Patent GB-923232-A (GB)
Title: Parallax interval sensing device
IPC: G01B11/16
Filed: 1959-04-21
Abstract: 923,232. Photo-electric measurement of the positions of moving bodies. CHICAGO AERIAL INDUSTRIES Inc. April 21, 1959 [April 23, 1958], No. 13574/59. Class 40 (3). [Also in Groups XXXVI and XL (c)] The invention relates to equipment for making parallax interval measurements of cyclically moving objects in which spaced, directional, radiation-energy sensitive devices produce electric signals when radiation-energy beams defined by said devices are interrupted or reflected by the objects, data relating to the objects being determined from the intervals between the signals from the devices. The invention is described as applied to rotating bodies, and particularly the rotor blade assembly of a helicopter, embodiments being given for measuring (1) the relative and absolute heights of the track positions of the blades in relation to a reference plane containing the measuring apparatus, (2) the displacements of the blades from their nominal positions in their plane of rotation and (3) the displacement of a single rotor blade from its nominal position in its plane of rotation. Height determination.-The measurement is based on the use of spaced sensitive devices 10, 10<SP>1</SP>, Fig. 3, each comprised of an optical system 12, 12<SP>1</SP> defining a beam 36, 39 and an associated photo-electric cell 19, 19<SP>1</SP>' The beams are mutually inclined at an angle # whereby a moving body 48 intercepts the beams to produce impulses from the photoelectric cells which are spaced by an interval determined by the time t occupied by the body in travelling the distance S 3 . If the velocity V of the body is known, the height S 2  of the body above the cross-over point may be determined (for small angles #) from the expression S 2 =   Vt/#À As applied to height measurements of the rotor blades of a helicopter, Fig. 5, the blade linear velocity V is obtained from the rotor angular velocity # by multiplication by the radial distance R to the measuring device 57. The calculation of blade height data and its display is effected by the arrangement shown in Fig. 6. Impulses and waveforms throughout the arrangement are shown in Fig. 7. Impulses A and B from the devices 10, 10<SP>1</SP> are applied to a generator 59 which produces impulses C determined by the time separations t, successive pulses being due to the passage of successive rotor blades 1, 2, 3. The generator also produces constant duration pulses D and F for timing operations in other parts of the arrangement. The pulses D are applied to a computer 70 which receives also impulses G of a frequency determined by the angular velocity # of the rotor assembly and is pre-set by a potentiometer 72 in accordance with radial distance R to the measuring device 57. The computer determines the linear velocity V of the rotor blades at the point where they intercept the radiation beams from the product of # and R. Impulses G are derived from a magnetic pick-up device 71 and armature 73, or a capacitive or photo-electric device, associated with the rotor shaft 56. The computer output, which is in the form of a D.C. voltage, is then applied as input to a further computer 96 which also receives impulses C, information relating to the angle # and timing pulses D. Computer 96 functions to produce pulses J, the amplitudes of which are   determined by the expression #Rt/#. The # information is obtained from the setting of a potentiometer 44, Fig. 3, which is rotated in accordance with angular setting of device 10 by gears 41, 44. The pulses J are applied to a recorder gate 121 and a meter storage gate 122 which are controlled by a ring counter 120 which is cycled in step with the measurements of successive rotor blades by timing pulses F and is correctly phased in the cycle by pulses H occurring once in each cycle and derived from computer 70. The apparatus may be arranged for measurements on rotors having more than three blades, ring counter 120 being illustrated as providing for six outputs. Recorder gate 121 is controlled by the combination of counter outputs M and functions to produce a signal N indicating the heights of all the rotor blades and intended for display on an oscilloscope or for recording. Meter storage gate 122 receives inputs K and L from the first position and a selected one of positions two to six of the counter and functions to control a meter 124 to display the relative height of a selected rotor blade with respect to blade 1 which is used as a reference blade. A feature of the invention is the provision of a servo control system 123 which responds to the height measurement of the lowest blade, i.e. the lowest amplitude level in signal N, and controls a motor 45, Fig. 3, to adjust the angular position of device 10 so as to move the cross-over point of the beams so as to maintain the height measurement within predetermined limits selected to minimise the effects of vibrational disturbances on the measurements. A plurality of height measurements may be made at different radial distances with the aid of a plurality of measuring devices or with one device selectively movable to different distances. Measurement of blade displacement, Fig. 14.- For this purpose the two sensitive devices 214, 215 are spaced apart at the nominal angular displacements of the blades and are disposed with their beams substantially normal to the plane of rotation of the blades. The time interval t between the pulses from devices is determined by the relative blade displacement S 2  at the point of measurement and may be calculated from the expression #Rt, where # equals the angular velocity and R the radial distance from the rotor shaft to the measuring devices. The calculation to be made is similar to that made in the height measuring apparatus (see above) if the quantity # is replaced by a constant. The calculation may thus be performed in the apparatus shown in Fig. 6 by replacing potentiometer 42 by a fixed resistor. In this arrangement the servo system is arranged to respond to the smallest of the relative lag distances and to control motor 45 to adjust the beam angle of device 214 so as to maintain the distance within predetermined limits. Measurement of displacement of a single blade, Figs. 17 and 18.-For this measurement the two sensitive devices are again spaced apart, device 221 being located so as to receive an impulse once for each revolution of the rotor, the impulse being produced by light from a source 225 reflected by a mirror or prism 224 mounted on the rotor hub, and device 227 being located at a radial distance R and such that its beam 228 is tangential to the leading edge 232 of the blade when the blade is at rest. The device 227 is also positioned so that the impulses from the two devices are normally in time coincidence. When the blade is rotated any lag S is revealed as a time displacement t in the pulses. A signal of frequency determined by the angular velocity of the rotor is provided by a magnetic pick-up or similar device 233, 234. The actual magnitude of the lag may then be computed from the expression #Rt by the arrangement shown in Fig. 18 which is generally similar to Fig. 6 (see above) differing only in that (a) the potentiometer providing the # information is dispensed with and replaced by a fixed resistor, (b) an inverter 235 is placed in the output from device 221 to allow for the use of reflected light and (c) the omission of the ring counter as a consequence of need to provide for only a single measurement during each rotor revolution. Detailed circuits of generator 59, computers 70, 96, recorder gate 121, meter storage gate 122 and servo control 123 are described with reference to Figs. 8-13 (not shown). Specification 847,901 is referred to.