Abstract:
A system and method for controlling a robot ( 1 ), includes at least three setting devices ( 2, 3, 4 ) which can be extended or shortened in the longitudinal direction. Each setting device being directly or indirectly secured in a fixed frame ( 6 ) via a first joint ( 20, 30, 40 ) so that each setting device is pivotable in all directions in relation to the frame and that each setting device is attached at one end in a movable position head ( 8 ) via a second joint ( 21, 31, 41 ). Each sensor is provided with a length sensor (LS 1 , LS 2 , LS 3 ), said sensors forming a part of control system (S 1 ) for controlling the location (X, Y, Z), wherein the control system cooperates with a feedback control system (R 2 ) arranged to correct the location (X, Y, Z) and feedback control system (R 2 ) operate in accordance with different coordinate systems.

Description:
TECHNICAL FIELD 
     The present invention relates to controlling and correcting the spatial location of the positioning head of a robot. In the following, the term “robot” also covers machine tools of NC type, for instance, since in certain applications the present robot acts as a numerically controlled machine tool. 
     BACKGROUND ART 
     According to Patent specification SE 8502327-3 (452 279) a robot with a positioning head, the robot comprising at least three setting devices which can be extended and shortened in longitudinal direction, is already known. Each setting device is secured in a fixed frame via a first joint so that each setting device is pivotable in all directions in relation to the frame. Each setting device is also attached at one end in a movable positioning head via a second joint. A rigid arm is also joined to and emanates from the positioning head between the setting devices, the arm being radially guided but axially displaceably journalled in a universal joint relative to and rigidly connected to the frame. 
     A similar robot is also known in which the setting devices are secured in a displaceable runner via a first joint, the runner being secured in a fixed frame, so that each setting device is pivotable in all directions in relation to the frame. The joint is thus indirectly secured in the frame. 
     These known robots are controlled by each setting device being provided with a motor to extend or shorten the setting device. Control is effected by the setting devices being controlled linearly to a predetermined location of the positioning head or the positioning head being controlled along a predetermined pattern of movement. There is thus no control of the set location of the positioning head, which means that the influence of force and temperature on the setting devices, joints, positioning head, etc. results in a difference between the desired set point and the actual position. 
     This difference between set point and actual position is unsatisfactory and leads to reduced accuracy in the use of the robot. 
     OBJECT OF THE INVENTION 
     The object of the present invention is to increase the accuracy of a robot of the type described above. 
     SUMMARY OF THE INVENTION 
     This object is achieved by arranging a feedback control system to supplement the existing control system. Characteristic of the invention is first that the existing control system and the feedback control system indicate the coordinates of the positioning head in different coordinate systems. Secondly, the sensors of the feedback control system are placed for measurement on a part which is movable in the robot and accompanies its movement, but which is free of tension. The part is thus not influenced by temperature or force. 
     The invention thus relates to a control system for a robot, which robot comprises at least three setting devices which can be extended or shortened in the longitudinal direction. Each setting device is secured in a fixed frame via a first joint so that each setting device is pivotable in all directions in relation to the frame. Each setting device is attached at one end in a movable positioning head via a second joint. A rigid arm is joined to, emanates from the positioning head and is arranged between the setting devices, the arm being radially guided but axially displaceably journalled in a universal joint relative to and rigidly connected to the frame. Each setting device is provided with a length sensor LS 1 , LS 2 , LS 3 , said sensors forming a part of a control system S 1  for controlling the location (X, Y, Z) of the positioning head in the work space of the robot. The control system thus cooperates with a feedback control system R 2  arranged to correct the location (X, Y, Z) of the positioning head in the work space. The control system S 1  and feedback control system R 2  operate in accordance with different coordinate systems. 
     The invention also relates to a method for controlling the location of the positioning head of a robot, wherein 
     a) a control system controls the positioning head in accordance with a cartesian coordinate system, 
     b) a feedback control system senses the location of the positioning head in accordance with a spherical coordinate system, 
     c) a coordinate transformation is performed of the location of the positioning head from the spherical coordinate system to the cartesian coordinate system, 
     d) the deviation obtained between set point and actual position in the cartesian coordinate system is added to the previous set point of the positioning head, thus producing a new set point 
     e) the location of the positioning head is altered to this new set point, 
     f) steps a)-e) are repeated to achieve continuous control of the set point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention will be described in more detail by means of embodiments with reference to the accompanying drawings. 
     FIG. 1 shows a front view of the robot in accordance with the invention. 
     FIG. 2 shows a view in perspective of a fix holder for a universal joint in accordance with the invention. 
     FIG. 3 shows a view in perspective of a central arm controlled in the universal joint, where the joint is provided with sensors operating in a separate coordinate system in accordance with the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention relates directly to a robot  1  having at least three setting devices  2 ,  3 ,  4  which can be extended or shortened in the longitudinal direction, each setting device being secured in a fixed frame  6  via a first joint  20 ,  30 ,  40  so that each setting device  2 ,  3 ,  4  is pivotable in all directions in relation to the frame  6 . Each setting device is also attached at one end in a movable positioning head  8  via a second joint  21 ,  31 ,  41 . An intermediate arm  10  is also joined between the setting devices  2 ,  3 ,  4  and emanates from the positioning head  8 . In FIG. 1 the setting device  3  with its first joint  30  and second joint  31  is hidden by this intermediate arm  10 , but these parts are indicated by arrows. Similar to the manner shown in patent specification SE 8502327-3 (452 279), each of the setting devices is in the form of a piston movable in a cylinder. 
     The arm  10  is connected to the positioning head  8  and extends up between the setting devices. It is radially guided in a universal joint  12  secured to the frame  6 . The universal joint  12  permits the arm to slide axially through the joint while being radially guided. The universal joint  12  also permits the arm  10  to be turned an angle α about a first axis and to be turned an angle β about a second axis. The first and second axes intersect each other under the angle 90°. 
     Each setting device  2 ,  3 ,  4 , is operated by a motor  22 ,  32 ,  42 , preferably of electric type, in accordance with a predetermined pattern of movement. The movement pattern forms the basis of a control system S 1  which activates respective motors  22 ,  32 ,  42  to extend or shorten the setting devices  2 ,  3 ,  4 , thus causing displacement of the positioning head  8  to a specific location in its work space or to continuously move the positioning head  8  along a given curve in the work space. Each setting device is provided with an angle transducer measuring the motor position, which represents the longitudinal location of the setting device and is thus synonymous with a length sensor LS 1 , LS 2 , LS 3 , which sensors form a part of the control system S 1  for controlling the location X, Y, Z of the positioning head in the work space of the robot. The positioning head  8  is thus guided to a position X, Y, Z in a cartesian coordinate system, as indicated by the coordinate system in FIG.  1 . As is also clear in FIG. 1, a tool head  14  is connected to the positioning head  8  by connecting means  16 , which determine whether the robot is to have 4, 5 or 6 axes. More than six axes is also possible. 
     FIG. 2 shows a part of the robot with the arm  10  and a universal joint housing  13  from which the inner part of the universal joint  12  has been dismantled. The universal joint housing  13  also carries the first joints  20 ,  30 ,  40  externally. As is clear in FIG. 2, all these first joints are of universal type, i.e. the setting devices can turn in the joint above two perpendicular axes. However, the joints do not allow the setting devices to be displaced axially through the joints. As described earlier, the movement in axial direction is achieved by the setting device being extended or shortened. 
     FIG. 3 shows the arm  10  and the internal parts of the universal joint  12 , i.e. the parts mounted in the universal joint housing  13 . These inner parts consist of an inner ring  18  and an outer ring  19 . The arm  10  can slide in the inner ring  18  but is locked against turning by slide rails  50  placed opposite each other on the arm  10  and running in holders  51  in the inner ring. Externally the inner ring is provided with fixed opposite bearing pins  60  pivotably journalled in the outer ring  19 . The outer ring is also provided with fixed opposite bearing pins  70  journalled in the universal joint housing  13 . All bearing pins are arranged in one and the same plane but an axial line through the bearing pins of the inner ring intersects an axial line through the bearing pins of the outer ring at an angle of 90°. 
     The pattern of movement for the arm  10  in the universal joint  12  is such that it can be turned an angle α about a first universal axis and an angle β about a second universal axis. The arm can also slide the distance l in the inner ring  18 . Since the arm is joined to the positioning head  8  this means that it can assume all the positions X n , Y n , Z n  in a defined work space which is only limited by the construction of the robot. 
     FIG. 3 also shows that the outer ring  19  is provided with a first angle sensor  71 , see also FIG. 1, which detects the angle α of the arm about the first universal axis. The outer ring is also provided with a second angle sensor  72  which detects the angle β of the arm about the second universal axis. Furthermore, a length sensor  73 , preferably in the form of a glass scale, is arranged along the arm to detect the length location l of the arm. These three sensors produce coordinates corresponding to the location of the positioning head  8  in a spherical coordinate system indicated in FIG.  3 . This location in the spherical coordinate system represents the actual location of the positioning head  8 . This actual location is obtained in the above-mentioned cartesian coordinate system by means of a coordinate transformation. 
     The coordinate transformation is obtained by the equations: 
     
       
           X   a =l sin α cos β 
       
     
     
       
           Y   a =l sin α sin β 
       
     
     
       
           Z   a =l cos α cos α 
       
     
     The positioning head  8  is thus controlled in the cartesian coordinate system to the location (X b , Y b , Z b ) representing a set point. The location is thereafter detected by the sensors  71 ,  72 ,  73  which indicate an actual location (α, β, l) in the spherical coordinate system. The coordinate transformation is performed, whereupon an actual location (X a , Y a , Z a ) is obtained in the cartesian coordinate system. After that the difference (ΔX, ΔY, ΔZ) is calculated between the set point and the actual position in the cartesian coordinate system. This difference is finally added to the previous set point (X b , Y b , Z b ) in order to obtain a new set point (X n , Y n , Z n ). These detections and calculations are repeated continuously for feedback control, and increased accuracy of the location of the positioning head is achieved through compensation for force, temperature and mechanical deviations in the machine. 
     The deviation (ΔX, ΔY, ΔZ) between set point (X, Y, Z) and actual location (X a , Y a , Z a ) is thus added in the cartesian coordinate system to the previous set point (X b , Y b , Z b ), thus giving a new set point X n =X b +ΔA, Y n =Y b +ΔY, Z n =Z b +ΔZ. This calculation and correction is then repeated for continuous control of the set point. 
     FIG. 3 also shows that a tool head  14 , movable in at least two axes, is connected to the positioning head  8 . Turning about these axes is also detected by sensors  81 ,  82  which may also be included in the control system for correcting the set point. If the number of axes for the tool head is two, a robot with 5 axes will be obtained. More axes are also possible.