The Design of Drift-Free, Fabrication-Fault-Tolerant Fluidic Sensors by Closed Loop Error Suppression

This paper discusses the design of fluidic sensor/gainblock circuits to minimize the effects of finite component common-mode-rejection. Output drift in an open loop circuit is the direct result of supply and ambient variations operating on imperfectly fabricated fluidic components. Direct coupled sensor power supply is replaced by the output of a regulator circuit. The regulator output is determined by the sensor differential null offset output, and feedback from the high gain amplifier circuit drives the circuit to an equilibrium condition. In this manner, fluidic components with poor offset characteristics actually enhance power supply variation rejection as long as sufficient forward gain exists. For conventionally photo-chemically etched fluidic components the analysis shows that almost two orders of magnitude improvement in supply variation rejection is possible. In addition, the fundamental open loop time constant is reduced by the feedback and the frequency response is improved. For a laminar jet angular rate sensor significant improvements in threshold and frequency response are possible.