Patent GB-635633-A (GB)
Title: Improvements in means for measuring, indicating, or utilising very small displacements
IPC: G01D5/20
Filed: 1947-06-13
Abstract: 635,633. Gauging-apparatus. WIGAN, E. R. June 13, 1947, No. 15580. [Class 106 (ii)] [Also in Group XXXVIII] Apparatus for measuring, indicating or utilizing a small displacement of a relatively rigid body comprises a mechanical vibratory system arranged to be driven at a frequency near to but below a free resonant frequency of the system, means for causing the small displacement of the body to change the amplitude of oscillation by an amount depending on the magnitude of the small displacement, and means for measuring, indicating or utilizing the change in amplitude of the system so produced. In a modification, the system may be driven at a frequency which is near to but above a free resonant frequency of the system, and " negative feedback " applied to the system to prevent instability. The invention may be applied to apparatus for comparing standard length gauges, to barographs, thermostats, or any apparatus wherein variations of some physical quantity are converted into small displacements. As shown, a vibratory system 1 comprises a reed 2 flexibly mounted between the poles of a permanent magnet, and is carried by a flexible member 4 secured to a bar 5 rigidly clamped at one end to a baseplate 5a. A probe 6 mounted in the baseplate at 7 for parallel motion, is moved by a body 8 whose displacement is to be measured, so that it restricts the vibrations of the reed 2. Initial fine adjustment of the probe so that it just touches the reed is effected by rotating a dial 9 on a screw 9a which is connected by a spring 10 to an adjustable sleeve 11a on a blade 11. The stress on the blade bends the relatively rigid bar 5 about the point at which it is clamped to the plate 5a, whereby the reed 2 moves towards the probe 6. By substituting a micrometer for the body 8 and calibrating the scale of dial 9 in thousandths of an inch, movements of the assembly 1 of the order of millionths of an inch can be estimated. A winding 3 exciting the magnet in the assembly 1 forms one arm of a bridge 12 supplied with A.C. from a source 13. A D.C. meter 14 fed from a amplifier and rectifier 15 indicates the degree of unbalance of the bridge 12. When the frequency of the source 13 is just below a resonance frequency of the reed 2, and the probe 6 encroaches on the free vibration of the reed 2, the impedance of the winding 3 passes through a series of stable values in a regular manner, and the reading of the meter 14 will rise from zero through a series of values approximately proportional to the corresponding displacement of the rigid body 8. If the frequency of the source 13 is slightly higher than a resonance frequency of the reed 2, to prevent the reed 2 vibrating in an uncontrollable manner, " negative feedback " is employed comprising an electromagnet 16 polarized by a permanent magnet 16c and fed with D.C. derived by rectifying the " unbalance " current from the bridge 12 by the device 15. The magnet attracts the arm 4 and system 1 away from the probe 6. Negative feedback may also be used when the reed 2 is driven at a frequency slightly below a resonance frequency. In this case the negative feedback is used to reduce the change in amplitude of the reed 2 resulting from a given displacement of object 8, and so extend the range of measurement of the apparatus. " Positive feedback " by reversing the connections 16a, 16b or by reversing a switch 18, whereby the bar 4 and system 1 is deflected towards the probe 6 as the rectified current increases, may be used to increase the sensitivity of the device. Electromagnetic excitation of the system 1 may be replaced by electrostatic, piezo-electric, or magnetostrictive means. Separate means to oscillate the system 1 and indicate changes in the amplitude thereof may be used, in which case the bridge 12 is not required. In this case two coils, one above and the other below the reed 2, are used, one directly supplied with current and the other having current generated therein by the vibrations of the reed 2. Specification 443,485 is referred to.