How does a sonic sensor measure distance for variable rate application?

Sonic sensors measure distance by emitting a high-frequency sound wave and measuring the time it takes for the sound wave to travel to a target and return. This is often referred to as time-of-flight measurement. For variable rate application, these sensors are primarily used to maintain a consistent boom height above the crop canopy or soil surface. The sonic sensor emits a short burst of ultrasonic sound. This sound wave travels through the air until it encounters a surface, such as the top of the crop canopy or the ground. The sound wave is then reflected back towards the sensor. The sensor measures the time elapsed between the emission of the sound wave and the reception of the echo. Knowing the speed of sound in air, the distance to the target can be calculated using the formula: Distance = (Speed of Sound x Time) / 2. The division by 2 is necessary because the sound wave travels to the target and back. The speed of sound in air varies with temperature, so the sensor typically incorporates a temperature sensor to compensate for temperature changes. This distance measurement is used by the variable rate application system to adjust the boom height, ensuring that the spray nozzles are at the optimal distance from the target. Maintaining a consistent boom height is crucial for achieving uniform application rates and minimizing drift. For example, a sonic sensor mounted on a sprayer boom can measure the distance to the top of the crop canopy. If the distance decreases, the boom will automatically raise. If the distance increases, the boom will automatically lower, maintaining a consistent spray height.