Abstract:
Industrial robot arranged with a helical spring based balancing system, which is able to stand up to high load.

Description:
TECHNICAL FIELD 
   The present invention relates to an industrial robot with an equalizing device, comprising a helical spring, and a method for balancing and use of the robot. 
   BACKGROUND 
   In industrial robots, comprising two robot parts pivotally arranged relative each other, powerful power consuming motors, which execute the pivoting of the robot, are required. Powerful power consuming motors are large, heavy and expensive, which commands a need of alternative solutions. One alternative is to supplement the robot with a device, which in the pivoting of the robot participates in the pivoting by absorbing the torque during the pivoting from a rest position/initial position, i.e. when the robot starts a work cycle. The concept pivoting from a rest position/initial position refers to a pivoting in a direction where the attraction of gravity contributes to the pivoting. The arrangement is of such a nature that it during the pivoting from the rest position generates a torque, which acts to restore the robot to its rest position/initial position and thereby helps/relieves the driving motor concerned during the lifting/pivoting back. The concept pivoting back to the rest position/initial position refers to a pivoting which counteracts and thereby compensates for the attraction of gravity, which pivoting in the following is designated balancing. The arrangement according to the foregoing is thus considered a balancing arrangement. 
   By arranging industrial robots with balancing arrangements, which help and relieve the driving motors, the robot manufacturer is not forced to install unnecessarily large and powerful motors in the robot. The opposite also applies, a powerful driving motor in combination with a powerful balancing arrangement increases the lifting capacity in the wrist of a large industrial robot. However, this leads to an increase in dead weight of both the motor and the balancing arrangement, which in turn calls for even bigger demands on the driving motor in question. 
   A balancing arrangement thus helps the motor concerned to counterbalance the applied handling weight as well as the present dead weight when pivoting occurs during operation of the robot. Balancing arrangements generally consist of weights, gas-hydraulic devices or spring devices in the form of helical springs, torsion springs and/or gas-based balancing cylinders. Apart from the counterweights the above mentioned devices are expensive, heavy and sensitive constructions. Gashydraulic devices are space demanding and are furthermore marred by density problems. 
   In helical spring-based balancing cylinders there is always a risk for obliquity of the piston rod in relation to the cylinder, the so-called drawer effect, which when it arises leads to wear of the cylinder device and drastically shortened life-time. The alternative with counterweights also results in disadvantages, because a robot with a counterweight can not be as compact and space-saving. The counterweight also restrains the freedom of movement of the robot. When the robot performs overly limited motion cycles, i.e. the robot is moving too little, problems with poor lubrication in the integral bearings arise. 
   The Japanese patent JP 10015874 discloses a robot arranged with a gravitation compensating spring device. The device comprises a spring housing, which includes a helical spring, a spring seat and a pull rod attached to the spring seat. Three guide pins are each arranged through a separate hole in the spring seat, which glides along the guide pins when the pull rod is pulled out and the helical spring by that is compressed. The aim is to prevent damage on the pull rod. 
   Industrial robots usually consist of a robot foot, a stand and a robot arm. The stand is rotatably arranged on the robot foot. The robot arm is pivotably arranged in a joint on the stand. The robot arm is composed of arm parts pivotably arranged in relation to each other. The robot arm comprises e.g. a first and a second arm part and also a wrist arranged with a tool attachment. The arm in its initial position/rest position is oriented with the first arm part almost vertical. When the robot is moving/in operation the arm pivots in relation to the stand at the same time as the arm parts pivots in relation to each other. 
   The total load on the robot consists of the applied handling weight in the wrist on one hand and the present dead weight of the robot on the other. In pivoting, the motor in question pivots the robot arm, and the the gravity acting on the arm loads/influences the balancing arrangement, whereby the balancing arrangement generates a torque. 
   The balancing arrangement then facilitates for the motor to pivot the arm back to its initial position/rest position. The pivot motor in question must thus, in pivoting the robot back, be able to handle a remaining torque, which is the sum of the moment from the total load as well as the oppositely directed torque generated in the balancing arrangement. The torque generated by the balancing arrangement and the power of the concerned pivot motor are thus in a state of dependence. 
   The development of industrial robots is moving towards ever larger robots. 10 years ago large robots managed to lift up to 100 kg with the wrist. The further development has made lifts of 200 kg possible and now there is a need to increase the lifting capacity in the wrist to extremely high loads of approximately 250 kg. With as high loads as that in the wrist, it is immensely important that a balancing arrangement works in the right manner. 
   In balancing arrangements comprising helical springs the helical spring is compressed or extended. When a helical spring is compressed there is always the risk that it deflects sideways i.e. bends/collapses. It must thus be prevented that the helical spring bends. 
   With loads on the robot of up to 250 kg in the wrist a balancing arrangement is forced to work with very large torque forces and is easily damaged. The damages usually originate through imbalance in the load of the balancing arrangement. In the case with a helical spring, spring housing, spring seat and a pull rod an imbalance in the load of the pull rod, so-called drawer effect, leads to obliquity of the pull rod in the spring housing, wear appears and the expected life-time of the balancing arrangement is reduced to an unacceptably low level. This leads to unwanted and expensive production interruptions. Furthermore there will also be extra unwanted cost for spare equipment. 
   In the arrangement according to the above mentioned Japanese patent the spring seat cannot turn axially in the spring housing. Load leads to large bending moments in the pull rod part in the spring housing, which results in very high strain in the construction, high surface pressures are generated and all this taken together results in the deflection of the pull rod. 
   Consequently, when producing industrial robots of the kind described above the need of a balancing arrangement arises, which can manage loads up to 250 kg and at the same time has as long expected life-time as the industrial robot. Thereby unwanted production interruptions and the need of spare parts is eliminated. 
   Those needs cannot be fulfilled by the balancing arrangement according to the above mentioned Japanese patent. 
   DESCRIPTION OF THE INVENTION 
   An industrial robot, comprising a manipulator with a control system, presents a robot foot, a stand and a robot arm with a wrist and a tool. The stand is pivotably arranged on the robot foot. The robot arm is pivotably arranged on the stand in a joint. The robot arm is composed of at least a first and a second arm part and also the wrist, all of which are pivotably arranged in relation to each other. A balancing arrangement is arranged to exert, when the robot is pivoted, a pulling force between a first and a second robot part and thereby to compensate for/balance the attraction of gravity when the relative position of the robot parts changes. The balancing arrangement is attached to the corresponding robot part with fastening devices. 
   The aim of the invention is to, arrange a helical spring based balancing arrangement on a robot as above, where the robot manages to lift 250 kg with its wrist without damaging the balancing arrangement. The aim of the invention is also to provide the robot with a balancing arrangement, which has as long life expectance as the robot. Consequently, the object of the invention is to improve, in a balancing arrangement according to above, the guiding of a pull rod in a spring housing and by that eliminate the risk of obliquity of the pull rod, the so-called drawer effect. 
   The solution according to the invention is characterized by the device specified in patent claim  1  with a balancing arrangement in the form of a helical spring based telescopic unit. A pull rod together with a guide tube form a telescopic unit., which is arranged between the robot parts and constitutes a support and a guide for the helical spring. When pivoting the robot the telescopic unit is extended or shortened at the same time as the guiding of the pull rod is improved in accordance with the independent method claim. Furthermore, the invention prevents that torque forces from the helical spring/springs spread to the telescopic unit, as the pull rod can pivot freely around its longitudinal axis, in accordance with the dependent claims. A robot according to the invention can be equipped with one or more balancing arrangements and preferably be arranged with a vertical robot arm in accordance with the independent utilization claim. 
   It is within the scope of the invention that the telescopic unit comprises more than two telescopic parts. 
   It is within the scope of the invention that the balancing arrangement according to the invention is arranged between arm parts in the robot, which are not directly connected. 
   It is within the scope of the invention that the robot is mounted in the ceiling or angularly mounted. 
   It is also within the scope of the invention that a robot is arranged in such a manner that the balancing arrangement is provided with spring seats, which are rigidly mounted on the respective attachments of the balancing arrangement, and that the telescopic unit extends coaxially through the spring set. When pivoting the robot both the telescopic unit and the helical spring unit are longitudinally pulled out. 
   In the solution according to the invention is also included that the robot is provided with one or more balancing arrangements. 
   It is within the scope of the invention that the spring housing is provided with aerating holes to eliminate pumping effects from the movement of the piston back and forth. 
   It is within the scope of the invention that a ring fastener is rotatingly arranged through a roller bearing. 
   It is within the scope of the invention that the described piston is replaced by another type of spring seat. 
   It is within the scope of the invention that the pull rod is a piston rod. 

   
     DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in detail through a description of an embodiment of the invention in reference to the accompanying drawing, where 
       FIG. 1  discloses a balancing arrangement according to the present invention with the pull rod retracted. 
       FIG. 2  discloses a balancing arrangement according to  FIG. 1  with the pull rod pulled out. 
       FIG. 3  discloses a balancing arrangement according to  FIG. 1  without a helical spring and pull rod. 
       FIGS. 4   a  and  4   b  disclose a guide ring arranged in a spring housing opening. 
       FIG. 5  discloses an alternative embodiment of the invention. 
       FIG. 6  discloses an industrial robot provided with a balancing arrangement according to the invention. 
   

   DESCRIPTION OF AN EMBODIMENT 
   An industrial robot  1  ( FIG. 6 ) comprises a robot foot  2 , a stand  3  pivotably arranged on the robot foot  2  and a robot arm  5  connected to a joint  4  on the stand  3 , which robot arm  5  comprises a first and a second arm part  6  and  7 , respectively. The robot arm  5  is pivoted around a horisontal axis  4   a  of rotation in the joint  4 . A balancing arrangement  8 , comprising a telescopic unit  9  and a helical spring unit  10 , is mounted on the robot  1  (FIG.  1 ). The helical spring unit  10  is coaxially arranged on the telescopic unit  9 . The balancing arrangement  8  comprises in its first end  11  a first attachment  12  for pivoted mounting on the stand  3  and in its second end  13  a second attachment  14  for pivoted mounting on the first arm part  6 . 
   The telescopic unit  9  comprises a first spring seat  15  and a second spring seat  16  between which the helical spring unit  10  is arranged (FIG.  2 ). The first spring seat  15  comprises a spring housing  15   a , which is arranged with a first end  17 , a cylindrical envelope surface  18  and also a second end  19 , provided with an opening  20 . A mounting  12  in the form of a first ring fastener  21  is arranged on the outside of the first gable  17 . The second spring seat  16  comprises a piston  22 , which is rigidly arranged at the first end  23   a  of a pull rod  23 . The pull rod  23  together with the piston  22  are displaceably arranged inside the spring housing  15   a . The tube-formed pull rod  23  extends from the piston  22 , through a part of the spring housing  15   a  and out through the opening  20  in the second gable  19  of the spring housing  15   a . The pull rod  23  is in its second end  23   b  provided with an attachment  14  in the form of a second ring fastener  24 . The helical spring unit  10  comprises a spring set  25  in the form of two helical springs  25   a  and  25   b  which are arranged inside the spring housing  15   a  between the piston  22  and the second gable  19  of the spring housing  15   a.    
   When the pull rod is pulled out of the spring housing  15   a  the spring set  25  is compressed and thereby generates a spring force, which strives to extend the helical spring set and thus retract the pull rod  23  back into the spring housing  15   a . The generated spring force is used for the balancing. 
   Coaxially inside the spring housing  15   a  on the inside of the first gable  17  a guide-tube  26  is arranged. The guide tube  26  extends inside the spring housing  15   a  from the first gable  17  and almost to the second gable  19 . The guide tube  26  thus has a length smaller than that of the spring housing  15   a . The guide tube  26  has an outside diameter somewhat smaller than the inside diameter of the tube-formed pull rod  23 . 
   When the pull rod  23  is displaced along the guide tube  26  the pull rod  23  will glide with very good guiding and minimal friction along the guide tube  26 . This is accomplished by a first and a second bushing  27  and  28 . The first bushing  27  is rigidly arranged coaxially with and on the inside of the pull rod  23  and of an opening  29  in the spring seat  22  to form a longitudinal continous first guide surface  30  (FIG.  1 ). The second bushing  28  is rigidly arranged on the outside of the free end  31  of the guide tube  26 , in order to form a longitudinal second guide surface  32  on the guide tube  26  (FIG.  3 ). When displacing the pull rod  23  the spring seat  22  slides along the guide tube  26   
   When the pull rod  23  is displaced through the opening  20  in the gable  19  it slides telescopically on the outside of the guide tube  26 , which thus together form a telescopic unit  9 . The movement is stabilized by the pull rod  23  being supported by the first  30  and the second  32  guide surfaces, which are arranged at a distance from each other longitudinally. The first  30  and the second  32  guide surfaces guide the pull rod  23  with a slip fit, which together with the guide tube  26  form a rigid unit  33  between the first  21  and the second  24  ring fasteners (FIG.  2 ). 
   A guide ring  34  is rigidly arranged in opening  20  of the second gable  19  of the spring housing  15   a . The guide ring  34  is shaped with a third longitudinal guide surface  35  in the spring housing  15   a . The third guide surface  35  guides and acts as a slide bearing to the pull rod  23  in its movement out of and into the spring housing  15   a  through the opening  20  (FIG.  2 ). 
   The possibility to compress the helical springs  25   a  and  25   b  arranged between the piston  22  and the second gable  19  of the spring housing  15   a  determines how far the pull rod  23  can be pulled out from the spring housing  15   a . From the  FIG. 2  it is clear that the guide tube  26  and the pull rod  23  are telescopically arranged within each other in sufficient degree to provide exact guiding and a good stability when the pull rod  23  is maximally pulled out. To secure that the pull rod  23  can be freely pivoted in the spring housing  15   a  the second ring fastener  24  is rotatably arranged in the second end  23   b  of the pull rod  23  through an angular ball bearing  36 . 
   The guide ring  34  is detachably mounted coaxially in the opening  20  of the gable  19  ( FIGS. 4   a  and  4   b ). The function of the guide ring  34  is primarily to guide the pull rod  23  through the third guide surface  35  and secondarily to seal or mark off the spring housing  15   a . The guide ring  34  is easily replaceable. The guide ring  34  has an opening  37 , which is slotted without material loss. It can thus easily be slipped on the pull rod  23  and guided axially to its intended position in the opening  20 . A clamp ring  38  locks the guide ring  34  against axial displacement in the opening  20 .