For example, subsequent to stripping down an industrial robot owing to service requirements, it is necessary to again meas- ure-up and calibrate the robot reference system in order to be able to maintain the accuracy of its set measuring system.

A device and a method for such calibration are known, for instance, from SE Patent 9501124-3, Publication No. 508 176.

This known method enables a co-ordinate to be determined in the three-dimensional geometry of the robot with the aid of two length and angle measurements. In this case, a given reference point is taken in the robot and the distance and angle to a point on the arm of the robot is measured. Measur- ing is effected with the aid of a telescopic measuring stick equipped with an electronic measuring head in respect of length measurements, and with the aid of two electronic spirit levels. in respect of measuring the angle relative to the vertical. Alternatively, only one spirit level can be used, provided that it can be rotated through precisely 90° between the measurements.

However, measuring processes of the aforesaid kind are still encumbered with problems related to measuring accuracy due to geometric faults in the measuring stick used, by virtue of the fact that there is no possibility of controlling rota- tion/twisting of the stick about its own axis.

Accordingly, an object of the present invention is to provide a novel and improved ball guide, particularly for use with such a measuring stick, such as to prevent the ball-carrying element of the ball joint from rotating about its own axis.

This object is achieved with a guide according to the present invention, in which either the ball or the ball-carrying element includes a guide element which projects out from the ball or from the ball-carrying element for engagement in and co-action with a groove in a ball-accommodating body.

Another object of the invention is to provide a guide that enables the measuring stick to be rotated through exactly 180° about its own axis between two measurements.

This further object of the invention is achieved with a guide where the ball-accommodating body is provided on its outer side with two grooves that are mutually displaced through 180°, and where the guide element is spring-loaded so as to lie against the outer side of the ball-accommodating body.

The invention will now be described in more detail with ref- erence to a non-limiting embodiment illustrated in the accom- panying drawings, in which Fig. 1 illustrates schematically a first embodiment of the lower part of a measuring stick pro- vided with an inventive guide; Fig. la illustrates the actual measuring stick and ball; Fig. lb is a schematic view of the ball in a ball-accommodating body; Fig. lc is a schematic section view through the ball-accommodating body in a direc- tion perpendicular to the view shown in Fig. lc ; Fig. 2 is a schematic view of a second embodiment of the lower part of a measuring stick provided with an inventive guide, and also shows the stick fitted in an accommodating body; Fig. 2a shows the measuring stick in a position in which it is gener- ally vertical; Fig. 2b shows the measuring stick inclined in one direction at an angle of 30° ; Fig. 2c shows the measuring stick inclined at an angle of 30° in another direction; Fig.

3 shows the lower part of the measuring stick and the accom- modating body according to the embodiment of Fig. 2 in greater detail; Fig. 4 is a perspective view of one embodi- ment of the accommodating body according to the embodiment shown in Fig. 2 ; Fig. 5 illustrates schematically a further

embodiment of the lower part of a measuring stick provided with an inventive guide and inserted in an accommodating body, said body having a different design than the body shown in Fig. 2; and Fig. 6 illustrates yet another embodiment of the lower part of a measuring stick provided with an inven- tive guide and inserted in an accommodating body, where the accommodating body in particular has a design different to that shown in Fig. 2.

Shown in Fig. la is the lower end of an elongated shaft 1, which carries at its end a ball 2 that forms one part of a ball joint. The elongated shaft 1 may, for instance, be a measuring stick of the aforedescribed kind. The ball 2 car- ries two radially and outwardly projecting guide pins 3, which function as ball guide elements when the ball is in- serted in the ball-accommodating body to form a ball joint.

The guide pins 3 are disposed at right angles to the geomet- ric axis 4 of the elongated shaft 1, i. e. so that the exten- sions of the centre lines of the guide pins 3 to the centre of the ball 2 meet each other in the centre of said ball.

Fig. 1b shows the ball 2 and its guide pins 3 received in an accommodating body 5 that includes a recess 6 for receiving and supporting the ball 2 to form a ball joint. Fig. 1c shows the body 5 in a view that has been turned through 90° in relation to the view of Fig. lb. In addition to showing the recess 6 that accommodates the ball 2, said figure also shows an open groove 7 on both sides of the recess, said grooves extending out from the recess 6 in mutually opposite direc- tions. These grooves 7 are configured so as to accommodate exactly the guide pins projecting out from the ball 2, and extend slightly below the centre of the ball 2 when said ball is located in the accommodating body 5. This prevents the shaft 1 from rotating about its own geometric axis, although the shaft is able to tilt in any direction relative to the vertical. The maximum angle to which the shaft 1 may tilt is

determined by the contact of the shaft 1 with the upper side of the accommodating body 5 and the depth of the grooves 7.

As mentioned above, the elongated shaft 1 may be a measuring stick. However, the shaft may, of course, be any type of shaft where it is desired to be able to tilt the shaft in any direction in the ball joint but where rotation of the shaft 1 about its own geometric axis shall be prevented.

As mentioned above, Fig. 2 illustrates another example of an embodiment of an inventive ball joint guide. As in the ear- lier case, the ball joint includes the lower end of an elon- gated shaft 11, which carries at its end a ball 12 for form- ing one part of a ball joint. As in the earlier case, the ball is guided by a guide element 13, although in this case said guide element prevents the shaft 11 from rotating about its geometric axis 14 by virtue of abutment with the outside of an accommodating body 15. The body 15 includes an inner recess 16 for receiving the ball 12. In this case, however, the ball 12 is guided for co-action with the guide element 13 through the medium of grooves 17 provided on the outside of the accommodating body and extending parallel with the verti- cal direction of the elongated shaft 11 and with the axial direction of the body 15. The accommodating body 15 has an at least hemispherical outer surface with two mutually opposed part-spherical surfaces 18 in which two mutually opposing grooves 17 are arranged. The opposing grooves are displaced through precisely 180° relative to each other so as to be able to accommodate the guide element 13, which is intended for displacement in one longitudinal direction of the groove 17 so as to enable the shaft 11 to be rotated about the cen- tre of the ball 12 when said ball is in position in the body 15. The grooves 17 therefore have a radius of curvature with its centre located in the centre of the ball 12, and the contact point of the guide element 13 in the groove 17 as the guide element 13 moves up or down in said groove also de-

scribes an arc with which the centre of its radius of curva- ture also coincides with the centre of the ball 12.

As will be evident from Fig. 2, there is also attached to the lower end 11 of the shaft a spacer element 19 to which the guide element 13 is, in turn, attached with the aid of a fastener means 20. The fastener means 20 extends from the spacer element 19 parallel with the shaft 11 to a position in which it is on a level with the centre of the ball 12, said guide element 13 being thus disposed at this position. When a measuring stick, or some other elongated device that includes the shaft 11, is inclined, the guide element 13 will move up or down in the groove 17 whilst preventing the shaft 11 from rotating about its own geometric axis. Figs. 2b and 2c illus- trate the shaft in two positions in which it is inclined at an angle of about 30° in different directions relative to the vertical. It will be evident from these figures that the guide element 13 has been moved along the groove 17 in the part-spherical outer surface 18 of the accommodating body 15.

The fastener means 20 is preferably comprised of a bendable, flexible material, for example has the form of a leaf spring.

This enables the guide element 13 to be snapped out of the groove 17 on one side of the accommodating body 15, therewith enabling the whole of the shaft 11 to be turned through 180°. so that the guide element 13 snaps into the groove 17 on the opposite side of the part-spherical surface 18 of the accom- modating body. The guide element 13 thus enables the shaft 11 to be localised precisely, in co-action with the two grooves 17 in the outer surface 18 of the accommodating body, there- with enabling the distance between two points to be measured at the ends of a measuring stick while compensating for defi- ciencies in its geometry.

As before mentioned, Fig. 3 is a view in larger scale of the lower part of the elongated shaft 11 and the accommodating

body 15 forming the ball joint according to the embodiment shown in Fig. 2.

Fig. 4 is a perspective view of the accommodating body 15 for the ball joint shown in Figs. 2 and 3. These figures show the recess 16 of the accommodating body, in which the ball on the end of a shaft can be accommodated to form a ball joint. It will be seen that the accommodating body 15 is part-spherical and has two part-spherical outer surfaces 18, each of which includes a groove 17 for receiving a guide element 13, as mentioned above. The grooves 17 are smoothed down on both sides to provide ramp surfaces 21 which facilitate insertion of the guide element 13 into the groove and also enable the guide element to snap readily out of said groove. The guide element 13 is preferably a guide ball that can be readily displaced in the longitudinal direction of the groove 17 and which can, nevertheless, be caused to snap out of the groove 17 when this is the intention.

Fig. 5 illustrates a variant of the embodiment according to Fig. 2, in which the accommodating body 15 has instead a cylindrical outer surface 22 and also four grooves 17 dis- posed along the periphery of the cylinder, said grooves being mutually separated through 90°. This embodiment is similar to the Fig. 2 embodiment in other respects, and has in the cen- tre of the accommodating body 15 a recess 16 for accommodat- ing the ball 12 at the end of an elongated shaft 11. The elongated shaft of this embodiment also includes a spacer element 19 on which a guide element 13 is attached by means of a preferably resilient fastener means 20. The guide ele- ment 13 is arranged to lie in abutment with the cylindrical surface 22 through the medium of the fastener means 20 and to snap into the precisely defined grooves 17.

Fig. 6 illustrates a further variant in which the accommodat- ing body has a cylindrical outer surface 22, similar to the Fig. 5 embodiment. In this embodiment, however, two grooves 17 are disposed radially from the centre of the cylinder along the upper axial delimiting surface 23 towards its pe- riphery, said grooves also being separated through an angle of 90°. It will be understood that the Fig. 6 variant may also include four grooves. A spring-loaded fastener means 20 is also provided for carrying the guide element 13, although the fastener means has been attached to the shaft 11 and directed generally radially outwards from said shaft in this case.

The inventive guide thus functions to prevent a shaft, which is freely rotatable in general and which carries a ball at its one end, from rotating about its own axis. The inventive guide can also be used in areas other than the described exemplifying embodiment where corresponding requirements nevertheless exist.