Planter Section Control Overlap and Skip at Headlands
A 24-row 60-foot planter is pulled by a 300 hp tractor equipped with RTK auto-steer and ISOBUS section control. The operator reports that at headlands and point rows the planter leaves 1.5 m gaps at the start and doubles seed population for about 2 m at the end of each pass. Population on the display is 34,000 seeds/acre target, but the seed monitor shows strips of 60,000. The seed meter calibration is correct, and vacuum is steady at 4.5 in H2O. The RTK correction age occasionally reaches 8 seconds, and the radio link has a 45% signal strength in one field corner. Section control is set to 2-second off delay and 0.5-second on delay. The machine is 60 ft wide, 24 rows at 30 in spacing, and operates at 5.5 mph. The customer wants to finish 800 acres before a forecast rain.
1. Calculate the distance travelled during the section delays and relate it to the observed overlap and gap.
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At 5.5 mph, the planter travels 5.5 × 5,280 ÷ 3,600 = 8.07 ft/s, or about 2.46 m/s.
- A 2-second off delay means the section stays on for 2 × 8.07 = 16.1 ft, or 4.9 m, after the command to shut off. That would produce up to 4.9 m of overlap if the valve and seed meter respond instantly at the end of the delay. The operator sees about 2 m of doubled population, which is less than the full calculated distance. That suggests the controller may already have some look-ahead, or the valve closes before the full delay expires, or the overlap is being measured only where the population monitor is sensitive enough to show it.
- A 0.5-second on delay means the section starts 0.5 × 8.07 = 4.0 ft, or 1.2 m, after the command to turn on. The observed gap is about 1.5 m, so the on-delay accounts for most of the skip. The extra 0.3 m could come from GPS latency, valve response, or the seed meter filling time.
- RTK correction age of 8 seconds is not the same as an 8-second position lag, but it does mean the correction is stale. At 8.07 ft/s, 8 seconds of travel is 64.5 ft, so if the receiver were simply extrapolating a stale correction, the position error could be large. RTK systems usually hold the last correction and degrade gracefully, but 8 seconds is long enough to reduce accuracy at headlands.
The observed 2 m overlap and 1.5 m gap are consistent with the delay settings plus some latency, not with a meter calibration error.
2. How would you determine whether the remaining error is GPS latency, controller processing, or valve response?
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I would separate the system into command, position, and mechanical response.
- Check the RTK correction age and signal strength in the corner. If the age reaches 8 seconds and the radio strength is 45%, the position solution is suspect. I would record the correction age while the planter is working and compare it with the overlap pattern. If the overlap occurs only in the low-signal corner, radio coverage is a major contributor.
- Use a static repeatability test: park the planter, mark a point, and watch the guidance position for several minutes. A drifting position with a steady correction age points to the receiver or base station, not the section valves.
- Command a single section on and off manually at a known ground speed and measure the distance from the command to the seed drop. This isolates controller and valve response from GPS. If the measured distance matches the 2-second off delay, the controller is doing what it is told; if it is longer, valve or meter response is slow.
- Use the ISOBUS diagnostics or a scope to check the section command signal and the valve current. A valve that is slow to close will show a decaying current or a delayed pressure response.
- Compare the seed monitor population strips with the as-applied map. If the double population is behind the actual section boundary, the delay is in the mechanical response; if it is ahead, the controller or GPS is predicting the boundary incorrectly.
I would not change the delay settings until I knew which of these three is dominant.
3. What settings and operational changes would you make, and would you release the planter for the 800 acres?
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I would first improve the RTK signal, because an 8-second correction age is not acceptable for accurate section control at 5.5 mph. Raising the radio antenna, adding a repeater, or moving the base station to improve line-of-sight in the weak corner is a priority. If the age can be held below 2 seconds, the position error shrinks and the delay settings become meaningful.
Then I would tune the section delays using the measured response:
- Reduce the off delay from 2 seconds to about 0.8 second to target roughly 2 m of overlap at 5.5 mph, then verify with a seed drop test.
- Increase the on delay from 0.5 second to about 0.6 second if the measured gap is still 1.5 m, or leave it if the valve response accounts for the difference.
- Enable any look-ahead or boundary compensation the display offers, and calibrate the section valve response if the controller supports it.
- Slow down at headlands and point rows; at 4.5 mph the same delay covers less ground.
I would release the planter only after a test pass in the weak-signal corner shows the gap and overlap are within the customer's tolerance, and after documenting the correction age. If the RTK signal cannot be stabilized, I would tell the customer that section control will not be accurate in that corner and recommend manual control or a different pass pattern. The 800-acre deadline is not a reason to plant with an 8-second correction age and uncorrected delays.