SE455281B - DEVICE BY INDUSTRIROBOT - Google Patents

Description

455 281 ' 10 15 20 25 30 It is an additional desire to before each welding or continuously during welding be able to sense, for example, the width of a gap to be welded or the volume of a joint in order to be able to set suitable ones accordingly welding parameters, eg welding current.

A further requirement is that the detection of the joint must take place with high resolution. so that joints even in thin sheet metal can be safely detected.

It is previously known to use different types of optical sensors to feel the position of a weld. These have the advantage of allowing non-contact sensing. of the position of the joint, but in robot welding previously used sensors of this types - see for example the U.S. patent specification U 306 1HU - have disadvantages in the form of low resolution, slow sensing or so high sensitivity to disturbing light from a welding arc that the sensor becomes unusable during the actual welding process.

The invention intends to provide a device of the kind initially indicated, which has the following advantageous properties: 1) it enables fast and accurate positioning of the robot to a desired one starting point for a work operation even with large variations between success workpieces in terms of, for example, position and orientation, 2) it enables continuous sensing even during ongoing work operation, eg arc welding and thus accurate tracking in two or three dimensions of, for example, a weld.

What characterizes a device according to the invention is stated in the accompanying patent claims.

The invention will be described in the following in connection with the accompanying figures 1-10. Fig. 1 schematically shows an industrial robot according to the invention arranged to perform a welding operation on a workpiece. Pig 2 shows an example of how the connection between the robot's control system and the distance sensor according to the invention can be made. Pig 3 shows the principle of a distance sensor of it according to the invention use the species. Fig. U shows how the sensor according to Fig. 3 can arranged for searching for a weld. Figs. Sa and Sb show the output signal from the sensor. clean when following a weld and the principle of generating one correction signal for the robot's control system. Figures 6a and 6b show the appearance of the output signals from the distance sensor when passing an edge joint and a 15 20 25 30 455 281 kä1f0g_ Fig. 7 shows the principle of an increase and follow-up process at welding of a joint on a workpiece. Fig. 8 shows the principle of how to measure the signals from the distance sensor are filtered and stored. Fig. 9 shows a flow diagram for controlling the height increase according to Fig. 7. Fig. 10 shows in the same way a flow chart for the first page search phase according to Fig. 7.

Pig 1 shows an industrial robot of previously known type with a foundation 1, on which a column 2 rotatable about a vertical axis is arranged. In the upper pillar end, a forearm U is mounted in a joint 3, around which the arm can be rotated around one horizontal axis of rotation. At the other end of the forearm there is a joint 5, around the an upper arm 6 can be rotated about an equally horizontal axis of rotation. In over- the outer end of the arm 6 is a joint 7, around which the robot hand 8 can be rotated a horizontal axis of rotation. The robot hand is thus in the example shown in only a degree of freedom relative to the upper arm 6, but in a known manner the hand is made movable in one or two more degrees of freedom relative to armen. The robot is in a manner known per se provided with drive means for them different movements, such as electric DC motors. The robot hand carries a schematically shown welding electrode 9 for arc welding and a spacer welding sensor S, which will be described in more detail below.

The robot is arranged for performing welding of one or more welds on a workpiece 10. In a typical application, the workpiece may be formed of a car body, a number of substantially identical car bodies one by one is transported to the robot and kept stationary at it during the execution of the welding process.

The robot has a per se known control system RC, which via a multi-channel connection 11 is connected to the drive motors for the various robot shafts as well to position and speed sensors arranged on these.

The sensor S is connected via a connection 12 to the sensor controller SC. Via led- 12, a signal m is applied to the control device SC, which is a measure of the distance d between the sensor and the workpiece 10. The connection 12 between the sensor and the control The device may, as will be described below, also contain lines for feeding the sensor and for controlling and sensing a oscillating movement of the sensor sub-joint tracking. The sensor control unit SC is connected to the robot control unit RC via a connection 13 via which the sensor and the robot are made to cooperate in the manner described in more detail below. 455 10 15 20 25 30 35 2281 The industrial robot and its control means are previously well known per se type. For example, the control means may be designed in any of the ways that described in U.S. Patents 3 WOOD 032, 3,661,051, 3,306 H71, 4 1H0953 or 39H3 3N3. The robot is programmed by the operator to control it one by one of a plurality of points along the desired path of movement, wherein these points are stored in the robot's program memory, and brought into operation the robot then by its controller to move between the programmed the points, possibly with interpolation between the points.

Fig. 2 shows a block diagram of the sensor controllers and signal processing unit SC and its interconnection with the sensor and the robot's control means RC. Sensor S is as shown in Fig. 1 placed on the robot hand and measures the distance between and a workpiece 10, which has a weld 22 to be welded off the robot. The sensor emits an analog measurement signal m, which is a measure of the distance d between the sensor and the workpiece. In an analog-to-digital converter 21 is converted the signal m to a digital measuring signal M, which is applied to a central unit MP, which may consist of a microprocessor and a semiconductor memory. The central unit analyzes in the manner to be described below the measurement signal M to detect when the robot hand and the sensor during search pass over a weld. At passage of the weld joint, the central unit emits a search stop signal SS to the robot control means for interrupting the search. The central unit further emits a filter rated average Ma of the measurement signal M, which can be used directly by the robot.

To enable automatic tracking of a weld joint during the actual welding In the process, the sensor is equipped with a oscillating sensor PM, which is connected with the sensor via a shaft 210. The oscillation sensor imparts to the measuring spot a and reciprocating motion with a frequency of one or a few Hz. Under pend- The sensor measuring point 23 is moved back and forth in a direction having one component perpendicular to the longitudinal direction of the weld 22. The amplitude of the the periodic movement of the point can be, for example, a few mm or a few tens mm. The sensor's controller SC emits a control signal PC to the oscillation sensor for switching on this following a weld. From the commuter sensor a feedback signal PP is provided, which at any given moment is a measure of the position of the point 23 laterally relative to a neutral position. This signal is applied the position detector LD, which will be described in more detail below, and which emits correction signals ds and dh for correction of the lateral position of the robot hand and height relative to the weld.

The device works briefly in the following way. On arrival at a objects to the robot's workplace, a search operation is started by the robot's control means RC. The search operation may consist of a first stage, in which the robot moves the hand and the sensor in a vertical direction in the figure towards the work object until 10 20 25 30 35 455 281 the sensor and the hand are at a predetermined distance from the object.

In a subsequent stage, the robot hand is moved substantially parallel to the work object. surface, wherein in one or more steps the position of the object and the position of the weld determined by the sensor and its signal processing means detecting them distance discontinuities that occur at an edge or a weld. At detecting such a distance discontinuity, the central unit MP emits one search stop signal Sš to the robot's control means RC, which then interrupt the search movement and initiates a new search movement in another direction or also initiates the work operation itself. When by searching in this way a point has been reached when the work operation is to be started, it is started by the robot's control means RC.

If necessary, for example with longer welds, the robot can during work is brought to follow the weld automatically. The sensor's controller SC emits thereby a signal PC, which starts the oscillating movement of the sensor. From the center-- The signal ds is obtained, which is a measure of the deviation of the robot hand in laterally from the weld. This signal is applied to the control means of the robot, which turns it to correct the position of the robot hand laterally relative to a pre- programmed path that approximately indicates the direction of the weld. The central unit MP further emits a height correction signal dh, which is a measure of the robot hand deviation in height from a desired working distance relative to the workpiece. This signal is also applied to the robot's control means RC, which on themselves known method uses it to correct the position of the robot hand in height relative a pre-programmed path.

The control means further outputs a control signal LC to the sensor, which signal strikes to the sensor at the beginning of a workflow and switches it off when the the process is complete.

Fig. 3 shows in more detail the construction of the sensor itself S. The sensor consists of a housing, in which a light source 30, a photodetector 33, an amplifier 35 and two lenses 31 and 32 are mounted. The light source 30 may conveniently be stand by an LED or laser LED. The light emitted from the light source focuses by means of a lens 31 to a small light spot 23. It reflected the light from this spot of light is projected by a lens 32 onto the photodetector 33.

In a preferred embodiment, this consists of a so-called one-dimensional lateral photodetector. Such may suitably consist of a narrow and elongate photo- diode, for example made by Hamamatsu 81352. The center of the detector is the ten to a fixed voltage uo, and its endpoints are connected to plus- and the negative inputs of an amplifier 35. The output signal m from the amplifier 35 is output makes known in a known manner the position of the projected light spot BU on the If the distance of the workpiece 10 from the sensor is changed, the projector 10 15 20 25 30 35 455 281 The 3U position of the light spot on the detector is also to be changed. To illustrate this is indicated in Fig. 3 by broken lines by another position 10 'of the working paragraph. The light spot 23 from the light source 30 will then be located in the point denoted by 23 ', and the projected light spot will be find themselves in the position marked with 3U 'on the detector 33. The output signal m from the the amplifier 35 will thus constitute an unambiguous measure of the distance between sensor and workpiece 10. This procedure is commonly referred to as optical angulation. The use of a detector 33 of the type described above provides several significant benefits. Because the detector is one-dimensional, it can be made very narrow and thus takes in a minimum of disturbing light, ie such light which does not come from the projected light spot 23. This is of very large importance in certain applications, such as arc welding, where an intensive light is generated during the actual work operation. In order to further reduce the The similarity of interfering light can, if desired, be modulated from the light source 30 with a certain modulation frequency. In this case, a bandpass filter is provided, which measures the measurement signal m so that only components with a frequency equal to the modulus the ring frequency is passed through. This procedure for reducing noise sensitivity is not possible to use with certain other detector types, e.g. wherein the detector consists of a number of separate arranged in a row one after the other photodiodes, as these diodes must be scanned periodically at a certain frequency and this scan easily conflicts with the modulation used. the frequency. The sensitivity to interference can be further reduced in a known manner by that the light source 30 either by itself or by means of an optical filter caused to emit light within a narrow frequency range. A filter, which only transmits light with this frequency range, is arranged in front the detector 33.

An additional advantage of this preferred type of detector is that the analog the measuring signal m at any moment gives immediate information about the measuring distance, thereby avoiding the delay that arises due to the scanning of e.g. such detectors containing a series of separate photodiodes.

As mentioned above, the sensor S is permanently mounted on the robot hand. Searching for determining the position of a work object or, for example, a weld joint relative the robot hand moves the robot hand and thus the sensor in relation to the workpiece while sensing the distance between the sensor and the workpiece.

During the actual work operation, however, the robot hand must carefully follow a certain path, for example following a weld, and this procedure can therefore 10 15 20 25 30 35 HO 7 455 281 not be used if automatic tracking of an edge or eg a weld is required during the work operation itself. If automatic tracking is desired, this can is achieved by arranging the sensor to periodically scan the distance following a direction that forms an angle with the joint or edge to be followed during the work operation. As shown in Fig. H, this can be done by healing the sensor housing is imparted a periodic reciprocating motion by means of a commuting transmitter PM. The oscillating sensor is connected to the sensor via a shaft 210, and it gives the sensor housing a reciprocating motion so that the light spot 23, which is shown in the figure in broken lines, moves, for example, perpendicular to one longitudinal direction of welds 22. The frequency of this movement may be any Hz and the amplitude of the motion of the light spot 23 may be a few or some tens mm. When the light spot 23 is swept back and forth over the one shown in Fig. U. weld joint 22, at each passage of the joint a discontinuity occurs in the known distance d. Fig. 5a shows how the sensed distance d varies with light the position s of the spot, which for example may be so defined that it has its zero point when the light spot is vertically below the light source 30 of the sensor.

The oscillating movement can be effected, for example, by an electric motor, which via an eccentric disc affects the sensor, or by means of a reciprocating running electric motor.

Fig. Bb shows how in the central unit MP correction signals in side and height can generated to achieve automatic tracking during the performance of a Operation. A signal PP from the commuting sensor, starts at each cycle start a clock CL, whose output signal via a gate Ä1 is supplied to a buffer UH.

When the search stop signal SS is obtained from the central unit MP, the clock is read. At each passage of the current edge or joint is thus added to that of the clock output the buffer NOW where it is stored for the next passage. At the same time it is fed previously stored the value from the buffer HU to a second buffer Uü '. The difference In the time between two successive passages of the joint in different directions are formed in a difference generator ÄH ". The output signal from the difference generator UN" will then by continuously constituting a measure of the deviation between the edge or joint, to be followed, and the neutral or zero position of the sensor. This signal ds is applied to the robot's control means and is used in a known manner to correct the robot the position of the hand laterally so that the signal ds is brought to zero. A blanket- measured and averaged measurement signal Ma is applied to a second gate Å2, whose output signal via a buffer US is applied to a negative input of an amplifier H3. To a non-negated input of the amplifier a reference value Mr, corresponding to the desired distance between the sensor and the workpiece during conducting the work operation. Gate H2 is controlled by the search stop signal SS, which means that the measured value Ma at each passage of, for example, the weld joint 22 in fig Ä is loaded to and stored in buffer H5. The output of the buffer corresponds to 10 20 25 30 35 455 281 8 thereby the latest value of the sensor's distance from the workpiece. The difference between this value and the reference value Mr is formed in the amplifier H3 and constitutes of its output signal dh. This signal is applied to the robot's controller RC on such way that it controls the position of the robot hand in height relative to the workpiece on the way that the sensor is kept at a distance equal to Mr from the workpiece during the execution of the work operation. An alternative method is to store the the values over a half period and at the end calculate the side and height position.

No immediate action is required for follow-up. The sensitivity is reduced In this way.

It has been described above how during tracking the sensor is kept in a relatively constant position the workpiece, and in this way also the one placed on the robot hand the work tool, such as a welding electrode, to be closely monitored desired working path, e.g. the weld 22 in Fig. U.

It has been described above how, during follow-up, the required scan can be accomplished. is obtained by imparting a periodic movement to the entire sensor S. Alternatively the sensor can be fixedly mounted on the robot hand, whereby per se known the periodic search movement of the measuring point (light spot 23) can come through mirrors or prisms, which are imparted a periodic forward and reciprocating or continuously rotating motion.

Fig. 6a schematically shows the far left of the sensor S during measurement of stand for a workpiece. The latter consists of, for example, two plates 10 'and 10 ", which at 22 form an overlap joint. The conditions become analogous in it In this case, the sensor is used to detect an edge of a sheet or other material. goal 10 ", which is placed on a base 10 '. The diagram marked with SM shows the measured distance m as a function of the time t during the equipment search mode, ie when the sensor of the robot is moved along the surface of the workpiece in it with the arrow showed the search direction. At time t1 the edge of the plate 10 'is passed, i.e. joint 22, and a discontinuity is obtained at the measured distance. As below to be described, a so-called search stop signal is emitted at t1, which stops it with using the robot arm performed the search operation. The diagram to the right, which is marked with TM, shows the output signal m of the sensor as a function of time t during technical follow-up, eg when the robot moves along during the execution of a weld his t and t 1 '2 3 marked times denote the moments when the measuring point passes the joint 22 the joint 22 in a direction perpendicular to the plane of the paper and sweeps measuring point periodically back and forth over the joint 22. Those with t whereby discontinuities are obtained in the measured distance. Under follow-up as described in connection with Figs. H and 5, a correction signal can be recovered which automatically causes the robot hand to follow the joint 22. 15 20 25 30 35 455 281 Fig. 6b shows the corresponding figure and diagram for the case where the robot is arranged. detecting a keel joint 22 between two plates 10 'and 10 ". The left, with The SM marked diagram shows the measured distance m as a function of time during the search mode, ie when the sensor S is moved by the robot arm in the direction of the arrow.

The measuring point of the sensor places the joint at time t1 and which is to be determined below. is written, a search stop signal is emitted. The right diagram shows the sensor measurement signal as a function of time during the tracking mode in the same way as in fig Ba. At times t1, t2 and t3 Also in this case, of course, as described above, a correction signal can be the sensor's measuring point passes the joint center. is guided and supplied to the robot for automatic control of the robot hand along the joint.

Fig. 7 schematically shows a complete search process for an equipment according to the invention. The equipment is arranged to seek out an overlap joint 22 between two plates 10 'and 10 "mounted on a substrate. Of the robot, one the sensor S mounted on the robot hand is shown. The robot is programmed to move to a starting point, such that the sensor's measuring point is in the point marked with I. When this point is reached, the control means of the robot in the program to the first search stage, to which the robot arm touches substantially vertically down towards the surface of the workpiece along it with a dashed line denoted the path. At the point indicated by II, the sensor has and thus the robot hand has reached a predetermined desired distance from the workpiece, whereby a first search stop signal is emitted. The search stop signal interrupts the robot movement, the robot makes a jump but goes back to point II.

The search stop signal further causes the robot's control means to continue next search stage. In this, the robot hand is caused to move in a programmed direction from point II. This direction is essentially parallel. with the surface of the workpiece and perpendicular to the joint 22. When the sensor place in point III when the joint gives a second search stop signal. This search stop- signal interrupts the robot's movement. The measuring spot is moved in a piece on the surface to point IV. The search stop signal further initiates the next search stage, in which the robot moves along a pre-programmed direction from point IV. This direction is suitably substantially parallel to the longitudinal direction of the joint 22. When the measuring point of the sensor at point V reaches the edge of the workpiece, a third search stop signal, which interrupts the robot's movement. The measuring spot is in point V. From this point a programmed motion is made so that the welding electrode positioned to point VI. 10 15 20 25 30 -35 455 281 10 The third search stop signal emitted in point V initiates (possibly after correction of the initial position) commencement of the work operation. Robots control means are then arranged to control the robot hand so that it follows a programmed direction which substantially corresponds to the longitudinal direction of the joint 22. If the weld is short and / or the deviations in orientation between on successive work objects are small, the operation can be performed along the programmed the course without the use of automatic tracking. For longer welding joints or in the case of major variations between the workpieces in However, automatic tracking can be used. In the latter case, robotic the program is arranged to start oscillating the sensor and to correct the programmed path with the corrections received from the sensor signal processing unit. the signals (see description above in connection with Figs. 4-6).

In the following, the working method of the central unit MP will be described in more detail in connection with Fig. 8. The digital measuring signal incoming to the unit M is first subjected to a filtration. The H0 most recently filtered measured values are constantly stored in a memory MU, which contains the filtered ones the measurement signals Mo, Mi, räè. The value M0 is then the latest filtered measurement. the value. The memory MU is shown schematically in Fig. Bb. Fig. 8a schematically shows one flow chart for the filtration. This is performed at intervals of 0.6 ms, ie every other ms a start signal is obtained. M is the current measured value and u is a constant, which can be 0.6, for example.

At intervals of 3.5 ms, a calculation is performed in the central unit MP to detect height, an edge or a joint. The basis for this calculation is thereby the contents of the memory MU. Fig. 9 shows the MP operating mode of the central unit at height increase (distances I-II in Fig. 7). Every 3.5 ms a start signal is obtained.

First, a calculation is made of the average value MV2 of the five most recent measured values in memory MU. This mean value is then compared with a height reference value do.

If the calculated mean value MV2 is greater than the reference value, the central device and waiting for a new start signal. If, on the other hand, the mean value MV2 is smaller than or equal to the reference value, a search stop signal SS is emitted.

Fig. 10 shows the operation of the central unit during side search (section II ~ III or IV-V in Fig. 7). This calculation is also performed at intervals of 3.5 ms.

Every 3.5 ms, a start signal is thus obtained, which starts the calculation process.

First, the two mean values MV2 and MV17 are calculated and then based on these averages a direction signal RI and then a forecast value PR.

The forecast value is the measured value that can be expected at the current moment if the surface of the workpiece is flat, ie if the measured distance is linear 10 15 20 25 30 35 455 281 ' 11 _ and continuous operation of time. By calculating the direction signal RI account is also taken of the case where the sensor does not move in parallel during the search. lellt with the surface of the workpiece. It is then examined whether the forecast value PR deviates with more than a predetermined amount K from the most recent measured value M. If the then even less than K sets the central unit a quantity N to zero and returns and waiting to get a new start signal after 3.5ms. The quantity K is chosen so that it is not exceeded for minor irregularities in the surface of the workpiece but so small that it will certainly be exceeded in the occurrence of an edge, fog e d. If the absolute value of the deviation PR - M is greater than K, the program runs further and increases the quantity N by one. It is then examined whether N has reached the number 5. If this is not the case, the memory area MU is updated with the forecast value PR i instead of with the latest measured value and the program returns to wait a new start signal. About the detected deviation between forecast value and measured value only been temporary, eg caused by a scratch in the workpiece or a dirt particle, the deviation between the forecast value and measured value to be less than K, the quantity N to be reduced by 1 to at least 0 and the memory area MU is updated with the measured value M. If the deviation is instead due to an edge or joint is detected, a deviation will also occur in subsequent calculations to occur between forecast value and measured value, the quantity N will be increased by one at each calculation and when the quantity N reaches the value 5, a search stop signal is emitted.

The invention has been described above in connection with one used for arc welding robot, but a device according to the invention is of course also applicable to other purposes. A device according to the invention is generally useful there there is a requirement to be able to accurately determine the position and orientation of workpieces in order to determine the point and / or the path there a work operation must be started or performed.

In some welding applications, the weld joint consists of a gap. That is often the case of importance to be able to determine the dimensions or cross-sectional area of the gap for to be able to choose optimal welding parameters. The device according to the invention can be used for this purpose. Since the sensor included in the device is a distance sensing sensor, a measure of the depth of the gap is easily obtained, and of The method described above can be used during both searching and tracking the position of the column edges and thus its width is determined. - A device according to the invention offers significant advantages over known devices of a similar kind. Because the sensor is arranged on the robot hand and by using the robot for the initial search, can in a simple way, even large areas are quickly scanned. When the robot hand reaches the the searched place on the workpiece is both the robot hand and the sensor directly 10 15 20 25 455 281 12 in the right position, either to initiate an additional search phase or to start spirit of a work operation. This allows the robot to work with minimal time delays. In that the device according to the invention comprises a spacer sensing sensor, the function of the device becomes to a very large extent independent of surface condition, reflectivity etc of the workpiece. The device has therefore a great deal of flexibility and a wide range of uses and can be used das during operations on the workpieces of the most diverse types and performed by the most diverse materials. The distance measuring sensor also entails, as mentioned above the essential advantage that, if necessary, the welding volume of a gap joint or corresponding can easily be calculated and used for setting welding parameters.

Because the detecting means of the sensor used only "consider" it a lot small light spot that the sensor generates on the workpiece, the sensor becomes a lot insensitive to disturbing light. This insensitivity contributes significantly the fact that according to a preferred embodiment a one-dimensional photodetector tor was used. Hereby the device according to the invention becomes useful in e.g. electric arc welding or other operations, which produce a powerful disturbing light. The device according to the invention hereby entails very essential advantages over prior art equipment, which have not been useful during welding or other operations, which produce a highly disturbing light.

A further significant advantage thus obtained is that the measurement of the sensor point can be arranged very close to the working point of, for example, the one carried by the robot welding equipment, whereby on the one hand it is achieved that the dimensions of the hand-carried equipment becomes smaller, thereby increasing weight and accessibility, partly that the accuracy of search and tracking will be great. The device according to the invention has been found in experiments to be very reliable and useful.

It can thus, for example, with great certainty detect edge joints in very thin sheet metal, which has been a major problem with prior art equipment.

Claims (9)

- 455 281 13 PATENT REQUIREMENTS Device in an industrial robot, which is arranged to perform a work operation on a workpiece (10) - The device is characterized in that the robot is provided with a sensor (S) placed on the robot hand (8), which is arranged to sense the distance between the robot hand and the workpiece using optical triangulation. The robot is provided with control means (RC) arranged to control the robot hand in two successive pre-programmed search movements (eg II-IV; IV-V) in different directions. At least one direction of the search movement has a component parallel to the surface of the workpiece facing the robot hand. The device has signal processing means (SC) arranged to determine the position of the workpiece in the search direction relative to the robot on the basis of the distance sensed by the sensor during the search. The signal processing means (SC) comprises means (MP) arranged to detect the discontinuity in the distance sensed by the sensor which occurs when the robot hand (8) during an search movement with a component parallel to the surface passes an edge, joint or equivalent ( 22) of the workpiece (10) and thereby emit an indicator signal (SS). The sensor (S) comprises a light source (30) arranged to project a light spot (23) onto the workpiece (10), and optical means (32) arranged to generate on a one-dimensional detector (33) an image (3H) of the light spot, wherein the sensor is arranged to emit an output signal (m) which is dependent on the position of the detector of said image (3ß) and thus on the distance between the sensor and the workpiece. Device according to claim 1, characterized in that the signal processing means are arranged to calculate a forecast value (PR) for the distance on the basis of values sensed during the search operation, to compare the sensed distance (M) with this forecast value and that , if the sensed distance (M) deviates from the forecast value (PR) by more than a predetermined amount (K), emit said indication signal (SS). Device according to claim 1, characterized in that the indication signal (SS) is arranged to be supplied to the robot's control means (RC) for interrupting an ongoing search operation. U. Device according to claim 3, characterized in that the control means of the robot are arranged to perform a second pre-programmed search movement (IV-V) with a direction after the interruption of a first search movement 455 281 '14 «(e.g. II-IV). forming an angle with the direction of said first search movement, and that the signal processing means (SC) are arranged to output to the control means an indication signal (SS) for interrupting the second search movement when detecting a distance discontinuity. Device according to claim 1 or H, characterized in that the robot's control means (RC) are arranged to control the robot to perform said work operation after the interruption of the search operation. Device according to claim 5, characterized in that the control means are arranged to control the robot along a pre-programmed path during the working operation. Device according to claim 5 or 6, characterized in that the sensor (S) is arranged to periodically scan the distance between the robot hand and the workpiece along the search operation which forms an angle with the movement of the robot hand, that the signal processing means (SC) are arranged to sense the position along the search line of a distance discontinuity caused by, for example, an edge or a joint and to supply the robot control means a correction signal (ds) dependent on said position to cause the robot to follow said distance discontinuity. Device according to one of the preceding claims, characterized in that the distance sensed by the sensor (S) during the working operation is supplied with means (MP) arranged to form the deviation between the sensed distance (Ma) and a reference value. (Mr) and to supply the deviation of the robot (RC) with said deviation for controlling the distance between the robot hand and the workpiece to said reference value. Device according to claim 1, characterized in that the detector (33) is an analog lateral photodetector.