What are MEMS Pellistors and How Do They Work?

In mining applications, the MP-7217 (VQ548MP) pellistor combustible gas sensor (Figure 1) can be used to ensure the protection of facilities and personnel. Thanks to its innovative structural design, the gas sensor offers excellent performances. The changeover from a standard platinum coil to a micro-machined diaphragm (MEMS) including embedded planar heater meander provided with a cycled operating mode guarantees significant power savings.

The MP-7217 (VQ548MP) pellistor combustible gas sensor.

Figure 1. The MP-7217 (VQ548MP) pellistor combustible gas sensor.

The MP-7217 (VQ548MP) MEMS pellistor structure is provided with two planar metal heaters coated with a layer of noble metal catalyst for Detector structure and an inert layer for Compensator structure. This heater pair is ideally designed for thermal insulation and assembled onto membranes, thus reducing the operating power down to 60 mW per structure during continuous operation.

The meander functions as an electrical heater and a resistance thermometer. The pellistor dual-structure is die-connected on a PCB and electrically connected by means of wire bonding. The sensor chip is surrounded by a plastic can provided with a flame arrestor at the end exposed to the atmosphere.

The sensor is further heated by the energy released during the oxidation of a flammable gas when the sensor is heated to approximately 400–500 °C. The higher temperature is detected with an increase in the heater meander’s resistance. The measurement is carried out by employing the pellistor dual-structure in a circuit to identify the variations in the resistances of the structures (Comp, Det).

Measurement Circuit

In general, a Wheatstone bridge is used to carry out pellistor measurements (Figure 2). Power consumption can be reduced by using a duty cycle operation represented by the ratio of the time over which the sensor is powered over the cycle period (TON/T) (Figure 3).

Pellistor measurements are commonly performed by using a Wheatstone bridge.

Figure 2. Pellistor measurements are commonly performed by using a Wheatstone bridge.

The use of duty cycle operation enables power savings.

Figure 3. The use of duty cycle operation enables power savings.

Operation Principle

It is necessary for the CPU used in the instrument to control the cycled mode (Phase 1) to realize sensor operation synchronization with the measurement (Phase 2). Reliable measurement can be performed by averaging the measurements performed toward the end of the operation period over different samples according to the acquisition frequency and ADC performances (Phase 3). The instrument has the ability to calculate and display the gas concentration based on the calibration and measurement (Figure 4).

Operation principle.

Figure 4. Operation principle.

Cycle Mode Definition

The period length and duty cycle are related to the constraints of the application in terms of T90%. The sensor has the ability to reach the operating temperature in just 30 ms. Upon achieving the operating temperature, it is possible for users to observe the catalytic reaction in the presence of a gas and achieve excellent signal response after approximately 250 ms. The minimal operating period is represented based on this value.

The response time is given by the non-operation period and the duration between two measurements. T90% is approximately 12 seconds for detection of methane during continuous operation; this value stays constant in the cycle mode operation since the major portion of the response time is associated with diffusion time through the cap.

Performances

The value reached following an operation for 200–1000 ms is better when compared to that in continuous mode. Consequently, in cycled mode, sensitivity is considerably higher (Figure 5). This suggests a side effect phenomenon caused by the non-continuous combustion of the gas that contacts the heated catalytic layer.

Sensitivity curve vs operating mode.

Figure 5. Sensitivity curve vs operating mode.

The sensor response in the continuous operation mode is related to the equilibrium between the catalytic reaction and the gas diffusion through the cap. In cycled mode, a higher sensitivity is realized by quantifying the output of the sensor before this equilibrium is reached. Thanks to the possibility of increasing battery life, the cycled operation mode feature is appropriate for portable instruments.

This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.

For more information on this source, please visit SGX Sensortech (IS) Ltd.

 

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