The housing 101, the airlock 202 and the convergent nozzle 201 thus have longitudinal axes substantially merged.

Seals 202c, for example O-rings, are placed in grooves formed in the housing 101, to provide a seal between the hollow volume 201c and the outer medium separated by the lock 202 and the housing 101, as shown in FIGS. 3a and 3b.

In an alternative embodiment, the housing 101 is glued into the central hole 202a or snugly inserted into said central hole.

In the embodiment of Figure 1, the housing 101 is sealed and a front portion of said housing 101 protrudes inside the hollow volume 201c of the convergent nozzle 201 so that the optical window 101b is kept inside. said hollow volume and that its immersion in the fluid flowing in the hollow volume is ensured.

In a variant of the embodiment of the device, illustrated in Figure 4, the convergent nozzle 201, the lock 202 and the central hole 202a of said lock have substantially rectangular sections with rounded corners.

In the illustrated example, four eccentric holes 202b of fluid inlet, substantially circular section and passing through the chamber 202 from one side to another along an axis parallel to the longitudinal axis of said lock, are distributed in a regular manner around the hole central 202a.

As shown in detail (a) of Figure 4, the eccentric holes 202b are placed in the corners of the airlock 202 and the centers of said eccentric holes 202b form a rectangle.

The housing 101 of the optical apparatus has a section substantially identical to the section of the central hole 202a, so that said housing is inserted and held in said central hole, as shown in Figures 5a and 5b.

As in the previous embodiment, two seals 202c sealing between the hollow volume 201c and the external medium are placed in grooves formed in the housing 101 to ensure a seal between the hollow volume 201c and the outer medium separated by the airlock 202 and the housing 101.

During the implementation of the device, a homogeneous and transparent liquid for a range of wavelengths of interest is delivered by the source 204.

The meaning of the expression "transparent for a range of wavelengths of interest" will be better understood in the remainder of the description. However, for the sake of simplification of the presentation, it will be admitted later that the transparent liquid will be for the range of wavelengths of interest. In other words, the expression "transparent liquid" will be substituted for the expression "transparent liquid for the range of wavelengths of interest".

The homogeneous and transparent liquid is, for example, water. The water is conveyed via the ducts 203 from the source 204 to the airlock 202. The water passes through the airlock 202 through the eccentric holes 202b and continuously fills the hollow volume 201c of the convergent nozzle 201 communicating directly with the The seal provided by the seals 202c makes it possible to prevent leakage of the liquid between the hollow volume 201c and a rear zone of the generator 20. The water in the hollow volume 201c of the convergent nozzle 201 is ejected by the outlet 201b of the convergent nozzle 201, outside in the form of a jet 30, said hollow volume being kept full of water at the inlet pressure Pa at the flow rate accepted by the source 204.

The shape of the jet 30 depends on the speed of said jet. For a sufficiently high speed, the jet 30 is characterized by a break length Lbu, as illustrated in FIG. 7. This length corresponds to the distance, measured from the outlet section 201b of the convergent nozzle 201, at which appears a change in the flow regime in the jet 30, for example the appearance of a bubble of gas in the liquid, generally preceding a loss of cohesion of said jet.

This notion of break length is illustrated in FIG. 6. In this example, the break length Lbu is the distance between the outlet section 201b of the convergent nozzle 201 and the position of a first gas bubble inside. transparent liquid.

The rupture length Lbu is dependent on the geometry of the convergent nozzle 201 and the velocity Vjet of the liquid in the jet 30. The curve of FIG. 7 qualitatively shows the typical evolution of the Lbu rupture length as a function of the speed Vjet of the liquid in the jet 30. For very low speeds, less than VA, the flow regime corresponds to a drip regime for which there is no defined Lbu rupture length. For higher speeds, greater than Va, the break length Lbu can be defined and the curve reaches two maxima, including an overall maximum in B. It is advantageous to work at speeds close to Vb.

In the remainder of the discussion, the flow rate of the liquid is considered sufficient for the jet 30 to be produced with a sufficiently high speed not to be in a trickle mode.

The jet 30 thus obtained is therefore solid, homogeneous, cylindrical and transparent at least over a distance less than or equal to its break length Lbu. It also has, because of the relative arrangement and shapes of the convergent nozzle 201 and the optical apparatus 10, a longitudinal axis substantially coincident with that of the housing 101 of said optical apparatus.

The water jet formed reaches a subject 40, placed at a distance from the outlet section 201b of the convergent nozzle 201 less than the break length Lbu, on which it determines an observed surface 401 at each instant which corresponds to the intersection of the jet 30 with a surface of said subject at the moment considered. Any pollutant elements 402 located on the observed surface 401 or in the air between the convergent nozzle 201 and said observed surface, for example liquid sheets or aerosols, are dispersed or spread by the jet 30.

In the embodiment of FIG. 1, the housing 101 of the optical apparatus 10 is a waterproof digital camera connected to an observation terminal 103 via a digital link 102 which also provides a power supply for the camera, and the subject 40 and the convergent nozzle 201 are placed at a relative distance such that the liquid jet which reaches it is solid and cylindrical, thus a distance less than the rupture length Lbu.

Optical rays 501 from the points of the observed surface 401 at each instant propagate through the jet 30 in a direction opposite to the direction of flow of the liquid and in the direction of the axis of said jet, as illustrated in FIG. 3a. . Since the liquid is homogeneous and transparent, the optical rays 501 undergo no substantial refraction, occultation or diffusion and propagate to the convergent nozzle 201 following a rectilinear trajectory, then to the camera 101.

The camera then makes it possible to photograph or film the scene observed at the level of the observed surface 401 at each instant. The data is then transmitted to the observation terminal 103 via the link 102.

Advantageously, the velocity Vjet of the liquid in the jet 30 will be substantially equal to Vb. This relatively low speed makes it possible to reach a maximum break length Lbu of the jet 30 and thus more freedom as to the distance of the optical apparatus 10 to the subject 40.

In a variant of the first embodiment not illustrated, the device is applied to water jet cutting. This cutting process consists of mixing an abrasive in powder form with water, and to spray this mixture under very high pressure.

The jet 30 is then advantageously used to monitor the evolution of the shape and quality of the cutting line. The optical device according to the invention makes it possible to overcome the annoying optical effects of splashing water and abrasive, thanks to the liquid jet it produces, and thus achieve an undisturbed optical observation.

In a second mode of implementation, the device is used to perform distance measurements.

The device is assembled in the embodiment of FIG. 1. The optical apparatus 10 integrated in the device is a laser distance meter in flight time measurement.

As illustrated in FIG. 3b, the housing 101 of the laser range finder sends a laser beam 502a, in a direction identical to that of the axis of the jet 30, said laser beam passes through the immersed volume of the convergent nozzle 201 and then propagates substantially following the axis of said jet 30. The water being homogeneous, the laser beam 502a follows a rectilinear direction trajectory parallel to a line of sight of the laser rangefinder to a subject 40. The reflected ray 502b then returns to the laser rangefinder by the the liquid of the jet 30, allowing a measurement of the distance between the housing 101 and the subject 40 obtained by a measurement of propagation time and or phase.

The need to have a solid and homogeneous liquid jet takes all the more sense here that a measurement by laser telemetry is very sensitive to the inhomogeneity of the medium in which it is performed. In addition, the rangefinder implemented will take into account a refractive index of the liquid used to form the jet that determines the speed of propagation of light in the medium.

The two embodiments described above make it possible to clarify the expression "transparent liquid for a range of wavelengths of interest". It is necessary, during the implementation of the device, that the wavelengths of interest, that is to say the wavelengths of the optical domain implemented can propagate in the liquid, for example the wavelengths of the spectrum in which the images are taken by the camera to allow good observation in the case of the first embodiment; the wavelength of the telemetry laser in the case of the second mode of implementation.

In a variant of the second mode of implementation, the convergent nozzle 201 and the lock 202 are of rectangular shape as in the embodiment of FIG. 4.

In this variant, the housing 101 of the laser range finder is a range triangulation rangefinder substantially identical to the section of the central hole 202a in which it is integrated.

In this embodiment, a laser beam 502a is emitted, through the jet 30, towards the observed surface 401 at each instant of the subject 40, which surface reflects the light. A reflected laser beam 502b received by a detector forms a base with the emitted laser, making it possible to determine the distance between the subject 40 and the optical apparatus 10 by triangulation.

In a non-illustrated embodiment, the optical apparatus 10 is an observation apparatus incorporating a laser range finder. It is thus possible in one embodiment to control the distance between the convergent nozzle 201 and the subject 40. The invention thus has several advantages: it allows to work in polluted air environments, for example by smoke or dust; it is possible to work away from the subject 40 by placing said subject at a distance from the convergent nozzle 201 less than the break length LbU. as shown in FIG. 7, for a given nozzle configuration, an Lbu break length can be defined irrespective of the flow rate, since the flow rate is large enough to leave the drip system, which is the case in the applications covered by the invention. The invention is therefore versatile in that it is applicable to various types of jets.

The device according to the invention is particularly intended for cutting operations by transparent liquid jet. The machines implemented are highly disturbed environment generators that do not allow direct shooting for real-time tracking. However, this parameter can be essential in industrial as well as medical applications: cutting of materials, cutting of tissues, organ dissections, repair of bone microcracks, for example.

The device also allows precise distance measurements, necessary for example to control the position of an equipment with respect to a face of a workpiece during a machining operation, the convergent nozzle 201 of the device then being carried by the equipment to be enslaved in distance.

Claims (15)

An optical device comprising an optical observation and / or measuring apparatus arranged so that a line of sight of said optical apparatus, along or near which line of sight propagates light rays, passes through a medium. transparent liquid, characterized in that: - said liquid medium is confined in an aerial jet (30) of liquid produced by a jet generator (20) of the optical device; a longitudinal axis of said jet is substantially coincident with the line of sight of the optical apparatus (10); said jet is produced with a velocity Vjet of the liquid in the jet (30) sufficient to produce a homogeneous and transparent jet over a non-zero length Uu in the direction of the longitudinal axis of said jet. 2. Device according to claim 1, characterized in that the generator (20) jet comprises: - a convergent nozzle (201) having an inlet section (201a) greater than an outlet section (201 b) and delimiting a hollow volume (201c); an airlock (202) arranged on said convergent nozzle at said inlet section, said airlock and said convergent nozzle being assembled together so as to ensure a seal between the hollow volume (201c) and an external medium; a source (204) delivering the liquid under a pressure Pa of admission into the hollow volume (201c) and with a flow rate adapted to obtain the speed Vjet of the liquid in the jet (30) desired at the outlet of the convergent nozzle ( 201); - At least one conduit (203) for conveying the liquid from said source to said lock. 3. Device according to claim 2, characterized in that the lock (202) comprises at least: - a central hole (202a) passing through said airlock and wherein is maintained all or part of the optical apparatus (10); at least one eccentric hole (202b) for admitting the liquid into the hollow space (201c) of the convergent nozzle (201) connected to the at least one duct (203). 4. Device according to claim 3, characterized in that a housing (101) of the optical apparatus (10) is held in the central hole (202a) so that an end of said housing is immersed in the hollow volume (201c) of the convergent nozzle (201). 5. Device according to claim 4, characterized in that the housing (101) comprises a camera whose line of sight is substantially aligned with the axis of the convergent nozzle (201) and the jet (30). 6. Device according to claim 4 or claim 5, characterized in that the housing (101) comprises an optical portion of a laser rangefinder flight time. 7. Device according to claim 4 or claim 5, characterized in that the housing (101) comprises an optical portion of a triangulation laser rangefinder. 8. Device according to any one of claims 1 to 7, characterized in that the jet (30) is of substantially circular section. 9. Device according to any one of claims 1 to 7, characterized in that the jet (30) is of substantially rectangular section. 10. Device according to any one of the preceding claims, characterized in that the liquid is water. 11. Device according to any one of the preceding claims characterized in that the optical device (10) has a dimmable illumination according to the scene to be observed. 12. An assembly comprising a machining device characterized in that it comprises an optical device according to one of the preceding claims taken in combination with claim 5, wherein together a position of the optical device is slaved to the machining device so observing by means of the camera an area whose position is defined relative to a working area of the machining device. 13. The assembly of claim 12, characterized in that the machining device is a water jet cutting device, and the area observed by means of the camera is located behind the water jet of the cutting device. following a direction of movement of said water jet relative to a workpiece when the cutting device is in operation. 14. An assembly comprising a machining or control device characterized in that it comprises an optical device according to one of the preceding claims taken in combination with claim 6 or claim 7 wherein the optical apparatus (10) is attached to an effector of the machining or control device so as to restore a distance measurement of a reference point of said effector with respect to a workpiece during machining or control, when the machining device or control is in operation. 15. The assembly of claim 14, comprising means for controlling the position of the effector to maintain the measured distance to a constant value predefined in a predefined tolerance.