803,975. Photo-electric indicating systems. GISHOLT MACHINE CO. Jan. 2, 1956 [Nov. 2, 1955], No. 41/56. Class 40 (3). [Also in Group XXXVIII] In an unbalance measuring machine in which compensation for unbalance can be made at certain angular positions only of the body under test, unbalance is resolved to equivalent unbalance at the various angular positions so that the total correction is equivalent to the single correction necessary. Fig. 10 shows an arrangement for determining unbalance in two planes 12, 13 on a rotating body 1 and for detecting how much of the unbalance can be located at two of three possible positions equally spaced around the circumference of the body. The body rotates in bearing 2, 3 which are flexibly supported to allow free lateral movement of the workpiece in a horizontal plane. Vibration pick-ups 4, 5 each consisting of a coil vibrating in a magnetic field are connected to the bearing. Each pair 4, 4 or 5, 5 is associated with one of planes 12, 13 and is connected to known circuits 8, 10 for combining the outputs of one of the pick-ups at each end to represent the equivalent unbalance in one of the planes. The output from circuit 8 representing the unbalance in one of the correction planes only, will be considered. This output is amplified at 9 and fed to a triggering and phase shift device to fire a lamp 15 each time the unbalance is in the horizontal direction. The workpiece is driven by a motor 17 which also drives a pair of generators 98 each supplying alternating currents whose phases correspond to the three possible correction positions, and are so chosen that one of the currents is opposite in phase to the output signal from amplifier 9 due to unbalance at each of the unbalance positions. Motor 18 also drives, through 1: 1 gearing, a shaft 21 to which are attached a pair of slotted rotating masks 60; one mask corresponds to each unbalance plane and one only will be considered. The mask rotates around flashing stroboscopic lamp 15 the instant of flashing of which depends on the direction of the resultant unbalance in the plane under consideration. Surrounding the mask is a double housing (Fig. 12) one of which contains photocell 64, 65, 66 and other, cells 68, 69, 67. The rotating mask is provided with an outer slit 23 which illuminates cells 64, 65, 66 successively through slots 67 in a fixed shroud 63 (Fig. 13) and an inner slot 61 which can illuminate two of cells 68, 69, 70 at a time. The two sets of cells are aligned with each other. Cells 68-70 are used to determine between which of the pairs of cells the unbalance is located, and cells 64-67 to resolve the unbalance into its components at the two angular positions between which the unbalance lies and at which correction is to be made. In operation, assume the unbalance to be situated between correction lines C and D. Each time the unbalance passes the horizontal direction (arrow 27) lamp 15 will flash and because of the position of slot 61, photo-cells 69 and 70 will be instantaneously energized. Cells 69 and 70 close relays 76 and 77 respectively which, through relays 81 and 82, energize a pair of motors 84 and 85 and prevent energization of a further motor 86. Motors 84, 85 and 86 are connected to one of the wipers of potentiometers 80, 81, 82 respectively each of which is supplied with one of the phases of generators 98. In the present case both motors 84 and 85 supply increasing signals of the two chosen phases to buck the output of amplifier 9 and thus to shift the instant in the rotation of mask 10 at which lamp 15 fires. This instant continues to charge until one of cells 64 and 66 receives light through slot 67 according as the unbalance is nearer point D or point C. If cell 64 receives light first it energizes relay 72 and in turn relay 88 which transfers control of motor 85 to relay 72 and stops motor 84. Motor 85 continues to run, however, until the light falling on cell 64 is zero when relay 72 de-energizes and motor 85 stops. Each of the motors 84-88 carries a cam 93 on its shaft the angular position of which when the motor stops indicates the unbalance at the angular position to which the motors correspond and may be used to operate recording gear. If photo-cell 66 receives light before cell 64 the unbalance is nearer correction line C. Motor 85 stops first and motor 84 is stopped afterwards. Whenever the unbalance occurs a pair of cells 64-67 is used to detect its approximate position and one of cells 68-70 to determine the components in the two chosen correction lines. The Specification also describes an arrangement in which correction can take place at two angular positions only (Fig. 6). In this arrangement it is assumed that the workpiece is such that the unbalance will always lie between the chosen correction directions. A single photo-cell outside a rotating mask with a single slot in it is necessary and a pair of motors which in turn supply correctly phased alternating currents to balance out the component of the unbalance in the direction with which they are associated.