Christoph Will

Six-Port Based Interferometry for Precise Radar and Sensing Applications.

Microwave technology plays a more important role in modern industrial sensing applications. Pushed by the significant progress in monolithic microwave integrated circuit technology over the past decades, complex sensing systems operating in the microwave and even millimeter-wave range are available for reasonable costs combined with exquisite performance. In the context of industrial sensing, this stimulates new approaches for metrology based on microwave technology. An old measurement principle nearly forgotten over the years has recently gained more and more attention in both academia and industry: the six-port interferometer. This paper reviews the basic concept, investigates promising applications in remote, as well as contact-based sensing and compares the system with state-of-the-art metrology. The significant advantages will be discussed just as the limitations of the six-port architecture. Particular attention will be paid to impairment effects and non-ideal behavior, as well as compensation and linearization concepts. It will be shown that in application fields, like remote distance sensing, precise alignment measurements, as well as interferometrically-evaluated mechanical strain analysis, the six-port architecture delivers extraordinary measurement results combined with high measurement data update rates for reasonable system costs. This makes the six-port architecture a promising candidate for industrial metrology.

Microwave interferometer radar-based vital sign detection for driver monitoring syst

Autonomous driving is a topic of great importance for our society and is expected to be introduced in the automotive market in a couple of decades. Today, vehicles can already autonomously brake and accelerate in order to avoid crashes with obstacles detected by camera and radar technology. The next step towards autonomous driving is the introduction of driver health monitoring systems. Intensive research is being conducted in this field with the goal of developing a ”biometric driver seat”. In this paper, an interferometric radar-based vital sign detection is presented. The aim is to investigate the applicability of radar technology for vital sign monitoring such as heartbeat and breath rate of the driver with sensors embedded in the seat. The proposed interferometric radar is based on the Six-Port receiver technique and shows excellent performance for the addressed application.

Substrate integrated waveguide fed antenna for 61 GHz ultra-short-range interferometric radar systems

Radar sensors play a key role in todays industrial automation. Being an alternative to frequency modulated continuous wave radar, Six-Port based radar sensors recently attracted the interest of the community as well as of the industry. This paper presents a 61 GHz Six-Port radar sensor for ultra-short range applications and a dedicated substrate integrate waveguide fed tapered slot antenna design. Due to an optimized design, the antenna allows for measurements in the direct near-zone of 0mm to 10mm without ambiguity effects. Making use of a spiral fitting operation, the presented measurement results show an absolute distance error of less than 100 μm.

Segmental polynomial approximation based phase error correction for precise near field displacement measurements using Six-Port microwave interferometers

Six-Port microwave interferometers are a low-cost as well as low-power type of radar sensor with a high phase accuracy, which can be used for precise displacement measurements. Near field effects strongly influence the signal characteristics of a reflection of the electromagnetic wave near the antenna, especially if the target is low reflective. In this paper a calibration procedure based on phase error correction by segmental polynomial approximation is proposed that utilizes these effects. After validating the functionality of the calibration algorithm and its improvement by comparison to a comparable state-of-the-art procedure, two further near field measurements are presented. A cardboard as well as a plastic plate are used as low reflecting targets to show the applicability of the proposed calibration procedure for diverse measurement scenarios.

Detector nonlinearity in Six-port radar

This paper presents an in-situ detector characterization and calibration method for differential I/Q Six-port radar systems. A separate calibration of the nonlinear detectors and the remaining linear RF front-end enables an independent analysis of errors caused by detector nonlinearity and by the measurement scenario. A linearity analysis of various detectors in Six-port radar systems shows the necessity of a detector calibration for accurate displacement measurements. The calibration is evaluated with three different Six-port based radar systems at 24 GHz in diverse measurement scenarios. An error reduction of up to 50% has been achieved.

Local Pulse Wave Detection Using Continuous Wave Radar Systems

Although a lot of research has been done into radar-based heartbeat detection over the past few years, it is still unknown which physiological effects truly underlie these measurement signals. This paper investigates the cardiovascular system, as well as the effects due to the antenna characteristics, and establishes a connection to the variety of heartbeat signals that can be found in comparable publications. For the first time, the cause of the diverging signal shapes is researched, revealing the locally specific pulse wave curves named sphygmograms. Three different types are investigated: the carotid, the venous, and the ventricle sphygmogram. Verification of the measured signal shapes is ensured by a laser sensor. Afterward, the influence of the filtering on the sphygmograms is researched, as well as the influence of the antenna characteristics on the radar signal. Finally, all novel insights are combined to substantiate the variety of published heartbeat signal curves.

Advanced template matching algorithm for instantaneous heartbeat detection using continuous wave radar systems

Instantaneous heartbeat detection is a key parameter in modern vital sign monitoring. Continuous wave (CW) radar systems enable contactless measurements of the vibrations on the human skin effected by heartbeats. Since a high accuracy as well as robustness of the measurement sensor is appreciated, the belonging signal processing routine has to deal with challenging requirements. In this paper, an advanced template matching (ATM) algorithm is proposed to enhance the performance regarding instantaneous heartbeat detection using CW radar systems. Compared to common template matching algorithms, multiple heterogeneous templates are utilized in this approach, at which the appropriate template type is determined by prior feature detection. A 24 Ghz Six-Port microwave interferometer is used for vital sign measurements of a person-under-test. The functionality of the proposed algorithm is verified by a synchronous reference electrocardiogram (ECG) and its enhancement is shown by reducing the root-mean-square error (RMSE) of the interbeat intervals (IBI) compared to an ordinary template matching algorithm.

Instantaneous heartbeat detection using a cross-correlation based template matching for continuous wave radar systems

Instantaneous heartbeat detection is necessary for an optimal patient monitoring in healthcare centers. Whereas electrocardiogram (ECG) and ballistocardiogram (BCG), for instance, are established methods for real time monitoring nowadays, contact-free systems are appreciated. Herewith, a 24 GHz continuous wave (CW) radar system with an intelligent signal processing is presented. Common heartbeat detection algorithms use the fast Fourier transform (FFT), and therefore require an adequate observation time window and rather detect an averaged heart rate. The proposed algorithm in contrast accomplishes the detection of single heart beats directly in the time domain with an insignificant delay. The core of that algorithm is a template matching using the cross-correlation method. In this paper, after describing the radar system with the novel signal processing, the presented system is compared to a commercial ECG product.

Intelligent signal processing routine for instantaneous heart rate detection using a Six-Port microwave interferometer

Instantaneous heart rate detection is one of the key parameters in medical vital parameter monitoring. In medical centers e.g., real time monitoring of the vital signs of a patient under surveillance is necessary. Nowadays, the dominant technologies are electrocardiogram (ECG) or ballistocardiogram (BCG), but the required direct contact to the person-under-surveillance is a common drawback of these sensors. In this paper, a Six-Port microwave interferometer is presented and used to detect the current heart rate of a person-under-test. An intelligent signal processing routing is proposed that avoids the fast Fourier transform (FFT) due to the implicated longsome observation window and operates directly in the time domain instead. A commercial ECG product is used to proof the reliability of the presented signal processing routine to establish Six-Port microwave interferometers for instantaneous heart rate detection.

Radar-Based Heart Sound Detection

This paper introduces heart sound detection by radar systems, which enables touch-free and continuous monitoring of heart sounds. The proposed measurement principle entails two enhancements in modern vital sign monitoring. First, common touch-based auscultation with a phonocardiograph can be simplified by using biomedical radar systems. Second, detecting heart sounds offers a further feasibility in radar-based heartbeat monitoring. To analyse the performance of the proposed measurement principle, 9930 seconds of eleven persons-under-tests’ vital signs were acquired and stored in a database using multiple, synchronised sensors: a continuous wave radar system, a phonocardiograph (PCG), an electrocardiograph (ECG), and a temperature-based respiration sensor. A hidden semi-Markov model is utilised to detect the heart sounds in the phonocardiograph and radar data and additionally, an advanced template matching (ATM) algorithm is used for state-of-the-art radar-based heartbeat detection. The feasibility of the proposed measurement principle is shown by a morphology analysis between the data acquired by radar and PCG for the dominant heart sounds S1 and S2: The correlation is 82.97 ± 11.15% for 5274 used occurrences of S1 and 80.72 ± 12.16% for 5277 used occurrences of S2. The performance of the proposed detection method is evaluated by comparing the F-scores for radar and PCG-based heart sound detection with ECG as reference: Achieving an F1 value of 92.22 ± 2.07%, the radar system approximates the score of 94.15 ± 1.61% for the PCG. The accuracy regarding the detection timing of heartbeat occurrences is analysed by means of the root-mean-square error: In comparison to the ATM algorithm (144.9 ms) and the PCG-based variant (59.4 ms), the proposed method has the lowest error value (44.2 ms). Based on these results, utilising the detected heart sounds considerably improves radar-based heartbeat monitoring, while the achieved performance is also competitive to phonocardiography.