Patent GB-760678-A (GB)
Title: Improvements relating to the balancing of rotating bodies
Inventor: WILLIAMSON DENIS
IPC: G01M1/22
Filed: 1952-10-30
Abstract: 760,678. Electric control systems. FERRANTI, Ltd. Jan. 29, 1954 [Oct. 30, 1952], No. 27266/52. Class 40 (1). In rotor dynamic balancetesting apparatus, a pulsed reference signal, derived by a pick-up from a reference mark on the rotor, is fed to a multivibrator, one valve of which passes current for constant periods of duration equal to one half the period of rotation of the rotor at a predetermined test speed; the current pulses through the other valve being longer, equal to, or less than said constant period according to whether the rotor speed is respectively lower, equal to, or higher than the test speed. The rotor speed may be measured by a comparison of the outputs of the two valves, and the cyclic output of the vibrator is compared with the vibration pick-up outputs in the usual way to derive the unbalance. As shown, Figs. 1, 2, the fixed spindle 11-12 of a gyro rotor 10 to be tested, is secured horizontally in metal clamps 13, 14 attached by rubber blocks 15, 16 to frame members 17, 18 adjustable axially of the rotor and secured to a base 20. Moving coil pick-offs 31, 32 are actuated by the horizontal vibrations of the respective clamps 13, 14 through wires 33, 34 disposed horizontally and sufficiently close to the end face planes of the rotor as to be regarded as responding to the out-of-balance forces in these, the balancing planes. A reference signal is derived, e.g. photo-electrically or electromagnetically, by a pick-off 23, responsive to an axial reference mark on the rotor periphery, the phase of this reference signal being adjustable by manual rotation of a wheel 25, coaxial with the rotor axis, and carrying the pick-off by means of an arm 24. Circuiting, Figs. 3, 4. The reference pick-off output is applied over a transformer 40 and amplifier stages 41, 42, the latter of which suppresses unwanted signals of lesser amplitude, to a multivibrator 43, Fig..4. In its free running state, the multivibrator triode 62 conducts during each cycle for a time equal to that for the rotor 10 to make one half a revolu. tion at the predetermined test speed, and the triode 61 conducts for a longer period. The reference output pulses are applied over capacitor 78 to the grid of triode 61 and take over control thereof as the rotor runs up towards its test speed. At the test speed the triodes conduct for equal times in' each cycle, the triode 61 conducting for a longer or lesser time according to whether the rotor speed is less than or greater than this test speed. A meter 44 connected across the cathodes of these triodes responds to the difference of their load currents and is graduated to indicate the rotor speed or the speed deviations thereof from the test speed. The vibration pick-offs 31, 32 are connected through a switch 50 and parallel channels 52-53, 54 to an integrator 51, the switch being arranged to connect the pick-off 32, e.g. directly, by way of input transformer 54, to the integrator when an out-of-balance test is being made in the corresponding " left " balancing plane. At the same time the other pick-off 31 is connected to the integrator through an attenuator 53 to compensate for the effect of the out-of-balance masses at the righthand rotor end of the measurement at the lefthand end. Similarly, for a measurement in the " right " balancing plane, the pick-off 31 is connected directly to the integrator and the pick-off 32 is connected thereto through the attenuator. The integrator output is fed through a lowpass filter 55, amplifier 56 and output transformer 57 to one diagonal of a bridge 72-76, Fig. 4, in series with a centre zero meter 58 for indicating the out-of-balance magnitude, the other diagonal of the bridge being fed from the cathodes of the multivibrator triodes 61, 62. Calibration and operation. Using a balanced gyro rotor, its speed as indicated by a stroboscopic tachometer, is brought to the test speed, when resistors 67, 70 are adjusted in turn to equalize the durations and amplitudes of the pulses through triodes 61, 62, thus bringing the speed meter 44 to its centre zero. The meter 44 is then calibrated for various speeds of the rotor. Next, with a small weight attached to, say, the right-hand (RH) rotor face in the same radial position as the reference mark on the rotor, switch 50 is thrown " left " to test the left-hand (LH) rotor end, thereby connecting the RH pick-off 31 through the attenuator 53 which is adjusted, at the test speed, until the out-of-balance meter gives a minimum reading. The switch 50 is now thrown to the RH test position and at the test speed, the wheel 25 is adjusted until the meter 58 is at centre zero, when a fixed pointer which indicates the rotational setting of the wheel 25 is set to read zero, since the small weight on the RH face of the rotor is coincident with the reference mark. Meter 58 is then calibrated for out-of-balance mass by throwing switch 50 for RH testing, say, and noting the maximum readings on one side, e.g. the positive side, of the centre zero for various values of out-of-balance weights secured to the RH face of the balanced rotor. To test an unbalanced rotor, e.g. at the RH end the wheel 25 is rotated in the direction in which the gyro is rotating at the test speed until the out-of-balance meter 58 indicates a maximum on the positive side of its zero. The meter then indicates the mass of the unbalance in the RH balancing plane. Wheel 25 is then rotated in the same direction until meter 58 reading is at its centre zero in passing from the positive to the negative side of the scale, when the angle indicated by the wheel 25 and cooperating pointer gives the radial position of the out-of-balance mass with respect to the reference mark. The LH end is then similarly tested. Modifications. One vibration pick-off only may be used and disposed under the rotor, the operating wire therefor being coupled to a rigid horizontal arm located beneath and parallel to the rotor axis and supported by a rigid arm extending radially downwards from the block 13 or 14. To test a rotor at e.g. the RH end, the point of attachment of the pickup wire to the horizontal arm is moved along the arm and the pick-off itself is slid along its mounting to maintain the coupling wire vertical, until the nodal point with respect to unbalance at the other end face is reached, i.e. that point at which there is no transverse vibration due to unbalance at the end face. This point is determined using a balanced rotor and a known unbalance weight. The test procedure is the same as before except that the rotor must be reversed in the clamps 13, 14 to test other end. Alternatively, two pick-offs disposed at the two nodal points could be used with a switch to determine which face is to be tested. The nodal point may be transferred to a position beyond one axial end of the rotor by clamping weights thereto, to simplify the mounting of the vibration pick-off. Such an arrangement could be used with the first embodiment to transfer the nodal points to the positions of the clamps 13, 14 in which case only the pick-off 31, 32 connected with the end-face under test would be connected to the integrator 51. The integrator stage may be dispensed with if the vibration pick-offs are of the type responsive to displacement rather than velocity e.g. of the crystal type.