Patent GB-1408890-A (GB)
Title: Oscillating crystal force transducers
IPC: H04R17/00
Filed: 1971-11-23
Abstract: 1408890 Measuring force electrically J P CORBETT 17 Oct 1972 [23 Nov 1971] 54410/71 Heading G1N A force transducer comprises a plate-like Piezo-electric crystal 1 arranged in an upright position between the base 20 of a frame 43 and a spigot 19 by which the force to be measured is applied, a resilient diaphragm 16 being arranged between the member 19 and the frame 43 so as to hold the member 19 aligned with the crystal thus holding the crystal in position. Electrical connections are made to the crystal. As shown in Fig. 1, the crystal is mounted on a brass shim 4 mounted in turn on the base 20 through a stainless steel platform 5 and a mica shim 7. A brass shim 8 is provided at the top of the crystal. The platform 5 may be arranged to swivel in the base 20 to prevent the crystal twisting, Fig. 10 (not shown). Electrical connections are made by clips 10, one being connected to shim 4 via a copper wire 12, and the other to the shim 8 which is earthed. Alternatively, the crystal may be mounted on shim 4 through silverimpregnated lead borate or similar conducting cement, Fig. 3 (not shown), or the copper wire may be connected to the crystal using silverimpregnated lead borate, Fig.4 (not shown). The crystal has bevelled edges and flattened portions, as shown. In Fig. 5, a second resilient mounting is provided by a diaphragm 18, although a flat spring may be used. A pointed circuit board 33, containing the oscillatory circuit in which the crystal is arranged, is provided, the electrical output connection being shown at 37. The electrical connection at the bottom of the crystal is connected to the circuit board 33 via a nut 40 and stud 42. The enclosure may be filled with nitrogen via a tube 39.  A solenoid and armature may be provided to lift the member 19 and thus relieve load on the crystal, Fig. 6 (not shown). As described with reference to Fig. 7 (not shown), the output of the crystal oscillatory circuit is compared with a reference frequency to provide a difference beat frequency signal which is applied to a phase-locked loop. The latter provides an output signal to a reversible counter, the output signal having a frequency which is a multiple of the difference beat frequency. In operation, the load applied to the crystal is relieved for a pre-set period and the multiple frequency output signal is applied to a downcounter which stores an appropriate valve which is transferred to an up-counter. The load is then applied to the crystal for the same pre-set period and the multiple frequency output signal is applied to the up-counter, the up-counter at the end of this period then giving an indication of the difference between the load and no-load conditions. Means may be provided for positioning the crystal between the member 19 and the base 20, Fig. 8 (not shown). An ideal position for the crystal is as shown in Fig. 1 where the load acts through the sensitive point A of the crystal, the line of action of the force making an angle with the X-axis of the crystal such that the temperature coefficient of the force/frequency constant is substantially zero.