How to Implement Accurate Temperature and Humidity Sensing in ADAS Sensor Modules

Highly sensitive optics require accurate diagnostics to maintain excellent performance over time, much like RF ADAS modules. This necessitates the use of accurate external temperature sensors, a necessary building block for ADAS modules that are rapidly becoming the safety-critical systems of the future.

Your first car, like mine, may lack the sensor modules like cameras, radar, and lidar that make modern advanced driver assistance systems (ADAS) safety features such as blind spot detection, parking assist, and collision avoidance) becomes possible. Since the data collected by these sensor modules is directly related to passenger safety, it is important to ensure that they are always functioning properly. Unfortunately, a common cause of damage is prolonged overheating or exposure to moisture.

Accurate temperature sensors in cameras, radar, and lidar help extend their lifespan and enhance safety and reliability. First, let’s look at the effect of temperature on a car’s camera module.

Camera

Figure 1 shows that each car may have as many as six cameras. These cameras require high dynamic range and fast response times as well as excellent low light sensitivity. To meet these requirements, designers must avoid prolonged operation of image sensors at high temperatures.

How to Implement Accurate Temperature and Humidity Sensing in ADAS Sensor Modules
Figure 1: Overview of ADAS sensors in modern cars

As shown in Figure 2, automotive cameras are typically small (1.4in³) enclosed cubes with no active cooling, making them very susceptible to heat build-up and rapid warming. Image sensors are typically rated at C40°C to 125°C (junction temperature) and C40°C to 105°C (ambient temperature). If the upper or lower limits of these ranges are reached, the Electronic control unit (ECU) will have to reduce power to the image sensor or turn the sensor off completely until the temperature returns to normal operating conditions. Therefore, it is very important to accurately obtain the temperature of the camera.

How to Implement Accurate Temperature and Humidity Sensing in ADAS Sensor Modules
Figure 2: Camera module of a small car

Image sensors typically employ embedded temperature sensors with an error range of ±6°C. Such a large error means that the ECU may limit the use of the camera by shutting down earlier or later. These miscalculations can cause damage to the image sensor, temporarily limiting ADAS functionality until it is serviced.

The solution is to add a separate temperature sensor that provides accurate temperature measurements with an error of less than ±1°C.The application note “Improving System Reliability for Automotive and Industrial Cameras with Accurate Temperature Sensing” is helpful for selecting temperature sensors for specific camera topologies

radar

The receiver (RX) sensitivity, gain, input noise, and even output transmitter (TX) power of millimeter-wave (mmWave) sensors can vary with temperature. In Figure 3, the host processor attempts to mitigate the effects of temperature variations by periodically adjusting the circuit configuration during operation to keep the RX gain and TX power as close to the configured settings as possible.

How to Implement Accurate Temperature and Humidity Sensing in ADAS Sensor Modules
How to Implement Accurate Temperature and Humidity Sensing in ADAS Sensor Modules
Figure 3: RX gain (a) and TX power (b) as a function of temperature

The need for high-accuracy temperature measurement is a balance between maximizing radar performance as much as possible and preventing thermal damage from high temperatures. To achieve this balance, the radar sensor must operate near the temperature limit while being able to reliably shut down as close as possible to the limit. Achieving this can be difficult because:

1. OEMs begin to demand higher ambient temperatures.

2. To reduce costs, manufacturers started to use plastic module housings instead of metal housings. Metals are better conductors of heat and are often used as heat sinks to dissipate the heat generated inside the module.

3. The power consumption of the radar chip is high, which will cause self-heating.

4. The embedded temperature sensor on the radar chip has a maximum error range of ±7°C, which limits the performance of the radar chip. Because of this error, to be on the safe side, you must turn off ±7°C from the operating limit to prevent damage.

Today, designers aim to achieve ±1°C temperature accuracy of the die temperature inside a radar chip. For this you can use two separate temperature sensors to measure the temperature difference, or use an ultra-thin temperature sensor under the radar chip, such as the TMP114. To learn more about implementing differential temperature measurements, read the application note “Component Temperature Monitoring Using Differential Temperature Measurements”.

For more details on under-component temperature monitoring, read the application note “Under-Component Monitoring Using Ultra-Small Temperature Sensors”.

Lidar

As shown in Figure 4, LiDAR sensors can capture short-, mid-, and long-range data, providing in-depth point clouds as a key element in enabling ADAS functional safety. Lidars contain laser arrays, time-of-flight (ToF) sensors, and controllers, all of which require temperature compensation to maintain their performance. Temperature changes can affect lidar distance measurements, and above 70°C, the performance of the laser array may degrade. ToF sensors have high power consumption, which can cause self-heating, and the controller often needs to reduce its clock frequency or shut down completely around 105°C to prevent thermal runaway.

How to Implement Accurate Temperature and Humidity Sensing in ADAS Sensor Modules
Figure 4: Automotive lidar range

An important design consideration for lidar systems is the target automotive safety integrity level (ASIL).The application note “Using Remote Temperature Sensors to Meet ASIL Requirements for LiDAR Systems” provides some ideas for quickly implementing redundant and diverse temperature sensors

Both lidar and camera modules have lenses that can break, so moisture can damage the optics inside. Automotive-grade humidity sensors such as the HDC3020-Q1 measure relative humidity and temperature. It detects moisture (which could indicate a leak) and calculates when the dew point is exceeded (which causes condensation on the lens), allowing the system to notify the user to take corrective action.

How to choose a temperature sensor

When evaluating your next temperature sensor, consider its maximum accuracy, whether you need alarms or other features, and your communication channels. For example, if you don’t have any ADC channels available (often found in surround view and low-end driver monitoring cameras), you can connect a digital temperature sensor to the I2C or SPI channel of the FPD-Link serializer. If you just want a threshold alarm with hysteresis, you can use a temperature switch connected to a general purpose input/output. When you do have ADC channels available, the output voltage of an analog temperature sensor is proportional to temperature without being affected by the tolerances of external components like a discrete thermistor solution. If you do need a thermistor, consider a silicon-based linear thermistor, which solves the accuracy and reliability problems of negative temperature coefficient (NTC) thermistors, while maintaining their low cost and small size.

Epilogue

Highly sensitive optics require accurate diagnostics to maintain excellent performance over time, much like RF ADAS modules. This necessitates the use of accurate external temperature sensors, a necessary building block for ADAS modules that are rapidly becoming the safety-critical systems of the future.

The Links:   QM150DY-24BK CM300DY-12NF