CSEM Scientific and Technical Report 2008

A Noninvasive Sensor for Fluidic Process Monitoring
T. Burch, N. Schmid, R. Limacher, D. Fengels
A high performance, low cost capacitive sensor has been developed for process monitoring in liquid handling applications.
Liquid handling is predominant in application fields such as life
science and clinical diagnostics. Both market segments have
a strong demand for miniaturized devices which can sense
and control small volumes of liquid samples precisely and
reliably.
The presented non-invasive sensor for process monitoring in
liquid handling applications is based on two components: a
two pin-electrode structure and a high resolution,
Σ-Δ capacitance-to-digital converter to transfer the capacitive
signal into an electric output signal. The two electrodes
(Figure 1, objects 1 and 3) are located close but noninvasively
to a micro channel of 1 mm2 cross section (Figure 1,
object 2). The dielectric fluid quantity changes the permittivity
between the electrodes and results in a change of the
capacitance in the range of 1 to 2 Femtofarad per µl. An
additional pair of electrodes is used to shield the main
electrodes so that external interferences are minimized.
A) Fluid channel empty B) Fluid channel filled
Figure 1: Capacitive electrodes modeling – cross section view.
1) Active electrode, 2) Fluid channel, 3) Measurement electrode
In this arrangement the sensor provides low cost and high
performance measurement of the liquid level inside a micro
channel. The sensor performance depends on the permittivity
as well as the conductivity of the liquid inside the channel. The
presented sensor was designed for liquids with low
conductivity. With purified water the following performance
has been achieved:
• Measurement range 50 mm (50 µl)
• Measurement resolution 0.025 mm (0.025 µl)
• Rel. measurement accuracy 0.1 mm (0.1 µl)
The sensor performance can be further improved through
segmentation of the pin-electrodes. After measuring the
capacitances between the various segments the level can be
determined in a two-step coarse-fine approach. In addition,
the optimization of the shielding electrodes and the
introduction of a dedicated guard electrode can further reduce
the electromagnetic and electrostatic interference effect.
To prove reliability and performance of the measurement
principle a liquid handling demonstrator has been developed
(Figure 2).
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1 2 3 1 2 3
Figure 2: Liquid handling demonstrator with integrated liquid level
sensor
A mini pump [1] driven by a DC Motor of 8 mm in diameter is
connected to two reservoirs filled with a liquid with low vapour
pressure and low viscosity (Figure 3). The first reservoir has
atmospheric pressure. The second reservoir is connected via
a third reservoir to the micro channel of 1 mm 2 cross section.
The capacitive sensor is integrated as described above. The
sensor signal output is directly used as the actual value for the
closed control loop.
Figure 3: Closed control loop for liquid handling demonstrator
This arrangement allows fast and precise positioning and
movement of the liquid piston inside the micro channel. The
achieved control accuracy is 0.25 µl and the maximal flow
rate is 500 µl per second.
The non-invasive capacitive sensing system is currently under
optimization for a range of potential applications in the life
sciences and lab automation business.
[1] N. Schmid, et al., “High Performance Minipump”, in this report,
page 105