Instrumentation & Measurement Magazine 25-8 - 12

Fig. 5. Illustration of the principle of electrical capacitance tomography (ECT) for
flow measurement in pneumatic conveying. Based on capactive measurements
the material distribution of the bulk material can be determined. This can also be
used for evaluate flow parameters [31].
sensing could be demonstrated. Experiences from operation
on high-voltage power lines are also positive [25].
Capacitive Process Tomography for Pneumatic
Conveying
Bulk material transport by means of pneumatic conveying is
considered as the most widely used transport technique for
pulverized materials in industry. Examples can be found in
all sectors of industry, e.g., in agriculture, the food industry,
the chemical industry, the plastics industry and even heavy
industry. Instruments for the characterization of these flows
are highly desired for the optimization and process control
of pneumatic conveying systems [26], [27]. This refers to flow
monitoring [28], i.e., how the bulk material is distributed inside
the pipe as well as flow metering [29], [30].
From the field of capacitive measurement technology, electrical
capacitance tomography offers a possible approach.
Fig. 5 depicts the scheme of electrical capacitance tomography
(ECT). Electrodes are mounted on the outside of a nonconductive
pipe. The capacitances between the electrodes depend on
the material distribution of the transport process inside the
pipe. Measurement of the capacitances can be used to compute
images for the material distribution inside the pipe. Fig. 6
shows a lab demonstrator for ECT for pneumatic conveying
[32]. The two reconstruction images represent the material distribution
from the conveying process.
However, transferring a technique from laboratory experiments
to a field application poses a challenge. For example, for
the application of ECT in a harsh environment, the pressure
or the heat of the transport goods have direct influence on the
Fig. 7. Different assembly steps and installed ECT sensor for a pneumatic
conveyor system in heavy industry.
measurement capacitances. Hence, a holistic research strategy
as proposed for the ice sensor has to be applied. Fig. 7 shows
different assembly steps and the installed ECT sensor for a
pneumatic conveyor system in heavy industry. Following the
proposed strategy, all components and elements of the sensor
and the measurement system have been modeled/characterized
to understand all mutual independencies [33]. This again
extends towards dedicated descriptions of the material distributions
[34], the material properties [35], or models for the
occurring flow regimes [37]. Based on this data, suitable signal
processing techniques and compensation methods can be
developed.
Fig. 8 shows an exemplary reconstruction result for the
flow in the industrial plant. A so-called slug flow scheme is
present. The results prove the applicability of ECT-based technology
in heavy industries. Based on these results, ECT offers
the capability for the development of mass flow metering instruments
[39].
Outlook: Where Will Capacitive Sensing
Go?
The two examples discussed in the previous section give an
impression for the capabilities of capacitive measurement
technology. " Capacitive sensing is sensitive with respect to
almost everything " is a catchphrase which researchers have
to consider. It opens up the possibility of solving many measurement
problems, but on the other hand makes capacitive
sensing prone to cross-sensitivities. Hence, it can often be the
case that the actual sensing effect is only weakly pronounced
within the measurement signal. It is therefore essential to understand
all relevant effects contributing to the measurement
Fig. 6. Lab demonstration of ECT for flow imaging [28], [32].
Fig. 8. Reconstructed flow pattern from the industrial plant.
12
IEEE Instrumentation & Measurement Magazine
November 2022

Instrumentation & Measurement Magazine 25-8

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-8

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