Dynamic Weighing for accurate Fertilizer Application

The mass flow of fertilizer spreaders must be calibrated for different types of fertilizer. To obtain an accurate fertilizer application a manual calibration procedure has to be repeated frequently.
An automatic calibration is possible when actual fertilizer mass flow is known. One method to determine mass flow is to monitor the weight decrease of the spreader during fertilizer application. When spreader weight differs significantly from the applied amount of fertilizer according to the current calibration, the dose controller calibration is adjusted.
In this research a dynamic weighing system for automatic flow controller calibration is presented (figure 1). Monitoring of spreader weight also offers the opportunity to log spreader performance, which is important for on farm evaluation of crop yield to the applied fertilizer strategy.
figure 1

Materials and Method

Two strain gauge load cells are mounted on a frame between tractor hitch and spreader as sketched in the figure below. The entire spreader rests vertically on load cell A, suspended by a reversed parallellogram. Load cell B measures a known load which gives information on vertical acceleration and on inclination of the spreader. figure 2


The dynamic weighing filter algorithm consists of a low pass filter for the load cell signals and a proportional correction from load cell B on load cell A. Figure 3 shows the results of filtering and correction of a dynamic signal acquired at 2 m/s driving velocity on rough surface. figure 3
The filter and correction procedure delay is 1 second and the standard deviation of the determined mass is 1.8 kg for this experiment.

The algorithm to adjust the calibration of the flow controller is based on a linear regression between predicted weight decrease at current calibration and weight decrease measured by dynamic weighing.
A calibration adjustment is permitted when the level of the 99% confidence interval of this linear regression coefficient is equal to or below the accuracy of the current calibration. figure 4
With an initial calibration accuracy preset at 10%, a simulation shows how the algorithm starts to adjust calibration after a mass decrease of 35 kg. A calibration accuracy of 1% is reached after spreading approximately 150 kg. of fertilizer.

From research environment to farm practice

For the 1997 season the above described dynamic weighing system has been implemented on a farmer's equipment. The objectives where to record actual applied amount of fertilizer to be able to relate yield maps to fertilizer strategy in an on farm evaluation of precision farming.

A dual spinning disc type fertilizer spreader for mineral fertilizers is weighed continuously. Fertilizer rate settings and applied amount are recorded by equipment mounted in the tractor. Tractor operating conditions like motor RPM, radar and wheel velocities and position (DGPS) complete the dataset. See tractor and spreader while loading fertilizer:

tractor & spreader

weight derrick

A detailed view on the weight measuring implementation; the orange painted weighing derrick equipped with load cells between the spreader and tractor.

cab1 cab2

A view inside the tractor cabine. Two computers reside on board; leftside the Mueller Elektronik equipment to control the fertilizer rate dependent of driving speed and to record task data. On the righthand the touchscreen of the datalogging equipment. The datalogging computer is mounted beside the chair; yes, it 's becoming busy in the cab, but luckily a place was found for the strain gauge amplifier on top of the former passenger seat. Finally, the DGPS system was small enough to reside near the computer.

Post processing of the obtained data yielded maps of applied fertilizer. The detailed measurements indicated fertilizer flow actuator malfunctioning which the farmer previously had noticed too. Additional EMC suppression was needed to increase accuracy of the fertilizer rate control.

For the 1998 growing season the strain gauge amplifier is going to be replaced by an integrated sigma delta converter. The system will be modified to be able to connect an Agricultural BUS for electronic data transfer. A Controller Area Network (CAN) node on the spreader performs dynamic weighing and control of the spreader and is connected through CAN to the boardcomputer running a virtual terminal and taskcontroller.

The picture below shows the single board computer to be used as electronic heart of the fertilizer spreader. It features a Siemens SAB167 16-bit microcontroller with integrated CAN bus controller. rmb167

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