Rain SensorsDefinition and Principle of a Rain Sensor

Rain sensors enhance safety and comfort while driving, as they avoid the need of a driver to adjust the wiper setting on the windshield in order to ensure clear view via the windshield without rattling of the wiper blades. These sensors automatically switch on the wiper in case of a water splash on the windshield. Upon installation of the ‘assistant light switch’, the headlamps of the vehicle will be turned on automatically without the need to have the lights on throughout. Bosch RSM3ALS is the commonly used rain sensor having an assistant light switch.

The optical system of the rain sensor comprises of an electronic control unit (ECU), ambient light sensor, lenses, a light receiving diode (LRD) and a light-emitting diode (LED). Outer surface of the windshield reflects the light produced from the LED at an angle of total reflection, which is in the range of 42° to 63°. Some amount of light is coupled out if the windshield contains water. As a result, LRD generates less amount of current, which is estimated by the electronics. The area where the light is reflected from the windshield to LRD is known as the ‘sensitive area’ of the sensor. Detection of rain by the sensor depends on the hitting of water drops on the sensitive area. A reliable and sensitive system is achieved when the ratio between the windshield area and sensitive area is considerable.

Functional Principle

The infrared (IR) transmitter unit radiates IR light, which is directed to the windshield through the lens. The IR receiver located at another end of the lens measures the intensity of the light reflected from the windshield.

The light is completely reflected if the sensitive area of the windshield is dry. As a result, the intensity of light measured by the IR receiver is high.

A part of light from the windshield is scattered if the sensitive area of the windshield is wet. Thus, the intensity of light is reduced.

Measurement of light intensity is associated with the amount of water present on the windshield. If the measured light intensity is small, the amount of water on the windshield is large.

Rain Sensors for Irrigation and Types of Application

Rain sensors are major components of an automatic system used in regions where rainfall is observed during the months of irrigation. These devices automatically turn off the irrigation system upon receiving a desired amount of rainfall and enable the system to resume its preset conditions when the device is dried. In Florida, the installation of rain sensors in irrigation system has become mandatory since 1991. However, the utilization of rain sensors in the region where the rainfall occurs in winter is limited due to minimal or non-existence of irrigation. The following video describes the functional principle to the Rain Bird RSD Series Rain Sensors, which turns off a sprinkler system to conserve water. 

The key benefits of rain sensors used in irrigation system are as follows:

Inexpensive Less installation time Improved performance and reliability.

Rain Sensors for Automotive

The windshield is mounted with rain sensor to detect the movement of wiper and rain without disturbing the view of the driver. This sensor device can control the wiping actions at variable speeds when the system is switched on. Generally, the following two different sensor systems are used:

Rain sensors in a network: These sensors are linked to a bus system through which all information and commands can be sent and received.

Stand-alone rain sensors: These rain sensors are directly connected to the wiper motor park signal, wiper motor relays and wiper column switch.

Evaluation of Analog Signal

The following section will discuss in detail about the components used for evaluating analog signal.

Transmitter

The transmitter comprises a current source, a digital-analog converter (DAC) and LEDs. It is necessary to regulate the transmitter current as the light conversion efficiency is variable. In an attempt to increase the current and reduce power loss at LEDs, light from the LEDs is pulsed.

Receiver

The receiver is provided with an amplifier, a current-voltage converter, LRDs, a filter to avoid low frequency or dc offsets and an analog-digital converter (ADC), which is included in a microcontroller. The microcontroller switches on or off the LRD to control the optical path. A variable current- voltage converter is used for wide transmission ranges. The signal is filtered after conversion and then amplified. The amplified signal is converted using ADC.

Microcontroller

The entire system is controlled by the microcontroller, which also performs evaluation of the signal. The best operating point is determined before performing the measurements. A signal is generated by the microcontroller between the upper and lower limits at the receiver. At this point, the sensor starts functioning. The signal and disturbance is evaluated repeatedly. Further, an extra sensor measures ambient light for identifying day/night conditions.