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Session 2259

Micro-controller based Heater Control for Gas Sensors Michael Amos, Dr. Bruce Segee

University of Maine Department of Electrical and Computer Engineering Instrumentation Research Laboratory

Abstract

Semiconductor Metal Oxide (SMO) Gas Sensors have emerged as a dominant sensor technology in recent years. These sensors are now able to detect compounds ranging from greenhouse gasses to chemical weapon agents. The behavior of the sensor is temperature dependent and the sensor typically operates at elevated temperature (200 °C to 600 °C). Laboratory temperature controllers used high cost, high power, large size electronics and limited the potential usefulness of the sensor. In conjunction with a local company, The University of Maine Instrumentation Research Laboratory has developed a small, low power, cost effective heater controller. The solution utilizes Pulse Width Modulation (PWM) to control the average power dissipated by the heater. Interestingly, the heater changes conductivity as a function of temperature, so the heater element is also used to measure sensor temperature. This eliminates the need to affix an external temperature measurement device (such as a thermocouple) to the sensor. The temperature controller can be used stand-alone or with a host computer. When used stand-alone, a bank of DIP switches specifies the desired temperature, making this system very portable for deployable applications. When used with a host computer, this platform is able to communicate via RS-232 to receive commands and relay feedback about the system performance. Additionally, the temperature controller communicates over a Serial Peripheral Interface (SPI) bus to an LCD display unit. The ability to “mix and match” temperature controllers, display units, and host computers provides for a flexible, low cost, deployable solution suited to a variety of needs.

I. Introduction

Problem Description

A local company designs, produces, and tests Semiconductor Metal Oxide (SMO) Gas Sensors and needs to simplify the heater control requirements of these sensors. SMO sensors are highly temperature dependent and need to operate at an elevated temperature of 200 to 600 degrees Celsius. Due to this temperature dependency, each sensor is mounted on top of a heating element. The resistance of the heating element is temperature dependant, and can be used to accurately measure the temperature, eliminating a separate temperature measuring device, such as a thermocouple.

Unfortunately, the heater has a relatively large power consumption (several watts) and needs specialized equipment to drive the analog power to the heater. The analog signal is generated by a high power analog PC card that resides inside a full tower desktop PC case. This analog card is able to determine both the voltage and the current that is being supplied to the heater. The PC

Proceedings of the 2001 American Society for Engineering Education Annual Conference & Exposition Copyright © 2001, American Society for Engineering Education

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Segee, B., & Amos, M. D. (2001, June), Micro Controller Based Heater Control For Gas Sensors Paper presented at 2001 Annual Conference, Albuquerque, New Mexico. 10.18260/1-2--9557