OCR Text |
Show modulator started, the variable length link is sent to a position of excess air which is the first point used in the generation of a characterization curve. This characterization curve consists of detector signal (temperature) versus jack shaft position (excess air). The reference signal from the modulator and the detector signal are captured by the microprocessor; and, the modulation frequency is precisely determined from the reference signal. Yith this frequency information, phase sensitive WIDE ICAH ICAH TU'U. 0" MOD. - _________ ..r-- TUIIIH OH MOD. FIG . 6 Control Loop Algorithm for fuel/a i r trim system . 202 detection is performed on the detector signal. After the result of the phase sensitive detection and the shaft position are stored the variable link is adjusted to a less lean position . This process is repeated until sufficient data are accumulated to construct a wide scan characterization curve. A cubic fit to the data is performed and the link is adjusted to a position just lean of the position at which maximum emission occurs. An additional set of data are then acquired in the region of the maximum. This is a fine tuning procedure. The result of the fine tuning is checked to ensure that the maximum lies within a predetermined range as might be accomplished in a analog system by adjusting the loop gain . The variable link is then set to the correct length. The process described above is then repeated for several fuel flow settings so that a family of characterization curves are produced. The lengths at which maximum emission occurs become part of a library of data which permits the modulator to be turned off. In a traditional analog control system, this would not be possible. As fire level is varied, it is tracked by the computer via the fire level potentiometer. The optimum link length is then determined by interpolation from the data in the library. RESULTS The control method described above has been demonstrated on a laboratory scale (Zabielski, 1986). An evaluation of the method on a commercial liquid/gaseous fuel boiler is in progress . All of the major elements of the hardware and software described above have been tested and installed on a boiler. The results of infrared signature measurements employing the butterfly modulator are shown in Figure 7. The data were obtained viewing the flame from the rear sight port . The oxygen concentrations were measured with a Scott Model 150 paramagnetic oxygen analyzer. Because of a mechanical stop l~cated within the burner assembly, the alr damper cannot be adjusted sufficiently to produce the lean side of the characterization curve . The fact that the oxygen concentration is not 2-4% is related to the mixing characteristics of the burner . Yhen the boiler is fired on oil, the oxygen concentrations are 1-3% . |