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Show inexpensive amplifier stage. This architecture allows the analog inputs to use a common voltage reference to perform continuous calibration while using inexpensive amplifiers in the input circuit. One should note that the combustibles sensor input requires an additional stage of amplification due to its significantly lower output. This stage uses more precise amplifiers to provide acceptable accuracy for the combustibles measurement. The multiplexed analog signal is then fed through a voltage (V) to frequency (F) converter. The V to F output is a frequency directly proportional to the voltage at its input. This frequency is fed to the microprocessor for measurement. The microprocessor chosen for the controller is the Motorola 68HC11, a recently developed, highly integrated device. Included on the chip are significant amounts of Read Only Memory (ROM), Random Access Memory (RAM), and Electrically Erasable Programmable Read Only Memory (EEPROM), a serial InputlOutput (1/0) port, and a fairly sophisticated timer system. Due to the high degree of integration of the 68HC11, the complexity of the low cost combustion controller's electronics is reduced. Many of the functions which would have been performed by a large number of Integrated Circuits (IC's) or a semi-custom chip are now performed by one standard part. The microprocessor, controlled by the system's firmware, performs the complex calculations required to efficiently control a combustion process. The microprocessor is also responsible for continuously monitoring the controller's keyboard and updating its Liquid Crystal Display (LCD). The LCD display chosen, a 20 character by 4 line model, is used to display relevant process conditions and operating instructions. The 68HC11 timer system is used to provide four independent pulse trains which serve to drive the analog output system. The pulse trains are of constant period and variable duty cycle and drive circuitry which converts the information represented by the duty cycle into an analog signal. Using this method, which lends itself to providing ground isolation, two sets of two isolated outputs are provided. It is felt that this isolation method, which is a compromise between the low cost of no isolation and the higher performance of complete isolation, will provide suitable performance at an acceptable cost. The controller's internal power supply, which accepts standard 120VAC, 50/60 Hz line voltage at its input, provides power to both the controller's electronics and the sensor heaters located in the sensor enclosure. The total power requirements of the controller electronics are currently estimated to be in the 10 Watt (W) range. This power will be supplied by a conventional linear supply. Current estimates for the sensor heater power supply are in the 200 - 300W range. As the electronics industry has grown, more and more functions have been incorporated into 216 smaller spaces and fewer circuits. This has lead to fewer parts and lower cost to achieve more operations. In direct relation to the lower number of parts involved electronically, is the increase in produdct reliability which impacts the overall cost in the longrun. Another definite advantage of highly integrated circuitry such as microprocessors and microcontrollers is the ability for them to calibrate themselves. This can be done as often as needed, with no external interface required. The next logical extension would seem to be on autocalibration setup for the entire system. This would minimize operator interaction and greatly minimize the opportunity for errors. The configuration for combustion control is based on the sensor inputs. This configuration is used to implement the desired control strategy of holding a temperature set point while maintaining an efficient fuel to air ratio. A simplified diagram of the configuration is shown in Figure 3. The configuration can be divided into three sections. A thermocouple input for process temperature is used to control the air damper output for an approximate fuel to air ratio. The excess air portion of the configuration reads the output from the controller's oxygen sensor and adjusts, within certain limits, the air damper position so that the desired and actual values of excess air are the same. The signal from the controller's combustibles sensor is used to adjust the oxygen setpoint. The percentage of oxygen in the flue gas is not used to directly control the position of the air damper. Rather, it adjusts the air damper position within certain limits (usually ± 10%). This control method, known as O2 Trim, is safer than allowing the oxygen signal to directly control the air damper. If a sensor should fail, the controller does not allow the fuel to air ratio to become dangerously rich. PACKAGING In the area of combustion control systems, packaging plays a very important role. Here, packaging consists of much more than just the two external enclosures. The housing for the sensors, along with the manifolding and passages are a small part of packaging. A better description of the packaging function could be system integration. This task seeks to integrate or combine the various parts into a working system. Externally, the enclosures for the electronics and the sensor assemblies are the start of the system integration. The electronics assembly has a membrane keyboard and a 20 character 4 line liquid crystal display on the cover. This comprises the user interface and allows communication with the outside world, consisting of current system values and operator entered set points. This information is visible on the screen and can be scrolled for Viewing. |