Patent GB-815336-A (GB)
Title: Means for determining the sense of movement of a moving member
Inventor: BANE WILLIAM THOMAS
IPC: H03K5/26
Filed: 1956-05-15
Abstract: 815,336. Photo-electric apparatus. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 13, 1957 [May 15, 1956], No. 15071/56. Class 40 (3). [Also in Group XIX] An apparatus for determining the direction of movement of an object in opposite senses, comprises means for producing two alternating signals in response to movement of the object, the signals being of the same frequency but with one signal leading or lagging the other according to the direction of motion of the object, and means for applying a separate one of the signals to each of two inputs of an electrical circuit such that when only the first signal is instantaneously of effective value the circuit lies in a first stable condition, when only the second signal is instantaneously of effective value the circuit lies in a second stable condition and when both the first signal and the second signal have effective values the circuit lies in an unstable condition, there being means for detecting the order in which a change is made from one stable condition to the other to provide an indication of direction of movement. In one embodiment the moving object is a rotating shaft 4, Fig. 10, carrying a disc 3 with alternate opaque and transparent areas for interrupting a light beam 6 directed at two photo-electric cells 1 and 2 arranged so that the cyclic variation of the quantity of light falling on one cell is partially out of step with that of the other cell. The photo-electric cells 1, 2 may each comprise a photo-transistor Tl, T2, in the circuit shown in Fig. 3, the operation of which can best be explained with reference to Fig. 5 showing diagrammatically how the two light signals change as the disc 3 revolves. The sections A and B correspond to the illumination periods of the photo-transistors T1 and T2 and the sequence of operations in the direction marked " positive rotation" is :-(a) during the period 3 to 4 only signal B is active so that transistor T2 conducts and transistor T1 is cut off, this being the first stable condition of the circuits; (b) at the point 4 the signal B is no longer active but the transistor T2 continues to conduct since the circuit is still in its first stable condition; (c) at the point 1 the signal A becomes active with the result that the circuit in Fig. 3 is triggered in the opposite direction, the transistor T1 now conducting and the transistor T2 being cut off, this being the second stable condition of the circuit; (d) at the point 2 the signal B also becomes active so that both transistors T1 and T2 conduct, this being the unstable condition of the circuit; (e) at the point 3 the signal A is no longer active so that the transistor T1 is then cut off since the signal B to the transistor T2 is still active. For positive rotation, the point 1 is the only point at which the circuit triggers from one stable position to the other and this causes the terminal (i), Fig. 3, to go positive and at the same time causes the terminal (ii) to go negative and this event is used to give a positive count. For negative rotation the point 4 is the only one at which the whole circuit triggers from one stable position to the other and this causes the terminal (i) to go negative and the terminal (ii) to go positive and this event is used to give a negative count. Modiftcations.-In an alternative arrangement the electrical circuit includes two thermionic valves connected as a trigger circuit, a separate diode being connected to the control grid of each of the valves, whereby the application of a positive signal to a diode causes the valve to the control grid of which it is connected to conduct. In alternative arrangements the marks on the moving disc may consist of conducting segments in an insulating plate (or vice versa) sensed by a mechanical contact, magnetic marks sensed by a specially designed magnetic head, or they may be produced by optical interference methods using ruled gratings. In use the outputs of the circuit described may be applied to a reversible counter, so that a count of the difference between the number of marks passed in the positive and negative directions is obtained, this count giving an indication of the amounts by which the moving member has moved. Specification 760,321 is referred to.