Transmission Lines in Capacitance Measurement Systems: An Investigation of Receiver Structures

Abstract

Dielectric sensing based on capacitive measurement technology is a favourable measurement approach in many industries and fields of application. From an electrical point of view, a coupling capacitance must be measured in the presence of stray capacitances. Different receiver circuit structures have been proposed for the underlying displacement current measurement. Ideally, the sensor assembly is directly connected to the sensor circuitry to minimize the influence with respect to these parasitic capacitances. However, under harsh operating conditions, e.g., at high temperatures, the sensor and the receiver circuit must be separated in order to protect the electronics. Consequently, the receiver circuit and the sensor have to be connected by cables, e.g., coaxial cables. The measurement setup differs significantly from the ideal design with a direct connection. In this paper, we investigate the behaviour of three common measurement circuits for capacitive measurements in instrumentations with cables. We study the interaction between the sensor and the electronics and analyse the operating behaviour of the circuit, as well as the operating states of the amplifiers used. We also address cross-sensitivities in the sensor design due to stray capacitances. The analyses are carried out for different cable lengths and measuring frequencies, and conditions for the usability of the circuit are deduced. In addition to the operational behaviour, we also evaluate the circuits by means of a noise analyses. Based on this analysis, we show a direct comparison of the circuits. The analysis is based on simulation studies, as well as collaborative measurements on test circuits where all circuit parameters are provided. The test circuits are realized with dedicated state-of-the-art circuit elements and, together with the analysis approach and the results, thus provide a basis for future developments.

ECT in a large scale industrial pneumatic conveying system

“…Hence the signal propagation is not well defined and standing waves are formed due to the reflections, which can also harm the measurement circuits. The λ/4-line presents a worse case scenario, yet even small lines lengths can lead to significant impedance transformation effects by the coaxial cables [37], which also increases the sensitivity with respect to parasitic capacitances.…”

“…Due to this length the coaxial lines have a significant influence on the behaviour of the measurement system. In order to overcome this issue, the cable length of the system was set to be λ/2 [37]. For the λ/2-line, the aforementioned effects disappear, leading to a well defined signal propagation.…”

“…From an electrical point of view the low-Z measurement circuitry appears to be directly connected to the sensor, while they are actually separated. Attenuation effects due to the lines can be reduced by calibration [37].…”

“…The use of a rectangular signal wave form instead of a sinusoidal signal is possible with the used coaxial cables, as the addressed signal propagation with the λ/2-line also applies for the harmonics of the signal. For the current to voltage converters AD8000 operational amplifiers are used [37,67]. The output signals are bandpass filtered.…”