Patent GB-816370-A (GB)
Title: Improvements in and relating to position control servo mechanisms
Inventor: WHITELEY AUSTYN LESLIE
IPC: G05B19/35
Filed: 1955-02-08
Abstract: 816,370. Programming for machine tools; gauging devices. BRITISH THOMSONHOUSTON CO. Ltd. May 8, 1956 [Feb. 8, 1955], No. 3734/55. Class 83 (3). [Also in Group XXXVIII] A position-control servo-system, e.g. for giving sequential displacements to the table of a machine-tool, contains several input-responsive members displaceable in accordance with different degrees of fineness which move independently of one another; the members may control separate transmitting selsyns whose respective receivers are geared together and control the same servo amplifier and motor in succession, or alternatively the input members may be coupled to the same transmitter, e.g. via differential gears; a combination of these methods may be used. There are also provided means for automatically returning the output member to a datum position before displacement of the transmitter. The system is particularly suitable for dealing with inputs in digital form, e.g. punched tape, magnetic tape or marks scanned by a photo-electric cell, and may be so used to control the linear motion of a machine-tool part. In Fig. 1A, four input devices are shown responding to the first four significant figures of a quantity recorded on punched tape, in order to move the member 11 through a distance represented by this quantity. With the figures shown the solenoids 3, 13, 23, 33 are given 3, 4, 5 and 6 impulses respectively, so that the transmitting selsyns 6, 16, 26, 36 are rotated through angles corresponding to the respective figures on the tape. Initially, switch 10 is closed while switches 20, 30, 40 are open, so that only the output of receiving selsyn 7 is effective to operate the motor 9 via amplifier 8. This motor therefore drives member 11 through a distance equal to the first significant figure of the quantity, and in doing so cancels out the error voltage from selsyn 7. When this happens, switch 10 opens and 20 closes, transferring control of the motor to receiving selsyn 17, which rotates it by an amount corresponding to the second significant figure of the quantity. The error voltage from 17 is cancelled out by the motor via the reduction gearing G1. Similar operation of selsyns 27, 37 provides the third and fourth significant figures. In the case of the fifth and sixth figures, the arrangement of Fig. 1B is preferable to avoid adding further stages of reduction gearing. Here the ratchet wheel 45 has 100 teeth instead of 10 so that the differential 46 is driven by shaft 45A to an extent representing one tenth of the direct drive from the wheel 35. The impulses representing the last figure are given a further 10: 1 reduction by gearing 57, 58 before driving the differential 56. Before operations are commenced, all the selsyns are returned to the datum position (by means not shown), this position for each successive selsyn being displaced in the negative direction by an amount slightly less than five times the unit of the succeeding stage. Thus, if the unit of the last selsyn is 0.01 inches, the datum for this is zero and that of selsyn 26 is 0.045 inches, i.e. the output voltage is zero when the corresponding positions of the rack 11 and the ratchet 25 differ by this amount. Similarly the datum position of selsyn 16 is 0.45 inches and that of selsyn 6 is 4.5 inches. By this means, any position from 0 to 49 inches can be set up to the indicated degree of accuracy. If the " wasted movement " of 4.5 inches is undesirable, the selsyns 6, 16 may be directly geared together as in previous systems (e.g. that of Specification 581,743, [Group XXXVIII]), leaving the latter selsyns in independent relationship. In a modification (Fig. 5, not shown), the input data are set up manually on dials which directly cause rotation of the selsyns 6, 16, 26, 36. Manual setting may also be used with the arrangement of Fig. 1B.