Patent GB-1181515-A (GB)
Title: Improvements in or relating to Force-Measuring Apparatus.
Inventor: AGAR JORAM
IPC: G01L1/16
Filed: 1966-05-18
Abstract: 1,181,515. Measuring force or fluid-pressure electrically. SOLARTRON ELECTRONIC GROUP Ltd. May 2, 1967 [May 18, 1966], No. 22146/66. Heading G1N. A force or fluid-pressure transducer comprises two resilient struts 11, 12, Fig. 1, whose ends are connected to end-pieces 13, 14 to one of which the force is applied via rod 15 and disc 17, the other being rigid with the transducer casing 19, or having a second force applied for differential measurements. The struts are kept in vibration at a common resonance frequency which depends on the force applied and which is measured electrically to give an indication of the applied force. Construction. The interior of the casing is sealed by a diaphragm 21 and is kept either at a constant reference pressure or evacuated. In a modification, a reference pressure is introduced into a chamber formed by connecting a further disc 32, Fig. 3, via rod 33 to a disc 31 corresponding to the disc 17, the chamber being sealed by diaphragm 36, and the housing surrounding the struts 11, 12 being evacuated. In this embodiment the ratio of the area of disc 31 to that of disc 32 is made as small as possible to minimize the response of the transducer to the sum of the reference pressure and pressure to be measured. Alternatively, to eliminate this response, the end-piece 14 of the transducer is connected via rods (16, 43, Fig. 4 not shown), to a similar arrangement of discs (41), (42) defining a chamber for the reference pressure, the area of disc (42) being larger than that of disc (41) by an amount equal to the area of disc (31). In a modification of the first embodiment for measuring the difference between two pressures, an end-piece (51, Fig. 5 not shown) is connected via a rod (54) to two discs (55), (56) to which the two pressures are applied, an annular second endpiece (52) being connected to the casing (53). The struts may be rectangular in cross-section or hollow tubes, they may be straight or concave, Fig. 6 (not shown), or the struts and end-pieces may take the form of a ring, the pressure being applied across a diameter. In an alternative form the struts are formed from a hollow cylinder of ferromagnetic material in the walls of which wide longitudinal slots have been made to leave a plurality of struts. Vibration excitement and detection. In one embodiment struts of non-ferromagnetic material are connected in parallel across the output of an A. C. amplifier 25 and are situated in the D.C. magnetic field of bar magnets (23, 24, Fig.2 not shown), so that they vibrate in a traverse direction. The vibration is detected by strain gauges 26, 27, Fig. 1, whose outputs are fed via a pre-amplifier 28, a phase shift network 29 and an amplifier 25 to maintain the vibration of the struts at their resonant frequency and is measuredby a meter 30 to give an indication of the applied pressure. Alternatively the struts may be of ferromagnetic material, internal or external drive and pick-up coils being provided which are connected to amplifiers 25 and 28 respectively. Alternatively vibration of the struts and detection of the vibration may be performed by Piezoelectric plates (101-104, Fig.9 not shown), bonded to the ends of the struts, plates (101), (102) being energized by amplifier 25 and plates (103), (104) feeding pre-amplifier 28. In a modification the struts themselves are cut from a quartz Piezoelectric plate and are provided with pairs of electrodes (114-117, Fig 10 not shown), on both sides, the electrode pairs (115), (117) being connected to pre-amplifier 28 and electrode pairs (114), (116) to the amplifier 28.