Displacement coder
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to displacement coders which, in robots, automata, machining centers and other automatic machines, are associated with the different mobile members with a view to locating the exact position of each of them in the sequence of displacements. 
     2. History of the Related Art 
     Between the absolute direct-reading coders and the incremental coders having relative count-down, intermediate systems are known in which the precise reading of the positioning is effected by a so-called operation of calibration which consists in displacing the mobile member in order to find the position thereof by counting pulses. 
     U.S. Pats. Nos. 4,363,026 to SALMON and 4,041,484 to GROFF and British Pat. No. 1,565,400 400 to OTIS ELEVATOR CO., for example, propose displacement coders comprising a first disc angularly connected to the displacement of the member in question, a second disc rotatably connected to the first in a ratio close to 1 but different from this value, and pulse counting means associated with each of the discs for revealing the angular shift existing therebetween. 
     In this way, precise data are obtained at a cost much lower than that of the absolute coders, by means of a prior operation of calibration of much reduced amplitude, not capable of being hindered by the surroundings of the mobile member to be verified. However, the dimensions of these coders which may be qualified as &#34;differential&#34; are large and hinder installation thereof, and their structure is complicated. 
     SUMMARY OF THE INVENTION 
     It is a principal object of the present invention to overcome the aforementioned drawbacks, by providing a displacement coder of the differential type, characterized in that the first disc is fitted on a principal shaft secured to the mobile member being monitored while the second disc is mounted idly on the principal shaft, so that the two discs are disposed coaxially thereon. The disks are connected together via two toothed wheels which are themselves disposed coaxially on the principal shaft and which cooperate with a common pinion carried by a secondary shaft oriented parallel to the principal shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be more readily understood on reading the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic axial section illustrating the arrangement of the coder according to the invention. 
     FIGS. 2 and 3 are views in elevation showing each of the two rotating discs of the apparatus according to FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, in FIG. 1, reference 1 designates a casing inside which axially rotates a principal shaft 2 supported in two opposite bearings 3. One of the ends of this shaft 2 projects outside the casing 1 and is shaped at 2a so as to allow coupling thereof with the shaft 4 which ensures control of the angular or linear displacement of the mobile member of the machine whose position it is desired to locate at any moment. 
     On the principal shaft 2 are fitted a disc 5 and a toothed wheel 6 both of which mesh with a pinion 7 secured to a secondary shaft 8 mounted idly inside bearings 9 provided in the casing 1. The pinion 7 meshes simultaneously with a second toothed wheel 10 secured with a hub 11 which rotates freely on shaft 2 to the rear of disc 5. On this hub 11 is fitted a second rotating disc, referenced 12. 
     It should be observed that the wheels 6 and 10 comprise very slightly different numbers of teeth (a difference of one tooth suffices) so that, due to this differential linkage, the two discs 5 and 12 are rendered rotatably secured in a ratio close to 1, but different from this value. 
     In order to fix ideas by a precise example, which, however, presents no limiting character, it may be supposed that the toothed wheel 6 connected to shaft 2 and to disc 5 has 125 teeth, while toothed wheel 10 connected to hub 11 and to disc 12 includes only 124, with the result that, when disc 5 has made 124 revolutions, disc 12 will have made 125. 
     As shown in FIG. 2, the disc 5 comprises three concentric scales or tracks referenced 5a, 205b and 5c respectively. The two tracks 5a and 5b comprise the same number of marks, but the track 5a are offset angularly from one track to the other; track 5c may comprise a variable number of marks, one only in the embodiment shown. In the manner illustrated in FIG. 3, disc 12 comprises only one track 12a having a number of marks equal to that of track 5c of disc 5, viz. one in the example shown. 
     With each of these discs 5 and 12 is associated a detector 13, 14, respectively, comprising a number of detection members obviously equal to that of the tracks on the disc which corresponds thereto. The nature of these detection members depends on that of the marks which constitute the tracks of discs 5 and 12; they may be photoelectric cells, magnetic sensors or any other apparatus adapted to generate an incremental pulse upon passage of each mark of the corresponding track during rotation of the discs 5 and 12, which pulse is transmitted to a counting apparatus enclosed in the control cabinet of the machine in question. 
     Operation of the coder described above is similar to that of the conventional apparatus of the same type. It will be assumed that the coder is fixed to the rear of the motor which ensures linear displacement of the arm of a robot or automation. Discs 5 and 12 were previously adjusted so that, at half the stroke of the arm in question, the marks of tracks 5c and 12a are simultaneously in register with the corresponding detection member of one and the other of the two detectors 13 and 14. 
     In the course of operation of the machine, the arm in question will of course move on either side of this central position, ensuring rotation of the two discs 5 and 12. However, further to the differential linkage effected by the toothed wheels 6 and 10 and the common pinion 7, the marks of tracks 5c and 12a will be offset with respect to one another, so that, at any moment, in order to determine the position of the arm of the robot, it suffices to displace this arm over a maximum distance corresponding to one revolution and to read the angular shift between the two tracks or marks 5c and 12a, such reading being effected by counting pulses at the level of tracks 5a and 5b of disc 5. 
     Counting of pulses generated by the detectors 13 and 14 upon passage of the marks of tracks 5c  and 12a of discs 5 and 12 does not involve any analog processing, with the result that it is simple and reliable. Moreover and in particular, the apparatus is of much reduced dimensions and is consequently capable of being very easily installed on the machine at each of the mobile members thereof.