Patent GB-984526-A (GB)
Title: Improvements in or relating to apparatus operated by variations of capacitance
Inventor: HILL WILLIAM FRANK
IPC: G01F23/26
Filed: 1961-01-12
Abstract: 984,526. Capacitance measurements. LANCASHIRE DYNAMO ELECTRONIC PRODUCTS Ltd. Jan. 12, 1962 [Jan. 12, 1961], No. 1407/61. Headings G1N and G1U. Apparatus responsive to capacitance variations (Fig. 1 not shown) comprises an operational amplifier having a capacitance feedback path from output to the input which is in series with a variable capacitance and a controlled alternating voltage source; the voltage drop across the variable capacitance being maintained equal to the controlled voltage by the feedback current, and phase discriminating means being provided for obtaining a signal measurable to indicate capacitance values which is determined by the component of the voltage drop in phase with the controlled voltage, while variations of the loss resistance of the variable capacitance produce only signals in quadrature with the controlled voltage to which the discriminator is unresponsive. In Fig. 2 a variable capacitance Z i  due e.g. to a probe inserted in material contained in an earthed hopper is connected through feedback capacitance Z f  to the output of operational amplifier A energized by the error signal V S  from a difference circuit B receiving a controlled alternating voltage V i  and a feedback voltage V f  from the function of Z i  and Z f  so that V i  is held approximately equal to V f  if the gain is large, and variations of capacitance Z i  (or capacitance Z f ) produce variations of output voltage V o  in phase with V i  while variations of the loss resistance of either capacitor produce variations of output voltage which are in quadrature therewith as before. In Fig. 3 a radio frequency oscillator 2 drives a cathode follower amplifier 3 to excite transformer 5, one of whose secondaries energizes the cathode of pentode 6 with voltage V i  while voltage V f  is applied to the control grid over capacitance C (constituting impedance Z f ) from the cathode of amplifier 7 excited from the anode of pentode 6, and loaded by inductance 9 and resistance 8. A probe tapped down autotransformer 10 introduces a virtual capacitance (constituting impedance Z i ) in series with feedback capacitance C. An output voltage V o  coupled from the cathode of amplifier 7 over capacitance C16 is applied to phase sensitive rectifier PSR together with a reference signal from a secondary of transformer 5 to develop a direct voltage representing the in phase component and the variations of probe capacitance, which is coupled for amplification at 7 and separated by inductance 9 and capacitance C16 to deflect meter V indicating the capacitance between the probe and a bin containing material in which it is immersed, and thus the level of material. The quadrature component of output voltage due to the loss resistance of the variable capacitance is disregarded by the phase rectifier and the amplified direct voltage may also be applied to operate an automatic control system. Bias for amplifier 7 is derived from potentiometer 12 and the phase of the alternating output of pentode 6 is adjustable by its resistance/ reactance load circuit 15. The Provisional Specification refers to a control system responsive to capacitance variations wherein a variable capacitor responsive e.g. to level of material in a bin is connected in a self balancing bridge wherein a reference capacitor is adjusted automatically to balance the variable capacitance and a resistance which may shunt the reference capacitance is adjusted to balance the resistive component of the variable capacitance. Rebalancing control signals may be obtained from phase sensitive rectifiers respectively responsive to quadrature and in phase unbalance signals, and the value of the variable capacitance may be derived from the current flow in the reference capacitor at balance, which may operate a control or indicating system.