H2TD Series Anti-Fog/Defogging Sensor

Ensuring clear visibility is essential for safe driving, particularly in changing weather conditions. This need has been enforced globally through government regulations, such as the Commission Regulation (EU) No 672/2010, which mandates that all vehicles include a system to remove mist from the interior glazed surface of the windscreen. An effective fog prevention strategy allows automotive manufacturers to help reduce the risk of fog build-up, minimize potential distractions, meet global regulations and improve overall driver awareness.

Maintaining efficiency is a significant challenge in battery electric vehicles (BEVs) and other energy-conscious designs. The ability to predict and prevent fogging conditions reduces the need for energy-intensive, reactive defogging. Accurate fog condition detection allows systems to activate only when necessary, preemptively adjusting surface temperatures to prevent condensation from forming. This approach requires less energy than converting existing condensation back to vapor, which helps conserve battery power and can help improve the vehicle’s overall efficiency and range.

Predicting a fogging condition relies on three main measurements:

• relative humidity
• temperature measurement of ambient environments
• surface temperature of the target surface (typically the windshield or camera lens)

Condensation forms when the windshield surface temperature falls below the dew point. The dew point (°C) is a function of relative humidity and ambient temperature, expressed as Dew Point(°C) = ƒ(RH, T), and can be estimated using various algorithms. By analyzing these inputs, the system can assess the likelihood of condensation by comparing the dew point temperature to the windshield surface temperature.

Establishing an optimal threshold between the windshield temperature and dew point is critical, as a small, precisely calibrated threshold enables efficient HVAC system performance. If the threshold is set too large, the HVAC system may activate too soon, leading to unnecessary energy consumption. Conversely, fog may appear on the windshield if the threshold is too small, leading to manual intervention and increased energy consumption to convert the condensation back to a vapor.

Figure 1 illustrates an ideal scenario where the HVAC is set to activate just before the windshield temperature meets the dew point. Minimizing this threshold while accounting for potential errors is important to help improve the driver experience and extend the possible range of electric motors.

Setting an ideal threshold and delivering accurate fog detection depends on:

• high-precision dew point measurements
• fast response time for windshield temperature
• long-term measurement stability
• minimal thermal interference from the environment

Location, location, location. Just as in real estate, location is critical for sensor placement. For windshield fog prevention, the ideal location is the center top of the windshield—commonly used for rearview mirrors, cameras, and antennas. This position is furthest from boundary conditions and minimizes the influence of external factors such as sunload and HVAC system thermal interference.

Placing sensors in less optimal locations can result in measurements being affected by hot or cold spots and other thermal interference, compromising the sensor's accuracy and the effectiveness of the fog prevention strategy.

While this strategy offers benefits for sunload measurement, there are notable drawbacks and considerations for fog prevention applications:

• Heat generation: combination sensors typically contain several active components that support the rain and sunlight functionality and generate heat, which can affect measurement accuracy¹.
• Size: combination sensors are typically around 60% larger than standalone humidity sensors².
• Consolidation of RLS: vision-based camera sensing is rapidly evolving to meet the requirements for Rain, Light, and Solar detection, signaling a shift in the need for traditional combination sensors and their role in vehicle architectures.
  
  

The sensor boasts a compact form factor (29.5 mm x 21.6 mm x 17.4mm), allowing precise placement. Its customizable housing and small size make it easy to integrate into modern vehicle designs, particularly within the center top of the windshield.

The sensor provides accurate dew point measurements (±1.5°C) isolated from the windshield, enabling reliable data unaffected by environmental influences. By isolating the windshield temperature measurement from ambient interference, the sensor provides precise readings, helping enhance the predictive capability of the system and helping allow for optimized activation of the HVAC.

Forwarding-facing cameras installed near the windshield may utilize the standalone anti-fog sensor from the rearview mirror area to support anti-fog measures. However, cameras installed in alternate locations must have a separate strategy to maintain a clear line of sight. TE also offers chip-level solutions which can be integrated into the camera system to support predictive condensation countermeasures.