GB904734A - Improvements in or relating to electric control means for controlling the position of a power driven movable object - Google Patents

Abstract

904,734. Electric selective signalling systems. AIRMEC Ltd. Sept. 8, 1958 [Sept. 16, 1957], No. 29148/57. Class 40(1). A shaft positioning system is characterized in that the shape is always driven to a new position by the shortest route. The device described angularly positions a shaft under the control of manually set ten-position switches by using the gapswitch principle, that is each ten position control switch Ac, Fig. 1, has ten fixed contacts 19 connected to ten conductive segments 11 on a fixed disc As which is scanned by a brush 12 controlled by the shaft being positioned, and the arrangement is such that for each position of the control switch every contact except one has a potential applied to it so that unless the shaft being positioned is in such a position that the brush 12 is on the unmarked or floating segment a motor circuit is completed which rotates the shaft until it is in that position. The circuit shown in Fig. 1 also has the feature that half of the marked contacts on the switch are given a positive potential and the remainder a negative potential which results in the shaft being driven to its desired position by the shortest route. This feature, however, can introduce errors or hunting in systems, such as that shown in Fig. 1, where the angular position of a shaft is controlled over a range of more than one revolution by a multiplicity of control switches having their associated scanning brushes geared together by 10 to 1 reduction gearing. For example, in a system having two switches, if a shaft starts from its 00 position and a desired position of, say, 29 is set on the switches, the shaft will first move under the control of the tens switch to position 20 and then, under control of the units switch back to position 19 since as far as the units switch is concerned the quickest way of reaching position 9 from 0 is to go backwards. It is clear that to prevent this error, before control is transferred from the tens to the units switch, the shaft must reach a position which is between 25 and 34. That is to say, the subsidiary shaft controlled by the units switch must be within half a revolution of its desired final position before control is transferred to the units stage. This, in turn, means that the shaft controlled by the tens switch must be within one-twentieth of its desired final position before control is transferred from it. This is achieved, in the main embodiment, Fig. 3, which contains four control switches Ac, Bc, Cc and Dc and corresponding discs As, Bs, Cs and Ds, by sensing each disc with seven brushes (as opposed to one as shown in Fig. 1) and by selecting, for each disc, e.g. Cs, two brushes which are actually used by a rotary switch 30b ganged to the control switch Bc of the immediate lower order. The two brushes selected by each rotary switch 30 are connected in parallel and in effect form a single " wide " brush. The rotary switch 30 which selects the sensing brushes for the lowest order disc As forms a fine position control. The circuit shown in Fig. 3 also includes controlling relays A, B, C, D having their contacts (shown with relays deenergized) so arranged in a chain that the highest order stage not in its desired position (its relay will then be energized) is effective to control relays E and F which, in turn, control the motor circuit. In a modification, successively lower order stages, as they assume control, engage different gear ratios, electromagnetically in the motor driving circuit, so that the shaft slows down as it approaches its desired position. In a second modification, the final approach to the desired position is always in the same direction. In a third modification, when the shaft being positioned is a lead-screw, inaccuracies therein are compensated for by means of a cam, attached to the highest order subsidiary shaft, which is so shaped and arranged as to impart small correcting rotations to the lowest order switch.