A generic smart sensing system utilizing a multi-element gas analyzer.

Abstract

Smart microsensors can offer features such as self-testing, autocalibration, digital compensation, and bus-compatibility to improve system reliability and performance while reducing cost. A distributed control/instrumentation system architecture with up to 256 smart sensing nodes is described. The smart sensing nodes are self-testing, addressable, programmable, compatible with a standard digital bus, capable of handling commands sent by the host controller, offer 12 bits of accuracy using internally-stored compensation coefficients, and are capable of operating up to 32 sensors and actuators per node. Each sensing node is microprocessor controlled and has been implemented using discrete commercial components as well as designed and simulated in monolithic form. Monolithic sensing node integration appears well within the range of current technology. A description is also provided for the parallel and serial bus structures which were developed as part of this work and which are currently used at the University of Michigan. This system organization makes sensor interrogations appear much like memory accesses, yielding very fast response times to the host controller. Solid-state gas sensors are in high demand, especially for automotive, medical, and process control applications. As a vehicle for smart sensing system definition, a multi-element high-performance gas analyzer has been developed. It is realized in silicon as the hybrid of two dual-element gas detector chips and a control/interface chip. The conductivity-type gas detectors utilize ultra-thin metal sensing films supported by a micromachined dielectric window. The detector control chip contains interface circuitry to control and monitor four such detectors simultaneously and independently. Window temperature is controlled from ambient to 1200$\sp\circ$C with eight-bit accuracy by controlling a boron-diffused silicon heater under each window. Film conductivity is measured to eight-bit accuracy for full-scale resistances from 5$\Omega$ to 1.3M$\Omega$ over a frequency range from dc to 2MHz. The control chip utilizes a standard eight-line interface to users, allowing the implementation of the analyzer as a smart peripheral. The detectors are realized using a six-mask process while the control chip is implemented in an eleven-mask 2$\mu$m CMOS process in a die size of 4.4mm $\times$ 6.6mm.