Patent GB-864049-A (GB)
Title: Registering and repeating device for the stop positions of the mobile member of a precision machine-tool or of a measuring machine
IPC: G05B19/39
Filed: 1957-09-06
Abstract: 864,049. Programmed electric control systems; recording positions. SOC. GENEVOISE D'INSTRUMENTS DE PHYSIQUE. Sept. 6, 1957 [July 18, 1957], No. 28219/57. Classes 38 (6) and 40 (1). [Also in Group XL (b)] Desired stop-positions of a machine-tool relative to a workpiece are stored on a magnetic drum in separate tracks, the angular position of a record spot in each track corresponding to the associated tool linear stop position. For subsequent repetition control purposes the drum is rotated and each track is read to produce pulses whose phase corresponds to the angular position of the associated recorded stop-position relative to a datum generatrix of the drum. The phase of these pulses is compared with that of similar pulses, whose phase position corresponds to the actual position of the tool, to produce an error signal which drives a servomotor to move the machine-tool into the desired stop position. Coarse and fine control are provided. Recording stop positions; Figs. 1, 4, 5, 6.- A workpiece P, which has, e.g., to be drilled at positions 5, 6, 7, is secured to a table 1 movable along guide-rails 2, 3, Fig. 5, by a lead-screw 35 and nut 41, Fig. 6, driven by a motor E1. A two-way switch 28, Fig. 1, is operated to close the motor circuit in the appropriate sense to drive the table to within 1 mm. of the first desired stop-position 5, as indicated by an index 27, Fig. 6, carried by the table 1 against a fixed mm. scale 26. A micrometer drum F, Figs. 4, 6, carried by the machine frame 4, is then set according to the exact stop position required and through a pinion 23 and rack 22 sets an index 21 to a corresponding position. The table 1 carries a mm. scale 13 and the graduation thereon substantially opposite a fixed measuring position is optically projected in known manner, e.g. as described in Specification 864,050, by a lamp 15, condensers 16, 18, a semi-transparent mirror 17 and a further mirror 19 on to a screen 20. The table 1 is now moved manually, through a hand wheel 29, Fig. 5, until the image of the appropriate mm. graduation on the screen 20 coincides with the micrometer index 21. The table is now in the exact stop position desired and through a rack 44 and step-down gearing rv has set the shaft 43 of a " coarse " phase-shifter ES to an angular position corresponding thereto. A " fine " phase-shifter es has been similarly set through a rack 45 rigid with the rack 22 in correspondence with the micrometer F setting. A switch IP is now closed to start synchronous motors MS, ms which drive respective magnetic drum stores M, m and alternators A, a. A selector N is then adjusted to a position corresponding to the operative stop position 5 to set selector switches SE, SL, se, sl and then a button K is pressed to close switches J, j. The phase-shifter ES is fed from the alternator A and delivers to a unit AE a single-phase output ct whose phase relative to any one phase of the alternator A corresponds to the angular setting of the phase shifter ES and hence to the table position. The output ct is converted at AE into short pulses it occurring at the beginning of each cycle, and these pulses are fed over the selector SE to a writing head TE 1 which records them as a single spot p on the associated track of drum M, the angular position of the spot p relative to a datum generatrix nl corresponding to the table setting. In like manner the setting of the micrometer F is recorded on the " fine " drum m by the writing head te1. The switches J, j are now released and the table adjusted successively to the other desired stop positions 6, 7, which are then recorded on the " coarse " and " fine " drum stores M, m, as above described, but on separate tracks by using the selector N to switch in successive writing-heads TE1, TE2 . . . te1, te2 . . . &c. Repetitive control by recorded signals.-A further workpiece to be drilled having been positioned on the table 1, and switches J, j opened, the selector N is set to the position corresponding to " stop position " 5, the synchronous motors MS, ms, are switched on, and switches U, u are closed. The pulses it now derived in the unit AE from the phase-shifter ES correspond in phase to the actual position of the table 1 and these pulses are compared in a phase comparator CP with pulses iM corresponding to the desired stop-position 5 of the table and derived from the drum store M by the appropriate reading-head TL1 selected by the selector SL. The output of the phase comparator CP is a sinusoidal error signal dM proportional in magnitude to the phase difference of the pulses it, iM, which is amplified at RC and fed through a control unit CT to drive the table feed-motor E1 at high speed and in a sense to reduce the error voltage. When the table has approached to within, say, 8 mm. of the desired stop-position the control unit CT operates to switch in an alternative drive-motor E2 so that the table is then driven at reduced speed. When the table has approached to within 0.5 mm. of the desired stop position, the reduced error voltage causes the control unit to switch over control of the table feed motor E2 to " fine " control. If the micrometer F is not initially in the exact position corresponding to stop-position 5, then, when switch u is closed a servomotor mf is immediately operated to bring the micrometer into this position under control of an error signal dm derived from a comparison at cp of the phase of pulses if corresponding to the angular setting of the phase-changer es, and hence to the actual position of the micrometer and pulses im corresponding to the desired position of the micrometer and derived from the associated recorded spot on the drum store m via the reading head tl1 selected by the selector switch sl. Thus, when the control unit CT switches to fine control the micrometer is already in its correct position. Fine control is effected by a photo-electric reader L, Fig. 6, carried by a carriage C actuated by the racks 22, 45. The reader comprises an optical projection system which projects an image of the appropriate graduation of the above-mentioned mm. scale 13 carried by the table 1 on to a screen 100 via the mirror 19, prisms 96, 97, and motor-oscillated mirror 99 which causes the image to oscillate over a slot 101 in the screen 100 and pulse a photo-cell 47. The pulses so produced are equi-spaced only when the micrometer index 21 coincides with the corresponding image projected as above-described on to the screen 21, i.e. when the table is in the exact stop position. When the fine control is first switched in these pulses therefore have unequal spacing which is sensed at CE. The output of CE is proportional to this inequality and controls the table feed motor E2 via the control unit CT until the " fine " error signal reaches a predetermined small magnitude when a relay RE again reduces the feed speed by modifying the gear ratio in the table gear drive. Finally, as the error voltage reaches zero a relay RV operates a brake FR to stop the table at the desired stop position. For " centering " workpieces on the table 1, the rack 44 is mounted for limited linear movement in slides 74, relative to the table 1, under control of a knob 32, and the precision ruler 13 is likewise mounted in a slide 52 for limited linear movement relative to the table 1 under control of a knob 33. Erasure of the recordings on the drum M, m is effected by energizing the reading-heads with direct current over switches W, w. Modifications.-In one modification, Fig. 8 (not shown), the optical projection systems and the photo-electric reader are dispensed with. For recording, the work-table is set to a desired stop-position through its lead-screw by a hand wheel and vernier and the positions of the " coarse " and " fine " phase-shifters ES, es, set thereby, recorded on a single magnetic drum store. For repetitive control, the table feed motor is initially controlled as above described by the coarse error signal during which time the " fine " phase-shifter es is mechanically uncoupled from the feed motor, this coupling being resumed when the " fine " control takes over. In another embodiment, Fig. 9, where both coarse and fine stop positions are also recorded on a single drum M, the alternator A and phaseshifter ES of Fig. 6 are replaced by a photo-electric pulse generator comprising a photo-cell 85 carried by a crank geared to the rack 44 so as to be set angularly relative to the drum axis, in correspondence with the work-table position, the cell being pulsed by light from a lamp 83 mounted in a casing AT slotted at 82 and driven coaxially with the drum M, the phase of the pulses thus produced relative to a datum drum generatrix varying in correspondence with the table position. A similar arrangement could be used in lieu of the " fine " phase comparator es and alternator. An electromagnetic generator, Fig. 10 (not shown), may be used instead of the photo-electric device of Fig. 9.