New Sensor Automates Calibration for Granular Fertilizer Spreaders

Tony Grift

To ensure that granular fertilizer is evenly distributed in their fields, farmers must calibrate their spreaders to determine fertilizer output and spread pattern. But the standard method of calibration is a tedious process. It involves placing 25 collection trays in a row, spreading fertilizer in the trays, weighing the trays’ contents, and computing the overlapped uniformity and application rate at a certain swath width.

Now, there’s good news on the calibration front. After years of intensive research, AAES researchers in the Department of Biosystems Engineering at Auburn University have developed a system that automates the process. Instead of collecting fertilizer, this system predicts where individual fertilizer particles will land. Using a special built-in optical sensor, the device measures the velocity and diameter of a sampling of fertilizer granules and uses a mathematical ballistic model to predict where the granules will end up on the ground.

This model takes into account the air density, the temperature, and the mass of each fertilizer particle. Accumulation of several thousand landing spots reveals the shape of the spread pattern. Using this pattern, a custom computer program calculates a simulated overlapped pattern that would emerge on the ground at a preset swath width.

Figure 1. Optical measurement device.

The newly developed electronic calibration sensor (Figure 1) works like this: Fertilizer particles that are thrown from the disc pass through the square hole in the back plate. As they pass the sensor, they interrupt a double light beam, one in the front and one in the back. The time between the two interruption processes is measured and used to compute the velocity of the particles. In addition, the amount of time a particle blocks either sensor is measured. Combined, these two measurements yield the diameter of the particle.

Field tests of the optical calibration sensor were conducted using a Lowery 300 broadcast spreader (Figure 2) with the hopper half filled. As the spreader worked, the measurement device rotated around the spreader rim to sample the entire spreading zone. A separate encoder measured the angle of the sensor with respect to the spreader.	Figure 2. Field testing of calibration instrument.
All signals were fed into a computer that stored the data. These data were then used as input for the computer model that predicted the landing positions of the particles.

Figure 3. Landing spots of approximately 1,000 fertilizer particles

Results of the field test (Figure 3) show that the spread pattern is skewed and that there is a gap at 90 degrees. That occurred because this kind of spreader has two feed holes that can be adjusted separately. Using this information, the operator could compensate for the gap by changing the swath width or by adjusting the setting for the right or left feed holes.

Ultimately, the new calibration device will lead to significant improvements in fertilizer distribution in a field. The principle, however, can readily be upgraded to larger fertilizer application equipment. The device also can be used as a basis to measure mass flows of particles, which is of interest in aerial fertilizer application. In addition, the information from the sensor can also be tied in with spatial information from Global Positioning System (GPS) receivers to build site-specific fertilizer application equipment (“smart spreader systems”). The system as developed here can help to make precision agriculture an everyday reality.