Design of an Automatic Control Algorithm for Energy-Efficient ...
Design of an Automatic Control Algorithm for Energy-Efficient ...
Design of an Automatic Control Algorithm for Energy-Efficient ...
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2 <strong>Control</strong> objectives 5<br />
2.1 Windscreen defogging<br />
Fogging occurs when the air at the windscreen is saturated with water. Water precipitates<br />
on the glass <strong>an</strong>d limits the view. The amount <strong>of</strong> water in the air leading to condensation<br />
is dependent on the air temperature. The saturation vapour pressure (maximum partial<br />
vapour pressure) c<strong>an</strong> be well approximated within the relev<strong>an</strong>t limits <strong>for</strong> a car by the<br />
following equation taken from DIN 4108-5 [6]. The resulting curve is shown in Figure 2.1.<br />
� ����� ��� � ¡<br />
� ���<br />
�<br />
� � �<br />
�<br />
��� ���℃<br />
Water vapour saturation pressure [Pa]<br />
� Water vapour temperature [℃]<br />
� �, � �, � �<br />
Const<strong>an</strong>ts<br />
Table 2.1: Const<strong>an</strong>ts used in equation 2.1 <strong>for</strong> the different temperature r<strong>an</strong>ges. [6]<br />
Phase Temperature r<strong>an</strong>ge � � [Pa] � � � �<br />
Ice 4.689 1.486 12.30<br />
Water 0 ℃ - 30 ℃ 288.68 1.098 8.02<br />
(2.1)<br />
There exist several solutions to detect fogging. Some are based on optic effects<br />
(infrared or a camera). The majority <strong>of</strong> implementations use a humidity sensor coupled<br />
with temperature measurements on the (glass) surface. The following will focus on a<br />
system where the absolute humidity in the cabin as well as the windscreen temperature<br />
is known (i.e. measured) . With this data the position in Figure 2.1 c<strong>an</strong> be determined<br />
<strong>an</strong>d a “fogging risk” c<strong>an</strong> be calculated. In this way <strong>an</strong> impairment <strong>of</strong> the driver’s vision<br />
c<strong>an</strong> be avoided.<br />
Relative humidity is not a good indicator to determine how close to fogging the<br />
conditions are since it depends a lot on the temperature. In the paper Prediction <strong>of</strong> Water<br />
Vapor Condensation on Automobile Windscreens [6] C. Gühm<strong>an</strong>n defines <strong>an</strong> absolute<br />
dist<strong>an</strong>ce <strong>of</strong> a point to the saturation curve in the temperature-water vapour pressure<br />
diagram. To compensate <strong>for</strong> the different units <strong>of</strong> both axes in the graph a factor is