Abstract
A primary concern in a multitude of industrial processes is the precise monitoring of gaseous substances to ensure proper operating conditions. However, many traditional technologies are not suitable for operation under harsh environmental conditions. Radar-based time-of-flight permittivity measurements have been proposed as alternative but suffer from high cost and limited accuracy in highly cluttered industrial plants. This paper examines the performance limits of low-cost frequency-modulated continuous-wave (FMCW) radar sensors for permittivity measurements. First, the accuracy limits are investigated theoretically and the Cramér-Rao lower bounds for time-of-flight based permittivity and concentration measurements are derived. In addition, Monte-Carlo simulations are carried out to validate the analytical solutions. The capabilities of the measurement concept are then demonstrated with different binary gas mixtures of Helium and Carbon Dioxide in air. A low-cost time-of-flight sensor based on two synchronized fully-integrated millimeter-wave (MMW) radar transceivers is developed and evaluated. A method to compensate systematic deviations caused by the measurement setup is proposed and implemented. The theoretical discussion underlines the necessity of exploiting the information contained in the signal phase to achieve the desired accuracy. Results of various permittivity and gas concentration measurements are in good accordance to reference sensors and measurements with a commercial vector network analyzer (VNA). In conclusion, the proposed radar-based low-cost sensor solution shows promising performance for the intended use in demanding industrial applications.
Highlights
On-line gas monitoring is an increasingly important element in a multitude of industrial applications, for example, in the process [1] or petrochemical industry [2,3], for food processing [4]as well as for waste treatment plants [5,6]
This paper focuses on the theoretical and practical performance limits of low-cost frequency-modulated continuous-wave (FMCW) radar sensors for monitoring binary gas mixtures
A constant flow rate of 10 L/min is maintained until the inclusion gas volume fraction ζ i starts to settle at a minimum of 90% as measured by the MOX concentration sensors
Summary
On-line gas monitoring is an increasingly important element in a multitude of industrial applications, for example, in the process [1] or petrochemical industry [2,3], for food processing [4]as well as for waste treatment plants [5,6]. On-line gas monitoring is an increasingly important element in a multitude of industrial applications, for example, in the process [1] or petrochemical industry [2,3], for food processing [4]. Various gas sensing technologies, for example, metal-oxide (MOX) semiconductor, calorimetric and optical. Sensors 2019, 19, 3351 sensors, for instance nondispersive infrared (NDIR), are widely used [7,8]. Invasive sensor technologies such as MOX and calorimetric detectors require direct contact to the medium-under-test (MUT). They might not be applicable for harsh industrial environments containing hot or corrosive gases. NDIR optical sensing is a potentially contactless technique with real-time capability but detects only infrared-active gases [9,10]
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