EP0216644B1 - Guiding device for sewing three-dimensional soft work pieces upon sewing machines - Google Patents

Description

The invention relates to an assembly for guiding sheets of flexible material intended to form a three-dimensional assembly, said assembly comprising a drive device formed of at least one support plate, a drive wheel pressing a sheet to be guided on the support plate, drive wheel control means, and a sheet edge position detector for controlling the drive device, said detector comprising at least one photoelectric detector disposed in front of the edge, serving guiding the piece of fabric and the electronic circuits, transforming the signal delivered by the photoelectric detector into a control signal from the drive device.

The assembly of flexible parts in a three-dimensional shape is mainly encountered in the clothing industries. Indeed, the making of clothing consists in assembling pieces of more or less deformable fabric to form a surface intended to cover a volume. The assembly is carried out for fabrics, whatever their texture, by sewing. Two cases can arise depending on whether or not the pieces can be superimposed before sewing.

In the first case, it is a classic seam, which, when made, leaves the assembled pieces, flat. In the second case, it is necessary to guide each of the pieces separately under the needle and once the sewing has been carried out, the assembled pieces can no longer be laid flat without folds occurring on at least one of the pieces. This assembly gives a three-dimensional surface and is found, for example, in the cups of bras.

Conventional sewing machines generally include a mechanism providing the inter-looping of the sewing thread (s) and a mechanism for advancing the fabric. This advance mechanism consists of an elliptical movement claw and a presser foot between which the fabric passes.

The longitudinal amplitude of the claw movement determines the length of the stitch and is generally adjustable.

The parts to be assembled are guided relative to their edges which the operator superimposes as exactly as possible.

Normally a sewing machine is only able to make a straight seam. To obtain a curved seam it is necessary to superimpose in advance, given by the claw of the machine, a rotational movement around the needle.

These trajectory modifications must be carried out simultaneously on each of the parts to be assembled.

Automatic machines are known which make it possible to sew at a given distance from a curved edge of a piece of fabric. These machines include an actuator which comes into contact with the part to be guided and gives it a movement, a component of which has the effect of causing it to rotate around the needle.

Actuators can take many forms, however it seems that only claw and wheel actuators have had some developments, with a preference for the latter, the implementation of which is easier.

The wheel actuators of the known technique can be divided into two groups: those with free wheel and fixed direction, and those with controlled wheel and fixed direction.

French Patent No. 2,241,648 describes an actuator of the first group whose free wheel makes an angle with the direction of advance of the piece of fabric so as to impart thereto a rotational movement, limited by the abutment of the edge of the fabric along a guide whose curvature corresponds to the orientation necessary for the formation of a three-dimensional assembly seam, the second piece of fabric, possibly being guided by the same device.

This process has a certain number of drawbacks among which it is necessary to point out a use limited to fabrics sufficiently rigid so that their edge can be effectively guided by the guide, and the need to have as many guides as models of making and sizes.

These drawbacks are not found with an actuator with a controlled wheel, as described, for example, in French patent No. 2,518,134. The wheel, the axis of rotation of which is parallel to the direction of advance of the fabric, is likely to be rotated in one direction or the other according to the information transmitted by a position detector of the edges of the piece of fabric. The guide device also comprises means for adjusting the pressure of the wheel on the fabric so as to ensure a contact pressure which is a function, inter alia, of the material, the thickness and the weaving.

The position detector comprises at least one photoelectric detector disposed in front of one of the faces of a plate intended to separate, to allow their positioning, two pieces of fabric to be assembled by sewing.

The photoelectric detector is made up of two pairs of light-emitting diodes and photo-transistors. The reference position, from which the position of the edge of the fabric is checked, is that for which the edge is centered between the two diodes. Thus, in the reference position, there is the light emitted by the first diode and reflected by the separation plate which is interrupted by the fabric before being received by the corresponding photo-transistor while that emitted by the second diode and reflected by the plate is received by the corresponding photo-transistor. A second similar photoelectric detector is mounted on the other side of the separation plate to detect the edge of the second piece of fabric.

The electrical signals supplied by the photoelectric detectors are sent into an electronic processor device which compares the signals received for each piece of fabric with reference values stored in memory and acts on the drive circuits of the motors of a device for guiding pieces of fabric.

Controlling the rotation of the wheel by the position detector, based on an all-or-nothing response, produces relatively large trajectory changes and short amplitudes which create a

wrinkling of the fabric when changing direction and does not allow to obtain curved seams of small radius.

The use of a photoelectric detector using a wide light beam would allow the measurement of the occulted surface rate and would consequently supply an analog signal which would be proportional to the position of the edge under two conditions: that the fabric be uniform transparency and that the light ray is distributed uniformly according to an elongated area perpendicular to the edge of the fabric.

The analog device, no more than the logic system (all or nothing) does not allow the treatment of fabrics of irregular transparency such as lace, tulle, or printed fabrics with strong variations in density.

The openwork fabrics have, after cutting, very jagged edges and, in the limit, in the case of tulle, threads separated by spaces greater than their diameter. The presence of gaps inside the fabric in the field of the photoelectric detector prevents the use of simple analog sensors measuring the level of obscured surface. The scrolling of the edge of an openwork fabric in front of a supposed sensor capable of detecting the precise position of the apparent edge and not of the cut will give an analog signal reproducing the fluctuations of this apparent edge. These fluctuations are due to the displacement of the cutting line in front of the sensor and to the presence of gaps in the fabric along this line. We must therefore be able to distinguish these two causes in order to reconstruct the cutting line that the seam must follow.

The subject of the invention is a guide assembly the drive device of which comprises a drive wheel which can be oriented as a function of the desired curvature, the speed of rotation of which is mechanically controlled by its orientation and the edge position detector of which allows, from the detection of the apparent edge of the fabric, to reconstruct the cutting line parallel to which the seam is to be executed.

• La figure 1 représente une vue de face d'une machine à coudre équipée de l'ensemble de guidage, comprenant un dispositif d'entraînement et un détecteur de bord, selon l'invention.
• La figure 2 est une vue à plus grande échelle du dispositif d'entraînement.
• La figure 3 est une vue selon III-III de la figure 2.
• La figure 4 est un schéma de principe montrant la variation de vitesse de la roue en fonction de son orientation.
• La figure 5 montre une construction géométrique de principe expliquant le fonctionnement de l'ensemble de guidage.
• La figure 6 est un exemple de courbes selon lequel est réalisé un assemblage.
• La figure 7 représente le schéma synoptique du dispositif électronique destiné à traiter les signaux du détecteur de bord pour le suivi du contour de la découpe et, dans le cas d'étoffes ajourées, à corriger les signaux de ceux des contours des lacunes avant leur utilisation pour la commande du dispositif d'entraînement des pièces d'étoffe.
• La figure 8 présente les chronogrammes des différents signaux obtenus à la sortie des circuits représentés sous forme de blocs à la figure 2.
• La figure 9 est un schéma du circuit de commande du dispositif d'entraînement.
• Les figures 1 à 3 représentent une machine à coudre équipée d'un ensemble de guidage comportant un dispositif d'entraînement M et un détecteur de bord N.

The explanations and figures given below by way of example will make it possible to understand how the invention can be implemented.

• FIG. 1 represents a front view of a sewing machine equipped with the guide assembly, comprising a drive device and an edge detector, according to the invention.
• Figure 2 is an enlarged view of the drive device.
• Figure 3 is a view along III-III of Figure 2.
• Figure 4 is a block diagram showing the speed variation of the wheel as a function of its orientation.
• Figure 5 shows a geometric construction in principle explaining the operation of the guide assembly.
• FIG. 6 is an example of curves according to which an assembly is produced.
• FIG. 7 represents the block diagram of the electronic device intended to process the signals of the edge detector for monitoring the contour of the cutout and, in the case of openwork fabrics, to correct the signals of those of the contours of the gaps before their use for controlling the device for driving the fabric pieces.
• FIG. 8 presents the timing diagrams of the various signals obtained at the output of the circuits represented in the form of blocks in FIG. 2.
• Figure 9 is a diagram of the drive device control circuit.
• Figures 1 to 3 show a sewing machine equipped with a guide assembly comprising a drive device M and an edge detector N.

The drive device is mounted on a larger scale, FIG. 2, mounted on a machine head containing the mechanism for actuating the needle rod 2 at the end of which the needle 3 is fixed and the lowering mechanism and presser foot developer 4.

The head further includes the mechanism for feeding and guiding the needle thread (not shown).

In known manner, the table 5 (FIG. 3) carries the needle plate 6 in which are provided openings for the passage of the feed means constituted by the claw 7 and a needle passage hole 8. The claw is animated with an alternating movement which causes the teeth to periodically protrude from the plate simultaneously with their movement from the front to the rear of the machine (for an observer looking at the machine according to FIG. 1), the return of the claw s performing, the teeth being below the plate. The table also carries the stitch formation mechanism by interlocking the needle thread with the bobbin thread.

The pieces of the fabric to be assembled by sewing are introduced once positioned on the needle plate where they are held by the pressure of the lifting presser foot.

The synchronization of the various movements of the mechanical parts of the machine is designed so that when the needle passes through the pieces of the fabric to loop the sewing threads, these are kept stationary. When this operation is finished and the needle goes up and out of the fabric, the claw protrudes from the needle plate and presses the fabric against the presser foot and drives it backwards bringing between the foot presser and needle plate a length of fabric corresponding to the length of the stitch.

When the guiding of the pieces of fabric must result in the formation of a non-rectilinear seam, it is necessary to move the piece or pieces of fabric around the needle by taking this one as center of rotation. This guidance can only be carried out when the needle is in the low position, in the phase of forming the looping of the threads.

According to the embodiment shown, the drive device according to the invention is positioned in front of the needle plate, and comprises a spherical wheel 9 which presses on one of the pieces of fabric to be guided. The wheel is freely movable around an axis 10, parallel to the surface of the fabric, fixed by one of its ends to a wing 11 of a bracket 12 whose other wing 13 is fixed to a shaft 14, mobile around its axis in bearings 15, 16 fixed to a fixed frame 17. The shaft 14 and the axis 10 of the wheel are, according to the embodiment perpendicular and the axis of the shaft 14 passes through the center of the spherical wheel.

The shaft 14 is provided with an angular position measurement device 18 consisting, for example, of a circular potentiometer and carries at its free end rotation control means consisting of a servo motor 19 controlled by a device edge detection electronics which will be described later. The stators of the potentiometer and servo-motor are fixed by a spacer 20 to the fixed frame 17.

The wheel 9 is rotated about its axis 10 by a roller 21 whose axis 22 is in a plane containing the axis 10 of the wheel, preferably in the horizontal plane orthogonal to the shaft 14. The roller occupies a fixed position relative to the frame 17 on which it is held by a bearing.

According to the embodiment shown, the roller is rotated by a mechanical transmission consisting at least in part of a set of pulleys 23, 24 and belt 25. This set is itself driven by a transmission 26 connected directly or indirectly to a power take-off 27 on the main shaft of the machine. The speed of rotation thus communicated to the roller is linked to that of the machine and therefore is a function of the speed of advance of the parts under the seam forming mechanism.

The roller 21 drives the spherical wheel 9 in circles, the diameter of which depends on the orientation thereof. As the piece of cloth is always driven at the same point situated on a large circle of the wheel, the speed at this point will vary inversely proportional to the diameter of the circle along which the roller will drive the wheel.

FIG. 4 makes it possible to understand the principle of the variation of the drive speed as a function of the position of the wheel in front of the roller whose speed is considered to be constant.

The first position 1 (solid line) shows the spherical wheel 9 oriented so that its plane of symmetry coincides with the plane of symmetry of the roller 21. The roller attacks the wheel according to its large circle of radius R for it communicate a tangential speed, represented by the vector V _o , equal to a coefficient near k at the constant speed Vg of the roller (k Vg = V _o ).

In a second position II (in broken lines) the vertical plane of symmetry of the wheel is turned, for example to the right, by an angle a. The axis of the wheel, perpendicular to the plane of symmetry, also rotates by the same angle, presenting before the roller 21 a circle of radius R 1 less than R and equal to R cos a. It follows that the tangential speed at the point of contact of the large circle with the piece of fabric increases and becomes V, = V _o / cos a. V _o can be considered as a component of the speed V, and was chosen, for reasons which will be developed later, equal to the speed of advance of the claw and in the same direction.

According to a variant, the synchronous mechanical drive of the movement of the main shaft of the machine is replaced by a servomotor 230 (shown in broken lines) making it possible to communicate to the wheel, via the roller, a tangential speed producing a drive of the part to be guided at a speed of which a component parallel to the direction of movement of the claw is constant and equal to the speed of advance of the parts to be assembled.

In order to allow the easy orientation of each piece of fabric before their assembly, the guide assembly comprises a separation device 28 consisting of a support plate 29 comprised between two support plates 30, 31 disposed respectively above and below the plate 29 and at a distance from the support plate, corresponding approximately to the thickness of the pieces of fabric 32, 33 to be assembled.

The support plates carry an opening 34 ensuring the passage of the wheel of the device for driving the piece of fabric, the wheel being in abutment against said piece of fabric.

According to an application of the drive device to a guide assembly of a sewing machine, provision is made for guiding each of the pieces of fabric to be assembled. The piece of fabric 33 passing between the support plate 31 and the support plate 29 is guided by a device at all points similar to the first, which is suggested in FIG. 2 by a portion of wheel 35 shown in phantom, arranged possibly opposite the first with respect to the support plate.

The pieces of fabric are guided, as known, by their edges: at least one of the edges being cut along the curve, projected onto the plane of the fabric, of the desired assembly and serving as a reference. The guide assembly also includes an edge detector N which comprises edge detection means formed by a proportional photodetector which will be described later.

The signals supplied by the edge detector N are sent to an electronic device which detects the source of the signals and generates a signal which is sent to a comparator, one of the inputs of which receives a reference signal taking into account the orientation of the wheel given by the potentiometer 18. The resulting signal controls a servo device comprising the servomotor 19 which controls the orientation of the wheel 9 in front of the roller 21.

Figure 5 is a geometric construction for understanding the operation of the guide device.

The straight lines D and E correspond respectively to a straight seam obtained by keeping constant the distance e from the straight edge E of the piece of fabric to the needle A.

This piece of fabric is driven by the claw disposed upstream of the needle, at a speed V _o which is represented in intensity and direction by the vector V _o applied approximately at point A, as will be explained later.

The tracking of a curved line is done by superimposing a rotational movement of the fabric in advance given by the claw. The supposedly rigid fabric, is endowed with a speed field which can be expressed by the rotation around an instantaneous center of rotation Ci, 0 ₂ , Os ... This instantaneous center is located at the intersection of all perpendicular to the velocity vectors at each point of the fabric. In particular, the

perpendicular XX 'to the speed vector V _o given by the claw is a constant whatever the curvature of the seam since this speed is tangent to the seam by definition. The instantaneous center of rotation is therefore always on this perpendicular. It will be assumed that this perpendicular passes through the needle A, which is only true approximately.

To define the desired speed field, a point B, other than the needle A, is applied via the wheel 9, a speed defined in orientation and intensity by a vector V _i for example.

In order to avoid a deformation of the plane of the fabric between the drive points A and B, the vector must be V ₁ has a component V ' _o with the same orientation and intensity as the vector V _o applied to point A. This component V _o is V, cos a = V _o . The angle a is the angle made by the diametrical plane of the wheel passing through the point of application B, it is also the angle made by the horizontal axis of the wheel with the line XX ', the point d intersection O ₁ being an instantaneous center of rotation. The point O ₁ is the center of a curve Ci of radius equal to O, F, F being the point of intersection of the edge of the fabric with the line XX '.

A modification of the radius of curvature of the seam will be obtained by modification of the orientation and intensity of the speed vector V _i . For example, the curve C ₂ will be obtained by the application of a new speed vector V ₂ which will cause a displacement of the instantaneous center of rotation in O ₂ .

It should be noted that, according to the exemplary embodiment, the orientation of the wheel limited on either side of the direction BH to an angle less than 45 ° depending on the importance of the horizontal axis around which rotates wheel. According to other embodiments, this angle can be greater or less, depending on the size of the spherical part of the wheel.

When the speed vector is oriented in the right part of the portion of the plane limited by the line BH, the curves obtained will be convex. When oriented in the left part, the curves obtained are concave.

An example of the latter case corresponds to the vector V ₃ .

The instantaneous center of rotation is then at 0 ₃ and the concave curve C ₃ is obtained. It is obvious that the latter case has an exemplary value and can only be applied to a flat, not three-dimensional, assembly seam. Figure 6 illustrates the curved seams obtained with the different instantaneous centers of rotation.

The edge detector N must be positioned so that the photodetector cells observe positions 36 located on either side of the theoretical path of the edge of the part to be guided. Their position determines the distance "e" between the seam and the edge of the fabric. If the seam must have a succession of different curvatures, the position of the cells must theoretically be modified according to the progress of the seam to keep e constant. An equivalent result can be obtained using a row of cells of which only the two cells surrounding the theoretical path of the edge for the desired curvature are activated. In practice, the precision sought does not often require this correction.

The choice of training point B is dictated by the seam to be made and depends on the distance from this point to the nearest edge. Depending on the curvature of the seam, it can be on the other side of the straight line D passing through the needle.

This position is easily determined by the skilled person.

The angular position of the drive wheel 9 is identified by the potentiometer 18 while the position of the edge of the fabric is identified by the edge position detector N comprising, according to the embodiment shown, a photoelectric sensor arranged in an enclosure 37 fixed to the arm of the machine. A mirror 38 transmits a light beam on the positions 36 of the polished support plate 29 on which it is reflected. The reflected beam falls on the photoelectric sensor consisting of a strip of diodes mounted on an integrated circuit, in a pitch of 0.1 mm. The bar used, in the application described has 64 diodes, which allows a viewing range of 6.4 mm deemed to be sufficient. The size of the diodes (0.1 mm) was chosen so that their width is less than the diameter of the threads constituting the fabric and in particular those constituting the mesh network of tulle or lace. The place of the thread can therefore be detected to the nearest 0.1 mm, which is sufficient to obtain a sewing precision of ± 0.5 mm.

The light rays which fall on the fabric are diffused with a weak intensity while those which fall at the side or in the gaps are reflected and concentrated by the optics towards the strip of diode. The result is a set of weak signals in front of the fabric and strong outside.

These signals are processed to find the edge and then the cut, taking into account that the shape of the contour due to gaps can be distinguished from that due to the cut due to the difference in the radii of curvature (a few millimeters for a gap against at least 30 millimeters for cutting) when scrolling the edge in front of the diode array, scrolling due to the advance movement imparted to the fabric by the claw.

The state of the different diodes of the detector 37 (FIG. 7) is observed by a cyclic scan clocked by signals 40 from a clock and defined by synchronization signals 41 from a multiplexer 42 to form a video signal. The timing diagrams of the signals, represented in FIG. 8, correspond, in order to simplify the diagram, to a strip of ten photodiodes, while the embodiment uses a strip of 64 photodiodes.

On the video signal, each level represents the illumination of a photodiode. The diagram corresponds to three successive scans l, II, III triggered by the three synchronization signals A, B, C and to a single movement of the fabric, which explains the modification of the signal between each synchronization.

In the first scan I, two low levels can be observed between higher levels, the low levels correspond to a hole in a mesh of a lace.

The absence of the material should give a zero level but practically the level is not zero. In order to clearly separate the signals corresponding to the presence or absence of fabric, a threshold 43 is defined against which the signals are compared. The comparator 44, which receives the threshold signal and the video signal, separates the levels and makes it possible to obtain an all-or-nothing signal 45.

The output of comparator 44 is sent to a bi-stable flip-flop circuit 45. When the bar is scanned, the first presence of the fabric encountered generates a signal which switches the bistable to state 1 shown in FIG. 8, this is only reset to state 0 at the end of scanning, that is to say on the arrival of the next synchronization signal A. Consequently, only the edge of l is taken into account. 'fabric and not any holes. A gate circuit 46 controlled by the output signal of the bistable allows a number of pulses of the clock signal 40 proportional to the position of the first photodiode marked by the edge of the fabric to pass.

A counting circuit 47, triggered by a synchronization signal 41, determines the number of pulses contained in each output signal of the bistable passing through the gate circuit 46, between two synchronization signals 41. The result of the counting is then transferred in a transfer register 48 by a transfer control circuit 49, which also ensures the reset of the counter 47, controlled by the synchronization signal after time shift.

The count value previously obtained is translated into voltage by a digital analog converter 50. A signal proportional to the distance separating the edge of the fabric from the end of the bar is thus obtained.

This edge signal faithfully follows the shape obtained after cutting, including the contour of the gaps cut by the cutting line in the openwork fabrics.

A second signal processing is necessary to reconstruct the cutting line, and will consist in making a peak-to-peak follow-up of the edge signal, so as to skip the gaps.

It has been found that the apparent curvature of the fabric at the level of the sensor generates very slow signal variations compared to those caused by the gaps. It is therefore possible to limit the variations in the signal in the direction of the trough of the gaps according to a slope value corresponding to that which is observed in the event of maximum apparent curvature which is in the embodiment 30 mm.

The analog signal obtained in the first processing is sent to a correction circuit 51 which transforms it into a peak signal. This correction is obtained from two time constants, a short one which quickly takes the maximum value, a longer one which makes it possible to follow the minimum acceptable curve by guiding the fabric.

The signal obtained at the output of the correction circuit 51 is sent to a framing circuit 52 where it is calibrated and exits in the form of a signal e ₂ .

The signal from the angular position sensor 18 is also sent to a framing circuit 53 where it is calibrated in the form of a signal e _l .

The signals e _i and e ₂ are sent in a comparison and control circuit 54 and (FIG. 9) consisting, according to the example, of at least three comparator circuits 55, 56, 57 (for example SIEMENS TCA 965) of which the outputs are associated with three relays 58, 59, 60. The circuit 58 makes it possible to limit the speed of the servomotor 19 by interposing a series resistor R, and the circuits 59 and 60 define the direction of rotation ROT + and ROT- and l 'engine stop by presence at its terminals of the same polarity.

The comparator 55 receives the signal e, which it compares to two maximum and minimum stop values. These two values are sent to the comparator 57 which receives the values of e _i and e ₂ . One of the relays 59 of the motor circuit is actuated by the signal e _i > e ₂ + ε ₁ , ε ₁ being the tripping value, the other relay 60 is actuated by the signal ei <e ₂ - ε ₁ .

If the two relays are in the same state, the motor finds the same polarity at its terminals and remains stationary, otherwise it turns in one direction or the other.

The comparator 56 compares the absolute value of the difference of the signals e _l and e ₂ with a trigger value s ₂ to put or remove a speed limitation resistor on the motor circuit.

It was admitted in this exemplary embodiment that the tangential speed of the drive wheel varied automatically with its orientation, that is to say that the drive device included a spherical wheel driven by a roller 21 at constant speed, as previously described.

It is obvious that a device which does not include this feature would require a power supply circuit for the servo-motor 230 of the drive wheel, which is capable of delivering a variable supply voltage as a function of the orientation a wheel.

Claims (7)

1. Unit for the guidance of sheets of flexible material for the purpose of forming a three-dimensional assembly, the said unit including a drive device formed from at least one bearing plate, a drive wheel pressing a sheet to be guided onto the bearing plate, means of control of the drive wheel, and a detector of the position of the edge of the sheet, the said detector including at least one photoelectric detector responsive to the position of the edge used in the guidance of the sheet and electronic circuits converting the signals provided by the detector into a control signal for the drive device, characterized in that the device includes a spherical drive wheel (9) which moves freely around a spindle (10) parallel to the bearing plate (29) fixed by one of its ends to one end of the shaft (14), moving about its axis, whose extension approximately orthogonal to the bearing plate passes through the center of the wheel; a drive roller (21), of axis parallel to the bearing plate, controlling the rotation of the wheel, the shaft (14) and the roller (21) being supported by a same frame (17, 20); means of measurement of angular position (18) and means of rotation control (19) are provided at the other end of the shaft (14).

2. Unit as claimed in claim 1, characterized in that the drive wheel (9) has, at its point of contact with the material, a tangential velocity (Vi) which is variable according to the cosine of the angle (a) that its plane of symmetry, perpendicular to its axis, makes with the plane of symmetry of the drive roller (21 ) at constant velocity (Vg).

3. Unit as claimed in claim 2, wherein one component V'o of the velocity vector V of the drive wheel (9) is parallel to and equal to the velocity vector V_o of the feed means (7).

4. Unit as claimed in claim 1, characterized in that the detector of the position of the edge (N) of the sheet includes a photo-electric detector comprising a sensor formed by a strip of photo-electric diodes and a light source, a light beam emitted by the source is reflected on a bearing plate (29) over which the edge of the material passes, a multiplexer (42) forming a video signal from the signals of each diode, a comparator (43) to separate the levels of the video signal into all or nothing, a bistable flip-flop circuit (41) going from the 1 state by the first presence of material encountered, to the 0 state at the end of the scan of the strip, a gate circuit (46) allowing a number of clock pulses to pass which are proportional to the position of the first diode masked by the edge of the material, a counting circuit (47), a transfer register (48) and a digital-analog converter (50), the output voltage corresponding faithfullyto the contour of the observed edge, a hole-correction circuit (51) converts the output signal produced by perforated materials into a peak-to-peak signal, a comparison and control circuit (54) comparing the processed signal coming from the correction circuit and the position signal coming from the position sensor (18) in order to control the orientation and velocity of the drive device.

5. Unit as claimed in claim 4, characterized in that the photodiodes forming the strip have a maximum diameter of less than that of the threads forming the stitches of the perforated material.

6. Unit as claimed in claim 4, characterized in that the comparison and control circuit (54) of the motor includes three integrated comparator circuits (55, 56, 57), a first comparator circuit (55) receives the processed signal (e₂) from the diode strip and compares it with two stop values, maximum and minimum (S_i, S₂), a second comparator circuit (57) receives the two previous stop values and the signals (ei, e₂) coming from the diode strip (19) and from the position sensor (18) the outputs of the comparator circuits (55, 57) giving two signals, greater than or less than a reference value (e₂) increased or reduced by a trigger value (ε₁), activating relays controlling a servo-motor (19) for the orientation of the drive device; the third comparator circuit (56) comparing the signal (e₁) coming from the position sensor (18) and the signal (e₂) from the diode strip with a reference value (ε₂) in order to include a resistor in the power supply circuit of the servo-motor (19) in order to limit its speed.

7. Unit as claimed in claims 3 or 4, characterized in that the drive wheel (9) is driven in rotation by a servo-motor (230) and wherein the edge detector includes a circuit for powering the servo-motor (230) providing a power supply voltage variable according to the orientation (a) of the wheel.

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