Capacitance-level/density monitor for fluidized-bed combustor

Claim 1

A capacitance based density/level monitor for a fluid-like medium within an electrically conductive containment vessel comprising: a capacitance probe adapted to be vertically disposed in said medium and including an electrically conductive housing, a plurality of electrically conductive segments dispposed vertically of said housing, each of said segments separately insulated from said housing; means for applying a predetermined frequency excitation voltage between said containment vessel and said probe housing, thereby generating a current through said medium; a capacitance detection circuit means for separately detecting the quadrature component of said current flowing through said medium in the presence of a resistive current component which is in phase with said excitation voltage and generating a voltage at an output thereof indicative of the selected probe segment-to-vessel wall capacitance, including an operational amplifier having an inverting input forming a current summing junction for receiving said selected probe segment current and a feedback current, means for applying said selected probe segment current to said current summing junction, synchronous detection circuit means operable in response to said excitation voltage for separately synchronously detecting the capacitive and resistive voltage components of the voltage signal at the output of said operational amplifier corresponding to the capacitive and resistive components of said selected probe segment current and generating voltages at separate outputs thereof corresponding to said capacitive and resistive component voltages respectively, a gate-controlled amplifier means for generating said feedback current at an output thereof coupled to said current summing junction of said operational amplifier and further including a gate control input connected to said resistive component voltage output of said synchronous detection circuit means for varying the gain of said amplifier in response to the amplitude of said resistive component voltage and a phase-adjusting circuit means responsive to said excitation voltage for generating a selected phase voltage input to said gate-controlled amplifier to provide the proper phase for said feedback current to cancel the resistive component current of said selected probe segment current applied to said summing junction; segment selection circuit means responsive to a control signal for separately connecting each of said plurality of segments of said probe in a selected sequential order to the input of said capacitance detection circuit means; sampling circuit means having a plurality of sampling channels corresponding to said plurality of probe segments for sampling the output voltage from said detection circuit means from corresponding ones of said plurality of probe segments in response to said control signal; and control circuit means for generating and applying said control signal to control inputs of said segment selection circuit means and said sampling circuit means to select and sample the corresponding segment detected capacitance signal at a preselected sampling rate.