The combine automates yield monitoring by gathering data from various sensors, including speed, position, header height and width, mass-flow and moisture. Each of these sensors contributes an essential piece of data necessary to the production of an accurate yield map.

 

The first piece of information needed is the area harvested. Various machines solve this problem differently, but generally the yield monitor knows that the harvest has commenced by first verifying the separator is on and then if the header height is in the harvest position. This brings us to the first important adjustment. Different operators and varying harvest conditions require positioning the header higher or lower. The operator must inform the yield monitoring system when the header is at the harvesting height so it can determine if the machine is harvesting or making another maneuver (e.g. headland turn). The header position assigned to harvesting works in conjunction with the activated separator, like an on/off switch for the yield monitor. The value is usually set through the monitor itself and can be represented as a percent of height or angle measured between the header attachment point and the ground.

 

Now that the yield monitor knows that the operator is serious about harvesting (separator on, header down) the monitor must know the width being gathered into the combine. Surprisingly, most machines have not automated this process. Therefore, the operator must enter the number of rows being harvested or, in the case of a cutting platform, width of the header used (e.g. feet).

 

Harvest width combined with forward travel speed allows the combine to calculate area harvested per unit time, usually represented as acres per hour. For example, a 6-row (15ft) corn head, fully utilized, traveling at 5 miles/hour would result in an area productivity of about 9 acres/hour (5 MPH multiplied by 15ft and divided by 8.25 to convert the units). For those using GPS there is no need for speed calibration; however, those using a speed pickup or doppler-shift system need to calibrate their speed sensor. Once again, the procedure varies by machine, but in general it requires traveling a known, measured distance under field conditions (proper header attached, half-tank of grain). This distance is used in conjunction with the sensor’s output to correct its calibration.

With area covered precisely known, we just need to measure the amount of grain harvested for that area. This could be done by simply weighing the grain, but engineers have had difficulty coming up with cost-effective, on-board weighing systems. So this task is accomplished indirectly with a mass-flow sensor. Various mass-flow sensors have been tried throughout the past, but today two types are being utilized. The first type of sensor measures the height of the grain as each paddle of the clean grain elevator pass by. Using this height, the volume of grain is estimated and, in-conjunction with a density (mass of grain per volume) assumption, the weight of the grain is estimated. The density is calculated by adding the volumes up in the last calibration load you ran. So the weight you entered from the scale ticket divided by the cumulative volume from each paddle gives the sensor a density reading of weight per volume. With this calibration, as each paddle passes the sensor, the volume is measured and the weight is estimated using your last calibration data.

 

This can be accurate if the relationship between weight and volume are constant, but unfortunately, as in most biological products, nothing stays the same for too long. Changes in corn variety, moisture content and individual kernel density can lead to measurement error. It is recommended that a calibration load be entered every 2-3 weeks, or more often, if there are noticeable changes in crop condition.

 

Another, more common type of mass-flow sensor uses an impact-plate to measure the force of the grain as it is ejected from the clean grain elevator into the bubble-up auger. The idea is to add up the force over time to determine the accumulated weight. This, too, is an indirect measurement of weight requiring calibration to ensure proper function. Before considering calibration, it is prudent to inspect not only the sensor for wear but also the clean grain elevator paddles. Bent or broken paddles could result in grain falling before being thrown against the impact plate. Also, worn paddles may not cause the grain to follow the top of the conveyer to the impact plate, resulting in low readings. Some machines allow for an adjustment of paddle height at the top of the clean grain elevator to give the operator an opportunity to fine-tune how the grain impacts the sensor.

 

The impact-plate sensors usually use a non-linear relationship to relate sensor output to grain weight. What this means is that the sensor output isn’t directly proportional to grain weight. For example if the two were linearly related, then an output of 1 volt (V) would be 5 lbs of grain; 2V, 10 lbs; 3V, 15 lbs; etc. However, due to friction and grain falling off the paddles at high flow-rates, a non-linear calibration is necessary. So, in our previous example, 1V would be 3.5 lbs; 2V, 10lbs and 3V could be 25 lbs. What this means for the operator is that multiple calibration loads are necessary to ensure yields collected at very high and very low mass-flow rates are accurate.

 

Calibration load recommendations vary depending on manufacturer. Some may have the operator find a consistently yielding, level area of the field, harvesting at an average rate while others have the operator harvest at varying rates throughout the calibration period. Those using non-linear calibrations may require the operator to harvest more than one calibration load, each time varying the harvest rate. Most operators’ manuals provide step-by-step instructions. It is important to follow the instructions specified by your operator’s manual. This process may seem involved, but most machines allow the operator to continue harvesting until the calibration load weigh ticket returns so harvesting is not impeded by a calibration update.

 

If you already weigh all of the loads from the field, you still need to calibrate the mass-flow sensor as the yield map won’t accurately account for the highs and lows, even after being corrected with actual loads. Again, this is a result of the sensor’s non-linearity.

 

The final piece of the yield map data is moisture. Grain yields must be corrected for moisture, otherwise wetter, heavier grain will skew the yields higher while drier, lighter grain will appear to decrease yield. The moisture sensor’s calibration also needs periodic adjustment as conditions change. This process usually includes taking a few representative samples from the grain tank to the elevator for analysis; the values from the elevator are then used to update the combine’s calibration.

 

Although time investment may seem significant, calibrating your monitor is necessary to ensure accurate yield maps and subsequent management decisions. For more grain production related information please visit the University of Wisconsin – Extension Team Grain website at: http://www.uwex.edu/ces/ag/teams/grains/