Abstract:
A method for controlling a manipulator includes determining by a control device one or more contact force values between the manipulator and a first workpiece. Each of the contact force values is based on an actual driving force of the manipulator and a drive force according to a dynamic model of the manipulator. The method also includes at least one of a) measuring in multiple stages an orientation and location of the first workpiece based on at least one of the one or more determined contact force values or b) joining a second workpiece and the first workpiece under a compliant regulation, where a joining state of the first and second workpieces is monitored based on at least one of an end pose of the manipulator obtained under the compliant regulation, a speed of a temporal change of the manipulator, or at least one of the one or more determined contact force values.

Description:
TECHNICAL FIELD 
     The present invention concerns a method and a device for controlling a manipulator, in particular a robot, and a retaining tool for a manipulator, used according to the invention. 
     BACKGROUND 
     Industrial robots are to be used for, among other things, the joining of components. For this, position tolerances, in particular, of workpieces that have been supplied, which are to be joined to the workpieces that are held by the robot, complicate the automation. 
     Aside from structural compliances, such as a flexible robot, or a flexible end effector connection, for example by means of a so-called “remote center of compliance” (“RCC”), diverse theoretic solution approaches for such so-called “pin/hole” problems have been proposed, e.g. the detection of contact forces by means of additional contact sensors, and the determination of the correct joining trajectory based on said contact forces. These approaches, however, have rarely been put into practice due to a variety of problems. 
     The objective of the present invention is to improve the control of a manipulator. 
     SUMMARY 
     A method or, respectively, a device according to the present invention is provided, in particular, for a single or multiple, e.g. six-axis or redundant robot, such as, for example, an industrial robot or a lightweight construction robot, such as the lightweight construction robot “LBR” of the applicant. 
     For this, a control can also be understood to mean regulation, i.e. the specification of control dimensions based on given guidance, and detected actual, dimensions. 
     According to a first aspect of the present invention, a contact force is detected, based on the actual driving force and driving forces of a dynamic model of the manipulator. 
     For the purpose of condensing the depiction, for the present, a torque, i.e. a non-parallel pair of forces, is referred to in general as a force, such that, for example, the detection of a contact force is to be understood as the detection of force and/or torque components along one or more axes, in particular Cartesian axes, or articulation axes, and a driving torque, such as that of an electric motor, is referred to as a driving force. 
     Actual driving forces can be detected directly, by means of force sensors on a single axle or actuator of the manipulator, and/or indirectly, e.g. based on power consumption or output of a actuator. 
     A dynamic model describes in general the connection between kinematic dimensions, in particular articulation coordinates q, speeds dq/dt and accelerations d 2 q/dt 2 , and forces, in particular drive, weight and frictional forces, such that
 
 Md   2   q/dt   2   +h ( q,dq/dt )=τ modell  
 
with the mass matrix M and the generalized forces h. If the kinematic dimensions such as weight and frictional forces are known, then differences result between the drive forces τ modell  of the model and the actual drive forces τ, aside from model and measurement errors, in particular from contact forces not provided in the model, which can be determined therefore due to the difference between the actual measured drive forces and the model drive forces, such that:
 
 F   kontakt   =F (τ modell −τ)
 
     This detection of contact forces between the manipulator and a workpiece, based on the actual drive force and drive forces of a dynamic model, eliminates additional force sensors and enables, in an advantageous manner, a measurement that will be explained later, a joining monitoring, and/or a switching to a compliant regulation. 
     According to one aspect of this first aspect of the present invention, one or more poses, in particular for the measuring or joining of workpieces, is initiated with the rigidly regulated manipulator. A rigid regulation is understood, in particular, to mean a position regulation, for example a proportional, differential and/or integral regulation, of a Cartesian position of a reference point such as the TCPs, for example, or in the articulation coordinates of the manipulator. 
     For this, one or more contact forces are detected continuously, or at discreet points in time, which act on the manipulator when contact is made with a supplied workpiece. 
     If a detected contact force exceeds a given value, which in particular can also be zero, if applicable, taking into consideration the corresponding tolerances, then, according to the invention, the switching to a compliant regulation is carried out. Ideally, a switching of this type is executed within a maximum of 3 milliseconds after the establishment of contact, preferably within a maximum of 1.5 milliseconds. 
     A compliant regulation, or a switching to said regulation, can be obtained, for example, by means of reducing the regulation coefficients of a P or PD regulation, and/or the elimination of an integral portion of a PID regulation, such that the manipulator does not exert a high level of drive force even with larger regulation deviations, or tracking errors, respectively, between the target and the actual pose. In addition, or alternatively, a drive force can be limited to a, preferably low, maximum value, such that in turn, no high level of drive forces can be generated, even with larger tracking errors. In a preferred embodiment, a compliant regulation is designed as a so-called impedance regulation, in particular, a force based impedance regulation. In general, a compliant regulation, in contrast to a rigid regulation, indicates, in the present case in particular, any regulation in which the drive forces of the manipulator are generated such that with contact to a workpiece, neither said workpiece, nor the manipulator are damaged, even if a given target pose demands a more extensive engagement by the manipulator with the workpiece. 
     In a preferred embodiment, the workpiece is then joined using the compliant regulation and/or its position is measured based on the detected contact force in a multi-stage procedure. In this manner, it is possible, in particular in connection with a lightweight construction robot, having limited inertia, to reduce the cycle times in an advantageous manner, as this then allows for a measurement or joining pose to be initiated, by means of a rigid regulation, quickly and precisely, without the risk of damaging the workpiece or manipulator in the course of measuring or joining the workpiece, because when contact is initiated, a switching over to the compliant regulation occurs immediately. 
     According to another aspect of this first aspect of the present invention, a workpiece is joined while subject to a compliant regulation, and a joined state of the workpiece based on a detected contact force is monitored. By means of monitoring a contact force, a snapping in place of a workpiece, joined while subject to an elastic or plastic deformation, can occur, due to a significant increase or decrease of said force, and a correct joining procedure can thereby be ascertained. In this manner, a contact force is then increased in the course of the joining by means of an elastic deformation, in particular in opposition to the direction of the joining. When the workpiece then settles, for example, after overcoming a bead, this force decreases significantly. Likewise, the state of tension of a screw can be checked and determined by a torque, in the form of a contact force applied to the manipulator, detected according to the first aspect, which is exerted by the inserted screw, as to whether or not the screw has been tightened too tightly, its head has been twisted off, or the screw is too loose, due to a settling of the threading. 
     Additionally, or alternatively, a joining state of the workpiece can be monitored, based on an end pose of the manipulator obtained under the compliant regulation. It is possible, by means of a compliant regulation, to obtain that the manipulator, independently of having reached a target joining end pose, no longer moves the workpiece in the joining direction. If the end pose obtained through the compliant regulation is compared with the target joining end pose, which can be determined by means of instructing a robot, it is possible to determine whether the joining procedure has been correctly executed. 
     Furthermore, additionally or alternatively, a joining state of the workpiece can be monitored based on the temporal change to the pose of the manipulator resulting under the compliant regulation as well, in particular, based on the speed of said change. If a speed of the TCP, or the workpiece guided by the manipulator, respectively, falls below a limit value, for example, it is possible to determine that the workpiece is correctly joined. 
     If this is not the case, for example, because the workpiece that is to be joined is impeded by an interfering contour, then a joining strategy, in particular a joining position, can be changed, in that, for example, it is attempted to insert a bolt from a new starting position, shifted from the original starting joining position. 
     According to a preferred design, in the joining of a workpiece using a compliant regulation, both one or more target forces as well as one or more target movements are to be provided. If the compliant regulation is obtained, for example, by means of a force-based impedance regulation, in which the drive force τ is defined as
 
τ= J   T   [c ( x   soll   −x )+ F )]
 
by means of which the TCP is determined, as a result of the transported Jacobian matrix J in the space of the articulated coordinates, the rotational angle of an articulated arm robot, the projected spring tension of a virtual spring having the spring constant c, between a Cartesian target (soll) and actual (ist) position x soll  or x, then the target movement, according to
 
τ= J   T   [c ( x   soll   −x )+ F] 
 
can have a force defined in Cartesian space applied to it. This can occur selectively, preferably, depending on the already executed joining procedure, or the joining strategy for the respective workpiece, for example. The target force can be given as constant, inclining, alternating or swelling, and in particular, sinusoidal, for example, in order to overcome, by way of example, stick/slip phenomena, ridges, or small elastic tolerances that occur in the joining.
 
     In particular, in order to join workpieces to one another, it is necessary to recognize a position, i.e. a location and/or orientation of a workpiece, such as a base body, for example, in relation to the manipulator, that a workpiece that is to be joined, such as a bolt, a clamp, or similar items, is in. Thus, according to a second aspect of the present invention, a position of a workpiece is measured by means of the manipulator in a multi-stage procedure, whereby, preferably this second aspect is combined with the first aspect, explained above. 
     According to this second aspect, positions of preferably at least two non-aligned contours, in particular, edges, of the workpiece are determined, in that poses of the manipulator, and thereby the contact force applied to said robot, are detected. If a contact force increases significantly when a pose is assumed, resulting from a feeler of the manipulator coming into contact with a contour of the workpiece, then the contact position can be determined from the associated pose of the manipulator. 
     If, in this manner, the position of the workpiece in relation to the manipulator is roughly known, then reference points of the workpiece can be assigned, which can be defined, for example, by recesses, in particular, holes. When a feeler makes contact with a reference point, such as, by inserting a feeler in a recess, for example, then, due to an increase in the corresponding contact force, which acts against further movement of the feeler, the position of the reference point can be determined. Based on these reference points, a workpiece coordination system can be determined, which enables, in contrast to the rough orientation of referenced contours, a precise determination of, for example, joining positions and suchlike. 
     Preferably, a reference point is approached by a feeler in a perturbative manner, i.e. the manipulator moves the feeler to the immediate vicinity of the position, estimated in advance, of the reference point, along a predetermined tracking path, preferably along a single plane, which is oriented substantially tangential to the contour of the workpiece at the reference point, and in this manner, searches the region for the previously estimated position of the reference point, until, due to the contact force resulting from the slipping of the feeler into a hole, it is prevented from moving further. In doing so, the advantage is demonstrated, when according to the first aspect, the position, estimated in advance, of the reference point is approached according to a compliant regulation. 
     In particular, in order to ensure the slipping of the feeler into a reference point, which defines the recess, the manipulator can exert a standard force on the workpiece, perpendicular to the plane explained above, said plane being oriented substantially tangential to the contour of the workpiece at the reference point. In a preferred design of the present invention, a feeler and a recess for defining a reference point are constructed such that the feeler centers itself when slipping into the recess. By way of example, the feeler can be a cone shaped point, and the recess can have a circular perimeter. 
     The contacting of contours for determining the rough position of the workpiece, in order to be able to reliably make contact with reference points, and the contacting of reference points can take place successively such that, first, all predefined contours are contacted, before, subsequently, the reference points, the approximate positions of which are known, are approached. At the same time, these steps can be carried out alternatingly, such that first, the respective contours, preferably in the vicinity of the reference points, are contacted, and subsequently the reference points, the approximate locations of which with respect to said contour, are known, before subsequently, other contours are contacted in turn, in order to determine the approximate positions of other reference points, and to reliably contact said reference points subsequently. 
     A third aspect of the present invention concerns the joining of a workpiece by means of the manipulator. This can advantageously be combined with the first and/or second aspects explained above. 
     According to the third aspect, the manipulator holds a workpiece that is to be joined, first in at least two, preferably non-collinear, in particular substantially perpendicular to one another oriented, force-contacts, when it places said workpiece in a joining starting position. For this, a force-contact indicates a contact between the workpiece and the manipulator, in particular a retaining tool, with which the manipulator exerts a retaining force on the workpiece, oriented in a manner corresponding to the direction of the respective retaining force. 
     By way of example, a first force-contact can be realized in a force-locking manner by means of an electromagnet, which attracts the workpiece when actuated. A second force-contact can, for example, be realized in an interlocking manner by means of a lip, on which the workpiece is supported. 
     After being placed in a joining position, the manipulator moves the workpiece in the direction of the joining to a joining end position. According to the invention, one of the force-contacts, for example, the first one, is opened by means of a force-contact established by an electromagnet when its supporting or retaining function is taken over by a workpiece, with which the workpiece held by the manipulator is to be joined. 
     This enables a reorientation of the manipulator that is holding the workpiece, such that a conflict with contours of a workpiece, with which the workpiece held by the manipulator is to be joined, can be prevented. 
     In particular, in order to enable shorter cycle times for a measurement and a subsequent joining of workpieces according to at least one of the aforementioned aspects, a retaining tool according to the invention for a manipulator exhibits a retaining region for holding, in particular in a force-locking or form-locking manner, a workpiece, and also a feeler for making contact with a workpiece. The retaining region can, for example, depict, with one or more electromagnets and/or lips, the first or second force-contact described above, having feelers, preferably having cone shaped ends for self-centering in a recess under a compliant regulation. Preferably, the feeler extends at a substantially right angle to, or in the opposite direction of the position or retaining region of the retaining tool, in order to prevent conflicts between a workpiece and the retaining region during the measuring, or the feeler during joining. A feeler as set forth in the present invention can be a bolt, a pin, or similar item, designed, in particular, to be rigidly connected to a retaining tool of the manipulator, or as an integral part of said tool, which is preferably rotationally symmetrical, and/or having a point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and characteristics can be derived from the dependent Claims and the embodiment examples. For this, the drawings show, in part, schematically: 
         FIG. 1 : A top view of a workpiece being measured in accordance with a method according to one design of the present invention; 
         FIG. 2 : A cut along the line II-II in  FIG. 1 ; 
         FIG. 3A : A side view from the upper left perspective in  FIG. 1 , during the joining of a workpiece in accordance with a method according to one design of the present invention; 
         FIG. 3B : A cut along the line IIIB-IIIB in  FIG. 1 ,  3 A, with a workpiece placed in the joining starting position; 
         FIG. 3C : A view according to  FIG. 3B  with a workpiece in the joining starting position; 
         FIGS. 4A-4D : A side view during the joining of another workpiece in accordance with a method according to one design of the present invention; 
         FIG. 5 : The sequence of the measurement show in  FIGS. 1 ,  2 ; 
         FIG. 6 : The sequence of the change of a joining position shown in  FIG. 3A ; and 
         FIG. 7 : The sequence of a joining shown in  FIGS. 4A-4D . 
     
    
    
     DETAILED DESCRIPTION 
     Based on  FIGS. 1 ,  2 , and  5 , a multi-stage measurement of a position of a workpiece  2  is explained, on the basis of contact forces detected in accordance with a method according to one design of the present invention. 
     For this, the workpiece  2 , by way of example, an instrument panel, is first positioned after delivery within certain tolerances somewhere in the working region of a lightweight construction robot, wherein, only a part of a retaining tool  1  is shown in the figures; in  FIGS. 1 and 2 , only a cylindrical feeler  1 A with a cone-shaped point (cf.  FIG. 3A ) configured at a right angle to a retaining region  1 . 1 ,  1 . 2  with which in the following, with respect to  FIGS. 3 and 4 , engagement will be made. A base coordination system B of the robot is defined in the work region, the x and y axes of which are indicated in  FIG. 1 . By means of the, explained below in increasingly greater detail, use of a retaining tool  1  according to the invention, having a retaining region  1 . 1 ,  1 . 2  (cf.  FIG. 3B ) and a feeler  1 A (cf.  FIG. 3A ), a tool change between the measurement and joining can be avoided, and thus, the cycle period can be reduced. 
     In a first stage of a multi-stage measurement, the robot moves its feeler  1 A, regulated by a rigid PID, along the x or y axis of the coordinate system B, until said feeler successively contacts the edges  2 . 1 ,  2 . 2  of the workpiece  2  at the points indicated in  FIG. 1  (cf.  FIG. 5 , step S 10 ). In doing so, a control device (not shown) compares the drive torque r measured on the robot with theoretical drive torques τ modell , that are theoretically necessary ( FIG. 5 : “S 20 ”), according to a dynamic, contact-free model, for generating the determined movement of the robot. If the difference between these drive torques, in particular in terms of the amount of torque, exceeds a predetermined limit value τ kontakt , it is determined therefrom that the feeler  1 A has made contact with an edge  2 . 1  or  2 . 2  of the workpiece  2 . As a result, a switching to a compliant force-based impedance regulation is carried out immediately, within 1 millisecond ( FIG. 5 : “S 30 ”), in which the feeler  1 A is “pulled” by a virtual spring towards a target position x soll . 
     By making contact with the two un-aligned edges  2 . 1 ,  2 . 2 , the position of the workpiece  2  in the coordinate system B of the robot can already be roughly determined ( FIG. 5 : “S 40 ”). For this, the switching to the compliant regulation ensures that the robot can quickly approach the contours, first regulated rigidly, without damaging the workpiece or robot through contact. 
     In a second stage, three non-collinear reference holes  3 . 1 - 3 . 3  in the workpiece  2  are approached by the feeler. For this, the robot moves a feeler  1 A into positions  3 . 1 .sub.geschatzt and  3 . 2   geschätzt  [translator&#39;s note: geschätzt: estimated] ( FIG. 5 : “S 50 ”), that based on the position of the workpiece  2 , roughly known from the first stage, have been estimated, whereby the feeler  1 A is moved in a perturbative manner, e.g. meandering or in parallel tracks, about the estimated positions (cf.  FIG. 2 ). For this, the tip of the feeler is pressed against the workpiece with a standard force perpendicular to the surface plane of said workpiece, while, in particular in this plane (left-right in  FIG. 2 ) the robot is regulated in a compliant manner. 
     As soon as the feeler  1 A moves from a position lying adjacent to a reference hole, indicated in  FIG. 2  with a broken line and labeled  1 A″, to the reference point position defined by the reference hole  3 , it slides into said hole with the standard force exerted thereby, and centers itself, based on its cone-shaped tip and the compliant regulation in the surface plane, in this hole, as is depicted in  FIG. 2  in the expansion. 
     A further movement of the feeler  1 A by the compliantly regulated robot, which continues to attempt to follow the tracking path, acts against a significantly greater contact force to the feeler  1 A sitting in the hole  3 . 2  at this point, which is detected in step S 60  ( FIG. 5 ). In this manner, the positions of the reference holes  3 . 1 - 3 . 3  can be determined quickly, precisely, and reliably. From this, in particular, the position of a workpiece associated coordinate system W in relation to the robot coordinate system B can be determined. 
     At this point, by way of example, a clamp  4  (cf.  FIG. 3 ) is applied to the workpiece at the upper left edge in  FIG. 1 , whereby the multi-stage measurement, by means of switching to a compliant regulation, can also be eliminated. This joining procedure shall be explained using  FIGS. 3 and 6 . 
     A first joining starting position is indicated by a broken line in  FIG. 3A , at which point the robot first attaches the clamp  4 , and attempts to push said clamp onto the workpiece  2  in the joining direction (index ′ in  FIG. 3A ; step S 110  in  FIG. 6 ). In doing so, the clamp  4  collides, however, with a flank  2 . 3  of the workpiece  2  (cf.  FIG. 1  as well). As described previously, when contact is made, a switching to a compliant regulation occurs ( FIG. 6 : “S 120 ,” “S 130 ”), in which a target movement x soll  superimposes a given target force F, in particular, a constant force F xy  in the joining axis, and a force F z , having a sinusoidal curve perpendicular to the joining axis and plane, in order to make a pushing onto the edge easier. 
     Due to the compliant regulation, no damage occurs to the workpiece or robot, despite the collision with the flank  2 . 3 . Instead, as soon as the tool  1  of the robot, having the clamp  4  held by it, no longer moves due to the resistance, the end pose of the robot reached thereby (indicated with a broken line in  FIG. 3A ) is compared with a learned end position, in which the clamp  4  is correctly applied ( FIG. 6 : S 140 ). In the present case, due to the significant difference, the control device recognizes that the clamp  4  is not correctly applied. As a result, the joining position is changed ( FIG. 6 : S 150 ), in that the robot places the clamp  4 , shifted to one side, on the workpiece again, as is indicated in the expansion in  FIGS. 3A-3C . This procedure is repeated until the robot can apply the clamp  4  without a collision, or a predetermined number of erroneous attempts has been obtained. 
     A successful joining procedure shall be explained based on the  FIGS. 3B ,  3 C. For this, the robot places the workpiece, if applicable, after the erroneous attempts described above in reference to  FIG. 3A , in a joining starting position, and pushes it lightly towards the workpiece  2  ( FIG. 3B ). In doing this, it holds the clamp  4 , firstly, with a first force-contact in the vertical axis by means of an activated electromagnet  1 . 2 , and at the same time, supports it with a second force-contact against a lip  1 . 1  of the tool  1 , counter to the second force-contact acting against the application in the horizontal plane, in a form-locking manner. 
     In order to prevent a collision of the tool  1  with the flank  2 . 4  of the workpiece  2  during further joining, the control device releases the first force-contact while the tool is being applied, in that the electromagnet  1 . 2  is deactivated, which enables a re-orientation of the tool  1  ( FIG. 3C ) in relation to the partially applied clamp  4 . In this manner, the robot can fully slide the clamp  4  with the tool lip  1 . 1  onto the workpiece  2 . 
     Based on  FIGS. 4 and 7 , the inserting of another component, specifically an elastic plug  40 , in a hole  20  is explained. One sees that the robot first moves the plug  40  in the vertical plane towards the hole  20  (step S 210  in  FIG. 7 ). If a given limit value, based on the difference between the measured contact force and the model drive torque contact force that has been detected, is exceeded, then the control device detects the contact with the workpiece ( FIG. 7 : “S 220 ”) and switches to a compliant regulation ( FIG. 7 : “S 230 ”). Under this regulation, the plug is inserted further and the contact force is detected by means of the insertion advance z. 
     One sees in the image series, FIG.  4 B→ FIG. 4C , that the plug is deformed in an elastic manner thereby. As soon as its lower flank has fully passed through the hole  20 , and returned elastically to its starting state, the contact force reduces, acting counter to the insertion. 
     This decrease in force is detected by the control device in a step S 240  and it can, based on this, be checked, even in a compliant regulation, whether the plug  40  has been correctly inserted in the hole  20 . Alternatively, or in addition, the end position of the robot obtained at a standstill can be compared with a previously learned end position, in order to check whether the plug  40  has been fully inserted in the hole  20 . 
     In addition, or alternatively, a speed criteria can be used here. This is reasonable, in particular, if the end position in the direction of insertion is not precisely known, if for example, the position of the workpiece  20  varies, without its being measured, prior to the insertion. If, for example, the speed of the TCP, or the workpiece  40 , respectively, drops for a given period of time below a predetermined limit value, then the control device can detect that the plug  40  cannot be further pushed into the hole. The end pose obtained in this manner is then detected and compared with a pose that has been saved when contact to the workpieces  20 ,  40  has been established, which can be determined by the increase in force detected thereby. If the difference between the two poses lies within a predetermined tolerance range, then the joining procedure is determined to be successful. 
     In an embodiment that is not shown, the lightweight construction robot inserts a bolt into a threading by means of a given target torque and/or a target advance. Here too, a contact force, e.g. a torque in the direction of turning, is detected and the insertion state is monitored on the basis of said contact force. If the robot has reached its learned position, and there is an excessive torque at this point, then the bolt has been turned too tightly. If, however, the torque is to little, the bolt is not securely tightened, because, for example, a nut has been displaced, or a bolt head has been twisted off. 
     In addition, or alternatively, the inserted screw can be turned further after being tightened to a defined torque, through a predetermined angle of, for example, 90°. After this turning, the detected torque for a correctly tightened screw must lie within a predetermined range. 
     REFERENCE SYMBOL LIST 
     
         
           1 ;  10  Retaining tool 
           1 A Feeler 
           1 . 1  Lip (retaining region, form-locking force-contact) 
           1 . 2  Electromagnet (retaining region, force-locking force-contact) 
           2  Workpiece 
           2 . 1 ,  2 . 2  Edge (contour) 
           2 . 3 ,  2 . 4  Flank 
           3 . 1 - 3 . 3  Hole (reference point) 
           4  Clamp 
           20  Workpiece 
           40  Plug