Tag Archives: MEMS Technology

MEMS Technology for CO2 Sensing

Most technologies for detecting CO2 are based on photo-detection, where smoke particles reflect light that photo-sensors can detect. However, MEMS technology now offers a more sensitive technology for detecting CO2. Using their knowledge in sensors and MEMS technology, Infineon has now introduced a disruptive gas sensor for sensing CO2 gas.

Coming in a minuscule form factor, the XENSIV PAS CO2 from Infineon is a real CO2 sensor. Infineon has based it on the principle of photoacoustic spectroscopy or PAS. Infineon uses a MEMS microphone, which they have optimized for low-frequency operation. The sensor has a cavity that can detect pressure changes generated by CO2. An integrated microcontroller in the sensor then delivers the CO2 concentration in the form of a direct ppm readout. As the absorption chamber of the sensor is acoustically isolated from external noise, the sensor guarantees highly accurate readings of CO2.

XENSIV PAS CO2 has impressive features. Its operating range extends from 0 ppm to 10,000 ppm, with a linear response giving an accuracy of 30 ppm +3% of reading between 400 ppm and 5,000 ppm. The operating temperature range of the sensor is 0-50 °C at a relative humidity (non-condensing) of 0-85%.

The sensor requires two supply voltages, 12VDC for the emitter and 3.3VDC for its other components, and its average power consumption is typically 30 mW when operating at 1 measurement per minute. With a package dimension of 13.8 x 14 x 7.5 mm, the sensor offers three interface standards—I2C, UART, and PWM.

XENSIV PAS CO2 has several potential applications. On account of its high accuracy, SMD capabilities, and compact size, the sensor is ideally suitable for indoor air quality monitoring with numerous potential applications. For instance, the sensor is highly suitable for home appliances for air conditioners and air purifiers. It is also suitable for smart home IoT devices like smart lighting, indoor air quality monitors, personal assistants, baby monitors, speakers, and thermostats. Apart from use in in-cabin air quality monitoring in aircraft, the sensor is eminently suitable for city management and CO2 emission control in advertising billboards, bus stations, and outdoor lighting.

While measuring the CO2 concentration, the sensor operates in one of two modes—active state and inactive state. In the active state, the integrated CPU is in an operating state and performs tasks like running a measurement sequence or serving an interrupt. However, when the sensor has no specific task to perform, the CPU enters an inactive state. The device may enter an inactive state from an active state at the end of a measuring sequence.

During an inactive state, the CPU controlling the device can enter a sleep mode to optimize the consumption of power. Several events can wake up the CPU from its inactive state—a falling edge on the PWM pin, reception of a message on the serial communication interface, or the internal generation of a measurement request when the device is in continuous measurement mode.

It is possible to program the sensor module via its serial communication interface to operate in one of three modes—idle mode, Continuous mode, and Single-Shot mode.

MEMS Technology Helps To Measure Flow

Smart technologies are creating compact and lightweight sensing elements. Apart from being optimal, fast, and efficient solutions, these are not limited to only the data input functions as the conventional sensing technologies are. Rather, they integrate the areas of sensing and control while offering high-value information that humans or systems can subsequently process. Several unique and advanced technologies such as MEMS form the concept of sensing and control expertise. For example, flow sensors use the ButterflyMEMS technology to operate.

Flow sensors using the MEMS technology operate with major advantages. For example, they can easily measure flow speed ranging from 1 mm per second to 40 m per second. To understand this better, ButterflyMEMS technology can sense the fluttering of the wings of a butterfly and the roar of a typhoon with equal ease. A tiny MEMS flow sensor does all the work and it is the size of a 1.5 mm square chip, which is only 0.4 mm thick.

Conventionally, flow sensors have been using the method of resistance measurement. The method senses the change in electrical resistance of a filament because of a change in temperature caused by the flow of material across the filament. Balancing the resistance of the filament is a time-consuming method, which forms the major disadvantage of this method and makes it expensive.

In contrast, the MEMS flow sensor utilizes a thermopile, an element that converts thermal energy into electrical energy. This technology offers several advantages not seen earlier. For instance, MEMS technology offers cheaper operation, only a few adjustments, high sensitivity, and low power consumption.

This advanced sensor can even sense the direction of flow. The chip has two sets of thermopiles located on either side of a tiny heater element. The thermopiles measure the deviations in heat symmetry that the gas flow causes. The chip senses the direction of flow based on a positive or a negative deviation. A thin layer of insulating film covers the sensor chip and protects it from being exposed to the gas.

In the absence of flow, temperature distribution remains uniform around the heater and there is no differential voltage between the two thermopiles. With even the smallest flow, the heat symmetry collapses, as the thermopile on the side of the heater facing the flow shows a lower temperature, while the thermopile on the other side is warmer. This temperature difference causes a differential voltage to appear between the two thermopiles. This voltage is proportional to the mass flow rate.

The superb characteristic of the sensing chip comes from an unusual shape created by a unique etching technology. Compared to the conventional silicon etching, this unique etching technology creates a larger sensing area in the same volume. This results in a cavity design enabling heating with greater efficiency while keeping the power consumption low. Additionally, the cross-point of temperature characteristic can be factory adjusted, which results in high output stability even when the ambient temperature fluctuates.

Within the actual sensor, a set of screens in the sensor inlet produces a uniform, laminar flow through the sensor offering optimal mass flow readings. An orifice in the outlet side of the sensor buffers against pulsing flows.