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Yield monitors are a recent development in agricultural machinery that allow grain producers to assess the effects of weather, soil properties, and management on grain production. They are a logical first step for those who want to begin practicing site-specific crop management or “precision agriculture.” The accuracy of these devices depends on appropriate installation, calibration, and operation. Therefore, it is essential that grain producers understand how the yield monitor works to improve their grain enterprise.
This publication is intended to help equipment operators and farm managers select, install, and operate a yield monitor. It explores the costs of establishing a yield monitoring system, the essential components of yield monitors and the functions of those components and methods for ensuring accuracy of the data.
The yield monitor is intended to give the user an accurate assessment of how yields vary within a field. Although a yield monitor can assist grain producers in many aspects of crop management, the device was never intended to replace scales for marketing grain.
A yield monitor by itself can provide useful information and enhance on-farm research. Yield data can be accumulated for a specific load or field, thereby facilitating the comparison of hybrids, varieties, or treatments within test plots. For example, all yield monitors can measure grain mass and harvested area on a load-by-load or field-by-field basis. This feature allows an operator to get instantaneous readout in the field of accumulated grain weight, harvested area, and average yield. With many yield monitors, these values can be exported to a personal computer and stored in nonvolatile memory for further analysis or printing via specialized software packages or more standard word-processing and spreadsheet software. Season summaries of harvested areas might then be used to settle custom harvesting charges or to keep track of production from individual fields when it is impractical to scale grain trucks. With a yield monitor, a producer also can conduct on-farm variety trials or weed control evaluations without the need of a weigh wagon. Such on-farm comparisons help producers fine-tune crop production practices to their soils.
The Differential Global Positioning System (DGPS) is an integration of space- and ground-based segments that together comprise a radio-navigation facility. Initially developed for national security interests, a portion of the DGPS system is available to civilian users. (Figure 1 is a diagrammatic representation of DGPS.) When yield data are used with information generated by a DGPS receiver, a producer can generate yield maps that provide a quick visualization of crop performance within a particular crop production unit. Ultimately, any increased profit realized from incorporating a yield monitor into an operation will come from changes in management practices that result from the identification of problem areas using such yield maps. With DGPS, the benefits of using a yield monitor are even more evident.
When you make this first move into precision agriculture, your choices are limited by what is available and the level at which you intend to enter the technology. Capital outlays for yield monitoring systems range from $2,000 for a user-installed yield monitor alone to more than $8,000 for a complete turn-key system that includes a yield monitor, DGPS receiver, mapping software package and installation with the purchase or lease of a new combine. Potential purchasers and users of a yield monitoring system may want to consider the following questions:
Yield monitors are a combination of several components, as shown in Figure 2. They typically include several different sensors and other components, including a data storage device, user interface (display and key pad), and a task computer located in the combine cab, which controls the integration and interaction of these components. The sensors measure the mass or the volume of grain flow (grain flow sensors), separator speed, ground speed, grain moisture, and header height. The sensors are interfaced with both analog to digital and direct digital inputs. Yield is determined as a product of the various parameters being sensed. It is important to understand the function of these components to further understand the interaction of the yield monitor, the combine operator, and the combine dynamics.
Figure 2. Components of a grain yield monitoring system on a typical combine installation.
The most essential component of any yield monitoring system is the sensor used to measure the mass or volume of clean grain moving through the separator of a combine. While the nature of mass-flow or volumetric-flow sensing may vary by manufacturer, these devices are almost always located at or near the top of the clean grain elevator, as shown in Figure 3.
Figure 3. Typical installation of a mass-flow sensor in the clean grain elevator housing.
The most common approach is mass-flow sensing, which is accomplished by assessing the impact force of grain hitting a plate. The paddles of the clean grain elevator accelerate grain as the chain makes a 180° turn at the top of the elevator. The centrifugal acceleration generated as the paddles make this turn causes the grain to separate from the paddles. Grain then contacts the elevator enclosure and falls toward the base of the bin loading auger. It is at this location, where grain impacts the elevator housing, that impact-type sensors are positioned.
An impact sensor measures the mass flow rate of grain through a combine by sensing the force of grain as it impacts a plate or beam, using either a strain gage or a linear potentiometer to measure deflection of the beams or various members holding the plate. Because the impact sensor is the most prevalent type of grain flow sensor in the United States, it is the primary focus of this publication. Three other sensors—radiation-based, photoelectric, and paddle wheels—are also explained because they have been incorporated into a number of commercially viable products.
Radiation sensors measure the mass flow of grain passing between a radiation source and sensor, usually mounted on the top of the clean grain elevator. The presence of grain between the source and sensor attenuates or diminishes the amplitude of radiation incident on the sensor and is a function of the mass of grain between them.
Photoelectric sensors measure the volume of grain contained on each paddle of the clean grain elevator. One or more light sources and paired sensors are placed opposite one another across the clean grain elevator housing, and the volumetric flow of grain is correlated with the timing of the light circuit interruptions. Timing these interruption periods provides an estimate of volumetric grain flow. A drawback to this system is the potential for errors if the combine is operated on hillsides, whereby grain is forced to one side of the elevator paddles.
Volumetric flow can also be measured by using a paddle wheel at the base of the loading auger tank. The paddle wheel is rotated at a controlled speed to ensure that the region between adjacent paddles is filled with grain. The wheel rotates to a new position, allowing grain to accumulate in the space between the next two paddles. Grain volume is then determined by recording the number of paddle wheel revolutions.
Today, many combines can be purchased with factory-installed yield monitors. After-market yield monitors are also available for installation on late-model combines. The flow-measuring device (impact plate) is placed in the service door at the top of the clean grain elevator, or in an opening at the top of the elevator. In some cases, the impact sensor can be mounted only by cutting a hole in the top of the clean grain elevator housing.
The proper installation of a yield monitor requires positioning the mass-flow sensor relative to the position of the upper elevator sprocket. A new adjustment mechanism must also be installed on the lower end of the clean grain elevator. After calibration, only the lower sprocket should be changed to adjust elevator chain tension. Any movement of the top elevator sprocket changes the impact angle of grain, thereby requiring a new calibration. Calibration of the flow sensor refers to the correlation of the mass flow rate of grain, in pounds per second, to the magnitude and frequency of the impact force. The geometry of the impact plate relative to the elevator paddles must be maintained after calibration of the yield monitor to assure accurate data collection.
Essential to the yield monitoring process is the determination of both separator speed and ground speed. Separator speed is determined using a simple magnetic sensor on one of the separator shafts. This sensor generates a square wave with a frequency that is proportional to the speed at which a ferrous block passes the magnetic pick-up. These sensors were originally installed on combines to monitor cylinder and fan speeds. The clean grain elevator speed is a multiple separator speed, and it must be known for mass flow rate calibration. By monitoring the speed of a shaft directly coupled to the elevator drive, a simple ratio can be used to determine the frequency of paddles passing the impact plate and can be used for mass flow rate signal conditioning. Most after-market yield monitor manufacturers provide cables that allow the installer to tap into this speed signal.
Distance traveled is simply a product of ground speed (velocity) and sampling time. Ground speed can be easily measured using the existing magnetic pick-up provided by the combine manufacturer. A square wave is generated as ferrous gear teeth within the transmission pass the magnetic pick-up. Ground speed is a constant multiple of this generated frequency and is determined during the calibration of the yield monitor.
Once ground speed is known, the distance traveled by the combine during a sampling period is determined by multiplying the ground speed by the time of the sampling period. As an example, assume a combine is traveling 4.0 miles (253,440 inches) per hour. If a sampling period of 1 second is used to store yield data, the combine has traveled 70.4 inches within that sampling period. As will be illustrated later, this value—termed “distance traveled”—is stored as part of the yield data file and is needed for determining “harvested area,” which is a product of distance traveled and the effective header width during one cycle.
One of the problems with this approach is measurement error due to wheel slippage. Radar provides an alternative method for measuring ground speed and is preferable when compared to the magnetic sensor pick-up. It is not subject to wheel slip errors, which can be an important factor when combines are operated in unfavorable conditions such as on wet soil. However, radar is also susceptible to errors, especially when operating in high residue environments. Wind-induced movement of weeds or plant stubble may generate erroneous radar reflections.
More recently, yield monitor manufacturers have begun using the velocity determined by a DGPS receiver, but there are some drawbacks to this method. A functioning GPS receiver must be present if using it for speed determination. Operation of the yield monitor may not be possible when GPS signals are not available. Also, GPS speed determination is subject to the same errors as position determinations. This is not a great concern, however, because GPS velocity is always more accurate than position determinations.
Measuring the moisture content of the grain is also an important element of yield monitoring. Producers are concerned about moisture content from the perspective of harvesting, drying, and storing grain, plus the cost of drying or dockage at the elevator. Sensing this moisture content at the time of harvest allows yield data to be corrected to reflect a true marketable weight of grain. Moisture sensing is also useful at harvest to allow the combine operator to determine the suitability of a particular field for harvest and perhaps the delivery destination of the grain—storage, drying, or the local elevator.
Moisture content is determined by sensing the dielectric properties of the harvested grain. The level of moisture within the grain affects the grain’s capacitance (its ability to store an electrical charge), which is sensed by confining a predetermined volume of grain between two conductive metal surfaces. For most yield monitors, this is accomplished by installing a moisture sensor either on the tank-loading auger or on the side of the clean grain elevator, as shown in Figure 4.
Figure 4. Moisture sensor installation on the clean grain elevator for yield monitoring.
Today, most moisture sensors are mounted on the side of the clean grain elevator. This location is better than mounting the sensor on the tank-loading auger because the sensors encounter less grain flow, reducing the buildup of debris on the sensor plates. This buildup introduces bias into the moisture measurements. The moisture sensor is essentially a conductive shell or metal plates with an electrically isolated internal metal fin. As grain rises in the clean grain elevator, a small amount enters the top of the moisture sensor and moves between the metal plates. A small paddle wheel located in the bottom of the sensor housing ensures that grain always covers the plates. The paddle wheel also controls the rate at which grain reenters the clean grain elevator.
Users should check the moisture sensor periodically and clean the plates when the combine is operated in weedy or wet grains. These conditions can cause a buildup of dirt or plant residue on the sensing elements, which interferes with grain moisture measurements.
A task computer located in the cab of the combine serves several functions: it integrates and calibrates the sensors; converts their output signals into data for storage, display, and later use; contains the DGPS receiver interface, external data storage devices, and user interface (display and keyboard); and controls the interaction of these devices. Downloading a new program or changing the program chip in the yield monitor reconfigures and upgrades the yield monitor.
Data are downloaded or transferred from the yield monitor to a desktop or laptop computer using a Personal Computer Memory Card Industry Association (PCMCIA) standard memory card. Industry has moved toward adopting the term “PC card” or “PC card interface” for the PCMCIA-type devices. These cards can be divided into two types: static random access memory (SRAM) and dynamic random access memory (DRAM). DRAM cards require continual refreshing of the charged memory locations to maintain data integrity. Unfortunately, the continual refreshing of volatile memory locations requires power, making this type of card unsuitable for data transfer. SRAM cards enjoy a long read/write cycle life, making them a better choice for precision agriculture applications. As with the DRAM cards, the memory of SRAM cards is also volatile; therefore, a battery is required to preserve data stored on the card when it is removed from a powered system. Most PC card users should periodically check battery levels to prevent data loss. However, most yield monitors provide a battery voltage level check after formatting a PC card. The manufacturer usually recommends a replacement interval of two years.
Choosing the right type of PC card, as identified by its thickness, is as important as choosing the right category of card. PC card standards allow for Type I, II, III, and IV cards. All types of card measure 2.13 inches in width, 3.37 inches in length, but they vary in thickness: 0.15 inches for Type I, 0.20 inches for Type II, or 0.41 inches for Type III. The connectors’ pin spacings and support rails are all the same size. However, the spacing between adjacent cards determines which card type a device can accept. Figure 5 shows the differences in the types of cards and card construction. Memory access times for SRAM cards vary from 80 to 250 nanoseconds (ns), or 0.00008 to 0.00025 seconds.
Figure 5. PCMCIA card dimensions and typical cross section.
The data transfer medium used in most yield monitors is a static, battery-backup SRAM Type II PC card, with a storage capacity of one to two megabytes (Mb). It is essential to check with yield monitor manufacturers before purchasing these cards, as their devices may be limited to a specific type and speed.
The majority of data collected during field operation of the combine is stored directly on the PC card. Therefore, a card with adequate storage space must be present in the yield monitor or mapping processor boxes whenever yield data are to be logged. More than 30 hours of yield data can be logged at one-second intervals on a 2-Mb card. Most grain producers believe that it is important to have two cards so that they can be interchanged on a daily basis.
To generate yield maps, yield data must be geographically referenced (in the form of a latitude and longitude position) using a DGPS receiver. In the interest of national security, errors are intentionally incorporated into the civilian GPS signal information (C/A Code) rendering GPS coordinate fixes too inaccurate for generating yield maps. Therefore, civilians must obtain differential correction data from a fixed second source to reduce the error of the GPS positioning services used. Availability and accuracy of correction signals may limit the type of correction service many producers can use. The U.S. Coast Guard is one source of free correction information. The radio beacon signals are considered “local” correction signals. Errors associated with these correction services are proportional to the distance from the base station and signal transmitter. Producers can also subscribe to one of several private-sector correction service providers, which use geostationary communication satellites to broadcast their correction signal; these correction services are considered “wide area” in that they are available over a significant portion of the North American continent. Subscription fees accompany these services, ranging up to $800 per year. Wide-area correction signals are transmitted via C-Band (3.75 to 4.5 GHz) or L-Band (1.6 GHz) carriers. A diagrammatic representation of wide-area DGPS is presented in Figure 1.
When you purchase an integrated package that includes a yield monitor, a DGPS receiver, software, and a personal computer, the supplier must ensure that all components are compatible with one another. However, if you opt to move into yield monitoring component by component rather than buying an integrated system, you will need to know how to select and integrate a DGPS receiver into your system. Use extreme caution when swapping components between systems, and read all manuals and make sure you understand the implications of connecting components.
Currently, most yield monitor manufacturers utilize an RS-232 serial interface (a common interface for data transfer in the personal computer industry) to communicate data from the DGPS receiver to the yield monitor. An RS-232 interface typically uses a DB9 connector that has nine pins, but only two wires are required when transferring data between the DGPS receiver and the yield monitor. Most DGPS receivers used in mobile applications require a 12-volt DC supply. However, receiver manufacturers allow a sizable deviation from this power specification by typically allowing receivers to operate between 8 and 12 volts DC. Some system integrators now use a CAN (Controller Area Network) local-area network for transfer of data. Communications remain serial in nature; however, the RS-232 protocol is no longer applicable.
Data output from DGPS receivers intended for use with yield monitors must
conform to the NMEA (National Marine Electronics Association) 0183 data format.
This data format consists of several different character strings. All strings
begin with a “$” and five-character code and end with both a carriage
return (
$GPRMC,164259.00,A,3819.2665,N,08510.6114,W,000.1,157.8, 270296,0.000,E*25<CR><LF> The format of this string is:
$GPRMC,hhmmss.ss,a,ddmm.mmmm,n,dddmm.mmmm, w,z.z,y.y,ddmmyy,d.d,v,CC<CR><LF> where:
hhmmss.ss = Universal Time Coordinate (UTC) of position fix Two options exist when investing in a yield monitor. If you are purchasing or
leasing a new combine, you may order a factory-installed yield monitor with it.
Whether buying or leasing, the customer should expect a fully functioning system
and reasonable support from the manufacturer installing the system.
The second option is to purchase a yield monitor from an after-market
supplier. While these yield monitors may be installed on either new or used
combines, most yield monitor manufacturers support mainly late-model combines
and tend to shy away from adapting yield monitors to older combines. They
perceive this as a shrinking market and therefore cannot justify development
costs for these models. Late-model combines warrant their attention, and, to
this end, they have responded quite well.
Installing your own yield monitor can result in substantial savings. With the
savings, however, comes the potential for frustration. Be certain you are
willing to accept some risk and frustration when you decide to go it alone. The
following discussion is a summary of several users’ experiences and
suggestions and is intended to reduce your frustration level.
Prior to installation, read the instructions several times.
Installation requires cutting several openings and drilling several holes in
the sheet metal of your combine. Depending on the type of moisture sensor used,
you also may be required to remove a section of auger flighting from the
tank-loading auger. Yield monitor installation also affords you the opportunity
to replace elevator flights and chain, bearings, or augers. Acquire the
necessary replacement parts in advance of the installation. Yield monitor
manufacturers prepackage several tools, templates, and replacement bearings to
make your job easier. Familiarize yourself with all of the parts in the
installation kit prior to cutting or drilling holes.
Manufacturers provide templates to help accurately locate required openings
for both the mass-flow sensor and the moisture sensor. If your combine has a
service door for the clean grain elevator that is close to the desired location
of the mass-flow sensor, a new door may be provided to mount this sensor.
Otherwise, you must locate a template and cut a new opening.
The mass flow sensor must be centered on the paddles of the clean grain
elevator, so use caution in positioning the template before cutting or drilling.
Two openings on the side of the clean grain elevator are required for the
installation of most moisture sensors. Some manufacturers fabricate openings on
the side of the clean grain elevator for moisture sensors along with the proper
mounting area for the flow sensor. If you use a cutting torch to make these
openings, extreme caution is advised. Most areas in the cleaning and
separating regions of the combine tend to accumulate dust, dirt, and plant
material, and this accumulation, combined with grease and oil, creates an
extreme fire hazard. A much better alternative is to use a jigsaw with a
fine-tooth metal cutting blade or a high-speed air tool with a circular cut-off
blade. In either case, holes must be drilled at the corners of the rectangular
opening. Hand filing may be required to remove burrs around the opening.
After installing the moisture sensor and the mass-flow sensor, the remaining
modification is to move the chain tensioning device on the lower end of the
clean grain elevator. This procedure is more machine-specific than the other
modifications. The fixed position of the upper chain tensioner is crucial to
the long-term calibration and operation of your yield monitor. Any movement
or readjustment of the upper tensioning device will require recalibration of the
yield monitor. Provisions are made by manufacturers to install a new chain
tensioning device at the lower end of the elevator. While this resolves the
adjustment problem after fixing the position of the upper shaft, the amount of
adjustment is limited. You should keep half-links on hand for periodic
adjustments due to wear and stretching of the elevator chain.
The remainder of the yield monitor installation process involves routing the
wiring harnesses and mounting both the user interface and the mapping processor.
Proper location of the user interface is crucial for keypad operation and
display visibility. The display must be viewed on a regular basis, and the
operator may wish to observe several monitor features at one time. A series of
keystrokes is usually required to change the display mode and enter new data. As
the user becomes familiar with the various monitor functions, using the yield
monitor becomes routine. Some operators rely on the mass flow readout values to
determine how full the cylinder is and whether the ground speed of the combine
should be adjusted to match the capacity of the threshing and separating
mechanisms.
Cable routing seems to be straightforward; however, this activity is perhaps
the single greatest source of trouble in most installations. One of the weak
links on agricultural machinery is electrical connections. The quality of the
connectors themselves has increased considerably in the last 20 years. However,
the best connectors cannot make up for poor cable routing. The combine has
numerous moving parts that come in contact with the wiring harnesses. Rotating
parts quickly wear away insulation and cause short circuits. Cables routed where
excessive vibration or continual bending exists can result in open circuits as
the braided copper wires harden and break. Flowing grain within the grain tank
generates large frictional forces that pull at the cabling. Over time, these
forces tend to pull the wires from the connector or separate the pins within the
connector. In either case, the general rule is to appropriately route wires and
use plenty of wire ties to secure cables in place. Leaving slack at the cable
connectors enables them to be easily separated for troubleshooting and reduces
strain at the connector. When installing cables, resist the temptation to
shorten them as this may affect system performance. Often the electrical
resistance of a conductor must be considered in the design of sensors and
antennas. Therefore, changing the length of a wire/cable could alter the
electrical response of a component. Pre-harvest checks of combines with yield
monitors should include cable inspection.
Yield monitor manufacturers make every effort to build accuracy into their
systems; however, every combine and installation may have different errors.
Sources of errors in the data produced by the yield monitor include material
transport delays within the combine, effective swath width, moisture content,
and mass flow determinations. What has evolved are systems that must be
calibrated for each grain type. Four elements must be considered when
calibrating a new yield monitor:
Using the wheel speed sensor in the transmission to determine ground speed
requires calibration to relate the actual distance traveled to a specific number
of pulses from the sensor. The sensor is calibrated by operating the combine
over a known distance (e.g., 400 feet or length specified by the manufacturer)
at typical harvest speed and field condition. The yield monitor then generates a
scale factor to correct recorded speed. When calibrating the speed sensor, you
should match the actual operating conditions of the combine at harvest as
closely as possible. For example, operate a loaded combine on soft ground or a
hillside if this is typical of field conditions at harvest. Ground speed radar
must be calibrated in the same manner. However, yield monitors that rely on the
use of GPS for ground speed determination do not require calibration for speed.
Header height determination is important as it establishes the beginning and
ending of data logging and area accumulation. There are principally three
different methods for sensing header height. One method is a magnetic sensor
that opens a contact when the header reaches a predetermined position. A second
method uses a rotary potentiometer for sensing the angle or elevation of the
header. At the option of the combine operator, the start and stop positions
determined by the potentiometer can be adjusted on a control panel located in
the combine cab. The third method involves tracking the length of time the
header height control switch is in the “up” or “down” positions. Once
the actuation time exceeds a predetermined value (e.g., 1.5 seconds), area
accumulation and data logging is either turned on or off, depending on whether
the header is being lowered or raised.
Regardless of the methodology used to initiate or terminate data logging and
area accumulation, combine operators are urged to read the operator’s manual
and thoroughly understand the operation of this feature because the quality and
integrity of yield data depend heavily on its use.
To calibrate the mass flow sensor, producers must collect data regarding the
weight of grain harvested over a certain interval and then enter the actual
weight of grain harvested into the yield monitor. This interval might consist of
one to several combine tank loads. Based on a defined approach, the yield
monitor uses this information to fit a calibration curve, or a series of
factors, to the particular impact sensor, grain type, and combine/sensor
geometry. If any of these factors change, the system must be recalibrated.
Changes in grain properties such as test weight and moisture content may require
more frequent calibration.
Although the approach is similar from manufacturer to manufacturer, the
quantity and nature of the internal calibration approach differs. While the
factory or default calibration numbers provide a reasonable starting point, they
are not a substitute for on-farm calibration. At the very least, one truckload
of grain must be weighed. One manufacturer recommends weighing several
individual combine tank loads of grain. This process is greatly simplified when
a weigh wagon with digital readout is available for obtaining load weights.
Moisture sensor calibration is limited to providing an offset or bias to
correct the moisture content of the grain. The offset is a constant that can be
either positive or negative. It is difficult to find a suitable method of
determining the actual moisture content so that an offset can be entered into
the yield monitor. This is further compounded by considering the variability
that exists as the combine moves through the field. More importantly, a sample
of grain from the moisture sensor or tank-loading auger must be collected,
analyzed, and then compared with the moisture sensor reading from the instant in
time when the grain passed over the sensor to arrive at an accurate offset. In
short, this is not a simple operation because of the moisture content variation
at harvest and the transport delay within the combine separator.
Operators are urged to use caution when adjusting moisture offsets,
particularly when considering the accuracy of the moisture-measuring device that
will be used to determine the reference moisture content of the grain. Offsets
vary with grain type, and each grain type requires calibration to determine the
appropriate offset.
Once a yield monitor has been properly installed and calibrated, it allows
producers to observe many parameters “on the go” during harvest.
Instantaneous yield, average yield, mass flow rate through the combine, area
harvested, and moisture content can all be displayed on the monitor with the
touch of a button. Yield monitors also store a summary of this data for each
load and field. These data summaries can be downloaded via the PCMCIA card or an
RS-232 serial link to the office or a laptop PC. These exported summary files
can be opened in spreadsheets for further data analysis, used for record
keeping, or entered into income tax programs. These data are obtained without
DGPS.
When the yield monitor is linked to a DGPS receiver, geographically
referenced data is available for download to the office PC using a PCMCIA card.
This data can be expanded and exported to several software packages. The nature
of the data recorded by the yield monitor and how these data are related to the
sensors should be considered. The following “Advanced” export data format is
nearly universal:
ddd.dddddd,dd.dddddd,mm.mm,ttttttttt,n,lll,www,cc.c,kk,ppppp,ssssss,Fnn:bbbbbbbb,Lnn:bbbbbbbb,ggggggggggg,sss,
ppp,aaaa
where:
ddd.dddddd = longitude (degrees, + East and - West) A typical exported data string might look as follows,
-85.238446,38.308450,8.36,838257850,1,54,348,28.3,33,0,
950304,”F4:901",”L1:CLARK”,”Wheat”,12,64,813
It is obvious from this character string that wheat is the grain being
harvested in field 901 and that the operator has chosen “Clark” as a load
ID. Not as obvious is the yield in bushels per acre. As you look at the
character string, you can determine that 8.36 pounds of wheat at a moisture
content of 28.3 percent was harvested as the combine traveled 54 inches in a
one-second cycle. The effective combine header width was set by the operator at
348 inches (29 feet). To arrive at the marketable yield in bushels per acre for
this one-second cycle, the mass flow rate of grain at a particular moisture
content must be corrected to reflect marketable bushels. To accomplish this, the
wet mass flow value must be multiplied by a ratio of moisture content
differences,
where mmarket is the corrected mass flow rate of grain in
pounds/second, mharvest is the mass flow of moist grain at a moisture
content of MCharvest (%). MCmarket represents the moisture
content used as a basis for marketing, i.e., 13.0 percent for soybeans, 13.5
percent for wheat, 14.5 percent for barley, or 15.0 percent
for corn. Using this approach, we find the marketable mass flow rate of grain to
be:
Next, we must determine the yield on a per acre basis. To accomplish this,
the corrected mass flow rate is multiplied by the data logging interval and then
divided by the area harvested by the combine during the logging interval. The
form of this calculation is:
where Y is the corrected yield (bushels per acre), tsample
is the data logging interval (seconds), d is distance traveled in the
data logging period (inches), w is the effective width of the combine
header (inches), and rgrain
is the mass density of a particular grain (pounds per bushel). Typical mass
densities used for conversion from mass or weight to the volumetric measure of
bushels are 60.0 pounds per bushel for soybeans and wheat, 48.0 pounds
per bushel for barley, and 56.0 pounds per bushel for shelled corn.
Substituting in the appropriate numbers and conversion factors results in:
Thus, for the previously exported yield data string, we find the average
yield for this logging interval to be 34.9 bushels per acre. The reported yield
has been corrected to a marketable volume of grain per acre.
The following shows a popular exported file format referred to as
“Basic”:
ddd.dddddd,dd.dddddd,yyy.y,cc.c,ssssss,Fnn:bbbbbbbb,Lnn:bbbbbbbb,ggggggggggg
where:
ddd.dddddd = longitude (degrees, + East and - West) The “Basic” format is a simplified version of the “Advanced” format
with the estimated marketable yield (bushels per acres) presented rather than
the mass flow (pounds per second) of grain.
Properly calibrated yield monitors exhibit good correlation with scales used
for marketing. Conversely, larger errors can be expected with poorly calibrated
yield monitors. As previously mentioned, it is important to spend time
calibrating a yield monitor. Carefully consider all factors involved in the
process. Grain carts with load cells offer an easy in-field determination of
grain weights. Yield monitor calibration will be no more accurate than the
methods used during the calibration process. If the grain cart load cells
are out of calibration, this error will be transferred to the yield monitor.
Another source of error is the moisture sensing method. This is not to say
that the sensing technology is at fault. Rather, it is a question of whether a
consistent volume of grain makes good electrical contact with the sensing
plates. Anything altering this situation skews the moisture content measurement.
As an example, a buildup or accumulation of plant residue on the sensing plate(s)
can result in significant errors in moisture content determination. This type of
error is common when harvesting wheat or soybeans in fields where actively
growing broadleaf weeds and grasses are present. You should periodically check
the moisture-sensing plates for residue when harvesting in wet or weedy fields.
Errors in ground speed calibration carry over into yield determination as
they affect the calculated harvested area. The only positive note about speed
calibration errors is that they tend to be consistent and proportional, thereby
enabling correction at a future date.
Effective header width is also an important source of error when determining
yield. When harvesting with a corn head, this determination is easy, as each row
is within a specified distance of adjacent rows and the combine header width is
a multiple of this value.
For soybeans and other small grains requiring the use of a platform header,
operators have greater latitude to harvest these crops using various header
widths while traveling in directions other than that in which the crop was sown.
Both factors combine to create a situation where the complete cutter-bar width
may not be fully utilized. In these cases the operator must estimate the
effective swath width and enter this number in the yield monitor. The effective
swath width will also vary between equipment operators. It is possible to
decrement the effective header width through the user interface when needed.
However, this process requires the operator to divert attention from the harvest
operation and is therefore undesirable. Combines operating in point rows or
partial header widths skew the data toward lower point and average yield values.
In view of the efforts to reduce or eliminate errors in yield monitor data,
several factors must be considered when moving to the next step, which is the
generation of yield maps. An accurate relationship of mass flow to field
position is essential. Unfortunately, dynamics associated with material flow at
the header and within the combine create a delay between when the crop is
gathered and when the grain arrives at the mass flow sensor. A typical delay
between when the header engages a crop to when grain reaches the top of the
clean grain elevator is 12 seconds. Similarly, when the combine reaches the end
of a pass, grain flow continues at the mass flow sensor for another 8 seconds.
While these transport delays do not affect grain mass determination, difficulty
is encountered when trying to register yields with field positions determined
using DGPS. To facilitate registration, material transport delays present in all
yield monitors can be corrected using post-processing software where the user
specifies “start of pass delay” and “end of pass delay” values in
seconds. Mass flow rates are then moved back in time to conform to the position
at which the header engages the grain.
Yield monitors provide a new and powerful tool for grain production. Yield
monitor installation, calibration, operation, and purchase considerations are
discussed in this publication. Yield monitors have many benefits. One is that
operators can quickly view crop performance during harvest. A second benefit is
that this yield data can be transferred to a personal computer and summarized on
a field-by-field or total-farm basis for tax or record keeping purposes. A third
benefit is that this information can be geographically referenced for the
generation of yield maps, which provide year-to-year comparisons of high- and
low-yielding areas of a field throughout a crop rotation sequence.
Applications Mapping Inc. 1997. AL 2000, Ver 1.10 User’s Guide. Roswell,
Georgia.
Bigelow, S.J. 1994. Troubleshooting and Repairing PC Drives and Memory
Systems. Windcrest/McGraw-Hill Inc., New York, GreenStar: Combine Yield Mapping System. 1997. Operator’s Manual. Deere and
Company, Moline, Illinois.
Leick, A. 1995. GPS Satellite Surveying. 2nd ed. John Wiley & Sons
Inc., New York, New York.
Morgan, M. and D. Ess. 1997. The Precision-Farming Guide for
Agriculturists. Deere and Company, Moline, Illinois.
Myers, A. 1996. Yield Monitor 2000: Operator’s Manual. Ag Leader
Technology, Ames, Iowa.
Pierce, F.J., N.W. Anderson, T.S. Colvin, J.K. Schueller, D.S. Humburg, and
N.B. McLaughlin. 1997. Yield Mapping. pp. 211-243 Shearer, S.A. 1996. Course Notes, AEN 599—Site-Specific Agriculture.
University of Kentucky, Lexington
hh = hours (00 ... 24)
mm = minutes (00 … 59)
ss.ss = seconds (00.0000 … 59.9999)
a = status (A - valid or V - invalid)
ddmm.mmmm = latitude of position
dd = degrees (00 … 90)
mm.mmmm = minutes (00.0000 … 59.9999)
n = direction (N - North or S - South)
dddmm.mmmm = longitude of position
ddd = degrees (000 … 180)
mm.mmmm = minutes (00.0000 … 59.9999)
w = direction (E - East or W - West)
z.z = Ground Speed in knots (0.0 …..)
y.y = Track Made Good or reference to true North (0.0 … 359.9)
ddmmyy = Universal Time Coordinate date of position fix
dd = day (01 … 31)
mm = month (01 … 12)
yy = year (00 … 99)
d.d = magnetic variation (0.0 … 180.0)
v = variation sense (E - East or W - West)
CC = check sum (hex 00 … 7F)
Installation Considerations
Instructions
Installation Kit
Templates
Moving the Chain Tensioning Device
Routing the Wiring Harnesses and Mounting the User Interface
Calibrating a Yield Monitor
Distance
Header Height
Mass Flow Rate of Grain
Grain Moisture Content
Yield Monitor Data
dd.dddddd = latitude (degrees, + North and - South)
mm.mm = grain mass flow (pounds per second)
ttttttttt = GPS time (seconds)
n = cycle period (seconds)
lll = distance traveled in cycle period (inches)
www = effective swath width (inches)
cc.c = moisture content (percent wet basis)
kk = status (bits 0 thru 4 - header down)
ppppp = pass number
ssssss = yield monitor serial number
Fnn:bbbbbbbb = field ID (number and name)
Lnn.bbbbbbbb = load ID (number and name)
ggggggggggg = grain type
sss = GPS status
ppp = point dilution of precision (PDOP)
aaaa = altitude (feet)
dd.dddddd = latitude (degrees, + North and - South)
yyy.y = grain yield (marketable bushels per acre)
cc.c = moisture content (percent wet basis)
ssssss = yield monitor serial number
Fnn:bbbbbbbb = field ID (number and name)
Lnn.bbbbbbbb = load ID (number and name)
ggggggggggg = grain type.
Sources of Yield Measurement Errors
Generating Yield Maps
Summary
References
New York.
. IN: F.J. Pierce and E.J. Sadler, eds. The State of
Site-Specific Management for Agriculture. ASA misc. publ., ASA, CSSA,
and SSSA, Madison, Wisconsin.
