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COOPERATIVE EXTENSION SERVICE
UNIVERSITY OF KENTUCKY COLLEGE OF AGRICULTURE, FOOD AND ENVIRONMENT,
LEXINGTON, KY, 40546
Technology to Improve Sprayer
Accuracy
Tim Stombaugh, Biosystems and
Agricultural Engineering
A number
of new technologies have
been introduced over the last several
years aimed at improving the accuracy
of spray application, but do they really
work? The purpose of this document is
to highlight the most common causes
of application errors then discuss the
array of new sprayer technologies that
are becoming available, how they might
affect application accuracy, and pitfalls
involved in using them. |
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What are we really
trying to do?
Before talking about gizmos and gad-
gets, all operators should really under-
stand what they are trying to accomplish in the field. The main goal
of any spray application is to apply a certain amount of a
concentrated chemical product to a field as uniformly as possible
within the target area. Along with that goal, it is often
critical to precisely control the dilution of that product as well
as the droplet size of the diluted product as it is deposited on
plants or soil. These together will affect the efficacy or
effectiveness of the chemical that is applied. This all must be done
while minimizing the amount of material
that does not hit the target either by drift or errant placement.
The fundamental mathematical rela-
tionship that governs application rate is
shown in Figure 1.
Application rate, which is the amount
of material we want to apply, is often expressed as gallons per acre
or quarts per acre. Flow rate is the amount of material that is
coming out of a nozzle per unit of time; it is typically measured
with a cup and a stopwatch during calibration pro- |
cedures and is reported in
units such as
gallons per minute. Flow rate is a function primarily of the size of
the orifice (hole) in the nozzle and the fluid pressure at the
nozzle. Speed is simply how fast the machine is moving. Flow rate
and speed must be controlled together to maintain accuracy, e.g.
slowing the machine without reducing flow rate will cause an
over-application of chemical.
Flow rate can be increased by increas-
ing the pressure, but that also decreases the droplet size in the
spray pattern.
Smaller droplets are sometimes needed.
For example, many contact fungicides
and herbicides are more effective with
lots of small droplets covering as much
of the surface of the target as possible
while some soil-applied herbicides are
still effective with fewer larger drops.
The problem with smaller droplets is
that they are more susceptible to drift.
Because of all these conflicting factors,
the configuration of the sprayer in terms
of nozzle type and size, operating pres-
sure, and forward speed is often chal-
lenging and expensive. Several of the new technological developments
are meant to help give the operator more flexibility in |
controlling droplet sizes,
flow rates, and
operating speeds.
Causes of Application Errors
Five fundamental issues cause inac-
curacy of spray applications:
1. System calibration
2. Off-target application
3. Response of flow controllers to on/off
and rate change commands
4. Speed differential across the boom
during turns
5. Boom height
1. System Calibration
Calibrating the sprayer is the first and
most fundamental step that should be
taken to insure accurate application. The
goal when calibrating a sprayer is to cor-
relate the actual flow rate and speed with
machine settings and/or sensor outputs.
Sprayer controller manufacturers typi-
cally give clear explanations of calibra-
tion procedures, and there are a number
of good resources to help with calibration
techniques and calculations. In general,
the calibration will involve measuring the
actual flow rate from the nozzles with a
“bucket” and stopwatch and measuring
the actual machine speed with a tape
measure and stopwatch. The calibration
should be periodically checked as nozzles
and other components will wear and
change performance over time |
2. Off-Target
Application
Most farmers do not have the luxury
of working in perfectly rectangular fields
with no internal obstructions, so most
everyone must deal with point rows.
Especially as machinery gets bigger, field
irregularities will cause some kind of off-
target application of material. Off-target
application (Figure 2) could be double
coverage into a previously treated area
that might occur when spraying into an
angled headland or when overlapping
adjacent swaths because of imprecise
steering control. It could also be applica-
tion outside the field boundary into an
area such as a waterway, access road, or
fencerow.
Off-target application is obviously a
waste of input, which represents a direct
cost to producers or service providers.
One study showed that off-target appli-
cations could be as much as 25 percent
of the field area in small and irregularly
shaped fields. Beyond that, off-target ap-
plication can have a detrimental effect
on the crop since double application
of some chemicals can damage plants.
Extra chemical not used by the cropping
system could adversely impact the envi-
ronment. Application outside boundar-
ies could damage vegetative buffers and
other critical protective features |
3. Response of Flow
Controllers to On/
Off and Rate Change Commands
Another major cause of application
inaccuracy lies in the performance of
the flow control system on the sprayer.
Most modern flow control systems are
capable of automatically compensating
for a number of factors that affect desired flow rate such as
vehicle speed or desired application rate, but sometimes there is a
time delay in the response. For example, if a boom section is turned
on or off, there
will often be an abrupt pressure change
throughout the rest of the boom that will cause the output to
change, and it may take some time for the control system to settle
back to the desired flow rate. An abrupt speed change will cause the
same kind of behavior because the system must adjust to a new
operating flow rate that matches the new speed.
The article “Real-time Pressure and
Flow Dynamics Due to Boom Section
and Individual Nozzle Control on Agri-
cultural Sprayers” evaluated the response time of various flow
controllers to abrupt system changes by installing extra pressure
sensors near different nozzles across a boom to estimate actual flow
rates through the nozzles. They showed several
examples of control performance, one of which was presented in
Figure 3. When part of the boom was turned off at time 0, there was
an immediate pressure increase in the rest of the boom and it took
almost 30 seconds for that pressure to settle back to the desired
value. While this particular |
example may be one of the
more extreme examples, in general there will be a pressure spike or
dip in the rest of the boom when a boom section is turned off or
back on, and the system will take some time to settle back to the
correct operating point. Overall, they observed flow rate
increases of 3.7 percent to 10.6 percent that lasted up to 25
seconds when turning boom sections off. The percentage of rate
increase was roughly proportional to the percentage of the boom that
was turned off, i.e. if more of the boom is shut off, the rest of
the boom will see a higher rate increase spike.
Different controllers will respond
differently—some will settle more quickly than others, some may
allow larger spikes than others. The magnitude of the error spikes
and the time required for the performance to settle back to the
desired operating point will be dependent on the quality of the
sensors and components in the system, the location of the sensors in
the system, and the quality of the controlalgorithms designed by the
manufacturers.
Some modern spray equipment can
operate at extremely high field speeds. At 20 mph, a vehicle will
cover about 30 feet every second. If the rate controller takes even
5 seconds to respond to a change (which is not uncommon), that would
mean an area the width of the boom and 150 feet long would receive
the wrong application rate. With a 90-foot boom, that is more than
three tenths of an acre. Figure 2. Examples of off-target
application errors caused by overlap of adjacent passes, application
outside of field boundaries, or spray into previously treated
headland areas.
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4.
Speed Variations Across
Boom During Turns
The rate control systems currently
available on application equipment rely exclusively on the forward
speed of the machine measured either by a radar sensor, transmission
speed sensor, or GNSS data. They attempt to output the same flow
rate at every nozzle across the boom, which is appropriate if the
vehicle
GNSS: Global Navigation Satellite System GPS has become a
staple in
agriculture, but many people do
not realize that GPS is only one of
many Global Navigation Satellite
Systems maintained by different
countries around the world. Many
of the higher-end receivers used
in agriculture today are dual
system receivers that receive
both GPS and GLONASS signals.
GLONASS is a Russian satellite
system very similar to the U.S.
GPS. Thus the more generic GNSS
term was derived to describe all
of these satellite systems. |
is going straight. If the
vehicle is turning, the nozzles toward the outside end of the boom
are moving much faster than those on the inside of the turn. The
turning effect gets more pronounced as the booms get longer and as
the steering angles get
tighter. This means that the outer limits of the boom will not be
applying the desired amount of material per acre.
The graph in Figure 4 illustrates the
anticipated error across the boom at 1
degree, 3 degrees, and 5 degrees steer angles, which is the angle of
the front steering wheels relative to the vehicle chassis. It is
based on the “bicycle” model for vehicle movement and it assumes no
boom whip, no wheel slip, boom located over rear axle of a front
steered machine,
and 150-inch wheelbase, which is typical for larger high-clearance
sprayers.
Even very small turning angles will
create significant application errors.
The dotted lines in Figure 4 indicate
10 percent error, which is often held as an acceptable threshold for
application accuracy. The steering limits for different common boom
sizes that will keep errors below 10 percent are listed in Table 1.
For the larger 120-foot booms |
Table 1. Tightest allowable turn to keep
application errors at boom tip below 10
percent.
the wheels cannot be steered more than
about 1 degree to keep errors below 10%.
In field practice, turning maneuvers are
often coupled with speed changes. Since
speed changes also perpetuate applica-
tion inaccuracy, these theoretical turning
errors represent the minimum that will
probably be observed in the field while
turning.
5. Boom Height
Nozzles are carefully designed to
produce a given angle of spray with a
controlled flow profile across the spray
pattern. That flow profile is designed to
require a certain amount of overlap by
adjacent nozzles to produce a uniform
application pattern across the boom. |
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Educational programs of Kentucky Cooperative
Extension serve all people regardless of race, color, age, sex,
religion, disability, or national origin. Issued in furtherance of
Cooperative Extension work, Acts of May 8 and June 30, 1914, in
cooperation with the U.S. Department of Agriculture, Nancy M. Cox,
Director of Cooperative Extension Programs, University of Kentucky
College of Agriculture, Food and Environment, Lexington, and
Kentucky State University, Frankfort. Copyright © 2014 for materials
developed by University of Kentucky Cooperative Extension. This
publication may be reproduced in portions or its entirety for
educational or nonprofit purposes only. Permitted users shall give
credit to the author(s) and include this copyright notice.
Publications are also available on the World Wide Web at
www.ca.uky.edu.
Issued 3-2014
Exactrix®
Global Systems LLC
4501 East Trent Ave.
Spokane, WA 99212
www.exactrix®.com
509 995 1879 cell, Pacific.
exactrix@exactrix.com
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