GB1353779A - Converter for converting displacement into electric current - Google Patents

Abstract

1353779 Measuring displacement electrically: measuring mutual inductance COMPTEURS SCHLUMBERGER 30 June 1971 30739/71 Headings G1N and G1U Apparatus for measuring displacement, due for example to fluid pressure in a chamber 5, comprises a transformer 1 with a core 2, movable in response to variations of said pressure so as to vary the coupling between primary and secondary windings 11, 21 respectively, the secondary voltage Vs being rectified at 22 and applied to a comparator 24 producing the difference voltage from a reference value Vr' which difference acts through amplifier 6 on field-effect transistor 14 to vary current I, in the primary winding so as to nullify the difference voltage which current IP is measured at 13 and represents core displacement and therefore fluid pressure. It is shown that IP is independent of supply voltage variations and temperature changes. The transformer is energized by an A. C. source 12 and the reference voltage source is a further transformer 15 coupled to rectifier diode 25a and a voltage divider 28, 29. In an alternative embodiment Fig.2 (not shown) the transformer is energized at a frequency of 400 Hz by a chopper circuit including a multi-vibrator (16) driving two pairs of F. E. Ts and supplied from a D. C. source (12'). A shunt circuit is provided across the series connection of the F.E.T 14 and primary winding 11 with a fixed resistance (34) of value Rp, which shunt includes an F. E. T (31) in series with a fixed resistance (32) of value Rd and a Zener diode (33). A differential amplifier (35) controls the F.E.T. (31) to maintain the potential drop across the resistance Rp equal to that across the series combination of Rd and the Zener diode. It is shown that there is then a linear relation between the current IP through the primary winding and the total primary current Iz, which is measured. As before IP is varied by means of F.E.T. 14 to maintain the secondary voltage at a constant value. The reference voltage is derived from a voltage divider (28, 29) shunted across the D.C. source (12'). In a further embodiment Fig.3 (not shown), the supply is A. C. , the Zener diode (33) is eliminated and instead a reference voltage Vo is taken from a voltage divider (28', 29'), in parallel with that 28, 29 shown in Fig. 1, which reference voltage is summed with the voltdrops across resistors Rd Rp and the resultant applied to a differential amplifier (37) controlling a shunting F. E. T. (31).