Special liquid level sensors are of great importance especially for measuring levels in small containers. The radar system makes it possible to measure very exactly.
In the past, certain application scenarios in the field of distance measurement in the existing radar systems presented considerable challenges. The developed iSYS-5001 is a compact system from InnoSenT that accurately meets these high demands. Distance measurements are required in a variety of applications.
In an industrial environment for example, the exact positioning and tracking of machines and production robots enables a smooth and fully automated production process. Special liquid level sensors are of great importance especially for measuring levels in small containers. The smart system architecture of the iSYS-5001 eliminates the dead zone in front of the sensor and gives it an unbeatable advantage over traditional radar systems.
Distance measurement sensors can even be used in the field of vibration and oscillation analysis, since a vibration can be considered as a periodic change in the distance between the sensor and test object. The iSYS-5001 delivers exact measurement results through the high update rates that far exceed the frequency of oscillation to be examined and allows fast data acquisition and signal processing.
A microwave interferometer based on an alternative receiver structure, the six-port circuit, forms the core of the system and its concept which was already proposed in the 1970s by G.F. Engen and C. A. Hoer as a reflectometer. A first industry-mature system has now emerged from a project of InnoSent and the University of Erlangen-Nuremberg which is based on the sixport principle.The construction of the sensor is schematically shown in the following.
Schematic system structure of the radar measurement
The VCO (voltage-controlled oscillator) generates a stable 24 GHz signal using a phase-locked loop (PLL). The signal is split and partially sent as a reference signal to the six-port and the other part is fed to the antenna (in the case of the iSYS-5001, into a patch antenna) via a radar coupler. This transmission signal is reflected from an object at a distance d and again received as a signal by the antenna.
The two input signals (the reference signal of P1 and the signal reflected by the object P2) are superimposed in the six-port and generate four output signals P3...P6. These output signals are converted in the baseband and can then be digitised and evaluated as voltages B1 to B4.
The evaluation is done via an equation system which corresponds to the argument of the ratio of the complex input signals of the six-port structure of the phase difference of Δφ between the two input signals. Δφ can be calculated from the baseband voltages by forming the quotients and use of the arc tangent function.
If the six gate structure is used in a radar circuit as a receiver as shown here, the distance of the target being observed determines the phase of the input signal. Therefore the distance d of the target can be calculated from the measured phase difference Δφ.
HF front-end and the DSP board are installed together
The entire high-frequency front-end was produced on a dielectric, multilayer substrate in microstrip line technology with the antenna realised as a patch array. The sensor has an output power smaller 20dBm which allows worldwide operation within the license free 24 GHz ISM band. The signal processing is integrated into the system. The HF front-end and the DSP board are installed together in a 40 mm × 60 mm × 44 mm housing and can be connected via a RS232 interface to machines or via a USB adapter directly to a PC. A graphical user interface is available that allows an easy setup procedure of the device with a windows PC.
Micrometer accuracy and medical heart beat detection
Several measurements were performed on a high precision linear stage to evaluate the accuracy of this compact distance measurement device. A corner reflector was used as a test object (RCS 1m²) which was moved to a distance of 1.7 m in 100 μm increments by a total of 150 mm. After a calibration, the maximum error is ±400 µm.
To demonstrate the repeatability, the standard deviation of the measurement data is displayed in the following graph. Here, a total of 10,000 measurement values were recorded and evaluated over a period of 100 s. 83.8% of the measured values are within ±20 μm around the actual distance value of the test object. 99.6% are within ± 40 μm around the setpoint value. Industrial positioning applications, distance measurements and motion detection with an accuracy in the micrometer range, quick mechanical vibration analysis of industrial equipment, acoustic fingerprints or in the medical field of non-contact respiratory and heart beat detection are just a few examples of iSYS-5001’s versatility.
The sensor offers advantages for many more innovative and already existing applications. By combining high accuracy with the robustness of the radar sensors, the iSYS-5001 will be able to assert its position over the next few years on the radar-based sensor market. A real prototype test system with many applications. //LD
* * Stefan Bäuerlein is working in Marketing of InnoSent, Donnerstdorf Germany.