Patent GB-880361-A (GB)
Title: Dual string force transducer
IPC: G01P15/10
Filed: 1958-05-12
Abstract: 880,361. Force transducers; accelerometers; pressure gauges. BORG-WARNER CORPORATION. May 12, 1958 [May 17, 1957], No. 15220/58. Class 40 (1). [Also in Groups XIX and XL (c)] A force-measuring apparatus comprises a force-responsive mass with two non-magnetic wires extending from opposite sides of it, each wire being maintained in oscillation at its resonant frequency under tension in a magnetic field, means for suspending the mass so that it can move axially relative to the wires and thus differentially alter the tensions in the wires, an electronic amplifier having feedback to sustain the oscillations in the wires at a constant amplitude, and means to heterodyne the outputs of the two amplifiers and measure the difference frequency which is a measure of the axial force applied to the mass. Such a system is substantially unaffected by forces at right angles to the wires or by changes in ambient temperature. The force to be measured may for example be due to acceleration or fluid pressure. Accelerometer (Fig. 1).-A mass 16 is supported in a housing by an annular flexible diaphragm 18 so that it can move axially as far as stops 20. On each side of the housing is a cylindrical shell 24 carrying a cap 25 in which is mounted an insulated terminal post 26. A wire 28 is stretched between this post and a tab 27 on the mass 16 and passes through a semicircular magnet 29. Each wire is non-magnetic and is preferably made of tungsten. Axial displacement of the mass will produce an increase of tension in one wire and a decrease in the other so that the difference between their resonant frequencies (originally equal) is a measure of the force exerted on the mass. Pressure gauge (Fig. 4).-The mass 16 of the accelerometer is here replaced by a pin 102 which is attached to the centre portion of a diaphragm 94 and to hubs 98, 100 which are connected by thin webs 99 to mounting plates 92, 93. The centre portion of the diaphragm can move sideways by virtue of a thin portion 97. A cavity 95 on one side of the diaphragm communicates with a pressure source via a port 103, and a similar cavity 96 on the other side communicates with another pressure source via a port 104, so that axial movement of the pin 102 is a measure of the difference between the two pressures. One pressure source can be omitted if the absolute pressure of the other is to be determined. Modifications.-The two wires 28 can be unequal in length, density, cross-sectional area or modulus of elasticity if it is intended that the difference between their resonant frequencies shall be finite for zero applied force. The wires can be circular, ribbon-like or polygonal in cross-section. Conductively-coated strings can be used instead of wires. Circuitry (Fig. 3).-Each wire 28 is connected to a separate amplifier of the type shown here and is joined in a Wheatstone bridge with a similar wire 36 which is not in a magnetic field. This wire is preferably in the same housing as the wire 28 so that both are at the same temperature. Any change in the impedance of the wire 28, due to a change in the back E.M.F. induced in it by the magnetic field, alters the signal applied to three cascaded amplifier valves 37, 50, 62. A voltage is fed back from the anode of valve 50 to maintain oscillations in the wire 28, the amplitude being kept constant by an A.G.C. circuit fed from the output of valve 62 via a rectifier bridge 75. The output signal at terminal 72 is mixed with a similar signal from the other amplifier in a conventional heterodyning circuit, and the difference signal is selected by a low-pass filter and displayed on a meter.