Texas Instruments | Quantifying Ice and Frost Buildup Using Capacitive Sensors | Application notes | Texas Instruments Quantifying Ice and Frost Buildup Using Capacitive Sensors Application notes

Texas Instruments Quantifying Ice and Frost Buildup Using Capacitive Sensors Application notes
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Quantifying Ice and Frost Buildup Using Capacitive
Sensors
[Introduction]
Ice and frost buildup is a common problem for many
cooling systems in which conventional solutions do not
efficiently solve. The accumulation of ice and frost can
disrupt heat exchange in a system, resulting in
excessive power consumption. Traditional methods for
defrosting do not accurately quantify the amount of ice
built up on a surface which may result in overheating
the cooling system and therefore consume excessive
power. This may also potentially spoil food in a
refrigerator environment due to the lack of air
circulation. Conversely, inaccurate defrosting methods
may not heat the system enough to properly melt all of
the ice, increasing the risk of disrupting the airflow of a
cooling system.
One conventional solution seeks to melt ice and frost
by turning on the heating elements after a
predetermined time interval. This does not measure
the amount of ice and frost collected and therefore
assumes that it accumulates at a constant rate, which
is seldom the case. Another traditional method uses a
temperature sensor to defrost ice based on a detected
change in temperature. However, this requires an
extremely sensitive sensor and the measurement is
not directly correlated to ice thickness or distribution so
it is inherently inaccurate. Furthermore, more active
ice monitoring systems exist that aim to detect the
usage of the cooling system and actively decide when
to initiate the heating elements. These methods are
often complex, and ice buildup may not always be
proportional to the amount of activity of the cooling
system. Figure 1 shows the ideal ice frosting and
defrosting process in which all of the ice melts after
the heater is turned on.
The capacitive sensing technique solves ambiguity
issues concerning how much ice forms on the cooling
surface in phase 2 by directly quantifying the amount
of ice rather than following assumptions similar to the
other methods. This approach measures the
capacitance of the system which is unique at each
phase due to the arrangement and thickness of
materials and dielectric constants. As can be seen in
Table 1, the dielectric constant varies significantly
depending on the properties of the substance.
Table 1. Material Dielectric Constants
MATERIAL
DIELECTRIC CONSTANT (εr)
Air
1
Water (at 20°C)
80
Glass
7.6-8.0
Paper
2.3
Ice
3.2
FDC devices operate with a narrowband resonantbased measurement which minimizes noise compared
to the traditional broadband charge-based
measurement. In an application where accuracy and
noise rejection is important, such as the measurement
of ice and frost, TI’s EMI-resistant, capacitive sensing
portfolio can provide resolution up to 28 bits and is
reliable in temperatures as low as -40°C.
[Theory of Operation]
The FDC1004 and FDC2x1x devices measure the
capacitance between two parallel plates consisting of
the metal surface of a cooling body and an added
electrode at a fixed distance acting as the sensor. As
the height of the ice formation begins to increase, the
resonance in the LC tank also changes due to the
change in capacitance, resulting from the dielectric
change between the electrode and metal surface.
Figure 1. Ice and Frost Buildup Process
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Figure 2. FDC Principle of Operation
The area of the sensor corresponds to the degree of
sensitivity of the system—the larger the better.
Sensors can simply be an electrode around a cooling
tube or a sheet resting on top or side of a finned
cooling body similar to the image on the right in
Figure 3. TI recommends using a mesh electrode to
ensure that the sensor does not impede the natural
frost buildup between the electrode and cooling body
because this could cause an inconsistent reading on
the sensor compared to the rest of the system.
Specific sensor design considerations can be found in
Section 2.3.6 of the TIDA-01465 Capacitive Frost or
Ice Detection Reference Design (TIDUD79).
Figure 4. Capacitance Curve
[FDC2214]
The FDC2214 is an EMI-resistant, 28-bit, capacitanceto-digital converter. Unlike traditional capacitive
sensing technologies, the narrowband architecture of
the FDC2x1x series supports a wide range of
excitation frequencies and maintains performance
even in high-noise environments.
[FDC1004]
The FDC1004 is a 4-channel, high resolution,
capacitance-to-digital converter. This device includes
shield drivers for sensor shields that can reduce EMI
interference and focus in on the sensing direction of
the sensor.
Table 2. Alternative Device Recommendations
DEVICE
OPTIMIZED
PARAMETERS
PERFORMANCE
TRADE-OFF
FDC2114
High Speed
12Bit resolution
Figure 3. Capacitive Sensor on Cooling Body
Table 3. Recommended Collateral
The capacitance of the frost forming and defrosting
process can be seen in Figure 4 as well as Figure 1.
Observed in phase 1, capacitance stays constant until
frost begins to form in phase 2. At this phase, the
measured capacitance is proportional to the thickness
of ice accumulated. Once the defrosting process
begins in phase 3, water is introduced into the system
that causes a spike in the capacitance before it begins
to decrease rapidly as the water drains. Once all of the
water drains or evaporates, the capacitance returns to
the initial phase 1 value and the system is now free of
ice.
2
COLLATERAL
DESCRIPTION
TIDA-01465
Capacitive Frost or Ice Detection
Reference Design, Resolution of
<1mm, Temperature Drift <0.25%
(TIDUD79) –FDC2214
Application Report
Ice Buildup Detection Using TI’s
Capacitive Sensor Technology
(SLLA355) – FDC1004
Quantifying Ice and Frost Buildup Using Capacitive Sensors
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