Automotive Exhaust Gas Analyzer
For this project, we designed all the hardware and software for an automotive exhaust gas analyzer. These analyzers are called a "gas bench" in the industry. Gas benches generally don't have user interfaces as they are all designed to go into a larger piece of inspection, test and repair equipment.
Infra-red detectors are used to measure the three primary gases of interest. These are unburned hydrocarbons (HC), carbon monoxide (CO) and carbon dioxide (CO2). The other two gases of interest are oxygen (O2) and a group of related chemicals collectively known as oxides of nitrogen (NOx). O2 and NOx do not absorb infra-red, and so must be detected with separate electro-chemical cells. These cells are essentially variable batteries, and generate a voltage proportional to concentration. The output is very linear, but is very low (less than 10 mV) and varies considerably from unit to unit.
The infra-red detector has to be cooled to have enough sensitivity for carbon monoxide. We used a Peltier cooler for that purpose. Peltier coolers are solid state heat pumps. This design used a thermistor and PID loop controlling a PWM output. The tricky part of Peltier coolers is that if you run too much current through them, they go into runaway and both sides get hot.
The filters needed to be held at a constant temperature regardless of ambient conditions. Since this assembly was too large to be cooled, it was heated to 118 degrees F by a PID loop driving a PWM output.
The key to this project was maintaining the flexibility of the product to suit various markets, while simultaneously keeping the manufacturing costs low. There are requirements for three gas (HC, CO, CO2) four gas (adding O2) and five gas (adding NOx) benches. In addition there are 2 cycle engines with much higher HC. There are also requirements for different fuels, particularly in South Korea which seems to allow almost anything flammable to be run through a car. There are even requirements for printers and displays for very low cost units.
At the time, the leading competitor had split the tasks between three microcontrollers, and still had difficulty completing all the calculations in the required time. We thought that they had reused too much of the previous generation design. Although our client also had existing products, we started with a clean sheet of paper.
A single microcontroller controlled every function. Some optional features, such as the printer and NOX and oxygen sensors, were moved to daughter boards, but the same micro handled them also. All of the PWM, data acquisition and communication functions were handled with interrupts. In fact, the product used 12 of the 17 available interrupts. The product was about half the volume of competing products, and significantly less expensive. As is true of virtually all our designs, there were no trim pots, and a relatively simple calibration procedure.
Well, actually, just when the product was about finished, the company was sold to another company who decided not to pursue this market. But still, it was a very impressive design while it lasted.
Toucan Technology 2444 Lexington Dr. Ventura, California 93003 on the beach between Los Angeles and Santa Barbara.