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Show quickly, alert the operator, and put the combustion process in a safe state. For example, the "intelligence" of the microprocessor-based controller allows detection of a failed 02 or CO sensor or an open thermocouple. If position feedback is available from the actuators, it also can detect a stuck fuel valve or air damper. This capability will improve the safety of process operation during failure conditions. 4) The logic capabilities of the low cost combustion controller will permit the implementation of process startup procedures that are safer than before. For example, the controller can be configured so that it goes through a timed air purge process before it allows the burner lightoff sequence to begin, thus ensuring that no combustible gases are present in the furnace or boiler at lightoff. By means of these approaches and others, the safety features now available using expensive control equipment will be made affordable through the use of the low cost combustion controller. SENSORS To achieve these benefits, the development program focused on the three key areas of sensors, electronics and packaging. All three of these are important to the success of developing a low cost combustion controller but the sensor area is probably the most critical. The sensors are required to measure the key process variables that relate to combustion efficiency. The strategy of using a combination oxygen and carbon monoxide analyzer to provide the vital information needed for optimum combustion control was employed. The technology most widely favored for measuring the 02 content is the use of the solid electrochemical zirconium oxide cell. The sensor cell is comprised of a membrane having specific ion conductivity for oxygen at temperatures above 11000F. Zirconium dioxide, with a stabilizer such as calcia, yttria, or hafnia, is a material used for such cells. This sensor cell has a characteristic that gives it a significant safety-related advantage over other methods: it indicates the "net" oxygen that remains in the sample after any combustibles (such as methane) are burned. Therefore, if any combustibles are present, it will cause the control system to increase the air flow to compensate. Another advantage is that as the 02 concentration decreases, the sensor output increases to produce a more accurate and repeatable signal. These cells require that the sensor temperature be reasonably high before the electrolyte starts to conduct an adequate supply of oxygen ions (available from the reference air supply). The output of the sensor is directly related to the product of the absolute temperature and to the logarithmic difference in 215 the partial pressures of the 02 concentration across the membrane. Specific work was directed toward the new 02 sensors which have been introduced in the automotive industry. Alt hough these automotive 02 sensors are not designed for use as continuous 02 measuring devices, they have been shown to be quite adequate when used under the right conditions of voltage, current and temperature. A number of these sensors have been tested under various conditions and the results have been very encouraging. More work and tests are required before finalizing the 02 sensor design but initial results are very promising. There are several different methods for determining the CO content in a sample str'eam, however, there is only one which easily lends itself to be incorporated into a single sensor assembly with an 02 sensor. This technique is based upon catalytIc technology and has been used by Bailey Controls for the last eight years. The exact measuring principle, whether it is a differential thermocouple, RTD or electrochemical has not been finalized. A number of sensors employing these measurement technologies have been tested. Only the electrochemical approach did not meet our initial test criteria for accuracy, repeatability and linearity. The two other approaches are gOing through extensive testing to determine interferences from other gases and sensor life. Both of these techniques meet our criteria with respect to size, ease of maintenance and mass producibility. ELECTRONICS The electronics package consists of the microcomputer and support electronics, user interface hardware, communications port, power output circuitry and a power supply. A portion of the electronics section handles the inputs from the sensors. This information is "processed" by solving equations and comparing the results with a known standard. The differences are translated into output signals to correct the deficiency. A microprocessor based architecture was chosen to provide the extensive computations and overall intelligence required to perform combustion control. Extensive input/output and operator interface circuitry are provided to perform functions specific to a low cost combustion controller. Five to seven low level analog signals are accepted by the electronic assembly. These signals, typically in the 0-50 mV range, must be amplified and converted to digital form to be useful to the microprocessor. The analog inputs are multiplexed, with approximately a 10Hz sampling rate, through a common amplifier stage. This architecture allows the analog inputs to use a common voltage reference to perform continuous calibration while using an |