Agriculture Reference
In-Depth Information
Others can be installed on agricultural machines to conduct noncontact and on-the-
go measurements.
Optical sensors are a big category among crop sensing techniques. They pro-
vide measurements of spectral reflectance of canopies at different wavebands that
are used to calculate the normalized difference vegetation index (NDVI) value.
The NDVI values are often used to determine the nutrient level, leaf chlorophyll
content, leaf area index, etc. Three prominent commercial sensors in this category
include GreenSeeker™ (http://www.trimble.com/agriculture), the Crop Circle™
(http://hollandscientific.com), and CropSpec™ (http://topcon.com). They are all
based on real-time, on-the-go measurements of multiple optical sensors to deter-
mine crop growth status and make decisions on nitrogen applications. The primary
differences between these sensors are the wavelengths that each uses for sensing.
Skye Instruments Ltd (UK) provides a wide range of portable sensors and systems
for plant and crop monitoring including plant moisture potential, leaf area, and
root length. A French company, Force-A (http://www.force-a.eu), provides sensors
to measure polyphenol and chlorophyll concentration in leaves based on the plant's
intrinsic fluorescence characteristics.
Sudduth et al. (2000) designed an electromechanical sensor to count corn plants.
Cotton plant height was measured using mechanical fingers and infrared light beams
(Searcy and Beck, 2000). An infrared thermometer was used to measure canopy
temperature to control irrigation events (Evans et al., 2000). An online, real-time
spectrophotometer developed by Anom et al. (2000) was used to map plant water,
nutrient, disease, and salinity stresses. It was projected that development in gene
manipulation of crop plants may further enable differentiation between these stress
types (Stafford and Evans, 2000). Michels et al. (2000) designed an infrared plant-
temperature transducer to sense plant temperature changes caused by water stress.
Thermography is a new technology used to detect water stress in crop plants based
on canopy temperature (Ondimu and Murase, 2008). Traditional thermometer mea-
surements provide an average temperature reading over a single target area. Ther-
mal imaging systems can provide high-spatial-resolution, multipoint, temperature
measurements.
13.3.1.3 YieldSensor
Grain yields are measured using various types of yield sensors. The impact or mass
flow sensors measure the force of grain hitting an impact plate attached to a load cell.
The duration and magnitude of this force is well correlated with the mass flow of the
grain. The volumetric-flow sensor operates on a volumetric measurement principle.
Grain flows into a paddle wheel with a fixed volume. When the grain reaches a pre-
define threshold, a level sensor is activated, and the paddle wheel dumps the grain
into the combine grain tank. The total yield is estimated based on the number of
dumps and the volume of the paddle wheel. The conveyor belt load sensor measures
the weight of the grain passing through a specific location (with a load cell) on the
conveyor for a period of time. Optical sensors are becoming widely used because of
their noncontact measurements. The attenuation of light intensity through the grain
stream can be correlated to the amount of grain. Most major agricultural equip-
ment companies provide yield-mapping systems for their combine harvesters. Yield
