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  • #5 - Windshield Ice Detector

Labs Idea #5: Windshield Ice Detection

  • Motivation/Summary
  • Approach/Measurement
  • Implementation

Author: Sensirion Innovations Team
December 2017


Motivation/Summary

In areas where the temperature drops close to freezing or lower during the night, the windshield of a car is very often covered with a layer of ice in the morning. Knowing if the windshield has ice allows a notification to be sent to the driver advising them to get to the car a little bit earlier to manually de-ice the windshield or, in a more automated car, the windshield to be automatically de-iced in the morning before the driver gets to the car to eliminate the wait. Since the formation of ice is dependent on very local conditions (see details below), a local sensor is necessary.

The formation of ice can be estimated well with an RH/T sensor. This can be either an existing sensor in an anti-fogging module or a dedicated sensor.

Physical Background

Two conditions need to be fulfilled in order for ice to form on the windshield (or on any other body). First, the surface temperature needs to be below freezing. Second, the relative humidity of the surrounding air needs to be close to 100% at the temperature of the surface. Both conditions can be measured with an RH/T sensor.

In the case of the windshield, the temperature is often not the same as the temperature of the surrounding air. Obviously, during a sunny day, the windshield will be warmer due to the absorbed heat radiation. See figure 1:

Figure 1: Radiation balance of a body and its surrounding

The opposite situation can also occur. When there is very little incoming radiation, as is usually the case during a cloudless night, the temperature of the windshield falls below the temperature of the surrounding air. This can lead to surface temperatures below freezing although the ambient temperature is well above this level.

Since a car has always some natural ventilation (leakage and overpressure vent) and the rate of change in absolute humidity is rather slow in the atmosphere, a sensor on the inside of the windshield is feasible for this application. Either a dedicated sensor or the RH/T sensor of the anti-fogging module.


Approach/Measurement

Here is a sample measurement from a car parked outside. The formation of ice can be seen in the time-lapse video. Various sensors were placed in the car, on the car and around the car to monitor the conditions.

Ice starts to form the moment the surface temperature drops close to the dew point of the ambient air so that deposition can occur.

Video: Windshield ice time lapse

Figure 2: Sensor location, inside of the windshield, close to the anti-fogging module location (rear-view mirror)
Figure 3: Sensor location reference on the outside, at the same location as the sensor inside
Figure 4: Temperatures over time. The drop in body temperature (AFM = anti-fogging module location on the windshield) was much faster in the night when there was no cloud cover (see Fig. 5). The temperature stopped dropping further as soon as there was closed cloud cover. The windshield temperature dropped well below freezing and below the dew point of the ambient air (which means that the relative humidity was approx. 100% at the surface).
Figure 5: Cloud cover over the proximity of the experiment location at 1:50 a.m. white = clouds, other colors = no cloud coverage (source: meteo.search.ch)
Figure 6: Cloud cover over the proximity of the experiment location at 3:50 a.m. white = clouds, other colors = no cloud coverage (source: meteo.search.ch)

Implementation

relative_humidity, temperature =  read_sensor()

# check if condensation/deposition might occur
If relative_humidity > 85:

    # check if  it is condensation (water) or deposition (ice)
    If temperature <0:
        # Ice on windshield very likely
        inform_user()


Patent Information

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Basic evaluation kits to test 3.3V humidity sensors (EK-H5 for SHT21 and SHT31) and 1.8V humidity sensors (EK-H5 1.8V for SHTC1, SHTW2, STSC1).

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