Instrumentation & Measurement Magazine 24-8 - 38

Fig. 1. Stretchable electrode layer and schematic illustration of the setup.
be analytically approximated using a calculation of such a
form where A/d is replaced by a geometric-factor specific for
the considered setup. Such an analytical representation, however,
assumes a homogenous E-field between the conductors
and is consequently only an approximation. Finite element
method (FEM) simulations, shown for a classic touchpad design
in Fig. 2b, also cover fringing fields at conductor edges
and yield much more precise results for real-world setups.
Measurement Modes
Capacitive sensors can be driven either in so-called singleended
or differential configuration. In single-ended mode, the
displacement current is measured in the powered electrode.
Here, measurements can be done with a single electrode, yet
often an active guard electrode is included to shape the sensitive
region. In differential mode, at least two electrodes are
necessary: one is kept on or close to ground potential (receiving
electrode), while the other one is excited with an alternating
potential (transmitting electrode). Then, the displacement current
is measured at the receiving electrode. Capacitive sensors
in the differential mode can again operate based on two different
modes [5]: shielding (Fig. 3a) and coupling (Fig. 3b). Which
mode is active depends on the distance of the object of interest
to the sensor surface and the respective coupling to ground.
Both modes are schematically captured in Fig. 3 for proximity
sensing of a human hand acting on the sensor front-end and
considering a differential configuration: due to the high impedance
related to the coupling capacitances, the human hand
itself can be considered as a conductor, which itself has a certain
38
Fig. 2. (a) Simulation setup and (b) finite element method simulations with
force applied on the sensor surface, with the rightmost indicator giving the
displacement of the suface in mm, and the other indicator giving the respective
field strength at the points of the arrows.
coupling impedance ZHGND to ground. For lower excitation
frequencies and adult humans, this impedance is mainly represented
by a coupling capacitance in the order of 100 pF between
an isolated human and ground. With electrode 1 (E1
cific electric potential and electrode 2 (E2
) at a spe),
field lines emanate
from E1
and terminate at E2
. First, as in Fig. 3a, the human hand
is comparatively far away from the sensor: in this situation, the
capacitances between the hand and electrodes E1
,E2 (CHE1
). Thus, the displacement current between E1
,CHE2
)
are smaller than the capacitance between the hand and ground
(CHGND
both electrodes (IE1E2
and ground
(IE1H) is large compared to the displacement current between
), and the sensor is in shielding mode.
When the hand further approaches the sensor, as in Fig. 3b, as
soon as CHE1
and CHE2
are much larger than CHGND
enters the coupling mode. In this mode, IE1E2
in this case). Thus, as IE1H
, the sensor
increases. In most
cases, the considered output signal is the charge at the grounded
electrode (E2
increases, the output decreases.
At a certain distance, which depends on the geometry of
the electrodes, the sensor mode changes from shielding to coupling.
Thus, further reducing the distance between hand and
sensor leads to an increase in the capacitance between transmitter
and receiver and to an increased displacement current. In
shielding, the output signal increases with increasing distance,
whereas in coupling mode it decreases with increasing distance.
Based on an analysis of the sensor response using, e.g., finite element
method (FEM) analysis, the sensor can be specifically
designed to work in the desired operation mode.
IEEE Instrumentation & Measurement Magazine
November 2021
Instrumentation & Measurement Magazine 24-8

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