A novel remote respiration and heartbeat monitoring sensor is presented. The device is a monostatic radar based on a six-port interferometer operating a continuous-wave signal at 24 GHz and a radiated power of less than 3 $\mu\hbox{W}$ . Minor mechanical movements of the patient's body caused by the respiration as well as hearbeat can be tracked by analyzing the phase modulation of the backscattered signal by means of microwave interferometry with the six-port network. High-distance measurement accuracy in the micrometer scale as well as low system complexity are the benefits of the six-port receiver. To verify the performance of the system, different body areas have been observed by the six-port radar. The proposed system has been tested and validated by measurement results.

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.

Industrial automation today is an essential technology underlying our modern society. Advanced positioning and sensor feedback tasks in automation processes often require distance displacement detection, e.g., to measure and track the movement of robots. Furthermore, the detection of mechanical stress in complex industrial machinery through an accurate vibration analysis is often a task of major interest. Therefore, high-resolution distance measurements with short- and long-range positioning are important for a large number of sensing applications and can also be used as a precondition for vibrometer applications. Several automation technologies rely on high precision positioning sensors to track linear as well as rotational movements of various machinery.

Displacement detection and vibration measurements are important requirements for automation and mechanical stress diagnosis in diverse industrial environments. Laser interferometry is a widely used technique for these kinds of measurements since it features high-resolution, wide dynamic range and wide frequency response. Another advantage of the laser interferometry technique is contactless diagnosis/sensing which allows remote operations. However this technique is not suitable in harsh environments for instance with the presence of dust or fog which occur in several industrial production sites. In this paper a new contactless radar based vibration measurement technique is presented. It features an RF frontend based on a passive six-port interferometer working at 24 GHz. A dedicated signal processing unit is also presented. The proposed hardware allows to measure with a position accuracy of 0.5 µm.

Contactless sensing plays an important role in today's highly automated industrial environment, as modern sensors allow for an increased product quality in high-speed manufacturing lines. Hereby, a measurement system for sheet thickness monitoring in aluminum and steel rolling mills is presented. The presented concept is based on a differential measurement principle using two cooperating six-port radar systems. The radio-frequency front ends are designed in a high-density substrate-integrated waveguide technology at a frequency of 61 GHz. Furthermore, a detailed analysis of nonideal behavior of monostatic six-port radar front ends is presented. Moreover, the measurement principle is evaluated in a simplified measurement setup and the results are compared with the previously presented theory. Finally, an overview of modern sensing technology for steel rolling mills is presented and compared with the proposed measurement system considering the important aspects, i.e., update rate, precision, and thickness limit.

Six-port interferometers are of great interest for displacement and vibration measurements in industrial as well as medical applications. However, they show offsets as well as amplitude and phase imbalances in their differential in-phase and quadrature signals, leading to inaccuracies of the measured displacement. If a very slow moving or vibrating target has to be monitored, an additional phase shifter for calibration is needed, so that the system's errors can be compensated. A method for calculation and compensation of errors is presented and a compact and cost-efficient implementation of a phase shifter for 24 GHz Six-port interferometers is shown.