ChemiresisTIve sensors provide a low-cost means of measuring various gas concentrations for industrial control, HVAC systems, and health and safety applications. Because they rely on heating elements, developers must ensure accurate measurement of the sensor's resistance while also controlling the heating element to maintain the proper temperature.
For these two requirements, developers can use a variety of techniques to balance design complexity and measurement accuracy.
This article reviews the properties of chemiresisTIve sensors and their role in various applications. It then introduced the Integrated Gas Technology (IDT) chemical gas sensor device and then focused on the requirements for using these sensors and the analog design alternatives that support their operation.
Finally, a general MCU-based design methodology is introduced and the relevant boards and software for evaluating and developing gas sensor designs are presented.
Precise sensor
Qualitative and quantitative measurements are becoming increasingly important in professional and more common applications. Methane detectors provide critical warnings in mining operations, hydrogen measurements can alert users to problems with batteries, and accurate gas sensors can be used as "electronic noses" in medical applications. In residential and commercial buildings, the ability to monitor various gas levels can alert users to toxic gases and provide fire warnings.
Among the available gas sensors, chemiresisTIve metal oxide sensors offer a cost-effective solution that delivers reliable results even in demanding applications. In these sensors, changes in the molecular concentration of the airborne gas cause changes in the resistance of the sensor. This change in resistance can reach several orders of magnitude within the operating range of the sensor. This relationship between the sensor resistance (RS) and the gas concentration C is represented by a simple formula that contains only two additional constant factors: A and α.
Or written in equivalent form:
Equation 2 demonstrates the linear relationship between the gas concentration logarithm and the sensor resistance logarithm. In fact, the equation shows that these sensors will exhibit rapid changes in resistance at low concentrations, but at much higher concentrations (Fig. 1).
Figure 1: IDS's SGAS701 hydrogen sensor and other chemical sensor sensors exhibit a linear log-log relationship between sensor resistance and gas concentration, but the support circuit can cause nonlinearities in the measurement results. (Source: Integrated Device Technology)
A range of chemiresisTIve sensors from IDT provide accurate measurements of a wide range of gases, including:
Hydrogen, using IDT SGAS701 sensor
Measurement of volatile organic compounds (VOC) using SGAS707 sensors, including formaldehyde, toluene, acetone and alcohol
Flammable gases using SGAS711 sensors, including hydrocarbons, methane, propane, natural gas
Together with the sensor components, IDT's four-pin devices integrate a resistive element that heats the sensor to the optimum measurement temperature.
For developers, the challenge is to ensure accurate measurement of the sensor's resistance while maintaining the heating element at the proper temperature. For these two requirements, developers can use a variety of techniques to balance design complexity and measurement accuracy.
Type of failure
1. ONU with time slot drift
This kind of failure is usually caused by the aging of the laser and the drift of the time slot for the upstream data transmission of the ONU. It is also possible that an error occurs in the data of the bandwidth map sent by the downstream OLT, which causes an error in the transmission time slot of the ONU. This kind of faulty ONU is difficult to detect. Generally, the OLT needs to monitor the upstream data of the ONU in real time. Determine whether it sends data in the time slot allocated by the OLT. For this type of abnormally luminous ONU, because it mainly affects two adjacent ONUs, in addition to isolation, the protection bandwidth between time slots can also be increased to alleviate its impact on other ONUs.
2. Long light ONU
This failure is caused by the ONU's laser for a long time. As a result, the ONUs under the entire PON port cannot communicate normally. If the ONU emits strong light, the abnormal light emission of the ONU can be determined by testing the optical power at the optical splitter during troubleshooting. However, if it is a weakly emitting ONU, the judgment will be more difficult, and the abnormal light emission of the ONU cannot be measured at the optical splitter. Only through the intervention of the OLT and the use of specific detection algorithms can the abnormal ONU be judged. Usually, the business under the entire PON port will be affected, but because of its failure phenomenon has always existed, it has brought convenience to the troubleshooting to a certain extent.
3. Intermittent glow
The judgment of this kind of failure is the most difficult and irregular. It is also the biggest fault in the current network that accounts for the abnormally luminous 0NU. It is not only impossible to measure the abnormal light emission of the ONU at the beam splitter. In addition, it is necessary to observe and monitor faulty PON ports for a long time, which brings great challenges to network maintenance.
There are many fault phenomena in the network of abnormally luminous ONUs. There is no complete mechanism and process in the industry to solve this fault. Other operators have no controllable experience. In the operation of the actual network. The author summarized a set of procedures and experience for troubleshooting abnormal light-emitting ONUs. Can effectively solve most of the existing network failures.
cause of issue
1. Long light-emitting problem: The photoelectric converter is connected to the branch fiber by mistake, and the photoelectric converter emits light for a long time; the ONU optical module has an abnormal shutdown signal. Cause the ONU to emit light for a long time.
2. Coverage interference problem: For some reasons (such as insufficient extinction ratio), individual ONUs emit light in unlicensed time slots, which affects the normal services of other ONUs.
3. Overlapping interference problem: The inconsistency of the parameters of different ONU optical modules causes partial overlap of the upstream optical signals, which affects each other's services.
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