Precision Drilling

Basically, the metering of the seeds with precision drilling consists of two functions: singling of the seeds and transporting them to the furrow. In many cases, singling of the seeds is accomplished purely in a mechanical way; however, sometimes the singling process is supported by airflow. Accordingly, mechanical as well as air-assisted precision drills must be dealt with.

Cereal drills with a series of vacuum feed seeder units on a wide tractor toolbar can sow as little as 25 kg wheat per hectare. At the optimum seed rate of 50 kg/ha the drills plant approxi­mately 100 seeds per square metre.

Seed from the main hopper is blown to cyclones mounted above a series of independ­ently mounted seeder units. A metering device in each cyclone supplies seed to the seeder units which deliver single seeds to the ground-level coulters. The metering devices consist of hydraulically driven vertical discs with holes around the circumference which revolve in the seed hop­pers. Vacuum on one side of the disc pulls seed into the holes, and with any surplus seed removed, only one will arrive at the coulter where the vacuum is released and the seed falls to the ground. An in-cab computer controls the speed of the hydraulically driven feed wheels. It also sets the seed rate and varies the speed of the seeder discs to compensate for changes in tractor forward speed and maintain a constant seed rate.

Mechanical Precision Drilling. Singling methods are illustrated in Fig. 5. In most cases, a rotating disk with holes or cupped fingers at the circumference picks single kernels from the supply. Sometimes instead of a disk, a circulating belt with seed holes is used.

The shape as well as the dimensions of the holes and cups must be adapted to those of the seeds. Too large or too small dimensions promote doubles or skips, respectively. In most cases it is not possible to avoid doubles or skips completely. Therefore, the adaptation should aim at an equal number of doubles as well as skips. This means that a balance in the total number of seeds is achieved.

It is important that the seeds get to the furrow in the original sequence. A long, free falling distance from the delivery point to the furrow is detrimental; it deteriorates an originally even seed sequence. This especially applies to small seeds.

In this respect, the orientation of the metering disk is relevant. With a vertically oriented metering disk, the falling height can be smaller than with an inclined disk and especially smaller than with a horizontally located disk. This explains the predominance of vertically oriented metering disks with small vegetable seeds and sugar beet seeds. A disadvantage of vertical disks with larger seeds compared to horizontal disks can be the rather short peripheral sector available for filling. Therefore, in the United States for large seeds such as corn, horizontal disks still are in use despite their principal disadvantage in the falling height.

Inclined disks normally deliver the seeds at their highest point. Therefore, in order to get a low, free-falling height, they often operate with a parallel rotating chamber plate in the background. The seeds pass from the singling disk into the chambers of the parallel plate, which then delivers them closely above the furrow. In most cases, precision metering requires devices that remove doubles and triples from the respective cells. Counter-rotating rollers, stationary cut offs, or brushes are used for this purpose. Careful adjustment of these devices is essential. Despite this, there still is an influence of the peripheral seed disk speed and thus of the travel speed on the singling process. Increasing the speed raises the share of skips and reduces the percentage of doubles. Inclined disks often operate without special devices for removing doubles and triples. Excess seeds lie on top of other seeds. The rotation of the disk moves the gravity center of the excess seeds beyond the rim of the cup; thus, they fall back into the supply. However, this advantage of inclined disks is associated with less precise singling on sloped fields. Their use is not recommended on fields with more than 10% slope (1).

There can be uneven seed spacings in the furrow even if the kernels hit the ground in regular intervals. The main reason for this is bouncing of seeds along the furrow bottom, which occurs especially with spherical seeds if the trajectories are not vertical to the soil.

To some extent, these irregular spacings due to bouncing of the seed depend on the shape of the furrow-bottom. An acute-angled furrow-bottom often prevents excessive rolling and bouncing of the seeds, whereas a smooth rounded bottom promotes it. With new machines an acute-angled furrow-bottom, which is adapted to the seed dimensions, generally is formed. Yet wear on the lower pointed side of the opener can change the shape of the furrow-bottom and thus increase irregularities due to bouncing.

Figure 5. Mechanical precision-drilling.

The travel speed imparts the seeds a velocity component in the direction of travel. Vertical seed trajectories, therefore, require that this velocity component is offset by an opposing velocity component, such as the peripheral velocity of vertical seed plates. If the traveling speed and the opposing peripheral velocity are of the same magnitude, the seeds drop vertically into the furrow, and detrimental bouncing along the furrow bottom decreases. However, with seed plates that are filled from the outside (Fig. 5), the peripheral velocities are much lower than the usual travel speeds. Peripheral velocities in the order of the travel speeds with these seed plates would be associated with centrifugal forces, which prevent most seeds from entering the cells.

This problem can be overcome by filling the vertical seed plates from the inside (Fig. 5). Now centrifugal forces support gravitational forces in the filling process. Therefore, a much higher peripheral velocity–downright in the order of the travel speed– can be used (Fig. 6). The seeds drop vertically into the furrow (1).

Figure 6. Outside- or inside feeding with vertical metering disks

(Courtesy of W. Brinkmann, altered).

However, this method of vertical seed dropping requires peripheral cell distances at the seed disk, which correspond exactly to the desired seed distances in the furrow. Thus, the traditional method of varying the seed-spacing by adjusting the rotational velocity of the disk can be used only to a very limited extent; for major adjustments changing the seed disk is necessary.

Therefore, vertical seed dropping mainly is used with strictly spherical seeds such as sugarbeet pills, which tend to roll easily along the furrow-bottom.

Another approach to eliminate bouncing of the seeds along the furrow bottom is plac­ing them into a hole instead of in a furrow. For this purpose so-called punch-seeders have been developed (Fig. 7).

Figure 7. Operating principle of punch-seeder.

They are not yet used commercially except for punching through and seeding underneath a plastic mulch. Varying the seed spacing within the row with punch-seeders is even more difficult than with the vertical seed dropping method (2, 3).

Generally, mechanical precision drilling requires a much closer adaptation in the di­mensions of the singling elements to those of the seeds than air-assisted precision drilling. This applies especially to vertical or horizontal metering disks equipped with cells and slightly less with inclined disks equipped with cupped fingers. The cell diameters should be about 10% more than the largest diameters of the respective nearly spherical seeds (1).

Figure 8. Precison-metering of corn by cam-operated fingers.

There have been attempts to avoid the need of closely adopting the hole or cup dimensions to those of the seeds. A principle used with mechanical precision drilling is shown in Fig. 8. The vertical singling disk is equipped with cam-operated fingers. Within the seed supply these pointed fingers move close to the disk and respectively seize a seed. At the highest point, the seeds again pass from the singling disk respectively into the chambers of a parallel plate for delivery closely above the furrow.

Figure 9. Air-assisted precision-drilling.

Air-assisted Precision-drilling. Either the suction or the compression by air is used to assist in singling the seeds. For singling by suction, a vertical disk with suction holes passes through the seed supply (Fig. 9). Excess seeds per hole usually are brushed off. At the highest point the seeds pass to a sectioned wheel, which brings them to the dropping point.

Compressed air is used for singling the seeds within rather large conical cells on the periphery of a disk (Fig. 9). The pointed tips of the conical cells have open access to the atmosphere. The compressed air removes all seeds from the cells except the lowest, which covers the hole at the pointed tip. Thus, each cell keeps one seed and transports it to the dropping point.

Air-assisted precision drilling needs less precise seed calibration than all purely me­chanical methods and therefore is used extensively for irregularly and non-spherically shaped seeds such as corn, sunflower, and some bean varieties. Custom operators espe­cially appreciate that the seed disks seldom must be exchanged when moving from farm to farm with different seed varieties during the season. The investment for air-assisted precision drilling, however, is higher because of the PTO-driven blower.

The general concept of precision drilling is, that a metering unit is placed on each opener. This method allows for rather small distances from the delivery point to the furrow and thus favors the exact seed placement. However, this concept requires rather troublesome filling and monitoring of many seed hoppers. In case all openers of a machine are supplied with seeds from a central seed hopper, the supply can be refilled in bulk and monitoring it is facilitated.

But this concept of replacing the row of small hoppers and metering units with one central big hopper and one central metering system requires long seed tubes for gravi­tational and pneumatic seed transport to the openers. An originally even seed sequence deteriorates substantially during this seed transport (1, 4). Therefore, sometimes a central hopper provides for seed supply to the standard system of a small hoppers and a metering unit for each row. The investment for this method of twofold seed storage is high, yet it allows bulk filling as well as metering in place.

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