Elimination of Drift and Asymmetry Effects in Fluidic Sensors by Feedback

Abstract

This paper presents a feedback approach to the design of fluidic sensor circuits that will minimize drift and other unwanted offset effects, that are due to component asymmetries, and at the same time will improve the overall sensor frequency response. In this approach sensor supply pressure is generated by one output of a high gain differential regulator amplifier rather than by the raw system supply pressure. As a result, variations in system pressure, that normally would appear as sensor output drift (time varying null offset), are decoupled from the sensor. The amount of effort required to keep the sensor nulled becomes the sensor system output. When this feedback is implemented the overall dynamic response can be improved by a reduction in the fundamental circuit time constant, that is governed by the sensor dynamics, by introducing phase lead and so reducing the low frequency phase shift. Since the error signal driving the high gain regulator is proportional to the degree of sensor asymmetry, output performance of the feedback system is actually enhanced by having significant null offset characteristics. The offsets generally found in photochemically etched amplifiers and laminar jet angular rate sensors are shown to be reduced by two orders of magnitude and frequency response improved by as much as a factor of five. This technique has been applied to rate sensors, resistance bridges, pressure regulators, and airspeed sensors.