An Ultra-Low Power Fast Response TCD Sensor for Airborne Measurements Using Unmanned Aerial Vehicles

Unmanned aerial vehicles (UAVs) carry sensing instruments to interesting, often remote areas with difficult access to collect in situ data. Atmospheric sensors can providing critical real-time data for environmental monitoring and other earth science applications. However, there remains a significant technology gap in the development of high performance trace gas sensors that meet the stringent requirements for UAV deployment. Most gas sensors are costly, bulky, and too slow to capture data in a speeding aircraft. In order to integrate with UAVs, airborne sensing systems should meet the UAV payload size, weight and power limitations. In addition, the UAV-carried sensors should be able to perform sensitive measurements with large variations of temperature, pressure and altitude and be compatible with the UAV’s cruise speed.

This paper presents the investigation of a miniature, lightweight, ultra-low power (microwatts), ultra-fast, thermal conductivity (TCD) sensor platform to enable high resolution, fast analysis of methane (CH₄) in the atmosphere. KWJ’s MEMS TCD sensor consists of a microfabricated polysilicon element  that is low mass for rapid response and low power operation. The sensor elements are heated to a specified temperature by constant voltage or constant current. The thermal conductivity of the target gas in which the device resides will be reflected in the rate of cooling and equilibrium temperature of the device. TCD sensors with different geometries and dimensions were exposed to CH₄ in air mixtures over a range of concentrations (100ppm - 1000ppm). The results showed that the sensor’s response is linearly dependent on the CH₄ concentration. A lower limit of detection (LLD) for CH₄ of ~100 ppm in dry air was obtained using the TCD sensors.

Fabrication of MEMS TCD sensors. KWJ fabricated sensor elements are comprised of passivated polysilicon and fabricated with multiple sensing structures on one chip. Some of the elements are designed to accommodate a coating for high sensitivity and selectivity but these are not discussed herein. Figure 1 below is an SEM image of a 50 x1 µm TCD element suspended over a cavity.  

Characterization of TCD sensors. A benchtop measurement system, including testing chamber, environmental control system, gas delivery system, sensor control/measurement electronics and data acquisition system, was used for characterizing the response of the TCD sensors. The sensor element was placed in the test chamber and exposed to CH₄/air gas mixtures with different concentrations. A pulse current waveform is applied to excite the sensor. The typical pulse comprised a hot excitation (1.5mA) applied for 40 ms and the cold excitation (0.25mA) for 4960 ms to provide a total pulse and measurement cycle of 5 seconds. Both the sensor control and data acquisition were timed in a coordinated fashion via the PC interface (Labview) such that the TCD resistance was acquired during the correct portion of the repetitive heating and cooling cycles. With the pulse excitation, the power consumption is very low (1.5mA*4V = 6mW continuous and 6*0.04/5=48 µW) per reading duration of 40 ms. The UAV would only move about 2.3 feet at a 40 mph air speed and thus the airborne readings at 40 ms would provide a resolution of a few feet. Of course, the response time of the device is measured in microseconds and so resolution of a few inches or less could be obtained with optimization of this device.

The TCD sensor element was exposed to dry CH₄ with different concentrations (1000ppm, 750ppm, 500ppm, 250ppm and 100ppm) in air at ambient temperature.  The linear sensor response is illustrated below. These results provide a Lower Limit of Detection (LLD) for CH₄ of about ~100 ppm CH₄.  

Because of its small size, low mass, rapid response, and low power consumption, the novel sensor platform can be easily adapted to be deployed with different UAV platforms (balloons or different types of aircrafts). The structure of the novel sensor elements can accommodate metal  electrodes and, therefore, can be used with selective and high sensitivity coatings for specialized applications. These features will be explored in future work.  The MEMS approach used here provides a platform that can bridge the gap between the current large gas sensing equipment and the requirements for tiny payload UAV carrying gas sensor instrumentation for environmental measurements and earth science applications.

Figure 1.  MEMS TCD bridge (50 x 1 μm) and CH₄ calibration curve.